Nanostructured and ultrafine-grained metals are of great interest as they show improved mechanical and physical properties compared to their coarse-grained counterparts. However, the majority of studies reports inferior fatigue crack growth behavior, with only few exceptions, when the grain size is reduced. This could be a technical limitation for the use of these new materials when damage tolerance is demanded. Therefore in this contribution a comparative study on the fatigue crack growth behavior of microcrystalline (mc) Ni, ultrafine-grained (ufg) Ni produced by high pressure torsion and nanocrystalline (nc) Ni produced by electrodepositon was performed.
Quasi-static experiments on ufg and nc metals have shown a strong influence of the grain size, the grain shape and the grain orientation on mechanical properties like strength and ductility. By performing fatigue crack growth tests on high-purity metals with different grain sizes and grain shapes it is possible to study if the same influences exist under cyclic loading.
To learn more about the different contributions, crack propagation measurements are combined with fracture surface studies and microstructural analyses. In order to get information about the damage mechanisms, in-situ experiments are performed in the scanning electron microscope. Tests are made for different materials to see also the effect of inherent material properties.
The results should help to get a better understanding for the crack propagation process and to optimize the architecture of nanostructured materials in respect to fatigue.