Abstract: Materials with a large magnetocaloric response are highly desirable for magnetic cooling applications. It is suggested that a strong spin-lattice coupling tends to generate a large magnetocaloric effect, but no microscopic mechanism has been proposed. In this Letter, we use spin-lattice dynamics simulation to examine the lattice contribution to the magnetocaloric entropy change in bcc iron (Fe) and hcp gadolinium (Gd) with exchange interaction parameters determined from ab initio calculations. We find that indirect Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange interaction in hcp Gd leads to longer-range spin-lattice coupling and more strongly influences the low-frequency long-wavelength phonons. This results in a higher lattice contribution toward the total magnetocaloric entropy change as compared to bcc Fe with short-range direct exchange interactions. Our analysis provides a framework for understanding the magnetocaloric effect in magnetic materials with strong spin-lattice couplings. Our finding suggests that long-range indirect RKKY-type exchange gives rise to a larger lattice contribution to the magnetocaloric entropy change and is, thus, beneficial for magnetocaloric materials. read less

Abstract: Alexis Front, ∗ Georg Daniel Förster, 2, † Van-Truong Tran, Chu-Chun Fu, Cyrille Barreteau, François Ducastelle, and Hakim Amara 5, ‡ Laboratoire d’Etude des Microstructures, ONERA-CNRS, UMR104, Université Paris-Saclay, BP 72, Châtillon Cedex, 92322, France Interfaces, Confinement, Matériaux et Nanostructures (ICMN), CNRS, Université d’Orléans, Orléans, France Université Paris-Saclay, CEA, Service de Recherches de Métallurgie Physique, 91191 Gif-sur-Yvette, France DRF-Service de Physique de l’Etat Condensé, CEA-CNRS, Université Paris-Saclay, F-91191 Gif-sur-Yvette, France Université de Paris, Laboratoire Matériaux et Phénomènes Quantiques (MPQ), CNRS-UMR7162, 75013 Paris, France read less

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Abstract: In the present work, atomistic modeling based on the quasiparticle approach (QA) was performed to establish general trends in the segregation of solutes with different atomic size at symmetric 〈100〉 tilt grain boundaries (GBs) in α-Fe. Three types of solute atoms X1, X2 and X3 were considered, with atomic radii smaller (X1), similar (X2) and larger (X3) than iron atoms, respectively, corresponding to phosphorus (P), antimony (Sb) and tin (Sn). With this, we were able to evidence that segregation is dominated by atomic size and local hydrostatic stress. For low angle GBs, where the elastic field is produced by dislocation walls, X1 atoms segregate preferentially at the limit between compressed and dilated areas. Contrariwise, the positions of X2 atoms at GBs reflect the presence of tensile and compressive areal regions, corresponding to extremum values of the σXX and σYY components of the strain tensor. Regarding high angle GBs Σ5 (310) (θ = 36.95°) and Σ29 (730), it was found that all three types of solute atoms form Fe9X clusters within B structural units (SUs), albeit being deformed in the case of larger atoms (X2 and X3). In the specific case of Σ29 (730) where the GB structure can be described by a sequence of |BC.BC| SUs, it was also envisioned that the C SU can absorb up to four X1 atoms vs. one X2 or X3 atom only. Moreover, a depleted zone was observed in the vicinity of high angle GBs for X2 or X3 atoms. The significance of this research is the development of a QA methodology capable of ascertaining the atomic position of solute atoms for a wide range of GBs, as a mean to highlight the impact of the solute atoms’ size on their locations at and near GBs. read less

Abstract: A unified model of molecular and atomistic spin dynamics is presented enabling simulations both in microcanonical and canonical ensembles without the necessity of additional phenomenological spin damping. Transfer of energy and angular momentum between the lattice and the spin systems is achieved by a phenomenological coupling term representing the spin-orbit interaction. The characteristic spectra of the spin and phonon systems are analyzed for different coupling strength and temperatures. The spin spectral density shows magnon modes together with the uncorrelated noise induced by the coupling to the lattice. The effective damping parameter is investigated showing an increase with both coupling strength and temperature. The model paves the way to understanding magnetic relaxation processes beyond the phenomenological approach of the Gilbert damping and the dynamics of the energy transfer between lattice and spins. read less

