Journal of the American Ceramic Society-Carelli et al. Vol. 85. No. 3 pm 50 um 10画(d) 10 um Fig. 7. SEM micrographs of the fracture surfaces of the 45 specimens:(a)in as-processed condition, and(b) after aging for 1000 h at 1200.C stress level but rapidly diminishes with increasing displacement, with the 1200C aging. These results reveal yet again that changes complete rupture ensuing at a relatively small displacement. The occur in the state of the matrix as a result of the aging treatment corresponding fracture energy is reduced significantly, by more than a factor of 2 relative to that after the 1 100C heat treatment. This reduction in fracture energy can be attributed to the transition in the IV. Discussion and conclusions fracture mechanisms, from one of matrix cracking, delamination and fiber scissoring, to one involving extensive fiber fracture. This trend The mullite/alumina matrix undergoes some degree of sintering suggests the existence of an optimum matrix condition(e.g, strength) during the aging treatments. The main manifestations are increases at which the fracture energy in the +45 orientation attains a in modulus and hardness, by as much as a factor of 2. These maximum changes cause similar elevations in the modulus and the tensile The changes in matrix hardness with aging temperature ar strength of the composite in the +45 orientation, but with no plotted in Fig. 8(a). The hardness was essentially constant (50 change in the failure strain. Additionally, there is a noticeable g/mm)up to 1100 C. It subsequently increased and reached increase in the propensity for fiber fracture in this orientation aft value of s 100 kg/mm- following the 1200C aging. This twofold the highest temperature aging treatment and evidence of reduced increase in hardness is consistent with the twofold increase in the damage tolerance in the post-load-maximum regime By contrast, composite tensile strength in the +45 orientation following the in the 0/90 orientation, the composite modulus increases only same aging treatment. The corresponding changes in the matrix slightly and the tensile strength and the failure strain remain Youngs modulus are plotted in Fig. &(b). In light of the presence unchanged. Perhaps the most notable change in the latter orienta- of the processing-induced matrix cracks(Fig. 1), this modulus tion is the increase in the spatial correlation in the fiber failure sites presents an average value that incorporates the effects of the within an individual tow and the increased amount of matrix racks and is therefore expected to be somewhat lower than that of material adhered to the fibers. Nevertheless, the failure sites appea the porous matrix itself. Nevertheless, since the density of cracks to be sufficiently decorrelated from one another to suggest that th does not change appreciably during aging, the relative changes matrix largely continues to serve its role of mitigating stress the inferred matrix modulus are expected to reflect the changes due concentrations around fiber breaks. In light of the known sintering to matrix sintering. For the pristine composite, the inferred matrix kinetics of mullite and alumina, it is surmised that the sintering modulus the range 5-9 GPa(more than an order of within the matrix is associated predominantly with the Al O3, both magnitude lower than that of fully dense mullite). Furthermore, the from the particulates and that derived from the precursor value inferred from the +45 tensile tests is consistently higher The rather large differences in the effects of the matrix changes than that from the 0/90tests. It is surmised that this difference is on the 0/90 and +45 composite properties can be rationalized due to the different effects of the processing-induced matrix cracks with the aid of the schematic in Fig. 9. Since the properties in the (Fig. 1)on the average matrix modulus in the two testing +45 orientation are dominated by the matrix, it follows that orientations. The modulus increased with increasing aging temper- changes in the matrix properties will be translated in a roughly ature, especially above about 1 100%C, and essentially doubled after proportionate amount in the composite properties. This behaviorstress level but rapidly diminishes with increasing displacement, with complete rupture ensuing at a relatively small displacement. The corresponding fracture energy is reduced significantly, by more than a factor of 2 relative to that after the 1100°C heat treatment. This reduction in fracture energy can be attributed to the transition in the fracture mechanisms, from one of matrix cracking, delamination and fiber scissoring, to one involving extensive fiber fracture. This trend suggests the existence of an optimum matrix condition (e.g., strength) at which the fracture energy in the 45° orientation attains a maximum. The changes in matrix hardness with aging temperature are plotted in Fig. 8(a). The hardness was essentially constant (50 kg/mm2 ) up to 1100°C. It subsequently increased and reached a value of 100 kg/mm2 following the 1200°C aging. This twofold increase in hardness is consistent with the twofold increase in the composite tensile strength in the 45° orientation following the same aging treatment. The corresponding changes in the matrix Young’s modulus are plotted in Fig. 8(b). In light of the presence of the processing-induced matrix cracks (Fig. 1), this modulus represents an average value that incorporates the effects of the cracks and is therefore expected to be somewhat lower than that of the porous matrix itself. Nevertheless, since the density of cracks does not change appreciably during aging, the relative changes in the inferred matrix modulus are expected to reflect the changes due to matrix sintering. For the pristine composite, the inferred matrix modulus is in the range 5–9 GPa (more than an order of magnitude lower than that of fully dense mullite). Furthermore, the value inferred from the 45° tensile tests is consistently higher than that from the 0°/90° tests. It is surmised that this difference is due to the different effects of the processing-induced matrix cracks (Fig. 1) on the average matrix modulus in the two testing orientations. The modulus increased with increasing aging temper￾ature, especially above about 1100°C, and essentially doubled after the 1200°C aging. These results reveal yet again that changes occur in the state of the matrix as a result of the aging treatment. IV. Discussion and Conclusions The mullite/alumina matrix undergoes some degree of sintering during the aging treatments. The main manifestations are increases in modulus and hardness, by as much as a factor of 2. These changes cause similar elevations in the modulus and the tensile strength of the composite in the 45° orientation, but with no change in the failure strain. Additionally, there is a noticeable increase in the propensity for fiber fracture in this orientation after the highest temperature aging treatment and evidence of reduced damage tolerance in the post-load-maximum regime. By contrast, in the 0°/90° orientation, the composite modulus increases only slightly and the tensile strength and the failure strain remain unchanged. Perhaps the most notable change in the latter orienta￾tion is the increase in the spatial correlation in the fiber failure sites within an individual tow and the increased amount of matrix material adhered to the fibers. Nevertheless, the failure sites appear to be sufficiently decorrelated from one another to suggest that the matrix largely continues to serve its role of mitigating stress concentrations around fiber breaks. In light of the known sintering kinetics of mullite and alumina, it is surmised that the sintering within the matrix is associated predominantly with the Al2O3, both from the particulates and that derived from the precursor. The rather large differences in the effects of the matrix changes on the 0°/90° and 45° composite properties can be rationalized with the aid of the schematic in Fig. 9. Since the properties in the 45° orientation are dominated by the matrix, it follows that changes in the matrix properties will be translated in a roughly proportionate amount in the composite properties. This behavior is Fig. 7. SEM micrographs of the fracture surfaces of the  45° specimens: (a) in as-processed condition, and (b) after aging for 1000 h at 1200°C. 600 Journal of the American Ceramic Society—Carelli et al. Vol. 85, No. 3