On the role of anomalous twinning in the plasticity of magnesium

Abstract

Specially oriented Mg single crystals were deformed at ambient and elevated temperatures in channel-die compression with the c-axis inclined at 45° to the compression direction and 〈101¯0〉 parallel to the lateral constraint direction. The specimens deformed by basal slip, entailing a rotation of the c-axis towards the compression direction. At room temperature, starting already during early stages of deformation, macroscopic bands comprising a mesh of anomalous {101¯2} extension twins formed parallel to the constraint direction. These twins were characterized by a negative Schmid factor and produced a strain opposite to the imposed deformation. At the final true strain of −1, a significant volume fraction of the matrix was consumed by twins, causing a reorientation of the c-axis farther away from the compression direction, and hence delaying the formation of a hard basal texture component. An analysis of the displacement gradient tensors for the six possible extension twin variants showed that the selected twin variants, of which the bands were composed, involved a lattice rotation opposite to the one caused by basal slip. Less twinning was observed at elevated temperatures with the macroscopic bands disappearing at 370 °C. The obtained results are discussed with respect to deformation heterogeneity and the role of anomalous twinning for the deformation behavior of magnesium.