Kinetics of B2- and D0₃-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 D0₃ 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 D0₃ 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 D0₃-type ordering as well as concurrent ordering and phase separation to disordered A2+D0₃. The simulated results provide a good reproduction of the formation processes of B2 and D0₃ ordered domains in an Fe₃Al alloy.