Hf含量对Nb-Si基超高温合金组织的影响

Effects of Hf Addition on the Microstructure of Nb-Si Based Ultrahigh Temperature Alloys

采用真空非自耗电弧熔炼的方法制备了4种名义成分为Nb-22Ti-16Si-3Cr-3Al-x Hf(x=0,2,4,8,at%)的合金,并于1450℃保温50 h进行了均匀化处理,研究Hf含量对Nb-Si基超高温合金电弧熔炼态及热处理后组织的影响。结果表明:不含Hf的合金的组织主要由初生α(Nb,X)5Si3,Nbss枝晶以及2种共晶(即Nbss/α(Nb,X)5Si3和Nbss/γ(Nb,X)5Si3)构成;加入2 at%Hf,促进了初生γ(Nb,X)5Si3的形成并增加了Nbss/γ(Nb,X)5Si3共晶的含量;当Hf含量增加至4 at%和8 at%时,α(Nb,X)5Si3的形成受到完全抑制,合金中的硅化物仅以γ(Nb,X)5Si3的形式存在。经1450℃/50 h热处理后,原电弧熔炼态的Nbss枝晶与典型的共晶组织形貌基本消失,合金的组织均匀分布。对Hf含量为0 at%和2 at%的合金来说,热处理后的相组成没有发生改变,但α(Nb,X)5Si3的含量减少;而对Hf含量为4 at%和8 at%的合金来说,热处理后Cr2Nb Laves相消失,且在后一种合金中还观察到少量棒状的(Nb,X)3Si相。

Four Nb-Si based ultrahigh temperature alloys with compositions of Nb-22Ti-16Si-3Cr-3Al-x Hf（x = 0, 2, 4 and 8, at%） were prepared by vacuum non-consumable arc melting and then they were heat-treated at 1450 °C 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 °C/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）5Si3in the two heat-treated alloys decrease. In addition, the Cr2 Nb 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.