New developments in the modeling and analysis of radiation damage in materials II

Resume : Modification of materials using ion beams has become a widespread route to improve or design materials for advanced applications, from ion doping for microelectronic devices to emulation of nuclear reactor environments. Yet, despite decades of studies, some questions regarding irradiation-induced microstructural effects are not comprehensively addressed. One such question is the lattice-strain development process in irradiated crystals. To address this issue, we implemented a consistent approach that combines X-ray diffraction (XRD) measurements with molecular dynamics (MD) simulations, and applied that approach to study UO2, c-ZrO2 and MgO. We demonstrate that these materials exhibit a common behavior with respect to the strain development process1. In fact, strain build-up followed by strain relaxation is observed in the three investigated cases. The strain variation is unambiguously ascribed, thanks to MD simulation data, to a change in the defect configuration: strain development is first due to the clustering of interstitial defects into dislocation loops, while the strain release is associated with the disappearance of these loops through their integration into a network of dislocation lines. Another topic that is worth deeper investigation is the response of materials to irradiation at varying temperature. To address this question, we considered c-ZrO2 and MgO because, whereas these materials exhibit similar damage accumulation process under ion irradiation at fixed temperature, they, in contrast, display an unexpected opposite damaging rate to changes in the irradiation temperature. Combining XRD experiments with rate equation cluster dynamics (RECD) simulations, we showed that, with increasing temperature, defect clustering is favored over defect recombination in c-ZrO2, but the situation is reversed for MgO, explaining the observed opposite response to temperature of the two materials2. This contrasting behavior can be rationalized in terms of non-equivalent interstitial versus vacancy defect migration energies in MgO, behavior that is not present in c-ZrO2. In the two cases addressed in this presentation, the use of computational techniques allowed for unraveling the large quantity of experimental data and provided better insight into the basic irradiation-induced disordering processes. This methodology can be applied to any other insulating material, given some (sometimes strong) simplifying assumptions.

Resume : We report here the effect of crystallite (grain) size on the radiation tolerance against simultaneous dual beam irradiations. Yttria stabilized zirconia pellets with two different crystallite sizes (nano-crystalline and bulk-like) are simultaneously irradiated with high energy 27 MeV Fe (electronic energy loss (Se) dominant) and low energy 900 keV I (nuclear energy loss (Sn) dominant) ions. XRD and Raman spectroscopy indicate that the nano-crystalline sample exhibits lesser radiation damage (viz. degradation in crystallinity) after the simultaneous irradiations i.e. the nanocrystalline sample is found to be more radiation tolerant, when compared with the bulk-like sample,under simultaneous irradiation with the high energy and low energy ions. This is interpreted within the frame-work of the thermal spike model after considering the fact that there is essentially no spatial and time overlap between the damage events of the two ‘simultaneous’ ion beams. The current work presents a keen interest for both fundamental understanding of ion-material interactions and the potential application of nano-crystalline materials in the nuclear industry. EXAFS measurements suggest a similar trend in the local disorder/damage as that in the long range (shown by XRD).

Resume : Nowadays, a big effort is being done for the Scientific Community in order to understand the complex mechanisms that lie in the relationship between irradiation damage, stress and high temperature. That synergy is observed in the formation of dislocation loops, swelling, complex chemical changes such as chemical segregation or secondary phase transformations which modify completely the material. All the insight gain in this matter comes from experiments that sometimes are quite complex due to the technical requirements to be performed, but very impressive advances have been achieved in terms of characterization of such microstructural evolution and converted afterwards into models that would be able to predict the behaviour of materials subjected in such harsh environment. However unfortunately, a huge gap is found between the real structural steel microstructures and the computational models available today. Trying to move a bit forward, a set of experiments that combine strained samples, irradiation damage and high temperature are performed. Small specimens were subjected to a strain gradient (supported by a simple FEM analysis) during self-ion irradiation at high temperature to study the effect of a strain field and temperature on irradiation defects. Afterwards, specimens were extracted from the irradiated sample and a deep TEM characterization was carried out. In order to fully understand the evolution of the defects during such experiment, strain-free specimens were also irradiated and characterized to establish properly a starting point irradiated microstructure. The materials that were taken into consideration were both a model alloy such as EFDA Fe10Cr (because is the base of the complex promising structural RAFM materials) and an well known EUROFER97. In the beginning, since the lack of knowledge about the possible synergy between strain effect during irradiation in the defects fate, high irradiation damage was achieved, far from overlapping dose, to easily determine the possible effect or even if strain field act as bias for defect generation. So far, there is not extensive literature studying the issues concerning these experiments, for that reason, it is believed that this research is of a great interest to the Nuclear Fusion Community.

