Effect of stress level on hydrogen-induced nanohardness variations in CoCrNi medium-entropy alloy

The present study systematically investigated the influence of stress levels on hydrogen-induced hardening in CoCrNi medium-entropy alloys using nanoindentation rate jump experiments. Two different pyramidal indenters of varying sharpness, namely Berkovich and cube-corner indenters, were employed. With the Berkovich indenter, noticeable hydrogen-induced hardening was observed, demonstrating a considerable increase in hardening as the strain rate increased. In contrast, the cube-corner indenter revealed a comparatively subdued hydrogen effect on hardness. Through finite element simulations, it was found that the underlying mechanism responsible for these distinctive indenter sharpness-related effects was related to the hydrostatic pressure and stress-induced redistribution of hydrogen beneath the indenter. We expect that our study will provide insights into the complex interplay among hydrogen concentration, stress levels, strain rates, and dislocation motion in metallic materials.