Thermal stability of the lightweight 2099 Al-Cu-Li alloy: Tensile tests and microstructural investigations after overaging

The thermal stability of the lightweight, T83 heat treated 2099 Al-Cu-Li alloy was assessed in the temperature range 200–305 °C, through both hardness and tensile tests after overaging. After prolonged thermal exposure, the alloy exhibited a better performance compared to aluminium alloys specifically developed for high temperature applications, with the advantage of a considerable lower density. The tensile behaviour was modelled through Hollomon's equation as a function of residual hardness. The changes in the alloy performance were explained through both SEM and STEM investigations. Microstructural analyses gave evidence of Ostwald ripening, while fractographic analyses revealed a transition from an intergranular to a ductile fracture mechanism in the overaged alloy. STEM investigations highlighted the superior thermal stability of the T1 phase compared to ϑ and S strengthening phases, which dissolved during overaging at 245 °C. The study underlines the need to enhance the formation of T1 precipitates when high temperature strength is required. The results of the present study suggest that the 2099 alloy is a very promising candidate for automotive engine components, which are extremely demanding in terms of both thermal resistance and lightweight.