Showing papers in "Modelling and Simulation in Materials Science and Engineering in 2012"

Automated identification and indexing of dislocations in crystal interfaces

Abstract: We present a computational method for identifying partial and interfacial dislocations in atomistic models of crystals with defects. Our automated algorithm is based on a discrete Burgers circuit integral over the elastic displacement field and is not limited to specific lattices or dislocation types. Dislocations in grain boundaries and other interfaces are identified by mapping atomic bonds from the dislocated interface to an ideal template configuration of the coherent interface to reveal incompatible displacements induced by dislocations and to determine their Burgers vectors. In addition, the algorithm generates a continuous line representation of each dislocation segment in the crystal and also identifies dislocation junctions.

Structure identification methods for atomistic simulations of crystalline materials

Abstract: We discuss existing and new computational analysis techniques to classify local atomic arrangements in large-scale atomistic computer simulations of crystalline solids. This article includes a performance comparison of typical analysis algorithms such as common neighbor analysis (CNA), centrosymmetry analysis, bond angle analysis, bond order analysis and Voronoi analysis. In addition we propose a simple extension to the CNA method that makes it suitable for multi-phase systems. Finally, we introduce a new structure identification algorithm, the neighbor distance analysis, which is designed to identify atomic structure units in grain boundaries.

Atomic structures of symmetric tilt grain boundaries in hexagonal close packed (hcp) crystals

Abstract: Molecular dynamics simulation and interface defect theory are used to determine the relaxed equilibrium atomic structures of symmetric tilt grain boundaries (STGBs) in hexagonal close-packed (hcp) crystals with a \( [0\bar{1}10] \) tilt axis. STGBs of all possible rotation angles θ from 0 deg to 90 deg are found to have an ordered atomic structure. They correspond either to a coherent, defect-free boundary or to a tilt wall containing an array of distinct and discrete intrinsic grain boundary dislocations (GBDs). The STGBs adopt one of six base structures, \( P_{B}^{(i)} \), i = 1, …, 6, and the Burgers vector of the GBDs is related to the interplanar spacing of the base structure on which it lies. The base structures correspond to the basal plane (θ = 0 deg, \( P_{B}^{(1)} \)); one of four minimum-energy, coherent boundaries, \( (\bar{2}111),\;(\bar{2}112),\;(\bar{2}114) \), and \( (\bar{2}116)\;\left( {P_{B}^{(2)} - P_{B}^{(5)} } \right) \); and the \( \left( {11\bar{2}0} \right) \) plane (θ = 90 deg, \( P_{B}^{(6)} \)). Based on these features, STGBs can be classified into one of six possible structural sets, wherein STGBs belonging to the same set i contain the same base boundary structure \( P_{B}^{(i)} \) and an array of GBDs with the same Burgers vector \( b_{\text{GB}}^{(i)} \), which vary only in spacing and sign with θ. This classification is shown to apply to both Mg and Ti, two metals with different c/a ratios and employing different interatomic potentials in simulation. We use a simple model to forecast the misorientation range of each set for hcp crystals of general c/a ratio, the predictions of which are shown to agree well with the molecular dynamics (MD) simulations for Mg and Ti.

A simple finite element model of diffusion, finite deformation, plasticity and fracture in lithium ion insertion electrode materials

Abstract: We describe a finite element method for modeling deformation, diffusion, fracture and electrochemical reactions in materials used as lithium ion insertion electrodes. With a view to modeling high-capacity composite electrode materials such as silicon or tin, the model accounts for finite deformations and plastic flow, and models the evolving electrochemical boundary conditions resulting from the creation of new fracture surfaces using a cohesive zone. In addition, a simple mixed element is used to account for the driving force for diffusion arising from stress gradients. In this approach, the equations for diffusion and deformation are fully coupled, and can be integrated using a stable implicit time-stepping scheme. The method is illustrated by modeling plastic flow and fracture during cyclic lithiation of a thin-film Si electrode and a simple model of a battery microstructure.

