Nano-structural engineering through Mg/Cu ratio optimization for strength-ductility balance in Al-Li alloys

In this work, the effect of the Mg/Cu ratio ranging from 0 to 1 on the microstructural evolution and mechanical behavior of cast Al-Li-Cu-Mg alloys was systematically investigated using multi-scale characterization. The results reveal that increasing the Mg/Cu ratio enhances constitutional supercooling during solidification, thereby inducing significant grain refinement. The Mg/Cu ratio governs the competitive precipitation among δ′, T 1 , θ′, and S′ nanoprecipitates. Quantitative analysis shows that as the Mg/Cu ratio increases, the volume fraction of the δ′ phase decreases monotonically, whereas the T 1 and θ′ phases display complementary variations, and the S ′ precipitation occurs at higher Mg/Cu ratios. The strength mainly originates from Orowan strengthening by nanoprecipitates and short-range order strengthening by Cu-Mg clusters. Meanwhile, grain refinement and the narrowing of the δ′ precipitate-free zone improve ductility. An optimal balance of mechanical properties was achieved at an Mg/Cu ratio of 0.31 in the Al-2.5Li-1.8Cu-0.6Mg-0.15Zr alloy (YS = 363 MPa, UTS = 461 MPa, EL = 4.0 %). This work establishes a quantitative framework for understanding Mg/Cu-regulated nanoprecipitation, providing theoretical guidance and an experimental basis for the design of high-performance cast Al-Li alloys. • Mg/Cu ratio is a critical parameter controlling competitive precipitation of δ′, T 1 , θ′, and S′ phases. • Strengthening is dominated by nanoprecipitates and clusters, while ductility arises from grain refinement and PFZ narrowing. • An optimal Mg/Cu ratio (∼0.31) achieves superior strength-ductility synergy.