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In the present work, we have evaluated the performance of different embedded atom method (EAM) and second-nearest neighbour modified embedded atom method (2NN-MEAM) potentials based on their predictive capabilities for modelling fracture in single-and bicrystalline tungsten. As part of the study, a new 2NN-MEAM was fitted with emphasis on reproducing surface, unstable stacking fault and twinning energies as derived from density functional theory (DFT) modelling. The investigation showed a systematic underestimation of surface energies by most EAM potentials, and a significant variation in unstable stacking and twinning fault energies. Moreover, the EAM potentials in general lack the ability to reproduce the DFT traction-separation (TS) curves. The shorter interaction length and higher peak stress of the EAM TS curves compared to the 2NN-MEAM and DFT TS curves result in one order of magnitude higher lattice trapping than for cracks studied with 2NN-MEAM. These differences in lattice trapping can lead to significant qualitative differences in the fracture behaviour. Overall, the new 2NN-MEAM potential best reproduced fracture-relevant material properties and its results were consistent with fracture experiments. Finally, the results of fracture simulations were compared with analytical predictions based on Griffith and Rice theories, for which emerging discrepancies were discussed.

International audience; In this work, the Cu clustering in Fe under irradiation is investigated using experiments, cluster dynamics and atomistic kinetic Monte Carlo (AKMC) simulations. In experiments, cast iron and model FeCu alloy samples were irradiated with 2 MeV electrons for 143 h at 140 °C. The post-irradiation microstructure was characterized using atom probe tomography. Cluster dynamics and AKMC methods were used to simulate the Cu clustering under the same irradiation conditions. Both simulation methods show satisfactory agreement with experiments, lending strength to the validity of the models. Finally, the Cu clustering in spent-fuel repository conditions for 10$^5$ years at 100 °C was simulated using both methods. The results indicate that potential hardening by Cu clustering is insignificant over 10$^5$ years.

Publisher Copyright: © 2022 The Authors Tungsten is considered to be used in the future fusion reactors as plasma-facing material. In such ex-treme environments, defects are induced in materials that modify their macroscopic properties such as the mechanical ones. It is of paramount importance to be able to determine concentration and size of the vacancy defects, from the mono vacancy to the large cavities, to validate the models developed to predict the evolution of the microstructure of irradiated materials. Positrons are very useful non-destructive probes that can characterize vacancy-type defects in materials. We present a combined ex-perimental and theoretical study on detecting and estimating the sizes of vacancy clusters that are invis-ible with electron microscopy in tungsten, using positron annihilation spectroscopy. We here model the positron annihilation in the tungsten lattice and in vacancy-type defects using state-of-the-art first prin-ciples methodology. The Doppler broadening spectra and positron lifetimes in tungsten are calculated with two-component density functional theory with local density approximation and weighted density approximation. Our calculations are in excellent agreement with our experimental results. We show that the sizes of vacancy clusters in tungsten can be well estimated by combining both positron lifetimes and Doppler broadening spectra. We also determine the limit of validity of the canonical calculation method, which here is shown to fail when the vacancy clusters grow beyond their nucleation stage. This work is a first step needed to better interpret the measured positron annihilation characteristics (Doppler and lifetime) in tungsten and then extract quantitative data on small vacancy defects required to improve the understanding of early-stage vacancy defect evolution in tungsten. The method used in this paper could be used to study other metallic materials.(c) 2022 The Authors. Published by Elsevier B.V.This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ )

Modelling of UO2 mechanical behavior requires detailed knowledge of the local stresses and strains during the fuel’s operation in normal and accident conditions. Therefore, a crystal plasticity formulation is proposed for polycrystalline UO. The model contains a dislocation-density-based formulation including three slip families and their interactions. The model is parametrized with single crystal and polycrystal experimental data using an optimization scheme. The model’s capability to represent yield point, strain hardening behavior, temperature and strain rate dependencies are evaluated. Finally, different approaches to include porosity at the polycrystal are analyzed to assess the effect of porosity on homogenized macroscopic stress-strain behavior, and stress/strain localization at the grain level.

