Showing papers in "Modelling and Simulation in Materials Science and Engineering in 2009"
TL;DR: In this article, the authors reviewed the application of the phase-field method in different fields of materials science, including elastic interactions and fluid flow in multi-grain multi-phase structures in multicomponent materials.
Abstract: The phase-field method is reviewed against its historical and theoretical background. Starting from Van der Waals considerations on the structure of interfaces in materials the concept of the phase-field method is developed along historical lines. Basic relations are summarized in a comprehensive way. Special emphasis is given to the multi-phase-field method with extension to elastic interactions and fluid flow which allows one to treat multi-grain multi-phase structures in multicomponent materials. Examples are collected demonstrating the applicability of the different variants of the phase-field method in different fields of materials science.
1,004 citations
TL;DR: In this article, the extended and generalized finite element methods are reviewed with an emphasis on their applications to problems in material science: fracture, dislocations, grain boundaries and phase interfaces.
Abstract: The extended and generalized finite element methods are reviewed with an emphasis on their applications to problems in material science: (1) fracture, (2) dislocations, (3) grain boundaries and (4) phases interfaces. These methods facilitate the modeling of complicated geometries and the evolution of such geometries, particularly when combined with level set methods, as for example in the simulation growing cracks or moving phase interfaces. The state of the art for these problems is described along with the history of developments.
718 citations
TL;DR: This paper presents a unified framework in which fourteen leading multiscale methods can be represented as special cases and tests the accuracy and efficiency of the fourteen methods on a test problem; the structure and motion of a Lomer dislocation dipole in face-centered cubic aluminum.
Abstract: A partitioned-domain multiscale method is a computational framework in which certain key regions are modeled atomistically while most of the domain is treated with an approximate continuum model (such as finite elements). The goal of such methods is to be able to reproduce the results of a fully atomistic simulation at a reduced computational cost. In recent years, a large number of partitioned-domain methods have been proposed. Theoretically, these methods appear very different to each other making comparison difficult. Surprisingly, it turns out that at the implementation level these methods are in fact very similar. In this paper, we present a unified framework in which fourteen leading multiscale methods can be represented as special cases.We use this common framework as a platform to test the accuracy and efficiency of the fourteen methods on a test problem; the structure and motion of a Lomer dislocation dipole in face-centered cubic aluminum. This problem was carefully selected to be sufficiently simple to be quick to simulate and straightforward to analyze, but not so simple to unwittingly hide differences between methods. The analysis enables us to identify generic features in multiscale methods that correlate with either high or low accuracy and either fast or slow performance.All tests were performed using a single unified computer code in which all fourteen methods are implemented. This code is being made available to the public along with this paper.
381 citations
TL;DR: In this article, a scaling of the Madelung-like screened first-order correction term is proposed to correct the formation energy of charged defects in semiconductors, by potential alignment.
Abstract: The theoretical description of defects and impurities in semiconductors is largely based on density functional theory (DFT) employing supercell models. The literature discussion of uncertainties that limit the predictivity of this approach has focused mostly on two issues: (1) finite-size effects, in particular for charged defects; (2) the band-gap problem in local or semi-local DFT approximations. We here describe how finite-size effects (1) in the formation energy of charged defects can be accurately corrected in a simple way, i.e. by potential alignment in conjunction with a scaling of the Madelung-like screened first order correction term. The factor involved with this scaling depends only on the dielectric constant and the shape of the supercell, and quite accurately accounts for the full third order correction according to Makov and Payne. We further discuss in some detail the background and justification for this correction method, and also address the effect of the ionic screening on the magnitude of the image charge energy. In regard to (2) the band-gap problem, we discuss the merits of non-local external potentials that are added to the DFT Hamiltonian and allow for an empirical band-gap correction without significantly increasing the computational demand over that of standard DFT calculations. In combination with LDA + U, these potentials are further instrumental for the prediction of polaronic defects with localized holes in anion-p orbitals, such as the metal-site acceptors in wide-gap oxide semiconductors.
317 citations
TL;DR: In this article, the authors used the Consistent Valence Force Field (CVFF) model to characterize the force-separation behavior between carbon nanotubes and the polymer matrix.
