Compositional softening in Mg-Al spinel

Deviations from stoichiometry in Mg-Al spinel (MgO . nAl(2)O(3), n > 1) cause a marked decrease in both the high-temperature critical resolved shear stress (CRSS) and the steady-state flow stress for both {111} [(1) over bar)01] and {101}[(1) over bar 01] slip. However, Arrhenius plots give activation energies and stress exponents which are essentially the same for both stoichiometric and nonstoichiometric crystals. Dislocations observed after deformation in non-stoichiometric specimens undergoing {101} slip are predominately edge in character, while 60 degrees climb-dissociated dislocations are found in specimens which have undergone (111) slip. In contrast, edge and 30 degrees dislocations are found in stoichiometric (n = 1) spinel which has undergone {111} slip (Donlon et al., 1982, Phil. Mag. A, 45, 1013). Most of the dislocations in the non-stoichiometric crystals are dissociated by climb, although some partial dislocations are seen bounding widely separated glide faults, and some dislocations have segments alternately dissociated by a combination of glide and climb. Further analysis of the CRSS data shows a linear relationship between log(CRSS) and temperature which is at least as good as the usual Arrhenius plot. In addition, the CRSS decreases as [V-c](-2) where the concentration [V-c] of cation vacancies is given by (n - 1)/[3(3n + 1)1. These relationships imply that the CRSS is controlled by a Peierls stress which is reduced by kink nucleation at cation vacancies. The activation energy is slightly lower for {101} [(1) over bar 01] slip, so that this system is favoured in non-stoichiometric compositions at lower temperatures.