Dynamics of mesoscopic precipitate lattices in phase-separating alloys under external load

– We investigate, via three-dimensional atomistic computer simulations, phase separation in an alloy under external load. A regular two-dimensional array of cylindrical precipitates, forming a mesoscopic precipitate lattice, evolves in the case of applied tensile stress by the movement of mesoscopic lattice defects. A striking similarity to ordinary crystals is found in the movement of “meso-dislocations”, but new mechanisms are also observed. Point defects such as “meso-vacancies” or “meso-interstitials” are created or annihilated locally by merging and splitting of precipitates. When the system is subjected to compressive stress, we observe stacking faults in the mesoscopic one-dimensional array of plate-like precipitates. The coarsening of precipitates in a phase-separating alloy usually proceeds via an Ostwald ripening process where large particles grow at the expense of smaller ones [1]. The driving force for this process is the tendency to reduce the total amount of interface between matrix and precipitates. The coarsening kinetics follows the laws of dynamical scaling, with a characteristic length R(t) growing like t, as first described by the theory of Lifshitz, Slyozov and Wagner (LSW theory) when the density of precipitate particles is small [2, 3]. The situation changes radically when a difference in lattice spacing between matrix and precipitates (lattice misfit) gives rise to long-range elastic interactions. The precipitates then tend to be cuboidal or plate-like instead of spherical and start to align themselves into more or less regular arrays to minimize the elastic energy [4–8]. Changes in the growth kinetics of precipitates, especially a slowing-down in the later stages, have also been reported [9, 10]. When a uniaxial external stress is applied, the cubic symmetry is broken and precipitates arrange themselves into arrays of cylinders parallel to or of plates perpendicular to the direction of external stress (rafting) [11–16].