Grain Boundary Dislocations Research Articles

Electric current-induced directional slip of dislocation and grain boundary ordering

The evolution of lattice dislocations in metals can be induced by electric current. However, the effect of electric current on the interaction between lattice dislocations and grain boundaries (GBs) remains unclear. Herein, we investigate the evolution of lattice dislocations and grain boundary dislocations (GBDs) induced by electric current in M50 steel via in-situ TEM characterization, density functional theory (DFT) calculations, and molecular dynamics (MD) simulations. Our findings reveal that electric current induces the directional slip of lattice dislocations towards GBs, while the reduction of GB contrast is attributed to the decomposition and reorganization of GBDs. DFT calculations demonstrate that electric current enhances the force on atoms in defect regions. These in-situ TEM observations are well reproduced by MD simulations incorporating electric current, which include a virtual temperature rise at defect regions to reflect the electric current force on defect atoms. Based on these observations, we propose a novel GB accommodation model under electric current. These results provide valuable insights into the mechanisms by which electric current tailors lattice dislocation behavior and enhances GBDs ordering in metallic materials.

Grand canonically optimized grain boundary phases in hexagonal close-packed titanium

Grain boundaries (GBs) profoundly influence the properties and performance of materials, emphasizing the importance of understanding the GB structure and phase behavior. As recent computational studies have demonstrated the existence of multiple GB phases associated with varying the atomic density at the interface, we introduce a validated, open-source GRand canonical Interface Predictor (GRIP) tool that automates high-throughput, grand canonical optimization of GB structures. While previous studies of GB phases have almost exclusively focused on cubic systems, we demonstrate the utility of GRIP in an application to hexagonal close-packed titanium. We perform a systematic high-throughput exploration of tilt GBs in titanium and discover previously unreported structures and phase transitions. In low-angle boundaries, we demonstrate a coupling between point defect absorption and the change in the GB dislocation network topology due to GB phase transformations, which has important implications for the accommodation of radiation-induced defects.

Dislocation penetration in basal-to-prismatic slip transfer in Mg: A fracture mechanics criterion

Understanding the transition mechanism from microscopic to macroscopic yielding is crucial for developing high-strength metallic materials. Basal-to-prismatic slip transfer at grain boundaries (GBs) of Mg alloys is known to trigger macroscopic yielding. Here, the penetration behavior of pileup dislocations at a simple basal–prismatic (BP) boundary was analyzed using molecular dynamics simulations. Edge dislocation penetration creates a step in the BP boundary at the intersection with the slip plane, widened via subsequent penetration of dislocations. Many GBs, including the BP boundary, have a regular GB dislocation network that corresponds to a coincidence site lattice. Dislocation penetrations must gradually destroy the initial GB dislocation network, requiring increasing driving force for subsequent dislocation penetrations; thus, slip transfer at the boundary occurs sporadically and stably via the penetration of individual lattice dislocations. As the width of the step created by dislocation penetration increases, a GB dislocation network can be formed in the step, which can stabilize the energy of the step with specific widths. When the energy of step decreases with increasing step width, a smaller driving force is required for the subsequent penetration of dislocations, and thus slip transfer occurs unstably via the penetration of several sequential lattice dislocations. Once the step has a low energy at a specific width, it strongly obstructs dislocation penetration. This formation and disruption of the GB dislocation network cause intermittent penetration of dislocations. Finally, dislocation penetration at the BP boundary is discussed based on a fracture mechanics approach, which well-reproduces the transition between penetration behaviors and the macroscopic yield stress.

