Abstract: The process of deriving an interatomic potentials represents an attempt to integrate out the electronic degrees of freedom from the full quantum description of a condensed matter system. In practice it is the derivatives of the interatomic potentials which are used in molecular dynamics, as a model for the forces on a system. These forces should be the derivative of the free energy of the electronic system, which includes contributions from the entropy of the electronic states. This free energy is weakly temperature dependent, and although this can be safely neglected in many cases there are some systems where the electronic entropy plays a significant role. Here a method is proposed to incorporate electronic entropy in the Sommerfeld approximation into empirical potentials. The method is applied as a correction to an existing potential for titanium. Thermal properties of the new model are calculated, and a simple method for fixing the melting point and solid-solid phase transition temperature for existing models fitted to zero temperature data is presented. 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

Abstract: Reducing hydrogen embrittlement in the low‐cost Fe─C based steels have the potential to significantly impact the development of hydrogen energy technologies. Molecular dynamics studies of hydrogen interactions with Fe─C steels provide fundamental information about the behavior of hydrogen at microstructural length scales, although such studies have not been performed due to the lack of an Fe─C─H ternary interatomic potential. In this work, the literature on interatomic potentials related to the Fe─C─H systems are reviewed with the aim of constructing an Fe─C─H potential from the published binary potentials. We found that Fe─C, Fe─H, and C─H bond order potentials exist and can be combined to construct an Fe─C─H ternary potential. Therefore, we constructed two such Fe─C─H potentials and demonstrate that these ternary potentials can reasonably capture hydrogen effects on deformation characteristics and deformation mechanisms for a variety of microstructural variations of the Fe─C steels, including martensite that results from γ to α phase transformation, and pearlite that results from the eutectic formation of the Fe3C cementite compound. read less

Abstract: The adhesion feature of graphene on metal substrates is important in graphene synthesis, transfer and applications, as well as for graphene-reinforced metal matrix composites. We investigate the adhesion energy of graphene nanosheets (GNs) on iron substrate using molecular dynamic (MD) simulations. Two Fe–C potentials are examined as Lennard–Jones (LJ) pair potential and embedded-atom method (EAM) potential. For LJ potential, the adhesion energies of monolayer GN are 0.47, 0.62, 0.70 and 0.74 J/m2 on the iron {110}, {111}, {112} and {100} surfaces, respectively, compared to the values of 26.83, 24.87, 25.13 and 25.01 J/m2 from EAM potential. When the number of GN layers increases from one to three, the adhesion energy from EAM potential increases. Such a trend is not captured by LJ potential. The iron {110} surface is the most adhesive surface for monolayer, bilayer and trilayer GNs from EAM potential. The results suggest that the LJ potential describes a weak bond of Fe–C, opposed to a hybrid chemical and strong bond from EAM potential. The average vertical distances between monolayer GN and four iron surfaces are 2.0–2.2 Å from LJ potential and 1.3–1.4 Å from EAM potential. These separations are nearly unchanged with an increasing number of layers. The ABA-stacked GN is likely to form on lower-index {110} and {100} surfaces, while the ABC-stacked GN is preferred on higher-index {111} surface. Our insights of the graphene adhesion mechanics might be beneficial in graphene growing, surface engineering and enhancement of iron using graphene sheets. read less

