Kyushu University Academic Staff Educational and Research Activities Database
List of Papers
Kaneaki Tsuzaki Last modified date:2019.10.18

Professor / Material Strength / Department of Mechanical Engineering / Faculty of Engineering


Papers
1. Motomichi Koyama, Natsuki Terao, Kaneaki Tsuzaki, Revisiting the effects of hydrogen on deformation-induced γ-ε martensitic transformation, Materials Letters, 10.1016/j.matlet.2019.04.093, 249, 197-200, 2019.08, Hydrogen effects on deformation-induced γ-ε martensitic transformation were revisited in terms of morphology, number density, and fraction of ε-martensite in strained austenitic steel. Hydrogen charging suppresses the stress-assisted nucleation of the martensite in stress criterion, which is consistent with a previous finding that hydrogen suppresses thermally induced γ-ε martensitic transformation. At an identical strain (e.g., 4%), hydrogen charging decreases the average thickness of ε-martensite plates, but increases their number density. As a result, hydrogen charging promotes deformation-induced γ-ε martensitic transformation when compared at the identical strain. These facts provide the following two conclusions: (1) hydrogen increases the required elastic strain energy for martensite nucleation and (2) hydrogen promotes a dislocation-motion-related nucleation process..
2. Y. Kuroki, S. Kawano, S. Iikubo, H. Ohtani, M. Koyama, Kaneaki Tsuzaki, First-Principles Study of Chemical Driving Force for Face Centered Cubic to Hexagonal Close Packed Martensitic Transformation in Hydrogen-Charged Iron, Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science, 10.1007/s11661-019-05237-6, 50, 7, 3019-3023, 2019.07, This study uses first-principles calculations to investigate the effect of hydrogen on the chemical driving force of the transformation of iron from the face centered cubic (FCC) to hexagonal close packed (HCP) phase. The minimum energy path from FCC to HCP phases shows that FCC becomes stable with increasing hydrogen content. Furthermore, the energy difference between the FCC and HCP phases is observed to be smaller in Fe2H than in Fe throughout the temperature region. These results clearly explain the observed anomalous suppression of the martensitic transformation in the hydrogen-charged steel..
3. A.Forsström, S.Bossuyt, Y.Yagodzinskyy, K.Tsuzaki, H.Hänninen, Strain localization in copper canister FSW welds for spent nuclear fuel disposal, Journal of Nuclear Materials , 523, 347-359, 2019.07.
4. Evolution of Quasi-Brittle Hydrogen-Assisted Damages in a Dual-Phase Steel.
5. Motomichi Koyama, Cemal Cem Tasan, Kaneaki Tsuzaki, Overview of metastability and compositional complexity effects for hydrogen-resistant iron alloys:
Inverse austenite stability effects, Engineering Fracture Mechanics, 10.1016/j.engfracmech.2019.03.049, 214, 123-133, 2019.06, The main factors affecting resistance to hydrogen-assisted cracking are hydrogen diffusivity and local ductility. In this context, we note fcc (γ) to hcp (ε) martensitic transformation, instead of γ to bcc (ά) martensitic transformation. The γ-ε martensitic transformation decreases the local hydrogen diffusivity, which thereby can increase strength without critical deterioration of hydrogen embrittlement resistance. Furthermore, ε-martensite in a high-entropy alloy is extraordinary ductile. Consequently, the metastable high-entropy alloys showed lower fatigue crack growth rates under a hydrogen effect compared with those of conventional metastable austenitic steels such as type 304..
6. Akinobu Shibata, Masanori Enoki, Nahoko Saji, Hirotaka Tai, Motomichi Koyama, Hiroshi Ohtani, Nobuhiro Tsuji, Kaneaki Tsuzaki, Effect of Hydrogen on the Substructure of Lenticular Martensite in Fe-31Ni Alloy, Metallurgical and Materials Transactions A, 1-10, 2019.06.
7. Ibrahim Burkay Tuğluca, Motomichi Koyama, Yusaku Shimomura, Burak Bal, Demircan Canadinc, Eiji Akiyama, Kaneaki Tsuzaki, Lowering Strain Rate Simultaneously Enhances Carbon- and Hydrogen-Induced Mechanical Degradation in an Fe-33Mn-1.1C Steel, Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science, 10.1007/s11661-018-5080-7, 50, 3, 1137-1141, 2019.03, We investigated the strain rate dependency of the hydrogen-induced mechanical degradation of Fe-33Mn-1.1C steel at 303 K within the strain rate range of 10
−2
to 10
−5
s
−1
. In the presence of hydrogen, lowering the strain rate monotonically decreased the work hardening rate, elongation, and tensile strength and increased the yield strength. Lowering the strain rate simultaneously enhanced the plasticity-related effects of hydrogen and carbon, leading to the observed degradation of the ductility..
8. Ryohei Kakimoto, Motomichi Koyama, Kaneaki Tsuzaki, EBSD and ECCI based assessments of inhomogeneous plastic strain evolution coupled with digital image correlation, Tetsu-To-Hagane/Journal of the Iron and Steel Institute of Japan, 10.2355/tetsutohagane.TETSU-2018-072, 105, 2, 100-108, 2019.02, Synopsis: We measured local grain orientation gradient and dislocation density by electron backscatter diffraction (EBSD) measurement and electron channeling contrast imaging (ECCI) to obtain strain maps near a stress concentration source in a pure nickel as a FCC model specimen. In particular, we obtained relationship among grain orientation spread (GOS), dislocation density, and equivalent plastic strain on the specimen surface, which were obtained by EBSD, ECCI, and digital image correlation (DIC), respectively. After obtaining GOS-strain and dislocation density-strain relations, the strain distribution in the specimen interior was also determined by measuring the GOS and dislocation density. Both of the GOS and dislocation density showed a linear correlation, and the dislocation density-strain relation showed a relatively small deviation..
9. Shunsuke Mizumachi, Motomichi Koyama, Yoshihiro Fukushima, Kaneaki Tsuzaki, Growth behavior of a mechanically long fatigue crack in an FeCrNiMnCo high entropy alloy:
A comparison with an austenitic stainless steel, Tetsu-To-Hagane/Journal of the Iron and Steel Institute of Japan, 10.2355/tetsutohagane.TETSU-2018-073, 105, 2, 93-99, 2019.02, Synopsis: Fatigue crack growth characteristics of an Fe20Cr20Ni20Mn20Co high entropy alloy (HEA) were investigated by ΔK increasing compact tension test in comparison with a SUS316L. Fatigue crack growth rate of the HEA was lower than that of the SUS316L. The predominant crack growth path was grain interior for both alloys. A difference was found to be in the crack roughness, namely, the fatigue crack growth path of the HEA was more distinctly deflected compared with that of the SUS316L. This fact indicates that roughness-induced crack closure is a key factor decreasing the crack growth rate of the HEA. Another key is a non-crystallographic transgranular crack growth mechanism. The SUS316L shows crack growth via crack blunting/re-sharpening, while the HEA shows transgranular crack growth associated with dislocation substructure alignment..
10. Kishan Habib, Motomichi Koyama, Eisaku Sakurada, Nobuyuki Yoshimura, Tatsuo Yokoi, Kohsaku Ushioda, Kaneaki Tsuzaki, Hiroshi Noguchi, Fatigue behavior in an Fe-N binary ferritic steel
Similarity and difference between carbon and nitrogen, isij international, 10.2355/isijinternational.ISIJINT-2018-535, 59, 186-191, 2019.01, Fatigue crack initiation and propagation behavior of a water-quenched fully ferritic nitrogen steel were investigated by means of tension-compression fatigue tests. The Fe-0.011 mass% N steel showed no serrated flow associated with dynamic strain aging, and showed a fatigue limit of 150 MPa alongside a non-propagating fatigue crack. The major mode of crack initiation was at the grain boundaries, and the cracks propagated along the grain boundaries and interiors at and above the fatigue limit. The Fe-N steel did not exhibit a significant level of coaxing effect. The results were compared with our previous findings in Fe-0.006 and 0.017C steels, and the similarity and difference were discussed..
11. Motomichi Koyama, Kenshiro Ichii, Kaneaki Tsuzaki, Grain refinement effect on hydrogen embrittlement resistance of an equiatomic CoCrFeMnNi high-entropy alloy, International Journal of Hydrogen Energy, 10.1016/j.ijhydene.2019.04.280, 44, 31, 17163-17167, 2019.01, We investigated the grain refinement effect on the resistance to hydrogen embrittlement in an equiatomic CoCrFeMnNi high-entropy alloy. In a 100 MPa hydrogen gas pre-charging condition, grain refinement to 1.9 μm improved the hydrogen embrittlement resistance in terms of the strength-elongation balance. In particular, the tensile strength of the grain refined high-entropy alloy did not decrease by hydrogen uptake. However, when the σ phase formed at a relatively low temperature, both the strength and elongation decreased due to hydrogen charging..
12. X. Lin, Motomichi Koyama, S. Gao, N. Tsuji, Kaneaki Tsuzaki, Hiroshi Noguchi, Resistance to mechanically small fatigue crack growth in ultrafine grained interstitial-free steel fabricated by accumulative roll-bonding, International Journal of Fatigue, 10.1016/j.ijfatigue.2018.09.002, 118, 117-125, 2019.01, Effects of ultrafine grain refinement on fatigue crack growth were investigated using an interstitial-free (IF) steel with a grain size of 590 nm produced by accumulative roll bonding. The fatigue properties and associated microstructures were characterized by fully reversed bending fatigue tests, replica method coupled with optical microscopy, and electron backscattering diffraction measurements near the fracture surfaces. Compared with a coarse-grained IF steel tested at the same stress amplitude, the fatigue strength of ultrafine-grained steel was higher, which was attributed to an increase in hardness. Compared with the coarse-grained steel at the same ratio of stress amplitude to hardness, the crack growth rates in the ultrafine grained steel were higher in the short-crack regime because of the smaller crack roughness and perhaps the difference in the strain gradient at the crack tip..
13. T. Kumamoto, M. Koyama, K. Sato, Kaneaki Tsuzaki, Strain-rate sensitivity of hydrogen-assisted damage evolution and failure in dual-phase steel:
From vacancy to micrometer-scale void growth, Engineering Fracture Mechanics, 10.1016/j.engfracmech.2019.106513, 2019.01, Micro-damage quantification and associated microstructure characterization in a ferrite/martensite dual-phase (DP) steel were performed after tensile tests with different strain rates of 10−2 and 10−4 s−1 in order to understand the strain rate sensitivity of damage initiation resistance and damage arrestability. The results indicated the following two conclusions: (1) The damage nucleation rate at martensite increases with decreasing strain rate, and (2) lowering strain rate reduced the critical strain for fracture by shortening the damage arrest regime. However, the failure mode was ductile manner at both of the strain rates..
14. Motomichi Koyama, Kenji Hirata, Yuji Abe, Akihiro Mitsuda, Satoshi Iikubo, Kaneaki Tsuzaki, An unconventional hydrogen effect that suppresses thermal formation of the hcp phase in fcc steels, Scientific reports, 10.1038/s41598-018-34542-0, 8, 1, 2018.12, Iron and steels are extensively used as structural materials, and have three primary phase structures: Body-centered cubic (bcc), face-centered cubic (fcc), and hexagonal closed-packed (hcp). Controlling phase stabilities, especially by the use of interstitials, is a universal method that provides a diverse variety of functional and mechanical properties in steels. In this context, hydrogen, which can act as an interstitial species in steels, has been recognized to promote phase transformation from fcc to hcp. However, we here report a dramatic effect of interstitial hydrogen that suppresses this hcp phase transformation. More specifically, the fraction of hcp phase that forms during cooling decreases with increasing diffusible hydrogen content. This new finding opens new venues for thermodynamics-based microstructure design and for development of robust, strong, and ductile steels in hydrogen-related infrastructures..
15. K. Hirata, S. Iikubo, Motomichi Koyama, Kaneaki Tsuzaki, H. Ohtani, First-Principles Study on Hydrogen Diffusivity in BCC, FCC, and HCP Iron, Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science, 10.1007/s11661-018-4815-9, 49, 10, 5015-5022, 2018.10, The hydrogen diffusion behavior in BCC, FCC, and HCP iron has been investigated by means of first-principles calculations. Diffusion coefficients were estimated quantitatively from the migration energy derived by the Nudged elastic band method, and phonon calculations including the vibrations of all atoms at every stable and metastable site. Our calculations on the BCC structure show good agreement with those in the previous report. In the FCC structure as well, the calculated diffusion coefficients are in good agreement with experimental data. Our results suggest that the consideration of the antiferromagnetic state in FCC is important for the reproduction of experimental results. For the HCP structure, although there was a lack of systematic experimental results, our calculations predict that the diffusion coefficient is smaller than that in the case of the FCC sample. In the HCP lattice, there are two diffusion paths: one parallel to the c-axis and the other in the c-plane. The direction and the diffusion coefficient can be controlled by the tuning of c/a, which is the ratio of the lattice constants..
16. Motomichi Koyama, Takahiro Kaneko, Takahiro Sawaguchi, Kaneaki Tsuzaki, Microstructural damage evolution and arrest in binary Fe–high-Mn alloys with different deformation temperatures, International Journal of Fracture, 10.1007/s10704-018-0307-6, 213, 2, 193-206, 2018.10, We investigated the damage evolution behaviors of binary Fe–28–40Mn alloys (mass%) from 93 to 393 K by tensile testing. The underlying mechanisms of the microstructure-dependent damage evolution behavior were uncovered by damage quantification coupled with in situ strain mapping and post-mortem microstructure characterization. The damage growth behaviors could be classified into three types. In type I, the Fe–28Mn alloy at 93 K showed premature fracture associated with ductile damage initiation and subsequent quasi-cleavage damage growth associated with the ε -martensitic transformation. In type II, the Fe–28Mn alloy at 293 K and the Fe–32Mn alloy at 93 K showed delayed damage growth but did not stop growing. In type III, when the stacking fault energy was >19 mJ/m 2, the damage was strongly arrested until final ductile failure..
17. Motomichi Koyama, Daisuke Yamasaki, Kaneaki Tsuzaki, Surface orientation dependence of hydrogen flux in lenticular martensite of an Fe-Ni-C alloy clarified through in situ silver decoration technique, Materials Letters, 10.1016/j.matlet.2018.06.022, 228, 273-276, 2018.10, An in situ silver decoration technique was applied to investigate the effect of microstructure on hydrogen flux in an austenite/α′-martensite dual-phase Fe-32Ni-0.2C alloy. Using time-resolved hydrogen mapping, the surface orientation of the body-centered cubic lenticular martensite was found to have a significant effect on hydrogen flux. The hydrogen flux was particularly high at the near-〈0 0 1〉 while it was the lowest at the near-〈1 1 1〉 surface. This dependence of hydrogen flux on the surface orientation is attributed to hydrogen trapping at dislocations in the martensite. Particularly, 〈1 1 1〉 twinning-shear in the martensite enhances hydrogen trapping at dislocations, suppressing hydrogen diffusion in the shortest path of grains orientated to 〈1 1 1〉..
18. Motomichi Koyama, Yuji Abe, Kaneaki Tsuzaki, Split and shift of -martensite peak in an X-ray diffraction profile during hydrogen desorption
A geometric effect of atomic sequence, isij international, 10.2355/isijinternational.ISIJINT-2018-260, 58, 9, 1745-1747, 2018.09, Cryogenic X-ray diffraction measurements demonstrated a split of the -martensite peak at 193 K in a hydrogen-charged austenitic steel. Only the higher angle peak remained after aging at room temperature. This phenomenon can be interpreted by a change in the interstitial hydrogen position. Particularly, the motion of the leading partial involved in -martensitic transformation can move interstitial hydrogen from a tetrahedron to an octahedron site, expanding the lattice. Subsequently, the hydrogen can move back to the tetrahedron site, which relatively shrinks the lattice. The two different hydrogen positions cause the peak to split..
19. Z. Zhang, Motomichi Koyama, M. M. Wang, Kaneaki Tsuzaki, C. C. Tasan, Hiroshi Noguchi, Microstructural mechanisms of fatigue crack non-propagation in TRIP-maraging steels, International Journal of Fatigue, 10.1016/j.ijfatigue.2018.04.013, 113, 126-136, 2018.08, In contrast to conventional martensitic steels, transformation-induced plasticity (TRIP)-maraging steels exhibit exceptional high ductility without sacrificing strength and excellent fatigue property owing to the retained austenite/maraging martensite laminated structure. In this study, TRIP-maraging steel (Fe-9Mn-3Ni-1.4Al-0.01C, wt.%) with fine grained austenite was used to investigate the mechanism of high cycle fatigue resistance. Our analyses revealed that soft austenite region acts as a preferential crack propagation path, but the plastic deformation during crack opening involves martensitic transformation, resisting subsequent crack growth via transformation-induced local hardening or crack closure. Moreover, crack growth along the laminates and across the block boundary forms a zigzag crack path, which would act as roughness-induced crack closure. The combined effect of these factors plays an important role in resisting fatigue crack growth at high cycle fatigue..
20. Yuri Nishikura, Motomichi Koyama, Yusuke Yamamura, Takuro Ogawa, Kaneaki Tsuzaki, Hiroshi Noguchi, Non-propagating fatigue cracks in austenitic steels with a micro-notch
Effects of dynamic strain aging, martensitic transformation, and microstructural hardness heterogeneity, International Journal of Fatigue, 10.1016/j.ijfatigue.2018.04.027, 113, 359-366, 2018.08, The non-propagation limit of a microstructurally small fatigue crack was investigated with respect to dynamic strain aging (DSA), martensitic transformation, and microstructural hardness heterogeneity. In this study, we selected four model alloys: Fe-19Cr-8Ni-0.05C, Fe-19Cr-8Ni-0.14C, Fe-23Mn-0.5C, and as-hot-rolled Fe-30Mn-3Si-3Al steels. Transformation-induced cyclic hardening results in the most significant improvement of the non-propagation limit, i.e., in the case of the Fe-19Cr-8Ni-0.05C steel. Within different contexts, DSA, transformation-induced crack closure, and hardness-heterogeneity-enhanced plasticity-induced crack closure could also realize superior non-propagation limits. The effects of DSA and hardness heterogeneity can be combined with the effects of transformation, which is expected to create a new venue of material design and selection in terms of the crack non-propagation limit..
21. B. Bal, Motomichi Koyama, D. Canadinc, G. Gerstein, H. J. Maier, Kaneaki Tsuzaki, On the Utility of Crystal Plasticity Modeling to Uncover the Individual Roles of Microdeformation Mechanisms on the Work Hardening Response of Fe-23Mn-0.5C TWIP Steel in the Presence of Hydrogen, Journal of Engineering Materials and Technology, Transactions of the ASME, 10.1115/1.4038801, 140, 3, 2018.07, This paper presents a combined experimental and theoretical analysis focusing on the individual roles of microdeformation mechanisms that are simultaneously active during the deformation of twinning-induced plasticity (TWIP) steels in the presence of hydrogen. Deformation responses of hydrogen-free and hydrogen-charged TWIP steels were examined with the aid of thorough electron microscopy. Specifically, hydrogen charging promoted twinning over slip-twin interactions and reduced ductility. Based on the experimental findings, a mechanism-based microscale fracture model was proposed, and incorporated into a visco-plastic self-consistent (VPSC) model to account for the stress-strain response in the presence of hydrogen. In addition, slip-twin and slip-grain boundary interactions in TWIP steels were also incorporated into VPSC, in order to capture the deformation response of the material in the presence of hydrogen. The simulation results not only verify the success of the proposed hydrogen embrittlement (HE) mechanism for TWIP steels, but also open a venue for the utility of these superior materials in the presence of hydrogen..
22. Kenshiro Ichii, Motomichi Koyama, Cemal Cem Tasan, Kaneaki Tsuzaki, Comparative study of hydrogen embrittlement in stable and metastable high-entropy alloys, Scripta Materialia, 10.1016/j.scriptamat.2018.03.003, 150, 74-77, 2018.06, We investigated the effects of hydrogen on the mechanical response and embrittlement behavior of Fe20Mn20Ni20Cr20Co and Fe30Mn10Cr10Co (at.%) alloys precharged with 100 MPa hydrogen gas. These alloys can be classified as stable and metastable austenite-based high entropy alloys (HEAs), respectively. We carried out tensile tests at initial strain rates of 10−4 and 10−2 s−1 at ambient temperature. Both HEAs revealed hydrogen embrittlement associated with localized plasticity-assisted intergranular crack initiation. It is important to note that hydrogen-assisted cracking of the metastable HEA occurred via localized plasticity for both the austenite and ε-martensite phases – an unusual observation for the latter..
23. W. T. Geng, Vei Wang, Jin Xu Li, Nobuyuki Ishikawa, Hajime Kimizuka, Kaneaki Tsuzaki, Shigenobu Ogata, Hydrogen trapping in carbon supersaturated α‑iron and its decohesion effect in martensitic steel, Scripta Materialia, 10.1016/j.scriptamat.2018.02.025, 149, 79-83, 2018.05, Our first-principles calculations demonstrate that hydrogen is more stable in carbon supersaturated martensite than in α‑iron, due to the carbon-induced tetragonality in martensite lattice. The trapped hydrogen leads to remarkable decohesion between (110) planes both inside the martensite and along the martensite/ferrite interface, with the former being more significant than the latter. This decohesion can explain recent precise observations that in martensite/ferrite dual-phase steels the hydrogen-promoted crack was initiated in the martensite region and that in lath martensite steel it propagated not on lath boundaries but showed quasi-cleavage feature along (110) planes at very high hydrogen concentration..
24. Motomichi Koyama, Takahiro Sawaguchi, Kaneaki Tsuzaki, Overview of dynamic strain aging and associated phenomena in Fe-Mn-C austenitic steels, Tetsu-To-Hagane/Journal of the Iron and Steel Institute of Japan, 10.2355/tetsutohagane.TETSU-2017-089, 104, 4, 187-200, 2018.04, This paper presents an overview of the recent works on dynamic strain aging (DSA) of Fe-Mn-C austenitic steels including Hadfield and twinning-induced plasticity (TWIP) steels. First, a model of the DSA mechanism and its controlling factors are briefly explained in terms of Mn-C coupling and dislocation separation. Then, we introduce the effects of DSA on mechanical properties such as work hardening capability, uniform elongation, post-uniform elongation, and fatigue strength. Specifically, we note the pinning effect on extended dislocation for the work hardening, the Poretvin-Le Chatelier banding effect on damage evolution for the elongation, and the crack tip hardening/softening effect on crack resistance for the fatigue strength. We believe that this overview will help in designing advanced high-strength steels with superior ductility and fatigue resistance..
25. Motomichi Koyama, Yusuke Yamamura, Takahiro Sawaguchi, Kaneaki Tsuzaki, Hiroshi Noguchi, Microstructural hardness heterogeneity triggers fatigue crack non-propagation in as-hot-rolled Fe-30Mn-3Si-3Al twinning-induced plasticity steel, International Journal of Fatigue, 10.1016/j.ijfatigue.2017.11.009, 108, 18-24, 2018.03, We investigated the fatigue properties and associated crack growth behaviors of as-hot-rolled and solution-treated Fe-30Mn-3Si-3Al twinning-induced plasticity (TWIP) steels. The as-hot-rolled TWIP steel showed a higher fatigue strength than that of the solution-treated steel. Moreover, the as-hot-rolled steel showed a non-propagating fatigue crack at the fatigue limit, while the solution-treated steel did not. The high resistance to fatigue crack growth cannot only be attributed to an increase in hardness, but is also associated with heterogeneous hardness distribution. The effect of hardness heterogeneity was numerically supported by Dugdale model-based simulations..
26. Ibrahim Burkay Tuğluca, Motomichi Koyama, Burak Bal, Demircan Canadinc, Eiji Akiyama, Kaneaki Tsuzaki, High-concentration carbon assists plasticity-driven hydrogen embrittlement in a Fe-high Mn steel with a relatively high stacking fault energy, Materials Science & Engineering A: Structural Materials: Properties, Microstructure and Processing, 10.1016/j.msea.2018.01.087, 717, 78-84, 2018.02, We investigated the effects of electrochemical hydrogen charging on the mechanical properties of a Fe-33Mn-1.1C austenitic steel with high carbon concentration and relatively high stacking fault energy. Hydrogen pre-charging increased the yield strength and degraded the elongation and work-hardening capability. The increase in yield strength is a result of the solution hardening of hydrogen. A reduction in the cross-sectional area by subcrack formation is the primary factor causing reduction in work-hardening ability. Fracture modes were detected to be both intergranular and transgranular regionally. Neither intergranular nor transgranular cracking modes are related to deformation twinning or simple decohesion in contrast to conventional Fe-Mn-C twinning-induced plasticity steels. The hydrogen-assisted crack initiation and subsequent propagation are attributed to plasticity-dominated mechanisms associated with strain localization. The occurrence of dynamic strain aging by the high carbon content and ease of cross slip owing to the high stacking fault energy can cause strain/damage localization, which assists hydrogen embrittlement associated with the hydrogen-enhanced localized plasticity mechanism..
27. Motomichi Koyama, Y. Tanaka, Kaneaki Tsuzaki, Micrographic Digital Image Correlation Coupled with Microlithography
Case Study of Strain Localization and Subsequent Cracking at an FIB Notch Tip in a Laminated Ti-6Al-4V Alloy, Experimental Mechanics, 10.1007/s11340-017-0336-5, 58, 2, 381-386, 2018.02, This study presents a microlithography-based approach to increase the spatial resolution of strain mapping by micrographic digital image correlation. A micro-mesh with a lattice size of 500 nm was added on the surface of a Ti-6Al-4V alloy specimen with a coarse lath size of 1.1 μm. Although the micro-mesh pattern was not random, a combination of the laminated microstructure and the micro-mesh enabled sub-micrometer strain mapping through digital image correlation even for coarse lath larger than 1 μm. Specifically, the strain mapping technique used in this study was applied to characterize the strain component and distribution near an artificial sharp micro-stress concentration site introduced by a focused ion beam. The strain characterization under tensile deformation clarified that cracking occurred via shear strain localization at the micro-stress concentration site, indicating that accumulation of damage (such as vacancy or dislocation) plays an important role in the cracking mechanism of the Ti-6Al-4V alloy..
28. M. Koyama, Y. Tanaka, K. Tsuzaki, Micrographic Digital Image Correlation Coupled with Microlithography: Case Study of Strain Localization and Subsequent Cracking at an FIB Notch Tip in a Laminated Ti-6Al-4V Alloy, Experimental Mechanics, DOI 10.1007/s11340-017-0336-5, 58, 2, 381-386, 2018.02.
29. Takuro Ogawa, Motomichi Koyama, Yuri Nishikura, Kaneaki Tsuzaki, Hiroshi Noguchi, Fatigue behavior of Fe-Cr-Ni-based metastable austenitic steels with an identical tensile strength and different solute carbon contents, Tetsu-To-Hagane/Journal of the Iron and Steel Institute of Japan, 10.2355/tetsutohagane.TETSU-2017-058, 104, 2, 88-97, 2018.01, Fatigue properties of Fe-19Cr-8Ni-0.05C and Fe-19Cr-8Ni-0.14C steels were investigated using a rotating bending fatigue test machine. Fatigue limit of both of the steels were dominated by critical stress amplitude for crack initiation. Because of the austenite stability, the fatigue limit of the Fe-19Cr-8Ni-0.05C steel was 200 MPa higher than that of the Fe-19Cr-8Ni-0.14C steel. Although occurrence of dynamic strain aging in the Fe-19Cr-8Ni-0.14C was expected to improve fatigue limit, the effect did not appear due to the remarkable increase of phase stability that deteriorates positive effects of transformation-induced plasticity and transformation-induced crack closure..
30. Nao Uehata, Motomichi Koyama, Shusaku Takagi, Kaneaki Tsuzaki, Optical microscopy-Based damage quantification
an example of cryogenic deformation of a dual-Phase steel, ISIJ International, 10.2355/isijinternational.ISIJINT-2017-468, 58, 1, 179-185, 2018.01, We evaluated the availability of an optical-microscopy-based damage quantification method in a ferrite/ martensite dual-phase steel, and interpreted the obtained results toward screening damage evolution behavior under various test conditions. In this study, we employed this method for tensile deformation at 20, −100, and −180°C to analyze the temperature dependence of damage evolution in cryogenic regime as a case study. The damage evolution behavior was classified into regimes of damage nucleation, damage arrest, and damage growth to fracture, irrespective of the deformation temperature in a cryogenic temperature range. Coupled with some high-resolution observations, the damage nucleation and damage arrest sites were identified to be martensite and ferrite, which are common regardless of the deformation temperatures. This indicates that ferrite acted as a damage arrest site even at −180°C. However, a critical strain for damage growth to fracture decreased drastically by decreasing the temperature to −180°C. The distinct reduction in the critical strain is attributed to the transition of ferrite cracking mode from ductile to brittle mechanisms..
31. Yongjie Zhang, Kunio Shinbo, Takahito Ohmura, Takuya Suzuki, Kaneaki Tsuzaki, Goro Miyamoto, Tadashi Furuhara, Randomization of ferrite/austenite orientation relationship and resultant hardness increment by nitrogen addition in vanadium-microalloyed low carbon steels strengthened by interphase precipitation, ISIJ International, 10.2355/isijinternational.ISIJINT-2017-537, 58, 3, 542-550, 2018.01, Interphase precipitation of nano-sized alloy carbides is recently used to strengthen low carbon steels for its excellent contributions to strength and formability. The effects of nitrogen addition on the hardness of vanadium-microalloyed low carbon steels were investigated by considering both the dispersion of interphase precipitation and the ferrite/austenite crystallography. Three-dimensional atom probe analysis reveals that interphase precipitation of vanadium carbide is hardly affected by increasing the nitrogen content, although the nanohardness of ferrite is slightly increased. Another important factor determining the overall hardness of ferrite is found to be the ferrite/austenite crystallography. At lower transformation temperature, nitrogen addition reduces the amount of Widmanstatten ferrite and bainite, which are formed in absence of interphase precipitation. Instead, relatively harder allotriomorphic and idiomorphic grain boundary ferrite without Kurdjumov-Sachs orientation relationship against austenite are formed extensively..
32. A Belyakov, M Odnobokova, A Kipelova, K Tsuzaki, R Kaibyshev, Microstructural Evolution and Strengthening of Stainless Steels During Cold Rolling, Frontiers in Materials Processing, Applications, Research and Technology, https://doi.org/10.1007/978-981-10-4819-7_29, 341-347, 2018.01.
33. Motomichi Koyama, Yusaku Shimomura, Aya Chiba, Eiji Akiyama, Kaneaki Tsuzaki, Room-temperature blue brittleness of Fe-Mn-C austenitic steels, Scripta Materialia, 10.1016/j.scriptamat.2017.07.017, 141, 20-23, 2017.12, Fe-33Mn-xC (x = 0, 0.3, 0.6, 0.8, and 1.1 mass%) fully austenitic steels showed ductility degradation owing to dynamic strain aging (DSA). The elongation increased with increasing carbon concentration at a strain rate of 10− 2 s− 1. However, in the steels with carbon contents of 0.6%, 0.8%, and 1.1%, the elongation decreased with increasing carbon concentration at a strain rate of 10− 5 s− 1 where the DAS effect is distinct. Although all specimens showed ductile fracture with the formation of dimples, the work hardening-true stress relation of the Fe-33Mn-1.1C steel demonstrated fracture before satisfying Considère's criterion even at high strain rates..
34. Kentaro Wada, Adeyinka Abass, Saburo Okazaki, Yoshihiro Fukushima, Hisao Matsunaga, Kaneaki Tsuzaki, Fatigue Crack Threshold of Bearing Steel at a Very Low Stress Ratio, Procedia Structural Integrity, https://doi.org/10.1016/j.prostr.2017.11.104, 7, 391-398, 2017.12.
35. Motomichi Koyama, Daisuke Yamasaki, Kaneaki Tsuzaki, Reply to comments on the paper “In situ observations of silver-decoration evolution under hydrogen permeation
Effects of grain boundary misorientation on hydrogen flux in pure iron” by Gavriljuk and Teus, Scripta Materialia, 10.1016/j.scriptamat.2017.07.019, 140, 91-92, 2017.11, As a response to a comment by Gavriljuk and Teus, we discuss the effects of hydrogen-charging-induced crack formation and plastic deformation on a high hydrogen flux along the grain boundaries in pure iron. Because no cracks were observed in the microstructure of the pure iron specimen used in our study, the effects of crack formation could be ruled out. In contrast, dislocation localization near grain boundaries was observed, possibly due to a hydrogen concentration gradient; therefore, the presence of dislocations may assist hydrogen segregation and cause the high hydrogen flux..
36. Yun Byum Ju, Motomichi Koyama, Takahiro Sawaguchi, Kaneaki Tsuzaki, Hiroshi Noguchi, Effects of ε-martensitic transformation on crack tip deformation, plastic damage accumulation, and slip plane cracking associated with low-cycle fatigue crack growth, International Journal of Fatigue, 10.1016/j.ijfatigue.2017.06.040, 103, 533-545, 2017.10, Fatigue crack propagation behavior and associated plastic strain evolution in the vicinity of crack planes were investigated at different crack lengths for Fe-30Mn-6Al, Fe-30Mn-4Si-2Al, and Fe-30Mn-6Si alloys. In particular, fractographic analyses and electron backscatter diffraction measurements underneath the fracture surfaces were carried out. It was found that austenite of the Fe-30Mn-6Al alloy was completely stable at ambient temperature, and the Fe-30Mn-6Si and Fe-30Mn-4Si-2Al alloys showed deformation-induced ε-martensitic transformation. Both the Fe-30Mn-4Si-2Al and Fe-30Mn-6Si alloys showed γ/ε interface cracking. However, ductile cracking was observed in the former, while the latter showed brittle-like cracking. Additionally, both the Fe-30Mn-4Si-2Al and Fe-30Mn-6Al alloys showed ductile fatigue striation when the cracks became long, but the critical crack length to induce the striations in the Fe-30Mn-4Si-2Al alloy was longer than that in the Fe-30Mn-6Al. In contrast, the Fe-30Mn-6Si alloy did not show striation, not even just before failure. These observations are all related to ε-martensite transformation. In terms of the crack tip deformation, the key roles of ε-martensitic transformation are (1) brittle-like cracking along the γ/ε interface, (2) inhibition of fatigue damage accumulation, and (3) geometrical constraint of ε-martensite crystallographic structure at a fatigue crack tip. When ε-martensite is ductile, such as in the case of the Fe-30Mn-4Si-2Al alloy, the brittle-like cracking does not occur. Because of the roles (2) and (3) mentioned above, the Fe-30Mn-4Si-2Al alloy showed the lowest fatigue crack growth compared to the other tested alloys. This paper presents the proposed ε-martensite-related crack growth mechanism in detail..
37. Motomichi Koyama, Takuro Ogawa, Dingshun Yan, Yuya Matsumoto, Cemal Cem Tasan, Kenichi Takai, Kaneaki Tsuzaki, Hydrogen desorption and cracking associated with martensitic transformation in Fe-Cr-Ni-Based austenitic steels with different carbon contents, International Journal of Hydrogen Energy, 10.1016/j.ijhydene.2017.08.209, 42, 42, 26423-26435, 2017.10, The hydrogen embrittlement behavior of Fe-19Cr-8Ni-0.05C and Fe-19Cr-8Ni-0.14C metastable austenitic steels was investigated using tensile tests under hydrogen-charging, cryogenic thermal desorption spectroscopy, and in situ deformation experiments. Coupled with post-mortem microstructure characterization, the cracking paths were clarified to be transgranular along {110}α and {100}α in the Fe-19Cr-8Ni-0.05C steel and {100}α in the Fe-19Cr-8Ni-0.14C steel. Intergranular cracking also occurred in the Fe-19Cr-8Ni-0.05C steel when α′-martensite thoroughly covered the grain boundaries. Occurrence of the transgranular and intergranular hydrogen-assisted cracking in the steels is assisted by (1) an increase in the hydrogen-affected zone associated with presence of thermally induced α′-martensite, and (2) an increase in the local mobility of hydrogen that occurs with the deformation-induced α′-martensitic transformation. Additionally, (3) the transgranular hydrogen-assisted cracking is assisted by the intersection of deformation bands driven by the maximum Schmid factor and the stress concentration at the crack tip..
38. Motomichi Koyama, Michael Rohwerder, Cemal Cem Tasan, Asif Bashir, Eiji Akiyama, Kenichi Takai, Dierk Raabe, Kaneaki Tsuzaki, Recent progress in microstructural hydrogen mapping in steels
quantification, kinetic analysis, and multi-scale characterisation, Materials Science and Technology, 10.1080/02670836.2017.1299276, 33, 13, 1481-1496, 2017.09, This paper gives an overview of recent progress in microstructure-specific hydrogen mapping techniques. The challenging nature of mapping hydrogen with high spatial resolution, i.e. at the scale of finest microstructural features, led to the development of various methodologies: thermal desorption spectrometry, silver decoration, the hydrogen microprint technique, secondary ion mass spectroscopy, atom probe tomography, neutron radiography, and the scanning Kelvin probe. These techniques have different characteristics regarding spatial and temporal resolution associated with microstructure-sensitive hydrogen detection. Employing these techniques in a site-specific manner together with other microstructure probing methods enables multi-scale, quantitative, three-dimensional, high spatial, and kinetic resolution hydrogen mapping, depending on the specific multi-probe approaches used. Here, we present a brief overview of the specific characteristics of each method and the progress resulting from their combined application to the field of hydrogen embrittlement. This paper is part of a thematic issue on Hydrogen in Metallic Alloys..
39. Naoki Fukumura, Bochuan Li, Motomichi Koyama, Tomohiro Suzuki, Shigeru HAMADA, Kaneaki Tsuzaki, Hiroshi Noguchi, Material property controlling non-propagating fatigue crack length of mechanically and physically short-crack based on Dugdale-model analysis, THEORETICAL AND APPLIED FRACTURE MECHANICS, 10.1016/j.tafmec.2017.04.012, 90, 193-202, 2017.08.
40. Takuro Ogawa, Motomichi Koyama, Cemal Cem Tasan, Kaneaki Tsuzaki, Hiroshi Noguchi, Effects of martensitic transformability and dynamic strain age hardenability on plasticity in metastable austenitic steels containing carbon, JOURNAL OF MATERIALS SCIENCE, 10.1007/s10853-017-1052-3, 52, 13, 7868-7882, 2017.07.
41. Z. Zhang, Motomichi Koyama, M. M. Wang, Kaneaki Tsuzaki, C. C. Tasan, Hiroshi Noguchi, Effects of lamella size and connectivity on fatigue crack resistance of TRIP-maraging steel, International Journal of Fatigue, 10.1016/j.ijfatigue.2017.03.040, 100, 176-186, 2017.07, The effect of austenitization time on the fatigue crack resistance of transformation-induced plasticity (TRIP)-maraging steel was investigated by observing the crack initiation site, propagation path and fracture surface. Our analyses show that austenitization for a longer time increases austenite/martensite lamella size and connectivity of austenite. Simultaneously, increasing lamella size leads to a reduction in austenite hardness; higher austenite connectivity accelerates crack propagation. In addition, remarkable roughness on the crack surface associated with the laminated structure was observed in both steels, which caused roughness-induced crack closure..
42. Qinghua Wang, Shien Ri, Hiroshi Tsuda, Motomichi Koyama, Kaneaki Tsuzaki, Two-dimensional Moire phase analysis for accurate strain distribution measurement and application in crack prediction, OPTICS EXPRESS, 10.1364/OE.25.013465, 25, 12, 13465-13480, 2017.06.
43. Motomichi Koyama, Keita Yamanouchi, Qinghua Wang, Shien Ri, Yoshihisa Tanaka, Yasuaki Hamano, Shigeto Yamasaki, Masatoshi Mitsuhara, Masataka Ohkubo, Hiroshi Noguchi, Kaneaki Tsuzaki, Multiscale in situ deformation experiments
A sequential process from strain localization to failure in a laminated Ti-6Al-4V alloy, Materials Characterization, 10.1016/j.matchar.2017.04.010, 128, 217-225, 2017.06, The microscopic factors causing tensile failure of an α/β laminated Ti-6Al-4V alloy were investigated through in situ scanning electron microscopy and sampling moiré at an ambient temperature. Specifically, multiscale in situ microscopic observations were conducted to extract the most crucial factor of the failure. Slip localization in the vicinity of an intergranular α-sheet was clarified to be the primary factor that causes failure of the Ti-6Al-4V alloy. In addition, no relationship between interfacial strain localization and macroscopic shear localization at 45 degrees against the tensile direction was observed..
44. Motomichi Koyama, Eiji Akiyama, Young-Kook Lee, Dierk Raabe, Kaneaki Tsuzaki, Overview of hydrogen embrittlement in high-Mn steels, INTERNATIONAL JOURNAL OF HYDROGEN ENERGY, 10.1016/j.ijhydene.2017.02.214, 42, 17, 12706-12723, 2017.04, Hydrogen and fuels derived from it will serve as the energy carriers of the future. The associated rapidly growing demand for hydrogen energy-related infrastructure materials has stimulated multiple engineering and scientific studies on the hydrogen embrittlement resistance of various groups of high performance alloys. Among these, high-Mn steels have received special attention owing to their excellent strength – ductility – cost relationship. However, hydrogen-induced delayed fracture has been reported to occur in deep-drawn cup specimens of some of these alloys. Driven by this challenge we present here an overview of the hydrogen embrittlement research carried out on high-Mn steels. The hydrogen embrittlement susceptibility of high-Mn steels is particularly sensitive to their chemical composition since the various alloying elements simultaneously affect the material's stacking fault energy, phase stability, hydrogen uptake behavior, surface oxide scales and interstitial diffusivity, all of which affect the hydrogen embrittlement susceptibility. Here, we discuss the contribution of each of these factors to the hydrogen embrittlement susceptibility of these steels and discuss pathways how certain embrittlement mechanisms can be hampered or even inhibited. Examples of positive effects of hydrogen on the tensile ductility are also introduced..
45. Motomichi Koyama, Zhao Zhang, Meimei Wang, Dirk Ponge, Dierk Raabe, Kaneaki Tsuzaki, Hiroshi Noguchi, Cemal Cem Tasan, Bone-like crack resistance in hierarchical metastable nanolaminate steels, SCIENCE, 10.1126/science.aal2766, 355, 6329, 2017.03, Fatigue failures create enormous risks for all engineered structures, as well as for human lives, motivating large safety factors in design and, thus, inefficient use of resources. Inspired by the excellent fracture toughness of bone, we explored the fatigue resistance in metastability-assisted multiphase steels. We show here that when steel microstructures are hierarchical and laminated, similar to the substructure of bone, superior crack resistance can be realized. Our results reveal that tuning the interface structure, distribution, and phase stability to simultaneously activate multiple micromechanisms that resist crack propagation is key for the observed leap in mechanical response. The exceptional properties enabled by this strategy provide guidance for all fatigue-resistant alloy design efforts..
46. Motomichi Koyama, Daisuke Yamasaki, Tatsuya Nagashima, Cemal Cem Tasan, Kaneaki Tsuzaki, In situ observations of silver-decoration evolution under hydrogen permeation: Effects of grain boundary misorientation on hydrogen flux in pure iron, SCRIPTA MATERIALIA, 10.1016/j.scriptamat.2016.10.027, 129, 48-51, 2017.03.
47. Nobuaki Sekido, Ohmura Takahito, Kaneaki Tsuzaki, Application of Radio Frequency Glow Discharge Sputtering for Nanoindentation Sample Preparation, Journal of Materials Engineering and Performance, Vol. 26, Issue 3, 1245-1250, 2017.03.
48. T. Nagashima, Motomichi Koyama, A. Bashir, M. Rohwerder, C. C. Tasan, E. Akiyama, D. Raabe, Kaneaki Tsuzaki, Interfacial hydrogen localization in austenite/martensite dual-phase steel visualized through optimized silver decoration and scanning Kelvin probe force microscopy, Materials and Corrosion , Vol. 68, No. 3, 306-310, 2017.03.
49. P. Kusakin, A. Kalinenko, Kaneaki Tsuzaki, A. Belyakov, R. Kaibyshev, Influence of cold forging and annealing on microstructure and mechanical properties of a high-Mn TWIP steel, KOVOVE MATERIALY-METALLIC MATERIALS, 10.4149/km_2017_3_161, 55, 3, 161-167, 2017.03.
50. Toshinobu Omura, Motomichi Koyama, Yasuaki Hamano, Kaneaki Tsuzaki, Hiroshi Noguchi, Generalized evaluation method for determining transition crack length for microstructurally small to microstructurally large fatigue crack growth: Experimental definition, facilitation, and validation, INTERNATIONAL JOURNAL OF FATIGUE, 10.1016/j.ijfatigue.2016.10.010, 95, 38-44, 2017.02.
51. Motomichi Koyama, Huichao Li, Yasuaki Hamano, Takahiro Sawaguchi, Kaneaki Tsuzaki, Hiroshi Noguchi, Mechanical-probabilistic evaluation of size effect of fatigue life using data obtained from single smooth specimen: An example using Fe-30Mn-4Si-2Al seismic damper alloy, Engineering Failure Analysis , Vol. 72, 34-47, 2017.02.
52. Motomichi Koyama, Yusuke Yamamura, Rinqing Che, Takahiro Sawaguchi, Kaneaki Tsuzaki, Hiroshi Noguchi, Comparative study on small fatigue crack propagation between Fe-30Mn-3Si-3Al and Fe-23Mn-0.5C twinning-induced plasticity steels: Aspects of non-propagation of small fatigue cracks, INTERNATIONAL JOURNAL OF FATIGUE, 10.1016/j.ijfatigue.2016.09.003, 94, 1-5, 2017.01.
53. Pavel Kusakin, Kaneaki Tsuzaki, Dmitri A. Molodov, Rustam Kaibyshev, Andrey Belyakov, Advanced Thermomechanical Processing for a High-Mn Austenitic Steel, METALLURGICAL AND MATERIALS TRANSACTIONS A-PHYSICAL METALLURGY AND MATERIALS SCIENCE, 10.1007/s11661-016-3794-y, 47A, 12, 5704-5708, 2016.12.
