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  • 1. Abrikosov, I. A.
    et al.
    Olsson, Pär
    Department of Materials and Mechanics of Components, EDF RandD, les Renardières, Moret-sur-Loing, France.
    Ponomareva, A. V.
    Correlation between electronic structure, magnetism and physical properties of Fe-Cr alloys: Ab initio modeling2008In: MATERIALS ISSUES FOR GENERATION IV SYSTEMS: STATUS, OPEN QUESTIONS AND CHALLENGES, 2008, p. 153-168Conference paper (Refereed)
    Abstract [en]

    We review recent developments in the field of ab initio electronic structure theory and its application for studies of phase stability of alloy systems. Basic ideas behind state-of-the-art techniques for first-principles theoretical simulations of the phase stabilities and properties of intermetallic compounds and alloys based on the density functional theory are outlined. We concentrate on methods that allow for an efficient treatment of disorder effects, and illustrate their predictive power for the case of Fe-Cr system. We show that in the ferromagnetic alloys there are peculiarities of the mixing enthalpy in the low-Cr region in the bee phase. Thus the stability of the Cr containing steels stems from the negative mixing enthalpy at low concentrations of chromium. We explain the effect by the strong concentration dependence of the interatomic interactions in Fe-Cr system.

  • 2. Bakaev, A.
    et al.
    Terentyev, D.
    Chang, Z.
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Posselt, M.
    Olsson, Pär
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Zhurkin, E. E.
    Effect of isotropic stress on dislocation bias factor in bcc iron: an atomistic study2018In: Philosophical Magazine, ISSN 1478-6435, E-ISSN 1478-6443, Vol. 98, no 1, p. 54-74Article in journal (Refereed)
    Abstract [en]

    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. 

  • 3. Becquart, C. S.
    et al.
    Ngayam Happy, R.
    Olsson, P.
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Domain, C.
    A DFT study of the stability of SIAs and small SIA clusters in the vicinity of solute atoms in Fe2018In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 500, p. 92-109Article in journal (Refereed)
    Abstract [en]

    The energetics, defect volume and magnetic properties of single SIAs and small SIA clusters up to size 6 have been calculated by DFT for different configurations like the parallel 〈110〉 dumbbell, the non parallel 〈110〉 dumbbell and the C15 structure. The most stable configurations of each type have been further analyzed to determine the influence on their stability of various solute atoms (Ti, V, Cr, Mn, Co, Ni, Cu, Mo, W, Pd, Al, Si, P), relevant for steels used under irradiation. The results show that the presence of solute atoms does not change the relative stability order among SIA clusters. The small SIA clusters investigated can bind to both undersized and oversized solutes. Several descriptors have been considered to derive interesting trends from results. It appears that the local atomic volume available for the solute is the main physical quantity governing the binding energy evolution, whatever the solute type (undersized or oversized) and the cluster configuration (size and type).

  • 4.
    Björkas, C.
    et al.
    Accelerator Laboratory, University of Helsink.
    Nordlund, K.
    Accelerator Laboratory, University of Helsink.
    Malerba, L.
    SCKCEN, The Belgian Nuclear Research Centre.
    Terentyev, D.
    SCKCEN, The Belgian Nuclear Research Centre.
    Olsson, Pär
    Departément Matériaux et Méchanique des Composants, Electricité de France.
    Simulation of displacement cascades in Fe90Cr10 using a two band model potential2008In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 372, no 2-3, p. 312-317Article in journal (Refereed)
    Abstract [en]

    Molecular dynamics (MD) simulations of displacement cascades, with recoil energies up to 50 keV, have been performed in Fe90Cr10 and Fe using a recently developed two band embedded atomic model (2BM) potential that correctly describes the mixing enthalpy and the binding energy of the mixed dumbbell configurations. Comparisons between results obtained with the 2BM potential fitted to different data sets, a one band model (1BM), and another existing FeCr-potential previously used for similar calculations were done, showing differences in the vacancy clustered fraction and the Cr content in interstitials predicted by the potentials. The 2BM potential resulted in roughly the same concentration of Cr in interstitial positions as in the matrix, and the 1BM, which incorrectly predicts a positive heat of mixing, predicted even smaller concentrations. The calculated short range order parameter is around zero for the 2BM, and takes positive values within the 1BM, as expected from the mixing enthalpies.

  • 5. Bonny, G.
    et al.
    Bakaev, A.
    Olsson, Pär
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Domain, C.
    Zhurkin, E. E.
    Posselt, M.
    Interatomic potential to study the formation of NiCr clusters in high Cr ferritic steels2017In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 484, p. 42-50Article in journal (Refereed)
    Abstract [en]

    Under irradiation NiSiPCr clusters are formed in high-Cr ferritic martensitic steels as well as in FeCr model alloys. In the literature little is known about the origin and contribution to the hardening of these clusters. In this work we performed density functional theory (DFT) calculations to study the stability of small substitutional NiCr-vacancy clusters and interstitial configurations in bcc Fe. Based on DFT data and experimental considerations a ternary potential for the ferritic FeNiCr system was developed. The potential was applied to study the thermodynamic stability of NiCr clusters by means of Metropolis Monte Carlo (MMC) simulations. The results of our simulations show that Cr and Ni precipitate as separate fractions and suggest only a limited synergetic effect between Ni and Cr. Therefore our results suggest that the NiCrSiP clusters observed in experiments must be the result of other mechanisms than the synergy of Cr and Ni at thermal equilibrium.

  • 6. Bonny, G.
    et al.
    Castin, N.
    Domain, C.
    Olsson, Pär
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Verreyken, B.
    Pascuet, M. I.
    Terentyev, D.
    Density functional theory-based cluster expansion to simulate thermal annealing in FeCrW alloys2017In: Philosophical Magazine, ISSN 1478-6435, E-ISSN 1478-6443, Vol. 97, no 5, p. 299-317Article in journal (Refereed)
    Abstract [en]

    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.

  • 7. Bonny, G.
    et al.
    Pasianot, R. C.
    Malerba, L.
    Caro, A.
    Olsson, Pär
    vermore, CA 94550, United States f Department Matériaux et Mécanique des Composants, Electricité de France.
    Lavrentiev, M. Yu.
    Numerical prediction of thermodynamic properties of iron-chromium alloys using semi-empirical cohesive models: The state of the art2009In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 385, no 2, p. 268-277Article in journal (Refereed)
    Abstract [en]

    In this work the capability of existing cohesive models to predict the thermodynamicproperties of Fe-Cr alloys are critically evaluated and compared. The two-band model and the concentration-dependent model, which are independently developed extensions of the embedded-atom method, are demonstrated to be equivalent and equally capable of reproducing the thermodynamic properties of Fe-Cr alloys. The existing potentials fitted with these formalisms are discussed and compared with an existing cluster expansionmodel. The phase diagram corresponding to these models is evaluated using different but complementary methods. The influence of mixing enthalpy, low-energy states and vibrational entropy on the phase diagram is examined for the different cohesive models.