Resume : One of the main structural candidate materials for advanced nuclear fusion reactors over the last decade are oxide dispersed strengthened reduced activation ferritic steels (ODS RAFS). A fine, homogeneous, dispersion of stable nanoparticles such as Y2O3 would allow increasing their upper operating temperatures and radiation damage resistance. However, currently available results require further research to clarify the effect of irradiation on these materials. The main objective of this work is to investigate the microstructural stability of different ODS RAFS with nominal composition Fe–14Cr–2W–0.3Ti–0.3Y2O3 (wt%) subjected to simultaneous dual (Fe3+, He+) ion irradiation at 400 °C to 30 dpa. The characterization of the steels has been accomplished using Transmission electron microscopy (TEM), Atom Probe Tomography (APT) and Positron Annihilation Spectroscopy (PAS). The stability of the nanoparticles has been investigated. The irradiation induced damage has been characterized, focusing in the study of open volume defects. There is a slight change in the chemical composition of nanoparticles after irradiation. The mean size and mean distance of nanoclusters is lower after the irradiation pointing to dissolution of larger nanoclusters and nucleation of new, smaller ones. PAS investigations show a limited increase of open volume defects after the irradiation suggesting a possible entrapment of all injected He ions into the open-volume defects simultaneously created by the Fe irradiation or/and recombination of vacancies and interstitials due to the intermediate temperature of the irradiation. CDB profiles show no clear relation between positron traps and He.

Resume : In the study of the evolution of Fe microstructures and their alloys under irradiation, there is a large body of work focused on the defect formation energies calculated through ab initio simulations. In general, the minimum-energy configuration of defects is used in kinetic models to simulate the long-term evolution of the microstructure. In this work, our goal is to study the evolution of the Fe microstructure under irradiation when the presence of non-conventional configurations of interstitial clusters is taken into account. We calculated with ab initio the formation energies of several configurations of different interstitial and vacancy structures in Fe. Additionally, for each defect, the transformation energy between the two most stable configurations was calculated. In order to determine the influence of non-conventional configurations on the microstructure evolution, we reproduced a Resistivity Recovery (RR) experiment in electron-irradiated Fe using Object kinetic Monte Carlo (OkMC) with two different models. The first set of simulations uses the conventional configurations for interstitial clusters, which correspond to the most stable configurations. The second set of OkMC simulations takes the two most stable configurations for each cluster into consideration and the probability of transformation from one configuration to the other.

Resume : Positron annihilation spectroscopy (PAS) is a well-established technique for characterizing materials. It is based on the positron specific properties in matter. It can be trapped in vacancies and annihilates with electrons leading to the emission of gamma rays with energy depending on the momentum of the electron-positron annihilated pair. Two annihilation characteristics give the signature of the defects and allow determining some of their properties such as their nature, concentration, chemical environment : the positron lifetime related to the electron density experienced by positron before annihilating and the momentum distribution of the electron-positron annihilated pairs. By using slow positrons, the defects depth profile can be studied in thin layers. PAS has numerous advantages among them, it is non-destructive and can be used for conducting as well as insulating materials crystalline or amorphous. The annihilation characteristics can be predicted by first principle calculations and these data when available can help to identify defects. Sensitive to the single vacancy, PAS can allow to determine the size of the vacancy clusters up to a maximum of about 1 nm in the concentration range from 1015 to a few 1018 cm-3. In this talk the advantages of PAS an d its complementarity with other techniques and modelling will be illustrated with examples based on recent studies in the spintronic or nuclear (fusion, fission) applications.

Resume : Bombardment of semi-conductors induces several perturbations, chemical (nature, network and stoichiometry), morphological, optical… This study focuses on the perturbation evaluation induced on InP by an Ar bombardment. Both Ar ionic sputtering performed directly inside the X-Ray photoemission spectroscopy (XPS) chamber using an ion gun by means of a plasma source through Glow Discharge Optical Emission Spectroscopy (GD-OES) set-up are considered. Electrochemistry is a very efficient probe and quantitative tool to provide information about bulk and surface properties of any electrode material. Its capabilities will be illustrated and associated to XPS analyses to get a better insight on the damages induced by those bombardment which can strongly impact the material properties and the integrity of the information collected during XPS sequential profiling for example. One of the possible investigations using the modified electrochemical response concerns the differential abrasion effect induced by the ions/surface interaction. Ionic bombardment of InP surfaces generates a surface stoichiometry loss, with an In enrichment. We will discuss how the XPS and electrochemical characterizations combination is able to provide complete and complementary diagnostic of the stoichiometry loss and its electrochemical reactivity. Those modifications will be compared to an electrochemistry cathodic decomposition of InP. Anodic dissolution will be also presented in order to recover the surface integrity of InP surface. Characterizations of the perturbations will be performed with XPS measurements, Scanning electronic microscopy (SEM) but mostly, with electrochemical measurements.