Abstract: Interatomic potentials based on neural-network machine learning (ML) approach to address the longstanding challenge of accuracy versus efficiency in molecular-dynamics simulations have recently attracted a great deal of interest. Here, utilizing Pd-Si system as a prototype, we extend the development of neural-network ML potentials to compounds exhibiting various types of bonding characteristics. The ML potential is trained by fitting to the energies and forces of both liquid and crystal structures firstprinciples calculations based on density-functional theory (DFT). We show that the generated ML potential captures the structural features and motifs in Pd82Si18 and Pd75Si25 liquids more accurately than the existing interatomic potential based on embedded-atom method (EAM). The ML potential also describes the solid-liquid interface of these systems very well. Moreover, while the existing EAM potential fails to describe the relative energies of various crystalline structures and predict wrong ground-state structures at Pd3Si and Pd9Si2 composition, the developed ML potential predicts correctly the groundstate structures from genetic algorithm search. The efficient ML potential with DFT accuracy from our study will provide a promising scheme for accurate atomistic simulations of structures and dynamics of complex Pd-Si system. Disciplines Condensed Matter Physics Authors Tongqi Wen, Cai-Zhuang Wang, Matthew J. Kramer, Yang Sun, Beilin Ye, Haidi Wang, Xueyuan Liu, Chao Zhang, Feng Zhang, Kai-Ming Ho, and Nan Wang This article is available at Iowa State University Digital Repository: https://lib.dr.iastate.edu/ameslab_manuscripts/ 663 PHYSICAL REVIEW B 100, 174101 (2019) Development of a deep machine learning interatomic potential for metalloid-containing Pd-Si compounds Tongqi Wen ,1,2 Cai-Zhuang Wang,2,3,* M. J. Kramer ,2 Yang Sun ,2 Beilin Ye,4 Haidi Wang,2 Xueyuan Liu,2 Chao Zhang ,2,5 Feng Zhang,2 Kai-Ming Ho,2,3 and Nan Wang 1,† 1MOE Key Laboratory of Materials Physics and Chemistry under Extraordinary Conditions, School of Natural and Applied Sciences, Northwestern Polytechnical University, Xi’an 710072, China 2Ames Laboratory-USDOE, Iowa State University, Ames, Iowa 50011, USA 3Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA 4School of Materials Science and Engineering, South China University of Technology, Guangzhou 510641, China 5Department of Physics, School of Opto-electronic Information Science and Technology, Yantai University, Yantai, 264005, China (Received 16 July 2019; revised manuscript received 8 October 2019; published 4 November 2019) Interatomic potentials based on neural-network machine learning (ML) approach to address the long-standing challenge of accuracy versus efficiency in molecular-dynamics simulations have recently attracted a great deal of interest. Here, utilizing Pd-Si system as a prototype, we extend the development of neural-network ML potentials to compounds exhibiting various types of bonding characteristics. The ML potential is trained by fitting to the energies and forces of both liquid and crystal structures first-principles calculations based on density-functional theory (DFT). We show that the generated ML potential captures the structural features and motifs in Pd82Si18 and Pd75Si25 liquids more accurately than the existing interatomic potential based on embedded-atom method (EAM). The ML potential also describes the solid-liquid interface of these systems very well. Moreover, while the existing EAM potential fails to describe the relative energies of various crystalline structures and predict wrong ground-state structures at Pd3Si and Pd9Si2 composition, the developed ML potential predicts correctly the ground-state structures from genetic algorithm search. The efficient ML potential with DFT accuracy from our study will provide a promising scheme for accurate atomistic simulations of structures and dynamics of complex Pd-Si system. DOI: 10.1103/PhysRevB.100.174101 read less

Abstract: It has long been known that iron undergoes a phase transformation from body-centered cubic/ α structure to the metastable hexagonal close-packed/ ε phase under high pressure. However, the interplay of line and planar defects in the parent material with the transformation process is still not fully understood. We investigated the role of twins, dislocations, and Cottrell atmospheres in changing the crystalline iron structure during this phase transformation by using Monte Carlo methods and classical molecular dynamics simulations. Our results confirm that embryos of ε -Fe nucleate at twins under hydrostatic compression. The nucleation of the hcp phase is observed for single crystals containing an edge dislocation. We observe that the buckling of the dislocation can help to nucleate the dense phase. The crystal orientations between the initial structure α -Fe and ε -Fe in these simulations are 110 b c c | | 0001 h c p . The presence of Cottrell atmospheres surrounding an edge dislocation in bcc iron retards the development of the hcp phase. read less

