Cu-Au L1₀型規則合金の相安定性について

抄録

Stability of the L1₀-type ordered phase appearing in the Cu_1−cAu_c alloy system near the composition of c=1⁄2 is studied by the use of pseudopotential treatment which is a conventional method extended previously by the author to the noble-simple or noble-noble metal alloy systems. The structure-dependent energy U is conveniently composed of three terms; the energy of conduction electrons within an alloy crystal field (U_el), the electrostatic energy of ions occupying lattice sites (U_es) and the repulsive energy of ions due to overlap of d-electrons (U_rp).
The ordering energy of L1₀-type ordered phase is calculated for various c’s, and the energy change in U due to the tetragonal distortion for the CuAu alloy is obtained as a function of the axial ratio x. Numerical results for the ordering energy show negative values in the composition range 0.4≤c≤0.6 and explain successfully the fact that the ordered phase with L1₀-type exists at a low temperature as a stable phase. An estimation of the transition temperature gives a value of 1060 K for the CuAu alloy which is higher than the observed one (658 K). On the other hand, for the CuAu alloy, energy U calculated as a function of x has a minimum at x\simeq0.60, which is far from the observation (x=0.926), although it shows another minimum at x\simeq0.90.