Unraveling the Distinct Roles of Al and Ca in Microstructure Evolution and Tensile Response of Extruded Mg–Al–Ca Alloys

Mg-Al-Ca alloys are attractive low-cost wrought Mg alloys. However, the distinct roles of Al and Ca in regulating deformation-processed microstructures and mechanical properties remain unclear. In this work, Mg–6Al–0.5Ca, Mg–9Al–0.5Ca, and Mg–9Al–1.3Ca (wt. %) alloys were extruded at 250 °C and 300 °C to clarify the composition-dependent microstructure evolution and strengthening mechanisms. Increasing the Al content from 6 to 9 wt. % markedly promoted the formation of fine Mg17Al12 (f-Mg17Al12) and coarse Mg17Al12 particles, whereas increasing the Ca content from 0.5 to 1.3 wt. % promoted the formation of coarse Al2Ca particles while reducing the density of f-Mg17Al12. Quantitative analysis revealed that f-Mg17Al12 particles refined dynamically recrystallized grains by promoting recrystallization nucleation and pinning grain boundaries while also contributing to Orowan strengthening. The Mg–9Al–0.5Ca alloy exhibited the best strength–ductility balance, with a yield strength of 338 ± 4 MPa, ultimate tensile strength of 396 ± 5 MPa, and elongation of 8.7 ± 1.6% after extrusion at 250 °C. Strengthening calculations indicated that grain-boundary strengthening was the dominant strengthening contribution, while the strength advantage of Mg–9Al–0.5Ca originated from the dual role of f-Mg17Al12 in grain refinement and dislocation obstruction. These findings provide a practical strategy for designing high-strength non-rare-earth Mg–Al–Ca extrusion alloys.