F. Bergner

Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Saxony, Germany

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Publications (49)43.99 Total impact

  • A. Gokhman, M. Caturla, F. Bergner
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    ABSTRACT: A comparison between object kinetic Monte Carlo (OKMC) and cluster dynamics (CD) simulations of damage accumulation has been made in the domain of low (77–150 K) temperatures for electron-irradiated and then annealed pure iron as well as at 573 K temperature for neutron-irradiated high concentrated Fe–12.5 at% Cr alloy. Findings indicate that an increase in the size of the simulation box up to 300 nm×300 nm×300 nm in OKMC simulation and direct integration of master equation of CD provide the quantitative agreement results of these methods among themselves as well as with the experimental data of microstructure evolution of Fe–12.5 at% Cr alloy and qualitative agreement for the case of pure iron electron irradiated at 77 K and then annealed at temperatures between 77 and 150 K.
    Radiation Effects and Defects in Solids 01/2014; 169(3). · 0.50 Impact Factor
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    ABSTRACT: An attempt is made to quantify the contributions of different types of defect-solute clusters to the total irradiation-induced yield stress increase in neutron-irradiated (300 °C, 0.6 dpa), industrial-purity Fe–Cr model alloys (target Cr contents of 2.5, 5, 9 and 12 at.% Cr). Former work based on the application of transmission electron microscopy, atom probe tomography, and small-angle neutron scattering revealed the formation of dislocation loops, NiSiPCr-enriched clusters and α′-phase particles, which act as obstacles to dislocation glide. The values of the dimensionless obstacle strength are estimated in the framework of a three-feature dispersed-barrier hardening model. Special attention is paid to the effect of measuring errors, experimental details and model details on the estimates. The three families of obstacles and the hardening model are well capable of reproducing the observed yield stress increase as a function of Cr content, suggesting that the nanostructural features identified experimentally are the main, if not the only, causes of irradiation hardening in these model alloys.
    Journal of Nuclear Materials. 01/2014; 448(s 1–3):96–102.
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    ABSTRACT: Neutron irradiation at 300 °C up to 0.6 dpa of an industrial purity Fe-12at%Cr alloy gives rise to the formation of Cr-rich precipitates of radii of about 1 nm. Small-angle neutron scattering (SANS) and atom probe tomography (APT) applied to the same material should reveal consistent characteristics of the irradiation-induced features. They roughly do so with respect to size and volume fraction, but they do not with respect to the composition of the precipitates or clusters. The discrepancy was expressed in terms of the Porod invariant of nuclear SANS. This quantity can be determined directly by integrating the measured nuclear difference scattering cross section or, alternatively, estimated from the APT results. Both estimates were compared taking into account all potential sources of deviation including error propagation. We have found that the deviation is significant and can be progressively removed by artificially reducing the Fe fraction in the Cr-rich clusters with respect to the measured value. A well-known effect of this kind is the different evaporation fields of Cr-rich clusters and the Fe-rich matrix and resulting ion trajectory overlaps in APT. State-of-the-art consideration of this effect indicates, however, that it is not sufficient to remove the observed discrepancy.
