LOW-TEMPERATURE MECHANICAL LOSS SPECTROSCOPY OF 5N LEAD

By internal friction measurements in 5N lead, a relaxation peak is obs erved in the vicinity of 40 K at a frequency of 250 Hz, and in the vic inity of 150 K at a frequency of 18 MHz (ultrasonic attenuation). This thermally activated relaxation demonstrates all the characteristics o f a Bordoni peak. Using the ultrasonic low-frequency coupling method, new high-temperature signatures were observed on each temperature side of the high frequency Bordoni peak (150 K). These signatures can only be interpreted if one assumes that a thermally activated kink pair fo rmation (KPF) mechanism is responsible for the high-frequency Bordoni peak. Low-frequency internal friction measurements performed after low -temperature plastic deformation have shown that a modulus softening e ffect, similar to the softening already observed in aluminium after pl astic deformation or after electron irradiation, occurs in lead. This result confirms the existence of the dislocation lubrication process r ecently proposed to explain the discrepancy between the low-temperatur e behaviour of the critical resolved shear stress in FCC metals and me chanical spectroscopy measurements, and the signatures observed at ver y low temperatures suggest that a short-circuited KPF mechanism is res ponsible for this lubrication process.