Twist Grain Boundary Research Articles

Grain boundary segregation transitions and critical phenomena in binary regular solutions: A systematics of complexion diagrams with universal characters

A systematics of grain boundary (GB) segregation transitions and critical phenomena has been derived to expand the classical GB segregation theory. Using twist GBs as an example, this study uncovers when GB layering vs. prewetting transitions should occur and how they are related to one another. Moreover, a novel descriptor, normalized segregation strength (ϕseg), is introduced. It can represent several factors that control GB segregation, including strain and bond energies, as well as misorientation for small-angle GBs (in a mean-field approximation), which had to be treated separately in prior models. In a strong segregation system with a large ϕseg, first-order layering transitions occur at low temperatures and become continuous above GB roughing temperatures. With reducing ϕseg, the layering transitions gradually merge and finally lump into prewetting transitions without quantized layer numbers, akin to Cahn's critical-point wetting model. Furthermore, GB complexion diagrams with universal characters are constructed as the GB counterpart to the classical exemplar of Pelton-Thompson regular-solution binary bulk phase diagrams.

Effects of local elemental ordering on defect-grain boundary interactions in high-entropy alloys

The way in which defects interact with grain boundaries (GBs) has profound influences on materials performance. In this work, we study defect-GBs interactions in a model CuNiCoFe high-entropy alloy (HEA) based on atomistic simulations. Five representative GBs are considered, namely Σ3 < 101 > {11−1} coherent twin GB, Σ5 < 103 > {010} twist GB, Σ11 < 1–10 > {113} symmetric tilt GB (STGB), Σ11 < 1–10 > {554} asymmetric tilt GB (ATGB), and Σ45 < 1–20 > {001} tilt GB. A particular focus is placed on the role of chemical disorder and local elemental segregation in influencing the defect-GBs interactions. Specifically, we compare the results obtained within an averaged atom model, the random HEA with randomly distributed elements, and the equilibrated HEA with Cu segregation after a combined Monte-Carlo/Molecular statics algorithm. For the pristine CuNiCoFe HEA without GBs, we find chemical occupancy fluctuations tend to lower the formation energies of defects, especially for interstitials because of the larger lattice distortion. For defect-GBs interactions, we find GBs strongly interact with interstitials over vacancies. We further reveal that elemental segregation can enhance the sink strength of GBs towards vacancies, but at the same time, reduce the sink strength toward interstitials. Therefore, the bias effects of GBs toward interstitials and vacancies are suppressed in HEAs due to local ordering, promoting efficient defect annihilation within the grain interiors. We highlight that the local ordering tendency and elemental segregation in HEAs play dominant roles in influencing the defect-GB interactions.

Exceptionally Wide Thermal Range Enantiotropic Existence of a Highly Complex Twist Grain Boundary Phase in a Pure, Single-Component Liquid Crystal Chiral Dimer.

Twist grain boundary (TGB) phases exhibiting highly frustrated and complex liquid crystal structures have aroused enormous interest because of their close resemblance to superconductors. The remarkable experimental demonstration of their occurrence by Goodby and co-workers paved the way for developing new research endeavors. However, of the several genuine concerns associated with these intriguing structures, their temperature range has been challenging. In this communication, we report the occurrence of the TGB phase with smectic C* blocks (TGBC*) over a vast, unprecedented thermal range of ∼170 °C in a newly synthesized chiral dimer derived from cholesterol. Detailed investigations covering synthesis, characterization, and evaluation of liquid crystallinity with the aid of optical, calorimetric, and X-ray diffraction are presented.

Optimal transportation of grain boundaries: A forward model for predicting migration mechanisms

It has been hypothesized that the most likely atomic rearrangement mechanism during grain boundary (GB) migration is the one that minimizes the lengths of atomic displacements in the dichromatic pattern. In this work, we recast the problem of atomic displacement minimization during GB migration as an optimal transport (OT) problem. Under the assumption of a small potential energy barrier for atomic rearrangement, the principle of stationary action applied to GB migration is reduced to the determination of the Wasserstein metric for two point sets. In order to test the minimum distance hypothesis, optimal displacement patterns predicted on the basis of a regularized OT based forward model are compared to molecular dynamics (MD) GB migration data for a variety of GB types and temperatures. Limits of applicability of the minimum distance hypothesis and interesting consequences of the OT formulation are discussed in the context of MD data analysis for twist GBs, general Σ3 twin boundaries and a tilt GB that exhibits shear coupling. The forward model may be used to predict atomic displacement patterns for arbitrary disconnection modes and a variety of metastable states, facilitating the analysis of multimodal GB migration data.