A model for physical dislocation transmission through grain boundaries and its implementation in a discrete dislocation dynamics tool

The mechanical behavior of most metals in engineering applications is dominated by the grain size. Physics-based models of the interaction between dislocations and the grain boundary are important to correctly predict the plastic deformation behavior of polycrystalline materials. Dislocation-grain boundary interaction is complex and a challenge to model. We present a model for simulating the physical transmission of dislocations through grain boundaries within Discrete Dislocation Dynamics tools. The properties (glide plane, Burgers vector, initial length) of the transmitted dislocation are chosen based on geometric criteria as well as a maximization of the resolved shear stress of the transmitted dislocation. Additionally, stress and displacement transparency as well as the discontinuity are ensured via a grain boundary dislocation – a butterfly-like geometry in the general case – whose properties are selected to minimize the residual Burgers vector at the interface. This additional ‘grain boundary dislocation’ allows a direct comparison as well as a calibration of the model with experiments on the macroscale particularly for neighboring grains with a high dislocation density contrast. Two basic examples illustrate the model and an application to a 40-grain polycrystal demonstrates the scalability of the approach.

Study of the mechanism of the strength-ductility synergy of α-Ti at cryogenic temperature via experiment and atomistic simulation

Alpha titanium (α-Ti) is a promising material for making high-performance components for applications in aerospace, marine, energy and healthcare fields. The excellent strength-ductility synergy has been observed for α-Ti at cryogenic temperature. Twinning is generally considered a key mechanism of outstanding cryogenic ductility. The dislocation-grain boundaries (GBs) interaction and void nucleation usually play crucial roles in the plastic deformation of polycrystalline materials, but their effects on the cryogenic ductility of α-Ti are rarely considered. To eliminate this confusion and gain an in-depth insight into the mechanism of the cryogenic strength-ductility synergy of α-Ti, in this work, a series of characterization experiments and molecular dynamics (MD) simulations were designed and carried out. 1) From uniaxial tension tests of the coarse-grained α-Ti sheets at the temperature from 25 to -180 °C, the uniform elongation and post-necking elongation were increased by 92 % and 20 %, respectively. The material maintained a larger strain hardening rate within a greater range of strain at cryogenic temperature compared with room temperature. 2) Via microstructure and fractography observations and the analysis of slip and geometrically necessary dislocation (GND) activities, the uniform plastic deformation was mainly accomplished by prismatic slip, whether at room temperature or at cryogenic temperature. The significantly increased uniform elongation is mainly attributed to the more uniform distribution of GND pile-ups at cryogenic temperature. 3) The MD simulations revealed that cryogenic temperatures made the GBs present a stronger barrier effect on dislocation transmission compared with that at room temperature, contributing to the more uniform distribution of GNDs and lower densities of GND pile-ups. The GBs at cryogenic temperature show a greater ability to resist void nucleation due to the decreased accumulation rate of excess potential energy and increased energy required to void nucleation. The larger strains were thus required to increase the densities of GND pile-ups to induce large stress concentrations for driving void nucleation. This made the uniform elongation of α-Ti increase significantly at cryogenic temperature. This study revealed that the enhanced barrier effect of GBs on dislocation transmission and the improved ability of GBs to resist void nucleation are key mechanisms besides twinning governing the cryogenic strength-ductility synergy of α-Ti. The understanding developed in this work can be useful for the development of new high-performance materials and the precise forming of complex components with high quality.

Dislocation descriptors of low and high angle grain boundaries with convolutional neural networks

Multiscale modeling of plasticity in polycrystalline metals is a long-standing challenge in part because of the lack of an accepted grain boundary descriptor, which has hindered the bridging of scales between atomistic simulations and meso-scale discrete dislocation dynamics (DDD) models. While grain boundary dislocations (GBDs) for low angle grain boundaries can be ascertained by Burgers circuit analyses, the dislocation structures of high angle grain boundaries have remained elusive because of overlapping dislocation core fields. Here, we use convolutional neural networks (CNNs) to establish the locations of GBDs responsible for the misorientations of <001> symmetrical-tilt Cu grain boundaries, from the local atomistic stress fields modeled with molecular dynamics (MD) simulations. We achieve accurate CNN predictions of GBDs with sub-angstrom resolution across an extensive set of low- to high-tilt-angle grain boundaries through a training dataset, generated from superposing continuum-representations of the MD stress fields of single dislocations that consider the contributions of both the Volterra and dislocation core fields. The approach paves the way for dislocation representations of the atomistic grain boundary structures modeled by MD simulations or density functional theory (DFT) calculations in mesoscale DDD models to elucidate multiscale plasticity effects.

