A. Bakaev

Dislocations mediate hydrogen retention in tungsten

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 materials modelling with experimental assessment of W and W-based alloys as candidates for plasma facing components.

Dislocation mechanism of deuterium retention in tungsten under plasma implantation.

We have developed a new theoretical model for deuterium (D) retention in tungsten-based alloys on the basis of its being trapped at dislocations and transported to the surface via the dislocation network with parameters determined by ab initio calculations. The model is used to explain experimentally observed trends of D retention under sub-threshold implantation, which does not produce stable lattice defects to act as traps for D in conventional models. Saturation of D retention with implantation dose and effects due to alloying of tungsten with, e.g. tantalum, are evaluated, and comparison of the model predictions with experimental observations under high-flux plasma implantation conditions is presented.

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.

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.

Molecular dynamics simulation of the interaction of dislocations with radiation-induced defects in Fe-Ni-Cr austenitic alloys

A classical molecular dynamics method is used to theoretically study the interaction of dislocations with typical radiation-induced defects in an Fe-Ni10-Cr20 austenitic alloy. As a result, a set of interactions and the corresponding values for the critical stress required for unpinning of a dislocation from an obstacle are obtained for different temperatures and interaction geometries.

Interaction between mobile dislocations and perfect dislocation loops in Fe-Ni-Cr austenitic alloy systems

The classical molecular dynamics method is employed to simulate the interaction of screw and edge dislocations with interstitial perfect dislocation loops (of 2 and 5 nm in diameter) in the austenitic model alloy Fe70Ni10Cr20 at temperatures of T = 300–900 K. Perfect loops can be created from Frank loops during the plastic deformation of irradiated austenitic steels applied in nuclear reactors. As a result, the dislocation-defect interaction mechanisms are established and classified. The loop absorption mechanisms, which are related to the formation of free channels capable of enhancing radiation-induced steel embrittlement, are revealed. The effectivenesses of loop absorption observed during their interaction with screw and edge dislocations, as well as unpinning stresses required for a dislocation to overcome the defect acting as an obstacle, are compared versus the material temperature, defect size, and interaction geometry.

Atomistic simulation of the interaction between mobile edge dislocations and radiation-induced defects in Fe-Ni-Cr austenitic alloys

The classical molecular dynamics method is employed to simulate the interaction of edge dislocations with interstitial Frank loops (2 and 5 nm in diameter) in the Fe-Ni10-Cr20 model alloy at the temperatures T = 300–900 K. The examined Frank loops are typical extended radiation-induced defects in austenitic steels adapted to nuclear reactors, while the chosen triple alloy (Fe-Ni10-Cr20) has the alloying element concentration maximally resembling these steels. The dislocation-defect interaction mechanisms are ascertained and classified, and their comparison with the previously published data concerning screw dislocations is carried out. The detachment stress needed for a dislocation to overcome the defect acting as an obstacle is calculated depending on the material temperature, defect size, and interaction geometry. It is revealed that edge dislocations more efficiently absorb small loops than screw ones. It is demonstrated that, in the case of small loops, the number of reactions accompanied by loop absorption increases with temperature upon interaction with both edge and screw dislocations. It is established that Frank loops are stronger obstacles to the movement of screw dislocations than to the movement of edge ones.

Deuterium accumulation in tungsten under low-energy high-flux plasma exposure

The accumulation of deuterium implanted in tungsten is simulated within the framework of kinetic diffusion theory. The influence of the tungsten microstructure (dislocation density and impurity concentration) on the process of deuterium capture and accumulation is considered. It is established that, under the chosen irradiation conditions, deuterium accumulation in the near-surface region is determined by capture at defects formed during implantation. The deuterium concentration gradient, together with the material microstructure, determines its accumulation in tungsten. Variation in the dislocation density and impurity concentration does not affect the simulation results, which is, first, related to the fact that the model used does not contain alternative mechanisms for the formation and growth of vacancy clusters under the subthreshold irradiation mode. The simulation results are compared with experimental data, and ways of improving the model are discussed in order to explain the deuterium-saturation effect for high fluences (more than 1023 m−2).

Simulation of doping and primary radiation damage to the SiC(111) surface under bombardment by SiN atomic and cluster ions (N = 1, 5, and 60) using classical molecular dynamics

The features of the cascade of atomic collisions, the spatial distribution of dopes, and primary radiation damage in a near-surface region of cubic silicon carbide under bombardment by SiN ions and clusters (N = 1, 5, and 60) in the case of the same energy per one atom of the particle-projectile (200 and 1000 eV/atom) are studied in this paper. The study is carried out using classical molecular dynamics. As a result, several features of the low-energy implantation of polyatomic clusters in SiC(111) are revealed, namely, a relatively weak effect of the size of the implanted cluster on the distribution of ranges of incorporated atoms, a low degree of nonlinear effects at the cascade and postcascade stages, and formation of amorphous regions in the target during cluster implantation.

Atomistic simulation of the segregation of alloying elements close to radiation-induced defects in irradiated Fe–Cr–Ni BCC alloys

Ferritic-martensitic steels alloyed with Cr and Ni are promising structural materials for the nuclear and thermonuclear power industry. Under the influence of neutron irradiation degradation of the plastic properties of these materials takes place as a result of the generation of extended defects such as dislocation loops, and the formation of new phases (precipitates). In this work the atomistic computer simulation of thermodynamic processes of the precipitation of alloying elements is carried out using the newest model of ternary Fe–Ni–Cr bcc (body-centered cubic) alloys and the Metropolis Monte Carlo method in combination with the method of classical Molecular Dynamics. The composition and microstructure of Cr–Ni clusters formed in defect-free alloys and alloys containing dislocation loops, depending on the temperature and concentrations of Ni and Cr, are studied. An increase in the Ni solubility limit in the presence of dislocation loops by 100–200 K, depending on the Ni concentration, is detected. The synergetic effect of Ni and Cr segregation near dislocation loops in ternary alloy is established: the presence of Ni weakens Cr segregation, whereas Cr can either attenuate or amplify Ni segregation depending on the concentration of Ni in the alloy, the temperature and the type (Burgers vector) of loop. In general, in ternary Fe–Cr–Ni alloys, the total segregation effect is less pronounced than in binary Fe–Ni and Fe–Cr alloys.