Resume : As UO2 is a widely used fuel in current nuclear reactors worldwide, it is imperative to have a comprehensive knowledge of irradiation-induced defects in this material. It has been shown that molecular dynamics (MD) simulations can be an effective approach to mimic irradiation ballistic damage occurring in UO2 [1]. MD simulations, thus, can provide reference damaged targets as input for computational simulations aiming at reproducing experimental signals of characterization techniques. In this paper, we present the investigation of MD cells of damaged UO2 with newly developed simulation codes [2,3] for two characterization techniques: Rutherford backscattering spectrometry in channeling mode (RBS/C) and X-ray diffraction (XRD). The fluence of the UO2 MD cells, represented by displacements per Uranium atom (dpU), ranges from 0 to ~10 dpU, which covers several defect evolution stages. Disordering kinetics and elastic strain kinetics were determined from RBS/C and XRD simulations. The computed kinetics exhibit qualitatively close agreements with those determined experimentally, indicating that the used methodology is valid. In addition, decomposition of the kinetics was performed in order to study the effect of each defect separately, which enables a quantitative description of the disordering and strain build-up processes [4]. References [1] A. Chartier, C. Onofri, L. Van Brutzel, C. Sabathier, O. Dorosh, and J. Jagielski. Early stages of irradiation induced dislocations in urania. Appl. Phys. Lett., 109(18), 2016. [2] S. Zhang, K. Nordlund, F. Djurabekova, Y. Zhang, G. Velisa, and T. S.Wang. Simulation of Rutherford backscattering spectrometry from arbitrary atom structures. Phys. Rev. E, 94(4):1-12, 2016. [3] X. Jin, S. Zhang, J.-P. Crocombette, K. Nordlund, F. Djurabekova, F. Garrido, and A. Debelle. New developments in the simulation of Rutherford backscattering spectrometry in channeling mode using arbitrary atom structures, Model. Simul. Mat. Sci. Eng., 28 (2020) 075005. [4] Xin Jin, Alexandre Boulle, Alain Chartier, Jean-Paul Crocombette and Aurélien Debelle, Analysis of strain and disordering kinetics based on combined RBS-channeling and X-ray diffraction atomic-scale modelling, Acta Materialia, 201 (2020) 63-71.

Resume : The main purpose of the McChasy code is to reproduce RBS/C experimental spectra by simulating He-ions traveling inside the crystal structures. Nuclear-encounter-probability approach and Monte Carlo method are used to reproduce the spectra. The new version of the code (v.2.0) provides possibility to simulate channeling spectra in large samples of solid crystalline materials with ca. 10^8 atoms. Such samples can be created on the basis of crystallographic data or produced by Molecular Dynamic calculations. In this work McChasy code was used to study the local flux of ions passing through the crystal with extended structural defects (dislocations and dislocation loops) thermalized by using MD - LAMMPs code. Influence of such defects on the flux and on channeling backscattering spectra is shown and discussed.

Resume : Molecular dynamics simulation of radiation damages in materials frequently refer to studies in which displacement cascades, threshold displacement energies and sometimes, thermal spikes are performed. Each of these techniques are meant to address the primary damage – i.e. single events – produced by electronic loss and/or ballistic interactions. Recent attempts have tried to explore the effect of the irradiation dose by accumulating these single events. However, this demarche is limited to low doses (0.1 dpa) since very cpu demanding. One may circumvent single events and access higher doses – in some specific cases – by shortcutting part of the story. Instead of repeating ‘single event’ one after the other to get the effect of dose, the final state can be used as a starting point. Here, we show that as long as single events end up with point defects or very small clusters, it is convenient to accumulate point defects to reach doses up to 10 dpa or more [1]. Such a simulation procedure opens highways for understanding complicated mechanisms at work when materials are in severe irradiation environments. For example, one can (i) identify the specific defect responsible for amorphization in titanate pyrochlores, (ii) shed light on the wrinkling responsible for the anisotropic swelling in graphite, (iii) exhibit Frank loops as part of the monitoring mechanism for UO2 swelling, or (iv) evidence that C15 clusters are seeds for both ½ <111> and <100> loops.

