Abstract: Dislocation motion and interactions determine mechanical properties in body-centered cubic (bcc) metallic materials. However, studying mechanisms for the screw dislocation interaction is fundamentally challenging since many underlying processes involve mesotimescales and atomistic resolution, currently inaccessible by either experimental techniques or continuum theoretical methods. In this paper, we develop a computational capability based on self-evolving atomistic kinetic Monte Carlo (SEAKMC) to sample the critical events and saddle point energies related to screw dislocations and their junctions. The method is first validated by calculating the stress dependence of Peierls barriers and formation energies of kink pairs and cross-slip kink pairs on a single screw dislocation in bcc iron. Then the method is applied to a binary junction of a pair of intersecting screw dislocations, the structure of which is crucial for low-temperature plastic deformation. We identify three important mechanisms: coplanar cross-slipping , jog-pinning, and a previously unknown unzipping mechanism during the evolution of the binary junction. The mechanisms are then further validated using classical molecular dynamics simulations. The computational capability developed in this paper provides an effective tool to evaluate screw dislocation related thermally activated events in complex stress conditions. The mechanisms discovered in this paper provide critical insights into temperature dependence of the anomalous slip, a breakdown of the Schmidt law, during the plastic deformation in bcc iron and can be generalized to other bcc metals. DOI: 10.1103/PhysRevMaterials.6.123602 read less

Abstract: Body-centred-cubic (BCC) transition metals (TMs) tend to be brittle at low temperatures, posing significant challenges in their processing and major concerns for damage tolerance in critical load-carrying applications. The brittleness is largely dictated by the screw dislocation core structure; the nature and control of which has remained a puzzle for nearly a century. Here, we introduce a universal model and a physics-based material index χ that guides the manipulation of dislocation core structure in all pure BCC metals and alloys. We show that the core structure, commonly classified as degenerate (D) or non-degenerate (ND), is governed by the energy difference between BCC and face-centred cubic (FCC) structures and χ robustly captures this key quantity. For BCC TMs alloys, the core structure transition from ND to D occurs when χ drops below a threshold, as seen in atomistic simulations based on nearly all extant interatomic potentials and density functional theory (DFT) calculations of W-Re/Ta alloys. In binary W-TMs alloys, DFT calculations show that χ is related to the valence electron concentration at low to moderate solute concentrations, and can be controlled via alloying. χ can be quantitatively and efficiently predicted via rapid, low-cost DFT calculations for any BCC metal alloys, providing a robust, easily applied tool for the design of ductile and tough BCC alloys. read less

Abstract: Pure zinc oxide (ZnO) nanoparticles have been synthesized by non-aqueous sol-gel route and electrochemical properties were studied for pseudocapacitor applications. The structural, morphological and functional studies were done by XRD, SEM and FTIR respectively. The results showed that the synthesized nanoparticles have good crystallinity, phase pure hexagonal wurtzite structure with highly dispersed nanoparticles with crystallite size about 85 nm. UV–vis DRS spectroscopy results depict that the absorption wavelength gets red shifted. The electrochemical analysis was carried out in three electrode system with the potential window from −0.4 to +0.6 V under 3M KOH aqueous electrolyte. The calculated specific capacitance is very much high about 776 F g−1 at 4 A g−1 current density with 98% capacity retention upto 2500 cycles. Electrochemical impedance spectrum exhibited low ESR value revealed the fast charge transfer kinetics with superior electrical conductivity. These enhanced electrochemical performance highlight the potential of ZnO electrode for the development of supercapacitor applications. read less

Abstract: We have computed Peierls stresses of dislocations in a variety of crystals by applying a new discretized Peierls-Nabarro model (D-PN model) of the dislocation using the calculated ab-initio γ-surface. In the D-PN model, the disregistry distribution of atomic-row pairs facing at the glide plane is determined by the balance between the restoring stress due to the disregistry and the internal stress by an array of small dislocations placed at every intermediate position of atomic-row pairs, instead of continuous distribution of infinitesimal dislocations in the original PN model. Peierls stresses have been obtained by applying the shear stress to the model under which the structure is allowed to relax. The calculated Peierls stresses are largely reduced compared with those previously obtained by the original PN model (Y. Kamimura et al., Acta Mater. 148 (2018) 355-362), although the dislocation widths of the two models are almost the same. They have a good correlation with those experimentally estimated by Y. Kamimura et al. (Acta Mater. 61 (2013) 294-306) except for some very soft crystals, though the calculated Peierls stresses are generally still larger than those of experiments by a few factor. read less

