Abstract: This paper proposes an approach for parameter estimation of the Classical and Generalized Morse Potential functions. A new potential which is a modification of the Generalized Morse Potential was proposed as parameter estimates yielded complex conjugate roots using gold atom for simulation. Existing methods of parameter estimation requires the provision of initial guess values of which convergence to the optimal solution is not always guaranteed. This makes provision of initial guess values that guarantees convergence to the optimum solution more of an art than a science. The proposed objective least squares function method does not require the provision of initial guess values and it involves the minimization of two formulated objective functions using the differential numerical approach and least squares method. The built-in “Minimize” function of Mathematica® is also used to minimize the formulated objective function. Potential energy curves were constructed by fitting estimated parameter values to experimental data sets of the gold atom using values of the proposed approach and Mathematica® for performance evaluation. Errors of each constructed potential energy curves were simulated. It was observed that the errors were very small for both the Classical and Modified Generalized Morse Potential. Hence the approximations of the proposed approach are very good. read less

Abstract: The Materials Genome Initiative seeks to significantly decrease the cost and time of development and integration of new materials. Within the domain of atomistic simulations, several roadblocks stand in the way of reaching this goal. While the NIST Interatomic Potentials Repository hosts numerous interatomic potentials (force fields), researchers cannot immediately determine the best choice(s) for their use case. Researchers developing new potentials, specifically those in restricted environments, lack a comprehensive portfolio of efficient tools capable of calculating and archiving the properties of their potentials. This paper elucidates one solution to these problems, which uses Python-based scripts that are suitable for rapid property evaluation and human knowledge transfer. Calculation results are visible on the repository website, which reduces the time required to select an interatomic potential for a specific use case. Furthermore, property evaluation scripts are being integrated with modern platforms to improve discoverability and access of materials property data. To demonstrate these scripts and features, we will discuss the automation of stacking fault energy calculations and their application to additional elements. While the calculation methodology was developed previously, we are using it here as a case study in simulation automation and property calculations. We demonstrate how the use of Python scripts allows for rapid calculation in a more easily managed way where the calculations can be modified, and the results presented in user-friendly and concise ways. Additionally, the methods can be incorporated into other efforts, such as openKIM. read less

Abstract: The paper presents the results of applying a new method, previously developed by the authors, based on precalculated sets of 3D distributions of a matter density. The method is designed to recognize the spatial 3D distributions of the synthesized intermetallic compounds in the volume of a nanoparticle. A set of 3D distributions of a matter density in the volume of a cubic Ti@Al core — shell nanoparticle corresponds to a set of successive time points. It is calculated based on the results of the computer molecular dynamics simulation of self-propagating high temperature synthesis in the nanoparticle. Computational experiments are performed using the LAMMPS software package. Based on the obtained results, thermal and microstructural analyses are performed, confirming the multistage mechanism for the formation of intermetallic phases during the SHS reaction in the Ti-Al reaction medium. The sets of 3D distributions of the matter density and 3D distributions of synthesized intermetallic compounds in the volume of a nanoparticle corresponding to the sequence of time points are calculated. The paper shows the advantage of the method for recognizing 3D distributions of synthesized intermetallic compounds, proposed by the authors, over the methods of similar analysis built into the OVITO software package. read less

Abstract: The subject of research: the methodological and functional capabilities of specialized software packages of free access LAMMPS and OVITO and author's programs used for simulation of SHS-microkinetics in heterogeneous systems. Purpose of research: to increase information content, 3D-visualization, and adequate interpretation of the processes of structural-phase transformations at interfaces of the layered, layered-block nanoscale Ni-Al and Ti-Al systems. Methods and objects of redearch: the molecular dynamics simulation method implemented in the basis of the LAMMPS software package simulates the process of initiation and propagation of the SHS combustion wave in the systems under study, and using the OVITO package, 3D-visualization of the processes of dissolution, diffusion, and motion of the SHS combustion wave front is carried out. Main results of research: it is shown that the development by the author of software modules for calculating one-dimensional temperature and density profiles of the synthesized material along the direction of combustion wave motion, as well as for calculating and visualizing the 3D-structure of the distribution of synthesized intermetallic interlayers at the interface in nanoparticles with the Ni-Al "core-shell" structure (and similarly for Ti-Al) allows, in contrast to the OVITO package, to detect heterophase intermetallic structures in such systems. As a result of a large series of computational experiments on simulation of microkinetics in the studied nanoscale Ni-Al and Ti-Al systems, it has been established that the software package created by the author with support for parallel computing, which, in addition to the licensed packages LAMMPS and OVITO, integrates author's software modules, has new broader functionality in the methodological aspect of the study of the problems of physical chemistry of the SHS process and is a vivid example of the created software engineering tools in this area of research. read less

Abstract: The wide application of titanium aluminum (Ti–Al) intermetallic compounds for aerospace and automotive fields has accelerated the research and development of Ti3Al alloy. In this study, simulation is adopted to investigate the crystallization behavior during rapid solidification of Ti3Al alloys using molecular dynamics at different cooling rates of 1010 K s−1, 1011 K s−1, 1012 K s−1, and 1013 K s−1. The evolution of microstructures is characterized by taking advantage of the average potential energy, the pair distribution function and visualization. The results show that the system has formed a microstructural configuration with the face-centered cubic structure as the main structure and the hexagonal close-packed structure as the supplement. An increase in the cooling rate will reduce the grain size of the alloy, which in turn will increase the number of grains. At the cooling rate at which the alloy can crystallize, the system forms five-fold twin structures. Meanwhile, we obtain a deeper insight into the properties of five-fold twins in terms of atoms on different sites, and establish a standard model of the same specification for comparison to get the commonality and differences of the five-fold twins between the standard and the solidified. In addition, the evolution of dislocation densities and distribution of dislocation lines in the system under different conditions are analyzed. This study further explores crystallization behavior on the atomic scale and it is hoped that this research will contribute to expanding the understanding of Ti3Al alloy during the growth process. read less