Abstract: Carbon nanotube (CNT) polymer-matrix composites exhibit promising properties as structural materials for which appropriate constitutive models are sought, to predict their macroscale behavior. The reliability of determining the homogenized response of such materials depends upon the ability to accurately capture the interfacial behavior between the nanotubes and the polymer matrix. In this work, molecular dynamics simulations, using the Consistent Valence Force Field (CVFF) to describe the atomistic interactions, are used to study nanoscale load transfer between polyethylene and a graphene sheet, a model system chosen to characterize the force-separation behavior between CNTs and the polymer matrix. Separation studies are conducted for both opening as well as sliding modes and cohesive zone parameters such as peak traction and energy of separation are evaluated for each mode. Studies are also carried out to investigate the effect of tension and compression on sliding mode separation. Size dependence studies are conducted utilizing different sizes of the computational domain and different boundary conditions, to obtain the representative volume element and connect to continuum level properties. These results set the stage for continuum length-scale micromechanical models which may be used in determining the overall material response, incorporating interfacial phenomena.
218 citations
TL;DR: In this paper, the core structures of screw and edge dislocations on the basal and prism planes in Mg, and the associated gamma surfaces, were studied using an ab initio method and the embedded-atom-method interatomic potentials developed by Sun et al and Liu et al.
Abstract: The core structures of screw and edge dislocations on the basal and prism planes in Mg, and the associated gamma surfaces, were studied using an ab initio method and the embedded-atom-method interatomic potentials developed by Sun et al and Liu et al. The ab initio calculations predict that the basal plane dislocations dissociate into partials split by 16.7 angstrom (edge) and 6.3 angstrom (screw), as compared with 14.3 angstrom and 12.7 angstrom (Sun and Liu edge), and 6.3 angstrom and 1.4 angstrom (Sun and Liu screw), with the Liu screw dislocation being metastable. In the prism plane, the screw and edge cores are compact and the edge core structures are all similar, while ab initio does not predict a stable prismatic screw in stress-free conditions. These results are qualitatively understood through an examination of the gamma surfaces for interplanar sliding on the basal and prism planes. The Peierls stresses at T = 0K for basal slip are a few megapascals for the Sun potential, in agreement with experiments, but are ten times larger for the Liu potential. The Peierls stresses for prism slip are 10-40MPa for both potentials. Overall, the dislocation core structures from ab initio are well represented by the Sun potential in all cases while the Liu potential shows some notable differences. These results suggest that the Sun potential is preferable for studying other dislocations in Mg, particularly the textless c + a textgreater dislocations, for which the core structures are much larger and not accessible by ab initio methods.
167 citations
Journal Article•
152 citations
TL;DR: In this article, the authors compare finite element and fast Fourier transform approaches for the prediction of the micromechanical behavior of polycrystals, using the same visco-plastic single crystal constitutive law.
Abstract: In this work, we compare finite element and fast Fourier transform approaches for the prediction of the micromechanical behavior of polycrystals. Both approaches are full-field approaches and use the same visco-plastic single crystal constitutive law. We investigate the texture and the heterogeneity of the inter- and intragranular stress and strain fields obtained from the two models. Additionally, we also look into their computational performance. Two cases—rolling of aluminum and wire drawing of tungsten—are used to evaluate the predictions of the two models. Results from both the models are similar, when large grain distortions do not occur in the polycrystal. The finite element simulations were found to be highly computationally intensive, in comparison with the fast Fourier transform simulations. Figure 9 was corrected in this article on the 25 August 2009. The corrected electronic version is identical to the print version.
136 citations
TL;DR: In this paper, the authors examined the use of crystal based continuum mechanics in the context of dynamic loading and examined model forms and simulations which are relevant to pore collapse in crystalline energetic materials.
Abstract: This work examines the use of crystal based continuum mechanics in the context of dynamic loading. In particular, we examine model forms and simulations which are relevant to pore collapse in crystalline energetic materials. Strain localization and the associated generation of heat are important for the initiation of chemical reactions in this context. The crystal mechanics based model serves as a convenient testbed for the interactions among wave motion, slip kinetics, defect generation kinetics and physical length scale. After calibration to available molecular dynamics and single crystal gas gun data for HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine), the model is used to predict behaviors for the collapse of pores under various conditions. Implications for experimental observations are discussed.
