non-wetting fluid, as a function of the applied pressure Pappl. The curves obtained can be fitted to phenomenological equations, introducing a threshold pressure (P0) which must be overcome to initiate infiltration, and a shape parameter α (115,113) : S = 1− 1 2 1 ( Pappl P0 ) − ⋅ + α 2 45 Equation 29 This equation is valid for Pappl>P0. The curve shape described by the shape parameter value varies with the pore size distribution, the size and type of reinforcement and with the wetting behaviour of the matrix on the reinforcement. Dopler et al. (113) proposed an equation for modelling the infiltration behaviour of Saffil fibre preforms: dS ∂p ⎛ Ks ⋅ Kr ⎞ ( 1− θ f ) ⋅ ⋅ − ∇⎜ ⋅∇P ⎟ = 0 Equation 30 dp ∂t ⎝ μ ⎠ In most metal melt / ceramic preform systems, only the volume fraction θf and the dynamic viscosity μ are known. Thus the following terms need to be determined experimentally: 1. the drainage curve S(p) using the parameter P0 and α 2. the relative permeability Kr. 3. the specific permeability tensor Ks. Dopler et al. (113) investigated fibre preform infiltration using constant pressure both experimentally and by the aforementioned model. The system parameters 1 to 3 were determined and the function P(x,t) could be found by solving the non-linear partial differential equation (Equation 30) with given initial and boundary conditions. The results of the two- dimensional numerical analysis were in good agreement with the experiments, both in terms of infiltration kinetics and porosity distribution.
2.4. Preform fabrication Composites of aluminium and its alloys with Al2O3 have the advantage of complete thermodynamic compatibility and exhibit no solubility of one phase in the other which may result in strong interfacial bonding (64) . As a result, most of the published work on oxide ceramic preforms used for MMC fabrication is based on alumina. Mainly fibres, like Saffil and others, were infiltrated with aluminium melts. Due to the multiple processing steps of fibre fabrication, the preforms made of chopped fibres are 30 to 50 times more expensive than particulate preforms. 2.4.1. Fibre preforms The fibre material contains usually about 3- 4 wt.% silica which serves to stabilize a fine grain structure with grain sizes of around 50 nm and to inhibit coarsening of the crystallites (116) . This silica is dispersed throughout the fibre section, but also tends to become slightly concentrated at grain boundaries and free surfaces. On excessive heating, small crystallites of mullite (3Al2O3·2SiO2) may appear. For preform fabrication, the bulk fibre material is dispersed in a fluid, usually water, containing an inorganic binder of the silicate type and an organic binder (e.g. starch). After thorough mixing the fibres are allowed to settle and excess water is decanted (117) . Vacuum forming over a fine mesh followed by the application of pressure results in the required preform density. The resulting green body is dried and fired at a temperature ranging from 800 to 1000°C. The amount of fibres in typical preforms is limited to a volume fraction of 0.08 to 0.30. At the lower level it is difficult to produce a coherent structure, whilst above 0.30 excessive fibre cracking may results in a significant reduction in aspect ratio and thus in reinforcing properties. 46