CONTRIBUTION OF THE VIBRATIONAL FREE-ENERGY TO PHASE-STABILITY IN SUBSTITUTIONAL ALLOYS - METHODS AND TRENDS

Recent studies show that the effect of the lattice vibrations on predi ctions of phase stability can be important. Because complete computati ons of the phonon density of states from first principles are difficul t, simpler models for the vibrational free energy have been used to in corporate these effects into phase diagram calculations. In this paper , we systematically investigate the accuracy of these approximations b y studying model systems for which the vibrational free energy can be computed exactly in the harmonic approximation. We found that the Deby e approximation, usually used in first-principles studies, fails to ca pture the configuration dependence of the vibrational free energy. We explain the reason for this failure. With the same model systems, we a nalyzed the effect of the lattice vibrations on the predicted phase di agrams as a function of size mismatch and chemical affinity. By fittin g our results to the available experimental information, we find that the effect of vibrations on phase transition temperatures may be signi ficant.