Toward hydrogen energy society, hydrogen-induced mechanical degradation, i.e., hydrogen embrittlement, have been a bottleneck. Therefore, finding or developing hydrogen-resistant high-strength alloys is an urgent issue. Other than cohesive strength, characteristic factors causing hydrogen embrittlement susceptibility are:
(1) thermodynamic stability of face-centred cubic (FCC) phase,
(2) dislocation-driven local stress,
(3) aggregation of lattice defects.

In terms of phase stability, the martensitic transformation from FCC to body-centred cubic (BCC) structure critically deteriorates the hydrogen embrittlement resistance. However, the effect of transformation is still unclear. In this talk, I will present recent understanding of hydrogen effects on martensitic transformation from FCC to BCC via hexagonal closed pack structure [1]. Based on the understanding of transformation kinetics with hydrogen, I propose a thermodynamics-based design of hydrogen-resistant alloys (e.g., [2]).

The effect of dislocation-driven local stress and aggregation of lattice defects synagistically affects the hydrogen embrittlement behavior, which are important particularly for alloys with thermodynamically stable phases. For instance, stable austenitic steels have been recognized as hydrogen-resistant materials. However, they also show hydrogen-assisted failure when exposed to a severe hydrogen atmosphere, e.g. 100-MPa hydrogen gas. The hydrogen embrittlement susceptibility is significantly dependent on dislocation planarity due to local stress evolution at grain boundaries and localized planar dislocation arrays at a specific crystallographic plane [3, 4]. The dislocation planarity, which depends on stacking fault energy and short-range ordering degree, causes hydrogen-related intergranular or quasi-cleavage fracture. Therefore, this talk also argues how to consider the dislocation planarity effect in the alloy design (e.g., [5]).

Additionally, we also note the presence of “critical tensile strength for drastic acceleration for fatigue crack growth by hydrogen” in conventional steels that are practically important for realistic structure designs. When the target materials have a higher tensile strength than 900 MPa, the fatigue crack growth rate becomes over 100 times by hydrogen and processes a remarkable frequency dependence [6], which is a critical problem for structure design. To prevent the serious hydrogen-induced acceleration of fatigue crack growth, we must develop a new alloy design strategy for martensitic steels. In this talk, I present the mechanism of the remarkable acceleration of fatigue crack growth [7] and possible pathway to solve this problem (e.g., [8]).

Acknowlegement
This work was supported by MEXT Program: Data Creation and UtilizationType Material Research and Development Project Grant NumberJPMXP1122684766.

References
[1] Y. Wen; M. Koyama; T. Hojo; S. Ajito; E. Akiyama ISIJ Int., 2024, 64, 474-481.
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