Symmetric Grain Research Articles

Silver and Cesium Diffusion Dynamics at the β-SiC Σ5 Grain Boundary Investigated with Density Functional Theory Molecular Dynamics and Metadynamics

The diffusion and release of silver-110m, a strong γ-radiation emitter, through silicon carbide in coated nuclear fuel particles has remained an unsolved topic since it was first observed 40 years ago. The challenge remains to explain why, contrary to other elements, silver is capable of escaping the ceramic diffusion barriers. The current work investigates the underlying differences in the diffusion of silver and cesium along a symmetric tilt Σ5 grain boundary of β-SiC through accelerated density functional theory molecular dynamics simulations. The energy barriers extracted from the simulations give diffusion coefficients that are in reasonable agreement with experiment for silver (2.19 × 10(-19) to 1.05 × 10(-17) m(2) s(-1)), but for cesium the equivalent calculated coefficients for this mechanism are much smaller (3.85 × 10(-23) to 2.15 × 10(-21) m(2) s(-1)) than those found experimentally. Analysis of the simulated structures and electron densities and comparisons with the calculations of other researchers suggest that diffusion of silver and cesium in β-SiC proceeds via different mechanisms. The mechanisms of cesium diffusion appear to be dominated by its relatively large size and repulsive interactions with the silicon and carbon atoms; β-SiC grain boundaries still offer higher energy barriers to diffusion. Silver, on the other hand, is not only smaller in size but, as we show for the first time, can also participate in weak bonding interactions with the host atoms where favorable geometries allow, thus reducing the energy barrier and enhancing the rate of diffusion.

A Computational Algorithm to Produce Virtual X-ray and Electron Diffraction Patterns from Atomistic Simulations

Electron and x-ray diffraction are well-established experimental methods used to explore the atomic scale structure of materials. In this work, a computational algorithm is developed to produce virtual electron and x-ray diffraction patterns directly from atomistic simulations. This algorithm advances beyond previous virtual diffraction methods by using a high-resolution mesh of reciprocal space that eliminates the need for a priori knowledge of the crystal structure being modeled or other assumptions concerning the diffraction conditions. At each point on the reciprocal space mesh, the diffraction intensity is computed via explicit computation of the structure factor equation. To construct virtual selected-area electron diffraction patterns, a hemispherical slice of the reciprocal lattice mesh lying on the surface of the Ewald sphere is isolated and viewed along a specified zone axis. X-ray diffraction line profiles are created by binning the intensity of each reciprocal lattice point by its associated scattering angle, effectively mimicking powder diffraction conditions. The virtual diffraction algorithm is sufficiently generic to be applied to atomistic simulations of any atomic species. In this article, the capability and versatility of the virtual diffraction algorithm is exhibited by presenting findings from atomistic simulations of 〈100〉 symmetric tilt Ni grain boundaries, nanocrystalline Cu models, and a heterogeneous interface formed between α-Al2O3 (0001) and γ-Al2O3 (111).

Twist, tilt, and symmetric grain boundaries in hexagonal materials

Grain boundaries of characteristic geometry, e.g., twist, tilt, and symmetric boundaries are often used as reference boundaries in analyses of boundary networks in polycrystalline materials. This article deals with the issue of proper identification of characteristic boundaries in the case of materials with hexagonal D6h symmetry. To identify all boundaries of characteristic types, both analytical calculations and numerical searches are used. The first approach provides exact parameters of the characteristic boundaries, whereas the second one gives boundaries which can be classified as characteristic if some tolerance is allowed. In both methods, all symmetrically equivalent boundary representations are taken into consideration. The obtained sets of twist, tilt, symmetric, and 180°-tilt boundaries are presented in the form of two-dimensional maps containing stereographic projections of the corresponding boundary plane normals for selected grain misorientations. These diagrams facilitate interpretation of experimental distributions of grain boundaries; with the representation used, they can be directly linked to experimental distributions. Examples of such diagrams for lattice parameter ratios c/a of \(\sqrt{5/2}\) and \(\sqrt{20/21}\) are presented. They are compared to example boundary distributions in Ti alloy and distributions of WC/WC boundaries in WC–Co composites available in the literature.

