Variant selection of primary extension twins in AZ31 magnesium deformed at 400 °C

Uniaxial compression tests were performed at 400 °C and a strain rate of 0.3 s−1 on samples of as-extruded AZ31 magnesium alloy. At a true strain of −0.1, many extension twins were detected. The orientations of selected twins and their host matrix grains were determined by electron backscattered diffraction (EBSD) techniques. The Schmid factors (SFs) and accommodation strains associated with both the observed and absent potential twins were calculated. Analysis of the data indicated that some of the selected twins obeyed a Schmid factor criterion and others did not. In the latter cases, it was the accommodation strain that controlled the formation of the non-Schmid twins. Potential high SF twins did not form if this would have required the operation of 'difficult' deformation modes (i.e. prismatic or pyramidal slip and twinning). Their place was taken by low SF twins whose formation required little accommodation work. Thus it appears that the variant selection criteria originally derived for ambient temperature straining continue to apply at 400 °C, despite the much lower values of the CRSS's of the difficult deformation modes.