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Wang, Y.*; 友田 陽*; 大村 孝仁*; Gong, W.; Harjo, S.
Materialia, 27, p.101685_1 - 101685_9, 2023/03
The evolution of internal stress during displacive transformation is a topic of continuous debate. Neutron diffraction was used to study the isothermal bainite transformation in a 0.4 C low alloyed steel from 773 to 623 K to provide a clearer basis for discussion regarding the change in the austenite lattice parameter. According to diffraction profile analysis, fresh bainite possesses a body-centered tetragonal structure, and its c/a ratio decreases rapidly over time. The austenite lattice parameter increases or decreases depending on whether the transformation temperature is above or below the nose of the Time-Temperature-Transformation (TTT) curve. This isothermal transformation behavior can be divided into two categories: above and below the nose of TTT curve, which correspond to the upper and lower bainites, respectively. The internal stresses caused by the transformation strains are relaxed by dislocation motion and vacancy formation. The yielded dislocations and vacancies not only affect the broadening of both austenite and bainitic ferrite diffraction peaks but also the lattice parameter.
Wang, Y.*; 友田 陽*; 大村 孝仁*; Gong, W.*; Harjo, S.; 田中 雅彦*
Acta Materialia, 196, p.565 - 575, 2020/09
被引用回数：31 パーセンタイル：90.48(Materials Science, Multidisciplinary)The continuous and discontinuous yielding behaviors in ferrite-cementite steels were complementarily investigated via nano- and macroscale deformation examinations. The results obtained by electron microscopy, synchrotron X-ray, and neutron diffractions indicate that the ferrite-cementite interface of the heat-treated specimen is semi-coherent with a high internal stress field, whereas that of the recrystallized one is incoherent with a low internal stress field. Moreover, coherency strain, which depends on the total area of the ferrite-cementite interface, and thermal strain, which is governed by temperature, are the two factors that influence peak broadening. The nanoindentation tests revealed that the critical loads are significantly lower near the semi-coherent interface than those near the incoherent interface and the ferrite grain boundary; this suggests that dislocations are easily emitted from the semi-coherent interface.
友田 陽*; 村上 俊夫*; Wang, Y. X.*; 大村 孝仁*; Harjo, S.; Su, Y. H.; 篠原 武尚
Materials Characterization, 163, p.110243_1 - 110243_8, 2020/05
被引用回数：4 パーセンタイル：32.96(Materials Science, Multidisciplinary)In this study, the effects of carbon concentration and magnetic transition on the austenite lattice parameter of Fe-30Mn-
C alloys were assessed. A linear relationship was obtained between the carbon content and the lattice parameter determined by neutron diffraction and neutron Bragg-edge transmission imaging for austenitic steels bearing 0.007-1.20 mass% C. The results obtained through both methods showed good agreement. In addition, carbon partitioning was evaluated in artificially piled-up discs with different carbon contents in order to develop a new technique for advanced multi-phase steels. The analysis of asymmetry and broadening of the diffraction line profile revealed that the carbon partitioning could be determined. In conclusion, the results presented herein expand the current knowledge on the properties of high Mn austenite.
友田 陽*; Wang, Y.*; 大村 孝仁*; 関戸 信彰*; Harjo, S.; 川崎 卓郎; Gong, W.*; 谷山 明*
鉄と鋼, 106(5), p.262 - 271, 2020/05
被引用回数：0 パーセンタイル：0(Metallurgy & Metallurgical Engineering)The phase transformation behavior from austenite upon cooling in a 1.5Mn-1.5Si-0.2C steel was 
monitored using dilatometry, X-ray and neutron diffractions. The starting temperature of ferrite transformation was in good agreement between dilatometry and neutron diffraction, whereas much higher in X-ray diffraction. Such a discrepancy in transformation temperature is attributed to the change in chemical composition near the surface of a specimen heated to elevated temperatures in a helium gas atmosphere for X-ray diffraction. 
neutron diffraction enables us to investigate the changes in lattice constants of ferrite and austenite, which are affected by not only thermal contraction but also transformation strains, thermal misfit strains and carbon enrichment in austenite. Pearlite transformation started after carbon enrichment in austenite reached approximately 0.7 mass% and contributed to diffraction line broadening.
Wang, Y.*; 友田 陽*; 大村 孝仁*; 諸岡 聡; Gong, W.*; Harjo, S.
Acta Materialia, 184, p.30 - 40, 2020/02
被引用回数：26 パーセンタイル：86.78(Materials Science, Multidisciplinary)A high-intensity and high-resolution neutron diffractometer with a thermomechanically controlled processing simulator was employed in-situ to investigate martensite transformation behavior with and without ausforming for a medium-carbon low-alloy steel. Serial neutron diffraction profiles have revealed that the transformation behavior could be successfully monitored during quenching with and without the ausforming process. The lattice parameter and the full width at half maximum of austenite peaks significantly decreases and increases upon martensite transformation, respectively. After ausforming, the data reveal that lattice parameters are larger in austenite whereas smaller in martensite compared with those in the non-ausformed case, which is ascribed to the introduced dislocations. The cutting-edge operant quantitative measurements with neutron diffraction for steel production is demonstrated.
植山 大地*; 齋藤 勇一; 石川 法人; 大村 孝仁*; 千星 聡*; 堀 史説*; 岩瀬 彰宏*
Nuclear Instruments and Methods in Physics Research B, 351, p.1 - 5, 2015/05
被引用回数：6 パーセンタイル：39.96(Instruments & Instrumentation)We have irradiated Al-Mg-Si alloy with 5.4 MeV-16 MeV several ion species at room temperature so far, and have found that ion irradiation is a useful tool for controlling the surface hardness for the alloy. In the present study, we tried several experiments as some applications of ion beam irradiation for hardness modifications of the alloy. Main results are as follows; (1) the combination of ion beam irradiation and the subsequent thermal aging are effective for the hardness modification of the alloy, and (2) designated regions and areas of the specimen can be hardened by changing the energy of ion beam and producing the irradiated area and unirradiated area of the surface. Then, we can expand the possibility of the ion beam irradiation as a new process for the three-dimensional hardness modification of Al-Mg-Si alloy.
ionsLee, C. G.; 大村 孝仁*; 武田 良彦*; 松岡 三郎*; 岸本 直樹*
Journal of Nuclear Materials, 326(2-3), p.211 - 216, 2004/03
被引用回数：4 パーセンタイル：29.26(Materials Science, Multidisciplinary)耐照射損傷材として注目されているMg-Alスピネルにおいて、大電流イオン照射による体積及び硬度の変化を調べた。イオン照射は、60keV銅負イオンを線量3
10
ions/cm
に固定し、最大線量率100
A/cmまで行った。ナノインデンテーションで測定した硬度は全体に照射に伴い大きな減少を示した。硬度のイオン線量率依存性は体積変化の線量率依存性と強い相関を示し、硬度が減少すると体積が増加することがわかった。照射後のRBS測定結果、非晶質化は起きていないことから、本研究での照射誘起軟化の原因は一般に知られている照射誘起非晶質化ではないことを意味する。一方、光吸収の測定結果から、スピネル結晶の陰イオン副格子での照射誘起点欠陥及びその集合体が、照射誘起スエリングに大きく関与していることがわかった。
