Abstract: Machine Learning (ML)-based force fields are attracting ever-increasing interest due to their capacity to span spatiotemporal scales of classical interatomic potentials at quantum-level accuracy. They can be trained based on high-fidelity simulations or experiments, the former being the common case. However, both approaches are impaired by scarce and erroneous data resulting in models that either do not agree with well-known experimental observations or are under-constrained and only reproduce some properties. Here we leverage both Density Functional Theory (DFT) calculations and experimentally measured mechanical properties and lattice parameters to train an ML potential of titanium. We demonstrate that the fused data learning strategy can concurrently satisfy all target objectives, thus resulting in a molecular model of higher accuracy compared to the models trained with a single data source. The inaccuracies of DFT functionals at target experimental properties were corrected, while the investigated off-target properties remained largely unperturbed. Our approach is applicable to any material and can serve as a general strategy to obtain highly accurate ML potentials. read less

Abstract: The structural stability of tight-pitched (18 nm and below) damascene interconnects for back-end-of-line (BEOL) technologies are analyzed using force-field-based molecular dynamics simulations and finite-element modeling. At these pitches surface energy-dominated effects come into the picture, which lead to structural instability. The candidate metals analyzed are beyond copper (Cu) interconnect metals-ruthenium (Ru), cobalt (Co), and tungsten (W); and Cu is analyzed for reference. Cohesive traction and normal bonding energy are calculated using force-field- based molecular dynamics simulations and then fed as input to a finite-element analysis (FEA) tool, where their dependence on the physical dimensions of the interconnect lines is studied. The parameters studied for the BEOL structures are sidewall angle, aspect ratio, the internal stress of the metal, and modulus of elasticity of the dielectric material around the metal to understand the sensitivity of these parameters to the structural stability of the interconnects. We observe that a lower aspect ratio and higher modulus of elasticity of the dielectric results in stable structures whereas, intrinsic stress of the metal and side wall angle have a minor impact on the overall stability. The stability is analyzed at the seed-layer deposition step and based on this study, Co is the most stable alternate metal amongst Ru, Co, and W. read less

Abstract: Ferritic-martensitic steels, such as T91, are candidate materials for high-temperature applications, including superheaters, heat exchangers, and advanced nuclear reactors. Considering these alloys’ wide applications, an atomistic understanding of the underlying mechanisms responsible for their excellent mechano-chemical properties is crucial. Here, we developed a modified embedded-atom method (MEAM) potential for the Fe-Cr-Si-Mo quaternary alloy system—i.e., four major elements of T91—using a multi-objective optimization approach to fit thermomechanical properties reported using density functional theory (DFT) calculations and experimental measurements. Elastic constants calculated using the proposed potential for binary interactions agreed well with ab initio calculations. Furthermore, the computed thermal expansion and self-diffusion coefficients employing this potential are in good agreement with other studies. This potential will offer insightful atomistic knowledge to design alloys for use in harsh environments. read less

Abstract: ABSTRACT Magnesium (Mg) activates 〈c + a〉 dislocation slip on the second order pyramidal slip plane. This slip mode is very complex compared to other modes including several metastable structures. Due to the complexity and very similar energies of the different structures, reliably modelling this slip mode is challenging. The problem is exacerbated when considering alloying, in which a combination of 1st order and 2nd order pyramidal slip is usually observed. Motivated by the need for a high fidelity potential for Mg alloys, we have developed first a highly accuracy potential for pure Mg. The new potential shows better agreement with density functional theory and experimental calculations than previous interatomic potentials for Mg. With the help of this new potential, we demonstrate that the basal dissociated 〈c + a〉 core is not sessile, as previously thought, and that constant stress molecular dynamics demonstrate clear preference for the 2nd order pyramidal system over the 1st order system. read less

Abstract: The melting mechanism of single crystal and polycrystalline Nb 20.6 Mo 21.7 Ta 15.6 W 21.1 V 21.0 RHEAs was investigated by the molecular dynamics (MD) simulation using the 2NN MEAM potential. For the single crystal RHEA, the density profile displays an abrupt drop from 11.25 to 11.00 g/cm 3 at temperatures from 2910 to 2940 K, indicating all atoms begin significant local structural rearrangement. For polycrystalline RHEAs, a two-stage melting process is found. In the first melting stage, the melting of the grain boundary (GB) regions firstly occurs at the pre-melting temperature, which is relatively lower than the corresponding system-melting point. At the pre-melting temperature, most GB atoms have enough kinetic energies to leave their equilibrium positions, and then gradually induce the rearrangement of grain atoms close to GB. In the second melting stage at the melting point, most grain atoms have enough kinetic energies to rearrange, resulting in the chemical short-ranged order (CSRO) changes of all pairs. read less

Abstract: Modeling the processes of forming contact regions (interface) of the multilayer niobium-cobalt nanosystem is carried out. The morphology and composition of a multilayer nanosystem interface is investigated. The layer boundaries morphology is shown to depend on the deposition substrate temperature and, largely, is determined by preparing the surface for deposition. The work considers the deposition surface modification by removing its defects. Simulation showed that surface preparation significantly affects the morphology and composition of a multilayer nanosystem interface, depending on the type of deposited atoms and atoms forming the deposition surface. read less

Abstract: High entropy alloys (HEAs) exhibit an unusual combination of high fracture strength and ductility. However, atomic mechanisms responsible for crack propagation in HEAs are still not clear, which limits further improving the damage tolerance. Here we investigate effect of crystal orientation on the crack-tip behaviors in single-crystal HEA CrMnFeCoNi using atomic simulations to explore fracture micromechanism. The formation of deformation twinning and activation of multislip systems are observed during the propagation crack with the $(001)\ensuremath{\langle}110\ensuremath{\rangle}$ orientation, consistent with the previous experiments. Under the $(\overline{1}10)\ensuremath{\langle}110\ensuremath{\rangle}$ orientation, the amorphous region takes place throughout the crack growth, and is difficult to occur in traditional metal materials. Dissimilarly, for the $(1\overline{1}\overline{1})\ensuremath{\langle}110\ensuremath{\rangle}$ orientation, the blunting and slip bands occur at the front of the crack tip by switching the slip mode from the planar to wavy slip, observed in recent transmission electron microscopy experiments. The chemical disorder leads to the obvious fluctuation of flow stress, but hardly affects the deformation mechanism at the crack tip. Compared to traditional metals and alloys, the high local stress concentration induced by coupling effect of severe lattice distortion and tension strain leads to the structure transformation from crystallization to amorphization at the crack tip in HEA. While the presented atomic simulations and the associated conclusions are based on CrMnFeCoNi HEA, it is believed that the current deformation mechanism at crack tip could also be applied to other face-centered-cubic HEA. read less

Abstract: According to experimental observations, the temperature dependence of self-diffusion coefficient in most body-centered cubic metals (bcc) exhibits non-Arrhenius behavior. The origin of this behavio ... read less

Abstract: In simulations of metallic interfaces, a critical aspect of metallic behavior is missing from the some of the most widely used classical molecular dynamics force fields. We present a modification of the embedded atom method (EAM) which allows for electronic polarization of the metal by treating the valence density around each atom as a fluctuating dynamical quantity. The densities are represented by a set of additional fluctuating variables (and their conjugate momenta) which are propagated along with the nuclear coordinates. This ``density readjusting EAM'' (DR-EAM) preserves nearly all of the useful qualities of traditional EAM, including bulk elastic properties and surface energies. However, it also allows valence electron density to migrate through the metal in response to external perturbations. We show that DR-EAM can successfully model polarization in response to external charges, capturing the image charge effect in atomistic simulations. DR-EAM also captures some of the behavior of metals in the presence of uniform electric fields, predicting surface charging and shielding internal to the metal. We further show that it predicts charge transfer between the constituent atoms in alloys, leading to novel predictions about unit cell geometries in layered $\mathrm{L}{1}_{0}$ structures. read less

Abstract: We use ab initio calculations to analyze the influence of 4d and 5d transition metal alloying elements on cohesive properties of the bulk and a representative grain boundary in Cr within the framework of the Rice–Thomson–Wang approach. The results obtained for Cr are combined with the analogous results for Ni to select Ta and Nb as promising alloying additions to dual-phase (α/γ) Cr-Ni-base high-temperature alloys. Ta and Nb are added to the alloying system of an existing alloy I (Cr-Ni-W-V-Ti) in an attempt to design a chemical composition of a new alloy II (Cr-Ni-W-V-Ti) + (Ta-Nb). Investigation of the microstructure of the Ta-bearing Cr-Ni-alloy reveals a Ta enrichment of large γ-areas near GBs in α-matrix that we consider as potency to increase the cohesive strength of GBs and the cohesive energy of the bulk in γ-phase. Mechanical testing of alloys I and II demonstrates that the alloy II has improved tensile strength and creep resistance at high temperatures. read less

Abstract: In this work, we present a highly accurate spectral neighbor analysis potential (SNAP) model for molybdenum (Mo) developed through the rigorous application of machine learning techniques on large materials data sets. Despite Mo's importance as a structural metal, existing force fields for Mo based on the embedded atom and modified embedded atom methods still do not provide satisfactory accuracy on many properties. We will show that by fitting to the energies, forces and stress tensors of a large density functional theory (DFT)-computed dataset on a diverse set of Mo structures, a Mo SNAP model can be developed that achieves close to DFT accuracy in the prediction of a broad range of properties, including energies, forces, stresses, elastic constants, melting point, phonon spectra, surface energies, grain boundary energies, etc. We will outline a systematic model development process, which includes a rigorous approach to structural selection based on principal component analysis, as well as a differential evolution algorithm for optimizing the hyperparameters in the model fitting so that both the model error and the property prediction error can be simultaneously lowered. We expect that this newly developed Mo SNAP model will find broad applications in large-scale, long-time scale simulations. read less

Abstract: As gold-nanoparticles–embedded in amorphous carbon films the sp3 carbon orbits near the interface will be partially transferred to sp2. The Raman spectrum measurements as well as the molecular-dynamics simulations used the second reactive empirical bond order (REBO) potential simulating the interatomic force between carbon atoms both confirm the orbital transformations. The amorphous carbon films are initially inert to gases, while the films embedded with gold nanoparticles exhibit the increase of resistance in ammonia atmosphere. Namely, gold-nanoparticles–embedded amorphous carbon films become the candidate for ammonia gas sensor materials. read less

Abstract: A new interatomic potential in the framework of the modified embedded atom method (MEAM) to model U metal is presented. The potential acceptably reproduces the lattice parameters and cohesive energy of the orthorhombic αU. The relative stability of the experimentally observed phase at low temperatures with respect to several other structures (bct, bcc, simple cubic, tetragonal β Np, fcc and hcp) is also taken into account. Intrinsic point defect properties compare reasonably well with data from the literature. To determine the quality of the interaction, the potential is used to study a number of properties for the pure metal at finite temperatures and the results are compared with the available data. The obtained allotropic αU ↔ γU transformation and melting temperatures are in good agreement with experimental values. Based on the simulations, a new αU ↔ γU transformation mechanism is proposed. read less

Abstract: A set of parameters for the modified embedded atom method (MEAM) potential was developed to describe the perovskite silver tantalate (AgTaO3). First, MEAM parameters for AgO and TaO were determined based on the structural and elastic properties of the materials in a B1 reference structure predicted by density-functional theory (DFT). Then, using the fitted binary parameters, additional potential parameters were adjusted to enable the empirical potential to reproduce DFT-predicted lattice structure, elastic constants, cohesive energy and equation of state for the ternary AgTaO3. Finally, thermal expansion was predicted by a molecular dynamics (MD) simulation using the newly developed potential and compared directly to experimental values. The agreement with known experimental data for AgTaO3 is satisfactory, and confirms that the new empirical model is a good starting point for further MD studies. read less

Abstract: We have developed a reactive force field within the ReaxFF framework to accurately describe reactions involving aluminum–molybdenum alloy, which are part parameters of Al–O–Mo ternary system metastable intermolecular composites. The parameters are optimized from a training set, whose data come from density functional theory (DFT) calculations and experimental value, such as heat of formation, geometry data, and equation of states, which are reproduced well by ReaxFF. Body-centered cubic molybdenum’s surface energy, vacancy formation, and two transformational paths, Bain and trigonal paths are calculated to validate the ReaxFF ability describing the defects and deformations. Some structures’ elastic constant and phonon are calculated by DFT and ReaxFF to predict the structures’ mechanics and kinetic stability. All those results indicate that the fitted parameters can describe the energy difference of various structures under various circumstances and generally represent the diffusion property but cannot reproduce the elasticity and phonon spectra so well. read less

Abstract: Large-scale simulations of plastic deformation and phase transformations in alloys require reliable classical interatomic potentials. We construct an embedded-atom method potential for niobium as the first step in alloy potential development. Optimization of the potential parameters to a well-converged set of density-functional theory (DFT) forces, energies, and stresses produces a reliable and transferable potential for molecular dynamics simulations. The potential accurately describes properties related to the fitting data, and also produces excellent results for quantities outside the fitting range. Structural and elastic properties, defect energetics, and thermal behavior compare well with DFT results and experimental data, e.g., DFT surface energies are reproduced with less than 4% error, generalized stacking-fault energies differ from DFT values by less than 15%, and the melting temperature is within 2% of the experimental value. read less

Abstract: Ab initio electronic structure calculations are employed to study the stability and mobility of mono-self interstitial atoms (SIA) in $\ensuremath{\alpha}\text{-Fe}$ under external deformation. The ab initio results indicate that the volumetric and uniaxial strain dependences of the SIA formation energy are different in the expansion and compression regimes, in contrast to the linear behavior in continuum elasticity theory. We find a $⟨111⟩\ensuremath{\rightarrow}⟨100⟩$ SIA reorientation mechanism induced by uniaxial expansion which proceeds via $⟨11x⟩{\ensuremath{\mid}}_{x=2.7}$ configuration. Volumetric and uniaxial deformations are also found to have a considerable influence on the migration paths and activation energy barriers for the $⟨110⟩{110}\ensuremath{\leftrightarrow}⟨100⟩{100}$ transformation and the $⟨111⟩\ensuremath{\leftrightarrow}⟨100⟩$ reorientation. The results reveal that (i) the volumetric expansion (compression) decreases (increases) substantially the migration energy barrier and renders the diffusion process three (one) dimensional, (ii) the uniaxial strain removes (decreases) the migration energy barrier for the $⟨111⟩\ensuremath{\rightarrow}⟨11x⟩{\ensuremath{\mid}}_{x=2.7}(⟨11x⟩{\ensuremath{\mid}}_{x=2.7}\ensuremath{\rightarrow}⟨100⟩)$ transformation, leading to spontaneous reorientation of the $⟨111⟩$ SIA, and (iii) the uniaxial deformation breaks the cubic symmetry of the system and in turn induces anisotropy of the migration rates along different directions. These calculations demonstrate that changes in the electronic structure induced by global elastic deformation lead to additional contributions to the formation and migration energies, which cannot be adequately accounted for neither by elasticity theory nor by empirical interatomic potentials. read less

