Experimental investigation and thermodynamic description of the Co–Nb–Ti system

Abstract

The solidification microstructures of as-cast alloys and the equilibria microstructure of annealed alloys at 1273 K in the Co–Nb–Ti system were determined by scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS) and X-ray diffraction (XRD), and the liquidus surface projection and partial isothermal section at 1273 K were constructed. Nine primary solidification regions, bcc(Nb,Ti), fcc(Co), Co3Ti, CoTi, CoTi2, λ1, λ2, λ3, and μ and five ternary invariant reactions, liq. → λ1 + λ2 + μ, liq. → CoTi + λ2 + μ, liq. → bcc(Nb,Ti) + CoTi + μ, liq. + fcc(Co) + λ3 → Co3Ti, and liq. + CoTi → bcc(Nb,Ti) + CoTi2 were experimentally determined or deduced in the liquidus surface projection. Three three-phase regions, CoTi + λ2 + μ, bcc(Nb,Ti) + CoTi + μ, and bcc(Nb,Ti) + CoTi + CoTi2 and six two-phase regions, Co3Ti + λ3, λ2 + μ, CoTi + λ2, CoTi + μ, bcc(Nb,Ti) + μ, and bcc(Nb,Ti) + CoTi at 1273 K were experimentally determined. The continuous compound λ2 with a C15 structure from the Co–Nb side to Co–Ti side was formed at 1273 K. The solubilities of Ti in μ and Nb in CoTi were determined to be ~11.1 at% and ~18.4 at% at 1273 K, respectively. And no new phases were found. According to the available experimental data from the present work and literatures, the Co–Nb–Ti system was optimized by CALPHAD (CALculatiuon of PHAse Diagram) method. Four solution phases (liquid, bcc, fcc, and hcp) were described using the substitutional solution model. A two-sublattice model (Co,Nb,Ti)2(Co,Nb,Ti)1 was used to described three compounds λ1, λ2, and λ3 according to their crystal structures and homogeneity ranges. Co7Nb2 and CoTi2 were treated as Co7(Nb,Ti)2 and Co(Nb,Ti)2 by a two-sublattice model, respectively. μ was treated as (Co,Nb,Ti)1(Nb,Ti)4(Co,Nb,Ti)2(Co,Nb,Ti)6 by a four-sublattice model. Co3Ti with L12 structure and CoTi with B2 structure were treated as the ordered phases of fcc and bcc solutions using the thermodynamic models (Co,Nb,Ti)0.75(Co,Nb,Ti)0.25Va1 and (Co,Nb,Ti,Va)0.5(Co,Nb,Ti,Va)0.5Va3, respectively. A set of self-consistent thermodynamic parameters of the Co–Nb–Ti system was obtained.