Resume : Corundum (α-Al2O3) is an important wide gap material with a great potential for optical applications in radiation environment, e.g. for components of diagnostic windows and breeder blankets. Radiation damage of α-Al2O3 caused by fast neutrons, energetic electrons and ions have been investigated for many years. The optical characteristics of the anion-vacancy-related elementary Frenkel defects—F+ and F centers have been thoroughly studied in irradiated and additively colored crystals. Nevertheless, the complementary Frenkel defect to the F-type center—a single interstitial oxygen—still remains the most hidden primary defect in α-Al2O3. Here we present ab initio calculations of the oxygen interstitials with the formation of O2 defects with the host oxygens in various oxidation states (OS = 1-, 2-, 3-) in α-Al2O3. It is in line with recent experimental EPR data on single oxygen interstitial in a form of superoxide ions (OS=1-) created in α-Al2O3 single crystals under fast neutron irradiation. Using LCAO approach and hybrid B3PW functional as implemented in the CRYSTAL17 computer code structural, electronic and vibrational properties are evaluated for each OS. Energy barriers for oxygen interstitial migration are calculated and compared with recent thermal annealing kinetics data. The DFT calculations confirm experimentally observed superoxide ion with spin S = 1/2, its orientation along a axis. The O2- center is characterized by a short O–O bond length of 1.34 Å and different atomic charges and magnetic moments of the two oxygens.

Resume : The effect of C and N interstitials as well as vacancies (Vac), the concentration of which is increased in irradiated materials, on the phase stability of Fe-Cr alloys have been investigated using a combination of density functional theory (DFT), cluster expansion (CE) and Monte Carlo (MC) simulations. To develop the CE model, we performed DFT calculations for more than 1500 structures including various size Cr-Vac-C-N clusters in Fe and Cr matrices, in ordered Fe-Cr alloys as well as in alpha-cementite-like X3Y structures, where X is Fe, Cr or their combinations; Y is C, N or their combinations. Since in Fe-Cr alloys the C and N interstitials occupy octahedral positions, the CE model involves two sublattices – one lattice for Fe and Cr at bcc positions and the other one for C and N atoms at octahedral positions. Vacancies are present on both bcc and octahedral positions. Two-body CE interactions between atoms in bcc-bcc, octahedral-octahedral and bcc-octahedral positions are taken into account up to the 5th nearest neighbor distance in bcc lattice. Our studies show that including three-body interactions is also necessary to reproduce the experimental results. The CE model is applied to parametrize exchange MC simulations to investigate the phase stability of Fe-Cr alloys with low Cr concentration and increased concentration of vacancies, as well as higher content of carbon and nitrogen impurities. MC simulations show the formation of Cr-enriched clusters in the presence of C and N interstitials and vacancies for the majority of considered alloy compositions at temperatures of 450 K and 650 K. The concentration of Cr in the clusters increases with increasing content of C and/or N impurities in an alloy. However, the structure of Cr-enriched clusters can be very different depending on the concentrations and the ratios of concentrations of each element. Our modelling results are supported by experimental studies. Cr concentration in Cr-enriched clusters at 650 K is in a qualitative agreement with our recent atom probe tomography (APT) results for the Fe-5.8%Cr alloy irradiated at 673 K. Increased concentration of C and N interstitials is also observed inside the Cr-enriched clusters via the APT analysis.

Resume : The emergence of alternative Si processes and devices has promoted applications outside the usual processing temperature window and the failure of traditional models of defect kinetics. They are based on Ostwald ripening mechanisms with parameters estimated by atomistic simulations, but generally pre-established defect configurations were assumed, formation energies were evaluated at 0 K and entropic contributions were neglected. We have carried molecular dynamics simulations of self-interstitial clustering in crystalline Si with no assumptions on preferential defect configurations. We have analyzed relevant defect configurations and we have characterized them in terms of their formation enthalpy and also their vibrational entropy calculated from their local vibrational modes. Our calculations illustrate that entropic terms are key to understand defect evolution in a wide temperature range. In addition, we show that, although different crystallographic planes favor specific defect symmetries and “magic numbers”, energy barriers make it difficult the transformation to the minimum energy configuration for each size. This allows the presence of non-expected small defects clusters that may lead to optical and electrical signals in low temperature processes. At very high temperatures, typical of sub-melting laser Si processing, we have found highly entropic liquid-like self-interstitial clusters embedded in the solid that dominate defect kinetics through a coalescence mechanism.