Abstract: The application of a high electrostatic field at the apex of monocrystalline diamond nanoscale needles induces an energy splitting of the photoluminescence lines of color centers. In particular, the splitting of the zero-phonon line of the neutral nitrogen-vacancy complex (NV0) has been studied within a laser-assisted tomographic atom probe equipped with an in situ microphotoluminescence bench. The measured quadratic dependence of the energy splitting on the applied voltage corresponds to the stress generated on the metal-like apex surface by the electrostatic field. Tensile stress up to 7 GPa has thus been measured in the proximity of the needle apex. Furthermore, the stress scales along the needle shank inversely proportionally to its axial cross section. We demonstrate thus a method for contactless piezo-spectroscopy of nanoscale systems by electrostatic field regulation for the study of their mechanical properties. These results also provide an experimental confirmation to the models of dielectrics surface metallization under high electrostatic field. read less

Abstract: New concepts on the irreversible crystal deformation as a structure transformation caused by a change in interatomic interactions at fluctuations of the electron density under loading are described. The change in interatomic interactions lead to the excitation of dynamical displacements of atoms. A model and a theory of a deformable pristine crystal taking into account the excitation of thermally activated and dynamical displacements of atoms are suggested. New mechanisms of the nucleation of plastic deformation carriers and destruction in pristine crystals at the real value of the deforming stress are studied. read less

Abstract: We use time-dependent HRTEM to reveal that stable dislocation pairs in graphene are formed from an initial complex multi-vacancy cluster that undergoes multiple bond rotations and adatom incorporation. In the process, it is found that the transformation from the formed complex multi-vacancy cluster can proceed without the increase of vacancy because many atoms and dimers are not only evaporated but also actively adsorbed. In tight-binding molecular dynamics simulations, it is confirmed that adatoms play an important role in the reconstruction of non-hexagonal rings into hexagonal rings. From density functional theory calculations, it is also found from simulations that there is a favorable distance between two dislocations pointing away from each other (i.e. formed from atom loss). For dislocation pairs pointing away from each other, the hillock-basin structure is more stable than the hillock-hillock structure for dislocation pairs pointing away from each other (i.e. formed from atom loss). read less

Abstract: An acetic-acid-based sol-gel method was used to deposit lead lanthanum zirconate titanate (PLZT, 8/52/48) thin films on either platinized silicon (Pt/Si) or nickel buffered by a lanthanum nickel oxide buffer layer (LNO/Ni). X-ray diffraction and scanning electron microscopy of the samples revealed that dense polycrystalline PLZT thin films formed without apparent defects or secondary phases. The dielectric breakdown strength was greater in PLZT thin films deposited on LNO/Ni compared with those on Pt/Si, leading to better energy storage. Finally, optimized dielectric properties were determined for a 3-μm-thick PLZT/LNO/Ni capacitor for energy storage purposes: DC dielectric breakdown strength of ∼1.6 MV/cm (480 V), energy density of ∼22 J/cc, energy storage efficiency of ∼77%, and permittivity of ∼1100. These values are very stable from room temperature to 150 °C, indicating that cost-effective, volumetrically efficient capacitors can be fabricated for high-power energy storage. read less

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Abstract: Tungsten is a promising candidate material in fusion energy facilities. Molecular dynamics (MD) simulations reveal the atomistic scale mechanisms, so they are crucial for the understanding of the macroscopic property deterioration of tungsten under harsh and complex service environments. The interatomic potential used in the MD simulations is required to accurately describe a wide spectrum of relevant defect properties, which is by far challenging to the existing interatomic potentials. In this paper, we propose a new three-body embedding descriptor and hybridize it into the deep-potential (DP) framework, an end-to-end deep learning interatomic potential model. The potential model for tungsten, named DP-HYB, is trained with a database constructed by the concurrent learning method. The DP-HYB model is able to accurately predict elastic constants, stacking fault energy, the formation energies of free surfaces, and point defects, which are considered in the training dataset. It also accurately evaluates the formation energies of grain boundaries and prismatic loops, the core structure of screw dislocation, the Peierls barrier, and the transition path of the screw dislocation migration, which do not explicitly present in the training dataset. The DP-HYB is a good candidate for the atomistic simulations of tungsten property deterioration, especially those involving the mechanical property degradation under the harsh fusion service environment. read less