129 citations
TL;DR: In this paper, a many-body interatomic potential for the Fe-Ni system is fitted, capable of describing both the ferritic and austenitic phase, and the mixing enthalpy and defect properties were fitted.
Abstract: A many-body interatomic potential for the Fe–Ni system is fitted, capable of describing both the ferritic and austenitic phase. The Fe–Ni system exhibits two stable ordered intermetallic phases, namely, L10 FeNi and L12 FeNi3, that are key issues to be tackled when creating a Fe–Ni potential consistent with thermodynamics. A procedure, based on a rigid lattice Ising model and the theory of correlation functions space, is developed to address all the intermetallics that are possible ground states of the system. While controlling the ground states of the system, the mixing enthalpy and defect properties were fitted. Both bcc and fcc defect properties are compared with density functional theory calculations and other potentials found in the literature. Finally, the potential is thermodynamically validated by constructing the alloy phase diagram. It is shown that the experimental phase diagram is reproduced reasonably well and that our potential gives a globally improved description of the Fe–Ni system in the whole concentration range with respect to the potentials found in the literature.
122 citations
TL;DR: In this paper, the intrinsic mechanical properties of various materials and systems through ab initio tensile and shear testing simulations based on density-functional theory are reviewed, and several problems and future directions in this research field are discussed.
Abstract: First-principles studies on the intrinsic mechanical properties of various materials and systems through ab initio tensile and shear testing simulations based on density-functional theory are reviewed. For various materials, ideal tensile and shear strength and features of the deformation of bulk crystals without any defects have been examined, and the relation with the bonding nature has been analyzed. The surfaces or low-dimensional nano-structures revealpeculiarstrengthanddeformationbehaviorduetolocaldifferentbonding nature. For grain boundaries and metal/ceramic interfaces, tensile and shear behaviors depend on the interface bonding, which impacts on the research of real engineering materials. Remaining problems and future directions in this research field are discussed. (Some figures in this article are in colour only in the electronic version)
TL;DR: In this article, a robust finite element model of interface motion in media with multiple domains and junctions is described, as is the case in polycrystalline materials, where each domain (grain) is represented with a single level set function, while avoiding the creation of overlap or vacuum between these domains.
Abstract: The paper describes a robust finite element model of interface motion in media with multiple domains and junctions, as is the case in polycrystalline materials. The adopted level set framework describes each domain (grain) with a single level set function, while avoiding the creation of overlap or vacuum between these domains. The finite element mesh provides information on stored energies, calculated from a previous deformation step. Nucleation and growth of new grains are modelled by inserting additional level set functions around chosen nodes of the mesh. The kinetics and topological evolutions induced by primary recrystallization are discussed from simple test cases to more complex configurations and compared with the Johnson–Mehl–Avrami–Kolmogorov theory.
TL;DR: The package reduces the need for user intervention, automating the method to reduce human error and judgment, and extends standard cluster expansion formalism to the more complicated cases of ternary compounds, as well as surfaces, including adsorption and inequivalent sites.
Abstract: We present a new implementation of the cluster expansion formalism. The new code, UNiversal CLuster Expansion (UNCLE), consolidates recent advances in the methodology and leverages one new development in the formalism itself. As a core goal, the package reduces the need for user intervention, automating the method to reduce human error and judgment. The package extends standard cluster expansion formalism to the more complicated cases of ternary compounds, as well as surfaces, including adsorption and inequivalent sites.
TL;DR: In this article, the motion of dislocations with Burgers' vector lying on the basal, prismatic and pyramidal slip planes in pure magnesium was investigated numerically under static and dynamic loading conditions.
Abstract: The motion of dislocations with Burgers' vector lying on the basal, prismatic and pyramidal slip planes in pure magnesium was investigated numerically under static and dynamic loading conditions. The analysis of the dislocation core structures revealed that the basal slip system was the most favorable energetically, and therefore a dislocation loop cannot extend on the pyramidal slip plane, because screw dislocations were not stable in this slip plane. In agreement with experimental data, a strong anisotropy between slip systems was observed. In both the basal and the prismatic slip planes, the dislocation velocity is consistent with phonon drag theory. In addition, the edge dislocation velocity was always larger than the screw dislocation velocity independent of the slip system, while the dislocation velocity on the prismatic slip plane was always lower than the dislocation velocity on the basal plane regardless of the dislocation character.