Bulk response and grain boundary microelectrical activity of high TC BaTiO3–(Bi1/2K1/2)TiO3-based positive temperature coefficient of resistance ceramics

Abstract

Interaction of minor alloying elements of high-Cr ferritic steels with lattice defects: An ab initio study

Basic properties of minor alloying elements, namely Mo, W, Nb, Ta, V, Mn, Si entering the conventional and reduced-activation structural Fe–(9–12)Cr steels have been analyzed using ab initio calculations. The electronic structure calculations were applied to study the interaction of minor alloying elements with a number of important and well defined lattice structures, such as point defects, the 1/2〈111〉 screw dislocation core, high angle symmetric grain boundaries and free surfaces. The studied elements were classified according to their similarities and discrepancies regarding the interaction with the above mentioned defects. The refractory alloying elements are found to follow the same trend whereas Mn and Si exhibit peculiar behavior with respect to the interaction with both point and extended lattice defects. The obtained results are discussed and compared with previously published ab initio and available experimental data.

Principal physical parameters characterizing the interactions between irradiation-induced point defects and several tilt symmetric grain boundaries in Fe, Mo and W

Using molecular-statics, we investigate principal physical parameters characterizing the binding of vacancies and interstitials with grain boundaries (GBs), and their annihilation near GBs in iron, molybdenum and tungsten. Binding energies strongly correlate with GB energies averagely and have a general level when scaled by the bulk defect formation energy. Defect diffusion is enhanced near the GB. The diffusion barrier of the vacancy gradually decreases as it approaches to the GB. For interstitials, there exist several layers near the GB in which the absorption of interstitials is spontaneous and out of which orientation-dependent. For the interstitial-rich GB, the vacancy near the GB can be annihilated at a low barrier, independent of the system. The GB influence range is limited of 1.0–2.0nm from the GB. Our obtained principal physical parameters may be applied to build the master framework for defects’ generation, transport and fate and thus to evaluate the damage rate in nano/poly-crystalline materials.

Molecular dynamics simulation of Cu atoms interaction with symmetrical grain boundaries of BCC Fe

The interaction between Cu atoms and symmetrical grain boundaries (GB) Σ3{112} in BCC iron have been simulated with molecular dynamics and Metropolis monte carlo methods. Positive binding energy has been found for substitutional Cu with a symmetrical GB structure, and this positive binding energy increases as the size of the Cu clusters increase. After absorption, the Cu clusters in the GB prefer to be in a state of large size and low density rather than having a high number of randomly distributed smaller sized clusters. The phase transition of Cu-rich precipitates has been observed from BCC to FCC near the GB. These results indicate the possible segregation of Cu atoms to the GB, and provide the information for understanding the precipitation of Cu atoms and its effect on the properties of BCC iron.

Extension of the universal erosive burning law to partly symmetric propellant grain geometries

This paper aims at extending the universal erosive burning law developed by two of the present authors from axi-symmetric internally burning grains to partly symmetric burning grains. This extension revolves around three dimensional flow calculations inside highly loaded grain geometry and benefiting from an observation that the flow gradients normal to the surface in such geometries have a smooth behavior along the perimeter of the grain. These are used to help identify the diameter that gives the same perimeter the characteristic dimension rather than a mean hydraulic diameter chosen earlier. The predictions of highly loaded grains from the newly chosen dimension in the erosive burning law show better comparison with measured pressure–time curves while those with mean hydraulic diameter definitely over-predict the pressures.

Phase field crystal study on grain boundary evolution and its micro-mechanism under various symmetry

Phase field crystal method is used to investigate the deformation process and mechanism of twined structure of a trigonal phase under uniaxial tensile deformation, and the evolution and corresponding micro-mechanism of low-angle symmetric and asymmetric grain boundaries (GB) as well as high-angle symmetric and asymmetric GB during deformation process are analyzed in detail. The deformation is performed under the condition that the direction of applied stress is parallel to that of initial GB. Results show that low-angle symmetric GB is composed of two kinds of edge dislocations with the angle made by Burgers vectors being around 60 During deformation, two kinds of dislocations in low-angle symmetric GB move along two opposite directions, then meet with the same kind of dislocation emitted from another GB leading to the annihilation of partial dislocations. As to the low-angle asymmetric GB, its only one kind of dislocation first climbs and moves along the horizontal direction of the applied stress, then each dislocation will break down into two dislocations with their Burgers vectors making an angle about 120, finally a perfect single crystal is formed via the movement and annihilation of dislocations. High-angle GBs first keep horizontal shape under the applied stress, then become serrated, and the dislocations are emitted from the cusps in GBs. Finally, the high-angle asymmetric GB will decompose with the movement and annihilation of dislocation, while the shape of high-angle symmetric GB becomes horizontal again. It can be seen that the high-angle symmetric GB is more stable than the high-angle asymmetric GB; this is in agreement with the results of experiments and molecular dynamics.

