Intermetallic compound TiAl has been expected to be one of the most promising candidates for high-temperature structural materials to replace conventional titanium alloys and nickel-based superalloys. This is because the TiAl has an attractive properties such as low density, high melting point, high strength-to-weight ratio and excellent creep resistance relative to conventional superalloys and titanium-based alloys. However, the poor ductility at ambient temperature must be overcome for the practical use of this compounds. In recent, therefore, there have been enormous research works to improve room temperature ductility through microalloying and microstructural control. As a result, in the last few years marked improvement in the mechanical properties of two-phase TiAl compounds have been achieved mainly through appropriate alloying and controlling their microstructure by thermomechanical processing. However, such treatments have tendency to reduce the desired mechanical properties such as toughness and creep resistance. Therefore, there has been a great demanding to develop new tow-phase alloys and fabrication processes in order to obtain the balanced mechanical properties.
Nowadays research interest has been focused on the two-phase Ti-rich TiAl compounds with the fully lamellar structure which are beneficial for toughness and high-temperature strength. Directional solidification (DS) processing of two-phase Ti-rich TiAl alloys with fully lamellar structure has great possibilities to avoid inverse relations between tensile properties and fracture/high-temperature capabilities, and to take advantage of the anisotropic nature of the lamellar microstructure. If the lamellar orientation can be aligned parallel to the growth direction, the combination of strength and toughness would be optimized in the resulting microstructure consisting of columnar grains with the lamellar orientation aligned parallel to the growth direction. The main difficulty in the lamellar microstructure control of the two-phase Ti-rich TiAl alloys by DS processing is that the lamellar microstructure is not formed from the liquid but from the solid state.
There are two ways to control the lamellar boundary orientation by the DS process: one is β-solidification technique without seeding procedure and the other is seeding technique. In DS process using a seed crystal, thermal stability of the lamellar microstructure of the seed alloy is very important. If the seed alloy does not have thermal stability until the melting point during heating, recrystallization may occur and the initial lamellar orientation will be changed. Thus, the lamellar orientation of the growing crystal will be different from the aimed A orientation. The important factors that control the thermal stability of lamellar microstructure was reported as phase relationship between β and α phases and α-phase volume fraction change during heating and cooling. To select effective element to increase the thermal stability of lamellar microsturcture, Si and B was added to TiAl-Mo alloys.
Thermal stability of TiAl-1.5Mo-Si and TiAl-1.5Mo-B alloys was investigated by partial melting in floating zone (FZ) furnace. The lamellar stability of TiAl-1.5Mo alloys, which proved to be thermally unstable, was improved dramatically by Si addition. Lamellar stability can be improved by preventing α-phase formation from β-phase due to the effect of shifting phase diagram to the low Al side by Si addition. However, it was found that B addition in TiAl-1.5Mo alloys had no affirmative effect on lamellar stability. Contrary to Si addition, phase diagram cannot be shifted by B addition. Therefore, α-phase formation from β-phase cannot be prevented by B addition. In the result of partial melting, it can be concluded that in order to have enough lamellar stability as a seed alloy, the elements that can shift phase diagram to low Al side must be added to Ti-Al-Mo alloy. In addition, lamellar orientation control was successfully performed by directional solidification using the Ti-46Al-1.5Mo-1Si alloy as a seed alloy.
However, the mechanical properties and oxidation resistance of Ti-46Al-1.5Mo-1Si alloys was very poor because of large amount of Si addition and Mo effect. Therefore, the directionally solidified ingots produced using Ti-46Al-1.5Mo-1Si alloy as a seed material showed poor mechanical properties and oxidation resistance. To increase mechanical properties and oxidation resistance, Mo should exchanged by Nb that is known as effective element to increase oxidation resistance, and the amount of Si addition should be limited under 0.6 at. % which is known as the solution limit of Si by Si and C addition. The thermal stability of the lamellar microstructure in TiAl-Nb alloys that contain Si and C investigated by partial melting in an optical floating zone furnace. As a result of partial melting, the lamellar microstructures of several compositions were stable enough to be used as a seed material for directional solidification. Moreover, the lamellar orientation was successfully controlled by directional solidification using Ti-44.5Al-3Nb-0.6Si-0.2C (at. %) alloy as a seed material. Directionally solidified ingots exhibited a good balance of yield strength and ductility at room temperature and excellent compressive yield stress at high temperature.
