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The phase compositions, microstructure and especially phase interfaces in the as-cast and heat-treated Nb–Ti–Si based ultrahigh temperature alloys have been investigated. It is shown that β(Nb,X)5Si3 and γ(Nb,X)5Si3 are the primary phases in the Nb–22Ti–16Si–5Cr–5Al (S1) (at%) and Nb–20Ti–16Si–6Cr–4Al–5Hf–2B–0.06Y (S2) (at%) alloys, respectively. The Nb solid solution (Nbss) is the primary phase in Nb–22Ti–14Si–5Hf–3Al–1.5B–0.06Y (S3) (at%) alloy. An orientation relationship between Nbss and γ(Nb,X)5Si3 was determined to be (11¯0)Nb//(101¯0)γ and [111]Nb//[0001]γ in the as-cast S2 and S3 alloys. Some original β(Nb,X)5Si3 transformed into α(Nb,X)5Si3 because Al and Cr diffused from the β(Nb,X)5Si3 to Nbss during heat treatment at 1500°C for 50h in the S1 alloy. Meanwhile, Ti diffused from Nbss to β(Nb,X)5Si3, which induced αTi to generate near the interface between Nbss and Ti-rich β(Nb,X)5Si3. The orientation relationship between the newly-formed αTi and previous Nbss was (110)Nb//(11¯01¯)αTi and [001]Nb//(123¯1¯)αTi. Among the (Nb,X)5Si3 phases, the contents of Cr and Al in β(Nb,X)5Si3 are nearly the same as those in γ(Nb,X)5Si3 but obviously higher than those in the α(Nb,X)5Si3, whereas the content of Si in α(Nb,X)5Si3 is nearly the same as that in γ(Nb,X)5Si3 but higher than that in the β(Nb,X)5Si3

Effects of alloying and high-temperature heat treatment on the microstructure of Nb–Ti–Si based ultrahigh temperature alloys