Dual migration modes of unfaulted disconnections on curved twin boundaries

Abstract

Grain boundary migration governs microstructural evolution in crystalline materials, directly influencing mechanical properties such as strength and thermal stability. Disconnections, which are line defects formed at grain boundaries in response to local curvature, have been identified as critical carriers of boundary migration. Here, we investigate the glide of unfaulted disconnections (UFDs) on a coherent twin boundary in aluminum at elevated temperatures using molecular dynamics simulations combined with the Nudged Elastic Band (NEB) method. Our results reveal a striking bifurcation in migration behavior depending on the disconnection core structure. UFDs with a pure edge Burgers vector migrate via a thermally activated double-kink mechanism, exhibiting a migration velocity that increases monotonically with temperature. In contrast, UFDs containing a screw dipole component possess an energy barrier approximately eight times lower, and their core structure undergoes a continuous transformation during glide, giving rise to stochastic, bidirectional motion with no systematic temperature dependence. These findings demonstrate that the disconnection core structure fundamentally dictates the migration mode and kinetics of twin boundaries, offering new mechanistic insights into disconnection-mediated grain boundary migration.

Figures (16)

• Figure 1: Atomistic structures of the $\Sigma 3\,(111)$ coherent twin boundary and associated symmetric unfaulted disconnection (UFD) configurations. (a--c) Atomistic configurations of the twin boundary containing (a) UFD1, (b) UFD2, and (c) UFD3 disconnections. Atoms are colored according to the structure (green:FCC; red:HCP; white:Other) (d) Schematic illustration of the coordinate systems and the projection of Thompson’s tetrahedra for the upper ($\lambda$) and lower ($\mu$) grains. (e--g) Burgers circuit analyses hirth_steps_1996 of the disconnection cores for (e) UFD1, (f) UFD2, and (g) UFD3. (h--j) Schematic representations of the step heights and core structures corresponding to UFD1, UFD2, and UFD3, respectively. Defects are identified using adaptive common neighbor analysis (a-CNA) stukowskiAutomatedIdentificationIndexing2012: stacking faults are shown in red, and other defects are shown in gray. Atoms are colored by their depth along the viewing direction, with brighter and darker shades indicating shallower and deeper atomic positions, respectively.
• Figure 2: Comparison between equivalent dislocation stress fields calculated from continuum elasticity solutions and atomistic stress fields obtained from simulations for (a) UFD1-, (b) UFD2-, and (c) UFD3-type disconnections.
• Figure 3: (a) Temperature dependence of the mean disconnection velocities ($\Delta x/$Time) for the UFD1, UFD2, and UFD3 configurations, calculated as the relative approach speed of the two disconnections along the grain boundary plane. (b) Corresponding mean grain boundary migration rate ($\Delta \bar{y}/$Time) as a function of temperature, determined from the step height of the disconnections divided by the closing time. (c,d) Time evolution of the grain boundary displacement at 600 K for the (c) UFD1 and (d) UFD3 configurations, obtained from five independent simulations with different random velocity seeds. The trajectories highlight the distinct migration behaviors and the tendency of the grain boundary to evolve toward a flattened configuration. Reference lines denote the initial twin boundary position (dash-dotted) and the final twin boundary position after disconnection annihilation (dashed)
• Figure 4: NEB calculations for UFD1-type grain boundary (GB) migration. (a) Minimum energy path (MEP) for UFD1-type GB migration over a distance of $\tfrac{1}{2}[112]$ at 600 K. (b) Top-view atomistic configurations of the UFD1-type disconnection along the MEP. (c) Displacement field of atoms along the $z$ direction, from $\mathbf{A}_1$ to $\mathbf{I}_1$.(d) Per-atom potential energy of the double-kink configuration, from $\mathbf{A}_1$ to $\mathbf{E}_1$
• Figure 5: Temperature dependence of UFD1-type disconnection velocity for two separation regimes: far-field ($\Delta x>100\mathring{A}$, blue) and near-field ($\Delta x<80\mathring{A}$, red). Symbols with error bars represent the mean velocities averaged over five independent simulations at 600 K and 700 K, while solid curves correspond to the predictions of the double-kink nucleation model. Shaded regions indicate the uncertainty in theoretical predictions, reflecting the variation of interaction stress over the dipole separation ranges in the far-field and near-field regimes