PHASE FORMATION DURING ALUMINOTHERMAL REDUCTION OF Ti, Nb, Gd (Y) FROM OXIDES

Abstract

Alloys based on titanium and aluminum with additions of niobium and rare-earth metals have unique mechanical and heat-resistant properties, and also it likely that such alloys would have increased corrosion resistance. The method of thermodynamic modeling using the HSC program was used to study a system with aluminum consumption varying in the range from 0 to 100% of the mass of the initial charge. The features of phase formation in Al–[50TiO2–5Nb2O5–1Y2O3 (Gd2O3)] systems have been studied. The calculation of the heat balance of the process at 1600°C and 44% of Al was – 0.196 MJ per 1 kg of charge, which indicates the possibility of its occurrence with only the aluminothermic reactions. The reduction of titanium and niobium can proceed by reactions through the formation of their oxides of lower valency – TiO, NbO2, NbO. The aluminothermic reduction of gadolinium is thermodynamically possible only at temperatures below 1200°C. The reduction of yttrium through the interaction of Y2O3 with aluminum with the formation of AlY, Al2Y3 AlY2 compounds for the range of 1000–1800°C is thermodynamically impossible. The results of thermodynamic modeling of interactions correlated well with the data of differential thermal and X-ray phase analysis using STA 449 F3 Jupiter (NETZSCH) synchronous thermal analysis and XRD-7000 diffractometer (Shimadzu) with automatic program control, respectively. It was found that the process enters the active phase after the appearance of liquid aluminum and, apparently, is accompanied by exothermic effects with the formation of double and triple intermetallic compounds of aluminum with rare (Nb, Ti) and rare earth (Gd, Y) metals. Transformation of titanium dioxide and niobium pentoxide in the process of transformations is likely carried out through successive and parallel stages of formation of simple and complex oxides with low oxidation states. At the initial stages of the interaction of aluminum with oxides, niobium and titanium aluminides are mainly formed. At subsequent stages, the formation of more complex compounds is observed. At temperatures above 1300°C, ternary intermetallic compounds Al43Nb4Gd6, Ti4Al20Gd and Ti4Al3Y6, Al3Ti, Al0.23Nb0.07Ti0.7 are formed. Gadolinium and yttrium tend to form complex intermetallic compounds in such systems.