The intermetallic compound TiAl is being considered for high temperature structural materials, particularly the two-phase lamellar alloys consisting of TiAl (γ) and small volume fraction of $Ti_3Al(\alpa_2)$. The development of these materials has reached the point where turbocharger rotors of TiAl alloys have started to be used in commercial cars. If the mechanical properties, particularly the combination of room temperature ductility and creep resistance, are further improved, TiAl-base alloys would be suitable for the use in a variety of structural applications.
In recent, the studies of PST (polysynthetically twinned) TiAl alloys have shown that the good combination of room temperature ductility and yield stress can be achieved when the lamellar orientation is aligned parallel to the tensile direction. Seeding technique is one of the directional solidification methods to control the lamellar orientation parallel to the growth direction. However, the lamellar structure of seed alloy should be stable without recrystallization up to its melting temperature.
Therefore, there have been enormous studies to improve the thermal stability of lamellar microstructure. One of the important factors in the thermal lamellar stability is alloy composition. The different lamellar stability is due to the change of phase diagram according to the variation of compositions. And kinetics such as reaction rate according to heating rate is also important factor of the thermal stability of the lamellar microstructure. In this study, the thermal stability of lamellar TiAl alloys in several systems was investigated. The effects of separate and complex additions such as C, Mo and Si on the thermal stability were investigated through partial melting and heat-treatment. The change of phase diagram (for example, shift of α / α + γ / γ boundaries) by the third element addition was examined to discuss the lamellar stability. Finally, thermal stability of lamellar microstructure with respect to heating rate was also investigated. Moreover, to investigate the possibility of practical application of directionally solidified TiAl alloys, rolling was carried out. As a result, the thermal stability of Ti-46Al-1.5Mo-1.0Si and Ti-46Al-1.5Mo-0.2C alloys was very stable upon heating and cooling. The phase diagram was shifted to Al-rich side by the addition of Mo, and shifted to Ti-rich side by the addition of Si. In the case of very high heating rate, recrystallization occurred at low temperature and short holding time. In the thermally-stable compositions (Ti-46Al-1.5Mo-1.0Si and Ti-46Al-1.5Mo-0.2C), during annealing after rolling process, recrystallization was retarded. As a conclusion, Ti-46Al-1.5Mo-0.2C turned out to be the best alloy for applying practical use among the above compositions.
