PREDICTIVE APPROACH TO THERMODYNAMIC PROPERTIES OF CO NITRIDES AND PHASE-STABILITY IN THE CO - N SYSTEM

Thermodynamic properties of the Co-N system are poorly known from experiments, and there is a need of information on the stability of its various phases and the phase diagram. This kind of information has been obtained by us. Our approach combines the methods of thermodynamic modelling developed in the so-called CALPHAD i.e. CALculation of PHase Diagrams) work with predictions of unknown thermodynamic quantities. The predictions of entropy and enthalpy of formation for nitrides are based on regularities in bonding properties and vibrational entropy of 3d-transition metal compounds. The excess Gibbs energy parameters for the cubic and hexagonal nitride phase are determined by interpolation, relying on trends in the data for 3d-transition metal-nitrogen systems. The properties of the liquid phase are evaluated from experiments and tested in calculations of N-solubility in the ternary systems Co-Cr-N, Co-Fe-N and Co-Ni-N. The Gibbs energy functions obtained by us are used as input information in the Hillert-Staffansson "Two Sublattice" phenomenological model to calculate the Co-N phase diagram. Our results are compared with information on the Fe-N and Ni-N systems, and with the predictions of Miedema's formula.