Kyushu University Academic Staff Educational and Research Activities Database
List of Papers
Motomichi Koyama Last modified date:2018.05.16

Assistant Professor / Faculty of Engineering / Department of Mechanical Engineering / Faculty of Engineering

1. 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,, 52, 13, 7868-7882, 2017.07, We investigated the effects of solute carbon concentration on the mechanical properties of Fe–19Cr–8Ni–0.05C and Fe–19Cr–8Ni–0.14C metastable austenitic steels. These steels showed an FCC(γ) → HCP(ε) → BCC(α′) martensitic transformation, resulting in transformation-induced plasticity (TRIP). The presence of excess solute carbon reduced the transformability because of an increase in the austenite stability. However, the work hardening capability was enhanced by a combined effect of the TRIP and dynamic strain aging (DSA). DSA requires a high diffusivity of carbon. Thus, the FCC (low diffusivity) to BCC (high diffusivity) transformation favors DSA. The hardening capability of BCC-martensite per volume is enhanced by the dislocation pinning and solution hardening effect of the carbon atmosphere, despite a decrease in the transformation rate per strain by carbon addition. Moreover, carbon addition stabilizes the deformation-induced HCP-martensite against the BCC-martensite, improving the hardening capability of the HCP-martensite through suppression of the window effect, which affects the plastic accommodation mechanism. According to our study, the steel with a low carbon content demonstrated extraordinary work hardening rates owing to a high transformation rate per strain. In contrast, the steel with a high carbon content showed sustained and high work hardening rates because of DSA. Both the steels showed approximately the same tensile strength, but completely different work hardening behavior..
2. 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,, 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..
3. Nobuyuki Yoshimura, Kohsaku Ushioda, Mitsuharu Yonemura, Motomichi Koyama, Masaki Tanaka, Hiroshi Noguchi, Effect of the state of carbon on ductility in Fe-0.017mass%C ferritic steel, Materials Science & Engineering A: Structural Materials: Properties, Microstructure and Processing,, 701, 120-128, 2017.07, The influence of the state of carbon on ductility in Fe-0.017mass%C alloy was systematically investigated. Moreover, the changes in dislocation density, dislocation substructure, and dislocation character with tensile strain were evaluated using X-ray line profile analysis together with transmission electron microscopy. The solute carbon maintains uniform elongation while simultaneously increasing the ultimate tensile strength by increasing the work-hardening rate; however, post-uniform elongation is significantly impaired by the occurrence of dynamic strain aging (DSA). The coarse intergranular cementite leads to superior uniform elongation; however, post-uniform elongation deteriorates presumably owing to the void formation in the cementite/matrix interface or the fracture of cementite. Meanwhile, the fine transgranular cementite increases the yield strength, thus impairing the uniform elongation; however, it maintains post-uniform elongation. This is because small cementites less than about 1 µm in size do not deteriorate the post-uniform elongation. The solute carbon prominently increases the dislocation density, particularly the edge dislocation density, with tensile strain owing to DSA, leading to the prohibition of dislocation cell structure formation..
4. Motomichi Koyama, Yosuke Onishi, Hiroshi Noguchi, Characteristics of hydrogen-assisted intergranular fatigue crack growth in interstitial-free steel
role of plastic strain localization, International Journal of Fracture,, 206, 1, 123-130, 2017.07, The behavior of intergranular fatigue crack growth in an interstitial-free (IF) steel in a hydrogen environment was investigated at different frequencies. Focusing on the plastic strain localization, we observed details of the striation-like feature on the intergranular fracture surface, slip behavior around microvoids, and crystallographic orientation gradient underneath the fracture surface. It was determined that the intergranular fatigue crack growth mechanism in the IF steel is microvoid formation at the crack tip and subsequent coalescence with the crack. Moreover, it was found that the grain boundaries, acting as propagation paths, suffer from pre-damage arising from plastic strain localization near the grain boundaries even before the main crack propagates to a certain location. Therefore, fatigue cracks in a hydrogen environment easily propagate to the grain boundaries. The frequency dependence of fatigue crack growth in the hydrogen environment is significantly smaller than that in a low carbon steel, probably because of the frequency dependence of the pre-damage evolution behavior..
5. 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,, 90, 193-202, 2017.08, Non-propagating fatigue crack lengths were analytically calculated under stress control conditions using plasticity-induced crack closure analysis with the Dugdale model. In addition, a non-dimensionalization method was applied in terms of the Burgers vector and a monotonic plastic zone size under small-scale yielding conditions, which was validated for various initial crack lengths and material properties. When the yield strength was increased, the non-propagating fatigue crack lengths were found to increase for a short crack and decrease for a long crack. The non-dimensionalization enabled the analytical derivation of a generalized non-propagating fatigue crack length, which can be utilized for fatigue designs. The material property controlling the threshold stress intensity factor range of mechanically and physically short-crack was discussed..
6. Bochuan Li, Motomichi Koyama, Shigeru Hamada, Hiroshi Noguchi, Threshold stress intensity factor range of a mechanically-long and microstructually-short crack perpendicular to an interface with plastic mismatch, Engineering Fracture Mechanics,, 182, 287-302, 2017.09, In this study, we investigated long fatigue crack growth perpendicular to a soft-hard material interface comprising the same Young's moduli but different yield strengths. We determined the degree of fatigue crack growth retardation attributable to the interface under the constant stress intensity factor range (ΔK) using the plasticity-induced crack closure analysis with the Dugdale model. We subsequently determined the threshold stress intensity factor range (ΔKth) under a constant stress amplitude (σa). Under a constant ΔK, the retardation degree was primary dependent on the two materials’ yield strength ratio. Under a constant σa, ΔKth was dependent on both the yield strength ratio and the distance between initial crack tip and interface..
