WHAT IS RECRYSTALLIZATION?

The energy of a crystalline material is raised during permanent or plastic deformation by the presence of dislocations and interfaces introduced during the deformation, and a material containing these defects is thermodynamically unstable. However, the necessary atomistic mechanisms are often very slow at room temperature, with the result that the unstable defect structures are retained after deformation.

If the material is subsequently heated to a high temperature (annealed), thermally activated processes such as solid state diffusion provide mechanisms whereby the defects may be removed or alternatively arranged in configurations of lower energy.

The microstructure and also the properties may be partially restored to their original values by processes of recovery in which annihilation and rearrangement of the dislocations occurs. The microstructural changes during recovery are relatively homogeneous and do not usually affect the boundaries between the deformed grains. Similar recovery processes may also occur during deformation, particularly at high temperatures, and this is known as dynamic recovery.

Recovery generally involves only a partial restoration of properties because the dislocation structure is not completely removed, but reaches a metastable state. A further restoration process called recrystallization may occur in which new dislocation-free grains are formed within the deformed or recovered structure. These then grow and consume the old grains, resulting in a new grain structure with a low dislocation density. Recrystallization may take place during deformation at elevated temperatures and this is then termed dynamic recrystallization.

Scanning electron micrograph taken using backscattered electrons, of a partly recrystallized Al-Zr alloy. The large defect-free recrystallized grains can be seen consuming the deformed cellular microstructure.