The M4F project brings together the fusion and fission materials communities working on the prediction of radiation damage production and evolution and their effects on the mechanical behaviour of irradiated ferritic/martensitic (F/M) steels. It is a multidisciplinary project in which several different experimental and computational materials science tools are integrated to understand and model the complex phenomena associated with the formation and evolution of irradiation induced defects and their effects on the macroscopic behaviour of the target materials. In particular the project focuses on two specific aspects: (1) To develop physical understanding and predictive models of the origin and consequences of localised deformation under irradiation in F/M steels; (2) To develop good practices and possibly advance towards the definition of protocols for the use of ion irradiation as a tool to evaluate radiation effects on materials. Nineteen modelling codes across different scales are being used and developed and an experimental validation programme based on the examination of materials irradiated with neutrons and ions is being carried out. The project enters now its 4th year and is close to delivering high-quality results. This paper overviews the work performed so far within the project, highlighting its impact for fission and fusion materials science. Peer Reviewed L. Malerba a,*, M.J. Caturla b, E. Gaganidze c, C. Kaden d, M.J. Konstantinovi´c e, P. Olsson f, C. Robertson g, D. Rodney h, A.M. Ruiz-Moreno i, M. Serrano a, J. Aktaa c, N. Anento j, S. Austin i, A. Bakaev e, J.P. Balbuena b, F. Bergner d, F. Boioli k, M. Boleininger l, G. Bonny e, N. Castin e, J.B. J. Chapman l, P. Chekhonin d, M. Clozel m, B. Devincre k, L. Dupuy g, G. Diego a, S.L. Dudarev l, C.-C. Fu g, R. Gatti k, L. G´el´ebart g, B. G´omez-Ferrer n, D. Gonçalves g, C. Guerrero a, P.M. Gueye n, P. H¨ahner i, S.P. Hannula o, Q. Hayat p, M. Hern´andez-Mayoral a, J. Jagielski m, N. Jennett p, F. Jim´enez a, G. Kapoor d, A. Kraych h, T. Khvan e,q, L. Kurpaska m, A. Kuronen r, N. Kvashin j, O. Libera s, P.-W. Ma l, T. Manninen o, M.-C. Marinica g, S. Merino a, E. Meslin g, F. Mompiou t, F. Mota a, H. Namburi s, C.J. Ortiz a, C. Pareige n, M. Prester u, R.R. Rajakrishnan t, M. Sauzay g, A. Serra j, I. Simonovski i, F. Soisson g, P. Sp¨atig v, D. Tanguy h, D. Terentyev e, M. Trebala o, M. Trochet g, A. Ulbricht d, M.Vallet g, K. Vogel d, T. Yalcinkaya w, J. Zhao r a Centro de Investigaciones Energ´eticas, Medioambientales y Tecnol´ogicas (CIEMAT), Madrid, Spain b Universidad de Alicante, San Vicente del Raspeig, Spain c Karlsruher Institut für Technologie (KIT), Karlsruhe, Germany d Helmholtz-Zentrum Dresden-Rossendorf Ev (HZDR), Rossendorf, Germany e Studiecentrum voor Kernenergie / Centre d’´Etude de l’´Energie Nucl´eaire (SCK CEN), Mol, Belgium f KTH Royal Institute of Technology, Stockholm, Sweden g Universit´e Paris-Saclay, Commissariat `a l’´Energie Atomique et aux ´Energies Alternatives (CEA), Gif-sur-Yvette, France h Institut Lumi`ere Mati`ere (ILM), Centre National de la Recherche Scientifique, Lyon, France i Joint Research Centre (JRC)- European Commission, Petten, the Netherlands j Universitat Polit`ecnica de Catalunya, Barcelona, Spain k Laboratoire d’Etude des Microstructures (LEM), Centre National de la Recherche Scientifique, Chˆatillon, France l United Kingdom Atomic Energy Authority (UKAEA), Culham, UK m Narodowe Centrum Badan Jadrowych (NCBJ), Swierk, Poland n Normandie Univ, UNIROUEN, INSA Rouen, CNRS, Groupe de Physique des Mat´eriaux, 76000 Rouen, France o Aalto University, Espoo, Finland p Coventry University, UK q Universit´e de Li`ege, Belgium r Helsingin Yliopisto, Helsinki, Finland s Centrum Vyzkumu ˇReˇz S.R.O. (CVR), ˇReˇz, Czech Republic t Centre pour l’´Elaboration Elaboration de Mat´eriaux et pour l’´Etude des Structures (CEMES), Centre National de la Recherche Scientifique, Toulouse, France u Institut za Fiziku, Zagreb, Croatia v Paul Scherrer Institut (PSI), Villingen, Switzerland w Middle East Technical University (METU), Ankara, Turkey

At the atomic scale, uranium-plutonium mixed oxides (U,Pu)O_2 are characterized by cationic chemical disorder, which entails that U and Pu cations are randomly distributed on the cation sublattice. In the present work, we study the impact of disorder on point-defect formation energies in (U,Pu)O_2 using interatomic-potential and Density Functional Theory (DFT+U) calculations. We focus on bound Schottky defects (BSD) that are among the most stable defects in these oxides. As a first step, we estimate the distance R_D around the BSD up to which the local chemical environment significantly affects their formation energy. To this end, we propose an original procedure in which the formation energy is computed for several supercells at varying levels of disorder. We conclude that the first three cation shells around the BSD have a non-negligible influence on their formation energy (R_{D} = 7.0 \{AA}). We apply then a systematic approach to compute the BSD formation energies for all the possible cation configurations on the first and second nearest neighbor shells around the BSD. We show that the formation energy can range in an interval of 0.97 eV, depending on the relative amount of U and Pu neighboring cations. Based on these results, we propose an interaction model that describes the effect of nominal and local composition on the BSD formation energy. Finally, the DFT+U benchmark calculations show a satisfactory agreement for configurations characterized by a U-rich local environment, and a larger mismatch in the case of a Pu-rich one. In summary, this work provides valuable insights on the properties of BSD defects in (U,Pu)O_2, and can represent a valid strategy to study point defect properties in disordered compounds., 33 pages, 20 figures

We present the idea and illustrate potential benefits of having a tool chain of closely related regular, unscreened and screened hybrid exchange-correlation (XC) functionals, all within the consistent formulation of the van der Waals density functional (vdW-DF) method [JPCM 32, 393001 (2020)]. Use of this chain of nonempirical XC functionals allows us to map when the inclusion of truly nonlocal exchange and of truly nonlocal correlation is important. Here we begin the mapping by addressing hard and soft material challenges: magnetic elements, perovskites, and biomolecular problems. We also predict the structure and polarization for a ferroelectric polymer. To facilitate this work and future broader explorations, we furthermore present a stress formulation for spin vdW-DF and illustrate use of a simple stability-modeling scheme to assert when the prediction of a soft mode (an imaginary-frequency vibrational mode, ubiquitous in perovskites and soft matter) implies a prediction of an actual low-temperature transformation., 23 Pages, 8 figures

In the present work, the impact of phosphorus impurities on the grain boundary strength of nickel has been investigated by means of density functional theory (DFT) modelling. Owing to different outcomes and trends previously reported in the literature, it is unclear whether P is strengthening or weakening the Ni grain boundary. To address this issue, we utilize three different DFT based methods: the excess-energy approach, rigid grain separation, and Rice–Wang’s thermodynamic approach. The results show that the commonly used rigid model predicts P to have an increasing effect on the peak stress of Ni of up to 14%, as opposed to a reduction, which is indicated by the excess-energy approach. Employment of the Rice–Wang approach, on the other hand, displays a slight reduction in work of separation. The results show that the discrepancies between previous works can be attributed not so much to the physics of the system, but to the applied model, the partition scheme and the interpretation of the outcomes. This underlines the importance of a proper description of the fracture process, and shows that common simplifications can have a decisive impact on the observed trends.

