Unravelling shear band formation and propagation in metallic glasses through 4D-STEM

Abstract text (incl. figure legends and references)

Metallic glasses (MGs) are an exciting new class of materials due to their unique mechanical and functional properties. Still, potential applications are hindered by their limited ductility caused by the formation of shear bands leading to catastrophic failure. A fundmental understanding of the shear banding process is key for improving the mechanical properties of MGs. While progress in the understanding of deformation localization was made using colloidal solids or atomistic simulations, highlighting the importance of nanoscale structural variations and local strains for shear banding, direct experimental studies are scarce due to limited characterization techniques.

In the current work we study the deformation of metallic glasses at the nanoscale by scanning transmission electron microscopy (STEM) imaging using a JEOL 2200FS equipped with an in-column energy filter. To quantify local density and strain, a map of precession nanodiffraction patterns is recorded using 4D-STEM. Scanning and precession of the electron beam is controlled by a TVIPS universal scanning generator and the nanodiffraction patterns are recorded using a fast CMOS camera (TVIPS XF416) and a direct electron detector (Quantumdetectors MerlinEM 4R). A Cu46Zr46Al8 bulk MG plate is made by suction casting and shear bands (SB) are introduced by uniaxial compression. Cross section specimens are extracted on major SBs using a focused ion beam dual-beam workstation. Precession nanodiffraction enables to map the strain at at sufficient spatial resolution (<2 nm) to quantify atomic density and strain distribution within an individual SB.

Figure 1 shows a HAADF image and a local density map, revealing a decrease in atomic density within the SB [1]. In addition, the SB shows density alternations from the atomic scale to the submicron scale. The nanoscale density alternations show good agreement with molecular dynamic simulations indicating that they are linked to the autocatalytic generation of shear transformation zones, while the density alternation at submicron scale results from the progressive propagation of the SB front. The current results demonstrate that a direct quantitative measurement of strain in amorphous nano-volumes is important for revealing fundamental deformation processes in MGs, eventually allowing to tune their mechanical properties in a more controlled fashion.

This work was supported by the Austrian Science Fund (FWF): Y1236-N37.

[1] H. Sheng, D. Şopu, S. Fellner, J. Eckert, C. Gammer PHYSICAL REVIEW LETTERS 128, 245501 (2022)