D. Terentyev

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.

Synergetic Effects of Mn and Si in the Interaction with Point Defects in bcc Fe

A. Bakaev, D. Terentyev, X. He, D. Van Neck
Journal of Nuclear Materials
455 (1-3), 5-9
2014
A1

Abstract 

The interaction of Mn, Si and Cr with a vacancy and self-interstitial defects in BCC Fe has been analyzed using ab initio calculations. While the interaction of the considered solute clusters with a single vacancy is linearly additive, there is a considerable synergetic effect in the case of self-interstitial atoms, found to bind strongly with Mn–Si pairs. The latter therefore act as deep trapping configurations for self-interstitials. At the same time, the presence of the point defects nearby weakly attractive Mn–Si pairs significantly enhances the solute–solute binding. The revealed effects are rationalized on the basis of charge density and local magnetic moment distributions.

Radiation-induced strengthening and absorption of dislocation loops in ferritic Fe-Cr alloys: the role of Cr segregation

D. Terentyev, A. Bakaev
Journal of Physics: Condensed Matter
25 (26), 265702
2013
A1

Abstract 

The understanding of radiation-induced strengthening in ferritic FeCr-based steels remains an essential issue in the assessment of materials for fusion and fission reactors. Both early and recent experimental works on Fe-Cr alloys reveal Cr segregation on radiation-induced nanostructural features (mainly dislocation loops), whose impact on the modification of the mechanical response of the material might be key for explaining quantitatively the radiation-induced strengthening in these alloys. In this work, we use molecular dynamics to study systematically the interaction of dislocations with 1/2 and loops in all possible orientations, both enriched by Cr atoms and undecorated, for different temperatures, loop sizes and dislocation velocities. The configurations of the enriched loops have been obtained using a non-rigid lattice Monte Carlo method. The study reveals that Cr segregation influences the interaction mechanisms with both 1/2 and loops. The overall effect of Cr enrichment is to penalize the mobility of intrinsically glissile 1/2 loops, modifying the reaction mechanisms as a result. The following three most important effects associated with Cr enrichment have been revealed: (i) absence of dynamic drag; (ii) suppression of complete absorption; (iii) enhanced strength of small dislocation loops (2 nm and smaller). Overall the effect of the Cr enrichment is therefore to increase the unpinning stress, so experimentally 'invisible' nanostructural features may also contribute to radiation-induced strengthening. The reasons for the modification of the mechanisms are explained and the impact of the loading conditions is discussed.

Interaction of a screw dislocation with Frank loops in Fe-10Ni-20Cr alloy

D. Terentyev, A. Bakaev
Journal of Nuclear Materials
442 (1-3), 208-217
2013
A1

Abstract 

The interaction of 2 and 5 nm Frank loops with a moving screw dislocation is studied in Fe-10Ni-20Cr alloy (a model of austenitic 304 and 316 steels) employing the newly developed Fe-Ni-Cr interatomic potential in molecular dynamics simulations. The applied potential ensures full stability of FCC phase and smooth evolution of stacking fault energy (SFE) as a function of chemical composition, fitted to be in a close agreement with the CALPHAD database. A model of Fe-10Ni-20Cr random alloy closely reproduces elastic properties and SFE of 316-type austenitic steels. The results reveal a number of interaction mechanisms depending on loop orientation and ambient temperature. Half of the observed reactions lead to loop unfaulting despite a low SFE of the alloy. The unfaulting reactions are enhanced with temperature and the critical stress for the unfaulting is regularly higher in comparison with the loop shear interaction. By comparing present results with a recent study done in a low SFE Fe-50Ni alloy, we reveal that a magnitude of local variation of SFE is an important factor controlling the formation of dislocation constrictions. In the Fe-50Ni alloy, characterized by strong variations of local SFE, the constrictions are almost never observed so that the loop shear interaction prevails, while absorption is rare. In the Fe-10Ni-20Cr alloy, characterized by small variations of local SFE, the constrictions are regularly formed resulting in frequent loop unfaulting.