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

Abstract: In carbon‐containing alloys, the energetic stability of vacancy‐carbon (v‐C) clusters plays a key role in the post‐irradiation damage recovery. So far, all interpretations of post‐irradiation annealing experiments are based on the theoretical knowledge established for the iron‐carbon alloy. For the interpretation of high‐Cr steels, the stability of v‐C clusters in the presence of a solid solution background is essential. Therefore, a simple and efficient model to estimate the stability of v‐C clusters in random solid solution alloys is proposed. The model is parameterized for the Fe‐Cr‐C alloy using density functional theory. The results are discussed accounting for available experimental evidences. read less

Abstract: A static and kinetic study of the interaction between a 19 ½ 〈1 1 1〉 self-interstitial atoms loop and C atoms in body-centred cubic iron is presented in this work. An empirical potential matching the density functional theory calculations is used to study the static properties of the system. The usual kinetic Monte-Carlo (KMC) on-lattice restriction is not valid when the material is highly distorted, especially in the presence of a dislocation loop. Therefore, the dynamics of the system are investigated using both molecular dynamics simulations and k-ART, a self-learning/off-lattice atomic kinetic monte-carlo. The presented work is thus a full study of the C-loop and the C2-loop systems. A good agreement is observed between the statics and the kinetics (e.g. the discovery of a zone of stability of the C atom around the Fe cluster where the C can almost freely move), even though the kinetics show some unexpected behaviours of the studied systems. The pinning time of the loop induced by the C atoms is also estimated. read less

Abstract: Using molecular dynamics simulation, local plasticity of bcc Fe (0 0 1) is studied at different density of Fe–H cluster. H-induced softening and hardening of Fe substrate are observed along with the tensile elongation at low and high density, respectively. The two contradictory phenomena are ascribed to H behaviours-related plastic deformation. At high H partial pressure, initial H aggregation would lead to the formation of many H-enriched clusters similar to hydride. Tensile strain-induced dislocations (TSID) prefer to be generated and grow at the weakening interface of clusters and iron substrate. At low H partial pressure, TSIDs are uniformly distributed in the whole substrate. Owing to the affinity between H and dislocations, the diffusion of H appears to be distinct under different spatial distribution of TSIDs. H aggregation and dispersion can be enhanced and produce nonuniform and uniform plastic deformation during the continuous tensile process at high and low Fe–H cluster density, respectively. The former can stimulate local failures and accelerate the degradation of mechanical property. The results are helpful for better understanding of Fe–H cluster-related hardening and softening considering external strain-altered H behaviours except for the mechanism of H-dislocation interaction. read less

Abstract: We show that the Gaussian Approximation Potential machine learning framework can describe complex magnetic potential energy surfaces, taking ferromagnetic iron as a paradigmatic challenging case. The training database includes total energies, forces, and stresses obtained from density-functional theory in the generalized-gradient approximation, and comprises approximately 150,000 local atomic environments, ranging from pristine and defected bulk configurations to surfaces and generalized stacking faults with different crystallographic orientations. We find the structural, vibrational and thermodynamic properties of the GAP model to be in excellent agreement with those obtained directly from first-principles electronic-structure calculations. There is good transferability to quantities, such as Peierls energy barriers, which are determined to a large extent by atomic configurations that were not part of the training set. We observe the benefit and the need of using highly converged electronic-structure calculations to sample a target potential energy surface. The end result is a systematically improvable potential that can achieve the same accuracy of density-functional theory calculations, but at a fraction of the computational cost. 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 (MD) is a technique of atomistic simulation which has facilitated scientific discovery of interactions among particles since its advent in the late 1950s. Its merit lies in incorporating statistical mechanics to allow for examination of varying atomic configurations at finite temperatures. Its contributions to materials science from modeling pure metal properties to designing nanowires is also remarkable. This review paper focuses on the progress of MD in understanding the behavior of iron — in pure metal form, in alloys, and in composite nanomaterials. It also discusses the interatomic potentials and the integration algorithms used for simulating iron in the literature. Furthermore, it reveals the current progress of MD in simulating iron by exhibiting some results in the literature. Finally, the review paper briefly mentions the development of the hardware and software tools for such large-scale computations. 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: It is known that M23C6(M = Cr/Fe) behavior in heat-resistant ferritic steels affects the strength of the material at high temperature. The ability to garner direct information regarding the atomic motion using classical molecular dynamics simulations is useful for investigating the M23C6 behavior in heat-resistant ferritic steels. For such classical molecular dynamics calculations, a suitable interatomic potential is needed. To satisfy this requirement, an empirical bond-order-type interatomic potential for Fe-Cr-C systems was developed because the three main elements to simulate the M23C6 behavior in heat-resistant ferritic steels are Fe, Cr, and C. The angular-dependent term, which applies only in non-metallic systems, was determined based on the similarity between a Finnis-Sinclair-type embedded-atom-method interatomic potential and a Tersoff-type bond-order potential. The potential parameters were determined such that the material properties of Fe-Cr-C systems were reproduced. These properties include... read less