  • 8. Bonny, G.
    et al.
    Domain, C.
    Castin, N.
    Olsson, Pär
    KTH, School of Engineering Sciences (SCI), Physics, Nuclear Engineering.
    Malerba, L.
    The impact of alloying elements on the precipitation stability and kinetics in iron based alloys: An atomistic study2019In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 161, p. 309-320Article in journal (Refereed)
    Abstract [en]

    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

  • 9. Castin, N.
    et al.
    Messina, L.
    Domain, C.
    Pasianot, R. C.
    Olsson, Pär
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Improved atomistic Monte Carlo models based on ab-initio -trained neural networks: Application to FeCu and FeCr alloys2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 21, article id 214117Article in journal (Refereed)
    Abstract [en]

    We significantly improve the physical models underlying atomistic Monte Carlo (MC) simulations, through the use of ab initio fitted high-dimensional neural network potentials (NNPs). In this way, we can incorporate energetics derived from density functional theory (DFT) in MC, and avoid using empirical potentials that are very challenging to design for complex alloys. We take significant steps forward from a recent work where artificial neural networks (ANNs), exclusively trained on DFT vacancy migration energies, were used to perform kinetic MC simulations of Cu precipitation in Fe. Here, a more extensive transfer of knowledge from DFT to our cohesive model is achieved via the fitting of NNPs, aimed at accurately mimicking the most important aspects of the ab initio predictions. Rigid-lattice potentials are designed to monitor the evolution during the simulation of the system energy, thus taking care of the thermodynamic aspects of the model. In addition, other ANNs are designed to evaluate the activation energies associated with the MC events (migration towards first-nearest-neighbor positions of single point defects), thereby providing an accurate kinetic modeling. Because our methodology inherently requires the calculation of a substantial amount of reference data, we design as well lattice-free potentials, aimed at replacing the very costly DFT method with an approximate, yet accurate and considerably more computationally efficient, potential. The binary FeCu and FeCr alloys are taken as sample applications considering the extensive literature covering these systems.

  • 10.
    Castin, N.
    et al.
    Studie Ctr Kerneenergie, Ctr Etud Energie Nucl SCK CEN, NMS Unit, Boeretang 200, B-2400 Mol, Belgium..
    Pascuet, M. I.
    Consejo Nacl Invest Cient & Tecn CONICET, Godoy Cruz 2290 C1425FQB CABA, Buenos Aires, DF, Argentina..
    Messina, Luca
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Domain, C.
    EDF R&D, Dept Mat & Mecan Composants, F-77250 Moret Sur Loing, France..
    Olsson, Pär
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Pasianot, R. C.
    Consejo Nacl Invest Cient & Tecn CONICET, Godoy Cruz 2290 C1425FQB CABA, Buenos Aires, DF, Argentina.;Comis Nacl Energia Atom CNEA, Gcia Mat, Av Gral Paz 1499, RA-1650 San Martin, Argentina..
    Malerba, L.
    Studie Ctr Kerneenergie, Ctr Etud Energie Nucl SCK CEN, NMS Unit, Boeretang 200, B-2400 Mol, Belgium..
    Advanced atomistic models for radiation damage in Fe-based alloys: Contributions and future perspectives from artificial neural networks2018In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 148, p. 116-130Article in journal (Refereed)
    Abstract [en]

    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.

  • 11.
    Chang, Zhongwen
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Messina, Luca
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Sandberg, Nils
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Yousfi, Amine
    Toijer, Elin
    Thuvander, Mattias
    Boizot, Bruno
    Brysbaert, Gauthier
    Metayer, Vincent
    Gorse-Pomonti, Dominique
    Olsson, Pär
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Electron irradiation accelerated Cu precipitation in cast iron and an FeCu model alloyManuscript (preprint) (Other academic)
  • 12.
    Chang, Zhongwen
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Olsson, Pär
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Sandberg, Nils
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Terentyev, Dmitry
    Interaction Energy Calculations of Edge Dislocation with Point Defects in FCC Cu2013Conference paper (Refereed)
    Abstract [en]

    In order to improve the dislocation bias (DB) model of swelling under irradiation, a large scale of atomistic simulation of the interaction in face centered cubic (FCC) Cu model lattice between an edge dislocation (ED) and point defects such as a vacancy, a self-interstital atom (SIA) have been performed for various configurations. It is found dislocation core splits into partial cores after energy relaxation. Interactions with any SIA conficurations is one order of magnitute larger than with a vacancy. The reason that SIA creats a larger dilatation volumn than the vacancy is directly observed from calculation. Furthurmore, within the interaction range, an octahedron position rather than dumbbell in <100> direction is observed in the stable state after relaxation in interactions between a edge dislocation and a dumbbell SIA. Comparision of interaction energy in analytical and atomistic calculation shows that analytical one has a stronger interaction in vacancy-ED systems, suggesting that the bias factor (BF) from analytical calculation is larger than from atomistic calculation.

  • 13.
    Chang, Zhongwen
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Olsson, Pär
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Terentyev, D
    Nuclear Materials Science Institute, Belgium.
    Sandberg, N
    Swedish Radiation Safety Authority, Sweden.
    Multiscale calculation of dislocation bias in fcc Ni and bcc Fe model lattices2014In: Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, ISSN 0168-583X, E-ISSN 1872-9584Article in journal (Refereed)
    Abstract [en]

    In order to gain more insights on void swelling, dislocation bias is studied in this work. Molecular staticsimulations with empirical potentials are applied to map the dislocation–point defects interactionenergies in both fcc Ni and bcc Fe model lattices. The interaction energies are then used to numericallysolve the diffusion equation and obtain the dislocation bias. The importance of the dislocation core regionis studied under a the temperature range 573–1173 K and the dislocation densities 1012—1015 m-2. Theresults show that larger dislocation bias is found in the fcc Ni than in the bcc Fe under differenttemperatures and dislocation densities. The anisotropic interaction energy model is used to obtain thedislocation bias and the result is compared to that obtained using the atomistic interaction model, thecontribution from the core structure is then shown in both the Ni lattice and the Fe lattice.

  • 14.
    Chang, Zhongwen
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Olsson, Pär
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Terentyev, Dmitry
    Sandberg, Nils
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Dislocation bias factors in fcc copper derived from atomistic calculations2013In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 441, no 1-3, p. 357-363Article in journal (Refereed)
    Abstract [en]

    Atomistic calculations were employed in order to calculate the interaction energy of an edge dislocation with different point defects. The bias factor was calculated by applying a finite element method on the interaction energy landscapes obtained from the atomistic calculations. A comparison of the calculated bias factor with a model based on elasticity theory reveals around 30% discrepancy under conditions representative for electron irradiation at 600 degrees C. Possible reasons are discussed. The bias factor dependence on dislocation density and ambient temperature is presented and discussed.

  • 15.
    Chang, Zhongwen
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Sandberg, Nils
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics. Swedish Radiation Safety Authority, Sweden.
    Dmitry, Terentyev
    Samuelsson, Karl
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Bonny, Giovanni
    Olsson, Pär
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Assessment of the dislocation bias in fcc metals and extrapolation to austenitic steels2015In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 465Article in journal (Refereed)
    Abstract [en]

    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.

