# The Strain Heterogeneity and Microstructural Shear Bands in AZ31B Magnesium Alloy
## 3 Results
### Microstructure Evolution  
To investigate the macroscopic deformation characteristics of magnesium alloys under tensile loading and gain deeper insights into the microstructural transformations, particularly the activation patterns of twins within the deformation zone, CHANNEL5 software was employed to scrutinize the scanning images and quantify the microscopic deformation responses of the samples across varying loading strains. Figure 7 illustrates these findings, with arrows clearly marking the direction of loading. At a true strain of 0.01, the material  
Figure 6: Failure morphology of the sample. (**a**) The appearance near the fracture; (**b**) The dense network of slip lines within the shear region; (**c**) Enlarged view of the crack region.  
initiates yielding, transitioning into the plastic deformation regime, wherein initial twinning within selected grains was observed as shown in Figure 7a. As the strain escalated to 0.02, a notable increase in the width and density of twins was evident, accompanied by a significant grain reorientation, primarily manifesting as a \(\sim\)57\({}^{\circ}\) rotation along the C-axis, indicative of a compressive twin as shown in Figure 7b. This information could be observed in the enlarged image to the right of Figure 7b. Further straining to 0.05 prompted an expansion in the twinning phenomenon, with a pronounced concentration of twins forming along the shear direction, constituting a distinct twin concentration zone as shown in Figure 7c. Upon reaching a strain of 0.12, the sample exhibited signs of damage, marked by severe plastic deformation within the twin bands. This underscores the proliferation of high-density twins in grains aligned along the shear direction of the AZ31 magnesium alloy during uniaxial tensile testing. The observed microstructural evolution was consistent with the failure morphology, highlighting the severity of deformation along the shear axis.  
To delve deeper into the microstructural transformations of the AZ31 magnesium alloy subjected to uniaxial tension, our analysis focused on the dislocation density distribution within the in-situ monitoring region. By processing the EBSD scanning outcomes from the in-situ experiments, we generated KAM (Kernel Average Misorientation) diagrams for varying loading strains, which provided a statistical portrayal of the local orientation variations among the grains, as shown in Figure 8. The result showed that the dislocation density escalated progressively as the loading strain intensified, with a pronounced concentration observed along the shear direction. Notably, at strains of 0.01 and 0.02, a high-density dislocation zone emerged, aligning seamlessly with the twin concentration zone depicted in Figure 7 in terms of both direction and width. As the loading strain advanced to 0.05, the dislocation density within this concentrated band intensified further. Ultimately, at a strain of 0.12, the average KAM value surpassed 1.1\({}^{\circ}\), indicative of a heightened dislocation density across the entire sample. These observations underscored the significant role played by the dislocation activity, particularly along the shear direction, in the microstructural evolution of the AZ31 magnesium alloy under uniaxial tension.  
Figure 7: EBSD map of a representative area under different strains. (**a**) At 0.01 strain; (**b**) At 0.02 strain; (**c**) At 0.05 strain; (**d**) At 0.12 strain.