Dislocations and Mechanical Properties of MgO‐Al2O3 Spinel Single Crystals

Abstract

MgO· n Al 2 O 3 spinel single crystals can be deformed plastically at high temperatures, displaying a range of interesting features. Stress‐strain curves often exhibit strong work hardening followed by prominent work softening due to glide and climb processes. The critical resolved shear stress (CRSS) at a given temperature decreases dramatically, by almost 2 orders of magnitude, 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 , where T is the temperature in kelvin, T 0 a characteristic temperature, and [ V c ] the concentration of charge‐compensating cation vacancies. The Burgers vector is 1/2<110>, and slip can occur on {111} and {110} planes. Slip on {111} planes is believed to occur between the Kagomé cation layer and the adjacent anion layer. Slip on {110} planes is slightly easier (and has a higher T 0 ), because the planes are more widely separated. The temperature dependence of the CRSS can be explained in terms of the Peierls stress for partial dislocations, either in terms of a steep and high Peierls potential or in terms of temperature and stress‐dependent kink diffusion. The dependence of CRSS on [ V c ] ‐2 can be explained in terms of kink nucleation at cation vacancies.