Iron and steels are extensively used as structural materials, and have three primary phase structures: Body-centered cubic (bcc), face-centered cubic (fcc), and hexagonal closed-packed (hcp). Controlling phase stabilities, especially by the use of interstitials, is a universal method that provides a diverse variety of functional and mechanical properties in steels. In this context, hydrogen, which can act as an interstitial species in steels, has been recognized to promote phase transformation

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... rom fcc to hcp. However, we here report a dramatic effect of interstitial hydrogen that suppresses this hcp phase transformation. More specifically, the fraction of hcp phase that forms during cooling decreases with increasing diffusible hydrogen content. This new finding opens new venues for thermodynamics-based microstructure design and for development of robust, strong, and ductile steels in hydrogen-related infrastructures. Hydrogen is a key resource for next-generation green energies 1 , which creates new demands of hydrogen-compatible infrastructures. A challenge to develop such hydrogen-related infrastructures is the significant effect of hydrogen on mechanical degradation of materials 2 . Specifically, the effect of hydrogen in structural steel components causes deterioration of ductility, delayed fracture, and acceleration of fatigue crack growth, all of which are critical problems requiring solutions to realize a hydrogen-energy based-society. Hydrogen-induced mechanical degradation stems from multiple factors such as hydrogen diffusivity/segregation 3 , cohesive energy 4 , number of vacancies 5 , dislocation mobility 6 , crack tip deformability 7 , and phase stability of fcc structures 8-10 . In particular, phase stability has been recognized as a critical factor, because it causes a diffusionless transformation at ambient temperature to bcc or hcp structures. This in turn causes significant changes to hydrogen-related factors such as hydrogen diffusivity 11 . Moreover, solute hydrogen significantly affects the phase stability of fcc 12 , and thus, the synergetic effect of hydrogen and phase stability dramatically alters susceptibility to hydrogen embrittlement 13, 14 . From a viewpoint of phase stability, the diffusionless transformation from fcc to hcp is key to understanding the hydrogen-related phase transformation. More specifically, the hcp phase acts as metastable state of bcc, or in other words, the relative phase stability of fcc compared to hcp affects not only hcp transformation but also bcc transformation in fcc steels 15, 16 . In this context, the effect of solute hydrogen on the phase stability of the fcc to hcp has been experimentally investigated for a half-century, and all such experimental studies have indicated that hydrogen promotes hcp transformation 12,17-21 . A possible reason for this is reported to be a reduction in the stacking fault energy 17, 20, 21 . However, in this study, we unexpectedly in contrast found that solute hydrogen in fact distinctly suppresses the hcp diffusionless transformation. This finding rewrites our basic understanding of the hydrogen effect on phase stability of fcc steels, and renovates alloy design strategy for hydrogen-resistant steels. Here we present X-ray diffraction (XRD)-based evidence on this phenomenon, along with a reliable quantification of hydrogen contents. For this study, we selected a solution-treated Fe-15Mn-10Cr-8Ni (mass%) alloy 22 for the following four reasons: (1) The initial constituent phase should be fully fcc at room temperature (RT). (2) The estimated Néel temperature should be near or lower than the starting temperature for the hcp diffusionless transformation 23,24 to