Watching High-Cycle Fatigue with Automated Scanning Electron Microscope Experiments

Fatigue is a multistep process where cyclic loading causes damage within materials that eventually leads to crack formation and propagation. In nanocrystalline metals, a dominant damage mechanism is the abnormal growth of grains up to 100 times their original size. Previous in situ synchrotron experiments have revealed that this grain growth process precedes crack formation and takes up a majority of the fatigue lifetime. The growth of nanocrystalline grains leads to the formation of protrusions on the surface of a material, which can be resolved in scanning electron microscopy. Based on this concept, an automated in situ scanning electron microscope tension–tension fatigue test method has been developed to observe the evolution of crack formation and propagation in materials. In this study, this method was applied to understand the high-cycle fatigue behavior in nanocrystalline Ni- and Pt-based alloys. Fatigue tests between 105 and 107 cycles were performed, and in combination with postmortem characterization through grain orientation mapping and transmission electron microscopy, we identified differences in resistance to damage and crack propagation in the various alloys, and observed varying damage levels prior to crack formation, strongly dependent on the number of cycles to failure.