Fine grain alumina–mullite–zirconium composites demonstrate high strain rate superplastic flow (10−2/s) under compression at 1400 to 1500C. Transmission electron microscopy studies reveal dislocation activity in mullite grains of the deformed material, indicating that dislocations were generated and propagated during deformation as an accommodation mechanism for superplastic deformation. To further study dislocation accommodated slip in mullite, polycrystalline mullite in ratios of 3Al2O3·2SiO2 and 2Al2O3·1SiO2 were fabricated by reactive sintering of nanocrystalline alumina and colloidal silica. The strain rate of the resultant mullite was four orders of magnitude lower than the alumina–mullite–zirconium composite material. Dislocation generation accommodated the deformation of nominally single-phase polycrystalline mullite compositions at 1450C under 40MPa. Three types of dislocations were observed, with a few dislocations having the character b = [001]. Dislocation accommodated deformation at high temperatures was significant in mullite and the complex structure of mullite may activate multiple slip systems at high temperatures.

Observation of Dislocation Assisted High Temperature Deformation in Mullite and Mullite Composites. L.Taherabadi, J.E.Trujillo, T.Chen, J.R.Porter, M.L.Mecartney: Journal of the European Ceramic Society, 2008, 28[2], 371-6