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 {10 (1) over bar1}-{10 (1) over bar2} 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 degrees (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 degrees 12 10) variant occurs most because it provides the closest match between the primary and secondary twinning planes, thus minimizing the compatibility strain. (c) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.