Resume : By means of the hybrid exchange-correlation functionals, as it is implemented in the CRYSTAL computer code, ab initio calculations for main ABO3 perovskite (001) surfaces, namely SrTiO3, BaTiO3, PbTiO3, CaTiO3, SrZrO3, BaZrO3, PbZrO3 and CaZrO3 were performed. For ABO3 perovskite (001) surfaces, with a few exceptions, all atoms of the upper surface layer relax inward, all atoms of the second surface layer relax outward, and all third layer atoms, again inward. The relaxation of (001) surface metal atoms for ABO3 perovskite upper two surface layers for both AO and BO2-terminations, in most cases, are considerably larger than that of oxygen atoms, what leads to a considerable rumpling of the outermost plane. The ABO3 perovskite (001) surface energies always are smaller than the (011) and especially (111) surface energies. The ABO3 perovskite AO and BO2-terminated (001) surface band gaps always are reduced with respect to the bulk values. The B-O chemical bond population in ABO3 perovskite bulk always are smaller than near the (001) and especially (011) surfaces [1,2]. We performed comparative ab initio calculations for polar ReO3 and WO3 (001) surfaces, and compared the results with ABO3 perovskite (001) surfaces. The systematic trends in our ABO3 perovskite bulk and (001) surface F-center calculations are analyzed and compared with relevant experimental data [3]. References: [1] R. I. Eglitis and A. I. Popov, J. Saudi Chem. Soc. 22, 459-468 (2018) [2] R. I. Eglitis, J. Purans, J. Kleperis, A. I. Popov, R. Jia, Journal of Materials science 55, 203-217 (2020) [3] R. I. Eglitis and A. I. Popov, J. Nano-Electron. Phys. 11, 01001 (2019)

Resume : We simulate the dynamics of silicon after its excitation by an X-ray laser in two-dimensional (2D) geometry. The approach combines classical Monte Carlo (MC) [1] with Two-Temperature-like model, nTTM [2]. The MC takes into account X-ray absorption and electron cascading and provides the initial conditions for the nTTM, which takes into account Auger recombination and impact ionization, band gap evolution, ambipolar carrier diffusion, electronic and atomic heat conduction, and electron-phonon coupling. To solve the nTTM system of equations in 2D, we developed a finite-difference integration algorithm based on Alternating Direction Implicit method with additional predictor-corrector algorithm, which takes care of the nonlinearities. We show the first results of the model and discuss its possible applications. In particular, the approach should be applicable also for metals, various radiation sources (e.g., heavy ion beams), and extendable to three dimensions. [1] V. Lipp, N. Medvedev, B. Ziaja, Proc. SPIE 10236, 102360H (2017). [2] H. van Driel, Phys. Rev. B 35, 8166 (1987).

Resume : A large database of displacement cascades calculated with Molecular Dynamics has been built up. Statistics studies reveal detailed features of the primary damage, particularly from the point of view of the defect production and the distributions of size of the vacancy and SIA defects at the end of the cascades. Several empirical potentials in Fe and W have been used, also differing by their short range part (called “soft” or “hard” potentials) and their long range (or equilibrium) part. The sensitivity of the defect distributions to the equilibrium part of the potential as well as its hardened part has been analyzed. In parallel, other simulations of the static non-equilibrium properties (quasi static drag (QSD)), threshold displacement energies (TDE), replacement collision sequences (RCS) along the <110> direction have been realized. Correlations between the primary damage and these new properties have been established. Along the<110> direction, the lower the TDE, the lower the QSD and the more energy is transmitted along the <110> direction during the RCS, i.e. the softest potentials are the ones for which the most energy is transmitted from the PKA to the first head-on atom in the direction of the RCS sequence. The morphology of the individual displacement cascades has been described defining 3 descriptors: the volume, the number of sub cascades and the sphericity. On the other side, 7 descriptors of the primary damage characterize the total number of defects, the number of SIA and vacancy clusters and their full size distributions. A multivariate multiple linear regression analysis based on 3 7 descriptors and the choice of the potential has been conducted. We find that the combination of the volume and the sphericity is meaningful. This analysis highlights several cascade properties, among them, that the large and spherical cascades create less defects and in particular, less mono defects than small and fragmented ones. The multivariate analysis also shows that the choice of potential has a limited influence on the total number of defects but a large one on the number of mono vacancies. On average, soft potentials create cascades of larger volume, smaller sphericity and producing more defects than hard potentials. Finally, our investigations demonstrate that the formation of vacancy clusters is different in Fe than in W. In Fe, the fraction of vacancies in clusters is larger than that of SIAs and larger vacancy clusters are created than SIA clusters. In W, it is the opposite. The reasons are the differences of nuclear stopping power and threshold displacement energies, which result in different spatial distributions of open volumes that form during the expansion stage of the cascade

