9 - Abnormal grain Growth

Abstract

Abnormal grain growth is a coarsening type of microstructure where some large grains grow unusually quickly in a matrix of fine grains with a very slow growth rate. In terms of microstructure, grain size distribution is bimodal, unlike in normal grain growth with a unimodal distribution. Sometimes, a distinction between normal and abnormal grain growth is ambiguous. Abnormal grain growth is characterized by fast growth of a few large grains in a matrix of fine grains of which the size change with annealing time is almost nil or at least negligible compared to that of the abnormal grains. Abnormal grain growth can also occur in high chemical in equilibrium, such as in the Y-SiAlON system. When the Y-β-SiAlON grains form from Y-α-SiAlON in an oxynitride glass, which is in equilibrium with Y-β-SiAlON, abnormally large grains can also form.