A full energy range primary radiation damage model is presented here. It is based on the athermal recombination corrected displacements per atom (arc-dpa) model but includes a proper treatment of the near threshold conditions for metallic materials. Both ab initio (AIMD) and classical molecular dynamics (MD) simulations are used here for various metals with body-centered cubic (bcc), face-centered cubic (fcc), and hexagonal close-packed (hcp) structures to validate the model. For bcc and hcp metals, the simulation results fit very well with the model. For fcc metals, although there are slight deviations between the model and direct simulation results, it is still a clear improvement on the arc-dpa model. The deviations are due to qualitative differences in the threshold energy surfaces of fcc metals with respect to bcc and hcp metals according to our classical MD simulations. We introduce the minimum threshold displacement energy (TDE) as a term in our damage model. We calculated minimum TDEs for various metal materials using AIMD. In general, the calculated minimum TDEs are in very good agreement with experimental results. Moreover, we noticed a discrepancy in the literature for fcc Ni and estimated the average TDE of Ni using both classical MD and AIMD. It was found that the average TDE of Ni should be \ensuremath{\sim}70 eV based on simulation and experimental data, not the commonly used literature value of 40 eV. The most significant implications of introducing this full energy range damage model will be for estimating the effect of weak particle-matter interactions, such as for \ensuremath{\gamma}- and electron-radiation-induced damage.

Prediction of material performance in fusion reactor environments relies on computational modelling, and will continue to do so until the first generation of fusion power plants come on line and allow long-term behaviour to be observed. In the meantime, the modelling is supported by experiments that attempt to replicate some aspects of the eventual operational conditions. In 2019, a group of leading experts met under the umbrella of the IEA to discuss the current position and ongoing challenges in modelling of fusion materials and how advanced experimental characterisation is aiding model improvement. This review draws from the discussions held during that workshop. Topics covering modelling of irradiation-induced defect production and fundamental properties, gas behaviour, clustering and segregation, defect evolution and interactions are discussed, as well as new and novel multiscale simulation approaches, and the latest efforts to link modelling to experiments through advanced observation and characterisation techniques. MRG, SLD, and DRM acknowledge funding by the RCUK Energy Programme [grant number EP/T012250/1]. Part of this work has been carried out within the framework of the EUROFusion Consortium and has received funding from the Euratom research and training programme 2014–2018 and 2019–2020 under grant Agreement No. 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. JRT acknowledges funding from the US Department of Energy (DOE) through grant DE-SC0017899. ZB, LY,BDW, and SJZ acknowledge funding through the US DOE Fusion Energy Sciences grant DE-SC0006661ZB, LY and BDW also were partially supported from the US DOE Office of Science, Office of Fusion Energy Sciences and Office of Advanced Scientific Computing Research through the Scientific Discovery through Advanced Computing (SciDAC) project on Plasma-Surface Interactions. JMa acknowledges support from the US-DOEs Office of Fusion Energy Sciences (US-DOE), project DE-SC0019157. Pacific Northwest National Laboratory is operated by Battelle Memorial Institute for the US Department of Energy (DOE) under contract DE-AC05-76RL01830. YO and YZ were supported as part of the Energy Dissipation to Defect Evolution (EDDE), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under contract number DE-AC05-00OR22725. TS and TT are supported by JSPS KAKENHI Grant Number 19K05338.

Iron based industrial steels typically contain a large number of alloying elements, even so-called low alloyed steels. Under irradiation, these alloying elements form clusters that have a detrimental impact of the mechanical properties of the steel. The stability and formation mechanisms of such clusters are presently not fully understood. Therefore, in this work, we study the thermal stability and formation kinetics of small solute clusters in the bcc Fe matrix. We use density functional theory (DFT) to characterize the binding energy of vacancy/solute clusters containing Cr, Mn, Ni, Cu, Si and P, thereby exploring >700 different configurations. The constructed DFT data base is used to fit a cluster expansion (CE) for the vacancy-FeCrMnNiCuSiP system. In turn, the obtained CE is applied in atomistic kinetic Monte Carlo simulations to study the effect of Mn, Ni, Cr, Si and P on the precipitation formation in the FeCu alloy. We conclude that the addition of Mn and Ni delay the precipitation kinetics by an order of magnitude. The additional alloying with traces of P/Si further delays the kinetics by an additional order of magnitude. We found that Si plays an essential role in the formation of spatially mixed MnNiCuSi cluster.

We study the adhesion and tensile behaviour of bi-layer interfaces comprising polyethylene, doped with carbonyl and hydroxyl functional groups emanating from ozone treatment, and α" role="presentation">-Al2O3" role="presentation"> by means of density functional theory and classical atomistic modelling. The results show that the deformations are localized within the polymer and comprise chain slip, disentanglement and detachment from the substrate, where only the latter is notably affected by the doping. The binding energies and excess forces associated with the detachment of functional groups from the alumina substrate are of the order of 1.7 eV and 1 nN, respectively, for both types. Although such forces do not affect the maximum peak stress notably, they give rise to spikes in the traction-separation curves following the fibril formation and promote increased total work of fracture.

International audience; Radiation-induced segregation (RIS) of solutes in materials exposed to irradiation is a well-known problem.It affects the lifetime of nuclear reactor core components by favoring radiation-induced degradation phenomena such as hardening and embrittlement. In this work, RIS tendencies in face centered cubic (fcc) Ni-X (X = Cr, Fe, Ti, Mn, Si, P) dilute binary alloys are examined. The goal is to investigate the driving forces and kinetic mechanisms behind the experimentally observed segregation. By means of ab initio calculations, point-defect stabilities and interactions with solutes are determined, together with migration energies and attempt frequencies.Transport and diffusion coefficients are then calculated in a mean-field framework, to get a full picture of solutedefect kinetic coupling in the alloys. Results show that all solutes considered, with the exception of Cr, prefer vacancy-mediated over interstitial-mediated diffusion during both thermal and radiation-induced migration. Cr, on the other hand, preferentially migrates in a mixed-dumbbell configuration. P and Si are here shown to be enriched, and Fe and Mn to be depleted at sinks during irradiation of the material. Ti and Cr, on the other hand, display a crossover between enrichment at lower temperatures, and depletion in the higher temperature range.Results in this work are compared with previous studies in body centered cubic (bcc) Fe, and discussed in the context of RIS in austenitic alloys.

