A time-dependent Ginzburg-Landau formulation has been developed for L1 2-type ordering in face-centered cubic-based binary alloy systems. The fundamental face-centered cubic lattice is divided into four simple cubic sublattices. The site occupation probabilities of constituent atoms on the sublattices are given as a function of three order parameters and a composition parameter. The mean-field bulk free energy is defined using these four parameters. Excess energies due to local variations of degrees of order and concentration are given in the gradient square approximation with cubic symmetry, so that the orientation dependence of off-phase boundaries of ordered domains is incorporated. Kinetic equations for the evolutions of the local order parameters and the local composition with time are derived from the Ginzburg-Landau-type potential. Three-dimensional numerical simulations are performed on the basis of the derived kinetic equations. The results are compared with the formation of L12 ordered domains in Cu 3Au and Pt3Co alloys.
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