Tailoring a Heterogeneous Bimodal Structure for Superior Strength–Ductility Synergy in Dilute Mg-0.4Al-0.3Ca-0.2Mn-xSn Alloy: The Critical Role of Trace Sn Microalloying

To achieve an optimal balance of mechanical properties in low-cost alloy systems, this study tailored a heterogeneous bimodal structure in dilute Mg-0.4Al-0.3Ca-0.2Mn-xSn alloys (x = 0, 0.1 wt.%) and systematically investigated the critical role of trace Sn microalloying during hot extrusion. Mg-0.4Al-0.3Ca-0.2Mn-xSn alloys were fabricated via melting, homogenization, and subsequent hot extrusion at 320 °C. Trace Sn addition induced the formation of uniformly distributed CaMgSn phases within the homogenized matrix, facilitating a synergistic enhancement of strength and ductility. Specifically, the extruded alloys exhibited a characteristic bimodal grain structure consisting of coarse un-dynamic recrystallized (unDRXed) grains and fine dynamic recrystallized (DRXed) grains. Sn microalloying effectively refined the DRXed grains from 2.66 μm to 2.11 μm and significantly boosted the elongation (EL) from 12.9% to 26.3% while maintaining an Ultimate Tensile Strength (UTS) of 274 MPa. The Sn-containing secondary phases served as potent sites for particle-stimulated nucleation (PSN), thereby promoting the DRX process and reducing the texture intensity from 20.89 to 9.99. Overall, the superior strength-ductility synergy is primarily governed by the formation of the heterogeneous bimodal structure, where trace Sn facilitates grain refinement and texture weakening through PSN mechanisms, providing a robust strategy for the design of high-performance dilute magnesium alloys.