ON THE INAPPLICABILITY OF GIBBS PHASE RULE TO COHERENT SOLIDS

A relationship is derived between the number of degrees of freedom, independent chemical components, and number of phases for a specially constructed nonhydrostatically stressed coherent solid in complete thermodynamic equilibrium. The results obtained are based on the thermodynamics of crystalline solids, and indicate that the degrees of freedom are independent of the number of phases present. This model is used to demonstrate that coherent solids do not obey Gibbs phase rule. It is also shown that domains of what are the same phase in the absence of all stress effects may occupy different states of deformation and hence possess different equilibrium compositions when the system is constrained to behave coherently. The results derived here are in complete agreement with recent work on coherent phase diagrams. © 1991 The Minerals, Metals and Materials Society, and ASM International.