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 Mg₃RE (Al-free) to petal-like Al₂RE₃ (0.5Al), followed by the coexistence of blocky Al₂RE and striped Al₁₁RE₃ (1.0Al). As the Al content increases, the Mg₁₂RE 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 ℃, the El increased while both YS and UTS decreased. At 300 ℃, 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 Mg₁₂RE 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 Mg₁₂RE 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.

Magnesium alloys are lightweight materials with great potential, and plasma electrolytic oxidation (PEO) is effective surface treatment for necessary improvement of corrosion resistance of magnesium alloys. However, the ~14 µm thick and rough PEO protection layer has inferior wear resistance, which limits magnesium alloys as sliding or reciprocating parts, where magnesium alloys have special advantages by their inherent damping and denoising properties and attractive light-weighting. Here a novel super wear-resistant coating for magnesium alloys was achieved, via the discontinuous sealing (DCS) of a 1.3 µm thick polytetrafluoroethylene (PTFE) polymer layer with an initial area fraction (A_f) of 70% on the necessary PEO protection layer by selective spraying, and the wear resistance was exceptionally enhanced by ~5500 times in comparison with the base PEO coating. The initial surface roughness (Sa) under PEO+DCS (1.54 µm) was imperfectly 59% higher than that under PEO and conventional continuous sealing (CS). Interestingly, DCS was surprisingly 20 times superior for enhancing wear resistance in contrast to CS. DCS induced nano-cracks that splitted DCS layer into multilayer nano-blocks, and DCS also provided extra space for the movement of nano-blocks, which resulted in rolling friction and nano lubrication. Further, DCS promoted mixed wear of the PTFE polymer layer and the PEO coating, and the PTFE layer (HV: 6 Kg·mm⁻², A_f: 92.2%) and the PEO coating (HV: 310 Kg·mm⁻², A_f: 7.8%) served as the soft matrix and the hard point, respectively. Moreover, the dynamic decrease of S_a by 29% during wear also contributed to the super wear resistance. The strategy of depositing a low-frictional discontinuous layer on a rough and hard layer or matrix also opens a window for achieving super wear-resistant coatings in other materials.