7. 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,, 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..
8. 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,, 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..
9. 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,, 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..
10. 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,, 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..
11. Kishan Habib, Motomichi Koyama, Toshihiro Tsuchiyma, Hiroshi Noguchi, Fatigue crack non-propagation assisted by nitrogen-enhanced dislocation planarity in austenitic stainless steels, International Journal of Fatigue,, 104, 158-170, 2017.11, Rotating bending fatigue tests were conducted to assess the fatigue crack propagation behavior of the Fe-25Cr-1N and Fe-18Cr-14Ni austenitic steels in terms of the microstructure, crack propagation paths, and non-propagating fatigue crack characteristics. The Fe-25Cr-1N steel exhibited a non-propagating fatigue crack at the fatigue limit (310 MPa), but this did not occur in the Fe-18Cr-14Ni steel at the fatigue limit (110 MPa). The non-propagating fatigue crack observed in the Fe-25Cr-1N steel was produced by roughness-induced crack closure. This phenomenon was caused by the enhanced planar dislocation and high dislocation pile-up stress resulting from the suppression of cross-slip, which inhibited the dislocation emission from the crack tip. The Fe-25Cr-1N steel exhibited a lower fatigue crack growth rate than the Fe-18Cr-14Ni steel because of the enhanced dislocation planarity produced by the Cr-N interaction. The Cr-N interaction affected the fatigue crack growth behavior as follows. The short crack region exhibited a planar glide dislocation pattern, but multiple slip systems were activated as the crack lengthened. As the dislocation pattern remained planar on each slip plane, the crack propagation occurred along the {1 1 1}γ slip planes, even in the long crack. Moreover, the dislocation pile-up at the grain boundaries caused grain boundary subcracks, which can induce crack toughening through mechanisms such as stress redistribution. These positive effects contributed to the lower fatigue crack growth rate in the Fe-25Cr-1N steel than the Fe-18Cr-14Ni steel..
12. Motomichi Koyama, Yusaku Shimomura, Aya Chiba, Eiji Akiyama, Kaneaki Tsuzaki, Room-temperature blue brittleness of Fe-Mn-C austenitic steels, Scripta Materialia,, 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..
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,, 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. Daisuke SaSaki, Motomichi Koyama, Hiroshi Noguchi, Influence of stress re-distribution on hydrogen-induced fatigue crack propagation, Tetsu-To-Hagane/Journal of the Iron and Steel Institute of Japan,, 104, 1, 46-53, 2018.01, In order to clarify influence of stress re-distribution effect on hydrogen-induced fatigue crack propagation, we investigated fatigue crack propagation rates and brittle-like fracture ratio. The experiments were conducted in nitrogen and hydrogen gas atmosphere with ferrite-pearlite steels having different pearlite ratio, respectively. The crack propagation rates and the brittle-like fracture ratio decreased as pearlite ratio increased. To explain the changes of crack propagation rates and fracture ratio, we proposed that the stress re-distribution effect causing stress and strain relaxation at a crack tip contributes to suppression of the hydrogen-induced fatigue crack propagation. As a verification, finite element methods were operated with models having different width of the hard phase and different distance between a crack tip and a hard phase in plane stress and strain conditions, respectively. The finite element method analysis showed that stress re-distribution effect was smaller in plane strain condition than that in plane stress condition, indicating that a large hardness difference is crucial in plane stress condition to suppress the hydrogen-induced fatigue crack propagation..
15. Abbas Mohammadi, Motomichi Koyama, Gregory Gerstein, Hans Jürgen Maier, Hiroshi Noguchi, Hydrogen-assisted failure in a bimodal twinning-induced plasticity steel
Delamination events and damage evolution, International Journal of Hydrogen Energy,, 43, 4, 2492-2502, 2018.01, The effect of the bimodal grain size distribution on the hydrogen susceptibility of a high-Mn fully austenitic twinning-induced plasticity (TWIP) steel was investigated by tensile testing under ongoing electrochemical hydrogen charging. Observation of the surface microstructure of the hydrogen-charged specimen yielded a correlation between the microstructure, crack initiation sites, and crack propagation path. The observed embrittlement arose from crack initiation/propagation along the grain and twin boundaries and delamination governed crack growth. In the present bimodal TWIP steel, the fine grained regions mostly showed intergranular cracking along the grain boundaries between the fine and coarse grains. By contrast, the coarse grained region exhibited transgranular cracking along the twin boundaries. The delamination cracking phenomena is rationalized by the evident nucleation, growth, and coalescence of microvoids in the tensile direction. The results reveal that the bimodal grain size distribution of TWIP steel plays a major role in hydrogen-assisted cracking and the evolution of delamination-related damage..
16. 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,, 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..
17. 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,, 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..
18. 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,, 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..
19. 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,, 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..
20. 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,, 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..
