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 S-A, S-B, S-C and S-D, consisting of variants that are geometrically equivalent. A statistical analysis revealed that the observed secondary twins were almost entirely of S-A and S-D type (misorientations of 37.5 degrees and 69.9 degrees, 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 S-D variant, in contrast, can be attributed to its being favored by the highest resolved shear stresses, i.e., it obeys Schmid's law. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.