  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.
