Effects of Hf addition on the microstructure of Nb-Si based ultrahigh temperature alloys

Four Nb-Si based ultrahigh temperature alloys with compositions of Nb-22Ti-16Si-3Cr-3Al-xHf (x=0, 2, 4 and 8, at%) were prepared by vacuum non-consumable arc melting and then they were heat-treated at 1450℃ for 50 h. The effects of Hf content on the microstructures of the alloys under both as-cast and heat-treated conditions were investigated. The results show that the microstructure of the as-cast alloy without Hf is mainly comprised of primary α(Nb,X)₅Si₃ blocks, Nbss dendrites and two types of eutectic structures, namely Nbss/α(Nb,X)₅Si₃ and Nbss/γ(Nb,X)₅Si₃. The addition of 2 at% Hf causes the formation of primary γ(Nb,X)₅Si₃ and increases the content of Nbss/γ(Nb,X)₅Si₃ eutectic. With the increasing of Hf content up to 4 at% or 8 at%, the formation of α(Nb,X)₅Si₃ is completely suppressed and the silicide is only present in the form of γ(Nb,X)₅Si₃. The Nbss dendrites and typical eutectic structures emerging in the as-cast alloys are almost no longer observable, and the microstructural homogeneity is improved after 1450℃/50 h heat-treatment. The constituent phases of the alloys without Hf or with 2 at% Hf do not change after the heat-treatment. However, the amounts of α(Nb,X)₅Si₃ in the two heat-treated alloys decrease. In addition, the Cr₂Nb Laves phase is not found any more in the heat-treated alloys with 4 at% or 8 at% Hf, while a small amount of (Nb,X)₃Si with a claviform shape is observed in the latter alloy.