Grain growth is a vague term used in the context of recrystallization, "normal" grain growth and secondary recrystallization. In each case, a difference in chemical free energy produces the diffusion of atoms into the growing grain. The chemical free-energy difference associated with recrystallization is easy to understand because the new grain is relatively free of the defects produced by cold working. During recrystallization, the new grain consumes the surrounding material and grows until impinging another recrystallized grain. Both subsequent normal grain growth and secondary recrystallization are associated with surface energy and are the focus of this discussion.
The term normal grain growth is used to describe the phenomenon associated with an increase in average grain size during the annealing process. The driving force for normal grain growth often is related to the net decrease in surface area as the average grain size increases. However, this logic cannot explain the unusual stability of nanophase materials or the effect of particle pinning on normal grain growth. For a discussion on particle pinning, see March 2000 Engineering Concepts. Normal grain growth is driven by grain curvature, which then is combined with surface energy to create the driving force. In a mixed grain structure, there are both smaller and larger than average grains. Larger grains have a larger than average number of grain faces, which have a center of curvature that lies outside the grain. In contact, smaller grains have centers of curvature within the grain. Like a soap bubble, surface tension produces a higher pressure within the smaller grains. Consequently, atoms within the smaller grains diffuse out in an attempt to equalize the pressure in each grain; the diffusion produces normal grain growth. The pressure difference also can be expressed as a change in chemical free energy. If all grains were the same size and shaped like Kelvin tetrakaidecahedrons (Fig. 1), there would be no grain curvature and the grain structure would be stable regardless of grain size.