Structures and electronic properties of symmetric and nonsymmetric graphene grain boundaries

The graphene grain boundaries with periodic length up to 18Å have been studied using density functional theory. Atomic structures, thermodynamic stabilities and electronic properties of 40 grain boundaries with symmetric and nonsymmetric structures were investigated. According to the arrangements of pentagons and heptagons on the boundary, grain boundaries were cataloged into four classes. Some nonsymmetric grain boundaries constructed here have identical misorientation angles to the experimentally observed ones. The formation energies of grain boundaries can be correlated with the misorientation angle and inflection angle. Nonsymmetric grain boundaries possess comparable formation energies to their symmetric counterparts when the periodic length along the defect line is larger than 1nm. Analysis of electronic density of states shows that the existence of a grain boundary usually increases the density of states near the Fermi level, whereas some symmetric grain boundaries can open a small band gap due to local sp2-to-sp3 rehybridization.

Irradiation effects on microstructure change in nanocrystalline ceria – Phase, lattice stress, grain size and boundaries

With a wide variety of applications in numerous industries, ranging from biomedical to nuclear, ceramics such as ceria are key engineering materials. It is possible to significantly alter the materials functionality and therefore its applications by reducing the grain size to the nanometer size regime, at which point the unique varieties of grain boundaries and associated interfaces begin to dominate the material properties. Nanocrystalline films of cubic ceria deposited onto Si substrates have been irradiated with 3MeV Au+ ions at temperatures of 300 and 400K to evaluate their response to irradiation. It was observed that the films remained phase stable. Following a slight stress relief stage at low damage levels, the overall lattice is extremely stable up to high irradiation dose of ∼34 displacements per atom. The grains were also observed to undergo a temperature-dependent grain growth process upon ion irradiation. This is attributed to a defect-driven mechanism in which the diffusion of defects from the collision cascade is critical. Formation of dislocations that terminate and stabilize at symmetric grain boundaries may be the limiting factor in the grain growth and overall energy reduction of the system. Utilizing ion modification, possible improvement of the adhesion of thin films and reduction of the probability of detrimental effects of stress-induced problems are discussed.

Thermal transport in grain boundary of graphene by non-equilibrium Green’s function approach

A method of combining lattice dynamics through the Brenner potential [D. W. Brenner et al., J. Phys.: Condens. Matter 14, 783 (2002)] with phonon transport through non-equilibrium Green’s function approach is applied to calculate the thermal conductance of graphene grain boundaries. The results show that all types of grain boundaries offer excellent thermal conductivity, in contrast to electronic transport which is structure-dependent. Zigzag-oriented symmetric grain boundaries present the highest thermal conductance, with weak dependence on the crystal alignment angle of graphene domains. The thermal conductance of grain boundaries increases while thermal ballisticity decreases with temperature. The out-of-plane mode is dominant in thermal conductivity of graphene grain boundaries.

Effect of LAB on the Stress Rupture Properties and Fracture Characteristic of DD6 Single Crystal Superalloy

The slabs of the second generation single crystal superalloy DD6 were cast using bi-seeded crystals with symmetric twist low angle grain boundaries. The influence of low angle boundary (LAB) on the stress rupture properties of the alloy was investigated at 760 °C/758 MPa, 850 °C/550 MPa and 980 °C/250 MPa. SEM was used to study the fracture surface and the failure mechanism. The results show that when the misorientation angle of LAB is relatively small it has a less effect on the stress rupture properties of the alloy. The stress rupture properties decrease with the misorientation increase on the same test conditions. The fracture characteristic of specimen shows intergranular rupture trend with the misorientation increases on the same conditions or with increasing of temperature and decreasing of stress for the same misorientation of LAB.

First-principles determination of the effect of boron on aluminum grain boundary cohesion

Despite boron being a common alloying element in aluminum, its segregation into the aluminum grain boundary and its effect on the grain boundary strength have not been studied. Here, the electronic structures of the boron-doped $\ensuremath{\Sigma}5(012)[100]$ symmetrical tilt grain boundary and (012) free surface systems for aluminum are investigated by means of first-principles calculations using the full-potential linearized augmented plane-wave method with the generalized gradient approximation, within the framework of the Rice-Wang thermodynamic model and the theoretical tensile test approach. We establish that boron has a large driving force to segregate from Al bulk to the symmetrical grain boundary hollow site, and its segregation significantly enhances the grain boundary strength. Through precise calculations on both the grain boundary and free surface environments, it is found that boron is a strong cohesion enhancer in aluminum with a potency of \ensuremath{-}0.19 eV/atom. An analysis in terms of the relaxed atomic and electronic structures and bonding characters shows that the aluminum-boron bond has mixed covalent and metallic character and is strong in both grain boundary and free surface environments. The strengthening effect of boron is due to creation of additional B--Al bonds across the grain boundary, which are as strong as existing Al--Al transgranular bonds and thus significantly increase grain boundary adhesion and its resistance to tensile stress and cracking.

