Kyushu University Academic Staff Educational and Research Activities Database
List of Papers
Kaneaki Tsuzaki Last modified date:2018.07.26

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

1. Motomichi Koyamaa, Daisuke Yamasakia, 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,, 228, 273-276, 2018.10.
2. 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..
3. 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..
4. 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..
5. 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..
6. 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..
7. 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..
8. 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..
9. 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..
10. 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..
11. 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.
12. 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..
13. 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..
14. 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..
15. 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,, 341-347, 2018.01.
16. 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..
17. 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,, 7, 391-398, 2017.12.
18. 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..
19. 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..
20. 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..
21. 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..
22. 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.
23. 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.
24. 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..
25. 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.
26. 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..
27. 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..
28. 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..
29. 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.
30. 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.
31. 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.
32. 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.
33. 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.
34. 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.
35. 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.
36. 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.
37. 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.
38. 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.
39. 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.
40. 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.
41. 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.
42. 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.
43. Combined multi-scale analyses on strain/damage/microstructure in steel: Example of damage evolution associated with ε-martensitic transformation.
44. 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.
45. 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.
46. 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.
47. 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.
48. 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.
49. 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.
50. 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.
51. Effects of Si on Tensile Properties Associated with
Deformation-Induced ε-Martensitic Transformation in High Mn Austenitic Alloys.
52. 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.
53. 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.
54. 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.
55. 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.
56. 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.
57. 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.
58. 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.
59. Hidetoshi Somekawa, Tadanobu Inoue, Kaneaki Tsuzaki, Effect of deformation twin on toughness in magnesium binary alloys, Philosophical Magazine , 95, 23, 2513-2526, 2015.05.
60. 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.
61. 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.
62. 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.
63. 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.
64. 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.
65. 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.
66. 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.
67. 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.
68. Effect of Additional Boron Amount on the Surface Roughness after Lathe Turning of h-BN Dispersed Type 304 Stainless Steel.
69. Hidetoshi Somekawa, Alok Singh, Tadanobu Inoue, Kaneaki Tsuzaki, Crack propagation behaviour in magnesium binary alloys, Philosophical Magazin, 2014.11.
70. Deformation Twinning Behavior of Twinning-Induced Plasticity Steels with Different Carbon Concentrations -Part 1: Analyses by Atomic Force Microscopy and Electron Backscatter Diffraction Measurements-.
71. Deformation Twinning Behavior of Twinning-Induced Plasticity Steels with Different Carbon Concentrations -Part 2: Proposal of Dynamic Strain Aging-Assisted Deformation Twinning-.
72. First-Principles Calculation of the Effects of Carbon on Tetragonality and Magnetic Moment of BCC-Fe.
73. 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.
74. Factors Affecting Static Strain Aging Under Stress at Room Temperature in a Fe-Mn-C Twinning-Induced Plasticity Steel.
75. Effects of Static and Dynamic Strain Aging on Hydrogen Embrittlement in TWIP Steels Containing Al.
76. 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.
77. Measurement of Martensitic Transformation of 9%Ni Steel during Welding Using ESPI System.
78. Hydrogen Embrittlement in Al-Added Twinning-Induced Plasticity Steels Evaluated by Tensile Tests during Hydrogen Charging.
79. 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.
80. 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.
81. 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.
82. 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.
83. K. Satoru, K. Kimura, Kaneaki Tsuzaki, Interphase precipitation of Fe2Hf Laves phase in a Fe-9Cr/Fe-9Cr-Hf diffusion couple, Intermetallics, 2014.01.
84. 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..
85. 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..
86. 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. .
87. 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..
88. 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. .
89. 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. .
90. 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..
91. 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..
92. 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. .
93. 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. .
94. 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..
95. 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). .
96. 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. .
97. 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. .
98. 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. .
99. 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. .
100. 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. .
101. 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. .
102. 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..
103. 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. .
104. 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. .
105. 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. .
106. 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. .
107. 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. .
108. 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..
109. 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. .
110. 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. .
111. 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. .
112. 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. .
113. 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. .
114. 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. .
115. 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. .
116. 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..
117. 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. .
118. 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. .
119. 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. .
120. 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. .
121. 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. .
122. 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..
123. 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. .
124. 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..
125. 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. .
126. 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. .
127. 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..
128. 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. .