  • 16.
    Chang, Zhongwen
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Terentyev, Dmitry
    Nuclear Materials Science Institute, Belgium.
    Samuelsson, Karl
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Olsson, Pär
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Dislocation bias calculations in fcc materialsManuscript (preprint) (Other academic)
  • 17.
    Chang, Zhongwen
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Terentyev, Dmitry
    Nuclear Materials Science Institute, Belgium.
    Sandberg, Nils
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics. Swedish Radiation Safety Authority, Sweden.
    Samuelsson, Karl
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Olsson, Pär
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Anomalous bias factors of dislocations in bcc iron2015In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 461, p. 221-229Article in journal (Refereed)
    Abstract [en]

    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.

  • 18. Chiapetto, Monica
    et al.
    Messina, Luca
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics. Université Paris-Saclay, Gif-sur-Yvette, France.
    Becquart, Charlotte S.
    Olsson, Pär
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Malerba, Lorenzo
    Nanostructure evolution of neutron-irradiated reactor pressure vessel steels: Revised Object kinetic Monte Carlo model2017In: Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, ISSN 0168-583X, E-ISSN 1872-9584, Vol. 393, p. 105-109Article in journal (Refereed)
    Abstract [en]

    This work presents a revised set of parameters to be used in an Object kinetic Monte Carlo model to simulate the microstructure evolution under neutron irradiation of reactor pressure vessel steels at the operational temperature of light water reactors (∼300 °C). Within a “grey-alloy” approach, a more physical description than in a previous work is used to translate the effect of Mn and Ni solute atoms on the defect cluster diffusivity reduction. The slowing down of self-interstitial clusters, due to the interaction between solutes and crowdions in Fe is now parameterized using binding energies from the latest DFT calculations and the solute concentration in the matrix from atom-probe experiments. The mobility of vacancy clusters in the presence of Mn and Ni solute atoms was also modified on the basis of recent DFT results, thereby removing some previous approximations. The same set of parameters was seen to predict the correct microstructure evolution for two different types of alloys, under very different irradiation conditions: an Fe-C-MnNi model alloy, neutron irradiated at a relatively high flux, and a high-Mn, high-Ni RPV steel from the Swedish Ringhals reactor surveillance program. In both cases, the predicted self-interstitial loop density matches the experimental solute cluster density, further corroborating the surmise that the MnNi-rich nanofeatures form by solute enrichment of immobilized small interstitial loops, which are invisible to the electron microscope.

  • 19.
    Claisse, Antoine
    et al.
    KTH.
    Adorno Lopes, Denise
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Olsson, Pär
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Investigation of the ground- and metastable states of AnN (An=Th..Pu)Manuscript (preprint) (Other academic)
  • 20.
    Claisse, Antoine
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Klipfel, Marco
    Lindbom, Niclas
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Freyss, Michel
    Olsson, Pär
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    GGA plus U study of uranium mononitride: A comparison of the U-ramping and occupation matrix schemes and incorporation energies of fission products2016In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 478, p. 119-124Article in journal (Refereed)
    Abstract [en]

    Uranium mononitride is studied in the DFT + U framework. Its ground state is investigated and a study of the incorporation of diverse fission products in the crystal is conducted. The U-ramping and occupation matrix control (OMC) schemes are used to eliminate metastable states. Beyond a certain amount of introduced correlation, the OMC scheme starts to find a lower total energy. The OMC scheme is chosen for the second part of this study. Furthermore, the influence of the magnetic ordering is studied using the U-ramping method, showing that antiferromagnetic order is the most stable one when the U parameter is larger than 1.75 eV. The effect on the density of states is investigated and elastic constants are provided for comparison with other methods and experiments. The incorporation energies of fission products in different defect configurations are calculated and these energies are corrected to take into account the limited size of the supercell.

  • 21.
    Claisse, Antoine
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Olsson, Pär
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    First-principles calculations of (Y, Ti, O) cluster formation in body centred cubic iron-chromium2013In: Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, ISSN 0168-583X, E-ISSN 1872-9584, Vol. 303, p. 18-22Article in journal (Refereed)
    Abstract [en]

    In the present work, the ab initio parametrization necessary for a Monte Carlo study of the (Y, Ti, O) clusters in a FeCr matrix is done. The cohesive, binding and migration energies of all the solutes have been calculated in the dilute limit in the framework of density functional theory. The special case of the strong interaction between an Y atom and a vacancy has been considered. In the dilute limit, Cr is transparent with respect to Y, Ti, O or vacancies. On the contrary, Y binds O strongly in 2NN configuration while not in 1NN. Ti binds O in 1NN and 2NN configurations. A vacancy binds strongly with Y and O in 1NN position which is resulting in a low diffusion coefficient for Y. The peculiar case of the binding attraction between two interstitial oxygen atoms has been studied and is believed to be the main reason for the planar (2D) symmetry of the cluster nuclei. A preferential cluster shape is determined for the early nucleation stage, up to 12 atoms.

  • 22.
    Claisse, Antoine
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Schuler, Thomas
    Lopes, Denise Adorno
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Olsson, Pär
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Transport properties in dilute UN(X) solid solutions (X = Xe, Kr)2016In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, no 17, article id 174302Article in journal (Refereed)
    Abstract [en]

    Uranium nitride (UN) is a candidate fuel for current GEN III fission reactors, for which it is investigated as an accident-tolerant fuel, as well as for future GEN IV reactors. In this study, we investigate the kinetic properties of gas fission products (Xe and Kr) in UN. Binding and migration energies are obtained using density functional theory, with an added Hubbard correlation to model f electrons, and the occupation matrix control scheme to avoid metastable states. These energies are then used as input for the self-consistent mean field method which enables to determine transport coefficients for vacancy-mediated diffusion of Xe and Kr on the U sublattice. The magnetic ordering of the UN structure is explicitly taken into account, for both energetic and transport properties. Solute diffusivities are compared with experimental measurements and the effect of various parameters on the theoretical model is carefully investigated. We find that kinetic correlations are very strong in this system, and that despite atomic migration anisotropy, macroscopic solute diffusivities show limited anisotropy. Our model indicates that the discrepancy between experimental measurements probably results from different irradiation conditions, and hence different defect concentrations.

  • 23. Costa, D.
    et al.
    Adjanor, G.
    Becquart, C. S.
    Olsson, Pär
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics. Etude et Modelisation des Microstructures Pour le Vieillissement des Materiaux, France .
    Domain, C.
    Vacancy migration energy dependence on local chemical environment in Fe-Cr alloys: A Density Functional Theory study2014In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 452, no 1-3, p. 425-433Article in journal (Refereed)
    Abstract [en]

    The first step towards the understanding and the modelling of the Fe-Cr alloy kinetic properties consists in estimating the migration energies related to the processes that drive the microstructure evolution. The vacancy's migration barrier is expected to depend on the vacancy-migrating atom pair atomic environment as pointed out by Nguyen-Manh et al. or Bonny et al. In this paper, we address the issue of the dependence on the vacancy's local atomic environment of both the vacancy migration energy and the configurational energy change Delta E that occurs when the vacancy jumps towards one of its nearest neighbour sites. A DFT approach is used to determine the ground state energy associated to a given configuration of the system. The results are interpreted in the light of the chromium-chromium and chromium-vacancy binding energies as well as the substitutional chromium atoms magnetic properties.

