Cyclic bending experiments on free-standing Cu micron lines observed by electron backscatter diffraction

Polycrystalline Cu samples 20 × 20 μm2 in size were cyclically bent inside a scanning electron microscope until fracture occurred. The microstructural changes were investigated by secondary electron imaging and electron backscatter diffraction. The in situ experiments revealed that, for the coarse-grained samples, it is not the external stress that dominates the cyclic deformation, but the local internal strength. This is in strong contrast to macroscopic bending samples, where deformation always happens near the fixed end of the bending beam and decreases constantly with increasing distance from the fixation. For micron-sized polycrystalline samples, the grain dimensions, dislocation density evolution and grain orientation (Taylor factor) can define the location of failure if the grain size and sample diameter become similar in size. A comparison with cyclic in situ tension-tension experiments (ratio of minimum stress to maximum stress R ≈ 0) reveals that cyclic bending experiments (R ≈ −1) undergo bulk-like fatigue deformation with extrusions/intrusions, in contrast to the experiments with R ≈ 0. Both the cyclic tension-tension and bending experiments can be described by a Basquin equation, although different mechanisms lead to failure of the samples.