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Motomichi Koyama Last modified date:2018.05.16

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




E-Mail
Academic Degree
Ph.D, Eng.
Field of Specialization
Materials science and engineering
Outline Activities
My research topic is to reveal mechanisms of work hardening, embrittlement, and fatigue phenomena in advanced high strength steels.
Excellent mechanical properties can be designed through enhancement of work hardening and suppression of embrittlement and fatigue failure. We are attempting to propose a new material design strategy by clarifying the mechanisms of work hardening, embrittlement, and fatigue.
Our main tools are mechanical testing machines and microscopes such as SEM with EBSD and ECCI. Our future goal is placed on proposing the mechanisms based on metallography and mechanics.
Research
Research Interests
  • Enhancing fatigue crack resistance through microstructural laminates
    keyword : laminated structrue, fatigue crack
    2015.04~2018.03.
  • Understanding I-S interaction-related dynamic strain aging: toward its application
    keyword : I-S interaction, dynamic strain aging
    2014.04~2018.03.
  • Analysis on damage evolution of dual phase steel
    keyword : dual phase steel, damage evolution
    2014.04~2018.03.
  • Clarifying mechanisms of hydrogen embrittlement of austenitic steels, specifically high Mn steels
    keyword : hydrogen embrittlement, austenite
    2013.12~2015.03.
Academic Activities
Papers
1. 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..
2. 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.
3. 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.
4. 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..
5. 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. .
6. 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. .
7. 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. .
8. 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. .
9. 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. .
10. 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. .
11. 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..
12. 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. .
13. 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..
14. 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. .
15. 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..
16. 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. .
17. 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..
18. 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. .
19. 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..
20. 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..
21. 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. .
22. 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. .
23. 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..
24. 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. .
Membership in Academic Society
  • The Japan Institute of Metals and Materials
  • The Iron and Steel Institute of Japan
  • The Japan Society of Mechanical Engineering