G. Bonny

Hardening due to dislocation loop damage in RPV model alloys: role of Mn segregation

D. Terentyev, X. He, G. Bonny, A. Bakaev, E. Zhurkin, L. Malerba
Journal of Nuclear Materials
457, 173–181
2015
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Abstract 

The exact nature of the radiation defects causing hardening in reactor vessel pressure steels at high doses is not yet clearly determined. While generally it is attributed to solute-rich clusters (precipitates) and point defects clusters (matrix damage), recent fine-scale experiments and atomistic simulations suggest that solute rich clusters, mainly containing Mn, Ni and Cu, might be the result of the segregation of these elements to small dislocation loops (heterogeneous nucleation), so that the distinction between precipitates and matrix damage becomes blurred. Here, we perform an atomistic study to investigate the interaction of a0/2〈1 1 1〉 dislocation loops with moving dislocations and specifically address the effect of solute segregation on the loop’s strength and interaction mechanism, focusing in particular on Mn, alone or with other crucial solute elements such as Cu and Ni. It is found that the enrichment of Mn in the core of dislocation loops causes significant increase of the unpinning stress, especially for small, invisible ones. At the same time, the solute segregation at the dislocation loops enhances their resistance against absorption by moving dislocations.

On the thermal stability of late blooming phases in reactor pressure vessel steels: An atomistic study

G. Bonny, D. Terentyev, A. Bakaev, E. Zhurkin, M. Hou, L. Malerba
Journal of Nuclear Materials
442 (1-3), 282–291
2013
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Abstract 

Radiation-induced embrittlement of bainitic steels is the lifetime limiting factor of reactor pressure vessels in existing nuclear light water reactors. The primary mechanism of embrittlement is the obstruction of dislocation motion produced by nanometric defect structures that develop in the bulk of the material due to irradiation. In view of improving the predictive capability of existing models it is necessary to understand better the mechanisms leading to the formation of these defects, amongst which the so-called “late blooming phases”. In this work we study the stability of the latter by means of density functional theory (DFT) calculations and Monte Carlo simulations based on a here developed quaternary FeCuNiMn interatomic potential. The potential is based on extensive DFT and experimental data. The reference DFT data on solute–solute interaction reveal that, while Mn–Ni pairs and triplets are unstable, larger clusters are kept together by attractive binding energy. The NiMnCu synergy is found to increase the temperature range of stability of solute atom precipitates in Fe significantly as compared to binary FeNi and FeMn alloys. This allows for thermodynamically stable phases close to reactor temperature, the range of stability being, however, very sensitive to composition.

Review of many-body central force potentials for tungsten

G. Bonny, D. Terentyev, A. Bakaev, P. Grigorev, D. Van Neck
Modelling and Simulation in Materials Science and Engineering
22, 053001
2014
A1

Abstract 

Tungsten and tungsten-based alloys are the primary candidate materials for plasma facing components in fusion reactors. The exposure to high-energy radiation, however, severely degrades the performance and lifetime limits of the in-vessel components. In an effort to better understand the mechanisms driving the materials' degradation at the atomic level, large-scale atomistic simulations are performed to complement experimental investigations. At the core of such simulations lies the interatomic potential, on which all subsequent results hinge. In this work we review 19 central force many-body potentials and benchmark their performance against experiments and density functional theory (DFT) calculations. As basic features we consider the relative lattice stability, elastic constants and point-defect properties. In addition, we also investigate extended lattice defects, namely: free surfaces, symmetric tilt grain boundaries, the 1/2〈1 1 1〉{1 1 0} and 1/2〈1 1 1〉 {1 1 2} stacking fault energy profiles and the 1/2〈1 1 1〉 screw dislocation core. We also provide the Peierls stress for the 1/2〈1 1 1〉 edge and screw dislocations as well as the glide path of the latter at zero Kelvin. The presented results serve as an initial guide and reference list for both the modelling of atomically-driven phenomena in bcc tungsten, and the further development of its potentials.

On the mobility of vacancy clusters in reduced activation steels: an atomistic study in the Fe-Cr-W model alloy

G. Bonny, N. Castin, J. Bullens, A. Bakaev, T.P.C. Klaver, D. Terentyev
Journal of Physics: Condensed Matter
25 (31), 315401
2013
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Abstract 

Reduced activation steels are considered as structural materials for future fusion reactors. Besides iron and the main alloying element chromium, these steels contain other minor alloying elements, typically tungsten, vanadium and tantalum. In this work we study the impact of chromium and tungsten, being major alloying elements of ferritic Fe-Cr-W-based steels, on the stability and mobility of vacancy defects, typically formed under irradiation in collision cascades. For this purpose, we perform ab initio calculations, develop a many-body interatomic potential (EAM formalism) for large-scale calculations, validate the potential and apply it using an atomistic kinetic Monte Carlo method to characterize the lifetime and diffusivity of vacancy clusters. To distinguish the role of Cr and W we perform atomistic kinetic Monte Carlo simulations in Fe-Cr, Fe-W and Fe-Cr-W alloys. Within the limitation of transferability of the potentials it is found that both Cr and W enhance the diffusivity of vacancy clusters, while only W strongly reduces their lifetime. The cluster lifetime reduction increases with W concentration and saturates at about 1-2 at.%. The obtained results imply that W acts as an efficient 'breaker' of small migrating vacancy clusters and therefore the short-term annealing process of cascade debris is modified by the presence of W, even in small concentrations.

