Addressing the strength-ductility trade-off in a cast Al-Li-Cu alloy—Synergistic effect of Sc-alloying and optimized artificial ageing scheme

Abstract

• The synergistic effect of Sc-alloying and decreased ageing temperature could greatly modify microstructures. • Excellent mechanical properties of YS (401 MPa), UTS (565 MPa) and EL (8.2%) were obtained. • The δ' and δ'-PFZs were extremely fine, accompanied by a large number of Cu-containing phases. • The mechanism of microstructure evolution in ageing process is analyzed in detail. A dramatic improvement of strength and ductility of cast Al-2.5Li-1.5Cu-1Zn-0.5Mg-0.15 Zr alloy was obtained by the collaboration of Sc-alloying and optimized ageing scheme. Joint and independent influence of Sc-alloying and different ageing temperatures were investigated. The results revealed that a substantial increase was realized in the hardness of Sc-containing alloy, and the ageing response time was only influenced by ageing temperature. Coarse and heterogeneous δ' (Al 3 Li), wide δ'-precipitation free zones (δ'-PFZs), and a large amount of T 1 (Al 2 CuLi) precipitates were observed in Sc-containing alloy aged at 175 °C, which resulted in superior yield strength and poor elongation. The Sc-containing alloy obtained an excellent combination of ductility (elongation = 8.2 %) and tensile strength (ultimate strength =565 MPa) suffered to 150 °C ageing for 64 h. The increase in the elongation was mainly due to the combined effect of grain refining, much finer δ', and extremely narrow δ'-PFZs (<10 nm), while the higher strength was mainly attributed to the formation of Al 3 (Sc, Zr, Li) composite particles and a large amount of S' (Al 2 CuMg) phase. However, the enhancement of the different ageing temperature (150 °C and 175 °C) on the mechanical properties of the alloys without Sc addition was not obvious.