Abstract: The effects of neutron irradiation on materials are often interpreted in terms of atomic recoils, initiated by neutron impacts and producing crystal lattice defects. In addition, there is a remarkable two-step process, strongly pronounced in the medium-weight and heavy elements. This process involves the generation of energetic {\gamma} photons in nonelastic collisions of neutrons with atomic nuclei, achieved via capture and inelastic reactions. Subsequently, high-energy electrons are excited through the scattering of {\gamma} photons by the atomic electrons. We derive and validate equations enabling a fast and robust evaluation of photon and electron fluxes produced by the neutrons in the bulk of materials. The two-step n-{\gamma}-e scattering creates a nonequilibrium dynamically fluctuating steady-state population of high-energy electrons, with the spectra of photon and electron energies extending well into the mega-electron-volt range. This stimulates vacancy diffusion through electron-triggered atomic recoils, primarily involving vacancy-impurity dissociation, even if thermal activation is ineffective. Tungsten converts the energy of fusion or fission neutrons into a flux of {\gamma} radiation at the conversion efficiency approaching 99%, with implications for structural materials, superconductors, and insulators, as well as phenomena like corrosion, and helium and hydrogen isotope retention. read less

Abstract: The objective of this study was to develop a micromechanical approach for determining the Mie–Grüneisen EOS parameters of iron under the Hugoniot states. The multiscale shock technique (MSST) coupled with molecular dynamics (MD) simulations was employed to describe the shocked Hugoniot relation of single-crystal (SC) and nanocrystalline (NC) iron under high pressures. The Mie–Grüneisen equation of state (EOS) parameters, the cold pressure (Pc), the cold energy (Ec), the Grüneisen coefficient (γ), and the melting temperature (Tm) are discussed. The error between SC and NC iron results was found to be less than 1.5%. Interestingly, the differences in Hugoniot state (PH) and the internal energy between SC and NC iron were insignificant, which shows that the effect of grain size (GS) under high pressures was not significant. The Pc and Ec of SC and NC iron calculated based on the Morse potential were almost the same with those calculated based on the Born–Mayer potential; however, those calculated based on the Born–Mayer potential were a little larger at high pressures. In addition, several empirical and theoretical models were compared for the calculation of γ and Tm. The Mie–Grüneisen EOSs were shown on the 3D contour space; the pressure obtained with the Hugoniot curves as the reference was larger than that obtained with the cold curves as the reference. read less

Abstract: The torsion of pristine α-Fe nanowires was studied by molecular dynamics simulations. Torsion-induced plastic deformation in pristine nanowires is divided into two regimes. Under weak torsion, plastic deformation leads to dislocation nucleation and propagation. Twisting-induced dislocations are mainly 12<111> screw dislocations in a <112>-oriented nanowire. The nucleation and propagation of these dislocations were found to form avalanches which generate the emission of energy jerks. Their probability distribution function (PDF) showed power laws with mixing between different energy exponents. The mixing stemmed from simultaneous axial and radial dislocation movements. The power-law distribution indicated strongly correlated ‘wild’ dislocation dynamics. At the end of this regime, the dislocation pattern was frozen, and further twisting of the nanowire did not change the dislocation pattern. Instead, it induced local amorphization at the grip points at the ends of the sample. This “melting” generated highly dampened, mild avalanches. We compared the deformation mechanisms of twinned and pristine α-Fe nanowires under torsion. read less

Abstract: A molecular dynamics (MD) simulation study was performed to investigate the effects of helium (He) on the mechanical properties of nanocrystalline body-centered cubic iron (BCC Fe). Simulated X-ray diffraction (XRD) was used to explore the relationship between the generation of cracks and the change of the crystal structure in nanocrystalline BCC Fe during tensile deformation. It is observed that the peak stress and the elastic modulus decrease with increasing concentration of He atoms, which are introduced into the grain boundary (GB) region of nanocrystalline Fe. The generation and connection of intergranular cracks are enhanced by He atoms. Significant peak separation, which is associated with the generation of cracks, is found in the simulated XRD patterns of nanocrystalline Fe during the tensile process. The lower diffraction angle of the {211}′ peak suggests a more serious lattice distortion during loading. For all nanocrystalline Fe deformed to 6% strain, the degree and fraction of the lattice distortion increases with the increasing loading stress. read less

Abstract: Keenan Lyon, ∗ Anders Bergman, Paul Zeiger, Demie Kepaptsoglou, 3 Quentin M. Ramasse, 4, 5 Juan Carlos Idrobo, and Ján Rusz † Department of Physics and Astronomy, Uppsala University, Lägerhyddsvägen 1, Uppsala, Sweden SuperSTEM Laboratory, SciTech Daresbury Campus, Daresbury WA4 4AD, United Kingdom Department of Physics, University of York, York YO10 5DD, United Kingdom School of Chemical and Process Engineering, University of Leeds, Leeds LS2 9JT, United Kingdom School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, United Kingdom Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN, USA (Dated: May 11, 2021) read less

Abstract: While the crystal–melt interface kinetic equation predicts various kinetic behaviors, the realization of these scenarios and the corresponding thermodynamic conditions remain unclear. In this work, six representative interface kinetic behaviors of Fe were modeled and examined by molecular dynamics simulations. For the flat interface, several models were designed to study the migration, fluctuation, and recovery of the interface. For the cylindrical or curved interface, different models were also designed to test the equilibrium, migration, and instability of the interface. By comparing the kinetic behaviors of the two types of interfaces, we can observe the effect of interface curvature. During the simulations, two crucial material-specific parameters, the crystal–melt interface free energy and kinetic coefficient, were determined and compared among different models. read less

Abstract: ABSTRACT Pop-in is a widely observed phenomenon in nanoindentation. In this paper, dislocation evolution in pop-in processes is analysed in detail through molecular dynamics (MD) simulations. We found that a large number of dislocations nucleate homogeneously at the initiation of pop-in, followed by extensive dislocation propagation, which is the dominant mode of plastic deformation during pop-in. Moreover, we noted that establishing the correct dislocation evolution mechanisms of pop-in can serve to explain the overshoot phenomenon observed in nanoindentation experiments. Through our MD analysis on the obtained dislocation structures, therefore, we were able to propose a model that can predict the total length of dislocations associated with the plastic processes underneath a spherical indenter. In addition, the Taylor model was used to verify that our proposed dislocation length model sits well with the MD simulated force-displacement curves of nanoindentation. In fact, the MD simulated linear relation between critical force and indentation depth during pop-in is consistent with the Hertzian and Taylor models. Our MD simulations, therefore, can provide significant insight into the experimentally observed pop-in phenomena. read less

Abstract: In this work, we conducted a high-throughput atomistic simulation of the interstitial solid solutions of hydrogen in α-Fe. The elastic constants and moduli were calculated. Through statistical analysis of structures and results, the influences of the microscopic distribution of hydrogen on the elastic moduli, as well as hydrogen-induced hardening and softening, are discussed. We found that even though the uniformly distributed hydrogen caused slight softening in α-Fe, the distribution of hydrogen at different adjacent positions significantly affected the elastic moduli. For example, hydrogen increased the Young’s modulus and shear modulus at the 5th and 10th nearest neighbors, resulting in hardening, but decreased the bulk modulus at the 7th nearest neighbor, making the material easier to compress. These phenomena are related to the distribution densities of the positions that hydrogen atoms can occupy on the two major slip families, {110} and {112}, at different nearest neighbors distinguished by distances. read less

Abstract: Volume changes accompanying the plastic deformation at 300 K of nanocrystalline samples of α-Fe with a columnar grain structure possessing a ⟨11¯0⟩ random fiber texture has been obtained from molecular dynamics (MD) simulations. The samples were strained in tension along the common axial direction of the columnar grains. After removal of the elastic volume change, the evolution of plastic volume strain was obtained. Small but non-negligible volume dilations or contractions are observed depending on size (samples of very small grain size show volume contraction). The rate of volume change is high during the first 10% plastic deformation and continues at a low pace thereafter; the first 10% deformation represents a transient in the stress–strain behavior too. The complex behavior observed is reasonably explained by the superposition of contributions from different plastically-induced structural changes to the mass density change: Mainly from changes of grain size, grain boundary structure, dislocation density and density of point-defects. The results are of interest for the development of crystal plasticity theories not restricted by the volume conserving assumption. read less

Abstract: NanoMaterialsCAD is a new open source tool dedicated to the creation, manipulation, and 3D visualization of crystalline structures at the nanoscale. It is designed for preprocessing atomistic configurations to be used as input for atomistic (e.g., molecular dynamics or Monte Carlo) or ab initio (e.g., density functional theory) computer simulations. It offers several tools for designing complex nanostructures (including nanoparticles, nanowires, nanotubes, nanoscrolls, etc., and combinations/permutations of them) which are either lacking or cumbersome in other existing packages. Through its intuitive graphical user interface (GUI) it enables facile ways to design and modify the size/shape and relative positions of nanoobjects while observing the changes in real time. NanoMaterialsCAD is written in C++, and exploits Open Graphics Library (OpenGL) (for the GUI), Win32API (for interaction with Windows), and Assembly (for fast data management). The source code and executable file are available for download from GitHub (https://github.com/cossphy/NanoMaterialsCAD). It is aspired that NanoMaterialsCAD will be adopted by the nanomaterials modeling community as a valuable resource; to this end it will be kept improving, incorporating more nanostructures, and adding extra functionalities to its toolbox. read less

Abstract: The atomic structure variations on cooling, vitrification and crystallization processes in liquid metals face centered cubic (FCC) Cu are simulated in the present work in comparison with body centered cubic (BCC) Fe. The process is done on continuous cooling and isothermal annealing using a classical molecular-dynamics computer simulation procedure with an embedded-atom method potential at constant pressure. The structural changes are monitored with direct structure observation in the simulation cells containing from about 100 k to 1 M atoms. The crystallization process is analyzed under isothermal conditions by monitoring density and energy variation as a function of time. A common-neighbor cluster analysis is performed. The results of thermodynamic calculations on estimating the energy barrier for crystal nucleation and a critical nucleus size are compared with those obtained from simulation. The differences in crystallization of an FCC and a BCC metal are discussed. read less

Abstract: In the current study, the effect of hydrogen atoms on the intergranular failure of α-iron is examined by a molecular dynamics (MD) simulation. The effect of hydrogen embrittlement on the grain boundary (GB) is investigated by diffusing hydrogen atoms into the grain boundaries using a bicrystal body-centered cubic (BCC) model and then deforming the model with a uniaxial tension. The Debye Waller factors are applied to illustrate the volume change of GBs, and the simulation results suggest that the trapped hydrogen atoms in GBs can therefore increase the excess volume of GBs, thus enhancing intergranular failure. When a constant displacement loading is applied to the bicrystal model, the increased strain energy can barely be released via dislocation emission when H is present. The hydrogen pinning effect occurs in the current dislocation slip system, <111>{112}. The hydrogen atoms facilitate cracking via a decrease of the free surface energy and enhance the phase transition via an increase in the local pressure. Hence, the failure mechanism is prone to intergranular failure so as to release excessive pressure and energy near GBs. This study provides a mechanistic framework of intergranular failure, and a theoretical model is then developed to predict the intergranular cracking rate. read less

Abstract: Carbon nanotube (CNT) reinforced metal matrix composites (MMCs) are gaining the attention of the researchers because of their demand in space and automobile industries for having low weight and high mechanical properties. Iron is the most used metal in all engineering fields. Therefore, reinforcing iron with CNT can reduce its required amount, which might have a positive economic impact due to the reduced cost of production. However, before the industrial application of any material the mechanical properties under different conditions must be known. In this study, the mechanical properties of iron reinforced separately with single, double and triple wall CNTs are investigated by Molecular Dynamics (MD) simulation. The study revealed that the strength and stiffness of pure iron could be enhanced up to 80.4 % and 57.4 %, respectively, by adding CNTs into iron. We also investigated the effect of fiber volume percentage and temperature on the mechanical properties of the composite having single, double and triplewalled carbon nanotubes individually. As the stone-wales and bi-vacancy defects are inherently introduced in CNTs during manufacturing, their effect on mechanical properties are also investigated in the present study read less

Abstract: Tungsten (W) and W-based alloys are potential candidates for next-generation fusion reactors, which would withstand both irradiation damages and heavy heat load. In this work, we employed the molecular dynamic method to simulate the behaviors of different radiation defects under the effect of the temperature gradient field, which is induced by heat load. The rotation of the ⟨ 111 ⟩ dumbbell and habit plane of 1/2 ⟨ 111 ⟩ interstitial loops is analyzed in detail. The results show that these two behaviors are not significantly affected by the temperature gradient. Contrary to the thermal equilibrium state, temperature gradient results in the directional diffusion of ⟨ 111 ⟩ dumbbell and 1/2 ⟨ 111 ⟩ interstitial loops in tungsten from the cold to the hot region. The energy barrier is also reduced in the temperature gradient field, which accelerates the defect diffusion. These results indicate that the accumulation of radiation defects in the high-temperature region is expected in temperature gradient fields, which would lead to more severe radiation damages and degradation of materials. read less

Abstract: In the present work, the mechanical properties of nanocrystalline body-centered cubic (BCC) iron with an average grain size of 10 Å were investigated using molecular dynamics (MD) simulations. The structure has one layer of crystal grains, which means such a model could represent a structure with directional crystallization. A series of uniaxial tensile tests with different strain rates and temperatures was performed until the full rupture of the model. Moreover, tensile tests of the models with a void at the center and shear tests were carried out. In the tensile test simulations, peak stress and average values of flow stress increase with strain rate. However, the strain rate does not affect the elasticity modulus. Due to the presence of void, stress concentrations in structure have been observed, which leads to dislocation pile-up and grain boundary slips at lower strains. Furthermore, the model with the void reaches lower values of peak stresses as well as stress overshoot compared to the no void model. The study results provide a better understanding of the mechanical response of nanocrystalline BCC iron under various loadings. read less

Abstract: Characteristics of contact surface are important for the operation of precision machines, which need to transmit force and torque, such as the interface contact between rotating disks of gas turbine rotor system. Meanwhile, the contact behavior of microscopic contact interface is proven to be different from that of macroscopic contact. This work presents molecular dynamic simulation of microscopic contact behaviors with single-asperity, including single-asperity contact with same asperity radius, single-asperity contact with different asperity radius and effect of overlapping ratio of the adjacent asperity. Through molecular dynamic simulation results, comparisons are conducted between microscopic contact and macroscopic contact behavior as well as evolution process of the dislocation. The simulation results show that dislocation has important effects on single-asperity contact, resulting in the reduction of contact force comparing to Hertz contact theory. read less

Abstract: ABSTRACT The interaction between the crack and the grain boundary has been investigated by molecular dynamics simulation. The focus of this research is to study fracture resistance of grain boundary in a three-dimensional pre-cracked Fe-bicrystal. The fracture resistance of Σ5 < 100> {013} symmetric tilt grain boundary (STGB) has been compared with Σ5 < 100> {012} STGB in terms of crack length-time curve, temperature per time diagram, and the stress–strain curve to understand the detailed mechanism of fracture in Fe-bicrystal. Crack delay time at the grain boundary is proposed as a parameter for comparing the fracture resistance of grain boundaries. The results show that the crack delay time at the grain boundary is inversely related to the grain boundary energy. Hence, crack delay time for Σ5{013} STGB with 986 energy is more than Σ5{012} STGB with 1098 energy. The findings show that Σ5{013} STGB resists more than Σ5{012} STGB against crack propagation. The required stress, which is needed to overcome the grain boundary resistance and cause the crack penetration to the adjacent grain, has been calculated by using stress–strain curve. Modified BCC defect analysis algorithm and centrosymmetry parameter are also employed to analyze propagated defects and their interaction with the grain boundary. read less

Abstract: We develop a set of machine-learning interatomic potentials for elemental V, Nb, Mo, Ta, and W using the Gaussian approximation potential framework. The potentials show good accuracy and transferability for elastic, thermal, liquid, defect, and surface properties. All potentials are augmented with accurate repulsive potentials, making them applicable to radiation damage simulations involving high-energy collisions. We study melting and liquid properties in detail and use the potentials to provide melting curves up to 400 GPa for all five elements. read less

Abstract: Using molecular dynamics simulation, we study the cutting of an Fe single crystal using tools with various rake angles α . We focus on the (110)[001] cut system, since here, the crystal plasticity is governed by a simple mechanism for not too strongly negative rake angles. In this case, the evolution of the chip is driven by the generation of edge dislocations with the Burgers vector b = 1 2 [ 111 ] , such that a fixed shear angle of ϕ = 54.7 ∘ is established. It is independent of the rake angle of the tool. The chip form is rectangular, and the chip thickness agrees with the theoretical result calculated for this shear angle from the law of mass conservation. We find that the force angle χ between the direction of the force and the cutting direction is independent of the rake angle; however, it does not obey the predictions of macroscopic cutting theories, nor the correlations observed in experiments of (polycrystalline) cutting of mild steel. Only for (strongly) negative rake angles, the mechanism of plasticity changes, leading to a complex chip shape or even suppressing the formation of a chip. In these cases, the force angle strongly increases while the friction angle tends to zero. read less

Abstract: Only few available interatomic interaction potentials implement the α ↔ γ phase transformation in iron by featuring a stable low-temperature bcc and high-temperature fcc lattice structure. Among these are the potentials by Meyer and Entel (1998 Phys. Rev. B 57 5140), by Müller et al (2007 J. Phys.: Condens. Matter 19 326220) and by Lee et al (2012 J. Phys.: Condens. Matter 24 225404). We study how these potentials model the phase transformation during heating and cooling; in order to help initiating the transformation, the simulation volume contains a grain boundary. For the martensitic transformation occurring on cooling an fcc structure, we additionally study two potentials that only implement a stable bcc structure of iron, by Zhou et al (2004 Phys. Rev. B 69 144113) and by Mendelev et al (2003 Philos. Mag. 83 3977). We find that not only the transition temperature depends on the potential, but that also the height of the energy barrier between fcc and bcc phase governs whether the transformation takes place at all. In addition, details of the emerging microstructure depend on the potential, such as the fcc/hcp fraction formed in the α → γ transformation, or the twinning induced in and the lattice orientation of the bcc phase in the γ → α transformation. read less

Abstract: The concept of "interfacial region" has long prevailed for over half century for describing the homogeneous solid-liquid (SL) interface of metals, but its intrinsic structure is still unclear due to the homogeneity. In this study, we reveal, for the first time, the intrinsic structure of these homogeneous SL interfaces consisting of two conjugated monoatomic layers of interfacial solid (IS) and interfacial liquid (IL) with a certain degree of corrugation via molecular dynamics simulations. We named it as the conjugated bilayer structure (CBS). In the framework of CBS, only the IS + IL bilayer plays stepwise transition roles from the solid to the liquid, which defines the four-terrace nature of the interface and act simultaneously as the boundaries of the bilateral bulk phases. The inherent diffuse nature of the "interfacial region" is proven originating from the corrugation of the IS + IL bilayer and its four-terrace nature. More importantly, the CBS also explains that the interfacial free energy originates mainly from the increase in the potential energy of the IS layer relative to its counterpart bulk solid instead of the previously argued entropy loss of the liquid phase. After all these verifications and interpretations, the CBS was verified as the intrinsic structure of the homogeneous SL interface of metals. Accordingly, we argue that the concept of CBS also resolves the volume-bearing flaw of the "interfacial region" concept and can definitively locate the intrinsic surface according to the capillary wave theory. read less

