Temperature-dependent stability of θ′-Al₂Cu precipitates investigated with phase field simulations and experiments

The upper limit of service temperature for many Al-Cu alloys is determined by the thermal stability of strengthening θ′ (Al₂Cu) precipitates. Above a certain temperature, θ′ precipitates will undergo morphological evolution and transform into the detrimental, equilibrium θ phase, leading to a rapid drop in strength. Certain alloying elements have recently been reported to increase the thermal stability of θ′ precipitates, by mechanisms that are yet unclear. Herein, we investigate the effect of modified interfacial energy and solute chemical mobility on the thermal stability of θ′ via high-throughput phase field study. We identify a critical θ′ aspect ratio to predict the onset of θ formation. Using this criterion, we predict the time required for θ′ to θ phase transformation as a function of temperature, Cu diffusivity, and the interfacial energy of θ′ precipitates. The predicted times compare favorably with reported times for θ formation under similar experimental conditions. These phase field simulations predict that a moderate reduction in Cu mobilityis adequate to stabilize the as-aged microstructure up to 300 °C, while substantial reductions to both interfacial energy and Cu mobility are needed to achieve similar stability at 400 °C. Experimental microstructural evolution results in commercial (319) and thermally stabilized (RR350) cast aluminum alloys are presented to complement the simulations.