Identification and rationalization of secondary twin variants in a magnesium alloy

The present work combines electron backscatter diffraction, transmission electron microscopy and Schmid analysis to investigate secondary twinning in the magnesium alloy Mg-3AI-1Zn. Inspection of the misorientations between the parent matrix and {1011} - {1012} doubly twinned volumes reveals that there are four possible variants. One of these variants characterized by 38°< 1210 > misorientation with the matrix is favoured much more than the others. This variant involves activation of the secondary twinning systems that are quite inconsistent with the Schmid-type behaviour. For the secondary twin to grow significantly it must take on a shape enforced by the primary twin, however, this is not optimal for strain compatibility. It is argued that the 38° < 1210 > variant occurs most frequently because it provides the closest match between the primary and secondaty twinning planes, thus minimizing the compatibility strain. This conjecture is confirmed by the simulations of twin activity m ellipsoidal grains performed using the visco-plastic self-consistent crystal plasticity model.