Dislocation microstructures in fine-grained Cu polycrystals fatigued at low amplitude

Abstract

It is well-known that Persistent Slip Bands (PSBs) play a central role in low amplitude fatigue of Cu and other fcc metal single crystals. PSBs are accompanied by crack-related surface morphologies like intrusions and extrusions, hence regarded as the main cause of fatigue failure. PSBs have a characteristic dislocation substructure, usually embedded in a matrix structure, consisting of a regular array of predominantly primary dislocation walls, frequently referred to as ladder structure due to its appearance in TEM. PSBs form also in Cu polycrystals, but less frequently than in single crystals. Such difference in frequency is well-understood in terms of constraints among grains in polycrystalline materials, since constraints tend to cause additional slip other than single slip on one system thus inhibiting PSB formation. Hence PSB formation is thought to be increasingly difficult with decreasing grain size. To the present authors` knowledge, the smallest grain diameter of Cu employed ever was 25 {micro}m with which Mughrabi and Wang made a systematic study under constant plastic strain amplitude. While the authors tried to make specimens with even finer grain size, it was very difficult to decrease the grain size of pure Cu by usual work-hardening and annealing method. They then followedmore » to the cold-twisting method employed by Thompson and Backofen who succeeded in refining the grain size to less than 10 {micro}m at least at the specimen surface with a specimen geometry suited for fully reversed fatigue testing. In this study the authors achieved an average grain diameter of about 8 {micro}m and studied its effect on PSB formation in the initial stage of fatigue.« less