Abstract: The atomic cluster expansion (Drautz, Phys. Rev. B 99, 014104 (2019)) is extended in two ways, the modelling of vectorial and tensorial atomic properties and the inclusion of atomic degrees of freedom in addition to the positions of the atoms. In particular, atomic species, magnetic moments and charges are attached to the atomic positions and an atomic cluster expansion that includes the different degrees of freedom on equal footing is derived. Expressions for the efficient evaluation of forces and torques are given. Relations to other methods are discussed. read less

Abstract: An Fe-W potential has been constructed by means of the embedded-atom method and has proven to be more realistic than the three published Fe-W potentials in the literature. Based on the constructed Fe-W potential, molecular dynamic simulation has been used to reveal structural stability, thermodynamic properties, and mechanical properties of BCC Fe-W solid solutions within the entire composition range. It is found that the Fe-W interaction in BCC Fe-W solid solutions should be weak and attractive with small and negative heats of formation, which agree well with those from the thermodynamic Miedema model and could clarify the controversy regarding heats of formation of Fe-W solid solutions in the literature. In addition, the derived coefficient of thermal expansion, heat capacity, elastic constants, and elastic moduli of Fe-W solid solutions from the present Fe-W potential are in good agreement with the corresponding data from ab initio calculation or experiments in the literature.An Fe-W potential has been constructed by means of the embedded-atom method and has proven to be more realistic than the three published Fe-W potentials in the literature. Based on the constructed Fe-W potential, molecular dynamic simulation has been used to reveal structural stability, thermodynamic properties, and mechanical properties of BCC Fe-W solid solutions within the entire composition range. It is found that the Fe-W interaction in BCC Fe-W solid solutions should be weak and attractive with small and negative heats of formation, which agree well with those from the thermodynamic Miedema model and could clarify the controversy regarding heats of formation of Fe-W solid solutions in the literature. In addition, the derived coefficient of thermal expansion, heat capacity, elastic constants, and elastic moduli of Fe-W solid solutions from the present Fe-W potential are in good agreement with the corresponding data from ab initio calculation or experiments in the literature. read less

Abstract: We introduce a number of extensions and enhancements to a genetic algorithm for crystal structure prediction, to make it suitable to study magnetic systems. The coupling between magnetic properties and crystal structure means that it is essential to take a holistic approach, and we present for the first time, a genetic algorithm that performs a simultaneous global optimisation of both magnetic structure and crystal structure. We first illustrate the power of this approach on a novel test system—the magnetic Lennard–Jones potential—which we define. Then we study the complex interface structures found at the junction of a Heusler alloy and a semiconductor substrate as found in a proposed spintronic device and show the impact of the magnetic interface structure on the device performance. read less

Abstract: It has been found that under certain conditions, hydrogen retention would be strongly enhanced in irradiated austenitic stainless steels. To investigate the effect of the retained hydrogen on the defect microstructure, AL-6XN stainless steel specimens were irradiated with low energy (100 keV) H2+ so that high concentration of hydrogen was injected into the specimens while considerable displacement damage dose (up to 7 dpa) was also achieved. Irradiation induced dislocation loops and voids were characterised by transmission electron microscopy. For specimens irradiated to 7 dpa at 290 °C, dislocation loops with high number density were found and the void swelling was observed. At 380 °C, most of dislocation loops were unfaulted and tangled at 7 dpa, and the void swellings were observed at 5 dpa and above. Combining the data from low dose in previous work to high dose, four stages of dislocation loops evolution with hydrogen retention were suggested. Finally, molecular dynamics simulation was made to elucidate the division of large dislocation loops under irradiation. read less

