Geometrically Necessary Twins in Bending of an AZ31 Sheet

Discrete arcs of twins were observed in an AZ31 rolled sheet Mg alloy after three point bending at room temperature. The arcs geometrically followed the contour of the bent specimens and were present in the tension zone where the stress state strongly disfavors the extension twinning. Electron backscatter diffraction analysis indicated that the twins in the arcs are { 10\bar 12} < 10\bar 1\bar 1 > extension twins, instead of { 101\bar 1} < 10\bar 1\bar 2 > contraction twins. Calculations showed that the twinned grains in the arcs have very low values of Schmid factor, even close to zero. The main tensile stress component in the tension zone was nearly perpendicular to the c-axis of the parent grains. Thus { 10\bar 12} < 10\bar 1\bar 1 > twinning in the tension zone was activated in a most unlikely scenario. The mechanism for such unusual twinning behavior was analyzed from the perspective of strain components that are generated by the { 10\bar 12} < 10\bar 1\bar 1 > twinning. It was demonstrated that the { 10\bar 12} < 10\bar 1\bar 1 > twins in the arcs in the tension zone of the bent specimen can be treated as "geometrically necessary twins", similar to geometrically necessary dislocations and boundaries.