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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

4. RESULTS 4.1.

4. RESULTS 4.1. Thermodynamic calculations One aim of the research reported here was to investigate whether chemical reactivity between the ceramic reinforcement and the molten alloy during the MMC processing influences the infiltration behaviour and the properties of the resulting material. A negative heat of reaction is presumed to be a necessary conditions for reactivity. Values were calculated on the basis of the MMC target volume fraction (0.35 ceramic, 0.65 metal alloy) and the infiltration with pure Al at 750°C. 4.1.1. Standard free energy of formation The Ellingham-Richardson-Jeffes diagram (89) given in Figure 2.11 shows that oxide ceramics may be divided into a group exhibiting higher energies of formation than Al2O3, for example Y2O3, CaO and MgO, and a group with lower free energies such as ZrO2, TiO2 and SiO2. ΔG (kJ/mole / kJ/mole Al) Al 0 -20 -40 -60 -80 -100 Y 2 O 3 CaO MgO Al 2 O 3 ZrO 2 TiO 2 SiO 2 91 Ellingham ΔG (Me/MexOy) < ΔG (Al/Al2O3) ΔG (Me/MexOy) > ΔG (Al/Al2O3) Figure 4.1 Free energy of formation standardized to 1 mole of pure Al of liquid aluminium-oxide ceramic systems at 750°C. Ceramic volume fraction of 0.35. The latter are reactive systems when in contact with Al whereas the others may be regarded as non-reactive systems. The free energies of formation ΔG for these systems at 750°C were

calculated using the methods outlined in 3.1 and were normalized to one mole of Al. The resulting free energies of formation are shown in Figure 4.1. The calculation shows that the free energies of CaO, MgO, TiO2 and SiO2 are smaller than that of the Al-Al2O3 system. Even though CaO and MgO were assumed to be non-reactive systems, free energies of -39 kJ/mole Al and -8 kJ/mole Al, respectively, were calculated. This is attributed to the predicted formation of intermetallics and ternary phases, which are not taken into account in the data for the Ellingham-Richardson-Jeffes diagram, which is only based on the reaction of metal to the metal oxide. Greater values of ΔG were calculated for the reactive systems Al-TiO2 and Al-SiO2 (-68 and -52 kJ/mole Al) respectively. The calculations predicted no reaction of liquid Al with Y2O3, Al2O3 and ZrO2, as indicated by zero values for ΔG. 4.1.2. Most stable product phases Knowing the free energy of the system it was possible to calculate the composition at equilibrium. The composite density and the density changes were calculated based on data of Gmelin (171) . In contrast to the pure ceramics, the densities as a function of temperature were not available for all solid solution compounds. Thus the calculations were based on room temperature values of the density. The predicted volume fractions of the product phases brought to equilibrium at 750°C are shown in Figure 4.2. The thermodynamic calculations of Y2O3, Al2O3 and ZrO2 showed no change from the initial composition already indicated in the previous section by zero ΔG values. In the system with CaO, the entire ceramic phase reacted with the aluminium to form the more stable phases of alumina and an intermetallic compound AlCa2, in combination with a low volume fraction of the initial liquid aluminium. As no density data for AlCa2 could be found in the literature and the Gmelin (171) database, the density of the mechanical mixture of 92

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