May 2004 Effect of YAG Additions on Porous-Alumina- Matrix composites Fiber (b) Fiber YAG /Fiber 500 nm| AG/S 500nm d Fiber YAG 500nm 2um」 Fig. 2. TEM images of composites (a) YO. I, (b) AY50, (c)YY50, and (d)Y 100 sintered for 5 h at 1200"C. Large matrix grains in(b) and (c)are YAG as noted. Note(a-c) are the same magnification, while(d) is imaged at a lower magnification Nextel 610 fiber average-100 nm in diameter. A comparison of matrix under these conditions. However, after heating at 1200C. tibers in composites A and Y Y50 indicated a larger alumina grain the retained strength is much greater for composites containing size in the all-alumina-matrix composite (Fig. 4) by about a factor YAG than for the all-alumina composite. The highest strength f 2. The extent of fiber grain growth in all composites containing composites have average strengths of over 150 MPa after 5 h of YAG was similar heating, compared with that of A, whose strength is degraded to -50 MPa. After 100 h at 1200C. the Y AG-containing composite (2) Retained Tensile Strength strengths are reduced to-100 MPa, while that of composite A is The average strength of as-received Nextel 610 fiber tows is further reduced to-35 MPa. The strengths obtained with YAGare 1500 MPa, which is reduced to 1250 MPa after heating for 5 h at comparable to those of a similar all-alumina-matrix composite 200 C and is further reduced to 1 100 MPa after heating for 100 h reinforced with Nextel 610 fibers that contain a monazite fiber at 1200 C. 0.l If it is assumed that only fibers are carrying the coating d in the porous matrix composites with a fiber volume fraction Despite the higher strengths of all composites with YAG in of 0.20, a strength of 300 MPa is predicted at full fiber strength the matrix, there does not appear to be a direct correlation However, after heating the fibers alone at 1200 C for 5 h, the between the amount of YAG and the tensile strength. There strength is degraded by-17%, so the strength of the composite does appear to be a correlation between the use of YAG as a would be expected to be -250 MPa if no other fiber degradation binder phase, compared with the use of alumina and strength ccurs due to the presence of a matrix. Similarly, after heating for after heating at 1200C. Figure 6 is a plot of the stress-strain 100 h at 1200oC the strength would be expected to be -220 MPa. behavior of the composites after heating at 1200C for 5 h Figure 5 contains the results of tensile strength tests after Composite A exhibited an average strain to failure of -0.1% heating the composites to 1100 and 1200C. The strengths of All the YAG-containing composites showed greater strains to composites A, AY50, and Y 100 are all very similar after heating failure (0. 2%e-0.3%) than composite A. Figure 7 contain at 1 100C for 5 h, with A having a slightly higher average strength. micrographs of fracture surfaces of composites A and Y Y50 Thus, no apparent benefit is derived by incorporating YAG into the showing very brittle failure of the all-alumina-matrix composite