54. Burak Bal, Motomichi Koyama, G. Gerstein, H.J. Maier, Kaneaki Tsuzaki, Effect of strain rate on hydrogen embrittlement susceptibility of twinning-induced plasticity steel pre-charged with high-pressure hydrogen gas, INTERNATIONAL JOURNAL OF HYDROGEN ENERGY, 10.1016/j.ijhydene.2016.06.259, 41, 34, 15362-15372, 2016.09.
55. Motomichi Koyama, Yuji Abe, Kei Saito, Eiji Akiyama, Kenichi Takai, Kaneaki Tsuzaki, Martensitic transformation-induced hydrogen desorption characterized by utilizing cryogenic thermal desorption spectroscopy during cooling, SCRIPTA MATERIALIA, 10.1016/j.scriptamat.2016.05.012, 122, 50-53, 2016.09.
56. Ilya Nikulin, Takahiro Sawaguchi, Atsumichi Kushibe, Yasuhiko Inoue, Hiroaki Otsuka, Kaneaki Tsuzaki, Effect of strain amplitude on the low-cycle fatigue behavior of a new Fe–15Mn–10Cr–8Ni–4Si seismic damping alloy, International Journal of Fatigue, 88, 132-141, 2016.07.
57. Motomichi Koyama, Shota Okazaki, Takahiro Sawaguchi, Kaneaki Tsuzaki, Hydrogen Embrittlement Susceptibility of Fe-Mn Binary Alloys with High Mn Content: Effects of Stable and Metastable ε-Martensite, and Mn Concentration, METALLURGICAL AND MATERIALS TRANSACTIONS A, 47, 6, 2656-2673, 2016.06.
58. Yun-Byum Ju, Motomichi Koyama, Takahiro Sawaguchi, Kaneaki Tsuzaki, Hiroshi Noguchi, In situ microscopic observations of low-cycle fatigue-crack propagation in high-Mn austenitic alloys with deformation-induced epsilon-martensitic transformation, ACTA MATERIALIA, 10.1016/j.actamat.2016.04.042, 112, 326-336, 2016.06.
59. Iaroslava Shakhova, Andrey Belyakov, Zhanna Yanushkevich, Kaneaki Tsuzaki, Rustam Kaibyshev, On Strengthening of Austenitic Stainless Steel by Large Strain Cold Working, ISIJ International, Vol. 56, No. 7, 1289-1296, 2016.06.
60. Combined multi-scale analyses on strain/damage/microstructure in steel: Example of damage evolution associated with ε-martensitic transformation.
61. Xiaohua Min, Satoshi Emura, Xuejiao Chen, Xueyin Zhou, Kaneaki Tsuzaki, Koichi Tsuchiya, Deformation microstructural evolution and strain hardening of differently oriented grains in twinning-induced plasticity beta titanium alloy, MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, 10.1016/j.msea.2016.01.105, 659, 1-11, 2016.04.
62. Masahiro Kawajiri, Satoshi Emura, Xiaohua Min, Shigeo Yamamoto, Kazuyuki Sakuraya, Kaneaki Tsuzaki, Effect of additional boron amount on surface roughness after lathe turning in h-BN dispersed type 304 stainless steels , ISIJ International, Vol. 56 , No. 6, 1031-1037, 2016.04.
63. Kaneaki Tsuzaki, Koki Fukuda, Motomichi Koyama, Hisao Matsunaga, Hexagonal close-packedMartensite-related Fatigue Crack Growth under the
Influence of Hydrogen: Example of Fe–15Mn–10Cr–8Ni–4Si Austenitic Alloy, Scripta Materialia, 113, 6-9, 2016.03.
64. Ilya Nikulin, Takahiro Sawaguchi, Kazuyuki Ogawa, Kaneaki Tsuzaki, Effect of gamma to epsilon martensitic transformation on low-cycle fatigue behaviour and fatigue microstructure of Fe-15Mn-10Cr-8Ni-xSi austenitic alloys, ACTA MATERIALIA, 10.1016/j.actamat.2015.12.002, 105, 207-218, 2016.02.
65. Takahiro Sawaguchi, Tadakatsu Maruyama, Hiroaki Otsuka, Atsumichi Kushibe, Yasuhiko Inoue, Kaneaki Tsuzaki, Design Concept and Applications of Fe­Mn­Si-Based Alloys-from Shape-Memory to Seismic Response Control, Materials Transactions, Vol. 57, No. 3, 283-293, 2016.02.
66. Xiaohua Min, Yuuji kimura, Takashi kimura, Kaneaki Tsuzaki, Delamination toughening assisted by phosphorus in medium-carbon low-alloy steels with ultrafine elongated grain structures, Materials Science&Engineering, 649, 135-145, 2016.01.
67. Motomichi Koyama, Tasan Cemal Cem, Nagashima Tatsuya, Akiyama Eiji, Raabe Dierk, Kaneaki Tsuzaki, Hydrogen-assisted damage in austenite/martensite dual-phase steel, PHILOSOPHICAL MAGAZINE LETTERS, 10.1080/09500839.2015.1130275, 96, 1, 9-18, 2016.01.
68. Takahiro Kaneko, Motomichi Koyama, Tomoya Fujisawa, Kaneaki Tsuzaki, Combined multi-scale analyses on strain/damage/microstructure in steel
Example of damage evolution associated with ϵ-martensitic transformation, Tetsu-To-Hagane/Journal of the Iron and Steel Institute of Japan, 10.2355/tetsutohagane.TETSU-2015-081, 102, 5, 227-236, 2016.01, We studied damage evolution behavior associated with ϵ-martensite in a Fe-28Mn alloy. Visible factors of damage evolution associated with ϵ-martensite are considered to be strain distribution, microstructure, micro-void and crack. Combinatorial use of replica digital image correlation, electron backscattering diffraction, and electron channeling contrast imaging enables to clarify the distributions of strain, microstructure and damage. Through quantitative damage analysis, damage evolution behavior was classified into three regimes: (i) incubation regime, (ii) nucleation regime, and (iii) growth regime. In the incubation regime, an interaction of ϵ/ϵ-martensite plates and impingement of ϵ-martensite plates on grain boundaries caused plastic strain localization owing to plastic accommodation. In the nucleation regime, accumulation of the plastic strain on the boundaries caused microvoid formation. The damage propagated along with the boundaries through coalescence with other micro-voids, but the propagation was arrested by crack blunting at non-transformed austenite. In the growth regime, the arrested damage grew again when a further plastic strain was provided sufficiently to initiate ϵ-martensite near the damage..
69. Takahiro Kaneko, Motomichi Koyama, Tomoya Fujisawa, Kaneaki Tsuzaki, Combined multi-scale analyses on strain/damage/microstructure in steel
Example of damage evolution associated with ε-martensitic transformation, ISIJ International, 10.2355/isijinternational.ISIJINT-2016-272, 56, 11, 2037-2046, 2016.01, We studied damage evolution behavior associated with ε-martensite in a Fe-28Mn alloy. Visible factors of damage evolution associated with ε-martensite are considered to be strain distribution, microstructure, micro-void and crack. Combinatorial use of replica digital image correlation, electron backscattering diffraction, and electron channeling contrast imaging enables to clarify the distributions of strain, microstructure and damage. Through quantitative damage analysis, damage evolution behavior was classified into three regimes: (i) incubation regime, (ii) nucleation regime, and (iii) growth regime. In the incubation regime, an interaction of ε/ε-martensite plates and impingement of ε-martensite plates on grain boundaries caused plastic strain localization owing to plastic accommodation. In the nucleation regime, accumulation of the plastic strain on the boundaries caused microvoid formation. The damage propagated along with the boundaries through coalescence with other micro-voids, but the propagation was arrested by crack blunting at non-transformed austenite. In the growth regime, the arrested damage grew again when a further plastic strain was provided sufficiently to initiate ε-martensite near the damage..
70. Takahiro Sawaguchi, Tadakatsu Maruyama, Hiroaki Otsuka, Atsumichi Kushibe, Yasuhiko Inoue, Kaneaki Tsuzaki, Design concept and applications of Fe-Mn-Si-based alloys-from shape-memory to seismic response control, Materials Transactions, 10.2320/matertrans.MB201510, 57, 3, 283-293, 2016.01, Fe-Mn-Si-based alloys exhibit a shape-memory effect associated with deformation-induced γ→η martensitic transformation and its reversion. The γ→ηmartensitic transformation also enhances mechanical properties, such as strength, hardness, wear-resistance and low-cycle fatigue lives of the alloys. In this article, we review fundamental researches on transformation behavior, microstructural and crystallographic characteristics, and functional and mechanical properties of the Fe-Mn-Si-based alloys, and introduce various examples of their practical applications. A special emphasis is placed on their new application as architectural seismic dampers, which were developed based on a new finding of the passive two-way martensitic transformations under cyclic tensile-compressive loading..
71. Kaneaki Tsuzaki, Koki Fukuda, Motomichi Koyama, Hisao Matsunaga, Hexagonal close-packed Martensite-related Fatigue Crack Growth under the Influence of Hydrogen
Example of Fe-15Mn-10Cr-8Ni-4Si Austenitic Alloy, Scripta Materialia, 10.1016/j.scriptamat.2015.10.016, 113, 6-9, 2016.01, We investigated the fatigue crack propagation associated with a martensitic transformation from the face-centered cubic to the hexagonal close-packed phase in a Fe-15Mn-10Cr-8Ni-4Si austenitic alloy with and without hydrogen exposure (pre-charging in hydrogen gas at 10 MPa and/or testing in a hydrogen gas environment at 0.7 MPa). Unlike the case of Type 304 austenitic steel, the fatigue crack growth rate of the alloy was not affected by the hydrogen uptake at the present two hydrogen-charging conditions, although secondary cracking was increased by hydrogen..
72. Effects of Si on Tensile Properties Associated with
Deformation-Induced ε-Martensitic Transformation in High Mn Austenitic Alloys.
73. Ilya Nikulin, Takahiro Sawaguchi, Kazuyuki Ogawa, Kaneaki Tsuzaki, Microstructure Evolution Associated with a Superior Low-Cycle Fatigue Resistance of the Fe-30Mn-4Si-2Al Alloy, METALLURGICAL AND MATERIALS TRANSACTIONS A-PHYSICAL METALLURGY AND MATERIALS SCIENCE, 10.1007/s11661-015-3127-6, 46A, 11, 5103-5113, 2015.11.
74. Xiaohua Min, Satoshi Emura, Ling Zhang, Kaneaki Tsuzaki, Koichi Tsuchiya, Improvement of strength-ductility tradeoff in β titanium alloy through pre-strain induced twins combined with brittle ω phase, Materials Science and Engineering A, 10.1016/j.msea.2015.08.040, 646, 279-287, 2015.10, This paper provided a novel approach for improving the strength-ductility tradeoff in β titanium alloy through a combination of mechanical {332}<113> twins with brittle isothermal ω phase. Effects of pre-strain induced twins and subsequent precipitated ω phase on deformation microstructures, and tensile properties were investigated in a β-type Ti-15Mo alloy (mass%). Mechanical twins increased with increasing pre-strain from 1% to 9%, while formation of isothermal ω phase at 523-673. K for 3.6. ks was not significantly affected by the pre-strain induced twins. Yield strength and uniform elongation characteristics depended strongly on degree of pre-strain and aging temperature. An effective combination of 1-4% pre-strain and subsequent aging at 523-573. K markedly enhanced the uniform elongation in range of 8-15% at the yield strength level of 800-1000. MPa. The enhanced uniform elongation was mainly attributable to further twinning activation during deformation in conjunction with pre-strain induced twins..
75. Motomichi Koyama, Takahiro Sawaguchi, Kaneaki Tsuzaki, Deformation Twinning Behavior of Twinning-induced Plasticity Steels with Different Carbon Concentrations - Part 1: Atomic Force Microscopy and Electron Backscatter Diffraction Measurements, ISIJ INTERNATIONAL, 10.2355/isijinternational.ISIJINT-2015-069, 55, 8, 1747-1753, 2015.09.
76. Motomichi Koyama, Takahiro Sawaguchi, Kaneaki Tsuzaki, Deformation Twinning Behavior of Twinning-induced Plasticity Steels with Different Carbon Concentrations - Part 2: Proposal of Dynamic-strain-aging-assisted Deformation Twinning, ISIJ INTERNATIONAL, 10.2355/isijinternational.ISIJINT-2015-070, 55, 8, 1754-1761, 2015.09.
77. Xiaohua Min, Satoshi Emura, Ling Zhang, Kaneaki Tsuzaki, Koichi Tsuchiya, Improvement of strength–ductility tradeoff in β titanium alloy through pre-strain induced twins combined with brittle ω phase, Materials Science & Engineering, A646, 279-287, 2015.08.
78. Motomichi Koyama, Asif Bashir, Michael Rohwerder, Sergiy V. Merzlikin, Eiji Akiyama, Kaneaki Tsuzaki, Dierk Raabe, Spatially and Kinetically Resolved Mapping of Hydrogen in a Twinning-Induced Plasticity Steel by Use of Scanning Kelvin Probe Force Microscopy, JOURNAL OF THE ELECTROCHEMICAL SOCIETY, 10.1149/2.0131512jes, 162, 12, C638-C647, 2015.08.
79. Hideyuki Ohtsuka, Van An Dinh, Takahisa Ohno, Kaneaki Tsuzaki, Koichi Tsuchiya, Ryoji Sahara, Hideaki Kitazawa, Terumi Nakamura, First-principles Calculation of Effects of Carbon on Tetragonality and Magnetic Moment in Fe–C System, ISIJ International, 55, 11, 2483-2491, 2015.07.
80. C. C. Tasan, M. Diehl, D. Yan, M. Bechtold, F. Roters, L. Schemmann, C. Zheng, N. Peranio, D. Ponge, Motomichi Koyama, Kaneaki Tsuzaki, D. Raabe, An Overview of Dual-Phase Steels
Advances in Microstructure-Oriented Processing and Micromechanically Guided Design, Annual Review of Materials Research, 10.1146/annurev-matsci-070214-021103, 45, 391-431, 2015.07, Dual-phase (DP) steel is the flagship of advanced high-strength steels, which were the first among various candidate alloy systems to find application in weight-reduced automotive components. On the one hand, this is a metallurgical success story: Lean alloying and simple thermomechanical treatment enable use of less material to accomplish more performance while complying with demanding environmental and economic constraints. On the other hand, the enormous literature on DP steels demonstrates the immense complexity of microstructure physics in multiphase alloys: Roughly 50 years after the first reports on ferrite-martensite steels, there are still various open scientific questions. Fortunately, the last decades witnessed enormous advances in the development of enabling experimental and simulation techniques, significantly improving the understanding of DP steels. This review provides a detailed account of these improvements, focusing specifically on (a) microstructure evolution during processing, (b) experimental characterization of micromechanical behavior, and (c) the simulation of mechanical behavior, to highlight the critical unresolved issues and to guide future research efforts..
81. Motomichi Koyama, Kaneaki Tsuzaki, ε→γ Reverse Transformation-induced Hydrogen Desorption and Mn Effect on Hydrogen Uptake in Fe–Mn Binary Alloys, ISIJ International, 55, 10, 2269-2271, 2015.06.
82. Hidetoshi Somekawa, Tadanobu Inoue, Kaneaki Tsuzaki, Effect of deformation twin on toughness in magnesium binary alloys, Philosophical Magazine , 95, 23, 2513-2526, 2015.05.
83. Huichao Li, Koyama M., Takahiro Sawaguchi, Kaneaki Tsuzaki, Hiroshi Noguchi, Importance of crack-propagation-induced ε-martensite in strain-controlled low-cycle fatigue of high-Mn austenitic steel, Philosophical Magazine Letters, 95, 6, 303-311, 2015.05.
84. Kensuke Yamada, Motomichi Koyama, Takahiro Kaneko, Kaneaki Tsuzaki, Positive and negative effects of hydrogen on tensile behavior in polycrystalline Fe–30Mn–(6–x)Si–xAl austenitic alloys, Scripta Materialia, 105, 54-57, 2015.05.
85. T. Sawaguchi, Ilya Nikulin, K. Ogawa, K. Sekido, S. Takamori, T. Maruyama, Y. Chiba, A. Kushibe, Y. Inoue, Kaneaki Tsuzaki, Designing Fe-Mn-Si alloys with improved low-cycle fatigue lives, SCRIPTA MATERIALIA, 10.1016/j.scriptamat.2014.11.024, 99, 49-52, 2015.04.
86. Motomichi Koyama, Takahiro Sawaguchi, Kaneaki Tsuzaki, Effects of Si on Tensile Properties Associated with Deformation-Induced epsilon-Martensitic Transformation in High Mn Austenitic Alloys, JOURNAL OF THE JAPAN INSTITUTE OF METALS, 10.2320/matertrans.M2015042, 56, 6, 819-825, 2015.04.
87. Zhang, Y. -J., Miyamoto, G., Shinbo, K., Furuhara, T., Ohmura, T., Suzuki, T., Kaneaki Tsuzaki, Effects of transformation temperature on VC interphase precipitation and resultant hardness in low-carbon steels, ACTA MATERIALIA, 10.1016/j.actamat.2014.10.049, 84, 375-384, 2015.02.
88. N. Fukumura, T. Suzuki, Shigeru Hamada, Kaneaki Tsuzaki, Hiroshi Noguchi, Mechanical examination of crack length dependency and material dependency on threshold stress intensity factor range with Dugdale model, ENGINEERING FRACTURE MECHANICS, 10.1016/j.engfracmech.2015.01.003, 135, 168-186, 2015.02.
89. I. SHURO, K. Satoru, T. Nakamura, Kaneaki Tsuzaki, The Effects of Si and Al Alloying on α/γ Phase Equilibria in Fe-Cr-Ni-Mn Based Ternary and Quaternary Systems, Journal of Phase Equilibria and Diffusion , 2015.01.
90. N. Kamikawa, K. Sato, G. Miyamoto, M. Murayama, N. Sekido, Kaneaki Tsuzaki, T. Furuhara, Stress strain behavior of ferrite and bainite with nano-precipitation in low carbon steels, ACTA MATERIALIA, 10.1016/j.actamat.2014.10.010, 83, 383-396, 2015.01.
91. Daisuke Sasaki, Motomichi Koyama, Kenji Higashida, Kaneaki Tsuzaki, Hiroshi Noguchi, Effects of hydrogen-altered yielding and work hardening on plastic-zone evolution
A finite-element analysis, International Journal of Hydrogen Energy, 10.1016/j.ijhydene.2015.05.187, 40, 31, 9825-9837, 2015.01, Abstract In the present paper, finite-element analysis of a cracked specimen was conducted using a unified model for the elastic-plastic deformation and hydrogen diffusion. We considered the effects of the hydrogen-reduced yielding strength and work-hardening coefficient and used a comparison parameter in the simulation of the hydrogen-localized plastic zone near a crack tip. We realized two important facts: (1) the normal component of the plastic strain in the direction of remote stress near the crack tip is significantly increased by the reduced work-hardening coefficient at the same stress-intensity factor; (2) the reduced work-hardening coefficient enhances the localization of the plastic zone when compared to the case using the normal component of the crack-tip plastic strain in the direction of remote stress, which probably determines the ductile-brittle transition of the fatigue-crack propagation mode under a hydrogen atmosphere. These results indicate that the reduction in work-hardening coefficient and the utilization of the crack-tip plastic strain as a parameter to organize the data play important roles in the prediction of the transition condition of hydrogen-accelerated fatigue-crack propagation..
92. Effect of Additional Boron Amount on the Surface Roughness after Lathe Turning of h-BN Dispersed Type 304 Stainless Steel.
93. Hidetoshi Somekawa, Alok Singh, Tadanobu Inoue, Kaneaki Tsuzaki, Crack propagation behaviour in magnesium binary alloys, Philosophical Magazin, 2014.11.
94. Deformation Twinning Behavior of Twinning-Induced Plasticity Steels with Different Carbon Concentrations -Part 1: Analyses by Atomic Force Microscopy and Electron Backscatter Diffraction Measurements-.
95. Deformation Twinning Behavior of Twinning-Induced Plasticity Steels with Different Carbon Concentrations -Part 2: Proposal of Dynamic Strain Aging-Assisted Deformation Twinning-.
96. First-Principles Calculation of the Effects of Carbon on Tetragonality and Magnetic Moment of BCC-Fe.
97. Raabe, D., Springer, H., Gutierrez-Urrutia, I., Roters, F., Bausch, M., Seol, J. -B., Koyama M., Choi, P. -P., Kaneaki Tsuzaki, Alloy Design, Combinatorial Synthesis, and Microstructure-Property Relations for Low-Density Fe-Mn-Al-C Austenitic Steels, JOM, 10.1007/s11837-014-1032-x, 66, 9, 1845-1856, 2014.09.
98. Factors Affecting Static Strain Aging Under Stress at Room Temperature in a Fe-Mn-C Twinning-Induced Plasticity Steel.
99. Effects of Static and Dynamic Strain Aging on Hydrogen Embrittlement in TWIP Steels Containing Al.
100. Koyama M., Tasan, Cemal Cem, Akiyama, Eiji, Kaneaki Tsuzaki, Raabe, Dierk, Hydrogen-assisted decohesion and localized plasticity in dual-phase steel, ACTA MATERIALIA, 10.1016/j.actamat.2014.01.048, 70, 174-187, 2014.05.
101. Measurement of Martensitic Transformation of 9%Ni Steel during Welding Using ESPI System.
102. Hydrogen Embrittlement in Al-Added Twinning-Induced Plasticity Steels Evaluated by Tensile Tests during Hydrogen Charging.
103. Koyama M., Springer, Hauke, Merzlikin, Sergiy V., Kaneaki Tsuzaki, Akiyama, Eiji, Raabe, Dierk, Hydrogen embrittlement associated with strain localization in a precipitation-hardened Fe-Mn-Al-C light weight austenitic steel, INTERNATIONAL JOURNAL OF HYDROGEN ENERGY, 10.1016/j.ijhydene.2013.12.171, 39, 9, 4634-4646, 2014.03.
104. Shuro, I., Kobayashi, S., Nakamura, T., Kaneaki Tsuzaki, Determination of alpha/gamma phase boundaries in the Fe-Cr-Ni-Mn quaternary system with a diffusion-multiple method, JOURNAL OF ALLOYS AND COMPOUNDS, 10.1016/j.jallcom.2013.11.095, 588, 284-289, 2014.03.
105. Meysam Jafari, Warren M. Garrison Jr., Kaneaki Tsuzaki, Effect of Initial Microstructure on Impact Toughness of 1200 MPa-Class High Strength Steel with Ultrafine Elongated Grain Structure, The Minerals, Metals & Materials Society and ASM International , 45A, 647-653, 2014.02.
106. Min, X. H., Emura, S., Tsuchiya, K., Nishimura, T., Kaneaki Tsuzaki, Transition of multi-deformation modes in Ti-10Mo alloy with oxygen addition, MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, 10.1016/j.msea.2013.10.010, 590, 88-96, 2014.01.
107. K. Satoru, K. Kimura, Kaneaki Tsuzaki, Interphase precipitation of Fe2Hf Laves phase in a Fe-9Cr/Fe-9Cr-Hf diffusion couple, Intermetallics, 2014.01.
108. Masahiro Kawajiri, Satoshi Emura, Xiaohua Min, Shigeo Yamamoto, Kazuyuki Sakuraya, Kaneaki Tsuzaki, Effect of additional boron amount on the surface roughness after lathe turning of h-BN dispersed type 304 stainless steel, Tetsu-To-Hagane/Journal of the Iron and Steel Institute of Japan, 10.2355/tetsutohagane.100.1542, 100, 12, 1542-1547, 2014.01, The authors have reported that the machinability (lathe turning, drilling and sawing) of Type 304 austenitic stainless steel has been improved by the precipitation of hexagonal boron nitride (h-BN). In the present study, the influence of h-BN precipitation on surface roughness of Type 304 steel after lathe turning was investigated. Surface roughness was degraded with increasing the amount of boron addition, especially at lower cutting speed of 22 m/min. However, Type 304 steel with boron addition up to 0.05 mass % exhibited similar or better surface roughness to that of Type 303 sulfur added free-cutting steel. Precipitation of h-BN enhanced the size of built up edge and degraded the surface roughness. compared to that of commercial Type 304 steel, and 0.1 mass % boron added Type 304 steel showed better surface roughness compared On the other hand, increased Vickers hardness values due to solute nitrogen decreased the separation size of built up edge and improved the surfaced roughness..
109. Hideyuki Ohtsuka, Van An Dinh, Takahisa Ohno, Kaneaki Tsuzaki, Koichi Tsuchiya, Ryoji Sahara, Hideaki Kitazawa, Terumi Nakamura, First-principles calculation of the effects of carbon on tetragonality and magnetic moment of BCC-Fe, Tetsu-To-Hagane/Journal of the Iron and Steel Institute of Japan, 10.2355/tetsutohagane.100.1329, 100, 10, 1329-1338, 2014.01, The effects of carbon content on tetragonality and magnetic moment of bcc iron have been evaluated by first-principles calculation. Three kinds of supercells, Fe54C1, Fe54C2 and Fe128C1 (which correspond to Fe-0.40C, Fe-0.79 and Fe-0.17C mass%, respectively) are used for the calculation of tetragonality and magnetic moment of Fe-C system. Main results obtained are as follows. (1) The total energy and mechanical energy of the Fe-C system with carbon atom at the octahedral sites are smaller than those with carbon atom at the tetragonal sites. The carbon atom at octahedral site produces fairly large expansion in one direction. (2) Tetragonality of Fe-C system obtained by first-principles calculation increases linearly with increasing carbon content and agrees well with experimental results. The average magnetic moment of an Fe atom increases with increasing carbon content. (3) The magnetic moment of an Fe atom at the nearest neighbor of carbon atom is lower than that of pure iron and increases with increasing distance between the iron and carbon atoms. The projected density of states shows a hybridization with main contributions from Fe d and C p states which leads to the above mentioned decrease of the magnetic moment of an Fe atom. (4) In Fe54C2, tetragonality and magnetic moment of iron atom change with the distance between two carbon atoms. The value of tetragonality is either 0.981, 1.036 or 1.090. When the dumbbell structure which consists of the first carbon atom and its two nearest neighbor iron atoms is perpendicular to the second dumbbell structure which consists of the second carbon atom and its two nearest neighbor iron atoms, the tetragonality is 0.981 and does not agree with experimental value. The mechanical energy is relatively large. On the other hand, when the first dumbbell structure is parallel to the second dumbbell structure, the tetragonality is 1.036 which agrees well with experimental data. The mechanical energy is relatively small. When straight C-Fe-C pair is formed, tetragonality is 1.090. (5) In Fe54C2, formation enthalpy is relatively low when the calculated tetragonality is 1.036, and the existence probability under the assumption of Boltzmann distribution is high. In other cases, the existence probability is nearly zero. (6) The average magnetic moment of an Fe atom is proportional to volume, but not in a clear relation with tetragonality. It is considered that the increase of magnetic moment of an Fe atom by the addition of carbon atom is mainly due to the magneto-volume effect but not due to the tetragonality effect..
110. Ilya Nikulin, Takahiro Sawaguchi, Kazuyuki Ogawa, Kaneaki Tsuzaki, Low-cycle fatigue behavior and microstructural evolution of the Fe- 30Mn-4Si-2Al alloy, 8th International Conference on Processing and Manufacturing of Advanced Materials, THERMEC 2013 THERMEC 2013, 944-949, 2014.01, Superior fatigue life of 8000 cycles at low-cycle fatigue with a total strain Δε=2% was found in the Fe-30Mn-4Si-2Al high-Mn alloy, as compared to Fe-30Mn-6Si-0Al and Fe-30Mn- 3Si-3Al alloys with fatigue life of 2×103 cycles. Examination of microstructural evolution and cyclic hardening/softening behavior was shown that high fatigue resistance of Fe-30Mn-4Si-2Al alloy associated with delayed development of the deformation induced martensite and inhibited dislocation slip as compared to Fe-30Mn-6Si-0Al and Fe-30Mn-3Si-3Al alloys, respectively. Cyclic strain softening followed by secondary strain hardening was observed in the Fe-30Mn-4Si- 2Al alloy after primary hardening. Primary hardening to about 40 cycles was associated with continuous increase in density of planar dislocations and the development of slip bands. The cyclic softening manifesting as the drop of the stress amplitude in the range of the cycles from 40 to 400 was accompanied by development of deformation induced ε-martensite in place of the slip bands. At the N>400 cycles further increase in the volume fraction of deformation ε-martensite leads to continuous hardening up to the failure. In the presentation we will discuss the details of microstructural evolution during LCF of the Fe-30Mn-4Si-2Al alloy..
111. Susumu Meguro, Satoru Kobayashi, Terumi Nakamura, Kaneaki Tsuzaki, Measurement of martensitic transformation of 9% Ni steel during welding using ESPI system, Tetsu-To-Hagane/Journal of the Iron and Steel Institute of Japan, 10.2355/tetsutohagane.100.632, 100, 5, 632-638, 2014.01, Displacement and strain during welding are important parameters for evaluation of weld joint. But in-situ measurement of these values needs non-conductive method because weld joint reaches to high temperature. So we applied electronic speckle pattern interferrometry (ESPI) system to measurement of displacement and strain during gas-tungsten-arc (GTA) welding of 9%Ni steel. This system detects interference of refected laser illumination from weld joint surface. The change of interference depends on displacement of weld joint surface. ESPI measurement starts simultaneously with GTA heating, and continues during cooling after heating fnished. ESPI system reveals behavior of two dimensional displacement distribution during welding and martensitic transformation in weld joint. Quantitated displacement record of certain point in GTA heating area shows detail of martensitic transformation. Martensite start temperature (Ms) analyzed by ESPI result shows higher value than conventional value measured by uniform heating and cooling process. This result shows stress-assisted martensitic transformation occurred in this area, because tensile stress is forced after solidifcation of weld metal at GTA heating area. Quantitated value of Ms raising corresponds to additional chemical driving force for martensitic transformation. This driving force is estimated using free energy calculation. In the case of this study, this driving force is added as tensile stress at GTA heating area and ESPI result is useful to estimate the value of stress..
112. A. Belyakov, M. Odnobokova, A. Kipelova, Kaneaki Tsuzaki, R. Kaibyshev, Nanocrystalline structures and tensile properties of stainless steels processed by severe plastic deformation, 6th International Conference on Nanomaterials by Severe Plastic Deformation, NanoSPD 2014 IOP Conference Series: Materials Science and Engineering, 10.1088/1757-899X/63/1/012156, 63, 1, 2014.01, The development of nanocrystalline structures in austenitic stainless steels during large strain cold rolling and their tensile behavior were studied. The cold rolling to total equivalent strains above 2 was accompanied by the evolution of nanocrystalline structures with the transverse grain size of about 100 nm. The development of deformation twinning and martensitic transformation during cold working promoted the fast kinetics of structural changes. The development of nanocrystalline structures resulted in significant strengthening. More than fourfold increase in the yield strength was achieved. The strengthening of nanocrystalline steels after severe plastic deformation was considered as a concurrent operation of two strengthening mechanisms, which were attributed to grain size and internal stress. The contribution of internal stresses to the yield strength is comparable with that from grain size strengthening..
113. Ilya Nikulin, Takahiro Sawaguchi, Kazuyuki Ogawa, Kaneaki Tsuzaki, Low-cycle fatigue behavior and microstructural evolution of the Fe- 30Mn-4Si-2Al alloy, 8th International Conference on Processing and Manufacturing of Advanced Materials, THERMEC 2013 THERMEC 2013, 10.4028/www.scientific.net/MSF.783-786.944, 783-786, 944-949, 2014, Superior fatigue life of 8000 cycles at low-cycle fatigue with a total strain Δε=2% was found in the Fe-30Mn-4Si-2Al high-Mn alloy, as compared to Fe-30Mn-6Si-0Al and Fe-30Mn- 3Si-3Al alloys with fatigue life of 2×103 cycles. Examination of microstructural evolution and cyclic hardening/softening behavior was shown that high fatigue resistance of Fe-30Mn-4Si-2Al alloy associated with delayed development of the deformation induced martensite and inhibited dislocation slip as compared to Fe-30Mn-6Si-0Al and Fe-30Mn-3Si-3Al alloys, respectively. Cyclic strain softening followed by secondary strain hardening was observed in the Fe-30Mn-4Si- 2Al alloy after primary hardening. Primary hardening to about 40 cycles was associated with continuous increase in density of planar dislocations and the development of slip bands. The cyclic softening manifesting as the drop of the stress amplitude in the range of the cycles from 40 to 400 was accompanied by development of deformation induced ε-martensite in place of the slip bands. At the N>400 cycles further increase in the volume fraction of deformation ε-martensite leads to continuous hardening up to the failure. In the presentation we will discuss the details of microstructural evolution during LCF of the Fe-30Mn-4Si-2Al alloy..
114. Ilya Nikulin, Takahiro Sawaguchi, Kaneaki Tsuzaki, Effect of alloying composition on low-cycle fatigue properties and microstructure of Fe-30Mn-(6-x)Si-xAl TRIP/TWIP alloys, Materials Science and Engineering A, 10.1016/j.msea.2013.08.061, 587, 192-200, 2013.12, The change in low-cycle fatigue (LCF) properties and deformation microstructure due to the alteration of aluminum and silicon contents was studied in relation with the tensile properties in Fe-30Mn-(6-x)Si-xAl (x=0, 1, 2, 3, 4, 5, 6wt%) alloys, which are high-Mn austenitic TRIP/TWIP alloys. Austenite to ε-martensite transformation took place during LCF deformation in the TRIP alloys with x≤2 while mechanical twinning was not observed by electron-backscattering diffraction (EBSD) analysis in the TWIP alloys with x>2 after LCF deformation. The fatigue resistance of the alloys was shown to be correlated with the tensile proof strength and the hardening rate. Superior fatigue life of 8×103 cycles at a total strain range δε=2% was found in the Fe-30Mn-4Si-2Al TRIP alloy with a low fraction of ε-martensite, high tensile proof strength and low hardening rate at both tensile and fatigue deformations. On the other hand, a considerable decrease in the fatigue properties was observed in the alloys with decreasing proof strength and increasing hardening rate. Proof strength provided by the solid solution of Al and Si, represents the hampering of plastic deformation, and the hardening rate reflects the strain reversibility affected by the stacking fault energy (SFE) through the rate of austenite to martensite transformation in the TRIP alloys and the substructure formation in the TWIP alloys..
115. Hidetoshi Somekawa, Tadanobu Inoue, Kaneaki Tsuzaki, Effect of solute atoms on fracture toughness in dilute magnesium alloys, Philosophical Magazine, 10.1080/14786435.2013.838008, 93, 36, 4582-4592, 2013.12, The effect of alloying elements on the toughness and the fracture behaviour was investigated on seven kinds of Mg-0.3 at.% X (X= Ag, Al, Ca, Pb, Sn, Y and Zn) alloys with a grain size of 3-5 μm. The fracture toughness and fracture behaviour in magnesium alloys were closely related to the segregation energy. The Mg-Al and -Zn alloys that had small segregation energy showed high toughness and ductile fracture in most regions, while the Mg-Ca alloy with large segregation energy exhibited low toughness and intergranular fracture. These different tendencies resulted from solute segregation at grain boundaries (GBs). The change in the lattice parameter ratio was the influential material parameter regardless of whether the GB embrittlement was for enhancement or suppression..
116. X. H. Min, T. Sawaguchi, K. Ogawa, T. Maruyama, F. X. Yin, Kaneaki Tsuzaki, An attempt to lower Mn content of Fe-17Mn-6Si-0.3C shape memory alloy, Journal of Alloys and Compounds, 10.1016/j.jallcom.2012.02.140, 577, SUPPL. 1, 2013.11, The Mn content of Fe-17Mn-6Si-0.3C (mass%) shape memory alloy, which was reported to show a superior shape memory effect without being subjected to the training treatment, is decreased by substituting various elements such as Ni, Cr, Cu and Al. The phase transformation behavior accompanied with the shape memory effect is investigated in the alloy with the Mn content from 17 to 9 mass%. The internal friction-temperature curves during cooling and heating indicate that the Ms temperatures of the alloys are below ambient temperature. The shape recovery strain of the alloy shows that there exists a large gap in the shape memory effect between 13 and 11 mass% Mn content. For decreasing the Mn content of the alloy, Ni and Cr are more effective substitutes than Cu and Al. The alloy, which contains Mn more than 13 mass% with Ni and Cr substitutes, exhibits a good shape recovery larger than 2% in strain..
117. Toru Hara, Koichi Tsuchiya, Kaneaki Tsuzaki, Xin Man, Tatsuya Asahata, Atsushi Uemoto, Application of orthogonally arranged FIB-SEM for precise microstructure analysis of materials, Journal of Alloys and Compounds, 10.1016/j.jallcom.2012.02.019, 577, SUPPL. 1, 2013.11, In order to improve microstructure analysis with electron microscopes, we have developed an instrument that is based on a combination of a scanning electron microscope (SEM) and a focused ion beam (FIB). The most characteristic point is that the SEM and FIB are arranged orthogonally. The advantages of this orthogonal arrangement are that high-resolution and high-contrast SEM images can be obtained because of the uniform background intensity and the short working distance (≈2 mm). Furthermore, since other analytical instruments (such as energy-dispersive spectroscopy (EDS), electron backscatter diffraction (EBSD), scanning transmission electron microscopy (STEM) equipment, etc.) can be located in their ideal positions, multiscale and versatile analyses can be performed with this single instrument. As an example, the observation of the distribution of precipitates in tempered martensitic steels is described. Because of the high contrast achieved by this instrument, different kinds of nanosized precipitates can be distinguished by the contrast in the 3D image reconstructed by the serial-sectioning method, even at low magnifications. Other features of the equipment are also described in this work..
118. Yuuji Kimura, Tadanobu Inoue, Kaneaki Tsuzaki, Tempforming in medium-carbon low-alloy steel, Journal of Alloys and Compounds, 10.1016/j.jallcom.2011.12.123, 577, SUPPL. 1, S538, 2013.11, Deformation of tempered martensitic structure, i.e., tempforming using a caliber rolling at 500 C was applied to a 0.4%C-2%Si-1%Cr-1%Mo steel to obtain ultrafine elongated grain structure with a strong 〈1 1 0〉//RD fiber deformation texture and then tensile and Charpy impact properties were investigated. An inverse temperature dependence of impact toughness was demonstrated in the tempformed (TF) samples in subzero temperatures, where conventionally quenched and tempered samples showed a ductile-to-brittle transition. The enhancement in the impact toughness of the TF sample was attributed to the occurrence of a delamination, where cracks branched to the longitudinal direction of the impact test bar. The delamination was shown to occur due to the microstructural anisotropy. The dominating factors controlling the delamination were considered to be the transverse grain size, the grain shape and the deformation texture..
119. Meysam Jafari, Kaneaki Tsuzaki, Transition in fracture mode from ductile to intergranular and cleavage in 0.05%P doped high strength steel, Journal of Alloys and Compounds, 10.1016/j.jallcom.2012.03.031, 577, SUPPL. 1, 2013.11, Quenched and tempered martensite (QTM) structures were produced by tempering at 550 C in the 1200 MPa-class high strength steel with phosphorous (P) contents of 0.001 and 0.053 wt%. Charpy impact tests were performed at temperature range of 350 to -196 C on the QTM samples. The 0.001%P-QTM structure showed upper shelf energy of 97 J and a low ductile to brittle transition temperature (DBTT) of around -150 C with a transition region limited to the -100 to -196 C was observed. In the 0.053%P-QTM structure the upper shelf energy decreased from 97 J to 64 J and high DBTT of around +80 C were obtained with an extended transition region at the temperature range of 150 to -196 C. The fracture mode in the 0.001%P-QTM structure changed from ductile to cleavage and in the 0.053%P-QTM structure from ductile to intergranular and then cleavage. Thus, one can say P segregation and intergranular fracture in the 0.053%P steel increase the DBTT and extend the transition region. The critical stress for intergranular fracture was similar to or somehow lower than that for ductile fracture, and both increased with decreasing temperature. The critical stress for intergranular fracture was obtained to be 3800 at 150 C and 6000 MPa at -196 C. That is why intergranular fracture in the 0.053%P steel made the transition region wide. However, the stress for cleavage fracture was almost independent of temperature, and hence the 0.001%P steel showed a limited transition region. The critical stress for cleavage fracture was obtained to be 6000 MPa..
120. Emura S., Kawajiri M., Min X., Yamamoto S., Sakuraya K., Tsuzaki K., Machinability improvement and its mechanism in SUS304 austenitic stainless steel by precipitated hexagonal boron nitride, ISIJ International , 53 , 10, 1841-1849, 2013.10, To improve the machinability of SUS304 (Type 304) austenitic stainless steels, specimens were prepared containing 0.016 mass% boron and 0.2 mass% nitrogen, and hexagonal boron nitride (h-BN) particles with a diameter of 1 to 5 ì m were precipitated. Precipitation of h-BN reduced the cutting force and tool wear during lathe turning with a cemented carbide tool insert, especially at cutting speeds of 40 m/min and higher. The reduction in cutting force appeared attributable to internal lubrication by h-BN in the chip shear region and the deformation flow layer, as well as to lubrication between the chip and carbide tool. Improved chip disposability and tool wear suppression were also achieved by h-BN precipitation. SUS304 steel with precipitated h-BN was found to exhibit good machinability in drilling and sawing operations with high-speed steel tools..
121. Koyama M., Akiyama E., Sawaguchi T., Ogawa K, Kireeva I.V., Chumlyakov Y.I., Tsuzaki K., Hydrogen-assisted quasi-cleavage fracture in a single crystalline type 316 austenitic stainless steel, CORROSION SCIENCE, 10.1016/j.corsci.2013.06.018, 75, 345-353, 2013.10, Hydrogen embrittlement properties were examined in a single crystalline type 316 austenitic stainless steel. Tensile tests were conducted along the 〈1. 1. 1〉 and 〈0. 0. 1〉 directions under hydrogen charging. Hydrogen-assisted {1. 1. 1} quasi-cleavage fracture was observed in both tensile orientations. The degradation of fracture stress and elongation, hydrogen uptake, and hydrogen-induced fracture surface were dependent on the tensile orientation. The tensile orientation dependence of the hydrogen embrittlement properties was shown to result from the deformation twinning behavior. In addition, Ag decoration technique clarified that hydrogen localizes on regions where hydrogen-assisted quasi-cleavage fracture appeared..
122. Koyama M., Sawaguchi T., Tsuzaki K., Microstructure characteristic and its effect on mechanical and shape memory properties in a Fe-17Mn-8Si-0.3C alloy, JOURNAL OF ALLOYS AND COMPOUNDS, 10.1016/j.jallcom.2013.03.289, 573, 15-19, 2013.10, The influences of the Si content on the mechanical and shape memory properties of Fe-17Mn-xSi-0.3C alloys (wt.%) were examined, and a specific focus was placed on the properties of the Fe-17Mn-8Si-0.3C alloy. The 8%Si addition changed the initial microstructure from an austenite to an austenite/ferrite dual phase. A sufficient amount of deformation-induced ε-martensite was observed despite the pre-existing ferrite. Mn and C were preferentially distributed to the austenite, while Si was localized in the ferrite. The formation of ferrite increased the yield strength and the shape recovery stress, and decreased the starting temperature for the shape recovery. .
123. Hisashi Sato, Kentaro Ota, Motoo Furukawa, Miki Azuma, Yoshimi Watanabe, Zougui Zhang, Kaneaki Tsuzaki, Grain Refinement of As-Cast Pure Al by Cold-rolled Al-Ti Alloy Refiner, The Japan Institute of Metals and Materials, 54, 9, 1554-1561, 2013.09, Addition of Al–Ti alloy refiners into an Al melt induces grain refinement of α-Al grains in as-cast pure Al. In this study, the effects of cold rolling for the Al–Ti alloy refiners on the grain refinement of as-cast pure Al were investigated. The Al3Ti particles in the refiners, which act as nuclei for the α-Al grains, were fragmented by cold rolling. Grain size of the α-Al grains is decreased as increasing the reduction ratio of the refiners. This decrease in grain size is due to the increase in the number of Al3Ti particles in the refiners. The mechanical properties of the as-cast pure Al are also improved as a result of its grain refinement. In particular, the wear properties of the as-cast pure Al were drastically improved. It is concluded that the cold rolling for the refiners is an effective process to enhance its grain refinement ability..
124. Min X., Tsuzaki K., Emura S., Sawaguchi T., Ii S., Tsuchiya K., {332}< 113 > Twinning system selection in a beta-type Ti-15Mo-5Zr polycrystalline alloy, MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, 10.1016/j.msea.2013.04.119, 579, 164-169, 2013.09, The orientational dependence of {332}〈113〉 twinning and its system was examined in a 4.0% tensile-strained Ti-15Mo-5Zr (mass%) polycrystalline alloy by electron backscatter diffraction analysis combined with Schmid factor analysis. Twinning and system selections were found to obey the Schmid law in grains with tensile axes close to the [1 11] and [001] directions, in which the maximum Schmid factors of an easily operative (233)[311] twinning system were larger than 0.46 and smaller than 0.34, respectively. However, when the maximum Schmid factor ranged from 0.34 to 0.46, both selections became complex and not entirely explainable by the Schmid law around the center of a stereographic triangle. Twinning systems other than (233)[311] were also activated in grains with a Schmid factor even below 0.1 and inside twins with a negative Schmid factor. We conclude that additional factors, specifically local stress concentrations and geometric constraints between neighboring grains, should also be considered in regard to {332}〈113〉 twin formation, even in a polycrystalline β-titanium alloy that has been only slightly deformed. .
