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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

Equation 20, reduces P0.

Equation 20, reduces P0. However, Beyer (53) and Mattern (114) proposed complete non-wetting conditions (cosθ = -1) in TiO2 and Al2O3 preform infiltration with Al alloys. The ΔG value in the systems at any given temperature and concentration is a measure of reactivity. The temperature dependencies of the standard free energies of formation ΔG0 of metal-metal oxide systems of the Me-MexOy type are usually presented in form of Ellingham- Richardson-Jeffes diagram (89) , of which an extract was given in Figure 2.11. At constant temperature, oxides of Me-MexOy systems with higher ΔG0 values than that of Al-Al2O3 are reduced by Al and therefore represent reactive systems. This is the case for the Si-SiO2, Ti- TiO2 and Ti-TixOy, which are rated as reactive, in contrast to Al-MgO and Al-CaO whose curves are below that of Al-Al2O3 and therefore no reaction with Al is expected. Experimental results reported by Niu et al. (187) did not confirm the passivity of the Al-CaO system at elevated temperatures. Here a CaO coating was dissolved partially by liquid Al, due to the formation of AlCa2 intermetallics. Therefore statements about the non-reactivity of a given metal - oxide ceramic system based on the position of the curve in Ellingham-Richardson- Jeffes diagram are not sufficient for a full classification of the system. Numerical thermodynamic modelling was considered to be a more accurate tool to classify reactive systems. Using metal and ceramic data bases, the modelling of metal-ceramic reactions with the FACTSAGE software led to accurate results for the Al-TiO2 system (94) . In the present work, this code was used to calculate the systems (section 4.1.1). For Al-CaO the phases found in the experiments (187) were confirmed to be the most stable phases, Figure 4.2. Further, the reactivity of the system was confirmed, indicated by a negative ΔG, Figure 4.1. As shown in the thermodynamic calculations and reported by others (7,77) , the Al-Al2O3 system is assumed to be non-reactive at temperatures below 800°C which are used for preform infiltration. Similarly, Al-ZrO2 and Al-Y2O3 were identified as non-reactive, 211

indicated by zero values of the free energy of formation ΔG, Figure 4.1. Only the Al-Al2O3 was taken as a reference non-reactive system and the other non-reactive systems (Al-Y2O3, Al-ZrO2) were not included in the present experiments. It has been shown that the reduction of SiO2 with liquid Al can be used to synthesise Al(Si)- Al2O3 composites (77,23,101) . The Si formed by reduction dissolved in the melt and subsequently solidified to an Al-Si alloy matrix between the reaction-formed Al2O3. The pure Al-SiO2 was (149) not investigated due to the low mechanical performance of SiO2 , presumably resulting in a low reinforcing effect of the resulting MMC. However, an alumina preform (AGPC15) with silicate binder surfaces was investigated. The thermodynamic calculations confirmed the high reactivity of the Al-TiO2 system. This has been reported by others (53,155,163) aiming to synthesise ceramic-intermetallic composites (CIC) consisting of reaction-formed Al2O3 in a TixAly matrix. In contrast to the Al-SiO2 systems, reduction to metallic Ti was not observed experimentally. This conforms with the thermodynamic calculations where TiO2 is partially reduced to form suboxides like TiO and Ti2O3 in combination with the aluminides TiAl3 and TiAl, see section 4.1.1. Beyer (53) found that the squeeze casting of an Al alloy into TiO2 preforms with a median grain size of 0.3 µm resulted in strong exothermic reactions and temperature increases of up to 1300°C inside the preform. Due to the drastic temperature increase, the viscosity and surface tension of the alloy, whose temperature dependency is described by Equation 1, decreased significantly. As a result of this heating, the die system could not be sealed by solidification and resulted in hazardous spurting of the melt out of the tool. Thus, the processing of fine grain TiO2 preforms was considered to be uncontrollable. Beyer (53) also showed that macroscopic reactions could be observed with TiO2 powders having a median grain size of 1 to 5 µm (155,163) . A subsequent heat treatment up to 800°C with low heating rates led to consistent 212

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