Symmetric Grain Research Articles

Grain boundary migration and mechanical properties altering in Fe – 10Ni – 20Cr alloy under irradiation

Mechanisms of ∑5(210)[001] and ∑5(310)[001] symmetrical tilled grain boundaries migration in bicrystall Fe – 10Ni – 20Cr samples under irradiation were investigated by means of molecular dynamics. The density of radiation defects grows quite quickly up to a dose of ~0.02 dpa and then reaches saturation. This is due to balancing of the radiation defects generation and annihilation rates. It is shown that at the early stage of irradiation, grain boundaries began to deviate stochastically from their initial positions due to interaction with cascades of atomic displacements and absorption of structural defects. During irradiation, the grain boundary region thickened and became rough. With an increase in the radiation dose, size of the clusters of point defects (tetrahedrons of stacking faults and dislocation loops) increased. Interaction with large clusters of point defects led to the formation of bends on initially flat surfaces of grain boundaries. At small distances between the boundaries, the high driving force between the curved surfaces of grain boundaries significantly increased the rates of their approach. The average migration rates of grain boundaries before their direct interaction with each other were approximately 0.8 m/s. As a result of their approach, the grain boundaries were annihilated, the potential energy of the sample decreased abruptly, and the grains merged. The annihilation of grain boundaries ∑5(310)[001] required twice the radiation dose compared to the grain boundary ∑5(210)[001]. The direct interaction of grain boundaries with each other abruptly increased the velocity of their migration due to the emergence of a driving force from the curved sections of the grain boundary surfaces. Influence of the radiation dose on deformation behavior features of the samples under uniaxial strains was studied. With an increase in the radiation dose, the elastic limit decreased rapidly and reached saturation at an irradiation dose of ~0.01 dpa.

The effects of twist angle and loading mode on the deformation behavior of bicrystal copper

The effects of twist angle and loading mode on deformation behavior of bicrystal Cu containing symmetric twist grain boundary are investigated by molecular dynamics. Grain boundary dislocation network changes from square-like to ring-like with increasing twist angle. Tensile strength under loading parallel to grain boundary is higher than that under perpendicular loading to grain boundary for the same model, and the larger twist angle is, the larger difference under two loading modes is. Under perpendicular and parallel loadings, initial plastic yield of bicrystal Cu with low angle is triggered by slipping of grain boundary partial dislocations, which results in almost complete dissociation of grain boundary and formation of pillar-like voids inside grains. However, initial plastic yields of bicrystal Cu with higher angle are triggered by explosive collective slipping of short grain boundary partial dislocation and nucleating of new lattice dislocations on grain boundary, respectively, which result in direct splitting of grain boundary and formation of sphere-like voids, respectively. The calculated increase rates of partial dislocation number increase with increasing twist angle, but the increase amplitude is significantly larger under parallel loading. In later stage, void volume of each model is almost the same, but void surface area is significantly different.

Influence of iron anisotropy on phase transition near grain boundary under shock

As is well known, iron plays an important role in the fields of national defense and industry, so it is of great significance to study its behavior under dynamic high pressure. As one of the most common defects in metals in nature, grain boundaries have an important influence on the mechanical properties and deformation of materials under shock. This work intends to use large-scale non-equilibrium molecular dynamics simulation to study the influence of iron anisotropy on the phase transition in bicrystal under shock loading. By tracking the local structure, shear stress distribution and morphology characteristics after shock on both sides of the grain boundaries, the factors affecting the response on both sides of the grain boundary are analyzed. Our research result shows that shocking along the non-centrosymmetric grain direction can cause significant differences in the phase transition threshold, path and mode on both sides of the symmetric grain boundary. Especially, the different phase transition dynamic processes on both sides of the sigma11 grain boundary are discussed in detail in this work, which have been rarely discussed in previous studies. Considering the symmetry of the microstructure on both sides of this type of grain boundary, the result of asymmetric shock response is different from people’s inertial cognition. Finally, it is found that the atoms in both models will shift along the direction perpendicular to the shocked direction under shock, indicating that the shock wave generated by the piston method should no longer be simply regarded as one-dimensional when shocked along the non centrosymmetric crystal direction, and the displacement of atoms along the direction perpendicular to the shocked direction is closely related to the symmetry of the crystal, which causes significant differences in shear stress on both sides of the grain boundary and ultimately affects the shock response. This study reveals that the anisotropy of lattice has an important effect on the phase transition on both sides of grain boundaries under shock loading, which can provide theoretical support for the experimental studies of polycrystalline metals and alloys under shock.