  • 24. De Backer, A.
    et al.
    Sand, A.
    Ortiz, C. J.
    Domain, C.
    Olsson, Pär
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Berthod, E.
    Becquart, C. S.
    Primary damage in tungsten using the binary collision approximation, molecular dynamic simulations and the density functional theory2016In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. T167, article id 014018Article in journal (Refereed)
    Abstract [en]

    The damage produced by primary knock-on atoms (PKA) in W has been investigated from the threshold displacement energy (TDE) where it produces one self interstitial atom-vacancy pair to larger energies, up to 100 keV, where a large molten volume is formed. The TDE has been determined in different crystal directions using the Born-Oppenheimer density functional molecular dynamics (DFT-MD). A significant difference has been observed without and with the semi-core electrons. Classical MD has been used with two different empirical potentials characterized as 'soft' and 'hard' to obtain statistics on TDEs. Cascades of larger energy have been calculated, with these potentials, using a model that accounts for electronic losses (Sand et al 2013 Europhys. Lett. 103 46003). Two other sets of cascades have been produced using the binary collision approximation (BCA): a Monte Carlo BCA using SDTrimSP (Eckstein et al 2011 SDTrimSP: Version 5.00. Report IPP 12/8) (similar to SRIM www.srim.org) and MARLOWE (RSICC Home Page. (https://rsicc.ornl.gov/codes/psr/psr1/psr-137.html) (accessed May, 2014)). The comparison of these sets of cascades gave a recombination distance equal to 12 angstrom which is significantly larger from the one we reported in Hou et al (2010 J. Nucl. Mater. 403 89) because, here, we used bulk cascades rather than surface cascades which produce more defects (Stoller 2002 J. Nucl. Mater. 307 935, Nordlund et al 1999 Nature 398 49). Investigations on the defect clustering aspect showed that the difference between BCA and MD cascades is considerably reduced after the annealing of the cascade debris at 473 K using our Object Kinetic Monte Carlo model, LAKIMOCA.

  • 25. Djurabekova, F.
    et al.
    Malerba, L.
    Pasianot, R. C.
    Olsson, Pär
    Dept. MMC, EDF-RD, Les Renardiéres, France.
    Nordlund, K.
    Kinetics versus thermodynamics in materials modeling: The case of the di-vacancy in iron2010In: Philosophical Magazine, ISSN 1478-6435, E-ISSN 1478-6443, Vol. 90, no 19, p. 2585-2595Article in journal (Refereed)
    Abstract [en]

    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.

  • 26.
    Ejenstam, Jesper
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Thuvander, Mattias
    Olsson, Pär
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Rave, Fernando
    Szakalos, Peter
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Microstructural stability of Fe–Cr–Al alloys at 450–550 °C2015In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 457, p. 291-297Article in journal (Refereed)
    Abstract [en]

    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 α–α′ phase separation and embrittlement when subjected to temperatures up to 500 °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 °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.

  • 27. Hepburn, D. J.
    et al.
    Ackland, G. J.
    Olsson, Pär
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Rescaled potentials for transition metal solutes in α-iron2009In: Philosophical Magazine, ISSN 1478-6435, E-ISSN 1478-6443, Vol. 89, p. 3393-3411Article in journal (Refereed)
    Abstract [en]

    We present semi-empirical potentials for dilute transition metal solutes in α-iron. They are in the Finnis–Sinclair form and are therefore suitable for billion atom molecular dynamics simulations. The potentials have been developed using a rescaling technique to provide solute–iron and solute–solute interactions from an existing iron potential. By fitting to first principles calculations, which show clear trends in the properties of transition metal solutes in iron across the series, we find trends in the rescaling parameters, which we model using simple functions of the occupancy of the d-electron band. We comment on the possibility of utilizing such relationships for the fundamental electronic properties of the solute to create multicomponent potentials for transition metal solutes in α-iron.

  • 28. Kissavos, A. E.
    et al.
    Simak, S. I.
    Olsson, Pär
    Department of Neutron Research, Ångström Laboratory, Uppsala University.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Abrikosov, I. A.
    Total energy calculations for systems with magnetic and chemical disorder2006In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 35, no 1, p. 1-5Article in journal (Refereed)
    Abstract [en]

    The accuracy of the exact muffin-tin orbitals method combined with the coherent potential approximation (EMTO-CPA) for total energy calculations for systems with magnetic and chemical disorder, which is present simultaneously, is investigated. The mixing enthalpy of ordered, as well as disordered FeCo, FeNi, and FeCu equiatomic ferromagnetic alloys is calculated with the EMTO-CPA method and with the full-potential projector augmented wave (PAW) method. The results are compared and found to be in excellent agreement with each other. The EMTO-CPA method, in combination with disordered local moment model, is then applied to calculate the mixing enthalpy of the random paramagnetic face-centered cubic (fcc) FeCo alloy, as well as body-centered cubic (bcc) FeCr and FeV alloys over the whole concentration range. The results are compared with experimental data and a very good agreement is found again.

  • 29. Klaver, T. P. C.
    et al.
    Bonny, G.
    Olsson, P.
    Department of Materials and Mechanics of Components, EDF-R and D, Les Renardières, F-77250 Moret-sur-Loing, France.
    Terentyev, D.
    Benchmarking FeCr empirical potentials against density functional theory data2010In: Modelling and Simulation in Materials Science and Engineering, ISSN 0965-0393, E-ISSN 1361-651X, Vol. 18, no 7, p. 075004-Article in journal (Refereed)
    Abstract [en]

    Three semi-empirical force field FeCr potentials, two within the formalism of the two-band model and one within the formalism of the concentration dependent model, have been benchmarked against a wide variety of density functional theory (DFT) structures. The benchmarking allows an assessment of how reliable empirical potential results are in different areas relevant to radiation damage modelling. The DFT data consist of defect-free structures, structures with single interstitials and structures with small di- and tri-interstitial clusters. All three potentials reproduce the general trend of the heat of formation (h.o.f.) quite well. The most important shortcomings of the original two-band model potential are the low or even negative h.o.f. for Cr-rich structures and the lack of a strong repulsion when moving two solute Cr atoms from being second-nearest neighbours to nearest neighbours. The newer two-band model potential partly solves the first problem. The most important shortcoming in the concentration dependent model potential is the magnitude of the Cr-Cr repulsion, being too strong at short distances and mostly absent at longer distances. Both two-band model potentials do reproduce long-range Cr-Cr repulsion. For interstitials the two-band model potentials reproduce a number of Cr-interstitial binding energies surprisingly well, in contrast to the concentration dependent model potential. For Cr interacting with clusters, the result can sometimes be directly extrapolated from Cr interacting with single interstitials, both according to DFT and the three empirical potentials.

