Types Of Grain Boundaries Research Articles

Direct Measurement of Electronic Band Structures at Oxide Grain Boundaries.

Grain boundaries (GBs) modulate the macroscopic properties in polycrystalline materials because they have different atomic and electronic structures from the bulk. Despite the progress on the understanding of GB atomic structures, knowledge of the localized electronic band structures is still lacking. Here, we experimentally characterized the atomic structures and the band gaps of four typical GBs in α-Al2O3 by scanning transmission electron microscopy and valence electron energy-loss spectroscopy (EELS). It was found that the band gaps of the GBs are narrowed by 0.5-2.1 eV compared with that of 8.8 eV in the bulk. By combing core-loss EELS with first-principles calculations, we elucidated that the band gap reductions directly correlate with the decrease of the coordination numbers of Al and O ions at the GBs. These results provide in-depth understanding between the local atomic and electronic band structures for GBs and demonstrate a novel electronic-structure analysis for crystalline defects.

Hydrogen sorption capacity of crystal lattice defects and low Miller index surfaces of copper

The effect of hydrogen on the physical–chemical properties of copper is directly dependent on the types of chemical bonding between H and lattice defects in Cu. In this work, we performed a systematic study of the bonding of H-atoms with crystal lattice defects of copper. This included three types of symmetric tilt grain boundaries (GBs), Σ3, Σ5 and Σ11, and the low Miller index surfaces, (111), (110) and (100). A comparison with literature data for the bonding of H-atoms with point defects such as vacancies was done. From the defects investigated and analyzed, we conclude that the bond strength with H-atoms varies in the decreasing order: surfaces [(111), (110) and (100)] > vacancy > Σ5 GB > Σ11 GB > bulk ≈ Σ3 GB. A study on the effects of the fcc lattice expansion on the binding energies of H-atoms shows that the main driving force behind the segregation of H-atoms at some GBs is the larger volume at those interstitial GB sites when compared to the interstitial bulk sites.

Study of grain boundary self-diffusion in iron with different atomistic models

We studied grain boundary (GB) self-diffusion in body-centered cubic iron using ab initio calculations and molecular dynamics simulations with various interatomic potentials. A combination of different models allowed us to determine the principal characteristics of self-diffusion along different types of GBs. In particular, we found that atomic self-diffusion in symmetric tilt GBs is mostly driven by self-interstitial atoms. In contrast, in general GBs atoms diffuse predominantly via an exchange mechanism that does not involve a particular defect but is similar to diffusion in a liquid. Most observed mechanisms lead to a significant enhancement of self-diffusion along GBs as compared to diffusion in the bulk. The results of simulations are verified by comparison with available experimental data.

Carbide effects on tensile deformation behavior of [001] symmetric tilt grain boundaries in bcc Fe

The mechanical performance of carbides on the grain boundary (GB) of ferritic steels is always concerned. In this paper, four symmetric tilt GBs in body-centered cubic Fe were studied under tensile tests at 0 and 300 K through atomistic simulations. The GBs contained various sized Cr23C6 precipitates. Stress–strain curves were obtained through the molecular dynamics simulations and the tensile strength varied with GB type. Centro-symmetry parameter, common neighbor and dislocation analyses were performed for defect characterization. GBs with precipitates showed lower tensile strength than pure GBs. As the number and size of carbides increased, the tensile strength decreased. Cavities were easily formed near dislocation loops with a Burgers vector parallel with loading direction during tensile deformation of pure GBs, while ruptures occurred around the precipitate in GBs with carbides. The simulations indicate that Cr23C6 carbides in Fe are a brittle phase where the dislocations and cracks nucleate.

Failure in Two-Dimensional Materials: Defect Sensitivity and Failure Criteria

Abstract Two-dimensional (2D) materials have attracted a great deal of attention recently owing to their fascinating structural, mechanical, and electronic properties. The failure phenomena in 2D materials can be diverse and manifested in different forms due to the presence of defects. Here, we review the structural features of seven types of defects, including vacancies, dislocations, Stone-Wales (S-W) defects, chemical functionalization, grain boundary, holes, and cracks in 2D materials, as well as their diverse mechanical failure mechanisms. It is shown that in general, the failure behaviors of 2D materials are highly sensitive to the presence of defects, and their size, shape, and orientation also matter. It is also shown that the failure behaviors originated from these defects can be captured by the maximum bond-stretching criterion, where structural mechanics is suitable to describe the deformation and failure of 2D materials. While for a well-established crack, fracture mechanics-based failure criteria are still valid. It is expected that these findings may also hold for other nanomaterials. This overview presents a useful reference for the defect manipulation and design of 2D materials toward engineering applications.

