Internal Friction of a High-Nb Gamma-TiAl-Based Alloy with Different Microstructures
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Internal Friction of a High-Nb Gamma-TiAl-Based Alloy with Different Microstructures M. Weller1, H. Clemens2, G. Dehm1, G. Haneczok3 , S. Bystrzanowski4, A. Bartels4, R. Gerling5, and E. Arzt1 1 Max-Planck-Institut für Metallforschung, Heisenbergstr. 3, D-70569 Stuttgart, Germany 2 Dept. of Physical Metallurgy and Materials Testing, Montanunivesität Leoben, Franz-JosefStrasse 18, A-8700 Leoben, Austria 3 Institute of Materials Science, Silesian University, Katowice, Poland 4 Technical University of Hamburg-Harburg, Dept. of Materials Science and Technology, Eisendorferstrasse 42, D-21071 Hamburg, Germany 5 Institut for Materials Research, GKSS Research Centre, Max-Planck-Strasse 1, D-21502 Geesthacht, Germany
ABSTRACT 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 viscoelastic 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.
INTRODUCTION Intermetallic γ-TiAl based alloys exhibit increasing technical importance for high-temperature applications in the automotive and aerospace industries (see, for example [1]. These are based on their properties at elevated temperatures, such as high yield strength, advanced creep properties, high stiffness, and good oxidation/corrosion resistance. The mechanical properties at high temperatures are strongly influenced by the creep deformation behavior, which is largely controlled by diffusion mechanisms occurring in the different phases of the TiAl-system (self and solute diffusion). As an alternative to creep tests – which are time consuming and require considerable technical effort – mechanical loss (internal friction) experiments can also give access to the high-temperature mechanical properties. This was demonstrated for various materials such as intermetallic compounds [2], quasicrystalline materials [3], and ceramics [4]. The basic source of information in mechanical loss studies is the high-temperature damping background, which exhibits viscoelastic behavior and requires measurements far below 1 Hz. It
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