Deformation substructure in polycrystalline alumina

Abstract

Transmission electron microscopy techniques were employed in a study of substructure in two sintered, 20μm alumina (≧99%Al₂O₃) bodies, each being examined in the as-annealed condition and after achieving 2% permanent strains in compression at 1420 and 1700° C. The as-sintered microstructures were found to contain dislocation networks which were often associated with intragranular porosity. Twinned structures of rhombohedral and basal types were also observed but were relatively infrequent. Amorphous second phases commonly located at triple points were characteristic of the less pure (99% Al₂O₃) material, and were also observed in the 99.9% purity material, but only in very limited amonts. The plastic deformation behaviour of these bodies at elevated temperatures (T ≦ 1420° C) illustrates that dislocation motion is an important mode of deformation in alumina. The observation of 〈11¯20〉 and 〈¯1011〉 glide dislocations further suggests that alumina can exhibit ductile behaviour at elevated temperatures. Grain boundary shearing was also observed to contribute to plastic deformation, being associated with either grain-boundary dislocations or interfacial impurities which can alter deformation behaviour.