Effect of irradiation on mechanical properties of symmetrical grain boundaries in BCC tungsten: an atomistic study

Effect of irradiation on mechanical properties of symmetrical grain boundaries in BCC tungsten: an atomistic study

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.

Suppression of dislocations and twins by inducing asymmetrical grain boundaries for casting high-quality monocrystalline silicon ingot

Cast monocrystalline silicon (CMC-Si) is a promising photovoltaic substrate owing to the advantage of low cost of directional solidification. However, the high dislocation density and other crystal defects, such as twins, have limited its further application. We proposed a new seed pavement method, namely, the construction of asymmetrical large angle grain boundaries, to suppress the generation of dislocations and other crystal defects. Two CMC-Si ingots, Ingot-A for the traditional method of seed pavement and Ingot-B for the new method of seed pavement, have been casted. The results show that, the crystal quality of Ingot-A is remarkably lower than that of Ingot-B. For Ingot-A, a large number of dislocations are induced, such as small angle grain boundaries, which come from the seed junctions due to size error of the seeds. And some twins are brought out at the symmetric grain boundaries, which reduces the proportion of the (100) crystal plane. For Ingot-B, there are large angle orientation relationships between the adjacent seeds. This asymmetrical large angle grain boundaries have not formed small angle boundaries, and successfully prevented the generation of dislocations and twins. The average conversion efficiency of Ingot-B (21.81%) is significantly higher than that of Ingot-A (20.63%).

Evolution of Symmetrical Grain Boundaries under External Strain in Iron Investigated by Molecular Dynamics Method

In the present work, the evolution of atomic structures and related changes in energy state, atomic displacement and free volume of symmetrical grain boundaries (GB) under the effects of external strain in body-centered cubic (bcc) iron are investigated by the molecular dynamics (MD) method. The results indicate that without external strain, full MD relaxations at high temperatures are necessary to obtain the lower energy states of GBs, especially for GBs that have lost the symmetrical feature near GB planes following MD relaxations. Under external strain, two mechanisms are explored for the failure of these GBs, including slip system activation, dislocation nucleation and dislocation network formation induced directly by either the external strain field or by phase transformation from the initial bcc to fcc structure under the effects of external strain. Detailed analysis shows that the change in free volume is related to local structure changes in these two mechanisms, and can also lead to increases in local stress concentration. These findings provide a new explanation for the failure of GBs in BCC iron systems.

Induction of compression wood inhibits development of spiral grain in radiata pine

Summary Spiral grain refers to the helical patterns formed by the wood grain in the trunks of many tree species. In most gymnosperms, grain near the pith is vertical but wood formed after several years of growth has a slight to pronounced left-handed twist. Grain changes presumably involve the slow rotation of cells within the vascular cambium, but the mechanisms that allow this reorientation to occur remain unclear. Understanding this process is, however, important as the presence of strong spiral grain within the corewood of gymnosperms is a major wood quality issue devaluing cut timber. In this study, we measured wood grain in stems of Pinus radiata (radiata pine) saplings through reconstructions of resin canals that follow the grain, visualised by serial sectioning and scanning with circularly polarised light, and through X-ray computed microtomography (μCT) and image analysis in ImageJ. Vertical trees retained a symmetrical grain pattern that was weakly right-handed near the pith, but which became progressively more left-handed during the first eight months of growth. In tilted trees, however, the development of left-handed grain was inhibited by the formation of compression wood on the lower side of the tree whereas the wood on the upper side of the tree developed increasingly more left-handed grain as in the vertical controls. These results demonstrate that a previously unidentified link exists between compression wood formation and the inhibition of grain development.