On the elastic–plastic decomposition of crystal deformation at the atomic scale

Abstract: Given two snapshots of an atomistic system, taken at different stages of the deformation process, one can compute the incremental deformation gradient field, F, as defined by continuum mechanics theory, from the displacements of atoms. However, such a kinematic analysis of the total deformation does not reveal the respective contributions of elastic and plastic deformation. We develop a practical technique to perform the multiplicative decomposition of the deformation field, F = FeFp, into elastic and plastic parts for the case of crystalline materials. The described computational analysis method can be used to quantify plastic deformation in a material due to crystal slip-based mechanisms in molecular dynamics and molecular statics simulations. The knowledge of the plastic deformation field, Fp, and its variation with time can provide insight into the number, motion and localization of relevant crystal defects such as dislocations. The computed elastic field, Fe, provides information about inhomogeneous lattice strains and lattice rotations induced by the presence of defects.

Extended smoothed boundary method for solving partial differential equations with general boundary conditions on complex boundaries

Abstract: In this paper, we describe an approach for solving partial differential equations with general boundary conditions imposed on arbitrarily shaped boundaries. A continuous function, the domain parameter, is used to modify the original differential equations such that the equations are solved in the region where a domain parameter takes a specified value while boundary conditions are imposed on the region where the value of the domain parameter varies smoothly across a short distance. The mathematical derivations are straightforward and applicable to a wide variety of partial differential equations. To demonstrate the general applicability of the approach, we provide four examples herein: (1) the diffusion equation with both Neumann and Dirichlet boundary conditions; (2) the diffusion equation with both surface diffusion and reaction; (3) the mechanical equilibrium equation; and (4) the equation for phase transformation with the presence of additional boundaries. The solutions for several of these cases are validated against numerical solutions of the corresponding sharp-interface equations. The potential of the approach is demonstrated with five applications: surface-reaction?diffusion kinetics with a complex geometry, Kirkendall-effect-induced deformation, thermal stress in a complex geometry, phase transformations affected by substrate surfaces and relaxation of a droplet on irregular surfaces.

Nucleation of elementary \{\bar {1}\,0\,1\,1\} and \{\bar {1}\,0\,1\,3\} twinning dislocations at a twin boundary in hexagonal close-packed crystals

Abstract: In this paper, we study the kinetics and energetics involved in the nucleation and propagation of and twinning dislocations (TDs) in Mg using atomistic simulations. We demonstrate that for both twins, a 2-layer TD of mixed character nucleates as a result of the interaction of a basal dislocation with a twin boundary. The favorability of the 2-layer TD over a 4-layer TD of edge character can be explained by its greater mobility. The twin boundary likely translates by nucleating and propagating 2-layer TDs with opposite-signed screw components in equal amounts on average.

Quantification of dislocation structure heterogeneity in deformed polycrystals by EBSD

Abstract: Plastic deformation in polycrystalline materials involves a complex interaction of dislocations with defects in the lattice. The geometrically necessary component of the dislocation density can be quantified to some extent using data obtained from automated electron backscatter diffraction scans over planar regions or volumes using the three-dimensional imaging techniques that are currently available. Reliable measurements require that the step size of the orientation data used in determination of geometrically necessary dislocation densities be on the scale of the microstructural information. Measurements were performed in deformed Cu, Al and steel specimens. Geometrically necessary dislocation density in Cu deformed 10% in compression was about 15–30% of the overall estimated dislocation density. Measurements in Al demonstrate that three-dimensional estimates are on the order of 1.2–2 times the values obtained from 2D measurements on the same structures. Analysis of interstitial free steel specimens shows an increase in average geometrically necessary dislocation density by an order of magnitude for specimens deformed to 12% tensile deformation elongation.

A model for Ti-6Al-4V microstructure evolution for arbitrary temperature changes

Abstract: This paper presents a microstructure model for the titanium alloy Ti-6Al-4V designed to be used in coupled thermo-metallurgical-mechanical simulations of, eg, welding processes The microstructur

The influence of the representative volume element (RVE) size on the homogenized response of cured fiber composites

