| Permanent deformation in metals at low temperatures occurs by two mechanisms: slip and twinning. Twinning depends on the orientation of the crystals which make up the alloy with respect to the applied stresses. It is critical in magnesium alloys, and is poorly understood. Polycrystalline magnesium alloys with strong crystallographic texture show considerable anisotropy in their room temperature mechanical properties, because of the limited number of slip and twinning systems available. In this work, in-situ optical microscopy and Electron Backscatter Diffraction (EBSD) have been used to observe twinning during static uniaxial testing of AZ31B extruded rod. The yield stress is higher in the alloy than in commercially pure magnesium having nominally the same grain size and crystallographic texture. The increase in the yield point of the alloy compared with the pure magnesium is due to solid solution strengthening from the Al and Zn. These elements increase the CRSS for basal slip considerably, which results in a large increase in the yield stress. However, the alloying additions do not contribute to enhanced work hardening. The loading and reloading curves in cyclic tension are non-linear, indicating that plastic deformation processes are taking place. The non-linearity is most likely due to the reversible movement of twinning dislocations under the action of residual intergranular stresses. (Abstract shortened by UMI.)... |
