Simulations of hydrogen diffusion at grain boundaries in aluminum

Long-timescale simulations of hydrogen atom diffusion in aluminum at low concentration were carried out to study the effect grain boundaries can have on the diffusion mechanism and diffusion rate. Three different grain boundaries were studied: R5 twist, R5 tilt and general, twist + tilt, grain boundaries. The adaptive kinetic Monte Carlo method was used to simulate the system over time intervals spanning tens of microseconds. A potential function of the effective medium form was parametrized to density functional theory calculations and used to describe the atomic interactions. The twist boundary turns out to block diffusion across the boundary – only a few high-energy sites are found in the boundary layer. However, diffusion parallel to the boundary is slightly enhanced because of the reduced configuration space. The twist and twist + tilt grain boundaries have strong binding sites within the boundary layer, up to 0.1 and 0.3 eV stronger, respectively, than binding sites in the crystal. In the latter case, the H atom spends more than 99% of the time in the grain boundary layer and diffusion parallel to the grain boundary occurs only when the H atom re-enters a crystal grain. Because of the trapping, the diffusion is reduced by more than an order of magnitude compared with the crystal.