Dislocation-mediated creep process in nanocrystalline Cu

Nanocrystalline Cu with average grain sizes ranging from ～24.4 to 131.3 nm were prepared by electric brush-plating technique. Nanoindentation tests were performed within a wide strain rate range, and the creep process of nanocrystalline Cu during holding period and its relationship to dislocation and twin structures were examined. It was demonstrated that creep strain and creep strain rate are considerably significant for smaller grain size and higher loading strain rate, and are far higher than those predicted by the models of Cobble creep and grain boundary sliding. The analysis based on the calculations and experiments reveals that the significant creep deformation arises from the rapid absorption of high density dislocations stored in loading regime. Our experiments imply that stored dislocations during loading are highly unstable and dislocation activity can proceed and lead to significant post-loading plasticity.