    Journal of Nuclear Materials 11/2013; 442(1):463-469. · 2.02 Impact Factor
  • Journal of Nuclear Materials 10/2013; 441(1-3):487-492. · 2.02 Impact Factor
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    ABSTRACT: The increasing age of European Nuclear Power Plants and envisaged extensions of reactor pressure vessel (RPV) lifetimes up to 80 years require the accurate prediction and management of RPV neutron irradiation embrittlement. LONGLIFE (“Treatment of long term irradiation embrittlement effects in RPV safety assessment”) is a collaborative project of the 7th Framework Programme of EURATOM. This project has been initiated as the next step forward towards obtaining an improved understanding of irradiation effects in RPV steels under conditions representative of long term operation (LTO) of RPVs. Phenomena which might become important at high neutron fluences (such as late-blooming effects and flux effects) must be considered in detail as part of the process of upgrading safety assessments and embrittlement surveillance procedures to underwrite the safety of LTO of RPVs. The work starts with the collection and evaluation of plant-specific information and data such as target neutron fluences for LTO and the chemical compositions of the materials. This includes a survey of available results of RPV materials data from decommissioned plants, of validating surveillance data, and of specific irradiation effects relevant for LTO. Microstructural data are obtained from different techniques with the aim of assessing the adequacy of current dose-damage models with respect to their relevance to the mechanisms of irradiation damage associated with LTO of RPVs. Complementary mechanical tests are performed in order to address gaps in existing experimental data. Microstructural data pertaining to the evolution of irradiation damage are correlated with changes in mechanical properties, and the most important influencing factors will be identified. Surveillance guidelines for LTO of RPV base materials and welds will be developed as one of the principal outputs of the project. The scope of work and the project structure are outlined in the paper. Two LTO relevant phenomena – late blooming effect and flux effect – are discussed in more detail.
    ASTM Spec. Tech. Publ. 01/2013; 1547.
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    ABSTRACT: Ferritic/martensitic high-chromium steels are leading candidates for fission and fusion reactor components. Oxide dispersion strengthening is an effective way to improve properties related to thermal and irradiation-induced creep and to extend their elevated temperature applications. An extensive experimental study focusing on the microstructural characterization of oxide-dispersion strengthened Fe–9wt%Cr model alloys is reported. Several material variants were produced by means of high-energy milling of elemental powders of Fe, Cr and commercial yttria powders. Consolidation was based on spark plasma sintering. Special emphasis is placed on the characterization of the nano-particles using transmission electron microscopy, small-angle neutron scattering and atom probe tomography. The microstructure of the investigated alloys and the role of the process parameters are discussed. Implications for the reliability of the applied characterization techniques are also highlighted.
    Journal of Nuclear Materials 09/2012; · 2.02 Impact Factor
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    ABSTRACT: The increasing age of European Nuclear Power Plants (NPPs) and envisaged extensions of plant lifetimes from 40 up to 80 years require an improved understanding of ageing phenomena of RPV components. The Network of Excellence NULIFE (Nuclear Plant Life Prediction) has been established to advance the safe and economic long-term operation (LTO) of NPPs by facilitating increased co-operation for applied R&D amongst members of the European nuclear community. The accurate prediction and management of RPV neutron irradiation embrittlement connected with long-term operation is an important aspect of this co-operation. Phenomena that might become important at high neutron fluences (such as flux effects and late blooming effects) have to be considered adequately in safety assessments. However, the surveillance database for prolonged irradiation times and low neutron fluxes is sparse. Consequently, there are significant uncertainties in the treatment of long-term irradiation effects. Therefore, the project LONGLIFE (Treatment of long-term irradiation embrittlement effects in RPV safety assessment) was initiated under the Euratom 7th Framework Programme of the European Commission as an umbrella project of NULIFE. LONGLIFE aims at 1) improved understanding of long-term irradiation phenomena that might compromise RPV integrity, and thereby the LTO of European NPPs, and 2) assessment of the adequacy of current prediction tools, codes, standards and surveillance guidelines for supporting long-term RPV operation. The scope of the work comprises the analysis of LTO boundary conditions; microstructural investigations and supplementary mechanical tests on RPV steels, including RPV steels from decommissioned plants; training activities; and elaboration of recommendations for RPV materials assessment and embrittlement surveillance under LTO conditions. A key part of the technical work is the selection of relevant materials for examination, e.g. which contain different weld and base metals originating from European LWRs. The scope of work covers different features of Western LWR and WWER type RPV materials. Against this background, the paper presents an overview of the project structure and related tasks and presents some intermediate results.