21. Bochuan Li, Motomichi Koyama, Eisaku Sakurada, Nobuyuki Yoshimura, Kohsaku Ushioda, Hiroshi Noguchi, Temperature dependence of transgranular fatigue crack resistance in interstitial-free steel and Fe-C steels with supersaturated carbon
Effects of dynamic strain aging and dynamic precipitation, International Journal of Fatigue,, 110, 1-9, 2018.05, Tension tests at different temperatures were conducted for two ferritic steels: Fe-0.017C with 0.017% solute carbon content and interstitial-free (IF) steels. It is verified the occurrence of dynamic strain aging (DSA) in Fe-0.017C at high temperatures. Tension tests for the Fe-0.017C, following corresponding heating treatments, demonstrated the transition process of dynamic precipitation from under-aging to over-aging. Moreover, the threshold stress intensity factor ranges (ΔKth) of a small crack for these two steels both decrease with temperature growth. However, the ΔKth of Fe-0.017C was always larger than that of IF steel owing to solid solution hardening and DSA at 293 and 333 K and roughness-induced crack closure and precipitation hardening at 433 K..
22. Kenshiro Ichii, Motomichi Koyama, Cemal Cem Tasan, Kaneaki Tsuzaki, Comparative study of hydrogen embrittlement in stable and metastable high-entropy alloys, Scripta Materialia,, 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. 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,, 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..
24. 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,, 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..
25. K. Habib, Motomichi Koyama, Hiroshi Noguchi, Impact of Mn-C couples on fatigue crack growth in austenitic steels: the attractive atomic interaction is negative or positive?, International Journal of Fatigue, 99, 1-12, 2017.01.
26. B. Li, Motomichi Koyama, Eisaku Sakurada, N. Yoshimura, K. Ushioda, Hiroshi Noguchi, Underlying interstitial carbon concentration dependence of transgranular fatigue crack resistance in Fe-C ferritic steels: the kinetic effect viewpoint, International Journal of Fatigue, 98, 101-110, 2017.01.
27. Motomichi Koyama, H. 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, 72, 34-47, 2017.02.
28. 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, 129, 48-51, 2017.02.
29. Motomichi Koyama, Zhao Zhang, Meimei Wang, Dirk Ponge, Dierk Raabe, Kaneaki Tsuzaki, Hiroshi Noguchi, Cemal Cem Tasan, Bone-like crack resistance in hierarchical metastable nano-laminate steels, Science, 355, 1055-1057, 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..
30. Takuro Ogawa, Motomichi Koyama, 野口 博司, Suppression Mechanism of strain-age-hardening in carbon steel associated with hydrogen uptake, ISIJ International, Accepted, 2016.10, We investigated the suppression mechanism of strain-age-hardening in a ferritic carbon steel
associated with hydrogen uptake. We considered hydrogen-related three factors suppressing the
strain aging: 1) solution softening, for instance, arising from a re.
31. Motomichi Koyama, Yachen Yu, Jia-Xi Zhou, NobuyukiYoshimura, EisakuSakurada, KohsakuUshioda, 野口 博司, Elucidation oftheeffectsofcementitemorphologyondamage formation duringmonotonicandcyclictensioninbinarylowcarbon steelsusinginsitucharacterization, Materials Science & EngineeringA, 667, 358-367, 2016.05, The effectsofthemorphologyanddistributionofcementiteondamageformationwerestudiedusing
in situscanningelectronmicroscopyundermonotonicandcyclictension.Toinvestigatetheeffectsofthe
morphology/distribution ofcementite,intergranularcementiteprecipitation(ICP).
32. 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, Accepted, 2016.06, Hydrogen desorption associated with thermally induced α′ and ε martensitic transformations was characterized by cryogenic
thermal desorption spectroscopy (C-TDS) during cooling. The utilization of a cooling process during C-TDS measurements
revealed that .
33. Yun-Byum Ju, Motomichi Koyama, Takahiro Sawaguchi, Kaneaki Tsuzaki, 野口 博司, In situ microscopic observations of low-cycle fatigue-crack propagation in high-Mn austenitic alloys with deformation-induced ε-martensitic transformation, Acta Materialia, 112, 326-336, 2016.04, In this study, the microstructural changes in Fee30Mne6Al, Fee30Mne4Sie2Al, and Fee30Mne6Si
alloys that were subjected to bending fatigue tests with a total strain amplitude of 0.7% were observed in
situ. The Fee30Mne4Sie2Al and Fee30Mne6Si alloys exhibit.
34. Yasuaki Hamano, Motomichi Koyama, Shigeru HAMADA, 野口 博司, Notch Sensitivity of the fatigue limit in high-strength steel, ISIJ International, 56, Accepted, 2016.10.
35. 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 e-Martensite, and Mn Concentration, METALLURGICAL AND MATERIALS TRANSACTIONS A, 47A, 2656-2673, 2016.06, To obtain a basic understanding of hydrogen embrittlement associated with e-martensite, we
investigated the tensile behavior of binary Fe-Mn alloys with high Mn content under cathodic
hydrogen charging. We used Fe-20Mn, Fe-28Mn, Fe-32Mn, and Fe-40Mn alloy.
36. Yosuke Onishi, Motomichi Koyama, Daisuke Sasaki, 野口 博司, Characteristic fatigue crack growth behavior of low carbon steel under low-pressure hydrogen gas atmosphere in an ultra-low frequency, ISIJ International, 56, 5, 855-860, 2016.05, In order to clarify an influence of hydrogen on the fatigue crack propagation in ultra-low frequency
region, we investigated the crack propagation rates of S10C at different frequencies in hydrogen and
nitrogen atmospheres. In the low-pressure hydrogen ga.
