COST 507 - Repositório Aberto da Universidade do Porto

and Seifert et al.[96Sei], which will also be considered in this paper. Some
modifications will be recommended.
The consequences for the other systems due to the recommended modifications of the
Ti-N and Ti-C systems will be examined, in particular Ti-C-N, Ni-Ti-C, Al-Ti-C, Al-
Ti-N and Fe-Ti-C-N. The various descriptions of the Fe-Ti system will also be
examined.
2. TiN and the binary Ti-N system
All the assessments of the Ti-N system, [96aJon], [96Zen] and [90Oht], begin with an
assessment of the stoichiometric TiN and therefore we will first consider C P and H-
H 2 98, which is closely related to C P .
2.1 TiN
JANAF[85JAN] gives an excellent description of C P data up to about 1200 K but then
their curve starts to bend upwards in a way not indicated by data. Ohtani et al. [90Oht]
used a simple power series G m - HSER = a + bT + cTlnT + dT 2 , representing
H - HSER rather well between 298 and 1000 K but not at higher temperatures.
Jonsson [96aJon] added more terms, G m - H SER = a + bT + cTlnT + dT 2 + eT''+ fT 3 ,
and obtained an excellent fit for all data above 200 K, except for a small deviation for
the highest experimental point (1750 K). The melting point of TiN is about 3600 K
and it is unfortunate that there is no experimental information above 1750 K. The
description given by Jonsson [96aJon] yields only a modest increase of C P above
1750 K, which may be preferable to the rapid increase according to JANAF [85JAN],
see Fig.l . Zeng et al. [96Zen] criticised Jonsson's [96aJon] assessment because it
cannot be used below 200 K. They claimed that this problem disappears if the T 3
term is omitted. Instead, they introduced a T +3 term but did not describe how they
evaluated its value. Anyway, the value they gave is rather small and even at 3600 K it
yields a contribution to H by only about - 1 kJ/mol. It does not seem justified to
introduce a new term for such a small effect in a region far above the experimental
range. On the other hand, the T" 3 term used by Jonsson [96aJon] is important inside
the experimental range. By omitting that term but accepting his value of the other
coefficients, Zeng et al. [96Zen] got a serious disagreement at low temperatures
which starts already at about 500 K. Evidently, they failed to realize that Jonsson
[96aJon] used the T" 3 term to get agreement with data all the way down to 200 K.
That term would not be needed if one was satisfied to describe data down to 298 K
but then one would have to reevaluate the other coefficients. Fig.l shows a
comparison between the three different assessments for the C P curve. It seems that
one should use Jonsson's [96aJon] expression.
S 2 98 and C P are thus established and the stability (i.e. G m ) of TiN will then be fixed by
choosing a value for AfH. It was measured by Humphrey [51 Hum] and G m would
thus be fixed. In addition it was measured by Morozova et al. [66Mor] who studied
not only TiN but also TiNi. x . Their values extrapolate well to Humphrey's [51 Hum]
values but could also support a value 10 kJ/mol of TiN higher or lower. Humphrey's
- 174 -