D. Terentyev

Interaction of an edge dislocation with voids in α-iron modelled with different interatomic potentials

Atomic processes and strengthening effects due to interaction between edge dislocations and voids in α-iron have been investigated by means of molecular dynamics with a recently developed interatomic potential (Ackland et al 2004 J. Phys.: Condens. Matter 16 S2629) and compared with those obtained earlier with an older potential (Ackland et al 1997 Phil. Mag. A 75 713). Differences between the interactions for the two models are insignificant at temperature T≥100 K, thereby confirming the validity of the previous results. In particular, voids are relatively strong obstacles because for large voids and/or low temperature, the initially straight edge dislocation is pulled into screw orientation before it breaks away at the critical shear stress, τc. Differences between the core structures and glide planes of the screw dislocation for the two potentials do not affect τc in this temperature range. The only significant difference between the dislocation–void interactions in the two models occurs at low temperature in static or pseudo-static conditions (T≤1 K). It arises from the influence of the dislocation segment in the 70°-mixed orientation with the (Ackland et al 2004 J. Phys.: Condens. Matter 16 S2629) potential and is seen in the critical line shape at which the dislocation breaks from the void. It affects τc for some combinations of void size and spacing. The effect on the line shape does not arise from anisotropy of the elastic line tension: it is due to the high Peierls stress of the 70° dislocation. When this effect does not control breakaway, the dependence of τc on void size and spacing follows an equation first found by modelling the Orowan process in the approximation of linear elasticity.

Fast three dimensional migration of He clusters in bcc Fe and Fe―Cr alloys

In this work, we perform atomistic molecular dynamics simulations to assess the properties of small helium vacancy (He-V) and pure He clusters in body-centered cubic Fe and in Fe90–Cr10 (Fe–10Cr) random alloy. The following two goals are pursued: determining diffusion mechanisms of He-V clusters occurring in dynamic simulations and revealing a possible influence of Cr on the mobility/stability of He-V clusters in the Fe–10Cr alloy. We also present a newly developed set of interatomic potentials for the Fe–Cr–He system, fitted to a set of specially performed density functional theory calculations. The obtained results show that the dissociation energies of the studied He-V clusters, as well as the migration energy of He interstitial, are not significantly affected in the alloy compared to pure Fe. It was found that small pure He clusters with sizes up to four atoms, that were assumed to be immobile in many previous studies devoted to He-release/accumulation kinetics, in fact, exhibit fast three dimensional...

On the correlation between self-interstitial cluster diffusivity and irradiation-induced swelling in Fe–Cr alloys

It is shown that the dependence of self-interstitial cluster diffusivity in Fe–Cr alloys on Cr concentration correlates with that of swelling in these alloys under neutron irradiation; namely, with increasing chromium concentration the cluster diffusivity first decreases and then increases. The origin of such behaviour lies in a relatively long-ranged, ∼1 nm, attractive interaction between Cr atoms and crowdions. The minimum diffusivity is realized for ∼11 at.% Cr, where all crowdions constituting the cluster interact with Cr atoms, but the interaction fields of different Cr atoms do not overlap.

Reactions between a 1/2⟨111⟩ screw dislocation and ⟨100⟩ interstitial dislocation loops in alpha-iron modelled at atomic scale

Interstitial dislocation loops with Burgers vector of type are observed in α-iron irradiated by neutrons or heavy ions, and their population increases with increasing temperature. Their effect on motion of a edge dislocation was reported earlier 1. Results are presented of a molecular dynamics study of interactions between a screw dislocation and loops in iron at temperature in the range 100 to 600 K. A variety of reaction mechanisms and outcomes are observed and classified in terms of the resulting dislocation configuration and the maximum stress required for the dislocation to break away. The highest obstacle resistance arises when the loop is absorbed to form a helical turn on the screw dislocation line, for the dislocation cannot glide away until the turn closes and a loop is released with the same Burgers vector as the line. Other than one situation found, in which no dislocation–loop reaction occurs, the weakest obstacle strength is found when the original loop is restored at the end of the reaction. The important role of the cross-slip and the influence of model boundary conditions are emphasised and demonstrated by examples.

Modelling of Radiation Damage in Fe-Cr Alloys

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.

Benchmarking FeCr empirical potentials against density functional theory data

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 w ...

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

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 V (2)-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.

Density functional theory-based cluster expansion to simulate thermal annealing in FeCrW alloys

Abstract 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.

Effect of carbon decoration on the absorption of 〈100〉 dislocation loops by dislocations in iron

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 〈 100 〉 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.

Interaction of Dislocations with Carbides in BCC Fe Studied by Molecular Dynamics

Abstract Iron carbide (Fe3C), also known as cementite, is present in many steels and has also been seen as nanosized precipitates in steels. We examine the interaction of edge dislocations with nanosized cementite precipitates in Fe by molecular dynamics. The simulations are carried out with a Tersoff-like bond order interatomic potential by Henriksson et al. for Fe-C-Cr systems. Comparing the results obtained with this potential for a defect free Fe system with results from previously used potentials, we find that the potential by Henriksson et al. gives significantly higher values for the critical stress, at least at low temperatures. The explanation was found to be the difference in the core structure of the edge dislocation. The results show that edge dislocations can unpin from cementite precipitates of sizes 1 nm and 2 nm even at a temperature of 1 K, although the stresses needed for this are high. On the other hand, a 4 nm precipitate is not sheared by edge dislocations at low temperatures (≤100 K) on our simulation timescale.