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

Abstract: Core–shell nanoparticles made from iron embedded in gold have a strong potential interest in nanomedicine, the Au shell providing an efficient biocompatible coating for the magnetic Fe core. With the aim of determining theoretically the equilibrium morphologies of Fe–Au nanoparticles in a broad size range and with different compositions, a semiempirical many-body Fe–Au potential was designed combining well-established models for the pure metals and introducing dedicated contributions for the interactions involving mixed elements. The potential was parametrized against various energetic properties involving impurities, intermetallics, and finite clusters obtained using density functional calculations in the generalized gradient approximation. The potential was tested to investigate Fe–Au nanoparticles near equiconcentration containing about 1000–2000 atoms at finite temperature using parallel tempering Monte Carlo simulations initiated from the core–shell structure. The core–shell nanoparticles are found t... read less

Abstract: The elastic constants of tetragonally distorted - crystallites are calculated for several available interatomic interaction potentials. Besides embedded-atom-method-type potentials also a simple pair potential, modified embedded-atom-method and bond-order potentials are investigated. Care is taken to minimise the crystal structure properly in the presence of the C interstitials; we verify that the influence of statistics, i.e. the randomness of the C positions in the lattice, affects the elastic properties only little, as long as C is not allowed to cluster. We find that both sign and order of magnitude of the tetragonal elastic constants vary strongly between the predictions of the available potentials. Recent experimental data are available for the orientation-averaged elastic moduli; in contrast to the tetragonal constants, they feature only a mild dependence on C content. The experimental data are well reproduced by several of the potentials studied here. Existing deviations between experiment and predictions are discussed. read less

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

Abstract: Atomistic calculations are performed to investigate plastic slip in the <1 1 1>{3 2 1} system in body-centred cubic iron. Several modern interatomic potentials, developed over the last decade, are applied to compute the stacking fault γ-line energy in the {3 2 1} plane and the results are compared with the ab initio prediction. The applied potentials have shown strong deviations, but several potentials acquired good qualitative agreement with the ab initio data. Depending on the applied potential, the lowest value of the Peierls stress for the edge dislocation (ED) is 50 MPa (Ackland and Bacon from 1997) and the highest is 550 MPa (Dudarev and Derlet from 2005), while for the screw dislocation it is much higher, in the range 1–2 GPa. At finite temperature, however, the flow stress of the ED is found to decrease exponentially reaching a negligible value at about 200 K, irrespective of the applied potential. On the basis of the data obtained using Ackland–Mendelev potential from 2004, we conclude that the slip resistance of the <1 1 1>{3 2 1} system is in between the resistance of the <1 1 1>{1 1 0} and <1 1 1>{1 1 2} slip systems. read less

Abstract: The interaction of discrete breathers with the primary lattice defects in transition metals such as vacancy, dislocation, and surface is analyzed on the example of bcc iron employing atomistic simulations. Scattering of discrete breathers on the lattice defects induces localized atomic excitations, with intensity and relaxation time depending on the defect structure and breather kinetic energy. The dissipation of the intrinsic breather energy due to the scattering is computed and analyzed. It is concluded that the breather-to-defect energy transfer may stipulate the activation of the lattice defects causing unexpected athermal effects such as enhanced mass transfer or electroplasticity, already experimentally reported but so far not fully understood at the atomic-scale level. read less

Abstract: The theory of analytic Bond-Order Potentials as applied to non-collinear magnetic structures of transition metals is extended to take into account explicit rotations of Hamiltonian and local moment matrix elements between locally and globally defined spin-coordinate systems. Expressions for the gradients of the energy with respect to the Hamiltonian matrix elements, the interatomic forces and the magnetic torques are derived. The method is applied to simulations of the rotation of magnetic moments in α iron, as well as α and β manganese, based on d-valent orthogonal tight-binding parametrizations of the electronic structure. A new weighted-average terminator is introduced to improve the convergence of the Bond-Order Potential energies and torques with respect to tight-binding reference values, although the general behavior is qualitatively correct for low-moment expansions. read less

