598 Journal of the American Ceramic Society-Carelli et al. Vol. 85. No. 3 deviations from 14 tests are g.= 145+8 MPa and E=0.26 Following aging at 1200.C, the fiber tow failures in the 0/90 +.03%. Also shown for comparison in Fig. 2(c)is the retained specimens remained largely uncorrelated with one another at the strength of a comparable all-oxide ceramic composite consisting of macroscopic level(Fig. 3(c). However, there was a noticeable the same Nextel 720 fibers in an aluminosilicate matrix(in place ncrease in the correlation in the fiber failure sites within each tow of the mullite/alumina matrix used in the present study ), after the(Fig. 4(c). The most highly correlated failure sites appeared in same aging treatments. The strength of the aluminosilicate-basee small clusters, each containing perhaps 3-10 fibers. Additionall material decreases rapidly for aging temperatures beyond 1000%C the amount of matrix material remaining adhered to the fiber eportedly due to extensive densification of the matrix and an surfaces was significantly greater than that in the as-processed attendant loss in damage tolerance. material. These features clearly indicate that both the matrix and the In the 0/90 as-processed tensile specimens, the locations of fiber-matrix interface have been strengthened as a consequence of the the tow failures were uncorrelated with one another, as evident in aging treatment, thereby reducing somewhat the extent of damage the macrophotograph in Fig. 3(a). Indeed, the tow failure sites tolerance. Nevertheless, the effects do not appear to be sufficiently were offset by distances up to several centimeters along the large to noticeably alter the 0/90% composite strength. loading direction. larly, highly uncorrelated fiber fra In the +45 orientation, the effects of matrix strengthening on were obtained within each longitudinal tow. An example of a aging were more pronounced(Fig. 5). In all cases, the tensile broken tow near the fracture surface is shown in Fig. 4(a). a response was characterized by elastic-plastic behavior, reminis- articularly striking feature is the seemingly large lateral separa- cent of metal plasticity (albeit at lower levels of strain). The tion between adjacent fibers. This feature is somewhat misleading transition from elastic to plastic behavior was gradual and the in the sense that there are large longitudinal separations between ultimate tensile strength was controlled by a plastic instability the fiber fracture sites and hence many of the broken fibers within analogous to necking in metals, at an average strain of 0.32+ a broken tow are well outside the field of view when imaging the 0.03%, independent of aging treatment. By contrast, Youngs tow at even modest magnifications. These observations attest to modulus and the tensile strength increased dramatically following the efficacy of the matrix in mitigating stress concentrations aging, by as much as a factor of 2 at the highest aging temperature around fiber breaks and hence yielding damage tolerant behavior. This trend reaffirms that some strengthening of both the matrix and Higher-magnification SEM observations revealed only small amounts of matrix particulates remaining adhered to the fiber off-axis composite strength, such changes may be beneficial surface (Fig. 4(b). This result suggests that failure involves In the as-processed +45tensile specimens and the ones aged at debonding and sliding either at or very near the fiber-matrix temperatures up to 1100C, failure occurred mainly through the interface during fiber fracture, analogous to that in dense-matrix matrix and was accompanied by extensive interply delamination CFCCs with weak interphases. Similar features were observed on and fiber" scissoring, but with minimal fiber fracture(Figs. 6(a) the specimens that had been aged at either 1000 or 1100C(.g, and(b). A consequence of this"scissoring"is through-thi g.3(b) swelling in the region near the fracture surface. Following th 5 50 um 10m (d) 0 Fig. 4. SEM micrographs of the fracture surfaces of the 0/90 specimens: (a, b) in as-processed condition, and (c, d) after aging for 1000 h at 1200deviations from 14 tests are u  145  8 MPa and εf  0.26%  0.03%. Also shown for comparison in Fig. 2(c) is the retained strength of a comparable all-oxide ceramic composite consisting of the same Nextel 720 fibers in an aluminosilicate matrix (in place of the mullite/alumina matrix used in the present study), after the same aging treatments.7 The strength of the aluminosilicate-based material decreases rapidly for aging temperatures beyond 1000°C, reportedly due to extensive densification of the matrix and an attendant loss in damage tolerance. In the 0°/90° as-processed tensile specimens, the locations of the tow failures were uncorrelated with one another, as evident in the macrophotograph in Fig. 3(a). Indeed, the tow failure sites were offset by distances up to several centimeters along the loading direction. Similarly, highly uncorrelated fiber fractures were obtained within each longitudinal tow. An example of a broken tow near the fracture surface is shown in Fig. 4(a). A particularly striking feature is the seemingly large lateral separa￾tion between adjacent fibers. This feature is somewhat misleading in the sense that there are large longitudinal separations between the fiber fracture sites and hence many of the broken fibers within a broken tow are well outside the field of view when imaging the tow at even modest magnifications. These observations attest to the efficacy of the matrix in mitigating stress concentrations around fiber breaks and hence yielding damage tolerant behavior. Higher-magnification SEM observations revealed only small amounts of matrix particulates remaining adhered to the fiber surface (Fig. 4(b)). This result suggests that failure involves debonding and sliding either at or very near the fiber–matrix interface during fiber fracture, analogous to that in dense-matrix CFCCs with weak interphases. Similar features were observed on the specimens that had been aged at either 1000° or 1100°C (e.g., Fig. 3(b)). Following aging at 1200°C, the fiber tow failures in the 0°/90° specimens remained largely uncorrelated with one another at the macroscopic level (Fig. 3(c)). However, there was a noticeable increase in the correlation in the fiber failure sites within each tow (Fig. 4(c)). The most highly correlated failure sites appeared in small clusters, each containing perhaps 3–10 fibers. Additionally, the amount of matrix material remaining adhered to the fiber surfaces was significantly greater than that in the as-processed material. These features clearly indicate that both the matrix and the fiber–matrix interface have been strengthened as a consequence of the aging treatment, thereby reducing somewhat the extent of damage tolerance. Nevertheless, the effects do not appear to be sufficiently large to noticeably alter the 0°/90° composite strength. In the 45° orientation, the effects of matrix strengthening on aging were more pronounced (Fig. 5). In all cases, the tensile response was characterized by elastic–plastic behavior, reminis￾cent of metal plasticity (albeit at lower levels of strain). The transition from elastic to plastic behavior was gradual and the ultimate tensile strength was controlled by a plastic instability analogous to necking in metals,4 at an average strain of 0.32  0.03%, independent of aging treatment. By contrast, Young’s modulus and the tensile strength increased dramatically following aging, by as much as a factor of 2 at the highest aging temperature. This trend reaffirms that some strengthening of both the matrix and the fiber–matrix interfaces occurs during aging. In the context of off-axis composite strength, such changes may be beneficial. In the as-processed 45° tensile specimens and the ones aged at temperatures up to 1100°C, failure occurred mainly through the matrix and was accompanied by extensive interply delamination and fiber “scissoring,” but with minimal fiber fracture (Figs. 6(a) and (b)). A consequence of this “scissoring” is through-thickness swelling in the region near the fracture surface. Following the Fig. 4. SEM micrographs of the fracture surfaces of the 0°/90° specimens: (a,b) in as-processed condition, and (c,d) after aging for 1000 h at 1200°C. 598 Journal of the American Ceramic Society—Carelli et al. Vol. 85, No. 3