COST 507 - Repositório Aberto da Universidade do Porto

approximate agreement with de Boer, would be needed in order to describe the
L+Al 3 Ti+Al 3 C 3 +TiC equilibrium in the Al-Ti-C system [96cJon]. To get a heat of
formation of TiC they considered reasonable they found it necessary to use five
independent parameters for the description of the liquid phase. Jonsson and
Albertsen et al. used only two. In a recent assessment Dumitrescu accepted Jonsson
description of C P and S 298 but for A f °ίί she chose a value as high as possible (i.e.
less negative) with respect to measurements, having the same arguments as Seifert.
4.2 Binary system Ti-C
So far we have only considered stoichiometric TiC. We may also like to present the
whole of the binary Ti-C system. Fig.8 shows a comparison between three of the
existing descriptions of the Ti-C system [96bJon], [97Dum] and [96Sei].
The solution properties of δ-TiC..,. according to Jonsson, Dumitrescu and Seifert et
al. are fairly similar but Albertsen et al. have a different description because they
wanted to fit their own data in preference to several other studies. For the liquid phase
Seifert et al. used five parameters in order to fit information primarily concerning
other phases. That may be a dangerous strategy. For example, their liquidus line
shows two inflexion points close to the Ti side. Jonsson and Dumitrescu used only
two parameters for the liquid. Albertsen et al. and Seifert et al. also differ from
Jonsson and Dumitrescu on the solubility of C in the β phase. Admittedly, the
information on the solubility of C in α and β may not be very well established
experimentally but it does not seem as a natural choice to trust the information from
Cadoff et al.[53Cad] on the α phase, Fig.9, but not on the β phase, Fg.10.
All the assessments show a very strong temperature dependence of the solubility of C
in the α phase, in agreement with information from Cadoff et al. By using only one
adjustable parameter Jonsson and Dumitrescu reproduced a similar temperature
dependence for the β phase, in agreement with information from Cadoff et al.
Albertsen et al. used two independent parameters and calculated a solubility almost
independent of temperature. Seifert et al. did the same but then they had to use a very
large third Redlich-Kister parameter in the liquid in order to adjust the eutectic
composition. Dumitrescu has now reassessed the Ti-C system using her higher value
of A f "H m (29S) for TiC and got a reasonable result showing a variation of the
solubility in α with temperature.
It is recommended that one should use the assessment by Dumitrescu, which is very
close to Jonsson's, except for the choice of a different value of A f °// m (298).
5. Application of the descriptions of TiC and Ti-C to higher order
systems
G m of TiC and the description of the Ti-C system should then be tested, as it was
done for TiN, against information on higher-order systems of direct interest.
5.1 Ti-C-N
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