Effect of microstructural stability on fatigue crack growth behaviour of nanostructured Cu

Constant amplitude fatigue crack growth tests were carried out on commercial and high purity nanostructured copper processed by High Pressure Torsion (HPT). Due to strong grain refinement the HPT processed materials show higher tensile strength but also faster crack growth rates when compared to coarse grained material. Crack growth curves of nanostructured copper determined at different stress levels, however, showed that the occurrence of grain coarsening at low stress amplitudes leads to a retardation of crack growth in commercial and high purity HPT Cu. This effect was not observed for high purity HPT Cu with a bimodal microstructure. Crack propagation rates depend significantly on the coarsening phenomenon which on the other hand depends on the applied stress amplitude. A comparison of these results with cyclic deformation tests in the high cycle fatigue regime suggests that grain coarsening during crack growth depends more on the stored energy of the materials while a similar coarsening during cyclic deformation depends more on the activation enthalpy for annealing of defects.