Abstract: Austenite ({\gamma}-Fe, face centered cubic (FCC)) to ferrite ({\alpha}-Fe, body centered cubic (BCC)) phase transformation in steel is of great significance from the point of view of industrial applications. In this work, using classical molecular dynamics simulations, we study the atomistic mechanisms involved during the nucleation and growth of the ferrite phase embedded in an austenite phase. We find that the disconnections present at the inter-phase boundary can act as the nucleation centers for the ferrite phase. Relatively small interface velocities (1.19 - 4.67 m/s) confirm a phase change via massive transformation mechanism. Boundary mobilities obtained in a temperature range of 1000 to 1400 K show an Arrhenius behavior, with activation energies ranging from 30 - 40 kJ/mol. read less

Abstract: We present the results of free 3D molecular dynamics (MD) simulations, focused on the influence of temperature on the ductile-brittle behavior of a pre-existing central Griffith through microcrack (1¯10)[110] (crack plane/crack front) under biaxial loading σA and σB in tension mode I. At temperatures of 300 K and 600 K, the MD results provide new information on the threshold values of the stress intensity factor K and the energy release rate G, needed for the emission of <111>{112} blunting dislocations that support crack stability. A simple procedure for the evaluation of thermal activation from MD results is proposed in the paper. 3D atomistic results are compared with continuum predictions on thermal activation of the crack induced dislocation generation. At elevated temperature T and biaxiality ratios σB/σA ≤ 0.8 dislocation emission in MD is observed, supported by thermal activation energy of about ~30 kBT. With increasing temperature, the ductile-brittle transition moves to a higher biaxiality ratios in comparison with the situation at temperature of ~0 K. Near the transition, dislocation emission occurs at lower loadings than expected by continuum predictions. For the ratios σB/σA ≥ 1, the elevated temperature facilitates (surprisingly) the microcrack growth below Griffith level. read less

Abstract: The authors demonstrate that the line tension expression for dislocation core energy and the regularization of elastic fields near the core fundamentally emerge from the periodicity of the discrete atomic lattice. A continuum model for the dislocation core is presented which addresses the problem of short wavelength instability of dislocation lines inherent to linear elasticity theory, and predicts configurational energies. read less

Abstract: Nonlinear ultrasonic technique is one of several promising nondestructive evaluation methods for monitoring the evolution of nanosized defects such as radiation-induced defects in nuclear materials. In this work, a microstructure-based phase-field model of dynamic deformation in elastically nonlinear materials has been developed for investigating the dynamic interaction between distributed defects and a propagating longitudinal sound wave. With the model, the effect of second phase precipitates’ size and properties on the nonlinearity parameter β that describes the magnitude of the 2nd harmonic wave was simulated. The results showed that (1) the nonlinearity parameter β increases as the elastic inhomogeneity increases regardless of whether the precipitates are softer or harder than the matrix; (2) β linearly increases with the increase of lattice mismatch strain; and (3) for a given volume fraction of second phase precipitates, β strongly depends on the precipitate size. The predicted precipitate size dependence of β agrees with the experimental data. These results demonstrate that the developed model enables one to predict the contributions of different nonlinear sources to β, to explain the signal physics behind the measured nonlinear ultrasonic response, and to guide the development of nonlinear ultrasound nondestructive detection of material defects in nuclear reactor materials.Nonlinear ultrasonic technique is one of several promising nondestructive evaluation methods for monitoring the evolution of nanosized defects such as radiation-induced defects in nuclear materials. In this work, a microstructure-based phase-field model of dynamic deformation in elastically nonlinear materials has been developed for investigating the dynamic interaction between distributed defects and a propagating longitudinal sound wave. With the model, the effect of second phase precipitates’ size and properties on the nonlinearity parameter β that describes the magnitude of the 2nd harmonic wave was simulated. The results showed that (1) the nonlinearity parameter β increases as the elastic inhomogeneity increases regardless of whether the precipitates are softer or harder than the matrix; (2) β linearly increases with the increase of lattice mismatch strain; and (3) for a given volume fraction of second phase precipitates, β strongly depends on the precipitate size. The predicted precipitate size ... 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: The interaction between an edge dislocation and a sessile vacancy cluster in bcc Fe is investigated over a wide range of strain rates from 10^{8} down to 10^{3}  s^{-1}, which is enabled by employing an energy landscape-based atomistic modeling algorithm. It is observed that, at low strain rates regime less than 10^{5}  s^{-1}, such interaction leads to a surprising negative strain rate sensitivity behavior because of the different intermediate microstructures emerged under the complex interplays between thermal activation and applied strain rate. Implications of our findings regarding the previously established global diffusion model are also discussed. 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: The atomistic processes that form the basis of thin film growth often involve complex multi-atom movements of atoms or groups of atoms on or close to the surface of a substrate. These transitions and their pathways are often difficult to predict in advance. By using an adaptive kinetic Monte Carlo (AKMC) approach, many complex mechanisms can be identified so that the growth processes can be understood and ultimately controlled. Here the AKMC technique is briefly described along with some special adaptions that can speed up the simulations when, for example, the transition barriers are small. Examples are given of such complex processes that occur in different material systems especially for the growth of metals and metallic oxides. 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: Metastable Fe-Cu alloys are of considerable scientific interest, and an efficient interatomic potential is crucial for reliable atomistic simulations. Interatomic potentials developed for pure Fe and pure Cu are difficult to mix for the Fe-Cu alloys since the analytic function form of pure Fe is not of the same type of pure Cu potential. Additionally, elemental potentials taken from alloy descriptions may not work well for the pure species. This is particularly true if the elements were fit for compounds instead of being optimised separately. In this article, we present compatible analytic function forms that work for pure species and are easily mixed with two adjustable parameters for their alloys. We tested the proposed potentials to make sure that the performance is adequate for pure species as well as their alloys. The predicted values were in good agreement with experimental results. read less

Abstract: ABSTRACT Various types of nanometric defects such as voids and helium (He) bubbles produced by high-energy neutron irradiations are known to degrade the mechanical properties of irradiated materials. In this study, we have evaluated the obstacle strength of He bubbles to the mobility of an edge dislocation in α-iron for 2 and 4 nm bubbles with He-to-vacancy (He/V) ratios ranging from 0 to 1 at 300 and 500 K, by molecular dynamics simulation. Results showed that as the He/V ratio increases, the obstacle strength needed for the release of a dislocation from the bubble becomes stronger up to a moderate He/V ratio (0.6 and 0.4 for 2 and 4 nm bubbles, respectively, at both temperatures), and a further increase in the He/V ratio leads to weakening of the obstacle strength. For He/V = 1, the obstacle strengths are 10–30% weaker than those at moderate He/V ratios depending on the bubble size and temperature. The extent of obstacle strength was found to be correlated with the dilation caused by He bubbles depending on the bubble size, He/V ratio, and temperature. read less

Abstract: Molecular dynamics (MD) simulations on a model of pure Fe have been used in the investigation of solid-state nucleation of a body-centered-cubic (BCC) phase from a polycrystalline face-centered-cubic (FCC) matrix. A neighbor vector analysis (NVA) method has been introduced and it is shown how the NVA can be used to determine the misorientation of grain or interphase boundaries. In particular, the NVA was utilized to identify the orientation relationships (ORs) of several BCC nuclei and three special ORs were tested, namely the Kurdjumov-Sachs (KS), Nishiyama–Wassermann (NW) and Pitsch (P). From several quasi-2D simulations, it was found that all stable nuclei at grain boundaries formed at least one orientation relationship with the parent grains that was consistent with either the KS or NW relationship. Several initial MD simulation cells, which prohibited the formation of special ORs, were also examined and in these simulations no nucleation was observed after long run times. In addition, the {1 1 1}γ//{1 1 0}α ?> orientation was detected in all mobile phase boundaries. Consistent with experimental findings, these observations demonstrate the importance of this high coherency atomic plane during both the nucleation and growth process. The nucleation and phase boundary characteristics identified here may provide important insights into the nucleation rate and grain orientation of more general solid state nucleation processes. read less

Abstract: The present work is devoted to classical molecular dynamics investigation into microscopic mechanisms of the bcc-hcp transition in iron. The interatomic potential of EAM type used in the calculations was tested for the capability to reproduce ab initio data on energy evolution along the bcc-hcp transformation path (Burgers deformation + shuffe) and then used in the large-scale MD simulations. The large-scale simulations included constant volume deformation along the Burgers path to study the origin and nature of the plasticity, hydrostatic volume compression of defect free samples above the bcc to hcp transition threshold to observe the formation of new phase embryos, and the volume compression of samples containing screw dislocations to study the effect of the dislocations on the probability of the new phase critical embryo formation. The volume compression demonstrated high level of metastability. The transition starts at pressure much higher than the equilibrium one. Dislocations strongly affect the probability of the critical embryo formation and significantly reduce the onset pressure of transition. The dislocations affect also the resulting structure of the samples upon the transition. The formation of layered structure is typical for the samples containing the dislocations. The results of the simulations were compared with the in-situ experimental data on the mechanism of the bcc-hcp transition in iron. read less

Abstract: Iron carbide (Fe3C), also known as cementite, is present in many steels and has also been seen as nanosized precipitates in steels. We examine the interaction of edge dislocations with nanosized cementite precipitates in Fe by molecular dynamics. The simulations are carried out with a Tersoff-like bond order interatomic potential by Henriksson et al. for Fe-C-Cr systems. Comparing the results obtained with this potential for a defect free Fe system with results from previously used potentials, we find that the potential by Henriksson et al. gives significantly higher values for the critical stress, at least at low temperatures. The explanation was found to be the difference in the core structure of the edge dislocation. The results show that edge dislocations can unpin from cementite precipitates of sizes 1 nm and 2 nm even at a temperature of 1 K, although the stresses needed for this are high. On the other hand, a 4 nm precipitate is not sheared by edge dislocations at low temperatures (≤100 K) on our simulation timescale. read less

Abstract: We propose a mechanism for glide motion, i.e. one-dimensional (1D) migration, of interstitial clusters in concentrated alloys driven by high-energy particle irradiation. Interstitial clusters are fundamentally mobile on their respective 1D migration tracks, but in concentrated random alloys they are stationary at the position where the fluctuating formation energy achieves a local minimum. Irradiation changes the microscopic distribution of solute atoms through atomic displacement and recovery of the produced Frenkel pairs, which causes cluster 1D migration into a new stable position. In molecular dynamics simulations of interstitial clusters up to 217i in Fe–Cu alloys, stepwise 1D migration was observed under interatomic mixing or shrinkage of the cluster: a single 1D migration was induced by two exchanges per atom or cluster radius change by two interatomic distances. The 1D migration distance ranged up to several nanometers. We compared the frequency and distance of 1D migration with those for in situ observation using high-voltage electron microscopy, allowing for the extremely large rate of interatomic mixing and cluster shrinkage in the present simulation. read less

Abstract: Interfacial energy calculated with an atomistic model is very sensitive to adding atoms to or removing atoms from the interface, especially when the interface has a general orientation. Therefore, it is crucial to construct an appropriate initial atomic configuration in order to obtain a reliable value for the interfacial energy from atomistic simulations. In this work, a simple method is proposed for constructing the atomic configuration of a general interface under the condition that the interface is virtually free of interstitial and vacancy. The validity of the method is demonstrated by using it to calculate the equilibrium morphology of precipitates with interfaces in irrational orientations, which shows good agreement with experimental observations. read less

Abstract: From a direct observation of dislocation-obstacle interaction utilizing in situ straining experiments in transmission electron microscope (TEM), the obstacle strength factor could be evaluated from pinning angles of dislocation cusps. We simulated this process: we produced a dislocation cusp by molecular dynamics simulation of interaction between an edge dislocation and a void or a hard precipitate in copper, and calculated the TEM image by multislice method. In two-beam conditions, cusp images showed inside-outside contrast depending on the sign of the diffracting vector and other variations with the specimen geometry. The pinning angles measured on TEM images ranged up to a few tens of degrees and were between the true angles for the two partial dislocations. Characteristics and contrast mechanisms of cusp images were discussed based on those of dislocation dipoles. (doi:10.2320/matertrans.MD201312) (Received September 3, 2013; Accepted October 2, 2013; Published November 15, 2013) Various crystal lattice defects induced by neutron irradi- ation, such as precipitates, voids, dislocation lines and loops are responsible for degradation in mechanical properties: increase in yield stress, loss of ductility, and increase in ductile-brittle transition temperature. A number of researches have been devoted for defect structural evolution and mechanical property changes under neutron irradiation for nuclear materials development. Understanding the mecha- nisms involved is necessary to construct models for estimating the lifetime of components of nuclear power plant. In an elementary process of dislocation-obstacle interac- tion, a gliding dislocation is pinned by obstacles and bows out to form arcs between the neighboring pinning points, which induces cusps on the dislocation at obstacles. The apex angle of the dislocation cusp is referred to as the pinning angle o. The dislocation breaks away by bypassing or cutting through the obstacle when the pinning angle reaches a critical value oc. Stronger obstacles have smaller critical angles. The obstacle strength factor i ¼ cosðoc=2Þ and the distance between the neighboring pinning points are the key parame- ters that relate the defect microstructure to the change in macroscopic mechanical properties. There have been re- ported a few attempts to evaluate the factor from a direct observation of dislocation-obstacle interaction utilizing in situ straining experiments in transmission electron micro- scope (TEM). 1­3) The method has not been widely applied so far, due to high technical levels required for in situ experiments. Another difficulty comes from TEM images with a limited resolution both in time and space for measuring pinning angles of radiation-induced obstacles, typically less than a few nanometers in size, at a moment of the breakaway. Alternatively, molecular dynamics (MD) simulations have been applied extensively for the dislocation- obstacle interaction. 4­7) In the present study, we performed TEM image simulation of dislocation cusps to examine whether TEM images reveal the cusp structure in the scale suitable for evaluating the obstacle strength factor. For this purpose, we stopped MD simulation of interaction between an obstacle and an edge dislocation just before the breakaway. We then calculated TEM images of dislocation cusps under various conditions using the multislice method. We compared apex angles between the cusp structure and on the calculated images, which would support the experimental evaluation of the obstacle strength factor. read less

