A third-generation Calphad description of pure Nb

The development of advanced thermodynamic descriptions for pure elements is essential for accurate modelling of multicomponent systems. The third-generation Calphad descriptions incorporate physical effects such as electronic, vibrational and anharmonic contributions. In this study, we have developed a third-generation Calphad description for pure niobium (Nb). Thermodynamic properties of key phases — bcc, fcc, hcp and liquid — are presented for pure Nb. The vibrational contribution to the heat capacity of the solid phases has been modelled with the Einstein model, and the liquid phase has been modelled with the two-state model. In addition, the modelling of unstable phases has been extensively analysed. The traditional Calphad approach is evaluated and compared with the ab initio approach, which has a stronger theoretical basis. The 0 K energies of the unstable phases, fcc and hcp, have been selected from ab initio calculations using the inflection–detection method. Good agreement has been achieved with the selected experimental and ab initio data.