Abstract: The influence of the representative volume element (RVE) size (in terms of fiber packing and number of fibers for a given fiber-volume fraction) on the residual stresses created during the curing process of a continuous fiber-reinforced polymer matrix tow is investigated with the ultimate goal of finding a minimum unit cell size that can be used later for a homogenization procedure to calculate the response of woven fiber textile composites and in particular, fiber tows. A novel network curing model for the solidification of epoxy is used to model the curing process. The model takes into account heat conduction, cure kinetics and the creation of networks in a continuously shape changing body. The model is applied to the curing of a fiber/matrix RVE. The results for the minimum size of the RVE, obtained on the basis of the curing problem, are compared with a similar RVE, modeled as an elastic-plastic solid subjected to external loads, in order to compare the minimum RVE sizes obtained on the basis of different boundary value problem solutions. (Some figures may appear in colour only in the online journal)

Developing a second nearest-neighbor modified embedded atom method interatomic potential for lithium

Abstract: This paper reports the development of a second nearest-neighbor modified embedded atom method (2NN MEAM) interatomic potential for lithium (Li). The 2NN MEAM potential contains 14 adjustable parameters. For a given set of these parameters, a number of physical properties of Li were predicted by molecular dynamics (MD) simulations. By fitting these MD predictions to their corresponding values from either experimental measurements or ab initio simulations, these adjustable parameters in the potential were optimized to yield an accurate and robust interatomic potential. The parameter optimization was carried out using the particle swarm optimization technique. Finally, the newly developed potential was validated by calculating a wide range of material properties of Li, such as thermal expansion, melting temperature, radial distribution function of liquid Li and the structural stability at finite temperature by simulating the disordered–ordered transition. (Some figures may appear in colour only in the online journal)

Molecular modeling of crosslink distribution in epoxy polymers

Abstract: Experimental studies on epoxies report that the microstructure consists of highly crosslinked localized regions connected with a dispersed phase of low-crosslink density epoxy. Because epoxies play a major role in many structural applications, the influence of the crosslink distribution on the thermo-mechanical properties must be determined. But as experiments cannot reliably report the exact number or distribution of crosslinked covalent bonds present in the molecular network, molecular modeling is a valuable tool that can predict the influence of crosslink distribution on thermo-mechanical properties. In this study, molecular dynamics are used to establish well-equilibrated molecular models of an EPON 862-DETDA epoxy system with a range of crosslink densities and distributions. Crosslink distributions are varied by forming highly crosslinked clusters within the epoxy network and then forming additional crosslinks that connect between clusters. Results of simulations on these molecular models indicate that elastic properties increase with increasing levels of overall crosslink density and the thermal expansion coefficient decreases with overall crosslink density, both above and below the glass transition temperature. It is also found that within the range of crosslink distributions investigated, there is no discernible influence of crosslink distribution on the elastic modulus and the linear thermal expansion coefficient of the epoxy.

A reactive force field for lithium–aluminum silicates with applications to eucryptite phases

Abstract: We have parameterized a reactive force field (ReaxFF) for lithium–aluminum silicates using density functional theory (DFT) calculations of structural properties of a number of bulk phase oxides, silicates and aluminates, as well as of several representative clusters. The force field parameters optimized in this study were found to predict lattice parameters and heats of formation of selected condensed phases in excellent agreement with previous DFT calculations and with experiments. We have used the newly developed force field to study the eucryptite phases in terms of their thermodynamic stability and their elastic properties. We have found that (a) these ReaxFF parameters predict the correct order of stability of the three crystalline polymorphs of eucryptite, α, β and γ, and (b) that upon indentation, a new phase appears at applied pressures ≥7 GPa. The high-pressure phase obtained upon indentation is amorphous, as illustrated by the radial distribution functions calculated for different pairs of elements. In terms of elastic properties analysis, we have determined the elements of the stiffness tensor for α- and β-eucryptite at the level of ReaxFF, and discussed the elastic anisotropy of these two polymorphs. Polycrystalline average properties of these eucryptite phases are also reported to serve as ReaxFF predictions of their elastic moduli (in the case of α-eucryptite), or as tests against values known from experiments or DFT calculations (β-eucrypite). The ReaxFF potential reported here can also describe well single-species systems (e.g. Li-metal, Al-metal and condensed phases of silicon), which makes it suitable for investigating structure and properties of suboxides, atomic-scale mechanisms responsible for phase transformations, as well as oxidation–reduction reactions.