Phase field crystal simulation of strain-induced square phase low-angle symmetric tilt grain boundary dislocation reaction

In this paper, the phase field crystal method is used to study the dislocation motion and reaction of the square phase symmetric tilt low-angle grain boundaries, and the dislocation configurations with different misorientation angles are analyzed under the action of applied strain. The geometric phase approach is used to characterize the strain field around the dislocations. The results show that after the solidification relaxation, the interfacial dislocations on both sides of the grain are distributed in parallel but opposite direction. With the increase of misorientation angle between grains, the number of dislocations increases, the spacing between them decreases, and the free energy of the system increases. Imposed by the applied strain, the grain boundary dislocations undergo climbing, launching, and reactive annihilation, with the free energy fluctuating. When the misorientation increases, the dislocation motion mode changes from climbing to climbing-sliping, resulting in more dislocation group configurations, and more reactions between dislocations and dislocation groups. For the dislocation reactions of different configurations, positive shear strain drives dislocations to approach, and negative shear strain drives dislocations to annihilate.

An insight into thermal properties of BC3-graphene hetero-nanosheets: a molecular dynamics study

Simulation of thermal properties of graphene hetero-nanosheets is a key step in understanding their performance in nano-electronics where thermal loads and shocks are highly likely. Herein we combine graphene and boron-carbide nanosheets (BC3N) heterogeneous structures to obtain BC3N-graphene hetero-nanosheet (BC3GrHs) as a model semiconductor with tunable properties. Poor thermal properties of such heterostructures would curb their long-term practice. BC3GrHs may be imperfect with grain boundaries comprising non-hexagonal rings, heptagons, and pentagons as topological defects. Therefore, a realistic picture of the thermal properties of BC3GrHs necessitates consideration of grain boundaries of heptagon-pentagon defect pairs. Herein thermal properties of BC3GrHs with various defects were evaluated applying molecular dynamic (MD) simulation. First, temperature profiles along BC3GrHs interface with symmetric and asymmetric pentagon-heptagon pairs at 300 K, ΔT = 40 K, and zero strain were compared. Next, the effect of temperature, strain, and temperature gradient (ΔT) on Kaptiza resistance (interfacial thermal resistance at the grain boundary) was visualized. It was found that Kapitza resistance increases upon an increase of defect density in the grain boundary. Besides, among symmetric grain boundaries, 5–7–6–6 and 5–7–5–7 defect pairs showed the lowest (2 × 10–10 m2 K W−1) and highest (4.9 × 10–10 m2 K W−1) values of Kapitza resistance, respectively. Regarding parameters affecting Kapitza resistance, increased temperature and strain caused the rise and drop in Kaptiza thermal resistance, respectively. However, lengthier nanosheets had lower Kapitza thermal resistance. Moreover, changes in temperature gradient had a negligible effect on the Kapitza resistance.

Grain boundary energy in <112-0>/φ pure ice bicrystals samples

This paper presents the experimental values of relative energy γ gb γ s for grain boundaries (GB) in < 11 2 - 0 > / φ bicrystal samples with different misorientations and inclinations at −5°C and −18 °C. The relative energy γ gb γ s depends on misorientation and inclination of grain boundary showing greater variability at −18 °C than at −5°C. All the samples studied show that γ gb γ s increases with grain boundary inclination at −18 °C and that it is higher in asymmetric than in symmetric grain boundaries. Finally, the values of γ gb γ s were compared to a grain boundary structure model based on the lattice of coincident sites (CSL) for ice.