Abstract: Molecular dynamics simulations are used to study the effects of tensile loading on nucleation and subsequent growth of bcc phase in pure fcc iron. The results show that orientation variant selection occurs during the stress-induced fcc-to-bcc transformation, which leads to the coalescence of neighbouring bcc platelets with identical orientation. The bcc phase nucleates mainly following Nishiyama–Wassermann and Kurdjumov–Sachs orientation relationships with the parent fcc phase. The present simulations contribute to a better understanding of mechanisms controlling mechanically induced martensitic transformation as well as coalescence of bcc platelets in steels. 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: The deformation behaviour of twinned FCC nanowires has been extensively investigated in recent years. However, the same is not true for their BCC counterparts. Very few studies exist concerning the deformation behaviour of twinned BCC nanowires. In view of this, molecular dynamics (MD) simulations have been performed to understand the deformation mechanisms in twinned BCC Fe nanowires. The twin boundaries (TBs) were oriented parallel to the loading direction [110] and the number of TBs is varied from one to three. MD simulation results indicate that deformation under the compressive loading of twinned BCC Fe nanowires is dominated by a unique de-twinning mechanism involving the migration of a special twin–twin junction. This de-twinning mechanism results in the complete annihilation of pre-existing TBs along with reorientation of the nanowire. Further, it has been observed that the annihilation of pre-existing TBs has occurred through two different mechanisms, one without any resolved shear stress and other with finite and small resolved shear stress. The present study enhances our understanding of de-twinning in BCC nanowires. read less

Abstract: ABSTRACT There are two types of pop-in mode that have been widely observed in nanoindentation experiments: the single pop-in, and the successive pop-in modes. Here we employ the molecular dynamics (MD) modelling to simulate nanoindentation for three face-centred cubic (FCC) metals, including Al, Cu and Ni, and two body-centred cubic (BCC) metals, such as Fe and Ta. We aim to examine the deformation mechanisms underlying these pop-in modes. Our simulation results indicate that the dislocation structures formed in single crystals during nanoindentation are mainly composed of half prismatic dislocation loops. These half prismatic dislocation loops in FCC metals are primarily constituted of extended dislocations. Lomer–Cottrell locks that result from the interactions between these extended dislocations can resist the slipping of half dislocation loops. These locks can build up the elastic energy that is needed to activate the nucleation of new half dislocation loops. A repetition of this sequence results in successive pop-in events in Al and other FCC metals. Conversely, the half prismatic dislocation loops that form in BCC metals after first pop-in are prone to slip into the bulk, which sustains plastic indentation process after first pop-in and prevents subsequent pop-ins. We thus conclude that pop-in modes are correlated with lattice structures during nanoindentation, regardless of their crystal orientations. read less

Abstract: New generation nuclear fission and future fusion reactors provide one approach to address the world's increasing energy requirements. The irradiation of fission/fusion components can lead to fundamental changes in material properties that affect the stability and performance of not only the material component but that of the entire reactor. How does the material state evolve with respect to the irradiation dose, and can there exist a microstructure resistant to further irradiation? The present work develops a new computationally efficient approach to answer these questions. read less

Abstract: Subsurface voids may strongly affect the response of materials to nanoindentation. We explore these effects for a bcc single-crystalline Fe sample using molecular dynamics simulation. Deformation occurs mainly by nucleation and propagation of dislocations. As dislocations impinge into the voids, these suffer a reduction in volume, consistent with mass transfer mechanisms. Our results show that voids act as highly efficient absorbers of dislocations, effectively limiting the extension of the plastic zone. Surprisingly, mechanical properties are marginally affected by the presence of voids in the range of sizes and spatial distributions tested, except for voids a few nanometers below the surface. Deformation twinning is observed as a transient effect in some cases; however, for voids close enough to the indentation area, no twinning was found. read less

Abstract: Significance We demonstrate that an accurate estimation of the displacements associated with the transformation of liquid into crystal is necessary to explain the striking variations in the temperature dependence of the addition rate of liquid atoms to the growing crystal interface during freezing. An assignment algorithm, adapted from operations theory, is shown to provide a good estimate of these atomic displacements. As the assignment algorithm requires only initial and target locations, it is applicable to all forms of structural transformations. In resolving a fundamental feature of the kinetics of freezing, a phenomenon of central importance to material fabrication, this paper also provides the tools to open lines of research into the kinetics of structural transformation. The atomic displacements associated with the freezing of metals and salts are calculated by treating crystal growth as an assignment problem through the use of an optimal transport algorithm. Converting these displacements into timescales based on the dynamics of the bulk liquid, we show that we can predict the activation energy for crystal growth rates, including activation energies significantly smaller than those for atomic diffusion in the liquid. The exception to this success, pure metals that freeze into face-centered cubic crystals with little to no activation energy, are discussed. The atomic displacements generated by the assignment algorithm allows us to quantify the key roles of crystal structure and liquid caging length in determining the temperature dependence of crystal growth kinetics. read less

Abstract: Studying the shape-dependent structural and magnetic properties of nanoparticles is one of the most necessary scientific challenges in order to match these nano-objects for adequate applications. In this research paper, the shape effect of iron nanoparticles (FeNPs) on structural and magnetic properties was investigated on the basis of a combination of Molecular Statics (MS) and Monte Carlo (MC) simulations. To this end, three kinds of FeNP shapes (such as spherical, planar and rod) in an equal volume have been considered. The coordination number distribution of FeNPs obtained from the data extracted by MS simulations was exploited for performing MC simulations on the familiar Ising model. The numerical findings obtained showed that the structural stability, the Curie temperature as well as the shape of the hysteresis loop are correlated with the FeNP shape. read less

Abstract: Using molecular dynamics simulation, we study the influence of tip adhesion on nanoindentation and scratching. By using a model pair potential between tip atoms and substrate atoms, we can arbitrarily change the adhesion strength. For the prototypical case of a diamond tip and a bcc Fe substrate, we find that with increasing adhesion strength, the indentation hardness and also the normal hardness during scratching decreases. Even more pronounced is a strong increase of the transverse force and hence of the friction coefficient during scratching. The indent pit becomes atomically rough, and the pileup produced during scratch increases with increasing adhesion strength. On the other hand, the length of the dislocations produced and the spatial extent of the plastic zone shrinks. read less

Abstract: Cyclic indentation is a technique used to characterize materials by indenting repeatedly on the same location. This technique allows information to be obtained on how the plastic material response changes under repeated loading. We explore the processes underlying this technique using a combined experimental and simulative approach. We focus on the loading–unloading hysteresis and the dependence of the hysteresis width ha,p on the cycle number. In both approaches, we obtain a power-law demonstrating ha,p with respect to the hardening exponent e. A detailed analysis of the atomistic simulation results shows that changes in the dislocation network under repeated indentation are responsible for this behavior. read less

Abstract: Plastic deformation may affect bulk diffusion in nanocrystalline materials by altering the rates of point defect production and annihilation. In the present work, a detailed analysis of this phenomenon is given by a series of molecular dynamics (MD) simulations to clarify the effect of preplastic strain on the diffusivity of iron atoms. The embedded atom method interatomic potential was used to perform MD simulations. The self-diffusion coefficient of iron atoms in unstrained and prestrained samples was measured over temperatures ranging from 600 to 1000 K and at total strains of 5%–20%. The results reveal that the diffusivity is indeed enhanced as a result of plastic straining, especially at low temperatures. The calculated diffusion coefficient of iron atoms in the prestrained samples is 10–80 times higher than that in the unstrained samples. The atomic structure analysis results indicated that the generation of excess vacancies and unstructured region by preplastic deformation contributes to the enhancement of self-diffusion under plastic straining conditions. At low temperatures, preplastic straining has a considerable effect in the peak shifting and broadening of the radial distribution function, which probably lowers the activation barrier height for diffusion.Plastic deformation may affect bulk diffusion in nanocrystalline materials by altering the rates of point defect production and annihilation. In the present work, a detailed analysis of this phenomenon is given by a series of molecular dynamics (MD) simulations to clarify the effect of preplastic strain on the diffusivity of iron atoms. The embedded atom method interatomic potential was used to perform MD simulations. The self-diffusion coefficient of iron atoms in unstrained and prestrained samples was measured over temperatures ranging from 600 to 1000 K and at total strains of 5%–20%. The results reveal that the diffusivity is indeed enhanced as a result of plastic straining, especially at low temperatures. The calculated diffusion coefficient of iron atoms in the prestrained samples is 10–80 times higher than that in the unstrained samples. The atomic structure analysis results indicated that the generation of excess vacancies and unstructured region by preplastic deformation contributes to the enhanc... read less

Abstract: Molecular dynamics simulations of the temperature dependent crystal growth rates of the salts, NaCl and ZnS, from their melts are reported, along with those of a number of pure metals. The growth rate of NaCl and the FCC-forming metals show little evidence of activated control, while that of ZnS and Fe, a BCC forming metal, exhibit activation barriers similar to those observed for diffusion in the melt. Unlike ZnS and Fe, the interfacial inherent structures of NaCl and Cu and Ag are found to be crystalline. We calculate the median displacement between the interfacial liquid and crystalline states and show that this distance is smaller than the cage length, demonstrating that crystal growth in the fast crystallizers can occur via local vibrations and so largely avoid the activated kinetics associated with the larger displacements associated with particle transport. read less

Abstract: An important aspect of atomistic simulations of dislocations is the construction of the initial dislocation configurations. However, limited configurations can be constructed by previous methods, impeding the simulations of a general dislocation configuration in real materials. In this paper, we develop a simple and general method for constructing dislocations with arbitrary shapes specified by the users, realising the Volterra process at the atomic level. Examples of its applications to a dislocation helix, the partial dislocations, the multi-dislocation configurations, and the dislocations in the imperfect crystal are presented, showing the capacity and robustness of the present method. read less

Abstract: ABSTRACT The effect of Cr and Ni content on thermo-mechanical properties of FeNiCr austenitic stainless steel under ambient and high pressure and temperature were investigated by MD simulations. The FCC structure was selected as optimum structure for FeNiCr system based on obtained MD results from Bonny EAM potential and valid experimental results. The structural and mechanical properties of pure Fe, Ni, and Cr were also estimated based on this potential, indicating good agreement with experimental results. These properties were computed for four experimental case studies which showed less than 10% error. Moreover, the elastic constants of the Fe–(8–18)Ni–(18–25)Cr systems were estimated. Results showed that bulk modulus increases by increasing the Ni and Cr contents, which can be connected to the changes in bonding electrons. The thermal properties of FeNiCr were calculated in ambient and high pressure. Although thermo-mechanical properties confirm good agreement with experimental results at the ambient condition, however, they indicate that FeNiCr Bonny potential is not applicable at high pressure. In order to tackle this issue, a hybrid potential was used at high Pressure/Temperature. The results illustrate enhanced mechanical properties, increase of melting point and reduction of LTE in high pressure and deteriorated mechanical properties at high temperature. read less

Abstract: ABSTRACT Bimetallic nanoparticles comprised of two elements which are immiscible in the bulk present a unique combination of physical–chemical properties that strongly depend on the atomic arrangement within the particle. In this study, molecular dynamics (MD) simulations of bimetallic Fe–Cu nanoparticles formation by high-velocity collision of individual metal nanoparticles (IMNPs) were performed. Physically these conditions model fast electrical explosion of two metal wires (Fe and Cu). By varying the size, temperature and velocity of colliding IMNPs, the conditions under which phase-segregated Janus nanoparticles are formed were determined. The model predictions showed good agreement with the experimental results. The present work is a step forward to understanding the formation mechanisms of bimetallic nanoparticles with different chemical configurations. read less

Abstract: Helium (He) effect on the microstructure of nanocrystalline body-centered cubic iron (BCC-Fe) was studied through Molecular Dynamics (MD) simulation and simulated X-ray Diffraction (XRD). The crack generation and the change of lattice constant were investigated under a uniaxial tensile strain at room temperature to explore the roles of He concentration and distribution played in the degradation of mechanical properties. The simulation results show that the expansion of the lattice constant decreases and the swelling rate increases while the He in the BCC region diffuses into the grain boundary (GB) region. The mechanical property of nanocrystalline BCC-Fe shows He concentration and distribution dependence, and the existence of He in GB is found to benefit the generation and growth of cracks and to affect the strength of GB during loading. It is observed that the reduction of tensile stress contributed by GB He is more obvious than that contributed by grain interior He. read less

Abstract: Friction reduction is more than ever a key point in saving natural resources and energy, and the question of how to achieve this concerns first and foremost every lubricated system. Among the observed phenomena related to lubricated friction, limiting shear stress (LSS) appears to be one of the most challenging to explain, since its origin is still uncertain. Various scenarios have been proposed involving a transition to a glassy state under high pressure, shear banding, shear localization, or even the occurrence of slip at the solid-liquid interface, none of which have proven conclusive. This work provides new insights into the mechanisms leading to LSS and the underlying flow organization. It bridges a gap between the scenarios previously discussed in the literature to explain the mechanisms behind LSS and friction experiments, which provide macroscopic results only. We first present some general molecular dynamics (MD) simulations developed to characterize molecular fluids in their bulk state and then proceed to study their response under severe shearing and confinement. Results from simulations are compared to experimental data derived from friction tests. A further analysis of the pressure and temperature involved hints that LSS and the physical state of the lubricant are strongly interconnected concepts. Additionally, the friction results were uncorrelated to the choice of surfaces, contrary to velocity distribution. read less

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

Abstract: Coincidence site lattice (CSL) grain boundaries (GBs) are believed to be low-energy, resistant to intergranular fracture, as well as to hydrogen embrittlement. Nevertheless, the behavior of CSL-GBs are generally confused with their angular deviations. In the current study, the effect of angular deviation from the perfect ∑3(111)[110] GBs in α-iron on the hydrogen diffusion and the susceptibility of the GB to hydrogen embrittlement is investigated through molecular static and dynamics simulations. By utilizing Rice-Wang model, it is shown that the ideal GB shows the highest resistance to decohesion below the hydrogen saturation limit. Finally, the hydrogen diff usivity along the ideal GB is observed to be the highest. read less

Abstract: We use DFT to compute core structures of $a_0[100](010)$ edge, $a_0[100](011)$ edge, $a_0/2[\bar{1}\bar{1}1](1\bar{1}0)$ edge, and $a_0/2[111](1\bar{1}0)$ $71^{\circ}$ mixed dislocations in bcc Fe. The calculations use flexible boundary conditions (FBC), which allow dislocations to relax as isolated defects by coupling the core to an infinite harmonic lattice through the lattice Green function (LGF). We use LGFs of dislocated geometries in contrast to previous FBC-based dislocation calculations that use the bulk crystal LGF. Dislocation LGFs account for changes in topology in the core as well as strain throughout the lattice. A bulk-like approximation for the force constants in a dislocated geometry leads to LGFs that optimize the cores of the $a_0[100](010)$ edge, $a_0[100](011)$ edge, and $a_0/2[111](1\bar{1}0)$ $71^{\circ}$ mixed dislocations. This approximation fails for the $a_0/2[\bar{1}\bar{1}1](1\bar{1}0)$ dislocation, so here we derive the LGF using accurate force constants from a Gaussian approximation potential. The standard deviations of dislocation Nye tensor distributions quantify the widths of the cores. The relaxed cores are compact, and the magnetic moments on the Fe atoms closely follow the volumetric strain distributions in the cores. We also compute the core structures of these dislocations using eight different classical interatomic potentials, and quantify symmetry differences between the cores using the Fourier coefficients of their Nye tensor distributions. Most of the core structures computed using the classical potentials agree well with DFT results. The DFT geometries provide benchmarking for classical potential studies of work-hardening, as well as substitutional and interstitial sites for computing solute-dislocation interactions that serve as inputs for mesoscale models of solute strengthening and solute diffusion near dislocations. read less

Abstract: Interpreting nonlinear ultrasonic signals detected in a nondestructive evaluation of radiation damage requires the knowledge of the correlation between defects and nonlinearity. In this work, molecular dynamics simulations are performed to study the effect of distributed vacancies, voids, Cu atoms, and Cu precipitates on the nonlinear ultrasonic response in body-centered cubic (bcc) Fe. The nonlinearity parameter calculated from the second harmonic amplitude in the perfect lattice is 2.73. Vacancies are found to increase the nonlinearity. However, clusters of vacancies in the form of spherical voids show an opposite effect. This finding can be used to conveniently distinguish vacancies from voids in the material. Unlike vacancies, individual Cu atoms decrease the nonlinearity. Clustering of Cu atoms into Cu precipitates further decreases the nonlinearity. Interestingly, precipitates with a diameter of 2 nm and larger exhibit a similar effect despite their different structure and coherency with the Fe matrix.Interpreting nonlinear ultrasonic signals detected in a nondestructive evaluation of radiation damage requires the knowledge of the correlation between defects and nonlinearity. In this work, molecular dynamics simulations are performed to study the effect of distributed vacancies, voids, Cu atoms, and Cu precipitates on the nonlinear ultrasonic response in body-centered cubic (bcc) Fe. The nonlinearity parameter calculated from the second harmonic amplitude in the perfect lattice is 2.73. Vacancies are found to increase the nonlinearity. However, clusters of vacancies in the form of spherical voids show an opposite effect. This finding can be used to conveniently distinguish vacancies from voids in the material. Unlike vacancies, individual Cu atoms decrease the nonlinearity. Clustering of Cu atoms into Cu precipitates further decreases the nonlinearity. Interestingly, precipitates with a diameter of 2 nm and larger exhibit a similar effect despite their different structure and coherency with the Fe matrix. read less

Abstract: ABSTRACT We conduct kinetic Monte Carlo simulations for the conservative climb motion of a cluster of self-interstitial atoms (SIAs) towards another SIA cluster in BCC–Fe; the conservative climb velocity is inversely proportional to the fourth power of the distance between them, as per the prediction based on Einstein’s equation. The size of the climbing cluster significantly affects its conservative climb velocity, while the size of the cluster that originates the stress field does not. The activation energy for the conservative climb is considerably greater than that derived in previous studies and strongly dependent on the climbing cluster size. The results presented in this study are the atomistic evaluation of the behaviour of SIA clusters through three-dimensional motion, which cannot be achieved using molecular dynamics techniques alone. read less

Abstract: By combining molecular dynamics (MD) simulations with phase-field (PF) and phase-field crystal (PFC) modeling we study collision-controlled growth kinetics from the melt for pure Fe. The MD/PF comparison shows, on the one hand, that the PF model can be properly designed to reproduce quantitatively different aspects of the growth kinetics and anisotropy of planar and curved solid-liquid interfaces. On the other hand, this comparison demonstrates the ability of classical MD simulations to predict morphology and dynamics of moving curved interfaces up to a length scale of about 0.15 μm. After mapping the MD model to the PF one, the latter permits to analyze the separate contribution of different anisotropies to the interface morphology. The MD/PFC agreement regarding the growth anisotropy and morphology extends the trend already observed for the here used PFC model in describing structural and elastic properties of bcc Fe. read less