Comparison of different homogenization approaches for elastic–viscoplastic materials

Abstract: Homogenization of linear viscoelastic and non-linear viscoplastic composite materials is considered in this paper. First, we compare two homogenization schemes based on the Mori–Tanaka method coupled with the additive interaction (AI) law proposed by Molinari et al (1997 Mech. Mater. 26 43–62) or coupled with a concentration law based on translated fields (TF) originally proposed for the self-consistent scheme by Paquin et al (1999 Arch. Appl. Mech. 69 14–35). These methods are also evaluated against (i) full-field calculations of the literature based on the finite element method and on fast Fourier transform, (ii) available analytical exact solutions obtained in linear viscoelasticity and (iii) homogenization methods based on variational approaches. Developments of the AI model are obtained for linear and non-linear material responses while results for the TF method are shown for the linear case. Various configurations are considered: spherical inclusions, aligned fibers, hard and soft inclusions, large material contrasts between phases, volume-preserving versus dilatant anelastic flow, non-monotonic loading. The agreement between the AI and TF methods is excellent and the correlation with full field calculations is in general of quite good quality (with some exceptions for non-linear composites with a large volume fraction of very soft inclusions for which a discrepancy of about 15% was found for macroscopic stress). Description of the material behavior with internal variables can be accounted for with the AI and TF approaches and therefore complex loadings can be easily handled in contrast with most hereditary approaches.

3D CAFE modeling of grain structures: application to primary dendritic and secondary eutectic solidification

Abstract: A three-dimensional model is presented for the prediction of grain structures formed in casting. It is based on direct tracking of grain boundaries using a cellular automaton (CA) method. The model is fully coupled with a solution of the heat flow computed with a finite element (FE) method. Several unique capabilities are implemented including (i) the possibility to track the development of several types of grain structures, e.g. dendritic and eutectic grains, (ii) a coupling scheme that permits iterations between the FE method and the CA method, and (iii) tabulated enthalpy curves for the solid and liquid phases that offer the possibility to work with multicomponent alloys. The present CAFE model is also fully parallelized and runs on a cluster of computers. Demonstration is provided by direct comparison between simulated and recorded cooling curves for a directionally solidified aluminum?7?wt% silicon alloy.

Computer simulation of heterogeneous polymer photovoltaic devices

Abstract: Polymer-based photovoltaic devices have the potential for widespread usage due to their low cost per watt and mechanical flexibility. Efficiencies close to 9.0% have been achieved recently in conjugated polymer based organic solar cells (OSCs). These devices were fabricated using solvent-based processing of electron-donating and electron-accepting materials into the so-called bulk heterojunction (BHJ) architecture. Experimental evidence suggests that a key property determining the power-conversion efficiency of such devices is the final morphological distribution of the donor and acceptor constituents. In order to understand the role of morphology on device performance, we develop a scalable computational framework that efficiently interrogates OSCs to investigate relationships between the morphology at the nano-scale with the device performance.In this work, we extend the Buxton and Clarke model (2007 Modelling Simul. Mater. Sci. Eng. 15 13–26) to simulate realistic devices with complex active layer morphologies using a dimensionally independent, scalable, finite-element method. We incorporate all stages involved in current generation, namely (1) exciton generation and diffusion, (2) charge generation and (3) charge transport in a modular fashion. The numerical challenges encountered during interrogation of realistic microstructures are detailed. We compare each stage of the photovoltaic process for two microstructures: a BHJ morphology and an idealized sawtooth morphology. The results are presented for both two- and three-dimensional structures.

Elastic and electronic properties of tI26-type Mg12RE (RE = Ce, Pr and Nd) phases

Abstract: A detailed theoretical study of structural, elastic and electronic properties of tI26-type Mg12RE (RE = Ce, Pr and Nd) phases has been carried out by means of first-principles calculations based on density functional theory. The optimized lattice parameters at T = 0 K are in excellent agreement with the available experimental value at room temperature, and the obtained formation enthalpies show that with increasing atomic number of RE, the stability of Mg12RE alloys becomes lower. The elastic constants of tI26 Mg12RE are further calculated, then the bulk modulus B, shear modulus G, Young's modulus E and Poisson's ratio ν of polycrystalline aggregates are derived, and the relevant mechanical properties of Mg12RE alloys are also discussed. The elastic anisotropy of the three alloys is studied in detail using several methods, and the three-dimensional directional representation reveals the variation of Young's modulus along the crystallographic direction visually and completely. Finally, electronic density of states, charge density distribution and Bader charges are also calculated to reveal the underlying mechanism of structural stability and mechanical properties.