Phase field modelling allotropic transformation of solid solution

Based on multiphase field conception and integrated with the idea of vectorvalued phase field, a phase field model for typical allotropic transformation of solid solution is proposed. The model takes the non-uniform distribution of grain boundaries of parent phase and crystal orientation into account in proper way, as being illustrated by the simulation of austenite to ferrite transformation in low carbon steel. It is found that the misorientation dependent grain boundary mobility shows strong influence on the formation of ferrite morphology comparing with the weak effect exerted by misorientation dependent grain boundary energy. The evolution of various types of grain boundaries are quantitatively characterized in terms of its respective grain boundary energy dissipation. The simulated ferrite fraction agrees well with the expectation from phase diagram, which verifies this model. (Less)

Effects of grain boundaries on the radiation-induced defects evolution in BCC Fe–Cr alloy: A molecular dynamics study

Abstract Molecular dynamics (MD) method was conducted on four types of symmetric tilt grain boundaries (GBs) in body-centered cubic (BCC) Fe–9Cr alloy. The formation energy and segregation energy of different types of defects (interstitial and vacancy) was firstly calculated in each system, and the GB energy and width were also calculated. It is found that the Fe interstitials have higher segregation energy than Cr interstitials for all GB systems, which can explain energetic drive of the defect behavior in GB systems. By analyzing the defect production in GB systems and comparing them with those in single crystal, it was found that more interstitials than vacancies disappeared due to the existence of GB. But the influence of GB varies between GB types. The migration of GB was only found in ∑25(430) possibly because of the temperature gradient during the cascade evolution.

Representative and statistical volume elements for grain boundary networks: A stereological approach

Experimental and computational studies have demonstrated that the structure of grain boundary networks (GBNs), e.g. the connectivity of certain types of grain boundaries (GBs), has a significant impact on macroscopic material properties. Because minor changes in GBN structure can cause large changes in effective properties, it is common to use large microstructures or report averages over many microstructural instantiations with the assumption that results become more representative as the size or number of samples increases. For crystallographic texture, and composite materials, such ideas have been made rigorous through the definition of representative volume elements (RVEs) and statistical volume elements (SVEs). However, for GBNs, the size of an RVE and the cardinality of a set of SVEs have not yet been determined. In this study, we employ stereological methods to evaluate the convergence of triple junction fractions in both idealized and realistic 2D GBNs to quantitatively define RVEs and SVE sets for GBNs. We compare these results to those for crystallographic texture in the same polycrystals and find that the trends for GBNs and texture are remarkably different. Because the vast majority of experimental work currently relies on 2D microstructure characterization, the results obtained for 2D systems have great practical value in and of themselves. However, the stereological approach employed also allows us to make quantitative predictions of RVEs and SVEs for fully 3D microstructures. These results are expected to aid future experimental and computational work in selecting appropriately sized material volumes to achieve robust quantitative results.

Molecular dynamics study of tilt grain boundary evolution during the growth of beryllium thin films

• Grain boundaries can maintain its type as the epitaxial layer growing. • Large incident angle results in the formation of more defects at the grain boundary. • Incident energy about 5 eV is desired to form improved Beryllium film. Beryllium films fabricated by magnetron sputtering deposition unavoidably contain grain boundaries (GBs), which have a substantial effect on the resulting material properties. The growth dynamics and evolution of various tilt GBs were explored using a direct molecular dynamics vapor deposition simulation. The effects of the incident energy, incident angle and substrate temperature on the film morphologies were also considered. The results show that GBs and dislocations were extended to the epitaxial layer during the growth of the beryllium thin films. The types of GBs were maintained and were closely related to the surface topography of the substrate. An increased incident energy and substrate temperature enabled the formation of good-quality beryllium films. However, large incident angles resulted in the formation of defect structures in the interior of the beryllium films. The low-angle GBs were more stable than the high-angle GBs. Large incident energies (above 10 eV) with large incident angles (above 38°) always accompanied increased numbers of reflected atoms. In brief, during the growth of the beryllium thin films, the GBs were extended to the epitaxial layer, and the types of GBs were maintained. In addition, incident energies about 5 eV with incident angles less than 30° are desired to form improved Be films.

