Origin of Anomalous Slip in Tungsten

Low-temperature deformation of body-centered cubic metals shows a significant amount of plastic slip on planes with low shear stresses, a phenomenon called anomalous slip. Despite progress in atomistic modeling of the consequences of complex stress states on dislocation mobility, the phenomenon of anomalous slip remained elusive. Using in situ Laue microdiffraction and discrete dislocation dynamics in micrometer sized tungsten single crystals, we demonstrate the occurrence of significant anomalous slip. It occurs as a consequence of cross kinks, topological configurations generated by prior dislocation interactions. This clearly identifies anomalous slip as a multidislocation process and not a property of isolated dislocations. The cross-kink mechanism also explains the ambiguous reporting of anomalous slip traces in the past and directs us to ways of including anomalous slip in continuum crystal plasticity formulations.

Figure 1

In situ microcompression results. (a) Mechanical results of tungsten pillars 1 and 2. The inset represents the compression axis plotted in a stereographic projection with the slip systems of interest marked with a color code. The primary ($-101$), conjugate (101), and anomalous slip ($0-11$) planes are indicated in purple, blue, and orange, respectively, the corresponding slip directions [111] and $[-111]$ in bold black. (b) Two-dimensional representation of the ($-10-1$) diffraction spot plotted together with the six possible $\left\{110\right\}$ rotation directions (black lines) of pillar 2, at five different stages of the load indicated on the stress-strain curve. (c) Path of the ($20-2$) diffraction spot of pillar 1 with the six possible $\left\{110\right\}$ rotation lines plotted in white at different stages of the load indicated in (a): “1” for initial and “105” for final. (d) Slip traces on the surface of pillar 2 together with the indication of the corresponding planes. See text for more details.

Figure 2

The $y$ component of the total plastic displacement superimposed onto the deformed pillar after total plastic strain of 0.8% in units of the lattice constant. The deformations of the mesh are amplified by a factor of 15.

Figure 3

Mechanism of anomalous slip. Snapshots [(a)–(d)] of small sections of our sample showing the generation of anomalous slip due to $[-111]$ dislocations on the ($0-11$) plane. (a) scenario where the segments $AB$ and $BC$ form a cross kink. $AB$ lies on the ($0-11$) plane, $BC$ on the (101) plane, and the remaining kink complex $M$ lies on different planes of the zone of the $[-111]$ Burgers vector. The free surface (FS) of the pillar is indicated in gray color. The glide planes are indicated in orange (anomalous) and blue (conjugate). Dislocation lines are colored according to their habit planes. Node $A$ is attached to a screw dislocation while node $B$ is connected to a section of several small mixed segments lying over several planes. The resultant direction of motion of the entire complex is indicated with the arrow $M$. In (b) node $A$ reaches the surface and is released from the screw dislocation. The dislocation section $AB$ can rapidly increase in length, generating slip on the anomalous plane shown in (c). (d) an overlay of the dislocation structure at four different time steps. The positions of the nodes $A$ and $B$ at the different time steps are marked as $A_1$ to $A_4$ and $B_1$ to $B_4$, respectively. The mixed segment $AB$ glides back in the direction of the orange arrow. Node $A$ moves along the intersection line of the anomalous plane II with the surface and $B$ moves along the screw direction given by the intersection of planes I and II. The segment $DE$ is screw oriented and lies on plane III (which is parallel to II). Segment $BC$ moves on plane I and $CD$ represents schematically the kink complex attached to $BC$ and moves noncrystallographically. The area swept by segment $AB$ on the anomalous plane II between time steps 1 to 4 is shown in orange.