Orbital-free density functional theory simulations of dislocations in magnesium

Abstract: Metal plasticity is controlled by nucleation and motion of dislocations. Key metrics determining the ease of these two events are stacking fault energies (SFEs) and dislocation structures. Here we study screw and edge dislocation structures on the basal, prismatic and pyramidal planes in hexagonal-close-packed magnesium (Mg) using orbital-free density functional theory (OFDFT) in order to gain insight into plastic deformation mechanisms in Mg. The accuracy of the method is first benchmarked against the more accurate Kohn–Sham DFT (KSDFT) with emphasis on testing OFDFT's main approximations, i.e. the kinetic energy density functional and the bulk-derived local pseudopotential by comparing predicted equilibrium bulk energies, elastic constants and various SFEs. Then we compare generalized SFEs for the basal, prismatic and pyramidal slip systems calculated by OFDFT versus two mainstream counterparts, KSDFT and the classical potential embedded atom method (EAM). The latter produces spurious minima along the generalized SFE surface on the prismatic plane whereas OFDFT agrees with qualitative experimental observations. Thereafter, we optimize isolated dislocation structures within periodic cells containing a few thousand atoms. We predict that on the basal plane, the screw and edge dislocations separate into partial dislocations with widths of ~12 and ~24 A, respectively. Screw dislocations on the prismatic and pyramidal planes preferentially cross-slip and dissociate on the basal plane although a local minimum exists for a dissociated prismatic screw dislocation with widths of ≥~5 A. By contrast, the edge dislocations on prismatic and pyramidal planes are predicted to remain undissociated. Such cross-slip behavior of screw dislocations is not reproduced by EAM simulations. We propose that the propensity for screw dislocations to remain on or cross-slip to Mg's basal plane, along with the compact nature of edge dislocations on non-basal planes, is likely to be responsible for its limited ductility.

Atomistic modeling of pure Li and Mg?Li system

Abstract: Interatomic potentials for pure Li and the Mg–Li binary system have been developed based on the second nearest-neighbor modified embedded-atom method formalism. The potentials can describe various fundamental physical properties of pure Li (bulk, point defect, planar defect and thermal properties) and alloy behaviors (thermodynamic, structural and elastic properties) in reasonable agreement with experimental data or higher-level calculations. The applicability of the potential to atomistic investigations on the deformation behavior of Mg alloys and the effect of Li is demonstrated.

Density functional theory calculations and ab initio molecular dynamics simulations for diffusion of Li+ within liquid ethylene carbonate

Abstract: We have performed the density functional theory (DFT) method to study the electronic structure of ethylene carbonate (EC) in the gas phase and ab initio molecular dynamics (AIMD) simulations for its liquid phase at T = 450 K to avoid freezing. The calculated characteristics of EC in both gas and liquid phases are found to be consistent with the experimental results. The coordination of one Li+ with 1–5 EC molecules is studied using the DFT method in the gas phase. At the same time, the formation of the first solvation shell of one Li+ within the EC solution at T = 450 K is investigated using the AIMD simulation technique. The calculated results reveal the stability of the [Li+(EC)4] solvation shell, which contains four strongly bound EC molecules in a tetrahedral arrangement both in gas and liquid phases. The diffusion coefficient of the EC crystal at T = 450 K is found to be 2.42 × 10−9 m2 s−1 and that of Li+ in liquid EC at T = 450 K is found to be 1.27 × 10−9 m2 s−1. The conductivity of Li+ is found to be 5.46 mS cm−1. Such diffusion and conduction of Li+ in liquid EC may be helpful in understanding the formation of the primary solvation shell of Li+ and the solid electrolyte interphase formation mechanism, and hence may be useful in applications of lithium ion batteries.

