High carbon solubility in a γ-TiAl-based Ti–45Al–5Nb–0.5C alloy and its effect on hardening

The C distribution within the γ-TiAl-phase of a Ti–45Al–5Nb–0.5C alloy with near-γ microstructure was studied by atom probe tomography. In most areas, the C atoms were homogeneously distributed, and only a few C-enriched features were detected, which were presumably Cottrell atmospheres surrounding dislocation cores. The C concentration within the γ-phase was measured to be ∼0.25 at.%, which is a factor of 10 higher than the solubility limit reported for other TiAl alloys. The reason for this unusually high C solubility is explained by an existing model, which relates the number of octahedral sites consisting of six Ti atoms to the solubility limit of interstitials. The large amount of C in solid solution can explain the results of a recent study which showed that the C-containing alloy had a ∼30% increase in yield strength compared with a C-free sheet containing the same Ti, Al and Nb concentrations.