The effect of grain boundary structure on the intergranular degradation behavior of solution annealed alloy 690 in high temperature, hydrogenated water

The environmental degradation of four different types of grain boundaries were investigated on alloy 690 following slow strain rate tensile tests in 360°C hydrogenated water. Random high angle boundaries (RHABs) support fast Cr diffusion that promotes the formation of a continuous surface oxide film and grain boundary migration. Surprisingly, coherent twin boundaries (CTBs) are susceptible to intergranular oxidation and do not exhibit Cr diffusion or grain boundary migration. When CTBs are changed to transformed twin boundaries (TTBs) by cold work, they behave like RHABs. Finally, incoherent twin boundaries (ITBs) undergo intergranular oxidation with limited Cr depletion but no boundary migration beyond the oxide. The Cr diffusivity along grain boundary in this alloy is directly related to the density of coincident site in the grain boundary plane and determines the morphology of oxide formed near the grain boundary. CTBs are still highly resistant to stress corrosion cracking (SCC) due to the semi-coherent interface between the intergranular chromia and grain matrix. In contrast, the intergranular oxides formed along RHABs inherit highly-disordered boundary structure from the original grain boundaries and show much higher SCC susceptibility. The grain boundary structure dependence of SCC resistance should be understood from its effects on solute diffusivity, structure of intergranular oxide and the local stress-strain state.

Study of Grain Boundary Self-Diffusion in Iron with Different Atomistic Models

We studied grain boundary (GB) self-diffusion in body-centered cubic iron using ab initio calculations and molecular dynamics simulations with various interatomic potentials. A combination of different models allowed us to determine the principal characteristics of self-diffusion along different types of GBs. In particular, we found that atomic self-diffusion in symmetric tilt GBs is mostly driven by self-interstitial atoms. In contrast, in general GBs atoms diffuse predominantly via an exchange mechanism that does not involve a particular defect but is similar to diffusion in a liquid. Most observed mechanisms lead to a significant enhancement of self-diffusion along GBs as compared to diffusion in the bulk. The results of simulations are verified by comparison with available experimental data.

Interaction between a [formula omitted] twin boundary and grain boundaries in magnesium

Deformation twinning is a common plastic deformation mode in polycrystalline magnesium (Mg), so the interaction between twin boundary (TB) and grain boundary (GB) plays an important role in the improvement of material strength and ductility. In this work, molecular dynamics (MD) method was employed to study the interaction between {101‾2} TB and GB. Four GB types were considered, i.e., symmetric tilt grain boundary (STGB), asymmetric tilt grain boundary (ATGB), basal twist grain boundary (BTGB, GB coincides with the basal planes of both grains) and prismatic twist grain boundary (PTGB, GB coincides the prismatic planes of both grains). MD simulations show that for both tilt GBs, the twin is able to penetrate through GB at small misorientation angle (<15°). At medium misorientation angle (15°–75°), the TB is absorbed by GB with stacking faults nucleated on the new GB, while at large misorientation angle (>75°), the TB is fully absorbed without any stacking faults. For twist GBs, the TB always penetrates through PTGBs, but is fully absorbed by BTGBs for all considered misorientation angles. All the GBs were observed to have a blocking effect on twinning deformation whether the twin penetrates through GB or not. Based on the interaction mechanisms mentioned above, we proposed an analytical twin thickening model via the Eshelby solutions to describe the GB strengthening effects or grain size effects on twinning, which shows that the material strength dominated by twinning is inversely proportional to the grain size. By comparison with the Hall-Petch model, we can see that the grain size effect on twinning deformation is stronger than that on dislocation slip, which agrees well with previous experimental observations. It is worth noting that this model would be valid for not only HCP (hexagonal close-packed) materials but also FCC (face-centered cubic) and BCC (body-centered cubic) materials. Current work provides new insights into the GB strengthening effects and grain size effects on twinning deformation.

Effect of grain boundary angle on the thermal conductivity of nanostructured bicrystal ZnO based on the molecular dynamics simulation method

ZnO is a widely used semiconductor material due to its excellent physical and mechanical properties. The thermal transport behavior across a single grain boundary (GB) of bicrystal ZnO with varying tilt angles from 5.45° to 67.38° was investigated using a nonequilibrium molecular dynamics simulation. The GB energy and Kapitza resistance as a function of tilt angle were determined and parameters of the extended Read-Shockley model were calculated. The Kapitza resistance varied monotonically with a GB angle <36° and was nearly constant when the angle was >36°. Furthermore, effective thermal conductivity and Kapitza resistance were found to depend strongly on the sample length and temperature. Finally, we compared the phonon density of states of the two types of GBs and found a mismatch in the low frequency that might explain the effect of the GBs structures on heat conduction.

