Surface Rebound of Relativistic Dislocations Directly and Efficiently Initiates Deformation Twinning

Under ultrahigh stresses (e.g., under high strain rates or in small-volume metals) deformation twinning (DT) initiates on a very short time scale, indicating strong spatial-temporal correlations in dislocation dynamics. Using atomistic simulations, here we demonstrate that surface rebound of relativistic dislocations directly and efficiently triggers DT under a wide range of laboratory experimental conditions. Because of its stronger temporal correlation, surface rebound sustained relay of partial dislocations is shown to be dominant over the conventional mechanism of thermally activated nucleation of twinning dislocations.

Figure 1

DT initiation in a [100] oriented square Cu nanowire via surface rebound process. The starting time for DT initiation is 53.858 ns (i.e., $+0.0\mathrm{ps}$). Red lines are partial dislocation cores, green planes are stacking faults or twin boundaries and short arrows represent directions of dislocation motion.

Figure 2

Trajectory of a partial dislocation being accelerated in a copper slab subjected to a shear stress $\tau \sim 1.55\mathrm{GPa}$ at 2 K. The dislocation line is colored corresponding to its instantaneous speed. The crystals directions are $[\overline{1}10]$, $[11\overline{2}]$, and $[111]$ along the dislocation motion direction, the tangent of dislocation front and the slip plane normal.

Figure 3

Kinetic energy of the dislocation core induces free surface dislocation rebound. (a) The partial dislocation core (white atoms) identified by common neighbor analysis (CNA). (b) The core region (the yellow box) used in evaluating the kinetic energy. The color bar indicates the atomic kinetic energy range from 0 (blue) to 0.056 eV (red). (c) The saddle configuration of a twinning dislocation. The imposed shear stress $\tau$ in (a)–(c) is 1.6 GPa. (d) The kinetic energy of the dislocation core and the activation energy of twinning dislocation nucleation. (e)–(f) The rebounded dislocation loops of a $\sim 70\mathrm{nm}$ long incident partial dislocation under the critical ${\tau }_{\text{reb}}=1.4\mathrm{GPa}$ at an initial temperature 2 K. The snapshots are taken at 1.3 ps after the impact. (a) and (b) share the same coordinate system: $X[1\overline{1}0]$, $Y[\overline{1}\overline{1}\overline{1}]$, and $Z[\overline{1}\overline{1}2]$. Coordinate systems in (c),(e), and (f) are $X[11\overline{2}]$, $Y[\overline{1}10]$, $Z[\overline{1}\overline{1}\overline{1}]$; $X[1\overline{1}0]$, $Y[\overline{1}\overline{1}2]$, $Z[111]$; and $X[\overline{1}10]$, $Y[\overline{1}\overline{1}2]$, $Z[\overline{1}\overline{1}\overline{1}]$, respectively. Atoms in (c), (e), and (f) are colored according to CNA. Red atoms represent stacking faults or twin boundaries, atoms on dislocation cores or free surfaces are white, and perfect FCC atoms are black.

Figure 4

Determination on the dominant mechanism underlying the strongly correlated DT initiation. (a) Delay time (or the temporal correlation) between successive dislocations. (b) The nucleation axial stress of the first dislocation via TAN at 300 K for normally accessible strain rates (as marked in the figure, ${10}^{-3}{\mathrm{s}}^{-1}$ is typical for the strain rates used in laboratories, and ${10}^8{\mathrm{s}}^{-1}$ is often the strain rate applied in MD simulations), the predicted nucleation axial stress falls in the range of [1.5 GPa, 2.75 GPa] within which $t_{\mathrm{SRS}}\le t_{\mathrm{TAN}}$.