125. Koyama M., Lee T., Lee C.S., Kaneaki Tsuzaki, Grain refinement effect on cryogenic tensile ductility in a Fe-Mn-C twinning-induced plasticity steel, MATERIALS & DESIGN, 10.1016/j.matdes.2013.01.061, 49, 234-241, 2013.08, Cryogenic mechanical properties of twinning-induced plasticity steels of various grain sizes were investigated by the tensile tests. Specimens with coarse grain sizes, e.g. 10, 23 and 37. μm, showed a brittle fracture at 123. K. The embrittlement was suppressed by a grain refinement to 3.5. μm by cold rolling and recrystallization. Furthermore, a grain refinement to 460. nm by warm caliber rolling suppressed the embrittlement more effectively compared with the grain refinement via cold rolling and recrystallization treatment. The grain refinements improved the tensile elongation, yield strength, and ultimate tensile strength significantly at 123. K. It was found that the suppression of the embrittlement was caused by the reductions in the amount of ε-martensite. .
126. Koyama M., Akiyama E., Tsuzaki K., Effects of static and dynamic strain aging on hydrogen embrittlement in twip steels containing al, ISIJ International , 53, 7, 1268-1274, 2013.07, Al effects on strain aging and resistance against hydrogen embrittlement were examined in Fe-18Mn- 0.6C-based twinning-induced plasticity steels deformed at different strain rates. These steels showed a hydrogen-induced fracture when they were pre-deformed at a strain rate of 1.7×10-6 s -1. This fracture was suppressed by increasing the strain rate and Al content. The two important factors for improving the resistance to hydrogen embrittlement from the viewpoint of material strengthening by strain aging were found to be (1) the suppression of dynamic strain aging by increasing the strain rate and Al content, and (2) the suppression of static strain aging under loading by the Al addition..
127. Koyama M., Akiyama E., Tsuzaki K., Raabe D., Hydrogen-assisted failure in a twinning-induced plasticity steel studied under in situ hydrogen charging by electron channeling contrast imaging, ACTA MATERIALIA, 10.1016/j.actamat.2013.04.030, 61, 12, 4607-4618, 2013.07, We investigated the hydrogen embrittlement of a Fe-18Mn-1.2%C (wt.%) twinning-induced plasticity steel, focusing on the influence of deformation twins on hydrogen-assisted cracking. A tensile test under ongoing hydrogen charging was performed at low strain rate (1.7 × 10-6 s -1) to observe hydrogen-assisted cracking and crack propagation. Hydrogen-stimulated cracks and deformation twins were observed by electron channeling contrast imaging. We made the surprising observation that hydrogen-assisted cracking was initiated both at grain boundaries and also at deformation twins. Also, crack propagation occurred along both types of interfaces. Deformation twins were shown to assist intergranular cracking and crack propagation. The stress concentration at the tip of the deformation twins is suggested to play an important role in the hydrogen embrittlement of the Fe-Mn-C twining-induced plasticity steel..
128. Koyama M., Akiyama E., Kaneaki Tsuzaki, Factors Affecting Static Strain Aging under Stress at Room Temperature in a Fe-Mn-C Twinning-induced Plasticity Steel, ISIJ INTERNATIONAL, 10.2355/isijinternational.53.1089, 53, 6, 1089-1096, 2013.06, Al effects on strain aging and resistance against hydrogen embrittlement were examined in Fe-18Mn- 0.6C-based twinning-induced plasticity steels deformed at different strain rates. These steels showed a hydrogen-induced fracture when they were pre-deformed at a strain rate of 1.7×10-6 s -1. This fracture was suppressed by increasing the strain rate and Al content. The two important factors for improving the resistance to hydrogen embrittlement from the viewpoint of material strengthening by strain aging were found to be (1) the suppression of dynamic strain aging by increasing the strain rate and Al content, and (2) the suppression of static strain aging under loading by the Al addition. .
129. Eiji Akiyama, Wang M., Li S., Zhang Z., Kimura Y., Uno N., Kaneaki Tsuzaki, Studies of Evaluation of Hydrogen Embrittlement Property of High-Strength Steels with Consideration of the Effect of Atmospheric Corrosion, METALLURGICAL AND MATERIALS TRANSACTIONS A-PHYSICAL METALLURGY AND MATERIALS SCIENCE, 10.1007/s11661-012-1403-2, 44A, 3, 1290-1300, 2013.03, Hydrogen embrittlement of high-strength steels was investigated by using slow strain rate test (SSRT) of circumferentially notched round bar specimens after hydrogen precharging. On top of that, cyclic corrosion tests (CCT) and outdoor exposure tests were conducted prior to SSRT to take into account the effect of hydrogen uptake under atmospheric corrosion for the evaluation of the susceptibility of high-strength steels. Our studies of hydrogen embrittle properties of high-strength steels with 1100 to 1500 MPa of tensile strength and a prototype ultrahigh-strength steel with 1760 MPa containing hydrogen traps using those methods are reviewed in this article. A power law relationship between notch tensile strength of hydrogen-precharged specimens and diffusible hydrogen content has been found. It has also been found that the local stress and the local hydrogen concentration are controlling factors of fracture. The results obtained by using SSRT after CCT and outdoor exposure test were in good agreement with the hydrogen embrittlement fracture property obtained by means of long-term exposure tests of bolts made of the high-strength steels. .
130. Koyama M., Sawaguchi T., Tsuzaki K., TWIP Effect and plastic instability condition in an Fe-Mn-C austenitic steel, ISIJ International , 53, 2, 323 -329, 2013.02, We investigated the correlation among deformation twin density, work hardening, and tensile ductility in an Fe-18Mn-1.2C twinning-induced-plasticity (TWIP steel, and discussed the correlation with the plastic instability condition. The deformation twin density was varied by changing the deformation temperature from 123 to 523 K. An important factor for the uniform elongation is the work hardening rate in a later deformation stage. The increase in the deformation twin density enhanced the work hardening rate significantly but not monotonically just before the fracture, since the deformation twin density is saturated against plastic strain. In addition, dynamic strain aging in a later deformation stage and ε-martensitic transformation were found to accelerate the fracture due to the localized deformation and the premature fracture, respectively. Accordingly, the relationship between uniform elongation and deformation twin density was not simple. The optimum conditions for the TWIP effect were concluded to be (1) considerable amount of deformation twinning in a later deformation stage, (2) suppression of dynamic strain aging in a later deformation stage, and (3) inhibition of ε-martensitic transformation..
131. Meysam Jafari, Yuuji Kimura, Kaneaki Tsuzaki, Toughening by the addition of phosphorus to a high-strength steel with ultrafine elongated grain structure, PHILOSOPHICAL MAGAZINE LETTERS, 10.1080/09500839.2012.750766, 93, 2, 109-115, 2013.02, Phosphorus-doped high-strength steels are typically brittle at room temperature. In contrast to the non-hardening embrittlement of body-centred cubic (bcc) steels which decreases toughness without increasing strength, we observed an increase in toughness of about 20% by adding a large amount (0.053 wt%) of phosphorus (P) to a high-strength bcc steel with an ultrafine elongated ferrite grain structure processed by warm calibre rolling at 500 °C which produced a 91% reduction in area. The enhanced toughness is attributed to P segregation, which causes grain boundaries to become feasible crack propagation paths, thereby enhancing delamination toughening. The 0.053% P steel showed a microstructure and tensile properties similar to those of 0.001% P steel (reference steel). .
132. Sitdikov O., Avtokratova E., Sakai T., Tsuzaki K., Ultrafine-grain structure formation in an Al-Mg-Sc alloy during warm ECAP, Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science , 44, 2, 1087-1100, 2013.02, Microstructural evolution taking place during equal-channel angular pressing was studied in a commercial Al-6Mg-0.3Sc alloy at 523 K (250 C) (∼0.5T m). The structural changes are mainly associated with development of microshear bands (MSBs) that are continuously formed by strain accumulation and microstructural heterogeneities in each pass, which result in fragmentation of coarse original grains. New ultrafine grains (UFGs) with moderate-to-high angle boundary misorientations are concurrently evolved in the interiors of MSBs accompanied by rigid body rotation at medium-to-large strains. Such strain-induced grain refinement process occurs very slowly and incompletely in the present heavily alloyed Al alloy, leading to formation of a mixed microstructure, i.e., the UFGs in colony and some weakly misoriented fragments of original grains. The microstructure evolved at ε ≈ 12 is characterized by a bimodal crystallite size distribution with two peaks at d 1 ≈ 0.2 to 0.3 μm and d 2 ≈ 0.6 to 0.7 μm, and the fraction of high angle boundaries of about 0.35 ± 0.05. The main factors promoting dynamic formation of UFGs and the effects of thermal processes on it during severe plastic deformation are discussed in detail. .
133. I. Shuro, S. Kobayashi, T. Nakamura, Kaneaki Tsuzaki, Determination of α / γ phase boundaries in the Fe-Cr-Ni-Mn quaternary system with a diffusion-multiple method, 8th Pacific Rim International Congress on Advanced Materials and Processing 2013, PRICM 8 8th Pacific Rim International Congress on Advanced Materials and Processing 2013, PRICM 8, 1, 577-582, 2013.01, The phase relationships between the ferrite phase (α) and the austenite phase (γ) were investigated in the iron-rich comer of the Fe-Cr-Mn-Ni quaternary system at 1000°C. The phase boundary between the α and γ phases was determined using a diffusion-multiple technique. In this technique, three end members, FeCr, FeCrMn and FeCrNi alloys, were coupled inside a metal tube, and then heat treated at 1300°C for 24h to create a microstructural state with a composition gradient of Cr, Ni and Mn within the composition range of the end members. This was followed by long time heat treatment at 1000°C for a period of 240 h to attain equilibrium states between the γ and α phase in the composition range. The phase boundary between the γ single phase field and the γ+α two-phase field was found to slightly shift in a way to expand the γ single phase field when the manganese content was increased from 0 to 5 wt%. The shift of the phase boundary was not obvious at chromium contents above 20 wt% and nickel contents above 8 wt%. These results indicate that manganese is a weak austenitizer at low chromium and nickel contents..
134. Ilya Nikulin, Takahiro Sawaguchi, Kaneaki Tsuzaki, Low-cycle fatigue properties of the Fe-30Mn-(6-x)Si-xAl TRIP/TWIP alloys, 8th Pacific Rim International Congress on Advanced Materials and Processing 2013, PRICM 8 8th Pacific Rim International Congress on Advanced Materials and Processing 2013, PRICM 8, 665-671, 2013.01, Low-cycle fatigue (LCF) properties of the Fe-30Mn-(6-x)Si-xAl alloys ascribed to TRIP/TWIP high-Mn austenitic alloys were examined in relation with tensile properties. It was found that fatigue resistance of these alloys correlates with tensile yield strength and strain hardening rate, while tensile ductility are not affects LCF properties of examined alloys. At total fatigue strain range of 2% the highest fatigue life of about 8000 cycles was found in the Fe-30Mn-4Si-2Al alloy with higher yield strength (YS) and lower hardening rate. Decreasing of YS and increasing of hardening rate lead to considerable decrease of fatigue life of examined alloys. In the presentation we will discuss the relation between LCF properties and deformation microstructure..
135. Xiaohua Min, Xuejiao Chen, Satoshi Emura, Kaneaki Tsuzaki, Koichi Tsuchiya, Quantitative analysis of twinning-induced plasticity (TWIP) in beta-titanium alloy, 8th Pacific Rim International Congress on Advanced Materials and Processing 2013, PRICM 8 8th Pacific Rim International Congress on Advanced Materials and Processing 2013, PRICM 8, 2, 1149-1156, 2013, Work hardening behavior of Ti-15Mo alloy (mass %) was examined through quantitative evaluation of the {332}〈113〉 twinning structural evolution at various plastic strain levels by optical microscope (OM) observations combined with electron backscatter diffraction analyses (EBSD). Based on analyzed images from the optical micrographs, area fraction of twins rapidly increases to 40% at strain of 0.05, and gradually reaches to 52% at strain of 0.12. With further deformation to a strain of 0.17, it has no significant change. The EBSD inverse pole figure maps exhibit that the twins become denser and thicker with increasing strain. A linear relationship is found between true stress and inverse square root of mean free path of dislocations at strain up to 0.12, indicating that the work hardening is mainly dominated by dynamic microstructure refinement induced by the twinning..
136. MIn X., Tsuzaki K., Emura S., Tsuchiya K., Enhanced uniform elongation by pre-straining with deformation twinning in high-strength β-titanium alloys with an isothermal ω-phase, Philosophical Magazine Letters , 92, 12, 762-732, 2012.12, It is shown that pre-straining with deformation twinning is a novel approach to enhancing the uniform elongation of a high-strength β-type Ti-15Mo alloy (mass%) with isothermal ω-phase precipitation. Pre-existent mechanical {332}〈113〉 twins hinder the early onset of plastic instability (necking) after yielding, which is often caused by the presence of the isothermal ω-phase, and enhance the uniform elongation markedly from 0% to 13% at a yield strength level of 900MPa. .
137. Koyama M., Akiyama E., Tsuzaki K., Hydrogen embrittlement in Al-added twinning-induced plasticity steels evaluated by tensile tests during hydrogen charging, ISIJ International , 52, 12, 2283-2287, 2012.12, Hydrogen embrittlement of a Fe-18Mn-0.6C-1.5Al steel was observed in tensile deformation during cathodic hydrogen charging. The fracture mode was quasi-cleavage fracture. The relationship between diffusible hydrogen content and fracture stress was arranged by the power law like that for ferritic and Al-free TWIP steels. The Al addition did not affect the magnitude of the degradation of hydrogen embrittlement property at the same current density in TWIP steels. However, the Al-added steel showed a suppression of hydrogen entry and a larger total elongation in comparison to those of the Al-free TWIP steel in the same environment, although the Al addition decreased fracture stress. The larger elongation is one of the reasons for why the Al addition improves the hydrogen embrittlement property of cup specimens. .
138. K. Tsuchiya, T. Hara, S. Emura, X. Min, S. Ii, Kaneaki Tsuzaki, X. Man, K. Tanaka, A. Uemoto, 3D observation of isothermal ω phase in β-Ti alloy by novel orthogonal FIB/SEM system, 12th World Conference on Titanium, Ti 2011 Ti 2011 - Proceedings of the 12th World Conference on Titanium, 1, 593-596, 2012.12, Aging of metastable β-Ti alloys leads to the fine precipitation of co phase, resulting in a marked increase in strength accompanied by a significant loss in ductility (omega brittleness). Due to their small size and high number density, the observation method for co phase has been limited to transmission electron microscopy (TEM). The present report describes the result of 3D observations of the isothermal co phase by a newly developed FIB/SEM system In this system, a SEM column and FIB column are arranged orthogonally; this arrangement ensures that the surface sectioned by FIB can be perpendicular to the incident electron beam It was possible to observe the co particles of less than 10 nm with a very high contrast by an in-lens secondary-electron (SE) detector. The ω particles appeared darker than the matrix in the SE images. This may be due to the lower Mo concentration in the co particles than in the surrounding matrix. It was revealed that the shape of the particle varies with the size. The larger particles (about 100 nm) are ellipsoidal while the smaller ones are spherical. In addition, by serial sectioning (cut &. see),it was possible to obtain a 3D image for the volume of a micrometer cube containing numerous co particles, which is much larger than the volume observable by a TEM As a summary, this new FIB/SEM system is an extremely powerful tool and provides an opportunity to conduct an accurate quantitative assessment on co phase formation, which has never been done..
139. Koyama M., Sawaguchi T., Tsuzaki K., Premature fracture mechanism in an Fe-Mn-C austenitic steel, Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science , 43, 11, 4063-4074, 2012.11, We investigated the cause for poor ductility in austenitic Fe-Mn-C steels under a specific condition. Tensile tests were performed on an Fe-17Mn-0.3C steel at 273 K, 294 K, 323 K, 373 K, 423 K, 473 K, and 523 K (0 °C, 21 °C, 50 °C, 100 °C, 150 °C, 200 °C, and 250 °C). Microstructural observations were conducted by optical microscopy, atomic force microscopy, scanning electron microscopy and the X-ray diffraction method. e-martensitic transformation was concluded to be the major cause for the poor ductility. The cracks were initiated from the annealing twin boundaries that interacted with the ε-martensite. .
140. Koyama M., Sawaguchi T., Tsuzaki K., Effect of deformation temperature on tensile properties in a pre-cooled Fe-Mn-C austenitic steel, Materials Science and Engineering A , 556, 30, 331 -336, 2012.10, We investigated the tensile deformation behavior of a Fe-17Mn-0.3C (wt%) steel containing thermally-induced HCP-martensite that was formed by cooling to 77. K beforehand from various temperatures. In the temperature range where deformation-induced HCP→FCC reverse transformation and deformation twinning occur, the yield strength was enhanced by the pre-existing HCP-martensite, and the brittle cracking associated with the HCP-martensite was suppressed by the deformation-induced HCP→FCC reverse transformation after yielding. Additionally, the work hardening was sustained by deformation twinning. As a result, the yield and flow stresses were enhanced without any loss in elongation at the specific temperatures that were used in this study. .
141. Kaoru Sekido, Takahito Ohmura, Toru Hara, Kaneaki Tsuzaki, Effect of dislocation density on the initiation of plastic deformation on Fe-C steels, Materials Transactions, 10.2320/matertrans.M2011356, 53, 5, 907-912, 2012.10, The effect of pre-existing dislocations and interstitial carbon on the initiation of plastic deformation in interstitial free (IF) steel and ultra low carbon (ULC) steel were investigated by the nanoindentation technique. The critical load, P c, of the pop-in phenomenon, which corresponds to plasticity initiation, appeared clearly on the loading curve. The P c in high dislocation density materials was smaller than that in low dislocation density materials with no difference between IF and ULC while the P c in low dislocation density materials was remarkably higher in ULC than in IF. These results indicate that the interstitial carbon in the matrix does not affect the pop-in phenomenon when there are pre-existing dislocations or dislocation sources, and we discuss the reason for their occurrence occurs in high dislocation density materials..
142. MIn X., Tsuzaki K., Emura S., Tsuchiya K., Heterogeneous twin formation and its effect on tensile properties in Ti-Mo based β titanium alloys, Materials Science and Engineering A , 554, 30, 53-60, 2012.09, The deformation modes and the tensile properties were investigated in a Ti-15Mo-5Zr alloy with different heat treatments. Both the {3. 3. 2}〈1. 1. 3〉 twinning and the dislocation slip occurred in the samples with the heterogeneous distribution of Mo and Zr atoms. On the other hand, the twinning disappeared when the elemental distribution became homogeneous. The high yield strength and significant uniform elongation resulted from the combination of the twinning and the slip, and the more significant work hardening rate was obtained by the enhancement of the twinning. The heterogeneous twin formation was mainly discussed based on the effects of the interval of heterogeneous elemental distribution and the grain orientation. .
143. X. H. Min, Kaneaki Tsuzaki, S. Emura, K. Tsuchiya, Heterogeneous twin formation and its effect on tensile properties in Ti-Mo based β titanium alloys, Materials Science and Engineering A, 10.1016/j.msea.2012.06.009, 554, 53-60, 2012.09, The deformation modes and the tensile properties were investigated in a Ti-15Mo-5Zr alloy with different heat treatments. Both the {3. 3. 2}〈1. 1. 3〉 twinning and the dislocation slip occurred in the samples with the heterogeneous distribution of Mo and Zr atoms. On the other hand, the twinning disappeared when the elemental distribution became homogeneous. The high yield strength and significant uniform elongation resulted from the combination of the twinning and the slip, and the more significant work hardening rate was obtained by the enhancement of the twinning. The heterogeneous twin formation was mainly discussed based on the effects of the interval of heterogeneous elemental distribution and the grain orientation..
144. Koyama M., Sawaguchi T., Tsuzaki K., Selective appearance of-martensitic transformation and dynamic strain aging in Fe-Mn-C austenitic steels, Philosophical Magazine , 92, 24, 3051-3063, 2012.08, The influence of stress-induced-martensitic transformation on the serrated flow behavior associated with dynamic strain aging was investigated. The-martensitic transformation was controlled by changing the deformation temperature and adding Si to Fe-17Mn-xSi-0.3C alloys. The addition of Si promoted the-martensitic transformation, and suppressed the slip deformation due to solution hardening. The initiation of serrations around room temperature was delayed by the promotion of-martensitic transformation which initiated plastic deformation. The critical stress for the occurrence of serrations and the critical stress for the occurrence of slip deformation were found to have a linear relationship..
145. Zhang L., Ohmura T., Sekido K., Hara T., Nakajima K., Tsuzaki K., Dislocation character transition and related mechanical response in a body-centered cubic single crystal, Scripta Materialia , 67, 4, 388-391, 2012.08, In situ compression of a body-centered cubic single crystal Fe-3% Si pillar in transmission electron microscopy (TEM) by scanning TEM mode was performed to observe a single dislocation moving through the pillar in real time. The deformation is characterized by a series of pronounced pop-ins, which are accompanied by increases in stress. It is shown that these stress increases are caused by the change from an edge dislocation dominated process to a screw dislocation dominated process. .
146. Ling Zhang, Takahito Ohmura, Kaoru Sekido, Toru Hara, Kiyomi Nakajima, Kaneaki Tsuzaki, Dislocation character transition and related mechanical response in a body-centered cubic single crystal, Scripta Materialia, 10.1016/j.scriptamat.2012.05.032, 67, 4, 388-391, 2012.08, In situ compression of a body-centered cubic single crystal Fe-3% Si pillar in transmission electron microscopy (TEM) by scanning TEM mode was performed to observe a single dislocation moving through the pillar in real time. The deformation is characterized by a series of pronounced pop-ins, which are accompanied by increases in stress. It is shown that these stress increases are caused by the change from an edge dislocation dominated process to a screw dislocation dominated process..
147. Motomichi Koyama, Takahiro Sawaguchi, Kaneaki Tsuzaki, Selective appearance of-martensitic transformation and dynamic strain aging in Fe-Mn-C austenitic steels, Philosophical Magazine, 10.1080/14786435.2012.685769, 92, 24, 3051-3063, 2012.08, The influence of stress-induced-martensitic transformation on the serrated flow behavior associated with dynamic strain aging was investigated. The-martensitic transformation was controlled by changing the deformation temperature and adding Si to Fe-17Mn-xSi-0.3C alloys. The addition of Si promoted the-martensitic transformation, and suppressed the slip deformation due to solution hardening. The initiation of serrations around room temperature was delayed by the promotion of-martensitic transformation which initiated plastic deformation. The critical stress for the occurrence of serrations and the critical stress for the occurrence of slip deformation were found to have a linear relationship..
148. Ohmura T., Zhang L., Sekido K., Tsuzaki K., Effects of lattice defects on indentation-induced plasticity initiation behavior in metals, Journal of Materials Research , 27, 13, 1742-1749, 2012.07, Plasticity initiation behavior that appears as a pop-in phenomenon on a loading process during indentation-induced deformation was investigated to reveal the effects of lattice defects such as grain boundary and solute element for various metallic materials including Fe alloys through instrumented nanoindentation techniques. The critical load P c of pop-in on a loading process is lower in the vicinity of the grain boundary than in the grain interior, but the relative hardness of the boundary is equal to or greater than that in grain interior. In-solution Si produces a larger increase in the P c for both the grain boundary and the grain interior in the Fe-Si alloy than in the interstitial-free steel. The maximum shear stress corresponding to the P c underneath the indenter is directly proportional to the shear modulus in single crystals with various crystallographic structures. Microstructural effects on the P c are considered based on some dislocation models. .
149. MIn X., Sawaguchi T., Zhang X., Tsuzaki K., Reasons for incomplete shape recovery in polycrystalline Fe-Mn-Si shape memory alloys, Scripta Materialia , 67, 1, 37-40, 2012.07, Quantitative surface relief analysis proved that the incomplete shape recovery of a polycrystalline Fe-28Mn-6Si-5Cr alloy was not caused by slip deformation on loading but by irreversible phase transformation on heating, under given conditions ([54̄1̄] tensile axis, 5.9% strain). The observed area showed a higher recovery strain than the macroscopic recovery strain, implying inherently high reversibility. However, the value was significantly lower than that of a single crystal, due to the geometric constraint from surrounding grains, which reflected different transformation dislocations between forward and reverse transformations. .
150. Jafari M., Kimura Y., Tsuzaki K., Enhancement of upper shelf energy through delamination fracture in 0.05 pct P doped high-strength steel, Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science , 43, 7, 2453-2465, 2012.07, An ultrafine elongated grain (UFEG) structure with strong (110)//rolling direction (RD) fiber deformation texture was produced by warm-caliber rolling at 773 K (500 °C) and final tempering at 823 K (550 °C), namely tempforming in the 1200 MPa-class, medium-carbon, lowalloy steel with phosphorus (P) content of 0.053 wt pct. Charpy impact tests and tensile tests were performed at a temperature range of 77 K (-196 °C) to 623 K (350 °C) on the tempformed (TF) samples along with a conventional quenched and tempered (QT) samples. The QT structure showed a low upper shelf energy of 70 J and a high ductile-to-brittle transition temperature (DBTT) of 373 K (100 °C) as a result of P segregation and intergranular fracture. A remarkable increase in the upper shelf energy to 150 J from 70 J and a low DBTT of approximately 103 K (-170 °C) were obtained in the UFEG structure. P segregation embrittlement disappeared completely in the UFEG structure, and ductile fracture on the planes normal to RD along with delamination fracture on the planes along RD were observed at a temperature range of 123 K (-150 °C) to 423 K (150 °C). The enhanced delamination occurred because of the microstructural anisotropy of the UFEG structure, a strong (110)//RD fiber deformation texture, and interfaces (i.e. ferrite grain boundaries and cementite particles-ferrite matrix interfaces) weakened by P segregation as feasible crack propagation paths. We studied the delamination (crack-arrester-type) fracture in 0.053 pct P doped high-strength steel along with upper shelf energy and DBTT obtained from the UFEG structure. .
151. T. Ohmura, L. Zhang, K. Sekido, Kaneaki Tsuzaki, Effects of lattice defects on indentation-induced plasticity initiation behavior in metals, Journal of Materials Research, 10.1557/jmr.2012.161, 27, 13, 1742-1749, 2012.07, Plasticity initiation behavior that appears as a pop-in phenomenon on a loading process during indentation-induced deformation was investigated to reveal the effects of lattice defects such as grain boundary and solute element for various metallic materials including Fe alloys through instrumented nanoindentation techniques. The critical load P c of pop-in on a loading process is lower in the vicinity of the grain boundary than in the grain interior, but the relative hardness of the boundary is equal to or greater than that in grain interior. In-solution Si produces a larger increase in the P c for both the grain boundary and the grain interior in the Fe-Si alloy than in the interstitial-free steel. The maximum shear stress corresponding to the P c underneath the indenter is directly proportional to the shear modulus in single crystals with various crystallographic structures. Microstructural effects on the P c are considered based on some dislocation models..
152. Meysam Jafari, Yuuji Kimura, Kaneaki Tsuzaki, Enhancement of upper shelf energy through delamination fracture in 0.05 pct P doped high-strength steel, Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science, 10.1007/s11661-012-1095-7, 43, 7, 2453-2465, 2012.07, An ultrafine elongated grain (UFEG) structure with strong (110)//rolling direction (RD) fiber deformation texture was produced by warm-caliber rolling at 773 K (500 °C) and final tempering at 823 K (550 °C), namely tempforming in the 1200 MPa-class, medium-carbon, lowalloy steel with phosphorus (P) content of 0.053 wt pct. Charpy impact tests and tensile tests were performed at a temperature range of 77 K (-196 °C) to 623 K (350 °C) on the tempformed (TF) samples along with a conventional quenched and tempered (QT) samples. The QT structure showed a low upper shelf energy of 70 J and a high ductile-to-brittle transition temperature (DBTT) of 373 K (100 °C) as a result of P segregation and intergranular fracture. A remarkable increase in the upper shelf energy to 150 J from 70 J and a low DBTT of approximately 103 K (-170 °C) were obtained in the UFEG structure. P segregation embrittlement disappeared completely in the UFEG structure, and ductile fracture on the planes normal to RD along with delamination fracture on the planes along RD were observed at a temperature range of 123 K (-150 °C) to 423 K (150 °C). The enhanced delamination occurred because of the microstructural anisotropy of the UFEG structure, a strong (110)//RD fiber deformation texture, and interfaces (i.e. ferrite grain boundaries and cementite particles-ferrite matrix interfaces) weakened by P segregation as feasible crack propagation paths. We studied the delamination (crack-arrester-type) fracture in 0.053 pct P doped high-strength steel along with upper shelf energy and DBTT obtained from the UFEG structure..
153. Xiaohua Min, Takahiro Sawaguchi, Xin Zhang, Kaneaki Tsuzaki, Reasons for incomplete shape recovery in polycrystalline Fe-Mn-Si shape memory alloys, Scripta Materialia, 10.1016/j.scriptamat.2012.03.015, 67, 1, 37-40, 2012.07, Quantitative surface relief analysis proved that the incomplete shape recovery of a polycrystalline Fe-28Mn-6Si-5Cr alloy was not caused by slip deformation on loading but by irreversible phase transformation on heating, under given conditions ([54̄1̄] tensile axis, 5.9% strain). The observed area showed a higher recovery strain than the macroscopic recovery strain, implying inherently high reversibility. However, the value was significantly lower than that of a single crystal, due to the geometric constraint from surrounding grains, which reflected different transformation dislocations between forward and reverse transformations..
154. Koyama M., Akiyama E., Tsuzaki K., Hydrogen-induced delayed fracture of a Fe-22Mn-0.6C steel pre-strained at different strain rates, Scripta Materialia , 66, 11, 947-950, 2012.06, Hydrogen-induced delayed fracture under loading was investigated in a Fe-22Mn-0.6C twinning-induced plasticity steel that had been pre-deformed at various strain rates. Hydrogen-induced delayed fracture was suppressed by increasing the strain rate of the pre-deformation. In this study on the strain-rate effect, factors affecting the delayed fracture were found to be the negative strain-rate sensitivity of flow stress, stress drop caused by the relaxation phenomenon, and the increase in material strength due to strain aging. .
155. Koyama M., Akiyama E., Tsuzaki K., Effect of hydrogen content on the embrittlement in a Fe-Mn-C twinning-induced plasticity steel, Corrosion Science , 59, 277-281, 2012.06, The hydrogen embrittlement of a Fe-18Mn-0.6C austenitic steel (wt.%) was examined using tensile tests under hydrogen charging at various current densities. The tensile properties deteriorated due to the occurrence of intergranular fracture above a specific current density. The work hardening behavior was not affected by the hydrogen charging, indicating that the embrittlement was independent of the change in behavior of slip deformation, martensitic transformation, and twinning deformation. The relationship between the fracture stresses for the intergranular fracture and the diffusible hydrogen content of the austenitic steel was approximated to the power law similarly to ferritic high strength steels..
156. Motomichi Koyama, Eiji Akiyama, Kaneaki Tsuzaki, Effect of hydrogen content on the embrittlement in a Fe-Mn-C twinning-induced plasticity steel, Corrosion Science, 10.1016/j.corsci.2012.03.009, 59, 277-281, 2012.06, The hydrogen embrittlement of a Fe-18Mn-0.6C austenitic steel (wt.%) was examined using tensile tests under hydrogen charging at various current densities. The tensile properties deteriorated due to the occurrence of intergranular fracture above a specific current density. The work hardening behavior was not affected by the hydrogen charging, indicating that the embrittlement was independent of the change in behavior of slip deformation, martensitic transformation, and twinning deformation. The relationship between the fracture stresses for the intergranular fracture and the diffusible hydrogen content of the austenitic steel was approximated to the power law similarly to ferritic high strength steels..
157. Motomichi Koyama, Eiji Akiyama, Kaneaki Tsuzaki, Hydrogen-induced delayed fracture of a Fe-22Mn-0.6C steel pre-strained at different strain rates, Scripta Materialia, 10.1016/j.scriptamat.2012.02.040, 66, 11, 947-950, 2012.06, Hydrogen-induced delayed fracture under loading was investigated in a Fe-22Mn-0.6C twinning-induced plasticity steel that had been pre-deformed at various strain rates. Hydrogen-induced delayed fracture was suppressed by increasing the strain rate of the pre-deformation. In this study on the strain-rate effect, factors affecting the delayed fracture were found to be the negative strain-rate sensitivity of flow stress, stress drop caused by the relaxation phenomenon, and the increase in material strength due to strain aging..
158. Lee T., Koyama M., Tsuzaki K., Lee Y.H., Chong S.L., Tensile deformation behavior of Fe-Mn-C TWIP steel with ultrafine elongated grain structure, Materials Letters , 75, 15, 169-171, 2012.05, An Fe-17Mn-0.6C steel with ultrafine elongated grain structure was successfully produced utilizing a multi-pass caliber-rolling process at 773 K. The uniform elongation of the developed steel was not severely degraded although its strength was greatly enhanced, leading to superior tensile properties. The result contrasted with most ultrafine-grained metals, including C-free TWIP steels, reporting the critical loss of uniform elongation. The peculiar phenomenon in the present work was attributed to the high work hardening capacity of C-added TWIP steels with the submicrocrystalline structure caused by dynamic strain aging, deformation twinning, and formation of stacking faults even after strong grain refinement. .
159. Nie Y., Kimura Y., Inoue T., Yin F., Akiyama E., Tsuzaki K., Hydrogen embrittlement of a 1500-MPa tensile strength level steel with an ultrafine elongated grain structure, Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science , 43, 5, 1670-1687, 2012.05, A deformation of a tempered martensitic structure (i.e., tempforming) at 773 K (500°C) was applied to a 0.6 pct C-2 pct Si-1 pct Cr steel. The hydrogen embrittlement (HE) property of the tempformed (TF) steel was investigated by a slow strain rate test (SSRT) and an accelerated atmospheric corrosion test (AACT). Hydrogen content within the samples after SSRT andAACT was measured by thermal desorption spectrometry (TDS). The tempforming at 773 K (500°C) using multipass caliber rolling with an accumulative are reduction of 76 pct resulted in the evolution of an ultrafine elongated grain (UFEG) structure with a strong h110i//rolling direction (RD) fiber deformation texture and a dispersion of spheroidized cementite particles. The SSRT of the pre-hydrogen-charged notched specimens and the AACT demonstrated that the TF sample had superior potential for HE resistance to the conventional quenched and tempered (QT) sample at a tensile strength of 1500 MPa. The TDS analysis also indicated that the hydrogen might be mainly trapped by reversible trapping sites such as grain boundaries and dislocations in the TF sample, and the hydrogen trapping states of the TF sample were similar to those of the QT sample. The QT sample exhibited hydrogen-induced intergranular fracture along the boundaries of coarse prioraustenite grains. In contrast, the hydrogen-induced cracking occurred in association with the UFEG structure in the TF sample, leading to the higher HE resistance of the TF sample. .
160. I. Shakhova, V. Dudko, A. Belyakov, Kaneaki Tsuzaki, R. Kaibyshev, Effect of large strain cold rolling and subsequent annealing on microstructure and mechanical properties of an austenitic stainless steel, Materials Science and Engineering A, 10.1016/j.msea.2012.02.101, 545, 176-186, 2012.05, The microstructural evolution of an S304H steel during bar rolling to a strain of 4 and subsequent annealing as well as its effect on the mechanical properties were investigated. The cold working was accompanied by a strain-induced martensitic transformation, leading to the development of lamellar-type microstructure consisting of highly elongated austenite/ferrite subgrains with a mean transverse size of approximately 50. nm; the austenite volume fraction was approximately 0.35. This material exhibited a yield strength above 2000. MPa. The subsequent annealing resulted in grain coarsening following the ferrite → austenite reversion, which led to almost full austenitization at temperatures above 700 °C. The formation of the austenite/ferrite lamellar structure that mixed with separate equiaxed grains occurred after annealing at temperatures of T≤ 700 °C. The grain coarsening was accompanied by a degradation in strength, although the yield strength of above 1000. MPa remained after 2. h of annealing at 700 °C. The discontinuous recrystallization of austenite resulted in the development of a relatively coarse-grained microstructure at T≥ 800 °C..
161. Yihong Nie, Yuuji Kimura, Tadanobu Inoue, Fuxing Yin, Eiji Akiyama, Kaneaki Tsuzaki, Hydrogen embrittlement of a 1500-MPa tensile strength level steel with an ultrafine elongated grain structure, Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science, 10.1007/s11661-011-0974-7, 43, 5, 1670-1687, 2012.05, A deformation of a tempered martensitic structure (i.e., tempforming) at 773 K (500°C) was applied to a 0.6 pct C-2 pct Si-1 pct Cr steel. The hydrogen embrittlement (HE) property of the tempformed (TF) steel was investigated by a slow strain rate test (SSRT) and an accelerated atmospheric corrosion test (AACT). Hydrogen content within the samples after SSRT andAACT was measured by thermal desorption spectrometry (TDS). The tempforming at 773 K (500°C) using multipass caliber rolling with an accumulative are reduction of 76 pct resulted in the evolution of an ultrafine elongated grain (UFEG) structure with a strong h110i//rolling direction (RD) fiber deformation texture and a dispersion of spheroidized cementite particles. The SSRT of the pre-hydrogen-charged notched specimens and the AACT demonstrated that the TF sample had superior potential for HE resistance to the conventional quenched and tempered (QT) sample at a tensile strength of 1500 MPa. The TDS analysis also indicated that the hydrogen might be mainly trapped by reversible trapping sites such as grain boundaries and dislocations in the TF sample, and the hydrogen trapping states of the TF sample were similar to those of the QT sample. The QT sample exhibited hydrogen-induced intergranular fracture along the boundaries of coarse prioraustenite grains. In contrast, the hydrogen-induced cracking occurred in association with the UFEG structure in the TF sample, leading to the higher HE resistance of the TF sample..
162. Taekyung Lee, Motomichi Koyama, Kaneaki Tsuzaki, You Hwan Lee, Chong Soo Lee, Tensile deformation behavior of Fe-Mn-C TWIP steel with ultrafine elongated grain structure, Materials Letters, 10.1016/j.matlet.2012.02.012, 75, 169-171, 2012.05, An Fe-17Mn-0.6C steel with ultrafine elongated grain structure was successfully produced utilizing a multi-pass caliber-rolling process at 773 K. The uniform elongation of the developed steel was not severely degraded although its strength was greatly enhanced, leading to superior tensile properties. The result contrasted with most ultrafine-grained metals, including C-free TWIP steels, reporting the critical loss of uniform elongation. The peculiar phenomenon in the present work was attributed to the high work hardening capacity of C-added TWIP steels with the submicrocrystalline structure caused by dynamic strain aging, deformation twinning, and formation of stacking faults even after strong grain refinement..
163. Andrey Belyakov, Kaneaki Tsuzaki, Rustam Kaibyshev, Ultrafine grain evolution in austenitic stainless steel during large strain deformation and subsequent annealing, 4th International Conference on Recrystallization and Grain Growth, ReX and GG IV Recrystallization and Grain Growth IV, 10.4028/www.scientific.net/MSF.715-716.273, 273-278, 2012.05, The structural changes in a 304-type austenitic stainless steel during large strain cold rolling and subsequent annealing were studied. The severe deformation resulted in the development of highly elongated grains/subgrains aligned along the rolling axis. The transverse grain/subgrain size rapidly decreased to its minimal value of about 50 nm at relatively small strains of ∼1 and then hardly changed upon following deformation. Such a structural response on cold working was associated with multiple twinning resulting in fast grain subdivision. The processing was accompanied by a partial martensitic transformation resulting in a decrease of austenite volume fraction to about 0.35 after straining to ε?= 4.0. Isochronal annealing for 30 min was characterised by a gradual coarsening of grains, the average size of which increased to about 200 nm after heating to 800°C. The high elongation of ferrite grains facilitated simultaneous homogeneous nucleation of austenite grains throughout the matrix upon heating; and, therefore, promoted the development of ultrafine grained structure with the size of structural elements well below 1 micron..
164. Koyama M., Akiyama E., Sawaguchi T., Raabe D., Tsuzaki K., Hydrogen-induced cracking at grain and twin boundaries in an Fe-Mn-C austenitic steel, Scripta Materialia , 66, 7, 459-462, 2012.04, Hydrogen embrittlement was observed in an Fe-18Mn-1.2C (wt.%) steel. The tensile ductility was drastically reduced by hydrogen charging during tensile testing. The fracture mode was mainly intergranular fracture, though transgranular fracture was also partially observed. The transgranular fracture occurred parallel to the primary and secondary deformation twin boundaries, as confirmed by electron backscattering diffraction analysis and orientation-optimized electron channeling contrast imaging. The microstructural observations indicate that cracks are initiated at grain boundaries and twin boundaries. .
165. Motomichi Koyama, Eiji Akiyama, Takahiro Sawaguchi, Dierk Raabe, Kaneaki Tsuzaki, Hydrogen-induced cracking at grain and twin boundaries in an Fe-Mn-C austenitic steel, Scripta Materialia, 10.1016/j.scriptamat.2011.12.015, 66, 7, 459-462, 2012.04, Hydrogen embrittlement was observed in an Fe-18Mn-1.2C (wt.%) steel. The tensile ductility was drastically reduced by hydrogen charging during tensile testing. The fracture mode was mainly intergranular fracture, though transgranular fracture was also partially observed. The transgranular fracture occurred parallel to the primary and secondary deformation twin boundaries, as confirmed by electron backscattering diffraction analysis and orientation-optimized electron channeling contrast imaging. The microstructural observations indicate that cracks are initiated at grain boundaries and twin boundaries..
166. Motomichi Koyama, Takahiro Sawaguchi, Kaneaki Tsuzaki, Influence of dislocation separation on dynamic strain aging in a Fe-Mn-C austenitic steel, Materials Transactions, 10.2320/matertrans.M2011342, 53, 3, 546-552, 2012.04, The influences of deformation temperature and strain rate on the serrated flow behavior of a Fe-17Mn-0.3C alloy with a low stacking fault energy were investigated by the tensile tests in a temperature range of 273 to 523 K. Three regions were found when the deformation temperature was plotted against the critical strains for the onset of serrations. The critical strain decreased in the region of 273 to 323 K, increased in that of 323 to 423 K, and decreased again in that of 423 to 523K with increasing temperature. The first two regions are well known. However, the third region corresponding to that of high temperature has not been reported, and this region could be interpreted by separately considering the interactions of solute atoms with leading and trailing partials. Since the velocity of the leading partials is assumed to be significantly higher than that of the trailing partials, the critical strains in the first and third regions were concluded to result from trapping the trailing partials and the leading partials, respectively..
167. Koyama M., Sawaguchi T., Tsuzaki K., Influence of dislocation separation on dynamic strain aging in a Fe-Mn-C austenitic steel, Materials Transactions , 53, 3, 546-552, 2012.03, The influences of deformation temperature and strain rate on the serrated flow behavior of a Fe-17Mn-0.3C alloy with a low stacking fault energy were investigated by the tensile tests in a temperature range of 273 to 523 K. Three regions were found when the deformation temperature was plotted against the critical strains for the onset of serrations. The critical strain decreased in the region of 273 to 323 K, increased in that of 323 to 423 K, and decreased again in that of 423 to 523K with increasing temperature. The first two regions are well known. However, the third region corresponding to that of high temperature has not been reported, and this region could be interpreted by separately considering the interactions of solute atoms with leading and trailing partials. Since the velocity of the leading partials is assumed to be significantly higher than that of the trailing partials, the critical strains in the first and third regions were concluded to result from trapping the trailing partials and the leading partials, respectively. .
168. Koyama M., Sawaguchi T., Tsuzaki K., Inverse grain size dependence of critical strain for serrated flow in a Fe-Mn-C twinning-induced plasticity steel, Philosophical Magazine Letters, 92, 3, 145-152, 2012.03, The grain size dependence of critical strain for serrations associated with dynamic strain aging has been examined in a twinning-induced plasticity steel. Tensile tests were conducted at various deformation temperatures and strain rates in a Fe-17Mn-0.6C steel (mass%) with grain sizes 3.5, 10, 23, 37, and 44 μm. In addition, the carbon concentration varied from 0.3 to 0.8 in the Fe-17Mn-xC steels with coarse grains. The critical strain for the onset of serrations was found to show an inverse grain size dependence, i.e., the critical strain increased with the decrease in grain size, the opposite of what occurs in conventional alloys. .
169. MIn X., Zhang L., Sekido K., Ohmura T., Emura S., Tsuchiya K., Tsuzaki K., Strength evaluation of α and β phases by nanoindentation in Ti-15Mo alloys with Fe and Al addition, Materials Science and Technology , 28, 3, 342-347, 2012.03, The strengths of the α precipitate and the β matrix were evaluated by nanohardness in the Ti215Mo21Fe and Ti215Mo23Al alloys and compared to those of the Ti215Mo alloy. The α phases with similar size (a long axis of a few micrometres and a short axis of a few hundred nanometres), distribution and volume fraction were obtained in three alloys by adjusting the aging temperature. Analyses by SEM-EDS confirmed that Fe and Mo were enriched in the β phase and depleted in the α phase, while Al was enriched in the α phase and depleted in the β phase. Tensile tests were carried out, and the tensile strength was shown to be higher in the Ti215Mo21Fe and Ti215Mo23Al alloys than in the Ti215Mo alloy. The nanohardness measurements indicated that the α phase was softer than the β phase in both Ti215Mo21Fe and Ti215Mo alloys, while it was harder in the Ti215Mo23Al alloy. The increased tensile strength was mainly caused by the strength of the Fe enriched β phase in the Ti215Mo21Fe alloy and by the strength of the Al enriched α phase in the Ti215Mo23Al alloy. .
170. Oleg Sitdikov, Elena Avtokratova, Rita Babicheva, Taku Sakai, Kaneaki Tsuzaki, Yoshimi Watanabe, Influence of processing regimes on fine-grained microstructure development in an AlMgSc alloy by hot equal-channel angular pressing, Materials Transactions, 10.2320/matertrans.MD201108, 53, 1, 56-62, 2012.03, Grain refinement during equal-channel angular pressing (ECAP) was studied in a commercial Al5.8% Mg0.3% Sc alloy at temperatures from 473 to 723K (̃0.50.8T m). The samples were quenched in water in every ECAP pass, which is a conventional cyclic process. ECAP to ε = 12 resulted in ultrafine-grained structures developed uniformly at high strains at 473 and 723 K, while processing at 573K led to the evolution of a duplex grain structure containing partially much coarser grains. In contrast, ECAP process continuously carried out to ε = 12 without interruption at 573 K, resulted in development of a uniform ultrafine-grained structure. Effects of processing regimes on microstructural evolution in the AlMgSc alloy are discussed..
