COST 507 - Repositório Aberto da Universidade do Porto

A comprehensive critical assessment has been made of the experimental constitution
data of the Ti­N system. The result differs from previous evaluation of the solid state
equilibria [93Jon] and is supported also by recent experimental data. Based on the
selected original experimental phase diagram and thermodynamic data from the
literature, a set of thermodynamic functions for the Ti­N 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 Ti­N system: gas, liquid, solid solution phases aTi, ßTi, and 5­TiN.. x , and
stoichiometric compounds: ε-Τί 2 Ν, η-Τϊ 3 Ν2, and ζ-Τί 4 Ν 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 δ 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. Details are given in a publication [96Zenl] (see
appendix I).
2.2 V-N system [97Dul]
A consistent thermodynamic data set for the V-N system is obtained by a computeraided
least squares method applied to all of the experimental phase diagram and
thermodynamic data available from the literature. The sublattice model V.(N,Va) a is
used to model the phases: fee (a=l), bec (a=3), and hep (a=0.5). The liquid phase is
described by the Redlich-Kister formula, and the gas phase is treated as an ideal gas.
Special attention is paid to the modeling of the fee phase with its exceptional bulk of
experimental data. This phase is first analyzed by an ideal solution, then by a regular,
and finally by a subregular interaction in the nitrogen sublattice. The other solution
phases are analyzed with similar modeling procedures. This step-by-step analysis
procedure permits insight into reliable estimations for the parameters at each of the
higher level models. Comparisons between the calculated and measured phase
diagram and thermodynamic quantities show that most of the experimental
information is satisfactorily accounted for by the thermodynamic calculation.
Inconsistent experimental information is identified and ruled out. The
thermodynamic properties of the fee and hep phases in the V-N system are compared
with those in the Cr-N and Ti-N systems and related to Neumann-Kopp's rule.
Details are given in a publication [97Dul] (see appendix II).
2.3 Ti-Al-N system [97Zenl, 96Zen2]
The Ti-Al-N phase diagram has been assessed and a consistent set of thermodynamic
functions has been developed. Three ternary line compounds, Ti-Ti 3 AlN 0 .56» χ ι-
Ti 2 AlN 0 .82> x 3 -Ti 3 Al 2 N 2 , and the ternary solubilities of aTi and ßTi have been
modeled. An estimated model for the metastable mutual solubilities of the nitrides
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