Theory of Shear Banding in Metallic Glasses and Molecular Dynamics Calculations

Abstract

The aged-rejuvenation-glue-liquid (ARGL) shear band model has been proposed for metallic glasses (Acta Mater. 54 (2006) 4293), based on small-scale molecular dynamics simulations up to 20,000 atoms and thermomechanical analysis. The model predicts the existence of a critical lengthscale $10 nm, above which melting could occur in shear-alienated glass. Large-scale molecular dynamics simulations with up to 5 million atoms have directly verified this prediction. When the applied stress exceeds the glue traction (computed separately before in a shear cohesive zone, or an amorphous-amorphous ''generalized stacking fault energy'' calculation), we indeed observe maturation of the shear band embryo into bona fide shear crack, accompanied by melting. In contrast, when the applied stress is below the glue traction, the shear band embryo does not propagate, becomes diffuse, and eventually dies. Thus this all-important quantity, the glue traction which is a property of shearalienated glass, controls the macroscopic yield point of well-aged glass. We further suggest that the disruption of chemical short-range order (''chemical softening'') governs the glue traction microscopically. Catastrophic thermal softening occurs only after chemical alienation and softening in our simulation, after the shear band embryo has already run a critical length.

How to identify dislocations in molecular dynamics simulations?

“…This scheme works extremely well in practice [33] for directly visualizing microstructures, including point defects, dislocations, stacking faults, and their strain fields with color encoding.…”

“…The atomic local strain tensor coloring is originally proposed by Shimizu et al [33], which is different from the von Mises shear strain invariant coloring in which two configurations are required, one current and one reference configuration. It is also much more powerful and robust than the former because it does not rely on assumptions of prior high lattice symmetry.…”

“…For the purpose of quantifying plastic deformation and visualizing defect evolution at the atomic level, the atomic local shear strain [33,34] [32] easy to extend and able identify a wide range of coordination structures expensive complex system dislocation extraction algorithm (DXA) [48] advance and with Burgers vectors can be calculated automatically complex and need an ideal reference configuration arbitrary lattices For each neighbor j of atom i, their current separation vector is…”

Influence of microstructure on the cutting behaviour of silicon

“…Dislocation glide (in contrast to dislocation climb) is always associated with local shearing of atoms, which is why local shear strain is a useful measure to detect such activity. Therefore, to quantify the plastic deformation of the atoms, a measure proposed by Shimizu et al [38] in the form of atomic local shear strain was chosen. A sample snapshot from the simulation result of silicon nanowire (at it its ultimate tensile strength) is shown in figure 3.…”

Fast evaluation of local elastic constants and its application to nanosized structures

“…This could be achieved by averaging the atomic coordinates after a sufficiently long simulation. At each step, the atomic deformation gradient matrix α J of the α th atom can be obtained through [31,32] …”