  • 30. Klaver, T. P. C.
    et al.
    Olsson, Pär
    EDF R&D, Dept Mat & Mech Components, F-77250 Les Renardieres, Moret Sur Loing, France .
    Finnis, M. W.
    Interstitials in FeCr alloys studied by density functional theory2007In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 76, no 21, p. 214110-Article in journal (Refereed)
    Abstract [en]

    Density functional theory calculations have been used to study relaxed interstitial configurations in FeCr alloys. The ionic and electronic ground states of 69 interstitial structures have been determined. Interstitials were placed in alloys with up to 14 at. % Cr. Cr atoms were either monatomically dispersed or clustered together within a periodically repeated supercell consisting of 4x4x4 cubes of bcc unit cells. The distance between the interstitials and Cr atoms was varied within the supercells. It is shown that Cr atoms beyond third-nearest-neighbor distance from the interstitial can still have an interaction with it of up to 0.9 eV. The multibody nature of the Cr-Cr interactions causes the Cr-interstitial interaction to be strongly concentration dependent. The Cr-Cr interaction in defect-free alloys is also dependent on the overall Cr concentration. The effective Cr-Cr repulsion is weaker in alloys than in an environment of pure Fe. Apart from the Cr concentration, the Cr-interstitial interaction also depends on the dispersion level of Cr atoms beyond third-nearest-neighbor distance from the interstitial. The formation energy differences between dumbbell interstitials with different orientations are independent of the Cr concentration. We show that the long-range influence of Cr atoms on the interstitial is not due to the interstitial strain field protruding into Cr-rich parts of the supercells. The Fermi-level and band energies were found not to be the sole governing parameter in determining the formation energies. Implications for the construction of empirical potentials are discussed.

  • 31.
    Lagerstedt, Christina
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Terentyev, D.
    Olsson, P.
    Wallenius, Janne
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Malerba, L.
    Cluster formation mechanisms during relaxation and post-relaxation cascade stages in α-Fe:: a molecular dynamics studyArticle in journal (Other academic)
  • 32.
    Lopes, Denise Adorno
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Claisse, Antoine
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Olsson, Pär
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Ab-initio study of C and O impurities in uranium nitride2016In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 478, p. 112-118Article in journal (Refereed)
    Abstract [en]

    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 of the fuel residence time in the reactor. Carbon and oxygen impurities play a key role in the UN microstructure, influencing important parameters such as creep, swelling, gas release under irradiation, compatibility with structural steel and coolants, and thermal stability. In this work, a systematic study of the electronic structure of UN containing C and O impurities using first-principles calculations by the Density Functional Theory (DFT) method is presented. In order to describe accurately the localized U 5f electrons, the DFT + U formalism was adopted. Moreover, to avoid convergence toward metastable states, the Occupation Matrix Control (OMC) methodology was applied. The incorporation of C and O in the N-vacancy is found to be energetically favorable. In addition, only for O, the incorporation in the interstitial position is energetically possible, showing some degree of solubility for this element in this site. The binding energies show that the pairs (C-N-vac) and (O-N-vac) interact much further than the other defects, which indicate the possible occurrence of vacancy drag phenomena and clustering of these impurities in grain boundaries, dislocations and free surfaces. The migration energy of an impurity by single N-vacancy show that C and O employ different paths during diffusion. Oxygen migration requires significantly lower energy than carbon. This fact is due to flexibility in the U-O chemical bonds, which bend during the diffusion forming a pseudo UO2 coordination. On the other hand, C and N have a directional and inflexible chemical bond with uranium; always requiring the octahedral coordination. These findings provide detailed insight into how these impurities behave in the UN matrix, and can be of great interest for assisting the development of this new nuclear fuel for next-generation reactors.

  • 33. Malerba, L.
    et al.
    Ackland, G. J.
    Becquart, C. S.
    Bonny, G.
    Domain, C.
    Dudarev, S. L.
    Fu, C. -C
    Hepburn, D.
    Marinica, M. C.
    Olsson, Pär
    Dept. MMC, EDF-RandD, Site des Renardires, F-77218 Moret-sur-Loing, France.
    Pasianot, R. C.
    Raulot, J. M.
    Soisson, F.
    Terentyev, D.
    Vincent, E.
    Willaime, F.
    Ab initio calculations and interatomic potentials for iron and iron alloys: Achievements within the Perfect Project2010In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 406, no 1, p. 7-18Article in journal (Refereed)
    Abstract [en]

    The objective of the FP6 Perfect Project was to develop a first example of integrated multiscale computational models, capable of describing the effects of irradiation in nuclear reactor components, namely vessel and internals. The use of ab initio techniques to study, in the most reliable way currently possible, atomic-level interactions between species and defects, and the transfer of this knowledge to interatomic potentials, of use for large scale dynamic simulations, lie at the core of this effort. The target materials of the Project were bainitic steels (vessel) and austenitic steels (internals), i.e. iron alloys. In this article, the advances made within the Project in the understanding of defect properties in Fe alloys, by means of ab initio calculations, and in the development of interatomic potentials for Fe and Fe alloys are overviewed, thereby providing a reference basis for further progress in the field. Emphasis is put in showing how the produced data have enhanced our level of understanding of microstructural processes occurring under irradiation in model alloys and steels used in existing nuclear power plants.

  • 34. Malerba, L.
    et al.
    Marinica, M. C.
    Anento, N.
    Björkas, C.
    Nguyen, H.
    Domain, C.
    Djurabekova, F.
    Olsson, Pär
    Dept. MMC, EDF-RandD, Site des Renardires, F-77218 Moret-sur-Loing, France.
    Nordlund, K.
    Serra, A.
    Terentyev, D.
    Willaime, F.
    Becquart, C. S.
    Comparison of empirical interatomic potentials for iron applied to radiation damage studies2010In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 406, no 1, p. 19-38Article in journal (Refereed)
    Abstract [en]

    The performance of four recent semi-empirical interatomic potentials for iron, developed or used within the FP6 Perfect Project, is evaluated by comparing them between themselves and with available experimental or, more often, density functional theory data. The quantities chosen for the comparison are of specific interest for radiation damage studies, i.e. they concern mainly properties of point-defects and their clusters, as well as dislocations. For completeness, an earlier, widely used (also within the Project) iron potential is included in the comparison exercise as well. This exercise allows conclusions to be drawn about the reliability of the available potentials, while providing a snapshot of the state-of-the-art concerning fundamental properties of iron, thereby being also useful as a kind of handbook and as a framework for the validation of future semi-empirical interatomic potentials for iron. It is found that Mendelev-type potentials are currently the best choice in order to "extend density functional theory" to larger scales and this justifies their widespread use, also for the development of iron alloy potentials. However, a fully reliable description of self-interstitial atom clusters and dislocations with interatomic potentials remains largely an elusive objective, that calls for further effort within the concerned scientific community.

  • 35. Malerba, L.
    et al.
    Terentyev, D. A.
    Bonny, G.
    Barashev, A. V.
    Bjrkas, C.
    Juslin, N.
    Nordlund, K.
    Domain, C.
    Olsson, Pär
    EDF-R and D Les Renardieres.
    Sandberg, Nils
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Wallenius, Janne
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Modelling of Radiation Damage in Fe-Cr Alloys2008In: EFFECTS OF RADIATION ON MATERIALS: 23RD INTERNATIONAL SYMPOSIUM / [ed] Lott, RG; Busby, JT, 2008, Vol. 1492, p. 159-176Conference paper (Refereed)
    Abstract [en]

    High-Cr ferritic/martensitic steels are being considered as structural materials for a large number of future nuclear applications, from fusion to accelerator-driven systems and GenIV reactors. Fe-Cr alloys can be used as model materials to investigate some of the mechanisms governing their microstructure evolution under irradiation and its correlation to changes in their macroscopic properties. Focusing on these alloys, we show an example of how the integration of computer simulation and theoretical models can provide keys for the interpretation of a host of relevant experimental observations. In particular we show that proper accounting for two basic features of these alloys, namely, the existence of a fairly strong attractive interaction between self-interstitials and Cr atoms and of a mixing enthalpy that changes sign from negative to positive around 8 to 10 % Cr, is a necessary and, to a certain extent, sufficient condition to rationalize and understand their behavior under irradiation. These features have been revealed by ab initio calculations, are supported by experimental evidence, and have been adequately transferred into advanced empirical interatomic potentials, which have been and are being used for the simulation of damage production, defect behavior, and phase transformation in these alloys. The results of the simulations have been and are being used to parameterize models capable of extending the description of radiation effects to scales beyond the reach of molecular dynamics. The present paper intends to highlight the most important achievements and results of this research activity.

