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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

The sintering

The sintering temperatures of the MOPC20 preforms were varied. The MMC microstructures resulting from preforms sintered at the two extreme temperatures, namely 1300°C and 800°C, and infiltrated with the alloy IS are shown in Figure 4.56 a) and b). The microstructure in terms of the size and shape of the MgO ceramic phase differed significantly. The structure of the MOPC20IS made with a preform sintered at 1300°C in Figure 4.56 a) exhibited a coarse ceramic phase with particle diameters in the range of 2 to 10 µm. (a) TOPC10IS (c) MOPC20IS 153 (b) TOPC20IS Figure 4.55 Optical micrographs of the MMCs based on reactive preforms: TOPC10IS, TOPC20IS and MOPC20IS. In contrast, the MOPC20IS fabricated using a preform sintered at 800°C (Figure 4.56 b) had a significantly finer ceramic structure, with grain sizes in the sub-micron range. The intragranular region showed some areas where the light metal phase infiltrated the small pores

etween the dark grey ceramic phases. However a significant portion of the porosity between the ceramic particles was not infiltrated, shown by the black regions. With the current infiltration set-up and parameters, the fine pores were not infiltrated, leaving residual porosity. (a) MOPC20IS_Ts=1300°C (b)MOPC20IS_Ts= 800°C Figure 4.56 Optical micrograph of MOPC20IS with preforms sintered at a) 1300° and b) 800°C. The MMC based on the FATO preform comprised alumina fibres (the black phase) embedded in titania particles (the dark grey phase) and the infiltration alloy with relatively coarse needle shaped Si precipitates in α-Al, Figure 4.57. (a) FATOIS (b) Figure 4.57 Optical micrographs of the FATOIS type MMC at low (a) and higher magnification (b). 154

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