Dislocations and mechanical properties of MgO-Al2O3 spinel single crystals

MgO . nAl(2)O(3) spinel single crystals can be deformed plastically at high temperatures, displaying a range of interesting features. Stress-strain cu rves often exhibit strong work hardening followed by prominent work softeni ng due to glide and climb processes. The critical resolved shear stress (CR SS) at a given temperature decreases dramatically, by almost 2 orders of ma gnitude, with increasing deviation from stoichiometry, i.e., as n increases from 1 to 3.5. The CRSS is proportional to exp(-T/T-0) and to [V-c](-2), w here T is the temperature in kelvin, T-0 a characteristic temperature, and [V-c] the concentration of charge-compensating cation vacancies. The Burger s vector is 1/2(110), and slip can occur on {111} and {110} planes. Slip on (111) planes is believed to occur between the Kagome cation layer and the adjacent anion layer. Slip on (110) planes is slightly easier (and has a hi gher T-0), because the planes are more widely separated. The temperature de pendence of the CRSS can be explained in terms of the Peierls stress for pa rtial dislocations, either in terms of a steep and high Peierls potential o r in terms of temperature and stress-dependent kink diffusion. The dependen ce of CRSS on [V-c](-2) can be explained in terms of kink nucleation at cat ion vacancies.