INFLUENCE OF THERMAL AGING ON MICROSTRUCTURAL DEVELOPMENT OF MULLITE CONTAINING ALKALIS

The influence of annealing treatments at temperatures of 900 degrees C up to 1630 degrees C on the microstructure of a 3Al(2)O(3). 2SiO(2) m ullite that contains a small amount of alkali (<3 wt%) has been studie d. Annealing treatments of a base mullite material at the sintering te mperature (1630 degrees C) and at two temperatures lower (900 degrees C) and higher (1200 degrees C) than the lowest invariant points of the SiO2-Al2O3-Na2O system have been performed. Microstructures have been characterized by using scanning and transmission electron microscopy, Special attention has been given to grain-boundary characteristics-pa rticularly the amount, composition, and distribution of the remaining glasses, Aging of this material at high temperature leads to a redistr ibution of the microstructure toward an equilibrium that involves the dissolution of the mullite grains, formation of a liquid phase, and li quid-phase grain growth. As the aging temperature increases, liquid-ph ase grain growth progressively overcomes the effect of the dissolution of mullite and a bimodal microstructure with an increasing number of large, tabular grains develops.