Discovery of multimechanisms of screw dislocation interaction in bcc iron from open-ended saddle point searches

Dislocation motion and interactions determine mechanical properties in body-centered cubic (bcc) metallic materials. However, studying mechanisms for the screw dislocation interaction is fundamentally challenging since many underlying processes involve mesotimescales and atomistic resolution, currently inaccessible by either experimental techniques or continuum theoretical methods. In this paper, we develop a computational capability based on self-evolving atomistic kinetic Monte Carlo (SEAKMC) to sample the critical events and saddle point energies related to screw dislocations and their junctions. The method is first validated by calculating the stress dependence of Peierls barriers and formation energies of kink pairs and cross-slip kink pairs on a single screw dislocation in bcc iron. Then the method is applied to a binary junction of a pair of intersecting screw dislocations, the structure of which is crucial for low-temperature plastic deformation. We identify three important mechanisms: coplanar cross-slipping, jog-pinning, and a previously unknown unzipping mechanism during the evolution of the binary junction. The mechanisms are then further validated using classical molecular dynamics simulations. The computational capability developed in this paper provides an effective tool to evaluate screw dislocation related thermally activated events in complex stress conditions. The mechanisms discovered in this paper provide critical insights into temperature dependence of the anomalous slip, a breakdown of the Schmidt law, during the plastic deformation in bcc iron and can be generalized to other bcc metals.

Figure 1

Schematics of the saddle point search (SPS)/molecular dynamics (MD) setup on the screw dislocation junction. (a) Simulation setup of SPS and MD. (b) Theoretic behaviors of screw dislocation interaction in infinite-sized and finite-sized grains. (i) For a pure screw dislocation gliding through another screw dislocation with a different Burgers vector, both dislocations leave a jog parallel to their own Burgers vector. Unless the jog diffuses away or is annihilated at sinks, further gliding of the dislocation will be pinned by the jog, which could substantially increase the stress. (ii) Screw dislocation junction at finite-sized grain, with the left arm located one atomic plane higher than the right arm. This is made possible by the annihilation of cross-slip kinks at free surfaces. (c) Spontaneous formation of the screw dislocation junction. (i) Two separate dislocations at a distance before the formation of the junction. (ii) Extra cross-slip kinks formed as a result of the dislocation interaction and are annihilated at free surfaces, which induces (iii) the formation of a stable junction. The blue shades indicate the position of the active volume used in SPS.

Figure 2

Saddle point configurations and their activation energies of a short periodic screw dislocation. (a) Initial state, saddle point, and the final state configurations, wherein the dislocation takes SP6 and moves rightward. SP1–6 represent six possible different nearby saddle points, each leading to a different final state. The arrows show the differential displacement (DD) fields of the dislocation, and the colored shade indicates the distribution of local screw components of the Nye tensor [32, 34]. (b) The effects of resolved shear stress on the energy barriers for different saddle points. The external applied resolved stress ranges from 0 to 1000 MPa, which drives the dislocation toward SP6. (c) DD map and differences in DD (DDD) maps between the six saddle points and the initial configuration.

Figure 3

(a) Schematics for the kink-pair mechanism. Only atoms around the dislocation core are shown. Atoms around the dislocation segment that do not move are colored green. Atoms around the dislocation segment that has moved forward by one lattice spacing are colored blue. Atoms connecting these segments (i.e., around the kinks) are colored orange. The shades beneath the saddle point configuration show the atomistic displacements relative to the initial configuration. The dislocation configuration at the saddle point is shown in the expanded figure below. (b) Atomistic configurations of six saddle points, corresponding to the kink-pair formation from the initial configuration toward six different $\langle 112\rangle$ directions. (c) The energies of the six saddle points as a function of applied stress.

Figure 4

Schematics on the formation of screw dislocation binary junction and possible scenarios of evolution under stress. (a) Dislocation configurations of binary junctions and those before the formation of the junction. (b) Coplanar cross-slip (CPCS) when both arms cross-slip downward. (c) Jog pinning occurs when neither the left nor right arm cross-slips. (d) The unzipping scenario occurs when only the left arm cross-slips downward by one plane. Then after the gliding dislocation unzips the junction, a jog is left on the intersecting dislocation that connects the yellow and red segments.

Figure 5

(a) The structure of a dislocation junction, which is also the initial configuration for saddle point searches (SPSs). The gliding dislocation [$\mathbf{b}=\frac{1}{2}\langle 111\rangle$, slip plane $\left(1\overline{1}0\right)$], zipped with an intersecting dislocation ($\mathbf{b}=\frac{1}{2}\langle 1\overline{1}1\rangle$). A junction consists of four arms (left and right arms for the moving dislocation and upper and lower arms for the intersecting dislocation). The yellow/cyan arrows indicate the direction of kinks and cross-slip kinks formed at the left (green) and right (blue) arms. (b) Structures of saddle points correspond to kink or cross-slip kink formation at both left and right arms. (c) Stress effect on the activation energies of the kinks or cross-slip kinks on the left and right arms. (d) The initial configuration, saddle point, and final configuration of the coplanar cross-slip mechanism following the structure shown in (b)(i).

Figure 6

Saddle point search (SPS) performed on a screw dislocation junction with a preexisting kink on the right arm. The initial configuration is obtained following a right-arm kink event $\left(K^{\mathrm{right}}\right)$ in Fig. 5(ii). (a) Directions of kinks or cross-slip kinks formed on the left arm. Due to the existing kink, other kinks or cross-slip kinks become harder to nucleate on the right arm near the junction. (b) Structures of saddle points correspond to kink or cross-slip kink formation on the left arm. (c) Influence of stress on the formation energy of the kinks or cross-slip kinks on the left arm.

Figure 7

Molecular dynamics (MD) [molecular statics (MS)] simulation revealing the jog-pinning, coplanar cross-slipping (CPCS), and unzipping mechanisms. (a) In the MS simulation, the junction takes the stress-driven jog-pinning mechanism. (b) In the MD simulation at 200 K, the gliding dislocation only takes the CPCS mechanism. At 300 K, the gliding dislocation first takes CPCS and cross-slips for a short distance, then the unzipping mechanism occurs (the gliding dislocation passes the intersecting dislocation).