Study on the microstructure and age hardening capability in Al–Cu–Li alloys with different Cu/Li ratio

Abstract

Abstract The microstructural evolution during natural ageing and artificial ageing treatment has been quantified in Al–Cu–Li alloys with Cu/Li ratios of 2.3 and 3.9. Methods including various ageing, hardness testing, transmission electron microscopy and differential scanning calorimetry were employed. The precipitation of T1 (Al2CuLi) phase was confirmed for the first time in the high Li content alloy under natural ageing treatment for 5 months, while the Li-lean alloy exhibits barely any precipitation at room temperature. Under artificial ageing, the Li-rich alloy exhibits a significant increase in hardness due to the formation of high density spherical δ′ phase. On the other hand, the increasing Cu/Li ratio promotes the precipitation of the Cu containing precipitates T1 and θ (Al2Cu), the high Cu/Li ratio (3.9) alloy shows a recovery of ductility, with a uniform elongation of ∼20 %, which is caused by the strong interactions between the dislocations and the nano-scale T1 and θ precipitates. Meanwhile, as the main strengthening precipitate changes from θ and T1 to T1 alone with prolonged ageing time, the alloy displays a double-peak age hardening behavior. This work sheds light on the design of high strength and ductile Al–Li alloys through the well-controlled T1 phase precipitation.