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Abstract: Nanoparticles are commonly used in various fields of applications such as electronics, catalysis or engineering where they can be subjected to a certain amount of stress leading to structural instabilities or irreversible damages. In contrast with bulk materials, nanoparticles can sustain extremely high stresses (in the GPa range) and ductility, even in the case of originally brittle materials. This review article focuses on the modeling of the mechanical properties of nanoparticles, with an emphasis on elementary deformation processes. Various simulation methods are described, from classical molecular dynamics calculations, the best suited method when applied to the modeling the mechanics of nanoparticles, to dislocation dynamics based hybrid methodologies. We detail the mechanical behaviour of nanoparticles for a large array of material classes (metals, semi-conductors, ceramics, etc.), as well as their deformation processes. Regular crystalline nanoparticles are addressed, as well as more complex systems such as nanoporous or core-shell particles. In addition to the exhaustive review on the recent works published on the topic, challenges and future trends are proposed, providing solid foundations for forthcoming investigations. read less

Abstract: We show here how density functional theory calculations can be used to predict the temperatureand orientation-dependence of the yield stress of body-centered cubic (BCC) metals in the thermallyactivated regime where plasticity is governed by the glide of screw dislocations with a 1/2 Burgers vector. Our numerical model incorporates non-Schmid effects, both the twinning/antitwinning asymmetry and non-glide effects, characterized through ab initio calculations on straight dislocations. The model uses the stress-dependence of the kink-pair nucleation enthalpy predicted by a line tension model also fully parameterized on ab initio calculations. The methodology is illustrated here on BCC tungsten but is applicable to all BCC metals. Comparison with experimental data allows to highlight both the successes and remaining limitations of our modeling approach. read less

Abstract: In this study, we report atomistic calculations of the core structure of screw dislocations with [001] Burgers vector in Mg2SiO4 olivine. Computations based on the THB1 empirical potential set for olivine show that the stable core configurations of the screw dislocations correspond to a dissociation in {110} planes involving collinear partial dislocations. As a consequence, glide appears to be favorable in {110} planes at low temperature. This study also highlights the difference between dislocation glide mechanism in {110} versus (010) or (100) for which glide is expected to occur through a locking–unlocking mechanism. Résumé. Dans cette étude, nous présentons les résultats de calculs atomiques de la structure de coeur de la dislocation vis de vecteur de Burgers [001] dans l’olivine (Mg,Fe)2SiO4. Nos simulations, reposant sur l’utilisation du potentiel semi-empirique THB1, montrent que la configuration stable de la dislocation vis correspond à une structure de coeur dissociée dans les plans {110}. A basse température, le glissement des dislocations de vecteur de Burgers [001] dans les plans {110} de l’olivine est donc favorisé. read less

Abstract: The temperature variation of the defect densities in a crystal depends on vibrational entropy. This contribution to the system thermodynamics remains computationally challenging as it requires a diagonalization of the system's Hessian which scales as $O({N}^{3})$ for a crystal made of $N$ atoms. Here, to circumvent such a heavy computational task and make it feasible even for systems containing millions of atoms, the harmonic vibrational entropy of point defects is estimated directly from the relaxed atomic positions through a linear-in-descriptor machine learning approach of order $O(N)$. With a size-independent descriptor dimension and fixed model parameters, an excellent predictive power is demonstrated on a wide range of defect configurations, supercell sizes, and external deformations well outside the training database. In particular, formation entropies in a range of $250{k}_{B}$ are predicted with less than $1.6{k}_{B}$ error from a training database whose formation entropies span only $25{k}_{B}$ (training error less than $1.0{k}_{B}$). This exceptional transferability is found to hold even when the training is limited to a low-energy superbasin in the phase space while the tests are performed for a different liquid-like superbasin at higher energies. read less

