Critical Assessment and Thermodynamic Modeling of the Ti-N System

Abstract

A comprehensive critical assessment has been made of the experimental constitution data of the Ti-N system. The result differs from previous evaluations of the solid state equilibria and is supported also by recent data. Based on the selected original experimental phase diagram and thermodynamic data from the literature, a set of thermodynamic functions for the Ti-N system has been chosen and the parameters were optimized by the least squares method. A stepwise optimization procedure is useful in this system and described in some detail. Four different analytical descriptions were used to model the four different types of stable phases in the Ti-N system: gas, liquid, solid solution phases αTi, βTi, and δ-TiN 1 -x, and stoichiometric compounds: e-Ti 2 N, η-Ti 3 N2, and ζ-Ti 4 N 3 . Most of the experimental information is in accordance with the modeling, especially the invariant equilibria. The identification of reliable experimental data is demonstrated to be a point of distinction in this system. No derived values like the tabulated Gibbs energy of formation were used in optimization. The phenomenon of congruent vaporization or sublimation is found to be restricted to the liquid and 6 phases. A phase diagram including also this information on the gas phase equilibria is presented and used to explain the discrepancies arising in experimental data due to the composition shift of samples during evaporation.