Abstract: A full energy range primary radiation damage model is presented here. It is based on the athermal recombination corrected displacements per atom (arc-dpa) model but includes a proper treatment of the near threshold conditions for metallic materials. Both ab initio (AIMD) and classical molecular dynamics (MD) simulations are used here for various metals with body-centered cubic (bcc), face-centered cubic (fcc), and hexagonal close-packed (hcp) structures to validate the model. For bcc and hcp metals, the simulation results fit very well with the model. For fcc metals, although there are slight deviations between the model and direct simulation results, it is still a clear improvement on the arc-dpa model. The deviations are due to qualitative differences in the threshold energy surfaces of fcc metals with respect to bcc and hcp metals according to our classical MD simulations. We introduce the minimum threshold displacement energy (TDE) as a term in our damage model. We calculated minimum TDEs for various metal materials using AIMD. In general, the calculated minimum TDEs are in very good agreement with experimental results. Moreover, we noticed a discrepancy in the literature for fcc Ni and estimated the average TDE of Ni using both classical MD and AIMD. It was found that the average TDE of Ni should be \ensuremath{\sim}70 eV based on simulation and experimental data, not the commonly used literature value of 40 eV. The most significant implications of introducing this full energy range damage model will be for estimating the effect of weak particle-matter interactions, such as for \ensuremath{\gamma}- and electron-radiation-induced damage. read less

Abstract: The stability, elastic and electronic properties of titanium aluminide compounds have been systematically studied by the first-principles calculation. The calculated lattice parameters are consistent with the results found in the literature. The three Ti-Al binary compounds are thermodynamically stable intermetallics depending on their negative formation enthalpy. It has been found that the Ti-Al binary compounds are composed of both metallic and covalent bonds. Elastic properties revealed that these alloys are more resistant to deformation along the a- and c-axis. Besides, the (001)[100] deformation would be easier than (010)[100] deformation for these alloys. The results found in this chapter give a reliable reference for the design of novel Ti-Al binary alloys. read less

Abstract: The dependence of the melting temperature of Ti and TiAl nanoparticles on their diameter in vacuum and in Al matrix was studied by the method of molecular dynamics using EAM potentials of Zope and Mishin. Particles with a diameter of 2.5 to 12 nm were considered. The obtained values of the melting point are in good agreement with the approximation curves constructed on the basis that the decrease in the melting temperature is proportional to the ratio of the surface area of the particle to its volume. Wherein the values of the melting temperature of Ti and TiAl particles in the aluminum matrix turned out to be lower than those of particles in vacuum, which is explained by the smearing and disordering of the interface due to mutual diffusion. As the size of particles in vacuum and in aluminum increased, the values of their melting points tended to the same value, which is explained by the decrease in the role of the diffusion-blurred interface with an increase in the particle diameter. The particles began to melt from the surface. The velocity of movement of the melting front depended on temperature and increased with increasing temperature. In the case of particles in aluminum matrix, at temperatures close to the particle melting point, mutual diffusion was significantly accelerated due to melting of the particle boundary layer. Al atoms penetrating into the particle accelerated the movement of the melting front, rapidly occupying the next destroyed layer of the particle. read less

Abstract: γ-titanium aluminide (TiAl) alloys with fully lamellar microstructure possess excellent properties for high-temperature applications. Such fully lamellar microstructure has interfaces at different length scales. The separation behavior of the lamellae at these interfaces is crucial for the mechanical properties of the whole material. Unfortunately, quantifying it by experiments is difficult. Therefore, we use molecular dynamics (MD) simulations to this end. Specifically, we study the high-temperature separation behavior under tensile loading of the four different kinds of lamellar interfaces appearing in TiAl, namely, the γ / α 2 , γ / γ PT , γ / γ TT , and γ / γ RB interfaces. In our simulations, we use two different atomistic interface models, a defect-free (Type-1) model and a model with preexisting voids (Type-2). Clearly, the latter is more physical but studying the former also helps to understand the role of defects. Our simulation results show that among the four interfaces studied, the γ / α 2 interface possesses the highest yield strength, followed by the γ / γ PT , γ / γ TT , and γ / γ RB interfaces. For Type-1 models, our simulations reveal failure at the interface for all γ/γ interfaces but not for the γ / α 2 interface. By contrast, for Type-2 models, we observe for all the four interfaces failure at the interface. Our atomistic simulations provide important data to define the parameters of traction–separation laws and cohesive zone models, which can be used in the framework of continuum mechanical modeling of TiAl. Temperature-dependent model parameters were identified, and the complete traction–separation behavior was established, in which interface elasticity, interface plasticity, and interface damage could be distinguished. By carefully eliminating the contribution of bulk deformation from the interface behavior, we were able to quantify the contribution of interface plasticity and interface damage, which can also be related to the dislocation evolution and void nucleation in the atomistic simulations. 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 molecular dynamics method is used to study the process of development of dynamic instability of a thin film, leading to its destruction. The calculations are performed for a thin aluminum film using the interatomic interaction potential tested by comparing the numerical results with the analytical ones from the elasticity theory. An original approach which allows one to calculate the dispersion law of long‐wave phonons in ultrathin films is developed. The temperatures (<600 K) at which the system remains stable for 0.6 ns are found. This makes it possible to analyze the low‐frequency part of the spectrum down to the minimum frequency νmin = 0.0166 THz, and to determine the vibration frequency of the longest bending wave ν0 = 0.033 THz which decreases with increasing temperature, and therefore, its period grows. Once the vibration period becomes comparable with the time of simulation, there occurs a continuous increase in the amplitude of this mode which will be referred to as “retarded mode.” It is shown that the film destruction begins with the attainment of a certain critical value of the bending wave amplitude. 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: Building on our previously introduced multicell Monte Carlo (MC)^{2} method for modeling phase coexistence, this paper provides important improvements for efficient determination of phase equilibria in solids. The (MC)^{2} method uses multiple cells, representing possible phases. Mass transfer between cells is modeled virtually by solving the mass balance equation after the composition of each cell is changed arbitrarily. However, searching for the minimum free energy during this process poses a practical problem. The solution to the mass balance equation is not unique away from equilibrium, and consequently the algorithm is in risk of getting trapped in nonequilibrium solutions. Therefore, a proper stopping condition for (MC)^{2} is currently lacking. In this work, we introduce a consistency check via a predictor-corrector algorithm to penalize solutions that do not satisfy a necessary condition for equivalence of chemical potentials and steer the system toward finding equilibrium. The most general acceptance criteria for (MC)^{2} is derived starting from the isothermal-isobaric Gibbs ensemble for mixtures. Using this ensemble, translational MC moves are added to include vibrational excitations as well as volume MC moves to ensure the condition of constant pressure and temperature entirely with a MC approach, without relying on any other method for relaxation of these degrees of freedom. As a proof of concept the method is applied to two binary alloys with miscibility gaps and a model quaternary alloy, using classical interatomic potentials. read less

