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 gamma-austenite to epsilon-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 gamma-{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. (C) 2019 Acta Materialia Inc Published by Elsevier Ltd. All rights reserved.