37. Bochuan Li, Motomichi Koyama, Eisaku Sakurada, Nobuyuki Yoshimura, Kohsaku Ushioda, 野口 博司, Potential resistance to transgranular fatigue crack growth of Fe–C alloy with a supersaturated carbon clarified through FIB micro-notching technique, International Journal of Fatigue, 87, 1-5, 2016.01, The threshold stress intensity factor range (DKth) of water-quenched Fe–0.017C (wt.%) fully ferritic steel
was determined using room-temperature fatigue tests on micro-notched specimens. The experimentally
determined DKth was approximately 40.5% higher th.
38. Motomichi Koyama, Cemal Cem Tasan, Tatsuya Nagashima, Eiji Akiyama, Dierk Raabe, Kaneaki Tsuzaki, Hydrogen-assisted damage in austenite/martensite dual-phase steel, Philosophical Magazine Letters, 96, 9-18, 2016.01, For understanding the underlying hydrogen embrittlement
mechanism in transformation-induced plasticity steels, the process
of damage evolution in a model austenite/martensite dual-phase
microstructure following hydrogenation was investigated through
39. 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, 2016.01, We investigated the fatigue crack propagation associated with a martensitic transformation from the facecentered
cubic to the hexagonal close-packed phase in a Fe–15Mn–10Cr–8Ni–4Si austenitic alloy with and
without hydrogen exposure (pre-charging in hydro.
40. 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, 162, C638-C647, 2015.12, The hydrogen distribution in a hydrogen-charged Fe-18Mn-1.2C (wt%) twinning-induced plasticity austenitic steel was studied by
Scanning Kelvin Probe Force Microscopy (SKPFM). We observed that 1–2 days after the hydrogen-charging, hydrogen showed a
higher .
41. Zhou-Jia Xi, Motomichi Koyama, Yuichi Yoshida, Nobuyuki Yoshimura, Kohsaku Ushioda, 野口 博司, Effects of cementite morphology on short-fatigue-crack propagation in binary Fe–C steel, Philosophical Magazine Letters, 95, 384-391, 2015.07, The effects of cementite morphology on short-fatigue-crack propagation were
investigated by subjecting smooth steel specimens to rotating-bending fatigue
tests. Intergranular and transgranular cementite precipitation steels free of
other phases were prepa.
42. Motomichi Koyama, Zhou-Jia XI, Yuichi YOSHIDA, Nobuyuki YOSHIMURA, Kohsaku USHIODA, 野口 博司, Intergranular Fatigue Crack Initiation and Its Associated Small Fatigue Crack Propagation in Water-quenched Fe–C Fully Ferritic Steel, ISIJ International, 55, 11, 2463-2468, 2015.11, The fatigue crack initiation and propagation behavior of a water-quenched binary Fe–C fully ferritic steel
was investigated though rotating-bending fatigue testing. Intergranular and transgranular crack initiation
and propagation were observed. The interg.
43. 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, 2015.10, The effects of ε-martensite and Mn on hydrogen uptake and
desorption were investigated through cryogenic thermal desorption
analysis. Increasing Mn content promoted hydrogen uptake, and
reverse transformation from ε to γ phases induced hydrogen
44. Daisuke Sasaki, Motomichi Koyama, Kenji Higashida, Kaneaki Tsuzaki, 野口 博司, Effects of hydrogen-altered yielding and work hardening on plastic-zone evolution:A finite-element analysis, International journal o f hydrogen energy, 40, 9825-9837, 2015.06, In the present paper, finite-element analysis of a cracked specimen was conducted using a
unified model for the elasticeplastic deformation and hydrogen diffusion. We considered
the effects of the hydrogen-reduced yielding strength and work-hardening coef.
45. Daisuke Sasaki, Motomichi Koyama, 野口 博司, Factorsaffectinghydrogen-assistedcrackinginacommercialtempered martensiticsteel:Mnsegregation, MnS, andthestressstatearound abnormal cracks, MaterialsScience&EngineeringA, 640, 72-81, 2015.05, The purposeofthispaperistorevealthedominantfactorsaffectingtensilefractureunderahydrogen
gas atmosphere.Tensiletestswereconductedinhydrogengaswithcircumferentially-notchedspecimens
of acommercialtemperedmartensiticsteel.Twospecimenswereexposedtohydrogenga.
46. Meimei Wang, Cemal Cem Tasan, Motomichi Koyama, Dirk Ponge, Dierk Raabe, Enhancing Hydrogen Embrittlement Resistance of Lath Martensite by Introducing Nano-Films of Interlath Austenite, METALLURGICAL AND MATERIALS TRANSACTIONS A, 47, 3797-3802, 2016.06, Partial reversion of interlath austenite nano-films isinvestigated as a potential remedy for hydrogen embrittlement susceptibility of martensitic steels. We
conducted uniaxial tensile tests on hydrogen-free and pre-charged medium-Mn transformation-induce.