Abstract: Habit planes between face-centered cubic (fcc)/body-centered cubic (bcc) phases usually exhibit irrational orientations, which often agree with the O-line criterion. Previously, energy calculation was made to test whether the habit planes were energetically favorable, but the values of the energy were found very sensitive to the initial atomic configuration in an irrationally orientated interface. In this paper, under the O-line condition, simple selection criteria are proposed to define and remove interfacial interstitials and vacancies in the initial atomic configuration. The criteria are proved to be effective in obtaining robust energy results. Interfacial energies of two types of O-line interfaces in fcc/bcc systems are calculated following the criteria. The observed transformation crystallography of precipitates in Ni–Cr and Cu–Cr systems can be explained consistently as the irrational habit plane in each system is associated with the lowest energy O-line interface. read less

Abstract: Fluctuation in microscopic distribution of solute atoms will act as a barrier for glide motion (i.e. 1D migration) of interstitial clusters in random alloys. We proposed an analytical model in which the total interaction energy between an interstitial cluster and solute atoms is a superposition of the interaction potential between the cluster and individual solute atom. Then we examined the nature of fluctuation in the total interaction energy of a gliding cluster. The average amplitude of the fluctuation was directly proportional to the square root of both the solute concentration and the cluster radius . The distance separating local peaks in the fluctuation was virtually independent of and , but showed dependence only on the range of the interaction potential. We proposed a model for another fluctuation in the interaction energy because of solute–solute interaction that is effective at high . The models interpreted the results of the molecular statics simulations of the fluctuating interaction energy for interstitial clusters (7i, 61i and 217i) in dilute and concentrated Fe–Cu alloys with random solute distribution. We proposed that the fluctuation in the interaction energy is responsible for the short-range 1D migration that is observed in various alloys in electron irradiation experiments. The distance between local peaks would give the characteristic length of 1D migration in concentrated alloys. read less

Abstract: We investigated dislocation nucleation and defect formation underneath a spherical indenter which possesses atomic steps on its surface. Atomic-scale simulations of Cu (111) nanoindentation were performed. Our simulation results reveal that dislocations nucleate from surface ledges formed by atomic steps on indenter surfaces. We found that stepped indenters promote concurrent activation of three inclined {111} planes, which leads to an increased probability of forming threefold symmetric defects and punching prismatic loops along threefold symmetric directions. A new junction structure was observed and found to unzip during the formation of prismatic loops. The formation and destruction of defect structures can be explained using a conventional theory of dislocation reactions. read less

Abstract: The total energy of bcc Fe containing Cu clusters with different sizes and number densities are calculated using the molecular dynamics (MD) method. The results indicate that the Cu atoms prefer to form Cu clusters instead of being uniformly distributed in the bcc Fe matrix. The binding energy of substitutional Cu to Cu clusters is also found to increase with the number of Cu atoms. For a large-sized Cu precipitate, the change of the local stress state is found to relate to the phase transition from bcc to fcc Cu based on MD and common neighbor analysis. read less

Abstract: The role of 5-fold twin boundary on the structural and mechanical properties of fcc Fe nanowire under tension is explored by classical molecular dynamics. Twin-stabilized fcc nanowire with various diameters (6-24 nm) are examined by tension tests at several temperatures ranging from 0.01 to 1100 K. Significant increase in the Young's modulus of the smaller nanowires is revealed to originate from the central area of quinquefoliolate-like stress-distribution over the 5-fold twin, rather than from the surface tension that is often considered as the main source of such size-effects found in nanostructures. Because of the excess compressive stress caused by crossing twin-boundaries, the atoms in the center behave stiffer than those in bulk and even expand laterally under axial tension, providing locally negative Poisson's ratio. The yield strength of nanowire is also enhanced by the twin boundary that suppresses dislocation nucleation within a fcc twin-domain; therefore, the plasticity of nanowire is initiated by strain-induced fcc→bcc phase transformation that destroys the twin structure. After the yield, the nucleated bcc phase immediately spreads to the entire area, and forms a multigrain structure to realize ductile deformation followed by necking. As temperature elevated close to the critical temperature between bcc and fcc phases, the increased stability of fcc phase competes with the phase transformation under tension, and hence dislocation nucleations in fcc phase are observed exclusively at the highest temperature in our study. read less

Abstract: A perturbation theory is developed to calculate solid–liquid interfacial free energies, including anisotropy. The method is applied to systems with inverse-power and Lennard-Jones pair potentials as well as to metal systems with embedded-atom model potentials. The results are in reasonable agreement with the corresponding ones obtained from molecular dynamics simulations. read less

Abstract: Molecular dynamics simulations have been performed to give an estimate on the solid–liquid interfacial properties of bcc iron, namely the kinetic coefficients and solid–liquid interfacial energy. The kinetic coefficients for different orientations were estimated from the propagation velocity of planar solid–liquid interfaces. The anisotropy of kinetic coefficients, μ, was confirmed to be μ(100)>μ(110), which is similar to the literatures using other interatomic potentials. Moreover, growing and shrinking behavior of the freestanding spherical crystal and semi-spherical crystal on the substrate in the undercooled liquid was examined. There is a critical temperature dividing shrink or growth of both the freestanding spherical crystal and semi-spherical crystal on the substrate. The solid–liquid interfacial energy was then estimated from Gibbs–Thomson relation in the critical temperature as a function of the inverse of crystal radius. 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: Strengthening due to voids can be a significant effect of radiation damage in metals, but treatment of this by elasticity theory of dislocations is difficult when the mechanisms controlling the obstacle strength are atomic in nature. Results are reported of atomic-scale modelling to compare edge dislocation–void interaction in fcc copper and bcc iron. Voids of up to 6 nm diameter in iron and 8 nm diameter in copper were studied over the temperature range 0 to 600 K at different applied strain rates. Voids in iron are strong obstacles, for the dislocation has to adopt a dipole-like configuration at the void before breaking away. The dipole unzips at the critical stress when the dislocation is able to climb by absorbing vacancies and leave the void surface. Dislocation dissociation into Shockley partials in copper prevents dislocation climb and affects the strength of small and large voids differently. Small voids are much weaker obstacles than those in iron because the partials break from a void individually. Large voids are at least as strong as those in iron, but the controlling mechanism depends on temperature. read less

Abstract: Generalized stacking fault energies and screw dislocation core structures are reported for two sets of models for iron: density functional theory (DFT) calculations and empirical potentials. A thorough comparison between various DFT approaches has been performed on {110} and {211} γ-lines, which give a first indication on dislocation properties: (i) the effect of the exchange-correlation functional, LDA versus GGA, is significant in the pseudopotential approximation but not in the PAW approximation or in paramagnetic calculations; and (ii) the discrepancy due to the basis set between SIESTA and plane-wave results is rather small. Three empirical potentials for iron have been benchmarked on these DFT results. They all yield similar energies, but different shapes for the γ-lines. Using the criterion suggested by Duesbery and Vitek, the γ-line results point to non-degenerate core structures for the DFT calculations and for the Ackland and Ackland–Mendelev potentials but not for the Dudarev–Derlet potential. The direct calculations of the dislocation core structures show that the Ackland potential is an exception to the Duesbery–Vitek rule. More insight into the stability of the core structure can be gained by looking at the response to the polarization of the core. The Dudarev–Derlet and Ackland potentials have similar polarizations, but the energy difference between degenerate and non-degenerate cores is much larger with the Dudarev–Derlet potential, as expected from the γ-lines. The polarizability of the non-degenerate core is smaller with the Ackland–Mendelev potential than in DFT, indicating that the energy landscape is flatter in this direction. read less

Abstract: The strengthening effect of nanosized Cu precipitates in bcc Fe has been studied by performing molecular dynamics simulations of the interaction between a screw dislocation and a coherent bcc Cu precipitate of 1–4 nm diameter in bcc Fe. The dislocation detachment mechanism changes from shear at a precipitate diameter of 4 and 2.5 nm in the twinning and anti-twinning directions, respectively, due to the coherency loss caused by the screw dislocation assisted martensitic transformation of the precipitate. The screw dislocation detachment mechanism with the larger, transformed precipitates involves annihilation-and-renucleation, or Orowan looping in the twinning vs. anti-twinning direction, respectively. The critical resolved shear stress (CRSS) of the screw dislocation-precipitate interaction increases with increasing precipitate size, and is strongly dependent on the precipitate structure and detachment mechanism. The CRSS is much larger in the anti-twinning direction. [doi:10.2320/matertrans.M2009040] read less

Abstract: The self-guided molecular dynamics (SGMD) method, which can enhance the conformational sampling efficiency in MD simulations, was applied in investigating the phase transformation of Cu precipitate in α-iron that took place during thermal aging. It was shown that the SGMD method can accelerate calculating the bcc to 9R structure transformation of a small precipitate (even 4.0 nm in size), enabling the transformation without introducing any excess vacancies. The size dependence of the transformation also agreed with that seen in previous experimental studies. read less

Abstract: We present the results of molecular dynamic simulations of ductile–brittle behavior at an edge [1 1 0] crack neighboring with a Cu nanoprecipitate in bcc iron. This crack in pure bcc iron emits dislocations in the ⟨1 1 1⟩{1 1 2} slip systems and it is stable up to very high loads, in qualitative agreement with experiments and continuum predictions. Our question is how a copper precipitate influences the dislocation emission from the crack front and how the dislocations interact with the precipitate. read less

Abstract: Molecular dynamics simulations of the interaction between a screw dislocation and a 3.5 nm diameter bcc Cu precipitate in bcc Fe have been performed in the twinning and antitwinning direction between 10 and 400 K. The results indicate a significant temperature dependence in the antitwinning direction, whereby the screw dislocation bypasses the Cu precipitate by Orowan looping below 200 K, but shears the precipitate above 300 K. The transition in interaction mechanism is caused by a screw dislocation assisted martensitic transformation of the Cu precipitate, which significantly diminishes above 300 K. The screw dislocation shears the precipitate at all temperatures between 10 and 400 K in the twinning direction. Thus, transformation of the precipitate induces additional precipitate strengthening below 200 K in the antitwinning direction, which drastically decreases with increasing temperature above 300 K. read less

Abstract: We present static and dynamic simulations to study the thermally activated motion of dislocations. The model employed is the two-dimensional Frenkel-Kontorova model. The main interest is to allow simulations of dislocation dynamics over long-time periods �600 ns, giving access to large ranges of applied stresses and temperatures. The kink-pair nucleation rates, determined from dynamical simulations, are studied as a function of the ratio between the kink-pair activation enthalpy and the thermal energy. The former is computed from static simulations based on the elastic nudged band method. We show here that the dislocation motion is composed of two regimes: a low-temperature regime where the nucleation rate follows a thermally activated exponential law and a high-temperature regime where the motion is slower than expected from the lowtemperature exponential law. We evidence a correlation between successive dislocation jumps that stems from the dynamics of internal modes of the dislocation and impedes high-frequency nucleation events. read less

Abstract: The influence of temperature, T, and strain rate, , on the reaction between the edge dislocation line and a periodic row of 4 nm interstitial dislocation loops with Burgers vector in α-Fe has been investigated by means of molecular dynamics, using a potential developed recently for body centred cubic Fe (Ackland et al 2004 J. Phys.: Condens. Matter 16 1). A dislocation segment with b = [010] is formed by favourable reaction in all cases: it is sessile in the glide plane and leads to the formation of a screw dipole on the line under increasing stress. The mechanism controlling line breakaway and the corresponding critical stress depend mainly on T rather than . At high T (300 and 600 K here) the length of the screw dipole is short (<10b) and the controlling mechanism is the glide of the [010] segment over the loop surface coupled with cross-slip of the short screws. The loop is totally absorbed on the line by transformation of b to . At low T, where thermal effects are negligible, a long (∼100b) screw dipole is drawn out and the controlling mechanism is annihilation of the dipole by screw cross-slip. This results in only partial absorption of the loop. By comparing the results with earlier ones obtained using an older interatomic potential, conclusions are drawn on the effects of interaction between edge dislocations and interstitial loops in iron. read less

Abstract: Pinning interaction between a screw dislocation and a void in fcc copper is investigated by means of molecular dynamics simulation. A screw dislocation bows out to undergo depinning on the original glide plane at low temperatures, where the behavior of the depinning stress is consistent with that obtained by a continuum model. If the temperature is higher than $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the motion of a screw dislocation is no longer restricted to a single glide plane due to cross slip on the void surface. Several depinning mechanisms that involve multiple glide planes are found. In particular, a depinning mechanism that produces an intrinsic prismatic loop is found. We show that these complex depinning mechanisms significantly increase the depinning stress. read less

Abstract: Atomic-scale computer simulation is used to study the interaction between a vacancy and a cluster of self-interstitial atoms in metals with hcp, fcc and bcc crystal structure: α-zirconium, copper and α-iron. Effects of cluster size, atomic structure, dislocation nature of the cluster side and temperature are investigated. A vacancy can recombine with any interstitial in small clusters and this does not affect cluster mobility. With increasing sizes clusters develop dislocation character and their interaction with vacancies depends on whether the cluster sides dissociate into partial dislocations. A vacancy recombines only on undissociated sides and corners created with undissociated segments. Vacancies inside the cluster perimeter do not recombine but restrict cluster mobility. Temperature enhances recombination by either increasing the number of recombination sites or assisting vacancy diffusion towards such sites. The results are discussed with relevance to differences in irradiation microstructure evolution of bcc, fcc and hcp metals and higher level theoretical modelling techniques. read less

