Effect of Rare-Earth Elements (Gd, Y) on the High-Temperature Oxidation of Al–Ti–(Nb,Ta) Alloys

The effect of gadolinium and yttrium on the high-temperature oxidation of the Al–Ti–(Nb,Ta) systems produced by aluminothermic reduction of Ti, Nb, Ta, Gd, and Y from their oxides is studied. The results of thermodynamic simulation (TDS) performed with the HSC 6.1 software are shown to agree with the experimental data obtained during oxidation of samples in air at 800°C for 100 h. The TDS results show that niobium and aluminum oxides form mainly in niobium alloys and in alloys with rare-earth metal (REM) additions. In tantalum alloys and in alloys with REM additions, titanium and tantalum oxides form; according to literature data, they substantially increase the corrosion resistance as compared to that of the alloys with niobium and aluminum oxides. TDS predicts the formation of Ta₂O₅; however, only unstable Ta₂O₃ and Ta_0.15O_0.85 phases are found experimentally; they favor a decrease in the corrosion resistance and are likely to be incompletely oxidized and, subsequently, to transform into a stable form. Gadolinium added to the base Nb-containing Al–Ti alloy weakly affects oxidation as compared to that of the base alloy. Yttrium additions to the above system decrease the formation of titanium oxide protective film and lead to an increase in the alloy oxidation. Gadolinium additions to the tantalum-containing Al–Ti base alloy lead to the formation of the nonstoichiometric Ti_4.5O₅ oxide, which favors oxidation of such alloys due to the formation of stacking faults. Yttrium additions cause the formation of Ti₂O₃; at the higher temperatures, it oxidizes to TiO₂ and, in perspective, increases the oxidation stability of the alloys. The results obtained highlight the importance of structural peculiarities of oxide phases in optimizing the properties of the alloys, which is relevant to the development of materials characterized by high oxidation resistance.