    3rd International Conference on NPP Life Management (PLIM) for Long Term Operations (LTO), Salt Lake City, UT, USA; 05/2012
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    ABSTRACT: The effects of lattice vibration on the thermodynamics of nanosized coherent clusters in bcc-Fe consisting of vacancies and/or copper are investigated within the harmonic approximation. A combination of on-lattice simulated annealing based on Metropolis Monte Carlo simulations and off-lattice relaxation by molecular dynamics is applied to obtain the most stable cluster configurations at T = 0 K. The most recent interatomic potential built within the framework of the embedded-atom method for the Fe-Cu system is used. The total free energy of pure bcc-Fe and fcc-Cu as well as the total formation free energy and the total binding free energy of the vacancy-copper clusters are determined for finite temperatures. Our results are compared with the available data from previous investigations performed using many-body interatomic potentials and first-principles methods. For further applications in rate theory and object kinetic Monte Carlo simulations, the vibrational effects evaluated in the present study are included in the previously developed analytical fitting formulae.
    Journal of Physics Condensed Matter 05/2012; 24(22):225402. · 2.22 Impact Factor
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    ABSTRACT: Small-angle neutron scattering (SANS) was applied to characterize the microstructure of weld material taken from the reactor pressure vessel (RPV) of the decommissioned VVER440 (230)-type nuclear power plant (NPP) Greifswald, Units 1, 2 and 4. The welding seam of highest neutron exposure of Unit 1 was subject to a large-scale annealing treatment in 1988 after about 11.5 effective years of operation. The same type of annealing was applied to Unit 2 in 1990 after about 11 effective years of operation. After final decommissioning of NPP Greifswald in 1990, RPV material was left in the reirradiated condition (Unit 1), in the as-annealed condition (Unit 2) and in the as-irradiated condition (Unit 4). Trepans of material from the highly irradiated RPV welds of these Units have recently become available for examination. The results of the SANS investigation are reported and compared with published results obtained for as-irradiated, post-irradiation annealed and reirradiated surveillance material of the same type. A general agreement was found indicating in particular the formation of irradiation-induced Cu-enriched clusters and efficient recovery as a result of the large-scale annealing treatments. The only essential difference was observed for the ratio of magnetic and nuclear scattering indicating differences of the cluster composition for the RPV wall and surveillance material.
    Journal of Nuclear Materials 01/2012; 416. · 2.02 Impact Factor
  • A. Gokhman, F. Bergner
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    ABSTRACT: Cluster dynamics (CD) is used to study the evolution of the size distributions of vacancy clusters (VC), self-interstitial atom (SIA) clusters (SIAC) and Cr precipitates in neutron irradiated Fe-9at.%Cr and Fe-12.5at.%Cr alloys at T = 573 K with irradiation doses up to 1.5 dpa and a flux of 140 ndpa/s. Transmission electron microscopy (TEM) and small angle neutron scattering (SANS) data on the defect structure of this material irradiated at doses of 0.6 and 1.5 dpa are used to calibrate the model. For both alloys a saturation behavior has been found by CD for the free vacancy and free SIA concentrations as well as for the number density of the SIAC for the doses above 0.006 dpa. The CD simulations also indicate the presence of VC with radii less than 0.5 nm and a strong SIAC peak with a mean diameter of about 0.5 nm, both invisible in SANS and TEM experiments. CD modeling of Cr precipitates has been made taking into account the deviation of this system from the ideal cluster gas. A specific surface tension of about 0.17 J/m2 between the α matrix and the Cr-rich α' precipitate and the rate at which Cr monomers are absorbed about 7.94 m-1 have been found as best fit values for reproducing the long-term Cr evolution in the irradiated Fe-12.5%Cr alloys observed by SANS. The change of the composition of Fe-Cr precipitates due to irradiation has been found.