Abstract: Stres alanlarindaki kusur kinetiginin anlasilmasi, nukleer maddelerin bozulmasinin cok boyutlu modellenmesi icin onemlidir. Molekuler dinamik (MD) simulasyonu ile, formasyon ve goc enerjisi enerjileri, alfa Fe'de kendiliginden olusan atom (SIA) ve SIA kumeleri (1 ~ 3 gecis reklamlari) icin degerlendirilmistir. % 0 ~ 3 tek eksenli surdurulebilir [100] gerilme etkileri SIAs ve halter konfigurasyonlari icin test edilmistir. Kararlilik ile ilgili olarak, halter konfigurasyonlari daha buyuk suslarda ve daha buyuk kumelerde daha kararli hale gelir. Hareketlilik icin, surdurulebilir gerilmeler altinda tek SIA kusurlarinin difuzyonu izlenmistir. Serbest gerilme kosullarinda, SIA kumelerinin difuzivitesi, doymus gerilmede uc boyutlu (3D) ile bir boyutlu (1D) asamali bir gecise sahiptir. 3D gecis kucuk kumeler ve alt gerilmeler icin iken ve buyuk olcude SIA hizalama konfigurasyonu icin sunulurken, 1D gecisi buyuk kumeler ve buyuk gerginlik icin gozlenmistir. Cekme gerilmesi altinda ve kucuk kumeler icin, difuzyon artirimi daha yuksek bir sicaklikta daha buyuktur. Bununla birlikte, sicaklik etkisi daha buyuk kumeler icin kucuktur. Gerinim alanlarinin bu etkileri, kusurlar ve uygulanan stres alanlari arasindaki elastik etkilesim ile aciklanabilir. read less

Abstract: Many heterogeneous reactions catalyzed by nanoparticles occur at relatively high temperatures, which may modulate the surface morphology of nanoparticles during reaction. Inspired by the discovery of dynamic formation of active sites on gold nanoparticles, we explore theoretically the nature of the highly mobile atoms on the surface of nanoparticles of various sizes for 11 transition metals. Using molecular dynamics simulations, on a 3 nm Fe nanoparticle as an example, the effect of surface premelting and overall melting on the structure and physical properties of the nanoparticles is analyzed. When the nanoparticle is heated up, the atoms in the outer shell appear amorphous already at 900 K. Surface premelting is reached at 1050 K, with more than three liquid atoms, based on the Lindemann criterion. The activated atoms may transfer their extra kinetic energy to the rest of the nanoparticle and activate other atoms. The dynamic studies indicate that the number of highly mobile atoms on the surface increas... read less

Abstract: Molecular dynamics simulations are conducted to study the effects of alloying elements on the primary damage behaviors in three Fe-based ferritic alloy systems: (1) a Fe-Cr system in which the heat of mixing changes its sign with the Cr concentration; (2) a Fe-Cu system that has a positive heat of mixing; and (3) an ideal but artificial Fe-Cr system that has a zero heat of mixing, which is used as a reference system to investigate solute interstitial formation based on probability. It is found that in these alloys, the solute type and concentration do not have a significant effect on the total number of surviving Frenkel pairs. However, the fraction of solute interstitials has distinct behaviors. In Fe-Cr, the Cr interstitial fraction is much higher than the Cr solute concentration and the Cr interstitial production efficiency decreases with the increasing Cr concentration. By contrast, in Fe-Cu, Cu interstitials are barely produced. In the ideal alloy, the solute interstitial fraction is close to the sol... read less

Abstract: Two types of dislocation loops are observed in irradiated α-Fe, the 1/2〈111〉 loop and the 〈100〉 loop. Atomistic simulations consistently predict that only the energetically more favourable 1/2〈111〉 loops are formed directly in cascades, leaving the formation mechanism of 〈100〉 loops an unsolved question. We show how 〈100〉 loops can be formed when cascades overlap with random pre-existing primary radiation damage in Fe and FeCr. This indicates that there are no specific constraints involved in the formation of 〈100〉 loops, and can explain their common occurrence. read less

Abstract: Complex states of nanoscale interstitial dislocation loop can be described by its habit plane and Burgers vector. Using atomistic simulations, we provide direct evidences on the change of the habit plane of a 1/2〈1 1 1〉 loop from {1 1 1} to {1 1 0} and {2 1 1}, in agreement with TEM observations. A new {1 0 0} habit plane of this loop is also predicted by simulations. The non-conservation of the Burgers vector is approved theoretically for: (1) dislocation reactions between loops with different Burgers vectors and (2) the transition between 〈1 0 0〉 loops and 1/2〈1 1 1〉 loops. The rotation from a 1/2〈1 1 1〉 to a 〈1 0 0〉 loop has also been explored, which occurs at 570 K for time on the order of 10 s. The dislocation-precipitate phase duality and change of habit plane are then proposed as new features for nano-scale dislocation loops. read less

Abstract: The Gibbs free energy is the fundamental thermodynamic potential underlying the relative stability of different states of matter under constant-pressure conditions. However, computing this quantity from atomic-scale simulations is far from trivial. As a consequence, all too often the potential energy of the system is used as a proxy, overlooking entropic and anharmonic effects. Here we discuss a combination of different thermodynamic integration routes to obtain the absolute Gibbs free energy of a solid system starting from a harmonic reference state. This approach enables the direct comparison between the free energy of different structures, circumventing the need to sample the transition paths between them. We showcase this thermodynamic integration scheme by computing the Gibbs free energy associated with a vacancy in BCC iron, and the intrinsic stacking fault free energy of nickel. These examples highlight the pitfalls of estimating the free energy of crystallographic defects only using the minimum potential energy, which overestimates the vacancy free energy by 60% and the stacking-fault energy by almost 300% at temperatures close to the melting point. read less

Abstract: Single‐impact tests and molecular dynamics (MD) simulations are performed to evaluate effects at energy‐ and momentum‐variable impact phenomena at two distinct scales and velocity ranges. Therefore, a carbon steel in various heat treatment conditions is examined using the single impact test to evaluate the influence of varying energies and momenta on the deformation behavior. Experimentally found material parameters “momentum‐sensitivity” ps(E) and “minimum deformation momentum” p0(E) are introduced for a better mathematical description of impact phenomena or deformation processes using energy and momentum. Empirical laws are found, where the minimum deformation momentum is a linear function of the impact energy (E) and the deformation at low momenta is an inverse function of E, which has significant influence on the deformation. The proposed empirical law are recalculated via down‐scaled molecular dynamics simulation (rigid indenter impacting on an iron block) and is found applicable for macro and nano scale impact phenomena. read less

Abstract: Radiation damage not only seriously degrades the mechanical properties of tungsten (W) but also enhances hydrogen retention in the material. Introducing a large amount of defect sinks, e.g. grain boundaries (GBs) is an effective method for improving radiation-resistance of W. However, the mechanism by which the vacancies are dynamically annihilated at long timescale in nano-crystal W is still not clear. The dynamic picture for eliminating vacancies with single interstitials and small interstitial-clusters has been investigated by combining molecular dynamics, molecular statics and object Kinetic Monte Carlo methods. On one hand, the annihilation of bulk vacancies was enhanced due to the reflection of an interstitial-cluster of parallel 〈111〉 crowdions by the GB. The interstitial-cluster was observed to be reflected back into the grain interior when approaching a locally dense GB region. Near this region, the energy landscape for the interstitial was featured by a shoulder, different to the decreasing energy landscape of the interstitial near a locally loose region as indicative of the sink role of the GB. The bulk vacancy on the reflection path was annihilated. On the other hand, the dynamic interstitial emission efficiently anneals bulk vacancies. The single interstitial trapped at the GB firstly moved along the GB quickly and clustered to be the di-interstitial therein, reducing its mobility to a value comparable to that that for bulk vacancy diffusion. Then, the bulk vacancy was recombined via the coupled motion of the di-interstitial along the GB, the diffusion of the vacancy towards the GB and the accompanying interstitial emission. These results suggest that GBs play an efficient role in improving radiation-tolerance of nano-crystal W via reflecting highly-mobile interstitials and interstitial-clusters into the bulk and annihilating bulk vacancies, and via complex coupling of in-boundary interstitial diffusion, clustering of the interstitial and vacancy diffusion in the bulk. read less

Abstract: In this paper, vibration behaviors of Fe nanowires are investigated by using the large-scale molecular dynamics (MD) simulations. It is observed that the vibration frequency of nanowires rises slightly and nonlinearly with the increase of initial actuation amplitude. Based on the atomic arrangement, a discrete spring-mass model is developed. Its nonlinear elastic relation is used to explain this phenomenon. In addition, Fe nanowires with different lengths and heights show different vibration properties in this work. The ratio between the length ([Formula: see text]) and the height ([Formula: see text]) of nanowires has a significant influence on vibration behaviors. The vibration properties of nanowires can be explained by the Euler–Bernoulli model when the ratio is relatively large, while they can be illustrated by the Timoshenko model when the ratio is relatively small. read less

Abstract: We consider a nanomachining process of hard, abrasive particles grinding on the rough surface of a polycrystalline ferritic work piece. Using extensive large-scale molecular dynamics (MD) simulations, we show that the mode of thermostating, i.e., the way that the heat generated through deformation and friction is removed from the system, has crucial impact on tribological and materials related phenomena. By adopting an electron-phonon coupling approach to parametrize the thermostat of the system, thus including the electronic contribution to the thermal conductivity of iron, we can reproduce the experimentally measured values that yield realistic temperature gradients in the work piece. We compare these results to those obtained by assuming the two extreme cases of only phononic heat conduction and instantaneous removal of the heat generated in the machining interface. Our discussion of the differences between these three cases reveals that although the average shear stress is virtually temperature independent up to a normal pressure of approximately 1 GPa, the grain and chip morphology as well as most relevant quantities depend heavily on the mode of thermostating beyond a normal pressure of 0.4 GPa. These pronounced differences can be explained by the thermally activated processes that guide the reaction of the Fe lattice to the external mechanical and thermal loads caused by nanomachining. read less

Abstract: Extensive deformation of an iron alloy powder increases the static disorder contribution to the thermal factor, with an increase of ∼20% in the Debye–Waller coefficient observed by both X-ray diffraction and extended X-ray absorption fine structure. Molecular dynamics simulations shed light on the underlying mechanisms, confirming the major role played by the grain boundary. read less

Abstract: An atomistic model of a fully 3D, nano-sized, pre-cracked cantilever beam has been made and MD simulations have been performed to deflect the beam and initiate crack growth. The crucial process zone in front of the crack has been investigated with respect to linear elastic and elastic-plastic fracture mechanics and plastic deformation mechanisms such as dislocations and twinning. The effect of crack geometry and loading rate has been studied. Two crack geometries were compared, one atomically sharp and one blunted. The sharper crack was shown to lead to a cleaner crack extension on (110)-planes, while the rounded crack showed extension along the initial (100)-plane in accordance with experiments on micro-sized 3 wt% Si α-Fe cantilevers. The effect of strain rate was also investigated, and it was found that lower strain rate correlated better with experimental observations. However, the strain rate used is still several magnitudes higher than for experiments, limiting the usefulness of strain rate observations for predicting behavior in experiments. A brief post-deformation comparison between simulations and SEM-images of focused ion beam-fabricated micro-cantilevers was also done, showing possible signs of similar deformation mechanisms and dislocation systems between them. read less

Abstract: For the first time, we report the formation of pentagonal atomic chains during tensile deformation of ultra thin BCC Fe nanowires. Extensive molecular dynamics simulations have been performed on 〈100〉/{110} BCC Fe nanowires with different cross section width varying from 0.404 to 3.634 nm at temperatures ranging from 10 to 900 K. The results indicate that above certain temperature, long and stable pentagonal atomic chains form in BCC Fe nanowires with cross section width less than 2.83 nm. The temperature, above which the pentagonal chains form, increases with increase in nanowire size. The pentagonal chains have been observed to be highly stable over large plastic strains and contribute to high ductility in Fe nanowires. read less

Abstract: Molecular dynamics simulations revealed significant difference in deformation behaviour of 〈1 0 0〉 BCC Fe nanowires with and without twist boundary. The plastic deformation in perfect 〈1 0 0〉 BCC Fe nanowire was dominated by twinning and reorientation to 〈1 1 0〉 followed by further deformation by slip mode. On the contrary, 〈1 0 0〉 BCC Fe nanowire with a twist boundary deformed by slip at low plastic strains followed by twinning at high strains and absence of full reorientation. The results suggest that the deformation in 〈1 0 0〉 BCC Fe nanowire by dislocation slip is preferred over twinning in the presence of initial dislocations or dislocation networks. The results also explain the absence of extensive twinning in bulk materials, which inherently contains large number of dislocations. read less

Abstract: The growth of voids has a great impact on the mechanical properties of ductile materials by altering their microstructures. Exploring the process of void growth at the nanoscale helps in understanding the dynamic fracture of metals. While some very recent studies looked into the effects of the initial geometry of an elliptic void on the plastic deformation of face-centered cubic metals, a systematic study of the initial void ellipticity and orientation angle in body-centered cubic (BCC) metals is still lacking. In this paper, large scale molecular dynamics simulations with millions of atoms are conducted, investigating the void growth process during tensile loading of metallic thin films in BCC α-Fe. Our simulations elucidate the intertwined influences on void growth of the initial ellipticity and initial orientation angle of the void. It is shown that these two geometric parameters play an important role in the stress–strain response, the nucleation and evolution of defects, as well as the void size/outline evolution in α-Fe thin films. Results suggest that, together with void size, different initial void geometries should be taken into account if a continuum model is to be applied to nanoscale damage progression. read less

Abstract: Using molecular dynamics simulation method, the plastic deformation mechanism of Fe nanowires is studied by applying uniaxial tension along the [110] direction. The simulation result shows that the bcc-to-hcp martensitic phase transformation mechanism controls the plastic deformation of the nanowires at high strain rate or low temperature; however, the plastic deformation mechanism will transform into a dislocation nucleation mechanism at low strain rate and higher temperature. Furthermore, the underlying cause of why the bcc-to-hcp martensitic phase transition mechanism is related to high strain rate and low temperature is also carefully studied. Based on the present study, a strain rate-temperature plastic deformation map for Fe nanowires has been proposed. read less

Abstract: Molecular dynamics simulations were performed to understand the role of twin boundaries on deformation behaviour of body-centred cubic (BCC) iron (Fe) nanopillars. The twin boundaries varying from 1 to 5 providing twin boundary spacing in the range 8.5–2.8 nm were introduced perpendicular to the loading direction. The simulation results indicated that the twin boundaries in BCC Fe play a contrasting role during deformation under tensile and compressive loadings. During tensile deformation, a large reduction in yield stress was observed in twinned nanopillars compared to perfect nanopillar. However, the yield stress exhibited only marginal variation with respect to twin boundary spacing. On the contrary, a decrease in yield stress with increase in twin boundary spacing was obtained during compressive deformation. This contrasting behaviour originates from difference in operating mechanisms during yielding and subsequent plastic deformation. It has been observed that the deformation under tensile loading was dominated mainly by twin growth mechanism. On the other hand, the deformation was dominated by nucleation and slip of full dislocations under compressive loading. The twin boundaries offer a strong repulsive force on full dislocations resulting in the yield stress dependence on twin boundary spacing. The occurrence of twin–twin interaction during tensile deformation and dislocation–twin interaction during compressive deformation has been discussed. read less

Abstract: Modeling isolated dislocations is challenging due to their long-ranged strain fields. Flexible boundary condition methods capture the correct long-range strain field of a defect by coupling the defect core to an infinite harmonic bulk through the lattice Green function (LGF). To improve the accuracy and efficiency of flexible boundary condition methods, we develop a numerical method to compute the LGF specifically for a dislocation geometry; in contrast to previous methods, where the LGF was computed for the perfect bulk as an approximation for the dislocation. Our approach directly accounts for the topology of a dislocation, and the errors in the LGF computation converge rapidly for edge dislocations in a simple cubic model system as well as in BCC Fe with an empirical potential. When used within the flexible boundary condition approach, the dislocation LGF relaxes dislocation core geometries in fewer iterations than when the perfect bulk LGF is used as an approximation for the dislocation, making a flexible boundary condition approach more efficient. read less

Abstract: Molecular dynamics simulations have been used to study the effects of different orientation relationships between fcc and bcc phases on the bcc/fcc interfacial propagation in pure iron systems at 300 K. Three semi-coherent bcc/fcc interfaces have been investigated. In all the cases, results show that growth of the bcc phase starts in the areas of low potential energy and progresses into the areas of high potential energy at the original bcc/fcc interfaces. The phase transformation in areas of low potential energy is of a martensitic nature while that in the high potential energy areas involves occasional diffusional jumps of atoms. read less

Abstract: The plastic deformation mechanism of iron (Fe) nanowires under torsion is studied using the molecular dynamics (MD) method by applying an external driving force at a constant torsion speed. We find that the deformation behavior depends on the orientation of the wire. The dislocations in 〈100〉 and 〈111〉 oriented nanowires propagate through the nanowires under torsion, whereas those in 〈110〉 oriented nanowires divide the wire into two parts. The situation that there is a low angle twist grain boundary (GB) in the nanowires is also under consideration. The results reveal that the dislocations are concentrated on the GB in the initial state, presenting different patterns of dislocation network. The networks change depending on the twist direction. They shrink with increase in twist angle but expand with the decreasing twist angle, presenting an asymmetric phenomenon. Our findings can help us more thoroughly understand the plastic deformation mechanism of Fe nanowires under torsion. read less

Abstract: Atom probe tomography (APT) is an extremely powerful technique for determining the three-dimensional structure and chemical composition of a given sample. Although it is designed to provide images of material structure with atomic scale resolution, reconstruction artifacts, well-known to be present in reconstructed images, reduce their accuracy. No existing simulation technique has been able to describe the origin of these artifacts. Here we develop a simulation technique which allows for atomistic simulations of the atom emission process in the presence of high electric fields in APT experiments. Our code combines hybrid concurrent electrodynamics—molecular dynamics and a Monte Carlo approach. We use this technique to demonstrate the atom-level origin of artifacts in APT image reconstructions on examples of inclusions and voids in investigated samples. The results show that even small variations in the surface topology give rise to distortions in the local electric field, limiting the accuracy of conventional APT reconstruction algorithms. read less

Abstract: Molecular dynamics simulations of bent [100] α‐Fe nanowires show the nucleation of twins and nanoscale interfaces that lead to pseudo‐elasticity during loading/unloading cycles. The new type of interfaces along {110} stems from the accumulation of individual <111>/{112} twin boundaries and stores high interfacial energies. These nonconventional interfaces provide a large part of the driving force for shape recovery upon unloading, while the minimization of surface energy is no longer the dominant driving force. This new pseudo‐elastic effect is not much affected by surface roughness, and can be extended over a wide range of wire diameters, if the sample is seeded with conventional twin boundaries, which will transform to the desired {110} interfaces under bending. read less

Abstract: We proposed a scheme to describe the spatial correlation between two atoms in metallic glasses. Pair distribution function in a model iron was fully decomposed into several shells and can be presented as the spread of nearest neighbor correlation via distance. Moreover, angle distribution function can also be decomposed into groups. We demonstrate that there is close correlation between pair distribution function and angle distribution function for metallic glasses. We think that our results are very helpful understanding the atomic structure of metallic glasses. read less

Abstract: To obtain a material with the desired performance, the atomic-level mechanisms of nucleation from the liquid to solid phase must be understood. Although this transition has been investigated experimentally and theoretically, its atomic-level mechanisms remain debatable. In this work, the nucleation mechanisms of pure Fe under rapid cooling conditions are investigated. The local atomic packing stability and liquid-to-solid transition-energy pathways of Fe are studied using molecular dynamics simulations and first-principle calculations. The results are expressed as functions of cluster size in units of amorphous clusters (ACs) and body-centered cubic crystalline clusters (BCC-CCs). We found the prototypes of ACs in supercooled liquids and successfully divided these ACs to three categories according to their transition-energy pathways. The information obtained in this study could contribute to our current understanding of the crystallization of metallic melts during rapid cooling. read less

