Effect of Al content on the microstructural characteristics and tensile and fatigue properties of extruded SEN Mg alloys

This study systematically investigates the effects of Al content on the microstructure, tensile properties, and high-cycle fatigue behavior of extruded Mg–xAl–Zn–Ca–Y (SENx) alloys. The results reveal that all extruded alloys exhibit fully recrystallized microstructures with undissolved second-phase particles enriched with Ca and Y. The average grain size varies non-monotonically with Al content due to the competing effects of recrystallization kinetics, solute drag, and particle band distribution. As Al content increases, tensile strength increases, while ductility decreases. Notably, despite its relatively coarse grain structure, the SEN9 alloy exhibits significantly higher yield strength than the SEN6 alloy. This improvement is mainly attributed to additional precipitation strengthening from fine Mg₁₇Al₁₂ discontinuous precipitates formed along grain boundaries. Meanwhile, the reduced elongation observed in the SEN9 and SEN11 alloys is attributed to premature fracture at coarse Mg₁₇(Al,Zn,Ca)₁₂ particle bands aligned along the extrusion direction. From the SEN1 to SEN9 alloys, the fatigue strength (FS) increases approximately linearly with ultimate tensile strength (UTS), reaching 170 MPa. However, this FS–UTS proportionality fails in the SEN11 alloy, where FS falls to 120 MPa despite the highest UTS. This deviation from the FS–UTS correlation that is observed for the lower-Al-content alloys is attributed to the clustered distribution of coarse Mg₁₇(Al,Zn,Ca)₁₂ particles in the SEN11 alloy, which promotes early crack initiation under cyclic loading. These findings emphasize the importance of controlled Al addition in enhancing both strength and fatigue resistance while also highlighting the adverse effects of excessive Al content owing to microstructural embrittlement.