Computer simulation of stress-oriented nucleation and growth of θ′ precipitates in Al-Cu alloys

Many structural transformations result in several orientation variants whose volume fractions and distributions can be controlled by applied stresses during nucleation, growth or coarsening. Depending on the type of stress and the coupling between the applied stress and the lattice misfit strain, the precipitate variants may be aligned parallel or perpendicular to the stress axis. This paper reports our studies on the effect of applied stresses on nucleation and growth of coherent θ′ precipitates in Al-Cu alloys using computer simulations based on a diffuse-interface phase-field kinetic model. In this model, the orientational differences among precipitate variants are distinguished by non-conserved structural field variables, whereas the compositional difference between the precipitate and matrix is described by a conserved field variable. The temporal evolution of the spatially dependent field variables is determined by numerically solving the time-dependent Ginzburg-Landau (TDGL) equations for the structural variables and the Cahn-Hilliard diffusion equation for composition. Random noises were introduced in both the composition and the structural order parameter fields to simulate the nucleation of θ′ precipitates. It is demonstrated that although an applied stress affects the microstructural development of a two-phase alloy during both the nucleation and growth stages, it is most effective to apply stresses during the initial nucleation stage for producing anisotropic precipitate alignment.