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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

has to be applied. In

has to be applied. In general, infiltration methods are subdivided into the two methods of liquid transport: the constant pressure (CP) and the constant flux (CF) methods. The former is widely used in gas pressure infiltration of particulate compacts and is the single method published to determine dynamic wetting angles, as reviewed by Garcia-Cordovilla et al. (103) . The infiltration times vary in the range from 30 seconds to several minutes. In the CF infiltration mode, experimentally performed using direct squeeze casting and high pressure die casting set-ups, the molten metal is forced into the ceramic network within relatively short times of less than 10 seconds (130) . In all published infiltration experiments, the preforms were preheated to temperatures up to 1000°C. Nagata and Matsuda (131) suggested a critical minimum preheating temperature (Tpct) which is independent of size, morphology of the reinforcement and melt temperature and pressure (Equation 36). T pct = T m 0. 233⋅ ρ M ⋅ H − V ⋅ ρ ⋅c f f pf M 53 Equation 36 The influencing factors are those of the melt (liquidus temperature TM, melt density ρM and heat of fusion of the melt HM) as well those of the fibre preform (volume fraction Vf, ρf and its specific heat capacity cpf). The critical preheating temperature is important in CF infiltration, where the temperature is usually below the liquidus temperature of the infiltration alloy which is a result of the handling of the preform and the infiltration temperatures. In contrast, in gas pressure infiltration, the preform is heated at the same time in the same cavity as the melt, thus the temperature of the porous media is normally the same temperature as the melt.

2.5.1. Gas pressure infiltration (GPI) For continuous fibre preforms, GPI is the predominant way to produce MMCs. This is mainly attributed to the possibility of using low pressurization rates which are necessary to prevent fibre breakage and destruction of the preforms (103) . The process is performed in an autoclave within which the alloy and the preform is placed. To prevent gas inclusion in the preform, it has to be evacuated prior to infiltration. The alloy and usually the preform are heated up subsequently. After melting of the alloy, the autoclave is pressurized using an inert gas like Ar or He up to a maximum pressure, P, of 15 MPa (132) . This is the upper threshold for safety reasons. When P reaches P0, the melt starts to infiltrate the preform. Garcia-Cordovilla et al. (103) used a unidirectional setup to infiltrate particulate compacts which were enclosed in tubes. The experiments were performed using relatively long contact times of the order of minutes. In reactive systems, this period exceeds the incubation time which is in general necessary to initiate thermodynamically preferred reactions. The reaction products sometimes block the preform entrance, preventing further infiltration (133) . To ensure that the cavities remain saturated with metal during solidification, the shrinkage has to be directed toward the unreinforced region. Therefore infiltration experiments were carried out using a chill in the furnace to impose directional solidification (132,134) . Knechtel et al. (68) infiltrated Al2O3 particle beds with pure Al using 15 MPa gas pressure and high processing temperatures of 1110°C. The fracture surface of the resulting Al2O3-Al composites exhibited a transgranular fracture of Al2O3 and no debonding of the metal phase. Even though the processing was done under extreme conditions, the interfacial reaction layer was only a few monolayers thick. Apart from Al-Al2O3 systems, a few carbide systems were also investigated (103) , but no other Al-oxide ceramic systems were investigated in the pertinent literature. 54

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