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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

calcium and aluminium in

calcium and aluminium in the stoichiometric composition of AlCa2 , which was calculated to be 2320 kg/m³, was used for the volumetric calculations. The predicted phases after reaction of liquid aluminium with MgO were a small amount of the spinel phase MgAl2O4, the initial ceramic MgO and a solution of magnesium in aluminium. As indicated, the calculated equilibrium was reached before all the MgO reacted to form the ternary MgAl2O4 phase. In the Al-TiO2 system, it was predicted that Al would react completely to result in a composite containing Al2O3, titanium suboxides and titanium aluminides. The titanium suboxides were mainly TiO and Ti2O3 and the aluminide phases were predominantly TiAl3 and a minor fraction of TiAl. In the system with SiO2, the ceramic was reduced completely to give a composite of Al2O3 and a solution of Si in liquid Al. Volume fraction 1.0 0.8 0.6 0.4 0.2 0.0 Y 2 O 3 Al Al 2 O 3 AlCa 2 Al MgAl 2 O 4 MgO Al-Mg Al 2 O 3 Al ZrO 2 Y 2 O 3 CaO MgO Al 2 O 3 ZrO 2 TiO 2 SiO 2 Al 93 Al 2 O 3 Al2 O 3 Ti x Al y Ti x O y Al-Si Al Al-Me solution initial ceramics reaction formed phase I reaction formed phase II Figure 4.2 Equilibrium composition of different liquid aluminium-oxide ceramic systems. The relative volume changes between the reactants and the reaction products (i.e. thermodynamically most stable phases as shown in Figure 4.2) were calculated and the results are shown in Figure 4.3. No reaction was predicted in the systems with Y2O3, Al2O3 and ZrO2 and thus the density was a result of the mechanical mixture of the ceramic phase with Al.

Positive volume changes were predicted for the Al-CaO and Al-MgO systems. The formation of the CaAl2 intermetallic resulted in a volumetric increase of 10%. A marginal positive volume change of 3% was calculated for the MgO system. According to Gmelin (171) , the density of MgAl2O4 is the same as that of magnesia MgO. Therefore the volume change is a result of the formation of the solid solution of Mg in Al, which results in a density reduction of the alloy. According to Quested et al. (172) , the density of the solid solution of Mg in Al is accurately given by calculating that of the mechanical mixture of both elements. Negative volume changes were calculated for the Al-TiO2 and Al-SiO2 systems. The formation of the ceramic-intermetallic composite by reaction in the Al-TiO2 system resulted in a relative volume change of -10%. The largest volume change of -15% was in the Al-SiO2 system, due to the formation of a solid solution of Si in Al and Al2O3 by reduction of SiO2 with Al. The highest densities of more than 3500 kg/m³ were obtained when combining Al with Y2O3, ZrO2 and TiO2. Density (kg/m 3 Density composite (kg/m³) ) / kg/m³ 5000 4500 4000 3500 3000 2500 2000 volume change non-reacted density reacted density YY2O3 CaO MgO Al2O3 ZrO2 TiO2 SiO2 2O3 CaO MgO Al2O3 ZrO2 TiO2 SiO2 Figure 4.3 Theoretical density of the non-reacted and reacted composite materials at 25°C and the resulting relative volume change from non-reacted to reacted composite material. 94 0.5 0.4 0.3 0.2 0.1 0.0 -0.1 -0.2 -0.3 -0.4 -0.5 Relative volume change Relative volume change () ()

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