TL;DR: In this article, an adaptive arbitrary Lagrangian-Eulerian formulation is developed to compute the material flow and the temperature evolution during the three phases of the friction stir welding (FSW) process.
Abstract: An adaptive arbitrary Lagrangian–Eulerian formulation is developed to compute the material flow and the temperature evolution during the three phases of the friction stir welding (FSW) process. It follows a splitting approach: after the calculations of the velocity/pressure and temperature fields, the mesh velocity is derived from the domain boundary evolution and from an adaptive refinement criterion provided by error estimation, and finally state variables are remapped. In this way, the unilateral contact conditions between the plate and the tool are accurately taken into account, so allowing one to model various instabilities that may occur during the process, such as the role played by the plunge depth of the tool on the formations of flashes, the possible appearance of non-steady voids or tunnel holes and the influence of the threads on the material flow, the temperature field and the welding efforts. This formulation is implemented in the 3D Forge3 FE software with automatic remeshing. The non-steady phases of FSW can so be simulated, as well as the steady welding phase. The study of different process conditions shows that the main phenomena taking place during FSW can be simulated with the right sensitivities.
TL;DR: In this article, a two-step approach is presented for determining interatomic potentials, where values of atomic volume and the second and third-order elastic constants measured at room temperature are extrapolated to T = 0 K using classical thermo-mechanical relations that are thermodynamically consistent.
Abstract: An accurate description of the thermoelastic response of solids is central to classical simulations of compression- and deformation-induced condensed matter phenomena. To achieve the correct thermoelastic description in classical simulations, a new approach is presented for determining interatomic potentials. In this two-step approach, values of atomic volume and the second- and third-order elastic constants measured at room temperature are extrapolated to T = 0 K using classical thermo-mechanical relations that are thermodynamically consistent. Next, the interatomic potentials are fitted to these T = 0 K pseudo-values. This two-step approach avoids the low-temperature quantum regime, providing consistency with the assumptions of classical simulations and enabling the correct thermoelastic response to be recovered in simulations at room temperature and higher. As an example of our approach, an EAM potential was developed for aluminum, providing significantly better agreement with thermoelastic data compared with previous EAM potentials. The approach presented here is quite general and can be used for other potential types as well, the key restriction being the inapplicability of classical atomistic simulations when quantum effects are important.
TL;DR: In this paper, a phase field model for void formation in metals with vacancy concentrations exceeding the thermal equilibrium values is presented, which allows for a unified treatment of void nucleation and growth under the condition of random generation of vacancies, which is similar to vacancy generation by collision cascade in irradiated materials.
Abstract: Motivated by the need to develop a spatially resolved theory of irradiation-induced microstructure evolution in metals, we present a phase field model for void formation in metals with vacancy concentrations exceeding the thermal equilibrium values. This model, which is phenomenological in nature, is cast in the form of coupled Cahn–Hilliard and Allen–Cahn type equations governing the dynamics of the vacancy concentration field and the void microstructure in the matrix, respectively. The model allows for a unified treatment of void nucleation and growth under the condition of random generation of vacancies, which is similar to vacancy generation by collision cascade in irradiated materials. The basic features of the model are illustrated using two-dimensional solutions for the cases of void growth and shrinkage in supersaturated and undersaturated vacancy fields, void–void interactions, as well as the spontaneous nucleation and growth of a large population of voids.
TL;DR: The concentration-dependent embedded atom method (CD-EAM) is a powerful model for atomistic simulation of concentrated alloys with arbitrarily complex mixing enthalpy curves as discussed by the authors.
Abstract: The concentration-dependent embedded atom method (CD-EAM) is a powerful model for atomistic simulation of concentrated alloys with arbitrarily complex mixing enthalpy curves. In this paper, we show that in spite of explicit three-body forces, this model can be implemented quite simply with a computational efficiency comparable to the standard EAM for molecular-dynamics (MD) simulations. Ready-to-use subroutines for the parallel MD code LAMMPS can be provided by the authors upon request. We further propose an improved version of this potential that allows for very efficient calculations of single-particle displacement/transmutation energies, while retaining the complexity implicit in the three-body interactions. This enables large-scale Monte-Carlo simulations of alloys with the interatomic interactions described by the CD-EAM model.
