Progress in theoretical modeling of fcc-L1_O-L1₂ phase equilibria

Progress in fcc-based phase diagram investigation has been made based on the cluster variation method. The authors have performed a prototype calculation in which pair interaction energies are given as constant parameters, and the importance of atomic correlations in the free energy formula has been pointed out. Although the resulting phase diagrams demonstrated advantages over the ones obtained by the Bragg-Williams approximation, a serious drawback is the completely symmetrical feature of the phase boundary around 50 at%. By the introduction of concentration dependency to the pair interaction energies, the asymmetry of Cu-Au phase diagram has been actualized. Together with the electronic structure calculation via the ASW method and statistical numerical calculation via the cluster variation method, a first principles calculation was attempted to draw the phase diagrams of three kinds of Cu-Au, Cu-Ag and Ag-Au. The physical origin of the different alloying behavior of these systems are partly elucidated by microscopic analysis based on the electronic structure calculations. The discrepancies between the calculated phase diagrams and experimental ones are pronounced for the system which has a large difference in atomic size. The importance of the local lattice relaxation is pointed out.