Abstract
The oxidation thermodynamics of Ti-Nb alloys were studied as a step towards improving our general understanding of the oxidation behavior in complex refractory alloys for high temperature applications. Density Functional Theory (DFT) was utilized and the phonon normal mode frequencies were calculated via the harmonic approximation to analyze the stability of several phases including mixed oxides as a function of both temperature and the oxygen chemical potential. Increasing the chemical potential did not favor the formation of mixed oxides but led to Ti2O3 followed by TiO2, and then Nb2O5. The presence of Ti was shown to favor the formation of TiO2 over Nb2O5 in large regions of the stability phase diagram thereby contributing to enhanced oxidation resistance in Nb-containing alloys. Rutile oxides of the form NbxTi1−xO2 had a larger region of stability than NbxTi1−xO. Finally, the oxygen chemical potential was related to temperature and pressure to facilitate direct application of the results to experiment.
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