Abstract: The conditions for the formation of 〈100〉 dislocation loops in body-centered cubic (BCC) iron were investigated via molecular dynamics simulations using a simplified model intended to mimic conditions in high energy collision cascades, focusing on the possible coherent displacement of atoms at the boundary of a subcascade. We report on the formation of 〈100〉 dislocation loops due to the fast displacement of a few hundred atoms with a coherent acceleration, in agreement with previous results for much larger cascade simulations. We analyze in detail the resulting atomic velocities and pressures, and find that they cannot be described within the usual formalism for a shock regime, since the pressure pulse only lasts less than 1 ps and does not match expected values from a Hugoniot shock. Our simulations include two interatomic potentials: Mendelev, which is extensively used for radiation damage simulations, and Ackland, which has been used for shock simulations because it can reproduce the experimentally observed transition from BCC to hexagonal close-packed structure at around 25 GPa, at high deformation rates. They both show similar evolution of defects, also indicating departure from a shock regime which is extremely different for these potentials. 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: 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: The possible synergistic mechanisms for enhancing the ORR performance of 1.0-HZPC-8. read less

Abstract: Cyclotetramethylene tetranitramine (HMX) is an energetic material commonly found in plastic bonded explosives. Its crystallographic sensitivity to initiation and detonation has been linked to plastic slip in the various crystalline planes; shock directions which cannot accommodate substantial crystalline slip are expected to be more sensitive. In this work, we carry out constant stress-constant temperature molecular dynamics simulations of the motion of dislocations and characterize their behavior in the two most probable slip planes, (101) and (011), of this crystal. We evaluate critical thresholds for activating steady-state dislocation motion and the associated dislocation mobilities at various temperatures below 400 K. This information can be used to develop and calibrate larger scale models of plastic deformation in this material. read less

Abstract: We combine density functional theory (DFT) formation energies and empirical potential calculations of vibrational free energies to calculate the free energies of formation of point defects and clusters of oxygen interstitials, and use a dilute defect model to calculate the concentrations of defects as a function of temperature and composition. We find that at high temperature oxygen interstitials are dominant, either in isolated form or in clusters depending on the deviation from stoichiometry. At temperatures lower than 1300 K we predict uranium vacancies to be dominant in the stoichiometric material. The disorder in UO 2 therefore changes from Schottky to Frenkel type with increasing temperature. Uranium vacancies remain dominant up to deviations form stoichiometry as large as 0.045 at 800 K. Moreover, the concentration of uranium vacancies proves to be non-monotonous as a function of composition. These results are consistent with some experimental data on the evolution with stoichiometry of lattice constant, diffusion coefficients of uranium, positron lifetime and dilatometry measurements. read less

Abstract: The line tension model of obstacle hardening is modified to account for the thermally activated, kink-limited glide of 1/2111 screw dislocations, allowing application to the plastic flow of bcc metals. Using atomistically informed dislocation mobility laws, Frenkel-Kontorova simulations, and a simplified dislocation-obstacle model, we identify a size effect for intermediate obstacle densities, where the activation energy for screw dislocation motion halves once the obstacle density falls below a critical value. Our model shows striking agreement with fracture experiments across a wide range of unirradiated and irradiated bcc metals. In particular, we demonstrate that the presence of defects in the crystal lattice can at most double the brittle to ductile transition temperature. read less

Abstract: This review paper focuses on dislocations and plastic deformation in magnesium oxide crystals. MgO is an archetype ionic ceramic with refractory properties which is of interest in several fields of applications such as ceramic materials fabrication, nano-scale engineering and Earth sciences. In its bulk single crystal shape, MgO can deform up to few percent plastic strain due to dislocation plasticity processes that strongly depend on external parameters such as pressure, temperature, strain rate, or crystal size. This review describes how a combined approach of macro-mechanical tests, multi-scale modeling, nano-mechanical tests, and high pressure experiments and simulations have progressively helped to improve our understanding of MgO mechanical behavior and elementary dislocation-based processes under stress. read less

