A Model for the Signatures Observed in 5N Lead by the Two Wave Acoustic Coupling Method

Abstract
The ultrasonic low-frequency coupling method was used to study the relaxation processes in cold worked and annealed lead, in particular the Bordoni relaxation. New high-temperature signatures were observed in 5N lead on each temperature side of the high-frequency Bordoni peak. The signatures, measured in a temperature range where a non-negligible internal friction background is observed at low frequency, present a kind of "mirror symmetry" around the temperature of the high-frequency Bordoni peak. These signatures can be interpreted by a model assuming (i) that the high frequency motion of the dislocations is controlled by a thermally activated KPF (Kink Pair Formation) mechanism and (ii) that the low-frequency motion of the dislocations is controlled by a dislocation-point defects interaction, involving a LDWRF (Limited Displacement Without Restoring Force) mechanism. In this model, the KPF mechanism is responsible for the ultrasonic attenuation and for the mirror symmetry of the signatures. The LDWRF mechanism is responsible for the low-frequency internal friction background and for the characteristic shape of the signatures. An excellent correspondence between the experimental signatures and signatures numerically simulated with this model strongly suggests (i) that the Bordoni peak is due to a KPF mechanism and (ii) that the internal friction background is due to a LDWRF mechanism.