Dislocation Dynamic Properties

One of the sequential multiscale methodologies to bridge between the length and time scale, also known as the bottom-up approach, is to obtain the laws required for each scale, from a calculation on a smaller scale, that is to say, rules for mesoscopic calculations are derived from microscopic ones and the former are used to derive constitutive laws for continuum mechanics. In particular, information on dislocations’ mobility and energy barriers for dislocation processes, deduced from Molecular Dynamics simulations, can be used in Dislocation Dynamics calculations. Accordingly, one of the key problems in quantitative analysis of material properties is the description of the dynamics of dislocations and their interaction. While the experimental methods are limited in their capability to study dynamic properties of dislocations in detail, atomic calculations serve as a powerful tool to study a few dislocations at short time scales.

In the figure: A jogged edge dislocation under the application of a glide stress. Only the atoms in partial dislocations are shown.

Dislocation Glide

Constant-temperature constant-stress molecular dynamics simulations are employed to analyze the dislocation reaction to an applied gliding stress, with an eye to extract rules for the dislocation mobility and structure. Upon applying a glide shear stress to the computational cell, the dislocation accelerates to a stress-dependent terminal velocity, from which a temperature-dependent drag coefficient is extracted. Based upon these results, elastic-continuum models of dislocation mobility are constructed. These models are suitable for implementation in mesoscopic scale calculations.

High Strain-rate Deformation – Dislocation Properties

Plastic deformation at high strain rates occurs in systems such as shocked metals, high-velocity impact etc. The mechanisms that control the deformation remain unknown due to the difficulty to perform experimental observations during the deformation. Experimental study of high velocity dislocations in metals, under high strain deformations, is performed ex post facto, and only little is known about the mobility and the structure of high velocity dislocations while gliding. Additionally, at high strain rate plastic deformation there is a high and rapid increase by several orders of magnitude in the dislocation density, which the common sources in literature cannot explain. Consequently, one of the key questions is what is the generation mechanism under high-strain rate deformation conditions.
Using MD calculations we studied screw dislocations in fcc single crystals under high stresses. Under high shear stresses we identified variation in the core structure, which even spread into two slip planes. At very high stresses, partial dislocations are emitted rapidly and dynamically onto the secondary slip plane, i.e. high velocity dislocations may serve as a rapid source for dislocation nucleation. The dislocation emission results in stacking faults along the secondary plane, which shear the crystal in this direction. These calculation results are combined with elastic-continuum models, to analyze the induced stress field by the gliding dislocation.

In the figure: The stress field around a sessile and a gliding edge dislocation.

Cross Slip

In addition, we studied the cross-slip mechanism in Cu, by simulating the annihilation process of two parallel screw dislocations. Both narrow and wide dislocation dipoles were considered. Spontaneous annihilation via the cross-slip mechanism was observed for short separation distances between the dislocations. At large separation distances, cross-slip was stimulated by applying an external Escaig stress, which decreases the activation energy to cross-slip. From the time until cross-slip occurs, which was found to be dependent on the temperature, the activation energy for cross-slip was derived.