Nanocrystalline (NC) thin films are a complex and interesting material system that find potential applications in many technologically relevant fields. They often display enhanced mechanical properties in comparison to their bulk counterparts. Recent experiments on NC Au thin films have, however, shown that the presence of a notch leads to strong localization of deformation leading to brittle failure in such structures; the deformation mechanisms leading to such localization, nevertheless, remain unclear.
In this work, we investigate the deformation behavior of fcc thin films using atomistic simulations. The initial columnar-grained samples are generated by a constrained Voronoi tessellation technique so as to obtain an optimal grain boundary (GB) and triple junction (TJ) network. Notches/slits of different radii are subsequently introduced and the samples are then loaded under tension. Under tensile loading, the simulations clearly show that both dislocation and GB mediated plasticity are localized in the region in front of the notch. Furthermore, we observe copious amounts of deformation twins directly in front of the notch, resulting in surface undulations and a reduction of film thickness. We compare these simulation results with bulge test experiments and simulations on single crystalline thin films, in order to obtain insights into the deformation mechanisms and localization behavior. Additionally, we perform detailed analysis of the deformation mechanisms and the stress state in individual grains and correlate them with the neighborhood of each grain. The deformation behavior and the stress states in individual grains are, furthermore, discussed in the context of mesoscale simulation frameworks.