Abstract: In this work we perform molecular dynamics simulations to quantify and parametrize the evolution of a bcc Fe work piece topography during nanometric grinding with multiple hard abrasive particles. The final surface quality depends on both the normal pressure and the abrasive geometry. We fit the time development of the substrate’s root mean squared roughness to an exponential function, allowing the definition of a run-in regime, during which the surface ‘forgets’ about its initial state, and a steady-state regime where the roughness no longer changes. The time constants associated with smoothing and material removal are almost inversely proportional to each other, highlighting the distinctiveness of these two simultaneously occurring processes. We also describe an attempt to reduce the time required to achieve the smoothest possible surface finish by periodically re-adjusting the normal pressure during the grinding process. read less

Abstract: Large-scale molecular dynamics (MD) simulations have been performed to investigate the tensile properties and the related atomistic deformation mechanisms of the gradient nano-grained (GNG) structure of bcc Fe (gradient grains with d from 25 nm to 105 nm), and comparisons were made with the uniform nano-grained (NG) structure of bcc Fe (grains with d = 25 nm). The grain size gradient in the nano-scale converts the applied uniaxial stress to multi-axial stresses and promotes the dislocation behaviors in the GNG structure, which results in extra hardening and flow strength. Thus, the GNG structure shows slightly higher flow stress at the early plastic deformation stage when compared to the uniform NG structure (even with smaller grain size). In the GNG structure, the dominant deformation mechanisms are closely related to the grain sizes. For grains with d = 25 nm, the deformation mechanisms are dominated by GB migration, grain rotation and grain coalescence although a few dislocations are observed. For grains with d = 54 nm, dislocation nucleation, propagation and formation of dislocation wall near GBs are observed. Moreover, formation of dislocation wall and dislocation pile-up near GBs are observed for grains with d = 105 nm, which is the first observation by MD simulations to our best knowledge. The strain compatibility among different layers with various grain sizes in the GNG structure should promote the dislocation behaviors and the flow stress of the whole structure, and the present results should provide insights to design the microstructures for developing strong-and-ductile metals. (C) 2015 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License. read less

Abstract: The recombination radius of a Frenkel pair is a fundamental parameter for the object kinetic Monte Carlo (OKMC) and mean field rate theory (RT) methods that are used to investigate irradiation damage accumulation in irradiated materials. The recombination radius in bcc Fe has been studied both experimentally and numerically, however there is no general consensus about its value. The detailed atomistic processes of recombination also remain uncertain. Values from 1.0a0 to 3.3a0 have been employed as a recombination radius in previous studies using OKMC and RT. The recombination process of a Frenkel pair is investigated at the atomic level using the self-evolved atomistic kinetic Monte Carlo (SEAKMC) method in this paper. SEAKMC calculations reveal that a self-interstitial atom recombines with a vacancy in a spontaneous reaction from several nearby sites following characteristic pathways. The recombination radius of a Frenkel pair is estimated to be 2.26a0 by taking the average of the recombination distances from 80 simulation cases. In addition, we apply these procedures to the capture radius of a self-interstitial atom by a vacancy cluster. The capture radius is found to gradually increase with the size of the vacancy cluster. The fitting curve for the capture radius is obtained as a function of the number of vacancies in the cluster. read less

Abstract: Electron microscopy plays an important role in motivating and testing our theoretical understanding of grain boundary structure and behavior across the atomistic and continuum length scales. One of the key foundational challenges in grain boundary science is to establish meaningful links between atomic-scale interfacial configurations, which can often be described in terms of sets of characteristic structural units [1], and more macro-scale descriptors of the interfacial geometry, such as grain misorientation and boundary inclination. A useful approach for establishing such linkages is to identify and to characterize the sets of interfacial line defects that accommodate departures from low-energy singular reference configurations [2]. read less

Abstract: In this paper, we investigated the interaction of an edge dislocation with Cu precipitates with a spherical geometry and with Cu–Ni precipitates that possess a Cu core with an outer Ni shell, commonly observed in reactor pressure vessel (RPV) steels. We applied molecular dynamics techniques to explore the critical stress required to unpin the dislocation (CSRUD), the breakaway dislocation line shape when the dislocation leaves the precipitates and the transition of Cu atoms within precipitates. The results indicate that the CSRUD of the Cu–Ni precipitates with a diameter less than 2.38 nm is larger than that of Cu precipitates that contain the same number of Cu atoms, while for a diameter larger than 2.38 nm, the CSRUD of Cu–Ni precipitates is weaker, which is related to the bcc to fcc-like or hcp-like atoms transformation in precipitates. The dislocations interact with Cu and Cu–Ni precipitates via the cut mechanism. 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: Despite a number of large-scale molecular dynamics simulations of shock compressed iron, the morphological properties of simulated recovered samples are still unexplored. Key questions remain open in this area, including the role of dislocation motion and deformation twinning in shear stress release. In this study, we present simulations of homogeneous uniaxial compression and recovery of large polycrystalline iron samples. Our results reveal significant recovery of the body-centered cubic grains with some deformation twinning driven by shear stress, in agreement with experimental results by Wang et al. [Sci. Rep. 3, 1086 (2013)]. The twin fraction agrees reasonably well with a semi-analytical model which assumes a critical shear stress for twinning. On reloading, twins disappear and the material reaches a very low strength value. read less

Abstract: This article discusses novel algorithms for molecular-dynamics (MD) simulations with short-ranged forces on modern multi- and many-core processors like the Intel Xeon Phi. A task-based approach to the parallelization of MD on shared-memory computers and a tiling scheme to facilitate the SIMD vectorization of the force calculations is described. The algorithms have been tested with three different potentials and the resulting speed-ups on Intel Xeon Phi coprocessors are shown. read less

Abstract: The influence of carbon at dislocation cores in α-Fe is studied to determine the Peierls stress, i.e. the critical stress required to move the dislocation at 0 K. The effect of carbon on both edge and screw dislocations is investigated. A coupled molecular statics (MS) and extended finite element method (XFEM) is employed for this study, where the dislocation core is modeled atomistically. The results on pure Fe are found to be in good agreement with a fully atomistic study. The coupled approach captures the right core behavior and significantly reduces the size of the atomistic region, while describing the behavior of a single dislocation in an infinite anisotropic elastic medium. Furthermore, mechanical boundary conditions can be applied consistently. It was found that the influence of carbon on edge dislocations is much stronger than that on screw dislocations, and that carbon causes a directionally dependent Peierls stress in the case of a screw dislocation. Even though the increase of the Peierls stress is much more pronounced for edge dislocations, the total value does not reach the level of the Peierls stress for screw dislocations, either with or without carbon at the core. Hence, we conclude that the motion of screw dislocations remains the rate limiting factor for plastic deformation of α-Fe. read less

Abstract: Cu-rich precipitation is regarded as one of the main issues causing embrittlement of ferritic steels. In the present work, the Cu segregation at Σ5 {012} symmetrical grain boundary (GB) in BCC iron is investigated by combining Metropolis Monte Carlo and molecular statics approaches. The segregation driven energies of Cu clusters decrease with increasing the distance from GB and also depend on the cluster size. The length scales associated with Cu segregation at GB are determined. All these results indicate that Cu atoms prefer to segregate at S5 GB, which may account for the embrittlement of ferritic steels. The present results provide important knowledge to understand the detailed mechanisms of Cu segregation at GB and also the possible effects on mechanical properties of α-Fe. read less

Abstract: Due to the industrial importance of α-iron-based polycrystalline materials, their grain boundary (GB) structures and properties need to be well characterized and understood in order to optimize the materials through effective GB engineering. In this study, a molecular dynamics (MD) simulation study was performed to investigate a series of 〈1 1 0〉 symmetric tilt grain boundaries (STGBs) and asymmetric tilt grain boundaries (ATGBs) in α-iron. It is shown that the GB energy is proportional to the GB volumetric expansion. During uniaxial deformation, 〈1 1 1〉{1 1 2} twinning appears to be more competitive or easier than 〈1 1 1〉{1 1 2} dislocation emission from the GB at yielding. For bicrystal systems containing STGBs the yield strength obeys the Schmid law, while for ATGB bicrystal systems the yield strengths are mainly determined by the local stress rather than overall stress and average GB energy. The higher degree of atomic disordering in the ATGB regions generates larger local stress fluctuation and thus facilitates local defect emission when subjected to external stresses. read less

Abstract: To reveal the fracture failure mechanisms of single crystals at elevated temperatures, a new temperature-dependent ideal tensile strength model for solids has been developed, based on the critical strain principle. At the same time, the uniaxial tensile strength model, based on the critical failure energy density principle for isotropic materials that was presented in the previous study, is generalized to multi-axial loading and to cubic single crystals. The relationship between the two models is discussed, and how to obtain the material properties needed in the calculations is summarized. The two well-established models are used to predict the temperature-dependent ideal tensile strength of W, Fe and Al single crystals. The predictions from the critical strain principle agree well with the predictions from the critical failure energy density principle. The theoretical values from the critical strain principle at 0 K is in reasonable agreement with the ab initio results. The study shows that the temperature dependence of the ideal tensile strength is similar to that of Young’s modulus; that is, the ideal tensile strength firstly remains approximately constant and then decreases linearly with the temperature. The fracture failure for single crystals at elevated temperatures has been identified, for the first time, as a strain-controlled criterion. read less

Abstract: We establish an overall energy expression to determine the twin migration stress in bcc metals. Twin migration succeeds twin nucleation often after a load drop, and a model to establish twin migration stress is of paramount importance. We compute the planar fault energy barriers and determine the elastic energies of twinning dislocations including the role of residual dislocations (br) and twin intersection types such as 1 1 0, 1 1 3 and 2 1 0. The energy expression derived provides the twin migration stress in relation to the twin nucleation stress with a ratio of 0.5–0.8 depending on the resultant residual burgers vector and the intersection types. Utilizing digital image correlation, it was possible to differentiate the twin nucleation and twin advancement events experimentally, and transmission electron microscopy observations provided further support to the modelling efforts. Overall, the methodology developed provides an enhanced understanding of twin progression in bcc metals, and most importantly the proposed model does not rely on empirical constants. We utilize Fe-50at.%Cr in our experiments, and subsequently predict the twin migration stress for pure Fe, and Fe-3at.%V from the literature showing excellent agreement with experiments. read less

Abstract: Substitutional alloying elements significantly affect the recrystallization and austenite-ferrite phase transformation rates in steels. The atomistic mechanisms of their interaction with the interfaces are still largely unexplored. Using density functional theory, we determine the segregation energies between commonly used alloying elements and the Σ5 (013) tilt grain boundary in bcc iron. We find a strong solute-grain boundary interaction for Nb, Mo, and Ti that is consistent with experimental observations of the effects of these alloying elements on delaying recrystallization and the austenite-to-ferrite transformation in low-carbon steels. In addition, we compute the solute-solute interactions as a function of solute pair distance in the grain boundary, which suggest co-segregation for these large solutes at intermediate distances in striking contrast to the bulk. read less

Abstract: We performed molecular dynamics simulations of boundary-lubricated sliding, varying the boundary lubricant type, its molecular surface coverage, the substrate roughness, and the load. The resulting load versus friction behavior was then analyzed to study how changes in lubricant type, coverage, and roughness affect the extrapolated friction force at zero load, the so-called Derjaguin offset. A smooth-particle-based evaluation method by the authors, applied here for the first time to visualize the sliding interface between the two layers of boundary lubricant, allowed the definition and calculation of a dimensionless normalized sliding resistance area, which was then related to the Derjaguin offset. This relationship excellently reflects the molecular surface coverage, which determines the physical condition of the lubricant, and can differentiate between some lubricant-specific frictional properties. read less

Abstract: Based on an orientation-dependent dislocation line energy, a solution to the acoustic nonlinearity parameter is obtained for pure and mixed dislocations in anisotropic crystals. The solution is validated by comparison with molecular dynamic simulations. Parametric studies using this new solution show that (i) elastic anisotropy can significantly change the nonlinear behavior of dislocations including “corners” in the bowed dislocation line, much reduced critical stress for instability, sharp peaks in the β versus applied shear relationship, etc., (ii) mixed dislocations may have distinct behavior that is not bounded by the pure edge and screw dislocations, (iii) asymptotic solutions of the acoustic nonlinearity parameter in terms of power series (as high as 5th order) may not be valid even for pure dislocations in isotropic solids. read less

Abstract: Atomistic simulations play an important role in advancing our understanding of the mechanical properties of materials. Currently, most atomistic simulations are performed using relatively simple geometries under homogeneous loading conditions, and a significant part of the computer time is spent calculating the elastic response of the material, while the focus of the studies lies usually on the mechanisms of plastic deformation and failure. Here we present a simple but versatile approach called FE2AT to use finite element calculations to provide appropriate initial and boundary conditions for atomistic simulations. FE2AT allows us to forgo the simulation of large parts of the elastic loading process, even in the case of complex sample geometries and loading conditions. FE2AT is open source and can be used in combination with different atomistic simulation codes and methods. Its application is demonstrated using the bending of a nano-beam and the determination of the displacement field around a crack tip as examples. read less

Abstract: Precipitate hardening is commonly used in materials science to control strength by acting on the number density, size distribution, and shape of solute precipitates in the hardened matrix. The Fe-Cu system has attracted much attention over the last several decades due to its technological importance as a model alloy for Cu steels. In spite of these efforts several aspects of its phase diagram remain unexplained. Here we use atomistic simulations to characterize the polymorphic phase diagram of Cu precipitates in body-centered cubic (BCC) Fe and establish a consistent link between their thermodynamic and mechanical properties in terms of thermal stability, shape, and strength. The size at which Cu precipitates transform from BCC to a close-packed 9R structure is found to be strongly temperature dependent, ranging from approximately 4 nm in diameter (similar to 2700 atoms) at 200 K to about 8 nm (similar to 22 800 atoms) at 700 K. These numbers are in very good agreement with the interpretation of experimental data given Monzen et al. [Philos. Mag. A 80, 711 (2000)]. The strong temperature dependence originates from the entropic stabilization of BCC Cu, which is mechanically unstable as a bulk phase. While at high temperatures the transition exhibits first-order characteristics, the hysteresis, and thus the nucleation barrier, vanish at temperatures below approximately 300 K. This behavior is explained in terms of the mutual cancellation of the energy differences between core and shell (wetting layer) regions of BCC and 9R nanoprecipitates, respectively. The proposed mechanism is not specific for the Fe-Cu system but could generally be observed in immiscible systems, whenever the minority component is unstable in the lattice structure of the host matrix. Finally, we also study the interaction of precipitates with screw dislocations as a function of both structure and orientation. The results provide a coherent picture of precipitate strength that unifies previous calculations and experimental observations. read less

Abstract: In this study, we calculate the interaction energy of intrinsic point defects vacancies and interstitials) with screw dislocations in body-centered cubic iron. First (we calculate the dipole tensor of a defect in the bulk crystal using molecular statics. Using a formulation based on linear elasticity theory, we calculate the interaction energy of the defect and the dislocation using both isotropic and anisotropic strain fields. Second, we perform atomistic calculations using molecular statics methods to directly calculate the interaction energy. Results from these two methods are compared. We verify that continuum methods alone are unable to correctly predict the interactions of defects and dislocations near the core. Although anisotropic theory agrees qualitatively with atomistics far from the core, it cannot predict which dumbbell orientations are stable and any continuum calculations must be used with caution. Spontaneous absorption by the core of both vacancies and dumbbells is seen. This paper demonstrates and discusses the differences between continuum and atomistic calculations of interaction energy between a dislocation core and a point defect. read less

Abstract: It has recently been shown that under severe plastic deformation processing bi-metal fcc/bcc composites develop a mechanically stable heterophase interface that joins the {112}fcc//{112}bcc planes in the Kurdjumov-Sachs orientation relationship. In this article, we study variations in the relaxed equilibrium atomic structure of this interface with changes in fcc stacking fault energy (SFE) and lattice mismatch between the two crystals. Using molecular statics/dynamics simulations for three fcc/bcc systems, Cu-Nb, Al-Fe, and Al-Nb, we find that the number of distinct sets of intrinsic interfacial dislocations and their core structures vary significantly among these three systems. The impact of these atomic-scale structural differences on interfacial properties is demonstrated through their interactions with point defects. The interfaces studied here are shown to exhibit a wide variation in ability, ranging from being a poor to an excellent sink for vacancies. read less

Abstract: The mechanical behaviour of steels is strongly related to their underlying atomistic structures which evolve during thermal treatment. Cu-alloyed α-Fe undergoes a change in material behaviour during the ageing process, especially at temperatures of above 300°C, where precipitates form on a large time-scale within the α-Fe matrix, yielding first a precipitation strengthening of the material. As the precipitates grow further in time, the material strength decreases again. This complex process is modelled with a multiscale approach, combining Kinetic Monte Carlo (KMC) with Molecular Dynamics (MD) simulations in a sequential way and exploiting the advantages of both methods while simultaneously circumventing their particular disadvantages. The formation of precipitates is modelled on a single-crystal lattice with a diffusion based KMC approach. Transferring selected precipitation states at different ageing times to MD simulations allows the performance of nano tensile tests and the analysis of failure initiation. The anisotropic tensile behaviour is investigated in the [100], [110] and [111] directions, showing monotonically decreasing tensile strengths and deformation strains. Hence precipitation strengthening is mainly due to dislocation–precipitate interactions which are non-existent at small tensile loadings in this scenario. At the point of ductile failure, dislocations are generated at the interfaces between precipitates and the Fe matrix. Straining in the [100] direction, they lie on {110} and {112} glide planes, as expected. With the method presented here, the changes of the anisotropic tensile moduli are related to different states of thermal ageing, i.e., to nucleation, growth and Ostwald ripening of Cu precipitates. read less

Abstract: The low-temperature plastic yield of α-Fe single crystals is known to display a strong temperature dependence and to be controlled by the thermally activated motion of screw dislocations. In this paper, we present molecular dynamics simulations of 12 〈111〉{112} screw dislocation motion as a function of temperature and stress in order to extract mobility relations that describe the general dynamic behavior of screw dislocations in pure α-Fe. We find two dynamic regimes in the stress-velocity space governed by different mechanisms of motion. Consistent with experimental evidence, at low stresses and temperatures, the dislocations move by thermally activated nucleation and propagation of kink pairs. Then, at a critical stress, a temperature-dependent transition to a viscous linear regime is observed. Critical output from the simulations, such as threshold stresses and the stress dependence of the kink activation energy, are compared to experimental data and other atomistic works with generally very good agreement. Contrary to some experimental interpretations, we find that glide on {112} planes is only apparent, as slip always occurs by elementary kink-pair nucleation/propagation events on {110} planes. Additionally, a dislocation core transformation from compact to dissociated has been identified above room temperature, although its impact on the general mobility is seen to be limited. This and other observations expose the limitations of inferring or presuming dynamic behavior on the basis of only static calculations. We discuss the relevance and applicability of our results and provide a closed-form functional mobility law suitable for mesoscale computational techniques. read less

Abstract: Diffusion of interstitial hydrogen atoms in a-iron was investigated using molecular dynamic simulation. In particular, hydrogen diffusivities in bulk, on (001) surface and within a Σ5 [100]/(013) symmetric tilt grain boundary (STGB) were estimated in a temperature range of 400 and 700 K. Furthermore, hydrogen diffusivities in a series of Σ5 [100] tilt grain boundaries with different inclinations were also determined as a function of temperature. The inclination dependence of activation energy for diffusion exhibits two local maxima, which correspond to two STGBs. Additional calculation of inclination dependence of boundary energy and boundary specific excess volume shows two local minima at the same STGBs. This suggests hydrogen diffusion into and within a grain boundary might be assisted by grain boundary excess volume and stress. Simulation of effects of hydrostatic pressure on diffusion shows tensile stress can promote hydrogen diffusion in lattice into grain boundary or surface traps, while compressive stress leads to a decrease in diffusivity, and a slower rate of filling these traps. read less

