Progress in Theoretical Modeling of fcc-L1₀-L1₂ Phase Equilibria

抄録

The progress in fcc-based phase diagram investigation has been made based on the Cluster Variation Method. We first performed the prototype calculation in which pair interaction energies are given as constant parameters, and the importance of atomic correlations played in the free energy formula has been pointed out. Although the resulting phase diagrams demonstrated overwhelming advantages over the ones obtained by the Bragg-Williams approximation, serious drawback is the completely symmetrical feature of the phase boundary around 50at%. We, then, proceeded on the phenomenological model of which parameters are fitted to the thermodynamic data. By the introduction of concentration dependency to the pair interaction energies, the asymmetry of Cu-Au phase diagram has been actualized. Finally, together with the electronic structure calculation via ASW method and statistical numerical calculation via Cluster Variation Method, the first-principles calculation was attempted to draw the phase diagrams of three kinds of noble-metal alloys; Cu-Au, Cu-Ag and Ag-Au. The physical origin of the distinctly 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.