Interaction of minor alloying elements of high-Cr ferritic steels with lattice defects: An ab initio study

A. Bakaev, D. Terentyev, G. Bonny, T.P.C. Klaver, P. Olsson, D. Van Neck
Journal of Nuclear Materials
444 (1-3), 237-246
2014
A1

Abstract 

Basic properties of minor alloying elements, namely Mo, W, Nb, Ta, V, Mn, Si entering the conventional and reduced-activation structural Fe-(9-12)Cr steels have been analyzed using ab initio calculations. The electronic structure calculations were applied to study the interaction of minor alloying elements with a number of important and well defined lattice structures, such as point defects, the 1/2 screw dislocation core, high angle symmetric grain boundaries and free surfaces. The studied elements were classified according to their similarities and discrepancies regarding the interaction with the above mentioned defects. The refractory alloying elements are found to follow the same trend whereas Mn and Si exhibit peculiar behavior with respect to the interaction with both point and extended lattice defects. The obtained results are discussed and compared with previously published ab initio and available experimental data.

Interatomic potential to study plasticity in stainless steels: the FeNiCr model alloy

G. Bonny, D. Terentyev, R. C. Pasianot, S. Poncé, A. Bakaev
Modelling and Simulation in Materials Science and Engineering
19 (8), 085008
2011
A1

Abstract 

Austenitic stainless steels are commonly used materials for in-core components of nuclear light water reactors. In service, such components are exposed to harsh conditions: intense neutron irradiation, mechanical and thermal stresses, and aggressive corrosion environment which all contribute to the components' degradation. For a better understanding of the prevailing mechanisms responsible for the materials degradation, large-scale atomistic simulations are desirable. In this framework we developed an embedded atom method type interatomic potential for the ternary FeNiCr system to model movement of dislocations and their interaction with radiation defects. Special attention has been drawn to the Fe–10Ni–20Cr alloy, whose properties were ensured to be close to those of 316L austenitic stainless steel. In particular, the stacking fault energy and elastic constants are well reproduced. The fcc phase for the Fe–10Ni–20Cr random alloy was proven to be stable in the temperature range 0–900 K and under shear strain up to 5%. For the same alloy the stable glide of screw dislocations and stability of Frank loops was confirmed.

On the thermal stability of vacancy–carbon complexes in alpha iron

D. Terentyev, G. Bonny, A. Bakaev, D. Van Neck
Journal of Physics: Condensed Matter
24, 385401
2012
A1

Abstract 

In this work we have summarized the available ab initio data addressing the interaction of carbon with vacancy defects in bcc Fe and performed additional calculations to extend the available dataset. Using an ab initio based parameterization, we apply object kinetic
Monte Carlo (OKMC) simulations to model the process of isochronal annealing in bcc Fe doped with carbon to compare with experimental data. As a result of this work, we clarify that a binding energy of ~0.65 eV for a vacancy–carbon (V–C) pair fits the available experimental
data best. It is found that the V2–C complex is less stable than the V–C pair and its dissociation with activation energy of 0.55 + 0.49 eV also rationalizes a number of experimental data where the breakup of V–C complexes was assumed instead. From the summarized ab initio data, the subsequently obtained OKMC results and critical discussion, provided here, we suggest that the twofold interpretation of the V–C binding energy, which is believed to vary between 0.47 and 0.65 eV, depending on the ab initio approximation, should be removed. The stability and mobility of small and presumably immobile SIA clusters formed at stage II is also discussed in the view of experimental data.

Early stages of α-α′ phase separation in Fe-Cr alloys: An atomistic study

G. Bonny, D. Terentyev, L. Malerba, D. Van Neck
Physical Review B
79 (10), 104207
2009
A1

Abstract 

The thermal aging of Fe-Cr alloys was simulated using atomistic kinetic Monte Carlo techniques. The study was performed varying the Cr content in the range of 12–18 at. % Cr and at temperatures within the miscibility gap, where α-α′ phase separation occurs. The evolution of the phase-separation process was characterized in terms of precipitate shape, composition, density, and mean size. The results offer a description of α-α′ phase separation in its early stage, which is hardly accessible to experiments and of key importance in understanding the change in mechanical properties of Fe-Cr alloys under thermal aging. The critical size for a stable precipitate was estimated from the simulation data in the framework of Gibbs's homogeneous nucleation theory. The obtained results are compared, whenever possible, with available experimental data and the reliability, as well as the shortcomings, of the applied method is discussed accordingly. Despite strong oversimplifications, the used model shows good agreement with experimental data.

Quasiparticle properties in a density-functional framework

D. Van Neck, S. Verdonck, G. Bonny, P.W. Ayers, M. Waroquier
Physical Review A
74 (4), 042501
2006
A1

Abstract 

We propose a framework to construct the ground-state energy and density matrix of an N-electron system by solving a self-consistent set of single-particle equations. The method can be viewed as a nontrivial extension of the Kohn-Sham scheme (which is embedded as a special case). It is based on separating the Green’s function into a quasiparticle part and a background part, and expressing only the background part as a functional of the density matrix. The calculated single-particle energies and wave functions have a clear physical interpretation as quasiparticle energies and orbitals.

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