47. Huichao Li, Motomichi Koyama, Takahiro Sawaguchi, Kaneaki Tsuzaki, 野口 博司, Importance of crack-propagation-induced HCP-martensite in strain-controlled low cycle fatigue of high Mn austenitic steel, Philosophical Magazine Letters, 95, 303-311, 2015.05, Importance of crack-propagation-induced ε-martensite in strain-controlled low-cycle fatigue of high-Mn austenitic steel.
48. Daisuke Sasaki, Motomichi Koyama, Shigeru HAMADA, Hiroshi Noguchi, Tensile properties of precracked tempered martensitic steel specimens tested at ultralow strain rates in high-pressure hydrogen atmosphere, Philosophical Magazine letters, 95, 260-268, 2015.05, Tensile properties of precracked tempered martensitic steel specimens tested at ultralow strain rates in high-pressure hydrogen atmosphere.
49. Kensuke Yamada, Motomichi Koyama, 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, We investigated the tensile properties of Fe–30Mn–(6 x)Si–xAl (in wt.%, x = 0, 2, 3, 6) austenitic alloys precharged with hydrogen gas at
10 MPa. The Fe–30Mn–6Al alloy showed hydrogen-improved elongation through enhancement of work hardening capacity in.
50. 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, 55, 8, 1754-1761, 2015.08.
51. Motomichi Koyama, Takahiro Sawaguchi, Kaneaki Tsuzaki, Deformation twinning behavior of twinning-induced plasticity steels with different carbon concentrations -Part 1: Analyses by atomic force microscopy and electron backscatter diffraction measurements-, ISIJ International, 55, 8, 1747-1753, 2015.08.
52. Motomichi Koyama, Takahiro Sawaguchi, Kaneaki Tsuzaki, Effects of Si on tensile properties associated with deformation-induced ε-martensitic transformation in high Mn austenitic alloys, Materials Transactions, 56, 6, 819-825, 2015.06.
53. Atsushi Nishimoto, Motomichi Koyama, Shigeru Yamato, Yasuji Oda, Torhu Awane, Hiroshi Noguchi, Detection of charged hydrogen in ferritic steel through cryogenic secondary ion mass spectrometry, ISIJ International, 55, 335, 2015.01, Hydrogen uptake in a ferritic steel was investigated through secondary ion mass spectrometry (SIMS) at 83 K, where hydrogen diffusion is sufficiently suppressed. Additionally, the SIMS was operated with cold trap and Si sputtering to reduce the back ground effect. Thanks to the suppression of hydrogen diffusion during the measurements, the cryogenic SIMS could demonstrate reproducible results which showed a significant difference in hydrogen content between hydrogen-charged and uncharged specimens. Namely, hydrogen in the ferritic steel was successfully detected similarly to austenitic steels..
54. Shigeru HAMADA, Tomoya Fujisawa, Motomichi Koyama, Norimitsu Koga, Nobuo Nakada, Toshihiro Tsuchiyama, Masaharu Ueda, Hiroshi Noguchi, Strain mapping with high spatial resolution across a wide observation range by digital image correlation on plastic replicas, MATERIALS CHARACTERIZATION, 10.1016/j.matchar.2014.10.010, 98, 140-146, 2014.12.
55. Mayu Muramatsu, Motomichi Koyama, Ikumu Watanabe, Tensile testing with cyclic strain holding to analyze dynamic recrystallization of pure lead, Advances in Materials Science and Engineering, Accepted, 2014.05, In this paper, we analyzed the dynamic recrystallization of pure lead by tensile testing with cyclic strain holding at room temperature. The specimens were held at an identical strain and subsequently reloaded, providing the strength before and after the strain holding process. The difference in strength enables factors affecting dynamic recrystallization behavior to be analyzed through mechanical testing. For instance, the effects of strain rate on dynamic recrystallization were analyzed by comparing the results obtained from tensile tests with and without strain holding. This experimental technique demonstrated some parts of contribution of elastic strain, dynamic recovery, dynamic recrystallization, and necking to stress strain responses..
56. Motomichi Koyama, Cemal Cem Tasan, Eiji Akiyama, Kaneaki Tsuzaki, Dierk Raabe, Hydrogen-assisted decohesion and localized plasticity in dual phase steel , Acta Materialia, 10.1016/j.actamat.2014.01.048, 70, 174-187, 2014.02, Hydrogen embrittlement affects high strength ferrite/martensite dual phase (DP) steels. The associated micro-mechanisms leading to failure have not been fully clarified yet. Here we present a quantitative micro-mechanical analysis of the microstructural damage phenomena in a model DP steel in the presence of hydrogen. A high-resolution scanning electron microscopy based damage quantification technique has been employed to identify strain regimes where damage nucleation and damage growth take place, both, with and without hydrogen pre-charging. The mechanisms corresponding to these regimes have been investigated by employing post-mortem electron channeling contrast imaging and electron backscatter diffraction analyses, as well as additional in-situ deformation experiments. The results reveal that damage nucleation mechanism (i.e. martensite decohesion) and the damage growth mechanisms (e.g. interface decohesion) are both promoted by hydrogen, while the crack arresting capability of the ferrite is significantly reduced. The observations are discussed on the basis of the hydrogen-enhanced decohesion (HEDE) and hydrogen-enhanced localized plasticity (HELP) mechanisms. We discuss corresponding microstructure design strategies for better hydrogen-related damage tolerance of DP steels..
