Available online at www.sciencedirect.com Journal of the European Ceramic Society 28 (2008) 371–376 Observation of dislocation assisted high temperature deformation in mullite and mullite composites Lili Taherabadi, Joy E. Trujillo, Tiandan Chen, John R. Porter, Martha L. Mecartney ∗ University of California, Irvine, Department of Chemical Engineering and Materials Science, Irvine, CA 92697-2575, United States Available online 22 May 2007 Abstract 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 disloca- tions 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 3Al 2 O 3 ·2SiO 2 and 2Al 2 O 3 ·1SiO 2 were fabricated by reactive sintering of nanocrys- talline 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 accommo- dated deformation at high temperatures is significant in mullite and the complex structure of mullite may activate multiple slip systems at high temperatures. © 2007 Elsevier Ltd. All rights reserved. Keywords: Creep; Mullite; Dislocations 1. Background Superplastic ceramics have the ability to deform over 100% without fracture at high temperatures. Ceramics that have demonstrated superplasticity include single phase sys- tems such as yttria stabilized tetragonal zirconia polycrystals (Y-TZP), 1 two phase systems such as zirconia–mullite, 2 Y-TZP with silica 3 and, yttria cubic stabilized zirconia (Y- CSZ) with silica additions. 4 More recently, three phase systems such as alumina–spinel–TZP 5 and alumina–mullite (3Al 2 O 3 ·2SiO 2 )–TZP 6 have shown high strain rate potential. The key to superplastic deformation in all of these systems is the fabrication of a material with a fine grain size (usually less than 1 m for ceramics) and limited grain growth upon high temperature deformation. 7 Applications of superplastic forming require a high strain rate to make the process commercially feasible, thus achiev- ing as high a strain rate as possible is one goal of superplastic research. An empirical equation that links the strain rate (ε/t) that can be achieved with the applied stress (σ ), grain size (d), ∗ Corresponding author. Tel.: +1 949 824 2919; fax: +1 949 824 2541. E-mail address: martham@uci.edu (M.L. Mecartney). and temperature (T) is given in Eq. (1), where A is a material constant, n the stress exponent (usually 1–3), Q the activation energy, and R is the gas constant. It can be seen that the strain rate is inversely proportional to the grain size d, to the power of the grain size exponent p (usually 2 or 3): ˙ ε = A σ n d p exp  -Q RT  (1) Superplastic deformation has some other unique characteristics, which differentiate it from high temperature creep. In super- plastic deformation, the grains remain the same shape, and do not elongate, as would be observed in Coble creep. Deforma- tion occurs primarily by grain boundary sliding in superplastic deformation. Yet some accommodation for grain boundary slid- ing is required, either the formation of cavities (which would lead to premature failure), grain boundary migration, diffusional accommodation, liquid/viscous phase accommodation, or dis- location generation. 8 It has been generally accepted that most ceramic superplastic systems that are primarily single phase have diffusional accommodation. Diffusional accommodation, however, is difficult in three- phase ceramics. These materials maintain their fine grain size due to limited grain growth as a result of the microstructure, 0955-2219/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2007.03.005