Abstract: Using molecular-dynamics simulation, we study the austenitic and martensitic phase transformation in Fe–C nanowires with C contents up to 1.2 at%. The transformation temperatures decrease with C content. The martensite temperature decreases with wire diameter towards the bulk value. During the transformation, the bcc and fcc phases obey the Kurdjumov–Sachs orientation relationship. For ultrathin wires (diameter D ⩽ 2.8 nm), we observe wire buckling as well as shape-memory effects. Under axial tensile stress the martensite transformation is partially suppressed, leading to strong plastic deformation. Under the highest loads, the austenite only partially back-transforms while the crystalline phases in the wire re-orient giving the multi-phase mixture a high tensile strength. read less

Abstract: We employ a recently developed iron–carbon orthogonal tight-binding model in calculations of carbon in iron grain boundaries. We use the model to evaluate the properties of carbon near and on the Σ5 (3 1 0)[0 0 1] symmetric tilt grain boundary (GB) in iron, and calculations show that a carbon atom lowers the GB energy by 0.29 eV/atom in accordance with DFT. Carbon segregation to the GB is analyzed, and we find an energy barrier of 0.92 eV for carbon to segregate to the carbon-free interface while segregation to a fully filled interface is disfavored. Local volume (via Voronoi tessellation), magnetic, and electronic effects are correlated with atomic energy changes, and we isolate two different mechanisms governing carbon's behavior in iron: a volumetric strain which increases the energy of carbon in interstitial α iron and a non-strained local bonding which stabilizes carbon at the GB. 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: We investigate the influence of C interstitials on the phase stability of Fe–C crystals. We employ the Meyer–Entel interatomic interaction potential which is able to reproduce the austenite-martensite phase transition for pure Fe, and supplement it by a simple pairwise Fe–C interaction potential. Using two different thermodynamic methods, we calculate the free energies of the martensite and austenite phases. We find that C destabilizes the ground-state bcc phase. The decrease in the equilibrium transformation temperature with increasing C content parallels the one found in the experiment. This destabilization is found even if C is added for a potential in which only the bcc phase is stable until the melting point; here, for sufficiently high C addition, a stable fcc phase is established in the phase diagram. read less

Abstract: Stainless steels found in real-world applications usually have some C content in the base Fe–Cr alloy, resulting in hard and dislocation-pinning carbides—Fe3C (cementite) and Cr23C6—being present in the finished steel product. The higher complexity of the steel microstructure has implications, for example, for the elastic properties and the evolution of defects such as Frenkel pairs and dislocations. This makes it necessary to re-evaluate the effects of basic radiation phenomena and not simply to rely on results obtained from purely metallic Fe–Cr alloys. In this report, an analytical interatomic potential parameterization in the Abell–Brenner–Tersoff form for the entire Fe–Cr–C system is presented to enable such calculations. The potential reproduces, for example, the lattice parameter(s), formation energies and elastic properties of the principal Fe and Cr carbides (Fe3C, Fe5C2, Fe7C3, Cr3C2, Cr7C3, Cr23C6), the Fe–Cr mixing energy curve, formation energies of simple C point defects in Fe and Cr, and the martensite lattice anisotropy, with fair to excellent agreement with empirical results. Tests of the predictive power of the potential show, for example, that Fe–Cr nanowires and bulk samples become elastically stiffer with increasing Cr and C concentrations. High-concentration nanowires also fracture at shorter relative elongations than wires made of pure Fe. Also, tests with Fe3C inclusions show that these act as obstacles for edge dislocations moving through otherwise pure Fe. read less

