A MODEL FOR THE SIGNATURES OBSERVED IN 5N LEAD BY THE 2 WAVE ACOUSTICCOUPLING METHOD

The ultrasonic low-frequency coupling method was used to study the rel axation processes in cold worked and annealed lead, in particular the Bordoni relaxation. New high-temperature signatures were observed in 5 N lead on each temperature side of the high-frequency Bordoni peak. Th e signatures, measured in a temperature range where a non-negligible i nternal friction background is observed at low frequency, present a ki nd 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 d islocation-point defects interaction, involving a LDWRF (Limited Displ acement Without Restoring Force) mechanism. In this model, the KPF mec hanism is responsible for the ultrasonic attenuation and for the mirro r symmetry of the signatures. The LDWRF mechanism is responsible for t he low-frequency internal friction background and for the characterist ic shape of the signatures. An excellent correspondence between the ex perimental signatures and signatures numerically simulated with this m odel strongly suggests (i) that the Bordoni peak is due to a KPF mecha nism and (ii) that the internal friction background is due to a LDWRF mechanism.