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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

The bimodal porosity in

The bimodal porosity in the preforms generated by the pyrolysis route was completely filled with the metal during infiltration. Therefore a bimodal metal ligament distribution was developed with the large metal regions visible at lower resolution and fine metal ligaments between the particles visible at higher magnifications This is a characteristic feature of this type of MMC. The grains in AOPC20IS exhibited a significant fraction with an aligned elongated structure which can be seen in the circled area of AOPC20IS in Figure 4.53 a). This structure was not apparent in the micrograph of AOPF20IS. Even though the sintering temperature of the latter preform type was 100°C less than that of AOPC20IS, the ceramic phase was apparently coarser. This was due to the pore former influencing the sintering kinetics of the alumina. Similar to the alumina preforms, the intragranular metal fractions in the titania MMCs were not clearly defined in the low magnification micrographs of TOPC10IS and TOPC20IS as shown in Figure 4.54 a) and b) respectively. The intergranular metal fractions are apparent at this low magnification in MOPC20IS, Figure 4.54 c). There is a strong anisotropy with a preferred metal ligament orientation along the X-axis in TOPC20IS, Figure 4.54 b). Additionally, agglomerates of the ceramic particles are visible. Figure 4.55 shows the materials at higher magnifications. TOPC10IS (Figure 4.55 a) exhibited a finer ceramic structure than TOPC20IS (Figure 4.55 b) which resulted from the higher sintering temperature used for the latter. In comparison to the AO type MMCs shown in Figure 4.53, the ceramic phase was rather more continuous with distinct sintering bridges between the particles. In the microstructure of TOPC20IS, sintering reached a stage where closed cell porosity was formed as indicated exemplarily by the arrows to the black features in Figure 4.54 b) which were non infiltrated porosity. In contrast to the titania-based MMCs, the interconnection of 151

the ceramic particles was not visible in the magnesia-based MMC MOPC20IS, Figure 4.55 c), where a granular structure of the ceramic phase was apparent. (a) TOPC10IS (c) MOPC20IS 152 (b) TOPC20IS Figure 4.54 Optical micrographs of the MMCs based on reactive preforms: TOPC10IS, TOPC20IS and MOPC20IS. Similar to the alumina-based MMCs, the Si precipitates in the alloy in the intergranular regions of the reactive MMCs shown in Figure 4.55 a) to c) were finer and more spherical compared to that of the pure alloy. The microstructure of MOPC20IS exhibited the finest Si precipitates of all MMCs investigated in the present research: the lengths of the individual precipitates were generally below 3 µm and their width sub-micron in scale.

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