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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

attributed to the finer

attributed to the finer reinforcement size of AGPC15IS compared to AOPC20IS, as shown by comparing Figure 4.54 a) and c). In the AOPC20 preform, the alumina particles were sintered at higher temperatures leading to coarser ceramic grains, whereas the liquid phase sintering at lower temperatures (1000°C) preserved the initial grain size in AGPC15. In general a small interparticle spacing is regarded as the main factor for improved mechanical properties. This is supported by comparing AOPC20IS to AODY30IS which have a similar chemical composition. The metal ligaments in AODY30IS were up to 300 µm long compared to less than 40 µm in the intergranular regions and less than 3 µm in the intragranular regions of AOPC20IS. For the latter composite, the strength and fracture toughness were more than 7 and 3 times higher respectively. The relatively low mechanical properties of MOPC20IS were a result of its coarser ceramic structure (Figure 4.56 a) which may be improved in the future by lowering the preform sintering temperature. The latter preform type offers a unique possibility to influence the grain size of the ceramics, which is discussed in section 5.2.2. The bend test and SEVNB bars measured 3 mm wide and 4 mm thick. The micrograph of AODY30 in Figure 4.58 indicates that the bubble diameters were up to 300 µm and the strut thickness between two single bubbles around 40 µm. Therefore, in the worst case of an agglomeration of larger bubbles in the stressed region, just 6 bubbles and struts were loaded on the tensile area of the beam sample, which makes the validity of the results questionable. Therefore, testing samples of larger volume would be preferable. Nevertheless, the Weibull modulus of 13 indicates a medium scatter in properties and therefore the results are sufficiently reliable for the present comparative study. 199

5.1.5 Influence of reactions No reactions were found in the microstructure of MOPC20IS when infiltrated (Figure 4.60 c) and after thermal analysis (Figure 4.64 b). In contrast, the titania-reinforced MMCs showed reactions in both the as-cast and the heat treated states, but with different amounts of reaction products. After DSQC infiltration with IS, a 50- 100 nm thick reaction layer was visible (Figure 4.60 c) whereas, when using the alloy IM in the reaction, the central area of the MMC (Figure 4.65 b) reached thermodynamic equilibrium similar to that obtained after the thermal analysis (Figure 4.63 a). As the oxidation of the melt surface was far from complete in the present preform configurations, the difference in reaction behaviour between IS and IM was not a result of the differences in melt oxide layer chemistry, which was reported to be MgO in IM-type alloys (11) and Al2O3 in IS (6) . It was attributed to the reactivity between the Mg in the alloy and the TiO2 of the preform. In the centre, the exothermic reaction released sufficient energy to maintain the reaction whereas, in the outer areas, the heat flow towards the die walls prevented a temperature rise, leaving unreacted areas. This shows that the preform technology, coupled with fast infiltration methods, enables the combination of phases far from equilibrium which would not be achievable by techniques where the reinforcement is in prolonged contact with the liquid alloy like gas pressure infiltration of the reinforcement, where contact times of more than 100 s are required (103,132) . The elastic modulus Edyn of TOPC10IS, which was higher than the predicted upper bound shown in Figure 5.2, was a further result of the reaction between TiO2 and Al, with a brittle interfacial phase exhibiting a higher Edyn than the unreacted constituents (53) . The metal ligaments in TOPC10IS were finer than those in AOPC20IS, yet its strength was lower. This is attributed to the reaction layer which consisted of TiAl3, titanium suboxides (TixOy) and reaction-formed Al2O3 (53,94) . This reaction layer reduced the strength and fracture toughness due to brittle failure of the interface, as shown in Figure 4.81. In the case of fully-reacted 200

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