Detwinning, damage and crack initiation during cyclic loading of Cu samples containing aligned nanotwins

High-purity Cu samples containing parallel columns of highly aligned nanotwins separated by coherent Σ3 twin boundaries (TBs) with median spacing of about 35 nm were subjected to tension-tension fatigue. It was found that the microstructure of highly aligned nanotwinned Cu is much more stable under deformation than that of nanocrystalline Cu. Hardness values underwent only modest drops, the greatest decrease being seen for fatigue under a maximum stress of 450 MPa. While the majority of the twins remained after fatigue at this stress, a significant number of the original nanotwinned structures were destroyed. In parts of some columns new dislocation structures formed with straight extended dislocation walls. The de-nanotwinned regions are expected to be softer than the surrounding twinned material. Intense dislocation build-up is seen at the intersection of two columns in which the microstructures and thus patterns of dislocation activity differ. Such high stress regions are sites for crack nucleation. The S-N curves for the nanotwinned Cu and ultrafine-grained Cu are remarkably similar, perhaps because in both cases the intersections of soft regions with the surface are sites for crack initiation.