Abstract: A thermodynamic formalization is developed for description of the nucleation and growth of helium bubbles in metals during irradiation. The proposed formalization is available for evaluating both microstructural changes in fusion first wall materials where helium is produced by (n, α) nuclear transmutation reactions, and those in fusion diverter materials where helium particles with low energy are directly implanted. The calculated nucleation barrier is significantly reduced by the presence of helium, showing that a helium bubble with an appropriate number of helium atoms depending on bubble size can nucleate without any large nucleation barriers, even at a condition where an empty void has very large nucleation barriers without helium. With the proposed thermodynamic formalization, the nucleation and growth process of helium bubbles in iron during irradiation is simulated by the kinetic Monte Carlo (KMC) technique. It shows the nucleation path of a helium bubble on the (N He, N V) space as functions of temperatures and the concentration of helium in the matrix, where N He and N V are the numbers of helium atoms and vacancies contained in the helium bubble, respectively. Bubble growth rates depend on the nucleation path and suggest that two different mechanisms operate for bubble growth: one is controlled by vacancy diffusion and the other is controlled by interstitial helium diffusion. read less

Abstract: Copper-rich precipitates can nucleate and grow in ferritic steels containing small amounts of copper in solution and this affects mechanical properties. Growth kinetics, composition and structure of precipitates under irradiation are different from those under thermal ageing, and also vary with type of radiation. This implies that the interaction between radiation defects, i.e. vacancies, self-interstitial atoms (SIAs) and their clusters, and precipitates is influential. It is studied here by atomic-scale computer simulation. The results are compared with those of elasticity theory based on the size misfit of precipitates and defects, and the modulus difference between bcc iron and bcc copper. It is found that SIA defects are repelled by precipitates at large distance but, like vacancies, attracted at small distance. Copper precipitates in iron can, therefore, be sinks for both vacancy and interstitial defects and hence can act as recombination centres under irradiation conditions. A tentative explanation for the mixed Cu–Fe structure of precipitates observed in experiment and the absence of precipitate growth under neutron irradiation is given. More generally, agreement between the simulations and elasticity theory suggests that the results are not artefacts of the atomic model: both vacancy and interstitial defects in metals may bind to precipitates with weaker cohesion than the matrix. read less

Abstract: Molecular dynamics simulations of the interaction between a screw dislocation and a coherent bcc Cu precipitate in bcc Fe indicate that the screw dislocation stress field assists a martensitic transformation into a close-packed structure for precipitate diameters larger than 1.8nm, resulting in a stronger obstacle to dislocation glide. The observed martensitic transformation mechanism agrees with the Nishiyama-Kajiwara [Jpn. J. Appl. Phys. 2, 478 (1963)] model. For coherent bcc Cu precipitates with diameter larger than 2.5nm, the screw dislocation bypass mechanism becomes Orowan looping due to the coherency loss of the precipitates during the transformation. read less

Abstract: The effects of the substitutional element copper in solution in α-iron on glide of a ½ ⟨111⟩ { 110 } edge dislocation are investigated by atomic-scale computer simulation. Under static conditions (temperature T = 0 K), single copper atoms and nearest-neighbour pairs in the first atomic plane below the dislocation slip plane provide the strongest barrier to slip, in partial agreement with continuum theory. This contrasts with recent simulation results for the Ni–Al fcc alloy (Rodary et al 2004 Phys. Rev. B 70 054111), where Al atoms displaced into nearest-neighbour coordination across the slip plane form the strongest obstacles. The dynamics of dislocation glide in Fe–Cu solid solution at T > 0 K are determined as a function of solute concentration. Parameters such as velocity, critical stress and drag coefficient are analysed. Again, there are differences from the Ni–Al system. The results are discussed in terms of the static strength of solute configurations and the different crystal structure of iron and nickel. read less

Abstract: Clusters of self-interstitial atoms (SIAs) are formed in metals by high-energy displacement cascades, often in the form of small dislocation loops with a perfect Burgers vector, b. Atomic-scale computer simulation is used here to investigate their reaction with an edge dislocation gliding in α-iron under stress for the situation where b is inclined to the dislocation slip plane. The b of small loops (37 SIAs here) changes spontaneously and the interstitials are absorbed as a pair of superjogs. The line glides forward at critical stress τc when one or more vacancies are created and the jogs adopt a glissile form. A large loop (331 SIAs here) reacts spontaneously with the dislocation to form a segment with b = ⟨100 ⟩, which is sessile on the dislocation slip plane, and as applied stress increases the dislocation side arms are pulled into screw orientation. At low temperature (100 K), the ⟨100⟩ segment remains sessile and the dislocation eventually breaks free when the screw dipole arms cross-slip and annihilate. At 300 K and above, the segment can glide across the loop and transform it into a pair of superjogs, which become glissile at τc. Small loops are weaker obstacles than voids with a similar number of vacancies, large loops are stronger. Irrespective of size, the interaction processes leading to superjogs are efficient for absorption of SIA clusters from slip bands, an effect observed in flow localization. read less

Abstract: Monte Carlo calculations of copper atom diffusion via the vacancy mechanism in bcc iron are presented. The activation energy of atomic jumps is taken from recent ab initio calculations. It is shown that the vacancy–copper atom cross-diffusion coefficient is positive at all temperatures, for which the bcc crystal structure is preserved. This is shown to be opposite to the description obtained using data calculated with an empirical interatomic potential for the Fe–Cu system. The sensitivity of the results to the values of the activation energy within the uncertainty of ab initio calculations is analysed. Implications of the results for the features of copper precipitation in ferritic steels under neutron irradiation are discussed. read less

Abstract: The interaction between copper-rich precipitates in α-iron and either vacancies or self-interstitial atoms and their clusters is studied by atomic-scale modelling. Results are compared with predictions of elasticity theory and interpreted in terms of size misfit of precipitates and defects, and the modulus and cohesive energy differences between iron and copper. Interstitial defects are repelled by precipitates at large distance but, like vacancies, attracted at small distance. Hence, copper precipitates in iron can be sinks for both vacancy and interstitial defects, and can act as strong recombination centres under irradiation conditions. This leads to a tentative explanation for the mixed Cu–Fe structure of precipitates and the absence of precipitate growth under neutron irradiation conditions. More generally, both vacancy and interstitial defects may be strongly bound to precipitates with weaker cohesion than the matrix. read less

Abstract: Clusters of self-interstitial atoms are formed in metals by high-energy displacement cascades, often in the form of small dislocation loops with a perfect Burgers vector. In isolation, they are able to undergo fast, thermally activated glide in the direction of their Burgers vector, but do not move in response to a uniform stress field. The present work considers their ability to glide under the influence of the stress of a gliding dislocation. If loops can be dragged by a dislocation, it would have consequences for the effective cross-section for dislocation interaction with other defects near its glide plane. The lattice resistance to loop drag cannot be simulated accurately by the elasticity theory of dislocations, so here it is investigated in iron and copper by atomic-scale computer simulation. It is shown that a row of loops lying within a few nanometres of the dislocation slip plane can be dragged at very high speed. The drag coefficient associated with this process has been determined as a function of metal, temperature and loop size and spacing. A model for loop drag, based on the diffusivity of interstitial loops, is presented. It is tested against data obtained for the effects of drag on the stress to move a dislocation and the conditions under which a dislocation breaks away from a row of loops. read less

Abstract: Self-interstitial atom (SIA) type dislocation loop is one of the possible candidates of the so-called matrix damage that causes hardening and embrittlement of blanket structural materials of fusion reactors and/or pressure vessel materials of light water reactors. We present in this paper molecular dynamics computer simulation results on the interactions between an edge dislocation and a SIA loop with Burgers vectors of b = a 0 /2 [111] and b = a 0 /2 [111], respectively, which are introduced in bcc-Fe crystal. Then shear stresses of several different magnitudes are applied so that the dislocation moves to meet the SIA loop. General observation is that the SIA loops with diameter of ∼2 nm can be obstacles to dislocation motion, and the strength as obstacles to dislocation motion depends on applied stress. The origin of the stress dependent strength can be explained athermally using the elastic theory of dislocation interaction. In most cases, the SIA loops are absorbed by the edge dislocations to form a large super-jog after the interactions. This suggests a possibility of localized deformation of irradiated bcc-Fe due to the formation of dislocation channeling. read less

Abstract: This review provides a brief survey of the computational methods which have been adopted to model fracture in materials, particularly polycrystalline metals and alloys, and considers in greater detail the use of geometrical modelling. The different modes of fracture, transgranular and intergranular brittle and ductile, are discussed, together with the modelling tools adopted for macroscale, microscale and nanoscale applications. The procedures which have been used for creating computer models of two– and three–dimensional polycrystalline materials and for propagating cracks through them are then outlined. Applications of these methods to investigate the propagation of cleavage cracks across grain boundaries, to study the influence of texture, grain shape, impurity segregation and prior creep cavitation, to examine the ductile–to–brittle transition region in ferritic steels and to consider the influence of work hardening on ductile fracture are then presented. The predictions are compared with experimental results and proposals for future studies are discussed. read less

Abstract: The interaction between an edge dislocation and a void in copper is investigated by means of a molecular dynamics simulation. The depinning stresses of the leading partial and of the trailing partial show qualitatively different behaviors. The depinning stress of the trailing partial increases logarithmically with the void radius, while that of the leading partial behaves in a different manner due to the interaction between two partials. The pinning angle, which characterizes the obstacle strength, approaches zero when the void radius exceeds 3 nm. No temperature dependence is found in the critical stress and the critical angle. This is attributed to an absence of climb motion. It is also found that the distance between the void center and a glide plane asymmetrically affects the pinning strength. read less

Abstract: Using molecular dynamics calculations we demonstrate that with decreasing the thickness of ultrathin body-centered-cubic (bcc) α‐Fe film with (001) surfaces, the biaxial strain results in first bcc(001)→face-centered-cubic (fcc) (001) transition along the inverse Bain path due to softening of C33, and then fcc(001)→bcc(011) because of shear modulus vanishing along fcc ⟨110⟩ directions. For the bulk fcc γ‐Fe, the tensile biaxial strain along the Bain path transforms fcc (001) into bcc (001) with fcc⟨110⟩‖bcc⟨100⟩, while compressive strain results in shear instability, in agreement with recent ab initio calculations. read less

Abstract: Clusters of self-interstitial atoms (SIAs) in the form of parallel crowdions are created directly in high-energy displacement cascades produced in metals by neutron irradiation. They are equivalent to small perfect dislocation loops and, in isolation in pure metals, undergo fast thermally-activated glide in the direction of their Burgers vector. Their strain field and ability to glide allows long-range interaction with other extended defects. Indeed, dislocations decorated by dislocation loops are commonly observed after neutron irradiation. Dislocations gliding under applied stress also encounter these mobile defects. These effects influence mechanical properties and require further investigation. This paper presents results from an atomic-scale study of copper and α-iron at either 0 K or 300 K. Loop drag and breakaway effects are investigated for an edge dislocation under applied stress interacting with a row of SIA loops below its glide plane. The maximum speed at which a loop is dragged is lower in copper than iron, and the applied stress at which this occurs is also lower. These differences in the dynamics of cluster-dislocation interaction are determined by the atomic structure of the defects and cannot be investigated by continuum treatment. read less

Abstract: Recently a model has been developed by Osetsky and Bacon to study edge dislocations moving over large distances on the atomic scale. It permits investigation of motion of a dislocation under different conditions of applied shear stress with constant or variable strain rate and temperature, and in the presence of obstacles. In this paper we apply the model to study the motion of an infinite straight but flexible edge dislocation through a row of either voids or coherent copper precipitates in bcc iron. Stress–strain curves, energy barrier profile and strength characteristics of obstacles and other dislocation configuration information have been obtained from the modelling and compared with continuum treatments. Some specific atomic-scale mechanisms associated with strengthening due to voids and precipitates over a range of size have been observed and discussed. read less

Abstract: A model for simulating the dynamic behaviour of edge dislocations in metals at the atomic level is presented. The model extends an earlier approach based on an array of edge dislocations periodic in the Burgers vector direction and allows the external action (either shear strain or resolved shear stress), crystal energy, plastic displacement and dislocation position and velocity to be determined unambiguously. Two versions of the model for either static or dynamic conditions, i.e. zero or non-zero temperature, are described. The model is tested for elastic response of a perfect crystal and the atomic properties of a ½111 edge dislocation in a model of bcc Fe. Several examples of dislocation glide behaviour and dislocation–obstacle interactions at zero and non-zero temperature are presented and discussed. read less

Abstract: Nucleation and kinetics of defects at the atomic scale provide the most fundamental information about the mechanical response of materials and surfaces. Recent advances in experimental and computational analyses allow us to study this phenomenon in the context of nanoindentation and localized mechanical probing of surfaces. Here, we present an analytical formulation of the elastic limit that predicts the location and slip character of a homogeneously nucleated defect in crystalline metals, and extend this formulation to the atomic scale in the form of an energy-based, local elastic stability criterion, termed the L criterion. We demonstrate that this approach can be incorporated efficiently into computational methods such as molecular dynamics and finite-element models. Furthermore, we validate and calibrate the L criterion directly through nanoindentation experiments and two-dimensional experimental analogs such as the bubble raft model. We outline explicitly a compact and efficient application of the L criterion within the context of a nonlinear, interatomic potential finite-element model~IPFEM!. Further, we report three-dimensional molecular dynamics simulations in several face-centered cubic systems that elucidate the transition from the initiation to the early stages of plasticity during nanoindentation of metals, as characterized by homogeneous and heterogeneous nucleation of up to hundreds of dislocations. Correlation of these simulations with direct observations from nanoindentation experiments provides atomistic insights into the early stages of plasticity. read less

Abstract: The damage rate dependence of the yield stress change in a neutron-irradiated Fe–Cu model alloy was analyzed by a model calculation. The model was based on the rate theory, and focused on the description of the nucleation and growth of point defect clusters and copper clusters. The binding between copper atoms and vacancies and the effect of cascade damage which directly creates small point defect clusters were incorporated in this model. The instability of small point defect clusters caused by thermal dissociation was also included. From the result of the calculation, the yield stress changes were estimated using the Orowan model and the Russel-Brown model. As a result of this calculation, it was clarified that copper clusters are the main factor of yield stress change in almost all irradiation stages below 0.1 dpa. The contribution of copper clusters to yield stress change increased with decreasing damage rate. The nature of the damage rate dependence is not affected by the copper-vacancy binding but by the sink strength, which changes dynamically throughout irradiation. read less

