Bicrystal System Research Articles

Uncovering origin of grain boundary resistance to irradiation damage in NiCoCr multi-principal element alloys

Multi-principal element alloys (MPEAs) demonstrate significant promise as structural materials for nuclear energy equipment owing to their exceptional mechanical properties and radiation-resistant performances. In these alloys, the grain boundary (GB) serves as a crucial microstructure that typically mitigates irradiation damage by absorbing the irradiation-induced defect. However, the micromechanisms governing the anti-irradiation performance of GBs in MPEAs remain unclear. In this study, we investigate the irradiation defect production during collision cascade in the model NiCoCr bicrystal system through atomic simulations, aiming to unveil the atomic-scale origin of GB to resist irradiation damage in MPEAs. The results reveal that GBs effectively serve as sinks for irradiation defects in NiCoCr. The sink efficiency depends on the GB energetic state, including GB excess energy and defect segregation energy, as well as the energetic difference between interstitial and vacancy segregation. Statistical analysis identifies a universally exponent function between the defect absorption rate at GB and GB energetic state. In NiCoCr, the GB-disorder-induced-entropy increase leads to a biased reduction in interstitial segregation energy, narrowing the gap between interstitial and vacancy segregation energies by approximately 11 % compared to Ni. This improvement enhances the overall resistance of GBs to irradiation damage. Additionally, preferential segregation of Ni interstitial atoms is notably enhanced in NiCoCr, contributing to a high defect absorption rate at GBs. This study provides new insights into the resistance of GBs to irradiation defects in MPEAs and suggests GB engineering as an effective strategy for developing advanced alloys with enhanced radiation tolerance.

Substantial toughening by thick nanoscale amorphous intergranular films in nanocrystalline materials

Amorphous intergranular films (AIFs) have been proven in experiments to improve the damage tolerance of nanocrystalline materials. However, a quantitative study is still lacking. Molecular dynamics simulations were performed here to investigate the effect of CuNb AIFs on the fracture toughness of nanocrystalline Nb. In order to clarify the role of AIFs, a bicrystal Nb model with one straight symmetrical tilt grain boundary and a mode-I crack in one of the grains was constructed, in which the AIF effect was introduced by replacing the normal grain boundary with a CuNb AIF. Then, AIF thickness-dependent tensile deformation of the bicrystal Nb samples was simulated. The work-of-fracture, which is defined as the released strain energy due to the newly generated unit area in the crack during stretching, was employed to quantify the fracture toughness of the bicrystal systems. The results show that the fracture toughness of the AIF sample can be tripled due to the blunted crack tip and the relieved stress concentration at the crack tip as compared to the AIF-free one that exhibits a brittle crack propagation behavior. Also, the thicker the AIFs, the more pronounced this reinforcing effect. More importantly, it is found that there exists a critical AIF width of 1.7 nm, below which the crack will eventually break through the AIF, and above which the crack failed to do this. It is revealed that the enhanced fracture toughness originated from the transformation of brittle crack propagation to abundant dislocation emission from AIFs.

STUDY OF THE TEMPERATURE DEPENDENCE OF THE SYMMETRICAL GRAIN BOUNDARY ENERGIES ON THE PLANE (110) IN ALUMINUM

In this work the energy of symmetric tilt and twist grain boundaries in the range of grain misorientation angles from 0 to 180◦ and temperatures from 100 to 700 K in pure aluminum is investigated. The bicrystal systems with different grain tilt/twist angles are maintained at constant temperatures of 100, 400, or 700 K by molecular dynamic method and the energy of each grain boundary is calculated. The results show that the minimum grain boundary energy decreases as the temperature increases from 100 to 400 K; but the energy may decrease, remain practically unchanged, or even increase with further heating to 700 K. The average grain boundary energy obtained by averaging the energies of the resulting grain boundary structure variations at constant temperature grows with increasing temperature from 100 to 700 K for random boundaries with initially high energies. In the case of special grain boundaries with small Σ values, the average energy will be practically unchanged. To describe the continuous energy dependence of symmetric tilt and twist boundaries on temperature, an approximation by an forward propagation of artificial neural network is proposed. The neural network is trained and tested on atomistic simulation data and shows high predictive ability on test data and to describe the boundary energy in the temperature range from 100 to 700 K.

