Dynamic recrystallization is the process of removing the internal strain that remains in grains during deformation. This happens by the reorganization of a material with a change in grain size, shape, and orientation within the same mineral. When recrystallization occurs after deformation has come to an end and particularly at high temperatures, the process is called static recrystallization or annealing. Dynamic recrystallization results in grain size-reduction and static recrystallization results in the formation of larger equant grains.
Dynamic recrystallization can occur under a wide range of metamorphic conditions, and can strongly influence the mechanical properties of the deforming material. Dynamic recrystallization is the result of two end-member processes: (1) The formation and rotation of subgrains (rotation recrystallization) and (2) grain-boundary migration (migration recrystallization).
# Rotation recrystallization (subgrain rotation) is the progressive misorientation of a subgrain as more dislocations move into the dislocation wall (a zone of dislocations resulting from climb, cross-slip, and glide), which increases the crystallographic mismatch across the boundary. Eventually, the misorientation across the boundary is sufficiently large enough to recognize individual grains (usually 10–15° misorientation). Grains tend to be elongate or ribbon-shape, with many subgrains, with a characteristic gradual transition from low-angle subgrains to high-angle boundaries.
# Migration recrystallization (grain-boundary migration) is the processes by which a grain grows at the expense of the neighboring grains. At low temperatures, the mobility of the grain boundary may be local, and the grain boundary may bulge into a neighboring grain with a high dislocation density and form new, smaller, independent crystals by a process called low-temperature grain boundary migration, or bulging recrystallization. The bulges produced can separate from the original grain to form new grains by the formation of subgrain (low-angle) boundaries, which can evolve into grain boundaries, or by migration of the grain boundary. Bulging recrystallization often occurs along boundaries of old grains at triple junctions. At high temperatures, the growing grain has a lower dislocation density than the grain(s) consumed, and the grain boundary sweeps through the neighboring grains to remove dislocations by high-temperature grain-boundary migration crystallization. Grain boundaries are lobate with a variable grain size, with new grains generally larger than existing subgrains. At very high temperatures, grains are highly lobate or ameboid, but can be nearly strain-free.