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

Abstract: The interaction of fusion reactor plasma with the material of the first wall involves a complex multitude of interlinked physical and chemical effects. Hence, modern theoretical treatment of it relies to a large extent on multiscale modelling, i.e. using different kinds of simulation approaches suitable for different length and time scales in connection with each other. In this review article, we overview briefly the physics and chemistry of plasma–wall interactions in tokamak-like fusion reactors, and present some of the most commonly used material simulation approaches relevant for the topic. We also give summaries of recent multiscale modelling studies of the effects of fusion plasma on the modification of the materials of the first wall, especially on swift chemical sputtering, mixed material formation and hydrogen isotope retention in tungsten. read less

Abstract: Atomistic simulations play a crucial role in advancing our understanding of the crack-tip processes that take place during fracture of semi-brittle materials like α-iron. As with all atomistic simulations, the results of such simulations however depend critically on the underlying atomic interaction model. Here, we present a systematic study of eight α-iron embedded atom method potentials used to model cracks subjected to plane strain mode-I loading conditions in six different crystal orientations. Molecular statics simulations are used to determine the fracture behavior (cleavage, dislocation emission, twinning) and the critical stress intensity factor KIc. The structural transformations in front of the crack tips, and in particular the occurrence of {1 1 0} planar faults, are analyzed in detail and related to the strain-dependent generalized stacking fault energy curve. The simulation results are discussed in terms of theoretical fracture criteria and compared to recent experimental data. The different potentials are ranked according to their capability to model the experimentally observed fracture behavior. read less

Abstract: Analytic bond-order potentials (BOPs) for magnetic transition metals are applied for pure iron as described by an orthogonal d-valent tight-binding (TB) model. Explicit analytic equations for the gradients of the binding energy with respect to the Hamiltonian on-site levels are presented, and are then used to minimize the energy with respect to the magnetic moments, which is equivalent to a TB self-consistency scheme. These gradients are also used to calculate the exact forces, consistent with the energy, necessary for efficient relaxations and molecular dynamics. The Jackson kernel is used to remove unphysical negative densities of states, and approximations for the asymptotic recursion coefficients are examined. BOP, TB and density functional theory results are compared for a range of bulk and defect magnetic structures. The BOP energies and magnetic moments for bulk structures are shown to converge with increasing numbers of moments, with nine moments sufficient for a quantitative comparison of structural energy differences. The formation energies of simple defects such as the monovacancies and divacancies also converge rapidly. Other physical quantities, such as the position of the high-spin to low-spin transition in ferromagnetic fcc (face centred cubic) iron, surface peaks in the local density of states, the elastic constants and the formation energies of the self-interstitial atom defects, require higher moments for convergence. read less

Abstract: Combining an atomistic many-body potential with a classical spin Hamiltonian parameterized by first principles calculations, molecular-spin dynamics computer simulations were performed to investigate phonon-magnon interactions in body centered cubic iron. Results obtained for spin-spin and density-density dynamic structure factors show that noticeable softening and damping of magnon modes occur due to the presence of lattice vibrations. Furthermore, as a result of the phonon-magnon coupling, additional longitudinal spin wave excitations are observed, with the same frequencies as the longitudinal phonon modes. read less

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

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

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

Abstract: Fusion has the potential for delivering safe, clean, low carbon power; however, significant scientific and engineering hurdles must first be overcome. One such hurdle is the design of materials that will withstand the harsh conditions. The materials which line the vessel walls will experience exceptionally high heat and particle fluxes, which will gradually erode the materials and contaminate the plasma. The deuterium–tritium fusion reaction will produce high energy neutrons, which will create defects and transmutation reactions in the vessel walls. These defects, along with the transmutation gasses, evolve over time and change the microstructure and properties of the material. In order to design suitable materials for fusion, the radiation damage, and its evolution over time, must be understood and evaluated for a broad class of materials. Modelling has a vital role to play because it can provide details about processes that occur on length and timescales that are inaccessible to experiment. In this review, the challenges that face designers of fusion power plants are discussed. The modelling techniques that are used to model radiation effects are described and the links between modelling and experiment are discussed. The review concludes with a discussion about the future direction for fusion materials research. read less

