The dislocation structure evolution in polycrystalline copper at constant strain amplitude during low cycle fatigue is well understood. Single crystal, ultra-large grain polycrystalline copper dislocation development has received little attention. Ultra-grain polycrystalline copper with 600 m average grain size was used in this study to investigate the dislocation development at different fatigue strain amplitudes. The results show that; (1) the stress curve vs. number of cycles (S-N curve) produces hardening in the first stage followed by softening regardless of the strain amplitude. At the same time, no plateau is found in the S-N curves. (2) The fatigue saturation stress increases consistently with increased grain size. (3) The special dislocation morphology of ultra-grain copper during fatigue displays a loop patch structure or veined structures embedded in a long band area in parallel dislocation. This is because the larger grain has larger saturation stress, producing a large area with the same slip system band that regulates the high saturation stress.

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