Atomistic investigation of the influence of hydrogen on dislocation nucleation during nanoindentation in Ni and Pd

The effects of hydrogen charging on the mechanical response of FCC Ni and Pd under nanoindentation are systematically investigated by molecular dynamics simulations. Simulations consider a random H distribution and time scales prevent any diffusion of H during the simulations. Hydrogen charging is then found to reduce the threshold load for dislocation nucleation, i.e., the pop-in load, but only to a limited extent. After pop-in, the indentation response is largely independent of the presence of H. Furthermore, the influence of hydrogen charging on the pop-in load originates only from the hydrogen-induced swelling of the lattice. That is, H does not directly influence dislocation nucleation, either in terms of facilitating initial slip or interacting with the nascent dislocation(s). These findings suggest that rate-dependent processes, either associated with fluctuating nucleation or H transport, are necessary to interpret experimental observations of hydrogen-influenced reductions in the pop-in load.