Measuring simulated hydrogen diffusion in symmetric tilt nickel grain boundaries and examining the relevance of the Borisov relationship for individual boundary diffusion

Grain boundaries are known to act as important diffusion pathways in metals. Here we examine how hydrogen diffusion in nickel grain boundaries is affected by different grain boundary types. We simulate 26 [100] symmetric tilt grain boundaries at several temperatures and develop a modified mean squared displacement method for isolating the grain boundary diffusivity from the bulk diffusivity. We analyze the diffusivity data for the different boundaries and find the grain boundary diffusivity to be higher than the bulk diffusivity up to a temperature where the two converge. We test a model developed by Borisov et al. (Phys. Met. Metallogr.17, 1964, 80-84) that relates grain boundary diffusion to grain boundary interfacial energy. Despite the fact that the Borisov relationship has never been used to examine solute interstitial diffusion before, it fits the simulated diffusion data well for temperatures where the grain boundary diffusivity is higher than the bulk diffusivity. Some parameters in the Borisov relationship, related to the geometry of the grain boundary and its activated states, have always taken on assumed values because they are hard to measure or acquire. The fit of the simulated data provides estimates for these values that are physically realistic and provides evidence that the use of the traditionally assumed values should be reconsidered. This work also demonstrates that while there is a need for further validation, the Borisov relationship can reasonably relate solute interstitial diffusivity of hydrogen to the interfacial energy of individual [100] symmetric tilt nickel grain boundaries, whereas in the past it has been used to relate self diffusion and interfacial energy of a grain boundary network as a whole.

AB INITIO МОДЕЛИРОВАНИЕ ВЗАИМОДЕЙСТВИЯ P, H, C, S С ГРАНИЦАМИ ЗЕРЕН В α-ЖЕЛЕЗЕ

The results of modeling from the first principles of interaction of nonmetallic impurities of interstitial (H, C) and substitutional (P, S) with grain boundaries in α-iron are presented. The modeling has been conducted within the framework of the density functional theory (DFT) by the full-potential linearized augmented plane waves (FP LAPW) method with consideration to the generalized gradient approximation (GGA'96) in the WIEN2k software package. Three grain boundaries of the slope Σ3 (111), Σ5 (210) and Σ5 (310) are studied. The supercells of the tilt grain boundaries using the coincidence site lattice model is constructed. The values of the energy characteristics of various grain boundaries with impurities are influenced by a number of factors, namely, the volume of the Voronoi polyhedron per impurity, magnetic moments, and the symmetry of the surrounding matrix. The results show that symmetric grain boundaries Σ3 (111) and Σ5 (310) are embrittled by phosphorus, hydrogen, and sulfur, while carbon strengthens interatomic bonds at the grain boundary, which coincides with the data available in the work. In the case of an asymmetric grain boundary Σ5 (210), phosphorus and hydrogen weaken bonds at the grain boundary, while sulfur strengthens them. This is primarily explained by the geometry of the surrounding matrix. The magnetic moments at the impurity atoms are very small and, in most cases, are antiparallel to the magnetic moments at the neighboring Fe atoms.

Traction-separation laws of graphene grain boundaries.

Molecular dynamics simulations are used to extract the traction-separation laws (TSLs) of symmetric grain boundaries of graphene. Grain boundaries with realistic atomic structures are constructed using different types of dislocations. The TSLs of grain boundaries are extracted by using cohesive zone volume elements (CZVEs) ahead of the crack tip. The traction and separation of each cohesive zone volume element are calculated during the crack growth. The traction and separation values obtained for the cohesive elements predict that the TSLs of grain boundaries have a bilinear form. The areas under the traction-separation curves are used to calculate the separation energy of the grain boundaries. The results show that as the grain boundary misorientation angle increases the separation energy of the grain boundaries decreases. The impact of temperature on the traction separation laws is studied. The results show that, with an increase of the temperature from 0.1 K to 300 K, the separation energy first increases to reach its peak at around 25 K and then slightly decreases.

