Mechanical spectroscopy of a high-Nb-bearing γ-TiAl-based alloy with near-gamma and fully lamellar microstructure

Intermetallic gamma-TiAl sheet material of composition Ti - 46 at.% Al - 9 at.% Nb and two different microstructures (fine-grained near-gamma and coarse-grained fully lamellar) was studied by mechanical- loss (internal-friction) measurements using two frequency ranges: (I) 0.01 - 10 Hz and (II) around 2 kHz. The mechanical spectra in range I show (i) a loss peak of Debye type at T approximate to 1000 K, which occurs only in fully lamellar samples; and (ii) a high-temperature damping background above approximate to 1100 K. The values of the activation enthalpy H of the high-temperature background, 4.3 eV (near-gamma) and 4.2 eV ( fully lamellar), which were determined from the frequency shift, are distinctly higher than those obtained for TiAl sheet material with low Nb content. The high-temperature damping background is assigned to diffusion-assisted climb of dislocations, and the 1000 K peak ( H = 2.9 eV) to local ( reversible) glide of dislocation segments anchored between lamella interfaces. From measurements of the eigenfrequency in range II, the variation of Young's modulus in the temperature range 300 - 1000 K was determined.