Solid-State Phase Transformations and Reactions

Abstract

A phase transformation occurs when one material changes its composition or structure. The transformation can be caused by a change in temperature so that no other material is involved or it may involve the reaction with another material, which may or may not be a ceramic, and may be in the liquid or gaseous phase. In this chapter, we will restrict the discussion to phase transformations in which the ceramic is in the solid state. Whenever a phase transformation occurs, a phase boundary must move.

Phase transformations occur at interfaces and require the interface to move. A solid-state phase transformation occurs when the interface between two grains that are chemically or structurally different moves. If the grains are chemically the same but have different structures, the process is referred to as a (structural) phase transformation and local atomic movements can induce the change; if the grains have a different chemistry, then long-range diffusion must occur and the process is most likely part of a solid-state reaction. Clearly there are many features in common with grain growth where the grains are chemically and structurally the same. In particular, the ideas of curvature and capillarity carry over. This chapter thus builds on our discussion of all types of interfaces.