171. Motomichi Koyama, Takahiro Sawaguchi, Kaneaki Tsuzaki, Inverse grain size dependence of critical strain for serrated flow in a Fe-Mn-C twinning-induced plasticity steel, Philosophical Magazine Letters, 10.1080/09500839.2011.640645, 92, 3, 145-152, 2012.03, The grain size dependence of critical strain for serrations associated with dynamic strain aging has been examined in a twinning-induced plasticity steel. Tensile tests were conducted at various deformation temperatures and strain rates in a Fe-17Mn-0.6C steel (mass%) with grain sizes 3.5, 10, 23, 37, and 44 μm. In addition, the carbon concentration varied from 0.3 to 0.8 in the Fe-17Mn-xC steels with coarse grains. The critical strain for the onset of serrations was found to show an inverse grain size dependence, i.e., the critical strain increased with the decrease in grain size, the opposite of what occurs in conventional alloys..
172. X. H. Min, L. Zhang, K. Sekido, T. Ohmura, S. Emura, K. Tsuchiya, Kaneaki Tsuzaki, Strength evaluation of α and β phases by nanoindentation in Ti-15Mo alloys with Fe and Al addition, Materials Science and Technology, 10.1179/1743284711Y.0000000052, 28, 3, 342-347, 2012.02, The strengths of the α precipitate and the β matrix were evaluated by nanohardness in the Ti215Mo21Fe and Ti215Mo23Al alloys and compared to those of the Ti215Mo alloy. The α phases with similar size (a long axis of a few micrometres and a short axis of a few hundred nanometres), distribution and volume fraction were obtained in three alloys by adjusting the aging temperature. Analyses by SEM-EDS confirmed that Fe and Mo were enriched in the β phase and depleted in the α phase, while Al was enriched in the α phase and depleted in the β phase. Tensile tests were carried out, and the tensile strength was shown to be higher in the Ti215Mo21Fe and Ti215Mo23Al alloys than in the Ti215Mo alloy. The nanohardness measurements indicated that the α phase was softer than the β phase in both Ti215Mo21Fe and Ti215Mo alloys, while it was harder in the Ti215Mo23Al alloy. The increased tensile strength was mainly caused by the strength of the Fe enriched β phase in the Ti215Mo21Fe alloy and by the strength of the Al enriched α phase in the Ti215Mo23Al alloy..
173. Sitdikov O., Avtokratova E., Babicheva R., Sakai T., Tsuzaki K., Watanabe Y., Influence of processing regimes on fine-grained microstructure development in an AlMgSc alloy by hot equal-channel angular pressing , Materials Transactions , 53, 1, 56-62, 2012.01, Grain refinement during equal-channel angular pressing (ECAP) was studied in a commercial Al5.8% Mg0.3% Sc alloy at temperatures from 473 to 723K (̃0.50.8T m). The samples were quenched in water in every ECAP pass, which is a conventional cyclic process. ECAP to ε = 12 resulted in ultrafine-grained structures developed uniformly at high strains at 473 and 723 K, while processing at 573K led to the evolution of a duplex grain structure containing partially much coarser grains. In contrast, ECAP process continuously carried out to ε = 12 without interruption at 573 K, resulted in development of a uniform ultrafine-grained structure. Effects of processing regimes on microstructural evolution in the AlMgSc alloy are discussed. .
174. Koyama M., Sawaguchi T., Tsuzaki K., Quasi-cleavage fracture along annealing twin boundaries in a FE-MN-C austenitic steel, ISIJ International , 52, 1, 161-163, 2012.01, A study was conducted to demonstrate quasi-cleavage fracture along annealing twin boundaries in a Fe-Mn-C austenitic steel. A steel with a chemical composition of Fe-16.8Mn-0.29C was prepared by vacuum induction melting. The steel was hot forged and rolled at 1 273 K and it was solution treated at 1 273 K for 3.6 ks under an argon atmosphere and water quenched. Microstructural observations were made by optical microscopy, atomic force microscopy (AFM), and scanning electron microscopy (SEM) to produce a fractograph. Electron backscatter diffraction (EBSD) analyses were also conducted at 20 kV with a beam step size of 1 μm to characterize the boundaries. The specimen for optical microscopy, AFM, and EBSD analyses was electrolytically polished at 277 K after mechanical polishing. The serrations at ambient temperature in the steels were reported to be attributed to dynamic strain aging..
175. Jafari M., Kimura Y., Tsuzaki K., Enhanced upper shelf energy by ultrafine elongated grain structures in 1100MPa high strength steel, Materials Science and Engineering A , 532, 15, 420-429, 2012.01, The ultrafine elongated grain (UFEG) structure with strong <1. 1. 0>//RD fiber deformation texture was produced by warm caliber-rolling at 500°C and final tempering at 550°C, namely tempforming in the 1100. MPa medium carbon-low alloy steel with ultralow phosphorus and sulfur concentrations. Charpy impact tests were performed at temperature range of -196°C to 150°C on the UFEG structure along with a conventional quenched and tempered (QT) structure. A remarkable increase in upper shelf energy of 150. J was obtained in the UFEG structure without delamination, while that of QT structure was 97. J. The UFEG structures tempered at higher temperatures of 625 and 700°C showed remarkable increase in Charpy absorbed energy from 150. J to 187. J and 203. J. Also, the QT structure absorbed almost same energy as UFEG structure at 700°C. The enhanced toughness was discussed with tensile ductility, void nucleation and growth, and their relations to microstructure including the <1. 1. 0>//RD fiber deformation texture. .
176. Koyama M., Akiyama E., Tsuzaki K., Hydrogen embrittlement in a Fe-Mn-C ternary twinning-induced plasticity steel , Corrosion Science , 54, 1, 1-4, 2012.01, The influence of hydrogen entry on ductility was evaluated in a ternary twinning-induced plasticity (TWIP) steel with a composition of Fe-18Mn-0.6C in wt.% using tensile tests. The samples with a thickness of 1.2. mm were charged with hydrogen galvanostatically during the tensile tests. Significant hydrogen content was introduced by the hydrogen-charging. The total elongation was significantly deteriorated from approx. 60% to 30% by the hydrogen-charging. A clear intergranular fracture surface was observed in a vicinity of the sample surface in the hydrogen-charged samples. .
177. Wei F.G., Enomoto M., Tsuzaki K., Applicability of the Kissinger's formula and comparison with the McNabb-Foster model in simulation of thermal desorption spectrum, Computational Materials Science , 51, 1, 322-330, 2012.01, The Kissinger's formula has been analyzed theoretically and numerically to be applicable in simulation of not only the detrapping-controlled thermal desorption but also the diffusion-controlled thermal desorption provided that a sufficient pre-exposure before thermal desorption is carried out in the diffusion-controlled desorption. The desorption activation energy or the binding energy can be evaluated by a single thermal desorption spectrum from a single type of trap site. In the case of detrapping-controlled desorption the constant parameter A in the Kissinger's formula approaches the pre-exponential factor of detrap parameter, p0, in the McNabb-Foster model as specimen size becomes smaller. In the case of diffusion-controlled desorption where local equilibrium may be maintained and an effective diffusivity can be expressed, the A value changes according to A = α2D0 where D 0 is the pre-exponential factor of effective diffusivity and α is a geometrical parameter of specimen. .
178. Fu Gao Wei, Masato Enomoto, Kaneaki Tsuzaki, Applicability of the Kissinger's formula and comparison with the McNabb-Foster model in simulation of thermal desorption spectrum, Computational Materials Science, 10.1016/j.commatsci.2011.07.009, 51, 1, 322-330, 2012.01, The Kissinger's formula has been analyzed theoretically and numerically to be applicable in simulation of not only the detrapping-controlled thermal desorption but also the diffusion-controlled thermal desorption provided that a sufficient pre-exposure before thermal desorption is carried out in the diffusion-controlled desorption. The desorption activation energy or the binding energy can be evaluated by a single thermal desorption spectrum from a single type of trap site. In the case of detrapping-controlled desorption the constant parameter A in the Kissinger's formula approaches the pre-exponential factor of detrap parameter, p0, in the McNabb-Foster model as specimen size becomes smaller. In the case of diffusion-controlled desorption where local equilibrium may be maintained and an effective diffusivity can be expressed, the A value changes according to A = α2D0 where D 0 is the pre-exponential factor of effective diffusivity and α is a geometrical parameter of specimen..
179. Andrey Belyakov, Nadezhda Dudova, Marina Tikhonova, Taku Sakai, Kaneaki Tsuzaki, Rustam Kaibyshev, Dynamic recrystallization mechanisms operating under different processing conditions, 7th International Conference on Processing and Manufacturing of Advanced Materials, THERMEC'2011 THERMEC 2011, 10.4028/www.scientific.net/MSF.706-709.2704, 2704-2709, 2012.01, Dynamic recrystallization (DRX) is one of the most important mechanisms for microstructure evolution during deformation of various metals and alloys. So-called discontinuous DRX usually develops in structural materials with low to medium stacking fault energy during hot working. The local migration, i.e. bulging, of grain boundaries leads to the formation of recrystallization nuclei, which then grow out consuming work-hardened surroundings. The cyclic character of nucleation and growth of new grains during deformation results in a dynamically constant average grain size. The dynamic grain size is sensitively dependent on temperature and strain rate and can be expressed by a power law function of flow stress with a grain size exponent of about -0.7 under conditions of hot working. Recent studies on DRX phenomenon suggest that a decrease in deformation temperature changes the structural mechanism for new grain formation. As a result, the grain size exponent in the relationship between the dynamic grain size and flow stress approaches about -0.25 under warm working conditions..
180. F. G. Wei, Kaneaki Tsuzaki, Hydrogen trapping phenomena in martensitic steels, Gaseous Hydrogen Embrittlement of Materials in Energy Technologies The Problem, its Characterisation and Effects on Particular Alloy Classes, 10.1533/9780857093899.3.493, 493-525, 2012.01, This chapter begins with a brief description of hydrogen trapping theories and the major hydrogen trapping evaluation techniques including thermal desorption spectrometry (TDS). It then illustrates the hydrogen trapping characteristics of elemental martensitic microstructural features using alloys with a simple chemical composition and well-defined microstructure which maximize the effect of a microstructure feature of interest..
181. Meysam Jafari, Yuuji Kimura, Kaneaki Tsuzaki, Role of delamination fracture for enhanced impact toughness in 0.05 %P doped high strength steel with ultrafine elongated grain structure, 7th International Conference on Processing and Manufacturing of Advanced Materials, THERMEC'2011 THERMEC 2011 Supplement, 10.4028/www.scientific.net/AMR.409.231, 231-236, 2012.01, Ultrafine elongated grain (UFEG) structures with strong 〈110〉// rolling direction (RD) fiber deformation texture were produced by warm caliber-rolling at 773 K, namely tempforming in the 1200 MPa-class medium-carbon low-alloy steel with phosphorous (P) contents of 0.001 and 0.053 mass%. Charpy impact tests were performed at temperature range of -196 to 150 °C on the UFEG structure. Regardless of P content, high upper shelf energy about 145 J and a very low ductile to brittle transition temperature (DBTT) of around -175 °C were obtained. P segregation embrittlement completely disappeared in the 0.053 %P steel and both steels showed ductile fracture on the planes normal to RD at temperature range of -150 to 150 °C. The main reason for the high upper shelf energy and very low DBTT in the 0.053 %P steel would be delamination fracture along RD when both 0.001 and 0.053 %P steels showed quite similar microstructures including texture. Since the occurrence of delamination requires relatively weak interfaces or planes, P segregated to the ferrite grain boundaries and interfaces of cementite particles-ferrite matrix and made them feasible paths for crack branching and consequently delamination occurred. We showed in this work the advantage of delamination (crack arrester-type) on the high absorbed energy obtained by 0.053 %P steel in comparison with 0.001 %P steel..
182. Iaroslava Shakhova, Andrey Belyakov, Rustam Kaibyshev, Yuuji Kimura, Kaneaki Tsuzaki, Submicrocrystalline structures and tensile behaviour of stainless steels subjected to large strain deformation and subsequent annealing, 7th International Conference on Processing and Manufacturing of Advanced Materials, THERMEC'2011 THERMEC 2011 Supplement, 10.4028/www.scientific.net/AMR.409.607, 409, 607-612, 2012.01, Tensile behaviour of two steels with submicrocrystalline structures, i.e. a 304-type austenitic steel and an Fe-27%Cr-9%Ni austenitic-ferritic steel, was studied. The starting materials were subjected to large strain rolling and swaging to a total strain of ∼4 at ambient temperature. The severe deformation resulted in a partial martensitic transformation and the development of highly elongated austenite/ferrite (sub)grains aligned along the deformation axis. In the cold worked state, the transverse grain/subgrain size was about 100 nm in the 304-type steel and about 150 nm in the Fe-27%Cr-9%Ni steel. The grain refinement by severe plastic deformation resulted in increase of ultimate tensile strength to 2000 MPa and 1800 MPa in 304-type and Fe-27%Cr-9%Ni steels, respectively. The phase transformation and recrystallization took place concurrently upon annealing, leading to the development of submicrocrystalline structure consisting of austenite and ferrite grains. No significant softening took place under annealing at temperatures below 600°C. The tensile strength was 1920 MPa in 304-type steel and 1710 MPa in Fe-27%Cr-9%Ni steel after annealing at 500°C for 2 hours..
183. Xiaohua Min, Satoshi Emura, Seiichiro Ii, Koichi Tsuchiya, Kaneaki Tsuzaki, Ductility improvement in high strength Ti-Mo allovs by transition of deformation mode, 12th World Conference on Titanium, Ti 2011 Ti 2011 - Proceedings of the 12th World Conference on Titanium, 2, 1034-1037, 2012, This study demonstrated a possibility of ductility improvement in high strength β titanium alloys in the as-solution treated condition through the investigation of tensile properties and deformation microstructures in the Ti-lOMo and Ti-l5Mo base alloys with various β phase stability. The Ti-10Mo-2Fe and Ti-15Mo-5Zr alloys with an intennediate β phase stability showed a combination of high yield strength and large uniform elongation due to the heterogeneous deformation by dislocation slip and {332}<113> twinning. It was suggested that the deformation transition occurs partially between the slip and the twinning caused by the inhomogeneous elemental distribution..
184. Satoshi Emura, Xiaohua Min, Seiichiro Ii, Kaneaki Tsuzaki, Koichi Tsuchiya, Improvement of mechanical properties in Ti alloys using heterogeneous elemental distribution, 12th World Conference on Titanium, Ti 2011 Ti 2011 - Proceedings of the 12th World Conference on Titanium, 1, 536-539, 2012, Most of practical titanium alloys contain some alloying elements such as Nb and Mo, which are prone to segregation. In this study, we have tried to change the distribution of these alloying elements in two types of titanium alloys through thermomechanical treatment. Ti-Al-Nb base inter-metallic alloy, Ti-25Al-14Nb-2Mo-1Fe (mol %), was hot-forged and bar-rolled at 1273K in the (B2+α2) two phase region. After the rolling, Nb and/or Mo-rich region forms swirly band structure. Due to the segregation of these β stabilizing elements, we can obtain heterogeneous distribution of α2 phase particles after the heat treatment at 1273K, Room temperature tensile elongation value was improved from nearly 0 to 6 %, which can be attributed to this heterogeneous distribution of α2 phase particles. We also applied similar thermomechanical treatment on corrosion resistant β-type Ti-12Mo (mass %) alloys, and obtained Mo-rich swirly band-like structures. This led to heterogeneous precipitation of ω phase after aging at temperatures lower than 723K due to the local variation in β phase stability. Room temperature tensile elongation value after aging at 523K for 3. 6ks was improved from 4% to 19% with tensile strength of around 1000MPa..
185. Sekido K., Ohmura T., Zhang L., Hara T., Tsuzaki K., The effect of interstitial carbon on the initiation of plastic deformation of steels, Materials Science and Engineering A , 530, 1, 396-401, 2011.12, Pop-in behavior in nanoindentation was studied as a mechanism for the initiation of plastic deformation in two kinds of steels with different interstitial carbon contents; interstitial free (IF) and ultra low carbon (ULC) steels. The critical load Pc at which the pop-in occurs is higher in ULC than in IF, and the Pc decreases with decreasing loading rate, indicating that the pop-in mechanism is based on a thermal activation process. The interstitial carbon is thought to yield higher friction stress against dislocation movement and have an influence on the critical stress for the activation of the dislocation source formed underneath the indenter. .
186. Koyama M., Sawaguchi T., Tsuzaki K., Work hardening and uniform elongation of an ultrafine-grained Fe-33Mn binary alloy, Materials Science and Engineering A , 530, 1, 659-663, 2011.12, The grain-refinement effect on uniform elongation was examined in an Fe-33. wt%Mn alloy. The uniform elongation was 30% even in 0.7. μm grain size. No deformation-induced martensite and twins were observed; the significant uniform elongation was found to arise from formation of numerous stacking faults in the ultrafine-grained structure. .
187. Sekido K., Ohmura T., Sawaguchi T., Koyama M., Park H.W., Tsuzaki K., Nanoindentation/atomic force microscopy analyses of -martensitic transformation and shape memory effect in Fe-28Mn-6Si-5Cr alloy, Scripta Materialia , 65, 11, 942-945, 2011.12, Nanoindentation was conducted to investigate the stress-induced -martensitic transformation behavior and the shape memory effect in a Fe-28Mn-6Si-5Cr alloy. The shape memory effect was evaluated from the volume change in an indent mark caused by annealing measured by atomic force microscopy. Using the load-displacement (P-h) data obtained from nanoindentation, the plot of P/h vs. h showed two types of slopes, corresponding to the -martensitic transformation and slip deformation, and exhibited a correlation with the shape recovery ratio..
188. Koyama M., Sawaguchi T., Lee T., Lee C.S., Tsuzaki K., Work hardening associated with epsilon-martensitic transformation, deformation twinning and dynamic strain aging in Fe-17Mn-0.6C and Fe-17Mn-0.8C TWIP steels, Materials Science and Engineering A , 528, 24, 7310-7316, 2011.09, The tensile properties of carbon-containing twinning induced plasticity (TWIP) steels and their temperature dependence were investigated. Two steels with carbon concentrations of 0.6% and 0.8% (w/w) were tensile-tested at 173, 223, 273, 294, and 373. K. Three deformation modes were observed during tensile testing: e{open}-martensitic transformation, deformation twinning, and dynamic strain aging. The characteristic deformation mode that contributed to the work hardening rates changed with the deformation temperature and chemical compositions. The work hardening rate in the carbon-containing TWIP steels increased according to the deformation modes in the following order: e{open}-martensitic transformation > deformation twinning > dynamic strain aging. .
189. Motomichi Koyama, Takahiro Sawaguchi, Taekyung Lee, Chong Soo Lee, Kaneaki Tsuzaki, Work hardening associated with e{open}-martensitic transformation, deformation twinning and dynamic strain aging in Fe-17Mn-0.6C and Fe-17Mn-0.8C TWIP steels, Materials Science and Engineering A, 10.1016/j.msea.2011.06.011, 528, 24, 7310-7316, 2011.09, The tensile properties of carbon-containing twinning induced plasticity (TWIP) steels and their temperature dependence were investigated. Two steels with carbon concentrations of 0.6% and 0.8% (w/w) were tensile-tested at 173, 223, 273, 294, and 373. K. Three deformation modes were observed during tensile testing: e{open}-martensitic transformation, deformation twinning, and dynamic strain aging. The characteristic deformation mode that contributed to the work hardening rates changed with the deformation temperature and chemical compositions. The work hardening rate in the carbon-containing TWIP steels increased according to the deformation modes in the following order: e{open}-martensitic transformation > deformation twinning > dynamic strain aging..
190. MIn X., Tsuzaki K., Emura S., Nishimura T., Tsuchiya K., Optimization of strength, ductility and corrosion resistance in Ti-Mo base alloys by controlling Mo equivalency and bond order, Materials Transactions , 52, 8, 1611-1616, 2011.08, The crevice corrosion resistance in high chloride and high acidic solution at high temperature in Ti-15Mo-5Zr and Ti-15Mo-5Zr-1Fe alloys and the tensile properties at ambient temperature in Ti-15Mo-5Zr-1Fe alloy were investigated in this study. The crevice corrosion resistance in Ti-15Mo-5Zr alloy with a Mo equivalency of 18.2 mass%, which was previously reported to show a combination of the high yield strength and the significant uniform elongation, was almost twice as high as that in Ti-15Mo alloy when the bond order (Bo) increased from 2.8126 to 2.8232. A linear correlation between the crevice corrosion resistance and the Bo was confirmed in the Ti-Mo base alloys with the Bo between 2.7900 and 2.8232. Although the high crevice corrosion resistance, Ti-15Mo-5Zr-1Fe alloy with a Mo equivalency of 21.2 mass% showed high yield strength but negligible uniform elongation due to the deformation by dislocation slip. This study suggested that the two parameters of the Mo equivalency and the Bo are useful for an optimization of strength, ductility and corrosion resistance in Ti-Mo base alloys..
191. Akiyama E., Matsukado K., Li S., Tsuzaki K., Constant-load delayed fracture test of atmospherically corroded high strength steels, Applied Surface Science , 257, 19, 8275-8281, 2011.07, Constant load tests of circumferentially notched round bar specimens of high strength steels after cyclic corrosion test and outdoor exposure have been performed to demonstrate that delayed fracture occurs when the hydrogen content from the environment, HE, exceeds the critical hydrogen content for delayed fracture, HC. During the constant load tests the humidity around the specimen was increased in stepwise manner to increase hydrogen entry. After fracture the specimen was kept at the humidity long enough to homogenize hydrogen in the specimen and to obtain more quantitative hydrogen content by thermal desorption analysis. HE of the fractured specimens was higher than HC, and HE of the specimens not fractured was lower than HC. This result confirms that the balance between HC and HE determines the occurrence of delayed fracture and that hydrogen-content-based evaluation of susceptibility to delayed fracture is reasonable. To certify the increase of HE with increase in humidity, electrochemical hydrogen permeation test was carried out. The hydrogen permeation current density was increased especially at 98%RH. Enhancement of hydrogen entry with increase in CCT number was also shown by the test. .
192. Min X., Sawaguchi T., Ogawa K., Maruyama T., Yin F.X., Tsuzaki K., Shape memory effect in Fe-Mn-Ni-Si-C alloys with low Mn contents, Materials Science and Engineering A , 528, 15, 5251-5258, 2011.06, An attempt was made to develop a new Fe-Mn-Si-based shape memory alloy from a Fe-17Mn-6Si-0.3C (mass%) shape memory alloy, which was previously reported to show a superior shape memory effect without any costly training treatment, by lowering its Mn content. The shape memory effect and the phase transformation behavior were investigated for the as-solution treated Fe-(17-2. x)Mn-6Si-0.3C- xNi (x = 0, 1, 2, 3, 4) polycrystalline alloys. The shape recovery strain exceeded 2% in the alloys with x = 0-2, which is sufficient for an industrially applicable shape memory effect; however, it suddenly decreased in the alloys between x = 2 and 3 although the significant shape recovery strain still exceeded 1%. In the alloys with x = 3 and 4, X-ray diffraction analysis and transmission electron microscope observation revealed the existence of α′ martensite, which forms at the intersection of the e{open} martensite plates and suppresses the crystallographic reversibility of the γ austenite to e{open} martensitic transformation. .
193. Min X., Tsuzaki K., Emura S., Tsuchiya K., Enhancement of uniform elongation in high strength Ti-Mo based alloys by combination of deformation modes, Materials Science and Engineering A , 528, 13-14, 4569-4578, 2011.05, A combination of different deformation modes, namely, dislocation slip and {3. 3. 2}〈1. 1. 3〉 twinning was found to be effective for achieving high yield strength and large uniform elongation in the β type Ti-15Mo-5Zr and Ti-10Mo-2Fe alloys in the as-solution treated condition, where the Mo equivalency was designed to be between 15.3 and 18.7. mass%. The high yield strength was caused mainly by the slip, and the large uniform elongation was caused by the twinning through significant work hardening. The change in the work hardening rate with strain correlated well with the formation of mechanical twins. The deformation was heterogeneous among the grains and the twins were not seen in some of the grains even after the tensile fracture. This heterogeneity was discussed based on the effects of the grain orientation and the segregation of alloying elements..
194. Zhang L., Ohmura T., Sekido K., Nakajima K., Hara T., Tsuzaki K., Direct observation of plastic deformation in iron-3% silicon single crystal by in situ nanoindentation in transmission electron microscopy, Scripta Materialia , 64, 9, 919-922, 2011.05, By making an indentation into a body-centered cubic single crystal of Fe-3% Si through the in situ nanoindentation in a transmission electron microscopy technique, we found that dislocations formed during the early stage involved not distinguishable pop-ins but small load fluctuations. The pop-in behavior was connected with the change of dislocation structures. The phenomenon was explained by the conventional strain rate model. .
195. Motomichi Koyama, T. Sawaguchi, Kaneaki Tsuzaki, Si content dependence on shape memory and tensile properties in Fe-Mn-Si-C alloys, Materials Science and Engineering A, 10.1016/j.msea.2010.12.065, 528, 6, 2882-2888, 2011.03, Fe-17Mn-xSi-0.3C alloys (x= 0, 2, 4, 6. mass%) were used to investigate the influence of Si on the tensile properties and the shape recovery strain. We considered three kinds of tensile properties: critical stress for e{open}-martensitic transformation, critical stress for dislocation gliding, and work hardening rate. A significant increase in the shape recovery strain was obtained in the 6%Si added alloy, when the alloys were heated to 873. K after a pre-straining of 8% in tension. The critical stresses for both the e{open}-martensitic transformation and the dislocation gliding increased with an increase in Si content from 0 to 4% but were similar in the 4%Si and 6%Si added alloys. However, the work hardening rate between the 4%Si and 6%Si added alloys was significantly different and was much smaller in the 6%Si added alloy. Hence, a 6%Si addition suppresses the plastic deformation due to the dislocation gliding through the decrease in the work hardening rate along with the solution hardening. As a result, e{open}-martensitic transformation occurs as the predominant deformation mode at smaller strains and improves the shape recovery strain..
196. Andrey Belyakov, Kaneaki Tsuzaki, Rustam Kaibyshev, Nanostructure evolution in an austenitic stainless steel subjected to multiple forging at ambient temperature, 5th International Conference on Nanomaterials by Severe Plastic Deformation, NanoSPD5 Nanomaterials by Severe Plastic Deformation, NanoSPD5, 10.4028/www.scientific.net/MSF.667-669.553, 667-669, 553-558, 2011.02, Deformation behavior and structural changes were studied in a 304-type austenitic stainless steel subjected to large strain multiple forging at an ambient temperature. The number of forging passes was 10, leading to the total cumulative strain of 4.0. The yield stress rapidly increased to about 1000 MPa after the first forging pass and then gradually approached a saturation level of about 2000 MPa in large strains. The grain/subgrain size decreased to about 50 nm at total strain of about 2. This grain/subgrain size reduced a little upon further processing; and comprised 35 nm after a total strain of 4.0. The fast kinetics for grain refinement was associated with deformation twinning and strain-induced martensitic transformation. The both of them resulted in fast grain subdivision at relatively small strains..
197. Eiji Akiyama, Songjie Li, Tadashi Shinohara, Zuogui Zhang, Kaneaki Tsuzaki, Hydrogen entry into Fe and high strength steels under simulated atmospheric corrosion, Electrochimica Acta, 10.1016/j.electacta.2010.09.043, 56, 4, 1799-1805, 2011.01, Electrochemical hydrogen permeation tests of Fe sheets under two cyclic corrosion test (CCT) conditions were performed to understand hydrogen entry behavior under atmospheric corrosions. Hydrogen entry into 1300 MPa-class high strength steels under two CCT conditions was also investigated using thermal desorption analysis. One CCT consisted of salt spray, dry and wet stages (Salt Spray CCT; SSCCT), and the other consisted of dry and wet stages after NaCl deposition (Dry-Wet CCT; DWCCT). The corrosion rates of Fe and the steels were almost constant under SSCCT and they decreased under DWCCT with time. Nevertheless, both CCTs resulted in increases in hydrogen permeation current and diffusible hydrogen content with time indicating enhancement of hydrogen entry. Corrosion current monitored by means of an atmospheric corrosion monitoring sensor consisting of Fe anode and Ag cathode decreased obviously under dry stage of the CCTs, whereas hydrogen permeation was high at the beginning of the dry stage. The discrepancy between hydrogen entry and corrosion rate indicates that the hydrogen entry is not directly controlled by corrosion rate. Increase in acidity of underlying rust layer with growth of rust layer monitored using a W/WO3 electrode is considered to be one of the factors affecting the hydrogen entry efficiency..
198. Meysam Jafari, Yuuji Kimura, Yihong Nie, Kaneaki Tsuzaki, Enhancement of impact toughness of 0.05% P doped high strength steel through formation of an ultrafine elongated grain structure, isij international, 10.2355/isijinternational.50.1660, 50, 11, 1660-1665, 2010.12, An ultrafine elongated grain (UFEG) structure with a strong 〈110〉//rolling direction (RD) fibre deformation texture was produced by warm calibre-rolling at 773 K in 1 200MPa-class medium-carbon low-alloy steels with phosphorous (P) contents of 0.001 and 0.053 mass%. Charpy impact tests were performed at room temperature on the UFEG structure along with a conventional quenched and tempered (QT) structure. P segregation embrittlement completely disappeared in the UFEG structure. The UFEG structure exhibited the same high absorbed energy of 150 J regardless of P content, although the absorbed energy of the QT structure was significantly decreased from 93 J for 0.001 % P steel to 23 J for 0.053% P steel due to occurrence of intergranular fracture. The high absorbed energy of 150 J for 0.053% P-doped UFEG structure was believed to be due to both delamination fracture and fine/deep ductile dimples. The present results emphasized that the detrimental effects of P grain boundary segregation can be suppressed and the upper shelf energy can be increased through the formation of the UFEG structure..
199. Q. S. Mei, L. Zhang, K. Tsuchiya, H. Gao, T. Ohmura, Kaneaki Tsuzaki, Grain size dependence of the elastic modulus in nanostructured NiTi, Scripta Materialia, 10.1016/j.scriptamat.2010.07.018, 63, 10, 977-980, 2010.11, Nanostructured NiTi with a graded surface nanostructure was produced by surface mechanical attrition treatment (SMAT). Nanoindentation measurements revealed that the elastic modulus of SMAT samples increased significantly with decreasing distance from the SMAT surface and grain size, reaching ∼85 GPa for a grain size of ∼6 nm at the surface. This grain size dependence of the elastic modulus was found to be due to suppression of the stress-induced martensitic transformation in nanostructured NiTi..
200. Satoshi Emura, Kaneaki Tsuzaki, Koichi Tsuchiya, Improvement of room temperature ductility for Mo and Fe modified Ti2AlNb alloy, Materials Science and Engineering A, 10.1016/j.msea.2010.09.003, 528, 1, 355-362, 2010.11, The effect of hot bar rolling and a subsequent annealing in the (B2+α2) two-phase region on the mechanical properties was investigated for the Ti-25Al-14Nb-2Mo-1Fe (mol%) orthorhombic phase base alloy. After this thermomechanical treatment in the (B2+α2) two-phase region, a 'Van Gogh's Sky (VGS)' structure, with wavy and curled bands of spherical α2 precipitates, was obtained. Electron probe micro-analysis (EPMA) examinations indicated that the VGS bands correspond to the Nb and Mo lean regions. The width and spacing of VGS bands varied with thermomechanical treatment conditions (forging temperature before hot bar rolling as well as annealing temperature after the rolling). The samples with the VGS structure exhibited the higher tensile elongation-to-failure at room temperature. The creep resistance at 923K decreased with the existence of the VGS structure. These mechanical properties were affected by the width and spacing of the VGS bands..
201. Songjie Li, Eiji Akiyama, Nobuyoshi Uno, Keiji Hirai, Kaneaki Tsuzaki, Boping Zhang, Evaluation of delayed fracture property of outdoor-exposed high strength AISI 4135 steels, Corrosion Science, 10.1016/j.corsci.2010.05.038, 52, 10, 3198-3204, 2010.10, Delayed fracture properties of AISI 4135 high strength steels with 1490 and 1310. MPa of tensile strength, represented as B15 and B13, respectively, have been studied by means of slow strain rate test (SSRT) of notched bar specimens after outdoor exposure at rural and coastal areas. The exposed specimens were kept at humid medium before SSRT to reproduce active hydrogen entry influenced by the rust layer and to homogenize hydrogen distribution. The influences of exposure site and exposure time on fracture stress have been investigated. The susceptibility of B15 to delayed fracture was obviously higher than that of B13..
202. F. Musin, A. Belyakov, R. Kaibyshev, Y. Motohashi, G. Itoh, Kaneaki Tsuzaki, Microstructure evolution in a cast 1421AI alloy during hot equal-channel angular extrusion, Reviews on Advanced Materials Science, 25, 2, 107-112, 2010.10, Microstructure evolution during equal channel angular extrusion at 400 °C was studied in a cast 1421Al alloy. The structural changes were investigated by using transmission and scanning electron microscopes incorporating an orientation imaging microscopy. The microstructure evolution is characterized by the development of new fine grains with a size of about 2 μm primarily at triple junctions and near original grain boundaries, leading to the formation of necklace-like microstructure. The interlayers consisting of new fine grains do enlarge with increasing the total strain that results in a gradual rise of the volume fraction occupied by fine grains. The mechanism for the fine-grained microstructure evolution is discussed as a kind of continuous dynamic recrystallization..
203. T. Nishimura, S. Tamilselvi, X. H. Min, Kaneaki Tsuzaki, Corrosion resistance of aging heat-treated Ti-8Mo-5Fe alloy in highly acidic chloride solution, Materials Transactions, 10.2320/matertrans.M2010093, 51, 9, 1553-1559, 2010.09, The corrosion resistance of an aging heat-treated Ti-8Mo-5Fe alloy was investigated by electrochemical impedance spectroscopy (EIS), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and energy dispersive X-ray analysis (EDAX). The sample subjected to solution heat treatment (ST) featured a single β-phase microstructure, and the samples subjected to aging heat treatments at 600-700°C showed a-phase precipitates in the α-phase matrix. EIS results showed that the corrosion resistance of the aging heattreated samples was lower than that of the ST sample, but much higher than that of pure Ti in a long-term immersion test in an acidic 10 mass% NaCl solution (pH 0.5) at 97°C. Laser micrographs of the aging heat-treated samples indicated susceptibility to selective corrosion of a-phase at the grain boundary and in the grains, causing selective dissolution in the NaCl solution. The results of TEM combined with EDAX analyses showed the presence of the βphase matrix composed of 5.3 mass% Mo and 4.8 mass% Fe, and a-phase of 0.7 mass% Mo and 0.1 mass% Fe in the sample aged at 600°C. Thus, the Mo-poor a-phase precipitates were selectively dissolved in the 10 mass% NaCl solution (pH 0.5) at 97°C. In the results, the ST sample of only β-phase microstructure showed the highest resistance, and the aging heat-treated samples containing a-phase precipitates (0.7 mass% Mo) showed higher values than that of pure Ti in the corrosion test. Addition of Fe did not decrease the corrosion resistance of the alloy under the ST condition. Moreover, as Fe was involved in the β phase with Mo which showed remarkable increase in corrosion resistance, the addition of Fe did not decrease the corrosion resistance of the aging heat-treated Mo-Fe-Ti alloy..
204. Eiji Akiyama, Katsuhiro Matsukado, Maoqiu Wang, Kaneaki Tsuzaki, Evaluation of hydrogen entry into high strength steel under atmospheric corrosion, Corrosion Science, 10.1016/j.corsci.2009.11.046, 52, 9, 2758-2765, 2010.09, Hydrogen entry into high strength steels by atmospheric corrosion has been investigated to evaluate their susceptibility to hydrogen embrittlement. High strength steels were corroded by dry/wet cyclic corrosion after NaCl deposition. The maximum diffusible hydrogen concentration around the surface was successfully obtained by means of thermal desorption analysis after keeping the specimens at high humidity to reproduce enhanced hydrogen entry influenced by the rust layer and to homogenize the hydrogen distribution. Despite decrease in corrosion rate, hydrogen content in specimens did not decrease. Decrease of pH in inner rust layer is responsible for the enhanced hydrogen entry into steel..
205. X. H. Min, S. Emura, T. Nishimura, K. Tsuchiya, Kaneaki Tsuzaki, Microstructure, tensile deformation mode and crevice corrosion resistance in Ti-10Mo-xFe alloys, Materials Science and Engineering A, 10.1016/j.msea.2010.06.016, 527, 21-22, 5499-5506, 2010.08, The microstructure, the tensile deformation mode at ambient temperature and the crevice corrosion resistance at a high temperature of 373. K were investigated in the Ti-10Mo-. xFe (x=0, 1, 3, 5) alloys. The stability of the β phase increased, and the formation of the α″ martensite and the athermal ω phase was suppressed by the increase in the Fe content. EPMA examinations indicated that the existence of the α″ martensite in the Ti-10Mo alloy was caused by the solidification segregation of Mo atoms. EBSD observations showed that the deformation mode changed from a {3. 3. 2}〈1. 1. 3〉 twinning to a slip by an increase in the Fe content, which coincided with the prediction by the electron/atom (e/a) ratio. The Ti-10Mo-3Fe alloy showed the highest yield strength of 935. MPa among all the alloys, while the Ti-10Mo-1Fe alloy showed the lowest value of 563. MPa due to the change in the deformation mode. On the other hand, all the alloys exhibited a high crevice corrosion resistance in a high chloride and high acidic solution at the high temperature, although the corrosion resistance decreased with an increase in the Fe content. The decrease in the corrosion resistance can be explained by the bond order (Bo). A good combination of tensile properties and crevice corrosion resistance may be obtainable through a further optimization of the Fe content by the e/a ratio and the Bo..
206. Songjie Li, Eiji Akiyama, Kimura Yuuji, Kaneaki Tsuzaki, Nobuyoshi Uno, Boping Zhang, Hydrogen embrittlement property of a 1700-MPa-class ultrahigh-strength tempered martensitic steel, Science and Technology of Advanced Materials, 10.1088/1468-6996/11/2/025005, 11, 2, 2010.07, The hydrogen embrittlement property of a prototype 1700-MPa-class ultrahigh-strength steel (NIMS17) containing hydrogen traps was evaluated using a slow strain rate test (SSRT) after cathodic hydrogen precharging, cyclic corrosion test (CCT) and atmospheric exposure. The hydrogen content in a fractured specimen was measured after SSRT by thermal desorption spectroscopy (TDS). The relationship between fracture stress and hydrogen content for the hydrogen-precharged specimens showed that the fracture stress of NIMS17 steel was higher, at a given hydrogen content, than that of conventional AISI 4135 steels with tensile strengths of 1300 and 1500 MPa. This suggests better resistance of NIMS17 steel to hydrogen embrittlement. However, hydrogen uptake to NIMS17 steel under CCT and atmospheric exposure decreased the fracture stress. This is because of the stronger hydrogen uptake to the steel containing hydrogen traps than to the AISI 4135 steels. Although NIMS17 steel has a higher strength level than AISI 4135 steel with a tensile strength of 1500 MPa, the decrease in fracture stress is similar between these steels..
207. Songjie Li, Zuogui Zhang, Eiji Akiyama, Kaneaki Tsuzaki, Boping Zhang, Evaluation of susceptibility of high strength steels to delayed fracture by using cyclic corrosion test and slow strain rate test, Corrosion Science, 10.1016/j.corsci.2010.02.005, 52, 5, 1660-1667, 2010.05, To evaluate susceptibilities of high strength steels to delayed fracture, slow strain rate tests (SSRT) of notched bar specimens of AISI 4135 with tensile strengths of 1300 and 1500 MPa and boron-bearing steel with 1300 MPa have been performed after cyclic corrosion test (CCT). During SSRT the humidity around the specimen was kept high to keep absorbed diffusible hydrogen. The fracture stresses of AISI 4135 steels decreased with increment of diffusible hydrogen content which increased with CCT cycles. Their delayed fracture susceptibilities could be successfully evaluated in consideration of both influence of hydrogen content on mechanical property and hydrogen entry..
208. X. H. Min, S. Emura, N. Sekido, T. Nishimura, K. Tsuchiya, Kaneaki Tsuzaki, Effects of Fe addition on tensile deformation mode and crevice corrosion resistance in Ti-15Mo alloy, Materials Science and Engineering A, 10.1016/j.msea.2009.12.050, 527, 10-11, 2693-2701, 2010.04, The tensile deformation mode at ambient temperature and the crevice corrosion resistance at a high temperature of 373 K were investigated in Ti-15Mo and Ti-15Mo-1Fe (mass%) alloys. The β phase stability increased, and the formation of an athermal ω phase was suppressed by the Fe addition. EBSD and TEM observations showed that the deformation mode in the Ti-15Mo alloy changed from a {3 3 2}〈1 1 3〉 twinning to a slip by the Fe addition, which coincided with the predictions from the electron/atom (e/a) ratio and the Mo equivalency. EPMA examinations indicated that the existence of twins in a few regions in the Ti-15Mo-1Fe alloy was caused by the solidification segregation of Mo and Fe atoms. The yield strength of the Ti-15Mo-1Fe alloy of 837 MPa was much higher than that of the Ti-15Mo alloy of 439 MPa due to the change in the deformation mode. The Ti-15Mo-1Fe alloy maintained an extremely high crevice corrosion resistance in a 10% NaCl water solution with a pH value of 0.5 at 373 K since there was no significant decrease in the average value of the bond order (Bo). A good combination of tensile properties, crevice corrosion resistance and cost is thought to be obtainable through further optimization of the chemical compositions by the e/a ratio, the Mo equivalency and the Bo..
209. Ling Zhang, Takahito Ohmura, Akinobu Shibata, Kaneaki Tsuzaki, Characterization of local deformation behavior of Fe-Ni lenticular martensite by nanoindentation, Materials Science and Engineering A, 10.1016/j.msea.2009.11.051, 527, 7-8, 1869-1874, 2010.03, A Fe-33 mass% Ni alloy with transformed lenticular martensite consisting of various lattice defects was investigated by the nanoindentation technique. The highly dislocated regions such as the untwinned region and the austenite phase near the M/A interface showed a relatively easy plasticity initiation and lower flow stress than that of the midrib region containing fine transformation twins. The different responses of plastic deformation in the ultra-fine twins and the dislocated regions provided an insight into the local deformation response in various substructures during the nanoscale mechanical measurements..
210. Yuuji Kimura, Tadanobu Inoue, Y. I N Fuxing, Kaneaki Tsuzaki, Delamination toughening of ultrafine grain structure steels processed through tempforming at elevated temperatures, isij international, 10.2355/isijinternational.50.152, 50, 1, 152-161, 2010.03, The deformation of tempered martensitic structures, namely tempforming treatments, were applied to a 0.6C-2Si-1Cr steel at 500, 600 and 700°C using multi-pass caliber-rolling with an accumulated area reduction of 80%. The tensile and Charpy impact properties were investigated to make clear the relation between the microstructure and the delamination behavior of the tempformed (TF) samples. The tempforming treatments resulted in the evolution of ultrafine grain structures with strong (110)//rolling direction (RD) fiber deformation textures and fine spheroidized cementite particles distributions. In contrast to the ductile-to-brittle transition of the conventional quenched and tempered (QT) samples, the TF samples exhibited inverse temperature dependences of the impact toughness due to the delaminations, where the cracks branched in the longitudinal direction (//RD) of the impact test bars. As a result, high strength with excellent toughness was achieved in the TF samples. A yield strength of 1 364MPa and a V-notch Charpy absorbed energy of 125 J were obtained at room temperature in the sample that was tempformed at 500°C. The delamination was shown to occur due to the microstructural anisotropy of the TF samples, and the dominating factors controlling the delamination toughening were the transverse grain size, the grain shape and the (110)//RD fiber deformation texture. The discussion also indicated that the ultra refinement of the transverse grain structure was the key to enhancing both the yield strength and the toughness of the TF steel while lowering the ductile-to-brittle transition temperature..
211. X. H. Min, S. Emura, T. Nishimura, L. Zhang, S. Tamilselvi, K. Tsuchiya, Kaneaki Tsuzaki, Effects of α phase precipitation on crevice corrosion and tensile strength in Ti-15Mo alloy, Materials Science and Engineering A, 10.1016/j.msea.2009.10.033, 527, 6, 1480-1488, 2010.03, The effects of the α phase precipitation on the crevice corrosion at 373 K and the tensile properties at ambient temperature were investigated in the Ti-15Mo alloy. Relatively coarse α phase precipitates with the depletion of Mo formed in the alloy after aging at 873 K. The aged alloy maintained high crevice corrosion resistance in the 10% NaCl water solution with a pH value of 0.5 at 373 K, and showed much higher yield strength than that of the solution treated alloy. This increase in yield strength was mainly attributed to the change in the deformation mode of the β phase from the {332}〈113〉 twinning to the slip caused by the enrichment of Mo in the β phase, and contributions of the precipitation/dispersion hardening and the grain refinement strengthening are negligible. The yield strength decreased with an increase in the volume fraction of the α phase since the α phase is softer than the β phase which was confirmed by nanohardness measurements..
