Correlations between short-range order parameters during short-range order reactions

Abstract

It is investigated by Monte Carlo simulations of short-range order reactions in an f.c.c. alloy A₇₅B₂₅ how the different short-range order parameters are correlated with each other. Thereby three alloys with an increasing number of energy parameters for the different coordination shells are studied at an annealing temperature, always 10% above the critical long-range ordering temperature. The values of the energy parameters are such that the long-range order below this temperature is always of Ll₂-type (Cu₃Au). The initial state of the crystal is always totally disordered. The correlations have been found to depend on the shell energies, on the atom occupations of those lattice sites, which directly influence the performance probability of the reaction step and on the crystallographic features of the operating diffusion mechanism. The latter point is investigated by employing besides the actual mechanism of vacancy interchanges with first neighbourhood atoms also e.g. the fictitious vacancy interchanges with atoms at random distances. The correlations, found by the Monte Carlo simulations, are quantitatively explained for the beginning of the reaction, where the atoms were randomly distributed onto the lattice sites.