Relaxed Monte Carlo Simulations on Au-Ni Alloy

Abstract

This work deals with statistical thermodynamics of binary systems including, in addition to the classical Ising model, the lattice relaxation (atomic and volume). More precisely, it is a contribution to the study of the consequences of the atomic size effect on thermodynamic properties of binary alloys. We have taken as an example the Au-Ni system in which the size difference between the two constituents is very large (≅15%). This system presents a miscibility gap below 1100K (Hultgren et al., 1963; Massalsky, 1986) which is strongly asymmetric, its maximum temperature corresponding to 70.6% Ni. The reason why this miscibility gap occurs has stimulated many thermodynamic and atomistic investigations (see for example Hansen, 1958; Hultgren et al., 1963, and references therein). It is well known now that the lattice-distorsion energy rather than the energetic favoring of bond between like atoms is responsible for the phase segregation (Wu et al., 1982, 1983, 1984). Recently, Lu and Zunger (1994) have calculated the two contributions of the formation energy of the alloy, a pure chemical one (reflecting charge transfert and atomic relaxation) and a volume deformation energy reflecting the size effect. Their results show that the chemical term is negative and then corresponds to an ordering tendency, whereas the second one, always positive but here very large is responsible for the phase separation. These features make Au-Ni a very convenient system for our purpose.