    Nanodevices and Nanomaterials for Ecological Security. 01/2012;
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    ABSTRACT: Reactor pressure vessel (RPV) steel, when exposed to neutron irradiation, induces the formation of nanosized features. Using small angle neutron scattering (SANS) we have studied the neutron fluence dependence of the precipitate volume fraction for high-Cu and low-Cu materials separately. Cu-rich precipitates have long been recognized to play the dominant role in embrittlement of Cu-bearing RPV steels. In contrast, Mn–Ni-rich precipitates seem to govern embrittlement in the case of low levels of impurity Cu. The objective is to work out the resulting differences from the microstructural point of view. For low- Cu materials, the volume fraction was found to be within the detection limit of SANS at fluences below an apparent threshold fluence, whereas the slope increases considerably beyond. The relationship between irradiation-induced yield stress increase and precipitate volume fraction was also considered. We have derived estimates of the obstacle strength for Cu-rich precipitates and for Mn–Ni-rich precipitates.
    Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms 01/2012; 280:98–102. · 1.27 Impact Factor
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    ABSTRACT: The most stable atomic configuration of coherent nanoclusters in bcc-Fe formed by vacancies, Cu and Ni as well as the corresponding energetics are determined by on-lattice simulated annealing and subsequent off-lattice relaxation. An interatomic potential recently designed for investigations of radiation-induced effects in the ternary Fe–Cu–Ni system is used in the atomistic simulations. Ternary vlCumNin clusters show a core–shell structure with vacancies in the core coated by a shell of Cu atoms, followed by a shell of Ni atoms. In binary CumNin clusters the Cu core is covered by a shell of Ni atoms. On the contrary, binary vlNin clusters consist of a pure vacancy cluster surrounded by an agglomeration of Ni atoms. The latter is similar to a pure Ni cluster (Nin) and consists of Ni atoms at the second nearest neighbor distance. Because of this special arrangement of atoms vlNin and Nin are also called quasi-clusters. In all clusters investigated Ni atoms may be nearest neighbors of Cu atoms but never nearest neighbors of vacancies or other Ni atoms. The atomic configurations found can be understood by the peculiarities of the binding between vacancies, Cu, Ni and Fe atoms. The structure obtained for CumNin clusters is in agreement with previous theoretical results and with indications from measurements while for the other clusters reference data are not available. It is shown that the presence of Ni atoms promotes the nucleation of clusters containing vacancies and Cu. This is in agreement with experimental observations and with recent results of atomic kinetic Monte Carlo simulations. Based on the specific atomic structure of the clusters and the capillary model, compact and rather accurate analytical formulae for the total binding energy have been derived from the results of the atomistic simulations. Other important energetic characteristics of clusters, e.g. monomer or dimer binding energies, can be easily obtained from these analytical expressions and can be used in rate theory or object kinetic Monte Carlo simulations of nanocluster evolution.
    Journal of Nuclear Materials 11/2011; 418(1–3):215-222. · 2.02 Impact Factor
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    ABSTRACT: A combination of on-lattice simulated annealing based on Metropolis Monte Carlo simulations and off-lattice relaxation by Molecular Dynamics is applied in order to determine the structure and energetics of coherent copper–vacancy clusters in bcc-Fe. The most recent interatomic potential for Fe–Cu alloys is used. About 150 clusters consisting of up to 200 monomers (vacancies or copper atoms) are investigated. The atomic structure and the formation energy of the most stable configurations as well as their total and monomer binding energy are calculated. All clusters show facets which correspond to the main crystallographic planes. In the case of mixed clusters a core–shell structure is found where Cu atoms coat the outer surface of vacancy clusters. These findings are in agreement with previous theoretical results and with indications from measurements. For small clusters the total binding energy determined in this work shows a good agreement with literature data obtained by first-principle calculations. For further application in rate theory and object kinetic Monte Carlo simulations compact and physically-based fit formulae are derived from the atomistic data for the total and the monomer binding energy. The fit is based on the classical capillary model and the core–shell structure of the mixed clusters is explicitly taken into account. An atomistic nucleation model is established, and for typical irradiation conditions the nucleation free energy of pure vacancy and pure copper clusters as well as the critical size for cluster formation are estimated.
