Effect of hydrogen on the collective behavior of dislocations in the case of nanoindentation

Abstract Most of the studies reported treats the effect of hydrogen on single dislocation line, while models that describe the collective interaction are missing. In this study, hydrogen-induced softening of metallic materials is studied from a perspective of collective behavior of dislocations. Building on the evolution of dislocation density, a hydrogen-informed expanding cavity model is developed for the first time to predict the dynamic evolution of load-displacement curve obtained from nanoindentation tests. Large-scale molecular dynamics simulations on the mechanical behavior of fcc Ni with and without hydrogen (H) charged are performed to calibrate the proposed continuum model. The results show that the H-induced decrease of indentation force is due to that the energy barrier for dislocation nucleation is lowered by the solute drag of the H atmosphere formed around dislocations. Envisioned as a complex non-equilibrium process, it is found that the power-law exponent of the self-organized criticality of dislocations increases due to the insertion of H atoms. Analysis also indicates that H can reduce the probability of dislocation pile-up, thus promote the delivery of dislocations to the surface of specimens during nanoindentation.