57. Motomichi Koyama, Hauke Springer, Sergiy V. Merzlikin, Kaneaki Tsuzaki, Eiji Akiyama, Dierk Raabe, 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, 4634-4646, 2014.01, Hydrogen embrittlement of a precipitation-hardened Fe-26Mn-11Al-1.2C (wt.%) austenitic steel was examined by tensile testing under hydrogen charging and thermal desorption analysis. While the high strength of the alloy (>1 GPa) was not affected, hydrogen charging reduced the engineering tensile elongation from 44 to only 5 %. Hydrogen-assisted cracking mechanisms were studied via the joint use of electron backscatter diffraction analysis and orientation-optimized electron channeling contrast imaging. The observed embrittlement was mainly due to two mechanisms, namely, grain boundary triple junction cracking and slip-localization-induced intergranular cracking along micro-voids formed on grain boundaries. Grain boundary triple junction cracking occurs preferentially, while the microscopically ductile slip-localization-induced intergranular cracking assists crack growth during plastic deformation resulting in macroscopic brittle fracture appearance. .
58. Motomichi Koyama, Eiji Akiyama, Takahiro Sawaguchi, Kazuyuki Ogawa, Irina V. Kireeva, Yuriy I. Chumlyakov, Kaneaki Tsuzaki, 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. .
59. Motomichi Koyama, Eiji Akiyama, Kaneaki Tsuzaki, Effects of static and dynamic strain aging on hydrogen embrittlement in twip steels containing Al, ISIJ International , 10.2355/isijinternational.53.1268, 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. .
60. Motomichi Koyama, Eiji Akiyama, 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, We investigated the factors affecting static strain aging under stress in a Fe-22Mn-0.6C twinninginduced plasticity steel at room temperature. The magnitude of strengthening by the static strain aging was estimated by tensile strain holding and subsequent re-loading. Strain holding time, pre-strain, strain rate, external stress, and diffusible hydrogen content were varied to clarify their effects on static strain aging, and the present static strain aging was found to be affected by all of these factors. In this paper, we show the phenomenological laws of the relationship among the factors and the stress increase due to the static strain aging. .
61. Motomichi Koyama, Taekyung Lee, Chong Soo Lee, Kaneaki Tsuzaki, Grain refinement effect on cryogenic tensile ductility in a Fe-Mn-C twinning-induced plasticity steel, Materials and 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. .
62. Motomichi Koyama, Eiji Akiyama, Kaneaki Tsuzaki, Dierk Raabe, 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. .
63. Motomichi Koyama, Takahiro Sawaguchi, Kaneaki Tsuzaki, 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, 2013.03, 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..
64. Motomichi Koyama, Takahiro Sawaguchi, Kaneaki Tsuzaki, TWIP Effect and plastic instability condition in an Fe-Mn-C austenitic steel, ISIJ International , 10.2355/isijinternational.53.323, 53, 2, 323-329, 2012.02, 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. .
65. Motomichi Koyama, Eiji Akiyama, Kaneaki Tsuzaki, Hydrogen embrittlement in Al-added twinning-induced plasticity steels evaluated by tensile tests during hydrogen charging, ISIJ International , 10.2355/isijinternational.52.2283, 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..
66. Motomichi Koyama, Takahiro Sawaguchi, Kaneaki Tsuzaki, Premature fracture mechanism in an Fe-Mn-C austenitic steel, Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science , 10.1007/s11661-012-1220-7, 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. .
67. Motomichi Koyama, Takahiro Sawaguchi, Kaneaki Tsuzaki, Effect of deformation temperature on tensile properties in a pre-cooled Fe-Mn-C austenitic steel, Materials Science and Engineering A , 10.1016/j.msea.2012.06.095, 556, 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..
68. Motomichi Koyama, Takahiro Sawaguchi, Kaneaki Tsuzaki, Selective appearance of epsilon-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. .
69. 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..
70. 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. .
71. Taekyung Lee, Motomichi Koyama, Kaneaki Tsuzaki, YouHwan 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, 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. .
72. 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, 2012.03, 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..
73. 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. .
74. 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..
75. Motomichi Koyama, Takahiro Sawaguchi, Kaneaki Tsuzaki, Quasi-cleavage fracture along annealing twin boundaries in a Fe-Mn-C austenitic steel, ISIJ International , 10.2355/isijinternational.52.161, 52, 1, 161-163, 2011.01, 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..
76. Motomichi Koyama, Eiji Akiyama, Kaneaki Tsuzaki, Hydrogen embrittlement in a Fe-Mn-C ternary twinning-induced plasticity steel , Corrosion Science , 10.1016/j.corsci.2011.09.022, 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. .
77. Motomichi Koyama, Takahiro Sawaguchi, Kaneaki Tsuzaki, Work hardening and uniform elongation of an ultrafine-grained Fe-33Mn binary alloy, Materials Science and Engineering A , 10.1016/j.msea.2011.10.038, 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. .
78. Kaoru Sekido, Takahiro Ohmura, Takahiro Sawaguchi, Motomichi Koyama, Hyoungwon Park, Kaneaki Tsuzaki, Nanoindentation/atomic force microscopy analyses of epsilon-martensitic transformation and shape memory effect in Fe-28Mn-6Si-5Cr alloy, Scripta Materialia , 10.1016/j.scriptamat.2011.08.010, 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..
