In-situ observation of martensitic transformation in an interstitial metastable high-entropy alloy during cathodic hydrogen charging

We show for the first time that a critical amount of dissolved hydrogen can induce a phase transformation from γ-austenite to ε-martensite in an interstitial metastable high-entropy alloy. This is demonstrated by in-situ hydrogen charging in combination with nanoindentation and scanning probe microscopy, plus further electron channeling contrast imaging, X-ray diffraction, and transmission Kikuchi diffraction techniques. The transformed martensites appear as bands on the surface along γ-{111} habit planes, leading to an irreversible increase of hardness. The hydrogen-induced internal stress together with the intrinsic hydrogen effects are proposed to be responsible for the martensitic transformation upon hydrogen charging.