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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

literature (149) . The

literature (149) . The moduli of the MMCs investigated (Edyn,MMC), those reported elsewhere (51, 54) (FA13.5IS, FA20.0IS, AODY25), the predicted range of Ec and the values calculated using the rule of mixture (EROM) are shown in Figure 5.2. FA13.5IS and FA20.0IS were Saffil fibre reinforced MMCs with a matrix similar to IS and Vf of 0.135 and 0.20 (51) respectively. The MMC based on a foamed preform (54) had a similar structure to AODY30IS of the present work but a different ceramic volume fraction of 0.25 and is designated as AODY25. As shown in Figure 5.2, the rule of mixtures overestimates (Ec,ROM) the moduli of all MMCs. Hashin and Shtrikman (55) initially found similar trend issues and therefore introduced their model which gave ranges of values which were lower than Ec,ROM. Elastic modulus E (GPa) E / GPa 250 200 150 100 50 0 AOPC20IS AGPC15IS TOPC10IS MOPC20IS AODY25 AODY30IS FATOIS FA20.0IS FA13.5IS 191 E dyn, MMC E c,upper E c,lower E c,ROM Figure 5.2 Comparison of experimental results of the MMCs moduli (Edyn,MMC) and predictions based on the rule of mixture (Ec,ROM) and upper (Ec,upper) and lower (Ec,lower) bound of Ec in accordance to Hashin and Shtrikman (55) The alumina reinforced MMCs, AOPC20IS and AGPC15IS, showed good agreement with the model of Hashin and Shtrikman (55) , as indicated by the experimental values lying between Ec,lower and Ec,upper. With a similar reinforcement chemistry, the model overestimates the modulus of AODY30IS which can be attributed to the morphology of the ceramic phase or

interfacial debonding: Peng et al. (54) found that in preforms similar to AODY30, a gap was formed along the metal-ceramic interface, attributed to feeding deficiencies during solidification. By variation of the pressure holding times and the cooling schedule, they could optimize the structural integrity of the MMC. In the present work, the optimization of the preform infiltration process enabled the melt feeding into the preform cavities during solidification to be maintained, resulting in dense MMCs as shown in Figure 4.52. Therefore interfacial debonding was not present in AODY30IS as further shown by the microstructure in Figure 4.58. As aforementioned the model matches the measured Ec for AOPC20IS and the deviation in AODY30IS has to be attributed to the reinforcement morphology and its distribution. 5.1.3 Influence of homogeneity The porosity was reduced significantly when a chill was used in the DSQC tool to promote directional solidification from the bottom punch towards the top tool. Therefore, the porosity of castings made in the tool prior to modification can be attributed to insufficient feeding during solidification. In HPDC the infiltration tool was not optimized in terms of directional solidification and similar feeding defects were observed. Typical defects of this type are shown in the macrostructures of AODY30IS in Figure 4.71 where the porosity was concentrated in the centre. It is interesting to note that, when using similar processing conditions, the AOPC20 preform gave a lower porosity in the MMC but large pores on top of it (Figure 4.71) but compressed significantly more (22%) than AODY30 (3%). The compression in AOPC20IS was at the expense of the larger pores, as shown exemplarily in Figure 4.72 b) and c). The absence of porosity in the MMCs produced with the optimized SQC tool, as well as in the AOPC20IS infiltrated in HPDC, is evidence that the gas in the preform does not influence the infiltration results. Infiltration occurs from the top and the four outer faces, forcing the air out of the preform through the bottom face. 192

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