79. Motomichi Koyama, Takahiro Sawaguchi, Teakyung Lee, Chong Soo Lee, Kaneaki Tsuzaki, 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 , 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..
80. Motomichi Koyama, Takahiro Sawaguchi, Kaneaki Tsuzaki, Effect of deformation temperature on work hardening behavior in Fe-17Mn-0.6C and Fe-17Mn-0.8C steels, Proceedings of the High Manganese Steels Conference 2011, 2011.05, Carbon containing twinning induced plasticity steels show dynamic strain aging and ε-martensitic transformation as well as deformation twinning. We investigated a relationship between work hardening behavior and the deformation modes in Fe-17Mn-0.6C and Fe-17Mn-0.8C steels (mass %). Each deformation mode preferentially occurred at the specific deformation temperature and composition. ε-martensitic transformation made the Fe-17Mn-0.6C steel brittle at 173K, although it gives a work hardening rate of approx. 4GPa. Deformation twinning, dynamic strain aging give work hardening rates of approximately 3GPa at 173K, 2.3~3GPa at 373K in the Fe-17Mn-0.9C steel, respectively. The high work hardening rate contributes to an enhancement of uniform elongation. For instance, the uniform elongation and ultimate tensile strength at 294K in the Fe-17Mn-0.9C steel are >70% and 1.1GPa..
81. Motomichi Koyama, Takahiro Sawaguchi, Kaneaki Tsuzaki, Si content dependence on shape memory and tensile properties in Fe-Mn-Si-C alloys , Materials Science and Engineering A , 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. .
82. Motomichi Koyama, Takahiro Sawaguchi, Kazuyuki Ogawa, Takehiko Kikuchi, Masato Murakami, Continuous transition of deformation modes in Fe-30Mn-5Si-1Al alloy , Materials Transactions , 51, 7, 1194-1199, 2010.02, Deformation modes at various stages of plastic deformation have been investigated at the same location in an Fe-30Mn-5Si-1Al (mass%) alloy specimen, which exhibits a good shape memory effect associated with the FCC(γ) → HCP(ε) martensitic transformation and a relatively high ductility caused by deformation twinning. The surface relief caused by the γ → ε martensitic transformation, deformation twinning and slip band formation has been analyzed by measuring the surface tilt angles corresponding to each deformation mode by atomic force microscopy. Although the ε-martensitic transformation is the dominant deformation mode at an early deformation stage, a part of the ε- martensite plates changes to deformation twins with increasing tensile strain. Slip deformation also occurs inside the same region under excessive strain. A continuous transition of these deformation modes occurs in other grains as well in the same order: ε martensite → deformation twins → slip bands. .
83. Motomichi Koyama, Takahiro Sawaguchi, Masato Murakami, Kaneaki Tsuzaki, Role of Si on the shape memory property of Fe-Mn-Si-C based alloy, Proceedings of the European Symposium on Martensitic Transformations, 2009.09, Si content dependence of the shape memory effect (SME) associated with the ε-martensitic transformation has been investigated in Fe-17Mn-xSi-0.3C (x=0,2,4,6) alloys (mass%). By conducting tensile deformations and subsequent
heating to above the Af, it has been found that reduction of mere 2% Si content from 6% to 4% obviously deteriorates the SME. The Neel and Ms temperatures in both Fe-17Mn4%Si-0.3C and Fe-17Mn6%Si-0.3C alloys are content with the
conditions concerning the good SME. By obtaining stress-strain curves at various temperatures, it has been found that the Fe-17Mn-4Si-0.3C alloy has similar critical stresses for ε-martensitic transformation and slip deformation to the critical stresses in the Fe-17Mn-6Si-0.3C alloy. As a consequence, the SME in Fe-Mn-Si-C system sometimes does not appear, even if ε-martensitic transformation preferentially occurs..
84. Motomichi Koyama, Takahiro Sawaguchi, Kazuyuki Ogawa, Takehiko Kikuchi, Masato Murakami, The improvement of shape recovery property through training treatment in an Fe-30Mn-5Si-1Al, Proceedings of the International Conference on Martensitic Transformations, 583-586, 2009.07, We have studied shape memory properties and training effects for Fe-Mn-Si-Al alloys. It was found that Fe-30Mn-5Si-1Al alloy has similar shape recovery properties to Fe-30Mn-6Si alloy in that γ(fcc)↔ε(hcp) reversible phase transformation is responsible for shape recovery. Fe-30Mn-5Si-1Al alloys possess unique shape recovery behavior and mechanical property due to twinning
deformation. With training treatment, preferred formation of ε-martensite was promoted, which was obviously reflected in the shape
of stress-strain curves. The change of deformation behavior through
training treatment was different for the alloys with and without Al
depending on the combination of ε-martensitic transformation and
twinning deformation..
85. Motomichi Koyama, Takahiro Sawaguchi, Kazuyuki Ogawa, Takehiko Kikuchi, Masato Murakami, The effects of thermomechanical training treatment on the deformation characteristics of Fe-Mn-Si-Al alloys, Materials Science and Engineering A , 497, 1-2, 353-357, 2008.12, We have studied the effects of the thermomechanical training treatment on the deformation mode for Fe-30Mn-(6 - x)Si-xAl (x = 0, 1, 2 and 3 mass%) alloys, which exhibit complicated deformation processes due to coexistence of the stress-induced fcc to hcp martensitic transformation and the mechanical twinning of the fcc matrix in addition to slip deformation. The stress-strain curves at each training cycle were analyzed for the alloys, and the deformation microstructures were observed with atomic force microscopy on a Fe-30Mn-5Si-1Al alloy before and after the training treatment. It was found that the deformation mode shifts from the mechanical twinning to the stress-induced martensitic transformation, as the training proceeds..