In this work, a computationally efficient law of a nuclear fuel property obtained from molecular dynamics calculations is implemented in a fuel performance code, bridging the gap between atomic and pellet scales. The Cooper-Rushton-Grimes potential is employed through molecular dynamics simulations to compute the heat capacity of stoichiometric mixed-oxide fuels U1-yPuyO2 from 1000 K to the melting temperature, and over the whole range of plutonium content from pure UO2 (y=0) to pure PuO2 (y=1). The heat capacity is found to exhibit a peak at the temperature TB = 0.84 Tm, with Tm the melting temperature, for all the compounds with a significant effect of the plutonium content solely at high temperatures (T > 1800 K), i.e. around the peak. This peak is related to the so-called Bredig transition known to occur around 0.8 Tm. An analytical law of heat capacity Cp(T,y) has been established from our molecular dynamics data, and is valid from 700 K to the melting temperature and for the entire range of plutonium content. Concerning UO2, a good agreement is found between our calculations and the most recent experimental measurements. The law we propose in the present study has been implemented in GERMINAL V2, the fuel performance code developed by CEA to predict the in-pile behavior of mixed-oxide fuel in sodium-cooled fast reactors, and tested through the simulations of transient power test operated in the CABRI reactor. The results show that our law yields a lower margin to melting in the case of Reactivity-Initiated Accident, but throughout power ramp transient tests, the results obtained with our new law are very consistent with the reference heat capacity law currently used in GERMINAL.

We study the stability, magnetic order, charge segregation, and electronic properties of a novel three-layered Fe3O4 film by means of Hubbard-corrected density functional theory calculations. The stable film is predicted to consist of close-packed iron and oxygen layers, comprising a center layer with octahedrally coordinated Fe sandwiched between two layers with tetrahedrally coordinated Fe. The film exhibits an antiferromagnetic type I spin order. A charge analysis confirms that the stable structure has distinct charge segregation, with Fe2+ ions in the center layer and Fe3+ in the tetrahedral surface layers. Examination of the electronic band structures and densities of states shows that the bandgap is substantially reduced, from 2.4 eV for the bulk rocksalt to 0.3 eV for the film. The reduction in the bandgap is a consequence of the 2+ to 3+ change in oxidation state of Fe in the surface layers.

Point defects in body-centred cubic Fe, Cr and concentrated random magnetic Fe-Cr are investigated using density functional theory and theory of elasticity. The volume of a substitutional Cr atom in ferromagnetic bcc Fe is approximately 18\% larger than the volume of a host Fe atom, whereas the volume of a substitutional Fe atom in antiferromagnetic bcc Cr is 5\% smaller than the volume of a host Cr atom. Elastic dipole $\boldsymbol{P}$ and relaxation volume $\boldsymbol{\Omega}$ tensors of vacancies and self-interstitial atom (SIA) defects exhibit large fluctuations, with vacancies having negative and SIA large positive relaxation volumes. Dipole tensors of vacancies are nearly isotropic across the entire alloy composition range, with diagonal elements $P_{ii}$ decreasing as a function of Cr content. Fe-Fe and Fe-Cr SIA dumbbells are more anisotropic than Cr-Cr dumbbells. Fluctuations of elastic dipole tensors of SIA defects are primarily associated with the variable crystallographic orientations of the dumbbells. Statistical properties of tensors $\boldsymbol{P}$ and $\boldsymbol{\Omega}$ are analysed using their principal invariants, suggesting that point defects differ significantly in alloys containing below and above 10\% at. Cr. The relaxation volume of a vacancy depends sensitively on whether it occupies a Fe or a Cr lattice site. A correlation between elastic relaxation volumes and magnetic moments of defects found in this study suggests that magnetism is a significant factor influencing elastic fields of defects in Fe-Cr alloys., Comment: 27 pages, 7 tables, 16 figures

Hydrogen uptake and accommodation into U3Si2, a candidate accident- tolerant fuel system, has been modelled on the atomic scale using the density functional theory. The solution energy of multiple ...

International audience; Neutrons and high energy ions create displacement cascades in materials, which have been simulated using Molecular Dynamics, for many decades now. The breakthrough of this work is to explore in a large statistics of more than 70 0 0 cascades the relation between early cascade morphology and the final primary damage using a multivariate multiple regression analysis. For two energies in Fe and W, the total number of defects, the number of SIA and vacancy clusters and their full size distributions have been characterized using a multivariate multiple linear regression analysis based on 7 descriptors of the primary damage and 3 morphology descriptors. We find that the combination of the volume and the sphericity is significant. This analysis highlights several cascade properties, among them, that the large and spherical cascades create less defects and in particular, less mono defects than small and fragmented ones. 13 interatomic potentials differing either by their equilibrium part or the way they were hardened have been included in this study and the multivariate analysis shows that the choice of potential has a limited influence on the total number of defects but a large one on the number of mono vacancies. On average, soft potentials create cascades of larger volume, smaller sphericity and producing more defects than hard potentials. Finally, the formation of vacancy clusters is different in Fe than in W. In Fe, the fraction of vacancies in clusters is larger than that of SIAs and larger vacancy clusters are created than SIA clusters. In W, it is the opposite. The reasons are the differences of stopping power and threshold displacement energies, which result in different spatial distributions of open volumes that form during the expansion stage of the cascade. ?

