Microstructure, mechanical properties and oxidation resistance of Nb silicide based ultrahigh temperature alloys with Hf addition

Abstract

Four Nb silicide based ultrahigh temperature alloys with compositions of Nb–22Ti–16Si–3Cr–3Al–2B–xHf (x=0, 2, 4 and 8) (at%) were prepared by vacuum non-consumable arc melting. The effects of Hf content on the phase selection, microstructure, room-temperature fracture toughness, compressive performance and isothermal oxidation behavior at 1250°C of the alloys have been investigated. The results show that the formation of α(Nb,X)5Si3 is suppressed and instead the formation of γ(Nb,X)5Si3 is promoted by Hf addition. The Hf addition (especially 8at%) improves the room-temperature fracture toughness of the alloy. The hardness of γ(Nb,X)5Si3 increases with its Hf concentration. The compression strength of the alloy is the highest at 4at% Hf, and then obviously reduces at 8at% Hf due to embrittlement of the alloy (especially that of γ(Nb,X)5Si3). The oxidation resistance of the alloy has been firstly ameliorated by the lower Hf addition (2at%) and then degraded by the higher Hf addition (especially 8at%). The details have been discussed.