Next, Si and C effect to the mechanical property especially fracture toughness was examined. The fracture toughness of directional solidified Ti-(45, 47)Al-3Nb, Ti-(45, 47)Al-3Nb-0.2Si-0.1C, Ti-(45, 47)Al-3Nb-0.3Si-0.2C alloys were investigated. It was found that the lamellar thickness of Si and C doped alloys was fine and uniform, and that of 45Al alloys was finer than that of 47Al alloys. Fracture toughness of type Ⅰ specimens (in which crack propagate against lamellar microstructure) of the alloys decreased slightly with increasing Si and C contents and increased rapidly with decreasing Al content. It was considered that α2-α2 spacing increased with increasing Si and C contents so that the macroscopic delaminations in α2 phases were not easily happened which contribute the high resistance of crack propagation for type Ⅰ specimens. In case of 45Al alloys, α2-α2 spacing decreased by decreasing Al content, therefore, fracture toughness increased rapidly compared to 47Al alloys.
Finally, colony boundary resistance to the crack growth was examined to apply the fracture result of single crystals to polycrystalline crystals. The colony boundaries were reported to offer crack growth resistance for porycrystalline TiAl alloys. The contribution to crack growth resistance of TiAl-Nb alloys was studied using Bi-PST (polysynthetically twinned) crystals produced by directional solidification in FZ (floating zone) furnace. Lamellar orientations in the individual colonies are described using two angles defined with respect to the notch orientation : an in-plane kink angle and a through-thickness twist angle. Therefore, lamellar misorientation across an individual colony boundary is quantified as differences in these angles across the boundary. Crack growth resistance in colony boundary was identified by three-point bend test and crack advance was monitored by interrupted in-situ test. From three-point bend test, it was found that the colony boundary could offer significant resistance to crack growth under large twist angle difference. Moreover, crack growth resistance could increase by Nb addition by minimizing α2-α2 spacing in which delamination-type separation occurred.
By this study, it was found that Si and C additions to TiAl-Nb alloys were very effective to increase the thermal stability of lamellar microstructures. Moreover, TiAl-Nb-Si-C alloys showed excellent mechanical property and fracture toughness. Therfore, it is possible to manufacture ideal ingots that can use as high-temperature structural materials by Si and C addition in TiAl-Nb alloys.
금속간화합물 TiAl 합금은 Ni기 초합금을 대체할 유망한 재료로서 주목받고 있다. 그것은 TiAl합금의 높은 비강도, 융점, 고온강도 및 크리프저항성등의 특성 때문이다. 그러나 상온에서의 연성의 부족이 개선되어야할 문제점으로 대두되고 있다. 최근, 이러한 연성을 개선하기 위하여 일방향응고법이 도입되고 있다. 일방향응고를 통해 층상방위를 성장방향과 평행하게 제어한다면 우수한 강도 및 연성을 동시에 얻을 수 있다. 이러한 일방향응고법 중에는 종자결정을 사용한 방법이 가장 유용한 방법이다.
종자결정을 사용한 일방향응고에서 가장 중요한 요구조건은 종자결정의 열적안정성이다. 본인은 TiAl합금의 열적안정성을 개선하기 위하여 Mo-Si 을 복합첨가하였다. 그 결과, 열적안정성을 저하시키는 고온 베타상의 생성억제 및 알파상 증가를 억제함으로써 열적안정성이 효과적으로 개선되었다. 그러나, Si의 첨가량이 고용한도를 초과함으로써 다량의 석출물이 생성되고, Mo첨가로 인해 다량의 B2상의 생성으로 기계적성질이 저하되는 문제점을 나타내었다.이러한 문제점을 개선하기 위하여 Si및 C을 복합첨가하고 Mo대신 Nb을 첨가함으로써 열적안정성 뿐만 아니라 기계적성질의 개선도 가능한 합금계를 개발하였다. 그 결과 일방향응고시 종자결정으로 사용이 가능한 열적안정성 및 상온 연성이 8% 이상으로 매우 우수한 합금을 개발하였다.
