The high-temperature damping background in intermetallic alloys

The high-temperature damping background (HTDB) in various intermetallic compounds, such as γ-TiAl based alloys, NiAl single crystals and quasicrystalline d-Ni–Al–Co exhibits viscoelastic behavior which can be observed as damping, increasing exponentially with temperature. The activation enthalpy H was determined from measurements of the HTDB at various frequencies. In γ-TiAl based alloys with various chemical composition the activation enthalpies are in the range of H = 4.2–4.3 eV for Ti–46 at.%Al–9 at.%Nb, and H = 3.8–3.9 eV for Ti–46.5 at.%Al–4 at.% (Cr, Nb, Ta, B). For a Ni49.5Al50.5 single crystal an activation enthalpy of H = 3.3 eV was determined. The H-values in TiAl and NiAl agree well with activation enthalpies from creep and self-diffusion experiments. Therefore, the HTDB in TiAl and NiAl is assigned to diffusion-assisted climb of dislocations. Decagonal Al–Ni–Co quasicrystals show different behavior. The HTDB is only observed in polycrystalline material, but not in a single crystal. The activation enthalpies, H = 2.4–3 eV, agree with values obtained by tracer diffusion experiments. This indicates that the HTDB in decagonal Al–Ni–Co quasicrystals is due to an intragrain diffusion process which is equivalent to Nabarro–Herring creep.