The average grain-size of a population of deformed crystals in any highly deformed aggregates tends to increase with time. Surface tension across a curved boundary between two grains gives rise to a pressure difference, with the grain that possesses the concave boundary having a higher energy state.
Molecules tend to move from the high energy side of the boundary to the low energy side.
The process decelerates with time because the size of the high energy crystal is reduced which increases the curvature of the boundary and the differential energy between to the two grains. Dislocations and other defects that stem from deformation of a crystal lattice, increase a grain's disorder and internal energy. When a boundary separates a strained crystal from an unstrained crystal, the free energy of the strained crystal is higher. The grain boundary begins to migrate to reduce the total energy of the system, with molecules moving from the strained grain to the unstrained grain. Eventually the strained grain is reduced in size to the extent that curved boundary effects become significant and the strained grain is rapidly consumed.