Void-induced cross slip of screw dislocations in fcc copper

Pinning interaction between a screw dislocation and a void in fcc copper is investigated by means of molecular dynamics simulation. A screw dislocation bows out to undergo depinning on the original glide plane at low temperatures, where the behavior of the depinning stress is consistent with that obtained by a continuum model. If the temperature is higher than $300\mathrm{K}$, the motion of a screw dislocation is no longer restricted to a single glide plane due to cross slip on the void surface. Several depinning mechanisms that involve multiple glide planes are found. In particular, a depinning mechanism that produces an intrinsic prismatic loop is found. We show that these complex depinning mechanisms significantly increase the depinning stress.

Figure 1

(Color online) (a) Schematic of the initial atomistic configuration for a screw dislocation. The solid lines represent rows of atoms that belong to the region $z>0$, while the dashed lines represent those in $z<0$. The vertical dot-dashed line in the middle denotes a screw dislocation. (b) Schematic of a boundary condition with respect to the $x$ direction. Adjacent cells with respect to the $x$ direction are displaced by $b∕2$ along the $y$ direction.

Figure 2

(Color online) An initial configuration of the computational system where the void radius is $2\mathrm{nm}$ here. Initial distance between the void surface and the dislocation is $3\mathrm{nm}$. Atoms are colored according to the number of the nearest neighbors (Ref. 20): blue, 13; red, 11; green, 10; and pink, 9. Atoms that have 12 nearest neighbors, which constitute a perfect crystal, are omitted in order to visualize defects.

Figure 3

(Color online) The stress-strain relations in pinning processes for several voids at $10\mathrm{K}$. The red solid line, the green dashed line, the blue dot-dashed line, and the pink dotted line represent those for $r=0.5$, 1, 2, and $3\mathrm{nm}$, respectively.

Figure 4

Void radius (Å) dependence of the depinning stress $\sigma$ (MPa). Different symbols denote the depinning stresses at different temperatures: The circles, the squares, the triangles, and the diamonds represent those at 10, 150, 300, and $450\mathrm{K}$, respectively. The solid line is Eq. (2) with $B=0.45\mathrm{nm}$. At temperatures higher than $300\mathrm{K}$, the depinning stress does not obey Eq. (2) because the underlying mechanism changes. The blank triangles at $r⩾2\mathrm{nm}$ correspond to a depinning mechanism that involves double cross slip, while the solid symbols correspond to a mechanism that yields an intrinsic prismatic loop (see Sec. 4).

Figure 5

(Color online) (a) The [111] projection of a void and a dislocation. Atoms that constitute a perfect crystal are omitted, while those that constitute a dislocation and the stacking fault are visualized by the centrosymmetric parameter. A dislocation undergoes constriction on the void surface, as is indicated by the circle. Two vectors, $b_1$ and $b_2$, denote the Burgers vectors of two Shockley partials: $b_1=\frac{a}{6}\left[2\overline{1}\overline{1}\right]$ and $b_2=\frac{a}{6}\left[1\overline{2}1\right]$. (b) A schematic figure of constriction on the surface. The vector $b$ denotes the Burgers vector of a perfect screw dislocation, $\frac{a}{2}\left[1\overline{1}0\right]$.

Figure 6

(Color online) Cross slip occurs on the left side of a pinned dislocation via screwlike constriction. On the right side, constriction is edgelike. The visualization method is the same as that in Fig. 5a. (a) The [111] projection. The (red) thick arrows represent the Burgers vectors of partial dislocations. (b) The $\left[11\overline{2}\right]$ projection. The area surrounded by the (red) dashed line is subjected to cross slip, which is on the $\left(11\overline{1}\right)$ plane.

Figure 7

(Color online) A pinned dislocation undergoes double cross slip. Edgelike constriction does not take place. (a) The [111] projection of the atomistic configuration where partial double cross slip occurs on the left side via screwlike constriction. The (red) dashed line indicates a region that undergoes double cross slip. (b) The $\left[11\overline{2}\right]$ projection of the same configuration as that in (a). The dot-dashed line represents the original glide plane. The superjog is on the $\left(11\overline{1}\right)$ plane. The visualization method is the same as those in the previous figures.

Figure 8

(Color online) A depinning mechanism that yields an intrinsic prismatic loop. The circles of red dot-dashed line indicate stacking fault ribbon on $\left(11\overline{1}\right)$ planes. (a) From the configuration shown in Fig. 7, screwlike constriction again occurs (indicated by the dotted circle). (b) Screwlike constriction leads to cross slip. The other segment is further bowing out, leaving a stacking fault ribbon on $\left(11\overline{1}\right)$, which is regarded as a superjog. (c) The segment on the left side eventually double cross slips onto the original glide plane, (111). The two segments recombine there to leave an intrinsic prismatic loop. (d) An intrinsic prismatic loop is left behind.