Abstract: The paper examines different definitions of the local atomic stress at zero temperature for homogeneous and inhomogeneous strain across an interface. It is shown that if inhomogeneous straining occurs within the range of interatomic interaction, then the interplanar concept (based on force balance) describes better the local stress in comparison with a definition of the volume stress derived from the energy density around an atom. read less

Abstract: Recent molecular dynamics (MD) computer simulations of pure copper and iron have shown that clusters consisting of up to a few tens of self-interstitial atoms (SIAs) are highly mobile along close-packed crystallographic directions. This effect has important consequences for microstructure evolution in irradiated metals and so it is desirable to investigate the mechanisms of cluster motion. In the present paper, results of MD modelling of the thermally-activated motion of clusters of three, nine and 17 SIAs in α-iron in the temperature range from 90 to 1400 K are analysed. The correlation between the motion of the centre of mass of a cluster and the individual jumps of its constituent SIAs is revealed. It is found that the SIAs in a cluster jump almost independently and their jump frequency depends on the number of SIAs in the cluster. This leads to a simple relationship between the jump frequency of a cluster and the number of SIAs in it. The reason for the deviation of the cluster jump frequency from a simple Arrhenius relationship is discussed. It is shown that such clusters only exhibit an effectively random walk, that is a correlation factor of one, when the jump length defining diffusion is taken to be 3b to 4b, where b is the magnitude of the vector ½(111). read less

Abstract: Anomalous structural evolution was induced by room-temperature 200 keV xenon ion irradiation and it results in the formation of various new metastable phases in the equilibrium immiscible Fe–Cu system. First, nanosized quasicrystals were formed in an amorphous matrix through a two-step transition of crystal to amorphous to quasicrystal in Fe70Cu30 multilayered films. The real compositions of the amorphous matrix and quasicrystals were determined to be close to Fe70Cu30 and Fe50Cu50, respectively. Moreover, the same icosahedral phase was also obtained in another similarly designed Fe50Cu50 multilayered sample upon 850 °C thermal annealing, confirming the existence of such a metastable state. Second, amorphous alloys were formed in a composition range of 30–50 at. % of Cu. Third, a Cu-based face-centered-cubic (fcc) solid solution was formed at an alloy composition of about Fe30Cu70 and, interestingly, another fcc structured metastable crystalline phase was obtained at a composition very close to that of Fe... read less

Abstract: We present large-scale molecular dynamic simulations of microcrack growth in -iron based on an N-body potential model which gives a good description of defect energetics, anisotropic elasticity and phonon frequency spectra. We demonstrate dynamic overshoot of a pre-existing microcrack under impact loading. We show that the basic behaviour of the simulations is in agreement with the predictions of continuum models. Dynamic phenomena, such as scattering of stress waves at crack faces, acoustic emission (due to bond breakage) and reflections of the stress waves from sample borders, are studied here in detail. The results on microcrack growth, unimpeded by the wave reflections from external sample borders, indicate that under fast loading or at high crack velocities, i.e. under high strain rates, transient twin formation is possible from the crack tip with later detwinning at free crack faces as the crack advances: a twinning equivalent to virtual Knott dislocations. read less

Abstract: Results are presented for modeling the deposition of Ag and rutile TiO2. The model can be used to examine the effect of varying experimental parameters, such as the substrate bias in the magnetron and the stoichiometry of the deposition species. We illustrate how long time scale dynamics techniques can be used to model the process over experimental time scales. Long time dynamics is achieved through an on-the-fly Kinetic Monte Carlo (otf-KMC) method, which determines diffusion pathways and barriers, in parallel, with no prior knowledge of the involved transitions. Using this otf-KMC method we have modeled the deposition of Ag and TiO2 for various plasma deposition energies, in the range 1 eV to 100 eV. It was found that Ag {111} produces the most crystalline growth when deposited at 40 eV. TiO2 growth showed that at energies of 1 eV and 100 eV a porous structure occurs with void formation. At deposition energies of 30 eV and 40 eV, a more dense and crystalline rutile growth forms. The results show that deposition energy plays an important role in the resulting thin film quality and surface morphology. read less

Abstract: Interaction between a 1/2⟨1 1 1⟩{1 1 0} edge dislocation and voids or coherent bcc Cu precipitates (diameter D = 2 or 4 nm) in Fe with their centre displaced by ±Δz from the dislocation glide plane is investigated by computer simulation for temperature T = 0 to 450 K. Voids provide the highest critical stress, τc, when Δz = 0. The dislocation climbs up on release when Δz ⩾ 0, but down when Δz < 0. Void-surface facets influence the sense of climb. There is no correspondence between τc and the sense or magnitude of climb. 2 nm precipitates give highest τc when Δz < 0 and lowest when Δz > 0, due to a combination of the modulus difference and size misfit between bcc Cu and Fe. 4 nm precipitates are partially transformed to fcc structure by the dislocation when T ⩽ 300 K and Δz ⩾ 0. Surprisingly, the transformed fraction of Cu at low T is highest for Δz = D/2, due to the compressive field of the dislocation. The geometries that produce large transformed fractions result in the lowest τc, in contrast to expectation from previous studies. read less

Abstract: The paper examines different definitions of the local atomic stress at zero temperature for homogeneous and inhomogeneous strain across an interface. It is shown that if inhomogeneous straining occurs within the range of interatomic interaction, then the interplanar concept (based on force balance) describes better the local stress in comparison with a definition of the volume stress derived from the energy density around an atom. read less

Abstract: Neutron hardening and embrittlement of pressure vessel steels is due to a high density of nanometer scale features, including Cu-rich precipitates which form as a result of radiation enhanced diffusion. High-energy displacement cascades generate large numbers of both isolated point defects and clusters of vacancies and interstitials. The subsequent clustering, diffusion and ultimate annihilation of primary damage is inherently coupled with solute transport and hence, the overall chemical and microstructural evolutions under irradiation. In this work, we present atomistic simulation results, based on many-body interatomic potentials, of the migration of vacancies, solute and self-interstitial atoms (SIA) in pure Fe and binary Fe-0.9 and 1.0 at.% Cu alloys. Cu diffusion occurs by a vacancy mechanism and the calculated Cu diffusivity is in good agreement with experimental data. Strain field interactions between the oversized substitutional Cu solute atoms and SIA and SIA clusters are predominantly repulsive and result in both a decreased activation energy and diffusion pre-factor for SIA and small (N directions, as well as the transition from to mobile configurations. The migration behavior ofmore » larger SIA clusters, which undergo only one-dimensional diffusion during molecular dynamics timescales, is largely unaffected by the Fe-Cu alloy, although SIA clusters are effectively repelled by coherent Cu precipitates.« less read less

Abstract: It has long been noticed that the effect of Cu solute atoms is important for the microstructural evolution of ferritic pressure vessel steels under neutron irradiation conditions. Despite the low concentration of Cu in steel, Cu precipitates form inside the a-Fe surrounding matrix and by impeding free dislocation motion considerably contribute to the hardening of the material. It has been suggested that Cu-rich clusters and combined Cu solute atoms-defect clusters that may act as initiating structures of further precipitates nucleate during annealing of displacement cascades. In order to assess the importance of the different mechanisms taking place during collision events in the formation and later evolution of these structures, a detailed Molecular Dynamics (MD) analysis of displacement cascades in a Fe-1.3% at. Cu binary alloy has been carried out. Cascade energies ranging from 1 to 20 keV have been simulated at temperatures of 100 and 600 K using the MDCASK code, in which the Ackland-Finnis-Sinclair many-body interatomic potential has been implemented. The behavior of metastable Cu self-interstitial atoms (SIAs) in the form of mixed Fe-Cu features is studied as well as their impact on the resulting defect structures. It is observed that above 300 K generated Cu SIAs undergo recombinationmore » with no substantial effect on the after-cascade microstructure while at 100 K Cu SIAs remain sessile and exhibit a considerable binding to interstitial and vacancy clusters, Finally, the effect that the production of vacancies via collision cascades may have on the self-diffusion of Cu solute atoms is quantitatively addressed by means of determining diffusion coefficients for Cu atoms under different microstructural conditions.« less read less

Abstract: ABSTRACT The austenite-ferrite transformation is the key metallurgical tool to tailor properties of low alloyed steels and remains an active area of research. Models have yet to be developed with truly predictive capabilities for phase transformations in multi-component commercial steels. This review provides a critical analysis of the various austenite-to-ferrite diffusional transformation model approaches that have been significantly broadened over the past decade by modelling at different length scales, i.e. classical macro-scale models have been augmented with simulations at the meso-scale and atomistic scale. Both semi-empirical and fundamental models are reviewed with an emphasis on polygonal ferrite formation in low and medium carbon steels. Formation of ferrite with more complex morphologies (i.e. irregular/bainitic/Widmanstätten ferrite) is also discussed. In particular, approaches to describe the interaction of alloying elements with the austenite-ferrite interface are critically analysed. The paper concludes with an outlook on the proposed austenite-ferrite transformation modelling work for the next decade. read less

Abstract: This paper is devoted to an experimental and 3D atomistic study of the influence of loading rate on fracture toughness in dilute Fe–Si alloys and in bcc iron. We analyze new and previous experimental results from fracture tests performed at room temperature on bcc iron–silicon single crystals with edge cracks [Formula: see text][110] (crack plane/crack front). The specimens of single edge notch-type were loaded in tension mode I under different loading rates. The ductile–brittle behavior at the crack front was monitored online via optical microscopy together with external force and prolongation of the specimens. About 30% decrease in fracture toughness was monitored in the new experiment under the highest loading rate. The nanoscopic processes produced by the crack itself were studied at room temperature via 3D molecular dynamics (MD) simulations in bcc iron under equivalent boundary conditions as in experiments to reveal (explain) the sensitivity of the crack to loading rate. For this purpose, this MD study utilizes the self-similar character of linear fracture mechanics. The results show that the emission of blunting dislocations from the crack is the most difficult under the highest loading rate, which leads to the reduced fracture toughness of the atomistic sample. This is in a qualitative agreement with the experimental (macro) results. Moreover, MD indicates that there may be some synergetic (resonant) effect between the loading rate and thermal activation that promotes dislocation emission. read less

Abstract: In our work we discuss features of atomic jump simulation. Then we present a new approach (approach of natural thermostat) that fully takes into account the fluctuating character of atomic jumps at the constant temperature Molecular Dynamics (MD) simulation. In addition the combination of MD and modified method of MS allowed within this model to take into account thermal expansion of the lattice and long-range elastic displacement of the atoms in the vicinity of defects. With the developed model, we study features of atom diffusion in iron by vacancy mechanism at different temperatures using a many-body potential. Migration energy and pre-exponential factor are obtained and a comparison of the calculated diffusion parameters with the results of the work of other authors who used various modelling methods is carried out. We then developed our approach to take into account the effect of pressure on diffusion jumps of atoms. This made it possible to calculate the temperature dependence of the migration volume. Graphical abstract read less

Abstract: ABSTRACT In accordance with the molecular dynamic simulation and nudged elastic band method, the surface diffusion and growth of Fe–Cu nanoparticles were studied, respectively, using Large-scale Atomic/Molecular Massively Parallel Simulator code. Results showed that a single Cu adatom diffused on the surface of the Fe substrate mainly via the hopping mechanism and nearly did not exchange with the substrate atoms. In the Cu substrate, when interfacet diffusion of a Fe adatom on the surface was activated, the system energy evidently decreased after the exchange mechanism was chosen. At low temperatures, the metastable core–shell structure of FeshellCucore nanoparticles was obtained by simulating the deposition of Fe on Cu substrate. For the growth of Cu on Fe substrate, FecoreCushell nanoparticles could be formed and they gradually evolved into Wulff-like structures with depositing Cu atoms. The Monte Carlo calculation further showed that the stable configuration of Fe–Cu nanoparticles is FecoreCushell when the concentration of Cu atoms is small. With the increase of Cu atomic ratio (enough to cover the surface), the quasi-Janus was the stable structure in Fe–Cu nanoparticles. read less

Abstract: 3D atomistic simulations via molecular dynamics (MD) at temperature of 0 K and 295 K (22 °C) with a high quasi-static loading rate dP/dt of 2.92 kN s−1 show that cleavage fracture is supported by surface emission of oblique dislocations 111¯011 and by their subsequent cross slip to {112} planes, which increases separation of the (001) cleavage planes inside the crystal. Under the slower loading rate by a factor 5, the crack growth is hindered by twin generation on oblique planes {112} and the fracture is ductile. The MD results explain the contribution of the crack itself to the ductile-brittle transition observed in our fracture experiments on Fe-3wt%Si single crystals of the same orientation and geometry, loaded at the same rates dP/dt as in MD. The loading rates are equivalent to the cross head speed of 5 mm min−1 and 1 mm min−1 used in the experiment. The MD results also agree with the stress analysis performed by the anisotropic LFM and comply with experimental observations. 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: ABSTRACT While significant research has been conducted on the various mechanisms of hydrogen embrittlement, there remains a lack of quantitative understanding on the effect of atomic hydrogen concentration on mechanical properties. Previous work suggests that an increased hydrogen concentration will degrade both the elastic modulus and yield stress. However, experimental samples often contain other atomistic defects that make it difficult to determine the role hydrogen alone plays on material behaviour. Further, experimental studies are often unable to directly quantify the effect of hydrogen concentration on modulus. The purpose of this study was to use molecular dynamics simulations to quantify the effect of interstitial hydrogen on the mechanical properties of iron. The potential type used was shown to significantly affected predicted results. Atomic hydrogen was shown to linearly degrade the elastic modulus and stress to initiate dislocations at all temperatures considered. Increasing hydrogen concentration was shown to promote the formation of dislocations at a lower stress, resulting in a higher density of dislocations and shorter slip distances. This study provides a foundation for better understanding of the role of hydrogen on the degradation of mechanical properties during loading. read less

