Variant selection during secondary twinning in Mg-3%Al

Secondary twin formation during the tensile deformation of Mg-3.4%Al-0.33%Mn was studied by means of the electron backscatter diffraction technique. This was employed to identify the particular variants that formed in each grain. For this purpose, the variants were characterized with respect to the orientation of the parent grain rather than that of its host primary twin. This approach led to a regrouping of the 36 possible variants into four sets, namely SA, SB, SC and SD, consisting of variants that are geometrically equivalent. A statistical analysis revealed that the observed secondary twins were almost entirely of SA and SD type (misorientations of 37.5 deg. and 69.9 deg., respectively). The former variant is shown to require the least accommodation strain within the parent grain and to have the greatest potential for growth. Formation of the SD variant, in contrast, can be attributed to its being favored by the highest resolved shear stresses, i.e., it obeys Schmid's law.