ABSTRACT
A useful manner of writing the general equation connecting the molar Gibbs free energy, Ga, of an a solid solution with the atom fraction of solute, x, in the A-B system, where A is defined (at least on the A-rich side of the phase diagram) as the solvent and B is taken to be the solute is
Ga ¼ (1 x)GaA þ xGaB þ DGM,a (1:1) where
G is equal to H TS, H is molar enthalpy S is molar entropy T is the absolute temperature
GAandGB are theGibbs free energies (hereafter simply referred to as ‘‘free energies’’) of pureAandpure B in the crystal structure ofa, andDGM is the free energy ofmixingA andB to form thea solid solution.
The two types ofGi in Equation 1.1,GA andGB, represent standard states. According to Lupis [1], there is no point to the assignment of absolute values of Gi since they are about 10-12 orders of magnitude greater than the free energy changes attending phase transformations usually encountered. In dealing with phase transformations in terms of free energy-composition curves, the free energy change of interest is that involved in passing from one curve to another-to keep matters initially as simple as possible by restricting consideration to a transformation taking place without a change in composition.
