Si-induced aging response acceleration for high strength-ductility in cast Al-Cu-Mg alloy

High strength-ductility cast aluminum alloys play a critical role in lightweight engineering applications. However, these high-strength alloys often exhibit low elongation, making it challenging to design alloys with a balanced combination of strength and ductility. In this paper, we found that the 0.3% Si-microalloying remarkably accelerates the age-response of the cast Al-Cu-Mg alloy and the optimized Si-containing alloy shows excellent strength-ductility with yield strength of 446.8 MPa, ultimate tensile strength of 509.2 MPa, representing increases of 86.2 MPa and 34.1 MPa compared to the Si-free alloy, while maintaining a high elongation of 11.6%. TEM observations show that the age-strengthening precipitates have a higher number density and smaller size in the Si-containing alloy. DSC analysis coupled with computational modeling reveals that the Si addition reduces the precipitation activation energy and precipitation temperatures for θ′ nano-strengthen phase and accelerates the aging strengthen. The addition of Si facilitates the transformation of S-phase into Q′′ precipitates, which exhibit superior strengthening efficacy compared to S-phase, while simultaneously accelerating θ′ precipitation kinetics. The nanoscale θ′ phase, acting as the primary strengthening constituent, inhibits dislocation accumulation by redistributing interfacial stress concentrations, thereby delaying crack initiation. This synergistic effect between Q′′ (replacing S-phase) and θ′ precipitates enhances strain hardening capacity while preserving ductility, ultimately elevating the alloy’s mechanical performance. This work proposes an effective microalloying strategy to overcome traditional strength-ductility compromises in cast Al-Cu-Mg alloys.