Microstructural Evolution and Mechanical Properties of Cast Al–2.6Li–2.8Mg–(0.5, 1.4)Cu–0.2Zr–0.2Sc Alloys during Heat Treatment

Cu is commonly used to improve the mechanical properties of Al–Li alloys, but its exact role in multicomponent Al–Li alloys is not fully understood. Herein, the microstructure and mechanical properties of two cast Al–2.6Li–2.8Mg–(0.5, 1.4)Cu–0.2Zr–0.2Sc (wt%) alloys aged at 165 °C are compared. The results show that Alloy 1.4Cu possesses finer grain size and better mechanical properties than Alloy 0.5Cu. The δ′ (Al₃Li), Al₃(Sc, Zr, Li) phases, and Guinier–Preston–Bagaryatsky zones are formed in both alloys. During the early stage of ageing (8 h), many dispersed S′ (Al₂CuMg) phases precipitate only in Alloy 1.4Cu and coarsen continuously. Instead, many rod-shaped S₁ (Al₂MgLi) phases start to precipitate in Alloy 0.5Cu up to ageing for 32 h and then coarsen continuously with ageing. Moreover, Alloy 1.4Cu shows narrower δ′-precipitate-free zones along grain boundaries than Alloy 0.5Cu. The higher strength and ductility with more uniform strain distribution for Alloy 1.4Cu than Alloy 0.5Cu is attributed to finer grain size and unique phase formation for the former.