Supersonic Screw Dislocations Gliding at the Shear Wave Speed

Shenyou Peng, Yujie Wei, Zhaohui Jin, and Wei Yang

Phys. Rev. Lett. 122, 045501 – Published 29 January 2019

Abstract

The motion of dislocations bridges the atomistic-scale deformation events with the macroscopic strength and ductility of crystalline metals. In particular, screw dislocations, whose Burgers vector is parallel to the line, play crucial roles on plastic flow. Nevertheless, their speed limit and its stress dependence remain controversial. Using large-scale molecular dynamics simulations, we reveal that full screw dislocations and twinning partial screw-type dislocations can glide steadily at the speed of shear wave velocity. Such a scenario is excluded in existing theories due to energy dissipation singularity. We conclude that both types of screw dislocations can move supersonically. We further observe that the motion of a screw dislocation also depends on the shear stress components, which do not contribute to the resolved shear stress (RSS), in contrast to the conventional Schmid’s law, which states that the motion of a dislocation is determined by the RSS.

Received 6 October 2018
Revised 15 November 2018

DOI:https://doi.org/10.1103/PhysRevLett.122.045501

Physics Subject Headings (PhySH)

Crystal defects Defects Disclinations & dislocations Plasticity

Crystalline systems

Molecular dynamics

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Shenyou Peng^1,4, Yujie Wei^1,4,*, Zhaohui Jin², and Wei Yang³

¹LNM, Institute of Mechanics, Chinese Academy of Sciences, Beijing 100190, China
²Light Alloy Net Forming National Engineering Research Center and School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
³Institute of Applied Mechanics and Center for X-Mechanics, Zhejiang University, Hangzhou 310027, China
⁴School of Engineering Sciences, University of Chinese Academy of Sciences, Beijing 100049, China

^*Corresponding author. yujie_wei@lnm.imech.ac.cn

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Issue

Vol. 122, Iss. 4 — 1 February 2019

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Images

Figure 1
Structures and stress fields of perfect screw dislocations and twinning screw partial dislocations. (a) Illustration of the generation of a SD dipole and the coordinates defined by the crystallographic orientations $x = [11 \bar{2}], y = [111], z = [1 \bar{1} 0]$ . (b),(c) Normalized displacement shift $ϕ$ between the gliding plane and the glided plane nearby the dislocation core (b) for the SD and (c) for the TSPD. Here $b_{e}$ and $b_{s}$ are the magnitude of the edge and screw part of the Burgers vector. The theory (solid lines) matches well with simulations (circles). (d)–(k) Stress fields of the full screw dislocation: $σ_{y z}$ from theoretical prediction (d) and simulation (f); $σ_{x z}$ from theoretical prediction (e) and simulation (g); Other four stress components $σ_{x x}, σ_{y y}, σ_{z z}, and σ_{x y}$ from simulation are shown in (h)-(k). (l) The respective stress fields $σ_{x x}, σ_{y y}, σ_{z z}, σ_{x y}, σ_{y z}, and σ_{x z}$ of the TSPD from theoretical prediction and (m) MD simulations. (Scale $bar = 15 nm$ .)
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Figure 2
Motion of screw dislocations under shear straining. (a) Travel distance vs strain (red) and velocity vs strain (blue) of the SD. (b) Motion of a TSPD. Here $v_{s 1}^{[110]} = 1.63$ , $v_{s}^{[111]} = 2.16$ , and $v_{s}^{[100]} = v_{s 2}^{[110]} = 2.92 km / s$ are three shear wave speeds in single crystal copper. (c) Stacking fault width $d$ (red) of the SD and the stress-strain relation (blue) as a function of straining, where $d_{0}$ is the stacking fault width at a stress-free state. (d) Snapshots of stacking fault width corresponding to points keyed in (c), and atoms are colored by the common neighbor analysis method.
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Figure 3
MD snapshots of the SD at different velocities. (a) Fast acceleration at $v = 0.8 km / s$ . (b) Slow acceleration at $v = 1.8 km / s$ . (c), (d) The first velocity jump across the shear wave speed $v_{s}^{[111]}$ : (c) trailing partial glides slower than leading partial, (d) trailing partial catches up with the leading partial. (e) Steady motion at $v = 2.36 km / s$ , which is faster than $v_{s}^{[111]}$ . (f) Supersonic motion with $v = 3.1 km / s$ , where a shock wave induced by the dislocation is clearly seen. The corresponding shear wave velocity of each Mach cone is marked. (Scale $bar = 8 nm$ .)
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Figure 4
A SD moves at the shear wave speed $v_{s}^{[100]}$ . (a) The blue line is the amplified velocity-strain curve from Fig. 2. Three red circles mark the velocities at which the dislocation could glide steadily. (b)–(d) Velocity fields $v$ induced by steadily moving dislocation at the three velocities keyed in (a), respectively. (Scale $bar = 8 nm$ .)
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Figure 5
Non-Schmid effect for a SD in FCC Cu. Shear stress $σ_{y z}$ contributes to the resolved shear stress $τ_{RSS}$ and dominates the dislocation motion according to Schmid’s law. One observes here that $σ_{x z}$ also bears a non-negligible effect on dislocation mobility even though it does not contribute to $τ_{RSS}$ .
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