312 W. Sinkler et al be corundum(double arrow ) Cordierite crystals exhibit a star-like structure similar to that seen for with more equiaxed morphologies could occasion- cordierite, but at a larger scale. Dark crystals,cor ally also be resolved (single arrow). A few excep- responding to corundum and cordierite (spinel tional crystals of yttrium silicate were also crystals are unlikely, since too small are also visi- detected, appearing as the bright areas in Fig. 9 their compositions were checked using X-EDS They are present in very small quantity, and were thus not resolvable using XRD. The microscopic appearance of cordierite precipitates at Step 1 is illustrated using TEM in Fig. 10. The radial leaf like form is consistent with the SEM image shown as Fig. 5(c), although the size is significantly smal ler, indicating that a more advanced stage in the microstructure development was reached in crystals(single arrow) exhibiting a specific sphe- rical morphology, and a corundum crystal (large crystal, double arrow) Step 2 is illustrated in Figs 11-13. In OM, using transmitted, polarised light, an obvious color change from magenta to orange-pink was observed I um with respect to the Step I matrix. This is consistent with a fine-crystalline matrix, in which optically Fig. 10. YMAS matrix in composite TI, at Step 1(see text) anisotropic domains, in addition to some remain- Low magnification TEM image Peculiar star-like morphology ing amorphous phase (isotropic= magenta),are for cordierite at a primary crystallization step, to be compared to similar textures revealed in the MAs-Y glass by seM superimposed in the specimen cross-section. In a [Fig. 5(c). Single arrow indicates a spinel crystal with SEM image shown in Fig. 1 l(back-scattered elec haracteristic spherical morphology, double arrow indicates a trons), large crystals of yttrium silicate are now evident, and appear white in the micrograph. They ∥m um Fig. 11. YMAS matrix in composite P5, at Step 2(see text) SEM image (back-scattered electrons). Round parts are Fig 9. YMAS matrix in composite P3, at Step I(see text). carbon fibers, which appear dark because of their low Z SEM image(back-scattered electrons). Round parts are car- number. Other dark (polyhedral) parts are cordierite or cor bon fibers, which appear dark because of their very low Z undum. Bright parts are yttrium-rich crystals (probably sili- number. Other dark parts are yttrium-free crystals (single cate)exhibiting the star-like morphology specific of a primary ordierite, double arrow =corundum, as deter- crystallisation state, as already observed for cordierite mined from X-EDS). Clear parts contain Y-SiO. Straight Figs 5(c)and 10]. The gray contrast of the bulk indicates that dark lines joining fibers are cracks. most of the yttrium is still disseminated in the matrix.be corundum (double arrow). Cordierite crystals with more equiaxed morphologies could occasion￾ally also be resolved (single arrow). A few excep￾tional crystals of yttrium silicate were also detected, appearing as the bright areas in Fig. 9; their compositions were checked using X-EDS. They are present in very small quantity, and were thus not resolvable using XRD. The microscopic appearance of cordierite precipitates at Step 1 is illustrated using TEM in Fig. 10. The radial leaf￾like form is consistent with the SEM image shown as Fig. 5(c), although the size is signi®cantly smal￾ler, indicating that a more advanced stage in the microstructure development was reached in Fig. 5(c). In Fig. 10 are also imaged small spinel crystals (single arrow) exhibiting a speci®c sphe￾rical morphology, and a corundum crystal (large crystal, double arrow). Step 2 is illustrated in Figs 11±13. In OM, using transmitted, polarised light, an obvious color change from magenta to orange-pink was observed with respect to the Step 1 matrix. This is consistent with a ®ne-crystalline matrix, in which optically anisotropic domains, in addition to some remain￾ing amorphous phase (isotropic = magenta), are superimposed in the specimen cross-section. In a SEM image shown in Fig. 11 (back-scattered elec￾trons), large crystals of yttrium silicate are now evident, and appear white in the micrograph. They exhibit a star-like structure similar to that seen for cordierite, but at a larger scale. Dark crystals, cor￾responding to corundum and cordierite (spinel crystals are unlikely, since too small), are also visi￾Fig. 9. YMAS matrix in composite P3, at Step 1 (see text). SEM image (back-scattered electrons). Round parts are car￾bon ®bers, which appear dark because of their very low Z number. Other dark parts are yttrium-free crystals (single arrow = cordierite, double arrow = corundum, as deter￾mined from X-EDS). Clear parts contain Y±Si±O. Straight dark lines joining ®bers are cracks. Fig. 10. YMAS matrix in composite T1, at Step 1 (see text). Low magni®cation TEM image. Peculiar star-like morphology for cordierite at a primary crystallization step, to be compared to similar textures revealed in the MAS-Y glass by SEM [Fig. 5(c)]. Single arrow indicates a spinel crystal with a characteristic spherical morphology, double arrow indicates a corundum crystal. Fig. 11. YMAS matrix in composite P5, at Step 2 (see text). SEM image (back-scattered electrons). Round parts are carbon ®bers, which appear dark because of their low Z number. Other dark (polyhedral) parts are cordierite or cor￾undum. Bright parts are yttrium-rich crystals (probably sili￾cate) exhibiting the star-like morphology speci®c of a primary crystallisation state, as already observed for cordierite [Figs 5(c) and 10]. The gray contrast of the bulk indicates that most of the yttrium is still disseminated in the matrix. 312 W. Sinkler et al