Compressibility of polycrystal and monocrystal copper: Acoustic-resonance spectroscopy

Abstract

Using a method used mainly by geophysicists for small specimens-acousticresonance spectroscopy (ARS)-we measured the elastic-stiffness constants of centimeter-size copper specimens with rectangular-parallelepiped shapes. The polycrystal consisted of heavily twinned 75-μm crystallites. From the specimens' macroscopic resonance-vibration frequencies (midkilohertz to low-megahertz), we calculated the least-squares elastic-stiffness coefficients, two and three for the two cases. Using the same specimens, we augmented the ARS measurements with conventional pulse-echo-method measurements. Using rod specimens, we measured the Young modulus E and torsional modulus G, and we calculated the bulk modulus B. The less direct and less familiar ARS method gives the same results as a usual pulse-echo method and a rod-resonance method. The small difference between polycrystal and monocrystal values may arise from mobile twin boundaries that contribute a small reversible plastic strain to the intrinsic elastic strain. We list 16 advantages of the ARS method to measure elastic constants.