EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Digital image correlation (DIC) is a powerful technique allowing detailed mapping of local strain distributions from the images of deformed surface. Spatial resolution of DIC is restricted only by the pixel size of the image and quality of random speckle pattern on the surface. Here we demonstrate the capabilities of DIC to capture propagation of shear bands in thin film metallic glasses with spatial resolutions down to few tens of nanometers.
PdSi metallic glass films were sputter deposited on polyimide substrate and then covered with randomly distributed nano-sized Indium islands. Additionally, the films were pre-structured with specific FIB-milled patterns in a way that the formation of cracks is locally prohibited but in-plane shear bands can freely propagate. Straining experiments were performed in-situ in SEM and GOM Correlate software was employed for DIC analysis. Shear bands propagating within the film plane do not necessarily lead to appearence of surface traces and thus cannot be detected on the SEM images with a naked eye. With DIC analysis shear bands are clearly visualized as bands of extremely localized strains, significantly exceeding the strains in the rest of the film. Local strains before and after generation of shear bands were carefully measured and an analogue of von Mises yielding criterion is formulated. Additionally, provided analysis proved that shear bands are “cold” during operation, i. e the temperature on the surface stays far below the glass transition temperature. Demonstrated technique of combining pre-patterned polymer-supported films with in-situ SEM straining and DIC is shown to be extremely effective to capture microplasticity phenomena with nanoscale resolution.