Enhanced ductility of surface nano-crystallized materials by modulating grain size gradient

Abstract: Surface nano-crystallized (SNC) materials with a graded grain size distribution on their surfaces have been attracting increasing scientific interest over the past few decades due to their good synergy of high strength and high ductility. However, to date most of the existing studies have focused on the individual contribution of three different aspects, i.e. grain size gradient (GSG), work-hardened region and surface compressive residual stresses, which were induced by surface severe plastic deformation processes, to the improved strength of SNC materials as compared with that of their coarse grained (CG) counterparts. And the ductility of these materials has hardly been studied. In this study, a combination of theoretical analysis and finite element simulations was used to investigate the role of GSG in tuning the ductility of SNC materials. It was found that the ductility of an SNC material can be comparable to that of its CG counterpart, while it simultaneously possessed a much higher strength than its CG core if the optimal GSG thickness and grain size of the topmost phase were adopted. A design map that can be used as a guideline for fabrication of SNC materials was also plotted. Our predictions were also compared with the corresponding experimental results.

Porosity evolution in a creeping single crystal

Abstract: Experimental observations on tensile specimens in Srivastava et al (2012 in preparation) indicated that the growth of initially present processing induced voids in a nickel-based single crystal superalloy played a significant role in limiting creep life. Also, creep tests on single crystal superalloy specimens typically show greater creep strain rates and/or reduced creep life for thinner specimens than predicted by current theories. In order to quantify the role of void growth in single crystals in creep loading, we have carried out three-dimensional finite deformation finite element analyses of unit cells containing a single initially spherical void. The materials are characterized by a rate-dependent crystal plasticity constitutive relation accounting for primary and secondary creep. Two types of imposed loading are considered: an applied true stress (force/unit current area) that is time independent; and an applied nominal stress (force/unit initial area) that is time independent. Isothermal conditions are assumed. The evolution of porosity is calculated for various values of stress triaxiality and of the Lode parameter. The evolution of porosity with time is sensitive to whether constant true stress or constant nominal stress loading is applied. However, the evolution of porosity with the overall unit cell strain is insensitive to the mode of loading. At high values of stress triaxiality, the response is essentially independent of the value of the Lode parameter. At sufficiently low values of the stress triaxiality, the porosity evolution depends on the value of the Lode parameter and void collapse can occur. Also, rather large stress concentrations can develop which could play a role in the observed thickness dependence.

Atomistic modeling of penny-shaped and through-thickness cracks in bcc iron

Abstract: Atomistic simulations of penny-shaped embedded cracks in body-centered cubic (bcc) iron are performed using molecular dynamics. The results reveal that the original circular crack geometry can change shape gradually upon loading, depending on the crystallographic orientation. This new geometry generally favors emission of dislocation loops instead of unstable fracture. A comparison is made between through-thickness cracks in six different orientations and penny-shaped cracks on the same crack planes. We find that changes in crack shape and the interaction of events in different directions play an important role in how fracture mechanisms evolve when cracks in full 3D simulations extend, and that dislocation emission and mechanical twins ‘win’ over unstable crack growth by bond breaking.

Predicting active slip systems in β-Sn from ideal shear resistance

Abstract: We analyse the ideal shear resistances of 15 nonequivalent slip systems in ?-Sn using first-principles density functional theory. From the ideal shear resistance and Schmid's law, the orientation dependence of active slip systems in a ?-Sn single crystal subjected to uniaxial tension is investigated. We find that has the lowest ideal shear resistance among the 15 slip systems. Our calculations indicate that, depending on crystal orientation, uniaxial tension activates seven nonequivalent groups of slip systems. The active slip systems for [1?0?0] and [1?1?0] orientations determined in this study agree with the experimental results.

3D phase field modelling of recrystallization in a low-carbon steel

Abstract: Intercritical annealing is a critical processing step to manufacture dual-phase (DP) steels. As part of modelling the microstructure evolution in an intercritical-annealing cycle, a 3D multi-phase field model has been employed to simulate recrystallization during heating of a low-carbon steel that is used to produce commercial DP600 grade. The cold-rolled microstructure obtained from metallographic observations is used as the initial structure in the model. The nucleation conditions and the effective interface mobility are employed as adjustable parameters to fit the experimentally measured kinetics of isothermal recrystallization and then applied to non-isothermal recrystallization. The model predictions are in good agreement with experimental data for recrystallization during continuous heating. The model provides realistic recrystallized microstructures as initial conditions for modelling the subsequent formation and decomposition of austenite.