TL;DR: In this article, the authors address the band-gap underestimation of standard density-functional methods with its harmful consequences for the positioning of defect-related levels in the band gap region, and the slow convergence of calculated defect properties when the periodic supercell approach is used.
Abstract: Recent advances in density-functional theory (DFT) calculations of defect electronic properties in semiconductors and insulators are discussed. In particular, two issues are addressed: the band-gap underestimation of standard density-functional methods with its harmful consequences for the positioning of defect-related levels in the band-gap region, and the slow convergence of calculated defect properties when the periodic supercell approach is used. Systematic remedies for both of these deficiencies are now available, and are being implemented in the context of popular DFT codes. This should help in improving the parameter-free accuracy and thus the predictive power of the methods to enable unambiguous explanation of defect-related experimental observations. These include not only the various fingerprint spectroscopies for defects but also their thermochemistry and dynamics, i.e. the temperature-dependent concentration and diffusivities of defects under various doping conditions and in different stoichiometries.
TL;DR: In this paper, a finite element model was developed to predict shear force versus shear angle for woven fabrics, based on the TexGen geometric modelling schema, developed at the University of Nottingham and orthotropic constitutive models for yarn behaviour, coupled with a unified displacement-difference periodic boundary condition.
Abstract: In this study, a finite element model to predict shear force versus shear angle for woven fabrics is developed. The model is based on the TexGen geometric modelling schema, developed at the University of Nottingham and orthotropic constitutive models for yarn behaviour, coupled with a unified displacement-difference periodic boundary condition. A major distinction from prior modelling of fabric shear is that the details of picture frame kinematics are included in the model, which allows the mechanisms of fabric shear to be represented more accurately. Meso- and micro-mechanisms of deformation are modelled to determine their contributions to energy dissipation during shear. The model is evaluated using results obtained for a glass fibre plain woven fabric, and the importance of boundary conditions in the analysis of deformation mechanisms is highlighted. The simulation results show that the simple rotation boundary condition is adequate for predicting shear force at large deformations, with most of the energy being dissipated at higher shear angles due to yarn compaction. For small deformations, a detailed kinematic analysis is needed, enabling the yarn shear and rotation deformation mechanisms to be modelled accurately.
TL;DR: In this article, the effect of multiple nonlinear material models for representing the elastic-plastic behavior of materials was investigated, and the performance of each model was compared with available experimental results.
Abstract: Advanced mechanical surface enhancement techniques have been used successfully to increase the fatigue life of metallic components. These techniques impart deep compressive residual stresses into the component to counter potentially damage-inducing tensile stresses generated under service loading. Laser shock peening (LSP) is an advanced mechanical surface enhancement technique used predominantly in the aircraft industry. To reduce costs and make the technique available on a large-scale basis for industrial applications, simulation of the LSP process is required. Accurate simulation of the LSP process is a challenging task, because the process has many parameters such as laser spot size, pressure profile and material model that must be precisely determined. This work focuses on investigating the appropriate material model that could be used in simulation and design. In the LSP process material is subjected to strain rates of 106 s−1, which is very high compared with conventional strain rates. The importance of an accurate material model increases because the material behaves significantly different at such high strain rates. This work investigates the effect of multiple nonlinear material models for representing the elastic–plastic behavior of materials. Elastic perfectly plastic, Johnson–Cook and Zerilli–Armstrong models are used, and the performance of each model is compared with available experimental results.
TL;DR: In this paper, the authors systematically review the known sources of error and suggest that reliable calculations of defect properties may be obtained with density functional theory (DFT) using the supercell approximation.
Abstract: Reliable calculations of defect properties may be obtained with density functional theory (DFT) using the supercell approximation. We systematically review the known sources of error and suggest ho ...
TL;DR: In this article, a phase-field model for void formation in polycrystalline metals with vacancy concentrations exceeding the thermal equilibrium values is presented, which captures several relevant processes including vacancy annihilation and nucleation at grain boundaries (GBs), vacancy diffusion toward sinks (including GBs and void surfaces) as well as void nucleation and growth due to vacancy supersaturations occurring in the grain interiors.
