Role of thermal vibrations in alloy phase transitions

Abstract

We resolve the fundamental contradiction that has existed from the earliest studies on order–disorder transitions between theoretical models that ignore the effect of thermal vibrations on the chemical interaction parameter, J(0), and the analysis of diffraction data that always incorporates the role of vibrations. New analysis of diffraction data shows that the temperature dependence of the order parameter, the central feature of order–disorder transitions, predicted by existing models is scientifically invalid. All models are constrained by the diffraction data to represent the interaction parameter as J(T) and the ordering energy as temperature dependent. The discrepancy between the experimental and theoretical ordering energies in Ni3V and Pd3V is direct evidence of their temperature dependence. Thermal vibrations influence order–disorder transitions through both the configurational and vibrational terms and not just the vibrational terms as hitherto believed. A modified Bragg–Williams model is proposed, which is the simplest model whose predicted order parameter can be compared with experiments.