Abstract: The short-range repulsive interactions of any force field must be modified to be applicable for high energy atomic collisions because of extremely far from equilibrium state when used in molecular dynamics (MD) simulations. In this work, the short-range repulsive interaction of a reactive force field (ReaxFF), describing Fe–Ni–Al alloy system, is well modified by adding a tabulated function form based on Ziegler–Biersack–Littmark (ZBL) potential. The modified interaction covers three ranges, including short range, smooth range, and primordial range. The short range is totally predominated by ZBL potential. The primordial range means the interactions in this range is the as-is ReaxFF with no changes. The smooth range links the short-range ZBL and primordial-range ReaxFF potentials with a taper function. Both energies and forces are guaranteed to be continuous, and qualified to the consistent requirement in LAMMPS. This modified force field is applicable for simulations of energetic particle bombardments and reproducing point defects' booming and recombination effectively. read less

Abstract: Accurate methods and an efficient workflow for computing and documenting dislocation core energies are developed and applied to $\frac{1}{2}\ensuremath{\langle}111\ensuremath{\rangle}$ and $\ensuremath{\langle}100\ensuremath{\rangle}$ dislocations in five body-centered cubic (bcc) metals W, Ta, V, Mo, and $\ensuremath{\alpha}$-Fe represented by 13 model interatomic potentials. For each dislocation type, dislocation core energies are extracted for a large number of dislocation characters thoroughly sampling the entire 2-space of crystallographic line orientations of the bcc lattice. Of particular interest, core energies of the $\frac{1}{2}\ensuremath{\langle}111\ensuremath{\rangle}{110}$ dislocations are found to be distinctly asymmetric with respect to the sign of the character angle, whereas core energies of $\ensuremath{\langle}100\ensuremath{\rangle}{110}$ junction dislocations exhibit marked cusps for line orientations vicinal to the closed-packed $\ensuremath{\langle}111\ensuremath{\rangle}$ directions. Our findings furnish substantial insights for developing accurate models of dislocation core energies employed in mesoscale dislocation dynamics simulations of crystal plasticity. read less

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Abstract: ABSTRACT Dissipative mechanism of formation of a modulated structure of vacancies in a body-centred cubic (b.c.c.) crystal is considered as its intrinsic property under an isothermal irradiation irrespectively of elastic-anisotropy factor sign. Conditions of self-organisation of a precursor of formation of a ‘superlattice’ of nanovoids, namely, the modulated structure in a spatial distribution of the diffusing vacancies generated by irradiation, because of the instability of their homogeneous distribution as a result of interaction between them in a b.c.c. host crystal under irradiation, are analysed. As shown for the first time, the ‘cohesive’ and ‘elastic’ couplings between interacting (and diffusing) vacancies have a crucial role in a pattern-selection process accordingly with different (positive or negative) elastic-anisotropy factors. read less

Abstract: ABSTRACT The dependence of the interactions of intermediate-size ½<111> self-interstitial atom (SIA) loops with an edge dislocation on strain rate and temperature was investigated by molecular dynamics (MD) simulations for the interatomic potential derived by Ackland et al. (A97). For low temperatures (T = 1 K), the mechanisms of the interactions were in agreement with recent literature. It was shown that a second passing of the dislocation through the loop led to a different mechanism than the one that occurred upon first passing. Since these mechanisms are associated with different SIA loop sizes, and since the loop lost a number of SIAs upon first interaction, it was deduced that the dividing threshold between large and small loops (rendering them strong or weak obstacles, respectively) is at the vicinity of the loop size studied (169 SIAs). For higher temperatures (T = 300 K), the strain rate dependence proved strong: for low strain rates, the dislocation absorbed the loop as a double super-jog almost immediately and continued its glide unimpeded. For a high strain rate, the dislocation was initially pinned due to the formation of an almost sessile segment leading to high critical stress. read less

Abstract: Indentation into a metastable austenite may induce the phase transformation to the bcc phase. We study this process using atomistic simulation. At temperatures low compared to the equilibrium transformation temperature, the indentation triggers the transformation of the entire crystallite: after starting the transformation, it rapidly proceeds throughout the simulation crystallite. The microstructure of the transformed sample is characterized by twinned grains. At higher temperatures, around the equilibrium transformation temperature, the crystal transforms only locally, in the vicinity of the indent pit. In addition, the indenter produces dislocation plasticity in the remaining austenite. At intermediate temperatures, the crystal continuously transforms throughout the indentation process.Indentation into a metastable austenite may induce the phase transformation to the bcc phase. We study this process using atomistic simulation. At temperatures low compared to the equilibrium transformation temperature, the indentation triggers the transformation of the entire crystallite: after starting the transformation, it rapidly proceeds throughout the simulation crystallite. The microstructure of the transformed sample is characterized by twinned grains. At higher temperatures, around the equilibrium transformation temperature, the crystal transforms only locally, in the vicinity of the indent pit. In addition, the indenter produces dislocation plasticity in the remaining austenite. At intermediate temperatures, the crystal continuously transforms throughout the indentation process. read less

Abstract: This work investigates how the interatomic surface potential influences molecular dynamics (MD)-derived thermal accommodation coefficients (TACs). Iron, copper, and silicon surfaces are considered over a range of temperatures that include their melting points. Several classes of potentials are reviewed, including two-body, three-body, and bond-order force fields. MD-derived densities and visualization of the surfaces are used to explain the differences in the parameterizations of these potentials within the context of gas–surface scattering. Finally, TACs are predicted for a range of gas–surface combinations, and recommended values of the TAC are selected that take into account the robustness and uncertainties of each of the considered parameterizations. Further, it is observed that there is a significant change in the TAC about phase changes that must be taken into account for applications with a large range of surface temperatures. read less

Abstract: Molecular dynamics simulations have been used to investigate the coupling process between plasticity and structural phase transformation in single-crystal iron under both shock and ramp compressions. In both cases, iron was found to yield via twinning. Then, the onset of the bcc-hcp phase transformation was shown to be tightly dependent on the plasticity history through a hardening-like effect, which in some conditions may inhibit the nucleation of the hcp phase. read less

Abstract: In the present work, molecular dynamics simulations were carried out to investigate the temperature distribution as well as the fundamental friction characteristics such as the coefficient of friction and wear in a disc-pad braking system. A wide range of constant velocity loadings was applied on metallic brake pads made of aluminium, copper and iron with different rotating speeds of a diamond-like carbon brake disc. The average temperature of Newtonian atoms and the coefficient of friction of the brake pad were investigated. The resulting relationship of the average temperature with the speed of the disc as well as the applied loading velocity can be described by power laws. The quantitative description of the volume lost from the brake pads was investigated, and it was found that the volume lost increases linearly with the sliding distance. Our results show that Archard's linear wear law is not applicable to a wide range of normal loads, e.g., in cases of low normal load where the wear rate was increased considerably and in cases of high load where there was a possibility of severe wear. In this work, a new formula for the brake pad wear in a disc brake assembly is proposed, which displays a power law relationship between the lost volume of the metallic brake pads per unit sliding distance and the applied normal load with an exponent of 0.62 ± 0.02. This work provides new insights into the fundamental understanding of the wear mechanism at the nano-scale leading to a new bottom-up wear law for metallic brake pads. read less

Abstract: ABSTRACT Molecular dynamics simulations using an embedded-atom method potential for pure iron (Fe) were performed to investigate the solid-state transformation of a body-centred-cubic phase from a polycrystalline face-centred-cubic matrix. A Kurdjumov–Sachs orientation relationship accompanied with a step-ledge disconnection structure was detected in high mobility phase boundaries of a full 3D classical and barrier-free nucleation specimen. The results imply that interface coherency is an important factor for both nucleation and growth processes, which may significantly affect the transition rate of the austenite–ferrite transformation. 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 effects of [001] uniaxial strain on the stable structures and structural evolution of vacancy clusters in fcc metals, Cu, Ni, Al and Fe, have been studied and compared. Under uniaxial strain, the clusters in all these metals tend to align parallel or perpendicular to the strain axis under tensile or compressive strain. Moreover, both the body cluster and the {001} planar cluster become the dominant types. In addition, the stacking fault tetrahedron cluster becomes another dominant type in Al under compressive strain. The cluster structures in Fe are disordered under strain possibly because the pure fcc Fe is thermodynamically unstable under the current simulation condition. read less

Abstract: In paper melting process of iron nanoparticles was investigated with molecular dynamics method. Melting temperatures was found for particles with radius from 1.5 to 4 nm. Results match with data of other authors. Heat capacity was calculated based on investigation of caloric curves. Dependence between heat capacity and temperature for different size of nanoparticles was approximated. Heat conductivity of iron nanoparticles was calculated. read less

Abstract: In order to improve the accident tolerance of light water reactor (LWR) fuel, alternative cladding materials have been proposed to replace zirconium (Zr)-based alloys. Of these materials, there is a particular focus on iron-chromium-aluminum (FeCrAl) alloys due to much slower oxidation kinetics in high-temperature steam than Zr-alloys. This should decrease the energy release due to oxidation and allow the cladding to remain integral longer in the presence of high temperature steam, making accident mitigation more likely. As a continuation of the development for these alloys, the material response must be demonstrated to provide suitable radiation stability, in order to ensure that there will not be significant dimensional changes (e.g., swelling), as well as quantifying the radiation hardening and radiation creep behavior. In this report, we describe the use of cluster dynamics modeling to evaluate the defect physics and damage accumulation behavior of FeCrAl alloys subjected to neutron irradiation, with a particular focus on irradiation-induced swelling and defect fluxes to dislocations that are required to model irradiation creep behavior. read less

Abstract: Using classical molecular dynamics simulations and the Meyer-Entel interaction potential, we study the martensitic transformation pathway in a pure iron bi-crystal containing a symmetric tilt grain boundary. Upon cooling the system from the austenitic phase, the transformation starts with the nucleation of the martensitic phase near the grain boundary in a plate-like arrangement. The Kurdjumov-Sachs orientation relations are fulfilled at the plates. During further cooling, the plates expand and merge. In contrast to the orientation relation in the plate structure, the complete transformation proceeds via the Pitsch pathway. read less

Abstract: Adsorption and dissociation of gaseous carbon monoxide (CO) on metal surfaces is one of the most frequently occurring processes of carburisation, known as primary initiator of metal dusting corrosion. Among the various factors that can significantly influence the carburisation process are the intrinsic surface defects such as single surface vacancies occurring at high concentrations due to their low formation energy. Intuitively, adsorption and dissociation barriers of CO are expected to be lowered in the vicinity of a surface vacancy, due to the strong attractive interaction between the vacancy and the C atom. Here the adsorption energies and dissociation pathways of CO on clean and defective Fe 110 surface are explored by means of density functional theory. Interestingly, we find that the O adatom, resulting from the CO dissociation, is unstable in the electron-deficit neighbourhood of the vacancy due to its large electron affinity, and raises the barrier of the carburisation pathway. Still, a full comparative study between the clean surface and the vacancy-defected surface reveals that the complete process of carburisation, starting from adsorption to subsurface diffusion of C, is more favourable in the vicinity of a vacancy defect. 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: We examine the effect of elastic stresses induced by growing voids on the diffusion vacancy fluxes using newly derived equations. One of the main goals of our work is to obtain a kinetic equation for the growth rate of voids in cubic metals. The diffusion equation for vacancies, in which the influence of elastic stress near the void on the flux is taken into account, is linearized and solved. Then after mathematical transformations that are similar to Lifshitz - Slyozov theory, kinetic equations for the growth rate of the voids in fcc and bcc metals are obtained. The kinetic equations contain additional terms due to developed strain. This feature distinguishes the present equation from known ones and changes the kinetic of void growth. The functional dependence on strain is determined by coefficients, which characterize the strain influence on diffusion (SID coefficients). These coefficients are very sensitive to the atomic structure in the nearest vicinity of the saddle-point configuration. We have built an advanced model to evaluate them. SID coefficient simulation is the next step of this work. Using the kinetic equations and the SID coefficients, we calculate the void growth rate in cubic metals under different conditions. 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: The development of vacancy clusters in α-Fe is essential for understanding the formation of irradiation damage. In this work, the structures and the development of vacancy clusters in α-Fe have been studied by atomistic computer simulation. It was found out that larger clusters can form by combinations of smaller cluster units including triangular trimers and square tetramers. Furthermore, the formation of stacking faults, cracks or voids can be explained by the development of the clusters. read less

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

Abstract: In this paper, we summarized our recent experimental results on Fe-Cu model alloys irradiated by Fe ion. Two kinds of Fe-Cu alloys with 0.3%Cu and 0.6%Cu were prepared and irradiated by 2.5 MeV Fe ion at 573 K. Irradiation dose is 0.1 dpa and 1.2 dpa for each type alloy respectively. Positron annihilation techniques of slow positron beam were used to investigate the irradiation induced defects. Results show that the S parameters are higher in the specimens with high irradiation dose, but the intensity of Cu peaks of CDB is lower. It indicates that the precipitation of Cu atoms formed easily as lower irradiation dose. read less

