In this study, research interest has been focused on the carbon-doped TiAl compounds with the fully lamellar microstructure which is beneficial for creep resistance.
The mother alloy had fully lamellar microstructure after heat treatment at 1390℃. However, the temperature for the fully lamellar microstructure increased 10℃ in the alloy containing 1.0at.%N and 20℃ in the alloy containing 1.0at.%C respectively. The addition of C and N was found to be effective in refining lamellar grains. This grain size refinement is due to precipitates which act as nucleation site. Transmission electron microscopy(TEM) observations revealed that the large precipitates are $Ti_2AlC$(H-phase) and small precipitates are $Ti_3AlC$(P-phase). H-phase is considered to act as nucleation site of lamellar grain. The effects of precipitates on mechanical properties were investigated using tensile test of the 1.0at.%C added TiAl alloy after aging at 900℃ and 1000℃ respectively. $Ti_3AlC$(P-phase) is considered to be stable at 900℃, but unstable at 1000℃. The alloy aged at 900℃ had better yield strength than the alloy aged at 1000℃. From these results, it is considered that the effects of P-phase on precipitation-hardening effect in TiAl alloy is important.
To compare the carbon and nitrogen addition effect, tensile tests of 1.0at.% C and N added TiAl alloy aged at 900℃ for 12 hours were investigated. The yield strength increased by the addition of carbon and nitrogen. The nitrogen addition was more effective in increasing yield strength, and less degraded elongation.
Moreover the creep properties drastically increased with the addition of 1.0at.% carbon and nitrogen. The carbon added TiAl alloy showed better primary regime creep properties than nitrogen added TiAl alloy.