Abstract: In this work, we seek to develop a new interatomic potential for α-Fe that is able to rationalize experimental flow stress data. We generate a series of potentials with similar bulk and point defect properties, but exhibit different energetic landscapes for the Peierls potential. The family of potentials all possess a compact core structure, which we find necessitates a camel-hump shaped Peierls potential. Within this constraint, we analyze the relationships between the Peierls potential, the 3-D kink nucleation energetics, and the resulting shape of the kink structures for the screw dislocation. We find that one of our models, labeled MPG20, gives very good agreement with experimental flow stress data over the entire stress range considered. read less

Abstract: Local atomic magnetic moments in crystalline Fe are perturbed by the presence of dislocations. The effects are most pronounced near the dislocation core and decay slowly as the strain field of the dislocation decreases with distance. We have calculated local moments using the locally self-consistent multiple scattering (LSMS) method for a supercell containing a screw-dislocation quadrupole. Finite size effects are found to be significant indicating that dislocation cores affect the electronic structure and magnetic moments of neighboring dislocations. The influence of neighboring dislocations points to a need to study individual dislocations from first principles just as they appear amid surrounding atoms in large-scale classical force field simulations. An approach for the use of the LSMS to calculate local moments in subvolumes of large atomic configurations generated in the course of classical molecular dynamics simulation of dislocationdynamics is discussed. read less

Abstract: Using molecular dynamics simulations, we study the wetting of liquid iron in a carbon nanotube and on a graphene sheet. It is found that the contact angle of a droplet in a carbon nanotube increases linearly with the increase of wall curvature but is independent of the length of the filled liquid. The contact angle for a droplet on a graphene sheet decreases with the increasing droplet size. The line tension of a droplet on a graphene sheet is also obtained. Detailed studies show that liquid iron near the carbon walls exhibits the ordering tendencies in both the normal and tangential directions. read less

Abstract: In this work, we examine the kink-nucleation process in BCC screw dislocations using atomistic simulation and transition pathway analysis, with a particular focus on the compact core structure. We observe the existence of a threshold stress, which results in an abrupt change in the minimum energy path of the kink-nucleation process, and hence, a discontinuity in the activation energy versus stress for the process. The magnitude of the discontinuity is found to be related to the degree of metastability of an intermediate split-core structure. This feature appears to be a direct consequence of the so-called ‘camel-hump’ nature of the Peierls potential, which manifests itself in the existence of a metastable, intermediate split-core structure. The effect is observed in a number of empirical EAM potentials, including Fe, Ta, V, Nb and Mo, suggesting a generality to the observations. read less

Abstract: A simple theoretical model proposed recently to evaluate the ability of bulk materials to form single crystals is further tested via vast molecular dynamics simulations of growth for fcc (Ni, Cu, Al, Ar) and hcp (Mg) crystals, especially applied to the growth of bcc (Fe) crystal, showing that the validity of the model is independent of crystal types and the interaction potentials of the constitute atoms. read less

Abstract: It is shown that the temperature effect on the variance of local shear stresses is the main factor pre-determining the temperature law of the yield stress of nano-sized crystals. The results of molecular dynamics simulations of uniaxial tension of Mo, α-Fe and W nanowires in three crystallographic directions ([100], [110] and [111]) over the temperature range 100–1000 K are presented. It is found that within this temperature range, the yield stress of nano-sized crystals varies not exponentially, as for bulk single crystals, but is a parabolic function of temperature. read less

Abstract: Molecular dynamics (MD) simulations of diffusion in Cu–Zr alloys in their liquid and supercooled liquid states were performed using a recently developed Finnis–Sinclair many-body interatomic potential. To help assess how well the interatomic potential describes the energetics of the Cu–Zr system, the liquid structure determined by MD simulations was compared with wide-angle X-ray scattering measurements of the liquid structure for a Cu64.5Zr35.5 alloy. Diffusion was examined as a function of composition, pressure and temperature. The simulations reveal that the diffusion exhibits strong compositional dependence, with both species exhibiting minimum diffusivities at ∼70% Cu. Moreover, the MD simulations show that the activation volumes for Zr and Cu atoms exhibit a maximum near 70% Cu. Evidence is obtained that the glass transition temperature also changes strongly with composition, thereby contributing to the diffusion behaviour. The relationship between this minimum in diffusion and the apparent best glass-forming composition in the Cu–Zr system is discussed. read less

Abstract: We simulate unsteady nanoscale thermal transport at a solid-fluid interface by placing cooler liquid-vapor Ar mixtures adjacent to warmer Fe walls. The equilibration of the system towards a uniform overall temperature is investigated using nonequilibrium molecular dynamics simulations from which the heat flux is also determined explicitly. The Ar–Fe intermolecular interactions induce the migration of fluid atoms into quasicrystalline interfacial layers adjacent to the walls, creating vacancies at the migration sites. This induces temperature discontinuities between the solidlike interfaces and their neighboring fluid molecules. The interfacial temperature difference and thus the heat flux decrease as the system equilibrates over time. The averaged interfacial thermal resistance Rk,av decreases as the imposed wall temperature Tw is increased, as Rk,av∝Tw−4.8. The simulated temperature evolution deviates from an analytical continuum solution due to the overall system heterogeneity. read less

Abstract: Body-centered-cubic iron develops an elastic instability, driven by spin fluctuations, near the alpha-gamma phase transition temperature T(c) = 912 degrees C that is associated with the dramatic reduction of the shear stiffness constant c' (c(11)-c(12))/2 near T(c). This reduction of c' has a profound effect on the temperature dependence of the anisotropic elastic self-energies of dislocations in iron. It also affects the relative stability of the a[100] and a/2[111] prismatic edge dislocation loops formed during irradiation. The difference between the anisotropic elastic free energies provides the fundamental explanation for the observed dominant occurrence of the a[100], as opposed to the a/2[111], Burgers vector configurations of prismatic dislocation loops in iron and iron-based alloys at high temperatures. read less

Abstract: Molecular dynamics simulations, based on embedded-atom method potentials, have been used to compute thermodynamic and kinetic properties of crystal–melt interfaces in the bcc metals Mo and V. The interfacial free energy and its associated crystalline anisotropy have been obtained with the capillary fluctuation method and for both metals the anisotropy and the value of the Turnbull coefficient are found to be significantly lower than for the case of fcc materials. The interface mobility, or kinetic coefficient, which relates the isothermal crystallization rate to interface undercooling, was computed by non-equilibrium molecular dynamics simulations. Mobilities in the range 9-16 cm s−1K−1 are obtained. For Mo the mobility in the (110) crystallographic growth direction is larger than in the (100) and (111) directions, whereas for V the growth is found to be isotropic within numerical uncertainty. The kinetic-coefficient results are discussed within the framework of a density-functional-based theory of crystal growth. read less

Abstract: A deviation from the Arrhenius law in ?-Fe self-diffusion and also in the diffusion of substitutional impurities is found experimentally. Below the Curie temperature the diffusion coefficients have lower values than those extrapolated from the paramagnetic region and the Arrhenius plot shows an upward curvature. In this work we attempt to understand this behaviour from first-principles calculations. Formation and migration energies for self-diffusion and also for the diffusion of some substitutional impurities are calculated. Spin-polarized and non-spin-polarized calculations are assumed to approximately represent ferromagnetic and paramagnetic ?-Fe respectively. The calculations were performed using the WIEN97 code, with a supercell of 36?atoms which allows us to include both vacancies and impurities and therefore to study the migration along the direction. The increment in the diffusion barrier due to the total magnetic alignment at 0?K, with respect to the paramagnetic case, is almost constant for non-magnetic impurities, as it is in the experiments, whereas for magnetic impurities it depends on the diffusing atom. read less

Abstract: As a sequel to the previous paper on the calculation of the crystal–melt interface free energy (2021 Materialia 15 100962), here we report the results on the kinetic coefficients using molecular dynamics simulations performed on six fcc metals and four bcc metals with the intention to compare the crystal structural influence. We found that the calculated kinetic coefficients are well described by the model by Broughton, Gilmer and Jackson (1982 Phys. Rev. Lett. 49 1496), and in particular, they exhibit varying degrees of anisotropy. We reveal that the anisotropies are related to the fluctuation of the crystal–melt interfaces, which causes the increase of the actual interface area in melting or solidification. The kinetic coefficients always display asymmetry between the solidification and melting process, and the difference is much more pronounced for the (111) interfaces in fcc metals which have the highest anisotropy. We found that the atomic mechanisms of the kinetic behaviors of these interfaces are closely related to the formation of twin-crystal domains during solidification, which delays the solidification process and consequently causes a decrease in the calculated kinetic coefficients. read less

Abstract: A detailed study of the mobility and kinetic properties of solid liquid interfaces (SLI) with different types of crystal lattices (fcc Al, Cu) and (bcc Fe) metals in a wide range of temperatures and pressures was carried out using atomistic modeling. The ranges of maximum permissible values of superheated/undercooled states for each metal have been determined. The ultimate goal of the study was to determine the temperature dependences of the stationary front velocity υsl(ΔT) describing the SLI mobility in each of the metals in an analytical form. The analytical dependence υsl(ΔT) was constructed by comparing the results of atomistic modeling in the area of maximum permissible superheating/undercooling values with the data of the main kinetic models of Wilson Frenkel (WF) and Broughton, Gilmer and Jackson (BGJ). An acceptable agreement was achieved by introducing appropriate correction parameters into the kinetic models using the least squares method. The influence of the crystallographic orientation of metals and external pressure on the SLI mobility is investigated. read less

Abstract: Nucleation is ubiquitous, from the formation of clouds to the preparation of pharmaceutical compounds, from metal casting to the tempering of chocolates, and from the growth of beautiful nautilus shells to the assembly of microtubules in cells. The first experiment for observing a nucleation event was performed by Fahrenheit in 1724, and it can be easily replicated in a kitchen: simply place a bottle of purified water in the freezer. The liquid water can be cooled to far below zero degrees Celsius without freezing due to the lack of microscopic nuclei, which are the embryos from which the freezing phase transition can occur. After that, shaking the bottle will induce nucleation which in turn prompts a rapid ice crystallization. Despite its pivotal importance and long history, we only have a rough idea about the underlying mechanism of nucleation. The classical nucleation theory says that, during the growth of a nucleus inside a bulk phase, an interface that surrounds the nucleus has to be created. This interface is associated with an energy penalty, which the system has to overcome for the nucleus to grow into a critical size which precedes an avalanche of structural transitions. It is analogous to being stuck in a valley on the Alps, so that a great deal of time and energy have to be spent to climb over a peak in order to reach another valley. And yet, we have not reached a quantitative understanding of how high the energy barrier is and how long is the waiting time of nucleation for specific systems. Taking again the example of ice nucleation from bulk liquid water, there has been a long-standing discrepancy by more than 10 orders of magnitude between the measured and the predicted expectation time of nucleation. While the classical nucleation theory is able to paint a physical picture of nucleation, for many systems it is insufficient and thus needs extension. Despite substantial improvements in recent years, experimental characterization of the dynamical nucleation processes is extremely difficult, which motivates atomistic modelling efforts that use numerical simulation techniques. However, atomistic simulations also faces a number of challenges: firstly the typical time scales accessible to atomistic simulations are confined to below microseconds, while nucleation can take hours or days to occur. We have mitigated the challenge by employing state-of-the-art enhanced sampling methods in the simulation studies of nucleation [1–5]. In a nutshell, instead of naively waiting for a rare event to happen, we place a bias to help the system overcome the energy penalty of nucleation. To return to the earlier analogy with Alpine hiking, we can flatten out the Alps by depositing (a lot of) sand into all the valleys, making the landscape level and easy to explore. Secondly, only microscopic quantities such as the coordinates and the velocities of each atom can be directly obtained from simulations. On the contrary, macroscopic observables read less

Abstract: In the search for new alloys with a great strength-to-weight ratio, magnesium has emerged at the forefront. With a strength rivaling that of steel and aluminum alloys --- materials which are deployed widely in real world applications today --- but only a fraction of the density, magnesium holds great promise in a variety of next-generation applications. Unfortunately, the widespread adoption of magnesium is hindered by the fact that it fails in a brittle fashion, which is undesirable when it comes to plastic deformation mechanisms. Consequently, one must design magnesium alloys to navigate around this shortcoming and fail in a more ductile fashion. However, such designs are not possible without a thorough understanding of the underlying mechanisms of deformation in magnesium, which is somewhat contested at the moment. In addition to slip, which is one of the dominant mechanisms in metallic alloys, a mechanism known as twinning is also present, especially in hexagonal close-packed (HCP) materials such as magnesium. Twinning involves the reorientation of the material lattice about a planar discontinuity and has been shown as one of the preferred mechanisms by which magnesium accommodates out-of-plane deformation. Unfortunately, twinning is not particularly well-understood in magnesium, and needs to be addressed before progress can be made in materials design. In particular, though two specific modes of twinning have been acknowledged, various works in the literature have identified a host of additional modes, many of which have been cast aside as "anomalous" observations. To this end, we introduce a new framework for predicting the modes by which a material can twin, for any given material. Focusing on magnesium, we begin our investigation by introducing a kinematic framework that predicts novel twin configurations, cataloging these twins modes by their planar normal and twinning shear. We then subject the predicted twin modes to a series of atomistic simulations, primarily in molecular statics but with supplementary calculations using density functional theory, giving us insight on both the energy of the twin interface and barriers to formation. We then perform a stress analysis and identify the twin modes which are most likely to be activated, thus finding the ones most likely to affect the yield surface of magnesium. Over the course of our investigation, we show that many different modes actually participate on the yield surface of magnesium; the two classical modes which are accepted by the community are confirmed, but many additional modes --- some of which are close to modes which have been previously regarded as anomalies --- are also observed. We also perform some extensional work, showing the flexibility of our framework in predicting twins in other materials and in other environments and highlighting the complicated nature of twinning, especially in HCP materials. read less

Abstract: We present the technique and results for finding norm-conserving pseudopotentials and EAM potentials that can be used to recover atomic and electronic structure of liquid iron at and above the melting point. Pseudopotentials were found by minimizing the energy differences of our results with all-electron reference methods; EAM potentials — by the modified hybridization method proposed earlier by Belashchenko. We show that these potentials are at least as accurate in describing liquid iron as the established potentials in the field. read less

Abstract: Sliding friction originating due to ploughing and adhesive wear significantly affects the performance of small-scale components, that is, nano-electromechanical systems/micro-electromechanical. To get a comprehensive understanding of the friction mechanisms, a comprehensive study of surface interactions at the nanoscale is crucial, particularly when dealing with nano-electromechanical and micro-electromechanical components. This study performed molecular dynamics simulation to explore the interactions between asperities (made of similar/dissimilar materials) at the nanoscale under dry sliding conditions. The research framework focuses on modelling the contact between two hemispherical asperities during dry sliding by considering three material combinations: soft-to-soft (Cu–Cu), hard-to-hard (Fe–Fe), and hard-to-soft (Fe–Cu). The study assesses plastic deformation and atomic wear at specific sliding speeds. Notably, the results indicate that the frictional force on the lower asperity increases as interference increases. Additionally, atomic wear rises with increased interference in the case of the Fe–Cu tribopair. Particularly high atomic wear is observed in the Cu–Cu tribopair due to the ease of slip within the face-centred cubic crystal structure of copper. read less

Abstract: The interactions between displacement cascades and three types of structures, dislocations, dislocation loops and grain boundaries, in BCC-Fe are investigated through molecular dynamics simulations. Wigner–Seitz analysis is used to calculate the number of point defects induced in order to illustrate the effects of three special structures on the displacement cascade. The displacement cascades in systems interacting with all three types of structure tend to generate more total defects compared to bulk Fe. The surviving number of point defects in the grain boundary case is the largest of the three types of structures. The changes in the atomic structures of dislocations, dislocation loops and grain boundaries after displacement cascades are analyzed to understand how irradiation damage affects them. These results could reveal irradiation damage at the microscale. Varied defect production numbers and efficiencies are investigated, which could be used as the input parameters for higher scale simulation. read less

Abstract: Slip or twinning is one of the fundamental questions in the deformation studies of metals and alloys. Internal parameters such as generalized stacking fault energy and size and external parameters such as pressure, strain rate, and temperature influence the competition between the full slip and twinning, thus dictating the predominance of one mechanism over the other. In the present investigation, we studied the influence of preloaded stress and temperature on the deformation behavior of BCC-Fe nanowires using molecular dynamics simulations and theoretical analysis. Based on detailed investigations into the energetics associated with slip and twinning, we observed that twinning is the preferred deformation mechanism in BCC-Fe. However, this has been modified by preloaded stresses applied in normal, transverse, and both directions on the nanowire. We observed a slip on {110}, on {112}, and even on {123} planes. The temperature did not alter the inherent twinning nature but linearly decreased the various fault energies. read less

Abstract: Metal targets irradiated with laser pulses have a wide range of applications in thin film preparation, nanomaterial synthesis, bio-medical imaging, and metal ablation. Here, using two-temperature model based molecular dynamics simulation, we investigate laser mediated ablation in copper. Ablation of the film starts with the formation of voids within it. This void forming mechanism at low laser fluences ([Formula: see text] mJ/cm[Formula: see text]) is studied using both picosecond and femtosecond pulses. At the same fluence, shorter laser pulse transfers more energy to the atoms generating temperatures greater than the melting temperature of the crystal. This increases the kinetic energy of the atoms and they start vibrating with different velocities. If these vibrations cross a threshold of 5 Å per picosecond (500 m/s), voids and faults start appearing in the system. At the same fluence, higher concentration of voids are also created at a faster rate with the femtosecond pulse. read less

Abstract: Fe–Cr–Al alloy is one of the candidate materials for reactor fuel cladding due to excellent high-temperature oxidation resistance; however, it has significant irradiation embrittlement and hardening. To understand the effect of Cr and Al and the defects (point defects, clusters, and nanocracks) produced from radiation damage on the mechanical properties, the uniaxial tensile property of single-crystal Fe–Cr–Al is investigated. The results show that, due to the presence of Cr and Al, the phase transformation from body-centered-cubic to face-centered-cubic is impeded and the formation of defects and amorphous structures is promoted, leading to the reduction of Young’s modulus and the ultimate tensile stress. Interstitials are the main factor in Frenkel pairs contributing to the reduction of mechanical properties due to the high shear stress and lattice distortion. The collapse of the nanocrack causes the increase of Young’s modulus and the decrease of the ultimate tensile strength. Graphical abstract read less

Abstract: Molecular dynamic simulation is used to simulate the corrosion process of Fe or Ni in liquid Cs by Large-scale Atomic/Molecular Massively Parallel Simulator. The embedded-atom method potential is used to describe the interaction of Fe–Fe, Ni–Ni, and Cs–Cs, and Morse two-body potential is used to describe the Fe–Cs and Ni–Cs atomic interaction. Temperature is considered as a critical condition in this work. Results indicate that corrosion is easy to occur in the systems. The increase in temperature can help the process of Cs corrosion. Compared to the Ni–Cs system, the Fe–Cs system has a higher atomic concentration function. The radial distribution function shows that Cs atoms are dissolved into the substrates, but the Fe and Ni substrates are still crystalline structures. Moreover, Cs in Fe or Ni is still a liquid phase. read less