Abstract: We extend the analytic bond-order potentials for transition metals [Phys. Rev. B 74, 174117 (2006)] to include ferro, antiferro, and noncollinear magnetism and charge transfer. This is achieved by first deriving a suitable tight-binding model through the expansion of the spin-density energy functional to second order with respect to magnetic and charge fluctuations. The tight-binding model is then approximated locally by the bond-order potential expansion, where the variational property of the bond-order potential expansion allows us to derive analytic expressions for the forces and torques on the atoms. From the bond-order potentials we then extract a hierarchy of multispin interactions beyond the conventional Heisenberg model. The explicit valence dependence of the bond-order potentials enables us to characterize the magnetic properties of the 3$d$ transition metals and to reproduce the trend from antiferromagnetic spin ordering close to the center of the $d$ band through noncollinear spin configurations to ferromagnetic ordering toward the edges of the $d$ band. The analytic representation of the energy within the bond-order potentials is then further expanded in the form of a Ginzburg-Landau expansion, deriving the prefactors explicitly from tight-binding and bond-order potentials. Thus, in this paper we present a coherent simplification from fundamental to empirical models of magnetism through coarse graining the electronic structure from spin-density functional theory to tight binding to bond-order potentials to the Ginzburg-Landau expansion. 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: A set of modified embedded-atom method (MEAM) potentials for the interactions between Al, Si, Mg, Cu, and Fe was developed from a combination of each element's MEAM potential in order to study metal alloying. Previously published MEAM parameters of single elements have been improved for better agreement to the generalized stacking fault energy (GSFE) curves when compared with ab initio generated GSFE curves. The MEAM parameters for element pairs were constructed based on the structural and elastic properties of element pairs in the NaCl reference structure garnered from ab initio calculations, with adjustment to reproduce the ab initio heat of formation of the most stable binary compounds. The new MEAM potentials were validated by comparing the formation energies of defects, equilibrium volumes, elastic moduli, and heat of formation for several binary compounds with ab initio simulations and experiments. Single elements in their ground-state crystal structure were subjected to heating to test the potentials at elevated temperatures. An Al potential was modified to avoid formation of an unphysical solid structure at high temperatures. The thermal expansion coefficient of a compound with the composition of AA 6061 alloy was evaluated and compared with experimental values. MEAM potential tests performed in this work, utilizing the universal atomistic simulation environment (ASE), are distributed to facilitate reproducibility of the results. read less

Abstract: Our recent model has been used to evaluate the point defect characteristics including those determining the effect of pressure on the concentration of vacancies, di-vacancies, interstitials and their diffusion mobility in set of BCC and FCC metals. Our model has been developed to calculate temperature dependences of mentioned features. In contrast to other studies, the vacancy migration volumes have been found for all the metals studied. read less

Abstract: The preference of the habit planes (HPs) developed from precipitation in the fcc/bcc system has been investigated. The interfacial energy of different interface orientations has been examined with variation of the orientation relationships (OR) and lattice parameters by a classical molecular dynamics method. The results show that interface has the lowest interfacial energy when it contains parallel Burgers vectors and a set of dislocations. The local minimum of interfacial energy may not associated with a maximum of dislocation spacing. It is also found that the near Kurdjumov-Sachs OR is more preferable than the near Nishiyama-Wasserman OR. Contrary to the previous interfacial energy calculations, which usually limit to rational ORs, the present work allows ORs to be irrational, which agrees with the observations. read less

Abstract: We present new results of molecular dynamic (MD) simulations in 3D bcc iron crystals with embedded central through crack (001)[110] of Griffith type loaded in mode I. Two different sample geometries of the same crystallographic orientation were tested with negative and positive values of the T-stress, which change the ductile-brittle behavior along the crack front in 3D. This phenomenon is explained in the framework of stress analysis, both on the continuum and atomistic level. read less

Abstract: Setting out from the results found in a set of small-angle neutron scattering (SANS) experiments for neutron-irradiated Fe-Cu model alloys, a rate theory model for the simulation of the irradiation-induced time-evolution of Cu-rich precipitates in these model alloys is presented which follows the idea that the precipitate clusters are mixed Cu-vacancy aggregates. This is done by explicitly allowing the defect clusters to absorb vacancies. The resulting Vacancy-Coupled Copper Clustering (V3C) model is calibrated by SANS experiments on two different Fe-Cu model alloys neutron-irradiated at four different doses. Quantitative agreement with the SANS experiments could be achieved by introducing a dependence of the Fe-Cu interface energy on the amount of vacancies in the mixed precipitate clusters. Phenomeno- logically, this energy can be seen as a function of the weight-percentage of Cu in the iron matrix. An empirical expression for this dependence is suggested. In addition, the new V3C model is used to gain some preliminary insight into the time-evolution of the chemical composition of the mixed Cu-vacancy clusters, confirming qualitatively the experimental findings. The relation of our ansatz to the heterogeneous Cu-precipitation mechanism proposed by others for neutron-irradiated Fe-Cu alloys of low Cu content is discussed. read less

Abstract: Experiments and atomic-scale computer simulations have shown that nano-scale voids and copper precipitates can be strong obstacles to the glide of dislocations in neutron-irradiated iron. Simulations have shown that voids are strong obstacles and that an edge dislocation climbs by absorbing vacancies at it breaks away from voids. The obstacle strength of copper precipitates is enhanced by a dislocation-induced structural transformation if they are large enough and the temperature is low enough. Most simulations have the centre of a spherical void or precipitate on the slip plane of an edge dislocation. The present work investigates how the strength of 2 and 4 nm voids and precipitates varies with the distance of their centre from the slip plane at temperatures across the range 0 to 450 K. The strength of voids is highest when their centre coincides with the slip plane, but this is not the case for small precipitates, which do not transform from the bcc structure. The strength of both type of obstacle, and the extent of climb at voids and transformation of large precipitates are not symmetric with respect to the position of their centre from the slip plane. The results are discussed in terms of the atomic mechanisms involved. read less

Abstract: In many ferrous austenitic alloys a strain induced martensitic transformation is important for determining bulk mechanical response. In this work molecular dynamic simulations have been performed in an attempt to further elucidate the mechanisms by which bcc-α' martensite may form at deformation induced hcp- martensite bands. The question of critical nucleus size and fault band arrangement are discussed in relation to the Olson-Cohen model for nucleation at fault band intersections as is the resulting orientation relationship. read less

Abstract: Given the time and length scales in molecular dynamics (MD) simulations of dislocation–defect interactions, quantitative MD results cannot be used directly in larger scale simulations or compared directly with experiment. A method to extract fundamental quantities from MD simulations is proposed here. The first quantity is a critical stress defined to characterise the obstacle resistance. This mesoscopic parameter, rather than the obstacle ‘strength’ designed for a point obstacle, is to be used for an obstacle of finite size. At finite temperature, our analyses of MD simulations allow the activation energy to be determined as a function of temperature. The results confirm the proportionality between activation energy and temperature that is frequently observed by experiment. By coupling the data for the activation energy and the critical stress as functions of temperature, we show how the activation energy can be deduced at a given value of the critical stress. 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: Ab initio electronic structure calculations are employed to study the stability and mobility of mono-self interstitial atoms (SIA) in $\ensuremath{\alpha}\text{-Fe}$ under external deformation. The ab initio results indicate that the volumetric and uniaxial strain dependences of the SIA formation energy are different in the expansion and compression regimes, in contrast to the linear behavior in continuum elasticity theory. We find a $⟨111⟩\ensuremath{\rightarrow}⟨100⟩$ SIA reorientation mechanism induced by uniaxial expansion which proceeds via $⟨11x⟩{\ensuremath{\mid}}_{x=2.7}$ configuration. Volumetric and uniaxial deformations are also found to have a considerable influence on the migration paths and activation energy barriers for the $⟨110⟩{110}\ensuremath{\leftrightarrow}⟨100⟩{100}$ transformation and the $⟨111⟩\ensuremath{\leftrightarrow}⟨100⟩$ reorientation. The results reveal that (i) the volumetric expansion (compression) decreases (increases) substantially the migration energy barrier and renders the diffusion process three (one) dimensional, (ii) the uniaxial strain removes (decreases) the migration energy barrier for the $⟨111⟩\ensuremath{\rightarrow}⟨11x⟩{\ensuremath{\mid}}_{x=2.7}(⟨11x⟩{\ensuremath{\mid}}_{x=2.7}\ensuremath{\rightarrow}⟨100⟩)$ transformation, leading to spontaneous reorientation of the $⟨111⟩$ SIA, and (iii) the uniaxial deformation breaks the cubic symmetry of the system and in turn induces anisotropy of the migration rates along different directions. These calculations demonstrate that changes in the electronic structure induced by global elastic deformation lead to additional contributions to the formation and migration energies, which cannot be adequately accounted for neither by elasticity theory nor by empirical interatomic potentials. read less

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

Abstract: Nano-scale defect clusters, such as voids, dislocation loops, stacking-fault tetrahedra and irradiation-induced precipitates, are produced in metals by irradiation with high-energy atomic particles. They are obstacles to dislocation glide and can give rise to substantial changes in the yield and flow stresses and ductility. Atomic-scale computer simulation is able to provide detail of how these effects are influenced by obstacle structure, applied stress, strain rate and temperature. Some recent results from modelling dislocations interacting with obstacles are described. Emphasis is placed on dislocation interaction with voids, copper precipitates and dislocation loops in the BCC metal iron and stacking fault tetrahedra in FCC copper. In the latter case, the importance of surfaces in reactions in TEM foils is highlighted. It is shown that while some atomic processes can be represented adequately by the continuum theory of crystal defects, others cannot. read less

Abstract: Existing interatomic potentials for the iron-carbon system suffer from qualitative flaws in describing even the simplest of defects. In contrast to more accurate first-principles calculations, all previous potentials show strong bonding of carbon to overcoordinated defects (e.g., self-interstitials, dislocation cores) and a failure to accurately reproduce the energetics of carbon-vacancy complexes. Thus any results from their application in molecular dynamics to more complex environments are unreliable. The problem arises from a fundamental error in potential design--the failure to describe short-ranged covalent bonding of the carbon p electrons. We describe a resolution to the problem and present an empirical potential based on insights from density-functional theory, showing covalent-type bonding for carbon. The potential correctly describes the interaction of carbon and iron across a wide range of defect environments. It has the embedded atom method form and hence appropriate for billion atom molecular-dynamics simulations. read less

Abstract: One-dimensional (1D) migration of small interstitial-type dislocation loops was studied for Fe specimens of different purities at room temperature under electron irradiation using a high-voltage electron microscope. Most 1D migration appeared as discrete jumps (stepwise positional changes) at irregular intervals, and sometimes involved back and forth motion between certain points. The distribution of jump distances extended to over 100 nm in high-purity specimens; it was less than 30 nm in low-purity specimens. Jump frequency was almost proportional to electron beam intensity and was on the same order as the rate of atomic displacement by electron irradiation. Molecular dynamics simulation suggested the suppression of 1D migration of an interstitial cluster (7i) by an oversized solute Cu atom located in the dilatational strain field of the cluster. We proposed that the 1D jump process occurs in the following sequence: (1) interstitial clusters are in a stationary state due to trapping effect by impurity atoms, (2) incident electrons hit and displace impurity atom to cause detrapping, (3) liberated clusters cause fast 1D migration at low activation energy, and (4) the cluster is trapped again by another impurity. Experimental results were analyzed and discussed in terms of the proposed model. read less

Abstract: The paper presents results of molecular dynamic (MD) simulations in 3D bcc iron crystals with edge pre-existing cracks (001)[110] and (110) [110] (crack plane/crack front) loaded uni-axially in tension mode I at temperature of 300 K. The iron crystals in MD have the same orientation and similar geometry as in our recent fracture tests performed at room temperature on iron (3wt.%Si) single crystals [1]. read less

Abstract: We have addressed two issues concerning the relative stabilities of various orienta- tions of interstitial clusters in iron by making a comprehensive comparison between four recent empirical potentials. First, we have investigated the effect of finite temperature on the com- petition between clusters made of a few dumbbells oriented along h111i or h110i. We show by quasi-harmonic calculations that h111i clusters have much larger vibrational formation en- tropies and that they are therefore stabilized with respect to h110i clusters at high temperature. Second, we have compared the formation energies of loops with several hundred atoms with Burgers vector 1 2 h111i or h100i. The 1 2 h111i loops are found to be always more stable, but the energy differences with h100i loops depend strongly on the potential. read less

Abstract: Molecular dynamics (MD) methods are utilized to study the displacement cascades in α-Fe containing different concentrations of substitutional He atoms. Primary knock-on atom (PKA) energies, Ep, from o.5 keV to 20 keV are considered at a temperature of 100 K and 600 K, and the results are compared with those performed in pure α-Fe. There are distinct differences in the number and size of defect clusters within displacement cascades with and without substitutional helium atoms. Particularly, the number and size of helium-vacancy clusters generally increase with increasing helium concentration and PKA energy. However, the number of He-vacancy (He-V) clusters increases with increasing temperature, the mean size of He-V clusters is independent on temperature for the same He concentration and energy recoils. 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: 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: By combining density functional theory, empirical potential, and atomic transport model approaches, we investigate the energetics and the diffusion properties of P interstitials in dilute Fe-P alloys. Although P is a substitutional impurity in {alpha}-iron, when a self-interstitial atom (SIA) approaches a substitutional P, the P atom becomes interstitial with an energy gain of up to 1.0 eV. The octahedral and the mixed dumbbell are the lowest-energy configurations with similar stabilities. The P atoms are highly mobile in both configurations. The transitions between these two configurations also require low activation energies. The most likely mechanisms leading to long-distance diffusion of a P interstitial are proposed by ab initio calculations. The resulting effective diffusion energy estimated by the transport model is 0.19 eV, which agrees with the result from resistivity recovery experiments, suggesting that the Fe-P mixed dumbbells are more mobile than the SIAs. The fast-migrating P interstitial can be deeply trapped by a substitutional P atom. The resulting complexes are very stable with a binding energy of around 1.0 eV. Their mobilities are investigated by means of the dimer method using an Fe-P empirical potential. A comparison between the present predictions and existing experimental results is also discussed. read less