The spectrum of atomic excess free volume in grain boundaries

The grain boundary (GB) excess volume is an important structural factor that is strongly correlated with various thermodynamic and kinetic properties of GBs such as GB energy, GB mobility, GB diffusivity, and GB segregation energy, etc. However, the excess volume is usually reported as an average value of the entire GB. Such simplification does not consider the spectral nature of the excess volume in a GB, which cannot be used to describe the atomic mechanisms of some kinetic process, such as GB migration, that involves only a few atoms at a time. Here, we explore the spectrum of atomic excess volume in representative nanocrystalline Ni and Al samples as well as 388 Ni bicrystals based on the Olmsted dataset by using atomistic simulations. It is found that the nanocrystalline Ni and Al models show a skew-normal distribution in the spectrum of both the atomic excess volume and the atomic excess energy in the GBs, which show a weak inverse correlation between them. This is in stark contrast to the widely reported positive correlation between GB energy and excess volume based on the average value. We further show based on the statistical analysis that the correlation between the atomic excess volume and excess energy strongly depends on the GB type and a universal trend between them does not exist. While low ∑ Ni GBs generally shows a strong inverse linear correlation between these two properties, such correlation is weak for high ∑ Ni GBs. Moreover, we find that the spectrum of the excess volume shows characteristics distribution in some special Ni GBs. For example, twist GBs generally show a symmetrical unimodal distribution while most surveyed ∑3 Ni GBs with anti-thermal behavior show an apparent bimodal distribution. Nevertheless, a strong correlation is found between the atomic excess volume and the segregation energy based on the nanocrystalline Al model with Mg impurity, which implies a possible universal trend between the two properties. The current study thus shows that the excess volume provides useful insights in revealing the elemental structure–property correlations in GBs, which may be used as a structural variable in future thermodynamic modeling of GBs.

Tensile deformation behavior of twist grain boundaries in CoCrFeMnNi high entropy alloy bicrystals

High entropy alloys (HEA) are a class of materials that consist of multiple elemental species in similar concentrations. The use of elements in far from dilute concentrations introduces a multi-dimensional composition design space by which the properties of metallic systems can be tailored. While the mechanical behavior of HEAs has been the subject of active research recently, the role of grain boundaries (GBs) in their deformation behavior remains poorly understood. Motivated by recent experiments on HEAs demonstrating that GBs act as nucleation sites for deformation twins, herein, we leverage atomistic simulations to construct a series of equiatomic CoCrFeMnNi HEA bicrystals with langle 110 rangle and langle 111 rangle symmetric twist GBs and examine their tensile behavior and underlying deformation mechanisms at 77 K. Simulation results reveal that plastic deformation proceeds by the nucleation of partial dislocations from GBs, which then grow with further loading by bowing into the bulk crystals leaving behind stacking faults. Variations in the nucleation stress exist as function of GB character, defined in this work by the twist angle. Our results provide future avenues to explore GBs as a microstructure design tool to develop HEAs with tailored properties.

Misorientation effect of twist grain boundaries on crack nucleation from molecular dynamics

Grain boundaries (GBs) have profound effects on the dislocation motion in materials so as to cause crack nucleation. However, the experimental observation on how the misorientation of twist GBs affects the crack nucleation is scarce. In this paper, we proposed a theoretical model to quantify the dislocation energy per unit length. With it, the critical resolved shear stress at GBs was calculated to assess the crack nucleation. It was found that crack nucleation is significantly influenced by the surface energy, grain boundary energy, system potential energy, boundary stress and misorientation. With the increase of the twist angle, high-angle GBs are more favorable for crack nucleation as general. Interestingly, some specific low-angle GBs possess comparable small critical resolved shear stress as the high-angle GBs and they are favorable for crack nucleation as well. Through the analysis of the resolved shear stress distribution of GBs, it is revealed that the crack nucleation mechanism of these specific low-angle GBs is similar to that of general high-angle GBs. It is inferred that the special low-angle GB systems could generate a crack under a relatively low resolved shear stress.