Abstract: R&D activities on fusion reactor materials in Switzerland focus on (1) the development of advanced metallic materials for structural applications in plasma-facing (first wall, divertor) and breeding blanket components of the future fusion power reactors, in particular oxide dispersion strengthened reduced activation ferritic steels and tungsten-base materials, (2) the modelling of radiation damage and radiation effects and (3) small specimen test technology for the future International Fusion Materials Irradiation Facility. The main objectives, examples of recent results and future activities are described in the case of these three R&D areas. read less

Abstract: A second generation of empirical potentials is produced for α-Fe within the framework of the magnetic potential formalism (Dudarev and Derlet 2005 J. Phys.: Condens. Matter 17 7097). A materials database that, in addition to ab initio-derived point defect formation energies, now includes third-order elastic constant and ab initio-derived string potential data controlling, respectively, the thermal expansion properties and the core structure of the 1/2⟨111⟩ screw dislocation. Three parameterizations are presented in detail, all of which exhibit positive thermal expansion and produce a non-degenerate configuration for the relaxed 1/2⟨111⟩ screw dislocation easy core structure. These potentials, along with two other published potentials, are investigated in terms of defect formation volume, early stage dislocation loop clustering energetics, ⟨110⟩ dumbbell interstitial diffusion, and the zero-stress 1/2⟨111⟩ screw dislocation Peierls barrier and its corresponding kink formation energies. read less

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

Abstract: We present an Fe–Cr interatomic potential to model high-Cr ferritic alloys. The potential is fitted to thermodynamic and point-defect properties obtained from density functional theory (DFT) calculations and experiments. The developed potential is also benchmarked against other potentials available in literature. It shows particularly good agreement with the DFT obtained mixing enthalpy of the random alloy, the formation energy of intermetallics and experimental excess vibrational entropy and phase diagram. In addition, DFT calculated point-defect properties, both interstitial and substitutional, are well reproduced, as is the screw dislocation core structure. As a first validation of the potential, we study the precipitation hardening of Fe–Cr alloys via static simulations of the interaction between Cr precipitates and screw dislocations. It is concluded that the description of the dislocation core modification near a precipitate might have a significant influence on the interaction mechanisms observed in dynamic simulations. read less

Abstract: Atomistic simulations are a primary means of understanding the damage done to metallic materials by high energy particulate radiation. In many situations the electrons in a target material are known to exert a strong influence on the rate and type of damage. The dynamic exchange of energy between electrons and ions can act to damp the ionic motion, to inhibit the production of defects or to quench in damage, depending on the situation. Finding ways to incorporate these electronic effects into atomistic simulations of radiation damage is a topic of current major interest, driven by materials science challenges in diverse areas such as energy production and device manufacture. In this review, we discuss the range of approaches that have been used to tackle these challenges. We compare augmented classical models of various kinds and consider recent work applying semi-classical techniques to allow the explicit incorporation of quantum mechanical electrons within atomistic simulations of radiation damage. We also outline the body of theoretical work on stopping power and electron–phonon coupling used to inform efforts to incorporate electronic effects in atomistic simulations and to evaluate their performance. read less

Abstract: Functional representations of the spin polarization and the exchange interaction in terms of the lattice configuration is necessary to model the dynamics of the coupled spin and lattice subsystems in large-scale atomistic simulation of magnetic materials. Data needed for this purpose have only existed in the regime of small displacements from the equilibrium perfect lattice configurations. In this paper, we report and discuss the results of our first-principles calculations for bcc iron over a wide range of lattice constants using the magnetic force theorem and the one-electron Green's function. Despite the relatively complex functional form of the exchange interaction function for bcc iron our results show that it can be expressed as a superposition of Bethe-Slater-type curves representing interatomic exchange interaction of the $3d$ electrons. read less

