Acoustic study of kinetics of vacancy diffusion toward dislocations in aluminum

Using ultrasonic shear waves, we study the kinetics of migration of vacancy to dislocations in aluminum. Ultrasonic attenuation is sensitive to the effective dislocation length, which is shortened by the segregation of point defects. Two principal measurements were done: (i) measurement of temperature dependence of decay behavior of attenuation after elastic deformation; (ii) continuous measurement of attenuation of a specimen quenched from 573 K to cold-water and then to liquid-nitrogen temperatures. The specimens were polycrystalline and monocrystal aluminum with 99.995% purity. The former measurement provided us with the activation energy 0.28 eV for migration of dominant point defects through the Granato-Hikata-Lücke theory. The latter measurement informed us of the kinetics of vacancy: the attenuation coefficient rapidly decreased twice at 125 and 250 K, corresponding to activation energies 0.28 and 0.61 eV for migration, respectively. Thus, the same mechanism occurs for the attenuation change after elastic deformation and at 125 K after quenching. We attributed the 0.28-eV migration as fast diffusion of vacancy along dislocations. The 0.61-eV migration is consistent with the bulk diffusion of vacancy in aluminum. Throughout this study, the electromagnetic acoustic resonance method was used for making noncontacting and highly accurate measurement of attenuation.