Kinetics of B2- and D0(3)-type ordering and formation of domain structures in Fe-Al alloys.

Time-dependent Ginzburg-Landau (TDGL) formulation has been developed for the ordering processes of B2 and D0(3) types in binary alloy systems. In the formulation, three order parameters are defined in order to describe the state of order. Equivalent variants of B2 and D0(3) structures are distinguished using these order parameters. The mean-field free energy is defined in the form of a Landau-type expansion using the order parameters and a composition parameter. Interface energies due to local variations in the degrees of order and concentration are given with a gradient square approximation. Kinetic equations are derived from the Ginzburg-Landau-type potential in order to describe the time-evolutions of the order parameters and the concentration. Numerical simulations of the kinetic equations have been performed for B2- and D0(3)-type ordering as well as concurrent ordering and phase separation to disordered A2+D0(3). The simulated results provide a good reproduction of the formation processes of B2 and D0(3) ordered domains in an Fe(3)Al alloy.