Clarifying the definition of 'transonic' screw dislocations

TL;DR

This paper clarifies the definition of transonic screw dislocation motion, demonstrating through analysis and simulations that dislocations can move above the shear wave speed at very low temperatures, challenging classical predictions.

Contribution

It provides an analytic derivation of a critical velocity separating subsonic and supersonic dislocation regimes, and presents MD simulations confirming the possibility of supersonic dislocations at low temperatures.

Findings

Elastic energy diverges at a velocity greater than shear wave speed.

Stable screw dislocation motion occurs below the derived critical velocity.

Supersonic dislocations are possible at very low temperatures.

Abstract

A number of recent Molecular Dynamics (MD) simulations have demonstrated that screw dislocations in face centered cubic (fcc) metals can achieve stable steady state motion above the lowest shear wave speed ($v_\text{shear}$) which is parallel to their direction of motion (often referred to as transonic motion). This is in direct contrast to classical continuum analyses which predict a divergence in the elastic energy of the host material at a crystal geometry dependent `critical' velocity $v_\text{crit}$. Within this work, we first demonstrate through analytic analyses that the elastic energy of the host material diverges at a dislocation velocity ($v_\text{crit}$) which is greater than $v_\text{shear}$, i.e. $v_\text{crit} > v_\text{shear}$. We argue that it is this latter derived velocity ($v_\text{crit}$) which separates `subsonic' and `supersonic' regimes of dislocation motion in the analytic solution. In addition to our analyses, we also present a comprehensive suite of MD simulation results of steady state screw dislocation motion for a range of stresses and several cubic metals at both cryogenic and room temperatures. At room temperature, both our independent MD simulations and the earlier works find stable screw dislocation motion only below our derived $v_\text{crit}$. Nonetheless, in real-world polycrystalline materials $v_\text{crit}$ cannot be interpreted as a hard limit for subsonic dislocation motion. In fact, at very low temperatures our MD simulations of Cu at 10 Kelvin confirm a recent claim in the literature that true `supersonic' screw dislocations with dislocation velocities $v>v_\text{crit}$ are possible at very low temperatures.