The main factors affecting resistance to hydrogen-assisted cracking are hydrogen diffusivity and local ductility. In this context, we note fcc (γ) to hcp (ε) martensitic transformation, instead of γ to bcc (ά) martensitic transformation. The γ-ε martensitic transformation decreases the local hydrogen diffusivity, which thereby can increase strength without critical deterioration of hydrogen embrittlement resistance. Furthermore, ε-martensite in a high-entropy alloy is extraordinary ductile. Consequently, the metastable high-entropy alloys showed lower fatigue crack growth rates under a hydrogen effect compared with those of conventional metastable austenitic steels such as type 304.
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