High fatigue strength in a Mg-Al-Zn-Mn alloy with bimodal microstructure

Mg-3Al-1Zn-0.3Mn alloy was fabricated by low-temperature extrusion to engineer a heterogeneous bimodal structure of coarse deformed grains and a fine dynamically recrystallized (DRXed) matrix. It exhibits a yield strength of 330 MPa, an ultimate tensile strength of 386 MPa, and a fatigue strength of 214 MPa at 107 cycles under R = 0, among the highest values reported for rare-earth-free Mg alloys. Mechanistic characterization reveals progressive participation of DRXed grains in cyclic deformation and activation of multiple dislocation slip modes, enabling distributed strain storage and accommodation. Meanwhile, the nearly fully-recrystallized counterpart shows localized basal slip and cleavage-dominated final fracture morphology.