Mobility of screw dislocations absorbed in symmetric tilt grain boundaries in Al

ABSTRACT The strength and ductility of polycrystalline metals are influenced by the interactions between dislocations and grain boundaries, particularly when the grain size is small. One possible reaction is the absorption of dislocations into the grain boundary structure; however, the mobility of the resulting grain boundary dislocations (GBDs) remains largely unexplored. Thus, the objective of this work is to determine the mobility of GBDs arising from the absorption of lattice screw dislocations into Σ3{111} and Σ11{113} <110> symmetric tilt grain boundaries (STGBs) in Al. Atomistic simulations reveal a reduction in Peierls stress of ∼80% and phonon damping coefficient of ∼65% for GBDs in the {111}<110> STGB compared to lattice screw dislocations. This significant mobility increase is caused by the complete dissociation of the absorbed dislocation. The Peierls stress of GBDs in the {113}<110> STGB is increased to almost 8 times that of the lattice screw dislocation due to the smaller interplanar spacing between slip planes. This work provides a structural justification for the absorption reactions and demonstrates that the mobility of an absorbed dislocation is highly sensitive to the structure of the host grain boundary. Ultimately, mobility laws are provided which can be used to model GBD motion in mesoscale simulations.

The sink efficiency of symmetric tilt grain boundary under displacement cascade in zirconium

Molecular dynamic (MD) simulation was performed to investigate the sink efficiency of [12¯10]symmetric tilt grain boundaries (STGBs) to irradiation point defects in zirconium (Zr). A total of 15 STGBs with the rotation angle ranged from 0 to 90° were studied during displacement cascade. The results show that the sink efficiency to point defects (especially interstitials) of GB is stronger than that of twin boundary (TB), since the atoms at grain boundary dislocations (GBDs) are loosely arranged with high atomic volume to form channels to absorb interstitials. The higher GB energy indicates the lower coherency of GB including more complex GBD structure, thus the GB has a stronger absorption ability to interstitials. The GB migration induced by displacement cascade is mainly attributed to the cavity formed by the remaining vacancies as the interstitials are absorbed and trapped by GBDs. This work provides new insights of relationship between GB sink efficiency and GB characters, which are important for understanding the irradiation effects in Zr alloys.

Potential regulation of strength-ductility by nucleation and growth mechanisms of shear bands in heterogeneous laminates

Shear bands (SBs) expand rapidly in hard domains and are suppressed in soft domains, effectively improving the tensile plasticity of the material. Significant evolution of SBs during elastoplastic deformation exists in heterogeneous laminates (HLs). Molecular dynamics (MD) simulation results indicate that the activities of grain boundary (GB) and dislocation jointly mediate the nucleation and growth of SBs. SBs tend to nucleate near the interface, and then grow rapidly under the promotion of GB activities and dislocation slips, resulting in a rapid increase in plastic strain gradient. Ultimately, the density and morphology of SBs determine the magnitude of the plastic strain gradient. In structures with lower layer thickness, the band-like SBs are densely distributed, which achieves higher strain gradient strengthening capability. However, the interface serves as an accumulation site for geometrically necessary dislocations (GNDs), too low a layer thickness will induce overlapping of the interface-affected-zone (IAZ), thereby weakening the strain hardening efficiency. The simulations track the nucleation and growth of SBs at the atomic scale, and discover that the structure can better balance HL's strength and ductility when the layer thickness is three times the grain size of the soft domain.

