Tailoring microstructure and mechanical properties of high-pressure die-cast Mg-RE-Gd alloys via trace Al additions

• Al addition triggers changes in Al-RE phase types and morphology distribution. • DFT calculations reveal Al-RE exhibits a higher modulus than Mg-RE. • The strength difference results from Mg 12 RE morphology and connectivity. • Al serves as the preferred element for guiding HPDC alloy design. A trade-off between strength and ductility often constrains the widespread application of high-pressure die-casting (HPDC) Mg-RE alloys. This study modulates the intermetallic compounds at the grain boundary (GB) in Mg-3.5RE-1.5Gd alloys through trace Al additions (0, 0.5, and 1.0 wt.%). Multiscale characterization and density functional theory (DFT) revealed a transition from metastable Mg 3 RE (Al-free) to petal-like Al 2 RE 3 (0.5Al), followed by the coexistence of blocky Al 2 RE and striped Al 11 RE 3 (1.0Al). As the Al content increases, the Mg 12 RE network remains the major phase, but its connectivity weakens. At room temperature (RT), yield strength (YS) decreases from 175 to 169 and 165 MPa, whereas ultimate tensile strength (UTS) increases from 180 to 200 and 205 MPa, and elongation (El) rises from 1.9% to 2.3% and 2.4%. At 250 °C, the El increased while both YS and UTS decreased. At 300 °C, the Al-containing alloy exhibited a comparatively high level of El, though this was lower than that observed in the Al-free alloy. This outcome is consistent with the weakened Mg 12 RE network connectivity. The fracture analysis revealed a mixed quasi-cleavage fracture with dimples at RT. At elevated temperatures, the predominant form of fracture is intergranular ductile fracture. DFT calculations confirm that Al-RE compounds exhibit more negative formation enthalpies and higher moduli than Mg-RE phases. However, the continuous Mg 12 RE framework phase provides superior GB pinning and load transfer capabilities. The present study elucidates the Al-mediated phase control mechanism, thus offering a viable alloy design pathway for the optimization of HPDC Mg-RE alloys.