COST 507 - Repositório Aberto da Universidade do Porto

An application of the data to the nitridation of Ti­Al alloys is in preparation. A key
point is that the stable and metastable "stability diagrams" can be calculated, using
the nitrogen pressure as a variable [97Sch].
3.2 Ti-V-N system
3.2.1 Solidification of Ti-V-N alloys [97Zen2]
Solidification of the Ti­V­N alloys have been simulated under the equilibrium and
Scheil conditions, respectively (Fig. 11 and 12 in Appendix IV). It has been shown
that in the high temperature range, say above 2200°C, the solidification process of
the alloys containing 5 at.% V is close to the equilibrium state, but it moved towards
the Scheil mode in the temperature range below 2200°C. The calculated composition
range of vanadium in the secondary α of the alloy 70TÌ­5V­25N (at.%) by the Scheil
model is very close to the experimental one. But that of the β phase by Scheil model
is larger and higher than the measured one, which indicates that the diffusion of
vanadium in β can still proceed down to at least 1600°C.
3.2.2 Nitridation of Ti-V alloys [97Zen2]
When a Ti­V alloy with higher than 50 at.% Ti is exposed to nitrogen at 1400°C, the
nitrogen atoms are first dissolved in the β phase. After β is saturated with nitrogen,
the α phase starts to form in the surface region. With the nitriding proceeding, the δ
phase appears. This nitriding process is presented in Fig. 13 in Appendix IV as
dashed line for the alloy of 80TÌ­20V (at.%). Therefore, the outer layer on the
samples nitrided for a long enough time is the Ti­rich fee phase (Ti,V)N|. x with a
few percent of vanadium. The morphology of the reaction zone depends on the
vanadium content of the alloy. The α layer can be avoided by increasing the
vanadium content or by increasing the nitriding temperature.
3.3 Ti-Ni-N system
The calculated phase diagrams are being used to analyze the denitrification and
phase formation during the sintering process of TiN­Ni mixed powder compacts. It is
found that the temperature at which the liquid phase appears strongly depends on the
pressure. The thermodynamic dataset can be applied to design the sintering
techniques for the TiN­Ni powder mixture. This work is in progress.
References
79Fuk M. Fukuhara and H. Mitani: Nippon Kinzoku Gakkaishi 43 (1979) 169­174.
82Wit M. Wittmer: J. Appi. Phys. 53 (1982) 1007­1012.
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