  • 36. Malerba, L.
    et al.
    Terentyev, D.
    Olsson, P.
    Chakarova, Roumiana
    KTH, Superseded Departments, Physics.
    Wallenius, Janne
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Molecular dynamics simulation of displacement cascades in Fe-Cr alloys2004In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 329-33, p. 1156-1160Article in journal (Refereed)
    Abstract [en]

    An embedded atom method (EAM) empirical potential recently fitted and validated for Fe-Cr systems is used to simulate displacement cascades up to 15 keV in Fe and Fe-10%Cr. The evolution of these cascades up to thermalisation of the primary damage state is followed and quantitatively analysed. Particular attention is devoted to assessing the effect of Cr atoms on the defect distribution versus pure Fe. Using the Wigner-Seitz cell criterion to identify point defects, first results show that the main effect of the presence of Cr in the system is the preferential formation of mixed Fe-Cr dumbbells and mixed interstitial clusters, with expected lower mobility than in pure Fe.

  • 37.
    Messina, Luca
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Castin, N.
    Domain, C.
    Olsson, Pär
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Introducing ab initio based neural networks for transition-rate prediction in kinetic Monte Carlo simulations2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 6, article id 064112Article in journal (Refereed)
    Abstract [en]

    The quality of kinetic Monte Carlo (KMC) simulations of microstructure evolution in alloys relies on the parametrization of point-defect migration rates, which are complex functions of the local chemical composition and can be calculated accurately with ab initio methods. However, constructing reliable models that ensure the best possible transfer of physical information from ab initio to KMC is a challenging task. This work presents an innovative approach, where the transition rates are predicted by artificial neural networks trained on a database of 2000 migration barriers, obtained with density functional theory (DFT) in place of interatomic potentials. The method is tested on copper precipitation in thermally aged iron alloys, by means of a hybrid atomistic-object KMC model. For the object part of the model, the stability and mobility properties of copper-vacancy clusters are analyzed by means of independent atomistic KMC simulations, driven by the same neural networks. The cluster diffusion coefficients and mean free paths are found to increase with size, confirming the dominant role of coarsening of medium- and large-sized clusters in the precipitation kinetics. The evolution under thermal aging is in better agreement with experiments with respect to a previous interatomic-potential model, especially concerning the experiment time scales. However, the model underestimates the solubility of copper in iron due to the excessively high solution energy predicted by the chosen DFT method. Nevertheless, this work proves the capability of neural networks to transfer complex ab initio physical properties to higher-scale models, and facilitates the extension to systems with increasing chemical complexity, setting the ground for reliable microstructure evolution simulations in a wide range of alloys and applications.

  • 38.
    Messina, Luca
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Castin, Nicolas
    Domain, Christophe
    Olsson, Pär
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Introducing ab initio-based neural networks for transition-rate prediction in kinetic Monte Carlo simulationsManuscript (preprint) (Other academic)
    Abstract [en]

    This work presents an innovative approach to kinetic Monte Carlo (KMC) simulations, in which atomic transition rates are predicted by an artificial neural network trained on ab initio migration barriers. The method is applied to the parameterization of a hybrid atomistic-object KMC model to simulate copper precipitation during thermal aging in iron. The stability and mobility of copper clusters containing one vacancy is analyzed by means of independent atomistic KMC simulations driven by the same neural network, with the aim of parameterizing the object KMC part of the model. Copper clusters are found to be more stable and mobile with respect to previous studies, and can cover longer diffusion paths, reaching up to a few lattice units. The mean free path increases with cluster size up to around 100 copper atoms. In addition, the emission of the vacancy often occurs concurrently with the emission of one or more copper atoms, because of strong vacancy-copper correlations and kinetic coupling. In the hybrid KMC simulations, the density of copper clusters is overestimated because of the excessively high solution energy predicted by the ab initio method. Nevertheless, this work proves the capability of neural networks to transfer detailed ab initio thermodynamic and kinetic properties to the KMC model, and sets the ground for reliable microstructure evolution simulations in a wide range of alloys.

  • 39.
    Messina, Luca
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Chang, Zhongwen
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Olsson, Pär
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Ab initio modelling of vacancy-solute dragging in dilute irradiated iron-based alloys2013In: Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, ISSN 0168-583X, E-ISSN 1872-9584, Vol. 303, p. 28-32Article in journal (Refereed)
    Abstract [en]

    The formation of solute-defect nanoclusters in RPV steels is the main cause of radiation induced embrittlement. Solute atoms may diffuse in the alloy by a vacancy drag mechanism, depending on the strength of interaction with point defects. A multifrequency model based on ab initio computed migration barriers was applied in order to investigate the possibility of solute drag in iron-based bcc binary alloys containing Ni, Cr, Cu or Mn, and the obtained solute diffusion coefficients were compared with previous experiments. The results show that Ni is expected to be dragged at temperatures below approximately 900 K, while Cr and Mn are not involved in the dragging mechanism. As for Cu, the results are controversial because the computed migration barriers are strongly affected by the particular choice of the ab initio method.

  • 40.
    Messina, Luca
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Chiapetto, Monica
    Olsson, Pär
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Becquart, Charlotte S.
    Malerba, Lorenzo
    An object kinetic Monte Carlo model for the microstructure evolution of neutron-irradiated reactor pressure vessel steels2016In: Physica Status Solidi (a) applications and materials science, ISSN 1862-6300, E-ISSN 1862-6319, Vol. 213, no 11, p. 2974-2980Article in journal (Refereed)
    Abstract [en]

    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, where the effect of solute atoms is seen exclusively as a reduction of the mobility of defect clusters. The same set of parameters yields a satisfactory evolution for two different types of alloys, in very different irradiation conditions: an Fe-C-MnNi model alloy (high flux) and a high-Mn, high-Ni RPV steel (low flux). A satisfactory match with the experimental characterizations is obtained only if assuming a substantial immobilization of vacancy clusters due to solute atoms, which is here verified by means of independent atomistic kinetic Monte Carlo simulations. The microstructure evolution of the two alloys is strongly affected by the dose rate; a predominance of single defects and small defect clusters is observed at low dose rates, whereas larger defect clusters appear at high dose rates. In both cases, the predicted density of interstitial loops matches the experimental solute-cluster density, suggesting that the MnNi-rich nanofeatures might form as a consequence of solute enrichment on immobilized small interstitial loops, which are invisible to the electron microscope.

