Slip line and dislocation structures in fatigued copper

Abstract

Large grained long life push-pull fatigue specimens of 99-999% purity copper have been examined at 10% and 50% of the expected life. Carbon replicas of slip striations were examined, interference microscopy was used to measure the displacement across the striations and dislocation structure was examined by transmission electron microscopy. The total length of slip striation in a given area was measured as a function of fatigue life. After polishing the surface the pattern of slip striations which appeared when cycling was resumed was identical to that removed by polishing. The observations indicate that the slip striations are produced by a very large number of related dislocation sources situated in the planes of the striations. Intrusions and extrusions may be built up by the operation of two dislocation sources near the surface on closely adjacent parallel planes, one operated by a tensile stress and the other by the compressive stress. It is suggested that annealing a specimen after half the expected life reduces the dislocation density and so facilitates dislocation movement. The poor fatigue properties of the sub-cell structure which is produced arise from the large number of dislocation sources present in the sub-cell structure as compared with the recrystallized metal.