Atomistic simulation of dislocation-assisted γ-precipitate nucleation in Mg-Al alloys

Magnesium has drawn increasing interests as a lightweight material for applications in transportation and aerospace industries. Mechanical processing methods like equal channel angular extrusion (ECAE) have shown potential in controlling the morphology of the precipitate particles for enhanced precipitation hardening performance. Understanding how deformation affects the precipitation process is crucial for processing condition optimization, property prediction, and materials design.

We study the dislocation-assisted precipitate nucleation of Mg₁₇Al₁₂ precipitates in Mg-Al alloys. We determine the critical nucleus size under different composition and stress conditions. The critical nucleus size is very sensitive to the solute concentration. The local stress introduced by dislocations can significantly change the nucleation barrier and the nucleation path. The local stress not only affects the misfit strain energy, but also significantly alters the relative stability of the phases. As a result, although γ precipitate is denser than the α matrix, counter-intuitively nucleation is favored under tension conditions rather than compression conditions. We incorporate our results into continuum model and compare with recent ECAE experiments.