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  • Print publication year: 1996
  • Online publication date: May 2018

15 - Recrystallization

Summary

Definitions

In previous chapters we have discussed processes which introduce lattice defects into metals: deformation generates dislocations, irradiation can cause displacement cascades, martensitic and diffusion-controlled transformations produce grain boundaries, etc. These processes change the microstructure creating a state of higher free energy. Recrystallization is the formation of a new microstructure with a lower free energy by a reaction in the solid state similar to the formation of the microstructure by the crystallization of a melt (chapter 4). In a typical recrystallization experiment, a heavily deformed metal is annealed at a temperature higher than one-half its melting point. This removes many of the lattice defects introduced by deformation and a new arrangement of grain boundaries is established. As was stated in chapter 3, the microstructure is, by definition, not in thermodynamic equilibrium. The grain boundaries remaining after recrystallization constitute a metastable arrangement separating grains with a certain orientation distribution. In other words the metal exhibits a characteristic recrystallization texture. Such textures have a vital influence on many of the physical properties of technological materials, e.g. magnetization losses in transformer sheet.

Recrystallization, the generation of a system of new grains, is fundamentally different from recovery which always precedes it and in which the lattice defects within a given arrangement of high-angle grain boundaries either anneal out or rearrange themselves. The recovery stages I to IV described in section 10.4 in which point defects created by irradiation, deformation or quenching anneal out are thus recovery in the true sense. Stage V is the recrystallization of deformed material.