314 W. Sinkler et al spherulitic aspect(Fig. 12, double arrow) were also of the aY Si2O, structure reported by Liddell and Thompson. It is likely that the change in mor- The composite representative of Step 3(P7)was phology and the structural aB transformation annealed for an extended time of 6 h at 1050 C(see are related as the developement of BY2Si2O7 would Table 1). The XRD spectrum did not differ at this change the cordierite/yttrium silicate interface point from that after Step 2, and no significant structure. This may result in a loss of coherency differences were detected in SEM. Figure 14 shows and an increase in the surface energy, which would a TEM image of the lamellar cordierite/yttrium increase the driving force for morphological silicate microstructure corresponding to that dis- change and/or coarsenin cussed above for Step 2. The wavy form of th The final step in the microstructure evolution of lamellae is indicative of the beginning of a mor- the YMAS matrix, Step 4, is represented by phology change for the lamellar microstructure due to the beginning of coarsening, and possibly driven by a minimization of the surface area asso- ciated with the microstructure. In addition the Y2Si2O7 crystals exhibit numerous stacking faults at this stage(Fig. 15) which are not observed at Step 2. This is presumably related to the gradual a-B transformation of Y2Si2O7. The funda mental planes parallel to the stacking disorder evi dent in Fig. 16 are consistent with the(011)planes nm Fig. 15. YMAS matrix in composite P7, at Step 3(see High resolution TEM image. The aY2Si2O7 lattice now bits a peculiar contrast revealing stacking faults. This :85m Fig. 16. YMAS matrix in composite Pll, at Step 4(see text) SEM image(back-scattered electrons). Both carbon fibers and 50 nm the remaining bulk have now a similar dark contrast, due to the widespread concentration of yttrium into large keiviite crystals. Most of the keiviite crystals now exhibit a round Fig. 14. YMAS matrix in composite P7, at Step 3(see text). Low magnification TEM image. The lamellar eutectic microstructure morphology, the sizes of which are large enough to be imaged by SEM. Elongated crystals are inherited from the previous deforms, and lamellar crystals tend to become sphericalspherulitic aspect (Fig. 12, double arrow) were also found. The composite representative of Step 3 (P7) was annealed for an extended time of 6 h at 1050C (see Table 1). The XRD spectrum did not di€er at this point from that after Step 2, and no signi®cant di€erences were detected in SEM. Figure 14 shows a TEM image of the lamellar cordierite/yttrium silicate microstructure corresponding to that dis￾cussed above for Step 2. The wavy form of the lamellae is indicative of the beginning of a mor￾phology change for the lamellar microstructure, due to the beginning of coarsening, and possibly driven by a minimization of the surface area asso￾ciated with the microstructure. In addition, the Y2Si2O7 crystals exhibit numerous stacking faults at this stage (Fig. 15) which are not observed at Step 2. This is presumably related to the gradual  !  transformation of Y2Si2O7. The funda￾mental planes parallel to the stacking disorder evi￾dent in Fig. 16 are consistent with the (0 11) planes of the Y2Si2O7 structure reported by Liddell and Thompson.9 It is likely that the change in mor￾phology and the structural ! transformation are related as the developement of Y2Si2O7 would change the cordierite/yttrium silicate interface structure. This may result in a loss of coherency and an increase in the surface energy, which would increase the driving force for morphological change and/or coarsening. The ®nal step in the microstructure evolution of the YMAS matrix, Step 4, is represented by Fig. 14. YMAS matrix in composite P7, at Step 3 (see text). Low magni®cation TEM image. The lamellar eutectic microstructure deforms, and lamellar crystals tend to become spherical. Fig. 15. YMAS matrix in composite P7, at Step 3 (see text). High resolution TEM image. The Y2Si2O7 lattice now exhi￾bits a peculiar contrast revealing stacking faults. This might precede the ! transformation. Fig. 16. YMAS matrix in composite P11, at Step 4 (see text). SEM image (back-scattered electrons). Both carbon ®bers and the remaining bulk have now a similar dark contrast, due to the widespread concentration of yttrium into large keiviite crystals. Most of the keiviite crystals now exhibit a round morphology, the sizes of which are large enough to be imaged by SEM. Elongated crystals are inherited from the previous lamellar microstructure. 314 W. Sinkler et al