Role of grain boundaries and substrate in plastic deformation of core–shell nanostructures

The mechanical properties of metallic systems at the nanoscale can be modified by varying their structure and composition. One example of a deformation-resistant structure is a core–shell nanostructure (CSN). In the present work, we use molecular dynamics simulations to perform nanoindentation and retractions to understand plastic deformation of core–shell nanostructures. The core consists of aluminum (Al), the shell is amorphous silicon (a-Si), and the substrate is either crystalline Al or a-Si. The unique aspect of this work is that we study the deformation behavior of CSNs that contain symmetric and asymmetric grain boundaries in the core with two different orientations; comparisons are made to deformation in a CSN with a single-crystal core. Nanoindentation on CSNs with 5 nm and 10 nm core radii shows that the elastic stiffness with and without a grain boundary is similar when the substrate material is the same. Five-nanometer-core-radius CSNs with a-Si and Al substrates and an asymmetric tilt grain boundary in the core show 100% recovery from dislocation plasticity, but damage within the core leads to reduction of ~ 50% of the atoms that no longer being identified as FCC crystal structure, which makes these CSNs non-deformation-resistant. CSNs with Al substrate (5 nm and 10 nm core radii) obtained 100% recovery from plasticity and retained its crystal structure after unloading when the core is single crystal or contains a symmetric tilt grain boundary. Moreover, a single-crystal core can withstand nanoindentation to 100% of the shell thickness, whereas a symmetric tilt grain boundary core can only withstand nanoindentation about 80% of the shell thickness and still have 100% recovery from plasticity. Therefore, CSNs with single-crystal core are more reliable for deformation-resistant behavior than those that contain grain boundaries.

MD simulation on crystallization mechanisms of rapidly supercooled Fe-Ni alloys

Abstract A molecular dynamics (MD) simulation has been conducted to investigate the crystallization mechanism during rapid solidification of Fe-Ni alloys. The microstructural evolution is analyzed in terms of the system energy, the pair distribution function (PDF), the largest standard cluster analysis (LSCA), the structure entropy, and visualization. Results indicate that Fe10Ni90, Fe20Ni80, Fe30Ni70 and Fe90Ni10 are rapidly cooled into crystalline solids. The effect of composition of alloys on chemical order of liquid is rather weak, while stronger on solids and Fe20Ni80 has the relative strongest chemical order. Among the Fe10Ni90 alloy, five-fold symmetrical grain boundaries including truncated decahedron atoms hamper the hcp → fcc transition due to their different orientations. But the parallel grain boundaries in Fe30Ni70 is easy covert, resulting in a pathway of supercooled-liquid → (fcc + hcp) → fcc states. Moreover, crystallization is accompanied with the reduction in the number of cluster kinds, the average potential energy, and the structure entropy. In addition, the transformation process of crystal structures at the atomic scale are unveiled by employing the tracing method. These findings will significantly advance the understanding on the crystallization of liquid alloy as well as other metals.

Modelling Hydrogen Embrittlement using Density Functional Theory: A theoretical approach to understanding environmentally assisted cracking in 7xxx series aluminium alloys

The effects of H segregation to a Σ11 symmetric tilt Al grain boundary are investigated using atomistic simulations, as part of a wider study on cracking in 7xxx series alloys. Density functional theory based simulations of uniaxial straining of grain boundaries containing 11 different concentrations of H were performed under the cohesive zone fracture mechanics framework. The theoretical strength of grain boundaries is shown to be supressed by H segregation, and the cause of this is attributed to the prevention of the formation of Al ligaments across grain boundaries. Segregated concentrations of relevant alloying elements (Zn, Mg, and Cu) show minimal impact on the H embrittlement process investigated, namely H enhanced decohesion (HEDE). Further modelling, of H transport and grain boundary precipitates, is required to confirm the validity of the HEDE mechanism in the case of 7xxx alloys.

