Internal friction of γ-TiAl alloys at high temperature

Intermetallic γ-TiAl based alloys of engineering interest with respect to high temperature applications are two-phase alloys consisting of γ-TiAl (ordered face-centered tetragonal structure) and ₂-Ti₃Al (ordered hexagonal structure). For this investigation a γ-TiAl based alloy with a composition of Ti-46.4 at%Al-4 at% (Cr, Nb, Ta, B) was studied utilizing a low frequency subresonance apparatus at frequencies between 0.01 and 10 Hz. Above 1000 K, the damping increases strongly up to Q⁻¹=0.2. The frequency and temperature dependence of the high-temperature background was analysed by applying a Maxwell rheological model for viscoelastic relaxation including a distribution of relaxation times. The activation enthalpy of H=3.9 eV agrees well with that from creep experiments (H=3.6 eV) carried out at temperatures between 973 and 1073 K at 200 MPa. The results are discussed in terms of diffusion controlled dislocation climb.