The effects of composition, growth rate and third element addition on the control of lamellar boundary of directionally solidified (DS) TiAl alloys were systematically investigated. From the DTA analysis on the binary TiAl alloys and the EDS analysis on the quenched dendrite tips, phase diagram near the stoichiometric composition could be established. It was found that β and α phase were formed as a primary phase from the dendritic morphologies of Ti-44Al and Ti-52Al alloys (at.%), respectively, during directional solidification at the growth rate of 180 mm/hr. From the above results, the composition in which β phase was fully transformed could be determined. When a directional solidification of Ti-44Al, Ti-46Al, Ti-48Al alloys (at.%) was conducted with changing growth rates, the lamellar boundary orientation tended to align to the growth direction with the slow growth rate and the low Al contents. Some β stabilizing elements such as Mo, Cr and Nb were added in order to increase β phase field in the binary system. In the case of Ti-46Al-2Mo, Ti-46Al-5Cr and Ti-46Al-2.5Nb DS ingots (at.%) grown at the rate of 45 mm/hr, it was found that the lamellar orientation was aligned nearly 0˚ to 45˚ to the growth direction. Therefore, it can be concluded that there is great possibility to control the lamellar orientation by adding some β stabilizing elements such as Mo, Cr and Nb. As growth rate increased, average lamellar spacings decreased. In case of adding third elements such as Mo, Cr and Nb, average lamellar spacings drastically decreased in low growth rate. Since β stabilizer such as Mo, Cr and Nb is effective to control lamellar boundary orientation as well as reduce the lamellar spacing, it can be considered that β stabilizer plays an important role in mechanical properties of TiAl alloy consisted of fully lamllar grains.