Abstract: The molecular dynamics simulation method is utilized to investigate the physical characteristics of nanoscale vanadium powder during 3D printing powder bed fusion laser sintering process. The radius of gyration, neck width, and root mean square of different powder size of nanoscale vanadium powder during 3D printing laser sintering process under different heating rate is analyzed and discussed. The neck width and internal lattice changes are also observed.It is observed that the temperature of solid state diffusion decrease with heating rate increasing, but the temperature of that increase with powder size decreasing. It is found that the coalescence temperature of nanoscale vanadium powder is range of between 1600K and 1950K, and the melting temperature of that is range of between 1850K and 1990K. read less

Abstract: Atomistic simulations are used to determine the resolved shear stress necessary for equilibrium and the resulting geometry of nanoscale dislocation shear loops in Al. Dislocation loops with different sizes and shapes are created via superposition of elemental triangular dislocation displacement fields in the presence of an externally imposed shear stress. First, a bisection algorithm is developed to determine systematically the resolved shear stress necessary for equilibrium at 0 K. This approach allows for the identification of dislocation core structure and a correlation between dislocation loop size, shape and the computed shear stress for equilibrium. It is found, in agreement with predictions made by Scattergood and Bacon, that the equilibrium shape of a dislocation loop becomes more circular with increasing loop size. Second, the bisection algorithm is extended to study the influence of temperature on the resolved shear stress necessary for stability. An approach is presented to compute the effective lattice friction stress, including temperature dependence, for dislocation loops in Al. The temperature dependence of the effective lattice friction stress can be reliably computed for dislocation loops larger than 16.2 nm. However, for dislocation loops smaller than this threshold, the effective lattice friction stress shows a dislocation loop size dependence caused by significant overlap of the stress fields on the interior of the dislocation loops. Combined, static and finite temperature atomistic simulations provide essential data to parameterize discrete dislocation dynamics simulations. read less

Abstract: Microstructure and mechanical properties are key parameters influencing the performance of structural multi-phase alloys such as those based on intermetallic TiAl compounds. There, the main constituent, a γ -TiAl phase, is derived from a face-centered cubic structure. Consequently, the dissociation of dislocations and generation of stacking faults (SFs) are important factors contributing to the overall deformation behavior, as well as mechanical properties, such as tensile/creep strength and, most importantly, fracture elongation below the brittle-to-ductile transition temperature. In this work, SFs on the { 111 ) plane in γ -TiAl are revisited by means of ab initio calculations, finding their energies in agreement with previous reports. Subsequently, stacking fault energies are evaluated for eight ternary additions, namely group IVB–VIB elements, together with Ti off-stoichiometry. It is found that the energies of superlattice intrinsic SFs, anti-phase boundaries (APBs), as well as complex SFs decrease by 20–40% with respect to values in stoichiometric γ -TiAl once an alloying element X is present in the fault plane having thus a composition of Ti-50Al-12.5X. In addition, Mo, Ti and V stabilize the APB on the (111) plane, which is intrinsically unstable at 0 K in stoichiometric γ -TiAl. read less

Abstract: A new process of shock wave consolidation of nanoparticles into a nanocrystalline coating is theoretically considered. In the proposed scheme, the nanoparticle layers, which are attached to the substrate surface by adhesion, are compacted by plane ultra-short shock waves coming from the substrate. The initial adhesion is self-arisen at any contact between the nanoparticles without a pre-compression. The absence of the nanoparticle ejections due to the shock wave action is connected with the strong adhesive forces, which allow nanoparticles to be attached to each other and to substrate while they are being compacted; this should be valid for small enough nanoparticles. Severe plastic deformation of the nanoparticles and the increased temperature due to collapse of voids between them facilitate their compaction into the monolithic nanocrystalline layer. We consider the examples of Cu and Ni nanoparticles on Al substrate using molecular dynamic simulations. We show the efficiency of the action of multiple sh... read less

Abstract: Machine-learning interatomic potential (MLIP) has been of growing interest as a useful method to describe the energetics of systems of interest. In the present study, we examine the accuracy of linearized pairwise MLIPs and angular-dependent MLIPs for 31 elemental metals. Using all of the optimal MLIPs for 31 elemental metals, we show the robustness of the linearized frameworks, the general trend of the predictive power of MLIPs, and the limitation of pairwise MLIPs. As a result, we obtain accurate MLIPs for all 31 elements using the same linearized framework. This indicates that the use of numerous descriptors is the most important practical feature for constructing MLIPs with high accuracy. An accurate MLIP can be constructed using only pairwise descriptors for most non-transition metals, whereas it is very important to consider angular-dependent descriptors when expressing interatomic interactions of transition metals. read less

