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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

Compared to the

Compared to the micrographs of the MMCs fabricated with preforms of the PFA pyrolysis route, e.g. the AOPC20IS in Figure 4.52, the alloy regions were significantly larger in the FATOIS. The length of the fibres aligned along the cutting plane of the micrograph varied in the range of 30 µm to 80 µm. At higher magnifications, fibre fragments shorter than 10 µm were visible. An example is marked with an arrow in Figure 4.57 b). The fibre diameter was in the range of 3 µm to 8 µm. The fibres at the polished surface were transparent allowing the sub-structure to be seen. The MMC fabricated with the AODY30 preform exhibited a cellular ceramic structure, as shown in Figure 4.58. Here the dark phase is the relatively dense sintered alumina struts and the light phase the alloy IS. The bubbles in the ceramic slurry were filled with the alloy during infiltration. The Si precipitates were comparatively coarse and dendritic growth of α-Al can be seen in Figure 4.58 a). The areas of metal had diameters in the range from 10 µm to more than 300 µm, which represented the bubble diameters in the preform. The ceramic struts between the metal regions showed some residual porosity. The partial filling of the pores in the struts was apparent at high magnification in Figure 4.58 b). The residual porosity consisted of isolated, closed cell pores which could not be infiltrated. (a) AODY30IS (b) Figure 4.58 Optical micrographs AODY30IS type MMC. 155

The windows, one of which is marked in Figure 4.58 b), in foamed preforms were connecting paths between two single bubble-shaped cavities. The light phase inside the struts is the metal intruded into the sub-micron pores. Therefore a small portion of the preform porosity consisted of open cell pores in the struts. The Si phase in the alloy was marginally coarser than that in FATOIS, Figure 4.57 b), but significantly coarser than in the MMCs fabricated using PFA-formed pores in the preform shown in Figure 4.53. 4.8.8 Interfacial microstructure of MMC with alloy IS matrix The interface between the unreinforced region on top of the MMC and the MMC itself was investigated in order to detect any oxide films originating from the alloy melt. These films might be prevented from entering the preform during infiltration as the preform inlets blocked the passing of solid particles into the body. This is the concept of metal filtration, where inclusions are collected at the surface of the ceramic. Al-Si alloy MMC Si Figure 4.59 SEM micrograph (backscattered electron image) of the interface between MMC and unreinforced material on the top of the MMC type AOPC20IS. The interface of the AOPC20IS MMC is presented in Figure 4.59. At the chosen magnification, there was no evidence of oxide film accumulation at the preform inlet. The silicon precipitates on top of the MMC exhibited some cracking. These cracks resulted from 156 Si

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eTheses Repository - University of Birmingham
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