Effect of alloying elements on oxidation behavior of Nb-Ti-Al-C alloys at high temperature

Nb-25Ti-8C alloys with different Al contents were fabricated by a vacuum non-consumable arc-melting method. Microstructure and oxidation behavior of the Nb-25Ti-8C-(0, 5, 10, 15)Al alloys were investigated. The results show that the Nb-25Ti-8C-(0, 5, 10)Al alloys consist of niobium solid solution (Nbss) and carbide of (Nb, Ti)C, and the Nb-25Ti-8C-15Al alloy consists of Nbss, (Nb, Ti)C, and Nb₃Al phases. In the range of 800 and 1000℃, the oxide layers of the alloys consist of Nb₂O₅, TiO₂, Al₂O₃, NbO₂ and TiNb₂O₇ phases. The oxidation resistance of the alloys improves with doping of Ti and Al due to the elements reaction with oxygen primarily, this causes the decrease of solubility and diffusion rate of oxygen into the Nbss matrix. Furthermore, the oxidation reaction of Al and oxygen becomes more obvious with increasing oxidation temperature. Nb-25Ti-8C alloy exhibits excellent oxidation resistance when oxidized at 800℃. In addition, the oxidation rate of Nb-25Ti-8C-xAl alloy is less than that of industrial niobium alloy C-103 at 1000℃. The fraction of Nb₂O₅ in the oxide scale increases with increasing oxidation temperature, which leads to the enhancement of the internal stress between the oxide film and alloy matrix, and the exfoliation of the outer oxide film. C element in the carbides volatilizes as CO₂ lead to formation of oxidation of empty on the surface of oxide film.