Abstract: Properties of ferritic Fe-based alloys are highly sensitive to the carbon content dissolved in the matrix because interstitial carbon is known to strongly interact with lattice point defects and dislocations. As a result, the accumulation of radiation defects and its impact on the change of mechanical properties is also affected by the presence of dissolved interstitial carbon. This work contributes to an understanding of how interstitial carbon atoms influence the properties of small dislocation loops, which form directly in collision cascades upon neutron or ion irradiation and are ‘invisible’ to (i.e. undetectable by) standard experimental techniques applied to reveal nano-structural damage in metals. We have carried out MD simulations to investigate how the trapping of 1/2〈111〉 dislocation loops at thermally stable carbon–vacancy complexes, known to form under irradiation, affects the interaction of these dislocation loops with dislocations in bcc Fe. We have considered loops of size 1 and 3.5 nm, which represent experimentally invisible and visible defects, respectively. The obtained results point at the strong suppression of the drag of carbon-decorated loops by dislocations. In the case of direct interaction between dislocation and carbon-decorated loops, invisible loops are found to act as obstacles whose strength is at least twice as high compared to that of undecorated ones. Additional strengthening due to the carbon decoration on the visible loops was also regularly registered. The reasons for the additional strengthening have been rationalized and discussed. It is demonstrated that carbon decoration/segregation at dislocation loops affects not only accumulation of radiation damage under prolonged irradiation but also alters the post-irradiation plastic deformation mechanisms. For the first time, we provide evidence that undetectable dislocation loops decorated by carbon do contribute to the radiation hardening. read less

Abstract: N atom is one of the most frequent foreign interstitial atoms in α-iron along with C atoms. The Fe–C potential has been well-developed and can reproduce many significant interactions of C with point defects present in α-iron. However, there exists no satisfactory Fe–N potential to describe the interactions of N with point defects. Here, we develop a many-body potential for N in α-iron. The potential parameters are determined by fitting to ab initio data, which includes energetics, configurations, and relaxations of Fe atoms close to N atom. This potential successfully describes the interactions of Fe–N across a wide range of defect environments. The potential employs the embedded atom method form and hence is appropriate for large-scale molecular dynamics simulation. read less

Abstract: In this work we have summarized the available ab initio data addressing the interaction of carbon with vacancy defects in bcc Fe and performed additional calculations to extend the available dataset. Using an ab initio based parameterization, we apply object kinetic Monte Carlo (OKMC) simulations to model the process of isochronal annealing in bcc Fe doped with carbon to compare with experimental data. As a result of this work, we clarify that a binding energy of ∼0.65 eV for a vacancy–carbon (V–C) pair fits the available experimental data best. It is found that the V 2–C complex is less stable than the V–C pair and its dissociation with activation energy of 0.55 + 0.49 eV also rationalizes a number of experimental data where the breakup of V–C complexes was assumed instead. From the summarized ab initio data, the subsequently obtained OKMC results and critical discussion, provided here, we suggest that the twofold interpretation of the V–C binding energy, which is believed to vary between 0.47 and 0.65 eV, depending on the ab initio approximation, should be removed. The stability and mobility of small and presumably immobile SIA clusters formed at stage II is also discussed in the view of experimental data. read less