Misorientation dependence grain boundary complexions in symmetric tilt Al grain boundaries

Since polycrystalline alloys consist of a complex network of various types of grain boundaries (GBs), detailed atomic-scale analysis of how some impurities are distributed at every type of GBs is necessary to fully understand the implications of GB segregation on material's performance. In this study, we present the atomic-scale structural combined with a chemical analysis of segregation induced GB complexions across the various types of Al alloy 7075 GBs using aberration-corrected microscopy and crystal orientation mapping assisted with precession electron diffraction. The result shows multilayer Cu GB segregation containing non-uniformly segregated mixed atomic columns across the interfaces. Two distinct types of Cu GB segregation behavior were observed, point and parallel array, analyzed by means of a displacement field obtained from the dichromatic pattern. Atomistic simulations were performed to test the energetic feasibility of the observed segregation behavior. As per the knowledge of the authors, this is the first report on experimental analysis of segregation induced periodic ordered structured GB complexions on Al alloy system. Furthermore, every GBs of the films were segregated uniquely forming ordered structures along the interface. The distance between two consecutive high segregated units was periodic for the point segregated GBs and followed a trend of a theoretical model of dislocation spacing. Based on the distance between two high segregated units, it is inferred that highly misorientated GBs are more segregated than low misoriented GBs. This study demonstrates that the misorientation between the neighboring grains significantly influences the segregation behavior across the interface and consequently, the structure of segregation-induced GB complexions.

Effect of slip transmission at grain boundaries in Al bicrystals

The effect of slip transfer on the deformation mechanisms of Al bicrystals was explored using a rate-dependent dislocation-based crystal plasticity model. Three different types of grain boundaries (GBs) were included in the model by modifying the rate of dislocation accumulation near the GB in the Kocks-Mecking law, leading to fully-opaque (dislocation blocking), fully-transparent and partially-transparent GBs. In the latter, slip transmission is only allowed in pairs of slip systems (SS) in neighbour grains that are suitably oriented for slip transfer according to the Luster-Morris parameter. Modifications of the GB character led to important changes in the deformation mechanisms at the GB. In general, bicrystals with fully-opaque (dislocation blocking) boundaries showed an increase in the dislocation density near the GB, which was associated with an increase in the Von Mises stress. In contrast, the dislocation pile-ups and the stress concentration were less pronounced in the case of partially-transparent boundaries as the slip in one grain can progress into the next grain with some degree of continuity. No stress concentrations were found at these boundaries for fully-transparent boundaries, and there was continuity of strain across the boundary, which is not typical of most experimentally observed GBs (Hémery et al., 2018; Bieler et al., 2019). Simulations of ideal bicrystals oriented for favorable slip transfer on the most highly favored slip system in grains with high Schmid factors for slip transfer depends on the number of active SS in operation in the neighborhood indicating that most boundaries will lead to nearly opaque conditions while some boundaries will be transparent. Finally, the model was applied to a particular experimentally observed GB in which slip transfer was clearly operating indicating that the model predicted a nearly transparent GB.

Atomic resolution analyses on defects in nanocrystalline Cu-based alloys generated by severe plastic deformation

Bulk nanocrystalline (NC) materials possess high volume fraction (up to 50%) of grain boundaries (GBs), which contributes significantly to their mechanical and physical properties. Nevertheless, high density of dislocations at GBs and the associated strain fields complicate the analyses on the fine GBs structures of NC materials. Here, taking NC Cu-based alloys as model systems, the atomic-scale configurations of dislocations and typical GBs triggered by severe plastic deformation are well investigated using aberration-corrected high-resolution transmission electron microscopy and geometric phase analysis. A periodic array of extended dislocations with a spacing of 2.2 nm is found to form low-angle GBs (LAGBs) in NC Cu-based alloys. The interaction between dislocations and twins play a crucial role in forming the GBs of NC. Twin boundaries (TBs) in NC Cu-based alloys can serve as barriers for trapping dislocations, which in turn tune the TBs directly evolving into LAGBs or high-angle GBs. In addition, a symmetric 6° [001] tilt LAGB is also analyzed to be composed of a/2〈100〉 edge dislocations, which are activated by the a/2〈110〉 full dislocation dissociation or simple shear deformation of {100} planes. Our study offers insights into the atomic resolution structures of dislocations and GBs in NC materials.

Atomically Sharp Dual Grain Boundaries in 2D WS2 Bilayers.