Coupling of alloy chemistry, diffusion and structure by grain boundary engineering in Ni–Cr–Fe

The diffusion–microstructure correlations for grain boundaries (GBs) in the technologically-relevant Ni-based 602CA alloy are investigated. Prolonged annealing treatments up to 744 h create distinct GB complexions with specific segregation–precipitation–structure states. Globular M23C6-type carbides at straight GBs and plate-like carbides together with NiAl-enriched (γ′-type) particles at hackly GBs are found to co-exist. Moreover, an atomic-scale GB spinodal-like decomposition, especially at straight GBs, is observed. The co-existence of the two distinct states of general high-angle GBs, indicated by tracer diffusion experiments and verified by a detailed structure examination, is explained via state-of-the-art measurements of local elastic strains. In a course of annealing at 873 K, the relatively “fast” diffusivities are found to increase by a factor of 10 or more as a result of a coupled evolution of the GB plate-like precipitates and the irregular GB structures, whereas the relatively ”slow” diffusivites remained practically unchanged representing the contributions of straight interfaces with spherical precipitates. Thus, the diffusion properties of high-angle GBs evolve together with characteristic changes of GB complexions distinguished by a growth of carbide- and γ′-type precipitates and a concomitant generation of GB dislocation networks. The obtained results provide novel insights into grain boundary tailoring by utilizing structure – kinetics correlations involving segregation, precipitation and the evolution of interface defects.

The Effect of Deformation Temperature on the Yield Stress of Ultrafine-Grained Al-Cu-Zr Alloy Containing Grain Boundary Nanoprecipitates

A theoretical model is suggested that describes the effect of deformation temperature on the yield stress of an ultrafine-grained (UFG) Al-Cu-Zr alloy structured with severe plastic deformation. Within the model, nanoprecipitates (NPs) of Al2Cu act as sources of lattice dislocations in the presence of a number of extrinsic grain-boundary dislocations (EGBDs) near the NPs. It is shown that the number of EGBDs near the NPs decreases with a drop in the deformation temperature that increases the yield stress of the Al-Cu-Zr alloy. The proposed model is in good quantitative agreement with available experimental results.

Revealing the deformation mechanisms of 〈110〉 symmetric tilt grain boundaries in CoCrNi medium-entropy alloy

The synergy of multiple deformation mechanisms responsible for the excellent mechanical properties attracts an increasing interest in CoCrNi medium entropy alloy (MEA). In this study, we employed molecular dynamics simulations to investigate the chemical properties of grain boundaries (GBs) and their influence on deformation mechanisms in CrCoNi MEA, with particular emphasis on role of lattice distortion and short-range order (SRO) in dislocation plasticity. After aging, we found Ni element segregate at GB through short-range diffusion driven by a more negative segregation enthalpy. The degree of SRO within the GB region is significantly correlated with the Ni concentration, and it markedly influences the structure and local stress distribution of the GB. Compared to the lattice distortion, SRO can effectively suppress the GB dislocations nucleation and slip, thereby increasing yield strength of the material. During the plastic stage, although the deformation microstructure is intimately linked to the active slip systems and grain orientation, the presence of SRO, as well as the resultant rugged dislocation pathways and increased slip resistance lowers the propensity for stacking faults and phase transformation. The current findings can enhance a fundamental comprehension of diverse deformation mechanisms in CoCrNi MEA and suggest that the chemical characteristics of GB could serve as a pivotal approach for modifying the mechanical properties of MEAs.

On the co-nucleation of adjoining twin pairs at grain boundaries in hexagonal close-packed materials

This work addresses the formation of adjoining twin pairs (ATPs) at grain boundaries (GBs) in hexagonal close-packed (hcp) metals from the perspective of the co-nucleation (CN) of pairs of deformation twins. A continuum defect mechanics model is proposed to investigate the energetic feasibility of the CN of ATPs resulting from the dissociation of GB dislocations. Application of the model to a set of GBs in Mg reveals that CN is largely preferred over the nucleation of a single twin variant for low misorientation angle GBs, which is consistent with previous experimental analyses. The CN events are subsequently analyzed considering the GB character, and the twin system alignment (m′) and Schmid factors to obtain better insights into the conditions governing the formation of ATPs via CN. The likelihood of CN even at low m′ values reveals that CN events could be responsible for the formation of ATP pairs in these scenarios.

