Experimental investigation, thermodynamic modeling and solidified microstructure of the Cu-Ti-Nb ternary system

The phase equilibria of the Cu-Ti-Nb ternary system were investigated via thermodynamic modeling coupled with key experiments. Fourteen equilibrated alloys were prepared to determine the isothermal sections of the Cu-Ti-Nb system at 600 and 700 degrees C combining X-ray diffraction (XRD) and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDS), and four as-cast alloys were prepared to study the alloys solidification behavior. The solubilities of Nb in binary compounds of the Cu-Ti system and Cu in the (beta Ti, Nb) phase were measured. No ternary compound was found. The thermodynamic assessment of the Cu-Ti-Nb system was carried out by the CALPHAD (CALculation of PHAse Diagrams) approach based on the thermodynamic descriptions of three constitutive binary systems as well as the experimental phase equilibria data obtained from the present work and literature. The substitutional model and sublattice model were adopted to describe the solution phases and intermediate phases, respectively. A set of self-consistent thermodynamic parameters of the Cu-Ti-Nb system was obtained. The calculated isothermal sections are in good agreement with most of the reliable experimental data. The liquidus projection and reaction scheme of the Cu-Ti-Nb system over the whole composition were also presented. The solidification behaviors of as-cast alloys were described under GulliverScheil non-equilibrium condition. The simulated and experimental results are consistent with each other.