On the relationship between shock and particle velocities in single and bicrystal systems of Aluminum: A molecular dynamics study

Molecular dynamics (MD) simulations are used to study shock wave propagation in various symmetric tilt and symmetric twist grain boundaries (STGB & STwGB) in Al for a range of impact velocities (1 to 3.5 km/s) using the open source LAMMPS package. The calculated shock velocity (US) to particle velocity (UP) relationship for Al single crystal (SC) matches well with the results of ab-initio and experiments. However for the shock simulations in bi-crystals (biC), the calculated shock velocity range fall in the same order of magnitude but does not show a linear, monotonic, US vs UP relationship. The interaction of the shock with the bi-crystal boundary, and shock propagation along different crystal directions on either side of the bi-crystal boundary are possible reasons for such behaviour. Common Neighbour Analysis (CNA) shows that there are more structural changes occurring in the bicrystal than in the single crystal. This is because the impact is along a different plane than the 〈0 1 0〉 direction as is the case of a single direction. We report effect of grain boundary on the calculated shock velocity.

Arrangement and Decomposition of Grain Boundary Dislocations: Two-Mode Phase-Field Crystal Simulation

The grain-boundary dislocation arrangement and decomposition during constant-volume deformation of a nanoscale bi-crystal system in fcc-structured materials were studied by using the two-mode phase-field crystal (2PFC) method. The effects of different grain boundary misorientations (GBMs) and tensile deformation directions on the dislocation arrangement and decomposition are analyzed. In three different symmetrical tilt grain boundaries evaluated by PFC, the atomic density profile of grain boundaries changed periodically at equilibrium. The initial grain boundary dislocation arrangement of the three samples is almost the same when tensile deformation is applied to the samples in the x- or y- direction, and all are symmetrically arranged in a “bowknot ”structure. The stress at the grain boundary is concentrated with the increase of strain, and dislocation decomposition can effectively reduce the stress concentration. The time steps of dislocation decomposition at grain boundaries decreases with increasing strain rate. This work facilitates the application of PFC in the analysis of grain-boundary mechanics in an extended range of materials.

Rapid dehydrogenation of metallic materials under external electric field

The interaction between hydrogen and metal materials can lead to catastrophic failure of metallic materials, and while some metal materials can use this effect as hydrogen storage materials to make better use of hydrogen energy. If dehydrogenation is easier, it will help control hydrogen-induced degradation and the utilization of metallic hydrogen storage materials. In this study, molecular dynamics simulations are used to apply an electric field in the bicrystal hydrogen-iron system to study the diffusion coefficient of hydrogen atoms. Simultaneously, using hydrogen-charged high-strength as a model system, the effect of electric field on hydrogen content and mechanical properties of hydrogen-charged high-strength steel was also studied. Experimental and molecular simulation results show that, compared with traditional tempering, the diffusion rate of hydrogen atoms is faster and the diffusion activation energy is smaller under the action of an electric field, which is more conducive to the recovery of the mechanical properties of hydrogen-charged high-strength steel. Due to the introduction of an electric field to cause additional electrical energy, the energy barrier for hydrogen atom diffusion is lowered. At the same time, due to the influence of electromigration, the diffusion of hydrogen atoms is driven by an electron wind force, which eventually leads to an acceleration of the dehydrogenation rate.

(Invited) Vertically Stacked Junction Devices Fabricated Using Single-Crystal Graphene on SiC Substrate

