Dual effect of Cu on the Al₃Sc nanoprecipitate coarsening

It is generally considered that the Al₃Sc nanoprecipitates are highly thermal stable, mainly due to quite slow Sc diffusion in the α-Al matrix. In this paper, we demonstrate in an Al-Cu-Sc alloy that the Cu atoms have dual effect on the coarsening of Al₃Sc nanoprecipitates. On the one hand, the Cu atoms with high diffusivity tend to accelerate the Al₃Sc coarsening, which results from the Cu-promoted Sc diffusion. On the other hand, some Cu atoms will segregate at the Al₃Sc/matrix interface, which further stabilizes the Al₃Sc nanoprecipitates by reducing the interfacial energy. Competition between these two effects is tailored by temperature, which rationalizes the experimental findings that the coarsening kinetics of Al₃Sc nanoprecipitate is greatly boosted at 300 °C-overaging while significantly suppressed at 400 °C-overaging.