Amplitude-dependent internal friction and induced modulus defect in purified iron

Abstract

The amplitude-dependent internal friction and associated modulus defect of pure iron were observed at 32 kHz between 77°K and 300°K in a search for features associated with dislocation motion. The samples were variously annealed, cold-worked and subjected to magnetic fields. The amplitude dependence was predominantly magneto-mechanical in nature and therefore attributed to the motion of domain walls. Another method for studying dislocation motion was developed. High amplitude oscillations induced a modulus defect which decayed after the exciting oscillations ceased. The decay was observed, at low amplitudes, as a function of temperature The strain amplitude necessary to induce the modulus defect was 5 × 10 −6 at 300°K and 1.5 × 10 −5 at 77°K in purified iron. The induced defect could be observed at amplitudes at least as low as 10 −7 at 77°K. We attribute the induced modulus defect to the motion of dislocations freed by the excitation oscillation, and conclude that, at 77°K, at least one class of dislocations will undergo further motion when acted on by stresses at least as small as 2 g/mm 2, until immobilized by another process possibly involving kink diffusion.