Graphene is a potential post-silicon nanomaterial because of its tremendous properties. Single-crystal graphene epitaxially grown on a SiC substrate is the most promising candidate for future electron devices. Single-crystal graphene with a 100 mm2 size has been successfully fabricated using a high-temperature rapid thermal annealing system [1]. In this study, two types of stacked graphene devices using graphene on SiC are demonstrated. The first device is a bi-crystal system consisting of two graphene samples as shown in Fig. 1 (a). These samples are directly bonded in a face-to-face manner. Contact pins are attached to the graphene surface for ohmic contact. The electrical characteristics of the stacked graphene junction were ohmic at a low bias voltage below 20 V. The linear characteristic at a low bias voltage is attributed to the linear dispersion relation in the band structure of graphene, Dirac cone. At a high bias voltage, the conductance of the graphene junction drastically increased. This nonlinear behavior originates from a semi-metallic property far from the Dirac point. Thermal imaging results indicated that electrical power was directly converted to infrared radiation. The radiation efficiency was estimated to be approximately 10%, which was high compared to that of a previously reported graphene blackbody emitter. The infrared emission spectrum measured using FT-IR was blackbody-like [2]. The peak wavelength of the measured spectrum was almost constant at 10.2 µm with changing applied power. The estimated junction voltage was approximately 5 V at a bias voltage of 80 V. Hot electrons were injected to the graphene layer. The results suggested that infrared emissions from the graphene–graphene stacked junction could be attributed to a nonthermal emission mechanism, such as phonon–plasmon coupling. The other vertically stacked graphene junction is a tunneling diode as shown in Fig. 1 (b). An insulative layer is sandwiched between two graphene samples. A structured water layer [3] formed by deionized water treatment on the epitaxial graphene sample acts as a tunneling barrier. Nonlinear I–V characteristics were observed. The dynamic range of the current change reached the sixth order of magnitude. The estimated thickness of the structured water layer was approximately 0.5 nm. As the tunneling barrier was sufficiently thin, a direct tunneling phenomenon could be observed in a low bias regime. In a high bias regime, the Fowler–Nordheim tunneling phenomenon was observed. A sandwich structure of a vertically stacked graphene junction with a sub-nm-thick structured water layer was constructed through simple water treatment and a direct bonding technique [4]. [1] T. Aritsuki, et al., Jpn. J. Appl. Phys. 55 (2016) 06GF03. [2] N. Murakami, et al., Jpn. J. Appl. Phys. 60 (2021) SCCD01. [3] M. Kitaoka, et al., Jpn. J. Appl. Phys. 56 (2017) 085102. [4] J. Du, et al., Jpn. J. Appl. Phys. 58 (2019) SDDE01. Figure 1

Deterministic modeling of solidification microstructure formation in directed energy deposition fabricated Ti6Al4V

Metal additive manufacturing (MAM) technology is now widely applied in the manufacturing and remanufacturing industry, among which microstructure simulation gradually shows its importance. Traditional solidification microstructure simulation methods all have their merits as well as drawbacks when applied in MAM cases. In this work, a deterministic solidification microstructure model, named “invasion model”, is established to avoid the essential defects of traditional methods. This model focuses on the interaction between the neighboring bi-crystals instead of simulating the growth kinetics of each columnar grain or deriving the field form of variables. Within a bi-crystal system, the grain boundary tilt from thermal gradient direction is understood as a transient invasion behavior of one grain to another, and the competitive grain growth behavior along the buildup process of MAM is a summary of all the invasions in bi-crystal systems. To fill the gaps in the rapid solidification theory, a database recording the anisotropic growth effect under a rapid directional solidification condition was established with the help of an artificial neural network (ANN). Wire feed directed energy deposition (DED) fabricated Ti6Al4V thin-wall samples with full dendritic columnar grains (prior beta) are used as a benchmark to test the validation of the novel simulation model. The grain geometrical structure of reconstructed prior β grains along the build-up direction has a good agreement with the simulation result. The model can also be applied to other cases of MAM or combine with various models to achieve real-time as-solidified crystallographic feature prediction when it meets the scope of application. • Traditional solidification microstructure simulation methods all have their merits and drawbacks when applied in MAM cases. • Invasion model is established to investigate the transient competitive grain growth behavior within bi-crystal systems. • Wire DED Ti6Al4V thin-wall samples are used as a benchmark to test the validation of the novel simulation model.

A crystal plasticity model with an atomistically informed description of grain boundary sliding for improved predictions of deformation fields

Crystal plasticity (CP) is a powerful meso-scale technique for deformation modeling in polycrystalline materials. Crystal plasticity models typically do not include an explicit description of grain boundary (GB) sliding, which could lead to inaccurate predictions of strain distributions, especially in the vicinity of GBs. In the present study, a CP model is developed that includes GB sliding as an additional deformation mechanism and models the interaction between slip and GB sliding. Atomistic simulations are used to formulate the constitutive model for GB sliding and its interaction with incident slip. The deformation fields and structure of the GBs are obtained directly from experimental characterization and are faithfully reproduced as bicrystal systems for molecular dynamics simulations. The GBs are modeled as random-type (non-coincidence, asymmetrical) boundaries as observed from experimental data. The underlying atomistic-scale mechanism for pure sliding of random GBs was found to be analogous to fluid-flow. The interaction between incident slip and a sliding GB caused local increases in stress concentration, which further led to a momentary increase in the local sliding rate. The displacement profiles at the sliding GBs computed from the CP model with sliding accommodation are 38% more accurate than the baseline CP model as quantified by the mean squared error between the simulations and experiment. These results help improve the sophistication and accuracy of deformation modeling by including physics-based descriptions for GB sliding and its interaction with slip, eventually leading to more reliable predictions of micromechanical quantities.

