  Coarsening kinetics is usually described using a linear gradient
approximation for the underlying interface migration (IM) rates, wherein the
migration fluxes at the interfaces vary linearly with the driving force. Recent
experimental studies have shown that coarsening of nanocrystalline interface
microstructures is unexpectedly stable compared to conventional parabolic
coarsening kinetics. Here, we show that during early stage coarsening of these
microstructures, IM rates can develop a non-linear dependence on the driving
force, the mean interface curvature. We derive the modified mean field law for
coarsening kinetics. Molecular dynamics simulations of individual grain
boundaries reveal a sub-linear curvature dependence of IM rates, suggesting an
intrinsic origin for the slow coarsening kinetics observed in polycrystalline
metals.