Strain-induced grain boundary migration and grain rotation in polycrystalline metals: Atomic-and meso-scale phase field simulations

The study reveals that in phase field (PF) simulations with over 300 grains, grain boundary (GB) migration exhibits anisotropic behavior and higher velocities under applied external loading. However, these simulations fail to provide detailed atomic-scale information about the evolution of grain orientation. Therefore, the researchers utilize phase field crystal (PFC) simulations to track the evolution of dislocation configurations and the annihilation of GB dislocations. The PFC simulations confirm a newly proposed plasticity process in nanocrystalline metals, where grain rotation is primarily dominated by dislocation climb and dislocation absorption at triple-junction points on GB, rather than GB sliding or diffusional creep as previously suggested. Furthermore, the PFC simulations demonstrate that the analysis of misorientation angles and GB dislocation evolution between neighboring grains can be quantitatively described using the Frank-Bilby equation. Thus, a hybrid approach of PF and PFC successfully describes in-situ transmission electron microscopy observations of GB migration and grain rotation for the first time. This multi-scale simulation method provides a deeper understanding of plasticity formation and development in nanostructured materials.

A Lie-algebra-based description of disclination densities and the quantification of partial disclinations in deformed polycrystalline metals

Crystalline defects (e.g., dislocations, disclinations and grain boundaries) play a critical role in determining the mechanical and functional properties of metallic materials. From a physical point of view, dislocations and disclinations are the two fundamental types of topological defects in crystals, which originate from the breaking of translational symmetry and rotational symmetry, respectively. In spite of extensive studies on dislocations in the literature, the effects of disclinations on mechanical properties of metals have not been well recognized, partially due to the lack of an appropriate description for the rotational nature of disclinations. Here we suggest a Lie-algebra-based method to quantify the rotational properties of disclinations, which leads to a convenient way to determine disclination density distribution directly from Electron Backscatter Diffraction data. Through quasi-in-situ Electron Backscatter Diffraction characterizations, three major formation mechanisms of disclinations have been identified in deformed polycrystalline Mg alloys, i.e., dislocation-grain boundary interaction, dislocation redistribution and twin-twin reaction, all of which can be treated as topological reactions among various types of defects under a general framework. Our work not only suggests a new mathematical tool to investigate the interactions and reactions among multiple types of crystalline defects, but also provides a new insight to understand the deformation behaviors of metals and alloys based on dislocation/disclination theory.

Role of Ag segregation on microscale strengthening and slip transmission in an asymmetric Σ5 copper grain boundary

Micropillar compression was used to investigate whether Ag segregation to an asymmetric Σ5[001] grain boundary will lead to measurable strength differences compared to the pure copper bicrystal. Ag segregation was accomplished by deposition and subsequent annealing of an Ag thin-film applied on the surface of the Cu bicrystal. Atom probe tomography analysis indicated Ag segregation at the grain boundary with a peak concentration of 2.3 at.%. While the pristine Σ5 grain boundary shows a yield strength of 288 ± 18 MPa when compressing 1 µm diameter pillars along 〈001〉, micropillars containing an Ag-segregated Σ5 grain boundary demonstrated an increased yield strength of 318 ± 17 MPa. In addition, post-deformation electron microscopy was carried out to examine the active slip systems and slip transmission across Ag-free and Ag-containing bicrystals. The results are compared to reference measurements of the adjacent single crystal grains. The 1 µm pillar diameter promoted deformation governed by dislocation-grain boundary interactions for the bicrystalline pillars. This is the first time that changes in flow stress associated with grain boundary segregation have been quantified locally without interference from other mechanisms such as solid solution strengthening, formation of precipitates or changes in stacking fault energy. The results clearly indicate that purely geometrical models for slip transmission are not sufficient as the local atomic structure and composition influence dislocation transmission through grain boundaries.

Microstructural Aspects of the Fabrication of Al/Al2O3 Composite by Friction Stir Processing.