Abstract: The extension of molecular mechanics to reactive systems, metals, and covalently bonded clusters with variable coordination numbers requires new functional forms beyond those popular for organic chemistry and biomolecules. Here we present a new scheme for reactive molecular mechanics, which is denoted as the valence-bond order model, for approximating reactive potential energy surfaces in large molecules, clusters, nanoparticles, solids, and other condensed-phase materials, especially those containing metals. The model is motivated by a moment approximation to tight binding molecular orbital theory, and we test how well one can approximate potential energy surfaces with a very simple functional form involving only interatomic distances with no explicit dependence on bond angles or dihedral angles. For large systems the computational requirements scale linearly with system size, and no diagonalizations or iterations are required; thus the method is well suited to large-scale simulations. The method is illustrated here by developing a force field for particles and solids composed of aluminum and hydrogen. The parameters were optimized against both interaction energies and relative interaction energies. The method performs well for pure aluminum clusters, nanoparticles, and bulk lattices and reasonably well for pure hydrogen clusters; the mean unsigned error per atom for the aluminum-hydrogen clusters is 0.1 eV/atom. read less

Abstract: Electronic structure calculations are employed in order to investigate the cohesive properties (lattice parameter, enthalpy of formation, and bulk modulus) of random Fe-Cr alloys as a function of composition and magnetic state, as well as to derive the chemical and magnetic exchange interactions of the constituent atoms. The calculations predict certain anomalies in the cohesive properties of ferromagnetic alloys at a concentration of about $7\text{ }\text{at}\text{ }%$ Cr; these anomalies may be related to the changes in Fermi-surface topology that occur with composition in this alloy system. The obtained interatomic interactions are used as parameters in the configurational (Ising) and magnetic (Heisenberg) Hamiltonians for modeling finite-temperature thermodynamic properties of the alloys. We discuss the approximations and limitations of similar modeling approaches, investigate the origin of existing difficulties, and analyze possible ways of extending the theoretical models in order to capture the essential physics of interatomic interactions in the Fe-Cr or similar alloys where magnetism plays a crucial role in the phase stability. read less

Abstract: When the surface energy of bottom layer is lower than top layer, the atoms of bottom layer should be pumped up at the triple point of the grain boundary. This phenomenon should be useful for the fabrication of well-defined nanobridge. To confirm the Atom Pumping-Up mechanism for metal/metal thin films, Ta(2 nm) on epitaxial MgO(001)/Fe(001) films were prepared by the electron beam deposition method. The Pumping-Up mechanism was successfully confirmed by the Auger Electron Spectroscopy, the in situ Scanning Tunneling Microscopy, and the Transmission Electron Microscope observation after heat treatment of 400 degCtimes30 min. The size of the Fe bridges was 2-5 nm. Furthermore, the magnetic exchange coupling effect on the magnetization process through the ferromagnetic bridges between two Fe layers in MgO/Fe/Ta/Fe after Atom Pumping-Up shows a good agreement to the micromagnetics simulation result from the assumption of ferromagnetic bridges with the size of 1-4 nm read less

Abstract: In this study we use a new topological structure measure to analyze the local environment of 923 atom gold clusters quenched from the melt, at various quench rates, by molecular dynamics. The crystallization and geometrical rearrangements of the core atoms upon freezing can be clearly observed using our structure measure which is based on planar graphs. Our results support the hypothesis that crystallization is initiated from the surface and proceeds into the cluster core. read less

Abstract: We propose an extended Finnis–Sinclair (FS) potential by extending the repulsive term into a sextic polynomial for enhancing the repulsive interaction and adding a quartic term to describe the electronic density function. It turns out that for bcc metals the proposed potential not only overcomes the ‘soft’ behaviour of the original FS potential, but also performs better than the modified FS one by Ackland et al, and that for fcc metals the proposed potential is able to reproduce the lattice constants, cohesive energies, elastic constant, vacancy formation energies, equations of state, pressure–volume relationships, melting points and melting heats. Moreover, for some fcc–bcc systems, e.g. the Ag–refractory metal systems, the lattice constants, cohesive energies and elastic constants of some alloys are reproduced by the proposed potential and are quite compatible with those directly determined by ab initio calculations. read less

Abstract: We report a detailed ab initio study of the stability and migration of self-interstitial atoms (SIAs) and di-interstitials (di-SIAs) in alpha-Fe. The <110> dumbbell is confirmed to be the most stable SIA configuration, 0.7 eV below the <111> dumbbell. The lowest-energy migration path corresponds to a nearest-neighbor translation-rotation jump with a barrier of 0.34 eV. The most stable configuration for di-SIAs consists of <110> parallel dumbbells. Their migration mechanism is similar to that for SIAs, with an activation energy of 0.42 eV. These results are at variance with predictions from existing empirical potentials and allow one to reconcile theory with experiments. read less

Abstract: Using the (re)modified embedded atom method, an extension of the embedded atom model that includes angular forces and second-nearest-neighbor interactions, we performed quenched molecular-dynamics simulations to compute the lowest-energy structures of Fe n clusters (n=2-20,25) supported on or embedded in the top few layers of the Al (001) surface. Embedded clusters were always more stable than adsorbed clusters, and formed either linear chains (for n=3 or 5) or warped single-layer close-packed islands (for n=4 or n≥6). Determination of the spin-polarized electronic structure using a self-consistent tight-binding method showed that, due to hybridization between the Al sp and Fe d states, the embedded Fe n clusters are nonmagnetic for n=2-10. However, for larger sizes they can sustain magnetic moments. read less

Abstract: Previously a new universal λ-integration path and associated methodology was developed for the calculation of “exact” surface and interfacial free energies of solids. Such a method is in principle applicable to any intermolecular potential function, including those based on ab initio methods, but in previous work the method was only tested using a relatively simple embedded atom method iron potential. In this present work we apply the new methodology to the more sophisticated and more accurate modified embedded atom method (MEAM) iron potential, where application of other free- energy methods would be extremely difficult due to the complex many-body nature of the potential. We demonstrate that the new technique simplifies the process of obtaining “exact” surface free energies by calculating the complete set of these properties for the low index surface faces of bcc and fcc solid iron structures. By combining these data with further calculations of liquid surface tensions we obtain the first complete set o... read less

Abstract: A new scheme of the modified embedded atom method (MEAM) is developed by modifying the analytic form of the embedding function. The new MEAM parameters for Mo, W, V, Nb, Ta and Fe have been determined by relating them to not only bulk properties but also some non-bulk properties. The new scheme was applied to calculate the elastic stiffness of the crystal, the vacancy formation energy, the lattice stability, the surface energies for low-index crystal faces and the bond length and the binding energy for the dimer. The results give a fairly good agreement with the experimental data. read less

Abstract: Modified embedded atom method (MEAM) potentials for fcc elements Cu, Ag, Au, Ni, Pd, Pt, Al, and Pb have been newly developed using the original first nearest-neighbor MEAM and the recently developed second nearest-neighbor MEAM formalisms. It was found that the original MEAM potentials for fcc elements show some critical shortcomings such as structural instability and incorrect surface reconstructions on (100), (110), and/or (111) surfaces. The newly developed MEAM potentials solve most of the problems and describe the bulk properties (elastic constants, structural energy differences), point defect properties (vacancy and interstitial formation energy and formation volume, activation energy of vacancy diffusion), planar defect properties (stacking fault energy, surface energy, surface relaxation and reconstruction), and thermal properties (thermal expansion coefficients, specific heat, melting point, heat of melting) of the fcc elements considered, in good agreement with relevant experimental information. It has been shown that in the MEAM the degree of many-body screening (C m i n ) is an important material property and that structural stability at finite temperatures should be included as a checkpoint during development of semiempirical potentials. read less

Abstract: A new scheme of modified embedded atom method (MEAM) is proposed in this paper. The analytic form of the embedding function is modified. All the parameters of MEAM have been reset by relating them with bulk properties and some non-bulk properties, for example, the bond length of a dimer and the change of surface interlayer distance. The new scheme has been applied to calculate the elastic stiffness of crystal, the vacancy formation energy and some properties of non-bulk systems such as the surface energies for low index crystal faces, the bond length and the binding energy for a dimer. The results are compared with the experimental data and get a fairly good agreement. read less

Abstract: We develop a many-body force field (FF) for tantalum based on extensive ab initio quantum mechanical (QM) calculations and illustrate its application with molecular dynamics (MD). As input data to the FF we use ab initio methods (LAPW-GGA) to calculate: (i) the zero temperature equation of state (EOS) of Ta for bcc, fcc, and hcp crystal structures for pressures up to ∼500 GPa, and (ii) elastic constants. We use a mixed-basis pseudopotential code to calculate: (iii) volume-relaxed vacancy formation energy also as a function of pressure. In developing the Ta FF we also use previous QM calculations of: (iv) the EOS for the A15 structure; (v) the surface energy bcc (100); (vi) energetics for shear twinning of the bcc crystal. We find that, with appropriate parameters, an embedded atom model FF (denoted as qEAM FF) is able to reproduce all this QM data. We illustrate the use of the qEAM FF with MD to calculate such finite temperature properties as the melting curve up to 300 GPa and thermal expansivity in a wide temperature range. Both our predictions agree well with experimental values. read less

Abstract: Constructing an accurate atomistic model for the high-pressure phases of tin (Sn) is challenging because properties of Sn are sensitive to pressures. We develop machine-learning-based deep potentials for Sn with pressures ranging from 0 to 50 GPa and temperatures ranging from 0 to 2000 K. In particular, we find the deep potential, which is obtained by training the ab initio data from density functional theory calculations with the state-of-the-art SCAN exchange-correlation functional, is suitable to characterize high-pressure phases of Sn. We systematically validate several structural and elastic properties of the {\alpha} (diamond structure), {\beta}, bct, and bcc structures of Sn, as well as the structural and dynamic properties of liquid Sn. The thermodynamics integration method is further utilized to compute the free energies of the {\alpha}, {\beta}, bct, and liquid phases, from which the deep potential successfully predicts the phase diagram of Sn including the existence of the triple-point that qualitatively agrees with the experiment. read less

Abstract: In this article, the compression simulation of AZ31 magnesium alloy is simulated by the molecular dynamics method. The effects of loading mode, temperature, and strain rate on the compression behavior are analyzed. The lattice distortion, mechanical behavior, structural evolution, and dislocation evolution in the compression process are deeply analyzed, and the results of different loading modes are obtained. The hexagonal close-packed (HCP) → face-centered cubic (FCC) phase transformation mechanism of AZ31 magnesium alloy during compression at temperature and strain rate, which is related to the mechanical behavior, has been studied completely. This article perfects the research on the compression behavior of magnesium alloys, excavates the application potential of magnesium alloys, and provides a new idea for improving the processing technology and developing high-performance magnesium alloys. read less

Abstract: In this paper, we investigate the thermostructural properties of a type of silicon-based nanomaterials, which we refer to as SiC@Si nanocomposites, formed by SiC crystalline nanoparticles (with the cubic phase), embedded within an amorphous Si matrix. We have followed an in silico approach to characterize the mechanical and thermal behaviour of these materials, by calculating the elastic constants, uniaxial stress-strain curves, coefficients of thermal expansion, and specific heats, at different temperatures, using interatomic potential calculations. The results obtained from our simulations suggest that this type of material presents enhanced thermal resistance features, making it suitable to be used in devices subjected to big temperature changes, such as heat sinks in micro and nanoelectronics, solar energy harvesters at high temperatures, power electronics, or in other applications in which good thermomechanical properties are required. read less

Abstract: The second nearest-neighbor modified embedded-atom method (2NN MEAM) potential parameters of the Ti–Cr binary and Ti–Cr–N ternary systems are optimized in accordance with the 2NN MEAM method. The novel constructed potential parameters can well reproduce the multiple fundamental physical characteristics of binary and ternary systems and reasonably agree with the first-principles calculation or experimental data. Thus, the newly constructed 2NN MEAM potential parameters can be used for atomic simulations to determine the underlying principle of the hardness enhancement of TiN/CrN multilayered coatings. read less

Abstract: An interatomic potential for the ternary Ag–Cu–Sn system, an important material system related to the applications of lead-free solders, is developed on the basis of the second nearest-neighbor modified embedded-atom-method formalism. Potential parameters for the ternary and related binary systems are determined based on the recently improved unary description of pure Sn and the present improvements to the unary descriptions of pure Ag and Cu. To ensure the sufficient performance of atomistic simulations in various applications, the optimization of potential parameters is conducted based on the force-matching method that utilizes density functional theory predictions of energies and forces on various atomic configurations. We validate that the developed interatomic potential exhibits sufficient accuracy and transferability to various physical properties of pure metals, intermetallic compounds, solid solutions, and liquid solutions. The proposed interatomic potential can be straightforwardly used in future studies to investigate atomic-scale phenomena in soldering applications. read less

Abstract: When the phase diagram of the element Fe is examined, it is seen that it has different crystal structures at different temperatures below its melting temperature. In this study, the solid-solid phase transformations occurring at different temperatures in the Fe model system consisting of 4000 atoms were investigated using molecular dynamic simulation method. The Embedded Atom Method(EAM), which includes many body interactions, was used to calculate interactions between atoms. For the element Fe, the α, γ and δ phases formed below the melting temperature and the transformation temperatures for these phases were determined and the results were compared with the experimental values. Radial distribution function, changes in thermodynamic quantities and Ackland-Jones analysis method were used in the structural analysis of the model system. read less

