THERMOMECHANICAL BEHAVIOR OF MULLITE

In the present work a complete analysis of the mechanical properties o f dense mullite compacts and their relationship with microstructural f eatures is made. The flexural strength curve shows three different reg ions corresponding to a low temperature region (20-600 degrees C) with a transgranular fracture of mullite grains, a medium temperature regi on (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 de fects due to a poor load transmission across the sample, and a high te mperature range (T> 1200 degrees C) where fracture appears at lower st ress, first by non-catastrophic intergranular propagation up to a crit ical size where transgranular Fracture takes place. Fracture toughness was found to be highly dependent on the deformation rate. The low val ue of the stress exponent in the creep law (n = 1 at 1200 degrees C) i s associated with diffusion assisted by a sliding process. The diffusi onal shape changes of the mullite grains are accommodated by grain bou ndary sliding assisted by viscous flow of the glassy phase. Due to the high sintering temperatures (>1600 degrees C) required to obtain dens e mullite compacts, a silica rich glassy phase is present at grain bou ndaries leading to plastic deformation under load at high temperatures . The thermomechanical behaviour of mullite is controlled by the visco sity of such a grain boundary amorphous phase and has been determined by using the internal friction technique. Copyright (C) 1997 Acta Meta llurgica Inc.