Ultrasonic control of purity in zone-refined aluminum

Amplitude-independent dislocation damping has been applied to control the total amount of impurities in zone-refined aluminum. A number of samples having the residual resistance ratio (RRR=R{sub 300K}/R{sub 4.2K}) from 13000 to 38000 have been investigated. The rod-shaped samples were 50 mm in length corresponding the fundamental mode frequency of about 50 kHz. Before the measurements of internal friction, the samples were stressed by a static load within the elastic range of deformation diagram. Then, after the load was released, computer-controlled set up with electrodynamic excitation of vibrations was used to follow the decrement versus time curves. The dependence of the decrement on the value of RRR at l=const. has been obtained from a set of the recovery curves. In the beginning of recovery, the decrement of vibrations has been concluded to be more sensitive to the total impurity content then the value of RRR. The recovery curves were analyzed within the framework of Granato-Luecke theory. According to the theory, it is possible to use both kHz and MHz data for the Control of purity. Advantages and disadvantages of both methods have been discussed.