The influence of dislocations on ultrasonic waves in single crystals

Abstract

Some properties of dislocations in aluminium and sodium chloride single crystals have been investigated by observing the generation of harmonics in an ultrasonic pulse travelling through a crystal. Pulses of 10 MHz longitudinal waves were generated at one end of a crystal with a quartz transducer and either 20 MHz or 30 MHz waves were detected at the opposite end of the crystal. The harmonic waves were detected either with a quartz transducer or a capacitative detector. Experiments on the generation of second harmonics due to dislocations were carried out on aluminium crystals. No dislocation contribution to the second harmonic was detected in NaCl crystals5 and this was attributed to a lack of purity in the crystals. However, experiments on third harmonic generation due to dislocations were performed on both aluminium and NaCl crystals. The amplitude of the third harmonics and the attenuation of the fundamental wave were measured as a function of impurity content, static bias stress, plastic deformation, temperature, the amplitude of the fundamental wave and, in the case of NaCl, X-irradiation also. The results of these experiments are compared with the behaviour predicted by the theory of Hikata and Elbaum (1966). Many of the results are found to be in good qualitative agreement with this theory. During the course of some third harmonic measurements in NaCl evidence was found of internal stresses remaining after plastic deformation. Compressive internal stresses were found after a plastic deformation in tension., and tensile internal stresses were found after further plastic deformation in compression. A previously unreported time dependence of the second harmonic and third harmonic amplitudes was noted in some crystals after a bias stress was applied. Thermal activation energies associated with the time dependent behaviour were measured. Such measurements provide a useful method of studying strain-ageing properties in crystals.