THERMODYNAMIC ANALYSIS OF THE FE-N SYSTEM USING THE COMPOUND-ENERGY MODEL WITH PREDICTIONS OF THE VIBRATIONAL ENTROPY

The thermodynamic properties and the phase diagram of the Fe-N system have been analysed using a theoretical method that combines the Compou nd-Energy Model for interstitial phases with a predictive approach to high-temperature entropy of stable and metastable Fe nitrides. The pre dictions are based on the regular behavior of the vibrational entropy and other cohesive properties, which has been established in recent st udies on transition metal compounds. A description of the Gibbs energy of the various phases is reported which accounts well for the observe d thermodynamic behavior, and reduces a long-standing discrepancy betw een calculations and experiments on the epsilon/gamma' + epsilon equil ibrium boundary of the phase diagram. The present study generates enth alpy of formation values for various nitrides of Fe, which are used in a new test of the predictions of Miedema's formula for metastable com pounds. The trends in the present and Miedema's values agree very well , but significant quantitative discrepancies are found.