The effect of externally applied stress on the dislocation bias factor (BF) in bcc iron has been studied using a combination of atomistic static calculations and finite element integration. Three kinds of dislocations were considered, namely, a0/2〈1 1 1〉{1 1 0} screw, a0/2〈1 1 1〉{1 1 0} edge and a0〈1 0 0〉{0 0 1} edge dislocations. The computations reveal that the isotropic crystal expansion leads to an increasing or constant dislocation bias, depending on the Burgers vector and type of dislocation. On the other hand, compressive stress reduces the dislocation bias for all the dislocations studied. Variation of the dislocation BF depending on dislocation type and Burgers vector is discussed by analysing the modification of the interaction energy landscape and the capture efficiency values for the vacancy and self-interstitial atom.

Injection-moulding is one of the most common manufacturing processes used for polymers. In many applications, the mechanical properties of the product is of great importance. Injection-moulding of thin-walled polymer products tends to leave the polymer structure in a state where the mechanical properties are anisotropic, due to alignment of polymer chains along the melt flow direction. The anisotropic elastic-viscoplastic properties of low-density polyethylene, that has undergone an injection-moulding process, are therefore examined in the present work. Test specimens were punched out from injection-moulded plates and tested in uniaxial tension. Three in-plane material directions were investigated. Because of the small thickness of the plates, only the in-plane properties could be determined. Tensile tests with both monotonic and cyclic loading were performed, and the local strains on the surface of the test specimens were measured using image analysis. True stress vs. true strain diagrams were constructed, and the material response was evaluated using an elastic-viscoplasticity law. The components of the anisotropic compliance matrix were determined together with the direction-specific plastic hardening parameters. © 2017 The Author(s) Open access

Nanoindentation, in combination with scanning probe microscopy, has been used to measure the hardness and Youngs modulus in the hydride and matrix of a high burn-up neutron-irradiated Zircaloy-2 cl ...

In this work, we develop a rigid lattice cluster expansion as an ultimate goal to track the micro-structural evolution of Eurofer steel under neutron irradiation. The fact that all (defect) structures are mapped upon a rigid lattice allows a simplified computation and fitting procedure, thus enabling alloys of large chemical complexity to be modelled. As a first step towards the chemical complexity of Eurofer steels, we develop a cluster expansion (CE) for the FeCrW-vacancy system based on density functional theory (DFT) calculations in the dilute alloy limit. The DFT calculations suggest that only CrW clusters containing vacancies are stabilised. The cluster expansion was used to simulate thermal annealing in Fe?20Cr?xW alloys at 773 K. It is found that the addition of W to the alloy results in a non-linear decrease in the precipitation kinetics. The CE was found suitable to describe the energetics of the FeCrW-vacancy system in the Fe-rich limit. Fil: Bonny, G.. Nuclear Materials Science Institute; Bélgica Fil: Castin, Nicolas. Nuclear Materials Science Institute; Bélgica Fil: Domain, C.. Les Renardières; Francia Fil: Olsson, P.. KTH Royal Institute of Technology; Suecia Fil: Verreyken, B.. Nuclear Materials Science Institute; Bélgica Fil: Pascuet, Maria Ines Magdalena. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Terentyev, D.. Nuclear Materials Science Institute; Bélgica

This work investigates solute transport due to self-interstitial defects and radiation induced segregation tendencies in dilute ferritic alloys, by computing the transport coefficients of each system based on ab initio calculations of binding energies and migration rates. The implementation of the self-consistent mean field method in the KineCluE code allows to extend the calculation of transport coefficients to arbitrary interaction ranges, crystal structures, and diffusion mechanisms. The results show that the diffusivity of P, Mn, and Cr solute atoms is dominated by the dumbbell mechanism, that of Cu by vacancies, while the two mechanisms might be in competition for Ni and Si, despite the fact that the corresponding mixed dumbbells are not stable. Systematic enrichment at defect sinks is expected for P and Mn solutes due to dumbbell diffusion, and for Si due mainly to vacancy drag. Vacancy drag is also responsible for Cu and Ni enrichment below 1085 K. The RIS behavior of Cr is the outcome of a fine balance between dumbbell enrichment and vacancy depletion. Therefore, for dilute Cr concentrations global enrichment occurs below 540 K, and depletion above. This threshold temperature grows with solute concentration. The findings are in agreement with experimental observations of RIS and clustering phenomena, and confirm that solute-defect kinetic coupling plays an important role in the formation of solute clusters in reactor pressure vessel steels and other alloys., Preprint accepted for publication

This work investigates solute transport due to self-interstitial defects and radiation induced segregation tendencies in dilute ferritic alloys, by computing the transport coefficients of each system based on ab initio calculations of binding energies and migration rates. The implementation of the self-consistent mean field method in the KineCluE code allows to extend the calculation of transport coefficients to arbitrary interaction ranges, crystal structures, and diffusion mechanisms. The results show that the diffusivity of P, Mn, and Cr solute atoms is dominated by the dumbbell mechanism, that of Cu by vacancies, while the two mechanisms might be in competition for Ni and Si, despite the fact that the corresponding mixed dumbbells are not stable. Systematic enrichment at defect sinks is expected for P and Mn solutes due to dumbbell diffusion, and for Si due mainly to vacancy drag. Vacancy drag is also responsible for Cu and Ni enrichment below 1085 K. The RIS behavior of Cr is the outcome of a fine balance between dumbbell enrichment and vacancy depletion. Therefore, for dilute Cr concentrations global enrichment occurs below 540 K, and depletion above. This threshold temperature grows with solute concentration. The findings are in agreement with experimental observations of RIS and clustering phenomena, and confirm that solute-defect kinetic coupling plays an important role in the formation of solute clusters in reactor pressure vessel steels and other alloys.