  • 41.
    Messina, Luca
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Malerba, Lorenzo
    Olsson, Pär
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Stability and mobility of small vacancy-solute complexes in Fe-MnNi and dilute Fe-X alloys: A kinetic Monte Carlo study2015In: Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, ISSN 0168-583X, E-ISSN 1872-9584, Vol. 352, p. 61-66Article in journal (Refereed)
    Abstract [en]

    Manganese and nickel solute atoms in irradiated ferritic steels play a major role in the nanostructural evolution of reactor pressure vessels (RPV), as they are responsible for the formation of embrittling nanofeatures even in the absence of copper. The stability and mobility of small vacancy solute clusters is here studied with an atomistic kinetic Monte Carlo approach based on ab initio calculations, in order to investigate the influence of Mn and Ni on the early life of small radiation-induced vacancy clusters, and to provide the necessary parameters for advanced object kinetic Monte Carlo simulations of the RPV long-term nanostructural evolution. Migration barriers are obtained by direct ab initio calculations or through a binding energy model based on ab initio data. Our results show a clear immobilizing and stabilizing effect on vacancy clusters as the solute content is increased, whereas the only evident difference between the two solute species is a somewhat longer elongation of the cluster mean free path in the presence of a few Mn atoms.

  • 42.
    Messina, Luca
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Nastar, Maylise
    Garnier, Thomas
    Domain, Christophe
    Olsson, Pär
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Exact ab initio transport coefficients in bcc Fe-X (X=Cr, Cu, Mn, Ni, P, Si) dilute alloys2014In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, no 10, p. 104203-Article in journal (Refereed)
    Abstract [en]

    Defect-driven diffusion of impurities is the major phenomenon leading to formation of embrittling nanoscopic precipitates in irradiated reactor pressure vessel (RPV) steels. Diffusion depends strongly on the kinetic correlations that may lead to flux coupling between solute atoms and point defects. In this work, flux coupling phenomena such as solute drag by vacancies and radiation-induced segregation at defect sinks are systematically investigated for six bcc iron-based dilute binary alloys, containing Cr, Cu, Mn, Ni, P, and Si impurities, respectively. First, solute-vacancy interactions and migration energies are obtained by means of ab initio calculations; subsequently, self-consistent mean field theory is employed in order to determine the exact Onsager matrix of the alloys. This innovative multiscale approach provides a more complete treatment of the solute-defect interaction than previous multifrequency models. Solute drag is found to be a widespread phenomenon that occurs systematically in ferritic alloys and is enhanced at low temperatures (as for instance RPV operational temperature), as long as an attractive solute-vacancy interaction is present, and that the kinetic modeling of bcc alloys requires the extension of the interaction shell to the second-nearest neighbors. Drag occurs in all alloys except Fe(Cr); the transition from dragging to nondragging regime takes place for the other alloys around (Cu, Mn, Ni) or above (P, Si) the Curie temperature. As far as only the vacancy-mediated solute migration is concerned, Cr depletion at sinks is foreseen by the model, as opposed to the other impurities which are expected to enrich up to no less than 1000 K. The results of this study confirm the current interpretation of the hardening processes in ferritic-martensitic steels under irradiation.

  • 43.
    Messina, Luca
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Nastar, Maylise
    Olsson, Pär
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Ab initio-based investigation of solute-dumbbell transport and radiation induced segregation in Fe-X (X=Cr, Cu, Mn, Ni, P, Si) dilute alloysManuscript (preprint) (Other academic)
    Abstract [en]

    In this work are analyzed the solute-transport mechanisms due to coupling with dumbbell-type defects in iron alloys, for selected impurities, by combining ab initio calculations of defect transition rates with a mean-field treatment yielding the transport coefficients of the alloy. Average radiation-induced segregation tendencies are determined based on these results and the vacancy-diffusion tendencies derived in a previous study. A new mathematical framework allows for such tendencies to be expressed in terms of vacancy-solute and dumbbell-solute flux-coupling, as well as the relative efficiency of the two mechanisms. The results show that P, Mn, and Cr to a lesser extent are transported by dumbbells thanks to the combination of high mixed-dumbbell stability and mobility, whereas Cu, Ni, and Si impurities are not. For the latter impurities the vacancy mechanism is dominant, which entails solute enrichment at low temperature and depletion above the drag transition temperature. For P and Mn, the mixed-dumbbell mechanism is dominant and leads to consistent enrichment at defect sinks, independently of temperature. Finally, the RIS tendency for Cr is the outcome of a balance between enrichment due to dumbbells and depletion due to vacancies, leading to a switchover between enrichment and depletion at 460 K. The results are in qualitative agreement with resistivity-recovery experiments and experimental RIS observations in ferritic alloys. 

  • 44.
    Messina, Luca
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Nastar, Maylise
    Sandberg, Nils
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics. Swedish Radiation Safety Authority, Sweden.
    Olsson, Pär
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Systematic electronic-structure investigation of substitutional impurity diffusion and flux coupling in bcc iron2016In: Physical Review B, ISSN 2469-9950, Vol. 93, no 18, article id 184302Article in journal (Refereed)
    Abstract [en]

    The diffusion properties of a wide range of impurities (transition metals and Al, Si, and P) in ferritic alloys are here investigated by means of a combined ab initio-atomic diffusion theory approach. The flux-coupling mechanisms and the solute-diffusion coefficients are inferred from electronic-structure calculations of solute-defect interactions and microscopic jump frequencies. All properties except the second-nearest-neighbor binding energy are found to have a characteristic bell shape as a function of the d-band filling for the 4d and 5d series, and an M shape for the 3d row because of the out-of-trend behavior of Mn. The solute jump frequencies are governed by compressibility, which makes diffusion of large solutes faster, although this effect is partially compensated for by lower attempt frequencies and larger correlations with the vacancy. Diffusion coefficients are predicted in a wide temperature range, far below the experimentally accessible temperatures. In accordance with experiments, Co is found to be a slow diffuser in iron, and the same behavior is predicted for Re, Os, and Ir impurities. Finally, flux-coupling phenomena depend on the iron jump frequencies next to a solute atom, which are mainly controlled by similar electronic interactions to those determining the binding energies. Vacancy drag and solute enrichment at sinks systematically arise below a solute-dependent temperature threshold, directly correlated with the electronic-level interactions at the equilibrium and the saddle-point states. Early transition metals with repulsive second-nearest-neighbor interactions also diffuse via vacancy drag, although they show a lower temperature threshold than the late metals. This confirms that drag is the most common solute-vacancy coupling mechanism in iron at low temperatures, and this is likely to be confirmed as well for impurity diffusion in other transition metals.

