Towards Understanding the Effect of Loading Direction on Fatigue Crack Behavior in Mg-3Al-Zn Magnesium Alloy

As the lightest structural metal material, magnesium alloy is increasingly widely used in aerospace, rail transportation and other fields. However, during its service, the action of alternating loads often induces fatigue fracture damage, which seriously threatens its service safety and stability. This study investigates the fatigue behaviors of Mg-3Al-Zn (AZ31) magnesium alloy along its rolling direction (RD) and normal direction (ND) under strain-controlled tension-tension cyclic loading. The strain-life curve reveals that ND specimens in which the deformation was dominated by twinning-detwinning exhibits higher fatigue life than RD specimens dominated by slip. Morphological analysis of crack patterns indicates many grain boundary fractures and long surface cracks in RD specimens, contrasting with the ND specimens' uniform distribution of short, linear cracks throughout the texture. Scanning Electron Microscopy (SEM) and Electron Backscatter Diffraction (EBSD) analyses suggest that crack initiation in RD specimens is likely at grain boundaries due to deformation incompatibility between the grains with soft and hard directions. Conversely, the cracks of ND specimen predominantly formed at the {10 $\overline{1}$ 2} twinning boundaries, owing to the strain incompatibility between the matrix and twin due to the discrepancy of basal dislocation activations.