G. Haneczok

Internal friction of γ-TiAl alloys at high temperature

Abstract 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 α2-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.

Interaction of interstitial solute atoms in bcc metals

Abstract The applicability of the random cooperative strain interaction model to the description of interaction of interstitial solute atoms (ISAs) in bcc metals (the Snoek relaxation) is discussed in detail. Based on this model in the mean-field approximation with the ‘Onsager correction’, a new formula describing the mechanical after-effect (creep) is derived and analysed. The results make it possible to formulate a hypothesis in which to a first approximation the interaction of ISAs in bcc metals does not depend on the solvent lattice nor, possibly, on the kind of solute atom. Some indications concerning future experiments leading to the verification of this hypothesis are presented.

The Snoek relaxation

Abstract Theory of the Snoek relaxation is presented and discussed by taking into account the interaction of interstitial solute atoms (i.e. random cooperative strain interaction model).

Internal Friction of a High-Nb Gamma-TiAl-Based Alloy with Different Microstructures

An intermetallic Ti-46Al-9Nb (at%) alloy with different microstructures (near gamma, duplex, and fully lamellar) was studied by internal friction measurements at 300 K to 1280 K using different frequency ranges: (I) 0.01 Hz to 10 Hz and (II) around 2 kHz. The loss spectra in range I show (i) a loss peak of Debye type at T ≈ 1000 K which is only present in duplex and fully lamellar samples; (ii) a high-temperature damping background above ≈ 1100 K. The activation enthalpies determined from the frequency shift are H = 2.9 eV for the loss peak and H = 4.1–4.3 eV for the high-temperature damping background. The activation enthalpies for the visco-elastic high-temperature damping background agree well with values obtained from creep experiments and are in the range of those determined for self-diffusion of Al in TiAl. These results indicate that both properties (high-temperature damping background and creep) are controlled by volume diffusion-assisted climb of dislocations. The loss peak is assigned to diffusion-controlled local glide of dislocation segments which, as indicated by transmission electron microscopy observations, are pinned at lamella interfaces.