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
A1

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.

Effect of carbon decoration on the absorption of < 100 > dislocation loops by dislocations in iron

D. Terentyev, A. Bakaev
Journal of Physics: Condensed Matter
26 (16), 165402
2014
A1

Abstract 

This work closes a series of molecular dynamics studies addressing how solute/interstitial segregation at dislocation loops affects their interaction with moving dislocations in body-centred cubic Fe-based alloys. We consider the interaction of interstitial dislocation loops decorated by different numbers of carbon atoms in a wide temperature range. The results reveal clearly that the decoration affects the reaction mechanism and increases the unpinning stress, in general. The most pronounced and reproducible increase of the unpinning stress is found in the intermediate temperature range from 300 up to 600 K. The carbon-decoration effect is related to the modification of the loop-dislocation reaction and its importance at the technologically relevant neutron irradiation conditions is discussed.

Dislocations mediate hydrogen retention in tungsten

D. Terentyev, V. Dubinko, A. Bakaev, Y. Zayachuk, W. Van Renterghem, P. Grigorev
Nuclear Fusion
54 (4), 042004
2014
A1

Abstract 

In this letter, a comprehensive mechanism for the nucleation and growth of bubbles on dislocations under plasma exposure of tungsten is proposed. The mechanism reconciles long-standing experimental observations of hydrogen isotopes retention, essentially defined by material microstructure, and so far not fully explained. Hence, this work provides an important link to unify material's modelling with experimental assessment of W and W-based alloys as candidates for plasma facing components.

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.

Interaction of dislocations with Frank loops in Fe–Ni alloys and pure Ni: An MD study

D. Terentyev, A. Bakaev, Y. Osetsky
Journal of Nuclear Materials
442 (1–3), S628–S632
2013
A1

Abstract 

Variation of the stacking fault energy in FCC alloys and austenitic steels is well known to influence the evolution of radiation damage and its effect on deformation mechanisms. The primary defects observed in austenitic steels under neutron irradiation are mainly Frank loops. Here, we study the interaction of edge and screw dislocations with Frank loops in low stacking fault energy Fe–Ni alloys. The interatomic potentials employed were specially developed to reproduce a number of properties of real austenitic steels. The influence of temperature and loop morphology on the interaction mechanism and the critical resolved shear stress for dislocations to overcome loops has been investigated. All investigated reactions have been subdivided into three classes depending on temperature, loop size and interaction geometry. It is shown that by decreasing stacking fault energy below a certain value the formation of constrictions on dislocations is suppressed so that loop unfaulting becomes a less favorable mechanism in comparison with loop shear. Additional effect of solid-solution alloying, causing a non-negligible friction stress, is expressed in the impedance of the propagation of dislocations in the secondary glide planes, which is another factor limiting the unfaulting process.

Energetics of radiation defects in Fe-based austenitic alloys: Atomic scale study

A. Bakaev, D. Terentyev, X. He, E. Zhurkin
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms
303, 33–36
2013
A1

Abstract 

Energetics of typical radiation defects observed in austenitic stainless steel of 304L type has been characterized in the model FeNi10Cr20 alloy by means of atomistic simulations employing a set of interatomic potentials specially derived to reproduce main features of 304L steel. The following defects have been considered: dislocation loops of both interstitial and vacancy nature, stacking fault tetrahedron, perfect loops and voids. The formation energy of these defects has been calculated at 0 K and the obtained results have been compared with the prediction of the elasticity theory. A good agreement has been found in all the cases except for the hexagonal Frank loop, whose sides have splitted into 1/6〈1 1 2〉 partial dislocations, thus lowering the total formation energy. High temperature annealing, performed using molecular dynamics simulations, has proven that the considered defects are thermally stable in the temperature range 300–1200 K.

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