COST 507 - Repositório Aberto da Universidade do Porto

value has been accepted in all three assessments discussed here.
2.2 Binary system Ti-N
So far we have only considered stoichiometric TiN. We may also like to make
recommendations on the whole of the binary system. Fig.2 shows a comparison
between the three existing descriptions of the Ti-N system [96aJon], [96Zen] and
[90Oht].
2.3 δ-ΤίΝ^ solution
This is an interstitial solution of N in fee Ti. The end-member, TiN, has just been
discussed. The other end-member is identical to pure fee Ti. Today it seems that one
should use the SGTE descriptions for pure elements published by Dinsdale [91 Din].
However, it should be realized that they contain an extrapolation of solid phases
above the melting point which was invented in order to avoid making them stable at
very high temperatures. It usually underestimates C P of overheated solid phases. This
is not a serious problem close to the melting point but Jonsson [96aJon] wanted to
avoid this in the case of Ti-N where the fee phase extends far above the melting
point, about 3600 K compared to 1941 K. Thus, he used the solid Ti descriptions for
1150-1941 K at higher temperatures, as well. As a consequence, he found that the bec
phase is predicted to become stable above 5133 K but that is no practical problem. On
the other hand, if one would insist on using Dinsdale's original description and
combine it with Jonsson's set of parameter values for the Ti-N system, the differences
would be relatively small and only occur in a high temperature region where there in
no reliable experimental information. The congruent melting point of TiNi_ x would
probably be about 20 K lower, which is within the experimental uncertainity.
There is much information on the partial pressures P Ti and P N2 over the δ phase but
the accuracy is probably not very high. Jonsson concluded that it may be "necessary to
reproduce only the general information such as temperature and composition
dependence...". Zeng et al. accepted this view. In spite of this, they made a new
assessment but their results are very similar to Jonsson's.
Eron'yan et al. [76Ero] have published a series of measurements of P N2 at the melting
of δ-TiN..j of different initial compositions which deserves special mention . When
plotting their data they found it necessary to use a line with a sharp break. Ohtani et
al. [90Oht] explained this by calculating a line with a very strong curvature but were
not able to get a very good fit. Another explanation was given by Jonsson [96aJon]
who proposed that the points in the lower temperature region concern the single-phase
δ below the melting range. Those points were found to fall close to a line calculated
for TiN 0 .g 2 . Zeng et al. did not accept this explanation but were not able to fit those
data to the δ/liquid equilibrium better than Ohtani et al. Thus, their assessment did not
differ much from Jonsson's in this respect.
2.4 Terminal Ti-N phases
Pure Ti exists as a(hcp) and ß(bcc). For the α phase Jonsson chose the model
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