Theory and Examples of Spinodal Decomposition in a Variety of Materials

Spinodal decomposition is observed in a variety of materials, such as metal alloys, oxide glasses, mineral solid solutions, steels, gels, ceramics and mixtures of polymers and liquids. It’s an irreversible process in which the transition of solid or liquid solutions is not triggered by nucleation and growth from a sudden concentration peak, but by local sinusoidal fluctuations in the concentration of the components. It occurs, when rapidly quenched, in the thermodynamically unstable region of the demixing area, beyond the spinodal. The spinodal which leads to spontaneous decomposition is the spinodal of the coherent free energy curve of the metastable coherent phase diagram. The early-stage kinetics of spinodal decomposition of a binary alloy A-B can be obtained with a nonlinear diffusion equation, the Cahn-Hilliard-Cook equation, with constant mobility. The equation with variable mobility, describes the later-stage and the coarsening process. In this report the process of spinodal decomposition is discussed phenomenological and mathematically. Also two examples of materials in which decomposition takes place are given, i.e. computer simulations in solids and mineral alkali feldspar solid solution in geology.