Thermodynamic assessment of the Nb-N system

The phase equilibrium and thermodynamic information of the Nb-N system was reviewed and assessed by using thermodynamic models for the Gibbs energy of individual phases. Although there was a large amount of experimental information of the system, heat capacity data of the Nb2N and NbN were not available either in low or high temperatures. In the present study, low-temperature heat capacity and the S-degrees(298) values were estimated using estimated entropy Debye temperatures. Only the Nb2N (hcp) and NbN (fee) nitrides were considered to be the true binary phases and were included in the present evaluation in addition to the N-2 gas, liquid, and alpha-solid solution (bcc). Three thermodynamic models were used: a two-sublattice model for the solid solution phases, a substitutional model for the liquid phase, and an ideal-gas model for the N-2 gas. The model parameters were evaluated by fitting to the selected data by means of a computer program. A consistent set of parameters was obtained which satisfactorily described most of the experimental and estimated data.