Abstract: A tungsten-rhenium (W-Re) classical interatomic potential is developed within the embedded atom method interaction framework. A force-matching method is employed to fit the potential to ab initio forces, energies, and stresses. Simulated annealing is combined with the conjugate gradient technique to search for an optimum potential from over 1000 initial trial sets. The potential is designed for studying point defects in W-Re systems. It gives good predictions of the formation energies of Re defects in W and the binding energies of W self-interstitial clusters with Re. The potential is further evaluated for describing the formation energy of structures in the σ and χ intermetallic phases. The predicted convex-hulls of formation energy are in excellent agreement with ab initio data. In pure Re, the potential can reproduce the formation energies of vacancies and self-interstitial defects sufficiently accurately and gives the correct ground state self-interstitial configuration. Furthermore, by including liquid structures in the fit, the potential yields a Re melting temperature (3130 K) that is close to the experimental value (3459 K).A tungsten-rhenium (W-Re) classical interatomic potential is developed within the embedded atom method interaction framework. A force-matching method is employed to fit the potential to ab initio forces, energies, and stresses. Simulated annealing is combined with the conjugate gradient technique to search for an optimum potential from over 1000 initial trial sets. The potential is designed for studying point defects in W-Re systems. It gives good predictions of the formation energies of Re defects in W and the binding energies of W self-interstitial clusters with Re. The potential is further evaluated for describing the formation energy of structures in the σ and χ intermetallic phases. The predicted convex-hulls of formation energy are in excellent agreement with ab initio data. In pure Re, the potential can reproduce the formation energies of vacancies and self-interstitial defects sufficiently accurately and gives the correct ground state self-interstitial configuration. Furthermore, by including liqu... read less

Abstract: Atomistic simulations using classical interatomic potentials are powerful investigative tools linking atomic structures to dynamic properties and behaviors. It is well known that different interatomic potentials produce different results, thus making it necessary to characterize potentials based on how they predict basic properties. Doing so makes it possible to compare existing interatomic models in order to select those best suited for specific use cases, and to identify any limitations of the models that may lead to unrealistic responses. While the methods for obtaining many of these properties are often thought of as simple calculations, there are many underlying aspects that can lead to variability in the reported property values. For instance, multiple methods may exist for computing the same property and values may be sensitive to certain simulation parameters. Here, we introduce a new high-throughput computational framework that encodes various simulation methodologies as Python calculation scripts. Three distinct methods for evaluating the lattice and elastic constants of bulk crystal structures are implemented and used to evaluate the properties across 120 interatomic potentials, 18 crystal prototypes, and all possible combinations of unique lattice site and elemental model pairings. Analysis of the results reveals which potentials and crystal prototypes are sensitive to the calculation methods and parameters, and it assists with the verification of potentials, methods, and molecular dynamics software. The results, calculation scripts, and computational infrastructure are self-contained and openly available to support researchers in performing meaningful simulations. read less

Abstract: The calculation of free energy differences for thermally activated mechanisms in the solid state are routinely hindered by the inability to define a set of collective variable functions that accurately describe the mechanism under study. Even when possible, the requirement of descriptors for each mechanism under study prevents implementation of free energy calculations in the growing range of automated material simulation schemes. We provide a solution, deriving a path-based, exact expression for free energy differences in the solid state which does not require a converged reaction pathway, collective variable functions, Gram matrix evaluations, or probability flux-based estimators. The generality and efficiency of our method is demonstrated on a complex transformation of C15 interstitial defects in iron and double kink nucleation on a screw dislocation in tungsten, the latter system consisting of more than 120 000 atoms. Both cases exhibit significant anharmonicity under experimentally relevant temperatures. read less

Abstract: The low temperature diffusivity of nanoscale crystal defects, where quantum mechanical fluctuations are known to play a crucial role, are essential to interpret observations of irradiated microstructures conducted at cryogenic temperatures. Using density functional theory calculations, quantum heat bath molecular dynamics and open quantum systems theory, we evaluate the low temperature diffusivity of self-interstitial atom clusters in tungsten valid down to temperatures of 1 K. Due to an exceptionally low defect migration barrier, our results show that interstitial defects exhibit very high diffusivity of order 10 3 μ m 2 s − 1 over the entire range of temperatures investigated. read less

