High-temperature oxidation behavior of Nb–Si-based alloy with separate vanadium, tantalum, tungsten and zirconium addition

The microstructure and oxidation behavior of directionally solidified (DS) Nb–15Si–24Ti–4Cr–2Al–2Hf alloys with separate vanadium, tantalum, tungsten and zirconium additions were investigated by X-ray diffraction (XRD), electron probe microanalyzer (EPMA) equipped with wave-dispersive spectroscopy (WDS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) equipped with an energy-dispersive spectroscopy (EDS). Results show that the five alloys are all composed of primary (Nb,Ti) solid solution ((Nb,Ti)_SS) phase and eutectic (Nb,Ti)_SS/(Nb,Ti)₅Si₃ structure. After oxidation at 1250 °C for 100 h in air, the surfaces of the five alloys are covered by the oxides of Nb₂O₅, TiNb₂O₇, Ti₂Nb₁₀O₂₉, TiO₂ and amorphous SiO₂. It is found that the alloying elements of V and W are detrimental for oxidation resistance and the addition of Ta has no obvious effect. Zr addition obviously benefits the oxidation resistance at high temperature, by decreasing the weight gain from 242.75 to 184.83 mg·cm⁻². The oxidation mechanism of Nb–Si-based alloys and the effects of different alloying elements on the oxidation resistance of Nb–Si-based alloys were discussed.