Thermomechanical behaviour of mullite

In the present work a complete analysis of the mechanical properties of dense mullite compacts and their relationship with microstructural features is made. The flexural strength curve shows three different regions corresponding to a low temperature region (20-600 degrees C) with a transgranular fracture of mullite grains, a medium temperature region (600-1200 degrees C) where an increase in sigma(f) is observed due to a lower stress intensity Factor value al the tip of the critical defects due to a poor load transmission across the sample, and a high temperature range (T> 1200 degrees C) where fracture appears at lower stress, first by non-catastrophic intergranular propagation up to a critical size where transgranular Fracture takes place. Fracture toughness was found to be highly dependent on the deformation rate. The low value of the stress exponent in the creep law (n = 1 at 1200 degrees C) is associated with diffusion assisted by a sliding process. The diffusional shape changes of the mullite grains are accommodated by grain boundary sliding assisted by viscous flow of the glassy phase. Due to the high sintering temperatures (>1600 degrees C) required to obtain dense mullite compacts, a silica rich glassy phase is present at grain boundaries leading to plastic deformation under load at high temperatures. The thermomechanical behaviour of mullite is controlled by the viscosity of such a grain boundary amorphous phase and has been determined by using the internal friction technique. Copyright (C) 1997 Acta Metallurgica Inc.