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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

Figure 4.68 Optical

Figure 4.68 Optical micrograph taken from the centre of FATOIM. There were no major differences in the ceramic phase when the MgO preform infiltrated with alloy IM, Figure 4.69, was compared to infiltration with IS (Figure 4.55 b). Therefore reactions between the reinforcement and the alloy were not evident. Light grey precipitates were found in the alloy areas of the MMC similar to those visible in the unreinforced alloy IM, Figure 4.44 a). The binary phase diagram in Figure 2.2 would suggest the precipitates in the alloy IM to be the intermetallic Al3Mg2. Figure 4.69 Optical micrograph of magnesia-reinforced MMC infiltrated with the aluminium-magnesium alloy IM (MOPC20IM). 167

4.9. High pressure die casting infiltration As well as squeeze cast infiltration, some preforms were infiltrated using high pressure die casting. Two different infiltration modes were used which were characterised by significantly different metal flow velocities at the ingate: the indirect squeeze casting (ISQC) and the high pressure die casting (HPDC) mode. For each mode, four velocities were selected using a natural logarithmic velocity graduation shown in Table 3.8. The resulting MMCs were characterised in terms of their homogeneity and the relative compression of the preforms. 4.9.1 Homogeneity of MMC The homogeneity of the MMCs was characterised using 3D X-ray computed tomography. In the resulting virtual cuts, the light areas were the MMC, the grey areas the non-reinforced metal alloy and the black areas were either porosity or the atmosphere surrounding the sample. Prior to the 3D analysis, approximately 2 mm were milled off the castings along the y-axis until the surface of the MMC was reached. As a result, no alloy coverage is visible on top of the MMC in the XZ-plane in the virtual cuts. Each figure shows the virtual cuts of the central layer in the MMC in all three cartesian coordinates, as defined in Figure 3.11. The virtual cut in Figure 4.70 shows the internal structure of the AODY30 preform infiltrated in the ISQC mode using a velocity v0 of 0.72 m/s. The parameter v0 is defined by Equation 24. In each plane, significant fractions of porosity and multiple cracking were detected in the MMC. There was also porosity in the unreinforced alloy on top of the MMC. Examination of the X-Y plane reveals the porosity in the MMC was predominantly concentrated in a rectangular area 6 mm from the edges of the preform. The porosity was subdivided into round pores with a diameter of up to 1.5 mm and pore agglomerations exhibiting a cellular arrangement. The cracking of the preform was most obvious in this plane. 168

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