Abstract: We present a phase-field model for void formation in polycrystalline metals with vacancy concentrations exceeding the thermal equilibrium values. By incorporating a coupled set of Cahn–Hilliard and Allen–Cahn equations, the model captures several relevant processes including vacancy annihilation and nucleation at grain boundaries (GBs), vacancy diffusion toward sinks (including GBs and void surfaces) as well as void nucleation and growth due to vacancy supersaturations occurring in the grain interiors. Illustrative results are presented that characterize the rate of annihilation of the vacancy population at the GB sinks, as well as the formation of void denuded zones adjacent to GBs in bicrystalline and polycrystalline samples, the width of which is found to depend on both the vacancy diffusivity and the vacancy production rate.
TL;DR: The maximization of posterior marginals (MPM) segmentation technique, originally developed for computer vision applications, is applied towards automated segmentation of microstructural images from Ti- and Ni-alloy systems.
Abstract: Several automated algorithms are presented for the segmentation of features of interest from microstructure images acquired with modern high-throughput electron microscopes. Specifically, the maximization of posterior marginals (MPM) segmentation technique, originally developed for computer vision applications, is applied towards automated segmentation of microstructural images from Ti- and Ni-alloy systems. The MPM technique classifies image pixels according to the most probable class to which they can belong. Three derivatives of the MPM algorithm are introduced and assessed: expectation maximization MPM (EM/MPM), EM/MPM with simulated annealing (EM/MPM/SA) and vector EM/MPM/SA. Example applications of all three approaches are given. The EM/MPM model allows for automated segmentation of α laths in a Ti-6242 sample and primary γ' in an IN100 superalloy, but has difficulty accurately locating the boundaries between regions. The EM/MPM/SA algorithm involves a gradual increase in the interface capillarity during segmentation and allows for pixel accuracy determination of boundaries between phases. The vector EM/MPM/SA method is capable of simultaneously segmenting a series of images acquired with differing imaging conditions. The limitations of the algorithms are discussed as well as potential future modifications.
TL;DR: In this article, a set of efficient numerical algorithms to accurately compute the forces on dislocations in free-standing thin films is presented, including the use of virtual segments and the associated uniqueness of their solutions.
Abstract: We present a set of efficient numerical algorithms to accurately compute the forces on dislocations in free-standing thin films. We first present a spectral method for computing the image stress field of dislocations in an isotropic elastic half space and a free-standing thin film. The traction force on the free surface is decomposed into Fourier modes by a discrete Fourier transform and the resulting image stress field is obtained by superimposing analytic solutions in the Fourier space. Dislocations intersecting free surfaces are discussed, including the use of virtual segments and the associated uniqueness of their solutions. The efficiency of the algorithm is enhanced by incorporating the analytical solutions for straight dislocations intersecting free surfaces. A comprehensive algorithm, including a flow diagram, is formulated and the numerical convergence of these algorithms discussed. As a benchmark, we compute the equilibrium orientation of a threading dislocation in a free-standing thin film. Good agreement is observed between the predictions from the dislocation dynamics model and those from molecular static simulations and the line tension model.
TL;DR: In this article, the cellular automaton method coupled fundamental metallurgical principles was used to simulate the initial microstructure and dynamic recrystallization (DRX) of 30Cr2Ni4MoV rotor steel.
Abstract: The cellular automaton (CA) method coupling fundamental metallurgical principles was used to simulate the initial microstructure and dynamic recrystallization (DRX) of 30Cr2Ni4MoV rotor steel. For the initial microstructure generation, reasonable transformation rules were established based on the thermodynamic mechanism, the activation energy and the curvature-driven mechanism. For the purposes of obtaining the material constants which were used in the CA model for DRX, including initial grain size, nucleation rate, softening parameter and activation energy, the hot deformation characteristics of 30Cr2Ni4MoV rotor steel were investigated by uniaxial hot compression tests on Gleeble-3500 machine. The effect of a wide range of thermomechanical processing parameters (temperature and strain rate) on the nucleation rate, the percentage of DRX and the final grain size were investigated. By comparison of the flow stress–strain curves and the metallographs, it was shown that the CA model coupling fundamental metallurgical principles can accurately simulate the microstructural evolution and the plastic flow behavior for 30Cr2Ni4MoV rotor steel at various deformation parameters.