Abstract: Lithium gets a new ground state For the past 70 years, the lowest-energy crystal structure of lithium was believed to be a relatively complex one called the 9R structure. Ackland et al. show that this is incorrect. The actual lowest-energy structure for lithium is the much simpler closest-packed face-centered cubic form. In addition, 6Li and 7Li isotopes have crystal phase transitions at slightly different pressures and temperatures. This difference is chalked up to large quantum mechanical effects between the isotopes. Lithium is the only metal that shows this type of quantum effect and presents a challenge for theoreticians to explain. Science, this issue p. 1254 Lithium’s ground state has a face-centered cubic structure, and quantum effects alter the phase diagram between the 6Li and 7Li isotopes. The crystal structure of elements at zero pressure and temperature is the most fundamental information in condensed matter physics. For decades it has been believed that lithium, the simplest metallic element, has a complicated ground-state crystal structure. Using synchrotron x-ray diffraction in diamond anvil cells and multiscale simulations with density functional theory and molecular dynamics, we show that the previously accepted martensitic ground state is metastable. The actual ground state is face-centered cubic (fcc). We find that isotopes of lithium, under similar thermal paths, exhibit a considerable difference in martensitic transition temperature. Lithium exhibits nuclear quantum mechanical effects, serving as a metallic intermediate between helium, with its quantum effect–dominated structures, and the higher-mass elements. By disentangling the quantum kinetic complexities, we prove that fcc lithium is the ground state, and we synthesize it by decompression. read less

Abstract: Crystal dislocations govern the plastic mechanical properties of materials but also affect the electrical and optical properties. However, a fundamental and quantitative quantum field theory of a dislocation has remained undiscovered for decades. Here we present an exactly-solvable one-dimensional quantum field theory of a dislocation, for both edge and screw dislocations in an isotropic medium, by introducing a new quasiparticle which we have called the ‘dislon’. The electron-dislocation relaxation time can then be studied directly from the electron self-energy calculation, which is reducible to classical results. In addition, we predict that the electron energy will experience an oscillation pattern near a dislocation. Compared with the electron density’s Friedel oscillation, such an oscillation is intrinsically different since it exists even with only single electron is present. With our approach, the effect of dislocations on materials’ non-mechanical properties can be studied at a full quantum field theoretical level. read less

Abstract: First-Principles Molecular Dynamics (FPMD) methods, although powerful, are notoriously expensive computationally due to the quantum modeling of electrons. Traditional FPMD approaches have typically been limited to a few thousand atoms at most, due to O(N3) or worse solver complexity and the large amount of communication required for highly parallel implementations. Attempts to lower the complexity have often introduced uncontrolled approximations or systematic errors. Using a robust new algorithm, we have developed an O(N) complexity solver for electronic structure problems with fully controllable numerical error. Its minimal use of global communications yields excellent scalability, allowing for very accurate FPMD simulations of more than a million atoms on over a million cores. At these scales, this approach provides multiple orders of magnitude speedup compared to the standard plane-wave approach typically used in condensed matter applications, without sacrificing accuracy. This will open up entire new classes of FPMD simulations, e.g. dilute aqueous solutions. read less

Abstract: A possible role of quantum effects, such as tunneling and zero-point energy, in the structural dynamics of supercooled liquids is studied by dielectric spectroscopy. The presented results demonstrate that the liquids, bulk 3-methyl pentane and confined normal and deuterated water, have low glass transition temperature and unusually low for their class of materials steepness of the temperature dependence of structural relaxation (fragility). Although we do not find any signs of tunneling in the structural relaxation of these liquids, their unusually low fragility can be well described by the influence of the quantum fluctuations. Confined water presents an especially interesting case in comparison to the earlier data on bulk low-density amorphous and vapor deposited water. Confined water exhibits a much weaker isotope effect than bulk water, although the effect is still significant. We show that it can be ascribed to the change of the energy barrier for relaxation due to a decrease in the zero-point energy upon D/H substitution. The observed difference in the behavior of confined and bulk water demonstrates high sensitivity of quantum effects to the barrier heights and structure of water. Moreover, these results demonstrate that extrapolation of confined water properties to the bulk water behavior is questionable. read less

Abstract: ABSTRACT The threshold displacement energy in iron is determined using ab initio molecular dynamics. This is the most fundamental input parameter for radiation damage assessments. The predictions agree well with the available experiments and provide a significantly lower average value for iron than the standard one. This result impacts radiation damage assessments in iron alloys and steels and especially so for dose estimations and conditions close to the threshold. The importance of using an appropriate description of the core and valence electrons is highlighted. Energy loss simulations provide important fitting parameters for improved interatomic potentials. IMPACT STATEMENT Ground-breaking ab initio calculations of the threshold displacement energies in iron show significant differences in the angular anisotropy and predicted average value with respect to previous literature. GRAPHICAL ABSTRACT read less

