S Gustafsson et al. / Journal of the European Ceramic Society 29(2009)539-550 SiC 500nm Fig. 10. A grain showing strain contours(arrowed) in the TEM thin-foil speci- men of the polycrystalline mullite crept under a stress of 13.0 MPa at 1400C 25 nm Strain contours were generally observed in the TEM bright field images of the microstructures of the crept mullite speci mens as shown in Figs. 10 and 11. These contours reflect the stresses that built up in the material during creep deformation An increased cavitation at multi-grain junctions was observed in some areas of the crept microstructures, see Fig. 11. This type of cavitation was most pronounced in the specimen that had been subjected to the highest stress (48.6 MPa) at 1400C, but so evident in th SiC at 1300C. The microstructure of the specimen crept under a stress of 13.0 MPa at 1400C resembled. on the other hand. that of the as-sintered material, with only a limited increase in the density of cavi A number of the intragranular cavities changed shape or size during creep testing, and in some cases thin channels connecting Sic nearby cavities had developed, see Fig. 12. The size of these cavities ranged from several hundred nanometers up to 1 um. The overall dislocation density was low also creep tested mullite specimens. Dislocations were present 25 nm Fig 9. Thin amorphous films separating intragranular SiC particles from the surrounding mullite grain in the as-sintered nanocomposite. (a) Defocus Fres- nel fringes(arrowed ).(b) Diffuse dark field image. The glassy films(arrowed) appear with bright contrast. glass volumes in the thin-foil TEM specimens were too small for quantitative analysis 4.3. The polycrystalline mullite after creep testing 200nm The average grain sizes of the creep tested mullite specimens were virtually the same as that of the as-sintered material, Fig. 11. Cavity formation in a multi-grain junction in the polycrystalline mullite Table 1. This indicates that grain growth was not significa creep tested under a stress of 48.6 MPa at 1400C. One of the grains shows strain during creep testing, see Fig. 2546 S. Gustafsson et al. / Journal of the European Ceramic Society 29 (2009) 539–550 Fig. 9. Thin amorphous films separating intragranular SiC particles from the surrounding mullite grain in the as-sintered nanocomposite. (a) Defocus Fres￾nel fringes (arrowed). (b) Diffuse dark field image. The glassy films (arrowed) appear with bright contrast. glass volumes in the thin-foil TEM specimens were too small for quantitative analysis. 4.3. The polycrystalline mullite after creep testing The average grain sizes of the creep tested mullite specimens were virtually the same as that of the as-sintered material, see Table 1. This indicates that grain growth was not significant during creep testing, see Fig. 2. Fig. 10. A grain showing strain contours (arrowed) in the TEM thin-foil speci￾men of the polycrystalline mullite crept under a stress of 13.0 MPa at 1400 ◦C. Strain contours were generally observed in the TEM bright field images of the microstructures of the crept mullite speci￾mens as shown in Figs. 10 and 11. These contours reflect the stresses that built up in the material during creep deformation. An increased cavitation at multi-grain junctions was observed in some areas of the crept microstructures, see Fig. 11. This type of cavitation was most pronounced in the specimen that had been subjected to the highest stress (48.6 MPa) at 1400 ◦C, but also evident in the specimen crept under a stress of 14.9 MPa at 1300 ◦C. The microstructure of the specimen crept under a stress of 13.0 MPa at 1400 ◦C resembled, on the other hand, that of the as-sintered material, with only a limited increase in the density of cavities. A number of the intragranular cavities changed shape or size during creep testing, and in some cases thin channels connecting nearby cavities had developed, see Fig. 12. The size of these cavities ranged from several hundred nanometers up to 1 m. The overall dislocation density was low also in the creep tested mullite specimens. Dislocations were present Fig. 11. Cavity formation in a multi-grain junction in the polycrystalline mullite creep tested under a stress of 48.6 MPa at 1400 ◦C. One of the grains shows strain contours (arrowed).