Thermal stability of $L1_2$ $(Al,Cr)_3Ti$ alloys was investigated. In order to investigate of thermal stability in $L1_2$-single phase, Al-25Ti-10Cr and Al-29Ti-11Cr alloys were exposed at 1150℃ and 800℃. $L1_2$ phase in Al-29Ti-11Cr was completely decomposed to Al2Ti and TiAlCr, while $L1_2$ phase in Al-25Ti-10Cr. As a result of it, it was known $L1_2$ phase field didn't only shrink, but also moves to Al-corner with decreasing temperature from 1150℃ to 800℃. Thermal stability of $L1_2$ -based two phase alloys was investigated. $L1_2$ phase in Al-30Ti-15Cr which was Ti-rich phase was completely decomposed and mechanical property of the alloy was greatly deteriorated. $L1_2$ phases in Al-21Ti-23Cr and Al-21Ti-15Cr which were Al-rich phase were also decomposed, and mechanical property of the alloys was deteriorated. To improve the thermal stability of Al-21Ti-23Cr, alloying elements were added to the alloy. $L1_2$ phase in Cu, Ag, Fe which substitute for Cr added alloy was decomposed, while $L1_2$ phase in Hf and Y which substitute for Ti added alloy was still stable after 800℃ 10 h exposure. Mechanical property of Hf added alloy was superior to the original alloy. It was suggested that Hf added Al-20Ti-23Cr -1Hf was suitable for oxidation-resistant coating alloys for TiAl in respect to thermal stability and mechanical property.
