Microstructure Development in In Situ Reinforced Reaction-Bonded Aluminum Niobate-Based Composites

Reaction-bonded aluminum niobate-based (AlNbO{sub 4}-based) composites with tailorable microstructures were fabricated through controlled nucleation and growth of needlelike AlNbO{sub 4} grains. Equiaxed to highly elongated grains were obtained by different heat treatments at 1,340 and 1,400 C in air. Scanning electron microscopy observations revealed that increasing the heating rate promoted the development of large, homogeneously distributed needlelike grains. Because of the elongated nature of the AlNbO{sub 4} grains and the high residual stresses at the interface, enhanced crack wake bridging occurred. This bridging accounted for the observed increase of fracture toughness.