The mean saturation along the x-axis calculated as the arithmetic mean of pore saturation along the z-direction is presented in Figure 4.43 for the AOPC20 preform. In general, the saturation decreased from the edge towards the centre of the preform. After infiltration at a constant pressure of 0.80 MPa, the saturation on the edge was 0.27 but this dropped to close to zero at a depth beyond about 12.5 mm. Saturation S S() 1.0 0.8 0.6 0.4 0.2 0.0 0 5 10 15 20 25 30 Position x (mm) x / mm 137 0.8 MPa 1.2 MPa Figure 4.43 Mean saturation along x-axis in AOPC20 infiltrated at constant gas pressures of 0.80 MPa and 1.20 MPa. At 1.20 MPa, the saturation on the outer edge was 0.77 and decreased to 0.22 close to the centre of the preform. The curve showed unsteady behaviour towards the centre. Starting from an infiltration depth of 26 mm, the saturation increased again to 0.39 in the centre at the x- position of 30 mm. 4.8. Squeeze cast preform infiltration The non-reinforced infiltration alloys were characterised in terms of their microstructure, thermal behaviour whilst cooling, and the thermal conditions during an infiltration cycle.
4.8.1 Unreinforced matrix properties The microstructures of the non-reinforced infiltration alloys are shown in Figure 4.44 a) and b) for squeeze cast specimens processed using same conditions as used for infiltration. The microstructure of IM (AlMg9) consisted of discontinuous chain-shaped precipitates in a light matrix as shown in Figure 4.44 a). According to the binary phase diagram of Al-Mg, Figure 2.2, the precipitates consisted of the β-phase, Al3Mg2. (a) (c) IM IS + 540°C/1h 138 (b) IS 2 20 2 20 µmµm Figure 4.44 Microstructure of the unreinforced matrix alloys a) Al-Mg IM ; b) Al-Si IS and c) IS heat treated at 540°C for 1 h. The AlSi12Fe alloy (IS) was examined in the as-cast condition (Figure 4.44 b) and after heat treatment at 540°C for 1h (Figure 4.44 c). In the as-cast condition, acicular precipitates and small Chinese-character shaped light grey precipitates were observed. The binary phase diagram of the Al-Si system in Figure 2.4 indicates no room temperature solubility of silicon in aluminium. Therefore the acicular precipitates consisted of pure silicon whereas the light