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Point defect avalanches mediate grain boundary diffusion

Grain boundary (GB) diffusion in polycrystalline materials is a physical phenomenon of great fundamental interest and practical significance. Although the accelerated ("short circuit") atomic transport along GBs has been known for decades, the current atomic-level understanding of GB diffusion remains poor. Experiments can measure numerical values of GB diffusion coefficients but provide little information about the underlying mechanisms. Previous atomistic simulations focused on relatively low temperatures when the GB structure is ordered or relatively high temperatures when it is highly disordered. Here, we report on molecular dynamics simulations of GB diffusion at intermediate temperatures, which are most relevant to applications. One of the surprising results of this work is the observation of strongly intermittent GB diffusion behavior and its strong system-size dependence unseen in previous work. We demonstrate that both effects originate from an intermittent, avalanche-type generation of point defects causing spontaneous bursts of GB diffusivity mediated by highly cooperative atomic rearrangements. We identify the length and time scales of the avalanches and link their formation to the dynamic heterogeneity phenomenon in partially disordered systems. Our findings have significant implications for future computer modeling of GB diffusion and mass transport in nano-scale materials.