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].