Uranium silicide, U3Si2, is considered as an advanced nuclear fuel for commercial light water reactors with improved accident tolerance as well as competitive economics. Nd is employed as a local burnup indicator for conventional oxide fuels due, among other reasons, to its low mobility in the UO2 fuel matrix and its high fission product yield. As part of the studies necessary to determine whether Nd can be considered as a candidate burnup indicator in the U3Si2 concept fuel, we investigate the mobility of Nd in U3Si2. In this work, density functional theory (DFT) calculations are performed to predict the most stable accommodation sites of Nd in U3Si2, found to be within the uranium sublattice. Based on DFT calculations of binding energies and migration activation energies, we investigate Nd diffusion by computing the transport coefficients within the framework of the self-consistent mean-field method. Our calculations predict that the diffusion ratio of Nd to U is smaller in U3Si2 than in UO2. Moreover, at the individual maximum centerline temperature of the fuel, the diffusion of Nd in U3Si2 is much slower than in UO2. From this perspective, Nd represents a good candidate burnup indicator, in similarity to that in UO2.

International audience; The primary damage in metallic alloys, i.e. the point defect distribution resulting from the interaction between an energetic particle and a metallic matrix has been investigated for more than 60 years using atomistic simulations. The defect distribution produced in cascades is sensitive to the equilibrium part of the potential as well as its hardened part. An analysis based on large statistics of molecular dynamics simulations and comparison with different embedded atom method potentials in Fe and W allows to rationalize the potential behavior. Correlations between static non-equilibrium properties (quasi static drag (QSD)), threshold displacement energies (TDE), replacement collision sequences (RCS) along the < 110 > direction have been revealed. Along this direction, the lower the TDE, the lower the QSD and the more energy is transmitted along the < 110 > direction during the RCS, i.e. the softest potentials are the ones for which the most energy is transmitted from the PKA to the first head-on atom in the direction of the RCS sequence.

We investigate the variation of elastic stiffness moduli and the thermodynamic properties of yttrium orthosilicate ( Y 2 SiO 5 , YSO) under various doping concentrations of Eu 3 + ions. The model is based on a low temperature approximation (T θ D ) , and the plane-wave density functional theory (DFT) is used to carry out the calculations. The results show that the Eu 3 + ions primarily occupy the Y 1 site of the basic molecule for all applied concentrations. The overall shear, bulk, and Young’s moduli exhibit a decreasing trend with increasing Eu 3 + concentration. The overall anisotropy shows a very small increase with increasing concentration. The Debye temperature as well as the Grunesien parameter for each concentration are predicted. Lastly, the predicted heat capacity at constant volume is calculated and compared to experimental values. Our study reveals that there is almost linear relationship between concentration and mechanical properties of YSO. The decrease of the Grunesien parameter with concentration increase might decrease the anharmonic effects in YSO, although this effect is small. In addition, the change in heat capacity with concentration rise is negligible.

Composite fuels such as UN-UO2 are being considered to address the lower oxidation resistance of the UN fuel from a safety perspective for use in light water reactors, whilst improving the in-reactor behaviour of the more ubiquitous UO2 fuel. An innovative UN-UO2 accident tolerant fuel has recently been fabricated and studied: UN microspheres embedded in UO2 matrix. In the present study, detailed oxidative thermogravimetric investigations (TGA/DSC) of high-density UN/U2N3-UO2 composite fuels (91-97 %TD), as well as post oxidised microstructures obtained by SEM, are reported and analysed. Triplicate TGA measurements of each specimen were carried out at 5 K/min up to 973 K in a synthetic air atmosphere to assess their oxidation kinetics. The mass variation due to the oxidation reactions (%), the oxidation onset temperatures (OOTs), and the maximum reaction temperatures (MRTs) are also presented and discussed. The results show that all composites have similar post oxidised microstructures with mostly intergranular cracking and spalling. The oxidation resistance of the pellet with initially 10 wt% of UN microspheres is surprisingly better than the UO2 reference. Moreover, there is no significant difference in the OOT (~557 K) and MRT (~615 K) when 30 wt% or 50 wt% of embedded UN microspheres are used. Therefore, the findings in this article demonstrate that the UO2 matrix acts as a barrier to improve the oxidation resistance of the nitride phases at the beginning of life conditions.

Uranium nitride (UN)-uranium dioxide (UO2) composite fuels are being considered as an accident tolerant fuel (ATF) option for light water reactors. However, the complexity related to the chemical interactions between UN and UO2 during sintering is still an open problem. Moreover, there is a lack of knowledge regarding the influence of the sintering parameters on the amount and morphology of the α-U2N3 phase formed. In this study, a detailed investigation of the interaction between UN and UO2 is provided and a formation mechanism for the resulting α-U2N3 phase is proposed. Coupled with these analyses, an innovative ATF concept was investigated: UN microspheres and UO2.13 powder were mixed and subsequently sintered by spark plasma sintering. Different temperatures, pressures, times and cooling rates were evaluated. The pellets were characterised by complementary techniques, including XRD, DSC, and SEM-EDS/WDS/EBSD. The UN and UO2 interaction is driven by O diffusion into the UN phase and N diffusion in the opposite direction, forming a long-range solid solution in the UO2 matrix, that can be described as UO2-xNx. The cooling process decreases the N solubility in UO2-xNx, causing then N redistribution and precipitation as α-U2N3 phase along and inside the UO2 grains. This precipitation mechanism occurs at temperatures between 1273 K and 973 K on cooling, following specific crystallographic grain orientation patterns.

In this work we have studied transgranular cleavage and the fracture toughness of titanium hydrides by means of quantum mechanical calculations based on density functional theory. The calculations ...

This work presents a full object kinetic Monte Carlo framework for the simulation of the microstructure evolution of reactor pressure vessel (RPV) steels. The model pursues a "gray-alloy" approach, ...

Uranium nitride (UN) has been considered a potential fuel for Generation IV (GEN-IV) nuclear reactors as well as a possible new fuel for Light Water Reactors (LWR), which would permit an extension ...

