Recrystallization (metallurgy)

In materials science, recrystallization is a process by which deformed grains are replaced by a new set of defect-free grains that nucleate and grow until the original grains have been entirely consumed. Recrystallization is usually accompanied by a reduction in the strength and hardness of a material and a simultaneous increase in the ductility. Thus, the process may be introduced as a deliberate step in metals processing or may be an undesirable byproduct of another processing step. The most important industrial uses are softening of metals previously hardened or rendered brittle by cold work, and control of the grain structure in the final product. Recrystallization temperature is typically 0.3–0.4 times the melting point for pure metals and 0.5 times for alloys. Recrystallization is defined as the process in which grains of a crystal structure come in a new structure or new crystal shape. A precise definition of recrystallization is difficult to state as the process is strongly related to several other processes, most notably recovery and grain growth. In some cases it is difficult to precisely define the point at which one process begins and another ends. Doherty et al. (1997) defined recrystallization as: "... the formation of a new grain structure in a deformed material by the formation and migration of high angle grain boundaries driven by the stored energy of deformation. High angle boundaries are those with greater than a 10-15° misorientation" Thus the process can be differentiated from recovery (where high angle grain boundaries do not migrate) and grain growth (where the driving force is only due to the reduction in boundary area). Recrystallization may occur during or after deformation (during cooling or subsequent heat treatment, for example). The former is termed dynamic while the latter is termed static. In addition, recrystallization may occur in a discontinuous manner, where distinct new grains form and grow, or a continuous manner, where the microstructure gradually evolves into a recrystallized microstructure.