Tensile-compression fatigue deformation tests were conducted on AZ31 magnesium alloy at room temperature. Electron backscatter diffraction (EBSD) scanning electron microscopy was used to scan the microstructure near the fatigue fracture surface. It was found that lamellar {10-11}-{10-12} secondary twins (STs) appeared inside primary {10-11} contraction twins (CTs), with a morphology similar to the previously discovered {10-12}-{10-12} STs. However, through detailed misorientation calibration, it was determined that this type of secondary twin is {10-11}-{10-12} ST. Through calculation and analysis, it was found that the matrix was under compressive stress in the normal direction (ND) during fatigue deformation, which was beneficial for the activation of primary {10-11} CTs. The local strain accommodation was evaluated based on the geometric compatibility parameter (m’) combined with the Schmid factor (SF) of the slip system, leading us to propose and discuss the possible formation mechanism of this secondary twin. The analysis results indicate that when the local strain caused by basal slip at the twin boundaries cannot be well transmitted, {10-11}-{10-12} STs are activated to coordinate the strain, and different loading directions lead to different formation mechanisms. Moreover, from the microstructure characterization near the entire fracture surface, we surmise that the presence of such secondary twins is not common.

The tensile and compression fatigue deformation tests of AZ31 magnesium alloy were carried out at room temperature, and the activation behavior of {10-12}-{10-12} secondary twins in different loading directions (ND normal direction, RD rolling direction) and different strain amplitude (0.5%, 1%) was studied. The results showed that {10-12}-{10-12} secondary twins were observed on the fracture surface of the fatigue samples in the ND at the strain of 1%. In contrast, no secondary twins were observed in the ND at the strain of 0.5% nor in the RD at the strain of 1% and 0.5%, and the fatigue life in the ND was greater than that in the RD. The analysis showed that the generation of {10-12}-{10-12} secondary twins was associated with the grain size and loading direction and that the fatigue deformation at 1% strain was more likely to generate {10-12}-{10-12} secondary twins due to the large lamellar thickness of the primary twins generated at relatively large strain conditions. The fatigue deformation along the ND tension was more favorable for the activation of multiple {10-12} primary twins, and the interaction of multiple twin variants was more favorable to activate {10-12}-{10-12} secondary twins.