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℃). 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 phase is expected to occur from the shift of the α+γ/γ phase boundary.
TiAl 합금의 내산화성 및 고온강도 개선에 매우 효과적인 것으로 알려진 Nb을 첨가하고, 열적안정성 개선에 효과적인 Si 및 C을 첨가하여 seed 합금으로 사용하기에 충분한 열적안정성과 우수한 기계적성질을 동시에 지니는 합금을 설계하고자 하였다. 이를 위하여 optical floating zone furnace(FZ로)을 이용한 부분용해를 수행함으로써 고온까지 열적안정성을 지니는 합금조성을 조사하였다. 또한 선택된 조성을 seed 합금으로 이용하여 동일조성합금의 층상경계방향을 제어하였으며, 인장시험 및 압축시험을 수행함으로써 일방향응고재의 기계적성질을 평가하였다.
