Observing High-Cycle Fatigue Damage in Freestanding Gold Thin Films with Bulge Testing and Intermittent Transmission Electron Microscopy Imaging

Bulge testing is a potent technique for measuring the mechanical properties of freestanding thin films, but in situ imaging is only possible in limited experimental configurations. This poses a serious limitation for unraveling nanoscale failure mechanisms, such as the deformation mechanisms induced by cyclic loading in freestanding gold thin films of 150 nm thickness. Herein, a new experimental workflow combining standalone bulge cyclic testing with intermittent high-resolution imaging by transmission electron microscopy (TEM) at specific positions of interest is introduced. The observed low dislocation activity in planar areas of the thin films is consistent with the slow strain accumulation during high-cycle fatigue testing. In contrast, notches in the films lead to localized plasticity with sustained dislocation activity, but also grain growth and subgrain formation. At a more advanced stage, cracks proceed along grain boundaries, with crack bridging seemingly slowing down their propagation. The presented setup can be used with a number of TEM-based characterization techniques and has the potential to reveal cyclic deformation mechanisms in several thin-film systems. Intermittent transmission electron microscopy investigation of freestanding gold thin films after custom cycle numbers reveals that dislocations are mainly sessile and thus do not contribute to fatigue deformation. Artificial notching leads to stress localization and thus crack propagation, inducing plasticity in the form of slip bands and dislocation structures near and in front of the crack.image (c) 2024 WILEY-VCH GmbH