Phase-field-crystal study on shear-induced coupled evolution of intragranular crack and grain boundary in nanoscale bicrystal system

Shear-induced coupled evolution behavior of intragranular crack and grain boundary (GB) in nanoscale bicrystal system is studied by using phase-field-crystal method. Evolution process of system with different crack sizes and grain boundary misorientations (GBMs) is analyzed. Instead of normally extending under applied strain, intragranular crack can partially heal and even disappear under shear strain. This healing process is realized by compression stress field provided by GB migration. Increasing GBM and decreasing crack size can both facilitate crack healing. Besides, with the existence of intragranular crack, centripetal GB migration can be accelerated. The acceleration effect will be decreased by large GBM and small crack size. Low GBM is a prerequisite, but decreasing crack size can only make this effect not obvious. Increasing GBM or crack size has opposite effects on crack healing and centripetal GB migration.

Phase-field modeling of crack propagation in polycrystalline materials

A phase-field model based on a modified form of the regularized formulation of Griffith’s fracture theory is presented to investigate intergranular and transgranular crack propagations in polycrystalline brittle materials. Grains and grain boundaries are incorporated in the crack initiation and propagation model based on a phase-field model for grain growth, in which the elastic anisotropy varies based on the grain orientation angle, and the grain boundary energy is related to the misorientation angle of the adjacent grains. Correction parameters are utilized in the total free energy functional and mechanical equilibrium equations to consider the effect of material strength on crack nucleation and propagation independent of the regularization parameter. This allows controlling the strength and crack surface energy along the grain boundaries as a function of the misorientation angle in order to mediate intergranular and/or transgranular crack propagation. To demonstrate the capability of the proposed model, intergranular and transgranular crack propagation in ZrB2 bicrystal systems under tensile loading are studied in detail. The effects of grain boundary misorientation angle, grain boundary inclination with respect to initial crack direction, and grain boundary strength (and/or crack surface energy) on the crack propagation path are investigated. Intergranular crack propagation can be promoted by specific combinations of grain boundary strength and crack surface energy, which can contribute to the fracture toughness of polycrystalline materials.

Grain Boundary Motion in Two-Dimensional Hexagonal Boron Nitride.

An in-depth understanding and precise controlling of grain boundary (GB) motion at the atomic scale are crucial for grain growth and recrystallization in polycrystalline materials. So far, the reported studies mainly focus on the GB motion in the ideal bicrystal system, while the atomic mechanisms of GB motion in polycrystals remain poorly understood. Herein, taking two-dimensional (2D) hexagonal boron nitride (h-BN) as a model system, we experimentally investigated the atomic-scale mechanisms of the GB motion in 2D polycrystals. Since GB motion is directly related to the GB structures, this article is organized following the configurations of GBs, which can be divided into straight (including symmetric and asymmetric GBs) and curved GBs. We revealed that (I) for symmetric GBs, the shear-coupled motion alone is insufficient to drive the continuous GB motion in polycrystalline materials, and GB sliding is also needed. (II) For asymmetric GBs, GB motion follows a defaceting-faceting process, in which dislocation reactions are crucial. (III) For curved GBs, shear-coupled GB motion (during grain shrinking) leads to grain rotation, and the rotation direction highly depends on the misorientation angles. (IV) Finally, we will discuss the characteristics of binary lattice h-BN and find that partial dislocations participate in the GB motion at high misorientation angles (>38°). Our results build up the framework of the atomic-scale mechanisms of the GB motion in 2D polycrystalline materials and will be instructive for technological applications such as grain growth and GB engineering.

Machine Learning for Competitive Grain Growth Behavior in Additive Manufacturing Ti6Al4V

Metal additive manufacturing (MAM) technology is now changing the pattern of the high-end manufacturing industry, among which MAM fabricated Ti6Al4V has been far the most extensively investigated material and attracts a lot of research interests. This work established a deep neural network (DNN) to investigate the grain boundary in competitive grain growth for a bi-crystal system, the column β grains of Ti6Al4V as an example. Because of the limited number of experimental samples, the DNN is trained based on the data coming from the Geometrical Limited criterion. A series of direct energy deposition experiment using Ti6Al4V is carried out under the Taguchi experimental design. The grain boundary angles between the column grains are measured in the experiment and used to evaluate the accuracy of DNN.

