Correlation of Oxygen and Aluminum Contents of Molten Titanium-Aluminum Alloys in Alumina and Calcia Crucibles

Abstract

As a key part of investigating a new proposed Ti-alloy production process, Ti-Al alloys were produced by aluminothermic reduction experiments at 1973 K in Al2O3 crucibles. Besides, pure Ti and Al were melted inside CaO crucibles at 1973 K to investigate the possibility of melt refining in equilibrium with CaO. The experiments were carried out in a vacuum induction furnace under argon gas at atmospheric pressure. The experimental results were proved by thermodynamic evaluations including also literature data. The correlation of oxygen and aluminum in Ti melt with 11 to 23 wt pct Al in contact with Al2O3 was assessed as $$ 3\ln X_{\text{O}} = -\,2\ln X_{\text{Al}} - 9.1 \pm 0.4. $$ This correlation in Ti melt with 3 to 10 wt pct Al in contact with CaO was $$ 3\ln X_{\text{O}} = - \,2\ln X_{\text{Al}} - 14.9 \pm 0.3. $$ Partial molar excess Gibbs free energies of mixing and combinations thereof in Ti-rich Ti-Al-O-Ca melt were assessed as $$ RT \ln (\gamma_{\text{Al}}^{2} \cdot \gamma_{\text{O}}^{3} ) = - 894 \pm 6\,({\text{kJ}}/{\text{mol}}) $$ , $$ RT \ln \left( {\gamma_{\text{Ca}} \cdot \gamma_{\text{O}} } \right) = - 195 \pm 4\,({\text{kJ}}/{\text{mol}}) $$ , $$ RT \ln \gamma_{\text{O}} = - 258 \pm 6\,({\text{kJ}}/{\text{mol}}) $$ , $$ RT \ln \gamma_{\text{Ca}} = 62 \pm 7\,({\text{kJ}}/{\text{mol}}). $$ The activity of Al2O3 in CaO-saturated slag was assessed as 0.003 ± 0.001. The changes of Gibbs free energy for the dissolution of 1 wt pct oxygen and 1 wt pct calcium in Ti-Al melt were estimated as −630 ± 6 and −11 ± 7 (kJ/mol), respectively. A model was developed and applied to calculate the basic data for refining Ti-Al alloy.