Nucleation and thermal stability of carbide precipitates in high Nb containing TiAl alloys

This work monitors the nucleation, growth and coarsening of carbides in powder metallurgically processed, high Nb containing TiAl alloys. The effects of carbon content, annealing conditions and internal defects on the precipitation and stability of carbides were systematically investigated by high energy X-ray diffraction and transmission electron microscopy. In general, at 800 °C the carbide microstructure can still change significantly up to 1000 h of annealing. It is found that a higher carbon concentration promotes the carbide precipitation process and increases the thermal stability of carbides. Internal interfaces and other crystallographic defects act not only as heterogeneous nucleation sites for perovskite Ti3AlC carbides but also as carbon sinks. This retards the carbide nucleation in the interior of γ-TiAl grains. By homogenising the carbon distribution through solution heat treatment the nucleation of carbides in the γ-TiAl matrix is significantly accelerated as an effect of higher matrix carbon content.