Point-defect avalanches mediate grain boundary diffusion

Grain boundary diffusion in polycrystalline materials is a physical phenomenon of great fundamental interest and practical significance. Although accelerated atomic transport along grain boundaries has been known for decades, atomic-level understanding of diffusion mechanisms remains poor. Previous atomistic simulations focused on low temperatures where the grain boundary structure is ordered or high temperatures where it is highly disordered. Here, we conduct molecular dynamics simulations of grain boundary diffusion at intermediate temperatures most relevant to applications. A surprising result of this work is the observation of intermittent GB diffusion behavior and its strong system-size dependence unseen in previous work. Both effects are found to originate from thermally activated point-defect avalanches. We identify the length and time scales of the avalanches and link their formation to dynamic heterogeneity in partially disordered systems. Our findings have implications for future computer modeling of grain boundary diffusion and mass transport in nano-scale materials.