Revisiting fatigue crack growth in various grain size regimes of Ni

The fatigue crack growth characteristics of high purity nickel with grain sizes ranging from the micro- down to the nanocrystalline regime were examined. Fatigue crack growth measurements for different load-ratios combined with extensive fracture surface analyses revealed that dislocation motion is the dominating damage mechanism in the near-threshold region up to the Paris regime, irrespective of the grain size. Grain boundaries serve as preferential crack paths in the entire investigated grain size range, depending only on the loading conditions. The controlling factor is mainly the size of the cyclic plastic zone in comparison to the grain size. As a further peculiarity crack closure disappears for microstructures with a grain size below one micron. Along with the grain size variation the influence of the grain aspect ratio was in focus, which can cause a strong orientation dependence of the fatigue crack growth behavior. Elongated microstructures aligned perpendicular to the crack growth direction can considerably improve the overall resistance against fatigue crack growth.