S Gustafsson et al. /Journal of the European Ceramic Sociery 29(2009)539-550 and silicon were determined. These were then converted into the but a limited number of larger and elongated sections was also equivalent mol fractions of alumina and silica. observed on the etched surfaces. The average grain size was Grain boundary films and amorphous regions were imaged in determined to 1. 5 um. A smaller amount of residual porosity diffuse dark field, and the presence of grain boundary films was was observed throughout the microstructure, consistent with the also established by defocus Fresnel imaging of edge-on grain density measurement (97% dense), see Fig. 3. Intergranular. boundaries.21-25The defocus Fresnel technique was used in esti- irregularly shaped, cavities with sizes in the range 0. 1-1 um mating the thickness of the amorphous grain boundary film in were present at some multi-grain junctions, while the grain around 20 randomly chosen grain boundaries in the as-sintered boundaries were free of cavities materials and in the mullite and nanocomposite specimens crept Largerelongated grain sections often contained faceted intra- at 1400C under a stress of 48.6 and 50.0 MPa, respectively. granular cavities, 50-500 nm in size, but only a limited number The grain boundaries were oriented in the edge-on position by of the equiaxed grain sections showed cavities. The intragranular oking at the reflection symmetry of the fringe intensity across cavities were often associated with single dislocations, or an odd the boundary. Through-focus series were then recorded and pro- dislocation network, as shown in Fig. 3b. The overall dislocation cessed with the Gatan DigitalMicrograph software. An intensity density in the microstructure was, however, low. Only occa profile across a boundary was obtained by integrating the image sional dislocation structures, such as pile-ups at grain boundaries over a distance of 10-15 nm along the boundary. The fringe sepa-(Fig 3)and low angle grain boundaries, were observed ration was then determined from the intensity profiles and plotted Thin glassy grain boundary films merging into amorphous as a function of defocus. The film thickness was estimated by pockets at triple grain junctions were present throughout the xtrapolating the data to Gaussian focus microstructure, see Fig 4. The amorphous triple grain junctions d a diameter(corresponding to the diameter of a circle of equivalent area)in the range 30-70 nm. All analysed amorphous grain boundary films were found to be in the range 0.6-0.9nm 4.1. The as-sintered polycrystalline mullite thick. The result of a TEM through-focus series is shown in The general microstructure of the as-sintered mullite material The mullite grains did not have a perfect 3: 2 mullite compo- is shown in Figs 2a and 3. Most grain sections were equiaxed, sition; the mol fraction Al2O3 was 57.6+2.0%, slightly lower 多 (c) Fig. 6. Thermally etched surfaces of the nanocomposite in the(a)as-sintered condition, and after creep testing under a stress of(b)50.0 MPa at 1400C, (c)12.1 MPa at1400°C,and(d)144 MPa at1300°C544 S. Gustafsson et al. / Journal of the European Ceramic Society 29 (2009) 539–550 and silicon were determined. These were then converted into the equivalent mol fractions of alumina and silica. Grain boundary films and amorphous regions were imaged in diffuse dark field, and the presence of grain boundary films was also established by defocus Fresnel imaging of edge-on grain boundaries.21–25 The defocus Fresnel technique was used in esti￾mating the thickness of the amorphous grain boundary film in around 20 randomly chosen grain boundaries in the as-sintered materials and in the mullite and nanocomposite specimens crept at 1400 ◦C under a stress of 48.6 and 50.0 MPa, respectively. The grain boundaries were oriented in the edge-on position by looking at the reflection symmetry of the fringe intensity across the boundary. Through-focus series were then recorded and pro￾cessed with the Gatan DigitalMicrograph software. An intensity profile across a boundary was obtained by integrating the image over a distance of 10–15 nm along the boundary. The fringe sepa￾ration was then determined from the intensity profiles and plotted as a function of defocus. The film thickness was estimated by extrapolating the data to Gaussian focus. 4. Results 4.1. The as-sintered polycrystalline mullite The general microstructure of the as-sintered mullite material is shown in Figs. 2a and 3. Most grain sections were equiaxed, but a limited number of larger and elongated sections was also observed on the etched surfaces. The average grain size was determined to 1.5m. A smaller amount of residual porosity was observed throughout the microstructure, consistent with the density measurement (97% dense), see Fig. 3. Intergranular, irregularly shaped, cavities with sizes in the range 0.1–1 m were present at some multi-grain junctions, while the grain boundaries were free of cavities. Larger elongated grain sections often contained faceted intra￾granular cavities, 50–500 nm in size, but only a limited number of the equiaxed grain sections showed cavities. The intragranular cavities were often associated with single dislocations, or an odd dislocation network, as shown in Fig. 3b. The overall dislocation density in the microstructure was, however, low. Only occa￾sional dislocation structures, such as pile-ups at grain boundaries (Fig. 3) and low angle grain boundaries, were observed. Thin glassy grain boundary films merging into amorphous pockets at triple grain junctions were present throughout the microstructure, see Fig. 4. The amorphous triple grain junctions had a diameter (corresponding to the diameter of a circle of equivalent area) in the range 30–70 nm. All analysed amorphous grain boundary films were found to be in the range 0.6–0.9 nm thick. The result of a TEM through-focus series is shown in Fig. 5. The mullite grains did not have a perfect 3:2 mullite compo￾sition; the mol fraction Al2O3 was 57.6 ± 2.0%, slightly lower Fig. 6. Thermally etched surfaces of the nanocomposite in the (a) as-sintered condition, and after creep testing under a stress of (b) 50.0 MPa at 1400 ◦C, (c) 12.1 MPa at 1400 ◦C, and (d) 14.4 MPa at 1300 ◦C.