The free energy barrier to grain splitting

Crystal grain coarsening is widely thought to proceed via grain rotation and grain shrinkage, but recent experimental studies have revealed a third mechanism, in which a single crystal grain splits into counterrotating regions themselves composed of small, individually rotating granules. These granules are so small that the Read-Shockley model for grain boundary energy does not apply to them. Instead, we present a particle-scale theoretical description of the free energy of the polycrystal during the grain splitting event. The free energy is calculated from the entropy, which in our hard sphere system is directly computed from the free space available to each particle. We determine that there is a free energy barrier to grain splitting, and that the barrier height decreases with decreasing grain size. Consequently grain splitting is likely to play an important role in polycrystalline systems with small grains, a broad-reaching discovery that can inform future mesoscale modeling of grain coarsening, in both colloidal and atomic polycrystals.