Dissolution of precipitates heated above the solubility Line: A Monte Carlo simulation

The use of precipitate hardened alloys at high temperatures is often limited by the thermal stability of the precipitates. To improve the understanding of precipitate dissolution after a rapid increase of temperature, we used computer simulation to study a binary A-5 at. % B model alloy. The system consisted of A and B atoms on a quadratic lattice with nearest-neighbor attractive interactions between like atoms. The dynamics was provided by a single vacancy moving across the lattice by exchanges with neighboring atoms. The evolution of the precipitates was studied as a function of time at various temperatures T_a, starting from a given initial configuration, which was prepared by "annealing" a random mixture of A and B atoms at a low temperature inside the miscibility gap of the alloy. Depending on the new temperature T_a, different processes were observed. For T_a inside the miscibility gap, the precipitates stayed compact and dissolved partially at first, but afterwards their average size increased again by a coarsening process. For T_a outside the miscibility gap, but below the critical temperature T_c, the precipitates stayed compact and dissolved completely by evaporation of atoms from their surfaces. Above T_c, they decomposed rapidly into many smaller particles in a process that resembles an explosion. A theoretical description of these various processes, based on a phase field model, is presented.