[Paper Review] Dual migration modes of unfaulted disconnections on curved twin boundaries

Hongrui He, Hao Lyu+1 more|arXiv (Cornell University)|Mar 26, 2026

Microstructure and mechanical properties0 citations

TL;DR

The study shows that unfaulted disconnections at a curved Σ3 (111) coherent twin boundary in Al migrate via two distinct modes determined by disconnection core structure: pure-edge disconnections follow a thermally activated double-kink mechanism, while screw-containing disconnections exhibit a low-energy, stochastic, non-Arrhenius motif.

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.

Motivation & Objective

• Understand how disconnection core structure influences migration modes on curved twin boundaries in aluminum.
• Characterize energy barriers and temperature dependence for different disconnection types (edge, mixed, screw).
• Determine whether disconnections migrate collectively to drive grain boundary migration or remain as stable dipoles.
• Assess how energy density differences across the boundary drive or hinder disconnection motion.

Proposed method

• Perform MD simulations of a Σ3 (111) coherent twin boundary in an Al bicrystal using an EAM potential.
• Construct unfaulted disconnection dipoles with symmetric cores and classify them by Burgers circuit analysis.
• Compute energy barriers and migration paths with the Nudged Elastic Band (NEB) method.
• Apply the ECO method to impose controlled energy density differences across the boundary.
• Track disconnection separation and twin boundary position to extract migration velocities.
• Model kink-pair nucleation and migration using a thermally activated double-kink framework with relevant parameters.

Experimental results

Research questions

• RQ1 How does the disconnection core structure (edge vs screw components) control the migration mechanism of curved CTBs in Al?
• RQ2 What are the energy barriers for UFD1-type and UFD3-type disconnections and how do these barriers change with temperature?
• RQ3 Do UFD dipoles annihilate to produce single-layer GB migration, or do they remain as stable dipoles under elevated temperatures?
• RQ4 How does an energy density difference across the boundary drive disconnection motion and overall grain boundary migration?

Key findings

• UFD1-type disconnections migrate via a thermally activated double-kink mechanism with an energy barrier of about 0.65 eV at 600 K.
• UFD1-type disconnection velocity and GB migration rate increase monotonically with temperature due to kink-pair nucleation enhanced by entropy.
• UFD2-type disconnections are immobile across the investigated temperature range due to locally zero net Burgers vector and low dipole energy.
• UFD3-type disconnections, containing a screw dipole component, have a much lower NEB barrier (~0.08 eV) and exhibit stochastic, bidirectional motion with no systematic temperature dependence.
• The core transformation in UFD3-type disconnections enables reversible migration steps, leading to reduced net migration velocity compared to UFD1.
• Energy density differences across the boundary influence migration direction and barriers, with the disconnections tending to annihilate at boundary sides rather than center under given conditions.