Abstract
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 γ-rich regions. Microstructural analysis revealed that the lamellar regions are composed of α₂(Ti₃Al) and γ(TiAl) lamellae, coarse B2 particles and fine needle-like B2 precipitates. The feathery regions contain γ-matrix with α₂, B2 and Ti₅Si₃ particles. Coarse Ti₅Si₃ particles are identified within the γ-rich regions. During ageing the α₂-phase in the lamellar and feathery regions transforms to the γ-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.

Key words
intermetallics, titanium aluminides, TiAl, heat treatment, microstructure, microhardness

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