Microstructural evolution under load and high temperature deformation mechanisms of a mullite/alumina fibre

A two-phase mullite alumina fibre, the 3M Nextel 720 fibre, has been studied in tension and creep. The fibre shows the highest creep resistance of all current commercial fine oxide fibres up to 1500 °C. The creep mechanisms involve progressive dissolution of mullite and simultaneous reprecipitation of alumina into elongated oriented grains and grain boundary sliding by a thin alumino-silicate liquid phase. The rate of grain growth in creep at a given temperature is dependant on the applied stress. The combination of sub-micron size mullite crystallites and alumina grains gives rise to a high sensitivity to alkaline contamination. Stress enhanced diffusion of the contaminants from the fibre surface results in crack nucleation, dissolution of mullite, formation of a liquid phase and slow crack growth. From 1200 °C, this process is coupled with a fast α-alumina grain growth at the fibre surface.