Microstructure evolution in thin Cu films during room temperature self-annealing is investigated by means of a mesoscale level set model. The model is formulated such that the relative, or collective, influence of anisotropic grain boundary energy, mobility and heterogeneously distributed stored energy can be investigated. Density functional theory (DFT) calculations are performed in the present work to provide the variation of grain boundary energy for different grain boundary configurations. The stability of the predominant (111) fiber texture in the as-deposited state is studied as well as the stability of some special low-Σ grain boundaries. Further, the numerical model allows tracing of the grain size distribution and occurrence of abnormal grain growth during self-annealing. It is found that abnormal grain growth depends mainly on the presence of stored energy variations, whereas anisotropic grain boundary energy or mobility is insufficient to trigger any abnormal growth in the model. However, texture dependent grain boundary properties, mobility in particular, contribute to an increased content of low-Σ boundaries in the annealed microstructure. The increased presence of such boundaries is also promoted by stored energy variations. In addition, if the stored energy variations are sufficient the coexisting (111) and (001) texture components in the as-deposited state will evolve into a (001) dominated texture during annealing. Further, it is found that whereas stored energy variations promote the stability of the (001) texture component, anisotropic grain boundary energy and mobility tend to work the other way and stabilize the (111) component at the expense of (001) grains.

In this work we report the results of an ab initio study of the influence of hydrogen filled vacancies on the mechanical properties of zirconium. The modelling shows that hydrogen filled vacancies contribute to a lowering of the surface energy and an increase in the unstable stacking fault energy, which implies a reduction in ductility. The increase in unstable stacking fault energy suggests that the defects promote a change in the dislocation glide mechanism from prismatic to basal slip. To investigate the cleavage energetics, we model the decohesion process. For describing the interplanar interaction we adopt an extended version of Rose's universal binding energy relation, which is found to reproduce the behaviour accurately. The results of the modelling imply that the work of fracture and peak stress decrease as a result of the presence of hydrogen filled vacancies.

In this study, the process involved in the fabrication of a potential accident tolerant fuel is described. Homogeneous uranium nitride microspheres doped with different thorium content were success ...

The journal of chemical physics 152(11), 114705 (1-9) (2020). doi:10.1063/1.5142558, We have investigated the structure of an ultrathin iron oxide phase grown on Ag(100) using surface x-ray diffraction in combination with Hubbard-corrected density functional theory (DFT+U) calculations. The film exhibits a novel structure composed of one close-packed layer of octahedrally coordinated Fe$^{2+}$ sandwiched between two close-packed layers of tetrahedrally coordinated Fe$^{3+}$ and an overall stoichiometry of Fe$_3$O$_4$. As the structure is distinct from bulk iron oxide phases and the coupling with the silver substrate is weak, we propose that the phase should be classified as a metastable two-dimensional oxide. The chemical and physical properties are potentially interesting, thanks to the predicted charge ordering between atomic layers, and analogy with bulk ferrite spinels suggests the possibility of synthesis of a whole class of two-dimensional ternary oxides with varying electronic, optical, and chemical properties., Published by American Institute of Physics, Melville, NY

Machine learning, and more specifically artificial neural networks (ANN), are powerful and flexible numerical tools that can lead to significant improvements in many materials modelling techniques. This paper provides a review of the efforts made so far to describe the effects of irradiation in Fe-based and W-based alloys, in a multiscale modelling framework. ANN were successfully used as innovative parametrization tools in these models, thereby greatly enhancing their physical accuracy and capability to accomplish increasingly challenging goals. In the provided examples, the main goal of ANN is to predict how the chemical complexity of local atomic configurations, and/or specific strain fields, influence the activation energy of selected thermally-activated events. This is most often a more efficient approach with respect to previous computationally heavy methods. In a future perspective, similar schemes can be potentially used to calculate other quantities than activation energies. They can thus transfer atomic-scale properties to higher-scale simulations, providing a proper bridging across scales, and hence contributing to the achievement of accurate and reliable multiscale models. Fil: Castin, Nicolas. Studie Centrum voor Kerneenergie; Bélgica Fil: Pascuet, Maria Ines Magdalena. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Messina, L.. Royal Institute of Technology; Suecia Fil: Domain, C.. Électricité de France; Francia Fil: Olsson, P.. Royal Institute of Technology; Suecia Fil: Pasianot, Roberto Cesar. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica; Argentina Fil: Malerba, L.. Studie Centrum voor Kerneenergie; Bélgica

The formation of a second phase in presence of a crack in a crystalline material is modelled and studied for different prevailing conditions in order to predict and a posteriori prevent failure, e.g. by delayed hydride cracking. To this end, the phase field formulation of Ginzburg-Landau is selected to describe the phase transformation, and simulations using the finite volume method are performed for a wide range of material properties. A sixth order Landau potential for a single structural order parameter is adopted because it allows the modeling of both first and second order transitions and its corresponding phase diagram can be outlined analytically. The elastic stress field induced by the crack is found to cause a space-dependent shift in the transition temperature, which promotes a second-phase precipitation in vicinity of the crack tip. The spatio-temporal evolution during nucleation and growth is successfully monitored for different combinations of material properties and applied loads. Results for the second-phase shape and size evolution are presented and discussed for a number of selected characteristic cases. The numerical results at steady state are compared to mean-field equilibrium solutions and a good agreement is achieved. For materials applicable to the model, the results can be used to predict the evolution of an eventual second-phase formation through a dimensionless phase transformation in the crack-tip vicinity for given conditions.

© 2018 Elsevier Ltd In the present work we have performed classical molecular dynamics modelling to investigate the effects of different types of force-fields on the stress-strain and yielding behaviours in semi-crystalline lamellar stacked linear polyethylene. To this end, specifically the all-atomic optimized potential for liquid simulations (OPLS-AA) and the coarse-grained united-atom (UA) force-fields are used to simulate the yielding and tensile behaviour for the lamellar separation mode. Despite that the considered samples and their topologies are identical for both approaches, the results show that they predict widely different stress-strain and yielding behaviours. For all UA simulations we obtain oscillating stress-strain curves accompanied by repetitive chain transport to the amorphous region, along with substantial chain slip and crystal reorientation. For the OPLS-AA modelling primarily cavitation formation is observed, with small amounts of chain slip to reorient the crystal such that the chains align in the tensile direction. This force-field dependence is rooted in the lack of explicit H-H and C-H repulsion in the UA approach, which gives rise to underestimated ideal critical resolved shear stress. The computed critical resolved shear stress for the OPLS-AA approach is in good agreement with density functional theory calculations and the yielding mechanisms resemble those of the lamellar separation mode. The disparate energy and shear stress barriers for chain slip of the different models can be interpreted as differently predicted intrinsic activation rates for the mechanism, which ultimately are responsible for the observed diverse responses of the two modelling approaches.

