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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

a) b) Figure 4.40 Cross

a) b) Figure 4.40 Cross sectional macrograph of AOPC20 infiltrated at 0.80 MPa: a) Reflected light grey scale image with metal (white), alumina and the residual porosities (dark). The frame in the microstructure indicates the position of the detailed view in Figure 4.42 b) Quantification of the relative saturation of a). a) b) Figure 4.41 Cross sectional macrograph of AOPC20 infiltrated at 1.20 MPa: a) Reflected light grey scale image: frame indicates the position of the detailed view in Figure 4.42 b) Quantification of the relative saturation of a). The preform infiltrated at 0.80 MPa had metal ligaments concentrated towards the side and the top areas of the preform. About 35% of the overall area of the preform showed metal 135 Z Z X X

intrusions and the other areas were free of metal. The saturation reached 0.4 on the outer edge and the top of the preform and about 0.2 towards the centre. a) 0.8 MPa b) 1.20 MPa 136 Z X 2 100 2 100 µm µm Figure 4.42 Microstructure of preforms AOPC20 infiltrated at constant pressures of a) 0.8 MPa and b) 1.20 MPa. Dark phase: residual porosity; grey phase: Al2O3 ceramic; light: alloy. In contrast, comparatively larger saturations were observed in the preform infiltrated at 1.20 MPa, as shown in Figure 4.41. Apart from a small area on the upper corner of the infiltrated preform, no other regions were fully infiltrated. Similar to the preform infiltrated at 0.80 MPa, the areas toward the outer edge had saturation values between 0.7 and 0.8. In the central area, the saturation reduced to 0.2 to 0.3 with a significant negative saturation gradient along the positive x-axis, from the outside to the centre of the preform. The microstructures at defined position (5 mm from the outside border along the x-axis and in the centre of the y-direction of the preform and marked with a square in Figure 4.40 a) and Figure 4.41 b) are shown in Figure 4.42 a) and b). At the lower pressure of 0.80 MPa, the coarse porosity resulting from the pyrolysis of PFA was filled while the pores between the finer particles remained empty. In contrast, at 1.20 MPa, the fine porosity between the particles was at least filled partly. This is in agreement with the overall saturation data.

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