Achieving 500 MPa strength in rare-earth free magnesium alloys: synergetic ultrafine graining and multi-scale precipitation

The development of high-strength rare-earth (RE)-free magnesium alloys with a desirable strength-ductility synergy remains a significant challenge. Here, we report a novel RE-free Mg-3.5Ag-0.4Mn-0.2Ca (MAMC) alloy processed via a secondary ultra-low speed hot extrusion (SHE) strategy, achieving an exceptional ultimate tensile strength of 518 MPa and a yield strength of 493 MPa, while maintaining an elongation of 8.7%. This superior performance originates from a meticulously engineered microstructure comprising ultrafine α-Mg grains (∼330 nm) and hierarchical multi-scale precipitates (Mg 54 Ag 17 , MgCaAg 2 with/without β-Mn cores, and Mn-rich clusters <5 nm). This superior performance originates from a meticulously engineered microstructure consisting of ultrafine α-Mg grains (∼330 nm) and a hierarchical distribution of multi-scale precipitates, including Mg 54 Ag 17 , MgCaAg 2 (with and without β-Mn cores), and Mn-rich clusters (<5 nm). Mechanistically, the exceptional grain refinement is driven by continuous dynamic recrystallization (CDRX) facilitated by the ultra-low strain rate, while the multi-scale precipitation architecture arises from a staged thermodynamic driving force combined with dislocation-mediated solute transport during the two-step extrusion. First-principles calculations further elucidate the role of β-Mn as a heterogeneous nucleation site for MgCaAg 2 , driven by strong interfacial electronic hybridization and low interfacial energy. Quantitative strengthening analysis demonstrates that Hall-Petch, modulus mismatch, dislocation, Orowan, and solid-solution mechanisms collectively contribute to the outstanding yield strength. This work provides a viable pathway for designing high-performance RE-free Mg alloys with strength levels comparable to advanced RE-containing counterparts, offering significant promise for lightweight structural applications.