Abstract: Regularities of the temperature effect on yield strength of nanosized tungsten crystals are ascertained. Experimental values of the yield strength of nanoneedles in the direction [1 1 0] are obtained using the high-field technology at the temperature range 17,4–862 K. It is found that in this range, decrease in strength of tungsten nanoneedles is about 30%. So low, compared with ordinary single crystals, susceptibility of nanoneedles’ strength to change in temperature is due to specific feature of mechanism of transition from elastic to plastic deformations in defect-free nanocrystals. To analyse this mechanism, molecular dynamic simulation of tensile of tungsten nanowire over the temperature range 3–2400 K has been utilized. The thermal fluctuation model of the temperature effect on the strength of nanosized crystals is offered. read less

Abstract: Based on β-NiMoO4 nanowire (NW) arrays grown on carbon cloth as the electrode materials, a 3D solid asymmetrical supercapacitor has been fabricated. The produced β-NiMoO4 NW arrays on carbon cloth compared to the power of NiMoO4 nanowires deposited directively on carbon cloth can increase the efficiency of nanomaterials participating in reactions. The cone-shaped NW arrays have a high specific surface area (99 m2 g−1), which can provide more electroactive sites for Li+ and enhance conductivity through providing short transport and diffusion paths for both ions and electrons. And the cylindrical supercapacitor can allow more β-NiMoO4 NW arrays to saturate with electrolyte to enhance the properties of the supercapacitor. Furthermore, the electrode has a highest energy density of 36.86 W h kg−1, a maximum power density of 1100 W kg−1, and a large capacitance of 414.7 F g−1 at a current density of 0.25 A g−1, all of which demonstrate excellent behavior. And the capacitance of the supercapacitor reached 65.96% of the initial capacitance over 6000 cycles. All of these results indicated that the β-NiMoO4 NW arrays grown on carbon cloth could be a promising candidate for high performance supercapacitors. read less

Abstract: Significant progress in our understanding of plasticity in body-centered cubic (bcc) metals during the last decade has enabled rigorous multiscale modeling based on quantitative physical principles. Significant advances have been made at the atomistic level in the understanding of dislocation core structures and energetics associated with dislocation glide by using high-fidelity models originating from quantum mechanical principles. These simulations revealed important details about the influence of non-Schmid (nonglide) stresses on the mobility of screw dislocations in bcc metals that could be implemented to mesoscopic discrete dislocation simulations with atomistically informed dislocation mobility laws. First applications of dislocation dynamics simulations to studies of plasticity in small-scale bcc single crystals have been performed. Dislocation dynamics simulations inspired the development of continuum models based on advanced 3D dislocation density measures with evolution equations that naturally track dislocation motion. These advances open new opportunities and perspectives for future quantitative and materials-specific multiscale simulation methods to describe plastic deformation in bcc metals and their alloys. read less

Abstract: Hexagonal dislocation networks (HDNs) formed by the reaction of <1 1 1>/2 screw dislocations are frequently observed in association with anomalous slip in body-centred cubic (bcc) metals. However, its role assigned in anomalous slip remains obscure due to the absence of quantitative description of its response to uniaxial loading. Here, systematic atomistic simulations are performed in molybdenum (Mo) to study the responses of a typical HDN to different applied loadings. The simulation results are used to develop a quantitative yield criterion for the HDN motion under uniaxial loading. Based on this criterion together with the yield equation that can account for the non-Schmid behaviours of an isolated <1 1 1>/2 screw dislocation, the transition from primary to anomalous slips with the loading direction is predicted to be consistent with the experimental observations in many bcc metals including Mo. This work also sheds light on other experimental results such as the lack of dead-band and the displacement accompanying anomalous slip. In addition, the reason for the absence of anomalous slip in bcc iron (Fe) is found by comparison of the reaction between <1 1 1>/2 screw dislocations in Mo and Fe. read less