Abstract: In the present work, the evolution of atomic structures and related changes in energy state, atomic displacement and free volume of symmetrical grain boundaries (GB) under the effects of external strain in body-centered cubic (bcc) iron are investigated by the molecular dynamics (MD) method. The results indicate that without external strain, full MD relaxations at high temperatures are necessary to obtain the lower energy states of GBs, especially for GBs that have lost the symmetrical feature near GB planes following MD relaxations. Under external strain, two mechanisms are explored for the failure of these GBs, including slip system activation, dislocation nucleation and dislocation network formation induced directly by either the external strain field or by phase transformation from the initial bcc to fcc structure under the effects of external strain. Detailed analysis shows that the change in free volume is related to local structure changes in these two mechanisms, and can also lead to increases in local stress concentration. These findings provide a new explanation for the failure of GBs in BCC iron systems. read less

Abstract: The lubricity of alkane is a research target for numerous tribological applications in either industrial area or fundamental scientific studies. In the current work, a comparative investigation using a classical molecular dynamics (MD) method is carried out to investigate the effect of pure iron and its oxide surfaces on structural properties, adsorption ability of hexadecane (C16H34). A reliable force field (FF) of condensed-phase optimized molecular potentials for atomistic simulation studies (COMPASS) is employed to describe the intra- and intermolecular interactions for hexadecane and its interaction with iron oxide surfaces, while the interaction between hexadecane and pure iron is derived from an ab initio result. Regarding the surfaces, the pure iron surfaces are considered using embedded-atom method/Finnis-Sinclair potential (EAM/FS), while the iron oxide surfaces are constructed using the traditional Buckingham force field. The results reveal that hexadecane shows preferential adsorption on iron oxide surfaces compared to pure iron. read less

Abstract: Summary. Hadfield steel, due to its excellent work hardening ability read less

Abstract: With the goal of improving data based materials design, it is shown that by a sequential design of experiment scheme the process of generating and learning from the data can be combined to discover the relevant sections of the parameter space. The application is the energy of grain boundaries as a function of their geometric degrees of freedom, calculated from a simple model, or via atomistic simulations. The challenge is to predict the deep cusps of the energy, which are located at irregular intervals of the geometric parameters. Existing sampling approaches either use large sets of datapoints or a priori knowledge of the cusps' positions. By contrast, the authors' technique can find unknown cusps automatically with a minimal amount of datapoints. Key point is a Kriging interpolator with Matérn kernel to estimate the energy function. Using the jackknife variance, the next point in the sequential design is a compromise between sampling the region of largest fluctuations and avoiding a clustering of datapoints. In this way, the cusps of the energy can be found within only a few iterations, and refined as desired. This approach will be advantageous for any application with strong, localized fluctuations in the values of the unknown function. read less

Abstract: ABSTRACT The elementary processes of friction in contact processes of two solid bodies occur on the nanoscale and are difficult to study experimentally. Therefore, molecular dynamics simulations are often used for their elucidation. In these studies, usually, only a single simulation is carried out for each scenario and the resulting observables are evaluated. In the present work, the reliability and reproducibility of measured observables from such nanoscopic contact process simulations are assessed by means of their statistical uncertainties. Therefore, the computer experiment is carried out not only once, but it is repeated eight times, with individual runs that only differ in the initial thermal motion. This set of replicas enables an assessment of observations for distinguishing reproducible physical phenomena from stochastic coincidence. In this way, a dry and a lubricated nanoscale scratching process were studied, in which a cylindrical indenter carried out two sequential movements: it first penetrates a substrate vertically and then scratches laterally, which causes elastic and plastic deformation of the substrate. In the lubricated case, the indenter was fully immersed in the fluid. Substrate, indenter, and fluid were described by suitably parametrised Lennard–Jones potentials. Various mechanical and thermodynamic process properties were monitored in all simulation runs. The results are compared and evaluated statistically. read less

Abstract: The screw dislocation mobility in bcc Nb has been studied by molecular dynamics (MD) simulations at different strain rates and temperatures using an embedded-atom method (EAM) potential. Static properties of the screw dislocation, as determined with the EAM potential, are in agreement with previous density-functional-theory calculations. The elementary slip plane of the screw dislocation remains (110) for all studied strain rates (in the range 6.3 × 107–6.3 × 109 s−1) and temperatures (5 to 550 K). However, the consecutive cross-slip on different symmetry-equivalent (110) planes leads to an effective glide on (112) planes. It is demonstrated that the screw dislocation trajectories, velocities and waviness of the screw dislocation depend on the crystallographic indices, (110) or (112), of the maximum resolved shear stress plane. The waiting time for the start of the screw dislocation motion increases exponentially with decreasing strain rate, substantiating the necessity to apply in future accelerated MD techniques in order to compare with macroscopic stress-strain experiments. read less

Abstract: To explore the thermal effect of interfacial friction at the nano/microscale, a solid-solid contact model of a rough surface with a single peak was established to research single-crystal Fe. The friction characteristics, stress distributions, temperature changes, and energy changes under different indentation depths and lattice orientations during the shearing process were analyzed. From the perspective of temperature and energy, the mechanism of the thermal effect was revealed. The relationship between the friction force, temperature, and energy at the atomic scale was clarified. The results showed that the temperature of the asperities gradually increased during the shearing process, and a stress concentration formed in the shearing zone. After contact, the asperities had undergone unrecoverable plastic deformation, and there was wear at the contact interface accompanied by the loss of atoms from the asperity. At each indentation depth, as the rotation angle of the crystal increased, the friction force, average temperature, and the sum of the changes in thermal kinetic and thermal potential energy all first increased and then decreased; the trends of the three parameters changing with the rotation angle of the crystal were consistent. The average decreases in the friction force, average temperature, and the sum of the changes in thermal kinetic and thermal potential energy were 52.47%, 30.91%, and 56.75%, respectively, for a crystal structure with a rotation angle of 45° compared to a crystal structure with a rotation angle of 0°. The methods used in this study provide a reference for the design of frictional pairs and the reduction of the thermal effect of interfacial friction. read less

Abstract: Lubricant has been widely applied to reduce wear and friction between the contact surfaces when they are in relative motion. In the current study, a nonequilibrium molecular dynamics (NEMD) simulation was specifically established to conduct a comprehensive investigation on the dynamic contact between two iron surfaces in a boundary friction system considering the mixed C4-alkane and nanoparticles as lubricant. The main research objective was to explore the effects of fluid and nanoparticles addition on the surface contact and friction force. It was found that nanoparticles acted like ball bearings between the contact surfaces, leading to a change of sliding friction mode to rolling friction mode. Under normal loads, plastic deformation occurred at the top surface because nanoparticles were mainly supporting the normal load. By increasing the number of C4-alkane molecules between two contact surfaces, the contact condition has been changed from partial to full lubrication. In addition, an attractive force from the solid–liquid LJ interaction between C4-alkane and surfaces was observed at the early stage of sliding, due to the large space formed by wall surfaces and nanoparticles. The findings in this paper would be beneficial for understanding the frictional behavior of a simple lubricant with or without nanoparticles addition in a small confinement. read less

Abstract: A characteristic region with vacancy concentration ranging from 0% to 2% was introduced into the single-crystal iron to investigate the effects of vacancies on plasticity and phase transformation of single-crystal iron under shock loading. The simulations were implemented by applying non-equilibrium molecular dynamics simulations with an excellent modified analytic embedded-atom method (MAEAM) potential. A fixed piston velocity of vp = 0.5 km/s was applied in our simulations, under which no plasticity or phase transformation occurred in the perfect single-crystal iron based on the description of the used MAEAM potential. The plasticity and phase transformation in iron were observably influenced by the vacancies as shown in this work. Significant anisotropy of shock response was distinctly exhibited. The nucleation and growth of dislocation loops emitting from the vacancy region were clearly observed in the sample that was shocked along the [110] direction, and the activated slip systems were determined as ( 11 2 ¯ )[111] and (112) [ 11 1 ¯ ]. The vacancies and the vacancies-induced dislocation loops provided preferential nucleation positions for the subsequent phase transformation, which resulted in the phenomenon that the phase transformation product (HCP phase) always preferentially appeared in the vacancy region. The influences of different vacancy concentrations on plasticity and phase transformation were also discussed. read less

Abstract: The tensile tests of body-centered cubic (BCC) Fe nanowires were simulated through molecular dynamics methods. The temperature and strain rate effects on the mechanical properties as well as the orientation-dependent plastic deformation mechanism were analyzed. For [001]-oriented BCC Fe nanowires, as the temperature increased, the yield stress and Young’s modulus decreased. While the yield stress and Young’s modulus increased as the strain rate increased. With the increase in temperature, when the temperature was less than 400 K, the twin propagation stress decreased dramatically, and then tended to reach a saturation value at higher temperatures. Under different temperatures and strain rates, the [001]-oriented Fe nanowires all deformed by twinning. The oscillation stage in the stress–strain curve corresponds to the process from the nucleation of the twin to the reorientation of the nanowire. For [110]-oriented Fe nanowires, the plastic deformation is dominated by dislocation slip. The independent events such as the nucleation, slip, and annihilation of dislocations are the causes of the unsteady fluctuations in the stress–strain curve. The Fe nanowires eventually undergo shear damage along the dominant slip surface. read less

Abstract: The development of classical interatomic potential for iron is a quite demanding task with a long history background. A new interatomic potential for simulation of iron was created with a focus on description of crystal defects properties. In contrast with previous studies, here the potential development was based on force-matching method that requires only ab initio data as reference values. To verify our model, we studied various features of body-centered-cubic iron including the properties of point defects (vacancy and self-interstitial atom), the Peierls energy barrier for dislocations (screw and mix types), and the formation energies of planar defects (surfaces, grain boundaries, and stacking fault). The verification also implies thorough comparison of a potential with 11 other interatomic potentials reported in literature. This potential correctly reproduces the largest number of iron characteristics which ensures its advantage and wider applicability range compared to the other considered classical potentials. Here application of the model is illustrated by estimation of self-diffusion coefficients and the calculation of fcc lattice properties at high temperature. read less

Abstract: Nanowires (NWs) have many enhanced properties such as high yield strength, high ductility and large elongation under tensile loading. Though many molecular dynamics simulations have been performed exhibiting the benefit of having grain boundaries in nanowires, this study focuses on the mechanical properties of a specific grain formation similar to a structure of bamboo that indicates the opposite phenomena. In this experiment, BCC Fe nanowires were strained under tensile loading and with the assistance of molecular dynamics simulation it was demonstrated that the behavior of polycrystalline Fe NW exhibits dramatic differences from their single crystalline NW counterparts. In single crystalline Fe NW, dislocations were generated from the surface in the directions of specific planes for BCC materials; whereas in the case of nanowire with bamboolike polycrystalline shape, dislocation were generated from grain boundary due to the localized high stress and the heterogeneous character of atoms in the grain boundary. Ultimate strengths for 〈111〉, 〈1-1-2〉, 〈102〉 and 〈110〉 oriented single crystal NWs were found 34.57 GPa, 19.15 GPa, 14.94 GPa and 25.48 GPa respectively. A substantial low strength (8.89 GPa) was observed for the polycrystalline Fe structure, which was mainly caused due to the deformation in the interface of the two subsequent grains. Necking was observed followed by fracture, resulting in an overall drop in toughness. These findings can be a matter of concern in certain applications where materials with high strength and stability are required. read less

Abstract: This study concerns the effect of bicrystal spatial orientation on the iron Σ5 boundary under nanoindentation tests. The orientation of the bicrystal is changed relative to the indenter by the parameter “nanoindentation angle” to create dislocation collisions with grain boundary in different directions and to determine the possibility of transmission of these dislocations through the grain boundary. Simulations were carried out for nanoindentation angles between −90° and 90° with a resolution of 18°. Molecular dynamics with EAM atomic potential functions are employed for the simulations. The effect of nanoindentation angle on the crystals Young’s modulus, shear stress, hardness, and Poisson’s ratio were determined. Due to the periodicity which has been observed in all of the aforementioned quantities of the rotated crystal, analytical equations were extracted by adapting the Fourier series expansion on the calculated data. Some explanations were provided for the dislocation behaviors based on the physics of the problem and also by the results reported in the literature. Simultaneous effects of Poisson’s ratio and pile-ups on each other were also investigated and interpreted. read less

Abstract: Molecular dynamics simulations were conducted to explore primary radiation damages on body-centered cubic (BCC) Fe nanoparticle. A series of 6 cascades for each primary knock-on atom (PKA) energy (5 keV, 10 keV, 20 keV, 30 keV and 40 keV) was simulated to assure statistical precision. It has been observed that defects created due to the interactions have stayed as a single to several size clusters. Among them, most of the clusters are either single interstitials (SIAs) or vacancies (Vs). In each of the energy cases, it produces one block of a big cluster. The block cluster of SIAs stays at the near surface of the nanoparticle, however, Vs stay inside the nanoparticle. The study has shown that the total number of vacancy defects is larger than the total number of interstitial defects because more PKA energy gives more mobility that some SIAs can get enough energy to leave the Fe nanoparticle. read less

Abstract: The remarkable lubricant properties of molybdenum dithiocarbamates (MoDTCs) make this class of oil additives well-known in the automotive industry. However, the mechanism of function of these compo... read less

Abstract: In this Letter, the structural stability, local structure characterization and magnetic properties of iron nanoparticles (FeNPs) were investigated by using a combination of Molecular Statics (MS) and Monte Carlo (MC) simulations. To do so, six kinds of spherical-shaped FeNPs with diameters in the range of 3.14 to 5.42 nm have been considered. The coordination number distribution of FeNPs obtained from the data extracted by MS simulations was exploited for performing MC simulations on the familiar Ising model. The numerical findings obtained in the current study show that the structural and magnetic properties correlate with the size of the FeNP. read less

Abstract: This study aims to investigate the underlying mechanisms which govern the development of dense defect networks in nanoscale crystal plasticity, either under contact and uniaxial loading conditions, with emphasis on the onset of intermittent avalanche phenomena. The investigation is based on a comprehensive set of massive molecular dynamics (MD) simulations performed with embedded-atom method potentials in face-centered cubic (FCC) and body-centered cubic (BCC) crystals. The first part of the thesis concerns the combined role of elasticity and plasticity in nanocontact loadings, where attention is given to the mechanisms leading to the formation of a permanent nanoimprint as well as to the onset of material pile-up at the contact vicinity. It is found that the topographical arrangement of the slip traces emitted at the surface into specific deformation patterns is a distinctive feature of the underlying dislocation glide and twinning processes occurring in FCC and BCC crystals as a function of temperature and surface orientation. A mechanistic analysis is made on the influence of the defect nucleation events in conjunction with the development of entangled defect networks upon the material hardness and its evolution towards a plateau level with increasing indenter-tip penetration. Complementary MD simulations of the uniaxial stress-strain curve of the plastically deformed region are carried out with the purpose of establishing a direct correlation between nanoscale material responses attaining under uniaxial and contact loading conditions. The results of this comparison illustrate on the key role played by defect nucleation processes on the formation of permanent nanoimprints, which differs from the conventional view in that in micro and macroscopic scales imprint formation is essentially governed by the evolutionary character of a preexisting (entangled) defect network: the greater the dislocation density, the larger the measured hardness. In overall, this work provides a fundamental insight into the understanding of why BCC surfaces are harder than FCC surfaces at the nanoscale. A statistical physics background is devised to investigate the influence of the dislocation mechanisms on the onset of avalanche events that are inherent to crystal plasticity. The analysis is predicated upon the notion in that the size distribution of such avalanches follows power-law scaling. To investigate the avalanche size distributions in cubic crystals, a group of novel MD simulations are performed where the computational cells containing a periodic arrangement of a preexisting dislocation network are subjected to uniaxial straining under displacement control at different strain rates and temperatures. Under sufficiently slow driving, the dislocation networks evolve through the emission of dislocation avalanches which do not overlap in time. This illustrates that the mobilized entangled dislocation arrangements exhibit quiescent periods during each plastic (dissipative) event, enabling comparison with experimental results which are also performed under strict displacement controlled conditions. The results illustrate on the attainment of a transitional slip size separating two power-law avalanche regimes as a function of the fundamental dislocation glide processes at the crossroads of self-organized and tuned criticality models. Detailed analyses of the MD simulations furnish specific mechanisms characterizing dislocation avalanche emission and propagation in FCC and BCC metals throughout a wide temperature range, which is central in supporting the onset of the aforementioned two power-law regimes. En este estudio se investigan los mecanismos fundamentales para el desarrollo de las densas redes de defectos que se producen durante la deformación plástica de metales mediante ensayos uniaxiales y de indentación en escalas nanométricas. Estos procesos de deformación plástica se caracterizan por la producción de eventos intermitentes o avalanchas de dislocaciones. La investigación se basa en un extenso grupo de simulaciones de dinámica molecular en las que se emplean potenciales interatómicos del tipo embedded-atom method en cristales cúbicos centrados en las caras (CCC) y cúbicos centrados en el cuerpo (CC). La primera parte de esta tesis discute el papel combinado de la elasticidad y plasticidad en los nanocontactos. Se presta una especial atención a los mecanismos que llevan a la formación de nanohuellas plásticas así como al desarrollo de apilamiento de material alrededor del nanocontacto. Se encuentra que los arreglos topográficos de trazas de deslizamiento (emitidas a la superficie) muestran patrones específicos de deformación, los cuales son a su vez un rasgo distintivo de los mecanismos de deslizamiento de dislocaciones y procesos de nanomaclado que ocurren en los materiales CCC y CC en función de la temperatura y la orientación de la superficie. Se presenta un estudio mecanístico sobre la influencia de los eventos de nucleación de defectos, que llevan al desarrollo de una compleja red de defectos, sobre la nanodureza y su convergencia hacia un valor relativamente constante a medida que el indentador penetra en la superficie La modelización del comportamiento uniaxial de la zona deformada debajo de las nanoindentaciones permite la correlación entre ambos tipos de ensayos. Los resultados de esta comparación ilustran el importante papel que juegan los procesos de nucleación de dislocaciones sobre la formación de nanohuellas plásticas, lo que difiere (en términos mecanísticos) del comportamiento plástico convencional encontrado en escales micro y macroscópicas, donde el carácter evolutivo de una red de defectos preexistente gobierna la formación de huella, cumpliéndose así que cuanto mayor es la densidad de defectos, mayores son también las macro y microdurezas. En general, este trabajo aporta un trasfondo fundamental para comprender la razón por la que las superficies CC son más duras que las CCC en la nano escala. En la última parte de esta investigación se utilizan modelos de física estadística para investigar la influencia de los mecanismos de propagación de dislocaciones sobre la emisión de avalanchas plásticas. El análisis se basa en la noción de que la distribución del tamaño de las avalanchas sigue una ley potencial universal. Para investigar esta distribución en cristales cúbicos, se realizan un grupo de simulaciones novedosas donde las celdas computaciones, que contienen arreglos periódicos de las redes de dislocaciones, son sometidas a cargas uniaxiales a diferentes temperaturas y velocidades de deformación. A velocidades de deformación suficientemente lentas, las redes de dislocaciones evolucionan a través de la emisión de avalanchas que no se sobreponen en el tiempo, lo que ilustra que la movilización de las redes ocurre de tal manera que se garantiza una alternancia entre periodos de inactividad y cada evento plástico. La comparación entre resultados experimentales y computacionales lleva a encontrar la existencia de una magnitud de deslizamiento crítico que separa a dos regímenes de avalanchas cuya distribución de tamaños obedece leyes potenciales. Este resultado demuestra que los procesos de avalanchas son claramente dependientes de los mecanismos de deslizamiento e interacción de dislocaciones presentes en el material; aspecto que describe la transición entre el modelo de criticalidad gobernada por la tensión y el de criticalidad auto-organizada. Las simulaciones muestran los mecanismos específicos que caracterizan la emisión y propagación de avalanchas en metales CC y CCC en un amplio rango de temperatura, lo que es de gran importancia para justificar la utilización de estos modelos de criticalidad. read less