Preshearing influences on sheet-bulk metal forming of aluminum gear

Abstract Shear-cutting methods are more efficient than machining methods for preparing billets from sheet metal. In this study, several shearing methods were applied to prepare semi-finished billets for sheet-bulk metal forming of duplex gears. The influences of shearing features such as fracture, roll over, and grain flow lines on subsequent forming were analyzed. The results indicated that conventional blanking billets with imperfect shapes in the fracture zone could cause defects in the duplex gear in the sheet-bulk metal forming (SBMF) stage. To avoid imperfect shapes in the fracture zone, five types of clean shear-cut billets were prepared. Different roll over values and grain flow lines were achieved for the five billets. SBMF analysis using the clean shear-cut billets indicated that grain flow lines had a major influence on the material flow in subsequent forming. Among the preformed billets, the semi-finished billets produced through backward closed die shearing provide symmetrical grain flow lines, which results in superior symmetry of the outer tooth filling.

Segregation behavior and embrittling effect of lanthanide La, Ce, Pr, and Nd at Σ3(111) tilt symmetric grain boundary in α-Fe**Project supported by the National Natural Science Foundation of China (Grant No. U1867217), the National Science and Technology Major Project of the Ministry of Science and Technology of China (Grant No. 2019ZX06004009), and the China National Nuclear Corporation Centralized Research and

The migration of lanthanide fission products to cladding materials is recognized as one of the key causes of fuel–cladding chemical interaction (FCCI) in metallic fuels during operation. We have performed first-principles density functional theory calculations to investigate the segregation behavior of lanthanide fission products (La, Ce, Pr, and Nd) and their effects on the intergranular embrittlement at Σ3(111) tilt symmetric grain boundary (GB) in α-Fe. It is found that La and Ce atoms tend to reside at the first layer near the GB with segregation energies of −2.55 eV and −1.60 eV, respectively, while Pr and Nd atoms prefer to the core mirror plane of the GB with respective segregation energies of −1.41 eV and −1.50 eV. Our calculations also show that La, Ce, Pr, and Nd atoms all act as strong embrittlers with positive strengthening energies of 2.05 eV, 1.52 eV, 1.50 eV, and 1.64 eV, respectively, when located at their most stable sites. The embrittlement capability of four lanthanide elements can be determined by the atomic size and their magnetism characters. The present calculations are helpful for understanding the behavior of fission products La, Ce, Pr, and Nd in α-Fe.

Shock-induced migration of asymmetry tilt grain boundary in iron bicrystal: A case study of Σ3 [110]**Project supported by the Fundamental Research for the Central Universities of China, the National Key Laboratory Project of Shock Wave and Detonation Physics of China, the Science and Technology Foundation of National Key Laboratory of Shock Wave and Detonation Physics of China, the National Key

Many of our previous studies have discussed the shock response of symmetrical grain boundaries in iron bicrystals. In this paper, the molecular dynamics simulation of an iron bicrystal containing Σ3 [110] asymmetry tilt grain boundary (ATGB) under shock-loading is performed. We find that the shock response of asymmetric grain boundaries is quite different from that of symmetric grain boundaries. Especially, our simulation proves that shock can induce migration of asymmetric grain boundary in iron. We also find that the shape and local structure of grain boundary (GB) would not be changed during shock-induced migration of Σ3 [110] ATGB, while the phase transformation near the GB could affect migration of GB. The most important discovery is that the shock-induced shear stress difference between two sides of GB is the key factor leading to GB migration. Our simulation involves a variety of piston velocities, and the migration of GB seems to be less sensitive to the piston velocity. Finally, the kinetics of GB migration at lattice level is discussed. Our work firstly reports the simulation of shock-induced grain boundary migration in iron. It is of great significance to the theory of GB migration and material engineering.