    Journal of Nuclear Materials 07/2011; 414(2):161-168. · 2.02 Impact Factor
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    ABSTRACT: Oxide dispersion strengthening of ferritic/martensitic chromium steels is a promising route for the extension of the range of operation temperatures for nuclear applications. The investigation of dedicated model alloys is an important means in order to separate individual effects contributing to the mechanical behaviour under irradiation and to improve mechanistic understanding. A powder metallurgy route based on spark plasma sintering was applied to fabricate oxide dispersion strengthened (ODS) Fe9Cr model materials. These materials along with Eurofer97 and ODS-Eurofer were investigated by means of small-angle neutron scattering (SANS) and TEM. For Fe9Cr–0.6wt.%Y2O3, TEM results indicate a peak radius of the size distribution of Y2O3 particles of 4.2nm with radii ranging up to 15nm, and a volume fraction of 0.7%, whereas SANS indicates a peak radius of 3.8nm and a volume fraction of 0.6%. It was found that the non-ODS Fe9Cr and Eurofer97 are suitable reference materials for ODS-Fe9Cr and ODS-Eurofer, respectively, and that the ODS-Fe9Cr variants are suitable model materials for the separated investigation of irradiation-Y2O3 particle interaction effects.
    Journal of Nuclear Materials 01/2011; 416(1):35-39. · 2.02 Impact Factor
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    ABSTRACT: Self-ion irradiation in combination with nanoindentation offers the possibility to characterize irradiation damage in a broad range of irradiation temperature and fluence. Nanoindentation results are reported for Fe–2.5at.%Cr, Fe–9at.%Cr and Fe–12.5at.%Cr irradiated at room temperature, 300°C and 500°C. Special features of this work are roughly rectangular damage profiles and exploitation of the full load dependence of hardness. The effects of Cr-content, fluence and irradiation temperature are discussed. Cases of both broad consistence with and deviations from reported trends are found. Hardening features were characterized by means of transmission electron microscopy also taking into account small-angle neutron scattering data reported for neutron-irradiated conditions of the same alloys. A tentative two-feature hardening model was applied.
    Journal of Nuclear Materials 01/2011; 417(1):980-983. · 2.02 Impact Factor
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    ABSTRACT: The effect of Cr on the irradiation-induced microstructure of neutron-irradiated Fe–Cr alloys is not yet known in detail. Small-angle neutron scattering was applied in order to provide the characteristics of nm-sized defects averaged over macroscopic volumes. Results are reported for a set of Fe–Cr alloys of Cr levels of 2.5, 5, 9 and 12.5at.%, irradiated at 300°C up to neutron exposures of 0.6 and 1.5dpa. We have found that the incoherent magnetic scattering of the unirradiated alloys exhibits a systematic variation with the Cr content and that there is an irradiation-induced increase of the coherent magnetic scattering for each of the irradiated conditions. The effect of Cr on size and type of irradiation-induced scatterers is discussed. For 12.5at.%Cr, the scatterers are unambiguously identified as α′ particles. For 2.5 and 5at.%Cr, the scatterers are tentatively interpreted as clusters enriched with alloying Cr and impurity C. For 9at.%Cr, a mixture of both kinds of scatterers explains the experimental findings.
    Journal of Nuclear Materials 01/2011; 409(2):106-111. · 2.02 Impact Factor
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    ABSTRACT: Available experimental results indicate that the addition of Cr to Fe and steels significantly influences the response of Fe–Cr alloys and ferritic/martensitic high-Cr steels to neutron irradiation. A level of 9 at%Cr is of particular interest because this composition is close to the boundary of the Fe–Cr miscibility gap. Furthermore, it corresponds to the composition of several candidate steels for application in nuclear technology. However, experimental evidence has been incomplete so far. The reported study by means of small-angle neutron scattering is devoted to the effect of neutron irradiation at 300 °C up to fluences of 0.6 and 1.5 dpa on the microstructure of an Fe–9 at%Cr alloy. We have observed a pronounced irradiation-induced increase of scattering cross-sections for both magnetic and nuclear scattering. Bimodal size distributions of irradiation-induced defect-solute clusters have been reconstructed. The restrictions on the composition of these clusters have been discussed in terms of the scattering contrast. We have found that vacancy clusters and α′-particles alone cannot explain the full set of experimental findings. The remaining inconsistency can be solved by taking into account a contribution of impurity carbon.
