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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

shear stresses in this

shear stresses in this brittle phase caused by the thermal mismatch at the interface. A light phase with a high specific mass was visible in the MMC microstructure in Figure 4.59. As already detected in the Al2O3 substrates for the sessile drop test (Figure 4.9), the light phase was ZrO2 which originated from debris of the milling vessel and balls worn during ceramic slurry preparation. (a) AOPC20IS (b) TOPC10IS (c) MOPC20IS C C M M S Figure 4.60 Backscattered electron micrographs of the metal-ceramic interfaces in the intragranular region of a) AOPC20IS b) TOPC10IS and c) MOPC20IS. Phase C, M and I: ceramic, metal alloy and interfacial phase, respectively. No significant interfacial reactions were observed in the microstructures of the MMCs presented in the previous section. The thermodynamic calculations predicted reactions when Al is in contact with the reactive preforms containing TiO2 or MgO. In order to detect 157 M C I

potential interfacial reactions, the microstructures close to the ceramic phases were investigated at high magnification using an SEM. At the resolution limit of the SEM employed, Figure 4.60 a) and c) show that no reaction products were observed on the inner surfaces of the alumina and magnesia MMCs, AOPC20IS and MOPC20IS. In the matrix alloy (M) of AOPC20IS, silicon precipitates (S) were observed as well as some debris resulting from metallographic preparation. Due to the significant hardness differences between alumina and the infiltration alloy, metallographic preparation led to preferential removal of the metal and alumina debris being embedded into the matrix alloy, as shown in the micrograph of AOPC20IS in Figure 4.60 a). In contrast, magnesia is comparatively soft and Figure 4.60 c) shows that the polishing results were significantly improved, indicated by the lack of debris in the metal areas. An intensive effort to optimize the SEM image quality was not successful, leading to a `fuzzy´ appearance of the microstructure. No reaction layer phase could be detected at the interface of MOPC20IS. The microstructure of the titania-reinforced MMC TOPC10IS is shown in Figure 4.60 b). The surface of the ceramic phase was covered with an interfacial layer with a thickness of 50 to 100 nm. The entrance to the bottle-neck shaped pore in the centre of the micrograph was entirely filled with the reaction product. As the micrograph was taken in the backscattered electron mode, the grey value of the interfacial phase indicates that its atomic mass was between that of the matrix alloy and titania. 4.8.9 Differential thermal analysis on MMCs with alloy IS matrix In order to investigate whether the predicted reactions could occur in the MMC, thermal analysis was performed on the final MMC in the temperature range between 100 and 800°C. Under these conditions the infiltrated alloy would remelt. Figure 4.61 shows the differential temperatures between sample and reference over the temperature range applied. Endothermic 158

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