material, similar to

material, similar to that resulting from the thermal analysis test, KIC was as low as 3 MPa·m 1/2 (163) . This was a result of the brittle character of the intermetallic TiAl3 (53,163) in combination with the presence of 11% porosity (Figure 4.63 a) resulting from the volumetric change due to the reaction products which were predicted by the thermodynamic calculations (Figure 4.3). The KIC values of the MMCs based on the self-fabricated preforms with a bimodal pore structure were higher than the MMCs made using the two purchased preforms: FATO, a hybrid preform with Al2O3 fibres (Saffil) and fine TiO2 particles, and AODY30 in which the predominant porosity was formed by bubbles in the ceramic slurry. The processing of the latter resulted in relatively large spherical porosity as shown in Figure 4.23 in the as- purchased condition (AODY30) and in the MMC (AODY30IS) in Figure 4.58. AODY30IS showed the lowest KIC and strength of all MMCs investigated, but gave superior tribological properties which were attributed to the differences in microstructure. Three main influencing factors were identified for tailoring the mechanical and tribological properties of MMCs at constant reinforcement volume fractions: a) Porosity in the MMC microstructure. b) Interface between the metal phase and the reinforcement c) Small interparticle spacings filled with the metal phase The porosity in the MMC has to be subdivided into pores generated by the melt solidification, which were discussed together with the interfacial aspects in this section, and those resulting from the saturation during infiltration which is discussed in section 5.3. The interparticle spacing is built into the preform during fabrication and is discussed in the following section. 201

5.2. Preform pore formation The target preform porosity range in the present work was between 60 and 70%. Neglecting compression, the size of the porosity measured in the ceramic preforms represents the size of the metal phase after infiltration. It has been shown that the reinforcing effect is superior in terms of mechanical properties for a fine distribution of metal ligaments compared to extensive ceramic and metal ligaments e.g. the MMC of the foamed preform AODY30IS. 5.2.1 Foamed preforms A window diameter of 10 µm and a cell diameter of 120 µm were measured in the microstructure of AODY30 (Figure 4.23). The window size, representing the bottleneck pathway between the single spherical cavities, was confirmed by the mercury intrusion and extrusion results: the main intrusion peak was at 9 µm, and 97% of the mercury was not extruded after pressure release to ambient pressure, which corresponded to a pore diameter of 11 µm, shown in Figure 4.32 and Table 4.2 respectively. The cell sizes assessed on the cross sections of the MMCs shown in Figure 4.58 ranged between 10 µm and more than 500 µm. The range of values resulted from the position of the section: as the bubbles in the ceramics were randomly distributed, the probability of cutting along the diameter of a single sphere was low. Nevertheless, there were larger spheres than the aforementioned 120 µm which indicates inhomogeneities in the preform blowing process. Peng et al. (125) claimed a uniform cell diameter distribution of about 150 µm, but bubbles of up to 350 µm in diameter were visible in their microstructures. This confirms that deviations from the mean cell size of more than 100% are characteristic for this pore forming method. In Equation 35 the relation between pore fraction Vpo and the ratio of window to cell size (k) of foamed preforms is given. According to this model at a k-value of zero, which means a window diameter of zero and therefore a closed cell foam, a Vpo of 0.74 results. Even when 202