Transmission electron microscopy investigation of ⟨c+a⟩ dislocations in Mg and α-solid solution Mg-Li alloys

The ductility of Mg alloys is limited due to a shortage of independent slip systems. In particular, c-axis compression cannot be accommodated by any of the easy slip or twinning modes. Basal-textured samples of pure Mg and Mg-15 at. pct Li were examined for the presence of [c + a] dislocations by post-mortem transmission electron microscopy (TEM) after a small deformation, which forced the majority of grains to compress nearly parallel to their c-axes. A higher density and more uniform distribution of [c + a] dislocations is found in the Li-containing alloy. Because the 1/3[1123] {1122} pyramidal slip mode offers five independent slip systems, it provides a satisfying explanation for the enhanced ductility of a-solid solution Mg-Li alloys as compared to pure Mg. The issue of [c + a] dislocation dissociation and decomposition remains open from an experimental point of view. Theoretically, the most feasible configuration is a collinear dissociation into two 1/2[c + a] partial dislocations, with an intervening stacking fault on the glide plane. It is speculated that Li additions may lower the fault's energy and, thereby, increase the stability of this glissile configuration.