The structure of 〈c + a〉 type dislocation loops in magnesium

Mg (hcp) single crystals exhibit an unusually high work-hardening rate when compressed along the c-axis at room temperature. Post-deformation investigation using bright-field and weak-beam dark-field techniques in a transmission electron microscope (TEM) and by high-angle annular dark-field imaging (HAADF) using a scanning transmission electron microscope confirms the presence of sessile c + a type dislocation loops on the basal plane whose density increases with increasing plastic strain; their presence is thought to account at least in part for the high work-hardening rate. These loops appear to be often arranged as rows along the 〈10–10〉 directions, are hexagonal in shape with their sides coinciding with the 〈10–10〉, are faulted and are composed of two 1/6〈20–23〉 type partials loops bounding a basal stacking fault. Their presence is attributed to the decomposition reaction 1/3[2-1-13] → 1/6[2-203] + SF(0001) + 1/6[20-23].