The purpose of this work was the examination of microstructural evolution during the fabrication of an Al/Al2O3 composite by friction stir processing (FSP). In order to obtain new insight into this process, a longitudinal section of the produced composite was studied, and advanced characterization techniques (including electron backscatter diffraction and microhardness mapping) were applied. It was found that the reinforcing particles rapidly rearranged into the "onion-ring" structure, which was very stable against the subsequent dispersion. Specifically, the remnants of the comparatively coarse-scale particle agglomerations have survived even after 12 FSP passes. Therefore, it was concluded that three or four FSP passes, which are often applied in practice, are not sufficient to provide a homogeneous dispersion of the reinforcing particles. It was also revealed that the gradual distribution of the nanoscale Al2O3 particles throughout the aluminum matrix promoted a subtle reduction in both the portion of high-angle boundaries and the average grain size. These observations were attributed to the particle pinning of grain-boundary migration and dislocation slip.

Atomic-scale inhomogeneous solute distribution in an ultrahigh strength nanocrystalline Al-8 Mg aluminum alloy

Exploring the redistribution mechanism of Mg from the perspective of experimental observation has been a long-term yet challenging attempt in Al-Mg alloys. Here we demonstrate a simple but effective approach to obtain non-uniform Mg solute distribution (i.e., Mg-enriched/depletion zones) around typical grain boundaries (GBs) in a nanocrystalline Al-8 Mg alloy. This abnormal segregation was detected by both high-angle annular dark-field scanning transmission electron microscopy and atom probe tomography. The results show that local strain and GB migration during deformation leads to spatially inhomogeneous Mg solute distribution around the GBs. Both the non-uniform distribution and the broadened GBs can hinder GB migration and dislocation motion, thus enhance the strength. An inhomogeneous solute distribution mechanism of Mg atoms is proposed based on the extensive investigations. This study may help with developing new strengthening mechanisms of nanocrystalline materials.

ATOMIC MECHANISMS OF &lt;100&gt; AND &lt;111&gt; TILT BOUNDARY MIGRATION ON THE EXAMPLE OF NICKEL

Fundamental differences in the understanding of the mechanism and values of the energy of activation of migration form a request for new studies of this scientific problem through clearly certified grain boundaries. The molecular dynamics method was used to analyze the dynamics of the atomic mechanism of migration of small-angle boundaries &lt;100&gt; and &lt;111&gt;, which showed that paired grain-boundary dislocations split during the boundary movement with the change of partner dislocations. The migration of small-angle slope boundaries &lt;100&gt; is realized by splitting and changing partner dislocations, as a result of the operation of this mechanism of displacement of atoms, a grid with square cells is formed. In the case of border migration &lt;111&gt;, there is also a mechanism of joint sliding of paired grain-boundary dislocations. Paired dislocations of boundaries &lt;111&gt;, unlike grain-boundary dislocations of boundaries &lt;100&gt;, have common sliding planes along which they can slide with a relatively low activation energy. During the migration of borders &lt;111&gt;, the combined action of both mechanisms was recorded: the joint sliding of paired grain-boundary dislocations and their splitting with the change of partner dislocations. In the process of migration, symmetrical sections are formed in the grain where the border was moving, which, by turning, "adjust" to the structure of another grain. Therefore, when migrating boundaries &lt;111&gt;, the cells of the atomic displacement grid have a hexagonal shape.

Direct Visualization of Localized Vibrations at Complex Grain Boundaries (Adv. Mater. 13/2023)

SrTiO3 Grain Boundaries Using an electron microscope with ultrahigh-energy-resolution enables direct imaging of local structure, nonstoichiometry, and chemical bonding, leading to a redistribution of vibrational states within the grain-boundary dislocation cores, as reported by by Eric R. Hoglund, Sokrates T. Pantelides, Patrick E. Hopkins, Jordan A. Hachtel, and co-workers in article number 2208920. Density-functional-theory calculations of thousand-atom supercells show that the unique environment of atoms in the dislocation cores results in new vibrational modes that are highly localized and have energies in the Restrahlen band.