212. V. Dudko, R. Kaibyshev, A. Belyakov, Y. Sakai, Kaneaki Tsuzaki, Creep behavior of an oxide dispersion strengthened iron with ultrafine grain structure, 6th International Conference on Processing and Manufacturing of Advanced Materials - THERMEC'2009 THERMEC 2009, 10.4028/www.scientific.net/MSF.638-642.3194, 3194-3199, 2010.02, The creep behavior of oxide-bearing Fe-0.6%O steel was studied in the temperature range of 550-700°C at stresses ranging from 100 to 400 MPa. The creep data showed high values of an apparent stress exponent n close to ∼16 for power-law creep. In addition the apparent experimental activation energy was much higher than that for the lattice diffusion in a-iron. Analysis of creep data revealed that the deformation behavior was strongly affected by the threshold stresses, which are associated with the interaction between moving dislocations and fine incoherent oxide particles. Analysis of deformation behavior in terms of threshold stress leads the true stress exponent of ∼8; the activation energy for creep became close to value of activation energy for lattice diffusion at 700°C and for pipe-diffusion in the temperature range of 550-650°C..
213. Tadanobu Inoue, Fuxing Yin, Yuuji Kimura, Kaneaki Tsuzaki, Shojiro Ochiai, Delamination effect on impact properties of ultrafine-grained low-carbon steel processed by warm caliber rolling, Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science, 10.1007/s11661-009-0093-x, 41, 2, 341-355, 2010.02, Bulk ultrafine-grained (UFG) low-carbon steel bars were produced by caliber rolling, and the impact and tensile properties were investigated. Initial samples with two different microstructures, ferrite-pearlite and martensite (or bainite), were prepared and then caliber rolling was conducted at 500 °C. The microstructures in the rolled bars consisted of an elongated UFG structure with a strong α-fiber texture. The rolled bar consisting of spheroidal cementite particles that distributed uniformly in the elongated ferrite matrix of transverse grain sizes 0.8 to 1.0 μm exhibited the best strength-ductility balance and impact properties. Although the yield strength in the rolled bar increased 2.4 times by grain refinement, the upper-shelf energy did not change, and its value was maintained from 100 °C to -40 °C. In the rolled bars, cracks during an impact test branched parallel to the longitudinal direction of the test samples as temperatures decreased. Delamination caused by such crack branching appeared, remarkably, near the ductile-to-brittle transition temperature (DBTT). The effect of delamination on the impact properties was associated with crack propagation on the basis of the microstructural features in the rolled bars. In conclusion, the strength-toughness balance is improved by refining crystal grains and controlling their shape and orientation; in addition, delamination effectively enhances the low-temperature toughness..
214. Andrey Belyakov, Rustam Kaibyshev, Yuuji Kimura, Kaneaki Tsuzaki, Recrystallization mechanisms in severely deformed dual-phase stainless steel, 6th International Conference on Processing and Manufacturing of Advanced Materials - THERMEC'2009 THERMEC 2009, 10.4028/www.scientific.net/MSF.638-642.1905, 1905-1910, 2010.02, The structural recrystallization mechanisms operating in an Fe - 27%Cr - 9% Ni dual-phase (ferrite-austenite) stainless steel after large strain processing to total strain of 4.4 were investigated in the temperature range of 400-700°C. The severe deformation resulted in the development of an ultrafine grained microstructure consisting of highly elongated grains/subgrains with transverse dimensions of 160 nm and 130 nm in ferrite and austenite, respectively. The annealing mechanism operating in ferrite phase was considered as continuous recrystallization, which involved recovery leading to the development of essentially polygonized microstructure. On the other hand, the mechanism of discontinuous nucleation took place at an early recrystallization stage in austenite phase..
215. A. Belyakov, Kaneaki Tsuzaki, R. Kaibyshev, Internal stresses in a 15%Cr ferritic stainless steel after large strain unidirectional processing, Journal of Physics: Conference Series, 10.1088/1742-6596/240/1/012115, 240, 2010.01, An interstitial free ferritic stainless steel was cold worked to a total strain of 4.6. The largely strained steel is characterized by a submicrocrystalline structure consisting of elongated grains/subgrains with the transverse size of about 210 nm; and the fraction of high-angle grain boundaries is about 0.6. Following a rapid rise at an early processing stage, the dislocation density in (sub)grain interiors unusually decreased after total strains of above 2. Nevertheless, the samples are characterized by high residual stresses that result in complex elastic distortions of the crystal lattice within the elongated crystallites. Such internal stresses are shown to be originated from deformation grain boundaries including low-angle subboundaries..
216. Fu Gao Wei, Toru Hara, Kaneaki Tsuzaki, Nano-precipitates design with hydrogen trapping character in high strength steels, 2008 International Hydrogen Conference - Effects of Hydrogen on Materials Proceedings of the 2008 International Hydrogen Conference - Effects of Hydrogen on Materials, 448-455, 2009.12, Nano-precipitates of alloy carbides TiC, NbC and VC in tempered martensite have been characterized by means of high-resolution transmission electron microscope (HRTEM) and correlated to the hydrogen trapping property. The examination of whether the amount of hydrogen absorbed by the TiC particles depends on their surface area or volume indicate that the coherent and semi-coherent TiC particles trap hydrogen at the precipitate/matrix interface at ambient temperature while the incoherent TiC particles trap hydrogen inside themselves only at high temperatures. Coherent and semi-coherent NbC and VC particles also demonstrate a surface area dependence of hydrogen trapping capacity with NbC > TiC >> VC. Contrary to TiC, incoherent NbC and VC particles are unable to trap hydrogen..
217. T. Shirane, S. Tsukamoto, Kaneaki Tsuzaki, Y. Adachi, T. Hanamura, M. Shimizu, F. Abe, Ferrite to austenite reverse transformation process in B containing 9%Cr heat resistant steel HAZ, Science and Technology of Welding and Joining, 10.1179/136217109X12505932584853, 14, 8, 698-707, 2009.11, Creep properties, HAZ microstructure Creep properties of the high Cr heat resistant steel welded joint can be improved by adding B due to prevention of the grain refinement in heat affected zone (HAZ). In the present study, phase transformation behaviour of the B steel HAZ has been investigated to understand suppression mechanism of the grain refinement. During reverse transformation, fine austenite was formed through diffusional transformation at the prior austenite grain boundary in the first stage, and then coarse austenite was formed at the same location of the original austenite. The volume fraction of the fine austenite increased with increasing perk temperature of the weld thermal cycle. This phenomenon can be explained if the coarse austenite contains high density of dislocations. Clear surface relief was observed during the reverse transformation by a confocal laser microscope. These results indicate that martensitic or displacive reverse transformation takes place during welding and it prevents the grain refinement in HAZ..
218. S. Tamilselvi, T. Nishimura, X. H. Min, Kaneaki Tsuzaki, The effect of microstructure on corrosion of molybdenum-bearing titanium alloys in high chloride and acidic solution at high temperature, Materials Transactions, 10.2320/matertrans.M2009267, 50, 11, 2545-2551, 2009.11, The effect of microstructure on corrosion of heat-treated Ti-15Mo alloys was investigated by Electrochemical impedance spectroscopy (EIS), Field emission scanning electron microscopy (FE-SEM), Transmission electron microscopy (TEM) and Energy dispersive X-ray analysis (EDAX). The sample subjected to solution heat treatment (ST) had a single β phase and samples subjected to aging heat treatment at 600°C had α phase precipitation in β phase. EIS results showed that the corrosion resistance of the aging heat-treated samples was lower than that of the ST sample, but much higher than that of pure Ti in 10% NaCl solution of pH 0.5 at 97°C. Laser micrographs and depth profile of the heat-treated samples indicated that a phase at the grain boundary and in the grain was selectively corroded and caused selective dissolution in NaCl solution. The results of TEM combined with EDAX showed that the Mo content was 18 mass% in the β phase and 0.8 mass% in α phase. Hence, less Mo α phase was selectively corroded in the NaCl solution. Moreover, the sample which had continuous precipitation of α phase had lower corrosion resistance than samples which had separated needle-shape α phases in the base β phase. Thus, it was also found that the form of precipitation of α phase affected the corrosion of these alloys. Finally, it was concluded that it is possible to maintain the high corrosion resistance of heat-treated Ti-Mo alloy by controlling the microstructure of α phase..
219. Ling Zhang, Takahito Ohmura, Satoshi Emura, Nobuaki Sekido, Fuxing Yin, Xiaohua Min, Kaneaki Tsuzaki, Evaluation of matrix strength in ultra-fine grained pure Al by nanoindentation, Journal of Materials Research, 10.1557/jmr.2009.0342, 24, 9, 2917-2923, 2009.09, Abstract Nanoindentation measurements of the grain interiors of an ultra-fine grained (UFG) pure Al produced by equal channel angular pressing were taken to evaluate the contribution of the matrix strength. Specimens were subjected to 0, 1, 2, 4, and 8 passes at ambient temperature. The nanohardness of the deformed samples was always higher than that of the undeformed sample OP in the range of the indentation depth that was investigated, suggesting a strengthening of the matrix in the UFG Al. The increase in hardness that was contributed by the matrix to the macroscopic scale hardness was significantly large in about 40% of the deformed samples. The microstructural characterization and the deformation response analysis with the pop-in event during indentation suggested that the strengthening of the matrix originated from dislocation strengthening and some other presumable factors in the grain interiors..
220. H. Li, F. Yin, T. Sawaguchi, X. Zhao, Kaneaki Tsuzaki, Texture characteristics controlled by single slip plane slipping in the warm-rolled Fe-14Mn-5Si-9Cr-5Ni shape memory alloy, Journal of Materials Research, 10.1557/jmr.2009.0228, 24, 6, 2097-2106, 2009.06, To clarify the texture evolution mechanism of Fe-Mn-Si-based shape memory alloys, the rolling texture of an Fe-14Mn-5Si-9Cr-5Ni shape memory alloy is investigated during rolling to a final reduction of 82% at 873 K. A new rolling texture caused by single slip plane slipping is observed from such alloy, which is different from the conventional copper-type and brass-type textures. By means of the {111} pole figure scattering analysis of the local deformation structure, we conclude that such single slip plane slipping results in the weakness of brass orientation in the a fiber and the great enhancement of β fiber connecting S′ {331}<213> and B′ orientations {110}<114>..
221. V. A. Dudko, R. O. Kaibyshev, A. N. Belyakov, Y. Sakai, Kaneaki Tsuzaki, Plastic flow of the mechanically alloyed Fe-0.6%O at temperatures of 550-700°C, Physics of Metals and Metallography, 10.1134/S0031918X09050135, 107, 5, 516-521, 2009.05, Creep of steel Fe-0.6%O produced by the method of powder metallurgy has been studied in a temperature range of 550-700°C at flow stresses from 100 to 400 MPa. It has been shown that the creep of the material is characterized by high values of the apparent activation energy for deformation, which considerably exceeds the value of the activation energy for self-diffusion in α iron, and by high values of the stress exponent in the power law of creep. An analysis of the deformation behavior of the alloy showed that there are observed high threshold stresses as a result of retardation of moving dislocations by small incoherent particles of oxides. Taking into account the threshold stresses and the temperature dependence of the shear modulus, it has been established that the deformation behavior of the powder material is described by a power law of creep. The true values of the stress exponent were found to be approximately 8, and the values of the true activation energy for deformation, to be close to the activation energy for bulk (at T = 700°C) and pipe (at T = 550-650°C) self-diffusion..
222. H. Li, F. Yin, T. Sawaguchi, K. Ogawa, X. Zhao, Kaneaki Tsuzaki, TEM observation of the restrained Goss-Brass orientation transformation in a warm-rolled Fe-28Mn-6Si-5Cr shape memory alloy, Philosophical Magazine Letters, 10.1080/09500830902902176, 89, 5, 348-357, 2009.05, A dominant Goss texture component instead of a Brass texture component has been observed in a warm-rolled Fe-28Mn-6Si-5Cr shape memory alloy [H. Li, F. Yin, T. Sawaguchi K. Ogawa, X. Zhao and K. Tsuzaki, Mater. Sci. Eng. A 494 (2008) p.217]. In order to clarify the orientation flow mechanism in these grains of Goss orientation, investigations on microtexture and microstructure were carried out by electron backscattering diffraction (EBSD) and transmission electron microscopy (TEM). It is concluded that the Goss-Brass orientation transformation is restrained in the Goss-orientated grains, which were widely observed in both the deformed matrix and shear bands..
223. Sergey Mironov, Yoshinobu Motohashi, Rustam Kaibyshev, Hidetoshi Somekawa, Toshiji Mukai, Kaneaki Tsuzaki, Development of fine-grained structure caused by friction stir welding process of a ZK60A magnesium alloy, Materials Transactions, 10.2320/matertrans.MRA2008192, 50, 3, 610-617, 2009.03, A fine-grained microstructure average grain size of which is around 2 μm was developed in ZK60A magnesium alloy subjected to a FSW process. Material flow arose mainly from basal slips and was governed by simple shear deformations caused by the tool rotation. The development of the fine-grained structure was a complex process driven by severe plastic deformations, formation of deformation bands, grain splitting, twinning and dynamic and/or static recrystallizations. It appears also that the microstructural evolution depended on morphology of the dispersoids..
224. A. Belyakov, Kaneaki Tsuzaki, Y. Kimura, Y. Mishima, Tensile behaviour of submicrocrystalline ferritic steel processed by large-strain deformation, Philosophical Magazine Letters, 10.1080/09500830902748298, 89, 3, 201-212, 2009.03, Mechanical tests have been carried out on Fe-15%Cr ferritic stainless steel with various microstructures. Ultrafine-grained microstructures with grain sizes of 0.2-0.3 m were developed by large-strain cold-working and light annealing. The effects of severe deformation on the mechanical behaviour of as-processed and recovered steel were evaluated with reference to the same material having conventional work-hardened and recrystallised microstructures. Despite the low dislocation density in the fine grain interiors in the as-processed state, the samples with strain-induced submicrocrystalline structure were characterised by high internal stresses that resulted in a higher strength than could be expected from simple grain-size strengthening. These internal stresses were associated with a non-equilibrium state of strain-induced grain boundaries after severe deformation..
225. Eiji Akiyama, Zuogui Zhang, Yoshimi Watanabe, Kaneaki Tsuzaki, Effects of severe plastic deformation on the corrosion behavior of aluminum alloys, Journal of Solid State Electrochemistry, 10.1007/s10008-007-0496-9, 13, 2, 277-282, 2009.02, Effects of severe plastic deformation on the corrosion behaviors of Al alloys containing precipitates have been investigated. Al and its alloys were severely deformed by equal-channel angular pressing (ECAP) processes and the corrosion behaviors of the Al alloys were evaluated by means of potentiodynamic polarization in a neutral buffer solution containing 0.002 M chloride ion. Introduction of huge plastic deformation to both of Al-5.4 wt% Ni and Al-5 wt% Cu alloys increased pitting potential. In contrast, ECAP treatment of 4N pure Al resulted in a decrease in open circuit potential, slight increase of passive current and shift of pitting potential to the negative direction. The influence of the change in microstructures caused by severe plastic deformation was investigated..
226. Eiji Akiyama, Songjie Li, Zuogui Zhang, Maoqiu Wang, Katsuhiro Matsukado, Kaneaki Tsuzaki, Boping Zhang, Hydrogen embrittlement of high strength steels and environmental hydrogen entry, 2008 International Hydrogen Conference - Effects of Hydrogen on Materials Proceedings of the 2008 International Hydrogen Conference - Effects of Hydrogen on Materials, 54-61, 2009, Delayed fracture properties of martensitic high strength steels have been evaluated by measuring the change in fracture stress of circumferentially notched bar specimens pre-charged with hydrogen by means of slow strain rate test (SSRT). The fracture stress of a steel with higher strength level showed sharper decrease indicating higher susceptibility to delayed fracture. It has been found that the decrease of fracture stress with hydrogen concentration follows a power law relationship when intergranular fracture mode is observed. Another finding is that the local stress and the local hydrogen concentration estimated by finite element analysis are determining delayed fracture. Hydrogen entry into steels caused by atmospheric corrosion has been evaluated as well by using cyclic corrosion test (CCT) simulating atmospheric corrosion. The circumferentially notched bar specimens were exposed to the CCT condition and SSRT was performed at humid environment to investigate both hydrogen entry and its influence on the decrease of fracture stress in the environment. The relation between fracture stress and the content of hydrogen in corrosive environment was compared with the relation of hydrogen pre-charged specimens. The SSRT combined with CCT was proposed as evaluation method considering both the effect of hydrogen in metals and hydrogen entry into metals..
227. Fu Gao Wei, Kaneaki Tsuzaki, Hydrogen trapping character of nano-sized NbC precipitates in tempered martensite, 2008 International Hydrogen Conference - Effects of Hydrogen on Materials Proceedings of the 2008 International Hydrogen Conference - Effects of Hydrogen on Materials, 456-463, 2009, The hydrogen trapping character of NbC nano-particles in a 0.05C-0.41Nb-2.0Ni tempered martensitic steel was investigated by hydrogen thermal-desorption spectrometry (TDS) analysis and atomic-level microstructural observation. The steel trapped the most hydrogen after tempering at 600 °C, and the trapped hydrogen decreased with increasing tempering temperature. The precipitation of NbC nanoparticles is responsible for the sharp increase at 600 °C. The coarsening of precipitates reduces the amount of trapped hydrogen. The TDS analysis of the same specimen charged with various levels of hydrogen concentration indicated that the apparent activation energy for desorption from NbC precipitates is dependent on hydrogen concentration. Hydrogen trap sites are discussed in terms of the character of the NbC/matrix interface and the crystal defects present at and around the interface..
228. Takahiro Shirane, Masatsugu Shimizu, Susumu Tsukamoto, Kaneaki Tsuzaki, Yoshitaka Adachi, Toshihiro Hanamura, Fujio Abe, Reverse transformation behaviour in B containing high Cr heat resistant steel HAZ, 8th International Conference on Trends in Welding Research Trends in Welding Research - Proceedings of the 8th International Conference, 10.1361/cp2008twr284, 284-291, 2009, Boron addition in the ferritic heat resistant steels significantly improve the creep strength of the welded joint due to prevention of the Type IV failure. It is attributed to improvement of the HAZ microstructure. Coarse grained prior austenite is formed in the B containing steel (B steel) HAZ due to reconstitution of the original austenite observed in the base metal, whereas the prior austenite is significantly refined in the conventional heat resistant steel HAZ. In the present study, ferrite to austenite reverse transformation process has been investigated to understand the suppression mechanism of the grain refinement in the B steel HAZ. 130 ppm B containing 9Cr-3W-3Co-Nb, V steel was used as the B steel. Some other steels with different reverse transformation process were also used to observe the typical feature of each reverse transformation process. HAZ thermal cycle simulation test was carried out for various peak temperatures to observe the microstructure evolution during weld thermal cycle. In addition, ferrite-to-austenite reverse transformation behaviour was directly observed using a laser microscopy with a high temperature stage. When the peak temperature was just above AC3, coarse prior austenite grains were observed in the B steel, but a few fine grains were also observed around the coarse austenite grain boundary. The volume fraction of the fine grains increased with increasing the peak temperature above AC3. This phenomenon can be explained if the coarse austenite contains high density of dislocations. Clear surface relief was confirmed during ferrite-to-austenite reverse transformation of the B steel by direct observation of the transformation process. This is a typical feature of martensitic or displacive transformation where a number of dislocations are introduced. It is thus considered that martensitic reverse transformation takes place during the weld thermal cycle in the B steel, and this prevents the grain refinement in the HAZ..
229. X. H. Min, S. Emura, L. Zhang, Kaneaki Tsuzaki, Effect of Fe and Zr additions on ω phase formation in β-type Ti-Mo alloys, Materials Science and Engineering A, 10.1016/j.msea.2008.06.018, 497, 1-2, 74-78, 2008.12, The effect of 1% Fe and/or 5% Zr (mass%) additions on ω phase formation was investigated for the Ti-15Mo alloy by means of X-ray diffraction analysis and hardness testing. Upon water quenching following solution treatment in the β phase region, the athermal ω phase formation could not be observed in all the alloys, regardless of Fe and Zr additions. The lattice parameter of the β phase decreases with Fe addition, while it increases with Zr addition. Solid solution strengthening by Fe and Zr is not recognized for the β phase. The isothermal ω phase formed after aging at 723 K and 773 K for 3.6 ks, which results in a decrease in the lattice parameter of the β phase and an increase in the hardness. The isothermal ω phase formation is suppressed with Fe and/or Zr additions. This is interpreted as the consequence of the increase in the average value of the bond order (Bo) for the Ti-15Mo-5Zr and Ti-15Mo-5Zr-1Fe alloys, and of the decrease in the average value of the metal d-orbital energy level (Md) for the Ti-15Mo-1Fe alloy. In addition, the degree of the suppression of isothermal ω phase can be predicted by the average values of Bo and Md..
230. Andrey Belyakov, Kaneaki Tsuzaki, Yuuji Kimura, Regularities of deformation microstructures in ferritic stainless steels during large strain cold working, isij international, 10.2355/isijinternational.48.1071, 48, 8, 1071-1079, 2008.11, The recent studies on grain refinement in several Cr-Ni ferritic stainless steels during large strain deformation at room temperature are critically reviewed. The paper is focussed on the mechanism of structural changes that is responsible for ultrafine grain development. It is concluded that the development of submicrocrystalline structures results from a kind of strain-induced continuous reaction; that is formation of deformation subboundaries and gradual increase in their misorientations up to typical values of ordinary grain boundaries. Following the rapid reduction at an early deformation, the transverse grain/subgrain size smoothly approaches its final value of the order of 0.1-0.2 μm, depending on alloying extent. The increase in average subboundary misorientation upon processing can be related to the strain as Δθ ∼ Kε, with K of about 5°. Therefore, the average misorientation between deformation grains/subgrains is about 20° after processing to total strain of 4 and the corresponding microstructure is composed of 50 % of high-angle grain boundaries. The structural refinement of starting material can significantly accelerate the kinetics of ultrafine grain development during subsequent plastic working, while the processing method has a little effect on developing microstructures..
231. Dorothée Dorner, Yoshitaka Adachi, Kaneaki Tsuzaki, Slip system partitioning as a possible mechanism for ultrafine grain formation in Fe-3%Si bicrystals, isij international, 10.2355/isijinternational.48.1102, 48, 8, 1102-1106, 2008.11, An Fe-3%Si bicrystal was deformed in compression to a strain of 0.9 at ambient temperature. In the interior of deformation bands, characteristic band structures with high orientation gradients and low and high angle boundaries were formed during straining. Furthermore, isolated ultrafine grains were found in the matrix of the deformation bands. The morphology and crystallography of the ultrafine grains indicate that they are caused by slip system partitioning, i.e. local activity of a highly stressed slip system that is different from the active slip system in the surrounding crystal volume..
232. H. Li, F. Yin, T. Sawaguchi, K. Ogawa, X. Zhao, Kaneaki Tsuzaki, Texture evolution analysis of warm-rolled Fe-28Mn-6Si-5Cr shape memory alloy, Materials Science and Engineering A, 10.1016/j.msea.2008.05.013, 494, 1-2, 217-226, 2008.10, Since texture control tends to be a promising way to improve the shape memory effect (SME) of polycrystalline Fe-Mn-Si shape memory alloys, rolling texture evolution of an Fe-28Mn-6Si-5Cr shape memory alloy was systematically investigated with orientation distribution functions (ODFs) and electron backscattering diffraction (EBSD) analysis. At the rolling temperature of 873 K, Copper-type texture components, including D, S, Goss, as well as a weak Brass, obviously develop before 44% rolling reduction. With increased rolling reduction to 57%, D orientation abruptly disappears, which indicates a texture transition has occurred. S orientation and α fiber texture except the Goss orientation undergo a decrease accompanying the intensification of γ fiber texture. In the whole deformation processes, Goss orientation is the dominant texture component while no pronounced Brass component is observed. The dominant Goss component can be attributed to the preferred Goss orientation both in shear bands and in matrix. When the rolling temperature is decreased to 573 K, even at the early deformation stage, 42% rolling reduction, both D and Brass orientations are not observed. EBSD analysis confirms that the texture evolution is promoted to the early deformation stages at lower rolling temperature..
233. T. Ohmura, K. Sawada, K. Kimura, Kaneaki Tsuzaki, Alteration in nanohardness of matrix phase associated with precipitation during long-term aging of type 316 stainless steel, Materials Science and Engineering A, 10.1016/j.msea.2007.12.003, 489, 1-2, 85-92, 2008.08, Mechanical characterization for a long-term aged type 316 austenitic stainless steel was carried out using the nanoindentation technique. Samples were aged isothermally at 973 K for various times up to 39,332 h (4.5 years). The precipitation behavior was characterized with transmission electron microscope (TEM), and the hardness of the matrix phase was evaluated by the nanoindentation technique. The nanohardness of the matrix starts to decrease simultaneously with the initiation of M23C6 precipitation and is reduced significantly after 100 h aging, while the macroscopic Vickers hardness shows a considerable increase. Since the reduction in the matrix hardness is synchronized with the M23C6 precipitation, the softening results from a depression of the solid-solution hardening by solute elements such as carbon, chromium and molybdenum, which transform into M23C6. An accurate amount of precipitation hardening is evaluated by considering the reduction in the matrix hardness. The strengthening by the second phase can be understood quantitatively based on the Orowan mechanism for 183 h aging and the composite strengthening for 39,332 h, respectively..
234. M. Murayama, T. Nishimura, Kaneaki Tsuzaki, Nano-scale chemical analysis of rust on a 2% Si-bearing low alloy steel exposed in a coastal environment, Corrosion Science, 10.1016/j.corsci.2008.05.022, 50, 8, 2159-2165, 2008.08, Corrosion products of the rust layer formed on a 2% Si-bearing low alloy steel during atmospheric exposure at a coastal area in Japan for three years were characterized. The inner layer of the rust, i.e., near oxide/metal interface, has a layer structure and the average grain size of corrosion products is finer than that in the upper region that is few micro meter above the interface. A series of nano probe energy disperse X-ray spectroscopy (EDS) and selected area electron diffraction (SAED) analyses performed using a field emission gun (FEG) transmission electron microscope (TEM) revealed that the inner layer consists of an iron rich, coarse grained β-FeOOH matrix and the silicon enriched narrow band composed a mixture of α-FeOOH and nano-meter size silicon oxide. On the other hand, the α-FeOOH single phase area exists where the layer structure did not develop. The result of detailed microstructural analyses indicated that the silicon plays an important role in modifying the rust layer structure..
235. Yuuji Kimura, Tadanobu Inoue, Fuxing Yin, Kaneaki Tsuzaki, Inverse temperature dependence of toughness in an ultrafine grain-structure steel, Science, 10.1126/science.1156084, 320, 5879, 1057-1060, 2008.05, Materials are typically ductile at higher temperatures and become brittle at lower temperatures. In contrast to the typical ductile-to-brittle transition behavior of body-centered cubic (bcc) steels, we observed an inverse temperature dependence of toughness in an ultrahigh-strength bcc steel with an ultrafine elongated ferrite grain structure that was processed by a thermomechanical treatment without the addition of a large amount of an alloying element. The enhanced toughness is attributed to a delamination that was a result of crack branching on the aligned {100} cleavage planes in the bundles of the ultrafine elongated ferrite grains strengthened by nanometer-sized carbides. In the temperature range from 60° to -60°C, the yield strength was greater, leading to the enhancement of the toughness..
236. T. Ohmura, Kaneaki Tsuzaki, Analysis of grain boundary effect of bulk polycrystalline materials through nanomechanical characterization, Journal of Physics D: Applied Physics, 10.1088/0022-3727/41/7/074015, 41, 7, 2008.04, Indentation-induced deformation behaviour was characterized in nano-scale for bulk polycrystalline materials to understand strengthening factors of macroscopic properties especially for the grain boundary effect. Compared with deformation behaviour in the vicinity of single grain boundary and grain interior of an interstitial free steel, plasticity initiation occurs at a lower applied stress near the grain boundary, which means the grain boundary is an effective dislocation source. The subsequent deformation after plasticity initiation is affected by the grain boundary as a barrier to dislocation motion. Strengthening factors of matrix and grain boundaries were evaluated separately for the Fe-C tempered martensite. The contribution of grain boundaries depends on the morphology of precipitates on grain boundaries. Combining the dislocation pile-up model and the Hall-Petch relation, the existence of the film-like carbide on the grain boundary remarkably affects the locking parameter k..
237. O. Sitdikov, T. Sakai, E. Avtokratova, R. Kaibyshev, Kaneaki Tsuzaki, Y. Watanabe, Microstructure behavior of Al-Mg-Sc alloy processed by ECAP at elevated temperature, Acta Materialia, 10.1016/j.actamat.2007.10.029, 56, 4, 821-834, 2008.02, Microstructural evolution taking place during equal channel angular pressing (ECAP) was studied in a coarse-grained Al-6% Mg-0.4% Mn-0.3% Sc alloy at 300 °C (∼0.6Tm). Samples were pressed to strain 12 and quenched in water after each pass. ECAP at moderate-to-high strains leads to formation of a bimodal grain structure, with grain sizes of about 1 and 8 μm and volume fractions of 0.3 and 0.6, respectively. The development of new-grained regions has been shown to result from the concurrent operation of the continuous dynamic recrystallization that occurs during deformation and the static recrystallization that occurs by the exposure of the as-deformed material in the die kept at 300 °C and/or reheating between pressings. The microstructural development is discussed in terms of the enhanced driving force for recrystallization, resulting from the evolution of high-density dislocation substructures due to localization of plastic flow and inhibition of recovery in the present alloy..
238. Masato Ohnuma, Jun ichi Suzuki, Fu Gao Wei, Kaneaki Tsuzaki, Direct observation of hydrogen trapped by NbC in steel using small-angle neutron scattering, Scripta Materialia, 10.1016/j.scriptamat.2007.09.026, 58, 2, 142-145, 2008.01, Small-angle neutron scattering has been measured for 97.62Fe-0.23C-0.25Nb-1.90Ni (at.%) steel with and without hydrogen charging. The intensity of the scattering from nanosized NbC is higher for samples with hydrogen than for those without hydrogen. The enhanced scattering intensity for samples with hydrogen disappears after hydrogen desorption heat treatment. This result provides direct structural evidence for the trapping of hydrogen by NbC..
239. O. Sitdikov, E. Avtokratova, T. Sakai, Kaneaki Tsuzaki, R. Kaibyshev, Y. Watanabe, Effect of processing temperature on microstructure development during ECAP of Al-Mg-Sc alloy, 4th International Conference on Nanomaterials by Severe Plastic Deformation Materials Science Forum - Nanomaterials by Severe Plastic Deformation 4 - 4th International Conference on Nanomaterials by Severe Plastic Deformation, 481-486, 2008.01, Microstructural evolution taking place during equal channel angular pressing (ECAP) was studied in a commercial coarse-grained Al-6%Mg-0.4%Mn-0.3%Sc alloy in a temperature interval 200-450°C (∼0.5-0.8 Tm). Samples were pressed using route A to a total strain of 12 and quenched in water after each ECAP pass. Uniform fine-grained microstructures with the average grain sizes of 0.7 and 2.5 μm, are almost fully evolved at high ECAP strains at 250°C and 450°C, respectively, while ECAP at 300°C (∼0.6 Tm) leads to the formation of bimodal grain structure with fine grains of around 1 μm and relatively coarse grains of around 8 μm. The latter are developed due to the occurrence of static recrystallization during "keeping" time in the ECAP channel and/or reheating between ECAP passes. The microstructural development under warm-to-hot ECAP conditions is discussed in terms of the large potential for grain boundary migration resulted from an overlapping of accelerated grain boundary mobility at high pressing temperatures and enhanced driving force for recrystallization, which is caused by a strong inhibition of dynamic recovery in a heavily-alloyed Al alloy..
240. K. Sawada, T. Ohmura, T. Ohba, Kaneaki Tsuzaki, F. Abe, K. Kimura, I. Nonaka, Elastic properties of heat resistant steels after long-term creep exposure, Materials at High Temperatures, 10.3184/096034008X357276, 25, 3, 179-185, 2008, In order to clarify the effect of microstructural change during ageing and creep on elastic modulus, the elastic moduli of aged, crept and serviced (service-exposed) materials were investigated. In ferritic steels, the elastic modulus obtained by ultrasonic pulse does not change after long-term ageing and creep although the formation of creep voids slightly decreases the elastic modulus. In SUS304 and SUS316, the elastic modulus evaluated by ultrasonic pulse increases during ageing due to precipitation of M23C6 carbides, Laves phase and σ phase though the elastic modulus decreases during creep because of the formation of creep voids on grain boundaries. The increased elastic modulus of aged material with a large amount of σ phase was regarded as that of composite consists of matrix and σ phase. For STBA24, the elastic modulus obtained by a high temperature tensile test decreases after long-term service. The tensile test at a higher strain rate was proposed to avoid underestimation of elastic modulus of serviced materials at high temperatures..
241. Yuuji Kimura, Tadanobu Inoue, Fuxing Yin, Kaneaki Tsuzaki, Enhanced toughness in a medium-carbon low-alloy steel processed by tempforming, 17th International Federation for Heat Treatment and Surface Engineering Congress 2008, IFHTSE 2008 Proceedings of the 17th IFHTSE Congress, 2, 430-433, 2008, Innovative Materials Engineering Laboratory, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan The deformation of a tempered martensitic structure, namely tempforming was applied to a high purity 0.4%C-2%Si-l%Cr-l%Mo steel at 500 °C using multi-pass caliber rolling with an accumulative reduction in area of 77%. Charpy impact properties were investigated for the tempformed sample. An excellent combination of strength and toughness without the addition of a large amount of alloying elements such as Ni was demonstrated in the ultrafine fibrous grain structure that was processed by the tempforming. The yield strength of 1.84GPa and the V-notch Charpy absorbed energy of 226J at room temperature and the ductile-to-brittle temperature of less than -60 °C that we obtained in the medium-carbon low-alloy steel are the most optimal values that have been reported in low-alloy steels..
242. Shusaku Takagi, Satoshi Terasaki, Kaneaki Tsuzaki, Tadanobu Inoue, Fumiyoshi Minami, Application of local approach to hydrogen embrittlement fracture evaluation of high strength steels, 5th International Conference on Processing and Manufacturing of Advanced Materials - THERMEC'2006 Supplement to THERMEC 2006, 5th International Conference on PROCESSING and MANUFACTURING OF ADVANCED MATERIALS, THERMEC 2006, 2155-2161, 2007.12, A new method for evaluating the hydrogen embrittlement (HE) susceptibility of ultra high strength steel was studied in order to propose a new method for assessing the delayed fracture property. The material used was 1400MPa tempered martensitic steel with the chemical composition 0.40C-0.24Si-0.81Mn-1.03Cr-0. 16Mo(mass%). The local approach originally used for evaluating the brittle fracture property was applied to HE susceptibility assessment after modifying the method to include the effect of hydrogen content. Critical HE data used in the modified local approach was obtained by a stepwise test in which alternating processes of stress increase and stress holding were repeated until the specimen fractured. The specimen used in the stepwise test was 10 mm in diameter and the stress concentration factor was 4.9. Assessment of HE susceptibility for specimens with other dimensions entailed the use of a critical hydrogen content for failure, Hc, representing the maximum hydrogen content among the unfractured specimens in the HE test with constant loading. Matters to be noted for obtaining the material parameters are discussed..
243. A. Belyakov, F. G. Wei, Kaneaki Tsuzaki, Y. Kimura, Y. Mishima, Incomplete recrystallization in cold worked steel containing TiC, Materials Science and Engineering A, 10.1016/j.msea.2007.04.022, 471, 1-2, 50-56, 2007.12, Development of primary recrystallization was studied in a steel containing 0.4 vol.% of fine TiC precipitations with an average size of 12 nm. After sufficiently large cold strains, the recrystallization developed readily upon annealing at temperatures above 600 °C. An increase in the cold strain as well as the annealing temperature resulted in the acceleration of recrystallization kinetics. However, a certain amount of cold worked microstructures of about 15 vol.% remained unrecrystallized even after annealing at a rather high temperature of 700 °C. The unrecrystallized portions were composed of grains with the 〈0 0 1〉 crystallographic direction parallel to the compression axis. Both the low stored energies in these grains and the pinning of recrystallizing grain boundaries by the dispersed carbides were discussed as crucial factors that resulted in the incomplete recrystallization..
244. T. Chandra, Kaneaki Tsuzaki, M. Militzer, C. Ravindran, Materials Science Forum
Preface, 5th International Conference on Processing and Manufacturing of Advanced Materials - THERMEC'2006 Materials Science Forum, 539-543, PART 1, 2007.12.
245. O. Sitdikov, T. Sakai, E. Avtokratova, R. Kaibyshev, Kaneaki Tsuzaki, Y. Watanabe, Microstructural evolution in a commercial Al-Mg-Sc alloy during ECAP at 300°C, 3rd International Conference on Recrystallization and Grain Growth, ReX GG III Recrystallization and Grain Growth III - Proceedings of the Third International Conference on Recrystallization and Grain Growth, ReX and GG III, 569-574, 2007.12, Microstructural evolution taking place during equal channel angular pressing (ECAP) was studied in a commercial coarse-grained Al-6%Mg-0.4%Mn-0.3%Sc alloy at a temperature of 300°C (∼0.6Tm). Samples were pressed using route A to a total strain of 12 and quenched in water after each ECAP pass. ECAP at moderate-to-high strains leads to the formation of a bimodal grain structure with grain sizes of around 1 and 8 μm and volume fractions of 0.3 and 0.6, respectively. The development of new-grained regions has been shown to result from a concurrent operation of continuous dynamic recrystallization that occurs during deformation and static recrystallization that occurs during each ECAP cycle by the exposure of the as-deformed material in the die kept at 300°C for around 1.5 minutes. The microstructural development during warm-to-hot ECAP is discussed in terms of the enhanced driving force for recrystallization, resulting from the evolution of high-density dislocation substructures due to the localization of plastic flow and inhibition of recovery in the present alloy..
246. Andrey Belyakov, Kaneaki Tsuzaki, Yoshisato Kimura, Yoshinao Mishima, Recovery in 15%Cr ferritic stainless steel after large strain deformation, 3rd International Conference on Recrystallization and Grain Growth, ReX GG III Recrystallization and Grain Growth III - Proceedings of the Third International Conference on Recrystallization and Grain Growth, ReX and GG III, 558-559, 119-124, 2007.12, 15%Cr ferritic stainless steel was machined in rectangular samples and then processed by multiple forging to a total cumulative strain of 7.2 at an ambient temperature. The large strain deformation resulted in almost equiaxed submicrocrystalline structure with a mean grain/subgrain size of 230 nm and about 2.2×1014 m-2 dislocation density in grain/subgrain interiors. The annealing at a relatively low temperature of 500°C did not lead to any discontinuous recrystallizations. The grain/subgrain size and the interior dislocation density slightly changed to 240 nm and 2.1×1014 m-2, respectively, after annealing for 30 min, while the Vickers hardness decreased from 3140 MPa in the as-processed state to 2900 MPa. This annealing softening was attributed to remarkable release (by 50%) of internal stresses, which are associated with a non-equilibrium character of strain-induced grain/subgrain boundaries..
247. Kaneaki Tsuzaki, Andrey Belyakov, Fuxing Yin, Texture invariant annealing in severely deformed steel, 3rd International Conference on Recrystallization and Grain Growth, ReX GG III Recrystallization and Grain Growth III - Proceedings of the Third International Conference on Recrystallization and Grain Growth, ReX and GG III, 558-559, 101-106, 2007.12, This paper showed an example of the phenomena that a strong deformed texture does not change after the annealing process in steels. An Fe - 22%Cr - 3%Ni ferritic stainless steel was processed by bar rolling/swaging to a total strain of 4.4 at an ambient temperature, and its annealing behaviour was studied in a temperature range of 400-700°C. The deformed sample showed a grain size of 200nm, a fraction of high-angle boundary (HAB) of about 0.6, and a strong fiber texture of 〈110〉{uvw}. This texture showed very little change after annealing which was characterised by the development of continuous recrystallization involving recovery processes and followed by a normal grain growth. On the other hand, by annealing a sample that was deformed to total strain of 2.0 containing rather fine grains (270nm) but without a large enough fraction of HAB (0.3), a discontinuous recrystallization took place, and its deformed texture changed considerably..
248. Dorothee Dorner, Yoshitaka Adachi, Kaneaki Tsuzaki, Stefan Zaefferer, Tracing the goss orientation during deformation and annealing of an FeSi single crystal, International Symposium on Fundamentals of Deformation and Annealing Fundamentals of Deformation and Annealing - Proceedings of the International Symposium held to coincide with the retirement of Professor John Humphreys, 10.4028/0-87849-434-0.485, 485-490, 2007.12, A Goss-oriented single crystal was cold rolled up to 89% thickness reduction, and subsequently annealed at 550°C or 850°C During deformation most of the initially Goss-oriented material rotated into the two symmetrical {111}〈112〉 orientations. In addition, Goss regions were observed related to microbands or microshear bands. Goss regions in microshear bands formed during straining, whereas Goss regions between microbands were retained from the initial Goss orientation. The recrystallisation texture for annealing temperatures of both 550°C and 850°C is characterised by a Goss texture. However, the origin of the Goss recrystallisation nuclei appeared to be different for the different annealing conditions. In the material annealed at 550°C, the Goss texture originated from the Goss regions in the microshear bands. In contrast, for an annealing temperature of 850°C, the Goss grains between the microbands are likely to form recrystallisation nuclei..
249. Andrey Belyakov, Kaneaki Tsuzaki, Y. Kimura, Y. Mishima, Annealing behavior of a ferritic stainless steel subjected to large-strain cold working, Journal of Materials Research, 10.1557/jmr.2007.0398, 22, 11, 3042-3051, 2007.11, Mechanisms of microstructure evolution during annealing after cold working were studied in an Fe-15%Cr ferritic stainless steel, which was processed by bar rolling/swaging to various total strains ranging from 1.0 to 7.3 at ambient temperature. Two types of recrystallization behavior were observed depending on the cold strain. An ordinary primary (discontinuous) recrystallization developed in the samples processed to conventional strains of 1.0-2.0. On the other hand, rapid recovery at early annealing resulted in ultrafine-grained microstructures in the larger strained samples that continuously coarsened on further annealing. Such annealing behavior was considered as continuous recrystallization..
250. Maoqiu Wang, Eiji Akiyama, Kaneaki Tsuzaki, Effect of hydrogen on the fracture behavior of high strength steel during slow strain rate test, Corrosion Science, 10.1016/j.corsci.2007.03.038, 49, 11, 4081-4097, 2007.11, The effect of hydrogen on the fracture behavior of the quenched and tempered AISI 4135 steel at 1450 MPa has been investigated by means of slow strain rate tests on smooth and circumferentially-notched round-bar specimens. Hydrogen was introduced into specimens by electrochemical charging and its content was measured by thermal desorption spectrometry (TDS) analysis. Results showed that the steel had high hydrogen embrittlement susceptibility. For both smooth and notched specimens, the fracture mode was changed from microvoid coalescence (MVC) to brittle intergranular (IG) fracture after the introduction of a small amount of diffusible hydrogen. Fracture initiated in the vicinity of the notch root for notched specimens, while it started from around the center in smooth specimens. The fracture stress decreased with increasing diffusible hydrogen content, and the decreasing trend was more prominent for specimens with a higher stress concentration factor. Taking into account the stress-driven hydrogen diffusion and accumulation in the vicinity of the notch root, the local diffusible hydrogen concentration and local fracture stress in notched specimens have been calculated. According to numerical results, the relationship between the local fracture stress and local diffusible hydrogen concentration was independent of stress concentration factor, which could account for the effect of hydrogen on the fracture stress of the steel..
251. Dorothée Dorner, Yoshitaka Adachi, Kaneaki Tsuzaki, Periodic crystal lattice rotation in microband groups in a bcc metal, Scripta Materialia, 10.1016/j.scriptamat.2007.06.048, 57, 8, 775-778, 2007.10, Fe-3mass%Si specimens with a columnar grain structure were deformed in compression at ambient temperature. Groups of microbands were found in the vicinity of a grain boundary. These microband groups are characterized by shear and a periodic change of crystal lattice orientation within distances of 1-2 μm. Furthermore, new grains with a size of 200-300 nm were formed in the microband groups during deformation up to 36% strain. A local lattice rotation model is given that explains the experimental observations..
252. Y. Kimura, T. Inoue, F. Yin, O. Sitdikov, Kaneaki Tsuzaki, Toughening of a 1500 MPa class steel through formation of an ultrafine fibrous grain structure, Scripta Materialia, 10.1016/j.scriptamat.2007.05.039, 57, 6, 465-468, 2007.09, A deformation of tempered martensite, namely tempforming at an elevated temperature, was applied to a medium-carbon low-alloy steel. This thermomechanical treatment led to an evolution of an ultrafine fibrous grain structure with a strong 〈1 1 0〉∥RD fiber deformation texture and to a remarkable improvement in Charpy impact strength (uE = 165 J) at a tensile strength of 1.5 GPa..
253. Zuogui Zhang, Eiji Akiyama, Yoshimi Watanabe, Yasuyuki Katada, Kaneaki Tsuzaki, Effect of α-Al/Al3Ni microstructure on the corrosion behaviour of Al-5.4 wt% Ni alloy fabricated by equal-channel angular pressing, Corrosion Science, 10.1016/j.corsci.2007.02.007, 49, 7, 2962-2972, 2007.07, The effect of microstructure on corrosion behaviour of an Al-5.4 wt% Ni alloy fabricated by equal-channel angular pressing (ECAP) was investigated by means of potentiodynamic polarization test. The Al-5.4 wt% Ni alloy samples were severely deformed by ECAP with two strain introduction methods of route A and route BC and the ECAP process was repetitively carried out up to 6 passes (strain 6). The anodic polarization measurements indicated that pitting potential of the ECAPed Al-Ni alloy samples in chloride containing neutral buffer solution increased with ECAP passes. The pitting corrosion resistance of Al-Ni alloy after ECAP by route BC was better than that by route A. It is attributable to that the size of α-Al crystal region was reduced with ECAP passes and route BC was able to obtain more homogeneous α-Al/Al3Ni structure than route A. On the other hand, pitting corrosion resistance of pure Al samples showed an obvious declining with increasing ECAP passes. It was indicated that more homogeneous and finer α-Al/Al3Ni structure obtained by ECAP played a vital role on improving the corrosion resistance of Al-5.4 wt% Ni alloy..
