Effects of Si and C additions on the thermal stability of directionally solidified TiAl–Nb alloys

The thermal stability of the lamellar microstructure in TiAl–Nb alloys containing Si and C has been investigated by partial melting experiments. The proper compositions, where the lamellar structure is stable enough to be used as a seed material are found for both TiAl–Nb–Si and TiAl–Nb–Si–C alloy systems. The lamellar microstructure of the Ti–44.5Al–3Nb–0.6Si–0.2C and Ti–45Al–2Nb–0.6Si–0.2C (at.%) alloys is indeed successfully aligned parallel to the growth direction by directional solidification (DS) with the seed material from the Ti–44.5Al–3Nb–0.6Si–0.2C alloy. The DS ingots of these alloys exhibit a good combination of room-temperature ductility (8.5%) and high-temperature yield strength (700 MPa at 800 °C). The proper composition range, where the lamellar structure is thermally stable enough to be used as the seed material for the TiAl–Nb–Si alloy is narrower than the corresponding range for previously investigated TiAl–Mo–Si and TiAl–Si alloys. The composition limits for such a region with the lamellar stability are discussed in terms of the critical volume fraction of the α (α2) phase for the Al-rich side limit and the α-peritectic composition for the Al-lean side limit. It is concluded that Nb is not an effective element to improve the lamellar stability because upon alloying with Nb, no significant change in the volume fraction of the α phase is expected to occur from the shift of the α+γ/γ phase boundary.