Clarifying the definition of ‘transonic’ screw dislocations

ABSTRACT

A number of recent Molecular Dynamics (MD) simulations have demonstrated that screw dislocations in face centred cubic (fcc) metals can achieve stable steady state motion above the lowest shear wave speed ($v_{\mathrm{s}\mathrm{h}\mathrm{e}\mathrm{a}\mathrm{r}}$) 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 $v_{\mathrm{c}\mathrm{r}\mathrm{i}\mathrm{t}}$. Within this work, we first demonstrate through analytic analyses that the elastic energy of the host material diverges at a dislocation velocity ($v_{\mathrm{c}\mathrm{r}\mathrm{i}\mathrm{t}}$) which is greater than $v_{\mathrm{s}\mathrm{h}\mathrm{e}\mathrm{a}\mathrm{r}}$, i.e. $v_{\mathrm{c}\mathrm{r}\mathrm{i}\mathrm{t}}>v_{\mathrm{s}\mathrm{h}\mathrm{e}\mathrm{a}\mathrm{r}}$. We argue that it is this latter derived velocity ($v_{\mathrm{c}\mathrm{r}\mathrm{i}\mathrm{t}}$) 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 $v_{\mathrm{c}\mathrm{r}\mathrm{i}\mathrm{t}}$. Nonetheless, in real-world polycrystalline materials $v_{\mathrm{c}\mathrm{r}\mathrm{i}\mathrm{t}}$ 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 $v>v_{\mathrm{c}\mathrm{r}\mathrm{i}\mathrm{t}}$ are possible at very low temperatures.

KEYWORDS:

• Dislocations in crystals
• transonic motion
• limiting velocity

Acknowledgments

The authors would like to thank R. G. Hoagland for related discussions.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Notes

1 We focus here on MD results that study the question whether supersonic dislocation motion can be achieved. For example, Ref. [64] studies screw dislocation motion in bcc α-Fe, but only below 1 km/s.

2 Ref. [65] also study subsonic dislocation motion in Al, putting their emphasis on dislocation character dependence, but do not comment on what happens beyond stresses of 1 GPa (the highest they study).

Additional information

Funding

This work was supported by the Institute for Material Science at Los Alamos National Laboratory. In particular, D.N.B. and J.C. acknowledge support by the IMS Rapid Response program.