First-Principles Study of Chemical Driving Force for Face Centered Cubic to Hexagonal Close Packed Martensitic Transform
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ogen can be effectively used for the diversification of energy sources and reduction of CO2 emission. However, hydrogen embrittlement in metals and alloys is a critical issue that needs to be resolved for the realization of a hydrogen energy society. Hydrogen embrittlement is influenced by several factors[1–10]: Chemically adsorbed hydrogen atoms on the surface of metals penetrate into the lattice and move through interstitial sites beyond the activation energy of
Y. KUROKI, S. KAWANO, and S. IIKUBO are with the Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, Kitakyushu 808-0196, Japan. Contact e-mail: [email protected] H. OHTANI is with the Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan. M. KOYAMA is with the Department of Mechanical Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan. K. TSUZAKI is with the Department of Mechanical Engineering, Kyushu University and also with the HYDROGENIOUS, Kyushu University, Motooka 744, Nishi-ku, Fukuoka 819-0395 Japan. Manuscript submitted January 14, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
potential energy surface. Practical metallic materials typically contain a large amount of lattice defects, such as dislocations and grain boundaries, which trap the penetrated hydrogen atoms. Furthermore, martensite formation affects the hydrogen embrittlement susceptibility, but the detrimental effect of hydrogen is smaller in hexagonal close packed (HCP) martensite than in body centered cubic (BCC) martensite in iron and steels. To understand this behavior, we have previously studied the hydrogen diffusivity in HCP iron[11] using first-principles calculations. In a more recent study, we observed the anomalous suppression of HCP-martensite formation in a face centered cubic (FCC) steel due to hydrogen charge using X-ray diffraction measurements.[12] This observation was unexpected considering the conventional dominant idea that the stacking fault energy is reduced by hydrogen charge and FCC–HCP martensitic transformation is promoted.[13–16] In fact, the conventional reports include effects of heterogeneity of hydrogen distribution, further transformation to BCC, and pre-existing dislocation substructure, which complicates the phenomena associated with hydrogen uptake. Our recent work solved all of these problems via controlling chemical composition and hydrogen charging condition, in which thereby we could uncover the underlying hydrogen effects on FCC–HCP martensitic transformation. Based on our experimental results, we proposed that the combined effect of hydrogen on the friction stress and free energy suppresses the martensitic transformation to HCP.[10] However, experimental approach is difficult to achieve simple and quantitative understanding of the hydrogen effects, because chemical composition and temperature are limited to form HCP-martensite and to prevent hydrogen desorption. In this context, theoretical calculation is an ideal solution of this p
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