Abstract: Using molecular-dynamics simulation, we determine the magnitude and anisotropy of the kinetic coefficient (mu) for the crystal growth from the melt for the hard-sphere system through an analysis of equilibrium capillary fluctuations in interfacial height. We find mu100 = 1.44(7), mu110 = 1.10(5), and mu111 = 0.64(3) in units of square root (kB/(mTm)), where kB is Boltzmann's constant, m is the particle mass, and Tm is the melting temperature. These values are shown to be consistent, with some exceptions, with those obtained in recent simulation results a variety of fcc metals, when expressed in hard-sphere units. This suggests that the kinetic coefficient for fcc metals can be roughly estimated from C square root (R/(MTm)), where R is the gas constant, M is the molar mass, and C is a constant that varies with interfacial orientation. 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: The iron-chromium alloy system has an unexplained anomaly: although there is a broad miscibility gap it appears to be favorable for chromium to dissolve in iron. This is consistent with ab initio calculation, but no simpler physically intuitive picture has been presented. Here it is shown that the Ising model, based on the bcc lattice with antiferromagnetic and ferromagnetic species, has the potential to exhibit similar behavior, with a skew miscibility gap arising from the solubility of antiferromagnetic species on nonadjacent sites. Essential characteristics of stainless steel (high Cr solubility and surface segregation) are correctly reproduced. Under some conditions, magnetization increases with temperature. The equilibrium miscibility gap due to mixed magnetism and segregation-driven positive dM/dT are fundamental features of the bcc Ising model itself, not just FeCr. read less

Abstract: Molecular dynamics (MD) methods are utilized to study the formation of vacancy clusters created by displacement cascades in α‐Fe containing different concentrations of substitutional He atoms. Primary knock-on atom energies, Ep, from 500eV to 20keV are considered at a temperature of 100K, and the results are compared with those performed in pure α‐Fe. There are distinct differences in the number and size of vacancy clusters within displacement cascades with and without substitutional helium atoms. It is found that many large vacancy clusters can be formed within cascade cores in α‐Fe with helium atoms, in contrast to a few small vacancy clusters observed in pure α‐Fe. The number and size of helium/vacancy clusters generally increase with increasing helium concentration and PKA energy. One of the striking results is that the number of self-interstitial atoms (SIAs) and the size of interstitial clusters are much smaller than those in pure α‐Fe. read less

Abstract: This work is devoted to simulation of the diffusion features of point defects in bcc metals. The properties of point defects have been investigated with the usage of many-body interatomic potentials. This approach, based on the density-functional theory, permitted us to derive more adequate diffusion features of solids. This investigation is carried out within the framework of the Finnis-Sinclair formalism, developed for an assembly of N atoms and represents the secondmoment approximation of the tight-binding theory. We used a new model, based on the molecular static method for simulating the atomic structure near the defect and vacancy migration in pure metals. This approach gives the opportunity to simulate the formation and the migration volumes of the point defects, taking into consideration the influence of pressure on structure and consequently on energy. The diffusion characteristics of bcc α-Fe and anomalous β-Zr have been investigated. read less

Abstract: This paper describes a computer simulation of thermal ageing process in Fe-Cu alloy. In order to perform accurate numerical simulation, firstly, we make numerical models of the diffusion and dissociation of Cu and Cu-vacancy clusters. This modeling was performed with kinetic lattice Monte Carlo method, which allows us to perform long-time simulation of vacancy diffusion in Fe-Cu dilute alloy. The model is input to the kinetic Monte Carlo method, and then, we performed the kinetic Monte Carlo simulation of the thermal ageing in the Fe-Cu alloy. The results of the KMC simulations tell us that the our new models describes well the rate and kinetics of the diffusion and dissociation of Cu and Cu-vacancy clusters, and works well in the kinetic Monte Carlo simulations. Finally, we discussed the further application of these numerical models. 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: Results of several parallel molecular dynamics crack simulations in bcc iron crystals with up to 128 million atoms are presented. The crack (001)[010] of Griffith type is loaded in Mode I. We observe dislocation emission and twinning near the free sample surfaces and later plastically induced crack initiation. read less

Abstract: When liquids solidify, the interface between a crystal and its melt often forms branching structures (dendrites), just as frost spreads across a window.The development of a quantitative understanding of dendritic evolution continues to present a major theoretical and experimental challenge within the metallurgical community. This article looks at key parameters that describe the interface—excess free energy and mobility—and discusses how these important properties relate to our understanding of crystal growth and other interfacial phenomena such as wetting and spreading of droplets and nucleation of the solid phase from the melt. In particular, two new simulation methods have emerged for computing the interfacial free energy and its anisotropy: the cleaving technique and the capillary fluctuation method. These are presented, along with methods for extracting the kinetic coefficient and a comparison of the results to several theories of crystal growth rates. read less

Abstract: For the immiscible Cu-Ta system, a Finnis-Sinclair potential is constructed and proven to be realistic in reproducing some static properties of the system. Applying the potential, molecular dynamics simulations reveal that among the nine $\mathrm{Cu}∕\mathrm{Ta}$ interfaces stacked by possible combinations of the (100), (110), and (111) atomic planes, the Ta (110) plane could remain stable up to a temperature of $600\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, while the Cu (111) plane could remain unchanged only if combined with the Ta (100) and (110) planes. Simulations also show that for the other $\mathrm{Cu}∕\mathrm{Ta}$ interfaces, the interface energy serves as the driving force for interdiffusion of the Cu and Ta atoms across the interface, resulting in solid-state amorphization. Interestingly, it is calculated that the amorphization energy of Cu is smaller than that of Ta, thus resulting in an asymmetric growth behavior of the amorphous interlayer, i.e., amorphization of the Cu lattice is easier and faster than that of the Ta lattice. In general, the agreement between the simulation results and experimental observations is fairly good. read less

Abstract: We present the computational approach for studying the microstructures of Cu clusters in Fe–Cu alloys by combining the molecular dynamics (MD) simulation and Monte Carlo methods. The MD simulation is used to characterize the primary damage resulting from the displacement cascade in Fe. Then, using the Metropolis Monte Carlo methods, the microstructure of the Cu clusters is predicted under the assumption that the system will evolve towards the equilibrium state. The formation of the Cu clusters is apparent for Fe–Cu alloys of a higher Cu content (1.0 w/o), whereas the degree of Cu clustering is not significant for the lower Cu content (0.1 w/o) alloys. The atomic configuration of the Cu–vacancy complex under irradiation, produced by this simulation, is in a fair agreement with the experiments. The simulation is expected to provide important information on the Cu-cluster morphology, which is useful for experimental data analysis. 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: We report a detailed ab initio study of the stability and migration of self-interstitial atoms (SIAs) and di-interstitials (di-SIAs) in alpha-Fe. The <110> dumbbell is confirmed to be the most stable SIA configuration, 0.7 eV below the <111> dumbbell. The lowest-energy migration path corresponds to a nearest-neighbor translation-rotation jump with a barrier of 0.34 eV. The most stable configuration for di-SIAs consists of <110> parallel dumbbells. Their migration mechanism is similar to that for SIAs, with an activation energy of 0.42 eV. These results are at variance with predictions from existing empirical potentials and allow one to reconcile theory with experiments. read less

Abstract: The behavior of carbon and nitrogen atoms in iron based solid solution is studied by ab initio densityfunctional-theory calculations. The interaction o faCo r aN atom in a-Fe with a vacancy, other C or N interstitials as well as self-interstitial atoms is discussed and compared to known experimental results. The migration of these two foreign interstitial atoms is determined in pure Fe or when a vacancy is present in the supercell. According to our results, there is a strong binding energy of C or N with vacancies, whereas a repulsion is observed with self-interstitial atoms. Furthermore, a vacancy can trap up to two C, and a covalent bonding forms between the two C atoms. The situation is not as clear for N atoms, and a competition between the formation of N-V pairs and NN-V triplets is very probable. read less

Abstract: This paper describes dislocation dynamics simulation of grain boundary effects on yield behavior of metals, such as α-Fe bcc metal. Since the stress field arising from the grain boundary has not been well understood yet, the geometrical effect of the grain boundary can be handled in the simulation by the use of rigid boundary condition. The dislocation pileups can be observed near the grain boundary in the result of the DD simulation. And the yield stress in the crystal having the grain boundary becomes larger than that in the crystal having free surface. This result tells us that the Hall-Petch effect can actually describe well the effects of the grain boundary on the yield behavior of metals. read less

Abstract: Molecular dynamics simulations are used to calculate the Gibbs free energy in the entire compositional range of Fe-Cu alloys described with a set of embedded atom potentials available in the literature. Thermodynamic integration and switching Hamiltonian techniques are used to obtain the phase diagram at high temperatures (neglecting phonon quantum effects and electronic contributions) with no further approximations. Limitations of the model were confirmed, such as the absence of the y and δ phases, a bcc to fcc transformation before melting for pure Fe, the unexpected existence of a stable bcc phase in pure Cu at high T, and consequently complete solid solubility of Fe in Cu in the bcc phase in some temperature range. This work seeds light on the power and limitations of the empirical description of complex systems. read less

Abstract: The recent progress on our multiscale modeling to understand radiation damage processes in materials during irradiation is reviewed. The energies of He-V cluster formation in Fe were evaluated using a molecular dynamics (MD) simulation technique that employed interatomic potentials partially developed by first-principle (FP) calculations. Using the calculated energies, the longer timescale behavior of He-V clusters in Fe was investigated using a kinetic Monte-Carlo (KMC) simulation technique. The FP-MD-KMC scheme provided us significant information on the thermal stability of a He-V cluster in Fe as a function of the helium-to-vacancy ratio of the cluster. read less

Abstract: The ability to predict the behavior of point defects in metals, particularly interstitial defects, is central to accurate modeling of the microstructural evolution in environments with high radiation fluxes. Existing interatomic potentials of embedded atom method type predict disparate stable interstitial defect configurations in vanadium. This is not surprising since accurate first-principles interstitial data were not available when these potentials were fitted. In order to provide the input information required to fit a vanadium potential appropriate for radiation damage studies, we perform a series of first-principles calculations on six different interstitial geometries and vacancies. These calculations identify the 〈111〉 dumbbell as the most stable interstitial with a formation energy of approximately 3.1 eV, at variance with predictions based upon existing potentials. Our potential is of Finnis–Sinclair type and is fitted exactly to the experimental equilibrium lattice parameter, cohesive energy, e... read less

Abstract: The primary damage formation in Mo is investigated using molecular dynamics (MD) simulation with embedded-defect (ED) and embedded-atom method (EAM) interatomic potentials. The former is similar in spirit to the latter but includes an approximate treatment of bond directionality in the many-body interaction. MD simulations are used to calculate threshold displacement energy as function of crystallographic orientation and displacement cascade evolution resulting from primary knock-on atoms (PKAs) with energies ranging from 0.5 up to 50 keV. The defect structures produced with increasing PKA energy are analysed and the results obtained are compared for the two potentials and with simulations from the literature for other bcc materials. The impact of temperature and inelastic losses on the cascade characteristics are also discussed. The ED approach is in better agreement with the experimental findings on threshold energy. It also predicts larger vacancy clusters as well as a larger fraction of clustered vacancies than the EAM does; this seems to be more consistent with experiment, however, on statistical grounds a definite assessment is not possible with the number of simulations performed. Inelastic losses coupled with the thermal spike affect the defect production in subtle ways, especially at the higher energies considered here. This does not seem to have been realized before and deserves to be studied more comprehensively. read less

Abstract: In this review article, first a brief summary is presented concerning the formation of amorphous alloys (or metallic glasses) in binary metal systems by solid-state reaction of metallic multilayers. Secondly, under the framework of Miedema's model, thermodynamic modelling of crystal-to-amorphous transition is developed with special consideration of the excess interfacial free energy in metallic multilayers. Thirdly, the results of molecular dynamics simulations in some representative systems are presented, revealing the detailed kinetics of the crystal-to-amorphous transition on the atomic scale, such as the temperature/time dependence of interfacial reactions, the asymmetric growth of amorphous interlayers, and the nucleation and/or presence of growth barriers resulting from the interfacial texture. Fourthly, the critical solid solubilities of some representative systems are directly determined from the inter-atomic potentials through molecular dynamics simulations and then correlated with the metallic-glass-forming ability of the systems as well as their asymmetric growth during solid-state amorphization observed in experiments and/or simulations. read less

Abstract: Static and molecular dynamics simulations have been used with different types of interatomic potentials to investigate the structure, properties and stability of self-interstitial atom (SIA) clusters produced during irradiation. In α-iron (Fe), faulted clusters of <110> dumbbells are unstable for all the potentials. The most stable SIA clusters are sets of parallel <111> crowdions. Large clusters of this type form perfect dislocation loops with Burgers vector b = ½⟨111⟩. Small clusters (less than 9 SIAs) of ⟨100⟩ crowdions are stable at 0K, but transform into a set of ⟨111⟩ crowdions on annealing. Larger ⟨100⟩ clusters are stable and form perfect dislocation loops with b = ⟨100⟩. Both types of loops are glissile. In copper (Cu), clusters of parallel ⟨100⟩ dumbbells and ⟨110⟩ crowdions are stable. Large clusters of these types form faulted and perfect dislocation loops with b = ⅓ ⟨111⟩ and ½ ⟨110⟩ respectively. Small faulted clusters (less than 7 SIAs) of irregular shape can transform into a set of ⟨110⟩ crowdions during annealing. Larger faulted clusters are stable as hexagonal ⅓ ⟨111⟩ Frank loops at temperatures of about up to 1050K for a period of several hundred picoseconds. All faulted clusters are sessile. Clusters of ⟨110⟩ crowdions and ½ ⟨110⟩ perfect loops are glissile and stable at all temperatures. When large enough (more than 49–64 SIAs) they can dissociate on their glide prism. Symmetric three-dimensional clusters of ⟨100⟩ dumbbells are stable at 0K but during annealing they transform into sets of ⟨110⟩ crowdions. The results for both iron and copper are discussed and compared with experimental data and provide a basis for investigating and explaining the observed differences in radiation damage accumulation behaviour between fcc and bcc metals. read less

Abstract: We present large scale atomistic simulations of crack growth in iron under quasistatic loading in mode I. We show that long cracks display a brittle character of extension, while the growth of smaller cracks is accompanied by emission of partial dislocations from the crack tip and subsequent transformation of the stacking faults behind the dislocations to multilayer twins. The competing shear processes at a crack tip are characterized in terms of the relative sliding of up to four adjacent atomic planes emanating from the crack tip region. The results are in agreement with a global energy balance derived from perfect samples, and with experimental observations that twinning and fracture are cooperating processes under sufficiently large quasistatic loading at low temperatures. read less