  • 45.
    Messina, Luca
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Nastar, Maylise
    Sandberg, Nils
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Olsson, Pär
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Systematic electronic-structure investigation of substitutional impurity diffusion and flux coupling in bcc ironManuscript (preprint) (Other academic)
    Abstract [en]

    The diffusion properties of a wide range of impurities (transition metals and Al, Si, and P) in ferritic alloys are here investigated by means of a combined ab initio-atomic diffusion theory approach. The flux-coupling mechanisms and the solute diffusion coefficients are inferred from electronic-structure calculations of solute-defect interactions and microscopic jump frequencies. All properties except the second nearest-neighbor binding energy are found to have a characteristic bell shape as a function of the d-band filling for the 4d and 5d series, and an M-shape for the 3d row because of the out-of-trend behavior of Mn. The solute jump frequencies are governed by compressibility, which makes diffusion of large solutes faster, although this effect is partially compensated for by lower attempt frequencies and larger correlations with the vacancy. Diffusion coefficients are predicted in a wide temperature range, far below the experimentally-accessible temperatures. In accordance with experiments, Co is found to be a slow diffuser in iron, and the same behavior is predicted for Re, Os, and Ir impurities. Finally, flux-coupling phenomena depend on the iron jump frequencies next to a solute atom, which are mainly controlled by similar electronic interactions to those determining the binding energies. Vacancy drag and solute enrichment at sinks systematically arise below a solute-dependent temperature threshold, directly correlated with the electronic-level interactions at the equilibrium and the saddle-point states. Early transition metals with repulsive second nearest-neighbor interactions also diffuse via vacancy drag, although they show a lower temperature threshold than the late metals. This confirms that drag is the most common solute-vacancy coupling mechanism in iron at low temperatures, and this is likely to be confirmed as well for impurity diffusion in other transition metals. 

  • 46. Ngayam-Happy, R.
    et al.
    Olsson, Pär
    EDF-R and D, Département Matériaux et Mécanique des Composants (MMC), Les Renardires.
    Becquart, C. S.
    Domain, C.
    Isochronal annealing of electron-irradiated dilute Fe alloys modelled by an ab initio based AKMC method Influence of solute-interstitial cluster properties2010In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 407, no 1, p. 16-28Article in journal (Refereed)
    Abstract [en]

    The evolution of the microstructure of dilute Fe alloys under irradiation has been modelled using a multiscale approach based on ab initio and atomistic kinetic Monte Carlo simulations In these simulations both self interstitials and vacancies isolated or in clusters are considered Isochronal annealing after electron irradiation experiments have been simulated in pure Fe Fe-Cu and Fe-Mn dilute alloys focusing on recovery stages I and II The parameters regarding the self interstitial - solute atom interactions are based on ab initio predictions and some of these interactions have been slightly adjusted without modifying the interaction character on isochronal annealing experimental data The different recovery peaks are globally well reproduced These simulations allow interpreting the different recovery peaks as well as the effect of varying solute concentration For some peaks these simulations have allowed to revisit and re-interpret the experimental data In Fe-Cu the trapping of self interstitials by Cu atoms allows experimental results to be reproduced although no mixed dumbbells are formed contrary to the former inter pretations Whereas in Fe-Mn the favorable formation of mixed dumbbell plays an important role in the Mn effect.

  • 47.
    Nouchy, Fabio
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Claisse, Antoine
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Olsson, Pär
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Carbon Effect on Thermal Ageing Simulations in Ferrite Steels2012In: Actinides And Nuclear Energy Materials / [ed] Anderson, D; Booth, CH; Burns, PC; Caciuffo, R; Devanathan, R; Durakiewicz, T; Stan, M; Tikare, V; Yu, SW, 2012, p. 49-55Conference paper (Refereed)
    Abstract [en]

    Two major causes of hardening and subsequent embrittlement in ferrite steels are the spinodal decomposition of the binary Fe-Cr solid solution and the carbide formation due to the presence of carbon as foreign interstitial atoms. In the present work, simulations of the microstructure evolution due to thermal ageing are performed by means of a kinetic Monte Carlo code and using a state-of-the-art interatomic potential based on density functional theory (DFT) predictions and experimental data. The main issues concern the possibility to perform thermal ageing simulations in an acceptable computational time frame and to reproduce a realistic behavior of carbon kinetics and carbide formation. The simulations on the binary system show the microstructural evolution during thermal ageing and allowed to find an exponential trend related to the acceleration as a function of temperature. With the insertion of carbon in the model, the chromium precipitation tends to accelerate. The carbon clustering, analyzed separately, is faster with higher C concentrations and in lattices with segregated chromium.

  • 48. Olsson, Pär
    Ab initio study of interstitial migration in Fe-Cr alloys2009In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 386-88, no C, p. 86-89Article in journal (Refereed)
    Abstract [en]

    The migration barriers for self-interstitial defects in Fe in the presence of solute Cr has been investigated using density functional theory calculations. It is seen that the dumbbell migration barrier is lower for a mixed interstitial than for a pure Fe one, in agreement with experiments. As a consequence, single self-interstitials in dilute Fe-Cr alloys will associate themselves to solute Cr atoms, and under irradiation conditions, the Cr can diffuse via both interstitial and vacancy mechanisms. Furthermore, the stability of self-interstitial defects and defect clusters in pure Cr have been calculated, showing that they should be significantly less stable in Cr rich precipitates than in the matrix. The most stable self-interstitial in Cr is shown to be a low-symmetry 〈2 2 1〉 configuration.

  • 49. Olsson, Pär
    et al.
    Abrikosov, I. A.
    Wallenius, Janne
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Electronic origin of the anomalous stability of Fe-rich bcc Fe-Cr alloys2006In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 73, no 10, p. 104416-Article in journal (Refereed)
    Abstract [en]

    The binary Fe-Cr alloy is a system with a miscibility gap. The decomposition occurs either via the nucleation and growth mechanism or as spinodal decomposition, depending on the Cr content. However, at low chromium concentrations the alloys are anomalously stable. This is shown to be true only for the ferromagnetic body centered cubic (bcc) phase. The stability stems from the negative mixing enthalpy at low concentrations of chromium. We show that the effect has an electronic origin, that is, it is directly related to variations of the electronic structure in the alloy with concentration. We also demonstrate that the variation in the state density of the majority channel at the Fermi level in the concentration interval below 20 at. % Cr indicates increasing tendency of the system towards the spinodal decomposition in the system. Moreover, in the equimolar concentration region, significant deviations of the spin up band from its canonical shape are observed, which destabilize the bcc phase.

  • 50. Olsson, Pär
    et al.
    Abrikosov, Igor A.
    Vitos, Levente
    Wallenius, Janne
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Ab initio formation energies of Fe-Cr alloys2003In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 321, no 1, p. 84-90Article in journal (Refereed)
    Abstract [en]

    We have calculated ab initio lattice parameters, formation energies, bulk moduli and magnetic moments of Fe-Cr alloys. The results agree well with available experimental data. In addition to body centered cubic (bcc) alloys, which are representative of ferritic steels used in fast neutron reactors, face centered cubic (fcc) and hexagonal close packed (hcp) phases were considered in order to complete a theoretical database of thermodynamic properties. Calculations were done for the ferromagnetic phase, as well as for a phase with local moment disorder, simulating the magnetic structure at high temperatures. For the latter case, the formation energy of the alloy is strictly positive smooth function of chromium concentration, in agreement with experiments performed at high temperature. In the ferromagnetic case, a negative mixing enthalpy is found for chromium concentrations below 6 %. Our observation is consistent with the experimentally observed inversion of the ordering trend, as well as with formation of the chromium rich alpha phase at Cr-concentrations above 9%, occurring at T < 900 K.

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