Phase-field modeling of corrosion kinetics under dual-oxidants

Abstract: A phase-field model is proposed to simulate corrosion kinetics under a dual-oxidant atmosphere. It will be demonstrated that the model can be applied to simulate corrosion kinetics under oxidation, sulfidation and simultaneous oxidation/sulfidation processes. Phase-dependent diffusivities are incorporated in a natural manner and allow more realistic modeling as the diffusivities usually differ by many orders of magnitude in different phases. Simple free energy models are then used for testing the model while calibrated free energy models can be implemented for quantitative modeling.

Influence of phase transformations on the asymptotic residual stress distribution arising near a sharp V-notch tip

Abstract: In this work, the residual stress distribution induced by the solidification and cooling of a fusion zone in the vicinity of a sharp V-notch tip is investigated. The intensity of the residual asymptotic stress fields, quantified by the notch stress intensity factors, was studied for two different V-notch specimen geometries under generalized plane-strain conditions. In order to analyze the influence of phase transformations on the obtained results, simulations with and without the effects of phase transformation were carried out on ASTM SA 516 steel plates. Thanks to the possibilities of numerical modelling, additional analyses were performed without taking into account the transformation plasticity phenomenon.It was found that phase transformation effects (both volume change and transformation plasticity) have a great influence on the intensity and sign of the asymptotic stress fields at the sharp V-notch tips. This result is believed to be very important for the correct numerical determination (and future applications) of notch stress intensity factors resulting from asymptotic residual stress distributions induced by transient thermal loads. The analyses were performed with the finite element code SYSWELD.

Predicting the optimal dopant concentration in gadolinium doped ceria: a kinetic lattice Monte Carlo approach

Abstract: Gadolinium doped ceria (GDC) is a promising alternative electrolyte material for solid oxide fuel cells that offers the possibility of operation in the intermediate temperature range (773?1073?K). To determine the optimal dopant concentration in GDC, we have employed a systematic approach of applying a 3D kinetic lattice Monte Carlo (KLMC) model of vacancy diffusion in conjunction with previously calculated activation energies for vacancy migration in GDC as inputs. KLMC simulations were performed including the vacancy repelling effects in GDC. Increasing the dopant concentration increases the vacancy concentration, which increases the ionic conductivity. However, at higher concentrations, vacancy?vacancy repulsion impedes vacancy diffusion, and together with vacancy trapping by dopants decreases the ionic conductivity. The maximum ionic conductivity is predicted to occur at ?20% to 25% mole fraction of Gd dopant. Placing Gd dopants in pairs, instead of randomly, was found to decrease the conductivity by ?50%. Overall, the trends in ionic conductivity results obtained using the KLMC model developed in this work are in reasonable agreement with the available experimental data. This KLMC model can be applied to a variety of ceria-based electrolyte materials for predicting the optimum dopant concentration.

Plate-impact loading of cellular structures formed by selective laser melting

Abstract: Porous materials are of great interest because of improved energy absorption over their solid counterparts. Their properties, however, have been difficult to optimize. Additive manufacturing has emerged as a potential technique to closely define the structure and properties of porous components, i.e. density, strutwidthandporesize; however, thebehaviourofthesematerialsatveryhigh impact energies remains largely unexplored. We describe an initial study of the dynamic compression response of lattice materials fabricated through additive manufacturing. Lattices consisting of an array of intersecting stainless steel rods were fabricated into discs using selective laser melting. The resulting discs were impacted against solid stainless steel targets at velocities ranging from 300 to 700ms −1 using a gas gun. Continuum CTH simulations were performed to identify key features in the measured wave profiles, while 3D simulations, in which the individual cells were modelled, revealed details of microscale deformation during collapse of the lattice structure. The validated computermodelshavebeenusedtoprovideanunderstandingofthedeformation processes in the cellular samples. The study supports the optimization of cellular structures for application as energy absorbers.