A rationale for modelling hydrogen-induced softening in fcc single crystals

Several atomistic-scale mechanisms have been proposed to explain the phenomenon of hydrogen-enhanced localized plasticity (HELP). However, the relative contributions of each of these atomistic mechanisms in determining the constitutive behaviour of fcc single crystals undergoing HELP are yet to be fully understood. In this study, we address this issue by presenting a constitutive model of H-induced softening for fcc single crystals. Here, we first compute H concentrations in the trapped state close to the dislocation cores and in the normal interstitial lattice sites (NILS), which are then used to determine the efficacy of different HELP mechanisms contingent on them. The H in NILS forms atmospheres around dislocations and shields the long-range elastic forces acting on them, while the H in deep traps around dislocations reduces the activation barrier associated with short-range obstacles and enhances the ease of dislocation nucleation. These mechanisms are incorporated in a coarse-grained form into a dislocation density-based constitutive model of fcc single crystals, and the constitutive response is simulated for each one of the underlying mechanisms. Our simulations reveal that the mechanism of enhancement of dislocation nucleation in the presence of H is the most potent to cause softening of the constitutive response. In comparison, the flow softening caused by the mechanisms of reduction of the activation barrier of short-range obstacles and the shielding effect of H appears to be insignificant. The presented constitutive model can be extended to simulate H-induced softening in polycrystalline fcc materials as well.