Impact of solute elements on detwinning in magnesium and its alloys

By combining finite element analysis, compressive tests, reverse tensile tests, in-situ electron-backscattering diffraction, and high-resolution transmission electron microscopy, we investigate systematically twinning and detwinning behaviors of {1012} twins in pre-compressed pure Mg and AZ31 and AZ91 alloys. We find that the solute elements Al and Zn and the resultant precipitates disrupt the synchronous motion of atoms in twinning boundaries (TBs) during twin formation, yielding a large amount of incoherent TBs and low-angle boundaries from the pre-existing TBs after detwinning. The detwinning in pure Mg and its alloys is found to be linked to both twin boundary (TB) mobility and the interaction of their boundaries with dislocations. At a low strain level (0-1%), comparable detwinning activities in both pure Mg and its alloys are observed due to the selective detwinning in grains with the highest SF, while at high strain level (1-4%), the contribution of detwinning in Mg alloy is higher. This is because a small twin size in Mg alloys leads to strong interactions between basal slip and TBs, and also because in pure Mg, twin size is much larger and its tip is pinned by grain boundaries. In addition, the secondary twins are also found to be formed remarkably in the interior of the primary twins in Mg alloys with a high solute-element concentration owing to the low mobility of incoherent TB and the variation of local stress state by precipitates. The ultimate strength of reverse tensile test is found to increase more substantially in the pre-compressed Mg alloys in comparison to pure Mg, especially in the AZ91 alloy, which is attributed to the hardening effect caused by the formation of many low-angle and secondary TBs and also for the reduced Schmid factor for the basal slip in Mg alloys during detwinning.