{10-12} Twinning Behavior And Twin-slip Interaction In AZ31 Magnesium Alloys

{10-12} extension twinning is one of the most common deformation mechanisms in Mg alloys due to its low critical resolved shear stress(CRSS). The activation ability of extension twinning and variant selection are intimately related to the mechanical properties and microstructure evolution of Mg alloys. In the present study, AZ31 Mg alloys were used as the model material. Electron backscatter diffraction(EBSD) technique and crystallographic calculation were employed to understand the twinning behaviors in two deformation modes(uniaxial tension and compression) and at two temperature. Specifically, the effects of initial texture and grain size on twinning and the interactions between twinning and slip were analyzed.Uniaxial tensile deformation was conducted at room temperature on five kinds of samples to comparatively examine the activation and variant selection of twinning. These five kinds of samples have the loading direction inclined by 0°, 30°, 45°, 60° and 90° to the normal direction(ND) of the Mg plate. It was found that the distribution of SF rank is relatively scattered in the 0° sample primarily because the six theoretical twin variants have close SF values. By contrast, most of the active twins have SF rank 1 or 2 in the 30°- 90° samples. This is because the largest SF difference among the six theoretical variants becomes more obvious from the 30° to 90° samples, which makes the Schmid law effect more evident on variant selection of twinning. The mechanical anisotropy of Mg alloys is associated with the critical resolved shear stress(CRSS) of the dominated deformation mechanism and the Schmid factor(SF) of various loading directions. The activities of twinning and dislocation slip determined the r-value of Mg alloys. The correlation between the macroscopic deformation characteristic(i.e., r-value > 1) and slip activity was revealed via crystal plasticity analysis.The interaction between basal slip and twinning was studied in the slip-dominated sample(i.e., the 45? sample). Based on EBSD and trace analyses, the strain compatibility factor m? between twinning and slip was statistically examined. It was found that most of the twins were activated to accommodate the local strain generated by basal slip in neighboring grains, indicating the slip induced the twin nucleation. For the twinning-dominated sample(i.e., the 0? sample), twinning preferred to occur in form of pairs at grain boundaries. A statistical analysis about the strain compatibility factor m? indicated that the paired twins generally had good strain accommodation, suggesting one twin was induced by the activation of another twin. The above mentioned slip induced twin and twin induced twin phenomena largely affected the selection of twin variants.Twinning behaviors were comparatively studied in the five kinds of samples subjected to compression deformation at 250?C. It was confirmed that twinning preferred to activate in the grains with relatively high SF. However, some non-Schmid twins were also observed activated in these samples. Here, the non-Schmid twins represent the variants with neither the first nor the second highest SF. By examing statistically the strain accommodation factor m? between twinning and slip, it was seen the slip band-grain boundary interaction was mainly responsible for the non-Schmid twinning behavior in the 90? sample. Numerous low angle grain boundaries(2-10?) were generated during deformation of the 60? sample, which became the preferential nucleation sites for twinning. The activation and variant selection of twinning were influenced by both SF and local strain accommodation.Twinning behaviors were comparatively studied in tension of fine- and coarse-grained Mg alloys. It was found that the nucleation of twinning was more largely affected by the local strain accommodation at grain boundaries. As a result, long twin chains or bands were likely formed through a few neighboring grains. However, more twin variants were activated in the coarse-grained sample because of their relatively long grain boundaries. Consequently, crossed twin lamellas were frequently formed in the samples. A friction stir welded(FSW) Mg alloy with fine grains was also examined. In particular, the effect of grain orientation on the propensity of twinning and slip and their competition were focused on. The correlation between the observed macroscopic strain anisotropy and basal slip activity was revealed.