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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

The low magnification

The low magnification micrographs of the alumina-reinforced MMCs AOPC20IS, AOPF20IS and AGPC15IS in Figure 4.52 a), b) and c) respectively show an apparently predominant fraction of the ceramic phase. The large metallic areas developed from the filling of the coarse pores in the preforms derived from the pyrolysed pore forming additive. In AOPF20IS, where carbon fibres were used to form the coarse porosity, the elongated shape of this PFA is clearly shown in the circled area in Figure 4.52 b). Additionally, the metal phase areas were larger and more distinct compared to AOPC20IS (a) and AGPC15IS (c), indicating that the initial green part contained carbon fibre agglomerates (arrows). (a) AOPC20IS (c) AGPC15IS 149 (b) AOPF20IS Figure 4.52 Optical micrographs (low magnification) of the MMC materials based on alumina preforms: AOPC20IS, AOPF20IS and AGPC15IS.

The metal filling the intragranular regions was visible at higher magnifications as shown in Figure 4.53 a) to c). Furthermore, the Si precipitates as well as the grey Fe-containing phases were resolved. The finest Si-precipitates were found in AGPC15IS where they were round and sub-micron in size. Here the interstices between the ceramic particles were relatively small in the sub-micron range and had a fuzzy appearance. It is presumed that the larger transparent areas were agglomerates of the glassy binder added to the ceramic slurry to bond the particles in this preform. The metal phase in the interstices of AOPF20IS (Figure 4.53 b) and AOPC20IS (Figure 4.53 a) was more distinct and the interstices were larger compared to AGPC15IS. (a) AOPC20IS (c) AGPC15IS 150 (b) AOPF20IS Figure 4.53 Optical micrographs (high magnification) of the MMC materials based on alumina preforms: AOPC20IS, AOPF20IS and AGPC15IS.

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