Flaw-Tolerance and Crack-Resistance Properties of Alumina-Aluminum Titanate Composites with Tailored Microstructures

The microstructures of alumina-aluminum titanate (A-AT) composites have been tailored with the intent of altering their crack-resistance ( R - or T -curve) behavior and resulting flaw tolerance. Specifically, two microstructural parameters which influence grain-localized crack bridging, viz., (i) internal residual stresses and (ii) microstructural coarseness, have been investigated. Particulate aluminum titanate was added to alumina to induce intense internal residual stresses from extreme thermal expansion mismatch. It was found that A-AT composites with uniformly distributed 20–30 vol% aluminum titanate ("duplex") showed significantly improved flaw tolerance over single-phase alumina. Coarsening of the duplex microstructure via grain growth scaling was relatively ineffective in improving the flaw tolerance further. Onset of spontaneous microcracking precluded further exploitation of this scaling approach. Therefore, an alternative approach to coarsening was developed, in which a uniform distribution of large alumina grains was incorporated within a fine-grain A-AT matrix ("duplex-bimodal"), via a powder processing route. The duplex-bimodal composites yielded excellent flaw tolerance with steady-state toughness of ∼8 MPa˙m^1/2. A qualitative model for microstructure development in these duplex-bimodal composites is presented.