A Nanoscale Study of Dislocation Nucleation at the Crack Tip in the Nickel-Hydrogen System

Strengthening and embrittlement are controlled by the interactions between dislocations and hydrogen (H)-induced defect structures that can inversely affect the deformation mechanisms in materials. Here we present a simulation framework to understand fundamental issues associated with H-assisted dislocation nucleation and mobility using Monte Carlo (MC) and molecular dynamics (MD). In order to study the interaction between H and dislocations and its effect on material failure, we extensively examined mode I loading of an edge crack using MD simulations. The MD calculations of the total structural energy in the nickel (Ni)-H system shows that H atoms prefer to occupy octahedral interstitial sites in the fcc Ni lattice. As H concentration is increased, the Young's modulus and the energy required to create free surface decreased, resulting in H-enhanced localized plasticity. The MD simulations also indicate that H not only facilitates dislocation emission from the crack tip but also enhances dislocation mobility, leading to softening of the material ahead of the crack tip. While the decrease in surface energy suggests H embrittlement, the increase in local plasticity induces crack blunting and prohibits crack propagation. The mechanisms responsible for transitioning from a ductile to brittle crack behavior clearly depend on the H concentration and its proximity to the crack tip. Enhanced plasticity will occur within a general field of H atoms that results in lower stacking fault and surface energies, yet H interstitials on preferential slip planes can inhibit dislocation nucleation.