Optimization of microstructure and mechanical properties for Al-Si hypoeutectic alloy via the strategic nucleation and evolution of multiscale heterogeneous phases

Squeeze casting and appropriate heat treatment was employed to prepare high-strength and heat-resistant Al-Si hypoeutectic alloy ingots. The prominent mechanical properties were synergistically achieved, including 361 MPa (YS of 25 °C), 394 MPa (UTS of 25 °C), 1% (EL of 25 °C), 85 MPa (YS of 350 °C), and 121 MPa (UTS of 350 °C). Consequently, approximately 23.5% of the YS was maintained at the elevated temperature of 350 °C, compared with the YS at 25 °C. It was attributed to high-stability and exceeding-strength precipitated phases of Al₈Mn₄Gd, Al₃Cu(Ni, Yb), and L1₂-Al₃(Zr, Ti, Gd, Yb) and extensive aggregation of nano-scale eutectic Si, Al₈Mn₄Gd, Mg₂Si, Al₃Cu(Ni, Yb), Al₂Cu, and Al₃(Zr, Ti, Gd, Yb). The performance of the designed alloy was additionally enhanced through three aspects. Firstly, second-phases strengthening of eutectic Si, Alpha phases (Al₆MnSiCuNi), and Al₈Mn₄Gd phases. Secondly, α-Al matrix strengthening of precipitated phases including Mg₂Si, Al₂Cu, Al₃Cu (Ni, Yb), Al₈Mn₄Gd, and L1₂-Al₃(Zr, Ti, Gd, Yb) phase. Thirdly, misalignment behaviors (including dislocations, stacking faults, and twins) presented widespread distribution among α-Al matrix, around second phases and at the boundaries. Fine-grain strengthening, dislocation strengthening, second-phase strengthening, and composite strengthening served as essential guarantees for the simultaneous enhancements of mechanical properties. Therefore, both room-temperature and high-temperature performances were guaranteed by the nucleation and aggregation evolution of multiscale heterogeneous phases, further ensuring the application prospect.