Resume : We developed a cluster dynamics model to study the evolution of He bubbles and dislocation loops in reduced activation ferrite/martensite Eurofer97 steel under Fe3 /He dual-beam irradiation. The model was firstly calibrated through experimental data of bubble size distributions. Simultaneously, the simulations also reasonably reproduced size distributions of dislocation loops concurrently formed during the irradiation. We show that with increasing irradiation dose, bubble nucleation was governed in sequence by irradiation-induced vacancies, implanted He atoms, and kicked-out He atoms. Among the three phases, the kicked-out phase provides a major contribution to bubble nucleation. Higher irradiation temperatures tend to weaken bubble nucleation in the phase driven by the kicked-out reaction due to a resulting stronger recapture effect on interstitial He atoms. The present cluster dynamics model also takes into account the widely reported formation of C15 clusters prior to loop formation. We show that as irradiation dose increases, the growth of dislocation loops follows a power law f(n) ~ a/nS, similar to the results found by molecular dynamics simulation of collision cascades. Enhanced growth of C15 clusters at 603K and 673K promotes C15 cluster loop transformation resulting in pronounced loop evolution. While at the higher irradiation temperature of 773K, the high (low) concentration of mobile vacancies (single, di- and tri-interstitials) keeps the loop evolution staying at the C15-loop complex formation stage, resulting in no loop observed experimentally. The effectiveness of the model developed in this study demonstrates its potential to be used in similar irradiation cases.

Resume : One of the problems within the framework of Dark Matter search experiments is the creation of a neutron absorption system - VETO. One of the most effective known neutron absorbers is gadolinium nuclei. The VETO system requires the development of a constructionl material in which gadolinium or its compounds would be uniformly distributed in the volume of the polymer matrix with Gd- concentration ~ 1 wt%. and total content of radioactive impurities less than 1 mBq/kg. In terms of Gd compounds, the U and Th contents should not exceed 50 ppt. We found commercial gadolinium containing preparations with chemical purity of 4N-5N in which the content of radioactive components varied by 3 orders of magnitude. The procedure of reduction of U and Th content to 50 ppt level has been developed. Several polymer matrixes were used to incorporate Gd nuclei and to achieve the homogeneity distribution. The mechanical properties of hybrid materials within 77-300 K temperature range were studied. The research was financially supported by the Ministry of Science and High Education of Russian Federation by the project RFMEFI60419X0238

Resume : Possible mechanisms of transformation of defects in thin films of GaN and GaAs under action of electromagnetic radiation in the microwave range and pulsed magnetic field have been analyzed. Electrical-resonance effects under non-thermal action of electromagnetic fields have been considered, namely: resonant detachment of dislocations and destruction of impurity complexes in semiconductor crystals, electrical-resonance transformation of defects in semiconductor crystals under action of weak pulsed magnetic fields; magnetic-resonance effects on defects in semiconductor crystals under action of weak magnetic electromagnetic fields. It has been shown that alternative interaction mechanisms should be used to explain a large number of reliably established magnetically induced effects and phenomena associated with the nonthermal effects of microwave fields. There are two most likely mechanisms: (i) spin-dependent reactions in paramagnetic defects of semiconductor crystals, as a result of which detachment and subsequent movement of dislocations in the field of internal stresses and (ii) resonant phenomena of various natures occur, which in generally, do not require high energies, and have been realized when the oscillation frequencies of the system and the external action are coincide. A sharp increase in the amplitude of oscillations leads to detachment of dislocations and destruct of impurity complexes with subsequent movement and diffusion under action of a mosaic of internal mechanical stresses of the crystal. The principal physical identity of the influence of a weak magnetic field and non-thermal action of microwave radiation on a semiconductor material has been shown. These results could be significant contribution to the “defect engineering” direction.

Resume : The determination of sub-surface strain and disorder profiles is an essential step in the characterization of ion irradiated materials. Among the few existing techniques that allow to reach this goal X-ray diffraction (XRD) has the advantage of being simultaneously sensitive to strain and disorder, while being a non-destructive technique that allows to determine these quantities on an absolute scale. The main obstacle on this path is that the determination of the depth-resolved strain/disorder profiles requires a careful modeling and simulation of experimental data, a task that is often restricted to experts of the XRD technique. In order to make the XRD determination of strain/disorder profiles more intuitive we previously developed the free and open-source software RaDMaX (https://aboulle.github.io/RaDMaX/). RaDMaX is written in Python and therefore requires the installation of a full Python development environment which can be a disincentive for a non-negligible amount of potential users. With this in mind, we rewrote RaDMaX in order to make it able to run in a web browser. This program, RaDMax-online, allows the users to upload XRD data, analyze and fit the data on the web, and download the simulation results. It can be found at: https://aboulle.github.io/RaDMaX-online/ . The program entirely relies on free / open-source libraries related to the Python ecosystem (NumPy, SciPy, Jupyter, ipywidgets, bqplot, voila). The website is deployed using Docker containers.

