Solution softening in spinel

The critical resolved shear stress of MgO.nAl sub 2 O sub 3 spinel at a given temperature decreases dramatically, by almost two orders of magnitude, with increasing deviation from stoichiometry, i.e., as n increases from 1-3.5. The CRSS is proportional to exp(-T/T sub 0 ) and to [V sub c ] exp -2 , where T sub 0 is a characteristic temperature and [V sub c ] is the concentration of charge-compensating cation vacancies. Slip on {111} planes is thought to occur between the Kagome cation layers and the adjacent anion layers. Slip on {101} planes is slightly easier (and has a higher T sub 0 ) because the planes are more widely separated. Slip in spinel does not require synchro-shear, unlike slip in the C15 AB sub 2 Laves phase, which has identical metal atom positions to the cation positions in spinel. The temperature dependence of the CRSS is analyzed in terms of a model based on partial dislocations overcoming a steep and high Peierls potential. Parameters for the model can be chosen to explain the observed values of T sub 0 and the CRSS extrapolated to T=0K for both spinel and sapphire. Other models are possible. The dependence of CRSS on [V sub c ] exp -2 can be explained in terms of kink nucleation at cation vacancies.