Atomic-Level Processes of Shear Band Nucleation in Metallic Glasses

The ability to control the plastic deformation of amorphous metals is based on the capacity to influence the percolation of the shear transformation zones (STZs). Despite the recent research progress, the mechanism of STZ self-assembly has so far remained elusive. Here, we identify the structural perturbation generated by an STZ in the surrounding material and show how such a perturbation triggers the activation of the neighboring STZ. The mechanism is based on the autocatalytic generation of successive strong strain and rotation fields, leading to STZ percolation and, ultimately, to the formation of a shear band.

Figure 1

Structural variations in the simulated ${\mathrm{Cu}}_{64}{\mathrm{Zr}}_{36}$ metallic glass during the early stages of shear banding. (a) Overall evolution of the atomic von Mises strain built up within consecutive time frames of 40 ps. von Mises strain, displacement, and rotation angle (radians) in the areas corresponding to the dotted boxes in (a) for the time frames (b) $\mathrm{\Delta }{t}_1=64-24\mathrm{ps}$ and (c) $\mathrm{\Delta }{t}_2=100-60\mathrm{ps}$.

Figure 2

(a) Representative vortex described by the displacement vectors and (b) displacement vectors and corresponding angle of rotation (color code). (c) The vortex is centered on a high population of FI clusters and are surrounded by distorted clusters (not shown here; only FI clusters colored according to their angle of rotation are displayed in the figure). (d) Average population of FI clusters across the shear band and characteristic contour map showing the angle of rotation (radians).

Figure 3

Values along the shear band (SB) direction of the deviation from affine deformation, $D_{\min }^2$ (a), von Mises strain (b), volumetric strain (c), von Mises stress (unit, $\mathrm{atm}\times {\AA }^3$) (d), and rotation angle (e) for $\mathrm{\Delta }{t}_2=100-60\mathrm{ps}$, averaged across the band. The dashed blue line in (a) marks the decreasing background used to identify the STZs. (f) Variation of the eigenvectors of the strain tensor across the shear band for the STZ-vortex combination at the shear band tip. The position of the shear band tip is defined where the monotonically decreasing background [dashed blue line in (a)] reaches $D_{\min }^2=0$.

Figure 4

Schematic representation of the STZ percolation mechanism. At small loads (a), the STZ near the notch experiences some degree of distortion with a negligible effect on the neighboring matrix. The STZ is activated at a higher load (b), generating a strong antisymmetric stress field in the FI-rich adjacent material, which in turn causes a collective vortexlike motion. This motion already induces some first deformation in the neighboring STZ, and, once the stress exceeds the threshold value, the following STZ is activated (c). The repetition of this two-unit STZ-vortex mechanism leads to STZ percolation along a specific direction and, ultimately, to the formation and propagation of a shear band. The nonactivated STZs are represented by purple circles, whereas STZ distortions are depicted by orange ellipses. The black arrows indicate atomic motion.