Abstract: Machine learning interatomic potentials (MLIPs) based on a large dataset obtained by density functional theory (DFT) calculation have been developed recently. This study gives both conceptual and practical bases for the high accuracy of MLIPs, although MLIPs have been considered to be simply an accurate black-box description of atomic energy. We also construct the most accurate MLIP of the elemental Ti ever reported using a linearized MLIP framework and many angular-dependent descriptors, which also corresponds to a generalization of the modified embedded atom method (MEAM) potential. read less

Abstract: The local atomic structure in aluminium monatomic metallic glass is studied using molecular dynamics simulations combined with the embedded atom method (EAM). We have used a variety of analytical methods to characterise the atomic configurations of our system: the Pair Distribution Function (PDF), the Common Neighbour Analysis (CNA) and the Voronoi Tessellation Analysis. CNA was used to investigate the order change from liquid to amorphous phases, recognising that the amount of icosahedral clusters increases with the decrease of temperature. The Voronoi analysis revealed that the icosahedral-like polyhedral are the predominant ones. It has been observed that the PDF function shows a splitting in the second peak, which cannot be attributed to the only ideal icosahedral polyhedron 〈0, 0, 12, 0〉, but also to the formation of other Voronoi polyhedra 〈0, 1, 10, 2〉 . Further, the PDFs were then integrated giving the cumulative coordination number in order to compute the fractal dimension (df). read less

Abstract: Today's research on the cathodic arc deposition technique and coatings therefrom primarily focuses on the effects of, e.g., nitrogen partial pressure, growth temperature, and substrate bias. Detailed studies on the morphology and structure of the starting material—the cathode—during film growth and its influence on coating properties at different process conditions are rare. This work aims to study the evolution of the converted layer, its morphology, and microstructure, as a function of the cathode material grain size during deposition of Ti-Al-N coatings. The coatings were reactively grown in pure N2 discharges from powder metallurgically manufactured Ti-50 at.% Al cathodes with grain size distribution averages close to 1800, 100, 50, and 10 μm, respectively, and characterized with respect to microstructure, composition, and mechanical properties. The results indicate that for the cathode of 1800 μm grain size the disparity in the work function among parent phases plays a dominant role in the pronounced... read less

Abstract: Empirical potential is vital to the classic atomic simulation, especially for the study of phase transitions, as well as the solid-interface. In this paper, we attempt to set up a uniform procedure for the validation among different potentials before the formal simulation study of phase transitions of metals. Two main steps are involved: (1) the prediction of the structures of both solid and liquid phases and their mutual transitions, i.e. melting and crystallization; (2) the prediction of vital thermodynamic (the equilibrium melting point at ambient pressure) and dynamic properties (the degrees of superheating and undercooling). We applied this procedure to the testing of seven published embedded-atom potentials (MKBA (Mendelev et al 2008 Philos. Mag. 88 1723), MFMP (Mishin et al 1999 Phys. Rev. B 59 3393), MDSL (Sturgeon and Laird 2000 Phys. Rev. B 62 14720), ZM (Zope and Mishin 2003 Phys. Rev. B 68 024102), LEA (Liu et al 2004 Model. Simul. Mater. Sci. Eng. 12 665), WKG (Winey et al 2009 Model. Simul. Mater. Sci. Eng. 17 055004) and ZJW (Zhou et al 2004 Phys. Rev. B 69 144113)) for the description of the solid–liquid transition of Al. All the predictions of structure, melting point and superheating/undercooling degrees were compared with the experiments or theoretical calculations. Then, two of them, MKBA and MDSL, were proven suitable for the study of the solid–liquid transition of Al while the residuals were unqualified. However, potential MKBA is more accurate to predict the structures of solid and liquid, while MDSL works a little better in the thermodynamic and dynamic predictions of solid–liquid transitions. read less

Abstract: New interatomic potentials describing defects, plasticity, and high temperature phase transitions for Ti are presented. Fitting the martensitic hcp-bcc phase transformation temperature requires an efficient and accurate method to determine it. We apply a molecular dynamics method based on determination of the melting temperature of competing solid phases, and Gibbs-Helmholtz integration, and a lattice-switch Monte Carlo method: these agree on the hcp-bcc transformation temperatures to within 2 K. We were able to develop embedded atom potentials which give a good fit to either low or high temperature data, but not both. The first developed potential (Ti1) reproduces the hcp-bcc transformation and melting temperatures and is suitable for the simulation of phase transitions and bcc Ti. Two other potentials (Ti2 and Ti3) correctly describe defect properties and can be used to simulate plasticity or radiation damage in hcp Ti. The fact that a single embedded atom method potential cannot describe both low and high temperature phases may be attributed to neglect of electronic degrees of freedom, notably bcc has a much higher electronic entropy. A temperature-dependent potential obtained from the combination of potentials Ti1 and Ti2 may be used to simulate Ti properties at any temperature. read less

Abstract: Thermophysical properties of liquid nickel (Ni) around the melting temperature are investigated by means of classical molecular dynamics (MD) simulation, using three different embedded atom method potentials to model the interactions between the Ni atoms. Melting temperature, enthalpy, static structure factor, self-diffusion coefficient, shear viscosity, and thermal diffusivity are compared to recent experimental results. Using ab initio MD simulation, we also determine the static structure factor and the mean-squared displacement at the experimental melting point. For most of the properties, excellent agreement is found between experiment and simulation, provided the comparison relative to the corresponding melting temperature. We discuss the validity of the Hansen-Verlet criterion for the static structure factor as well as the Stokes-Einstein relation between self-diffusion coefficient and shear viscosity. The thermal diffusivity is extracted from the autocorrelation function of a wavenumber-dependent temperature fluctuation variable. read less

