Thermodynamic description of the systems Co–Nb, Al–Nb and Co–Al–Nb

The Co–Al–Nb ternary system and its two binary subsystems Co–Nb and Al–Nb were thermodynamically modeled by the Calphad method based on own experimental results and critically assessed literature data. A characteristic of the Co–Nb and Co–Al–Nb system is the coexistence of three polytypes of Laves phases: hexagonal C14 and C36 as well as cubic C15. Two-sublattice models were used for the three Laves phases to enable a realistic description of their homogeneity ranges in the Co–Nb and Co–Al–Nb system. A re-assessment was needed for the Al–Nb system to obtain a better description of the homogeneity range of Nb2Al and the liquidus projection of the Co–Al–Nb system. The comparison between calculated and experimental values shows that the optimized set of thermodynamic parameters gives a very good description of most of the experimental data resulting in a complete, consistent thermodynamic description of the three systems Co–Nb, Al–Nb and Co–Al–Nb