In Situ Scanning Electron Microscopy Observations of Contraction Twinning and Double Twinning in Extruded Mg-1Mn (wt.%)

Abstract

The tensile deformation mechanisms of an extruded Mg-1Mn (wt.%) alloy at 323 K (50°C) was investigated by a combination of in situ tensile testing and electron backscatter diffraction analysis. The strong basal texture of the material resulted in placing the c-axis of most of the grains under compression during tensile loading parallel to the extrusion axis. Basal, prismatic, and pyramidal 〈c+a〉 slip activity was observed along with $\left\{ {10\overline{1}2} \right\}$ extension twinning. However, $\left\{ {10\overline{1}1} \right\}$ contraction twinning dominated the deformation. Although contraction twinning and pyramidal 〈c+a〉 slip both allow for c-component deformation, contraction twinning was preferred over pyramidal 〈c+a〉 slip, and this was expected to be due to the lower critical resolved shear stress (CRSS) value for the former mechanism at ambient temperatures. The contraction twins evolved into $\left\{ {10\overline{1}1} \right\} - \left\{ {10\overline{1}2} \right\}$ double twins with an increase in strain. The propensity of double twins to form shear bands due to shear localization within the double twinned region, which eventually resulted in cracks, led to the failure of the material. The shear localization in the double twins was expected to be due to the enhanced activity of basal slip in the twinned volume. The observations from the present study suggest that contraction twinning may play a critical role in the limited cold formability of magnesium and its alloys.