Abstract: An ab initio-based magnetic-cluster-expansion treatment developed for body- and face-centered cubic phases of iron and iron-chromium alloys is applied to modeling the $\ensuremath{\alpha}\text{\ensuremath{-}}\ensuremath{\gamma}$ and $\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\delta}$ phase transitions in these materials. The Curie, N\'eel, and the structural phase-transition temperatures predicted by the model are in good agreement with experimental observations, indicating that it is the thermal excitation of magnetic and phonon degrees of freedom that stabilizes the fcc $\ensuremath{\gamma}$ phase. The model also describes the occurrence of the $\ensuremath{\gamma}$ loop in the phase diagram of Fe-Cr alloys for a realistic interval of temperatures and Cr concentrations. read less

Abstract: Monte Carlo models are widely used for the study of microstructural and microchemical evolution of materials under irradiation. However, they often link explicitly the relevant activation energies to the energy difference between local equilibrium states. We provide a simple example (di-vacancy migration in iron) in which a rigorous activation energy calculation, by means of both empirical interatomic potentials and density functional theory methods, clearly shows that such a link is not granted, revealing a migration mechanism that a thermodynamics-linked activation energy model cannot predict. Such a mechanism is, however, fully consistent with thermodynamics. This example emphasizes the importance of basing Monte Carlo methods on models where the activation energies are rigorously calculated, rather than deduced from widespread heuristic equations. read less

Abstract: Many-body interatomic potentials play an important role in atomistic modelling of materials. For pure elements it is known that there exist gauge transformations that can change the form of the potential functions without modifying its properties. These same transformations, however, fail when applied to alloys. Even though different research groups may use the same potentials to describe pure elements, the gauges employed for fitting alloys will generally be different. In this scenario, it is a priori impossible to merge them into one potential describing the combined system, and thus no advantage is taken from state-of-the-art developments in the literature. Here, we generalise the gauge transformations applied to pure species in order to leave the properties of alloys invariant. Based on these transformations, a strategy to merge potentials developed within different gauges is presented, aiming at the description of the combined system. Advantage of existing state-of-the-art potentials is so taken, thus focusing the efforts on fitting only the missing interactions. Such a procedure constitutes a helpful tool for the development of potentials targeted to alloys of increased complexity, while maintaining the description quality of their constituents. read less

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

Abstract: 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: Voids and He bubbles are strong obstacles to dislocation, which induce hardening and loss of ductility. In Fe, molecular dynamics simulation is used to investigate the basic mechanisms of the interaction between a moving edge dislocation and a void or He bubble, as a function of its He content, temperature, interatomic potentials and interaction geometry. Different interatomic potentials for Fe–Fe and Fe–He interactions are used. It appears that temperature eases the dislocation release, due to the increased mobility of the screw segments appearing on the dislocation line upon bowing from the void or He bubble. The mobility includes the cross-slipping of these segments, which leads to the formation of a jog. It appears that the He bubble induces an inhomogeneous stress field in its surroundings, which strongly influences the dislocation passage depending on the geometry of the interaction. read less

Abstract: It is shown that semi-empirical potentials for bcc metals based on the non-directional second-moment Finnis–Sinclair approximation are able to predict, as a matter of routine, the non-degenerate core structure for the perfect ½⟨111⟩ dislocation if they correctly describe the inter-string pair potential of a rigid multi-string Frenkel–Kontorova model for the corresponding ideal bcc lattice. We prove this by inspecting the previously published empirical potentials, and also by performing an extensive search in functional parameter space for an optimal parameterisation of the magnetic potential formalism for bcc ferromagnetic Fe. read less

