Atomistic simulation study of the hydrogen diffusion in nickel

The fundamental mechanisms and the conditions in which hydrogen embrittlement (HE) occurs in pure nickel and its alloys has not been fully determined. Several models associated with hydrogen-induced deformation and fracture modes have been proposed. In these models, the transport and concentration of hydrogen play the rate-controlling role in delayed HE. In particular, the kinetics of the embrittlement process is driven by the diffusion of hydrogen. Extensive experimental studies have been performed to elucidate the diffusion of hydrogen in nickel. These investigations have determined a significant anisotropy in the diffusivity of hydrogen. However, the nature of the anisotropy is unclear and still needs to be clarified. In the present work, the diffusion of hydrogen in nickel is investigated using a combined approach involving density functional theory (DFT) and molecular dynamics (MD). The temperature-dependent diffusion coefficients of hydrogen in nickel single crystal, determined from simulations, is in excellent agreement with experimental data. Moreover, it is demonstrated that for a single crystal nickel, with no imposed stress, the computed diffusivities in the $\left(100\right)$, $\left(110\right)$ and $\left(111\right)$ directions did not show significant differences. The reported simulation results accurately describe the diffusion of hydrogen in nickel, and also suggest that stress fields may be the primary contributor to experimentally observed diffusion anisotropy.