Application of Molecular Dynamics Calculations to Elucidation of the Mechanism of Hydrogen-Induced Crack Initiation in Fracture Toughness Tests Using Tempered Martensitic Steels

It is well known that the presence of hydrogen deteriorates mechanical properties of steels, that appears as reduced fracture toughness, shorter fatigue lifetime, etc.; these phenomena are recognized as hydrogen embrittlement. The effect of hydrogen on crack initiation of fracture toughness test has been investigated using a combination of experimental and computational approaches. Tempered lath martensitic steel was subjected to fracture toughness test with monotonically rising load in air and high-pressure hydrogen gas. While crack propagated continuously within grains in air, cracking in hydrogen grew by linking isolated interface failure ahead of a main crack tip. Then, to understand the nucleation mechanism of isolated failure in the presence of hydrogen, the tensile simulations of twist grain boundaries (TGBs) rotated along <110> axis at various angles were conducted using molecular dynamics calculations. While the dislocation emission from TGB rotated 70° is dominant deformation mode in the absence of hydrogen, the rupture along TGB rotated 110° and 170° without stress relaxation due to dislocation emission is dominant deformation mode in the presence of hydrogen. As a consequence, it is indicated that the origin of hydrogen-induced isolated crack initiation in the vicinity of fatigue pre-crack is the rupture along the block boundaries within martensitic structure due to hydrogen-induced inhibition of dislocation emission from GBs.

The cholesteric and TGB phases under the applied electric field

ABSTRACT New lactic acid derivatives were prepared and their mesomorphic properties presented in comparison with analogous compounds we pursued previously. Studied materials exhibit cholesteric (Ch) and twist grain boundary (TGB) phases on cooling from the isotropic phase (Iso). We found the helical pitch in Ch phase was rather short, less than the visible light wavelength, and grew quickly on cooling, so at the Ch-TGBA phase transition, we were able to observe the selective reflection in visible range. For the longest homologue, the Ch-TGBA-TGBC phase sequence was detected. Due to the positive dielectric anisotropy, the applied electric field reoriented molecules along the field and the effect analogous to Frederiks transition was observed in cholesteric and TGBA mesophases. X-ray diffraction measurements were performed to unambiguously confirm the phase identification.

Molecular dynamics study on tensile strength of twist grain boundary structures under uniaxial tension in copper

Grain boundaries (GBs) play an important role in deformation responses and mechanical properties of materials. However, the lack of experimental observations of GBs restricts the in-depth understanding on the mechanical behavior of twist GBs and the continuum misorientation effect on the tensile strength of grains. In this paper, the <001>, <101> and <111> twist grain boundary structures for copper are studied by molecular dynamics simulations to obtain the tensile strength and intergranular stress. Over all the twist GB structures, <111> Σ3 GB structure is verified and analyzed to possess the maximum tensile strength, the lowest GBE (grain boundary energy) and the minimum CSL (coincident site lattice) value. The distribution of normal stress on GBs is further investigated to reveal the calculated results that the tensile strength is enhanced with increasing the twist angle for the <001> and <101> twist GB structures, but the misorientation has no significant influence on the tensile strength of almost all <111> twist GB structures, except Σ3. This work provides a fundamental information for understanding the effects of misorientation on interfacial stability and mechanical behaviors of grain boundary structures with different crystal directions.

Atomic Simulations of Interactions between Edge Dislocations and a Twist Grain Boundary in Mg

The roles of mechanical size and chemical bonding effect of segregated elements in grain boundaries (GBs) on the interactions between the GBs and dislocations are still not well understood. Because a twist GB tends to have a higher GB energy than a tilt GB owing to its random structure, the mechanical and chemical effects of solute elements on the dislocation-GB interactions in a twist GB are generally different from those in a tilt GB. In addition, dislocation emission from a GB in hcp metals is more complex than that in fcc metals because of the intense plastic anisotropy in hcp metals. In this study, interactions between basal 〈a〉 edge dislocations and a twist GB were studied in non-segregated and Al- and Fe-segregated twist Mg GBs by molecular dynamics simulations. The simulations showed that a prismatic dislocation was emitted from the GB after two basal dislocations were adsorbed into the GB. Dislocation adsorption into the GB was enhanced by Al and Fe segregation, but dislocation emission from the GB was suppressed by the segregation. Analyses of the GB width and potential energy suggested that the dislocation adsorption was mainly determined by mechanical effects, while dislocation emission strongly depended on chemical effects.