Abstract: extension. Interestingly, we find that the atomistic line tension is more than twice the usual elastic estimates. The calculations also show interesting group tendencies with the line tension and kink-pair width larger in group V than in group VI elements. Finally, the present kink-pair activation energies are shown to compare qualitatively with experimental data and potential origins of quantitative discrepancies are discussed. 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: Atomistic simulations play a crucial role in advancing our understanding of the crack-tip processes that take place during fracture of semi-brittle materials like α-iron. As with all atomistic simulations, the results of such simulations however depend critically on the underlying atomic interaction model. Here, we present a systematic study of eight α-iron embedded atom method potentials used to model cracks subjected to plane strain mode-I loading conditions in six different crystal orientations. Molecular statics simulations are used to determine the fracture behavior (cleavage, dislocation emission, twinning) and the critical stress intensity factor KIc. The structural transformations in front of the crack tips, and in particular the occurrence of {1 1 0} planar faults, are analyzed in detail and related to the strain-dependent generalized stacking fault energy curve. The simulation results are discussed in terms of theoretical fracture criteria and compared to recent experimental data. The different potentials are ranked according to their capability to model the experimentally observed fracture behavior. read less

Abstract: In this study, we model the core structure of screw dislocations with [1 0 0] and [0 1 0] Burgers vector in MgSiO3 perovskite, in the pressure range of Earth's lower mantle (25–130 GPa). We use a generalized Peierls–Nabarro model, called Peierls–Nabarro–Galerkin, based on generalized stacking-fault energy calculations. These stacking-fault energy calculations are performed using a pairwise potential parametrization and compared to ab initio results. The results of Peierls–Nabarro–Galerkin calculations demonstrate that [1 0 0] dislocation and [0 1 0] are, respectively, characterized by a planar core spreading in (0 1 0) and (1 0 0). Our results emphasize the role of [1 0 0](0 1 0) and [0 1 0](1 0 0) slip systems in the deformation mechanism of MgSiO3 perovskite. Furthermore, we validate the use of pairwise potential for further dislocation modelling of such minerals at the atomic scale. read less

Abstract: We have developed empirical interatomic potentials for studying radiation defects and dislocations in tungsten. The potentials use the embedded atom method formalism and are fitted to a mixed database, containing various experimentally measured properties of tungsten and ab initio formation energies of defects, as well as ab initio interatomic forces computed for random liquid configurations. The availability of data on atomic force fields proves critical for the development of the new potentials. Several point and extended defect configurations were used to test the transferability of the potentials. The trends predicted for the Peierls barrier of the 1 2 ⟨ 111 ⟩ ?> screw dislocation are in qualitative agreement with ab initio calculations, enabling quantitative comparison of the predicted kink-pair formation energies with experimental data. read less

Abstract: The plastic deformation of body-centered cubic iron at low temperatures is governed by slip behavior of screw dislocations. Their non-planar core structure gives rise to a strong temperature dependence of the yield stress and overall plastic behavior that does not follow the Schmid law common to most close-packed metals. In this work,we carry out a systematic study of the screw dislocation behavior in α-Fe by means of atomistic simulations using a state-of-the-art magnetic bond-order potential. Based on the atomistic simulations of the screw dislocations under various external loadings, we formulate an analytical yield criterion that correctly captures the non-Schmid plastic response of iron single crystals under general loading conditions. The theoretical predictions of operative slip systems for uniaxial loadings agree well with available experimental observations, and demonstrate the robustness and reliability of such atomistically based yield criterion. In addition, this bottom-up approach can be directly utilized to formulate dislocation mobility rules in mesoscopic discrete dislocation dynamics simulations. read less

Abstract: Dislocation, playing a crucial role in the plastic deformation of metals, can be significantly affected by introducing solute elements. Hydrogen (H) embrittlement is one such example, while the underlying mechanism for H affected dislocation structural stability and mobility remains unclear and the role of H has been controversial. Here, using first-principles calculations, we demonstrate that the effect of H on screw dislocation in bcc metals is H concentration-dependent, signified by a H-induced transition of SD core structure. At low concentrations of H segregation, dislocation maintains the intrinsic easy-core structure, and H atoms are attached to the "periphery" of dislocation to enhance dislocation motion. In contrast, at high H concentrations, dislocation transforms into a hard-core, metal hydride-like structure, as H atoms become the "body" of dislocation to significantly reduce the dislocation mobility. Further, such local easy-to-hard transition is found to be induced by just one solute of other elements, including helium, carbon, nitrogen and oxygen, independent of solute concentration. Our work sheds new light on the H-dislocation interactions in bcc metals, having broad implications in the interstitial solute-related phenomena. 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