Swetha Pemma | Searcharxiv

arXiv2026

Role of defects in the thermodynamic stability of grain boundary phases at asymmetric tilt boundaries in copper

Grain boundaries can exist as different grain boundary phases (also called complexions) with individual atomic structures. The thermodynamics of these defect phases in high-angle grain boundaries were studied mostly with atomistic and phase field computer simulations, but almost exclusively for special, symmetric boundaries. Here, we use molecular dynamics simulations combined with structure search methods, as well as scanning transmission electron microscopy experiments to take a step towards understanding more general grain boundaries. Using the example of $\Sigma$37c $[11\overline{1}]$ tilt boundaries in Cu, we show how the grain boundary phase transition on a symmetric boundary plane is changed by the geometrically necessary defects introduced in inclined, asymmetric boundaries. We analyze the disconnections - which are dislocation-like line defects of grain boundaries - both in the simulations, as well as in experimental Cu and Al samples. A main finding is that defect energies can have a major influence on the stability of grain boundary phases, even at small inclinations. Furthermore, some defects are not able to effect large inclinations. At that point, defective asymmetric GB phases compete with grain boundaries faceting into the adjacent symmetric GB phases.

cond-mat.mtrl-sci↗

arXiv2024

Effect of the atomic structure of complexions on the active disconnection mode during shear-coupled grain boundary motion

The migration of grain boundaries leads to grain growth in polycrystals and is one mechanism of grain-boundary-mediated plasticity, especially in nanocrystalline metals. This migration is due to the movement of dislocation-like defects, called disconnections, which couple to externally applied shear stresses. While this has been studied in detail in recent years, the active disconnection mode was typically associated with specific macroscopic grain boundary parameters. We know, however, that varying microscopic degrees of freedom can lead to different atomic structures without changing the macroscopic parameters. These structures can transition into each other and are called complexions. Here, we investigate $[11\overline{1}]$ symmetric tilt boundaries in fcc metals, where two complexions -- dubbed domino and pearl -- were observed before. We compare these two complexions for two different misorientations: In $\Sigma19$b $[11\overline{1}]$ $(178)$ boundaries, both complexions exhibit the same disconnection mode. The critical stress for nucleation and propagation of disconnections is nevertheless different for domino and pearl. At low temperatures, the Peierls-like barrier for disconnection propagation dominates, while at higher temperatures the nucleation is the limiting factor. For $\Sigma$7 $[11\overline{1}]$ $(145)$ boundaries, we observed a larger difference. The domino and pearl complexions migrate in different directions under the same boundary conditions. While both migration directions are possible crystallographically, an analysis of the complexions' structural motifs and the disconnection core structures reveals that the choice of disconnection mode and therefore migration direction is directly due to the atomic structure of the grain boundary.

cond-mat.mtrl-sci↗

arXiv2023

Stable nanofacets in [111] tilt grain boundaries of face-centered cubic metals

Grain boundaries can dissociate into facets if that reduces their excess energy. This, however, introduces line defects at the facet junctions, which present a driving force to grow the facets in order to reduce the total number of junctions and thus the system's energy. Often, micrometer-sized facet lengths are observed and facet growth only arrests for kinetic reasons. So far, energetically stable, finite-sized facets have not been observed, even though theoretical stability conditions have already been proposed. Here, we show a case where nanometer-sized facets are indeed stable compared to longer facets in [111] tilt grain boundaries in Cu by atomistic simulation and transmission electron microscopy. The facet junctions lack a Burgers vector component, which is unusual, but which removes the main energy cost of facet junctions. Only attractive interactions via line forces remain, which result from a discontinuity of grain boundary excess stress at the junction. Atomistic simulations predict that the same phenomenon also occurs in at least Al and Ag.

cond-mat.mtrl-sci↗

arXiv2023

Effect of the atomic structure of complexions on the active disconnection mode during shear-coupled grain boundary motion

The migration of grain boundaries leads to grain growth in polycrystals and is one mechanism of grain-boundary-mediated plasticity, especially in nanocrystalline metals. This migration is due to the movement of dislocation-like defects, called disconnections, which couple to externally applied shear stresses. While this has been studied in detail in recent years, the active disconnection mode was typically associated with specific macroscopic grain boundary parameters. We know, however, that varying microscopic degrees of freedom can lead to different atomic structures without changing the macroscopic parameters. These structures can transition into each other and are called complexions. Here, we investigate $[11\overline{1}]$ symmetric tilt boundaries in fcc metals, where two complexions -- dubbed domino and pearl -- were observed before. We compare these two complexions for two different misorientations: In $\Sigma19$b $[11\overline{1}]$ $(178)$ boundaries, both complexions exhibit the same disconnection mode. The critical stress for nucleation and propagation of disconnections is nevertheless different for domino and pearl. At low temperatures, the Peierls-like barrier for disconnection propagation dominates, while at higher temperatures the nucleation is the limiting factor. For $\Sigma$7 $[11\overline{1}]$ $(145)$ boundaries, we observed a larger difference. The domino and pearl complexions migrate in different directions under the same boundary conditions. While both migration directions are possible crystallographically, an analysis of the complexions' structural motifs and the disconnection core structures reveals that the choice of disconnection mode and therefore migration direction is directly due to the atomic structure of the grain boundary.

cond-mat.mtrl-sci↗