Helium bubbles diffusion in aluminum: Influence of gas pressure

The influence of helium pressure on the mechanisms of diffusion of the He-filled nanobubbles in fcc Al is investigated by the classical molecular dynamics. It is shown that at pressures < 20 kbar gas suppresses self-diffusion, while the concentration of adatoms, rather than their mobility, plays a key role. However, as the pressure increases to values at which the bubble emits self-interstitial atoms, the surface self-diffusion and bubble diffusion coefficients begin to increase. It is shown that the bubble diffusion coefficient is proportional to the concentration of self-interstitial atoms in the surface layer. At higher gas pressures, self-interstitial atoms form dislocation loops around the bubble and bubble diffusion coefficient drops significantly. The effect of surface self-diffusion suppression by gas pressure cannot explain the experimental data, while they can be explained by dislocation loops formation around bubbles. Two interatomic interactions models for Al-He system are considered. It is shown that the main findings of this work do not depend on the choice of the model.