Abstract: The plasticity of body-centered-cubic metals at low temperatures is substantially determined by the screw-dislocation kinetics. Because the core of screw dislocations in these metals has a non-planar structure, its motion is complex. For example, although density functional theory predicts slip on a {110} plane, the actual slip plane at elevated temperatures differs from the prediction. In this work, we explored state-of-the-art atomistic modeling methods and successfully reproduced the transition of the slip plane through a temperature increase. We then devised an algorithm to analyze the activation of dislocation jump over the Peierls barrier and discovered a possible origin of this unexpected phenomenon: thermal fluctuation leads to the kink-pair nucleation for cross slip jumps with no transition of the dislocation core structure. read less

Abstract: Carbon diffusion is a critical process to the manufacture of many industry products, such as iron carbides, stainless steels, and carbon materials. Here we investigate carbon diffusion induced by tensile, screw dislocation, edge dislocation, and polycrystal boundary through reactive molecular dynamics simulations with ReaxFF potentials. The temperature enhances the dynamics and therefore the carbon diffusion. The pre-existing defects promote the carbon diffusion with a linear relationship between carbon diffusion barrier and strain as well as line defect concentrations. Furthermore, we also observed a linear relationship between the carbon diffusion barrier and the volume fractions of the polycrystalline boundary, indicating that the grain boundary mechanism is prominent in carbon diffusion in the carbon iron. read less

Abstract: The understanding of radiation-induced microstructural defects in body-centered cubic (BCC) iron is of major interest to those using advanced steel under extreme conditions in nuclear reactors. In this study, molecular dynamics (MD) simulations were implemented to examine the primary radiation damage in BCC iron with displacement cascades of energy 1, 5, 10, 20, and 30 keV at temperatures ranging from 100 to 1000 K. Statistical analysis of eight MD simulations of collision cascades were carried out along each [110], [112], [111] and a high index [135] direction and the temperature dependence of the surviving number of point defects and the in-cascade clustering of vacancies and interstitials were studied. The peak time and the corresponding number of defects increase with increasing irradiation temperature and primary knock-on atom (PKA) energy. However, the final number of surviving point defects decreases with increasing lattice temperature. This is associated with the increase of thermal spike at high PKA energy and its long timespan at higher temperatures. Defect production efficiency (i.e., surviving MD defects, per Norgett-Robinson-Torrens displacements) also showed a continuous decrease with the increasing irradiation temperature and PKA energy. The number of interstitial clusters increases with both irradiation temperature and PKA energy. However, the increase in the number of vacancy clusters with PKA energy is minimal-to-constant and decreases as the irradiation temperature increases. Similarly, the probability and cluster size distribution for larger interstitials increase with temperature, whereas only smaller size vacancy clusters were observed at higher temperatures. read less

Abstract: The interactions of an edge dislocation (ED) with collision cascades induced by 5 keV primary knocked-on atoms (PKAs) towards the ED in bcc Fe are studied using classical molecular dynamics (MD) simulations. It is found that the number and distribution of the residual point defects are related to the distance between the initial PKAs and the ED. Based on this result, we provide a comprehensive summary of four characteristic phenomena for cascade–ED interactions, including few interactions, the formation of a vacancy cluster, the sink effect for point defects, and the sub-cascade area affection, depending on the overlap of the peak cascades' area with the ED line. Then a qualitative model is proposed to clearly elucidate the underlying mechanisms of the four situations. Considering that dislocations constitute an essential part of the micro-structure of crystalline solids, our work demonstrates that: the pre-existing dislocations in crystalline materials could induce diverse effects under irradiation environments, which should be taken into account for designing and improving the radiation resistant materials. read less

Abstract: В данном исследовании проверялась корректность работы трех межатомных потенциалов взаимодействия, доступных в научной литературе, в молекулярно-динамическом моделировании вакансионной диффузии в концентрированных сплавах Fe–Cr. Проведенная работа была необходима для дальнейшего детального исследования механизма вакансионной диффузии в данных сплавах с содержанием хрома 5–25 ат.% в температурном диапазоне 600–1000 К. Анализ был выполнен на моделях сплава с содержанием хрома 10, 20, 50 ат.%. Рассмотрение модели сплава с 50 ат.% хрома было необходимо для дальнейшего исследования диффузионных процессов в обогащенных хромом преципитатах данных сплавов. Для всех потенциалов были рассчитаны и проанализированы энергии формирования вакансии в сплавах и диффузионные подвижности атомов железа и хрома через искусственно созданную одиночную вакансию. В качестве основной характеристики для анализа подвижностей атомов была выбрана временная зависимость их среднеквадратичного смещения. Моделирование энергий формирования вакансий не выявило качественных различий между исследуемыми моделями потенциалов. Проведенное исследование атомных подвижностей показало плохое воспроизведение диффузии вакансии в исследуемых сплавах концентрационно-зависимой моделью (CDM), которая сильно занижала подвижность атомов хрома через вакансию во всем исследуемом диапазоне температур и концентраций хрома. Установлено, что двусвязная модель потенциала (2BM) в своей оригинальной и модифицированной версии подобных недостатков не имеет. Это позволяет использовать эти потенциалы в моделированиях вакансионного механизма диффузии в исследуемых сплавах. Для обоих 2BM-потенциалов была зафиксирована существенная зависимость соотношения подвижностей хрома и железа от температуры и содержания хрома в сплавах. Количественные данные коэффициентов диффузии атомов, полученные этими потенциалами, также существенно различаются. read less

Abstract: Molecular dynamics simulations have been performed to understand the influence of temperature on the tensile deformation and fracture behavior of ⟨111⟩ BCC Fe nanowires. The simulations have been carried out at different temperatures in the range 10–1000 K employing a constant strain rate of 1 × 108 s−1. The results indicate that at low temperatures (10–375 K), the nanowires yield through the nucleation of a sharp crack and fails in brittle manner. On the other hand, nucleation of multiple 1/2⟨111⟩ dislocations at yielding followed by significant plastic deformation leading to ductile failure has been observed at high temperatures in the range 450–1000 K. At 400 K, the nanowire yields through nucleation of crack associated with many mobile 1/2⟨111⟩ and immobile ⟨100⟩ dislocations at the crack tip and fails in ductile manner. The ductile-brittle transition observed in ⟨111⟩ BCC Fe nanowires is appropriately reflected in the stress-strain behavior and plastic strain at failure. The ductile-brittle transitio... read less

Abstract: Using magnetic potentials and a molecular statics approach, we study the changes in the magnetic structure of bcc Fe in the vicinity of grain boundaries (GBs). We focus on symmetric tilt GBs around the [0 0 1] axis with a tilt angle between 7 and 53. We find that immediately in the GB plane, the deviations in the magnetic moments from the bulk value are most pronounced. The distribution of moments in the GB plane is modulated according to the periodicity of the coincidence site lattice. In the direction perpendicular to the GB plane, the moments decay exponentially towards the bulk value; the decay length increases with decreasing tilt angle. This dependence can be explained by the well-known stress field around GBs. read less

Abstract: This paper presents molecular dynamic modeling and simulation of lubricant between sliding solids. Linear n-alkanes with united atoms were used to model lubricant while iron sliding solids were modeled with body-centered cubic crystal lattices. We employed various potential functions, including the embedded atom method, the multibody force field and the Lennard–Jones potential, to approximate the interatomic interactions in the molecular model. Hydrodynamic lubrication was considered in this paper. We found that the temperature and the chain length of alkanes had effects on the friction between lubricated sliding solids. In addition, one debris, modeled as a nanoparticle, was added in the lubricant to study its effect on the friction. It was observed that nanoparticles would increase the friction in hydrodynamic lubrication. read less

Abstract: Atomistic computer simulations are playing an increasingly important role in high temperature processes, bridging the gap between theory and experiment, and taking cutting edge research into unchartered territories. A fundamental understanding of the melting behaviour of BCC iron is of great significance in steelmaking operations. We report Monte Carlo simulations on the melting transition of iron using three well known interaction potentials, namely the Rosato potential, the Mendelev potential and the Sutton-Chen potential. Depending on the potential used, the interactions between iron atoms varied from 1/r2, as well as 1/r8.14 at short distances, and the range of applicability was found to vary from 3.5 A to 9.5 A. These atomic level computer simulations were carried out using a range of simulation variables such as the size of the simulation cell, step size, boundary conditions and statistical ensemble.  The melting transition was identified clearly in all cases through discontinuities in energy and increases in local disorder. The simulation data however showed a significant scatter. Interaction potentials of iron based on system characteristics at 0 K were found somewhat limited for quantitative high temperature simulations (~1800K). Iron potentials developed for earth science applications were not found to be suitable for these investigations. These computer simulations therefore point towards a significant gap in the knowledge base in the atomistic modelling of molten iron, especially for steelmaking applications. read less

Abstract: The evolution of atomic displacement cascades initiated near free surfaces with different crystallographic orientations in bcc iron specimens was studied on the base of the molecular dynamics approach. The craters surrounded by adatom mounds were formed in the case of the (111) surface irradiation. The dislocation loops consisted of vacancies were generated after the (110) surface irradiation. The dislocation Burgers vector was a/2 <111> or a <100>. It was shown that the type of structural damage is determined by the anisotropy of propagation of shock waves generated by atomic displacement cascades. For energies of the atomic displacement cascade lower than 20 keV the number of adatoms and survived point defects was higher for specimen with the (110) free surface due to the different character of surface damage. The increase in the cascade energy up to 20 keV results in formation of almost the equal number of survived point defects for the (110) and (111) free surfaces as displacement cascades were developed on larger distance from the irradiated surfaces. read less

Abstract: ABSTRACT Reduced-activation ferritic/martensitic steels of Cr concentration between 2.25 and 12 wt% are candidate structural materials for next-generation nuclear reactors. In this study, molecular dynamics (MD) simulation is used to generate the displacement cascades in Fe–Cr structures with different Cr concentrations by using different primary knock-on atom (PKA) energies between 2 and 10 keV. A concentration-dependent model potential has been used to describe the interactions between Fe and Cr. Single crystals (SCs) of three different coordinate bases (e.g. [310], [510], and [530]) and bi-crystal (BC) structures with three different [001] tilt grain boundaries (GBs) (e.g. Σ5, Σ13, and Σ17) have been simulated. The Wigner–Seitz cell criterion has been used to identify the produced Frenkel pairs. The results show a marked difference between collisions observed in SCs and those in BC structures. The numbers of vacancies and interstitials are found to be significantly higher in BC structures than those found in SCs. The number of point defects exhibits a power relationship with the PKA energies; however, the Cr concentration does not seem to have any influence on the number of survived point defects. In BC models, a large fraction of the total survived point defects (between 59% and 93%) tends accumulate at the GBs, which seem to trap the generated point defects. The BC structure with Σ17 GB is found to trap more defects than Σ5 and Σ13 GBs. The defect trapping is found to be dictated by the crystallographic parameters of the GBs. For all studied GBs, self-interstitial atoms (SIAs) are easily trapped within the GB region than vacancies. An analysis of defect composition reveals an enrichment of Cr in SIAs, and in BC cases, more than half of the Cr-SIAs are found to be located within the GB region. read less

Abstract: A nonequilibrium phase transition (melting) initiated by the volumetric heat source is investigated using a molecular dynamics method (MMD). Physical parameters characterizing the study process are computed. It is shown that at the critical level of the heat source power density the phase transition is realized under nonequilibrium conditions of considerable local overheating and may be accompanied by the formation of the locally unstable state of a microcrystal and complex dynamics: the origin of phase transformation fronts both on the surface and deep within the microcrystal. read less

Abstract: A Kinetic Monte Carlo simulation, using a modified version of the SPPARKS code, of simple defects and complex vacancy clusters was run on a bcc lattice. In this simulation the complexity of void formation was varied by introducing a detachment rate for individual vacancies leaving the void and either treating this value as constant for all size voids or having this value be dependent on the size of the void. Molecular Dynamics simulations were used to determine the binding energies of vacancies for voids of varying size. The simulation was then run over long time periods to determine the number of defects in the simulation under irradiation conditions. It was found that the additional complexity of size dependent void detachment rates had little effect on the defect concentrations and thus a constant barrier should be sufficient for simulations of voids in bcc metals. read less

Abstract: The effect of displacement cascade on Fe-Y2TiO5 bilayer is studied using classical molecular dynamics simulations. Different PKA species – Fe, Y, Ti and O – with the same PKA energy of 8 keV are used to produce displacement cascades that encompass the interface. It is shown that Ti atom has the highest movement in the ballistic regime of cascades which can lead to Ti atoms moving out of the oxide clusters into the Fe matrix in ODS alloys. read less

Abstract: In this paper, we investigate the dynamic shear strength of perfect monocrystalline metals using the molecular dynamics simulation. Three types of deformation (single shear, uniaxial compression and tension) are investigated for five metals of different crystallographic systems (fcc, bcc and hcp). A strong dependence of the calculated shear strength on the deformation type is observed. In the case of bcc (iron) and hcp (titanium) metals, the maximal shear strength is achieved at the uniaxial compression, while the minimal shear strength is observed at the uniaxial tension. In the case of fcc metals (aluminum, copper, nickel) the largest strength is achieved at the pure shear, the lowest strength is obtained at the uniaxial compression. read less

Abstract: Molecular dynamics investigation of metal crystallite with bcc lattice under nanoindentation was carried out. Potentials of interatomic interactions were calculated on the base of the approximation of the Finnis-Sinclair method. For clarity and simpler indentation data interpretation, an extended cylindrical indenter was used in the investigation. The features of the bcc iron structural response at nanoindentation of surfaces with different crystallographic orientations were revealed. Generation of structural defects in the contact zone always resulted in the decrease in the rate of growth of the reaction force. read less

Abstract: A comparative analysis of adsorption of six normal-alkanes (CNH2N+2, N = 4, 6, 8, 10, 12, 16) on Fe(110), FeO(110), and Fe2O3(0001) was carried out using classical molecular dynamics (MD) simulation. A realistic model system for adsorbed alkanes was employed using the COMPASS force field (FF), while the appropriate relaxed surfaces and an effective interfacial potential were obtained from ab initio calculations. The results show that butane molecules orient randomly on Fe(110) and Fe2O3(0001) surfaces, but they preferentially orient in the (010) direction on FeO(110) at low temperature. Additionally, alkanes adsorb physically on Fe(110), FeO(110), and Fe2O3(0001), in the following decreasing order Fe(110) > FeO(110) > Fe2O3(0001). The adsorption energies per saturated carbon site decrease with an increase of molecular chain length, and this propensity is similar for different surface potentials. In contrast, the saturated carbon density is insensitive to the surface potentials and shows an increasing tren... read less

Abstract: Nanofluids exhibit superior thermal properties to conventional fluid and particle-fluid suspensions and show a great potential as quenching media for quench hardening of steel components. The heat transfer mechanism in nanofluid is very complex and unclear. In this paper, molecular dynamics (MD) simulation method is used to theoretically study the heat transfer from a metal surface at different temperatures to a water-based nanofluid with functionalized carbon nanotubes (FCNTs). To model the quenching process, an initial temperature jump between the nanofluid and an iron slab is employed, and non-equilibrium molecular dynamics (NEMD) simulations are performed. The MD results reveal the heat transfer process in the initial stage of quenching and at the first moment of contact of a liquid nanofluid with a hot metal surface. The thermodynamics and transport properties of the nanofluid and the heat transfer characteristics are discussed with the atomistic details of the interactions of the FCNT with the iron atoms and the water molecules. read less

Abstract: (Received 10 July 2014; revised manuscript received 10 December 2014; published 12 January 2015) In this paper, molecular dynamics (MD) simulations based on the modified-embedded atom method (MEAM) and a phase-field crystal (PFC) model are utilized to quantitatively investigate the solid-liquid properties of Fe. A set of second nearest-neighbor MEAM parameters for high-temperature applications are developed for Fe, and the solid-liquid coexisting approach is utilized in MD simulations to accurately calculate the melting point, expansion in melting, latent heat, and solid-liquid interface free energy, and surface anisotropy. The required input properties to determine the PFC model parameters, such as liquid structure factor and fluctuations of atoms in the solid, are also calculated from MD simulations. The PFC parameters are calculated utilizing an iterative procedure from the inputs of MD simulations. The solid-liquid interface free energy and surface anisotropy are calculated using the PFC simulations. Very good agreement is observed between the results of our calculations from MEAM-MD and PFC simulations and the available modeling and experimental results in the literature. As an application of the developed model, the grain boundary free energy of Fe is calculated using the PFC model and the results are compared against experiments. read less

Abstract: A molecular dynamics simulation of confined n-alkanes with different chain length and thickness has been performed. It discusses the film structure and shear stress of thin film at the atomic scale. Specifically, layered structure in thin film is observed due to the strong liquid-solid interaction and small confinement. An explicit two-layer lubricant film that has ‘tetratic’ order is observed. Without crystalline bridges, the shear stress drops dramatically. For a thin film with up to three layers, the shear stress varies with thickness. This is due to that the number of crystalline bridges varies between ordered layers. When the film thickness increases further, the effect of the interfacial interaction towards the inner film becomes blunt, and the shear stress becomes stable and is dominated by the lubricant chain length. read less

Abstract: We investigate the influence of C interstitials on the phase stability of Fe–C crystals. We employ the Meyer–Entel interatomic interaction potential which is able to reproduce the austenite-martensite phase transition for pure Fe, and supplement it by a simple pairwise Fe–C interaction potential. Using two different thermodynamic methods, we calculate the free energies of the martensite and austenite phases. We find that C destabilizes the ground-state bcc phase. The decrease in the equilibrium transformation temperature with increasing C content parallels the one found in the experiment. This destabilization is found even if C is added for a potential in which only the bcc phase is stable until the melting point; here, for sufficiently high C addition, a stable fcc phase is established in the phase diagram. read less

Abstract: Molecular dynamics investigation of metal crystallite with bcc lattice under nanoindentation was carried out. Potentials of interatomic interactions were calculated on the base of the approximation of the embedded atom method. The potentials chosen make it possible to describe with a high accuracy the elastic and surface properties of the simulated metal and energy parameters of defects, which is important for solution of the task posed in the work. For clarity and simpler indentation data interpretation, an extended cylindrical indenter was used in the investigation and loading was realized by its lateral surface. The simulated crystallite had a parallelepiped shape. The loaded plane of crystallite was modeled as a free surface while the positions of atoms in the opposite plane of crystallite were fixed along the indentation direction. Other planes of crystallite were simulated as free surfaces. The indenter velocity varied from 5 to 25 m/s in different calculations. The loading of the model crystallite was realized at 300 K. Influence of interfaces (free surfaces and grain boundaries) on peculiarities of plastic deformation nucleation and interactions of generated structural defects with interfaces in simulated crystallite under nanoindentation were investigated. read less

Abstract: Molecular dynamics simulation has been performed to explore the structure, thermodynamics and dynamics properties of Cu-Co melt based upon embedded atom method (EAM). The pair correlation function of liquid Cu50Co50 show stronger interaction of homogeneous atom pairs. The coordination number (CN) of Cu-Cu and Co-Co in Cu50Co50 melt is a little higher than that of Cu-Co at temperature of 1800 K. The calculated enthalpy of mixing is positive in the whole concentration range and S CC(q) increases sharply at lower q, which are the typical features of dense fluid that exhibits phase segregation tendency. The interdiffusion coefficient shows same concentration dependence as that of demixing alloy. Our work indicates that Cu-Co melt exhibits weak demixing behaviour even at temperatures greater than those of bimodal curve. read less

