Study of the anisotropic effects of hydrogen on the subsurface mechanical behavior of nickel single crystal by nanoindentation

• Hydrogen’s impact on the subsurface mechanical properties of single-crystal nickel is highly dependent on crystallographic orientation, which is influenced by the anisotropic nature of hydrogen diffusion. • Hydrogen induces an anisotropic degradation of elastic properties, which is related to the production of point defects and the formation of hydrides. • Hydrogen induces both hardening and softening processes in plastic regime, depending on the occurrence of shielding and pinning elementary plastic mechanisms. Hydrogen ingress can significantly alter subsurface mechanical properties, often in a different manner than in the bulk. In this work, we investigate the effect of hydrogen on the subsurface mechanical behavior of face-centered cubic nickel single crystals using nanoindentation. Four crystallographic orientations (100), (110), (111), and (219) were examined before and after electrochemical hydrogen charging, with particular attention paid to surface preparation. The mechanical response is analyzed in terms of the nanoindentation reduced modulus (using the Hertz model), pop-in load and depth, and hardness. Our results show a reduction in elastic modulus that depends on crystallographic orientation and is associated with vacancy formation and potential hydride phases. Hydrogen alters the anisotropy of incipient plasticity through elastic shielding and enhanced dislocation pinning. The observed trends, orientation-dependent softening of modulus, depth-dependent pop-in behavior, and changes in critical shear stress are explained by the competing effects of softening (via vacancy and hydride formation) and hardening (via dislocation pinning), thus offering new insights into hydrogen-induced mechanical anisotropy in nickel