Direct Observation of Quadrupolar Strain Fields forming a Shear Band in Metallic Glasses

Abstract

For decades, scanning/transmission electron microscopy (S/TEM) techniques have been employed to analyze shear bands in metallic glasses and understand their formation in order to improve the mechanical properties of metallic glasses. However, due to a lack of direct information in reciprocal space, conventional S/TEM cannot characterize the local strain and atomic structure of amorphous materials, which are key to describe the deformation of glasses. For this work, 4-dimensional-STEM (4D-STEM) is applied to map and directly correlate the local strain and the atomic structure at the nanometer scale in deformed metallic glasses. Residual strain fields are observed with quadrupolar symmetry concentrated at dilated Eshelby inclusions. The strain fields percolate in a vortex-like manner building up the shear band. This provides a new understanding of the formation of shear bands in metallic glass.

Large-angle Lorentz 4D-STEM for Simultaneous Magnetic and Atomic Structure Mapping

“…Stress-induced distortions of the atomic structure of amorphous materials can be determined from the azimuthal elliptic distortion of the diffraction rings (Figure S2d-g). The residual elastic strain eld within the sample can be mapped by quantifying the elliptical deviation of all diffraction patterns in the 4D-STEM data and comparing it with a reference area, employing the data analysis process established by Gammer et al 29 and Kang et al 23 . In this work, we use the convention that the rst principal strain is oriented along the compression direction providing the compressive component ( , which in turn results in the tensile component ( as the second principal strain that is oriented perpendicular to the rst principal strain.…”

“…The asymmetrical strain elds with a sharp transition across shear bands are in line with the previous strain observations in deformed amorphous metallic alloys. 23,38,39 The strain also induces the variation of the relative atomic density which re ects the net volume change due to the hydrostatic stress. suffers a sudden change from positive to negative across the shear plane (Fig.…”

“…The diffraction patterns are tted with an algebraic method as illustrated in Ref. 23 , where the sensitivity of ~ 0.05% for the strain measurement was demostrated. From the tted ellipse,…”

“…Unlike techniques relying on highresolution atomic lattice imaging, the development of 4D-STEM has provided a means to map atomic structures over a large eld of view by recording information in reciprocal space by scanning a nanoscale electron probe across the samples. [22][23][24] In the conventional eld-free mode, the diffraction signal contains limited information about the atomic structure as the high-angle scattered electrons are physically blocked by the projection lenses. Our development enables simultaneous recording of both the unscattered electron beam and diffracted beams representing interatomic pairs, which provides at the same time the phase shift of the electron exit wave induced by the local magnetic eld and the information on the atomic structure of the sample.…”

First-principles atomic level stresses: application to a metallic glass under shear

“…The s k vM is useful in probing local shear response to a shear strain γ, because s xy k is not stress invariant. There have been simulation studies on the structural transformation of homogeneous systems using von Mises strain [67][68][69][70]. Moreover, the von Mises stress was proven effective for identifying triggers of the so-called β-process in CuZr glass under thermal activation [71] and for understanding the origin of local shear relaxation in liquids [72].…”