Tailoring microstructure and performance in cast Zr/Sc/Mn-modified Al-Li-Cu-X alloys through synergistic alloying and heat treatment

Abstract

In this study, the effects of Zr, Sc, and Mn, along with the heat treatment, on the microstructure and mechanical properties of the cast Al-Li-Cu-X alloys were systematically investigated. It was found that Mn has a minimal effect on grain size, while Zr and Sc significantly contribute to grain refinement. Notably, the simultaneous addition of Zr and Sc reduces the average grain size to below 30 μm. Furthermore, the introduction of Zr and Sc promotes the formation of spherical L1₂-type Al₃X (X=Ti, Zr, Sc) particles. The addition of Mn promotes the formation of Mn-containing phases at the grain boundaries, contributing to Zener pinning. As aging progresses, δ' precipitates gradually grow and coarsen, while T₁ precipitates nucleate and grow. Zr, Sc, and Mn significantly influence the growth of δ' precipitates and the broadening of the δ'-PFZ. Specially, Zr, Sc, and Mn solutes preferentially combine with vacancies to form clusters, reducing the concentration of effective free vacancies in the matrix. This, in turn, decreases the diffusion rate of Li atoms and vacancies, which ultimately slows the growth of δ' precipitates and the broadening of δ'-PFZ. The addition of Mn, Zr, and Sc significantly improves the yield strength of the alloys, with the Zr/Sc/Mn-modified alloy exhibiting an excellent combination of strength and ductility. Finally, the underlying mechanisms driving microstructural evolution and the resulting mechanical performance were thoroughly examined.