Effect of Cr content on microstructure and oxidation resistance of Nb-Ti-Si-Cr based ultrahigh-temperature alloy

Ultrahigh-temperature alloys with the composition of Nb-22Ti-14Si-xCr-3Al-5Hf-1.5B-0.06Y (x=0, 10, 14, 20, at%) were prepared by vacuum non-consumable arc-melting. The effect of Cr content on the phase constituents, microstructure and high temperature oxidation resistance of the alloys was investigated. The results show that microstructure of the alloy without Cr is composed of Nbss dendrites and Nbss/(Nb, X)₅Si₃ eutectic colonies. The primary phase is (Nb, X)₅Si₃ in the three alloys with different amount of Cr, while Nbss/Cr₂Nb eutectic colonies are also observed. With increaing Cr content, the amount of primary phase (Nb, X)₅Si₃ and Nbss/Cr₂Nb eutectic colonies increase, while the amount of Nbss/(Nb, X)₅Si₃ eutectic colonies decrease. As the Cr content increasing to 20 at%, the Nbss/(Nb, X)₅Si₃ eutectic colonies disappear. The results of oxidation test at 1250°C show that the amount of Ti_0.4Cr_0.3Nb_0.3O₂ and (Ti, Cr)NbO₄ increase in the oxide scale with increase of Cr content in the alloys, while the contents of Nb₂O₅ and Ti₂Nb₁₀O₂₉ in the oxide scale decrease. The high temperature oxidation resistance is improved due to the improvement of the binding ability of oxide scale with matrix.