Diffusion coefficients in ternary and quaternary intermetallic γ-TiAl-based alloys measured by mechanical spectroscopy

The use of light-weight intermetallic γ-TiAl alloys as turbine blades in the low-pressure turbines of aircraft engines is substantially contributing to decrease the CO 2 gas emission. At present the target is to enhance the high-temperature creep performances of the alloys in order to further increase the efficiency. This requires to guarantee the thermal stability of the microstructure and to reduce the creep rate caused by dislocation climb. Consequently, the precise knowledge of the atomic diffusion coefficients is a key factor. However, the existing data of diffusion coefficients were mostly obtained in binary γ-TiAl alloys, whereas the required performances move towards the design of ternary, quaternary and even more complex alloys. In the present work, the atomic diffusion coefficients of Ti and Al have been determined in several ternary and quaternary alloys through mechanical spectroscopy. The internal friction relaxation peaks associated with atomic diffusion were analyzed through a new methodology which allow to determine the diffusion coefficients between 950 and 1150 K, which is the expected temperature range for in-service working conditions. The methodology was validated in a pseudo-binary TiAl-Si alloy and a remarkably good agreement was found with the values measured in literature by the classical tracer diffusion technique conducted on binary TiAl alloys. The introduced methodology enables a relatively fast and easy way to determine diffusion coefficients in complex alloys. It was then applied to TiAl-Nb, TiAl-Nb-Mo and TiAl-Nb-Mo-C-Si alloys, analyzing the influence of Nb and C content on the diffusion coefficients of Ti and Al in such complex alloys. These results will contribute to the design of new generation γ-TiAl based alloys showing enhanced performance due to an extended temperature range.