Formula omitted] twinning behavior under biaxial tension of Mg–3Al–1Zn plate

Abstract

Although {101¯2} twinning behavior has been extensively studied, it has rarely been investigated under biaxial tension. In the present research, the mechanical response and {101¯2} twinning behavior of a Mg AZ31 sheet under biaxial tension with various stress ratios (σND:σTD) along the normal direction (ND) and transverse direction (TD) were systematically studied. The results indicate that the yield stress along the ND is nearly independent of σND:σTD. Schmid law applied to {101¯2} twinning under biaxial tension yields better efficiency than that for uniaxial tension, and a higher efficiency is observed with a larger stress ratio along the TD; the fraction of experimentally observed twin variants with Schmid factors (SFs) of ranks 1 and 2 is approximately 45% for uniaxial tension, whereas it is approximately 68% for σND:σTD=2:1 and approximately 88% for σND:σTD=1:2. This result contradicts with the previously reported result that Schmid law is more efficient for uniaxial stress states than under multiaxial stress conditions. It is found that biaxial tension will increase the difference in SF for the six twin variants and make Schmid law more efficient. Twin variant selection and volume fraction are highly dependent on σND:σTD. For uniaxial tension, the activities for the six variants are similar, and the (0002) poles of the twins are distributed evenly on a circle approximately 80°–90° away from the ND. In contrast, the variants (01¯12)[011¯1] and (011¯2)[01¯11] are more favorable under biaxial tension, and more (0002) poles of the twins concentrate around the rolling direction with a higher ratio of σTD. At a similar strain level, the twin fraction under biaxial tension is often different from that under uniaxial tension, and this difference varies with σND:σTD. This mechanism is associated with a stress-ratio-mediated detwinning behavior under biaxial tension. The findings in the present study provide a new and significant understanding of the twinning behavior of Mg alloys.