M 26Invited Talk S1-4 Abstract Hydrogen embrittlement issue has been regarded as a bottleneck for developing hydrogen-energy-related infrastructures. In this regard, austenitic steels, which are composed of a face-centered cubic (FCC) phase, show significant resistance to hydrogen embrittlement. However, the austenitic steels can also be susceptible to hydrogen embrittlement when the FCC phase is metastable against other phases such as body-centered cubic (BCC) phases. More specifically, when deformation-induced martensitic transformation occurs, hydrogen-related failure occurs. In this talk, we present hydrogen-assisted cracking and subsequent failure behaviors in metastable austenitic stainless steels toward understanding and controlling the hydrogen-related cracking behavior associated with martensitic transformation. The representative metastable stainless steel is SUS304. The martensitic transformation sequence during plastic deformation in SUS304 is FCC →most susceptible to hydrogen embrittlement in steels, it is generally known that hydrogen-related cracking occurs at a region where BCC martensite formation preferentially occurs. In this context, to control the hydrogen-related cracking, the hydrogen-transformation-cracking relationship must be understood. The BCC martensite-related cracking behavior was clarified by in situ deformation experiments with scanning electron microscopy. According to our analyses, hydrogen promotes FCC-HCP martensitic transformation, but the effect of hydrogen on HCP-BCC transformation depends on deformation temperature. Specifically, hydrogen promotes the formation of BCC martensite at ambient temperature, however, suppresses at cryogenic temperature. This implies that the hydrogen effect suppressing BCC martensite formation requires hydrogen diffusion from HCP/BCC phase interface to the BCC martensite region. To support this interpretation, we will present some experimental results in this talk, in terms of microstructure distribution and hydrogen desorption during deformation. Photo Interactions among Hydrogen-transformation-crack in Associate Professor, Tohoku University, Japan Session 1 ｜Motomichi Koyama obtained a PhD in Materials Science and Engineering (2012) at University of Tsukuba, Japan. In 2012, he received a postdoctoral fellowship (JSPS Research fellowship for young scientist PD). During his postdoc, he spent about 1 year and 3 months in the Max-Planck Institute for Iron Research in Düsseldorf, Germany and was visiting scientist in that institute (2012-2013) in the group of Prof. D. Raabe. In 2013, he became full time assistant professor at Kyushu University, Japan. In 2020, he moved to Tohoku University as associate professor. He joined Institute for Materials Research and attempted to establish alloy deign of hydrogen-resistant steels. In addition, he has developed crack-specific microstructure characterization methodologies towards mechanics-metallurgy-based understanding of hydrogen-assisted damage evolution. NIMS Award Symposium 2023etastable Austenitic Steels Motomichi Koyama Deformation and Fracture HCP → BCC. Because the BCC martensite is the
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