High-temperature microstructure evolution of an advanced intermetallic nano-lamellar γ-TiAl-based alloy and associated diffusion processes

Nano-lamellar advanced γ-TiAl based alloys doped with small amounts of C and Si are being developed to improve the creep resistance in order to increase the performances of this kind of alloys applied in the low-pressure turbine of aircraft engines. In order to extend the service temperature up to 1073 K or even above, the control of the microstructure stability is key. In this work, a complete study of the microstructure evolution during high-temperature exposure up to 1153 K has been approached through different electron microscopy techniques including HRTEM and HRSTEM with microanalysis. The nucleation and growth of the ordered βo precipitates and the ζ silicides inside the α2 lamellae has been carefully characterized and new orientation relationships and the misfit between all crystalline lattices has been determined, as well as the chemical concentration of the different atomic species on each phase. The electron microscopy study shows that βo and ζ precipitates inside α2 prevents or retards the dissolution of the α2 lamellae and its final disintegration in favor of the γ lamellae. This phenomenon has been discussed in terms of the phase coherence and diffusion processes. These important results allow conclude that: the coarsening of the γ lamellae is delayed because of the βo and ζ precipitation, allowing to explain the observed enhancement of the creep resistance in this γ-TiAl based alloy exhibiting a nano-lamellar microstructure.