Abstract: The effects of plateau width and step-edge kinking on carbon monoxide (CO)-induced restructuring of platinum surfaces were explored using molecular dynamics (MD) simulations. Platinum crystals displaying four different vicinal surfaces [(321), (765), (112), and (557)] were constructed and exposed to partial coverages of carbon monoxide. Platinum–CO interactions were fit to recent experimental data and density functional theory (DFT) calculations, providing a classical interaction model that captures the atop binding preference on Pt. The differences in Pt–Pt binding strength between edge atoms on the various facets were found to play a significant role in step-edge wandering and reconstruction events. Because the mechanism for step doubling relies on a stochastic meeting of two wandering edges, the widths of the plateaus on the original surfaces were also found to play a role in these reconstructions. On the Pt(321) surfaces, the CO adsorbate was found to assist in reordering the kinked step edges into st... read less

Abstract: The microstructures of titanium (Ti), an attractive tritium (T) storage material, will affect the evolution process of the retained helium (He). Understanding the diffusion behavior of He at the atomic scale is crucial for the mechanism of material degradation. The novel diffusion behavior of He has been reported by molecular dynamics (MD) simulation for the bulk hcp-Ti system and the system with grain boundary (GB). It is observed that the diffusion of He in the bulk hcp-Ti is significantly anisotropic (the diffusion coefficient of the [0001] direction is higher than that of the basal plane), as represented by the different migration energies. Different from convention, the GB accelerates the diffusion of He in one direction but not in the other. It is observed that a twin boundary (TB) can serve as an effective trapped region for He. The TB accelerates diffusion of He in the direction perpendicular to the twinning direction (TD), while it decelerates the diffusion in the TD. This finding is attributable to the change of diffusion path caused by the distortion of the local favorable site for He and the change of its number in the TB region. read less

Abstract: Ni–Al–Co is a promising system for ferromagnetic shape memory applications. This paper reports on the development of a ternary embedded-atom potential for this system by fitting to experimental and first-principles data. Reasonably good agreement is achieved for physical properties between values predicted by the potential and values known from experiment and/or first-principles calculations. The potential reproduces basic features of the martensitic phase transformation from the B2-ordered high-temperature phase to a tetragonal CuAu-ordered low-temperature phase. The compositional and temperature ranges of this transformation and the martensite microstructure predicted by the potential compare well with existing experimental data. These results indicate that the proposed potential can be used for simulations of the shape memory effect in the Ni–Al–Co system. read less

Abstract: We demonstrate the applicability of extended cluster expansion technique, GICE, to calculation of a potential energy surface (PES) at discrete position in terms of atomic arrangement with an example of Cu and Cu-Ti binary system on fcc lattice. We find that the proposed CE successfully predicts total energy within error of 0.5meV/atom for Cu and 1.2meV/atom for Cu-Ti with respect to DFT calculation, which indicates that this method can model the PES and possesses potential to formulate physical properties in terms of atomic arrangement. [doi:10.2320/matertrans.M2015024] read less

Abstract: An interatomic potential for Al is developed within the third generation of the charge optimized many-body (COMB3) formalism. The database used for the parameterization of the potential consists of experimental data and the results of first-principles and quantum chemical calculations. The potential exhibits reasonable agreement with cohesive energy, lattice parameters, elastic constants, bulk and shear modulus, surface energies, stacking fault energies, point defect formation energies, and the phase order of metallic Al from experiments and density functional theory. In addition, the predicted phonon dispersion is in good agreement with the experimental data and first-principles calculations. Importantly for the prediction of the mechanical behavior, the unstable stacking fault energetics along the direction on the (1 1 1) plane are similar to those obtained from first-principles calculations. The polycrsytal when strained shows responses that are physical and the overall behavior is consistent with experimental observations. read less

Abstract: Molecular dynamics simulations and micromechanics model analysis are performed to investigate the mechanical behaviours and interfacial effects of interpenetrating phase composites in the nanoscale. It is observed that the overall Young's modulus and ultimate strength of the nanocomposites vary nonlinearly with the cohesive energy of the interface. The cohesive properties affect the stiffness of the interface zone, and in turn, influence the effective Young's modulus of composites. The competition between interfacial failure and weak phase damage results in an optimal cohesive parameter of the interface, at which the composite possesses the maximal ultimate strength. The obtained results provide useful guidelines for the design and optimisation of advanced nanocomposites with superior mechanical properties. read less

Abstract: The annihilation of vacancy- and interstitial-type dislocation dipoles is investigated employing atomistic simulations and metadynamics in hexagonal close-packed (hcp) metals α-titanium and α-zirconium, with a variety of dipole heights, orientations and annealing temperatures. Molecular dynamics simulations reveal that depending on the dipole type, height, orientation, etc., dipolar configurations transform into specific reconstructed configurations at low temperature, while vacancy or interstitial clustering occurs at high temperature. The time of clustering and the lifetime of the resulting clusters are estimated through the search of the lowest-energy paths. Compared with previous knowledge on face-centered cubic (fcc) metals, the general processes of dislocation annihilation and point defect clustering are similar, however, due to the varied lattice symmetry: (1) the atomic structures of the reconstructed configurations at low temperature and the clusters at high temperature in hcp systems are different from those in fcc systems; and (2) in hcp systems the clustering process is shortened and the stability of the resulting clusters is enhanced compared with fcc systems. read less