It is shown that tilt grain boundaries (GBs) in bilayer 2D crystals of the transition metal dichalcogenide WS2 can be atomically sharp, where top and bottom layer GBs are located within sub-nanometer distances of each other. This expands the current knowledge of GBs in 2D bilayer crystals, beyond the established large overlapping GB types typically formed in chemical vapor deposition growth, to now include atomically sharp dual bilayer GBs. By using atomic-resolution annular dark-field scanning transmission electron microscopy (ADF-STEM) imaging, different atomic structures in the dual GBs are distinguished considering bilayers with a 3R (AB stacking)/2H (AA' stacking) interface as well as bilayers with 2H/2H boundaries. An in situ heating holder is used in ADF-STEM and the GBs are stable to at least 800 °C, with negligible thermally induced reconstructions observed. Normal dislocation cores are seen in one WS2 layer, but the second WS2 layer has different dislocation structures not seen in freestanding monolayers, which have metal-rich clusters to accommodate the stacking mismatch of the 2H:3R interface. These results reveal the competition between maintaining van der Waals bilayer stacking uniformity and dislocation cores required to stitch tilted bilayer GBs together.

Interactions of plasticity and phase transformation under shock in iron bicrystals

The interaction of plasticity and martensitic transformation in iron bicrystals under shock has been investigated via nonequilibrium molecular-dynamic simulations with our modified analytic embedded-atom model potential. Our results show that grain boundaries (GBs) can change the kinetics of α → ɛ martensitic transformations. The condition that GBs trigger strain induced transformation (SIT) is revealed, that is, the local structure of GBs can be converted to a hexagonal close packing (HCP) lattice with a lower potential barrier than the BCC → HCP transformation (stress assist transformation, SAT) because the driving energy of transition can be partly provided by the energy of GBs. It can explain why the threshold of the phase transition can be greatly reduced in samples containing some types of GBs. The threshold of SIT is lower than that of SAT, which agrees well with experimental results. Then, the nucleation kinetics of phase transition induced by dislocations are described. The propagation of dislocation provides the driving force for the nucleation of the phase transition. The dislocations can be directly emitted from the GBs under shock, which provides a new view that the phase transition is not always preceded by dislocations near GBs. In addition, dislocations can be induced by pre-existing dislocations under shock, which makes it understandable that GBs can emit dislocations. Our findings have an important significance for experimental studies and macroscopic and mesoscopic scale simulations regarding iron martensitic transformation.

Effects of twin boundary orientation on plasticity of bicrystalline copper micropillars: A discrete dislocation dynamics simulation study

Twin boundaries (TBs) constitute a special type of symmetric grain boundary (GB). TBs influence plastic deformation in a complex manner. They not only act as dislocation obstacles but can also accommodate twinning dislocations (TDs) whose motion enables twin boundary migration as an alternative deformation mechanism. Exploiting this dual effect offers interesting perspectives in view of designing materials that combine high strength and ductility. In the present work, we propose a model for dislocation-TB interactions in the framework of discrete dislocation dynamics (DDD) simulations, which we use to investigate the mechanical properties of bicrystalline copper micropillars containing a TB. We systematically investigate how the compressive response depends on the orientation of the TB with respect to the micropillar cross-section. The simulations show significant strengthening effects for TB orientation angles less than 45°, where the interaction between mixed dislocations and TBs plays an important role. For angles larger than 45°, the interaction between screw dislocations and TB dominates, leading to weak strengthening. At 45°, dislocations on the dominant slip systems glide parallel to the TB and no strengthening is observed.

Atomistic simulation of interaction of collision cascade with different types of grain boundaries in α-Fe

Presented here are atomistic simulations of the interaction of a collision cascade in α-Fe with different types of grain boundaries (GBs). The collision cascade was generated by a primary knock-on Fe atom of 3 keV kinetic energy. Four different types of symmetric tilt GB configurations (low angle and high angle GBs, Σ3 and Σ11 GBs) with two tilt axes [112¯] and [1¯10] were considered. Difference in potential energy, strength and average displacement of atoms between pristine and irradiated GB configurations were analysed to bring out the effect of irradiation. The vacancy and interstitial formation energies calculated using molecular statics simulations were found to be lower in the GB than in the grain. Collision cascade resulted in defect clusters in and around the GB. Low angle GBs were found to be better sinks for radiation induced defects. Tensile deformation of the irradiated configurations resulted in dislocation loops being nucleated from interstitial clusters. The loss in strength after irradiation was high for Σ3 GBs and negligible for other GBs. It was suggested that materials with low angle GBs and a high dislocation density would be preferred for use in radiation environment.