Microstructural stability and microhardness of a cast TiAl-based alloy for turbine blade applications

The effect of ageing on the microstructural stability and microhardness of samples prepared from a cast turbine blade with nominal chemical composition Ti-46Al-2W-0.5Si (at.%) was studied. Ageing experiments were performed at three temperatures of 973, 1023, and 1073 K for various times ranging from 10 to 7300 h in air. Microstructure of the ageing samples consisted of lamellar, feathery and gamma-rich regions. Microstructural analysis revealed that the lamellar regions are composed of alpha(2)(Ti3Al) and gamma(TiAl) lamellae, coarse B2 particles and fine needle-like B2 precipitates. The feathery regions contain gamma-matrix with alpha(2), B2 and Ti5Si3 particles. Coarse Ti5Si3 particles are identified within the gamma-rich regions. During ageing the alpha(2)-phase in the lamellar and feathery regions transforms to the gamma-phase and fine needle-like B2 precipitates. The microstructural instabilities lead to a softening of the alloy, which is characterized by a faster microhardness decrease of the lamellar regions comparing with that of the feathery ones. The measured time exponents and activation energies for softening are discussed from the point of diffusion-controlled transformations and coarsening of the coexisting phases. A possible effect of such microstructural changes on the kinetics of softening is analysed.