ABSTRACT
From the practical, experimental standpoint, it is often very difficult to know when equilibrium has been established in an oxide, silicate or ceramic system. This chapter discusses a few specific cases to illustrate the possibilities for non-equilibrium and their influence on the microstructure and properties of the system. In two-component systems there are many ways in which metastability or non-equilibrium can develop. The case of zircon in the system ZrO2-SiO2 is ideal for illustrating the effect of non-equilibrium superheating. Cooling a sample through a two-phase region consisting of a solid solution and a liquid under equilibrium conditions involves a continuous change in the compositions of both the solid and the liquid phases. Diffusion in the solid state is very slow and thus extremely slow cooling may be required to maintain equilibrium. Like binary compounds, many incongruently melting and dissociating ternary oxide compounds can be superheated beyond their equilibrium decomposition point at one atmosphere pressure.