Resume : To understand the physical phenomena responsible for irradiation damage of the materials used in Nuclear reactors, it is important to simulate the condition by exposing in harsh irradiation environment to study the operation life efficiency and crystallinity degradation. YSZ and CeO2 Inert matrix fuel concept have been proposed to reduce the further breeding of plutonium and Minor actinides. As to compensate the initial reactivity of fuels, Gadolinium is an excellent burnable poison with high neutron absorption cross-section. Gd2O3-CeO2 nano powders were synthesized and sintered at 800℃ , 1000℃ and 1300℃ to obtain different grain size. FESEM and TEM was done to visualize the grain size of pristine samples. At room temperature we irradiated pellets with 80 MeV Ag ions to emulate the nuclear reactor environment. For critical analysis of grain size effect and structural degradation post irradiation at different fluences we performed XRD and Raman Microscopy. Despite large damage due to 80 MeV Ag swift heavy ion at smaller grained size samples (S800) comparative to large grain size samples (S1000 &S1300) ;But no complete amorphization was observed whatsoever, as suggested by the presence of all peaks of Raman active T2g mode (centered at 462 cm-1) and XRD peak (111) of fluorite cubic structure upto highest fluence of 1×10^14. Thus Gd-doped ceria possess better radiation stability at room temperature in electronic energy loss regime against swift heavy ion irradiation.

Resume : Tungsten is used as plasma-facing wall in ITER and is also foreseen to be used in future fusion devices like DEMO, where it is subjected to considerable neutron irradiation. In this work, we study the damage formation in crystalline W(111) samples by neutron bombardment as function of temperature for a set of sample temperatures in the range of 300 - 1900 K. The consideration of temperature effects is important because most of the laboratory research on defect formation is performed at or near room temperature using ion-beam bombardments to emulate the kinetic aspects of neutron impacts. However, the operation temperature of a plasma-facing tungsten wall in a reactor is significantly above room temperature. In the simulation the neutron irradiation impact is emulated using primary knock-on atoms (PKA) of 10 keV. The molecular dynamics (MD) simulations are performed using a recently developed many-body tungsten potential derived within the Gaussian Approximation Potential (GAP) framework [1] by a ML ansatz which improves upon earlier W-potentials especially in the description of defects. The analysis of the induced damage is based on a rotation invariant mapping [2] of the local atomic neighborhoods allowing to extract, to identify and to visualize the atomic defect formation dynamically during and after the collision cascades. The evaluation allows to classify the various defect types and to quantify the defect evolution as function of time and initial sample temperature. The observed dependencies can be described reasonably by a sublinear scaling law. [1] J. Bygmaester et al Phys. Rev. B 100, 144105 (2019) [2] U. von Toussaint et al Comp. Phys. Comm. 107816 (2020). In Press.

Resume : Brushite CaHPO4.2H2O (Dicalcium Phosphate Dihydrate: DCPD) is considered as a sorbent of strontium (90Sr2+) which is a contaminant present in some acidic effluents produced by cleaning operations during the dismantling of formernuclear facilities. The idea is to check how electron - radiation – sensitive is this material, since structural and chemical changes caused by irradiation might affect significantly its ion exchange capacity. The energy deposited by the projectile electron into the target is known to be approximated as the sum of two contributions, ballistic and electronic. But only their order of magnitude is supposed to be known. As a result, the structural damage caused by electron irradiation cannot be even reasonably estimated and the radiation sensitivity of brushite has to be checked experimentally. In this paper, powder X-ray diffraction and spectroscopies are associated to analyze the damage effects to brushite following electron irradiation. The accelerator NEC Pelletron of the SIRIUS platform is used for electron irradiation at 2.5 MeV (dose rate of order 20 kGy/s). Amorphization of brushite is observed with increasing dose, and was found complete at 6 GGy. The spectroscopic analysis (Raman, IR, RMN) tells us that the ultimate compound at the highest dose could be of type calcium pyrophosphate. Consequences regarding the damage mechanisms are considered.

