Computer simulation of structural relaxations in intermetallics

The intermetallic compound Ni3Al, one of the well-known high-temperature L1 (2) (gamma (t)) superalloys is an excellent model system for the studies of long-range order (LRO) relaxation run by atomic jumps to nn vacancies in a homogeneous superstructure. Experimental studies of "order-order" kinetics in Ni3Al carried out by means of resistometry revealed that the relaxation isotherms of the Bragg-Williams LRO parameter eta were composed of two sin gle exponentials substantially differing in relaxation times. Later investi gations suggested that this effect may show up in systems with different ty pes of superlattices. The origin of the phenomenon was studied by means of Monte Carlo (MC) simul ations. Original studies were performed with a model system described by an Ising Hamiltonian with phenomenological pair-interaction energy parameters . Atomic jumps were simulated using the Glauber algorithm, in which their p robabilities depend on differences of system energies before and after the jumps. It was concluded that the fast component of the eta (t) relaxation c onsists of the efficient elimination/creation of nn antisite pairs resultin g from highly correlated ordering/disordering jumps of Al- and Ni-atoms. Th e process permanently competes with Al-antisite migration within Ni-sublatt ice, which is a mechanism of a slower elimination/creation of anti sites sh owing up as the slow eta -relaxation component. An important drawback of the kinetic model based on the Glauber algorithm i s the negligence of saddle-point energies, which in any case must be overco me by real atoms when jumping from lattice sites to nn vacancies. In the pr esent paper it is proposed to go beyond the Ising model and to approach the problem by directly implementing the "embedded-atom-method" (EAM) formalis m for lattice energy with the "activated-state-rate approximation" and MC s imulation. Assuming that atoms jump only to mi vacancies, "order-order" kinetics (and any other process controlled by atomic migration) is simulated by calculati ng on line changes of the system energy DeltaE caused by each atomic jump. It is important that, using EAM, it is possible to evaluate DeltaE being th e difference not only between the system energies before and after the jump , but also between the energy of a system with an atom on the saddle-point and its energy before the jump. Consequently, EAM may be implemented with a ny algorithm applied in MC simulation of atomic migration and postulating p articular formula for the jump probability. A systematic comparative study of "order-order" kinetics in Ni3Al is carrie d out by means of MC simulations involving Glauber and "residence-time" alg orithms implemented with EAM formalism. The results yield additional criter ia for the evaluation of EAM potentials for Ni3Al.