It was recalled that the ductility of Mg alloys was limited by a shortage of independent slip systems so that c-axis compression, in particular, could not be accommodated by any of the easy slip or twinning modes. Basal-textured samples of pure Mg and Mg-15at%Li were examined here, for the presence of <c+a> dislocations, by using post mortem transmission electron microscopy after a small deformation which forced the majority of grains to become compressed almost parallel to their c-axes. A higher density, and more uniform distribution, of <c+a> dislocations was found in the Li-containing alloy. Because

the 1/3<¯1¯1•3>{11••¯2} pyramidal slip mode offered 5 independent slip systems, it provided a neat explanation for the enhanced ductility of α-solid solution Mg-Li alloys as compared with that of pure Mg. The question of <c+a> dislocation dissociation and decomposition remained open from the experimental point of view. The most feasible configuration was, theoretically, a co-linear dissociation into two ½<c+a> partial dislocations; with an intervening stacking fault on the glide plane. It was suggested that Li additions could lower the fault energy and thereby increase the stability of this glissile configuration.

Transmission Electron Microscopy Investigation of <c+a> Dislocations in Mg and α-Solid Solution Mg-Li Alloys. S.R.Agnew, J.A.Horton, M.H.Yoo: Metallurgical and Materials Transactions A, 2002, 32[3A], 851-8