Continuous Recrystallization During and after Large Strain Deformation

Numerous low-angle boundaries are formed when a polycrystalline metal is deformed or when the material is subsequently recovered by annealing at low temperatures. On annealing at elevated temperatures, such a microstructure will usually recrystallize discontinuously, this process is driven mainly by the energy stored in the low-angle boundaries. After deformation to large strains, a microstructure consisting predominantly of high-angle grain boundaries may be formed. Minor boundary movements either during the deformation or on subsequent annealing may then result in a fine-grained microstructure consisting mainly of crystallites that are surrounded by high-angle boundaries. Such a microstructure is similar to that resulting from conventional recrystallization, but because no recognizable nucleation and growth of the recrystallized grains occurs, and the microstructure evolves relatively homogeneously throughout the material, the process can reasonably be classified as continuous recrystallization. The occurrence of continuous recrystallization during high temperature deformation, which is known as geometric dynamic recrystallization, has been recognized for some time. Severe cold-working of a metal can also result in a microstructure that consists almost entirely of high-angle grain boundaries, and that, on annealing, may undergo continuous recrystallization. The microstructures that evolve after such low temperature processing are often sub-micron grain (SMG) structures.