Revealing the Strain Effect on Radiation Response of Amorphous–Crystalline Cu-Zr Laminate

Nanocrystalline materials containing amorphous intergranular films (AIFs) exhibit excellent mechanical properties, radiation resistance, and thermal stability and may serve as promising candidate materials for use in advanced nuclear energy systems. The aim of this work is to reveal the effect of mechanical stress on the radiation damage behavior of AIF systems. Based on a bicrystal Cu system with Zr-doped AIFs, molecular dynamics is used to simulate the radiation process and examine the AIF sink efficiency, defect propensity, defect size distribution, and Zr mixing under uniaxial and hydrostatic strain conditions. The results show that the sink efficiency of the glue-like AIFs is not compromised under applied strains. The anisotropy resulting from the intrinsic microstructure and elastic deformation leads to a distinct radiation response, where extension (contraction) of the structure perpendicular to the AIFs increases (decreases) the vacancy density. The strain-dependent defect density, along with the cluster size distributions, can be interpreted based on the variations in the defect formation energy and anisotropic defect diffusion. Finally, the Zr mixing induced by collision cascades is found to be insensitive to the mechanical strains. These findings provide meaningful information towards understanding the stress effect on the radiation response of AIF systems.

Modeling the effects of helium-vacancy clusters on the stress-strain response of a grain boundary in iron by a mechanistic finite element approach informed by molecular dynamics data

The effects of helium (He)-vacancy clusters on the stress-strain behavior of polycrystalline iron (α-Fe) are investigated by a mechanistic finite element (FE) approach using a continuum damage mechanics (CDM) description of the material behavior informed by molecular dynamics (MD) data. First, MD analyses of a single crystal (loading normal to {332} plane) and a bicrystal system containing a Σ11<110>{332} grain boundary (GB) were performed to compute the uniaxial tensile response of an Fe single crystal and a system with a GB. MD results were then used in FE analyses of the same systems to identify parameters for the CDM constitutive relations for the crystal and the traction-separation law for the GB depicted by cohesive elements. Next, a 3D FE model of an α-Fe bicrystal system with an imperfect GB subjected to uniaxial tensile loading was developed. This model includes an equivalent hollow sphere under internal pressure in the middle of the GB to model the effects of pressurized He bubbles at 5 K, room temperature (RT) and 600 K on stress, strain and damage distributions. The radius of the equivalent sphere was determined assuming the presence of two vacancies in the system. Finally, MD stress/strain data of the same bicrystal system with He-vacancy clusters were compared to the corresponding FE results to validate this mechanistic approach that appears to be efficient in terms of computational time. FE model predictions of system strength and fracture strain are in fairly good agreement with the MD results at all three temperatures. Our results show that small and highly pressurized He-vacancy clusters reduce GB strength and fracture strain more significantly at 5 K than at RT and 600 K.

Phase field modeling of grain boundary migration and preferential grain growth driven by electric current stressing

A phase field model incorporating the electrostatic free energy and the grain orientation effect is developed and employed to study the grain boundary migration and preferential grain growth in widely used beta-tin (β-Sn) under electric current stressing. The directional migration of grain boundaries and the preferential growth of the grain with its orientation having low electrical resistivity along the electric current direction are theoretically clarified. In a bicrystal system containing a circular grain, the shrinkage velocity and morphology changes of grains are dominated by the competition effect between the grain boundary energy and the electrostatic free energy; in particular, the high-density electric current can induce the instability of grain morphology evolution. Moreover, grain morphology evolution leads to the change of the voltage across the β-Sn system; it is found that the voltage decreases over time in a tricrystal system, while the variation of the voltage across the bicrystal system is related to the above-mentioned competition effect. The proposed model and results provide insights into the orientation-related microstructure evolution under electric current stressing.

Modeling hydrogen solvus in zirconium solution by the mesoscale phase-field modeling code Hyrax

Zirconium-based alloys are common materials for light water reactor (LWR) fuel cladding. These alloys readily absorb hydrogen and are subjected to lose ductility due to hydride accumulation. A phase-field modeling code with Calphad-based free energy functions, Hyrax, has been used to model the hydrogen solvus in α-zirconium solution and the formation of the δ zirconium-hydride phase in the α-zirconium matrix. The modeled hydrogen solvus was compared against published experimental data; this is considered the first direct validation of Hyrax output. The effect of external stress on hydrogen solvus and hydride formation has also been modeled. A tensile stress was uniformly applied to a single zirconium crystal and a bi-crystal system. We observed that the stress does not affect hydrogen solvus but does cause hydride to accumulate in the crystalline which has the c-axis parallel to the stress direction. This is because the external stress creates a strain energy gradient across the system; the δ-hydride preferentially precipitates in the low strain energy region which yields more lattice misfit strain to compensate the gradient.

