FLAW-TOLERANCE AND CRACK-RESISTANCE PROPERTIES OF ALUMINA ALUMINUM TITANATE COMPOSITES WITH TAILORED MICROSTRUCTURES

The microstructures of alumina-aluminum titanate (A-AT) composites hav e 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 bridg ing, viz., (i) internal residual stresses and (ii) microstructural coa rseness, have been investigated. Particulate aluminum titanate was add ed to alumina to induce intense internal residual stresses from extrem e thermal expansion mismatch. It was found that A-AT composites with u niformly distributed 20-30 vol% aluminum titanate (''duplex'') showed significantly improved flaw tolerance over single-phase alumina. Coars ening of the duplex microstructure via grain growth scaling was relati vely ineffective in improving the flaw tolerance further. Onset of spo ntaneous microcracking precluded further exploitation of this scaling approach. Therefore, an alternative approach to coarsening was develop ed, in which a uniform distribution of large alumina grains was incorp orated within a fine-grain A-AT matrix (''duplex-bimodal''), via a pow der processing route. The duplex-bimodal composites yielded excellent flaw tolerance with steady-state toughness of almost-equal-to 8 MPa.m1 /2. A qualitative model for microstructure development in these duplex -bimodal composites is presented.