May 2004 Effect of YAG Additions on Porous-Alumina-Matrix Composites YAA 008 Fiber Matri a AYA Fiber sE Y100 Matrix 100nm Fig. 5. Normalized mean tensile stress for composites heated to (a) 1100 C for 5 h and (b)1200'C for 5 or 100 h. Error bars represent standard Fig. 4. TEM images of Nextel 610 fibers in composites(a) A and(b) YY50 showing the differences in grain size after heating for 100 h at 1200C. Images are at the same magnification in part, for the higher retained strengths. There is much work in the literature on the effect of rare-earth oxide dopants on sintering, grain growth, and creep. 20. 24-3Segregation of yttria and/or the retention of a weak matrix to ensure sequential fracture and lanthanide oxides to alumina grain boundaries generally of particle bonds and a subsequent tortuous crack path through the results in lowered rates of sintering and grain growth at matrix to protect the fiber temperatures of 1350-1650 C,2426,2729, 33 A recent study has Large differences in the initial densities of the processed also found that sintering and grain growth of alumina composites were comparable to differences in final sintered hindered by yttria doping at temperatures as low as 1200C. densities, which made it difficult to measure small changes in For yttria dopant levels of 3000 ppm, densities after sintering at density for comparison. Similarly, an experiment designed to 1200C for 5 and 100 h were 65. 7% and 69.8%, respectively. composite YY50 with the YAG precursor did not result in a direct undoped alumina. Under similar conditions, 50: 50 by volume correlation of density and strength due to the variability of compacts of alumina and YAG powder yielded densities of densities in the initial composites. However, there was a direct%, which can be attributed to a combination of the effects correlation between the increased number of infiltration cycles and of grain-boundary segregation and greater diffusion distances decreased strength. Mattoni et al. showed that matrix porosity imparted by the presence of two mutually insoluble phases could be reduced via multiple infiltration cycles, which resulted in While the magnitude of densification inhibition by the presence decreased composite strengths, where an -17% decrease in of YAG is likely to be less for a matrix in the presence of rigid porosity yielded an-65% reduction in strength. although their inclusi inclusions, as is the case with continuous fiber reinforcements infiltration process resulted in nonuniform infiltrant distribution.A in the present study, local reduction in sintering and grain nore uniform densification would not be expected to reduce growth is expected to retain uniform porosity and aid in the strength as dramatically retention of a weak matrix The effect of YAG and/or Y segregated to alumina A second reason for the increased strength retention of YAG- boundaries in these composites on the stability of the containing matrix composites is the probability of higher fiber microstructure at 1200C is believed to be responsible strengths for these composites compared with the all-alumina