Abstract: Embedded nanoparticles between a graphene coating and substrate often precipitate blisters generated in graphene, which may impede the application of graphene as a surface coating. Molecular dynamics (MD) simulations are performed to analyze the evolution of nanocoating morphology during the process of adhering the graphene onto a nanoparticle decorated metal surface. The simulation reveals that for a deformable nanoparticle, a blister rudiment with a tail formed and then changes into an irregular blister, but a rigid nanoparticle will be ejected from the surface of the substrate without any blistering. Substrate damage caused by the nanoparticles during coating is also analyzed. It is found that hexagonal close-packed atoms emerge from the contact point between the rigid particle and substrate. Understanding how blisters form in graphene coatings by an atomic-scale approach will help this promising material function more effectively and widely. read less

Abstract: Methods for and the results of the computer simulation of liquid metals are reviewed. Two basic methods, classical molecular dynamics with known interparticle potentials and the ab initio method, are considered. Most attention is given to the simulated results obtained using the embedded atom model (EAM). The thermodynamic, structural, and diffusion properties of liquid metal models under normal and extreme (shock) pressure conditions are considered. Liquid-metal simulated results for the Groups I–IV elements, a number of transition metals, and some binary systems (Fe–C, Fe–S) are examined. Possibilities for the simulation to account for the thermal contribution of delocalized electrons to energy and pressure are considered. Solidification features of supercooled metals are also discussed. read less

Abstract: An inverse “smaller is stronger” trend is predicted on the basis of molecular dynamics simulations of α-titanium (Ti) single-crystal nanopillars orientated for double prismatic slips when the nanopillars are less than 7 nm wide. This trend is attributed to a significant increase in the surface energy due to the nucleation and propagation of edge dislocations on the surface of the pillars. read less

Abstract: In terms of the fully relativistic screened Korringa–Kohn–Rostoker method we investigate the effect of stacking faults on the magnetic properties of hexagonal close-packed (hcp) cobalt. In particular, we consider the formation energy and the effect on the magnetocrystalline anisotropy energy (MAE) of four different stacking faults in hcp cobalt—an intrinsic growth fault, an intrinsic deformation fault, an extrinsic fault and a twin-like fault. We find that the intrinsic growth fault has the lowest formation energy, in good agreement with previous first-principles calculations. With the exception of the intrinsic deformation fault which has a positive impact on the MAE, we find that the presence of a stacking fault generally reduces the MAE of bulk Co. Finally, we consider a pair of intrinsic growth faults and find that their effect on the MAE is not additive, but synergic. read less

Abstract: Annihilation of vacancy-type non-screw dipolar dislocations is studied with molecular dynamics in fcc metals Al, Cu and Ni, and intermetallic γ-TiAl. Contrary to common belief, dipoles do not simply disappear. Instead, they transform into a series of defects depending on their height, orientation and temperature. At low temperatures, hollow structures, reconstructed configurations and faulted dipoles are formed. At high temperatures, with the help of short-range diffusion, isolated or interconnected vacancy clusters and stacking-fault tetrahedra are formed within a simulation time of 1 ns. Employing saddle-point-search methods, the formation of the above by-products is explained by accelerated diffusion paths along the dipole cores. read less

Abstract: Bulk properties of hcp-Ti, relevant for the description of dislocations, such as elastic constants, stacking faults and γ-surface, are computed using density functional theory (DFT) and two central force embedded atom interaction models (Zope and Mishin 2003 Phys. Rev. B 68 024102, Hammerschmidt et al 2005 Phys. Rev. B 71 205409). The results are compared with previously published calculations, except pair potential calculations, which are not appropriate for the description of the metallic bond. The comparison includes N-body central force (NB-CF) and N-body angular (NB-A) empirical potentials, tight-binding approximation to the electronic structure (TB), DFT pseudopotential (DFT-P) and all electron (DFT-A) calculations. None of the considered interaction models are fully satisfactory for the description of these properties. In particular, NB-CF, NB-A and TB interaction models are unable to describe the softening of the easy prismatic γ-surface leading to the appearance of a metastable stacking fault, as evidenced in all the DFT calculations. Most often, when the basal stacking fault excess energy is underevaluated, this leads to an inversion of the energetic stability between the I2 basal and the prismatic easy stacking faults. Even the DFT-pseudopotential calculations need to be improved regarding the description of the shear elastic constants. The implications of these results on the core structure and gliding properties of the screw dislocation are analyzed. The calculated dissociation lengths into Shockley partials in both the basal and prismatic planes for the different models compare well with the measured ones in the corresponding simulations of the dislocation core structure when available. Finally, the Peierls stress is also evaluated using the Peierls–Nabarro model and compared with the experimentally measured one. read less

Abstract: Using molecular dynamics (MD) simulations, the dislocation structures of symmetric tilt grain boundaries (STGBs) in hexagonal close packed (hcp) crystal structures are studied. STGBs over the entire range of possible rotation angles θ from 0° to 90° are found to have an ordered atomic structure. Formation energy calculations reveal four local minimum-energy boundaries that correspond to coherent grain boundaries (GBs). Deviations in tilt from the basal plane (θ = 0°, , prismatic plane (θ = 90°, , or one of these four minimum-energy boundaries, , result in the formation of a tilt wall (edge-type grain boundary dislocations, GBDs) superimposed on the nearest GB structure in θ-space. As θ deviates far from the rotation angle of one and draws closer to that of an adjacent , an abrupt transition in STGB base boundary structure and GBD Burgers vector occurs. For all θ, the sign and spacing of GBDs depend on θ, and their Burgers vector is either one or two times the interplanar spacing of PB. We present a simple model that generalizes the results to other c/a ratios. Subsequent MD simulations show that (1) the model forecasts the STGB structure to first-order and (2) STGBs with two distinct atomic structures can have remarkably different responses when interacting with basal lattice dislocations originating from the adjoining crystals. read less

