886 Journal of the American Ceramic Society--Cinibulk et al. Vol 87. No 5 the YAG-containing matrices, Si and Fe were present at the fiber grain boundaries at about the same level as Y No quantification of YY50 EDS spectra was performed in the present study, but Bunsell and Berger have reported levels of 0.35% SiO, and 0. 67% Fe,O, in Nextel 610. When these fibers were heated to temperatures above 1200C, greater grain growth with higher aspect ratio grains and a 150 三 silica-rich amorphous phase at multiple-grain junctions, which results from reduced grain-boundary area and consequent over- Y100 saturation of silica, are observed ( Fig. 8), Fibers with a 2-3-fold increase in grain size could be expected to be 30%0-40%o weaker. assuming that there is no change in fracture toughness and that flaw size is proportional to grain size; .e however, changes in grain morphology and the presence of Y could influence both A(control) fracture toughness and flaw size. Increased fiber strength alone cannot be responsible for the higher strengths of composites YY50 0.5 and AY50. which were nearly three times as strong as the control Strain [% composite A. However, a combination of increased fiber strength due to limited grain growth. and increased refractoriness of the Fig. 6. Representative stress versus strain curves for composites heated porous matrix is believed to be responsible for the improved for5hatl200°C strength retention of the YAG-containing composites composites following heating at 1200C. Alumina grains of the fibers in YAG-containing matrices were found to be 2-3 times smaller than the grains in fibers in the all-alumina matrices. A smaller grain size at 1200C as a result of Y doping has also been bserved in sintered compacts. The grain boundaries of fibers in both types of composites contained small amounts of Si and Fe In 1 um Grain Energy [ke V (b)