EFFECT OF LATTICE-VIBRATIONS ON THE ORDERING TENDENCIES IN SUBSTITUTIONAL BINARY-ALLOYS

The effect of lattice vibrations on the ordering tendencies and phase stability of substitutional binary alloys is analyzed. The alloy Hamiltonian is cast onto an Ising-like Hamiltonian through a cluster expansion of the vibrational free energy computed in the harmonic approximation. The temperature dependence of the vibrational effective cluster interactions (VECI's) is found for a general d-dimensional alloy. Analytic results for the VECI's for d = 1 are derived. It is argued that the vibrations can have pronounced effects on the phase diagram and on the short-range order at high temperatures.