Alloying effects on the microstructure and properties of Nb–Si based ultrahigh temperature alloys

•B or Cr promotes hypereutectic formation and Cr exerts no influence on silicides.•The combined additions of Hf and B has a strong effect of promoting γ(Nb,X)5Si3.•Cr is detrimental to fracture toughness but beneficial to macrohardness.•Macrohardness obviously decreases for Hf-containing alloys after heat-treatment.•In the presence of B and/or Cr, Hf addition degrades the oxidation resistance.

Eight multi-component Nb–Si based ultrahigh temperature alloys were prepared by vacuum non-consumable arc melting. The effects of Hf, B and Cr alloying on the phase selection, phase stability, both non-equilibrium and equilibrium microstructure, room-temperature fracture toughness, hardness and oxidation resistance at 1250 °C of the alloys have been investigated and estimated systematically. The results show that the addition of B or Cr promotes the formation of hypereutectic structures. The alloying with both Hf and B suppresses the formation of β(Nb,X)5Si3 and promotes the formation of α(Nb,X)5Si3 and γ(Nb,X)5Si3, while the alloying with Cr has no effect on the crystal structures of 5-3 silicides. The room-temperature fracture toughness of the alloys is always degraded by the addition of Cr but almost not influenced by the combined additions of Hf and B. The hardness of 5-3 silicides exhibits a tendency of γ > α > β. The macrohardness of the alloys increases with Cr addition, and it obviously reduces in the presence of Hf after 1450 °C/50 h heat-treatment. The best oxidation-resistant performance has been obtained for the alloy with both B and Cr additions. However, in the presence of B and/or Cr, the oxidation resistance of the alloys has been degraded by further addition of Hf.