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.