A model describing the variation in free energy in compound and single phase regions in binary systems containing miscibility gaps

Abstract

A model is derived to represent the variation of free energy of combination of one gram-atom of two components, represented by A and B, in intermediate single, or compound, phases in a binary system as a function of composition at constant temperature, and with a minimum of experimental data. The derivation of the model involves the, assumption that the straight lines representing the free energy of the two phase fields adjacent to a compound phase, on an isothermal integral free energy against atomic fraction diagram, intersect at the mid-point of the compound phase. A relation between loga_A, loga_B, and the atomic fraction N_B is developed so as to conform with the preceding requirement and yield an almost horizontal tangent to the curve representing the compound phase at N_B = ½ for a hypothetical symmetrical isothermal diagram. The equations developed on these bases are extended to non-symmetrical systems. These are shown to be successful in predicting the variation in free energy of compound phases, as a function of composition, in binary systems for which experimental data are available.