Elastic constants and internal friction of polycrystalline copper

Using ultrasonic-resonance spectroscopy (URS), we measured the elastic constant C and companion internal friction Q⁻¹ of isotropic polycrystalline copper. The annealed material was 0.9999 pure with equiaxed heavily twinned grains averaging about 75 μm diameter. The URS method offers the principal advantage of point contact or loose coupling, thus there was no contribution from a transducer-specimen bond and only small contributions from transducers and fixture. A second advantage is one measurement for all elastic constants and all associated internal frictions. The C's agree with established values. The Q⁻¹'s are much lower than pulse-echo-method values. Comparison of measured Q⁻¹ with the Koehler-Granato-Lücke model permits estimating an effective dislocation-loop length. Q⁻¹ (shear) exceeds Q⁻¹ (longitudinal) by a factor of about 1.5.