Genetic algorithm-guided deep learning of grain boundary diagrams: Addressing the challenge of five degrees of freedom

Grain boundaries (GBs) often control the processing and properties of polycrystalline materials. Here, potentially transformative research is represented by constructing GB property diagrams as functions of temperature and bulk composition, also called “complexion diagrams,” as a general materials science tool on par with phase diagrams. However, a GB has five macroscopic (crystallographic) degrees of freedom (DOFs). It is essentially a “mission impossible” to construct property diagrams for GBs as a function of five DOFs by either experiments or modeling. Herein, we combine isobaric semi-grand canonical ensemble hybrid Monte Carlo and molecular dynamics (hybrid MC/MD) simulations with a genetic algorithm (GA) and deep neural network (DNN) models to tackle this grand challenge. The DNN prediction is ∼108 faster than atomistic simulations, thereby enabling the construction of the property diagrams for millions of distinctly different GBs of five DOFs. Notably, excellent prediction accuracies have been achieved for not only symmetric-tilt and twist GBs, but also asymmetric-tilt and mixed tilt-twist GBs; the latter are more complex and much less understood, but they are ubiquitous and often limit the performance properties of real polycrystals as the weak links. The data-driven prediction of GB properties as function of temperature, bulk composition, and five crystallographic DOFs (i.e., in a 7D space) opens a new paradigm.

Nanoparticle-Stabilized Lattices of Topological Defects in Liquid Crystals

We present a review of nanoparticle (NP) stabilized lattices of topological defects (TDs) in liquid crystals (LCs). We focus on conditions for which chiral liquid-crystalline blue phases and twist-grain boundary A phases might be stable in bulk LCs. These phases exhibit lattices of disclinations and dislocations corresponding to line TDs in orientational and translational LC order, respectively. NPs of appropriate size and surface coating can assemble in the cores of defects and stabilize metastable or increase the temperature range of already stable lattices of TDs. The stabilization is achieved by the universal defect core replacement and adaptive defect core targeting mechanisms. Representative experiments revealing these phenomena and potential applications are discussed.

A survey of the crystallography-dependency of twist grain boundary mobility in Al based on atomistic simulations

The correlations between the mobility and crystallographic parameters of twist grain boundaries (GBs) are investigated based on a mobility dataset created by molecular dynamics simulations using a modified synthetic driving force method. Statistically, low-angle twist GBs are not less or more mobile than high-angle ones, but twist GBs with axes near 〈111〉 or boundary plane close to {111} have low mobilities. These tendencies are significantly different from those revealed for symmetrical tilt GBs. The mobility overall does not exhibit any visible correlation with the coincidence index or energy of a GB.

Grain boundary properties of elemental metals

The structure and energy of grain boundaries (GBs) are essential for predicting the properties of polycrystalline materials. In this work, we use high-throughput density functional theory calculations workflow to construct the Grain Boundary Database (GBDB), the largest database of DFT-computed grain boundary properties to date. The database currently encompasses 327 GBs of 58 elemental metals, including 10 common twist or symmetric tilt GBs for body-centered cubic (bcc) and face-centered cubic (fcc) systems and the Σ7 [0001] twist GB for hexagonal close-packed (hcp) systems. In particular, we demonstrate a novel scaled-structural template approach for HT GB calculations, which reduces the computational cost of converging GB structures by a factor of ~ 3–6. The grain boundary energies and work of separation are rigorously validated against previous experimental and computational data. Using this large GB dataset, we develop an improved predictive model for the GB energy of different elements based on the cohesive energy and shear modulus. The open GBDB represents a significant step forward in the availability of first principles GB properties, which we believe would help guide the future design of polycrystalline materials.

