COST 507 - Repositório Aberto da Universidade do Porto

Ti 2 (Va,N)! which yields Ti^ as a realistic end-member. This was accepted by Zeng
et al. The β phase was modelled as Ti.(Va,N) 3 in all assessments and that is the usual
choice for a bcc phase. The experimental information on α+β close to the transition
point for pure Ti seems to be well established by Palty [54Pal] but the peritectic
equilibria for L+δ—»a and L+a—»β are more uncertain.
The main difficulty in the high Ti region seems to be the Οί/δ equilibrium where
Jonsson chose to rely on data from Wolff et al. [85Wol], Etchessahar et al. [84Etc]
and Lengauer et al. [88Len], whereas Zeng et al. trusted Palty [54Pal] and two tielines
from a more recent unpublished work by Lengauer [91 Len]. They also claim to
have used the information from another study by Lengauer et al. but that is not
indicated by the result of their assessment. Both assessments yield fairly straight and
parallel phase boundaries. Jonsson's two-phase field is narrow and falls inside the
wider two-phase field by Zeng et al. In their discussion they seem to agree with
Jonsson on what the maximum Ν content of α should be at temperatures around 1300
K, where intermediary phases appear, but they would need a temperature dependent
interaction parameter in the α phase in order to reconcile such high Ν contents with
the lower Ν contents according to the two tie-lines by Lengauer [91 Len] which fall at
about 1600 and 1900 K. However, they did not try to do this. Instead, they claim that
their description is supported by information on two intermediary phases, ζ and η, and
unpublished information on the Ti-Ni-N system , Friec et al. [95Fri].
2.5 Intermediary Ti-N phases
An ε phase of an approximate composition Ti 2 N is stable below about 1338 K. It is
usually proposed to form by a peritectoid reaction but Wriedt et al. [87Wri]
emphasised that the probable position of the (α+δ)/δ phase boundary would result in a
congruent transition ε—>δ if the composition of ε is Ti 2 N. Evidently, the phase
diagram could be modified in three ways to yield a peritectoid reaction, (1) by
assuming that ε at its highest temperature has less Ν than according to TiN 2 , (2) by
moving the (α+δ)/δ phase boundary to higher Ν contents, (3) by introducing another
intermediary phase above the ε phase. Zeng et al. considered it a very important
conclusion that ε does not form congruently from δ and they wanted the phase
diagram to show this. Indeed, they were able to predict a peritectoid transformation by
accepting the higher Ν contents of the (α+δ)/δ boundary, indicated by Lengauer's
[91 Len] two tie-lines, and the existence of a new intermediary phase, η. On the other
hand, from a practical point of view, it does not seem very important to reproduce the
correct reaction for ε. It may be more important to establish the composition of ε by
new experiments. In particular, it does not seem justified to accept the higher Ν
content of the (α+δ)/δ phase boundary from Lengauer's [91 Len] two tie-lines just
because it contributes to the suppression of the congruent transformation point of ε
under the assumption that ε is Ti 2 N.
Lengauer and Ettmayer [88Len] first found the two new intermediary phases,
ζ-Τί 4 Ν 3 . χ and η-Τϊ 3 Ν 2 . χ , existing close to the maximum temperature of ε and reacting
with ε in two ways each. Zeng et al. reproduced the reported reaction temperatures
- 176 -