Observation of dislocation assisted high temperature deformation in mullite and mullite composites

Fine grain alumina–mullite–zirconia composites demonstrate high strain rate superplastic flow (10−2 s−1) under compression at 1400–1500 °C. Transmission electron microscopy (TEM) studies reveal dislocation activity in mullite grains of the deformed material, indicating that dislocations are 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–zirconia composite material. Dislocation generation accommodated the deformation of nominally single-phase polycrystalline mullite compositions at 1450 °C under 40 MPa. Three types of dislocations were observed, with a few dislocations having the character b = [0 0 1]. Dislocation accommodated deformation at high temperatures is significant in mullite and the complex structure of mullite may activate multiple slip systems at high temperatures.