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

The phase co mpositions, microstructure and especi ally phase i nterfaces 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 p hase s in the Nb–22Ti–16Si–5Cr–5Al (S1) (at%) and Nb–20Ti–16Si–6C r–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 –0.0 6Y (S3) (at%) alloy . An orientation relationship between Nbss and γ(Nb,X)5Si3 was determine d to be (1-10)Nb//(101-0)γ and 111]Nb//0001]γ in the as-cast S2 and S3 alloys. Some original β(Nb,X)5Si3 transfor med into α(Nb,X)5Si3 because Al and Cr diffused from the β(Nb,X)5Si3 to Nbss during heattreatment at 1500 °C for 50 h in the S1 alloy. Mean while, Ti diffused from Nbss to β(Nb,X)5Si3, which induced a Ti to generate near the interface between Nbss and Ti-rich β(Nb,X)5Si3. The orientation relationship between the newl y-formed a Ti and previous Nbss was (110 )Nb//(1-10-1) αTi and 001]Nb//(12-3-1)αTi. Among the ( Nb,X)5Si3 phases , the contents o f Cr and Al in β(Nb,X)5Si3 are n earl y the same as those in γ(Nb,X)5Si3 but obviously hi gher than those in the α(Nb,X)5Si3, where as the content of Si in α(Nb,X)5Si3 is nearly the same a s 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

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 50 h 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.