Abstract: We have studied the interfacial thermal conductance, G, of the flat Au(111)-water interface using non-equilibrium molecular dynamics simulations. We utilized two metal models, one based on the embedded atom method (EAM) and the other including metallic polarizability via a density readjusting EAM. These were combined with three popular water models, SPC/E, TIP4P, and TIP4P-FQ, to understand the role of polarizability in the thermal transport process. A thermal flux was introduced using velocity shearing and scaling reverse non-equilibrium molecular dynamics, and transport coefficients were measured by calculating the resulting thermal gradients and temperature differences at the interface. Our primary finding is that the computed interfacial thermal conductance between a bare metal interface and water increases when polarizability is taken into account in the metal model. Additional work to understand the origin of the conductance difference points to changes in the local ordering of the water molecules in the first two layers of water above the metal surface. Vibrational densities of states on both sides of the interface exhibit interesting frequency modulation close to the surface but no obvious differences due to metal polarizability. read less

Abstract: The presence of facets and line junctions connecting facets on grain boundaries (GBs) has a strong impact on the properties of structural, functional, and optoelectronic materials: They govern the mobility of interfaces, the segregation of impurities, as well the electronic properties. In the present paper, we employ density-functional theory and modified embedded atom method calculations to systematically investigate the energetics and thermodynamic stability of these defects. As a prototype system, we consider (cid:2) 3 tilt GBs in Si. By analyzing the energetics of different faceted GBs, we derive a diagram that describes and predicts the reconstruction of these extended defects as a function of facet length and boundary inclination angle. The phase diagram sheds light upon the fundamental mechanisms causing GB faceting phenomena. It demonstrates that the properties of faceting are not determined solely by anisotropic GB energies but by a complex interplay between geometry and microstructure, boundary energies as well as long-range strain interactions. read less

Abstract: ABSTRACT Molecular Dynamics (MD) was used to determine the accuracy of different force fields on predicting the elastic modulus of single crystal aluminum through nanoindentation tests. In this work, nanoindentation was performed using three different types of force fields (EAM, MEAM and ReaxFF) and the resulting elastic modulus was compared to the value obtained using elastic constants from standard small strain tensile simulations. When the predicted modulus of each force field was compared to the modulus via elastic constants, the ReaxFF resultant moduli were similar to that of nanoindentation, but for EAM and MEAM the two methods produced significantly different values. Therefore, even if a force field is parameterised for elastic modulus, it does not guarantee the force field will accurately predict the modulus from other procedures. As well, two different methods for calculating modulus from indentation curves were compared: The Hertz approximation and the Oliver and Pharr (O&P) method. For EAM and MEAM force fields, the Hertz method significantly under predicted modulus while the O&P method was in better agreement with the experimental modulus. read less

Abstract: We investigate the atomistic mechanism of homogeneous nucleation during solidification in molybdenum employing transition path sampling. The mechanism is characterized by the formation of a pre-structured region of high bond-orientational order in the supercooled liquid followed by the emergence of the crystalline bulk phase within the center of the growing solid cluster. This precursor plays a crucial role in the process as it provides a diffusive interface between the liquid and crystalline core, which lowers the interfacial free energy and facilitates the formation of the bulk phase. Furthermore, the structural features of the pre-ordered regions are distinct from the liquid and solid phases and preselect the specific polymorph that nucleates. The similarity in the nucleation mechanism of Mo with that of metals that exhibit different crystalline bulk phases indicates that the formation of a precursor is a general feature observed in these materials. The strong influence of the structural characteristics of the precursors on the final crystalline bulk phase demonstrates that for the investigated system, polymorph selection takes place in the very early stages of nucleation. read less

Abstract: An interatomic potential for the Ti–V binary alloy focusing on the evolution of defects, including ones arising as a result of the irradiation process, was constructed within the Lipnitskii–Saveliev approach, which accurately takes into account three-particle interactions and the sum of all multi-particle interactions of a higher order in the framework of the centrally symmetric approximation. In the new potential, Ti–V interactions were fitted to the density functional theory data on a set of model structures with different coordination numbers, including ones with vacancies. The properties used for fitting are accurately reproduced by the present potentials for both pure elements and alloy systems. The potential was tested on the binding energies between Ti atoms and self-point defects in bcc V, elastic moduli, thermal expansion and melting point of some alloys, and diffusion. We obtained qualitative agreement for these properties with available theoretical and experimental data. Finally, we investigated the evolution of excess vacancies in the V–4 at% Ti alloy at 700 K, which are typical conditions of vanadium-based alloys for fusion applications. We found that no vacancy loop is formed in the alloy in contrast to the pure V, which agrees with the experimental observations. The potential is expected to be especially suitable for irradiation simulations of vanadium-based V–Ti alloys. read less

Abstract: The interaction of glide dislocations with own interstitial atoms in α-Fe is studied. As a method of investigation the molecular dynamics simulation is used. The modelled sample is deformed with a rate close to the deformation rates under pulsed laser treatment. Mass transfer parameter for b.c.c. Fe under laser pulse irradiation is calculated. As established, the core of moving dislocation is a trap for interstitial atom. The influence of temperature and deformation rate on mass-transfer coefficient is studied. read less

Abstract: The martensitic transformation is one of the most important phenomena in metals science due to its essential contribution to the strength of steels and most engineering alloys. Yet the basic, atomistic mechanisms leading to martensite nucleation and twin morphology are not yet known. A detailed picture in this regard is required if the strengthening effects of martensite are to be properly understood. This work presents molecular dynamics (MD) simulations of the martensitic transformation using a model fcc/bcc semi-coherent interface with Nishiyama-Wasserman orientation relationship. Significant insight into this important phenomenon is detailed in this work which shows that the atomic displacements that cause nucleation and twin morphology formation of the martensitic phase originate at the fcc/bcc interface. The interface facilitates the initial atomic shear during the transformation which in turn causes the stress-induced homogeneous nucleation and twin morphology formation. The understanding of the atomistic processes leading to the twin morphology formation will allow the control of the twinning process for further enhancement of mechanical properties. read less

Abstract: Recent advances in biomedicine largely rely on the development in nanoengineering. As the access to unique properties in biomaterials is not readily available from traditional techniques, the nanoengineering becomes an effective approach for research and development, by which the performance as well as the functionalities of biomaterials has been greatly improved and enriched. This review focuses on the main materials used in biomedicine, including metallic materials, polymers, and nanocomposites, as well as the major applications of nanoengineering in developing biomedical treatments and techniques. Research that provides an in-depth understanding of material properties and efficient enhancement of material performance using molecular dynamics simulations from the nanoengineering perspective are discussed. The advanced techniques which facilitate nanoengineering in biomedical applications are also presented to inspire further improvement in the future. Furthermore, the potential challenges of nanoengineering in biomedicine are evaluated by summarizing concerned issues and possible solutions. Graphical abstract read less

Abstract: We developed new interatomic potentials, based on the second nearest-neighbor modified embedded-atom method (2NN-MEAM) formalism, for Ti, Ni, and the binary Ti–Ni system. These potentials were fit to melting points, latent heats, the binary phase diagrams for the Ti rich and Ni rich regions, and the liquid phase enthalpy of mixing for binary alloys, therefore they are particularly suited for calculations of crystal-melt (CM) interface thermodynamic and transport properties. The accuracy of the potentials for pure Ti and pure Ni were tested against both 0 K and high temperature properties by comparing various properties obtained from experiments or density functional theory calculations including structural properties, elastic constants, point-defect properties, surface energies, temperatures and enthalpies of phase transformations, and diffusivity and viscosity in the liquid phase. The fitted binary potential for Ti–Ni was also tested against various non-fitted properties at 0 K and high temperatures including lattice parameters, formation energies of different intermetallic compounds, and the temperature dependence of liquid density at various concentrations. The CM interfacial free energies obtained from simulations, based on the newly developed Ti–Ni potential, show that the bcc alloys tend to have smaller anisotropy compared with fcc alloys which is consistent with the finding from the previous studies comparing single component bcc and fcc materials. Moreover, the interfacial free energy and its anisotropy for Ti-2 atom% Ni were also used to parameterize a 2D phase field (PF) model utilized in solidification simulations. The PF simulation predictions of microstructure development during solidification are in good agreement with a geometric model for dendrite primary arm spacing. read less

Abstract: ABSTRACT Molecular dynamics simulations are more frequently being utilised to predict macroscale mechanical properties as a result of atomistic defects. However, the interatomic force field can significantly affect the resulting mechanical properties. While several studies exist which demonstrate the ability of various force fields to predict mechanical properties, the investigation into which is most accurate for the investigation of vacancies is limited. To obtain meaningful predictions of mechanical properties, a clear understanding of force field parameterisation is required. As such, the current study evaluates various many-body force fields to demonstrate the reduction in mechanical properties of iron and iron–chromium due to the presence of vacancies while undergoing room temperature atomistic uniaxial tension. Reduction was normalised in each case with the zero-vacancy elastic modulus, removing the need to predict an accurate nominal elastic modulus. Comparisons were made to experimental data and an empirical model from literature. It was demonstrated that accurate fitting to vacancy formation and migration energy allowed for accurate predictions. In addition, bond-order based force fields showed enhanced predictions regardless of fitting procedure. Overall, these findings highlight the need to understand capabilities and limitations of available force fields, as well as the need for enhanced parameterisation of force fields. read less

Abstract: In this work, we investigated tensile and compression forces effect on the thermal conductivity of silicon. We used equilibrium molecular dynamics approach for the evaluation of thermal conductivity considering different interatomic potentials. More specifically, we tested Stillinger-Weber, Tersoff, Environment-Dependent Interatomic Potential and Modified Embedded Atom Method potentials for the description of silicon atom motion under different strain and temperature conditions. Additionally, we extracted phonon density of states and dispersion curves from molecular dynamics simulations. These data were used for direct calculations of thermal conductivity considering the kinetic theory approach. Comparison of molecular dynamics and kinetic theory simulations results as a function of strain and temperature allowed us to investigate the different factors affecting the thermal conductivity of strained silicon. read less

Abstract: An interatomic potential for the Ni–Mo binary alloy focusing on irradiation has been constructed with the modified analysis embedded atom method. The newly developed interatomic (Ni–Ni and Mo–Mo) potentials and the Ni–Mo cross-interactions are fitted to the ab initio results and experimental data, including defect energies, formation energies of three stable phases. The properties used for fitting are accurately reproduced by the present potentials for both pure elements and alloy systems. Those properties beyond the fitting ranges are also well predicted, demonstrating its excellent transferability. The advantages and certain weaknesses of the new potential are also discussed in detail compared with other existing potentials. The potential is expected to be especially suitable for irradiation simulations of Ni–Mo alloys. read less

Abstract: ABSTRACT Molecular dynamics simulations, which take place on the atomistic scale, are now being used to predict the influence of atomistic processes on macro-scale mechanical properties. However, there is a lack of clear understanding on which potential should be used when attempting to obtain these properties. Moreover, many MD studies that do test mechanical properties do not actually simulate the macro-scale laboratory tension tests used to obtain them. As such, the purpose of the current study was to evaluate the various types of potentials for their accuracy in predicting the mechanical properties of iron from an atomistic uniaxial tension test at room temperature. Results demonstrated that while EAM and MEAM potentials all under predicted the elastic modulus at room temperature, the Tersoff and ReaxFF potentials were significantly more accurate. Unlike EAM and MEAM, both the Tersoff and ReaxFF potentials are bond order based. Therefore, these results demonstrate the importance of considering bonding between atoms when modelling tensile tests. In addition, the ReaxFF potential also accurately predicted the Poisson's ratio, allowing for complete characterisation of the material's behaviour. Overall, these findings highlight the need to understand the capabilities and limitations of each potential before application to a problem outside of the initial intended use. read less

Abstract: Modified embedded atom method potential parameters of beryllium oxide (BeO) have been developed, which can well reproduce the thermodynamic properties of beryllium oxide. To accurately describe the interactions between the atoms in the BeO structure, the density functional theory is used to calculate the fundamental properties such as the lattice constant, bulk modulus, and elastic constant, which are used for the potential fitting. The properties such as the enthalpy and specific heat are used to test the validity of the potential parameters. The calculated results by the developed potential parameters are compared with the experimental and other theoretical data as a function of temperature. The good agreement between the calculated results by the new potential and the experimental data verifies the potential parameters. The developed potential parameters have also been used to predict the thermal conductivity of BeO as a function of temperature for further applications of beryllium oxide. read less

Abstract: Lattice structures, defect structures, and deformation mechanisms of high-entropy alloys (HEAs) have been studied using atomistic simulations to explain their remarkable mechanical properties. These atomistic simulation techniques, such as first-principles calculations and molecular dynamics allow atomistic-level resolution of structure, defect configuration, and energetics. Following the structure–property paradigm, such understandings can be useful for guiding the design of high-performance HEAs. Although there have been a number of atomistic studies on HEAs, there is no comprehensive review on the state-of-the-art techniques and results of atomistic simulations of HEAs. This article is intended to fill the gap, providing an overview of the state-of-the-art atomistic simulations on HEAs. In particular, we discuss how atomistic simulations can elucidate the nanoscale mechanisms of plasticity underlying the outstanding properties of HEAs, and further present a list of interesting problems for forthcoming atomistic simulations of HEAs. read less

Abstract: We present a modified embedded atom method (MEAM) semi-empirical force-field model for the Ti1−xAlxN (0 ≤ x ≤ 1) alloy system. The MEAM parameters, determined via an adaptive simulated-annealing (ASA) minimization scheme, optimize the model’s predictions with respect to 0 K equilibrium volumes, elastic constants, cohesive energies, enthalpies of mixing, and point-defect formation energies, for a set of ≈40 elemental, binary, and ternary Ti-Al-N structures and configurations. Subsequently, the reliability of the model is thoroughly verified against known finite-temperature thermodynamic and kinetic properties of key binary Ti-N and Al-N phases, as well as properties of Ti1−xAlxN (0 < x < 1) alloys. The successful outcome of the validation underscores the transferability of our model, opening the way for large-scale molecular dynamics simulations of, e.g., phase evolution, interfacial processes, and mechanical response in Ti-Al-N-based alloys, superlattices, and nanostructures. read less