    Journal of Nuclear Materials 12/2010; 407(1):29–33. · 2.02 Impact Factor
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    ABSTRACT: The effect of neutron irradiation on the formation of defect/solute clusters in binary iron alloys is important for the understanding of the damage mechanisms in structural materials applied in the field of nuclear technology. The irradiation behaviour of four binary Fe-Cr alloys of commercial purity with chromium contents between 2.5 and 12.5at% and neutron exposure of 0.6 and 1.5 dpa at 300 °C has been studied by SANS. The analysis of the experimental results indicates that there are at least two populations of irradiation-induced features. The appearance of α-phase particles was shown for Fe-12.5at%Cr and to a less content in Fe-9at%Cr. We have observed a strong and complex effect of Cr and weak or insignificant effects of neutron exposure.
    Journal of Physics Conference Series 11/2010; 247(1):012035.
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    ABSTRACT: Setting out from the results found in a set of small-angle neutron scattering (SANS) experiments for neutron-irradiated Fe-Cu model alloys, a rate theory model for the simulation of the irradiation-induced time-evolution of Cu-rich precipitates in these model alloys is presented which follows the idea that the precipitate clusters are mixed Cu-vacancy aggregates. This is done by explicitly allowing the defect clusters to absorb vacancies. The resulting Vacancy-Coupled Copper Clustering (V3C) model is calibrated by SANS experiments on two different Fe-Cu model alloys neutron-irradiated at four different doses. Quantitative agreement with the SANS experiments could be achieved by introducing a dependence of the Fe-Cu interface energy on the amount of vacancies in the mixed precipitate clusters. Phenomenologically, this energy can be seen as a function of the weight-percentage of Cu in the iron matrix. An empirical expression for this dependence is suggested. In addition, the new V3C model is used to gain some preliminary insight into the time-evolution of the chemical composition of the mixed Cu-vacancy clusters, confirming qualitatively the experimental findings. The relation of our ansatz to the heterogeneous Cu-precipitation mechanism proposed by others for neutron-irradiated Fe-Cu alloys of low Cu content is discussed.
    Journal of Physics Conference Series 11/2010; 247(1):012011.
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    ABSTRACT: Irradiation-induced damage in reactor pressure vessel steels covers a multitude of different features at the nanometer size scale. The nature, formation kinetics and relative importance of these features are not yet well understood in detail. It also turned out that there is no single experimental technique capable of closing all the remaining gaps. The present approach is based on the idea that significant progress can be achieved by investigating the same set of neutron-irradiated model alloys of increasing complexity with several experimental techniques including transmission electron microscopy (TEM), atom probe tomography (APT), positron annihilation spectroscopy (PAS) and small-angle neutron scattering (SANS). The aim of the effort is to explore both complementarity and overlaps of the information gained from individual techniques and to close gaps by introducing proper models. In the present paper the results obtained by means of SANS are reported, self-consistent interpretation is given and the results are qualified for the discussion in combination with the other experimental techniques to be given in separate papers.
    Journal of Nuclear Materials 04/2010; · 2.02 Impact Factor

Publication Stats

156 Citations
43.99 Total Impact Points

Institutions

  • 2005–2014
    • Helmholtz-Zentrum Dresden-Rossendorf
      • Institute of Ion Beam Physics and Materials Research
      Dresden, Saxony, Germany
  • 2010
    • South Ukrainian National Pedagogical University
      Odessa, Odes’ka Oblast’, Ukraine
  • 2000–2003
    • Technische Universität Dresden
      • Institute of Materials Science
      Dresden, Saxony, Germany