Abstract: Using molecular dynamics (MD) simulations, the twinning mechanism in titanium (Ti) was studied by analyzing the interfacial structure at the twin boundary (TB). The simulation results reveal interesting twin growth controlled by interfacial dislocations at the TB. The elementary twinning dislocations (bT ) nucleate and glide in pairs but separately and sequentially on two neighboring planes, significantly different from conventional zonal dislocations, which spread over two or more twinning planes with each plane comprising one Burgers vector of an elementary twinning dislocation. The twin growth can be approximately described as These two separate elementary twinning dislocations amount to a net Burgers vector 2bT ≈ 0.16 nm along the twinning vector , with the components in the in-plane direction perpendicular to η 1 canceled out. These results support the classical twinning theory in which a homogeneous shear and local shuffling have to be involved. A mechanism taking into consideration local structure of the twinning plane for such extended zonal dislocations is discussed. read less

Abstract: It has been widely considered that Al3Ti is involved in the aluminium nucleation on TiB2, although the mechanism has not been fully understood. In this paper molecular dynamics has been conducted to investigate this phenomenon at an atomistic scale. It was found that a two-dimensional Al3Ti layer may remain on TiB2 above the aluminium liquidus. In addition, the results showed that this 2D Al3Ti undergoes interface reconstruction by forming a triangular pattern. This triangular pattern consists of different alternative stacking sequences. The transition region between the triangles forms an area of strain concentration. By means of this mechanism, this interfacial Al3Ti layer stabilizes itself by localizing the large misfit strain between TiB2 and Al3Ti This reconstruction is similar to the hdp-fcc interface reconstruction in other systems which has been observed experimentally [1]. read less

Abstract: It is usually difficult to undercool Ti–Al alloys on account of their high reactivity in the liquid state. This results in a serious scarcity of information on their thermophysical properties in the metastable state. Here, we report on the surface tension of a liquid Ti–Al alloy under high undercooling condition. By using the electromagnetic levitation technique, a maximum undercooling of 324 K (0.19 T L) was achieved for liquid Ti-51 at.% Al alloy. The surface tension of this alloy, which was determined over a broad temperature range 1429–2040 K, increases linearly with the enhancement of undercooling. The experimental value of the surface tension at the liquidus temperature of 1753 K is 1.094 N m−1 and its temperature coefficient is −1.422 × 10−4 N m−1 K−1. The viscosity, solute diffusion coefficient and Marangoni number of this liquid Ti–Al alloy are also derived from the measured surface tension. read less

Abstract: We construct an interatomic potential for the Ni-Al system within the embedded-atom method formalism. The potential is based on previously developed accurate potentials for pure Ni and Al. The cross-interactions are fitted to experimental cohesive energy, lattice parameter and elastic constants of B2-NiAl, as well as to ab initio formation energies of several real or imaginary intermetallic compounds with different crystal structures and chemical compositions. The potential accurately reproduces a variety of physical properties of the NiAl and Ni3Al phases, and shows reasonable agreement with experimental and ab initio data for phase stability across the Ni-Al phase diagram. Most of the properties reproduced by the new potential were not involved in the fitting process, which demonstrates its excellent transferability. Advantages and certain weaknesses of the new potential in comparison with other existing potentials are discussed in detail. The potential is expected to be especially suitable for simulations of heterophase interfaces and mechanical behavior of Ni-Al alloys. read less

Abstract: We perform first-principles computational tensile and compressive tests (FPCTT and FPCCT) to investigate the intrinsic bonding and mechanical properties of a γ-TiAl intermetallic compound (L 10 structure) using a first-principles total energy method. We found that the stress–strain relations and the corresponding theoretical tensile strengths exhibit strong anisotropy in the [001], [100] and [110] crystalline directions, originating from the structural anisotropy of γ-TiAl. Thus, γ-TiAl is a representative intermetallic compound that includes three totally different stress–strain modes. We demonstrate that all the structure transitions in the FPCTT and FPCCT result from the breakage or formation of bonds, and this can be generalized to all the structural transitions. Furthermore, based on the calculations we qualitatively show that the Ti–Al bond should be stronger than the Ti–Ti bond in γ-TiAl. Our results provide a useful reference for understanding the intrinsic bonding and mechanical properties of γ-TiAl as a high-temperature structural material. read less

Abstract: First-principles calculations based on a plane-wave pseudopotential method, as implemented in the VASP code, are presented for the formation energies of several transition-metal and non-transition-metal dopants in Ti‐ Al alloys. Substitution for either Ti or Al in g-TiAl, a2-Ti3Al, Ti2AlC, and Ti3AlC are considered. Calculated szero-temperatured defect formation energies exhibit clear trends as a function of the periodic-table column of transition metal solutes. Early transition metals in TiAl prefer the Ti sublattice, but this preference gradually shifts to the Al sublattice for late transition metals; the Ti sublattice is preferred by all transition metal solutes in Ti3Al. Partitioning of solutes to Ti 3Al is predicted for mid-period transition elements, and to TiAl for early and late transition elements. A simple Ising model treatment demonstrates the plausibility of these trends, which are in excellent overall agreement with experiment. The influence of temperature on formation energies is examined with a cluster expansion for the binary TiAl alloys and a low temperature expansion for dilute ternary alloys. Results for Nb-doped alloys provide insight into the relative sensitivity of solute partitioning to individual contributions to the free energy. Whereas the calculated formation energy of Nb ssubstitutiond at zero temperature favors partitioning to a2-Ti3Al, temperature-dependent contributions to the formation free energy, evaluated at 1075 K, favor partitioning to g-TiAl, in agreement with experiment. read less