Cohesive finite element modeling of the delamination of HTPB binder and HMX crystals under tensile loading

Accurately modeling the mechanical behavior of the polymer binders and the degradation of interfaces between binder and crystal is important to science-based understanding of the macro-scale response of polymer bonded explosives. The paper presents a description of relatively a simple bi-crystal HMX-HTPB specimen and associated tensile loading experiment including computed tomography imaging, the pertinent constitutive theory, and details of numerical simulations used to infer the behavior of the material during the delamination process. Within this work, mechanical testing and direct numerical simulation of this relatively simple bi-crystal system enabled reasonable isolation of binder-crystal interface delamination, in which the effects of the complicated thermomechanical response of explosive crystals were minimized. Cohesive finite element modeling of the degradation and delamination of the interface between a modified HTPB binder and HMX crystals was used to reproduce observed results from tensile loading experiments on bi-crystal specimens. Several comparisons are made with experimental measurements in order to identify appropriate constitutive behavior of the binder and appropriate parameters for the cohesive traction-separation behavior of the crystal-binder interface.This research demonstrates the utility of directly modeling the delamination between binder and crystal within crystal-binder-crystal tensile specimen towards characterizing the behavior of these interfaces in a manner amenable to larger scale simulation of polycrystalline PBX materials. One critical aspect of this approach is micro computed tomography imaging conducted during the experiments, which enabled comparison of delamination patterns between the direct numerical simulation and actual specimen. In addition to optimizing the cohesive interface parameters, one important finding from this investigation is that understanding and representing the strain-hardening plasticity of HTPB binder is important within the context of using a cohesive traction-separation model for the delamination of a crystal-binder system.

A phase field dislocation dynamics model for a bicrystal interface system: An investigation into dislocation slip transmission across cube-on-cube interfaces

In this work, we present a phase field dislocation dynamics formulation designed to treat a system comprised of two materials differing in moduli and lattice parameters that meet at a common interface. We apply the model to calculate the critical stress τcrit required to transmit a perfect dislocation across the bimaterial interface with a cube-on-cube orientation relationship. The calculation of τcrit accounts for the effects of: 1) the lattice mismatch (misfit or coherency stresses), 2) the elastic moduli mismatch (Koehler forces or image stresses), and 3) the formation of the residual dislocation in the interface. Our results show that the value of τcrit associated with the transmission of a dislocation from material 1 to material 2 is not the same as that from material 2 to material 1. Dislocation transmission from the material with the lower shear modulus and larger lattice parameter tends to be easier than the reverse and this apparent asymmetry in τcrit generally increases with increases in either lattice or moduli mismatch or both. In efforts to clarify the roles of lattice and moduli mismatch, we construct an analytical model for τcrit based on the formation energy of the residual dislocation. We show that path dependence in this energetic barrier can explain the asymmetry seen in the calculated τcrit values. Significantly, the analysis reveals that τcrit scales with a(2)G(2)a(1)+a(2)(a(1)a(2)−G(1)G(2))2, where G is the shear modulus, a is the lattice parameter, and the superscripts (1) and (2) indicate quantities for material 1 and material 2, respectively.

Effects of misorientation and inclination on mechanical response of 〈1 1 0〉 tilt grain boundaries in α-Fe to external stresses

Due to the industrial importance of α-iron-based polycrystalline materials, their grain boundary (GB) structures and properties need to be well characterized and understood in order to optimize the materials through effective GB engineering. In this study, a molecular dynamics (MD) simulation study was performed to investigate a series of 〈1 1 0〉 symmetric tilt grain boundaries (STGBs) and asymmetric tilt grain boundaries (ATGBs) in α-iron. It is shown that the GB energy is proportional to the GB volumetric expansion. During uniaxial deformation, 〈1 1 1〉{1 1 2} twinning appears to be more competitive or easier than 〈1 1 1〉{1 1 2} dislocation emission from the GB at yielding. For bicrystal systems containing STGBs the yield strength obeys the Schmid law, while for ATGB bicrystal systems the yield strengths are mainly determined by the local stress rather than overall stress and average GB energy. The higher degree of atomic disordering in the ATGB regions generates larger local stress fluctuation and thus facilitates local defect emission when subjected to external stresses.