Superplastic deformation of mullite composites and determination of Burger's vectors for mullite

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 of deformed fine grain superplastic alumina-mullite-zirconia composites reveal dislocation activity in mullite grains, 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. Dislocation generation resulted from the deformation of these single-phase polycrystalline mullite compositions at 1450°C under 40MPa. The strain rate of the single phase mullite was four orders of magnitude lower than the alumina-mullite-zirconia composite material.;In order to fabricate uniform and large grains for dislocation analysis by TEM, polycrystalline mullite (3Al2O3·2SiO 2) was prepared by sintering nanocrystalline alumina powder and colloidal silica sol gel at 1550°C for 10 hours and annealing at 1550°C for another 10 hours. Dislocations were present in the mullite prior to deformation but a higher dislocation density was observed after deformation to 30% strain at 1450°C and 40 MPa stress. TEM dislocation characterization of the deformed material revealed that there were 3 types of dislocations present in mullite. From dislocation analysis by the g · b technique, the Burger's vectors were determined to be [001], [010], and [101] with more [001] dislocations compared to the other types.;In order to attempt to determine slip planes in mullite, indentations were used on large grains of mullite prepared using pure mullite powders with an excess of silica and alumina (5% SiO2, 1% Al2O 3), annealed at 1550°C. Using electron backscatter microscopy (EBSD) the orientation of the grains was determined and indentations with two different orientations were applied to the surface of a large grain. Atomic force microscopy (AFM) was used to image the indentations and the plastic deformation on the sides of indentations. It is proposed that the slip planes of mullite can be (100), (010) and possibly (110).;Relative grain boundary sliding of six types of interfaces during the superplastic deformation of a fine grained alumina-zirconia-mullite ceramic composite was investigated by SEM and AFM. Deformation under 40MPa stress at 1350°C to 15% strain showed similar relative grain boundary sliding in all six types of interfaces. The zirconia-zirconia interfaces exhibited the most and mullite-mullite interfaces the least sliding mobility.