Liquid–solid phase equilibria in Nb-rich corner of the Nb–Ti–Si–B system

Nb–Ti–Si based alloys are considered as candidates of next-generation high temperature materials (i.e., working temperature >1200 °C). Boron has a beneficial effect in enhancing the oxidation resistance and reducing the anisotropy ratio ac/aa of the T2 phase in the Nb–Si alloys. The liquid–solid multiphase equilibria in the Nb–Ti–Si alloys with B additions have been investigated via an approach of integrating thermodynamic modeling with designed experiments. The present study suggests that Ti and Si additions increase the stability of Nb3B2, and the primary region of Nb3B2 will appear in the Nb–Ti–Si–B quaternary, while there is no primary solidification region of Nb3B2 in the constitute binaries and ternaries. The proposed liquidus surface of the Nb-rich Nb–Ti–Si–B system is associated with eight primary solidification regions of Nbss, T2, T1, D88, (Nb,Ti)3Si, NbB, TiB and Nb3B2. The direct eutectic solidification occurs between Nbss and T2 in the Nb-rich region in the Nb–Ti–Si–B liquidus projection, which could provide new opportunities for alloy design based on the Nb–Ti–Si–B system.

► The primary region of Nb3B2 exists in the Nb–Ti–Si–B quaternary, while there are no primary regions of Nb3B2 in the constitute binaries and ternaries.
► The proposed liquidus surface of the Nb-rich Nb–Ti–Si–B system is associated with eight primary regions of Nbss, T2, T1, D88, (Nb,Ti)3Si, NbB, TiB and Nb3B2 respectively.
► The knowledge of phase equilibria in the Nb–Ti–Si–B system is essential to understand the microstructure of these Nb–Nb silicide based alloys generated by liquid–solid processes.