Anisotropic grain growth and microstructural evolution of dense mullite above 1550°C

Mullite powder with a nearly stoichiometric composition was hot-pressed at 1550 degrees C to produce an almost fully dense microstructure of fine, nearly uniaxial grains. The grain growth of the dense mullite was investigated during subsequent annealing at temperatures in the range of 1550-1750 degrees C, Grain growth was relatively slow at 1550 degrees C and the microstructure remained nearly equiaxial, Annealing at temperatures above the eutectic temperature (similar to 1590 degrees C) produced fairly rapid anisotropic grain growth. At 1750 degrees C, the anisotropic grain growth can be divided into two stages. In the first stage, the initial microstructure with an anisometric shape factor of 1.7 evolved rapidly into a microstructure with a shape factor of 2.7, consisting of a significant fraction of highly elongated grains. In the second stage, the microstructure evolved slowly into a system consisting of somewhat "blocky" grains with a shape factor of 2.2. The Al2O3 content of the mullite grains increased slightly and reached an equilibrium value during the first stage of anisotropic grain growth. For the samples annealed at 1750 degrees C, the indentation fracture toughness (2.5 +/- 0.2 MPa . m(1/2)) was almost independent of the anisometric shape factor. The interaction between the indentation cracks and the microstructure showed a predominantly transgranular mode of crack propagation. The data indicate that while a network of highly elongated grains can be developed by the present approach, some further manipulation of the grain boundary chemistry is required for an improvement of the fracture toughness.