Abstract: The recent passing of the petaflop per second landmark by the Roadrunner project at the Los Alamos National Laboratory marks a preliminary peak of an impressive world-wide development in the high-performance scientific computing sector. Also, purely academic state-of-the-art supercomputers such as the IBM Blue Gene/P at Forschungszentrum Jülich allow us nowadays to investigate large systems of the order of 103 spin polarized transition metal atoms by means of density functional theory. Three applications will be presented where large-scale ab initio calculations contribute to the understanding of key properties emerging from a close interrelation between structure and magnetism. The first two examples discuss the size dependent evolution of equilibrium structural motifs in elementary iron and binary Fe–Pt and Co–Pt transition metal nanoparticles, which are currently discussed as promising candidates for ultra-high-density magnetic data storage media. However, the preference for multiply twinned morphologies at smaller cluster sizes counteracts the formation of a single-crystalline L10 phase, which alone provides the required hard magnetic properties. The third application is concerned with the magnetic shape memory effect in the Ni–Mn–Ga Heusler alloy, which is a technologically relevant candidate for magnetomechanical actuators and sensors. In this material strains of up to 10% can be induced by external magnetic fields due to the field induced shifting of martensitic twin boundaries, requiring an extremely high mobility of the martensitic twin boundaries, but also the selection of the appropriate martensitic structure from the rich phase diagram. read less

Abstract: S. Chiesa,1 P. M. Derlet,2 and S. L. Dudarev3,4 1NUM/ASQ-Materials Science and Simulation, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland 2NUM-Condensed Matter Theory Group, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland 3Culham Science Centre, EURATOM/UKAEA Fusion Association, Oxfordshire OX14 3DB, United Kingdom 4Department of Physics, Imperial College, Exhibition Road, London SW7 2AZ, United Kingdom Received 7 April 2009; revised manuscript received 22 May 2009; published 17 June 2009 read less

Abstract: combined application of the Langevin spin dynamics and the fluctuation-dissipation theorem. We investigate several applications of the method, performing microcanonical ensemble simulations of adiabatic spin-lattice relaxation of periodic arrays of 180° domain walls, and isothermal-isobaric ensemble dynamical simulations of thermally equilibrated homogeneous systems at various temperatures. The predicted isothermal magnetization curve agrees well with the experimental data for a broad range of temperatures. The equilibrium as well as time-correlation functions of spin orientations exhibit the presence of short-range magnetic order above the Curie temperature. Furthermore, short-range order spin fluctuations are shown to contribute to the thermal expansion of the material. Our analysis illustrates the significant part played by the spin degrees of freedom in the dynamics of motion of atoms in magnetic iron and iron-based alloys. It also shows that the spin-lattice dynamics algorithm developed in this paper offers a viable way of performing large-scale dynamical atomistic simulations of magnetic materials. read less

Abstract: By calculating free energies, several published interatomic interaction potentials for iron are investigated with respect to the stability of the low-temperature bcc phase and the high-temperature fcc phase. These are empirical many-body potentials for use in atomistic simulation. We find that in all of these potentials—except one—the bcc phase is the stable crystal structure for all temperatures up to the melting point. However, several potentials exhibit a metastable fcc phase in the sense that the fcc structure corresponds to a local minimum of the free energy. read less

Abstract: Low activation ferritic/martensitic steels are good candidates for the future fusion reactors, for, relative to austenitic steels, their lower damage accumulation and moderate swelling under irradiation by the 14 MeV neutrons produced by the fusion reaction. Irradiation of these steels, e.g. EUROFER97, is known to produce hardening, loss of ductility, shift in ductile to brittle transition temperature and a reduction of fracture toughness and creep resistance starting at the lowest doses. Helium, produced by transmutation by the 14 MeV neutrons, is known to impact mechanical properties, but its effect at the microstructure level is still unclear. The mechanisms underlying the degradation of mechanical properties are not well understood, despite numerous studies on the evolution of the microstructure under irradiation. This impedes our ability to predict materials' behaviour at higher doses for use in the future fusion reactors. Simulations of these effects are now essential. An overview is presented on molecular dynamics simulations of the primary state of damage in iron and of the mobility of a dislocation, vector of plasticity, in the presence of a defect. read less

Abstract: We describe a self-consistent magnetic tight-binding theory based in an expansion of the Hohenberg-Kohn density functional to second order, about a non-spin-polarized reference density. We show how a first order expansion about a density having a trial input magnetic moment leads to a fixed moment model. We employ a simple set of tight-binding parameters that accurately describes electronic structure and energetics, and show these to be transferable between first row transition metals and their alloys. We make a number of calculations of the electronic structure of dilute Cr impurities in Fe, which we compare with results using the local spin density approximation. The fixed moment model provides a powerful means for interpreting complex magnetic configurations in alloys; using this approach, we are able to advance a simple and readily understood explanation for the observed anomaly in the enthalpy of mixing. read less