Shock deformation and spallation of Cu bicrystals with (1 1 1) twist grain boundaries

The shock deformation and spallation of Cu bicrystals with (111) twist grain boundary (GB) of different misorientation angles are investigated by using molecular dynamics simulations. The selected GBs include low angle (LAGB), near twin (NTGB) and intermediate angle (IAGB) twist GBs, and the shock direction is along the GB normal. It is demonstrated that the bicrystals are of lower Hugoniot elastic limit than single crystal, as the twist GBs provide dislocation sources for deformation. At the shock compression stage, GB plasticity at the LAGBs and NTGBs is initiated via reaction and dissociation of GB dislocations, and can be triggered by elastic shock wave more readily than that at the IAGBs, on which dislocation emission is homogeneous and irrelevant to initial GB dislocations. At the tension stage, the incipient spall for different twist GBs occurs at an identical shock strength, but is of different details in dislocation and void evolutions. With varying shock strength and misorientation angle, spall strength of the Cu bicrystals depends on whether GB plasticity is produced at the compression stage, and the values are consistent with that of Cu single crystal under similar shock loading conditions.

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.

Atomic simulations of the formation of twist grain boundary and mechanical properties of graphene/aluminum nanolaminated composites

In this paper, generation of twist grain boundaries (TGBs) and their effects on the mechanical behaviors of graphene/aluminum (Gr/Al) nanolaminated composites were studied via molecular dynamics (MD) method. Crystallization of the Gr/Al composite was conducted to observe the microstructural evolution. The whole evolution process including nucleation and growth of crystals was presented. TGBs were formed between the adjacent crystals after crystallization. The fcc (1 1 1) plane of the Al matrix has three-fold symmetry, so that the TGB angles can range from 0° to 60°. Tensile tests on the Gr/Al composites with TGBs were carried out to study their deformation behavior. The results showed that the graphene-nanoplate (GNP) and TGBs greatly impact on the dislocation behavior of the Al matrix. The dependences of dislocation nucleation and evolution on the TGB angle were uncovered. The TGB also plays a significant role in blocking dislocation motion, and the effectiveness mainly depends on its strength. The variations of yield stress and strain of the Gr/Al composites with different TGB angles were also predicted. It was revealed that failure of the composite is caused by combined action of tensile deformation and shear deformation induced by defects at the GNP edge.

Crystal Plasticity Modeling of Void Growth on Grain Boundaries in Ni-Based Superalloys

In this work, we explore the effect of misorientation angles of crystal orientations between two grains along the grain boundary (GB) on void growth behavior in polycrystalline Ni-based superalloys by using a crystal plasticity finite element method. Quantitative analysis is conducted to study the coupled roles of the crystal orientation and stress triaxiality in void growth in bicrystals. Based on our simulation results, we find that, as the main loading axis perpendicular to the GB, voids grow more slowly on tilt GBs in bicrystals than those in single and bicrystal samples with twist GBs, while the void growth in single- and bicrystal samples with twist GBs exhibited almost the same rate and increased with the stress triaxiality levels. The interaction between two crystals bonded with the GB activates the effective Schmid factors in each crystal, which results in asymmetric distribution of the equivalent plastic strain around the void and induces distinct irregularly shaped voids during deformation.

Liquid crystal dimers containing Cholesteryl and Triazole-containing mesogenic units

ABSTRACT New liquid crystals categorised as cholesteryl dimers have been successfully synthesised through the reaction between cholesteryl 4-(prop-2-ynyloxy)benzoate moieties with n-azido(cholesteryloxy-carbonyl)alkane. All the dimers display enantiotropic mesophases. Whilst the odd-numbered dimers exhibit chiral nematic (N*), twisted grain boundary (TGB) and chiral smectic C (SmC*) phases, the even-numbered members from the same series show chiral smectic A and C. A detailed inspection on mesophase reveals that the chiral centres and the bent conformation of the odd-numbered members are essential for the induction of TGB phase. However, upon decreasing the temperature, the ratio of the transition temperatures (TSmC*-SmA*/TSmA*-I) is found to be 0.95, which indicate the second order transition according to the McMillan’s molecular theory. In addition, the X-ray diffraction study supports the presence of the smectic A phase on the even members rather than the N* by the appearance of the Bragg diffraction peaks at 190°C. A comparison study with the other analogues in which the cholesterol entity is substituted by azobenzene or biphenyl tails has been carried out to assess the relationship between the molecular structure and mesomorphic behaviour.