TL;DR: In this article, the Peierls-Nabarro model is used to model dislocation core properties in MgO as a function of pressure from ambient conditions to 100 GPa.
Abstract: We present in this study modelling of dislocation core properties in MgO as a function of pressure from ambient conditions to 100 GPa. The calculations are based on the Peierls–Nabarro model and we use first-principles calculations of the generalized stacking fault as input parameters. Our results confirm that {1 1 0} is the dominant slip system in MgO at ambient pressure and is easier than {1 0 0} as the pressure increases. Nevertheless, the {1 1 0} slip hardens with increasing pressure leading to a decrease in the difference in the Peierls stresses between {1 1 0} and {1 0 0}.
TL;DR: An anisotropic mesh refinement strategy based on the level set description is introduced and it is shown that it offers a good compromise between accuracy requirements on the one hand and computation time on the other hand.
Abstract: In finite element simulations dedicated to the modelling of microstructure evolution, the mesh has to be fine enough to: (i) accurately describe the geometry of the constituents; (ii) capture local strain gradients stemming from the heterogeneity in material properties. In this paper, 3D polycrystalline aggregates are discretized into unstructured meshes and a level set framework is used to represent the grain boundaries. The crystal plasticity finite element method is used to simulate the plastic deformation of these aggregates. A mesh sensitivity analysis based on the deformation energy distribution shows that the predictions are, on average, more sensitive near grain boundaries. An anisotropic mesh refinement strategy based on the level set description is introduced and it is shown that it offers a good compromise between accuracy requirements on the one hand and computation time on the other hand. As the aggregates deform, mesh distortion inevitably occurs and ultimately causes the breakdown of the simulations. An automatic remeshing tool is used to periodically reconstruct the mesh and appropriate transfer of state variables is performed. It is shown that the diffusion related to data transfer is not significant. Finally, remeshing is performed repeatedly in a highly resolved 500 grains polycrystal subjected to about 90% thickness reduction in rolling. The predicted texture is compared with the experimental data and with the predictions of a standard Taylor model.
TL;DR: In this article, the authors review several examples of successful modeling of electron and hole trapping in metal oxides, which demonstrate a breadth of polaronic behaviour, ranging from self-trapping in the perfect lattice to trapping by structural defects and impurities and illustrate the important phenomenon of charge localization.
Abstract: We critically review several examples of successful modelling of electron and hole trapping in metal oxides, which demonstrate a breadth of polaronic behaviour. The examples range from self-trapping in the perfect lattice to trapping by structural defects and impurities and illustrate the important phenomenon of charge localization. We present recent results in four different systems: nanoporous mayenite, amorphous SiO2, crystalline hafnia and MgO surfaces and interfaces. The complex nature of charge trapping and polaronic behaviour in these systems can go beyond traditional cases and illustrate the different challenges involved.
TL;DR: In this article, the structural properties and energy levels of simple intrinsic defects in gallium arsenide (GaAs) were investigated and the first quantitatively reliable survey of defect levels in GaAs, reassess the available literature and begin to decipher the complexity of GaAs defect chemistry.
Abstract: We investigate the structural properties and energy levels of simple intrinsic defects in gallium arsenide. The first-principles calculations (1) apply boundary conditions appropriate to charge defects in supercells and enable quantitatively accurate predictions of defect charge transitions with a supercell approximation, (2) are demonstrated to be converged with respect to cell size and (3) assess the sensitivity to model construction to Ga pseudopotential construction (3d core or 3d valence) and density functionals (local density or generalized gradient approximation). With these factors controlled, we present the first quantitatively reliable survey of defect levels in GaAs, reassess the available literature and begin to decipher the complexity of GaAs defect chemistry. The computed defect level spectrum spans the experimental GaAs band gap, defects exhibit multiple bistabilities with (sometimes overlapping) negative-U systems, express more extensive charge states than previously anticipated and collectively suggest that our atomistic understanding of GaAs defect physics needs to be reassessed.