Abstract: The thermal expansion and the phase transition of Fe-15.6 wt. %Co-12 wt. %Ni single-phase solid solution alloy were systematically investigated by thermal analysis experiments and molecular dynamics simulations. The coefficient of thermal expansion (CTE) was accurately measured in the temperature range of 300-1580 K. The eccentric changes of thermal expansion ranging from 900 to 1150 K were verified from the incomplete transformation of α-Fe phase to γ-Fe phase by differential scanning calorimetry (DSC) and in situ X-ray diffraction experiments. The CTE of α-Fe phase increases nonlinearly from 9.29 × 10−6 to 1.278 × 10−5 K−1 in the range of 300-900 K, which is in good agreement with the results obtained by molecular dynamics simulation, whereas the CTE of γ-Fe phase increases linearly from 2.024 × 10−5 to 2.398 × 10−5 K−1 in the range of 1150-1580 K. Meanwhile, the visual atomic positions at different temperatures indicate that thermal expansion is attributed to the anharmonic vibration and short-range diffusion of atoms when the temperature exceeds a certain value. Furthermore, the Curie temperature is determined as 725 K by the thermal expansion and DSC experiments. Additionally, the isothermal sections of the Fe-rich corner [Fe-5x wt. %Co-5y wt. %Ni(2 ≤ x + y ≤ 8)] in Fe-Co-Ni non-equilibrium ternary phase diagram at 300 K are derived by X-ray diffraction. Moreover, the CTE ranging from 300 to 1700 K of the Fe-rich corner in Fe-Co-Ni ternary phase diagram was predicted theoretically on the basis of the molecular dynamics method.The thermal expansion and the phase transition of Fe-15.6 wt. %Co-12 wt. %Ni single-phase solid solution alloy were systematically investigated by thermal analysis experiments and molecular dynamics simulations. The coefficient of thermal expansion (CTE) was accurately measured in the temperature range of 300-1580 K. The eccentric changes of thermal expansion ranging from 900 to 1150 K were verified from the incomplete transformation of α-Fe phase to γ-Fe phase by differential scanning calorimetry (DSC) and in situ X-ray diffraction experiments. The CTE of α-Fe phase increases nonlinearly from 9.29 × 10−6 to 1.278 × 10−5 K−1 in the range of 300-900 K, which is in good agreement with the results obtained by molecular dynamics simulation, whereas the CTE of γ-Fe phase increases linearly from 2.024 × 10−5 to 2.398 × 10−5 K−1 in the range of 1150-1580 K. Meanwhile, the visual atomic positions at different temperatures indicate that thermal expansion is attributed to the anharmonic vibration and short-range di... read less

Abstract: A new interatomic potential for the pure tin (Sn) system is developed on the basis of the second-nearest-neighbor modified embedded-atom-method formalism. The potential parameters were optimized based on the force-matching method utilizing the density functional theory (DFT) database of energies and forces of atomic configurations under various conditions. The developed potential significantly improves the reproducibility of many fundamental physical properties compared to previously reported modified embedded-atom method (MEAM) potentials, especially properties of the β phase that is stable at the ambient condition. Subsequent free energy calculations based on the quasiharmonic approximation and molecular-dynamics simulations verify that the developed potential can be successfully applied to study the allotropic phase transformation between α and β phases and diffusion phenomena of pure tin. read less

Abstract: Large-scale atomistic simulations with classical potentials can provide valuable insights into microscopic deformation mechanisms and defect-defect interactions in materials. Unfortunately, these assets often come with the uncertainty of whether the observed mechanisms are based on realistic physical phenomena or whether they are artifacts of the employed material models. One such example is the often reported occurrence of stable planar faults (PFs) in body-centered cubic (bcc) metals subjected to high strains, e.g., at crack tips or in strained nano-objects. In this paper, we study the strain dependence of the generalized stacking fault energy (GSFE) of {110} planes in various bcc metals with material models of increasing sophistication, i.e., (modified) embedded atom method, angular-dependent, Tersoff, and bond-order potentials as well as density functional theory. We show that under applied tensile strains the GSFE curves of many classical potentials exhibit a local minimum which gives rise to the formation of stable PFs. These PFs do not appear when more sophisticated material models are used and have thus to be regarded as artifacts of the potentials. We demonstrate that the local GSFE minimum is not formed for reasons of symmetry and we recommend including the determination of the strain-dependent (110) GSFE as a benchmark for newly developed potentials. read less

Abstract: An interatomic potential for the Mg–Y binary system is developed within the framework of the second-nearest-neighbor modified embedded-atom method (MEAM) based on a very good MEAM potential for pure Mg. The Mg–Y potential is fitted to a range of key physical properties, either experimental or computed by first-principles methods, including the Y interaction energy with basal and pyramidal stacking faults, and properties of the B2 Mg–Y intermetallic phase. Reasonable agreement is obtained—much better than existing potentials in the literature—but differences remain for subtle but important aspects of Y solutes in Mg. The predictions of the potential for Y misfit volume in Mg, Y solute interactions with the pyramidal II 〈 c + a 〉 edge dislocation and { 10 1 ¯ 2 } 〈 1 ¯ 011 〉 tension-twin boundary are then compared against recent density functional theory results, and reasonable accuracy is obtained. In light of the spectrum of results presented here, the applicability and limitations of this Mg–Y MEAM potential for investigating various plasticity phenomena in Mg–Y solid solution alloys are carefully discussed. read less

Abstract: In the present paper, we have developed a modified analytic embedded atom method potential for chromium. To solve the problem of a negative Cauchy relation for body-centered cubic metal at zero Kelvin temperature, a modification term is added to the total energy. Compared with available interatomic potentials, the present one is easier to be extended because of its analytic form. The fitted or input parameters are all well reproduced, and the properties beyond the fitting range (such as Cauchy relation, melting point and self-interstitial atom formation energies) are also well predicted. We have also studied the relation between the stacking faults energy and 1 2 〈 111 〉 screw dislocation core structure with different potentials. Especially, only our potential predicts a compact core structure, which agrees with the results obtained by density functional theory calculations. read less

Abstract: New interatomic potentials for pure Ti and Al, and binary TiAl were developed utilizing the second nearest neighbour modified embedded-atom method (MEAM) formalism. The potentials were parameterized to reproduce multiple properties spanning bulk solids, solid surfaces, solid/liquid phase changes, and liquid interfacial properties. This was carried out using a newly developed optimization procedure that combined the simple minimization of a fitness function with a genetic algorithm to efficiently span the parameter space. The resulting MEAM potentials gave good agreement with experimental and DFT solid and liquid properties, and reproduced the melting points for Ti, Al, and TiAl. However, the surface tensions from the model consistently underestimated experimental values. Liquid TiAl’s surface was found to be mostly covered with Al atoms, showing that Al has a significant propensity for the liquid/air interface. read less

Abstract: Using atomistic Monte Carlo simulations, we investigated the impact of the interface on the structural properties of iron and copper (Fe/Cu) magnetic multilayers grown by Voronoi diagram. Interest in magnetic multilayers has recently emerged as they are shown to be promising candidates for magnetic storage media, magneto-resistive sensors and personalized medical treatment. As these artificial materials show large differences in properties compared to conventional ones, many experimental and theoretical works have been dedicated on shedding light on these differences and tremendous results have emerged. However, little is known about the influence of the interfaces on magnetic layers. Using numerical approaches, we show that the structure of each layer depends on its thickness and the interface morphology. The Fe and Cu layers can adopt either the body-centered-cubic (bcc) or face-centered-cubic (fcc) structure, while the interface can assume amorphous, bcc, fcc, or a mixture of bcc and fcc structures depending on the layer thicknesses. These results are in good agreement with the experiments. They could be helpful in understanding effects such as giant magneto-resistance from the structural perspective. read less

Abstract: The objective of this work is to simulate the superelasticity and shape-memory effect in a single-crystalline nickel-titanium (NiTi) alloy through a molecular dynamics (MD) study. Cooling and heating processes for this material are reproduced to investigate the temperature-induced phase transformation in its microstructure. It is found that the martensitic transformation and its reverse process occur accompanied by an abrupt volume change, and the transformed variants lead to the appearance of the (001) type compound twin. In addition, the transform temperatures for martensite start ( ) and austenite finish ( ) are determined, respectively. The results indicate that when the temperature is beyond during the compressive loading-unloading, the superelastic behavior becomes pronounced, which is attributed to the role of nanotwins on the transformation from the austenitic phase (B2) to martensitic phase (B19′). Compared to existing experimental data, a reasonable agreement is achieved through the modeling results, highlighting the importance of the compound twins for dominating the superelasticity of nanostructured NiTi alloys. read less

Abstract: The study of the thermodynamics and structures of iron clusters has been carried on, focusing on small clusters and initial icosahedral and fcc-cuboctahedral structures. Two combined tools are used. First, energy intervals are explored by the Monte Carlo algorithm, called σ-mapping, detailed in the work of Soudan et al. [J. Chem. Phys. 135, 144109 (2011), Paper I]. In its flat histogram version, it provides the classical density of states, gp(Ep), in terms of the potential energy of the system. Second, the iron system is described by a potential which is called "corrected EAM" (cEAM), explained in the work of Basire et al. [J. Chem. Phys. 141, 104304 (2014), Paper II]. Small clusters from 3 to 12 atoms in their ground state have been compared first with published Density Functional Theory (DFT) calculations, giving a complete agreement of geometries. The series of 13, 55, 147, and 309 atom icosahedrons is shown to be the most stable form for the cEAM potential. However, the 147 atom cluster has a special behaviour, since decreasing the energy from the liquid zone leads to the irreversible trapping of the cluster in a reproducible amorphous state, 7.38 eV higher in energy than the icosahedron. This behaviour is not observed at the higher size of 309 atoms. The heat capacity of the 55, 147, and 309 atom clusters revealed a pronounced peak in the solid zone, related to a solid-solid transition, prior to the melting peak. The corresponding series of 13, 55, and 147 atom cuboctahedrons has been compared, underscoring the unstability towards the icosahedral structure. This unstability occurs clearly in several steps for the 147 atom cluster, with a sudden transformation at a transition state. This illustrates the concerted icosahedron-cuboctahedron transformation of Buckminster Fuller-Mackay, which is calculated for the cEAM potential. Two other clusters of initial fcc structures with 24 and 38 atoms have been studied, as well as a 302 atom cluster. Each one relaxes towards a more stable structure without regularity. The 38 atom cluster exhibits a nearly glassy relaxation, through a cascade of six metastable states of long life. This behaviour, as that of the 147 atom cluster towards the amorphous state, shows that difficulties to reach ergodicity in the lower half of the solid zone are related to particular features of the potential energy landscape, and not necessarily to a too large size of the system. Comparisons of the cEAM iron system with published results about Lennard-Jones systems and DFT calculations are made. The results of the previous clusters have been combined with that of Paper II to plot the cohesive energy Ec and the melting temperature Tm in terms of the cluster atom number Nat. The Nat-1/3 linear dependence of the melting temperature (Pawlow law) is observed again for Nat > 150. In contrast, for Nat < 150, the curve diverges strongly from the Pawlow law, giving it an overall V-shape, with a linear increase of Tm when Nat goes from 55 to 13 atoms. Surprisingly, the 38 atom cluster is anomalously below the overall curve. read less

Abstract: In this paper, we develop a new modified embedded atom method (MEAM) potential that includes the bond order (MEAM-BO) to describe the energetics of unsaturated hydrocarbons (double and triple carbon bonds) and also develop improved parameters for saturated hydrocarbons from those of our previous work. Such quantities like bond lengths, bond angles, and atomization energies at 0 K, dimer molecule interactions, rotational barriers, and the pressure-volume-temperature relationships of dense systems of small molecules give a comparable or more accurate property relative to experimental and first-principles data than the classical reactive force fields REBO and ReaxFF. Our extension of the MEAM potential for unsaturated hydrocarbons (MEAM-BO) is a step toward developing more reliable and accurate polymer simulations with their associated structure-property relationships, such as reactive multicomponent (organic/metal) systems, polymer-metal interfaces, and nanocomposites. When the constants for the BO are zero, MEAM-BO reduces to the original MEAM potential. As such, this MEAM-BO potential describing the interaction of organic materials with metals within the same MEAM formalism is a significant advancement for computational materials science. read less

Abstract: Zirconium nitride (ZrN) exhibits exceptional mechanical, chemical, and electrical properties, which make it attractive for a wide range of technological applications, including wear-resistant coatings, protection from corrosion, cutting/shaping tools, and nuclear breeder reactors. Despite its broad usability, an atomic scale understanding of the superior performance of ZrN, and its response to external stimuli, for example, temperature, applied strain, and so on, is not well understood. This is mainly due to the lack of interatomic potential models that accurately describe the interactions between Zr and N atoms. To address this challenge, we develop a modified embedded atom method (MEAM) interatomic potential for the Zr–N binary system by training against formation enthalpies, lattice parameters, elastic properties, and surface energies of ZrN (and, in some cases, also Zr3N4) obtained from density functional theory (DFT) calculations. The best set of MEAM parameters are determined by employing a multiobj... read less

Abstract: The interatomic potential for Fe–Cr–Ni–N system based on the second nearest-neighbour modified embedded-atom method has been developed in this work. The potential is based on those for the corresponding lower order systems. The potential parameters for the binary systems, Cr–N, Ni–N, Ni–Fe and Ni–Cr, were determined by fitting the lattice constants, elastic properties, heat of solution and defect binding energies. The potential parameters for the ternary systems were calculated based on the corresponding binary systems. Then, all of them were applied to the quaternary system Fe–Cr–Ni–N to confirm their validity by a simulation of the lattice constants of AISI 316 austenitic stainless steel with a range of nitrogen content. The results were in good agreement with the previous observations and calculations. read less

Abstract: Molecular dynamics (MD) is a technique of atomistic simulation which has facilitated scientific discovery of interactions among particles since its advent in the late 1950s. Its merit lies in incorporating statistical mechanics to allow for examination of varying atomic configurations at finite temperatures. Its contributions to materials science from modeling pure metal properties to designing nanowires is also remarkable. This review paper focuses on the progress of MD in understanding the behavior of iron — in pure metal form, in alloys, and in composite nanomaterials. It also discusses the interatomic potentials and the integration algorithms used for simulating iron in the literature. Furthermore, it reveals the current progress of MD in simulating iron by exhibiting some results in the literature. Finally, the review paper briefly mentions the development of the hardware and software tools for such large-scale computations. read less