Simulating interaction of two symmetric grain boundaries under shear strain conditions

The behavior of two large-angle grain boundaries of the Σ = 5 (210)[001] special type in a copper polycrystal under shear loading conditions has been numerically simulated. It is established that, similar to the case of a single boundary of this type, the grain boundaries shifts in the direction perpendicular to that of shear straining simultaneously with the relative slippage of grains in the direction of applied load. Features of the interaction of two symmetric defects that approach each other are considered. The obtained results provide a better understanding of the atomic mechanisms of plastic strain development in polycrystalline materials.

Microstructure stability of cold drawn AZ31 magnesium alloy during annealing process

The microstructural evolution of heavily cold drawn AZ31 magnesium alloy wires was investigated during a wide range of annealing temperature from 200 to 450°C. The results show that the mean grain size of the as-annealed material is sensitive to the annealing temperature and cold drawn area reduction. Upon annealing symmetrical grain growth takes place in the AZ31 wires of cold drawn area reduction 12.2% under all the investigated annealing conditions, while three different stages of grain size refinement, normal grain growth and abnormal grain coarsening occur gradually in the materials with cold drawn area reduction 23.0%–60.5% with annealing temperature increasing. The increase of cold drawn reduction leads to the decrease of critical annealing temperatures for the three periods, and also enhances the start of abnormal growing grains preferentially in the severely drawn materials from the outer surface where larger amount of shear deformation constitutes the driving force for growth.

Effect of Cu and Ag solute segregation on βSn grain boundary diffusivity

We investigate the effect of various amounts of Ag and Cu solute atoms on the self-diffusivity of Sn in the (101) symmetric tilt βSn grain boundary. Using molecular dynamics simulations over a temperature range of 300 K to 450 K, we show that both Ag and Cu decrease the grain boundary self-diffusivity of Sn as the amount of solute in the interface increases. Additionally, the presence of Ag at the grain boundary interface causes a greater reduction in the self-diffusivity of Sn when compared to Cu. We also analyze the solute effect on the diffusive width of the interface and find that low concentrations of both Ag and Cu shrink the width relative to the pure βSn interface. However, adding Cu in excess density greater than 5 × 10−3 atoms/Å2 causes the interface to expand to values almost twice the original width, possibly caused by the larger cohesive energy of Cu-Sn versus Sn-Sn.

Density Functional Theory Calculations of Properties of the Grain Boundaries in Aluminum

ABSTRACTThe Density Functional Theory has been used to analyze an inter-granular segregation of Cu and Mg. The stability of Cu and Mg atoms in the aluminum matrix, intermetallic phases and symmetric twist grain boundaries has been compared. The quantitative description of solubility of Cu and Mg atoms in the nano-crystalline aluminum has been proposed. The calculations have been carried out to investigate the properties of symmetric twist boundaries in aluminum with and without Cu/Mg atoms. The phenomena of are discussed and its effect on the stability of precipitates containing these elements.

Atomic structure, energy distribution, and short-range order in symmetric grain boundaries of Ni3Al intermetallide

We know that metals and alloys are polycrystalline in their regular state. The grain boundaries, which are a fundamental structural element of polycrystals, determine many of their physicomechanical proper� ties: plasticity, diffusion, recrystallization, decomposi� tion, strengthening, etc. The influence of the grain boundaries on the properties of polycrystals rests on

Topological defects in graphene: Dislocations and grain boundaries

Topological defects in graphene, dislocations and grain boundaries, are still not well understood despites the considerable number of experimental observations. We introduce a general approach for constructing dislocations in graphene characterized by arbitrary Burgers vectors as well as grain boundaries, covering the whole range of possible misorientation angles. By using ab initio calculations we investigate thermodynamic and electronic properties of these topological defects, finding energetically favorable symmetric large-angle grain boundaries, strong tendency towards out-of-plane deformation in the small-angle regimes, and pronounced effects on the electronic structure. The present results show that dislocations and grain boundaries are important intrinsic defects in graphene which may be used for engineering graphene-based nanomaterials and functional devices.