Mechanism of "order-order" kinetics in L12 superstructure studied by computer simulation

Monte Carlo simulations of the isothermal long-range order (LRO) relaxation in A(3)B system with LI, superstructure have been performed within a model based on a vacancy jump mechanism between nearest neighbour lattice sites. The studies aimed at the explanation of the origin of two simultaneous LRO relaxation processes experimentally observed in Ni,AI. An effect of pair interaction energies and saddle-point energies (assigned to jumping atoms) was studied. The preference of vacancies for the A-sublattice (face centres), commonly postulated for Ni,AI, occurred only for a narrow range of pair-interaction energies within the domain corresponding to LI, ordering. It was found that the appearance of the fast relaxation process is definitely correlated with the efficiency of B-atom jumps, as well as with the values of the saddlepoint energies. The results lead to a microscopic model of the "order-order" relaxation in Ll(2) superstructure and yield new indications for the choice of pair-interaction energy parameters applied in Monte Carlo simulations of atomic migration in Ll(2) intermetallic compounds.