Hydrogen–vacancy effects on the elastic and plastic behaviour of Ni<100> probed by nanoindentation

One of the fundamental aspects of hydrogen embrittlement is based on the impacts of hydrogen on the elementary mechanisms of plasticity. Even though it is well known that the solute hydrogen generally deteriorates the ductility of nickel, it highlighted the existence of antagonistic processes in the hydrogen effect as well, i.e., hydrogen-induced hardening and/or softening without a relevant universal explanation. These effects may also reflect an implication of hydrogen on the modification of the elasticity properties. In this work, the impact of hydrogen on elastic modulus, dislocation nucleation (i.e., pop-in), and hardness was investigated in nickel 〈100〉 single crystal using nanoindentation. The evolution of the different properties during hydrogen desorption offers the opportunity to distinguish the direct impact of hydrogen from those associated with solute-induced defects. The deformed sub-surfaces by nanoindentation were analyzed by TEM to characterise the development of dislocation structures and any other defects, and hence to establish the hydrogen-defect-elasticity-plasticity correlations. Hertz’s theory was used to model the elastic regime and Oliver and Pharr's model (Oliver and Pharr, 1992) was used to analyze the elastoplastic regime of the nanoindentation load-displacement curve. Hydrogen-induced impacts on maximum shear stress to activate dislocations, hardness and elastic modulus were observed. An irreversible reduction in elastic modulus with hydrogen absorption revealed the influence of hydrogen-induced vacancy clusters on elasticity. In addition, the increase in pop-in load and hardness with hydrogen absorption indicated a hardening behaviour in the plastic regime, resulting from the interaction of interstitial hydrogen and vacancy clusters with dislocation nucleation and mobility.