SUMMARY

Fatigue investigations of micro- and nanoscale structural metals have become of great significance due to the extensive use of microelectromechanical systems (MEMS) and metallic thin films. Many MEMS and thin films consist of movable small-scale components that are subjected to a large number of cycles throughout their lifetimes and exhibit a strong influence from size effects that deviates their behavior from bulk size materials. Therefore, there is a need to investigate and understand the fatigue behavior and mechanisms of small-scale metals under loading conditions relevant to their applications. This work investigates the fatigue behavior of Ni microbeams subjected to size effects (extreme stress/strain gradients and microstructurally small cracks) under fully reversed loadings. The microbeams are loaded under bending and subjected to in situ Scanning Electron Microscope (SEM) High and Very High Cycle Fatigue (HCF/VHCF) and Low Cycle Fatigue (LCF) testing. The in situ SEM HCF/VHCF tests reveal strong environmental effects on fatigue lives that are three orders of magnitude longer in a vacuum than in air. They also highlight crack propagation rates that are extremely low, indicating that the fatigue mechanism does not follow the common crack tip stress intensification. Instead, crack nucleation and propagation are caused by the formation of voids that nucleate from the condensation of vacancies. However, the LCF tests indicate that at high stress amplitudes the physical mechanisms of crack propagation are dominated by the large cyclic plastic deformation at the crack tip. These results showed that extreme stress gradients significantly reduce the driving force for crack growth, causing the void controlled fatigue mechanism in the HCF/VHCF regime. As the driving force is increased in the LCF regime, the fatigue mechanisms follow more closely the well documented conventional behavior. Additionally, fatigue-induced grain coarsening is investigated in the ultrafine grained region of the Ni microbeams. Results highlight cycle dependent abnormal grain growth driven by the reduction of elastic strain energy. These results are expected to provide a better understanding of the fatigue behavior in micro and nano scale devices and therefore allow for a more robust design of these devices.