Solid-solution theory and spinodal decomposition

Abstract

The free energy of solid solution varies with the atomic arrangement or degree of mixing in solution. The change of free energy with solution state in an alloy system with a miscibility gap was examined step by step from the random state toward the equilibrium state by a series of approximations provided by the cluster-variation method. As equilibrium is approached, the gap corresponding to the non-equilibrium state gradually shrinks to the equilibrium gap, while the chemical spinodal region expands and eventually approaches the solubility limits. The effects of these features on the spinodal decomposition are discussed. Several experimental observations of spinodal decomposition that cannot be explained satisfactorily by the current theory can be accounted for when the free energy is considered to vary with the state of solution.