Effects of Mo and Zr composite additions on the microstructure, mechanical properties and oxidation resistance of multi-elemental Nb-Si based ultrahigh temperature alloys

The Nb-Si based alloys adding Mo, Zr and Mo-Zr respectively were prepared by vacuum non-consumable arc melting. The alloys’ microstructure and comprehensive performances including microhardness, room temperature fracture toughness as well as compressive strength and oxidation resistance at 1250 °C were evaluated systematically. The results show that adding Zr in multi-elemental Nb-Si based alloys changes the microstructure from eutectic to hypereutectic, while further adding Mo in Zr-containing alloy decreases the content of primary silicide blocks. Both the single and composite additions of Mo and Zr in the alloys suppress the formation of α(Nb,X)₅Si₃ whilst promote the formation of γ(Nb,X)₅Si₃. Both the dissolved Mo (mainly in Nbss) and Zr (primary in γ(Nb,X)₅Si₃) have the effect of solid solution strengthening and improve the microhardness of phases. The room temperature fracture toughness of the alloys is ameliorated by Zr addition, while is reduced by further Mo addition. Alloying with Mo or Zr alone can enhance, and composite alloying with Mo and Zr continues to increase the compressive strength at 1250 °C of the alloys. The alloy with composite additions of Mo and Zr shows the best oxidation resistance at 1250 °C due to the formation of a denser and well adhesive inner layer of scale and the improvement of the oxidation resistance of silicides under the synergistic effects of Mo and Zr.