We study the phase stability and martensitic transformation of orthorhombic and monoclinic polyethylene by means of density functional theory using the non-empirical consistent-exchange vdW-DF-cx functional [Phys. Rev. B $\mathbf{89}$, 035412 (2014)]. The results show that the orthorhombic phase is the most stable of the two. Owing to the occurrence of soft librational phonon modes, the monoclinic phase is predicted not to be stable at zero pressure and temperature, but becomes stable when subjected to compressive transverse deformations that pin the chains and prevent them from wiggling freely. This theoretical characterization, or prediction, is consistent with the fact that the monoclinic phase is only observed experimentally when the material is subjected to mechanical loading. Also, the estimated threshold energy for the combination of lattice deformation associated with the T1 and T2 transformation paths (between the orthorhombic and monoclinic phases) and chain shuffling is found to be sufficiently low for thermally activated back transformations to occur. Thus, our prediction is that the crystalline part can transform back from the monoclinic to the orthorhombic phase upon unloading and/or annealing, which is consistent with experimental observations. Finally, we observe how a combination of such phase transformations can lead to a fold-plane reorientation from {110} to {100}-type in a single orthorhombic crystal.

A systematic study of dislocation bias has been performed using a method that combines atomistic and elastic dislocation-point defect interaction models with a numerical solution of the diffusion equation with a drift term. Copper, nickel and aluminium model lattices are used in this study, covering a wide range of shear moduli and stacking fault energies. It is found that the dominant parameter for the dislocation bias in fcc metals is the width of the stacking fault ribbon. The variation in elastic constants does not strongly impact the dislocation bias value. As a result of this analysis and its extrapolation, the dislocation bias of the widely applied austenitic stainless steels of 316 type is predicted to be about 0.1 at temperature close to the swelling peak (815 K) and typical dislocation density of 1014 m-2. This is in line with the bias calculated using the elastic interaction model, which implies that the prediction method can be used readily in other fcc systems even without EAM potentials. By comparing the bias values obtained using atomistic- and elastic interaction energies, about 20% discrepancy is found, therefore a more realistic bias value for the 316 type alloy is 0.08 in these conditions. Updated from submitted to published.QC 20151021

Dislocation bias factors in bcc Fe have been calculated based on atomistic interaction energy maps on three kinds of dislocations, namely the a0/2〈1 1 1〉{1 1 0} screw, a0/2〈1 1 1〉{1 1 0} and a0〈1 0 0〉{0 0 1} edge dislocations. The results show that the dislocation bias is higher for the a0/2〈1 1 1〉 edge dislocation than for the a0〈1 0 0〉 edge dislocation, even though the latter possesses a larger Burgers vector. This indicates the importance of the dislocation core contribution. For the a0/2〈1 1 1〉{1 1 0} screw dislocation, a negative dislocation bias has been obtained, which implies a more efficient absorption of vacancies than of SIAs. The effect of coexistence of both edge- and screw dislocations are assessed by a total bias. A possible complementary mechanism for explaining the long swelling incubation time in bcc metals is suggested and discussed. QC 20150331

The purpose of this work is to investigate how well the temperature dependence of the elastic constants of single crystal zirconium, magnesium and gold are reproduced by ab initio density functional theory (DFT). The modelling was conducted via the quasi-harmonic approximation with the exchange–correlation functional based on the local density approximation. For gold and magnesium, the low and high temperature dependency agrees well with measurements, whereas the transition between low and high temperature ranges occurs over a wider range of temperature than observed experimentally. For zirconium, the simulations qualitatively predict the temperature dependence of the isentropic elastic constants, where C 12 and C 13 increase with increasing temperature. Because this behaviour is absent for the isothermal elastic constants, the increase can be attributed to the transformation from isothermal to isentropic elastic constants.

We report the results of an ab initio-based density functional theory study of the thermodynamic and structural properties of titanium hydrides. The thermodynamic modelling contains contributions from both vibrational and electronic excitations to the free energy and is conducted using the quasi-harmonic approximation (QHA). The enthalpy, entropy and heat capacity are computed over a wide range of temperature ( 0 ≲ T ⩽ 1000 K) and found to concur well with available experimental data. The simulations show that the Debye temperature varies significantly with temperature below about 50 K demonstrating that the Debye model is too simplistic for thermodynamic modelling throughout the entire temperature spectrum. Comparing the Debye temperature from QHA with that calculated from low temperature elastic constants reveals limited agreement, which suggests that the actual frequency distribution characteristics do not comply with that of the Debye model. The calculated thermodynamic properties are found to show many similarities to those of zirconium hydrides, which are discussed.

Iron-Chromium-Aluminium (Fe-Cr-Al) alloys have been widely investigated as candidate materials for various nuclear applications. Albeit the excellent corrosion resistance, conventional Fe-Cr-Al alloys suffer from alpha-alpha' phase separation and embrittlement when subjected to temperatures up to 500 degrees C, due to their high Cr-content. Low-Cr Fe-Cr-Al alloys are anticipated to be embrittlement resistant and provide adequate oxidation properties, yet long-term aging experiments and simulations are lacking in literature. In this study, Fe-10Cr-(4-8)Al alloys and a Fe-21Cr-5Al were thermally aged in the temperature interval of 450-550 degrees C for times up to 10,000 h, and the microstructures were evaluated mainly using atom probe tomography. In addition, a Kinetic Monte Carlo (KMC) model of the Fe-Cr-Al system was developed. No phase separation was observed in the Fe-10Cr-(4-8)Al alloys, and the developed KMC model yielded results in good agreement with the experimental data.