Configuration Dependence of the Vibrational Free Energy in Substitutional Alloys and Its Effects on Phase Stability

The lattice vibrations influence the ordering tendencies in substitutional alloys through the configurational dependence of the vibrational free energy. In this paper we study the size of this effect and the trends with chemical affinity and size mismatch. By analyzing available experimental information and by studying simple model systems for which the phonon frequencies can be easily computed, we find that a) changes in transition temperatures of order-disorder transformations of 30% due to the lattice vibrations may not be uncommon; b) the transition temperatures are usually lowered when the vibrations are included in the modeling of the system, except for certain combinations of size mismatch and chemical affinity; and c) that the largest contribution to the configurational dependence of the vibrational free energy is due to the intermediate and high frequencies of the spectrum.