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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

In summary, the large

In summary, the large surface area of the particulate preforms is favourable in terms of clean aluminium melt surfaces and hence pure Al-reinforcement interfaces are formed, resulting in improved interfacial bonding and improved composite properties. This statement is probably valid for inert systems such as AOPC20IS fabrication and may be the main factor leading to its significantly higher strength compared to Saffil fibre-reinforced MMCs: AOPC20IS showed a σ0 value of 443 MPa (Figure 4.79 b) whereas Saffil fibre-reinforced MMCs, which were limited to a maximum ceramic volume fraction of 0.20 to 0.25, showed a maximum tensile strength of 295 MPa (Table 2.1). The lower value may further be attributed to the silicate binder obligatory in Saffil preform fabrication which is, as shown in Figure 2.13, distributed all along the fibre-matrix interface, leading to a brittle silicate interfacial phase preventing pure metal Al-Al2O3(f) contact during infiltration. Further, the binder is concentrated in the contact areas between the single fibres. Therefore the load transfer at the contact points is restricted to the silicate binder. This configuration led to brittle failure of the contact point and deterioration of the reinforcing network when the composite was extended. Porosity of less than 3% by volume in a MMC significantly influences its strength. When comparing the local strength of MMC before and after optimisation of the thermal conditions of the infiltration tool presented in Figure 4.76, the strength was lowered by 100 MPa to 300 MPa due to the pores. An example of the microstructure is shown in Figure 4.77 a) where metal filled the interparticle cavities but not the larger pores of the initial preform. Under constant pressure infiltration at two different pressures, the large pores were filled prior to the smaller ones as shown in Figure 4.43 a) and b). As similar behaviour was assumed under DSQC, the porosity in the MMC has to be attributed to a post-infiltration effect i.e. shrinkage of the metal during solidification. This is supported by the fact that the porosity was found in the centre of the MMC. This position represented the hot spot of the originally fully-infiltrated preform. The solidification started on the die walls and progressed toward the centre of the 195

MMC. Due to the solidification shrinkage, the melt extruded toward the direction of the advancing solidification front, toward the outer areas of the MMC. The larger pore extrusion phenomenon is in conflict with Washburn´s equation which is the basis of the mercury porosimetry (Equation 21) and valid for non-wetting fluids in a porous network. According to this, extrusion of the fluid would start from the smallest pores. If wetting occurs, the fluid is retained in both the small and the large cavities. Nevertheless, non-wetting of the investigated systems was demonstrated in the sessile drop experiments (Figure 4.6) as well in a spontaneous infiltration test of a metal droplet on a preform sample, where no intrusions were achieved during 30 min holding. In all experiments, wetting angles were larger than 90°. Premature solidification of the metal phase in the small pores may be further assumed. Even though a large surface area is offered to the melt in the intragranular region enabling easy nucleation, the temperature gradient between the melt in the large pore channels and the ceramics does not differ significantly. This inhibits the formation of nuclei on the surface of the ceramics and therefore prevents premature solidification in the intragranular region. The effect of hysteresis reported in the mercury intrusion (-extrusion) porosimetry (MIP) literature (110) seems applicable to the aforementioned extrusion. The hysteresis term in MIP describes the difference in the intrusion and extrusion curves shown in Figure 4.25 for the AO and AOPC20 preforms. Leon (110) attributed the effect mainly to the structural hysteresis characterised by the presence of bottle-neck shaped pores. The latter is characterised by the intragranular region in Figure 4.60 a) and b). The AOPC20IS and TOPC10IS MMCs had bottle-neck porosity which hinders extrusion of the melt out of the intragranular areas. Therefore the hysteresis is most likely to result in the large pore extrusion phenomenon. Some residual porosity was found in the ceramic struts of AODY30IS (Figure 4.58). To produce fully-infiltrated MMCs, the melt had to enter all pores during infiltration which is 196

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