Recently, TiAl alloy has been regarded as one of the strongest candidate materials for light and high temperature structural materials. Because of its low density and high creep resistance in this alloy, TiAl alloy is considered suitable for engine valve and turbine blades.
Therefore, TiAl alloys are used under conditions where fatigue and creep-fatigue interaction occurs. Most investigators have focussed mainly on fatigue crack growth resistance and fatigue fracture behavior at high cycle fatigue conditions.
In the present study, continuous low cycle fatigue and creep-fatigue tests have been conducted and effects of carbon addition are discussed. Firstly, as carbon is added, interlamellar spacings becomes narrow and carbides(P-phase) are precipitated at lamellar interface. When more than 0.3 at.%C is added, microstructure is not observed to be changed.
The continuous low cycle fatigue and creep-fatigue lives of TiAl with carbon are found to be longer than those of TiAl without carbon. In the cases of TiAl with carbon, dislocations are spread uniformly in each lamellar colony. This is problems due to lamellar spacing becoming narrow and the number of $α_2/γ$ interfaces increasing. Also, $α_2/γ$ interface acts an impediment for dislocation movement causing many dislocations difficult to move. Moreover, it is observed that dislocations are blocked by the carbide. Therefore, plastic deformations occur uniformly resulting in fatigue lives being increased. In creep-fatigue tests, a change in the microstructure of TiAl alloy without carbon is observed. Many dislocations are generated at $α_2/γ$ interface and $α_2$ phases become thinner. However, the microstructure of TiAl alloy with carbon is not observed to be changed. So carbides and many $α_2/γ$ interfaces can act effectively. Namely, TiAl alloy with carbon endures well creep damage.