Abstract: The crystallization process of a Ti–Al amorphous alloy during isothermal annealing was studied with molecular dynamics simulations. The structural development and phase transformation were analysed based on the variations of the internal energy, cell volume, radial distribution function, bond pairs, and atomic configuration. The crystal nucleation, grain growth, and grain coarsening during the crystallization process were studied. The three-stage feature of the crystallization process was identified. The simulation results also show that there are transformations from a metastable crystal phase to a more stable crystal phase during the crystallization. read less

Abstract: The rapid solidification of Ti3Al was studied with the constant-pressure and constant-temperature molecular dynamics (NPT-MD) technique to obtain an atomistic description of glass formation and crystallization in the alloy. The embedded atom method (EAM) potential for the Ti?Al binary system recently developed by Zope and Mishin (2003?Phys.?Rev.?B?68?024102) was applied in the simulations. The effects of different cooling rates on the glass formation and crystallization of liquid Ti3Al were studied. In addition, the crystallization of the amorphous Ti3Al as a function of increasing temperature was also studied. The calculated internal energy change and radial distribution function during cooling and heating processes provided a good picture of the structural transformations, and the results were consistent with the results obtained experimentally. 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: The continuing search for broadly applicable, predictive, and unique potential functions led to the invention of the multi-state modified embedded atom method (MS-MEAM) (Baskes et al 2007 Phys. Rev. B 75 094113). MS-MEAM replaced almost all of the prior arbitrary choices of the MEAM electron densities, embedding energy, pair potential, and angular screening functions by using first-principles computations of energy/volume relationships for multiple reference crystal structures and transformation paths connecting those reference structures. This strategy reasonably captured diverse interactions between atoms with variable coordinations in a face-centered-cubic (fcc)-stable copper system. However, a straightforward application of the original MS-MEAM framework to model technologically useful hexagonal-close-packed (hcp) metals proved elusive. This work describes the development of an hcp-stable/fcc-metastable MS-MEAM to model titanium by introducing a new angular function within the background electron density description. This critical insight enables the titanium MS-MEAM potential to reproduce first principles computations of reference structures and transformation paths extremely well. Importantly, it predicts lattice and elastic constants, defect energetics, and dynamics of non-ideal hcp and liquid titanium in good agreement with first principles computations and corresponding experiments, and often better than the three well-known literature models used as a benchmark. The titanium MS-MEAM has been made available in the Knowledgebase of Interatomic Models (https://openkim.org/) (Tadmor et al 2011 JOM 63 17). read less

Abstract: The numerical simulation of damage using phenomenological models on the macroscale was state of the art for many decades. However, such models are not able to capture the complex nature of damage, which simultaneously proceeds on multiple length scales. Furthermore, these phenomenological models usually contain damage parameters, which are physically not interpretable. Consequently, a reasonable experimental determination of these parameters is often impossible. In the last twenty years, the ongoing advance in computational capacities provided new opportunities for more and more detailed studies of the microstructural damage behavior. Today, multiphase models with several million degrees of freedom enable for the numerical simulation of micro-damage phenomena in naturally heterogeneous materials. Therewith, the application of multiscale concepts for the numerical investigation of the complex nature of damage can be realized. The presented thesis contributes to a hierarchical multiscale strategy for the simulation of brittle intergranular damage in polycrystalline materials, for example aluminum. The numerical investigation of physical damage phenomena on an atomistic microscale and the integration of these physically based information into damage models on the continuum meso- and macroscale is intended. Therefore, numerical methods for the damage analysis on the micro- and mesoscale including the scale transfer are presented and the transition to the macroscale is discussed. The investigation of brittle intergranular damage on the microscale is realized by the application of the nonlocal Quasicontinuum method, which fully describes the material behavior by atomistic potential functions, but reduces the number of atomic degrees of freedom by introducing kinematic couplings. Since this promising method is applied only by a limited group of researchers for special problems, necessary improvements have been realized in an own parallelized implementation of the 3D nonlocal Quasicontinuum method. The aim of this implementation was to develop and combine robust and efficient algorithms for a general use of the Quasicontinuum method, and therewith to allow for the atomistic damage analysis in arbitrary grain boundary configurations. The implementation is applied in analyses of brittle intergranular damage in ideal and nonideal grain boundary models of FCC aluminum, considering arbitrary misorientations. From the microscale simulations traction separation laws are derived, which describe grain boundary decohesion on the mesoscale. Traction separation laws are part of cohesive zone models to simulate the brittle interface decohesion in heterogeneous polycrystal structures. 2D and 3D mesoscale models are presented, which are able to reproduce crack initiation and propagation along cohesive interfaces in polycrystals. An improved Voronoi algorithm is developed in 2D to generate polycrystal material structures based on arbitrary distribution functions of grain size. The new model is more flexible in representing realistic grain size distributions. Further improvements of the 2D model are realized by the implementation and application of an orthotropic material model with Hill plasticity criterion to grains. The 2D and 3D polycrystal models are applied to analyze crack initiation and propagation in statically loaded samples of aluminum on the mesoscale without the necessity of initial damage definition. read less

Abstract: We have investigated site preference and bonding properties of alloying elements including Nb, Mo, Ni and Ag in TiAl with O impurity using a first-principles method based on the density functional theory. We found that the preferable sites for O are the Ti-rich octahedral interstitial ones, while those for the alloying elements are the substitutional ones. Among these elements which are beneficial to improve the mechanical properties of TiAl, Ni and Ag occupy the Al sites, while Nb and Mo occupy the Ti sites. We demonstrate that the presence of O alters the site preference of these alloying elements in TiAl, making these elements prefer to substitute Al that is the first nearest neighbor of O, because O prefers to bond with Ti rather than Al. We suggest that, according to the local density of states results, O can be deleterious to the ductility of TiAl with Nb and Mo, but has little effect on that of TiAl with Ni and Ag. read less