Local structural excitations in model glass systems under applied load

The potential-energy landscape of a model binary Lennard-Jones structural glass is investigated as a function of applied external strain, in terms of how local structural excitations (LSEs) respond to the load. Using the activation relaxation technique and nudged elastic band methods, the evolving structure and barrier energy of such LSEs are studied in detail. For the case of a tensile/compressive strain, the LSE barrier energies generally decrease/increase, whereas under pure shear, it may either increase or decrease resulting in a broadening of the barrier energy distribution. It is found that how a particular LSE responds to an applied strain is strongly controlled by the LSE's far-field internal stress signature prior to loading.

Figure 1

(a) Normalized distributions of barrier energies obtained via $\mathrm{ART}n$ for zero load and the maximum considered shear strain of $\gamma =0.03$. (b) Barrier-energy evolution as a function of XY shear-stress magnitude $(\gamma \times$ appropriate shear modulus) for several individual LSEs derived via a combination of $\mathrm{ART}n$ and NEB.

Figure 2

(a) Normalized distribution of barrier energies obtained via $\mathrm{ART}n$ for zero load and an isotropic strain of $\pm 0.7\%$. The distribution for the zero-load sample is also included. (b) Barrier-energy evolution as a function of isotropic stress magnitude $(\varepsilon \times$ bulk modulus) for several individual LSEs derived via a combination of $\mathrm{ART}n$ and NEB.

Figure 3

(a) Mean of the barrier energies derived from the $\mathrm{ART}n$ data and shown in Figs. 1 and 2, as a function of their corresponding strain magnitudes. (b) Normalized distributions of the estimated activation volume due to a pure shear and isotropic loading geometries.

Figure 4

Minimum-energy path of two representative LSEs shown as the energy change in going from the initial configuration to the activated and final configurations. Values are shown for the zero-load, XY, YZ, and ZX shear-strain $\left(2\gamma =0.03\right)$ configurations.

Figure 5

Atomic displacement fields of two representative local structural excitations (LSEs): (a) LSE1 and (b) LSE2. Only the largest atomic displacements are shown. For each case, the atomic displacement vector between the initial and activated, and the activ-ated and final, configurations is shown, with the color of the arrows reflecting the loading mode. The colored balls visualize the posi-tion of the initial zero-load configuration. Both LSEs display the common structure of sequential movement of atoms, in which one atom moves approximately a nearest-neighbor distance to replace another atom—see Ref. [20]. Graphical visualization is done using ovito [36].

Figure 6

For the two considered LSEs, i.e., (a) LSE1 and (b) LSE2, a plot of the cumulative barrier energy vs atomic displacement constructed by adding the change in local energy of those atoms with an atomic displacement less than the horizontal axis value. The point colors represent the number of atoms which have contributed thus far to the cumulative barrier energy, and the line colors represent the shear-strain geometries—see the coloring scheme in Figs. 4 and 5.

Figure 7

(a) Scatterplot of change in internal stress between the zero-load initial and activated configurations vs barrier energy, and (b) scatterplot of change in internal shear stress between the zero-load initial and activated configurations vs the change in barrier energy as a result of an imposed XY shear strain.