86. Kazuyuki Ogawa, Takahiro Sawaguchi, Takehiko Kikuchi, Motomichi Koyama, Masato Murakami, Influence of al concentration on deformation behavior and fracture mode of fe-30mn-6(SI, AL) alloys , Proceedings of the International Conference on Shape Memory and Superelastic Technologies, 645-652, 2008.12, Microstructural and fřactographic observations were systematically done on Fe-30Mn-(6-x)Si-xAl (x=0, 1, 2 and 4 mass %) alloys. Optical and transmission electron microscopic observations and X-ray diffractions revealed that the deformation mode continuously shifts from the stress induced fcc/hcp martensitic transformation to the mechanical twinning of the fee austenite as the Al content increases. It was also clarified by the scanning electron microscopic observations that the microstructural change depending on the Al content is accompanied by the change in the fracture mode from the quasi-cleavage fracture to the ductile fracture..
87. Motomichi Koyama, Masato Muramami, Kazuyuki Ogawa, Takehiko Kikuchi, Takahiro Sawaguchi, Shape memory characteristics of Fe-30Mn-5Si-1Al and Fe-28Mn-5Si-1Al-5Cr alloys, Proceedings of the International Conference on Shape Memory and Superelastic Technologies, 653-658, 2008.12, We have recently reported that the ductility of Fe-Mn-Si shape memory alloys can be
greatly improved by replacing a part of Si with a small amount of Al without decaying
the shape memory effect. Enhancement of the ductility is ascribed to the introduction of
deformation twins in addition to stress-induced hcp martensite on loading. In this work,
the shape recovery strains and recovery temperatures were measured for Fe-30Mn-5Si-
1Al alloy and corrosion-resistant Fe-28Mn-5Si-1Al-5Cr alloy. The maximum shape
recovery strain of Al-added alloys exceeded 2%, which was comparable to those of the
alloys without Al addition. The initial tensile strain required for obtaining the maximum
shape recovery strain was about 8% in Al-added alloys, which was about twice larger
than that of non-Al alloys, probably due to the intrusion of deformation twins. It was
also found that the shape recovery finishing temperature was as low as 470K..
88. Motomichi Koyama, Masato Muramami, Kazuyuki Ogawa, Takehiko Kikuchi, Takahiro Sawaguchi, AFM Observation of Microstructural Changes in Fe-Mn-Si-Al Shape Memory Alloy, Materials Transactions, 10.2320/matertrans.MRA2007321, 49, 4, 812-816, 2008.04, We analyzed the surface relief caused by stress-induced hcp martensitic transformation in Fe-30Mn-5Si-1Al shape memory alloy by atomic force microscopy. The alloy exhibits a good shape memory effect and an improved ductility due to a small addition of Al to a conventional Fe-30Mn-6Si shape memory alloy. The orientation of an austenite matrix was determined with surface traces of four f111gf planes, which enabled us to determine the surface tilt angles for all twelve variants of hcp martensites and deformation twins. On the basis of these values, stress-induced martensite and deformation twin coexisting in the same grain were identified by studying the surface tilt angles. The surface relieves caused by the stress-induced martensite recovered after heating above the reversed transformation temperature, however some
relieves originating from the deformation twin remained..
89. Takahiro Sawaguchi, Leandru-Gheorghe Bujoreanu, Takehiko Kikuchi, Kazuyuki Ogawa, Motomichi Koyama, Masato Murakami, Mechanism of reversible transformation-induced plasticity of Fe–Mn–Si shape memory alloys, Scripta Materialia, 59, 826-829, 2008.06, The mechanism of reversible transformation-induced plasticity of an Fe–30Mn–6Si (mass %) shape memory alloy, under tensile stress and subsequent compression, has been observed by optical and atomic force microscopy. The tensile stress-induced e (hexagonal close-packed) martensite reverts into c (face-centered cubic) austenite after compression to zero strain. Further compression to negative strains induces a different e martensite variant from that of the tensile-stress-induced e martensite..
90. Motomichi Koyama, Masato Muramami, Kazuyuki Ogawa, Takehiko Kikuchi, Takahiro Sawaguchi, Influence of Al on Shape Memory Effect and Twinning Induced Plasticity of Fe-Mn-Si-Al System Alloy, Materials Transactions, 10.2320/matertrans.MRA2007124, 48, 10, 2729-2734, 2007.10, We have investigated the shape memory effect and ductility of Fe-30Mn-(6-x)Si-xAl (x ¼ 0, 1, 2 and 3; in mass-%) alloys. The alloy with x ¼ 0 shows a good shape memory effect but suffers from poor ductility, while the alloys with x ¼ 2 and 3 are well-known TWIP (twinning induced plasticity) steels that show high ductility. In the present study, it was found that the shape memory effect is observed in the samples with x ¼ 0 and 1, but disappears when x exceeds 2. On the other hand, the ductility almost linearly increases with increasing the amount of Al. A good
combination of the shape memory effect and ductility was achieved in the alloy with x ¼ 1..