254. Andrey Belyakov, Kaneaki Tsuzaki, Yuuji Kimura, Yoshisato Kimura, Yoshinao Mishima, Comparative study on microstructure evolution upon unidirectional and multidirectional cold working in an Fe-15%Cr ferritic alloy, Materials Science and Engineering A, 10.1016/j.msea.2006.12.063, 456, 1-2, 323-331, 2007.05, Deformation microstructures of an Fe-15%Cr ferritic stainless steel during cold working to strains above 7 were studied. Two processing methods, unidirectional working consisting of bar rolling followed by swaging and multidirectional working involving multiple forging through three orthogonal axes, were used. The unidirectional deformation resulted in the evolution of a ribbon-like microstructure consisting of highly elongated grains. The mean grain size gradually decreased to about 0.4 μm during rolling/swaging to strains above 7. In contrast, the multiple forging resulted in the development of almost equiaxed fine grains with an average grain size of 0.45 μm at relatively small strains of about 4 followed by a slight grain refinement during further processing. On the other hand, both processing methods were characterised by almost the same strain dependence for the transverse grain and subgrain sizes, which were measured crosswise to the metal flow direction..
255. Takahito Ohmura, Kaneaki Tsuzaki, Plasticity initiation and subsequent deformation behavior in the vicinity of single grain boundary investigated through nanoindentation technique, Journal of Materials Science, 10.1007/s10853-006-0885-y, 42, 5, 1728-1732, 2007.03, The initiation of plasticity and the subsequent state in the vicinity of a single grain boundary during indentation-induced deformation were investigated to understand an elementary step of a stress-strain behavior of polycrystalline materials. Nanoindentation measurements on several points on a single grain boundary and the grain interior of an interstitial-free steel and an analysis on the pop-in behavior and the plastic nanohardness were carried out. The pop-in load P c that was obtained on the loading curve is different for each measurement. However, the loading curves overlap one another and the unloading curves coincide as well after the pop-in event. The nanohardness Hn has no dependence on the P c in the range of 150-550 μN. The relation between P c and Δh can be expressed as a simple cubic polynomial function based on a geometrically necessary dislocation loop model. The fitted function differed for various grains with different crystallographic orientations..
256. O. Sitdikov, T. Sakai, E. Avtokratova, R. Kaibyshev, Y. Kimura, Kaneaki Tsuzaki, Grain refinement in a commercial Al-Mg-Sc alloy under hot ECAP conditions, Materials Science and Engineering A, 10.1016/j.msea.2006.06.081, 444, 1-2, 18-30, 2007.01, Grain refinement taking place during equal-channel angular pressing (ECAP) was studied in a commercial Al-6% Mg-0.4% Mn-0.3% Sc alloy at a temperature of 450 °C (∼0.8T
m
) to demonstrate the feasibility of obtaining new fine-grained structure in a hard-to-deform Al-Mg-Sc alloy under hot intense plastic straining (IPS) conditions and investigate the evolution process of new grains. Inhomogeneous deformation occurring during hot ECAP leads to formation of deformation bands. Repeated ECAP results in mutual crossing and increase in number and the misorientation of deformation bands, followed by transformation of the boundaries of deformation bands into high-angle ones. As a result, a new fine-grained microstructure with an average crystallite size of 2.8 μm develops at large strains above 8. It is concluded that grain refinement occurs in accordance with deformation-induced continuous reactions; that is similar to in-situ or continuous dynamic recrystallization. The mechanisms of new grain evolution, as well as factors promoting grain refinement, are discussed in detail..
257. T. Chandra, Kaneaki Tsuzaki, M. Militzer, C. Ravindran, Preface, 5th International Conference on Processing and Manufacturing of Advanced Materials, THERMEC 2006 Materials Science Forum, 539-543, 2007.01.
258. T. Chandra, Kaneaki Tsuzaki, M. Militzer, C. Ravindran, Preface, 5th International Conference on Processing and Manufacturing of Advanced Materials - THERMEC 2006 Advanced Materials Research, 15-17, 2007.01.
259. Zuogui Zhang, Eiji Akiyama, Yoshimi Watanabe, Yasuyuki Katada, Kaneaki Tsuzaki, Corrosion behavior of Al-7wt% Sl-1.5wt% Cu severely deformed by equal-channel angular pressing, 5th International Conference on Processing and Manufacturing of Advanced Materials - THERMEC'2006 Supplement to THERMEC 2006, 5th International Conference on PROCESSING and MANUFACTURING OF ADVANCED MATERIALS, THERMEC 2006, 539-543, 2892-2897, 2007, In this study, an Al-7 wt% Si-1.5 wt% Cu alloy was subjected to severe plastic deformation (SPD) by an equal-channel angular pressing (ECAP) technique. The ECAP process was repetitively carried out up to 8 passes using a strain introduction method of route Bc, at a temperature of 25 °C and a pressing rate of 0.33 mm s-1. Microstructures of the samples before and after ECAP were observed by a scanning electron microscopy (SEM). Electrochemical properties of the Al-Si-Cu alloy fabricated by ECAP have been investigated in a borate-boric acid buffer solution containing Cl- ions at pH 8.3 and 25 °C by potentiodynamic polarization test. Corrosion pits on the sample surface after anodic polarization were investigated by means of SEM. The anodic polarization showed that as-cast Al-Si-Cu alloy with plate-shaped Si particles has poor resistance against pitting corrosion comparing to quenched sample without ECAP. Pitting potentials of ECAPed Al-Si-Cu alloy samples were higher than that of the sample without ECAP. In the Al-Si-Cu alloy, the corrosion pits were found in the region of Si particles and the size of pits formed on the ECAPed samples became smaller than that without ECAP. It is considered that the improvement of the pitting resistance of ECAPed Al-Si-Cu alloy is due to homogenous distribution of spherical Si particles generated during ECAP process..
260. Dorothée Dorner, Yoshitaka Adachi, Kaneaki Tsuzaki, Microband-to-microshear band transition near grain boundaries in bcc steel, 3rd International Conference on Recrystallization and Grain Growth, ReX GG III Recrystallization and Grain Growth III - Proceedings of the Third International Conference on Recrystallization and Grain Growth, ReX and GG III, 558-559, 873-878, 2007, Compression tests were performed on Fe-3%Si specimens with few grains. The deformation microstructure and microtexture were investigated by electron backscatter diffraction (EBSD) and related to the initial crystal orientation and grain boundary characteristics. Groups of microbands were found that are characterised by a periodic change in crystal orientation, shear at the grain boundary, and the formation of new grains. It is supposed that these microband groups represent an early stage of microshear band development..
261. Yoshitaka Adachi, Kaneaki Tsuzaki, Ultra rapid softening of high strength structural steels by thermomechanical treatment, 5th International Conference on Processing and Manufacturing of Advanced Materials - THERMEC'2006 Supplement to THERMEC 2006, 5th International Conference on Processing and Manufacturing of Advanced Materials, THERMEC 2006, 539-543, 4807-4812, 2007, This study aims to shorten the softening treatment period as possible in high strength structural steels. The steel used is SCM440 steel. As an initial microstructure, martensite, bainite, pearlite and complicated microstructure consisting of ultrafine polygonal, martensite and equiaxed cementite were extensively examined to understand their softening process on aging at 973K. These initial microstructures were prepared by heat or thermomechanical treatment. Their initial Vickers hardness (Hv(lOkgf)) were 634, 281, 219 and 238, respectively. It is noteworthy that within five minutes on aging hardness of the complicated microstructure reached lower than Hv200, while it took more than several hours for other initial microstructures. A quantitative evaluation of microstructures appears to help in understanding the mechanism of the softening kinetics..
262. T. Sundararajan, E. Akiyama, Kaneaki Tsuzaki, Acetate and chloride effects on hydrogen production across crevices, International Symposium on Advanced Structural and Functional Materials Design, 2004 Advanced Structural and Functional Materials Design - Proceedings of the International Symposium on Advanced Structural and Functional Materials Design, 2004, 10.4028/0-87849-996-2.97, 512, 97-102, 2006.12, Crevice corrosion experiments on pure iron were earned out in a 0.5 M acetate buffer with varied chloride concentrations. Changes in resultant currents and morphology due to crevice attack were explained by IR potential drop mechanisms. The specimens experienced potential drop inside the crevice, which resulted in the formation of passive, active, and hydrogen evolution regions. The passive region did not exist in the electrolyte containing 0.05 M and 0.5 M chloride. Hydrogen evolution, which occurred inside the crevice was measured on rear side of the specimen using hydrogen permeation test. The results suggest that the hydrogen produced inside the crevice is measurable using a permeation test. The entry of diffusible hydrogen showed a significant increase with the addition of chloride into the acetate buffer..
263. Kaneaki Tsuzaki, Andrey Belyakov, Yuuji Kimura, Deformation microstructures in a two-phase stainless steel during large strain deformation, 3rd International Conference on Nanomaterials by Severe Plastics Deformation, NanoSPD3 Nanomaterials by Severe Plastic Deformation, NanoSPD3 - Proceedings of the 3rd International Conference on Nanomaterials by Severe Plastics Deformation, 305-310, 2006.12, Deformation microstractures were studied in a two-phase (about 60% ferrite and 40% austenite) Fe - 27%Cr - 9%Ni stainless steel. Severe plastic working was carried out by rolling from 21.3×21.3 mm2 to 7.8×7.8 mm2 square bar followed by swaging from Ø7.0 to 0.6 mm rod at an ambient temperature, providing a total strain of 6.9. After a rapid increase in the hardness at an early deformation, the rate of the strain hardening gradually decreased to almost zero at large strains above 4. In other words, the hardness approached a saturation level, leading to an apparent steady-state deformation behaviour during cold working. The severe deformation resulted in the evolution of highly elongated (sub)grains aligned along the rolling/swaging axis with the final transverse (sub)grain size of about 0.1 μm and the fraction of high-angle (sub)boundaries above 60%. However, the kinetics of microstructure evolution in the two phases was different. In the ferrite phase, the transverse size of deformation (sub)grains gradually decreased during the processing and approached 0.1 μm at strains of about 6.0, while the transverse size of the austenite (sub)grains rapidly reduced to its final value of 0.1 μm after a relatively low strain about 1.0..
264. Maoqiu Wang, Eiji Akiyama, Kaneaki Tsuzaki, Effect of hydrogen on the fracture behavior of high-strength Cr-Mo steel, International Symposium on Advanced Structural and Functional Materials Design, 2004 Advanced Structural and Functional Materials Design - Proceedings of the International Symposium on Advanced Structural and Functional Materials Design, 2004, 10.4028/0-87849-996-2.55, 55-60, 2006.12, We examine the hydrogen embrittlement susceptibility of a high-strength AISI 4135 steel by means of a slow strain-rate test (SSRT) using notched round bar specimens. Hydrogen was introduced into the specimens by electrochemical charging and its content was measured by thermal desorption spectrometry (TDS). It was found that the maximum tensile stress decreased in a power law manner with increasing diffusible hydrogen content. Finite element method (FEM) calculations demonstrated that the peak value of the maximum principal stress and the peak value of the locally accumulated hydrogen concentration at the maximum tensile stress were in good agreement with one power law relationship for the specimens with different stress concentration factors..
265. O. Sitdikov, R. Kaibyshev, E. Avtokratova, Kaneaki Tsuzaki, Fine-grained structure formation in Al- Mg- Sc alloy during hot ECAP, 3rd International Conference on Nanomaterials by Severe Plastics Deformation, NanoSPD3 Nanomaterials by Severe Plastic Deformation, NanoSPD3 - Proceedings of the 3rd International Conference on Nanomaterials by Severe Plastics Deformation, 721-726, 2006.12, Grain refinement taking place during equal channel angular pressing (ECAP) was studied in a commercial Al-6%Mg-0.4%Mn-0.3%Sc alloy at a temperature of 4500C. Inhomogeneous deformation occurring during hot ECAP of the present alloy leads to formation of deformation bands. Repeated ECAP results in mutual crossing and increase in number and misorientation of deformation bands, followed by transformation of boundaries of deformation bands into high angle boundaries. As a result, a new fine-grained microstructure with an average crystallite size of 2.8 μm develops at large strains above 8. It is concluded that grain refinement occurs in accordance with deformation-induced continuous reactions; that is essentially similar to continuous dynamic recrystallization. The mechanisms of new grain evolution, as well as factors promoting grain refinement, are discussed in detail..
266. Hisashi Sato, Kentaro Ota, Yoshimi Watanabe, Zuogui Zhang, Kaneaki Tsuzaki, Grain refinement performance of aluminum cast-alloy by deformed Al-Al 3Ti alloy refiner, 10th International Conference on Aluminium Alloys, (ICAA-10) Aluminium Alloys 2006 Research Through Innovation and Technology - Proceedings of the 10th International Conference on Aluminium Alloys, 1859-1864, 2006.12, Grain refining experiments for casting of pure Al were conducted to evaluate the grain refinement performance of an Al-5mass%Ti alloy refiner before and after cold rolling. Al3Ti particles in the Al-Ti alloy refiner were efficiently fragmented by cold rolling. The size of the Al3Ti particles in cold rolled Al-Ti alloy refiner decreased from 280μm to 30μm with increasing reduction ratio of cold rolling. Mean size of α-Al grains in pure Al cast refined by the cold rolled Al-Ti alloy refiner decreased from about 500μm to 200μm with increasing the reduction ratio. In order to investigate mechanical property of the pure Al cast refined by the cold rolled Al-Ti alloy refiner, Vickers hardness test and tensile test were conducted. Strength of the pure Al cast refined by the cold rolled Al-Ti alloy refiner increased with increasing the reduction ratio. The strength improvement of pure Al cast refined by the cold rolled Al-Ti alloy refiner followed Hall-Petch relationship. From obtained results, it was concluded that cold rolling for refiner is useful practical application for pure Al cast..
267. Zuogui Zhang, Eiji Akiyama, Yoshimi Watanabe, Yasuyuki Katada, Kaneaki Tsuzaki, Influence of microstructure on electrochemical properties of severely deformed AI-5 wt% Cu alloy by ECAP, 10th International Conference on Aluminium Alloys, (ICAA-10) Aluminium Alloys 2006 Research Through Innovation and Technology - Proceedings of the 10th International Conference on Aluminium Alloys, 705-710, 2006.12, The influence of microstructure on the electrochemical properties of an Al-5 wt% Cu binary alloy which was severely deformed by equal-channel angular pressing (ECAP) has been investigated in a borate-boric acid buffer solution containing Cl- ions at pH 8.3 and 25°C by potentiodynamic polarization test. The anodic polarization results showed that pitting potentials of ECAPed Al-Cu alloy samples were higher than that of the sample without ECAP and increased with repetitive ECAP passes. The pitting corrosion attack of Al-Cu alloy samples took place in the surroundings of Al2Cu (9) phase. It is considered that the grain refinement of 9 phase and the solid solution generated in Al matrix during ECAP process are responsible for the improvement of the corrosion resistance of ECAPed Al-Cu alloy..
268. Andrey Belyakov, Yuuji Kimura, Kaneaki Tsuzaki, On structural mechanism of continuous recrystallization in ferritic stainless steel after large strain processing, 3rd International Conference on Nanomaterials by Severe Plastics Deformation, NanoSPD3 Nanomaterials by Severe Plastic Deformation, NanoSPD3 - Proceedings of the 3rd International Conference on Nanomaterials by Severe Plastics Deformation, 503-504, 323-328, 2006.12, The annealing behaviour of an Fe - 22%Cr - 3%Ni ferritic stainless steel processed by bar rolling/swaging to total strain of 4.4 at an ambient temperature was studied in the temperature range of 400-700°C. The annealing behaviour was characterised by the development of continuous recrystallization involving recovery processes followed by a normal grain growth. The large strain deformation caused the very fast recovery resulting in the development of almost equiaxed polygonized microstracture in place of the highly elongated deformation (sub)grains. The polygonization development was accompanied by some increase in the transverse (sub)grain size and the formation of many low-angle subboundaries. The latter ones could be composed from the dislocations, which were emitted by the strain-induced deformation (sub)boundaries. In spite of relatively large fraction of low-angle subboundaries, such polygonized microstructure was essentially stable against a discontinuous grain coarsening. Upon further annealing, therefore, the microstructure evolution was considered as a normal grain growth..
269. A. Belyakov, M. Murayama, Y. Sakai, Kaneaki Tsuzaki, M. Okubo, M. Eto, T. Kimura, Development of a high-strength high-conductivity Cu-Ni-P alloy. Part II
Processing by severe deformation, Journal of Electronic Materials, 10.1007/s11664-006-0306-7, 35, 11, 2000-2008, 2006.11, Deformation microstructures during large-strain plastic working were studied in the pure Cu and Cu-1.5%Ni-0.3%P alloy, the original microstructures of which have been detailed in our companion publication. Microstructural changes depended significantly on the initial structural state. Solution-treated and low-temperature (400°C) aged samples were characterized by rapid strain hardening with early deformation followed by steady-state-like behavior at large strains. Such a rapid increase in hardness might be responsible for the development of deformation twinning and microshear banding at relatively small strains. Deformation textures in these samples were associated with a large fraction of brass orientation, i.e., (011}〈211〉. On the other hand, the samples aged at elevated temperature (600°C) showed gradual continuous strain hardening upon cold working within the studied strain range. Twinning and microshearing were remarkably delayed, and the texture had no unique preferential orientation. After severe deformation, the samples aged at 600°C demonstrated hardness of ∼2500 MPa and electroconductivity >50% IACS..
270. Thiyagarajan Sundararajan, Eiji Akiyama, Yasuyuki Katada, Kaneaki Tsuzaki, Hydrogen entry in crevice region
Evaluation by hydrogen permeation technique, isij international, 10.2355/isijinternational.46.1081, 46, 7, 1081-1085, 2006.09, Pure iron exposed to acetate buffer showed both metal dissolution and hydrogen evolution inside the crevice region. Potential drop occurred inside the crevice led to both active dissolution and cathodic reactions. Hydrogen ingress into the specimen was monitored on the rear side using electrochemical hydrogen permeation technique. The results showed that magnitude of hydrogen entered inside the crevice could be detected by the permeation technique. Change in the applied potential significantly influenced the amount of permeated hydrogen. The results are correlated potential drop occurred inside the crevice, which influenced the magnitude of diffusible hydrogen..
271. Maoqiu Wang, Eiji Akiyama, Kaneaki Tsuzaki, Determination of the critical hydrogen concentration for delayed fracture of high strength steel by constant load test and numerical calculation, Corrosion Science, 10.1016/j.corsci.2005.07.010, 48, 8, 2189-2202, 2006.08, The critical hydrogen concentration for hydrogen induced delayed fracture of the AISI 4135 steel at 1320 and 1450 MPa has been determined by constant load tests in combination with numerical calculations, and thus the concept of a critical hydrogen concentration has been verified. The time to fracture was obtained for circumferentially notched round bar specimens under a constant load after electrochemically pre-charged with various hydrogen contents. A numerical model was then developed for calculating the accumulated hydrogen concentration in the vicinity of the notch root, taking into account the driving effect of the hydrostatic stress on hydrogen transport. The results showed that the delayed fracture of the steel occurred when a critical hydrogen concentration at the location of the stress peak was reached by accumulation, and that the time to fracture was related to the stress-driven hydrogen accumulation process. The critical hydrogen concentration was dependent not only on the strength level, but also on the stress concentration factor of the specimens..
272. Mao Qiu Wang, Han Dong, Wei Jun Hui, Jie Shi, Eiji Akiyama, Kaneaki Tsuzaki, Effect of hydrogen on notch tensile strength of high strength steel, Cailiao Rechuli Xuebao/Transactions of Materials and Heat Treatment, 27, 4, 57-60, 2006.08, Hydrogen embrittlement susceptibility of a high strength steel SCM 435 was investigated by means of slow strain rate tests. It is shown that the SCM 435 steel has high susceptibility to hydrogen embritdement when quenched and tempered to a tensile strength of 1450 MPa. The notch tensile strength of hydrogen-precharged specimens is lower than that of the uncharged specimen, and it decreases in a power law manner with the increase of diffusible hydrogen content. Results on notched specimens with different stress concentration factors indicate that hydrogen-induced fracture of the steel is dependent on the peak value of the locally accumulated hydrogen concentration and the peak value of the maximum principal stress..
273. M. Murayama, N. Makiishi, Y. Yazawa, T. Yokota, Kaneaki Tsuzaki, Nano-scale chemical analyses of passivated surface layer on stainless steels, Corrosion Science, 10.1016/j.corsci.2006.03.002, 48, 6, 1307-1318, 2006.06, A surface analysis technique using scanning transmission electron microscopy (STEM) combined with a unique needle shape specimen is proposed. The unique needle shape specimen is effective to reduce matrix convolution as well as improve peak to background ratio of spectroscopy. To reveal the performance of the proposed technique, HNO3 passivated surface layer on a type 436L ferritic and a type 304 austenitic stainless steels were analyzed. The results demonstrated that the proposed technique is able to quantify the distribution of alloying elements in the passivated surface layer approximately 3 nm in thickness, namely, the partitioning behaviour of alloying elements, i.e., Cr, Ni, Mo and Si in the passivated surface layer is revealed and agreed with that confirmed by Auger Electron Spectroscopy analysis. This proposed technique has potentialities for the analysis of nano scale local inhomogeneity, for example, pitting initiation process..
274. Takahito Ohmura, Kaneaki Tsuzaki, K. Sawada, K. Kimura, Inhomogeneous nano-mechanical properties in the multi-phase microstructure of long-term aged type 316 stainless steel, Journal of Materials Research, 10.1557/jmr.2006.0143, 21, 5, 1229-1236, 2006.05, Microstructure-related local deformation behavior was evaluated using nanoindentation techniques for a type 316 austenitic stainless steel used in energy generation plants. The sample was aged for 39,332 h (4.5 years) at 700 °C. The microstructure included the σ phase precipitated at grain boundaries and in grain interiors. The nanohardnesses of the σ phase and the matrix in the aged and virgin samples were evaluated quantitatively. The hardness of the σ phase was found to be extremely high in the order of 17 GPa, which is much higher than the value of about 3.4 GPa for the matrix in the aged sample. The hardnesses of the σ phase at the grain boundary and the adjoining matrix were the same as those in the grain interior. Moreover, the hardness of the matrix of the aged sample was about 30% lower than that of the virgin sample while the Vickers hardness as a macroscopic strength of the aged sample was about 45% higher than that of the virgin one. The deformation and fracture behavior in a local region was discussed in terms of the inhomogeneous mechanical properties in the multi-phase microstructure..
275. A. Belyakov, Y. Kimura, Kaneaki Tsuzaki, Microstructure evolution in dual-phase stainless steel during severe deformation, Acta Materialia, 10.1016/j.actamat.2006.01.035, 54, 9, 2521-2532, 2006.05, Deformation microstructures of an Fe-27% Cr-9% Ni dual-phase stainless steel, which was bar rolled/swaged to a total strain of 6.9 at ambient temperature, were studied. After a rapid increase in the hardness during early deformation, the rate of strain hardening slowed and produced a steady-state-like deformation behaviour at strains above 4. The severe deformation resulted in the evolution of similar microstructures in both austenite and ferrite consisting of elongated (sub)grains with a final transverse size of about 0.1 μm and about 70% of high-angle (sub)boundaries. However, the different phases were characterised by different structural change kinetics. The ferrite transverse (sub)grain size decreased continuously, approaching its minimum at large strains above 5.0, while the distinct grain subdivision in the austenite reduced the transverse (sub)grain size to its final value quickly at an early processing stage. The main mechanism of microstructure evolution during the large strain processing was considered to be micro-shearing with dynamic recovery..
276. M. Q. Wang, E. Akiyama, Kaneaki Tsuzaki, Fracture criterion for hydrogen embrittlement of high strength steel, Materials Science and Technology, 10.1179/174328406X86191, 22, 2, 167-172, 2006.02, The notch tensile strength of a boron bearing steel at 1305 MPa has been investigated by means of slow strain rate tests after electrochemical hydrogen charging. Results show that the notch tensile strength of the steel decreased with increasing diffusible hydrogen content and the decrease in the notch tensile strength was more pronounced for specimens at a higher stress concentration factor. Finite element analysis results show that the dependence of the notch tensile strength on stress concentration factor cannot be accounted for by the local peak stress and local peak hydrogen concentration; however, the equivalent plastic strain at the notch root at an applied stress equal to the notch tensile strength was independent of stress concentration factor. The equivalent plastic strain at the notch root can be used as a fracture criterion for hydrogen embrittlement of the steel..
277. Fugao G. Wei, Kaneaki Tsuzaki, Quantitative analysis on hydrogen trapping of TiC particles in steel, Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science, 10.1007/s11661-006-0004-3, 37, 2, 331-353, 2006.02, The change in the hydrogen-trapping behavior of a TiC particle accompanying its coherent to incoherent interfacial-character transition in a 0.05C-0.20Ti-2.0Ni steel that was quenched and tempered in a partially protective argon atmosphere and in ultrahigh vacuum (UHV) has been studied by thermal desorption spectrometry (TDS). The results indicated that (semi)coherent TiC precipitates demonstrate distinctly different hydrogen-trapping features from that of incoherent TiC particles with respect to hydrogen capacity, interaction energy with hydrogen, locations available for hydrogen occupation, and the capability of hydrogen absorption from the environment. The broad (semi)coherent interface of the disc-shaped (semi)coherent TiC precipitate does not trap hydrogen during tempering in a partially protected argon atmosphere, but traps hydrogen during cathodic charging at room temperature. The semicoherent interface traps 1.3 atoms/nm
2
of hydrogen at the core of the misfit dislocation with short-time charging (1 hour), which is characterized by a desorption activation energy of 55.8 kJ/mol. The side interface of the (semi)coherent TiC precipitate acts like the broad interface when the precipitate is small. As the precipitate grows, the side interface gradually loses its coherency and results in a simultaneous increase in the trapping activation energy and the binding energy. An increase in the trapping activation energy, i.e., the energy barrier for trapping, makes hydrogen trapping more difficult in cathodic charging at room temperature, while an increase in the binding energy enhances the capability of hydrogen absorption from the atmosphere during heat treatment. An incoherent TiC particle is not able to trap hydrogen during cathodic charging at room temperature due to its high energy barrier for trapping, but absorbs hydrogen during heat treatment at high temperatures. The amount of hydrogen that is trapped by incoherent TiC particles depends on their volume, which strongly indicates that incoherent TiC particles trap hydrogen within them rather than at the particle/matrix interface. Octahedral carbon vacancies are supposedly the hydrogen trap sites in incoherent TiC particles..
278. Takahito Ohmura, Kaneaki Tsuzaki, A new approach for interpretation of strengthening mechanism of martensitic steel through characterization of local deformation behavior, Tetsu-To-Hagane/Journal of the Iron and Steel Institute of Japan, 10.2355/tetsutohagane1955.92.5_295, 92, 5, 295-310, 2006.01, Mechanical characterization in a sub-micron scale by nanoindentation technique is applied to Fe-C base martensitic steels to discuss the strengthening mechanisms. Matrix strengths of block structures were probed to reveal a dependence on a carbon content, tempering temperature and metallurgical process. A grain boundary effect was also evaluated to consider the strengthening factors on the macroscopic strength. It has been found that the grain boundary effect is significantly large on the strength of the Fe-C based martensite due to a fine effective grain of block structure. A locking parameter in the Hall-Petch relation is reduced at tempering temperature above 450°C leading to a "hump" on the temper-softening curve because film-like carbides on grain boundaries are disappeared above 450°C. A microstructure in a conventional quench-tempered process includes high-density carbides on a grain boundary corresponding to a higher locking parameter than that of an ausform-tempered martensite. The lower locking parameter in the ausform-tempered martensite reduces an stress concentration at an interface between the carbide and ferrite phase, which contribute to the improved delayed fracture property of the ausform-tempered martensite..
279. M. Murayama, A. Belyakov, T. Hara, Y. Sakai, Kaneaki Tsuzaki, M. Okubo, M. Eto, T. Kimura, Development of a high-strength high-conductivity Cu-Ni-P alloy. Part I
Characterization of precipitation products, Journal of Electronic Materials, 10.1007/s11664-006-0158-1, 35, 10, 1787-1792, 2006.01, The precipitation products of a Cu-1.5Ni-0.32P (wt.%) alloy isochronally aged at 400°C and 600°C were investigated by transmission electron microscopy (TEM). There are two different sizes of precipitates formed in the 600°C aged specimen. Large precipitates appear to be lath, the length of which varies from 100 nm to 500 nm, whereas small dotty contrasts around 10 nm in diameter are uniformly distributed in the matrix. Only small dotty contrasts are observed after 400°C aging. In the composition range 0-33.3 at.%P, both the previous studies and the calculated Ni-P binary phase diagram indicated that the possible nickel phosphide phases are Ni
3
P, Ni
12
P
5
, Ni
5
P
2
, and Ni
2
P. However, selected area electron diffraction (SAED) pattern analysis in conjunction with energy dispersive x-ray spectroscopy (EDS) identified the large lath precipitates as Ni
12
P
5
phase. On the other hand, the small precipitates could not be analyzed by EDS due to thinness; thus, the structure is estimated to be a Ni
5
P
4
phase from the relationships between d-spacing and angles between planes measured from a selected area diffraction pattern..
280. Jinxu Li, Takahito Ohmura, Kaneaki Tsuzaki, Microstructure effect on nanohardness distribution for medium-carbon martensitic steel, Science in China, Series E: Technological Sciences, 10.1007/s11431-004-5228-0, 49, 1, 10-19, 2006.01, Nanoindentation measurement was performed to evaluate the local hardness distribution for quenched and tempered martensitic steel with ferrite-cementite constituent. The ratio of standard deviation to the average nanohardness was 15.4% when nanoindentation tests were carried out at the peak load of 1000 μN, while that of the Vickers hardness was only 1.5% at 9.8 N. Electron backscatter diffraction analysis and SEM observation all showed that the large scattering of nanohardness did not depend on the crystallographic orientation of each grain but from the inhomogeneous microstructure in the sub-micron scale such as cementite distribution. Compared with the results on tungsten single crystals with different surface orientations as well as another martensite processed by modified-ausforming with the same chemical composition but more homogeneous cementite distribution, the nanohardness showed smaller scattering, supporting the conclusion..
281. T. Ohmura, A. Minor, Kaneaki Tsuzaki, J. W. Morris, Indentation-induced deformation behavior in martensitic steel observed through in-situ nanoindentation in a transmission electron microscopy, 3rd International Conference on Nanomaterials by Severe Plastics Deformation, NanoSPD3 Nanomaterials by Severe Plastic Deformation, NanoSPD3 - Proceedings of the 3rd International Conference on Nanomaterials by Severe Plastics Deformation, 503-504, 239-244, 2006, Deformation behavior in the vicinity of grain boundary in Fe-0.4wt%C tempered martensitic steel were studied through in-situ nanoindentation in a TEM. Two types of boundaries were imaged in the dislocated martensitic structure: a low-angle lath boundary and a high-angle block boundary. In the case of a low-angle grain boundary, the dislocations induced by the indenter piled up against the boundary. As the indenter penetrated further, a critical stress appears to have been reached and a high density of dislocations was suddenly emitted on the far side of the grain boundary into the adjacent grain. In the case of the high-angle grain boundary, the numerous dislocations that were produced by the indentation were simply absorbed into the boundary, with no indication of pile-up or the transmission of strain..
282. Yoshitaka Adachi, Kazunari Hakata, Kaneaki Tsuzaki, Crystallographic analysis of grain boundary Bcc-precipitates in a Ni-Cr alloy by FESEM/EBSD and TEM/Kikuchi line methods, Materials Science and Engineering A, 10.1016/j.msea.2005.09.033, 412, 1-2, 252-263, 2005.12, Variant selection of intergranular bcc-Cr precipitates in a Ni-43 mass% Cr alloy was studied by electron backscattering diffraction (EBSD) in a scanning electron microscope equipped with a field emission gun (FESEM) and Kikuchi line analysis in transmission electron microscope (TEM). A single variant was invariably selected at the underlying grain boundaries (GB) when GB-precipitates were KS-related with respect to both the adjacent matrix grains. Meanwhile multiple variants were formed at the GB when GB-precipitates were KS-related to one of the adjacent matrix grains. Theses variant selections were examined with the tilt angle to underlying GB of low-energy interphase boundaries and the orientation relationship with respect to both the adjacent matrix grains. The underlying mechanism was discussed from the viewpoint of activation energy for nucleation of two-dimensional nuclei..
283. T. Sundararajan, E. Akiyama, Kaneaki Tsuzaki, Hydrogen mapping across a crevice
Effect of applied potential, Scripta Materialia, 10.1016/j.scriptamat.2005.08.016, 53, 11, 1219-1223, 2005.12, Iron exposed to acetate buffer in the presence of a crevice assembly showed metal dissolution and hydrogen evolution. Permeated hydrogen was successfully mapped on the rear surface of the crevice using the silver decoration technique. Magnitude of silver deposit varied with the change in the applied potential, with inhomogeneous distribution..
284. Maoqiu Wang, Eiji Akiyama, Kaneaki Tsuzaki, Crosshead speed dependence of the notch tensile strength of a high strength steel in the presence of hydrogen, Scripta Materialia, 10.1016/j.scriptamat.2005.05.014, 53, 6, 713-718, 2005.09, The notch tensile strength of quenched and tempered AISI 4135 steel after hydrogen charging decreases with decreasing crosshead speed. Numerical simulation results show that the accumulated hydrogen concentration in the vicinity of the notch root increases with decreasing crosshead speed, resulting in the decrease in the notch tensile strength..
285. T. Sundararajan, E. Akiyama, Kaneaki Tsuzaki, Hydrogen mapping across crevices, Electrochemical and Solid-State Letters, 10.1149/1.1979367, 8, 9, 2005.09, Crevice corrosion was investigated in pure iron exposed to a strong acetate buffer solution. The specimens experienced the potential drop inside the crevice, which resulted in the formation of passive, active, and hydrogen evolution regions. The distribution of hydrogen evolution occurring inside the crevice was mapped using a silver decoration technique applied on the other side of the specimen. The results suggest that the entry of absorbed hydrogen show a variation in the intensity across the crevice. The crevice mouth and bottom portions showed the least hydrogen entry, whereas the intermediate portion yielded the significant amount which resulted in the accumulation of silver deposits..
286. Takahito Ohmura, Kaneaki Tsuzaki, Fuxing Yin, Nanoindentation-induced deformation behavior in the vicinity of single grain boundary of interstitial-free steel, Materials Transactions, 10.2320/matertrans.46.2026, 46, 9, 2026-2029, 2005.09, Nanoindentation measurements are made on an interstitial-free steel to reveal the effect of single grain boundary on deformation behavior. Three different sites, "on grain boundary", "near grain boundary" and "grain interior", were probed to investigate the pop-in behavior on the initial loading curve and nanohardness. The typical pop-in load at the "grain interior" gives a maximum shear stress beneath the indenter as an order of ideal strength. The pop-in load at the "on grain boundary" is significantly smaller than that at the other sites, indicating that the grain boundary acts as an effective dislocation source with a lower applied shear stress. The nanohardness in the "grain interior" is about 20% lower than that at the other sites, suggesting that a single grain boundary has significant resistance to indentation-induced deformation..
287. Andrey Belyakov, Y. Kimura, Kaneaki Tsuzaki, Recovery and recrystallization in ferritic stainless steel after large strain deformation, Materials Science and Engineering A, 10.1016/j.msea.2005.05.057, 403, 1-2, 249-259, 2005.08, Recovery and recrystallization were studied in a Fe-22% Cr-3% Ni ferritic stainless steel processed by bar rolling/swaging at an ambient temperature. The annealing behaviour significantly depended on the preceding cold strain. The samples processed to relatively small strain of 2.0 did not show remarkable softening upon annealing at temperatures below 600 °C (0.5Tm), while the heating to higher temperatures resulted in the development of conventional primary (discontinuous) recrystallization leading to a rapid annealing softening. On the other hand, the annealing behaviour of the samples processed to strain of 4.4 was characterised by a continuous recrystallization involving a rapid polygonization followed by a normal grain growth that resulted in a gradual softening upon annealing at temperatures above 400 °C (0.38Tm). The continuously recrystallized microstructures consisted of much finer grains than those resulted from the discontinuous recrystallization..
288. Jinxu Li, Takahito Ohmura, Kaneaki Tsuzaki, Evaluation of grain boundary effect on strength of Fe-C low alloy martensitic steels by nanoindentation technique, Materials Transactions, 10.2320/matertrans.46.1301, 46, 6, 1301-1305, 2005.06, The grain boundary effect on the strength was evaluated through nanoindentation technique for Fe-0.4C-Cr-Mo steels that were produced by the ausform-tempered (AF) and conventional quench-tempered (QT) processes. A semiquantitative Hall-Petch plot was made to determine the locking parameter k for the two alloys using nanohardness, micro-Vickers hardness, and grain size. The k value for the QT sample is significantly larger than that for the AF sample and is attributed to the film-like carbides on the grain boundaries of the QT sample. The lower k value of the AF sample is one of the factors for the improved delayed fracture property in the AF compared to that of the QT sample..
289. Shusaku Takagi, Satoshi Terasaki, Kaneaki Tsuzaki, Tadanobu Inoue, Fumiyoshi Minami, A new evaluation method of hydrogen embrittlement fracture for high strength steel by local approach, isij international, 10.2355/isijinternational.45.263, 45, 2, 263-271, 2005.05, An evaluation method for hydrogen embrittlement (HE) susceptibility for ultra high strength steel was studied. The study showed that the evaluation results obtained by the method were independent of the stress concentration factor and the dimension of the specimen. A commercial 0.40C-0.24Si-0.81Mn-1. 03Cr-0.16Mo (mass%) steel with a tensile strength of 1 400 MPa was used. The Local Approach, which was originally used for the evaluation of brittle fracture property, was applied to evaluate the HE susceptibility. The method was modified to consider the effect of hydrogen content to evaluate the HE susceptibility. The critical HE data, which were required in the modified Local Approach, were obtained by the stepwise test in which the stress increasing process and the stress holding process were alternatively repeated until the specimen fractured. The dimensions of the specimen used in the stepwise test consisted of a diameter of 10mm and a stress concentration factor of 4.9. To evaluate the HE susceptibility for specimens with other dimensions, a critical hydrogen content for failure called He, which represents the maximum hydrogen content among the unfracture specimens on the HE test with constant loading, was used. It was found that the HE susceptibility could be evaluated by the modified Local Approach considering hydrogen content distribution in the specimens and that the probability of the HE fracture on the components could be designed by using the evaluation results..
290. Fu Gao Wei, Kaneaki Tsuzaki, Crystallography of [0 1 1]/54.7° lath boundary and cementite in tempered 0.2C steel, Acta Materialia, 10.1016/j.actamat.2005.02.004, 53, 8, 2419-2429, 2005.05, The crystallographic structures of the [0 1 1]/54.7° lath boundary and the intralath and interlath cementite precipitates in a 0.2C steel tempered at 400 °C have been observed by high resolution transmission electron microscopy and discussed in terms of the O-lattice model. The [0 1 1]/54.7° lath boundary which is composed of one Kurdjumov-Sachs variant (α1) and one Nishiyama-Wasserman variant (α2) is likely to facet on the (0 12 7)α1//(5 19 14)α2 irrational plane. The O-lattice analysis indicated that the facet coincides with a plane with a high density of O-points. Intralath cementite obeys the Isaichev orientation relationship (OR) with ferrite and has a habit plane of (1 0 1) θ//(1 2 1)α. Interlath cementite precipitating on the [0 1 1]/54.7° boundary maintains a strict Bagaryatskii OR with the ferrite on one side of the boundary and has a (1 0 0)θ habit plane..
291. Maoqiu Wang, Eiji Akiyama, Kaneaki Tsuzaki, Effect of hydrogen and stress concentration on the notch tensile strength of AISI 4135 steel, Materials Science and Engineering A, 10.1016/j.msea.2005.03.008, 398, 1-2, 37-46, 2005.05, The quantitative relationship between notch tensile strength and diffusible hydrogen content has been investigated for the AISI 4135 steel at 1320 MPa. The notch tensile strength was obtained by means of a slow strain rate test on circumferentially notched round bar specimens with stress concentration factors of 2.1, 3.3 and 4.9 after hydrogen charging, and the diffusible hydrogen content was then measured by thermal desorption spectrometry analysis. The diffusible hydrogen has been found to decrease the notch tensile strength in a power law manner, and the decrease is more prominent at a higher stress concentration factor. The finite element analysis results of stress and hydrogen distributions in the vicinity of the notch root have shown that the local fracture stress decreases with increasing local hydrogen concentration as the diffusible hydrogen content or stress concentration factor increases, resulting in the decrease in the notch tensile strength..
292. Eu Gao Wei, Kaneaki Tsuzaki, Toru Hara, A new method to determine the activation energy for hydrogen desorption from steels, Materials Science Forum, 475-479, I, 229-232, 2005.04, A new method has been developed to determine the activation energy for hydrogen desorption from steels by means of thermal desorption spectrometry (TDS). This method directly fits the Kissinger's reaction kinetic formula dX/dt=A(1-X)exp(-Ed/RT) to experimentally measured thermal desorption spectrum and best fit yields the activation energy (Ed) and the value of constant A. It has been proven that this new method is applicable to precise measurement of the activation energy for hydrogen desorption from incoherent TiC particle, coherent TiC precipitate, grain boundary and dislocation in 0.05C-0.20Ti-2.0Ni and 0.42C-0.30Ti steels..
293. Maoqiu Wang, Eiji Akiyama, Kaneaki Tsuzaki, Hydrogen degradation of a boron-bearing steel with 1050 and 1300MPa strength levels, Scripta Materialia, 10.1016/j.scriptamat.2004.10.023, 52, 5, 403-408, 2005.03, The maximum tensile stress of hydrogen pre-charged specimens during slow strain rate test decreased in a power law manner with increasing diffusible hydrogen content in the presence of intergranular fracture. The lower hydrogen degradation susceptibility at a lower strength level is due to delayed onset of intergranular fracture..
294. Fu Gao Wei, Kaneaki Tsuzaki, Response of hydrogen trapping capability to microstructural change in tempered Fe-0.2C martensite, Scripta Materialia, 10.1016/j.scriptamat.2004.11.008, 52, 6, 467-472, 2005.03, Thermal desorption spectrometry study of the hydrogen trapping capability of a tempered Fe-0.2C martensite indicated that dislocations in the martensitic substructure are the primary traps and new grain boundaries resulting from recovery and recrystallization of the ferrite matrix are the secondary traps for hydrogen. Cementite has a negligible effect on hydrogen trapping..
295. Yoshitaka Adachi, Kaneaki Tsuzaki, Light pre-deformation induced misorientation of grain boundary bcc-precipitates from the Kurdjumov-sacks relationship in a Ni-Cr alloy, Metals and Materials International, 10.1007/BF03027503, 11, 5, 341-349, 2005.01, Crystallography of grain boundary (GB) bcc-precipitates formed in a lightly pre-deformed fee-matrix phase was examined in a Ni-43Cr alloy by EBSD with emphasis on a misorientation from the Kurdjumov-Sachs orientation relationship. The misorientation of GB bcc-precipitates became somewhat larger when the matrix phase was lightly pre-deformed. The GB bcc-precipitates were preferentially formed at the intersection of geometrically necessary dislocation boundaries (GNB) with grain boundaries. In contrast, the pre-deformation did not influence the misorientation of intragranular bcc-precipitates. The preferential precipitation and the increased misorientation at the GNB-GB intersection were discussed..
296. Yoshitaka Adachi, Kaneaki Tsuzaki, Coherent-to-incoherent transition of intergranular bccprecipitates by pre-/post-deformations in a Ni-43Cr alloy, ISIJ International, 10.2355/isijinternational.45.1703, 45, 11, 1703-1710, 2005, Effects of light deformations before and after precipitation (pre-deformation and post-deformation) on orientation distribution of intergranular bcc-precipitates were examined in a Ni-43mass%Cr alloy by electron backscattered diffraction. It was found that the deviation angles from both the plane/direction parallel orientation relationships in the Kurdjumov-Sachs orientation relationship of intergranular precipitates were increased by both pre- and post-deformations, but post-deformation was more effective. Based on this result, microstructural evolution of intergranular precipitates formed dynamically was discussed with particular attention to coherent-to-incoherent transition of precipitates..
297. E. Akiyama, Kaneaki Tsuzaki, Depassivation and repassivation of titanium under particle impingements, 208th Meeting of the Electrochemical Society Corrosion and Electrochemistry of Advanced Materials, In Honor of Koji Hashimoto, 10.1149/1.2215527, 1, 437-446, 2005, Current transients of a titanium microelectrode corresponding to an impingement of a single zirconia particle were measured in a buffer solution. It was found that the repassivation after a particle impact followed high field model of oxide growth. The apparent repassivation rate evaluated by the relation between the charge and the peak height of a current transient showed dependence on applied potential, and the apparent repassivation rate increased with increase of potential. The influence of potential applied passivation prior to particle impingement on the apparent repassivation rate was more predominant than that of the potential applied during impingement of relatively large particle. This indicates that the apparent repassivation rate does not represent the actual repassivation kinetics. It is suggested that the passive film formed prior to a particle impact and repassivation give rise to resistance against surface deformation and film breakdown, and that it affects the apparent repassivation behavior. copyright The Electrochemical Society..
298. Jinxu Li, Takahito Ohmura, Fugao Wei, Kaneaki Tsuzaki, Effect of inhomogeneity of carbide precipitation on nanohardness distribution for martensitic steels, Materials Science Forum, 475-479, V, 4109-4112, 2005, Nanoindentation technique was applied to evaluate nanohardness distribution in a sub-micron scale for two kinds of martensitic steels: Fe-0.4C binary steel and Fe-0.05C-0.22Ti steel with a stoichiometric composition of TiC. AFM images showed that Fe-C steel includes relatively coarse cementite particles with about 100-200 nm in diameter and a couple of hundreds nanometer in average spacing, while high-resolution TEM observation showed that the Fe-C-Ti steel has fine TiC precipitates with 5 nm in diameter and 15 nm for average spacing. Nanoindentation results revealed that the standard deviation was much higher for the Fe-C than that for the Fe-C-Ti. Since the typical indent size was a couple of hundreds nanometer, which was about two orders larger than the size of the TiC and comparable to the cementite size, the small distribution of nanohardness of the Fe-C-Ti was attributed to the homogeneous microstructure in sub-micron scale, while the inhomogeneity of cementite particles in the Fe-C steel leaded to large nanohardness..