Abstract: We compare six lithium potentials by examining their ability to predict coexistence properties and liquid structure using molecular dynamics. All potentials are of the embedded-atom method type. The coexistence properties we focus on are the melting curve, vapor pressure, saturated liquid density, and vapor-liquid surface tension. For each property studied, the simulation results are compared to available experimental data in order to properly assess the accuracy of each potential. We find that the Cui second nearest-neighbor modified embedded-atom method potential is overall the most reliable potential, giving adequate agreement for most of the properties examined. For example, the zero-pressure melting point of this potential is shown to be around 443 K, while it is it known from experiments to be about 454 K. This potential also gives excellent agreement for the saturated liquid densities, even though no liquid properties were used in the fitting procedure. We conclude that even though this potential is the most reliable overall, there is still room for improvement in terms of obtaining more accurate agreement for some of the properties studied, specifically the slope of the melting pressure versus temperature. read less

Abstract: Interatomic potentials for the Ni–Co binary and Ni–Al–Co ternary systems have been developed on the basis of the second nearest-neighbor modified embedded-atom method (2NN MEAM) formalism. The potentials describe structural, thermodynamic, deformation and defect properties of solid solution phases or compound phases in reasonable agreements with experiments or first-principles calculations. The results demonstrate the transferability of the potentials and their applicability to large-scale atomistic simulations to investigate the effect of an alloying element, cobalt, on various microstructural factors related to mechanical properties of Ni-based superalloys on an atomic scale. read less

Abstract: A new interatomic potential for metals based on the embedded atom method is proposed in this paper. Some approximation of electron density distribution is suggested from the basic principles of quantum mechanics. The functional form of the electron density distribution includes two adjustable parameters. The form of this distribution defines the pair potential and, in part, the form of embedding energy function. The parameters are determined empirically by fitting to the equilibrium lattice constant, cohesion energy, vacancy formation energy, low index surface energy and elastic constants. Potential parameters for 27 metals (10 fcc metals, 9 bcc metals and 8 hcp metals) are presented. Potential is expressed by simple functions and can be used in molecular dynamics simulations of large atomic systems. PACS: 34.20.Cf, 61.50.Ah read less

Abstract: An approach to derive, from first-principles data, accurate and reliable potentials in the modified embedded-atom method in view of modeling the mechanical behavior of metals is presented in this work and applied to the optimization of a potential representative of lithium (Li). Although the theoretical background of the modified embedded-atom method was considered in this work, the proposed method is general and it can be applied to any other functional form. The main feature of the method is to introduce several path transformations in the material database that are critical for plastic and failure behavior. As part of the potential validation, path transformations different from the ones used for the parameterization procedure are considered. Applied in the case of Li, the material database was enriched with the generalized stacking fault energy curve along the  <1 1 1>  -direction on the {1 1 0}-plane, and with the traction-separation behavior of a {1 0 0}-surface. The path transformations used to enrich the material database were initially derived from first-principles calculations. For validation, the generalized stacking fault energy curves along the  <1 1 1>  -direction on the {1 1 2}- and {1 2 3}-planes were considered for plasticity, while traction-separation behavior of {1 1 0} and {1 1 1}-planes were considered for failure behavior. As part of the validation procedure, the predictions made in the MEAM framework were validated by first-principles data. The final potential accurately reproduced basic equilibrium properties, elastic constants, surface energies in agreement with first-principles predictions, and transition energy between different crystal structures. Furthermore, generalized stacking fault energy curves along the  <1 1 1>  -direction on the {1 1 0}, {1 1 2}, and {1 2 3}-planes, and tensile cohesive stress, characteristic length of fracture, and work of separation of a {1 0 0}, {1 1 0}, and {1 1 1} surfaces obtained in the MEAM framework compared well with first-principles predictions. It also predicts good elastic constants for a crystal structure different than the one used for the fitting of the potential and the other four path transformations. The potential was tested for failure behavior using a full atomistic setup, and in addition of being qualitatively correct, the stress intensity factor for different crack orientations was found to be in agreement with the theory of Rice (1992 J. Mech. Phys. Solids 40 239–71) within an error of 10%. Finally, the optimized Li-MEAM potential is expected to be transferable to different local environments encountered in atomistic simulations of lattice defects. read less

Abstract: The thermodynamics of iron clusters of various sizes, from 76 to 2452 atoms, typical of the catalyst particles used for carbon nanotubes growth, has been explored by a flat histogram Monte Carlo (MC) algorithm (called the σ-mapping), developed by Soudan et al. [J. Chem. Phys. 135, 144109 (2011), Paper I]. This method provides the classical density of states, gp(Ep) in the configurational space, in terms of the potential energy of the system, with good and well controlled convergence properties, particularly in the melting phase transition zone which is of interest in this work. To describe the system, an iron potential has been implemented, called "corrected EAM" (cEAM), which approximates the MEAM potential of Lee et al. [Phys. Rev. B 64, 184102 (2001)] with an accuracy better than 3 meV/at, and a five times larger computational speed. The main simplification concerns the angular dependence of the potential, with a small impact on accuracy, while the screening coefficients S(ij) are exactly computed with a fast algorithm. With this potential, ergodic explorations of the clusters can be performed efficiently in a reasonable computing time, at least in the upper half of the solid zone and above. Problems of ergodicity exist in the lower half of the solid zone but routes to overcome them are discussed. The solid-liquid (melting) phase transition temperature T(m) is plotted in terms of the cluster atom number N(at). The standard N(at)(-1/3) linear dependence (Pawlow law) is observed for N(at) >300, allowing an extrapolation up to the bulk metal at 1940 ±50 K. For N(at) <150, a strong divergence is observed compared to the Pawlow law. The melting transition, which begins at the surface, is stated by a Lindemann-Berry index and an atomic density analysis. Several new features are obtained for the thermodynamics of cEAM clusters, compared to the Rydberg pair potential clusters studied in Paper I. read less

Abstract: Tungsten (W) is considered to be one of the most promising plasma-facing materials (PFMs) for next-step fusion energy systems. However, as a PFM, W will be subjected to extremely high fluxes of low-energy hydrogen (H) isotopes, leading to retention of H isotopes and blistering in W, which will degrade the thermal and mechanical properties of W. Modelling and simulation are indispensable to understand the behaviour of H isotopes including dissolution, diffusion, accumulation and bubble formation, which can contribute directly to the design, preparation and application of W as a PFM under a fusion environment. This paper reviews the recent findings regarding the behaviour of H in W obtained via modelling and simulation at different scales. read less

Abstract: Tungsten and tungsten-based alloys are the primary candidate materials for plasma facing components in fusion reactors. The exposure to high-energy radiation, however, severely degrades the performance and lifetime limits of the in-vessel components. In an effort to better understand the mechanisms driving the materials' degradation at the atomic level, large-scale atomistic simulations are performed to complement experimental investigations. At the core of such simulations lies the interatomic potential, on which all subsequent results hinge. In this work we review 19 central force many-body potentials and benchmark their performance against experiments and density functional theory (DFT) calculations. As basic features we consider the relative lattice stability, elastic constants and point-defect properties. In addition, we also investigate extended lattice defects, namely: free surfaces, symmetric tilt grain boundaries, the 1/2〈1 1 1〉{1 1 0} and 1/2〈1 1 1〉 {1 1 2} stacking fault energy profiles and the 1/2〈1 1 1〉 screw dislocation core. We also provide the Peierls stress for the 1/2〈1 1 1〉 edge and screw dislocations as well as the glide path of the latter at zero Kelvin. The presented results serve as an initial guide and reference list for both the modelling of atomically-driven phenomena in bcc tungsten, and the further development of its potentials. read less

Abstract: Molecular dynamics (MD) and density functional theory (DFT) studies were performed to investigate the influence of vacancy defects on generalized stacking fault (GSF) energy of fcc metals. MEAM and EAM potentials were used for MD simulations, and DFT calculations were performed to test the accuracy of different common parameter sets for MEAM and EAM potentials in predicting GSF with different fractions of vacancy defects. Vacancy defects were placed at the stacking fault plane or at nearby atomic layers. The effect of vacancy defects at the stacking fault plane and the plane directly underneath of it was dominant compared to the effect of vacancies at other adjacent planes. The effects of vacancy fraction, the distance between vacancies, and lateral relaxation of atoms on the GSF curves with vacancy defects were investigated. A very similar variation of normalized SFEs with respect to vacancy fractions were observed for Ni and Cu. MEAM potentials qualitatively captured the effect of vacancies on GSF. read less

Abstract: Interatomic potentials for pure Y and the V–Pd–Y ternary system have been developed on the basis of the second nearest-neighbor modified embedded-atom method (2NN MEAM) formalism, with a purpose of investigating the interdiffusion mechanism and the role of yttrium in the palladium-coated vanadium-based hydrogen separation membranes. The potentials can describe various fundamental physical properties of pure Y (the bulk, defect and thermal properties) and the alloy behaviors (structural, thermodynamic and defect properties of solid solutions and compounds) of constituent systems in reasonable agreement with experimental data or first-principles calculations. read less

Abstract: Reduced activation steels are considered as structural materials for future fusion reactors. Besides iron and the main alloying element chromium, these steels contain other minor alloying elements, typically tungsten, vanadium and tantalum. In this work we study the impact of chromium and tungsten, being major alloying elements of ferritic Fe–Cr–W-based steels, on the stability and mobility of vacancy defects, typically formed under irradiation in collision cascades. For this purpose, we perform ab initio calculations, develop a many-body interatomic potential (EAM formalism) for large-scale calculations, validate the potential and apply it using an atomistic kinetic Monte Carlo method to characterize the lifetime and diffusivity of vacancy clusters. To distinguish the role of Cr and W we perform atomistic kinetic Monte Carlo simulations in Fe–Cr, Fe–W and Fe–Cr–W alloys. Within the limitation of transferability of the potentials it is found that both Cr and W enhance the diffusivity of vacancy clusters, while only W strongly reduces their lifetime. The cluster lifetime reduction increases with W concentration and saturates at about 1–2 at.%. The obtained results imply that W acts as an efficient ‘breaker’ of small migrating vacancy clusters and therefore the short-term annealing process of cascade debris is modified by the presence of W, even in small concentrations. read less

Abstract: In this paper we have developed a Finnis–Sinclair-type interatomic potential for W–W interactions that is based on ab initio calculations. The modified potential is able to reproduce the correct formation energies of self-interstitial atom (SIA) defects in tungsten, offering a significant improvement over the Ackland–Thetford tungsten potential. Using the modified potential, the thermal expansion is calculated in a temperature range from 0 to 3500 K. The results are in reasonable agreement with the experimental data, thus overcoming the shortcomings of the negative thermal expansion using the Derlet–Nguyen–Manh–Dudarev tungsten potential. The W–W potential presented here is also applied to study in detail the diffusion of SIAs in tungsten. We reveal that the initial SIA initiates a sequence of tungsten atom displacements and replacements in the 〈1 1 1〉 direction. An Arrhenius fit to the diffusion data at temperatures below 550 K indicates a migration energy of 0.022 eV, which is in reasonable agreement with the experimental data. read less

Abstract: In this work, we developed an interatomic potential for saturated hydrocarbons using the modified embedded-atom method (MEAM), a reactive semi-empirical many-body potential based on density functional theory and pair potentials. We parameterized the potential by fitting to a large experimental and first-principles (FP) database consisting of (1) bond distances, bond angles, and atomization energies at 0 K of a homologous series of alkanes and their select isomers from methane to n-octane, (2) the potential energy curves of H2, CH, and C2 diatomics, (3) the potential energy curves of hydrogen, methane, ethane, and propane dimers, i.e., (H2)2, (CH4)2, (C2H6)2, and (C3H8)2, respectively, and (4) pressure-volume-temperature (PVT) data of a dense high-pressure methane system with the density of 0.5534 g cc(-1). We compared the atomization energies and geometries of a range of linear alkanes, cycloalkanes, and free radicals calculated from the MEAM potential to those calculated by other commonly used reactive potentials for hydrocarbons, i.e., second-generation reactive empirical bond order (REBO) and reactive force field (ReaxFF). MEAM reproduced the experimental and/or FP data with accuracy comparable to or better than REBO or ReaxFF. The experimental PVT data for a relatively large series of methane, ethane, propane, and butane systems with different densities were predicted reasonably well by the MEAM potential. Although the MEAM formalism has been applied to atomic systems with predominantly metallic bonding in the past, the current work demonstrates the promising extension of the MEAM potential to covalently bonded molecular systems, specifically saturated hydrocarbons and saturated hydrocarbon-based polymers. The MEAM potential has already been parameterized for a large number of metallic unary, binary, ternary, carbide, nitride, and hydride systems, and extending it to saturated hydrocarbons provides a reliable and transferable potential for atomistic/molecular studies of complex material phenomena involving hydrocarbon-metal or polymer-metal interfaces, polymer-metal nanocomposites, fracture and failure in hydrocarbon-based polymers, etc. The latter is especially true since MEAM is a reactive potential that allows for dynamic bond formation and bond breaking during simulation. Our results show that MEAM predicts the energetics of two major chemical reactions for saturated hydrocarbons, i.e., breaking a C-C and a C-H bond, reasonably well. However, the current parameterization does not accurately reproduce the energetics and structures of unsaturated hydrocarbons and, therefore, should not be applied to such systems. read less

Abstract: A novel n-body potential for an Zr–Nb system was developed in the framework of the embedded-atom method. All the parameters of the constructed potential have been systematically evaluated by fitting to the ground state properties obtained from experimental measurements and first-principles calculations for pure elements and some alloys. It is shown that most of the static thermodynamics properties for Zr and Nb can be well reproduced by using the present potential. Some calculation results based on the present model are even closer to the experimental data than those based on previous potential models. The ground state properties of hypothetical Zr–Nb alloys were also calculated and found to be in agreement with first-principles calculations. Furthermore, the formation energies of random solid solutions of Zr–Nb with lattices of body centered cubic (bcc) and hexagonal close packed (hcp) type were calculated by fitting the energy–volume relations to Rose’s equation of state. These values were compared with those obtained by first-principles calculations based on special quasirandom structure models and the Miedema-ZSL-07 model (the improved Miedema model proposed by Zhang, Sheng and Liu in 2007). It is indicated that our n-body constructed potential for a Zr–Nb alloy provides an effective description for the interaction between the dissimilar ion interactions for hcp–bcc systems. read less

