S Gustafsson et aL. /Joumal of the European Ceramic Sociery 29(2009)539-550 400nm 200nm (a) Fig. 12. Intragranular cavities in the polycrystalline mullite crept under a stress of 14.9 MPa at 1300C. Thin channels(arrowed) are connecting, or extending out from. the enlarged cavities predominantly in the larger grain sections, and in general asso- ciated with intragranular cavities, as in the as-sintered material Glass pockets at multi-grain junctions and thin amorphous grain boundary films were present also in the crept microstruc tures. Measurements of the film thickness in the sample crept at 1400C under a stress of 48.6 MPa did not reveal any pronounced changes in the thickness as compared to the as- sintered mullite material; the films had a thickness of, typically, 0.6-0.9 nm also after creep testing 4.4. The nanocomposite after creep testing 200nm Grain size measurements on the creep tested mullite Fig growth during creep deformation, see Table 1 and Fig. 6. The 50.0 MPa at 14002 the nanocomposite after creep testing under a stress of nanocomposite specimens did not show any evidence of grain grain junctions (b) of location and distribution of the SiC particles was also appar ently unchanged, see Fig. 13. Around 80% of the Sic particles were located at the grain boundaries and multi-grain junctions in the specimen that had been crept at 1400C under a stress of 50.0 MPa. The mullite/mullite grain boundaries were often observed to bend near the intergranular SiC particles, see Fig. I 200mm An increased number of the multi-grain junctions, pre- dominantly junctions containing intergranular Sic particles, contained cavities after creep testing. This was particularly pro- nounced in the specimen crept under the highest stress, 50 MPa, at 1400 C, see Fig. 13b. Cavitation associated with the Sic par- ticles at the grain boundaries was rarely observed. The cavities (a) inside the mullite matrix grains seemed to have retained their ize and shape during creep testing. (200nm The dislocation densities in the creep tested nanocompos- e specimens were low and seemingly unchanged as compared to the as-sintered material. A limited number of grains with a locally increased dislocation density were observed in the crept microstructures. but these areas were not different from similar areas in the as-sintered nanocomposite microstructure. Thin intergranular glassy films were present also after creep testing Measurements of the grain boundary film thickness in Fig 14 Mullite matrix grain boundaries bending(arrowed)around intergranular the sample crept at 1400C under a stress of 50 MPa, showed SiC particles after creep testing under a stress of 50.0 MPa at 1400oC.S. Gustafsson et al. / Journal of the European Ceramic Society 29 (2009) 539–550 547 Fig. 12. Intragranular cavities in the polycrystalline mullite crept under a stress of 14.9 MPa at 1300 ◦C. Thin channels (arrowed) are connecting, or extending out from, the enlarged cavities. predominantly in the larger grain sections, and in general asso￾ciated with intragranular cavities, as in the as-sintered material. Glass pockets at multi-grain junctions and thin amorphous grain boundary films were present also in the crept microstruc￾tures. Measurements of the film thickness in the sample crept at 1400 ◦C under a stress of 48.6 MPa did not reveal any pronounced changes in the thickness as compared to the as￾sintered mullite material; the films had a thickness of, typically, 0.6–0.9 nm also after creep testing. 4.4. The nanocomposite after creep testing Grain size measurements on the creep tested mullite nanocomposite specimens did not show any evidence of grain growth during creep deformation, see Table 1 and Fig. 6. The location and distribution of the SiC particles was also appar￾ently unchanged, see Fig. 13. Around 80% of the SiC particles were located at the grain boundaries and multi-grain junctions in the specimen that had been crept at 1400 ◦C under a stress of 50.0 MPa. The mullite/mullite grain boundaries were often observed to bend near the intergranular SiC particles, see Fig. 14. An increased number of the multi-grain junctions, pre￾dominantly junctions containing intergranular SiC particles, contained cavities after creep testing. This was particularly pro￾nounced in the specimen crept under the highest stress, 50 MPa, at 1400 ◦C, see Fig. 13b. Cavitation associated with the SiC par￾ticles at the grain boundaries was rarely observed. The cavities inside the mullite matrix grains seemed to have retained their size and shape during creep testing. The dislocation densities in the creep tested nanocompos￾ite specimens were low and seemingly unchanged as compared to the as-sintered material. A limited number of grains with a locally increased dislocation density were observed in the crept microstructures, but these areas were not different from similar areas in the as-sintered nanocomposite microstructure. Thin intergranular glassy films were present also after creep testing. Measurements of the grain boundary film thickness in the sample crept at 1400 ◦C under a stress of 50 MPa, showed Fig. 13. The general microstructure (a) and cavity formation (arrowed) at multi￾grain junctions (b) of the nanocomposite after creep testing under a stress of 50.0 MPa at 1400 ◦C. Fig. 14. Mullite matrix grain boundaries bending (arrowed) around intergranular SiC particles after creep testing under a stress of 50.0 MPa at 1400 ◦C