299. K. Sawada, T. Ohmura, T. Ohba, Kaneaki Tsuzaki, F. Abe, K. Kimura, I. Nonaka, Elastic properties of heat resistant steels after long-term creep exposure, ECCC Creep Conference: Creep and Fracture in High Temperature Components - Design and Life Assessment Issues Proceedings - ECCC Creep Conference: Creep and Fracture in High Temperature Components - Design and Life Assessment Issues, 2005, 268-278, 2005, In order to clarify the effect of microstructural change during aging and creep on elastic modulus, the elastic moduli of aged, crept and serviced materials were investigated. In ferritic steels, the elastic modulus obtained by ultrasonic pulse does not change after long-term aging and creep although the formation of creep voids slightly decreases the elastic modulus. In SUS304 and SUS316, the elastic modulus evaluated by ultrasonic pulse increases during aging due to the precipitation of M 23C 6 carbides, Laves phase and a phase though the elastic modulus decreases during creep because of the formation of creep voids on grain boundaries. The increased elastic modulus of aged material with a large amount of a phase was regarded as that of composite consists of matrix and a phase. For STBA24, the elastic modulus obtained by a high temperature tensile test decreases after long-term service. The tensile test at a higher strain rate was proposed to avoid underestimation of elastic modulus of serviced materials at high temperatures..
300. T. Ohmura, Kaneaki Tsuzaki, Evaluation of grain boundary effect on the strength of Fe-C martensltic steels through nanoindentation technique, Materials Science Forum, 475-479, V, 4113-4116, 2005, Nanoindentation measurements were performed for Fe-C based martensitic steels, and then the strengthening factors such as grain boundary effect were evaluated. Nanohardness of the matrix of the martensite is lower than that expected from macroscopic hardness, indicating that the grain boundary effect is significant for the macroscopic strength of the Fe-C martensite. A remarkable decrease of the grain boundary effect was found at the tempering temperature of 673 K, which is due to a disappearance of film-like carbides on grain boundaries. These results will be discussed in light of the interpretations of grain boundary strengthening..
301. Kaneaki Tsuzaki, Fu Gao Wei, Origin of the hydrogen absorbed by incoherent TiC particles in iron, Materials Science Forum, 475-479, I, 233-236, 2005, Hydrogen absorption of incoherent TiC particles that were once reported to be strong hydrogen traps in iron at room temperature was investigated by means of thermal desorption spectrometry (TDS). The results indicated that incoherent TiC particles in iron do not trap hydrogen at all at room temperature even they are cathodically charged for a long time. Only at high temperatures and in atmosphere containing hydrogen source, incoherent TiC particles can trap hydrogen. The origin of hydrogen trapped by incoherent TiC particles was justified to be water vapor in the atmosphere during heat treatment..
302. Yoshitaka Adachi, Fuxing Yin, Kazunari Hakata, Kaneaki Tsuzaki, Variant selection in Fcc-to-Bcc precipitation at grain boundaries in Ni-43Cr alloy, Materials Science Forum, 475-479, I, 305-308, 2005, Variant selection of bcc-Cr at the grain boundaries in a supersaturated fee matrix was studied using a Ni-43Cr alloy. The preferentially selected variant was examined as a function of the grain boundary misorientation, the tilt angle between the {111}fcc plane and the grain boundary plane, and the orientation relationships with respect to both of the adjacent matrix grains..
303. A. Belyakov, Y. Sakai, T. Hara, Y. Kimura, Kaneaki Tsuzaki, Recovery and recrystallization in cold worked Fe - O steels, Proceedings of the Second Joint International Conferences on Recrystallization and Grain Growth, ReX and GG2, SF2M Materials Science Forum, 467-470, I, 229-234, 2004.12, Several Fe - O samples containing different fractions of dispersed oxides were processed by mechanical milling followed by consolidating rolling. The samples were annealed at 1000°C and then compressed to strains of 0.35, 1.2, and 1.9 at an ambient temperature. Dispersed oxides with size of about 20 nm were homogeneously distributed throughout the ferrite matrix and their volume fractions varied from about 0.3% to 2.0%. To study the annealing softening mechanisms, the cold-worked specimens were annealed for an hour at 700°C and 800°C. The fine dispersion of oxide particles was very effective to suppress any softening processes. Primary recrystallization fully developed in the samples with volume fraction of dispersed oxides of about 0.3%. Increase in the fraction of dispersed oxides resulted in decrease of the fraction recrystallized. In the samples containing 2.0 vol.% of dispersed oxides, only recovery was the annealing softening process irrespective of the preceding cold strain. The critical volume fraction of dispersed particles for development of the primary recrystallization is considered to range from 0.5 to 2.0%..
304. Fu Gao Wei, Toru Hara, Kaneaki Tsuzaki, High-resolution transmission electron microscopy study of crystallography and morphology of TiC precipitates in tempered steel, Philosophical Magazine, 10.1080/14786430310001638762, 84, 17, 1735-1751, 2004.06, In spite of the significance of NaCl-type transition-metal carbides in steels, their crystallography and morphology have not been understood on an atomic scale. High-resolution electron microscopy was employed in the present work to examine the crystallography and morphology of TiC particles that precipitated in the quenched and tempered 0.05wt%C-0.20wt%Ti-2.0wt%Ni steel. Plate-like TiC precipitates with a thickness ranging from a couple of atomic layers to 20 nm were observed in steels by tempering at 550, 600 and 800°C. It was found that the Baker-Nutting orientation relationship is always satisfied with the ferrite matrix within about 5° for both the nanosized and the coarse TiC particles. The habit plane of the TiC precipitate is the (100) ferrite plane. A moderate tendency for the faceting of lateral interfaces on the {001} and {011} ferrite planes was found. The presence of interfacial misfit dislocations was revealed by examining the excess lattice fringes terminating at the interfaces between the TiC platelet and the ferrite matrix. The location and number of the excess lattice fringes terminating at the broad plane and the lateral interface were consistent with the mismatch in the atomic arrangement between the Baker-Nutting orientation relationship related TiC and ferrite..
305. Andrey Belyakov, Yoshikazu Sakai, Toru Hara, Yuuji Kimura, Kaneaki Tsuzaki, Effect of nano-sized oxides on annealing behaviour of ultrafine grained steels, Materials Transactions, 10.2320/matertrans.45.2252, 45, 7, 2252-2258, 2004.01, The annealing behaviour of ultrafine grained steels containing nano-scale dispersed oxides was studied in a temperature range of 600-900°C by means of microstructural observations and hardness measurement. The starting materials with submicrocrystalline structures were developed by mechanical milling of Fe-Fe
3
O
4
powders followed by consolidating bar rolling at 700°C. Depending on the initial oxygen content and the mechanical milling time, the fraction of dispersed oxides varied from 0.3 to 3. 0 vol%. During the heating up to 800°C (i.e. within the ferrite region), the initial ultrafine grained microstructures were essentially stable against any discontinuous grain growth. The grain coarsening and the softening can be roughly expressed by power-law functions of annealing time. The main mechanism of microstructure evolution that operated during annealing is considered as a normal grain growth accompanied by recovery. The grain coarsening is characterized by a rather high value of the grain-growth exponent of about 20. The grain growth kinetics correlates with the oxide coarsening. The effect of dispersed oxides on the annealing behaviour of submicrocrystalline oxide bearing steels is discussed in some detail..
306. Shusaku Takagi, Satoshi Terasaki, Kaneaki Tsuzaki, Tadanobu Inoue, Fumiyoshi Minami, Evaluation of hydrogen embrittlement susceptibility of high strength steel based on local approach, Yosetsu Gakkai Ronbunshu/Quarterly Journal of the Japan Welding Society, 10.2207/qjjws.22.125, 22, 1, 125-131, 2004.01, In order to evaluate the hydrogen embrittlement susceptibility of high strength steel independently of stress concentration factor Kt and specimen size, Local Approach method modified to evaluate the hydrogen content distribution in the specimen was used. In the method it was considered that the size of micro cracks depends on the hydrogen content. The stepwise hydrogen embrittlement tests were carried out to obtain the hydrogen embrittlement susceptibility of the material which was 1400 MPa grade JIS SCM440 (0.40C-0.24Si-0.81Mn-1.03Cr-0.16Mo (mass%)). Stress was applied to the specimen, which is circumferentially notched round-bar type with the stress concentration factor of 4.9, after hydrogen pre-charging and homogenization treatments. The applied stress on the first step was set at 702 MPa and the stress of 14 MPa was increased on and after the second step. The holding time of stress was over 12 h on the first step and over 2 h on and after the second step. The diffusible hydrogen content was measured by thermal desorption analysis immediately after fracture. The distribution of stress and hydrogen content near notch root was calculated by FE-analysis. In the modified Local Approach method the new parameter Σw,cr was introduced including the stress factor and hydrogen content factor. The parameter Σw,cr obtained by the stepwise test obeyed the Weibull distribution as the Weibull stress in the conventional Local Approach. The hydrogen embrittlement susceptibility of any specimens with various stress concentration factors was able to be evaluated uniquely by the Local Approach considering the effect of hydrogen content distribution..
307. F. G. Wei, Kaneaki Tsuzaki, Hydrogen absorption of incoherent TiC particles in iron from environment at high temperatures, Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science, 10.1007/s11661-004-0060-5, 35 A, 10, 3155-3163, 2004.01, The effect of atmosphere in heat treatment on the hydrogen trapping of incoherent TiC particles in iron as been studied in order to clarify the origin of hydrogen trapped by incoherent TiC particles. The hydrogen trapped by incoherent TiC particles in iron after austenitizing and tempering treatments in air, m a nonprotective argon atmosphere, and in an ultrahigh vacuum (UHV) was identified and quantitatively measured by thermal-desorption spectrometry (TDS). Results showed that incoherent TiC particles in iron do not trap hydrogen at ambient temperature by a cathodic-charging method. It was justified that incoherent TiC particles trap hydrogen during high-temperature heat treatment in nonprotective atmospheres. The amount of hydrogen trapped by incoherent TiC particles decreases with increasing heat-treatment temperature, which is well explained by the equilibrium concentration of hydrogen trapped by incoherent TiC particles in iron under an atmosphere containing water vapor. The hydrogen is supplied through water-vapor oxidation of iron at high temperatures. According to this model, a binding energy between hydrogen and incoherent TiC of 53 kJ/mol was obtained. The energy barrier for hydrogen to jump into incoherent TiC was determined to range from 21 to 35 kJ/mol, which is too high for hydrogen to be trapped by incoherent TiC at low temperatures..
308. T. Ohmura, Kaneaki Tsuzaki, N. Tsuji, N. Kamikawa, Matrix strength evaluation of ultra-fine grained steel by nanoindentation, Journal of Materials Research, 19, 1, 347-350, 2004.01, Nanoindentation was performed to evaluate the matrix strength of the ultra-fine grained steel produced by accumulative roll-bonding and subsequently annealed. The nanohardness, associated with the matrix strength, decreases with increasing annealing temperature. Because the matrix strength corresponds to the first term σ0 of the Hall-Petch relation, this result suggests that the σ0 might not be constant for these steels between various grain sizes. Therefore, it is suggested that the change of the macroscopic strength during annealing is dominated by not only the grain coarsening leading to a reduction of grain refinement strengthening, but also the softening of the matrix strength..
309. T. Ohmura, T. Hara, Kaneaki Tsuzaki, H. Nakatsu, Y. Tamura, Mechanical characterization of secondary-hardening martensitic steel using nanoindentation, Journal of Materials Research, 19, 1, 79-84, 2004.01, Mechanical characterizations using nanoindentation technique were performed for the martensitic steel used as practical dies steel containing carbide-former elements of Cr, Mo, W, and V, which are responsible for secondary hardening by tempering. The nanohardness Hn corresponding to the matrix strength shows obvious secondary hardening, and the hardening-peak temperature coincides with that of the macroscale hardness Hv. By comparing the temper-softening behavior of the high-purity Fe-C binary martensite, the ratio of the nanohardness Hn of the dies steel to that of the Fe-C binary steel is approximately a factor of two, whereas the same ratio of the macroscopic hardness Hv is three at the secondary-hardening peak. These results suggest that the secondary hardening of the dies steel during tempering is attributed not only to the nanoscale strengthening factors such as precipitation hardening by the alloy carbides, but also to some other factors in larger scale. One of the strengthening factors in larger scale is a decomposition of 9% retained austenite to much harder phases, such as martensite and/or ferrite-cementite constituent..
310. Andrey Belyakov, Yuuji Kimura, Yoshitaka Adachi, Kaneaki Tsuzaki, Microstructure evolution in ferritic stainless steels during large strain deformation, Materials Transactions, 10.2320/matertrans.45.2812, 45, 9, 2812-2821, 2004.01, Deformation microstructures were studied in ferritic stainless steels during cold bar rolling and swaging to total true strains about 7. Two steels, i.e. Fe-22Cr-3Ni and Fe-18Cr-7Ni with coarse-grained ferritic and fine-grained martensitic initial microstructures, respectively, were selected as starting materials. Microstructure evolution in the both steels was characterized by the development of highly elongated (sub)grains aligned along the rolling/swaging axis. The transverse size of these (sub)grains in the Fe-22Cr-3Ni steel gradually decreased to about 0.1 μm with increasing the strain. On the other hand, the transverse (sub)grain size in the Fe-18Cr-7Ni steel decreased to its minimal value of 0.07 μm with straining to about 3 followed by a little coarsening under further working. The strengthening of worked steels that revealed by hardness tests correlated with the microstructure evolution. The hardness of the Fe-22Cr-3Ni steel increased with cold working within the studied strain range, while that of the Fe-18Cr-7Ni approached a saturation after fast work hardening at strains below 3, leading to an apparent steady-state behaviour. Development of strain-induced (sub)grain boundaries and internal stresses in the steels with different initial microstructures during severe deformation is discussed in some detail..
311. F. G. Wei, T. Hara, Kaneaki Tsuzaki, Precise determination of the activation energy for desorption of hydrogen in two Ti-added steels by a single thermal-desorption spectrum, Metallurgical and Materials Transactions B: Process Metallurgy and Materials Processing Science, 10.1007/s11663-004-0057-x, 35, 3, 587-597, 2004.01, Critical assessment of the existing models for the desorption rate of hydrogen dapped in steel indicated that the desorption rate can be described by the kinetic formula dX/dt = A(1 - X) exp (-Ed/RT). Good fit of the formula has been found to the hydrogen released during thermal-desorption spectrometry (TDS) analysis from the coherent and incoherent TiC particles in 0.05C-0.22Ti-2.0Ni and 0.42C-0.30Ti steels. The activation energy (E d) and the constant parameter A can be determined uniquely with high accuracy by a single spectrum simulation. The activation energy for hydrogen desorption from the incoherent TiC particle in the well-tempered 0.05C-0.22Ti-2.0Ni steel is 85.7 kJ/mol. In the 0.42C-0.30T1 steel, a higher activation energy of 116 kJ/mol was obtained for the coarse incoherent TiC when tempered at 650°C and 700°C. The activation energy decreased from 116 kJ/mol at 650°C to 68 kJ/mol at 500°C. The nanosized TiC coherent precipitates in the 0.42C-0.30Ti steel were found to have an activation energy ranging from 46 to 59 kJ/mol, depending on the tempering temperature. A low value of much less than 104 s-1 was obtained for the constant parameter A for most cases, which suggested that the retrapping of the released hydrogen is not important in the thermal-desorption analysis..
312. T. Ohmura, T. Hara, Kaneaki Tsuzaki, Evaluation of temper softening behavior of Fe-C binary martensitic steels by nanoindentation, Scripta Materialia, 10.1016/j.scriptamat.2003.08.025, 49, 12, 1157-1162, 2003.12, Temper softening behavior of Fe-C binary martensitic steels was studied. Matrix strength was evaluated by using nanoindentation method. Results showed the temperature dependence of micro-Vickers hardness which shows a 'hump' around 673 K. It was also found that natrix strength decreases with tempering temperature..
313. Y. Kimura, Y. Sakai, T. Hara, A. Belyakov, Kaneaki Tsuzaki, Hydrogen induced delayed fracture of ultrafine grained 0.6% O steel with dispersed oxide particles, Scripta Materialia, 10.1016/j.scriptamat.2003.08.006, 49, 11, 1111-1116, 2003.12, The hydrogen induced delayed fracture of ultrafine grained 0.6% O steel with dispersed oxide particles was discussed. The MM of 0.6% O steel powder followed by consolidation at 700 °C resulted in the ultrafine grained steel having an average ferrite grain size of 0.3 μm and the dispersion of Cr
2
O
3
particles of 10 nm. It was found that the grained steel exhibit high resistance to hydrogen embrittlement at the tensile strength of 1300 MPa by hydrogen trapping effect related to the nanosize oxide particles..
314. Kazuhiro Hono, Eiichiro Matsubara, Kaneaki Tsuzaki, Materials Transactions
Preface, Materials Transactions, 44, 10, 1899, 2003.10.
315. A. Belyakov, Kaneaki Tsuzaki, H. Miura, T. Sakai, Evolution of grain boundaries and subboundaries in stainless steel during dynamic recrystallization, Thermec 2003 Processing and Manufacturing of Advanced Materials Materials Science Forum, 426-432, 2, 1005-1010, 2003.07, The dynamic recrystallization (DRX) microstructures were studied in a 304-type stainless steel deformed at temperatures from 1023 K to 1223 K and at a strain rate of about 10-3 s-1. Both the dynamic grain size and the dynamic subgrain size were shown to depend on deformation conditions, and they could be expressed by different power-law functions of flow stresses. The subgrain size was related to the flow stress with a size exponent of about -1.0, while the grain size exponent was about -0.7. With increasing the deformation temperature, the fraction of the high-angle grain boundaries against all the strain-induced (sub)boundaries decreased from about 70% at 1023 K to 40% at 1223 K. The deformation microstructures are discussed in some detail with reference to the mechanisms of microstructure evolution that operated upon DRX..
316. A. Belyakov, Y. Sakai, T. Hara, Y. Kimura, Kaneaki Tsuzaki, Annealing softening mechanisms operating in cold worked oxide-bearing steels, Scripta Materialia, 10.1016/S1359-6462(03)00077-0, 48, 10, 1463-1468, 2003.05, Annealing softening mechanisms operating in cold-worked Fe-O steels were studied at 700 and 800 °C. Primary recrystallization develops only in 0.2%O samples after cold deformation of ε≥ 70%. In samples containing 0.6%O, static recovery is the only annealing softening process irrespective of cold deformation intensity..
317. A. Belyakov, Y. Sakai, T. Hara, Y. Kimura, Kaneaki Tsuzaki, Evolution of grain boundary assemblies in Fe-0.6%O under mechanical milling followed by consolidating rolling, Scripta Materialia, 10.1016/S1359-6462(02)00616-4, 48, 8, 1111-1116, 2003.04, The microstructure evolution in iron oxides processed by mechanical milling and consolidated warm bar rolling was studied. The milling was seen to promote the development of a submicrocrystalline structure with nano-sized dispersed oxides in the bulk materials. The structural changes were found to be dependent upon the annealed grain size..
318. A. Belyakov, Kaneaki Tsuzaki, H. Miura, T. Sakai, Effect of initial microstructures on grain refinement in a stainless steel by large strain deformation, Acta Materialia, 10.1016/S1359-6454(02)00476-7, 51, 3, 847-861, 2003.02, Initial grain size effect on submicrocrystalline structure evolution was studied in multiple compressions of a 304 stainless steel at 873 K (0.5 Tm). Four sets of specimens with different initial microstructures were used, i.e. annealed samples with grain sizes of D0 = 15 and 2.2 μm, and dynamically recrystallised ones with D0 = 3.5 and 1.5 μm. The new ultra-fine-grains (D = 0.25 μm) develop as a result of a continuous increase in the misorientations between the subgrains that evolved during deformation. In the samples with D0≤3.5 μm, the fraction of the strain-induced high-angle boundaries increases rapidly to more than 60% with a straining to about 1.5. On the other hand, their fraction does not exceed 20% at ε = 1.5 in the sample with D0 = 15 μm. The latter needs much more straining to around 6 to obtain 60% of high-angle (sub)grain boundaries..
319. Masao Hayakawa, S. Matsuoka, Kaneaki Tsuzaki, H. Hanada, M. Sugisaki, Erratum
Atomic force microscopy of induction- and furnace-heating-tempered prestressed steels with different delayed fracture properties (Scripta Materialia (2002) 47 (655-661) PII: S1359646202002634), Scripta Materialia, 10.1016/S1359-6462(02)00507-9, 48, 4, 2003.02.
320. A. Belyakov, Y. Sakai, T. Hara, Y. Kimura, Kaneaki Tsuzaki, Annealing behavior of submicrocrystalline oxide-bearing iron produced by mechanical alloying, Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science, 10.1007/s11661-003-0214-x, 34, 1, 131-138, 2003.01, The structural changes upon annealing of an ultrafine-grained iron containing dispersed oxides were studied. The starting material was subjected to mechanical milling followed by consolidating bar rolling. The fine-grained microstructures were essentially stable against discontinuous grain growth and/or primary recrystallization during annealing at temperatures up to 800 °C, where the specimens maintain a very fine ferrite grain size. The annealing behavior is mainly characterized by normal grain growth accompanied by recovery. The grain-growth kinetics correlates with the oxide coarsening. Both the grain growth and the oxide coarsening can be enhanced by an increase in the temperature. The static restoration/recrystallization processes operating in the ultrafine-grained matrices, as well as the effect of dispersed oxides particles, are discussed in some detail..
321. T. Ohmura, Kaneaki Tsuzaki, Evaluation of matrix strength of Fe-C as-quenched and quench-tempered martensite using nanoindentation techniques, International Conference on Martensitic Transformations Journal De Physique. IV : JP, 10.1051/jp4:2003880, 112 I, 267-270, 2003.01, Evaluation of the matrix strength of the Fe-C martensite is performed by nanoindentation techniques. As-quenched martensite was examined for five kinds of Fe-C alloys with various carbon contents in the range of 0.1 to 0.8 mass% while quench-tempered martensite was investigated for an Fe-0.4%C alloy. The nanohardness attributed to the matrix strength increases with the carbon content, and decreases with increasing tempering temperature. The ratio of the nanohardness to the macrohardness, Hn/Hv, was much smaller for the Fe-C martensite than those for the single crystals, indicating that there is a significant grain size effect for the martensite..
322. Fu Gao Wei, Toru Hara, Takehiro Tsuchida, Kaneaki Tsuzaki, Hydrogen trapping in quenched and tempered 0.42C-0.30Ti steel containing bimodally dispersed TiC particles, isij international, 10.2355/isijinternational.43.539, 43, 4, 539-547, 2003.01, The effect of tempering on hydrogen trapping in 0.42C-0.30Ti steel was studied by means of a hardness test, hydrogen thermal desorption spectrometry and high resolution transmission electron microscopy. In addition to the coarse undissolved TiC particles, fine TiC particles with a volume fraction up to 0.22 % precipitated during tempering at temperatures above 500°C. Coherent TiC square platelets with a diagonal length of about 2 nm and a thickness of less than 1 nm precipitated at 550 and 600°C, acting as reversible hydrogen traps. Tempering at 700°C caused the fine TiC particles to lose their coherency with the matrix, resulting in a very limited increase in reversibly trapped hydrogen content compared to the samples tempered below 500°C without TiC precipitation. On the other hand, the coarse undissolved TiC particles with an average diameter of 2 μm, acting as irreversible traps, greatly enhanced the irreversibly trapped hydrogen content when tempered at 500°C and were accompanied by a decrease in hydrogen desorption rate peak temperature. Comparison of both types of TiC particles with the same volume fraction indicated that the fine coherent TiC platelets were more effective in trapping hydrogen than the coarse incoherent undissolved TiC particles..
323. Y. Kimura, S. Takagi, T. Hara, S. Terasaki, Kaneaki Tsuzaki, Hydrogen-induced delayed fracture of a martensitic steel with fine prior-austenite grain size, International Conference on Martensitic Transformations Journal De Physique. IV : JP, 10.1051/jp4:2003911, 112 I, 403-406, 2003.01, The influence of the grain size of the prior-austenite on the hydrogen-induced delayed fracture property was investigated for a medium carbon tempered martensitic steel (JIS-SCM440) with a tensile strength of 1400MPa. Thermomechanical treatment, which consists of a warm working at 873K by multi-pass bar rolling of tempered martensite and subsequent rapid austenitizing at 1093K for 1s, resulted in the refinement of the prior-austenite grain to 3 μm. The hydrogen embrittlement susceptibility was examined using a conventional creep test machine under a constant load for hydrogen pre-charged notched specimens with a stress concentration factor of 4.9. The critical diffusible hydrogen content, below which the specimen never fracture, was approximately 0.24 mass ppm for the fine-grained specimen under an applied stress of 0.9TS, two times higher than that of the conventional QT sample (prior-austenite grain size; 17 μm). The immersion test in 0.1M HCl water solution (pH=1.0) at 298K showed that the intruded hydrogen content (HE) at 100h was 0.14 mass ppm for the fine-grained sample which was almost the same as that of the conventional QT sample (0.17 mass ppm)..
324. Tatsuaki Sawai, Saburo Matsuoka, Kaneaki Tsuzaki, Low- and high-cycle fatigue properties of ultrafine-grained low carbon steels, Tetsu-To-Hagane/Journal of the Iron and Steel Institute of Japan, 10.2355/tetsutohagane1955.89.6_726, 89, 6, 726-733, 2003.01, It has been well known that fatigue properties of steels are improved by grain refinement. In the STX-21 project, we have been developing low carbon steels with sub-micron meter grain sizes and reported that their tensile strength is 800 MPa and DBTT is around 77K. In this report, low- and high-cycle fatigue properties of ultrafine-grained steels with different grain diameters of 0.7 and 0.9 μm and different carbon contents of 0.05 and 0.15 mass% were investigated. The grain refinement improved high-cycle fatigue properties, whereas it did not affect low-cycle fatigue properties. Fatigue limit showed the similar Hall-Petch type relationship for several studies carried out hitherto..
325. T. Hara, T. Tsuchida, Kaneaki Tsuzaki, Relationship between microstructure and hydrogen absorption behavior in a V-bearing tempered martensitic steel, International Conference on Martensitic Transformations Journal De Physique. IV : JP, 10.1051/jp4:2003913, 112 I, 411-414, 2003.01, TEM observations and hydrogen thermal desorption analysis were performed to understand the relationship between microstructure and hydrogen absorption behavior in a V-bearing steel. The hydrogen absorption property was changed by controlling VC precipitation with changing tempering temperature from 573K to 973K. Hydrogen was absorbed into the specimen by cathode charging at a constant condition, and its content was examined by thermal desorption analysis. Energy-filtering and high-resolution TEM methods were employed to observe the size and distribution of VC precipitates. When the specimen was tempered at around 873K, the hydrogen absorption content was markedly increased; 4 times higher than that of the as-quenched specimen. TEM observations showed that this significant hydrogen absorption is attributed to nano-scale VC precipitates..
326. T. Ohmura, T. Hara, Kaneaki Tsuzaki, Relationship between nanohardness and microstructures in high-purity Fe-C as-quenched and quench-tempered martensite, Journal of Materials Research, 10.1557/JMR.2003.0201, 18, 6, 1465-1470, 2003.01, The relationship between the nanohardness and the microstructures in the Fe-C martensite was studied to understand the contributions of the matrix and the grain boundary to the macroscopic strength. As-quenched martensite was examined for five kinds of Fe-C alloys with various carbon contents in the range of 0.1-0.8 mass%, while quench-tempered martensite was investigated for an Fe-0.4% C alloy. The ratio of the nanohardness to the macrohardness Hn/Hv was much smaller for the Fe-C martensite than those for the single crystals, indicating that there is a significant grain-boundary effect for the martensite. The ratio Hn/Hv of the as-quenched martensite decreased with an increase in the carbon content since the size of the block structure decreased with increasing carbon content. For the quench-tempered specimens, a significant reduction of the grain-boundary effect occured at the tempering temperature of 723 K. It is mainly due to the depression of the locking parameter caused by the disappearance of the film-like carbides on the boundaries..
327. Kaneaki Tsuzaki, Development of submicrocrystalllne Fe - O steels under mechanical milling followed by consolidation, Materials Science Forum, 426-432, 3, 2771-2776, 2003, Microstructure evolution in Fe - O steels under mechanical milling followed by consolidating rolling was studied. The final microstructure that developed after consolidation was shown to depend on the volume fraction of the dispersed oxides. In the samples milled for 100 hours, the average grain size decreased from 0.7 to 0.2 μm with increase in the amount of oxygen from 0.2 to 1.5 mass%. The milling time had a similar effect on the final microstructure. The relatively short milling time of 20 hours resulted in the evolution of relatively coarse elongated grains with a large fraction of low-angle subgrain boundaries. In contrast, fine equiaxed grains (about 0.2 μm in size) with a near random boundary misorienations developed in the Fe - 0.6%O sample that was milled for 300 hours. The structural changes taking place under mechanical milling followed by consolidating rolling and the effect of dispersed oxides on final microstructures are discussed in some detail..
328. M. Hayakawa, S. Matsuoka, Kaneaki Tsuzaki, H. Hanada, M. Sugisaki, Atomic force microscopy of induction- and furnace-heating-tempered prestressed steels with different delayed fracture properties, Scripta Materialia, 10.1016/S1359-6462(02)00263-4, 47, 10, 655-661, 2002.11, Quantitative microstructure analyses by atomic force microscopy and delayed fracture tests were performed for two types of prestressed steel with a tensile strength of 1470 MPa. Size distributions of cementite particles were measured to characterize the relationship between microstructures and delayed fracture properties. This method superimposed two AFM images of the same area: one of an electropolished surface that showed carbides and blocks and the other of a picric acid-etched surface in which prior δ grain boundaries were selectively etched..
329. A. Belyakov, T. Sakai, H. Miura, R. Kaibyshev, Kaneaki Tsuzaki, Continuous recrystallization in austenitic stainless steel after large strain deformation, Acta Materialia, 10.1016/S1359-6454(02)00013-7, 50, 6, 1547-1557, 2002.04, Static restoration mechanisms operating during annealing were studied in a 304 steel with strain-induced submicron grain structures. The initial microstructure with an average grain size of about 0.3 μm was developed by large strain deformation at 873 K. Early annealing leads to a full relaxation of high internal stresses associated with non-equilibrium strain-induced grain boundaries, while their boundary misorientations and the average grain size barely change. Further annealing results in a transient recrystallization followed by a normal grain growth. The average grain boundary misorientation increases up to around 45° in the former and becomes constant in the latter. This is associated with the change in the grain boundary misorientation distribution from a characteristic strain-induced one to a near random distribution corresponding to a fully recrystallized state. The annealing processes operating in the strain-induced fine grains take place homogeneously in the whole matrix and can be called continuous recrystallization..
330. Shinji Sakashita, Eiji Akiyama, Kaneaki Tsuzaki, Shinsaku Matsuyama, Distribution of hydrogen occluded in bolts tightened beyond the yield strength and exposed at a seashore site, Tetsu-To-Hagane/Journal of the Iron and Steel Institute of Japan, 10.2355/tetsutohagane1955.88.12_849, 88, 12, 849-856, 2002.01, The distribution of hydrogen occluded in high strength bolts tightened beyond the yield strength and exposed for 9 years has been investigated to establish an evaluation method for delayed fracture property. Hydrogen thermal desorption analysis was employed for determining hydrogen content in specimens cut out from the exposed bolts. Diffusive hydrogen causing delayed fracture is not uniform in concentration within the bolt and enriched in the threaded portion where diffusive hydrogen concentration was about three times that in the body portion. It is reasonable that diffusive hydrogen intrudes according to corrosion reaction, but no clear correlation between rusting and hydrogen concentration was recognized. From evaluation of the role of stress and plastic strain on hydrogen concentration, it is concluded that high concentration of diffusive hydrogen in the threaded portion was mainly caused by plastic straining..
331. T. Ohmura, Kaneaki Tsuzaki, S. Matsuoka, Evaluation of the matrix strength of Fe-0.4 wt% C tempered martensite using nanoindentation techniques, Philosophical Magazine A: Physics of Condensed Matter, Structure, Defects and Mechanical Properties, 10.1080/01418610208235702, 82, 10, 1903-1910, 2002.01, Nanoindentation techniques were applied to evaluate the matrix strength of the as-quenched and the tempered martensite of an Fe-0.4 wt% C binary alloy. The matrix strength was compared with the micro Vickers hardness HvThe nanohardness Hnof the matrix decreases with increasing tempering temperature, which is similar to the temper softening observed in the micro Vickers hardness. The ratio Hn/HVwhich corresponds to the contribution of the matrix strength to the macroscopic strength, increases markedly when the specimen is tempered at 450°C. This is due to a reduction in the grain size effect caused by the grain growth of the blocks that are analogous to the effective grain. The pop-in behaviour that appears in the specimens tempered at or above 550°C is due to the reduction in the dislocation density caused by recrystallization..
332. Andrey Belyakov, Y. Sakai, T. Hara, Y. Kimura, Kaneaki Tsuzaki, Evolution of submicrocrystalline iron containing dispersed oxides under mechanical milling followed by consolidation, Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science, 10.1007/s11661-002-0310-3, 33, 10, 3241-3248, 2002.01, The changes an pct alloy during milling by structural in Fe-0.6 pet O mechanical milling followed consolidation through rolling were studied. The iron-iron oxide powders were mechanically milled in an argon atmosphere for various times from 20 to 300 hours. The powders were then canned into a steel pipe and multiple rolled at 700 °C for consolidation. The microstructure of the final product depended significantly on the milling time. The volume fraction of the dispersed oxides (10 nm in diameter) increased from about 0.3 to 2.5 pct when the milling time was increased from 20 to 300 hours. The relatively short milling time of 20 hours resulted in the evolution of elongated grains (an average size of about 1.2 μm) with a large fraction of low-angle grain boundaries after consolidation. In contrast, much finer grains (about 0.2 μm in size) with a near random grain-boundary misorientation distribution evolved in the samples milled for 300 hours..
333. Tatsuaki Sawai, Yuji Kimura, Kaneaki Tsuzaki, Etsuo Takeuchi, Saburo Matsuoka, High-Cycle Fatigue Properties of Modified Ausformed 1800 MPaClass Spring Steel, Nihon Kikai Gakkai Ronbunshu, A Hen/Transactions of the Japan Society of Mechanical Engineers, Part A, 10.1299/kikaia.68.49, 68, 665, 49-56, 2002.01, High-cycle fatigue properties up to 10
8
cycles were investigated for a modified-ausformed spring steel SUP 12 with the tensile strength σ
B
=l 772 MPa. The surface-type fracture occurred at the higher stress level. On the hand, some specimens had the fish-eye-type fracture origin at the inclusion, and others endured 10
8
cycling at the lower stress level. The fatigue limit at 10
8
cycles σw = 900 MPa for the fish-eye fracture was higher than σ
w
=886 MPa estimated from σ
w
=0.5σ
B
, which is the empirical equation for the surface-type fracture of ordinary quenched and tempered low alloy steels. The optically dark area was not found around the inclusion. Excluding the fish-eye-type fracture, the higher fatigue limit σ
w
= l 090 MPa for the surface-type fracture was expected. It corresponds to σ
W
= 0.62σ
B
. It is concluded from these results that the modified-ausforming improves the fatigue properties for both surface and fish-eye type fractures of the high strength steel..
334. Masao Hayakawa, Saburo Matsuoka, Kaneaki Tsuzaki, Microstructural analyses of grain boundary carbides of tempered martensite in medium-carbon steel by atomic force microscopy, Materials Transactions, 10.2320/matertrans.43.1758, 43, 7, 1758-1766, 2002.01, The microstructures of JIS-SCM440 steel (containing 0.4%C, 1%Cr and 0.2%Mo; mass%) that had been tempered at 723 K were observed by atomic force microscopy (AFM). In an AFM image of an electropolished surface, carbides and blocks were clearly distinguished, since the rate of electropolishing depends on the crystal phase and crystal orientation on the surface. However, the prior austenite (γ) grain boundaries could not be clearly recognized. Therefore, an AFM image of the surface that had been etched with picric acid was taken at the same location on the surface. Since picric acid selectively etches prior γ grain boundaries, the boundaries were clearly visible in the image. The two AFM images were then superimposed to reveal prior γ grain boundaries, carbides, and blocks. Finally, the number and sizes of the carbides on the prior γ grain boundaries were examined..
335. Masao Hayakawa, Satoshi Terasaki, Toru Hara, Kaneaki Tsuzaki, Saburo Matsuoka, Microstructural analyses of modified-ausformed medium-carbon steel with high resistance to hydrogen embrittlement by atomic force microscopy, Nippon Kinzoku Gakkaishi/Journal of the Japan Institute of Metals, 10.2320/jinstmet1952.66.7_745, 66, 7, 745-753, 2002.01, Quantitative microstructure analyses by atomic force microscopy (AFM) were performed for two kinds of JIS-SCM440 steel with the tensile strength of 1580 MPa; the modified-ausformed and tempered (MAQT) martensite and the conventional quench-tempered (CQT) martensite, where the critical diffusible hydrogen content for delayed fracture in the MAQT martensite was 0.53 mass ppm and higher than that of the CQT martensite (0.13 mass ppm), Size distribution of cementite particles and martensite blocks was measured to understand the relationship between microstructure and hydrogen embrittlement resistance. The average cementite size along prior austenite grain boundaries for the MAQT martensite was 0.51 times of that for the CQT martensite. Coarse film-like cementites above 200 nm in length markedly decreased for the MAQT martensite. As the result, occupied fraction of grain boundary cementite particles for the MAQT martensite was decreased to 41%o from 51%o of the CQT martensite. Inner-prior austenite grain cementite and block were refined for the MAQT martensite. The average cementite size of inner-prior austenite grains for the MAQT martensite was 0.58 times of that for the CQT martensite. The average block width was 0.38 μm for the MAQT martensite, while it was 0.49 μm for the CQT martensite. It is suggested that the refinement of the MAQT martensite structure along and inner-prior austenite grain, boundary plays an important role in improvement of hydrogen embrittlement resistance..
336. Takehiro Tsuchida, Toru Hara, Kaneaki Tsuzaki, Relationship between microstructure and hydrogen absorption behavior in a V-bearing high strength steel, Tetsu-To-Hagane/Journal of the Iron and Steel Institute of Japan, 10.2355/tetsutohagane1955.88.11_771, 88, 11, 771-778, 2002.01, Hydrogen thermal desorption analysis and TEM observation were performed to understand the relationship between microstructure and hydrogen absorption behavior in a V-bearing steel (0.4C-1.0Cr-0.7Mo-0.35V; mass%). The hydrogen absorption behavior was changed by controlling (V,X)C precipitation with change of tempering temperature from 300 to 700°C. Hydrogen was absorbed into the specimen by cathodic charging at a constant condition, and its content was examined by thermal desorption analysis. Energy-filtering and high-resolution TEM methods were employed to observe the size and distribution of (V,X)C precipitates. When the specimen was tempered at around 600°C, the absorbed hydrogen content was markedly increased; 3 times higher than that of the as-quenched specimen. TEM observation showed that this significant hydrogen absorption was attributed to nano-scale (V,X)C coherent precipitates. When the specimen was tempered at 700°C, the absorbed hydrogen content was decreased but was still as high as that of the as-quenched specimen. This was due to spherical (V,X)C incoherent precipitates with about 20 nm in diameter. It was estimated from hydrogen desorption profiles that activation energy of hydrogen evolution from trapping sites for the fine coherent precipitates was similar to that for dislocations and lower than that for the coarse incoherent precipitates..
337. Kaneaki Tsuzaki, Preface to the Special Issue on “Advanced Structural Steels”, ISIJ International, 10.2355/isijinternational.42.1325, 42, 12, 1325, 2002.
338. A. Belyakov, Y. Sakai, T. Hara, Y. Kimura, Kaneaki Tsuzaki, Thermal stability of ultra fine-grained steel containing dispersed oxides, Scripta Materialia, 10.1016/S1359-6462(01)01152-6, 45, 10, 1213-1219, 2001.11, The annealing behaviour of the Fe-1.5%O steel with a grain size of about 0.2 μm was investigated using mechanical milling techniques. The structural analysis of the given samples was conducted using transmission electron microscope (TEM) at 200 kV. It is found that ultra fine-grained microstructure is stable during rapid grain growth at the temperature less than 800°C while heating to higher temperatures results in sharp grain coarsening..
339. T. Ohmura, Kaneaki Tsuzaki, S. Matsuoka, Nanohardness measurement of high-purity Fe-C martensite, Scripta Materialia, 10.1016/S1359-6462(01)01121-6, 45, 8, 889-894, 2001.10, High-purity iron-carbon binary martensitic steels and body centered cubic single crystals were studied using nanohardness measurements. The effect of grain size on the strength of martensite was investigated. The ratio of nanohardness to micro Vickers hardness of the martensite was found to be smaller than those of the single crystals..
340. M. Otaguchi, S. Wanikawa, Kaneaki Tsuzaki, K. Nagai, Consolidation of sheathed pure iron powders by warm caliber rolling, Tetsu-To-Hagane/Journal of the Iron and Steel Institute of Japan, 10.2355/tetsutohagane1955.87.7_480, 87, 7, 480-485, 2001.01, Metal powders produced by mechanical milling show the specific microstructures and properties. With these merits preserved, the powders should be combined into a large size compact. Saito et al. and Kimura et al. have succeeded in consolidating the iron powders into 1 ∼ 2.5 mm thick sheets by sheath rolling at a relatively low temperature of 973K. Based upon their success, we have attempted to apply caliber rolling for consolidating the powders into a large bar (φ11 × 1000 mm). The sheath can was filled with pure iron powders, sealed in vacuum, and then rolled. When the rolling temperature was 973K, rolling reduction of more than 80% was enough to make sound compacts of >10 mm diameter with fine microstructure and relative density larger than 99.6%. When the rolling reduction was 90%, the consolidation was accomplished in a wide range of rolling temperature between 1273K and 823K. At 823K and 873K, the sound consolidation could be realized without recrystallization. The tensile propertie s were better than those of the HIP treated compact. Hence, the solid-state joining is confirmed to occur much easier in the present method than in the HIP method..
341. Shusaku Takagi, Tadanobu Inoue, Kaneaki Tsuzaki, Fumiyoshi Minami, Determination method of Weibull shape parameter for evaluation of the hydrogen embrittlement susceptibility of high strength steel, Nippon Kinzoku Gakkaishi/Journal of the Japan Institute of Metals, 10.2320/jinstmet1952.65.12_1082, 65, 12, 1082-1090, 2001.01, An experimental method to obtain the shape parameter m in the Weibull stress for evaluation of hydrogen embrittlement (HE) was examined in a commercial JIS SCM440 steel with the tensile strength of 1403 MPa. The stepwise HE tests were performed by using circumferentially notched round-bar specimens with the stress concentration factor of 4.9 after hydrogen pre-charging and homogenization treatments. The applied stress of the first step loading was set at 702 MPa and the stress increment from the second step was 14 MPa. The holding time was over 12 h for the first step and over 2 h for each step from the second step. It was conformed by acoustic emission measurement that the first cracking and final fracture occurred in the same step. The diffusible hydrogen content was measured by thermal desorption analysis immediately after fracture. The distribution of stress and hydrogen content near notch root was calculated by FE-analysis. By this procedure, it was able to obtain the distribution of critical Weibull stress under the condition that the local diffusible hydrogen content in fracture process zone was constant. The shape parameter m was determined to be 46 for the present steel. In the discussion part, it was confirmed that the evaluation of the HE susceptibility based on the local approach is appropriate and it was pointed out that the hydrogen content distribution in the fracture process zone must be taken into account for evaluating the HE..
342. A. Belyakov, Y. Sakai, T. Hara, Y. Kimura, Kaneaki Tsuzaki, Effect of dispersed particles on microstructure evolved in iron under mechanical milling followed by consolidating rolling, Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science, 10.1007/s11661-001-0153-3, 32, 7, 1769-1776, 2001.01, The microstructure and the strength of an iron mechanically milled with various amounts of oxygen (i.e., 0.2, 0.6, and 1.5 mass pct) were studied. The samples were subjected to a mechanical milling in an argon atmosphere for 100 hours followed by consolidating bar rolling to a total reduction of about 86 pct at 700°C. The microstructure of the steels sensitively changed depending on the oxygen content, i.e., on the volume fraction of the oxide particles. The average gain size decreased from about 0.7 to 0.2 μm with an increase in the amount of oxygen. Moreover, the misorientation distributions of the grain boundaries were different in the samples with various amounts of oxygen. A relatively large fraction of low-angle boundaries arranged crosswise to the rolling axis was registered in the samples with 0.2 and 0.6 pct oxygen, while the near random distribution of the boundary misorientations was obtained in the specimens with 1.5 pct oxygen. The effect of dispersed particles on the structure evolution and the relationship between microstructures and some mechanical properties are discussed..
343. T. Inoue, S. Torizuka, K. Nagai, Kaneaki Tsuzaki, T. Ohashi, Effect of plastic strain on grain size of ferrite transformed from deformed austenite in Si-Mn steel, Materials Science and Technology, 10.1179/026708301101509647, 17, 12, 1580-1588, 2001.01, The effect of plastic strain on the grain size of ferrite transformed from deformed and unrecrystallised austenite has been investigated for a low carbon Si - Mn steel. An explicit finite element technique was used to evaluate the plastic strain distribution introduced by deformation in a specimen. The specimen was compressed by a pair of anvils controlled to keep the strain rate constant. The contact condition between the anvil and the specimen was determined by the experimental result from an identically deformed screw set in the specimen. The interrelation between the equivalent plastic strain Eeq, numerically obtained in the range 0.1 < Eeq < 4.2, and the experimental grain size of 2 - 9 μm was analysed. The change in ferrite grain size caused by Eeq in the unrecrystallised austenite region has been discussed..