Abstract: The Phase-Field Crystal (PFC) model represents the atomic density as a continuous function, whose spatial distribution evolves at diffusional, rather than vibrational time scales. PFC provides a tool to study defect interactions at the atomistic level but over longer time scales than in molecular dynamics (MD). We examine the behavior of the PFC model with the goal of relating the PFC parameters to physical parameters of real systems, derived from MD simulations. For this purpose we model the phenomenon of the shrinking of a spherical grain situated in a matrix. By comparing the rate of shrinking of the central grain using MD and PFC we obtain a relationship between PFC and MD time scales for processes driven by grain boundary diffusion. The morphological changes in the central grain including grain shape and grain rotation are also examined in order to assess the accuracy of the PFC in capturing the evolution path predicted by MD. read less

Abstract: Isolated kinks on thermally fluctuating (1/2) screw, edge and (1/2) edge dislocations in bcc iron are simulated under zero stress conditions using molecular dynamics (MD). Kinks are seen to perform stochastic motion in a potential landscape that depends on the dislocation character and geometry, and their motion provides fresh insight into the coupling of dislocations to a heat bath. The kink formation energy, migration barrier and friction parameter are deduced from the simulations. A discrete Frenkel-Kontorova-Langevin (FKL) model is able to reproduce the coarse grained data from MD at a fraction of the computational cost, without assuming an a priori temperature dependence beyond the fluctuation-dissipation theorem. Analytic results reveal that discreteness effects play an essential r\^ole in thermally activated dislocation glide, revealing the existence of a crucial intermediate length scale between molecular and dislocation dynamics. The model is used to investigate dislocation motion under the vanishingly small stress levels found in the evolution of dislocation microstructures in irradiated materials. read less

Abstract: Phase boundary is an important kind of interfaces for the dual-phase metals. Orientation relationship (OR) is a crucial factor affecting the performance of the phase boundaries and the dual-phase metals. Molecular dynamics simulation is performed to examine the structures of the bcc/fcc iron phase boundaries in Nishiyama–Wassermann (N–W) and Kurdjumov–Sachs (K–S) orientation relationships, and the performances of the dual-phase model with these ORs. The structural relaxation shows that the phase boundary with N-W relation has the lower energy than that in K-S relation. Stress-strain curves show that dual-phase model in N-W relation has the higher stiffness and strength than that in K-S relation. Simulation results show that phase boundary in N-W relation has a more exellent performance, and is preferred to be processed in heat treatment. read less

Abstract: Understanding the interactions of hydrogen and helium within ferritic steels will aid in the development of materials appropriate for next generation nuclear reactors. We discuss interatomic potentials appropriate for simulating these elements, presenting a potential for the H-He interactions. Preliminary results for small H-He bubbles in bcc iron are given and discussed. read less

Abstract: The stability of hydrogen atoms trapped in vacancy clusters of a bcc iron structure is investigated by molecular statics calculations of the hydrogen binding energy to these clusters. The configurations having a minimum potential energy are obtained from the relaxation of a large number of different initial atomic configurations. Calculations of hydrogen binding energy to a mono-vacancy illustrate a relatively large gain of energy in trapping up to two hydrogen atoms in a monovacancy and the increasing difficulty to trap additional atoms due to hydrogen mutual repulsion. Comparison with ab-initio reference calculations of the hydrogen binding energy shows good agreement for up to three trapped hydrogen atoms. Based on the calculations conducted on the most stable vacancy-hydrogen complexes containing two to six vacancies, the maximum capacity of hydrogen atoms per vacancy was found to decrease with the size of vacancy cluster. The calculations of hydrogen binding energies to these clusters show that trapping two hydrogen atoms per vacancy is still a particularly favorable process for vacancy clusters. read less

Abstract: We report non-equilibrium Molecular Dynamics simulations providing a nanoscale view for the modeling of shock wave generation, propagation and melting in single crystalline materials Fe, Ta, W, of clear interest for Nuclear Fusion Technology. Our methodology successfully uses massive parallel molecular dynamics in an attempt to cover similar times and length scales as laser-shock experiments. Response of the materials are analyzed in terms of modern atomistic visualization and evolution of their structural properties. Preliminary results point that Wand Ta behave more efficiently in terms of uniformity under shock propagation than lighter materials like Fe. This kind of materials must attract our attention in the short term as possible designs in inertial confinement fusion (ICF) targets. read less

Abstract: Here we demonstrate and characterize the H-diffusion behavior around a screw dislocation in body-centered cubic (bcc) $\ensuremath{\alpha}$-Fe by performing path-integral molecular dynamics modeling and adopting an ab initio\char21{}based potential. Counterintuitively, our results indicate that the H diffusivity along the dislocation line is significantly lower than lattice diffusion. Thus, the ``fast'' pipe diffusion does not occur for H in $\ensuremath{\alpha}$-Fe. read less

Abstract: Molecular dynamics simulations, along with the modified analytic embedded atom method, have been employed to study the bcc → fcc phase transition of nanocrystalline iron. The Gibbs free energies of bulk fcc and bcc iron phases are calculated as a function of temperature, and used to determine the bulk phase-transition temperature. Furthermore, the transformation temperature in the nanocrystalline iron, with a mean grain size of 3 nm, is determined to be 975 ± 25 K using the bond-order parameter method. The radial-distribution function and common neighbor analysis are used to understand the phase structure of the nanocrystalline iron and the evolution of local atomic structure. The snapshots of a two atomic layer thick slice provide a visible scenario of structural evolution during phase transition. read less

Abstract: Molecular dynamics (MD) simulations were carried out to study the interaction between nanometric Cr precipitates and a 1/2 >{110} edge dislocation (ED) in pure Fe and Fe-9 at. % Cr (Fe-9Cr) random alloy. The aim of this work is to estimate the variation in the pinning strength of the Cr precipitate as a function of temperature, its chemical composition and the matrix composition in which the precipitate is embedded. The dislocation was observed to shear Cr precipitates rather than by-pass via the formation of the Orowan loop, even though a pronounced screw dipole was emerged in the reactions with the precipitates of size larger than 4.5 nm. The screw arms of the formed dipole were not observed to climb thus no point defects were left inside the sheared precipitates, irrespective of simulation temperature. Both Cr solution and Cr precipitates, embedded in the Fe-9Cr matrix, were seen to contribute to the flow stress. The decrease in the flow stress with temperature in the alloy containing Cr precipitates is, therefore, related to the simultaneous change in the matrix friction stress, precipitate resistance, and dislocation flexibility. Critical stress estimated from MD simulations was seen to have a strong dependence on the precipitate composition. If the latter decreases from 95% down to 80%, the corresponding critical stress decreases almost as twice. The results presented here suggest a significant contribution to the flow stress due to the alpha-alpha(') separation, at least for EDs. The obtained data can be used to validate and to parameterize dislocation dynamics models, where the temperature dependence of the obstacle strength is an essential input data. read less

Abstract: We formulate a multi-string Frenkel–Kontorova (MSFK) model for a a/2[111] screw dislocation in bcc iron, and investigate the occurrence of degenerate and non-degenerate dislocation core structures as functionals of the law of interaction between the [111] strings of atoms forming the crystal. By comparing the effective inter-string interaction laws derived from ab initio density functional calculations and from semi-empirical interatomic potentials for α-iron, we show that it is the form of the function determining how the atomic strings interact with each other as a function of their relative one-dimensional displacement in the [111] direction that determines whether a degenerate or a non-degenerate screw dislocation core configuration has lower energy. We show that by constructing a one-dimensional inter-string interaction law, and by solving the MSFK equations, it is possible to easily predict the nature of the screw dislocation core, hence providing a simple yet effective check to aid the development of short-range semi-empirical interatomic potentials for bcc transition metals. Finally, we analyse the relation between the inter-string interaction law, and the shape and the height of the Peierls energy barriers separating the adjacent equilibrium configurations for a migrating screw dislocation. read less

Abstract: Damage cascades representative of those that would be induced by neutron irradiation have been simulated in systems of pure iron and iron containing 0.01 at.% hydrogen. Results from molecular dynamics simulations using three different embedded-atom method (EAM) type potentials are compared for primary knock-on atom energies of 5, 10, and 20 keV to assess the effect of hydrogen on the primary damage state. We examine the influence of hydrogen on the primary damage state due to a single radiation cascade. These results can serve as an atomistic database for methods and simulations for long time scale evolution of radiation damage. read less

Abstract: In recent years, the development of atomistic models dealing with microstructure evolution and subsequent mechanical property change in reactor pressure vessel steels has been recognised as an important complement to experiments. In this framework, a literature study has shown the necessity of many-body interatomic potentials for multi-component alloys. In this paper, we develop a ternary many-body Fe–Cu–Ni potential for this purpose. As a first validation, we used it to perform a simulated thermal annealing study of the Fe–Cu and Fe–Cu–Ni alloys. Good qualitative agreement with experiments is found, although fully quantitative comparison proved impossible, due to limitations in the used simulation techniques. These limitations are also briefly discussed. read less

Abstract: Atomistic simulations of the interaction of a screw dislocation in (cid:1) -Fe with different size bcc Cu precipitates suggest two plausible strengthening mechanisms. For precipitate diameters in the range 1.5 nm (cid:2) d (cid:2) 3.3 nm, the dislocation core structure within the Cu precipitate undergoes a polarized to nonpolarized transformation, leading to the dislocation pinning at the precipitate-matrix interface and the bowing out of the dislocation line. The calculated bow-out angle and resolved shear stress required to detach the dislocation from the precipitate are in agreement with recent experiments. The structural transition of larger (cid:1) d (cid:3) 3.3 nm (cid:2) Cu precipitates under high shear stress is responsible for the loss of slip systems and hence for dislocation pinning. read less

Abstract: Helium is a ubiquitous impurity in nuclear materials that can have significant deleterious effects on mechanical properties, including deformation and fracture. To determine ways to mitigate the effects of helium it is necessary to understand the behavior of helium with respect to its interaction with various microstructural features. Toward that end, we have employed molecular statics, molecular dynamics, and the dimer method of potential surface mapping to study the fate of helium in the vicinity of dislocations and grain boundaries in alpha-iron. Even at very low temperatures interstitial helium atoms can migrate to dislocations and grain boundaries, where they are strongly bound. The binding energies of helium to these microstructural features relative to the perfect crystal and the migration energies of helium diffusing within them have a strong correlation to the excess atomic volume that exists in these extended defects. Helium atom migration energies within the dislocations and grain boundaries studied are in the range of 0.4–0.5 eV. Helium “kick out” mechanisms have been identified within dislocations and grain boundaries by which interstitial helium atoms replace an Fe lattice atom, creating a stable He-vacancy complex that may be a nucleation site for an He bubble. read less

Abstract: Understanding the liquid-solid phase transition has long been of scientific interest, owing to its singular importance in the processing of materials with desired microstructures. Presently, however, little is known about the atomic mechanisms controlling this process. Using molecular dynamics simulations, we find a surprising connection between the crystallization behavior of metals at extreme undercoolings and the properties of interstitial atoms in the crystalline phase. We show first that the activation energy of crystallization in a number of metals at the kinetically controlled regime is precisely the same as the migration energy of self-interstitials atom in the crystalline state. We then show, contrary to the present thought, that the advance of a planar solid-liquid interface in Fe at low temperatures is controlled by thermally activated jumps of a small fraction of the atoms on the liquid side of the interface, and remarkably these atoms have the same ⟨110⟩ dumbbell interstitialcy structure as observed for interstitials in crystalline Fe. read less

Abstract: The evolution of damage produced by collision cascades in Fe is studied using both kinetic Monte Carlo (kMC) and rate theory (RT) approaches. The initial damage distribution is obtained from molecular-dynamics simulations of 30 keV recoils in Fe. An isochronal annealing is simulated to identify the different thermally activated mechanisms that govern defect evolution. When clusters form during collision cascades, kMC simulations show that additional recovery peaks should be expected, in comparison to recovery curves obtained under electron irradiation conditions. Detailed kMC and RT simulations reveal that some of these recovery peaks are due to correlated recombinations at low temperature between defects. In particular, we show that under cascade-damage conditions it is possible to observe correlated recombinations between vacancies and self-interstitial clusters. These correlated recombinations cannot be reproduced with a RT model, and therefore kMC and RT differ at low temperature. However, for the conditions presented here, the contribution of correlated recombination is very small and therefore no significant differences are observed at high temperatures between these two models. read less

Abstract: The properties of Cr in alpha Fe have been investigated by ab initio calculations based on density functional theory. The intrinsic point defect formation energies were found to be larger in model ... read less

Abstract: Crystal-melt interfacial free energies $(\ensuremath{\gamma})$ are computed for hcp Mg by employing equilibrium molecular-dynamics (MD) simulations and the capillary-fluctuation method (CFM). This work makes use of a newly developed embedded-atom-method (EAM) interatomic potential for Mg fit to crystal, liquid, and melting properties. We describe how the CFM, which has previously been applied to cubic systems only, can be generalized for studies of hcp metals by employing a parametrization for the orientation dependence of $\ensuremath{\gamma}$ in terms of hexagonal harmonics. The method is applied in the calculation of the Turnbull coefficient $(\ensuremath{\alpha})$ and crystalline anisotropies of $\ensuremath{\gamma}$. We obtain a value of $\ensuremath{\alpha}=0.48$, with interfacial free energies for different high-symmetry orientations differing by approximately 1%. These results are compared to those obtained in previous MD-CFM studies for cubic EAM metals as well as experimental studies of solid-liquid interfaces in hcp alloys. In addition, the implications of our results for the prediction of dendrite growth directions in hcp metals are discussed. read less

Abstract: A modified embedded-atom method (MEAM) interatomic potential for the Fe-Cu binary system has been developed using previously developed MEAM potentials of Fe and Cu. The Fe-Cu potential was determined by fitting to data on the mixing enthalpy and the composition dependencies of the lattice parameters in terminal solid solutions. The potential gives a value of 0.65 eV for the dilute heat of solution and reproduces the increase of lattice parameter of Fe with addition of Cu in good agreement with experiments. The potential was used to investigate the primary irradiation defect formation in pure Fe and Fe-0.5 at. % Cu alloy by a molecular dynamics cascade simulation study with a PKA energy of 2 keV at 573 K. A tendency for self-interstitial atom-Cu binding, the formation of mixed (Fe-Cu) dumbbells and even Cu-Cu dumbbells was observed. Given a positive binding energy between Cu atoms and self-interstitials, Cu transport by an interstitial diffusion mechanism could be proposed to contribute to the formation of Cu-rich precipitates and irradiation-induced embrittlement in nuclear structural steels. read less

Abstract: We present an analysis, based upon atomistic simulation data, of the effect of Fe impurities on grain boundary migration in Al. The first step is the development of a new interatomic potential for Fe in Al. This potential provides an accurate description of Al–Fe liquid diffraction data and the bulk diffusivity of Fe in Al. We use this potential to determine the physical parameters in the Cahn–Lücke–Stüwe (CLS) model for the effect of impurities on grain boundary mobility. These include the heat of segregation of Fe to grain boundaries in Al and the diffusivity of Fe in Al. Using the simulation-parameterized CLS model, we predict the grain boundary mobility in Al in the presence of Fe as a function of temperature and Fe concentration. The order of magnitude and the trends in the mobility from the simulations are in agreement with existing experimental results. read less

Abstract: The structural dependence of crystal-melt interfacial free energies ( y) is investigated for fcc and bcc solids through molecular-dynamics calculations employing interatomic potentials for Fe. We compute 30-35 % lower values of y for the bcc structure, and find that our results cannot be explained simply in terms of differences in latent heats (L) or densities (p) for bulk bcc and fcc phases. We observe a strong structural dependence of the Turnbull coefficient a= γ/Lρ 2 / 3 , and find a trend towards lower crystalline anisotropies of y for the bcc structure relative to fcc. read less

Abstract: Alpha-Fe nanowires are seeded with twin walls (TBs) with orientations perpendicular to the wire direction. Twisting the wire generates topological defects in the twin walls, namely new twin boundaries (kinks) inside the twin walls for small twist angles, and junctions between kinks for large twist angles. During twisting the kink motion is jerky and uncorrelated at small twisting angles. The probability density function (PDF) of jerk energies follows approximately a Gaussian distribution, indicating a mild deformation mode. The kink dynamics transforms from mild to wild at larger twist angles when complex twin patterns with a high density of junctions are generated. The collective motion of kinks now shows avalanche behavior with the energy being power-law distributed. The wildness, which measures the proportion of strain energy relaxed through such avalanches, is correlated with the junction density, and controlled by the external length scale (wire diameter) as well as an internal length scale (twin boundary spacing). Good strain-stress recoverability is achieved when unloading the wire before the formation of complex twin patterns. We correlate the evolution of twin patterns with a statistical analysis of jerk dynamics, which identifies the unique mechanical properties governed by twin boundary motion in nanowires. read less

Abstract: We present a unified approach to describe the combined behavior of the atomic and magnetic degrees of freedom in magnetic materials. Using Monte Carlo simulations directly combined with first principles the Curie temperature can be obtained ab initio in good agreement with experimental values. The large scale constrained first principles calculations have been used to construct effective potentials for both the atomic and magnetic degrees of freedom that allow the unified study of influence of phonon-magnon coupling on the thermodynamics and dynamics of magnetic systems. The MC calculations predict the specific heat of iron in near perfect agreement with experimental results from 300K to above Tc and allow the identification of the importance of the magnon-phonon interaction at the phase-transition. Further Molecular Dynamics and Spin Dynamics calculations elucidate the dynamics of this coupling and open the potential for quantitative and predictive descriptions of dynamic structure factors in magnetic materials using first principles derived simulations. read less

Abstract: Numerous studies have reported that solute hydrogen atoms and lattice defects have strong interactions, and that hydrogen atoms significantly change the stability and/or mobility of lattice defects. Although molecular dynamics (MD) simulations can treat complicated interactions of various lattice defects, the time scale is insufficient to treat hydrogen diffusion so as to influence the lattice-defect generation and cooperative motion of hydrogen atoms and lattice defects. Here we developed an interatomic potential for Fe with pseudo-hydrogen effects on lattice-defect energies and performed MD simulations of tensile loading. First, we estimated the lattice-defect energies of Fe and hydrogen-trap energies of lattice defects by using first-principle calculations and evaluated the lattice-defect energies under a practical gaseous hydrogen environment. Second, we refitted the existing embedded-atom-method potential for Fe to represent the lattice-defect energies amended by hydrogen effects. Finally, we confirmed that our potential is applicable for various phenomena by estimating the reproducibility of grain-boundary energies that are not employed for potential fitting. Our tensile-loading simulations of a nano specimen show that hydrogen reduces elongation at rupture. read less

Abstract: Structural materials in the new Generation IV reactors will operate in harsh radiation conditions coupled with high levels of hydrogen and helium production, thus experiencing severe degradation of mechanical properties. The development of structural materials for use in such a hostile environment is predicated on understanding the underlying physical mechanisms responsible for microstructural evolution along with corresponding dimensional instabilities and mechanical property changes. As the phenomena involved are very complex and span in several length scales, a multiscale approach is necessary in order to fully understand the degradation of materials in irradiated environments. The purpose of this work is to study the behavior of Fe systems (namely a-Fe, Fe-Cr and Fe-Ni) under irradiation using both Molecular Dynamics (MD) and Dislocation Dynamics (DD) simulations. Critical information is passed from the atomistic (MD) to the microscopic scale (DD) in order to study the degradation of the material under examination. In particular, information pertaining to the dislocation-defects (such as voids, helium bubbles and prismatic loops) interactions is obtained from MD simulations. Then this information is used by DD to simulate large systems with high dislocation and defect densities. read less