Precipitates strengthening mechanism of a new squeeze-cast Al–Cu–Li–Mn alloy with high strength and ductility

In this paper, aging precipitates and their high effects on mechanical properties of squeeze-cast Al–5Cu-0.6Li-0.5Mn-0.3Mg-0.15Ti alloy (a novel Al–Cu–Li–Mn alloy) were investigated to reveal strengthening & toughening mechanism. After T6 heat treatment (i.e., solution treatment at 530 °C for 10 h + aging at 180 °C for 8 h), its ultimate tensile strength (UTS), yield strength (YS) and elongation (El.) are 465 MPa, 310 MPa and 16.5%, respectively. Compared with the as-cast Al–Cu–Li–Mn alloy, the UTS and YS are increased by 55% and 63.2%, respectively, with almost no loss in ductility. Interestingly, the product of UTS and El. (i.e., UTS•El.) of the T6-treated Al–Cu–Li–Mn alloy reaches 7.67 GPa%, which is 50.4% higher than that of the as-cast Al–Cu–Li–Mn alloy and better than that of most third-generation or fourth-generation Al–Li and Li-free 2xxx alloys prepared by casting or plastic deformation followed by heat treatment. The uniformly dispersed submicron-sized T (AlxMnyCuz), nano-sized T1 (Al2CuLi) and much smaller θ' (Al2Cu) phases precipitated in the T6-treated Al–Cu–Li–Mn alloy are coherent or semi-coherent with aluminum matrix, which not only enhances strength but also delays strain localization and fracture. In addition, the actual average thickness of T1 and θ′ precipitates are about 2 nm and 3.5 nm, which are much larger than their critical values for the conversion of their strengthening mechanisms. Therefore, the main strengthening mechanism of the squeeze-cast Al–Cu–Li–Mn alloy is the Orowan bypassing mechanism rather than the shearing mechanism.