Viscous Ultrasonic Attenuation in Metals

Using a pulse technique, the attenuation of longitudinal ultrasonic waves is measured in polycrystalline aluminum, cobalt, nickel, and copper, respectively, in the frequency range from 1 to 15 Mc/sec. The metals have previously received different treatments of loading and annealing. The measurement is extended, for nickel, in the temperature range from −78° up to 450°C. The results indicate that the attenuation-frequency curve can be divided into two parts: In the low frequency part, the (attenuation)2 varies linearly with frequency; the high-frequency part exhibits a constant attenuation corresponding to a diffusion loss as previously reported. The loss associated with the parabolic part is attributed to a viscous origin as that considered in boundary layers. The interaction of a dislocation-free volume and the surrounding dislocations (as a fluid) could explain qualitatively and quantitatively the parabolic viscous behavior.