Abstract: Interaction between metallic fuel and steel structures is one of the predominant phenomena in the progress of core disruptive accidents of Sodium-cooled fast reactor. In this study, the atomic diffusion across the interface between Pu and Fe was investigated by using molecular dynamics. The simulation was performed by using Modified Embedded Atom Method (MEAM). The interactions between plutonium and iron atoms were calculated by using the newly developed potential model determined so as to reproduce the material properties of PuFe2 and Pu6Fe. The material properties of the compounds predicted with the developed potential were in good agreement with the referenced data. The dissolution or melting at the interface between solid Fe and solid or liquid Pu were simulated by contacting semi-infinite slabs (or liquid layer) of them. Dissolution was observed for all the tested temperature conditions from 800 K up to 1700 K. The melting at the interface was also observed on the interface between solid Fe and PuFe2 slabs at the temperature approximately 100 K below the melting temperature of PuFe2 obtained based on the present model. read less

Abstract: The intergranular embrittlement in bcc iron by the grain boundary (GB) segregation of phosphorus is investigated using an atomistic simulation. The inhibition of the nucleation of dislocations near the crack tip is found to be the governing mechanism of the intergranular embrittlement in phosphorus-containing iron, in contrast to the conventional reasoning that focuses on the GB decohesion. The correlation between the nucleation of dislocations and dislocation transfer across a GB (GB strengthening) is discussed. Experimental evidence and supplementary simulation results that support the new finding in terms of the GB strengthening are also demonstrated. read less

Abstract: This one-year, study topic project will survey and investigate the known state-of-the-art of modeling and simulation methods suitable for performing fine-scale, fully 3-D modeling, of the growth of CZT crystals at the melt-solid interface, and correlating physical growth and post-growth conditions with generation and incorporation of defects into the solid CZT crystal. In the course of this study, this project will also identify the critical gaps in our knowledge of modeling and simulation techniques in terms of what would be needed to be developed in order to perform accurate physical simulations of defect generation in melt-grown CZT. The transformational nature of this study will be, for the first time, an investigation of modeling and simulation methods for describing microstructural evolution during crystal growth and the identification of the critical gaps in our knowledge of such methods, which is recognized as having tremendous scientific impacts for future model developments in a wide variety of materials science areas. read less

Abstract: Density-functional theory energies, forces, and elastic constants determine the parametrization of an empirical, modified embedded-atom method potential for molybdenum. The accuracy and transferability of the potential are verified by comparison to experimental and density-functional data for point defects, phonons, thermal expansion, surface and stacking fault energies, and ideal shear strength. Searching the energy landscape predicted by the potential using a genetic algorithm verifies that it reproduces not only the correct bcc ground state of molybdenum but also all low-energy metastable phases. The potential is also applicable to the study of plastic deformation and used to compute energies, core structures, and Peierls stresses of screw and edge dislocations. Molybdenum's high strength and high-temperature stability make this refractory metal very attractive for use in advanced process technologies. The motion of dislocations is generally accepted to be responsible for the complex deformation behavior of this transition metal. 1-8 In recent years progress has been made on the description of the properties of screw dislocations using density-functional theory (DFT), tight- binding calculations, and empirical potentials. 9-19 However, DFT and tight-binding techniques are limited to small system sizes, which is problematic due to the long-range strain field of dislocations, and current empirical potentials lack the required accuracy for the description of the dislocation structure. Simulations of dislocation motion and interactions require efficient interatomic potentials which accurately describe the dislocation energies, core structures, and motion. In this work we develop an empirical potential for Mo which predicts the ideal shear strength, generalized stacking fault en- ergies, energies of dislocations, and the Peierls stress and core structure of the � 111� /2 screw dislocation. The potential form is given by the modified embedded-atom method (MEAM) and the potential parameters are optimized usingabinitio energies, lattice parameters, forces, and elastic constants. Section II describes the calculations for the DFT database, the functional form of the MEAM potential, and the optimization of the potential parameters to the DFT database. The accuracy of the potential for structural, elastic, and defect properties is verified in Sec. III by comparison to DFT results and experiments. A genetic algorithm search of the energy landscape of the MEAM potential confirms that the potential reproduces the correct bcc ground state and predicts several low-energy metastable structures whose energies agree well with DFT results. Results of the MEAM potential for formation energies of point defects, phonon dispersion, thermal expansion, surface energies, ideal shear strength, and generalized stacking faults for the MEAM potential closely match DFT results and available experimental data. In Sec. IV we apply the potential to determine energies and Peierls stresses of the screw and edge dislocation in bcc Mo. The results show that the MEAM potential accurately describes the structural and mechanical properties of Mo and should be applicable to simulate the motion of dislocations and the plastic deformation of Mo. read less

Abstract: An interatomic potential for pure phosphorus, an element that has van der Waals, covalent and metallic bonding character, simultaneously, has been developed for the purpose of application to metal–phosphorus systems. As a simplification, the van der Waals interaction, which is less important in metal–phosphorus systems, was omitted in the parameterization process and potential formulation. On the basis of the second-nearest-neighbor modified embedded-atom method (2NN MEAM) interatomic potential formalism applicable to both covalent and metallic materials, a potential that can describe various fundamental physical properties of a wide range of allotropic or transformed crystalline structures of pure phosphorus could be developed. The potential was then extended to the Fe–P binary system describing various physical properties of intermetallic compounds, bcc and liquid alloys, and also the segregation tendency of phosphorus on grain boundaries of bcc iron, in good agreement with experimental information. The suitability of the present potential and the parameterization process for atomic scale investigations about the effects of various non-metallic impurity elements on metal properties is demonstrated. read less

Abstract: We develop two new modified embedded-atom method (MEAM) potentials for elemental iron, intended to reproduce the experimental phase stability with respect to both temperature and pressure. These simple interatomic potentials are fitted to a wide variety of material properties of bcc iron in close agreement with experiments. Numerous defect properties of bcc iron and bulk properties of the two close-packed structures calculated with these models are in reasonable agreement with the available first-principles calculations and experiments. Performance at finite temperatures of these models has also been examined using Monte Carlo simulations. We attempt to reproduce the experimental iron polymorphism at finite temperature by means of free energy computations, similar to the procedure previously pursued by Müller et al (2007 J. Phys.: Condens. Matter 19 326220), and re-examine the adequacy of the conclusion drawn in the study by addressing two critical aspects missing in their analysis: (i) the stability of the hcp structure relative to the bcc and fcc structures and (ii) the compatibility between the temperature and pressure dependences of the phase stability. Using two MEAM potentials, we are able to represent all of the observed structural phase transitions in iron. We discuss that the correct reproductions of the phase stability among three crystal structures of iron with respect to both temperature and pressure are incompatible with each other due to the lack of magnetic effects in this class of empirical interatomic potential models. The MEAM potentials developed in this study correctly predict, in the bcc structure, the self-interstitial in the 〈110〉 orientation to be the most stable configuration, and the screw dislocation to have a non-degenerate core structure, in contrast to many embedded-atom method potentials for bcc iron in the literature. read less

Abstract: An angular-dependent analytic bond-order potential (ABOP) for copper and Fe–Cu system was developed, based on the ABOP of pure iron introduced by Müller et al (2007 J. Phys.: Condens. Matter 19 326220). The potential parameters for the present ABOP model of copper were determined by fitting to the experimental data of the basic properties of fcc Cu and ab initio calculated properties of bcc Cu. The model predicts the vacancy formation energy in good agreement with the experimental result, although no vacancy formation information was used in the fitting of the model parameters. The melting point of Cu is also properly reproduced. The Fe–Cu binary system was described by adding two independent cross parameters in the potential model. The cross parameters were fitted using the ab initio data of the formation energies and lattice parameters of fictitious Fe–Cu alloys. The potential was applied to investigate the point defects and small defect clusters in dilute Fe–Cu alloys. The results were compared with the ab initio data and the values obtained with other potentials. read less

Abstract: The lattice-inversion embedded-atom-method (LI-EAM) interatomic potential we developed previously [J. Phys.: Condens. Matter 22 (2010) 375503] is extended to group-VA transition metals (V, Nb and Ta). It is found that considering interatomic interactions up to appropriate-distance-neighbor atoms is crucial to constructing accurate EAM potentials, especially for the prediction of surface energy. The LI-EAM interatomic potentials for group-VA transition metals are successfully built by considering interatomic interactions up to the fifth neighbor atoms. These angular-independent potentials drastically promote the accuracy of the predicted surface energies, which match the experimental results well. read less

Abstract: A novel Monte Carlo flat histogram algorithm is proposed to get the classical density of states in terms of the potential energy, g(E(p)), for systems with continuous variables such as atomic clusters. It aims at avoiding the long iterative process of the Wang-Landau method and controlling carefully the convergence, but keeping the ability to overcome energy barriers. Our algorithm is based on a preliminary mapping in a series of points (called a σ-mapping), obtained by a two-parameter local probing of g(E(p)), and it converges in only two subsequent reweighting iterations on large intervals. The method is illustrated on the model system of a 432 atom cluster bound by a Rydberg type potential. Convergence properties are first examined in detail, particularly in the phase transition zone. We get g(E(p)) varying by a factor 10(3700) over the energy range [0.01 < E(p) < 6000 eV], covered by only eight overlapping intervals. Canonical quantities are derived, such as the internal energy U(T) and the heat capacity C(V)(T). This reveals the solid to liquid phase transition, lying in our conditions at the triple point. This phase transition is further studied by computing a Lindemann-Berry index, the atomic cluster density n(r), and the pressure, demonstrating the progressive surface melting at this triple point. Some limited results are also given for 1224 and 4044 atom clusters. read less

Abstract: The design of multicomponent alloys used in different applications based on specific thermo-physical properties determined experimentally or predicted from theoretical calculations is of major importance in many engineering applications. A procedure based on Monte Carlo simulations (MCS) and the thermodynamic integration (TI) method to improve the quality of the predicted thermodynamic properties calculated from classical thermodynamic calculations is presented in this study. The Gibbs energy function of the liquid phase of the Cu-Zr system at 1800 K has been determined based on this approach. The internal structure of Cu-Zr melts and amorphous alloys at different temperatures, as well as other physical properties were also obtained from MCS in which the phase trajectory was modeled by the modified embedded atom model formalism. A rigorous comparison between available experimental data and simulated thermo-physical properties obtained from our MCS is presented in this work. The modified quasichemical model in the pair approximation was parameterized using the internal structure data obtained from our MCS and the precise Gibbs energy function calculated at 1800 K from the TI method. The predicted activity of copper in Cu-Zr melts at 1499 K obtained from our thermodynamic optimization was corroborated by experimental data found in the literature. The validity of the amplitude of the entropy of mixing obtained from the in silico procedure presented in this work was analyzed based on the thermodynamic description of hard sphere mixtures. read less

Abstract: Second nearest-neighbor modified embedded-atom method (MEAM) interatomic potentials for the Al–H and Ni–H binary systems have been developed on the basis of previously developed MEAM potentials of pure Al, Ni, and H. The potentials can describe various fundamental physical properties of the relevant binary alloys (structural, thermodynamic, defect, and dynamic properties of metastable hydrides or hydrogen in face-centered cubic solid solutions) in good agreement with experiments or first-principles calculations. The applicability of the present potentials to atomic level investigations of dynamic behavior of hydrogen atoms in metal membranes is also discussed. read less

Abstract: We review the studies on the thermophysical properties of undercooled metals and alloys by molecular simulations in recent years. The simulation methods of melting temperature, enthalpy, specific heat, surface tension, diffusion coefficient and viscosity are introduced and the simulated results are summarized. By comparing the experimental results and various theoretical models, the temperature and the composition dependences of the thermophysical properties in undercooled regime are discussed. read less

Abstract: The present work develops a physically reliable procedure for building the embedded-atom-method (EAM) interatomic potentials for the metals with fcc, bcc and hcp structures. This is mainly based on Chen–Möbius lattice inversion (Chen et al 1997 Phys. Rev. E 55 R5) and first-principles calculations. Following Baskes (Baskes et al 2007 Phys. Rev. B 75 094113), this new version of the EAM eliminates all of the prior arbitrary choices in the determination of the atomic electron density and pair potential functions. Parameterizing the universal form deduced from the calculations within the density-functional scheme for homogeneous electron gas as the embedding function, the new-type EAM potentials for Cu, Fe and Ti metals have successfully been constructed by considering interatomic interactions up to the fifth neighbor, the third neighbor and the seventh neighbor, respectively. The predictions of elastic constants, structural energy difference, vacancy formation energy and migration energy, activation energy of vacancy diffusion, latent heat of melting and relative volume change on melting all satisfactorily agree with the experimental results available or first-principles calculations. The predicted surface energies for low-index crystal faces and the melting point are in agreement with the experimental data to the same extent as those calculated by other EAM-type potentials such as the FBD-EAM, 2NN MEAM and MS-EAM. In addition, the order among the predicted low-index surface energies is also consistent with the experimental information. read less

Abstract: Molecular dynamics simulations were used to model grain boundary sliding in stressed Fe–Ni bicrystals exposed to low energy neutron irradiation. We studied how sliding stress thresholds and sliding mechanisms changed with variations in the Ni boundary morphology and boundary geometry. Simulations corresponding to ordered boundary Ni distributions and coincident-site lattice (CSL) geometries relaxed stress through a dislocation-mediated sliding mechanism. Such a mechanism was found to follow Orowan’s law, in which the stress relaxation rate is proportional to the dislocation velocity. Alternatively, simulations of disordered Ni distributions and non-CSL boundary geometries were described by a random shuffling process with time-dependent stress relaxation rate. Nevertheless, irrespective of the stress relaxation process followed by the bicrystals, after reaching equilibrium, the amounts of boundary displacement and stress relaxation were always found to be proportionally related. These observations might prove useful to groups working on building continuum (macroscopic) models of deformations in irradiated materials. read less

Abstract: Modified embedded-atom method (MEAM) interatomic potentials for Nb-C, Nb-N, Fe-Nb-C, and Fe-Nb-N systems have been developed based on the previously developed MEAM potentials for lower order systems. The potentials reproduce various fundamental physical properties (structural properties, elastic properties, thermal properties, and surface properties) of NbC and NbN, and interfacial energy between bcc Fe and NbC or NbN, in generally good agreement with higher-level calculations or experimental information. The applicability of the present potentials to atomic-level investigations to the precipitation behavior of complex-carbonitrides (Nb,Ti)(C,N) as well as NbC and NbN, and their effects on the mechanical properties of steels are also discussed. read less

