Prototype fcc-based binary-alloy phase diagrams from tight-binding electronic-structure calculations

Abstract

The generalized perturbation method (GPM), previously introduced to investigate the relative stability of simple and complex structures at T=0 K for transition-metal alloys, is applied to the calculation of fcc-based order-disorder phase diagrams with the use of the cluster-variation method (CVM). Results obtained with the GPM have shown that it was possible to expand the electronic configurational contribution of the total energy, namely the ordering energy, as a sum of concentration-dependent effective cluster interactions, taking the completely disordered state as a reference medium. These interactions are computed from a simple but realistic description of the electronic structure based on the tight-binding approximation appropriate for transition-metal alloys. Calculated energies of mixing and effective interactions are then introduced into the CVM free energy, thereby leading to the calculation of prototype phase diagrams describing phase equilibrium between fcc superstructures of type L$1_0$ and L$1_2$ and the corresponding disordered phase. Electronic parameters are varied systematically and their influence on the topology of the resulting phase diagrams is studied for the first time.