Grain Growth and Coarsening

1.6 Grain Growth and Coarsening
Grain growth is the term used to describe the increase in the average grain size of a polycrystalline material. The grain boundary is a region with a complex structure, about 1-2 unit cells wide, between two crystalline domains. It has a higher energy than the bulk crystal and, hence, a reduction in the grain boundary area will reduce the free energy of the system. The grain boundary moves by diffusion of atoms (ions) from one side of the boundary to the other so that atoms previously aligned with one grain becomes aligned with the other, causing it to grow at the expense of its neighbor. Growth occurs in such a way that the convex grain loses atoms while the concave grain gains atoms with the result that the boundary moves towards its center of curvature.
1.6.1 Normal Grain Growth
Normal grain growth in pure, dense, and single-phase materials has been analyzed by a number of different approaches. Mean field theories consider the change in size of an isolated grain embedded in a matrix that represent the average effect of the whole array of grains. In real systems grain growth, certain topological requirements of a space-filling array of grains must be balanced with the requirements of interracial tension.
1.6.2 Abnormal Grain Growth
Microstructures of polycrystalline ceramics that have been heated for some time at a sufficiently high temperature often show very large (abnormal) grains in a matrix of finer grains. It is important to understand and to be able to control abnormal grain growth for two main reasons.