Abstract: First principles calculations have given a new insight into the energies of point defects in many different materials, information which cannot be readily obtained from experiment. Most such calculation are done at zero Kelvin, with the assumption that finite temperature effects on defect energies and barriers are small. In some materials, however, the stable crystal structre of interest is mechanically unstable at 0K. In such cases, alternate approaches are needed. Here we present results of first principles calculations of austenitic iron using the VASP code. We determine an appropriate reference state for collinear magnetism to be the antiferromagnetic double-layer (AFM-d) which is both stable and lower in energy than other possible models for the low temperature limit of paramagnetic fcc iron. We then consider the energetics of dissolving typical alloying impurities (Ni, Cr) in the materials, and their interaction with point defects typical of the irradiated environment. We show that using standard methods there is a very strong dependence of calculated defect formation energies on the reference state chosen. Furthermore, there is a correlation between local free volume magnetism and energetics. The effect of substitutional Ni and Cr on defect properties is weak, rarely more than tenths of eV, so it is unlikely that small amounts of Ni and Cr will have a significant effect on the radiation damage in austenitic iron at high temperatures. read less

Abstract: Cold-drawn pearlitic steel wires are known to exhibit increasing strength with increasing elongation and are therefore highly interesting for a wide field of engineering applications. When isochronal heat treatment is performed at different temperatures, the tensile strength as well as the electrical resistivity decrease well before microstructural changes are observed. An Arrhenius analysis of both processes yield mean activation energies of about 0.3 eV. This is construed as interaction between carbon atoms and defects in ferrite, mainly vacancies and vacancy clusters. 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: unrelaxed structures are easily reproducible, enabling them to be used by other authors to readily test potential formalisms other than those considered in this work. Specifically, we will perform our calculations on unrelaxed bcc � -iron (ferrite) with interstitial carbon. This will enable us to make a contribution to the ongoing debate as to how the ferritic phase of steel might be best represented using a semi-empirical potential. 3–6) This is an important topic because many properties of steel originate from the interaction of carbon (and nitrogen) with point and extended defects. These interactions must be treated using a semiempirical or empirical approach, since the numbers of atoms involved in such interactions place them well out of reach of current ab-initio methods such as density functional theory (DFT). 7,8) read less

Abstract: An analytical bond-order interatomic potential has been developed for the iron-carbon system for use in molecular-dynamics and Monte Carlo simulations. The potential has been successfully fitted to cementite and Hagg carbide, which are most important crystalline polytypes among the many known metastable iron carbide phases. Predicted properties of other carbides and the simplest point defects are in good to reasonable agreement with available data from experiments and density-functional theory calculations. The potential correctly describes melting and recrystallization of cementite, making it useful for simulation of steels. We show that they correctly describe the metastability of cementite and can be used to model carbide growth and dissolution. read less

Abstract: Small concentrations of alloying elements can modify the α-γ phase transition temperature Tc of Fe. We study this effect using an atomistic model based on a set of many-body interaction potentials for iron and several alloying elements. Free-energy calculations based on perturbation theory allow us to determine the change in Tc introduced by the alloying element. The resulting changes are in semi-quantitative agreement with experiment. The effect is traced back to the shape of the pair potential describing the interaction between the Fe and the alloying atom. read less

Abstract: Molecular dynamics simulations are used to investigate the atomic effects of carbon (C) addition in Fe on the martensitic phase transformation in the presence of pre-existing defects such as stacking faults and twin boundaries. The pre-existing defect structures in Fe-C alloys have the same effect on the atomistic mechanisms of martensitic transformation as in pure Fe. However, C addition decreases the martensitic transformation temperature. This effect is captured by characterizing three parameters at the atomic level: atomic shear stresses, atomic energy, and total energy as a function of temperature for face-centered-cubic (fcc) and body-centered-cubic (bcc) phases. The thermodynamic effect of fcc phase stabilization by C addition is revealed by the atomic energy at a particular temperature and total energy as a function of temperature. The barrier for fcc-to-bcc transformation is revealed by analysis of atomic shear stresses. The analysis indicates that addition of C increases the atomic shear stresses for atomic displacements during martensitic transformation, which in turn decreases the martensitic transformation temperature. read less