On the selection of extension twin variants with low Schmid factors in a deformed Mg alloy

A statistical analysis is conducted on the formation of {1 0 -1 2} extension twin variants with low Schmid factors (SFs), based on large data sets acquired by electron backscatter diffraction on a deformed Mg AZ31 alloy. Two physical conditions of the analyzed low SF twins are characterized: those solely at grain boundaries (group 1) and those in a pair of cross-boundary twins (group 2). The presence of a majority of them can be explained by strain compatibility arguments. The twins in group 1 require not only the most or more accommodation through basal slip with very low critical resolved shear stress (CRSS), but also the least or less accommodation through pyramidal slip with very high CRSS, in their neighboring grains. Those in group 2 also require the least or less accommodations through pyramidal slip and/or contraction twinning with very high CRSSs, but in their neighboring twins. However, they require the most or more accommodation through prismatic slip and/or extension twinning with low CRSSs. This is due to the texture of the material with respect to the loading geometry that renders different low CRSS accommodation systems available to the two different groups of low SF twins. The shears induced by low SF twins can be accommodated by several different deformation systems. In such cases, twin variant selection criteria depending on only one deformation system are limited in the prediction of twin variants.