Abstract: An interatomic potential for the Fe–Al binary system has been developed based on the modified embedded-atom method (MEAM) potential formalism. The potential can describe various fundamental physical properties of Fe–Al binary alloys—structural, elastic and thermodynamic properties, defect formation behavior and interactions between defects—in reasonable agreement with experimental data or higher-level calculations. The applicability of the potential to atomistic investigations of various defect formation behaviors and their effects on the mechanical properties of high aluminum steels as well as Fe–Al binary alloys is demonstrated. read less

Abstract: We report an atomistic simulation study of the behavior of nanocomposite materials that are formed by incorporating single-walled carbon nanotubes SWCNTs, with three different diameters, and a multiwalled carbon nanotube MWCNT into a single-crystal nickel matrix. The interactions between carbon and nickel atoms are described by a modified embedded atom method potential. Mechanical properties of these nanocomposite materials are predicted by atomistic calculations and compared with that of fcc nickel and pristine CNTs. Our simulations predict that all Ni/CNT composites studied in this work are mechanically stable. Their elastic properties depend on the volume fraction and diameter of embedded CNTs. The single-crystal Young’s modulus E11 of Ni/SWCNT composites exhibit a large increase in the direction of CNTs alignment compared to that of a single-crystal nickel. However, a moderate but gradual decrease is seen for E22 and E33 in the transverse directions with increase in CNT diameters. As a consequence, Ni/SWCNTs show a gradual decrease for the polycrystalline Young’s, bulk and shear moduli with the increasing CNT diameters and volume fractions. These reductions, although moderate, suggest that enhancement of mechanical properties for polycrystalline Ni/SWCNT nanocomposites are not achievable at any CNT volume fraction. The Ni/MWCNT composite with high CNT volume fraction shows the highest increase in E11. Unlike the E22 and E33 for Ni/ SWCNTs, there is a significant increase in the E22 and the E33 for Ni/MWCNT. As a result, polycrystalline Ni/MWCNT composites show slight increase in the elastic properties. This suggests that nickel nanocomposites with enhanced mechanical properties can be fabricated using large volume fractions of larger diameter MWCNTs. Depending on type, alignment and volume fraction, Ni/CNT composites show varying degrees of elastic anisotropy and Poisson’s ratio compared to pure Ni. Simulation predicts strong adhesion at the Ni/CNT interface and a significant interfacial stress transfer between CNT and Ni matrix. read less

Abstract: This paper presents an inverse Monte Carlo method to reconstruct pair interaction potential from pair correlation function. This approach adopts an iterative algorithm on interaction potential to fit known pair correlation function by compelling deviations of canonical average to meet with Hamiltonian parameters on a basis of statistical mechanism. The effective interaction potential between particles in liquid Ag–Rh alloys has been calculated with the inverse Monte Carlo method. It demonstrates an effective and simple way to obtain the effective potential of complex melt systems. read less

Abstract: Modified embedded-atom method (MEAM) interatomic potentials for the Ga–N and In–N binary and Ga–In–N ternary systems have been developed based on the previously developed potentials for Ga, In and N. The potentials can describe various physical properties (structural, elastic and defect properties) of both zinc-blende and wurtzite-type GaN and InN as well as those of constituent elements, in good agreement with experimental data or high-level calculations. The potential can also describe the structural behavior of Ga1−xInxN ternary nitrides reasonably well. The applicability of the potentials to atomistic investigations of atomic/nanoscale structural evolution in Ga1−xInxN multi-component nitrides during the deposition of constituent element atoms is discussed. read less

Abstract: The structural and mechanical properties of single-and multi-walled carbon nanotubes filled with iron nanowires are studied using a recent parameterization of the modified embedded atom model. We have analyzed the effect of different crystal structures of iron (bcc and fcc) inside carbon nanotubes of different topographies. We have computed strain energy versus strain curves for pure systems: Fe nanowires, carbon and Fe-filled carbon nanotubes. A noticeable difference is found when these monatomic systems are joined to form iron-capped nanowires and where multi-layers of graphite are added to the nanotubes. read less

Abstract: We report melting points and other thermal properties of several semiconducting and metallic elements as they are modeled by different empirical interatomic potential models, including the Stillinger–Weber, the embedded-atom method, the Finnis–Sinclair and the modified-embedded-atom method. The state-of-the-art free energy methods are used to determine the melting points of these models within a very small error bar, so that they can be cross-compared with each other. The comparison reveals several systematic trends among elements with the same crystal structure. It identifies areas that require caution in the application of these models and suggests directions for their future improvement. read less

Abstract: A new analytic bond-order potential for iron is presented that has been fitted to experimental data and results from first-principles calculations. The angular-dependent functional form allows a proper description of a large variety of bulk, surface and defect properties, including the Bain path, phonon dispersions, defect diffusivities and defect formation energies. By calculating Gibbs free energies of body-centred cubic (bcc) and face-centred cubic (fcc) iron as a function of temperature, we show that this potential is able to reproduce the transitions from α-iron to γ-iron and δ-iron before the melting point. The results are compared to four widely used embedded-atom-method potentials for iron. read less

Abstract: Large-scale molecular dynamics simulations of tensile deformation of amorphous metals with nanocrystalline particles were performed in order to clarify the effects of particle size and crystal volume fraction on the deformation property and the strength. It was clarified that the size effects of the particle are very small, whereas the influences of the crystal volume fraction are large. Young’s modulus and the flow stress become large as the crystal volume fraction increases. Even after the yielding of the amorphous phase, the stress of the crystal phase still continues to increase. Thus, the flow stress of the composite increases after yielding, which prevents plastic localization and improves the ductility. When the crystal volume fraction is small, the stress distribution is homogeneous in the particle including near the amorphous-crystal interface. Therefore, possibility of deformation is small, and insideparticle plastic deformation is negligible. When the crystal volume fraction is high, the particle undergoes plastic deformation even with small global deformation. After the yielding of the crystal particle, the flow stress decreases because defects are introduced into the crystal. It is expected that there is an ideal crystal volume fraction that gives the maximum ductility. A Lennard-Jones potential modified to enforce the continuity at the cut-off distance was used as an interatomic potential. The potential parameters were defined based on Inoue’s three basic principles. read less

Abstract: A theoretical study has been performed to rationalize the strong evolution of (001) texture during postannealing of deposited Fe50Pt50 thin films on amorphous substrates, by comparing calculated strain energies of several crystals with different orientations under presumed strain conditions. An atomistic calculation method based on an empirical interatomic potential (MEAM) was used to calculate strain and surface energies and atomic force microscope experiments were carried out to confirm the surface energy calculation. The (001) texture evolution could not be explained using traditional factors, the surface energy anisotropy and the in-plane strain. It was found that the strain from the L10 ordering transformation that occurs during postannealing can make the (001) crystal (crystal with [001] crystallographic orientation into the surface normal) energetically most stable among those with various orientations. It is proposed that the occurrence of anisotropic strain due to ordering transformations should ... read less

Abstract: A semi-empirical interatomic potential formalism, the modified embedded atom method (MEAM), has been applied to obtain an interatomic potential for the Fe–Pt alloy system, based on the previously developed potentials for pure Fe and Pt. The potential can describe basic physical properties of the alloys (lattice parameter, bulk modulus, stability of individual phases, and order/disorder transformations), in good agreement with experimental information. The procedure for the determination of potential parameter values and comparisons between the present calculation and experimental data or high level calculation are presented. The applicability of the potential to atomistic studies to investigate structural evolution of Fe_50Pt_50 alloy thin films during post-annealing is also discussed. read less

Abstract: A reactive interatomic potential based on an analytical bond-order scheme is developed for the ternary system W–C–H. The model combines Brenner’s hydrocarbon potential with parameter sets for W–W, W–C, and W–H interactions and is adjusted to materials properties of reference structures with different local atomic coordinations including tungsten carbide, W–H molecules, as well as H dissolved in bulk W. The potential has been tested in various scenarios, such as surface, defect, and melting properties, none of which were considered in the fitting. The intended area of application is simulations of hydrogen and hydrocarbon interactions with tungsten, which have a crucial role in fusion reactor plasma-wall interactions. Furthermore, this study shows that the angular-dependent bond-order scheme can be extended to second nearest-neighbor interactions, which are relevant in body-centered-cubic metals. Moreover, it provides a possibly general route for modeling metal carbides. © 2005 American Institute of Physics. DOI: 10.1063/1.2149492 read less

Abstract: We fit a new gold embedded atom method (EAM) potential using an improved force matching methodology which included fitting to high-temperature solid lattice constants and liquid densities. The new potential shows a good overall improvement in agreement to the experimental lattice constants, elastic constants, stacking fault energy, radial distribution function, and fcc/hcp/bcc lattice energy differences over previous potentials by Foiles, Baskes, and Daw (FBD) [Phys. Rev. B 33, 7983 (1986)] Johnson [Phys. Rev. B 37, 3924 (1988)], and the glue model potential by Ercolessi et al. [Philos. Mag. A 50, 213 (1988)]. Surface energy was improved slightly as compared to potentials by FBD and Johnson but as a result vacancy formation energy is slightly inferior as compared to the same potentials. The results obtained here for gold suggest for other metal species that further overall improvements in potentials may still be possible within the EAM framework with an improved fitting methodology. On the other hand, we also explore the limitations of the EAM framework by attempting a brute force fit to all properties exactly which was found to be unsuccessful. The main conflict in such a brute force fit was between the surface energy and the liquid lattice constant where both could not be fitted identically. By intentionally using a very large number of spline sections for the pair potential, electron-density function, and embedding energy function, we eliminated a lack of functional freedom as a possible cause of this conflict and hence can conclude that it must result from a fundamental limitation in the EAM framework. read less

Abstract: The structure of small tantalum clusters is investigated by using molecular dynamics simulations. A structural evolution from polytetrahedral structures to layered Frank–Kasper-type structures is revealed as cluster size increases to N∼100 atoms. The lowest-energy structures have been located for clusters with N≤78. The bulk-like (bcc) structure becomes the most stable structure beyond N∼100. The stabilized structure strongly depends on the cooling rate. A structure similar to β-Ta, a σ-type Frank–Kasper structure, can be obtained by rapid cooling. The structural properties of small Ta clusters presented in the paper provide insight into the formation and origin of β-phase Ta. The growth of β-Ta films in practice may be due to the nucleation of Ta clusters with layered Frank–Kasper-type structure during the initial stage of film growth. read less

Abstract: We present a methodology to perform molecular static simulations of dislocations impinging on an interface between a metallic thin film and an amorphous substrate. The simulations show that the dislocations are absorbed in the interface and are not transmitted in the amorphous substrate. Computing the jump in atomic displacement across the interface reveals interface sliding in a region extending over 5 nm on each side of the dislocations, which can be interpreted in terms of spreading of the dislocation core. read less

Abstract: Molecular dynamics simulations have been performed on tantalum clusters using the embedded-atom-method potential. Melting simulations show that β-Ta clusters have a lower melting temperature than the same size clusters of α-Ta (bcc structure). Pure β-Ta clusters are quite stable and do not transform to the α-Ta on melting. Simulations on Ta clusters with mixed α- and β-phases reveal that inclusion of a bcc-Ta cluster within a β-Ta cluster induces the β-to-α-phase transformation at a temperature far below the melting point of a pure β-Ta cluster, depending on the cluster size and α-to-β-atom ratio. The results suggest that the observed phase transformation of β-Ta thin films is due to the presence of α-phase inclusions in the β-Ta film grains. read less

Abstract: Multiplane shear deformation behaviour in face-centred cubic metals, aluminium and copper, is studied and empirical many-body potential results are directly compared with ab initio electronic structure calculations. An analysis of stress–displacement, atomic relaxation, and gamma-surface for shear indicates that the potential for copper proposed by Mishin is able to capture the essential deformation behaviour. For aluminium the Mishin potential gives better results than the Ercolessi model in atomic relaxation and stress–displacement, although there remain details that neither are able to describe. Aluminium presents a greater challenge to empirical potential description because of the directional nature of its interatomic bonding. read less

Abstract: The formation of iron particles from the supersaturated gas phase is investigated by molecular dynamics simulation. The atomic interaction is modelled with a recent parameterization of the embedded atom method which is able to describe the bcc phase of bulk iron. The influence of the state conditions such as temperature and density on the growth mechanisms of the iron particles is analysed. With the common neighbour analysis method the structural changes of the particles over the course of the growth process is traced. Furthermore, the surface fraction is investigated as an order parameter for the morphology of the particles. It appears that in the early state of the growth process the atomic structure is dominated by icosahedral structures which are, over the course of the growth process, transformed into other structures such as fcc or hcp. Iron atoms in the bcc structure were not found for small particles. The structural reorganization depends mainly on the temperature of the system and on the magnitude of the heat removal from the system. The results are in agreement with experimental and other theoretical investigations on the structure of iron nanoparticles. read less

Abstract: Two procedures were developed to fit interatomic potentials of the embedded-atom method (EAM) form and applied to determine a potential which describes crystalline and liquid iron. While both procedures use perfect crystal and crystal defect data, the first procedure also employs the first-principles forces in a model liquid and the second procedure uses experimental liquid structure factor data. These additional types of information were incorporated to ensure more reasonable descriptions of atomic interactions at small separations than is provided using standard approaches, such as fitting to the universal binding energy relation. The new potentials (provided herein) are, on average, in better agreement with the experimental or first-principles lattice parameter, elastic constants, point-defect energies, bcc–fcc transformation energy, liquid density, liquid structure factor, melting temperature and other properties than other existing EAM iron potentials. read less

Abstract: An embedded-atom-method potential for tantalum (Ta) has been carefully constructed by fitting to a combination of experimental and density-functional theory (DFT) data. The fitted data include the elastic constants, lattice constant, cohesive energy, unrelaxed vacancy formation energy, and hundreds of force data calculated by DFT for a variety of structures such as liquids, surfaces, clusters, interstitials, vacancies, and stacking faults. We also fit to the cohesive energy vs volume data from the equation of state for the body-centered-cubic (bcc) Ta and to the calculated cohesive energy using DFT for the face-centered-cubic (fcc) Ta structure. We assess the accuracy of the new potential by comparing several calculated Ta properties with those obtained from other potentials previously reported in the literature. In many cases, the new potential yields superior accuracy at a comparable or lower computational cost. read less

