Internal friction and creep of titanium aluminides with different microstructure

A Ti-46.5 Al-4 (Cr, Nb, Ta, B) alloy (at.%) with different microstructures (fine-grained primary annealed or coarse-grained fully lamellar) was examined both by mechanical-loss (internal-friction) and creep experiments. The mechanical-loss spectra (0.01-10 Hz), ranging from 300 up to 1300 K, show two phenomena: (i) A loss peak at about 900 K (0.01 Hz) which occurs only in samples with a lamellar microstructure. The activation enthalpy, determined from the frequency shift, is 3.0 eV. The Debye-type peak is related to thermally activated reversible local movement of dislocation segments which are pinned at the lamellae interfaces and within gamma lamellae. (ii) A high-temperature damping background above 1000 K which is controlled by an activation enthalpy of 3.8 to 3.9 eV. This value agrees well with the activation enthalpy determined by creep tests and is in the range of values reported for self-diffusion of Al. These results indicate that both properties (high-temperature background and creep) are controlled by volume diffusion-assisted climb of dislocations.