344. Shusaku Takagi, Tadanobu Inoue, Kaneaki Tsuzaki, Fumiyoshi Minami, Evaluation of hydrogen embrittlement susceptibility of high strength steel by the Weibull stress, Nippon Kinzoku Gakkaishi/Journal of the Japan Institute of Metals, 10.2320/jinstmet1952.65.12_1073, 65, 12, 1073-1081, 2001.01, The parameters for levels of the evaluation of hydrogen embrittlement susceptibility of high strength steel, which are independent of stress concentration and applied stress, are examined using circumferentially notched round-bar specimens with different notch root radius. The hydrogen embrittlement tests are performed for quenched and tempered JIS SCM440 steel with the tensile strength of 1403 MPa. The applied stress is changed from 0.33 to 0.72 times the tensile strength of the notched specimen and the stress concentration factor (Kt) ranges from 2.1 to 6.9. The initiation of hydrogen-induced crack is detected by acoustic emission measurement. Specimens are unloaded immediately after detecting the first acoustic emission and the crack initiation points are observed with SEM. The results are summarized as follows. (1) The Weibull stress (σw) and diffusible hydrogen contents in fracture process zone (Hc*(ave.)) enable the hydrogen embrittlement susceptibility evaluation independently of stress concentration and applied stress levels of circumferentially notched round-bar specimens. (2) Initiation points of hydrogen embrittlement fracture are located in the region where the maximum principal stress exceeds 0.8 times its peak value. The significance of hydrogen embrittlement susceptibility evaluation using the σw-Hc* (ave.) relation is discussed in terms of the extent of fracture process zone ahead of the notch root..
345. A. Belyakov, T. Sakai, H. Miura, Kaneaki Tsuzaki, Grain refinement in copper under large strain deformation, Philosophical Magazine A: Physics of Condensed Matter, Structure, Defects and Mechanical Properties, 10.1080/01418610108216659, 81, 11, 2629-2643, 2001.01, Structure evolution taking place in pure polycrystalline copper was studied in multiple compressions at room temperature. Rectangular samples were compressed with consequent change in the loading direction from pass to pass. The deformation behaviour at high strains of above 2 shows an apparent steady-state flow following a rapid rise in the flow stress at an early stage of deformation. The structural changes are characterized by the evolution of many mutually crossing subboundaries at low to moderate strains, finally followed by the development of very fine grains with medium- to large-angle boundaries at large strains. These new grains are concluded to be evolved by a kind of continuous reaction, that is continuous dynamic recrystallization (DRX). The grains developed under continuous DRX are much finer than expected from the extrapolation of discontinuous DRX data for hot deformation. An average grain size of about 0.2 μm evolved at room temperature is roughly similar to that for subgrains developed at preceding strains..
346. T. Maki, T. Furuhara, Kaneaki Tsuzaki, Microstructure development by thermomechanical processing in duplex stainless steel, isij international, 41, 6, 571-579, 2001.01, In the case of two-phase alloys, a refinement matrix phase and a uniform distribution of second phase are important to improve mechanical properties. In this paper, microstructure change of α (ferrite)+γ (austenite) two-phase structure, especially microduplex structure consisting of α and γ in a fine-grained form (1-3μm), in the duplex stainless steel by various thermomechanical processings and heat treatments is briefly reviewed. Main topics are as follows, 1) formation process of fine-grained α+γ structure (microduplex structure) by various thermomechanical processings, 2) nature of microduplex structure and its stability during prolonged aging, 3) microstructure change of microduplex structure by annealing after heavy cold rolling and 4) microstructure change of microduplex structure during superplastic deformation..
347. M. Hayakawa, S. Matsuoka, Kaneaki Tsuzaki, Observations of prior austenite grain boundaries and carbides in the same area of tempered martensite in medium-carbon steel by atomic force microscopy, Nippon Kinzoku Gakkaishi/Journal of the Japan Institute of Metals, 10.2320/jinstmet1952.65.8_734, 65, 8, 734-741, 2001.01, Microstructures of JIS-SCM440 steel tempered at 723 K were observed on an electropolished and a picric-acid etched surfaces by atomic force microscopy (AFM). At first, AFM image was taken on an electropolished surface. Carbides and blocks were clearly distinguished on the image, because the electropolishing rate depends on the crystal phase and its surface orientation. However, prior austenite grain boundaries could not be completely recognized. Secondly, AFM image was taken in the same area on a picric-acid etched surface. Prior austenite grain boundaries were observed, because the boundaries are preferentially etched. Thirdly, the both of AFM images were overlapped. This procedure gave us AFM image where prior austenite grain boundaries as well as carbides and blocks were clearly recognized. Finally, number and sizes of the carbides along prior austenite grain boundaries were examined..
348. Kaneaki Tsuzaki, Preface to the special issue on "advances in physical metallurgy and processing of steels", isij international, 10.2355/isijinternational.41.519, 41, 6, 2001.01.
349. Shusaku Takagi, Tadanobu Inoue, Toru Hara, Masao Hayakawa, Kaneaki Tsuzaki, Toshihiko Takahashi, Parameters for the evaluation of hydrogen embrittlement of high strength steel, Tetsu-To-Hagane/Journal of the Iron and Steel Institute of Japan, 86, 10, 689-695, 2000.10, The influences of the applied stress and the stress concentration factor (Kt) on hydrogen embrittlement were investigated in the commercial JIS SCM440 steel with the tensile strength of 1403 MPa. The applied stress was changed from 0.33 times to 0.72 times the tensile strength of the notched specimen, while the Kt was altered from 2.1 to 6.9. The hydrogen embrittlement property was evaluated with the resistance to the diffusible hydrogen in the steel. The diffusible hydrogen concentrations were analyzed by thermal desorption analysis. The results are as follows. (1) The hydrogen embrittlement occurs under the lower diffusible hydrogen concentrations with increasing Kt. However, the HD-t (diffusible hydrogen concentration-time to failure) curves become almost the same when the Kt and the applied stress are high. (2) The criterion of the hydrogen embrittlement cannot be determined by the combination of the maximum local axial stress in the specimen and the maximum diffusible hydrogen concentration locally accumulated in the specimen. In the discussion part, it was proposed that it is very important to consider the stress distribution in the specimen for the evaluation of hydrogen embrittlement..
350. Satoru Yusa, Toru Hara, Kaneaki Tsuzaki, Grain boundary carbide structure in tempered martensitic steel with serrated prior austenite grain boundaries, Nippon Kinzoku Gakkaishi/Journal of the Japan Institute of Metals, 10.2320/jinstmet1952.64.12_1230, 64, 12, 1230-1238, 2000.01, An Fe-2%Mn-0.36%C martensite with serrated prior-austenite grain boundaries was derived from work hardened austenite deformed at 1023 K. The martensite was tempered and microstructure of grain boundary carbide was observed by scanning electron microscopy and transmission electron microscopy, and compared with a conventional quench-tempered martensite. Coarse carbide films formed along flat prior-austenite grain boundaries in the conventional quench-tempered martensite. These carbide films disappeared, and the fine and uniform carbide structure was obtained in the tempered martensite derived from the work hardened austenite. On a serrated prior-austenite grain boundary, carbide particles had various sizes and shapes, and some parts of the boundary were free from carbide precipitation. Although carbide particles with various orientations were observed on one serrated prior-grain boundary, almost identically oriented carbide particles existed at a locally flat region of the grain boundary. These changes of carbide structure can be accounted for by the fluctuation of orientation on the serrated prior-austenite grain boundary..
351. Masao Hayakawa, Toru Hara, Saburo Matsuoka, Kaneaki Tsuzaki, Microstructural observation of tempered martensite in a medium-carbon low-alloy steel by atomic force microscopy, Nippon Kinzoku Gakkaishi/Journal of the Japan Institute of Metals, 10.2320/jinstmet1952.64.6_460, 64, 6, 460-466, 2000.01, Microstructures of JIS-SCM440 steel tempered at 723 K were observed by atomic force microscopy (AFM). Two kinds of specimen surface were prepared by electrically polishing and by chemically etching. The quality of AFM images was much better on the electropolished surface because the high vertical resolution of AFM could be fully utilized. Since the electropolishing rate depends on the crystal phase and the surface orientation, prior austenite grains, packets, blocks and small precipitates were clearly distinguished on the electropolished surface, and the average block width was determined to be 0.6 μm. The density of the heterogeneously precipitated carbides along prior austenite grain boundaries was not uniform, suggesting that the nucleation of the carbides depends on the grain boundary characteristics. The apparent size of the carbides observed by AFM was larger than the actual size observed by TEM due to the effect of the tip curvature..
352. Shiro Torizuka, Osamu Umezawa, Kaneaki Tsuzaki, Kotobu Nagai, Shape, size and crystallographic orientation of the ferrite grains formed at grain boundaries of deformed austenite in a low carbon steel, Tetsu-To-Hagane/Journal of the Iron and Steel Institute of Japan, 10.2355/tetsutohagane1955.86.12_807, 86, 12, 807-814, 2000.01, The influence of plastic strain on the shape, size and crystallographic orientation of ferrite (α) grains formed at the grain boundaries of the deformed austenite (γ) was studied in a 0.17C-0.3Si-1.5Mn steel. Specimens with a coarse γ grain size of 300 μm were compressed at 1023 K and cooled at 10 K/s. When the plastic strain increased to 0.4, the shape of α grains changed from plate like to equiaxed, and the average length of α grains decreased from 12 to 6 μm. However, the average length did not change in the larger plastic strains up to 1.1. On the other hand, the average thickness of α grains was constant regardless of the plastic strain. The crystallographic orientations of the α grains formed at one γ grain boundary were almost the same when the plastic strain was smaller than 0.2 and the α grain shape was plate like. However, the orientations were widely distributed, and most of the α/α boundaries were high angle ones, when the plastic strain was larger than 0.4 and the α grain shape was equiaxed. The shape change and α grain refinement by the deformation resulted from the wide distribution of crystallographic orientations of α grains rather than from the increase in the nucleation rate. The wide distribution of crystallographic orientation of α grains is closely associated with the serrated austenite grain boundaries induced by the deformation..
353. Nobuo Nagashima, Kensuke Miyahara, Saburo Matsuoka, Kaneaki Tsuzaki, Toru Hara, Toshihiko Takahashi, Strength analysis of a ferrite+bainite steel by AFM ultra-micro hardness tester, Tetsu-To-Hagane/Journal of the Iron and Steel Institute of Japan, 10.2355/tetsutohagane1955.86.12_815, 86, 12, 815-821, 2000.01, AFM ultra-micro hardness test, micro Vickers hardness test and TEM observation were conducted for a ferrite+bainite steel, which consists of bainite and polygonal ferrite. The results obtained are in the following. (1) AFM ultra-micro hardness tests show that nanoscopic hardness is 155 near the center of bainitic ferrite grain and increases closer to its boundary. Nanoscopic hardness is kept constant at 151 in the polygonal ferrite grain. (2) Micro-Vickers hardness tests show that macroscopic hardness is 251 and 147 for the bainite and polygonal ferrite regions, respectively. (3) TEM observations show that many carbides precipitated along the grain boundaries of bainitic ferrite and the dislocation density was almost same in both grains of bainitic and polygonal ferrite. It is suggested from the above results that the strengthening by grain boundaries is important in the bainitic ferrite and this strengthening effect is enhanced by carbides along grain boundaries..
354. E. Sato, S. Furimolo, T. Furuhara, Kaneaki Tsuzaki, T. Maki, Microstructure control for superplasticity of an ultra-high carbon steel, Materials Science Forum, 304-306, 133-138, 1999.12, Relation between the (a+0) microstructure and the superplastic behavior in an ultra-high carbon steel (Fe-1.4Cr-1.0C) has been studied. Special attention was paid to the substructure and grain boundary character in a matrix. By 90% warm rolling of a pearlite structure, an (α+θ) microstructure with fine and equiaxed a grains of 0.4um in diameter and spheroidized 0 particles of 0.2um in diameter is obtained. Kikuchi pattern analysis has revealed that a matrix exhibits a recovered structure in which a large fraction of a grain boundaries are subgrain boundaries (low angle boundaries) not suitable for grain boundary sliding. When the 90% warm rolled specimens were austenitized in (y+9) two phase region, quenched and tempered at the temperature below Ai, an (a+0) microduplex structure with the a and 0 grain sizes equivalent to those in the as warm rolled specimen is formed. It was found that the fraction of high angle a grain boundary significantly increases in comparison with the as warm rolled specimen. The austenitizing, quenching and tempering treatment without warm rolling of pearlite structure also produces an .(a+9) microduplex structure with high angle a grain boundaries. The specimen austenitized, quenched and tempered shows much larger superplastic elongation in the tensile tests at 973K than the as warm rolled specimens..
355. S. Yusa, T. Hara, Kaneaki Tsuzaki, T. Takahashi, Refinement of grain boundary cementite in medium-carbon tempered martensite by thermomechanical processing, Materials Science and Engineering A, 10.1016/S0921-5093(99)00328-7, 273-275, 462-465, 1999.12, Microstructural observations of prior-austenite grain boundary cementite in tempered martensite of an Fe-2%Mn-0.36%C alloy were performed. Grain boundary cementite could be markedly refined by applying ausforming. Unlike the case of the non-ausformed specimen, no coarse cementite films were observed and fine cementite particles with various variants tended to form at a serrated prior-austenite grain boundary in the ausformed specimen..
356. Kaneaki Tsuzaki, E. Sato, S. Furimoto, T. Furuhara, T. Maki, Formation of an (α+θ) microduplex structure without thermomechanical processing in superplastic ultrahigh carbon steels, Scripta Materialia, 10.1016/S1359-6462(98)00486-2, 40, 6, 675-681, 1999.02, The effects of alloying elements and heat treatments on the formation of microduplex structure in ultrahigh carbon steels were investigated. A fully pearlitic structure without coarse pro-eutectoid θ particles was obtained by the addition of 2 mass% Al. The microduplex structure consisted of fine grained α matrix with a grain size of 0.7 μm. The structure exhibited superplasticity, elongation of 490%, and a maximum flow stress of 35 MPa at 973 K and at an initial strain rate of 5×10-4/s..
357. Hisashi Hirukawa, Saburo Matsuaka, Etsuo Takeuchi, Takahito Omura, Koji Yamaguchi, Kaneaki Tsuzaki, High resistance of fatigue crack growth for austenitic stainless steels containing nitrogen, Nihon Kikai Gakkai Ronbunshu, A Hen/Transactions of the Japan Society of Mechanical Engineers, Part A, 10.1299/kikaia.65.1343, 65, 634, 1343-1348, 1999.01, Fatigue crack growth properties for SUS 316 stainless steels containing nitrogen in the mass, percent from 0.001 to 0.66 were investigated in air at room temperature. The near-threshold fatigue crack growth properties for SUS 316 steel containing 0.001, 0.02 and 0.07 mass. % N were coincident each other. On the other hand, SUS 316 steels containing 0.24, 0.25, 0.40 and 0.66 mass. % N showed the remarkable decrease in fatigue crack growth rate and 50% increase in fatigue threshold. This behavior was related to the slip deformation at the fatigue crack tip. The slip deformation was localized for low nitrogen steels, while it was uniformed for high nitrogen steels..
358. S. Torizuka, O. Umezawa, Kaneaki Tsuzaki, K. Nagai, Refinement of ferrite-pearlite structures through transformation from heavily deformed austenite in a low carbon Si-Mn steel, Materials Science Forum, 284-286, 225-230, 1998.12, In a low-carbon Si-Mn steel without any other alloy ing elements, an ultra-fine ferritepearlite microstructure with a ferrite grain size of 2.3 μm was created through thermomechanical treatment. The feature of the process was one-pass heavy reduction at a low temperature in unrecry stallized austenite region followed by controlled cooling at 10 K/s. Crystallographic orientation of each ferrite grain was randomly distributed and most of the grain boundaries were high-angle ones..
359. Taku Sakai, Kaneaki Tsuzaki, Preface to the Special Issue on “Recrystallization and Related Phenomena”, isij international, 10.2355/isijinternational.38.509, 38, 6, 1998.01.
360. Kaneaki Tsuzaki, N. R. Ikeda, T. Maki, Effect of ferrite morphology on the cold-rolling and recrystallization textures in an ultra-low carbon steel, Scripta Materialia, 10.1016/S1359-6462(96)00495-2, 36, 8, 905-913, 1997.04.
361. Kaneaki Tsuzaki, Huang Xiaoxu, T. Maki, Mechanism of dynamic continuous recrystallization during superplastic deformation in a microduplex stainless steel, Acta Materialia, 10.1016/1359-6454(96)00080-8, 44, 11, 4491-4499, 1996.01, Microstructural evolution during the cyclic cold-rolling and annealing process in an (α + γ) microduplex stainless steel, which consists of α subgrains and fine γ particles, has been studied in detail with the aim of clarifying the mechanism of dynamic continuous recrystallization. A continuous increase in α subgrain boundary misorientation is obtained by the present processing where grain boundary sliding does not occur and the effect of increasing boundary misorientation with cumulative strain is comparable to those observed in dynamic continuous recrystallization of superplastic aluminium alloys. The increase in boundary misorientation is accounted for by the absorption of dislocations into subgrain boundaries during annealing, dislocations which had operated to accommodate the plastic strain incompatibility of the neighboring phases undergoing slip deformation. The present results show that grain boundary sliding is not indispensable but the difference in accommodation deformation between adjacent subgrains is of great importance for the dynamic continuous recrystallization during superplastic deformation..
362. Kaneaki Tsuzaki, Y. Natsume, Y. Tomota, T. Maki, Effect of solution hardening on the shape memory effect of FeMn based alloys, Scripta Metallurgica et Materiala, 10.1016/0956-716X(95)00337-U, 33, 7, 1087-1092, 1995.10.
363. Huang Xiaoxu, Kaneaki Tsuzaki, T. Maki, Effect of initial structure on recrystallization of the α matrix in an (α+γ) microduplex stainless steel, Scripta Metallurgica et Materiala, 10.1016/0956-716X(95)00198-5, 33, 3, 341-346, 1995.08.
364. N. Tsuji, H. Takebayashi, T. Takiguchi, Kaneaki Tsuzaki, T. Maki, Effect of initial orientation on the cold-rolling behavior of solidified columnar crystals in an Fe-36%Ni austenitic alloy, Acta Metallurgica Et Materialia, 10.1016/0956-7151(94)00264-I, 43, 2, 743-754, 1995.01, The cold rolling behavior of a solidified columnar-crystal specimen with the 001〈uv0〉 initial texture in an Fe-36%Ni austenitic (f.c.c.) alloy has been investigated. The rolled microstructure and the crystal rotation strongly depended on the initial orientation of columnar grains. 70% rolled microstructure of the initially (001)[100] oriented grains was composed of three kinds of deformed microstructures: the smoothly-etched matrix with the (001)[100] orientation, the wide deformation bands roughly parallel to the transverse direction (TD) with the (101)[101-] orientation and the narrow deformation bands parallel to the rolling direction (RD). The shear bands inclined by 30 deg from the rolling plane formed within the initially (001)[110] oriented grains, where the matrix regions rotated to the 113〈332〉 orientation. Each shear band was composed of many elongated cells whose width was less than 0.2 μm. The formation of each deformed microstructure was discussed in connection with the active slip systems.†Present address: Department of Materials Science and Engineering, Faculty of Engineering, Osaka University, Yamada-oka 2-1, Suita, Osaka, 565 Japan.‡Present address: Sumitomo Metal Industries Ltd, Hazaki-cho, Kashima-gun, Ibaragi, 314 Japan.§Present address: Nissan Motor Co. Ltd, Oppama, Yokosuka, Kanagawa, 273 Japan..
365. N. Tsuji, H. Takebayashi, T. Takiguchi, Kaneaki Tsuzaki, T. Maki, Recrystallization of solidified columnar crystals in an Fe-36%Ni austenitic alloy, Acta Metallurgica Et Materialia, 10.1016/0956-7151(94)00263-H, 43, 2, 755-768, 1995.01, The recrystallization behavior of the 21 columnar grains in an Fe-36%Ni austenitic (f.c.c.) alloy during annealing at 973 K after 70% cold-rolling, whose rolling behavior had been studied in the coupled paper, has been studied with emphasis on the effect of the initial orientation. The recrystallization rate and the recrystallized grain size of the columnar-crystal specimen were hardly dependent on the initial orientation. This result is in contrast to that reported in the Fe-19%Cr (b.c.c.) columnar crystals. The reason for this difference was discussed in connection with the easiness of cross-slip in the two alloys. On the other hand, the orientation of recrystallized grains was dependent on the initial orientation. The columnar-crystal specimen showed the (001)〈310〉 recrystallization texture. The (001)〈310〉 recrystallized grains preferentially appeared from the shear bands which formed in the (001)[110] initially oriented columnar grains. TEM observation showed that some elongated cells within the shear bands have the (001)〈310〉 orientation in the as-rolled state.†Present address: Department of Materials Science and Engineering, Faculty of Engineering, Osaka University, Yamada-oka 2-1, Suita, Osaka, 565 Japan.‡Present address: Sumitomo Metal Industries Ltd, Hazakicho, Kashima-gun, Ibaragi, 314 Japan.§Present address: Nissan Motor Co Ltd, Oppama, Yokosuka, Kanagawa, 273 Japan..
366. Huang Xiaoxu, Kaneaki Tsuzaki, T. Maki, Subgrain growth and misorientation of the α matrix in an (α + γ) microduplex stainless steel, Acta Metallurgica Et Materialia, 10.1016/0956-7151(95)00028-T, 43, 9, 3375-3384, 1995.01, The change of the (α + γ) microduplex structure during annealing in an Fe-26%Cr-7%Ni alloy has been investigated with emphasis on the change in the misorientation of the α matrix subgrain boundaries with subgrain growth in a large range of subgrain size. A long time holding up to 360 ks at 1273 K caused significant structure coarsening, and the growth of the α matrix subgrains and γ particles obeyed the third power law. On the other hand, the average misorientation of the α subgrain boundaries changed insignificantly, even though the α subgrain size markedly increased from 2.6 to 15.6 μm. This insignificance in the misorientation resulted from the fact that the variation of the local lattice rotation in the α matrix is not monotonic but periodic, and no long range lattice curvature exists..
367. Kaneaki Tsuzaki, Aki Kodai, Tactashi Maki, Formation mechanism of bainitic ferrite in an Fe-2 Pct Si-0.6 Pct C alloy, Metallurgical and Materials Transactions A, 10.1007/BF02649049, 25, 9, 2009-2016, 1994.09, The bainite transformation at 723 K in an Fe-2 pct Si-0.6 pct C alloy (mass pct) was investigated with transmission electron microscopy (TEM) and quantitative metallography to clarify the growth mechanism of the ferritic component of bainite. In early stages of transformation, the bainitic ferrite was carbide free. The laths of bainitic ferrite within a packet were parallel to one another and separated by carbon-enriched retained austenite. The average carbon concentration of the bainitic ferrite was estimated to be 0.19 mass pct at the lowest, indicating that the ferrite was highly supersaturated with respect to carbon. The laths did not thicken during the subsequent isothermal holding, although they were in contact with austenite of which the average carbon concentration was lower than the paraequilibrium value. In the later stage of transformation, large carbide plates formed in the austenite between the laths, resulting in the decrease in the carbon concentration of the austenite. Subsequently, the ferrite with a variant different from the initially formed ferrite in the packet was decomposed for the completion of transformation. The present results indicate that the bainitic ferrite develops by a displacive mechanism rather than a diffusional mechanism..
368. Nobuhiro Tsuji, Kaneaki Tsuzaki, Tadashi Maki, Effects of Rolling Reduction and Annealing Temperature on the Recrystallization Structure of Solidified Columnar Crystals in a 19% Cr Ferritic Stainless Steel, isij international, 10.2355/isijinternational.34.1008, 34, 12, 1008-1017, 1994.01, The cold-rolling and recrystallization behavior of solidified columnar crystals in a 19% Cr ferritic stainless steel has been investigated. As the rolling reduction increased, the recrystallization rate increased and the recrystallized grain size decreased. However, the (001)[110] initially-oriented columnar grains were hard to recrystallize without regard to rolling reduction so that the recrystallization structure was very inhomogeneous in grain size even in a heavily-rolled (90% rolled) specimen. The recrystallization rate of 70% rolled specimens increased as the annealing temperature increased. In particular, at the temperatures where carbides did not precipitate the recrystallization rapidly completed and a coarse-grained structure formed in a short time annealing because of rapid grain growth. As a result, in the Fe-19% Cr ferritic columnar crystals a favorable recrystallization structure with fine and uniform grain size cannot be obtained simply by increasing rolling reduction or by controlling annealing temperature..
369. N. Tsuji, T. Hyoue, Kaneaki Tsuzaki, T. Maki, Effect of constraint on the rolled microstructure of (001)[100] oriented single crystals in an Fe-19%Cr ferritic alloy, Scripta Metallurgica et Materiala, 10.1016/0956-716X(93)90151-H, 29, 4, 479-484, 1993.08.
370. Nobuhiro Tsuji, Kaneaki Tsuzaki, Tadashi Maki, Effect of Initial Orientation on the Recrystallization Behavior of Solidified Columnar Crystals in a 19% Cr Ferritic Stainless Steel, isij international, 10.2355/isijinternational.33.783, 33, 7, 783-792, 1993.01, The effect of the initial orientation of each grain on the recrystallization behavior at 973 K after 70% cold-rolling of solidified columnar crystals in a 19%Cr ferritic stainless steel have been investigated. It was demonstrated that the recrystallization kinetics, the recrystallization microstructure and the orientation of recrystallized grains in each columnar grain strongly depend on the initial orientation. The (001)[100] initially-oriented grains which showed the cold-rolled structure composed of fine elongated deformation bands readily recrystallized and formed fine recrystallized grains. On the other hand, the (001) [110] initially-oriented grains which showed a smooth-etching deformation structure were difficult to recrystallize and formed coarse recrystallized grains after long-time annealing. Because of this difference in the recrystallization behavior among columnar grains, the fully recrystallized structure of the columnar crystal specimen was extremely inhomogeneous in grain size. The fully recrystallized structure was also characterized by the “cube” texture (i.e., (001)[100] texture) which has been scarcely reported in polycrystalline bcc metals and alloys. Most of the recrystallized grains with cube orientation appeared in the (001) [100] initially-oriented grains. These features are discussed relative to the inhomogeneous cold-rolled microstructures of columnar crystals..
371. Kaneaki Tsuzaki, Y. Natsume, Y. Kurokawa, T. Maki, Improvement of the shape memory effect in Fe-Mn-Si alloys by the addition of carbon, Scripta Metallurgica et Materiala, 10.1016/0956-716X(92)90213-X, 27, 4, 471-473, 1992.08.
372. Y. Tomota, W. Nakagawara, Kaneaki Tsuzaki, T. Maki, Reversion of stress-induced ε{lunate} martensite and two-way shape memory in Fe-24Mn and Fe-24Mn-6Si alloys, Scripta Metallurgica et Materiala, 10.1016/0956-716X(92)90258-G, 26, 10, 1571-1574, 1992.05.
373. Nobuhiro Tsuji, Kaneaki Tsuzaki, Tadashi Maki, Effect of Initial Orientation on the Cold Rolling Behavior of Solidified Columnar Crystals in a 19% Cr Ferritic Stainless Steel, isij international, 10.2355/isijinternational.32.1319, 32, 12, 1319-1328, 1992.01, The cold rolling behavior of solidified columnar crystals in a 19%Cr ferritic stainless steel has been investigated in order to clarify the effects of the initial orientation of each columnar grain and of the interaction with adjacent grains in a polycrystalline specimen. The specimen showed {001}≪uv0≫ texture before rolling and {001}[110] texture after 70% rolling reduction. It was clearly shown that the rolled microstructure and the crystal rotation by rolling strongly depend on the initial orientation. The 70% rolled microstructure of (001) [100] oriented grains consisted of a large number of fine stringer deformation bands, whereas (001)[110] oriented grains showed a uniform and non-characteristic deformation structure. In the grains having intermediate orientations such as (001)[510]-[320], stringer deformation bands formed near grain boundaries. Both of the (001) [100] and (001) [110] oriented grains maintained their initial orientations even after 70% rolling reduction, while the (001)[510]-[320] oriented grains rotated toward (001) [110]. It was emphasized that the stability of (001)[100] oriented grains against rolling is contrast to the case of (001) [100] oriented bcc single crystals where the crystal rotation toward (001) (110) occurs. This difference was discussed from a viewpoint of the interaction with adjacent grains..
374. Kaneaki Tsuzaki, M. Ikegami, Y. Tomota, Y. Kurokawa, W. Nakagawara, T. Maki, Effect of thermal cycling on the martensitic transformation in an Fe-24Mn-6Si shape memory alloy, Materials Transactions, JIM, 10.2320/matertrans1989.33.263, 33, 3, 263-270, 1992.01, The effect of γ⇔ε cyclic transformation on the subsequent γ→ε martensitic transformation has been studied in an Fe-24%Mn-6%Si shape memory alloy in order to obtain fundamental information on the role of Si in the shape memory effect. The γ→ε martensitic transformation is markedly enhanced by the thermal cycling between 305 K (below Ms) and 573 K (above Af), although the Ms temperature slightly decreases. The amount of ε martensite at 305 K is less than 30% after a solution treatment and increases to 40% after 10 thermal cycles. Microstructural observation demonstrates that the Fe-Mn-Si alloy exhibits a good reproducibility of ε martensite plate formation. Moreover, there are few dislocations in the austenite of a thermal-cycled specimen unlike the case of Fe-Mn binary alloys. These results suggest that the addition of Si plays an important role in the reversible movement of transformation dislocations (i.e., Shockley partial dislocations) during γ⇔ε cyclic transformation. It is concluded that the role of Si in the shape memory effect is to make the movement of partial dislocations reversible as well as to restrict the permanent slip in austenite during a shape change..
375. Kaneaki Tsuzaki, Yuji Matsuzaki, Tadashi Maki, Imao Tamura, Fatigue deformation accompanying dynamic strain aging in a pearlitic eutectoid steel, Materials Science and Engineering A, 10.1016/0921-5093(91)90754-B, 142, 1, 63-70, 1991.08, The effect of dynamic strain aging on the cyclic stress response and the fatigue life in a pearlitic eutectoid steel was studied. The temperature ranges at which dynamic strain aging takes place during strain-controlled fatigue deformation and monotonic tensile deformation were also examined. Marked cyclic hardening due to dynamic strain aging took place at temperatures between 473 and 573 K, whereas cyclic softening occurred at room temperature. At 523 K and at a strain rate of 3.3 × 10-3 s-1, the cyclic hardening was the most prominent and the fatigue life was shorter than that at room temperature by a factor of more than 2. At the given strain rate, dynamic strain aging during fatigue deformation occurred at lower temperatures in comparison with that during tensile deformation..
376. Kaneaki Tsuzaki, Kazuki Fujiwara, Tadashi Maki, Bainite reaction in Fe-Ni-C alloys, Unknown Journal, 10.2320/matertrans1989.32.667, 32, 8, 667-678, 1991.01, In order to clarify the mechanism of bainite reaction, a metallographic study of isothermal-transformation products formed at various temperatures was made in Fe-9% Ni-0.08% C, -0.23% C, and -0.28% C alloys (mass%) where the solute drag-like effect does not occur. TTT diagrams were also examined. In all the alloys, the TTT diagram consists of two C-curves. The kinetic-Bs temperature is observed at 760 K in both of the 0.23 and 0.28% C alloys. In the 0.08% C alloy, the kinetic-Bs is not seen, but a 'bay' appears around 750 K. The microstructure changes markedly below the kinetic-Bs in the 0.23 and 0.28% C alloys. Proeutectoid ferrite and degenerate pearlite form at temperatures above the kinetic-Bs, whereas upper bainite characterized by ferrite laths forming in parallel becomes dominant below the kinetic-Bs. The upper critical temperature for the formation of bainite (i.e., the microstructural-Bs) corresponds to the kinetic-Bs and is hardly affected by carbon content. TEM microstructure of bainitic ferrite is similar to that of the lath martensite. These results strongly suggest that the bainite reaction has its own C-curve and occurs by a displacive mechanism. A model for the bainite reaction as a displacive transformation is proposed; the bainite reaction is the martensitic transformation of which nucleation is assisted by the formation of carbon poor zones in the parent austenite as a result of statistical fluctuation of carbon concentration. This model deduces a prediction that the Bs temperature of Fe-M-C alloys (M: a substitutional element) is independent of carbon content and corresponds to the Ms temperature of the Fe-M binary alloy. It is demonstrated that the Bs temperature of 760 K in the Fe-9% Ni-C alloys is in very good agreement with the Ms temperature of Fe-9% Ni alloys. The incomplete transformation and the Bf temperature of bainite reaction are also discussed..
377. Kaneaki Tsuzaki, Shin ichi Fukasaku, Yo Tomota, Tadashi Maki, Effect of prior deformation of austenite on the γ → ε martensitic transformation in Fe-Mn alloys, Unknown Journal, 10.2320/matertrans1989.32.222, 32, 3, 222-228, 1991.01, The influence of prior deformation of austenite (γ) on the formation of hcp (ε) martensite in Fe-Mn binary alloys during subsequent cooling was investigated. The formation of bcc (α′) lath martensite in deformed austenite was also examined for comparison. In both of the martensitic transformations to α′ in the Fe-9%Mn alloy and to ε in the Fe-16%Mn and Fe-24%Mn alloys, the transformation start temperature (Ms) monotonously decreases with increasing the reduction of austenite at 773 K. It is demonstrated that the decrease in Ms temperature for the γ → ε transformation is much larger than that for the γ → α′ transformation. The Ms temperature of the Fe-24%Mn alloy is lowered over 80 K by 40% reduction and the structure becomes fully austenitic at room temperature, whereas the decrease in Ms temperature of the Fe-9Mn alloy is 16 K. The decrease in Ms temperature by the prior deformation of austenite is quantitatively accounted for by the strengthening of austenite and the extra energy required for the transformation dislocations to advance through forest dislocations in austenite. The marked stabilization of austenite against the γ → ε martensitic transformation is mainly associated with the smaller increase in chemical driving force for the γ → ε transformation with decreasing temperature than that for the γ → α′ transformation..
378. Kaneaki Tsuzaki, Chikara Nakao, Tadashi Maki, Formation temperature of bainitic ferrite in Si-containing steels, Unknown Journal, 10.2320/matertrans1989.32.658, 32, 8, 658-666, 1991.01, A metallographic study of isothermal-transformation products formed at various temperatures was made in 2%Si-1%Mn-0.34%C and 2%Si-1%Mn-0.59%C steels with particular emphasis on the formation mechanism of bainitic ferrite. In the 0.34%C steel, the austenite first decomposes into ferrite and then into pearlite in the later stage of transformation at 973 K (just below the A1 temperature). As decreasing the decomposition temperature, the amount of proeutectoid ferrite decreases and the structure becomes fully pearlitic at 873 K. At lower temperatures of 848 K and 823 K, a mixed structure of pearlite and carbide-free bainitic ferrite is observed. At temperatures below 823 K and above the Ms temperature, pearlite does not form and the structure is fully bainitic. This indicates that carbide-free bainitic ferrite has its own formation temperature range different from that for proeutectoid ferrite. It is also demonstrated that at 823 K both pearlite and carbide-free bainitic ferrite exist in the initial stage of transformation and the amount of both phases increases with increasing the isothermal-holding time, unlike the formation process of proeutectoid ferrite and pearlite at 973 K. A similar competitive formation of pearlite and carbide-free bainitic ferrite is observed at 823 K in the 0.59%C steel. These results strongly suggest that the formation mechanism of bainitic ferrite is different from the diffusional mechanism for proeutectoid ferrite and pearlite. The upper critical temperature for the formation of bainite, i.e., the microstructural-Bs, is around 860 K and hardly changes with carbon content..
379. Kaneaki Tsuzaki, Tadashi Maki, Effect of Transformation Cycling on the ε Martensitic Transformation in Fe-Mn Alloys, ISIJ International, 10.2355/isijinternational.30.666, 30, 8, 666-673, 1990.01, The effect of γ⇆ε cyclic transformation on the subsequent forward and reverse transformations was studied in low carbon Fe-17%Mn and Fe-25%Mn binary alloys. A remarkable decrease in Ms temperature by the cyclic transformation was observed in the Fe-25%Mn alloy. After 7 thermal cycles between 305 and 573 K, Ms decreased by over 60 K and the structure became fully austenitic at room temperature. On the other hand, As and Af temperatures increased with an increase in the number of thermal cycles. As the upper peak temperature of thermal cycling increased, the changes in Ms, As and Af became smaller. The decrease in Ms by the cyclic transformation was smaller in the Fe-17%Mn alloy than in the Fe-25%Mn alloy. A large number of dislocations were observed in the austenite of the Fe-25%Mn alloy after the thermal cycling between 273 and 573 K, although dislocations were scarcely observed before the thermal cycling. The hardness of austenite at 573 K was markedly increased by the thermal cycling between 273 and 573 K. A microstructure memory, which is a phenomenon that martensite forms in the same position where the martensite formed in the last forward transformation, was not observed..
380. Kaneaki Tsuzaki, Hirohisa Matsuyama, Mamoru Nagao, Tadashi Maki, High strain rate superplasticity and role of dynamic recrystallization in duplex stainless steel, Nippon Kinzoku Gakkaishi/Journal of the Japan Institute of Metals, 10.2320/jinstmet1952.54.8_878, 54, 8, 878-887, 1990.01, Superplasticity and microstructural change during deformation of a 25Cr-7Ni-3Mo α/γ duplex stainless steel having a microduplex structure have been studied. The specimen of microduplex structure with fine α and γ grain sizes (approximately 1 μm) showed high strain rate superplasticity at 1273 K. Elongation over 1700% was obtained at a strain rate of 1.7 × 10-2S-1. Even at 1.7 × 10-1S-1 elongation was near 1000%. α/α boundaries were low angle ones before the superplastic deformation and were changed to high angle ones by dynamic recrystallization in the early stage of deformation. During deformation up to 1300% elongation after the recrystallization of the α phase, dislocations were scarcely observed in the whole of the specimen. It was concluded that the grain boundary sliding is a predominant deformation mode during superplastic flow in the present steel and the role of dynamic recrystallization is to make the structure suitable for the grain boundary sliding in the early stage of deformation. The mechanism of dynamic recrystallization was briefly discussed..
381. Kaneaki Tsuzaki, Hirohisa Matsuyama, Mamoru Nagao, Tadashi Maki, High-Strain Rate Superplasticity and Role of Dynamic Recrystallization in a Superplastic Duplex Stainless Steel, Materials Transactions, JIM, 10.2320/matertrans1989.31.983, 31, 11, 983-994, 1990.01, Superplasticity of a 25Cr-7Ni-3Mo ferrite/austenite duplex stainless steel has been studied with particular emphasis on the microstructural change during deformation, in order to clarify roles of dynamic recrystallization in the superplastic deformation. A microduplex structure consisting of fine ferritic subgrains and austenite particles (grain size: 1 μm) is obtained by annealing at 1273 K after 90% cold rolling of a fully ferrite structure. The steel with this fine-grained structure exhibits high-strain rate superplasticity at 1273 K in the ferrite/austenite two-phase region. A maximum elongation over 1700% is obtained at a strain rate of 1.7 x 10-2/s. Even at a higher strain rate of 1.7 x 10-1/s the elongation is near 1000%. Ferrite/ferrite boundaries are low-angle boundaries before the superplastic deformation. These low-angle boundaries are changed to high-angle ones by dynamic recrystallization of the ferrite matrix in the early stage of deformation. During the deformation after the recrystallization of the ferrite matrix, dislocations are scarcely observed in the whole of specimens. It is concluded that grain-boundary sliding is the dominant mode of the superplastic deformation, and that the role of the dynamic recrystallization is to make the fine structure suitable for the grain-boundary sliding in the early stage of deformation. A mechanism of the dynamic recrystallization is briefly discussed..
382. Kaneaki Tsuzaki, Kouji Tanaka, Tadashi Maki, Imao Tamura, Effect of phosphorus on hardenability and isothermal transformation behavior in high-purity 0.35% carbon steels, Tetsu-To-Hagane/Journal of the Iron and Steel Institute of Japan, 10.2355/tetsutohagane1955.75.1_128, 75, 1, 128-135, 1989.01, It was confirmed by Jominy end-quench testing, dilatometric measurement, and metallography that phosphorus increased the hardenability. The Jominy distance parabolically increased with phosphorus content, and the phosphorus effect tended to saturate at high phosphorus content around 0.04%. Metallography showed that the hardenability was limited by the pearlite (degenerate pearlite) transformation at around 820 K. The growth rate of pearlite nodules at 823 K decreased by addition of phosphorus, but the nucleation rate did not. It is concluded that the enhancement of hardenability by phosphorus addition was caused by the retardation of pearlite growth because of the solute drag effect..
383. Kaneaki Tsuzaki, Kenji Yamaguchi, Tadashi Maki, Imao Tamura, alpha yields gamma TRANSFORMATION BEHAVIOR DURING HEATING FROM ( alpha plus gamma ) REGION IN 0. 18% CARBON STEEL., Tetsu-To-Hagane/Journal of the Iron and Steel Institute of Japan, 74, 7, 234-241, 1988.07, The transformation was delayed with increasing an initial amount of proeutectoid alpha before heating. When the amount of proeutectoid alpha before heating was the same, the transformation kinetics was delayed with increases in initial gamma grain size and the gamma yields alpha transformation temperature in the alpha plus gamma region. Microscopic observation showed that transformation proceeded only by the growth of pre-existing gamma , namely, the dissolution of proeutectoid alpha without nucleation of new gamma grains in the proeutectoid alpha particles before heating, the experimental data lay on a straight line and T//0//. //9 increased with increasing particle size..
384. Kaneaki Tsuzaki, T. Maki, I. Tamura, Isothermal character and cooling rate dependence of lath martensitic transformation in Fe-15%Ni alloy, Scripta Metallurgica, 10.1016/0036-9748(87)90160-8, 21, 12, 1693-1698, 1987.01.
385. Kaneaki Tsuzaki, Takashi Hori, Tadashi Maki, Imao Tamura, Dynamic strain aging during fatigue deformation in type 304 austenitic stainless steel, Materials Science and Engineering, 10.1016/0025-5416(83)90107-6, 61, 3, 247-260, 1983.01, The test conditions (i.e. the test temperature and the strain rate) at which dynamic strain aging takes place during strain-controlled fatigue deformation in type 304 austenitic stainless steel were studied. When the strain rate was low (i.e. 5 × 10-4 - 1.7 × 10-3 s-1), the lower critical temperature for the appearance of serrations during fatigue deformation was much lower than that during monotonic tensile deformation. This result was considered to be caused by the longer arrest time of dislocations in fatigue deformation in comparison with that in monotonic tensile deformation. In such an arrest time the dislocations are locked by solute atoms. The longer arrest time in fatigue deformation is considered to be closely related to the cell-shuttling motion of dislocations in the saturated stress stage of the fatigue process..
386. Kaneaki Tsuzaki, Eizaburo Nakanishi, Tadashi Maki, Imao Tamura, LOW-CYCLE FATIGUE BEHAVIOR IN METASTABLE AUSTENITIC STEEL ACCOMPANYING DEFORMATION-INDUCED MARTENSITIC TRANSFORMATION., Transactions of the Iron and Steel Institute of Japan, 10.2355/isijinternational1966.23.834, 23, 10, 834-841, 1983.01, Sheet of type 304 austenitic stainless steel are used as the first wall (membrane) of LNG storage tanks. Since cyclic stress and strain are caused by the change in gas and liquid pressure due to variation of the liquid surface level, the low-cycle fatigue is an important problem for design. The effect of the deformation-induced martensitic transformation on strain-controlled low-cycle fatigue behavior in type 304 stainless steel at room temperature has been studied. The fatigue behavior in type 310 has been examined for comparison..
387. Kaneaki Tsuzaki, T. Maki, I. Tamura, KINETICS OF alpha prime -MARTENSITE FORMATION DURING FATIGUE DEFORMATION IN METASTABLE AUSTENITIC STAINLESS STEEL., Journal de Physique (Paris), Colloque, 43, 1982.12.
388. I. Tamura, Kaneaki Tsuzaki, T. Maki, MORPHOLOGY OF LATH MARTENSITE FORMED FROM DEFORMED AUSTENITE IN 18% Ni MARAGING STEEL., Journal de Physique (Paris), Colloque, 43, 1982.12.
389. M. Tokizane, N. Matsumura, Kaneaki Tsuzaki, T. Maki, I. Tamura, RECRYSTALLIZATION AND FORMATION OF AUSTENITE IN DEFORMED LATH MARTENSITIC STRUCTURE OF LOW CARBON STEELS., Metallurgical transactions. A, Physical metallurgy and materials science, 10.1007/BF02642875, 13 A, 8, 1379-1388, 1982.01, The investigation studied the effects of deformation of lath martensite on recrystallization and austenite formation from the point of view of austenite grain refinement. The recrystallization of lath martensite, which influences the subsequent alpha yields gamma transformation, was first examined, then the interrelationship between the recrystallization of lath martensite and the formation of austenite during austenitizing was studied..
390. Kaneaki Tsuzaki, Tadashi Maki, EFFECT OF COOLING RATE ON THE MORPHOLOGY OF LATH MARTENSITE IN Fe-Ni ALLOYS., Nippon Kinzoku Gakkaishi/Journal of the Japan Institute of Metals, 45, 2, 126-134, 1981.02, 18%Ni maraging steel, Fe-18%Ni and Fe-24%Ni alloys were cooled at different cooling rates using iced brine, water, oil, air or furnace cooling. The number of banded regions with parallel laths increases with increase in cooling rate. The final block width and packet size becomes smaller..