Local nanoscale strain mapping of a metallic glass during in situ testing

Abstract

The local elastic strains during tensile deformation in a CuZrAlAg metallic glass are obtained by fitting an elliptic shape function to the characteristic amorphous ring in electron diffraction patterns. Scanning nanobeam electron diffraction enables strain mapping with a resolution of a few nanometers. Here, a fast direct electron detector is used to acquire the diffraction patterns at a sufficient speed to map the local transient strain during continuous tensile loading in situ in the transmission electron microscope. The elastic strain in tensile direction was found to increase during loading. After catastrophic fracture, a residual elastic strain that relaxes over time was observed.

Direct measurement of nanostructural change during in situ deformation of a bulk metallic glass

“…The full experimental procedures can be found in the attached methods section. The diffraction patterns were then used to measure the spatially resolved evolution of both strain 41 and short and medium range order 48–52 at every probe position over a large area as the sample was mechanically deformed, with a spatial resolution (probe position step size) of 2.5 nm. It should be noted that these diffraction patterns arise through an interaction of the electron beam with a finite sized (1.47 nm FWHM) volume projected through the sample thickness, and therefore symmetry elements in the patterns can arise from interactions with multiple oriented clusters, making it impossible in this experiment to distinguish between singular oriented clusters (short range order) and cluster networks (medium range order).…”

“…These diffraction patterns were then used for strain mapping following the methods outlined in ref. 41 , in which an ellipse is fitted to every diffraction pattern using Eq. 1, and deviations from a reference radius are converted to strains.…”

“…This is done using the following equations, , , and , from ref. 41 . The reference radius was chosen to minimize strain at zero loading.…”

“…In situ experiments to date have been qualitative, too slow during acquisition, hard to interpret due to a lack of understanding of the contrast mechanisms in shear bands, or at too low of a spatial resolution to be comparable to MD models 40 . Recent advancements in techniques and hardware have, however, allowed for the observation of strain 41 and as we will show here, the evolution of locally resolved atomic short and medium range order, with nanometer resolution during in situ deformation, providing much more comparable information to the significant modeling efforts which have been performed.…”

Functional Materials Under Stress: In Situ TEM Observations of Structural Evolution

“…The application of 4D-STEM during in-situ experiments has been significantly expanded recently thanks to the deployment of fast DED camera with frame rates up to ∼thousands of frames per second level [46,47,51,68] . The fast readout speed is important, not just for increased scan speed, but also for the opportunity to probe dynamic events during in-situ STEM/TEM studies [48][49][50]69] . Figure 2(b) further demonstrates the advantages of NBED in in-situ experiments [70] , where multi-modal contrast can be achieved with a single dataset, simplifying the experimental workflow of in-situ nanomechanical testing and enabling more dynamic correlation analysis.…”