It was noted that deviations from stoichiometry, in Mg-Al spinels (MgO•nAl2O3), caused a marked decrease in the high-temperature critical resolved shear stress and in the steady-state flow stresses for both {111}<¯101> and {101}<¯101> slip. However, Arrhenius plots gave activation energies and stress exponents which were essentially the same for both stoichiometric and non-stoichiometric crystals. The dislocations which were observed after the deformation of non-stoichiometric specimens via {101} slip were of predominately edge type, while 60º climb-dissociated dislocations were found in specimens which had undergone {111} slip. On the other hand, edge and 30º dislocations were found in stoichiometric (n = 1) spinel which had undergone {111} slip. Most of the dislocations in non-stoichiometric crystals were dissociated by climb, although some partial dislocations were seen to be bounding widely separated glide faults. Some dislocations had segments which were alternately dissociated by a combination of glide and climb. Further analysis of the critical resolved shear stress revealed a linear relationship, between its logarithm and the test temperature, which was at least as good as the usual Arrhenius plot. Moreover, the critical resolved shear stress decreased with [Vc]-2. The concentration, [Vc], of cation vacancies was here given by (n - 1)/[3(3n + 1)]. These relationships implied that the critical resolved shear stress was controlled by a Peierls stress which was reduced by kink nucleation at cation vacancies. The activation energy was slightly lower for {101}<¯101> slip, so that this system was favored in non-stoichiometric samples at lower temperatures.

W.T.Donlon, A.H.Heuer, T.E.Mitchell: Philosophical Magazine A, 1998, 78[3], 615-41