Abstract: A semi-empirical interatomic potential for the post-transition metal, bismuth, is developed based on the second nearest-neighbor modified embedded-atom method (MEAM). The potential reproduces a range of physical properties, such as the lattice constant, cohesive energy, elastic constants, vacancy formation energy, surface energy, and the melting point of pure bismuth. The calculations are done for the rhombohedral ground state of Bi. The results show good agreement with density functional theory and experimental data. The developed MEAM potential for bismuth is useful for material and mechanical behavior studies of the pure material at different conditions and sets the stage for the development of interatomic potentials for bismuth alloys or other bismuth compounds. read less

Abstract: Der Fokus dieser Arbeit lag zunachst auf einer simulationsgestutzen Untersuchung uber die Entsteh- ungsmechanismen von Oxidteilchen in ODS-Stahlen. Hierbei bilden empirische Wechselwirkungs- potenziale von Eisen-Yttrium-Sauerstoff (Fe-Y-O) die Grundlage fur eine Beschreibung dieser Oxid- teilchen-Bildungs-Prozesse in Molekulardynamik (MD) Simulationen, die auch Eigenschaften von Versetzungen und anderen Bestrahlungs-Panomenen detailiert zur weiteren Aufklarung behandeln konnen. Zu diesem Zweck ist das speziell auf die Simulation zugeschnittene Anfitten der o.g. MD Potenziale (hier fur Fe-Y-O) notwendig. Hierzu dienen die zuvor durchgefuhrten ab-initio (DFT) Rechnungen als Daten- referenzgrundlage (z.B. von Phasen oder Defekten) zur Optimierung der Potenzialparameter wahrend des Anfittens, um ein moglichst exaktes MD Potenzial zu erzeugen, dass die ab-initio Daten auf groseren MD Skalen detailgetreu abbildet. Im ersten Drittel dieses Projektes wurden mehrere Potenziale fur die einzelnen Metall-Komponenten, Fe-Fe und Y-Y, erzeugt. Dabei stellte sich heraus, dass etablierte Standardmethoden nicht in der Lage sind genaue Fe-Y Potenziale als Teillosung fur das Fe-Y-O Problem zu erzeugen. Dabei wurde eine Kombination aus dem (M)EAM Modell und zur Optimierung eine LSM gestutzte Software (POTFIT) genutzt. Die Komplexitat des Problems liegt in den richtungsabhangigen Atombindungen, die die hier entwickelten fortgeschrittenen Simulations- und Fitmethoden benotigen. Im ersten Schritt von drei Schritten (chapter 3) wurden zunachst einmal die Defizite der Standard-Fittechniken evaluiert, indem die wahrend des Fitting-Prozesses gefundenen Parametersets im EAM Formalismus mit der flexiblen Software POTFIT auf ihre Eignung hin grundlich untersucht worden sind. Die hierfur genutzten Fitfunktionen wurden ursprunglich Anfang 2000 von Zhou und Wadley entwickelt. Hierbei liegt die Ursache fur die dann entdeckte Parameterset-Problematik darin, dass zur Beschreibung des Fe-Y Systems das Model aus drei Potentialkomponenten besteht: Fe-Fe, Y-Y und Fe-Y. Fur diese einzelnen Komponenten sind die Potentialparameter erfolgreich angefittet worden mit Bezug zur Gitterkonstante und Bindungs- bzw. Kohasionsenergie (beides mit 1% Genauigkeit bezgl. DFT Rechnungen) sowie zu allen elastischen Konstanten (5% Genauigkeit bezgl. Experimente). All dies unter Zuhilfenahme von Parametersuchraum-beschrankenden Techniken, die zur Einhaltung der oben genannten Eigenschaften dienen und urspurnglich von Johnson & Oh sind. Selbst kompliziertere Defekteigenschaften, wie Zwischengitter- und Leerstellenbildungsenergien wurden erfolgreich angefittet. Das hier entwickelte EAM Potenzial fur Y-Y ist z.B. in der Lage bei Eigenzwischengitteratomen die basal oktaedrische Position von Zwischengitteratomen (ZA) im Yttrium hcp-Gitter als Grundzustand und die Transition eines jeden ZAs aus einer anderen Position, wie zuvor in DFT berechnet, zu reproduzieren. Zur Bildung des angestrebten Fe-Y Potenzials wurden diese beiden Komponenten, Fe-Fe und Y-Y, zum weiteren Fitten in dem weitgefacherten und komplexen Fe-Y Potzenzialsuchraum genutzt. Die Parametersets wurden mit sogenannten hier entwickelten Hauptparameter (Key Driver) systematisch untersucht. Ein flexibleres Konzept statt der starreren Universal Binding Relations in Abhangigkeit von der Rose Gleichung. Dieser Hauptparameter zeigte eindeutig, dass die Nutzung der Rose Gleichung zur Parametersuchraum-Minimierung den Suchraum dahingehend einschrankt, sodass ein akkurates Anfitten der hier genutzten 900 DFT Datensets nicht mehr moglich ist. Allerdings ist die Orientierung im Parametersuchraum mit dieser Rose Gleichung bei standardmasigen Optimierungsmethoden (wie LSM) unabdingbar, da ohne diese die benotigten globalen Optima fur die Parameter nicht auffindbar sind. Als aufklarendes Testverfahren zur weiteren Ergrundung dieser Problematik und Prufung zur Eignung fur Fe-Y Potenziale und den anschliesenden Simulationen diente der Versuch, 9 verschiedene Bindungs-energien von Yttrium-Leerstellenclustern mit ansteigender Leerstellenzahl zu reproduzieren. Dieser Test konnte von diesen Potenzialen nur teilweise erfullt werden und wurde auf die fehlende Beschreibung der Bindungswinkelabhangigkeit im Modell zuruckgefuhrt. Die Erweiterung von EAM durch MEAM mit Winkelabhangigkeit ist jedoch keineswegs eine zufriedenstellende Losung, da MEAM alternativlos auf der irrefuhrenden Rose Gleichung beruht. Daher war die Benutzung des ubersichtlicheren EAM Typs aus zwei Grunden nutzlich: 1. MEAM braucht die Rose Gleichung um diesen komplexen Formalismus zu beherrschen mit denselben Problemen wie in EAM, aber dieses grundlegende Problem ist in MEAM deutlich schwerer zu identifizieren als in EAM. 2. Die mit EAM gefundenen, angefitteten Parameter sind eine hervorragende Startparameter-Grundlage fur den verbesserten darauffolgenden RF-MEAM Typ. Im zweiten Schritt wurde das Problem aus dem ersten Schritt gelost, indem ein modifizierter MEAM Spezialtyp im referenzlosen Format (RF-MEAM) angewandt worden ist. Im Gegensatz zum herkommlichen MEAM wird hier die Rose Gleichung durch mehr DFT Daten und insbesondere einer intelligenteren Machine Learning ahnlichen Genetic Algorithmus (GA) Optimiertechnik ersetzt, die allerdings eine bedachte Startparameterwahl vorraussetzt, womit Schritt 1 wieder ins Spiel kommt. Die genutzte fortgeschrittene MEAMfit Software, die per GA funktioniert, wurde zwischen 2016 und 2017 funktionierend eigens dafur implementiert. Mit den in Schritt 1 gefitteten Parametern und Set-Auswahltechniken konnten die weiterfuhrenden Fits mit optimalen Startparametern durchgefuhrt werden. Auf dieser Stufe waren diese Fits mit der speziell verbesserten Technik in der Lage ein detailgetreues Fe-Y Potenzial zu generieren, das sowohl alle Phasen (Fe2Y, Fe3Y, Fe5Y, Fe23Y6 und Fe17Y2 sowohl als auch reines Fe und Y) als auch die gesamte Defektdatenbasis mit einer durchschnittlichen Abweichung von ≈11% erfolgreich abbildet. Zusatzlich bestatigend zu dieser allgemeinen Ubereinstimmung wurde konsequenterweise der in Schritt 1 entwickelte Test hervorragend mit einmaliger Genauigkeit bestanden, mit max. 5% Abweichung von den komlexen o.g. Y-Leerstellen Bildungsenergien. Allerdings konnte ein systematischer Fehlertrend aufgespurt werden, der Schwachen in der Fe-Fe Komponente offenbarte. Als Folge dessen wurde umgehend diese Komponente durch ein anderes etabliertes Fe-Fe Potenzial von G. Ackland mit einer extrem genauen Schmelztemperatur (nur 3% Abweichung vom Exp.) ausgetauscht. Mit diesem genauen Potenzial konnte zum ersten Mal die Clusterbildung von gelosten Yttrium Atomen in einer Eisenschmelze erfolgreich per MD Simulation auf atomarer Ebene nachgestellt werden oberhalb von 1750 K. Temperaturen darunter hatten eine Ausscheidungsbildung von Y mit sehr geringer Y-Loslichkeit (<0.1%) in Ubereinstimmung mit den Experimenten zur Folge. Dies wurde durch den Pot. Typ A ermoglicht, der aber die energetische Reihenfolge bei den Fe-Y Phasen nicht ganz genau einhalt. Typ B hingegen halt diese ein, dort fehlt aber die Y-Clusterbildungsneigung. Durch den gebotenen Praxisbezug zur Metallurgie mussen die Loslichkeit und Clusterbildung gleichzeitig in der Simulation genau reproduzierbar sein, was aber weder Typ A noch B kann, was zum Typ A/B Dilemma fuhrt. Dieses Typ A/B Dilemma (Phasen oder Defekt Genauigkeit) fuhrt zum letzten dritten Schritt (chapter 5). Darin ist zusatzlich die Strukturaufklarung von der Fe17Y2 Phase mit Vergleichen zu exp. EXAFS Spektren unserer Kollaborationspartner vom ISSP (Riga) enthalten. Diese Aufklarung dient auch dazu die fehlenden magnetischen Abhangigkeiten im Potenzial zu kompensieren, da die Phasenreihenfolge mit sehr feinen Energieunterschieden wohl stark von magnetischen Wechselwirkungen gepragt ist. Obwohl Potenzial Typ B diesen (Magnetismus) nicht direkt beachtet, ist es in der Lage das tatsachlich gemessene EXAFS Spektrum grostenteils genau wiederzugeben. Allerdings offenbart eine einzige ausgepragte Phasenverschiebung, dass die angenommene hcp Struktur durch eine unterschwellige rhombohedrale Komponente, die sporadisch in der c-Gitterrichtung auftritt, korrigiert werden muss. AIMD (DFT) Berechnungen in Kooperation mit der University of Edinburgh bestatigen dies und zeigen sogar, dass magnetische Wechselwirkungen diese Strukturmischung stabilisieren. Endgultig bestatigt werden konnte dies mit der genauen EXAFS Spektren Reproduktion mit dem durch AIMD verbesserten nochmals gefitteten Potenzialtyp B, der als neuer Typ C durch AIMD indirekt den Einfluss der magnetischen Wechselwirkungen mit einschliest. Diese erstmalige nahezu deckungsgleiche MD Simulation eines EXAFS Spektrums von einem komplexen metallischen Alloy, hier Fe-Y, stellt eine bisher unerreichte Verbesserung dar. Schlieslich lost Typ C das Typ A/B Dilemma und ernoglicht eine genaue gleichzeitige MD Modellierung von Phasen- und Defekten in Fe-Y – ein Durchbruch in der MD-Potenzialentwicklung. read less

Abstract: Small concentrations of impurity atoms can affect the behavior of metal alloys in many ways of interest to the science and engineering community. In some cases, such as for oxygen (O) interstitials and twins or dislocations in titanium (Ti) alloys, these interactions can be difficult to study experimentally due to the small time and length scales of the governing mechanisms. Theory and atomic-scale modeling offers a path toward understanding materials at very high resolution using a variety of techniques ranging from ab initio methods such as density functional theory (DFT) to empirical potential methods such as the Modified Embedded Atom Method (MEAM). In this study we present the first published MEAM potential for the Ti–O system; the potential is fit to experimental measurements and DFT calculations for the lattice constants, elastic constants and thermodynamic characteristics of several Ti–O structures with O concentration between 0 and 50 at%. We validate the effectiveness of the new potential by successfully reproducing properties such as lattice constants, elastic constants and diffusion energy barriers for other structures. In doing so, we have calculated and report properties for two new stable structures of TiO: a CsCl structure and a zinc blende structure. We also report the first theoretical study on the effects of O concentration on the elastic constants of hexagonal close packed Ti without the confounding effects of changing supercell size. Overall, this new potential will enable interrogation of Ti and O interactions at time and length scales below those available experimentally and above the range accessible by DFT. read less

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. read less

Abstract: A selection of nanoscale processes is studied theoretically, with the aim of identifying themechanisms that could lead to selective carbon nanotube (CNT) growth. Only mechanisms relevant to catalytic chemical vapour deposition (CVD) are considered. The selected processes are analysed with classical molecular dynamics (MD) simulations and continuum modelling. The melting and pre-melting behaviour of supported nickel catalyst particles is investigated. Favourable epitaxy between a nanoparticle and the substrate is shown to significantly raise themelting point of the particle. It is also demonstrated that substrate binding can induce solid-solid transformations, whilst the epitaxy may even determine the orientation of individual crystal planes in supported catalysts. These findings suggest that the substrate crystal structure alone can potentially be used to manipulate the properties of catalyst particles and, hence, influence the structure of CNTs. The first attempt at modelling catalyst dewetting, a process where the catalyst unbinds from the inner walls of a nucleating nanotube, is presented. It is argued that understanding this process and gaining control over itmay lead to better selectivity in CNT growth. Two mutually exclusive dewetting mechanisms, namely cap lift-off and capillary withdrawal, are identified and then modelled as elastocapillary phenomena. The modelling yields an upper bound on the diameter of CNTs that can stem from a catalyst particle of a given size. It is also demonstrated that cap lift-off is sensitive to cap topology, suggesting that it may be possible to link catalyst characteristics to the structural properties of nucleating CNTs. However, a clear link to the chiral vector remains elusive. It is shown that particle size, as well as binding affinity, plays a critical role in capillary absorption and withdrawal of catalyst nanoparticles. This size dependence is explored in detail, revealing interesting ramifications to the statics and dynamics of capillary-driven flows at the nanoscale. The findings bear significant implications for our understanding of CNT growth from catalyst particles, whilst also suggesting new nanofluidic applications and methods for fabricating composite metal-CNT materials. read less