DISSOLUTION OF PRECIPITATES HEATED ABOVE THE SOLUBILITY LINE - A MONTE-CARLO SIMULATION

The use of precipitate hardened alloys at high temperatures is often l imited by the thermal stability of the precipitates. To improve the un derstanding of precipitate dissolution after a rapid increase of tempe rature, we used computer simulation to study a binary A -5 at.% B mode l 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 w as 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 m iscibility gap of the alloy. Depending on the new temperature T-a, dif ferent processes were observed. For T-a inside the miscibility gap, th e precipitates stayed compact and dissolved partially at first, but af terwards their average size increased again by a coarsening process. F or T-a outside the miscibility gap, but below the critical temperature T-c, the precipitates stayed compact and dissolved completely by evap oration of atoms from their surfaces. Above T-c, they decomposed rapid ly into many smaller particles in a process that resembles an explosio n. A theoretical description of these various processes, based on a ph ase field model, is presented.