Composite alloying effects of V and Zr on the microstructures and properties of multi-elemental Nb–Si based ultrahigh temperature alloys

Four multi-elemental Nb–Si based ultrahigh temperature alloys with compositions of Nb–22Ti–15Si–5Cr–3Al-2Hf-xV-yZr ((x, y) = (0, 0), (3, 0), (0, 4) and (3, 4)) (at.%) were prepared by vacuum non-consumable arc melting. The effects of single and combined additions of V and Zr on the microstructure, room temperature fracture toughness, microhardness and compressive yield strength and oxidation resistance at 1250 °C of the alloys have been investigated. The results show that the 0V–0Zr alloy is eutectic structure of Nb_SS/(Nb,X)₅Si₃, while the other three alloys are hypereutectic structure, mainly consisting of primary γ(Nb,X)₅Si₃ blocks and Nb_SS/(Nb,X)₅Si₃ eutectic. Alloying with V and Zr can suppress the formation of α(Nb,X)₅Si₃ and promote the formation of γ(Nb,X)₅Si₃. The room temperature fracture toughness of the alloys is improved by single addition of V or Zr, and is markedly improved by the composite additions of V and Zr. The microhardness of γ(Nb,X)₅Si₃ is enhanced by the solid solution strengthening of V and Zr addition, while that of Nb_SS is reduced by the solid solution softening of V addition. The compressive yield strength at 1250 °C of the alloys reduces with V addition while increases with Zr addition, and the alloy with composite additions of V and Zr shows the highest compressive yield strength. The single additions of V or Zr can ameliorate, but the composite additions of V and Zr degrade the oxidation resistant performance at 1250 °C.