Low-Temperature Mechanical Loss Spectroscopy of 5N Lead

Abstract
By internal friction measurements in 5N lead, a relaxation peak is observed in the vicinity of 40 K at a frequency of 250 Hz, and in the vicinity of 150 K at a frequency of 18 MHz (ultrasonic attenuation). This thermally activated relaxation demonstrates all the characteristics of 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 formation (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 effect, similar to the softening already observed in aluminium after plastic deformation or after electron irradiation, occurs in lead. This result confirms the existence of the dislocation lubrication process recently proposed to explain the discrepancy between the low-temperature behaviour of the critical resolved shear stress in FCC metals and mechanical spectroscopy measurements, and the signatures observed at very low temperatures suggest that a short-circuited KPF mechanism is responsible for this lubrication process.