Abstract
The effect of plastic deformation upon the internal friction of various grades of copper has been investigated with the aid of a torsion pendulum having a low background energy loss. Energy dissipated by deformed copper vibrating at frequencies of 1 - 2 cycles per second appears to be caused by relaxation phenomena rather than by Nowick's mechanism of static hysteresis. The cold work induced internal friction rises to a maximum for critical shear strains in the range 0.002 - 0.06 after which it decreases rapidly. Secondary maxima on the internal friction/strain curves are associated with preferred orientations in the recrystallised material. The magnitude of the internal friction effects produced by straining increases with the temperature at which the material has previously been annealed. The internal friction changes with time after straining. With commercially pure materials this change is invariably a decrease. With high purity materials the change might be either an increase or decrease, and is dependent upon the metallurgical history of the specimen. The impurity level marking the transition between these two types of behaviour is approximately 0.004% by weight. changes of elastic modulus accompany these changes of internal friction but are not directly related to them. The elastic modulus invariably increases with time after straining. The magnitude of this effect does not depend upon purity but is affected by the temperature at which the specimen was annealed. The internal friction induced by cold working con be virtually eliminated by annealing at 160°C. This temperature is well below that required to cause softening or appreciable recovery of elastic modulus. The experimental results have been interpreted in terms of a hypothesis involving the migration of dislocations to impurity atoms. Evidence in favour of this hypothesis is afforded by the results of electrical resistance measurements on strained copper wires.