Non-Schmid behaviour during secondary twinning in a polycrystalline magnesium alloy

The present work combines electron backscattering diffraction and Schmid analysis to investigate secondary twinning in the magnesium alloy Mg–3Al–1Zn. Inspection of the misorientations between the parent and {101¯1}–{101¯2} doubly twinned volumes reveals that there are four possible variants. One of these variants (the one that forms a misorientation with the matrix characterized by 38° 〈12¯10〉) is favoured much more than the others. This variant involves the activation of secondary twinning systems quite inconsistent with Schmid-type behaviour. For the secondary twin to grow significantly it must take on a shape enforced by the primary twin. This is non-optimal for strain compatibility. It is argued that the 38° 〈12¯10〉 variant occurs most because it provides the closest match between the primary and secondary twinning planes, thus minimizing the compatibility strain.