Abstract: Radiation-induced microstructural and compositional changes in solids are governed by the interaction between the fraction of defects that escape their nascent cascade and the material. We use a combination of molecular dynamics (MD) and kinetic Monte Carlo (KMC) simulations to calculate the damage production efficiency and the fraction of freely migrating defects in α-Fe at 600 K. MD simulations provide information on the nature of the primary damage state as a function of recoil energy, and on the kinetics and energetics of point defects and small defect clusters. The KMC simulations use as input the MD results and provide a description of defect diffusion and interaction over long time and length scales. For the MD simulations, we employ the analytical embedded-atom potential developed by Johnson and Oh for α-Fe, including a modification of the short-range repulsive interaction. We use MD to calculate the diffusivities of point defects and small defect clusters and the binding energy of small ... read less

Abstract: The authors present results of atomistic simulations of the interaction between self interstitial atoms and vacancies with edge dislocations in BCC iron. The calculations are carried out using molecular dynamics with an energy minimization scheme based on the quasi-Newton approach and use the Finnish-Sinclair interatomic potential for BCC iron developed by Ackland et al. large anisotropy in the strain field of self interstitials is observed and it causes strong interaction with edge dislocations even when the defect is located on the dislocation glide plane. For vacancies, the relaxation volume is smaller and much more isotropic, which results in a far weaker interaction with the dislocation. A temperature dependent capture radius for vacancies and self interstitials is extracted from the simulations. The difference between the capture radii of vacancies and self interstitials is used to define the sink strength of the dislocation. Large deviations are observed from the predictions of elasticity based on treating point defects as isotropic dilatational centers. Further, the capture radius of edge dislocations in BCC iron is observed to be small and is of the order of 1--3 nm for self interstitials. read less

Abstract: The kinetics of the recrystallization and austenite-ferrite (fcc-bcc) phase transformation in steels are markedly affected by substitutional alloying elements. Nevertheless, the detailed mechanisms of their interaction with the grain boundaries and interfaces are not fully understood. Using density functional theory, we determine the segregation energies of commonly used alloying elements (e.g. Nb, Mo, Mn, Si, Cr, Ni) in the Σ5 (013) tilt grain boundary in bcc and fcc Fe, and the bcc-fcc interfaces. We find a strong interaction between large solutes (e.g. Nb, Mo and Ti) and grain boundaries or interfaces 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 boundaries and interfaces, which suggest co-segregation for these large solutes at intermediate distances in striking contrast to the bulk. Besides the prediction of solute segregation, the selfand solute-diffusion in Febased system are also investigated within a framework combining density functional theory calculations and kinetic Monte Carlo simulations. Good agreement between our calculations and the measurements for selfand solute diffusion in bulk Fe is achieved. For the first time, the effective activation energies and diffusion coefficients for various solutes in the α-Fe Σ5 (013) grain boundary are determined. The results demonstrate that grain boundary diffusion is significantly faster than for lattice diffusion, confirming grain boundaries are fast diffusion paths. By contrast, the effective activation energy of self-diffusion in a bcc-fcc Fe interface is close to the value of fcc bulk self-diffusion, and the investigated bcc-fcc interface provides a moderate “fast diffusion” path. read less

Abstract: Abstract: Molecular dynamics simulation constitutes an appealing method to study, on an atomistic basis, the processes and mechanisms of martensitic phase transformations. Its use requires the existence of reliable interatomic potentials which adequately describe the properties of the phases. In this review we present a few recent examples demonstrating the application of this method to the study of the martensitic phase transition in iron. Besides phase changes in bulk materials, transformations in small systems (nanowires) are also considered. read less

Abstract: The size- and spacing- dependent obstacle strength due to the Cu precipitation in α-Fe is investigated by atomistic simulations, in which the effect on phase transformation of Cu precipitation is considered by a conventional self-guided molecular dynamics (SGMD) method that has an advantage to enhance the conformational sampling efficiency in MD simulations. A sequence of molecular statics simulations of the interaction between a pure edge dislocation and spherical Cu precipitation are performed to investigate the obstacle strength associated with phase transformation. It was shown that the SGMD method can accelerate calculating the bcc to 9R structure transformation of a small precipitate, enabling the transformation without introducing any excess vacancies. Such metallographic structures increase the obstacle strength through strong pinning effects as a result of the complicated atomic rearrangement within the Cu precipitation. read less

Abstract: Cu precipitation hardening behavior and mechanical properties were investigated in Cu added martensitic ultra-high strength steels. In this work, hardness measurement, TEM observation, lattice parameter measurement and tensile testing were conducted for 0.19%C–1.5%Mn steels with addition of up to 4% Cu those were water-quenched followed by aging at 250 through 550°C for 20 s through 360 min. Cu added steels exhibited higher hardness than a Cu free steel in each aging condition. Precipitated Cu content estimated by lattice parameter well corresponded to hardness increment in 4% Cu steels compared with Cu free steels aged at the same aging condition. Regarding the tensile properties of the aged Cu free and 4% Cu steels, 4% Cu steel exhibited superior balance of tensile strength and elongation with tensile strength level of 1300 MPa. Lower activation energy estimated by peak hardness increment than that of Cu diffusion in bcc-Fe matrix suggested that high dislocation density introduced by martensitic transformation accelerated the growth of Cu precipitates. And smaller Cu precipitation hardening in martensite matrix compared with that in ferrite matrix was also discussed taking into consideration nonlinear summation of dislocation hardening and precipitation hardening. read less

Abstract: The hydrogen trapping behavior at Cu precipitates was investigated in martensitic Fe-0.18%C-1.5%Mn-4.0%Cu steel having various precipitation conditions. A hydrogen desorption peak was observed around 373 K in all samples. Total amount of desorped hydrogen decreased first with aging period but increased later. The initial decrease has been attributed to the recovery of lattice defects, and the increase in the later stage to trapping at Cu precipitates. The amount of desorped hydrogen increased even after peak hardening. The crystal structure of Cu precipitates was observed by SR-XAFS to be basically bcc; however, indication of fcc structure was also observed. The intensity of fcc structure increases with aging time. This suggests that incoherency of matrix-precipitate interface increases with aging time especially after peak strength, and that hydrogen trapping is more pronounced at incoherent interface than coherent one at Cu precipitates. Since misfit strain of coherent Cu precipitate is smaller than other precipitates such as TiC, the ability of hydrogen trapping at coherent Cu precipitates is estimated to be small. read less

Abstract: Electronic structure calculations, especially those using density-functional theory have provided many insights into various materials properties in the recent decade. However, the computational complexity associated with electronic structure calculations has restricted these investigations to periodic geometries with small cell-sizes (computational domains) consisting of few atoms (about 200 atoms). But material properties are influenced by defects---vacancies, dopants, dislocations, cracks, free surfaces---in small concentrations (parts per million). A complete description of such defects must include both the electronic structure of the core at the fine (sub-nanometer) scale and also elastic and electrostatic interactions at the coarse (micrometer and beyond) scale. This in turn requires electronic structure calculations at macroscopic scales, involving millions of atoms, well beyond the current capability. This thesis presents the development of a seamless multi-scale scheme, Quasi-Continuum Orbital-Free Density-Functional Theory (QC-OFDFT) to address this significant issue. This multi-scale scheme has enabled for the first time a calculation of the electronic structure of multi-million atom systems using orbital-free density-functional theory, thus, paving the way to an accurate electronic structure study of defects in materials. The key ideas in the development of QC-OFDFT are (i) a real-space variational formulation of orbital-free density-functional theory, (ii) a nested finite-element discretization of the formulation, and (iii) a systematic means of adaptive coarse-graining retaining full resolution where necessary, and coarsening elsewhere with no patches, assumptions, or structure. The real-space formulation and the finite-element discretization gives freedom from periodicity, which is important in the study of defects in materials. More importantly, the real-space formulation and its finite-element discretization support unstructured coarse-graining of the basis functions, which is exploited to advantage in developing the QC-OFDFT method. This method has enabled for the first time a calculation of the electronic structure of samples with millions of atoms subjected to arbitrary boundary conditions. Importantly, the method is completely seamless, does not require any ad hoc assumptions, uses orbital-free density-functional theory as its only input, and enables convergence studies of its accuracy. From the viewpoint of mathematical analysis, the convergence of the finite-element approximation is established rigorously using Gamma-convergence, thus adding strength and validity to the formulation. The accuracy of the proposed multi-scale method under modest computational cost, and the physical insights it offers into properties of materials with defects, have been demonstrated by the study of vacancies in aluminum. One of the important results of this study is the strong cell-size effect observed on the formation energies of vacancies, where cells as large as tens of thousands of atoms were required to obtain convergence. This indicates the prevalence of long-range physics in materials with defects, and the need to calculate the electronic structure of materials at macroscopic scales, thus underscoring the importance of QC-OFDFT. Finally, QC-OFDFT was used to study a problem of great practical importance: the embrittlement of metals subjected to radiation. The brittle nature of metals exposed to radiation is associated with the formation of prismatic dislocation loops---dislocation loops whose Burgers vector has a component normal to their plane. QC-OFDFT provides an insight into the mechanism of prismatic dislocation loop nucleation, which has remained unclear to date. This study, for the first time using electronic structure calculations, establishes vacancy clustering as an energetically favorable process. Also, from direct numerical simulations, it is demonstrated that vacancy clusters collapse to form stable prismatic dislocation loops. This establishes vacancy clustering and collapse of these clusters as a possible mechanism for prismatic dislocation loop nucleation. The study also suggests that prismatic loops as small as those formed from a 7-vacancy cluster are stable, thus shedding new light on the nucleation size of these defects which was hitherto unknown. read less

Abstract: It is well known that nuclear pressure vessel steel shows embrittlement under thermal aging and strong neutron irradiation. We focus on nanoscale copper-rich precipitates and try to clarify the effect of the nanoscale copper-rich precipitates on embrittlement of reactor pressure vessel steels. Our final goal is to evaluate such embrittlement from microscopic viewpoint based on atomistic simulation. In this study, we simulate interaction between motion of an edge dislocation and copper clusters using Molecular Dynamics method with a PBC, where uniform shear strain is applied to the boundaries parallel to the slip plane (1 1 2) in the system. As the results, we clarify the effects of size, distance and pinning of the copper clusters on dislocation motion. read less

Abstract: Anomalous structural evolution was induced in the equilibrium immiscible Co-Cu and Fe-Cu systems by 100 or 200 keV xenon ion irradiation at 77 K or room temperature. In the Co 15 Cu 85 and Fe 70 Cu 30 multilayered films, nanosized quasicrystals were formed in an amorphous matrix, through a two-step transition of crystal-to-amorphous-to-quasicrystal. The obtained quasicrystals are Co-Cu dodecagonal and Fe-Cu icosahedral phases with twelve-fold and five-fold rotational symmetries, respectively. The real compositions of the amorphous matrix were determined to be close to Co 10 Cu 90 and Fe 70 Cu 30 , while those for quasicrystals are nearly Co 20 Cu 80 and Fe 50 Cu 50 , respectively. Moreover, the same dodecagonal and icosahedral phases were also obtained in the specifically designed Co 50 Cu 50 and Fe 50 Cu 50 multilayered samples upon thermal annealing at 500°C and 850°C, respectively, confirming the existence of these new metastable states in the respective systems. Besides, our molecular dynamics study showed that either Co or Fe could be mixed with Cu at an atomic scale in forming some metastable alloy phases. The amorphous-to-quasicrystal transition was discussed in terms of the similarity in the atomic configuration between the quasicrystal and amorphous short-range orders. read less

Abstract: The steel vessels of pressurized water reactors are embrittled by neutron irradiation. It is well known that copper atoms play an important role in the embrittlement and that different Cu-containing defects such as Cu-rich clusters (sometimes called atmospheres), Cu precipitates and Cu-vacancy complexes have been identified experimentally. It is still difficult to link the formation of these defects to the primary damage resulting from the neutron inducing displacement cascades. Therefore, the authors investigate the evolution of the primary damage in FeCu alloys using kinetic Monte Carlo simulations based on a vacancy diffusion mechanism. The calculations rely on adapted, phenomenological, n-body potentials that satisfactorily reproduce properties of FeCu. At room temperature, experimentally identified defects such as Cu-vacancy complexes (one Cu atom bound to three of four vacancies) form in the courses of the simulations. Furthermore, it appears that complex defects such as a Cu atom linked to two vacancies are very mobile and are responsible for the formation of small Cu clusters. read less

Abstract: The mobile defects created in displacement cascades can either interact within the cascade region or undergo long-range diffusion in the crystal. The kinetic Monte Carlo code ALSOME has been used in the present work to carry out annealing simulations of electron irradiation and single cascades with energy in the range of 2 to 40 keV in α-Fe as a function of temperature. Isochronal annealing of electron irradiation shows a temperature-dependence of the recovery stages that is reasonably close to experiment, but Stage I is controlled by the rotation energy of the dumbbell to the crowdion. The annealing of single cascades has demonstrated that nearly 60% of SIAs formed in the primary state of cascade damage escape from the cascade at temperatures above stage I. Most of the escaping SlAs are in clusters, for only 10% of them are mono-interstitials. Although the number of escaping defects increases with increasing cascade energy, the relative fraction is almost constant for the recoil energies considered. The results are compared with those for copper obtained using the same code [1]. read less

Abstract: A new empirical interatomic potential of the embedded atom type is developed for the Fe-Cu system. The potential for the alloy system was constructed to reproduce known physical parameters of the alloy, such as the heat of solution of Cu in Fe and the binding energy of a vacancy and a Cu atom in the matrix. The potential also reproduces first-principle calculations of the properties of metastable phases in the system. This atomic interaction model was used in simulation studies of the interface of small coherent Cu precipitates in and of dislocation core structure. The phase stability of the body-centred cubic Cu precipitates was also analysed. read less