Effect of Twinning on the Amplitude-Dependent Internal Friction in Tin Crystals

Abstract
Amplitude-dependent internal friction is measured on white tin single crystals to observe the change with the formation of mechanical twins. The material used is 99.9% pure, and the measurements are made by the transverse vibration method. After twinning, the internal friction, amplitude-independent at low amplitudes, rapidly increases above a breakpoint and then steeply increases as the driving force is increased. Above the breakpoint, the internal friction measured during the free decay is much higher than that measured before with increasing amplitude. The stress amplitude at which the internal friction steeply increases is independent of temperature, within the range observed, between 25°C and 80°C. It is also observed that above the breakpoint the internal friction increases with the time of excitation and decreases with the time of rest after the excitation. The results are interpreted in terms of the multiple-pole mechanism of twin dislocation discussed before. The high internal friction observed is due to the annihilation of twin dislocation loops of opposite signs, which sweep every twin layer at high speeds.