Abstract: We investigate the structure and mobility of single self-interstitial atom and vacancy defects in body-centered-cubic transition metals forming groups 5B (vanadium, niobium, and tantalum) and 6B (chromium, molybdenum, and tungsten) of the Periodic Table. Density-functional calculations show that in all these metals the axially symmetric self-interstitial atom configuration has the lowest formation energy. In chromium, the difference between the energies of the and the self-interstitial configurations is very small, making the two structures almost degenerate. Local densities of states for the atoms forming the core of crowdion configurations exhibit systematic widening of the "local" d band and an upward shift of the antibonding peak. Using the information provided by electronic structure calculations, we derive a family of Finnis-Sinclair-type interatomic potentials for vanadium, niobium, tantalum, molybdenum, and tungsten. Using these potentials, we investigate the thermally activated migration of self-interstitial atom defects in tungsten. We rationalize the results of simulations using analytical solutions of the multistring Frenkel-Kontorova model describing nonlinear elastic interactions between a defect and phonon excitations. We find that the discreteness of the crystal lattice plays a dominant part in the picture of mobility of defects. We are also able to explain the origin of the non-Arrhenius diffusion of crowdions and to show that at elevated temperatures the diffusion coefficient varies linearly as a function of absolute temperature. read less

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

Abstract: We perform large-scale molecular dynamics simulations to study the magnetic properties of amorphous iron under pressure. Simulations, exceeding by at least two orders of magnitude those accessible to density functional calculations, use the recently developed magnetic interatomic potential for iron. The distributions of the size of atomic magnetic moments and parameters characterizing the structure of amorphous iron, such as radial distribution functions, are calculated as a function of the applied hydrostatic stress. As the density increases, there is a reduction in the magnitude of the mean magnetic moment of individual atoms, accompanied by the transformation of an increasing proportion of atoms from a magnetic to a nonmagnetic configuration. Beyond a critical density the proportion of nonmagnetic atoms increases sharply, yet homogeneously. The local magnetic moment of an atom correlates with the local Voronoi volume via a logarithmic relation. In addition, we observe a complex dependence of the local magnetic moment on the topological arrangement of neighboring atoms. © 2007 American Institute of Physics. DOI: 10.1063/1.2715753 read less

Abstract: Iron-chromium alloys are characterized by a complex phase diagram, by the small negative enthalpy of mixing at low Cr concentrations in the bcc -phase of Fe, and by the inversion of the short-range order parameter. We present Monte Carlo simulations of the binary Fe-Cr alloy based on the cluster expansion approximation for the enthalpy of the system. The set of cluster expansion coefficients is validated against density functional calculations of energies of small clusters of chromium in bcc structure. We show that in the limit of small Cr concentration the enthalpy of mixing remains negative up to fairly high temperatures, and individual Cr atoms remain well separated from each other. Clustering of Cr atoms begins at concentrations exceeding approximately 10% at 800 K and 20% at 1400 K, with Cr-Fe interfaces being parallel to the 110 planes. Calculations show that the first and the second short-range order parameters change sign at approximately 10.5% Cr, in agreement with experimental observations. Semi-grand-canonical ensemble simulations used together with experimental data on vibrational entropy of mixing give an estimate for the temperature of the top of the - miscibility gap. We find that the complex ordering reactions occurring in Fe-Cr, as well as the thermodynamic properties of the alloy, can be reasonably well described using a few concentrationindependent cluster expansion coefficients. read less

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

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

Abstract: Some bulk properties of bcc iron were calculated. Structural and elastic properties such as cohesive energy, bulk modulus, typical elastic constants and vacancy formation energy were calculated for zero Kelvin temperature. All obtained results during the study were compared with the both previous experimental and theoretical results. Obtained results for the present study show well agreement with literature. read less