E. E. Zhurkin

Computer simulation of ion sputtering of polyatomic multilayered targets

Abstract The novel binary collision approximation Monte Carlo (BCA-MC) computer codes TRIRS and DYTRIRS for simulating ion sputtering of polyatomic nonuniform amorphous targets are presented. TRIRS simulates the collision cascade in a target and related secondary processes, including sputtering, damage generation etc., being more realistic than similar MC-BCA codes in modeling low-energy interatomic collisions. These improvements ensure better simulation of low-energy atomic collision processes in nonuniform targets, like sputtering, and ultra-low energy ion implantation. DYTRIRS is the extension of TRIRS which simulates the dynamics of the ballistic stage of ion-induced modification and sputtering for a target under high-fluence ion irradiation. The efficiency of DYTRIRS is verified by comparing the simulation of sputtering and secondary-ion mass-spectrometry in-depth compositional profiling of molecular-beam epitaxy grown two-dimensional (Al,Ga)AsGaAs (001) heterostructures, including structures with silicon and aluminum marker layers.

Copper precipitation in iron: a comparison between metropolis Monte Carlo and lattice kinetic Monte Carlo methods

Several variants are possible in the suite of programs forming multiscale predictive tools to estimate the yield strength increase caused by irradiation in RPV steels. For instance, at the atomic scale, both the Metropolis and the lattice kinetic Monte Carlo methods (MMC and LKMC respectively) allow predicting copper precipitation under irradiation conditions. Since these methods are based on different physical models, the present contribution discusses their consistency on the basis of a realistic case study. A cascade debris in iron containing 0.2% of copper was modelled by molecular dynamics with the DYMOKA code, which is part of the REVE suite. We use this debris as input for both the MMC and the LKMC simulations. Thermal motion and lattice relaxation can be avoided in the MMC, making the model closer to the LKMC (LMMC method). The predictions and the complementarity of the three methods for modelling the same phenomenon are then discussed.

Study of Si and C adatoms and SiC clusters on the silicon surface by the molecular dynamics method

Individual Si and C adatoms, as well as SiC clusters, on a Si surface are simulated by the molecular dynamics method in the course of investigation of the initial stages of formation of a SiC layer on silicon with the help of molecular beam epitaxy. The potential energy surfaces for Si and C adatoms on the (2 × 1) reconstructed Si(001) surface and on the nonreconstructed Si(111) surface, as well as on the Si(111) surface with a SiC cluster, are calculated and analyzed. The values of migration barriers for adatoms on these surfaces are calculated. The effect of the SiC cluster on deformation of the surface region of Si(111) and on the migration of adatoms is investigated. The deep minima observed on the potential energy surfaces immediately above a cluster and at its boundaries can trap diffusing adatoms. The distributions of stresses and strains in the silicon lattice under a cluster on the surface are studied and described.

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.

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.

Study of the Microstructure Induced by High-Flux Plasma via Transmission Electron Microscopy

The annealed and heavily deformed states of the tungsten microstructure are studied using transmission electron microscopy after irradiation by high-flux plasma. Exposure to plasma substantially increases the dislocation density in the surface layers of both samples, namely, by more than an order of magnitude as compared to the initial value. At a distance of more than 10–15 μm from the surface, the material microstructure is comparable with that observed in the bulk of the sample not exposed to plasma. The given observation indicates that high-flux plasma produces deep and localized plastic deformation in the subsurface layer regardless of the initial hardening and dislocation density.

Effect of Segregation of Ni and Cr at Dislocation Loops on Their Interaction with Gliding Dislocations in Irradiated Fe−Ni−Cr BCC Alloys

Ferritic–martensitic steels alloyed with chromium and containing nickel impurities are considered as promising structural materials for nuclear and thermonuclear power engineering. During operation, the plastic properties of such materials degrade under the influence of neutron irradiation caused by the generation of radiation defects of the crystal structure, in particular, dislocation loops and new phases (precipitates). In this paper, an atomistic computer simulation of the interaction of mobile edge dislocations with dislocation loops having the 〈100〉 and 1/2〈111〉 Burgers vectors forming a single extended defect with Ni−Cr precipitates is performed using the classical molecular dynamics method at various temperatures (300 and 600 K). Such composite radiation-induced defects cause a change in the plastic properties of the irradiated material due to radiation hardening. The results of studying the interactions of gliding dislocations with loops (both in pure iron and in the Fe−Ni−Cr alloy, taking into account the precipitation of Ni and Cr at dislocation loops) show that the presence of an increased concentration of chromium and nickel atoms near the dislocation-loop perimeter at 300 K either decreases the critical stress for passing the dislocation through a defect (by more than 50 MPa) for the 〈100〉 loops at 300 K or increases it for the 1/2〈111〉 loops at 300 K. At a high temperature (600 K), the presence of Ni and Cr impurities near the dislocation loop leads to an increase in the critical stress for both types of loops. It is shown that the presence of an increased concentration of Ni and Cr atoms near the loop perimeter facilitates or hinders (depending on the specific dislocation-loop configuration) the transverse gliding of dislocation segments, complicates the possibility of the resplitting of junction segments of the dislocation and loop in the plane of loop location, and causes the immobilization of the loop having the [111] Burgers vector parallel to the gliding plane of the dislocation at 300 K.

Thermal Desorption Spectroscopy of Deformed and Undeformed Tungsten after Exposure to a High-Intensity Plasma Flow

As a result of the exposure of tungsten to a high-intensity plasma flow, it is established that the exposure of recrystallized and plastically deformed samples leads to fundamentally different mechanisms of confinement of plasma particles and associated deformation of the surface. The surface of the exposed deformed samples contains micrometer-sized ruptured blisters: an indication of the formation of subsurface bubbles on a grid of dislocations forming during deformation. Desorption spectra of both types of sample are decomposed into three peaks, corresponding to the detachment of plasma–gas particles from dislocations, deuterium-vacancy clusters, and pores. Plastic deformation, which leads to an increase in the dislocation density, does not change the position of the three peaks in the desorption spectra but increases their amplitude in comparison with the recrystallized material.

Segregation effects in the formation of nanostructured NixAl1-x alloys : a computer simulation study

Cluster assembled materials represent a novel class of nanostructured solids which properties may strongly deviate from those of single crystal or amorphous solids with the same composition. At present time, most studies report about mono- elemental nanostructured materials. However, alloys and, in particular, bimetallic systems are well known for their technological interest and are of high innovative potentialities in their nanostructured form. The aim of the present work is to model on the atomic scale the structural and segregation properties in the NixAl1-x bimetallic nanostructured materials that are synthesized from isolated clusters. Therefore, we use classical Molecular Dynamics (MD) and Metropolis Monte-Carlo (MC) techniques. The combination of MC and MD codes allows modelling both the syntheses of these materials and the segregation phenomenon.