Resume : In alkali halide crystals under the action of low-temperature elastic deformation, the nature of luminescence and the mechanisms of radiation defect formation change significantly [1–2]. It is expected that the targeted effect of elastic deformation on the radiative relaxation of electronic excitations will improve the luminescence characteristics of CsI crystals, which are traditional objects as scintillation counters. In CsI crystal, the luminescence of a self-trapped exciton at 100 K consisting of two bands with maxima at 4.27 eV and 3.67 eV was studied under the action of elastic deformation and radiation, which selectively create excitons, electron-hole pairs, and X-rays. Under X-ray excitation, uniaxial compression (ε = 1%) of CsI crystal leads to the complete disappearance of the emission band with a maximum at 4.27 eV, and simultaneously, to a threefold increase in the radiation intensity with a maximum at 3.67 eV, which is important in the development of scintillation detectors. An analysis of the temperature dependence of two bands with maxima at 4.27 eV and 3.67 eV under the action of elastic deformation suggests that the emission band at 4.27 eV belongs to the on-configuration of the exciton, according to the exciton classification [3] and, therefore, is sensitive to uniaxial deformation, which lowers the symmetry of the lattice. For CsI-Na crystal, in addition to the radiation with maxima at 4.27 eV and 3.67 eV, an additional band at 2.95 eV was recorded, caused by the self-trapped excitons radiation near sodium, which disappears under the action of deformation at 100 K. In this case, the radiation at 4.27 eV is partially conserved, as evidenced by the intrinsic nature of the emission from CsI lattice. For CsI crystals under the action of elastic deformation, the luminescence of self-trapped excitons with an asymmetric configuration (on to weak off) with a maximum at 3.67 eV is more pronounced, and the emission of a self-trapped exciton with an on-configuration maximum of 4.27 eV disappears. This research has been funded by the Science Committee of the Ministry of Education and Science of the Republic of Kazakhstan (Grant No. AP08855672). References 1. А. Bekeshev, K. Shunkeev, E. Vasilchenko and A.Elango, Effect of uniaxial compression on the luminescence of self-trapped excitons in CsI at 80 K, Phys. Solid State 39, 75-76 (1997). 2. V. Babin, A. Elango, K. Kalder, A. Maaroos, K. Shunkeev, E. Vasil’chenko and S. Zazubovich, Luminescent defects created in alkali iodides by plastic deformation at 4.2K, J. Lumin. 81, 71-77 (1999). 3. K. Kan’no, H.Tanaka and T. Hayashi, New aspects of intrinsic luminescence in alkali halides, Rev. Solid State Sci. 4, 393-401 (1990).

Resume : Investigation of the relaxation processes of electronic excitations in alkali halide crystals in the field of disturbing factors is a very promising direction for the creation of materials with specified physical characteristics, as well as for the search for optical materials with a wide transparency region, even when exposed to ionizing radiation. In alkali halide crystals, primary radiation defects (F, H pairs) are created during the decay of anionic excitons at 4.2 K, and at temperatures above the delocalization temperature of interstitial halogen atoms (50 K), the main radiation defects complementary to F-centers are three halogen centers, which are formed as a result of the association of mobile halogen atoms with each other, as a form of their stabilization [1]. This work proposes an original approach to the study of three halogen centers in KCl matrix in the field of local, plastic, and elastic deformation. Local lattice deformation is carried out by purposeful doping of KCl crystal with light homologous cations (Li, Na), as well as bivalent impurities, which create dipole strontium complexes to maintain the electroneutrality of the lattice. Plastic deformation, which creates paired vacancy defects in the lattice - divacancies and vacancy quartets, is realized at room temperature within the degrees of relative deformation of 3−5%, then the deformed crystal is cooled to 80 K, and at this temperature it was irradiated with X-ray radiation. The elastic deformation of crystals was carried out in a cryostat at a temperature of 95K within the range of up to 1%, which corresponds to the linear section of the deformation dependence. The absorption spectra of crystals under three types of deformation and after the isodose regime of X-ray irradiation were recorded in a cryostat at a temperature of 95K [2]. The spectral characteristics and efficiency of formation of V-centers, lithium and strontium centers in the field of dipole complexes previously created in the lattice - divacancies, a quartet of vacancies , lithium and strontium complexes, respectively, have been established [3]. This research has been funded by the Science Committee of the Ministry of Education and Science of the Republic of Kazakhstan (Grant No. AP08855672). References 1. Lushchik Ch, Lushchik A. Evolution of Anion and Cation Excitons in Alkali Halide Crystals // Physics of the Solid State. – 2018. – Vol. 60. – P. 1487-1505. 2. Shunkeyev K., Sarmukhanov E., Bekeshev A., Sagimbaeva S., Bizhanova K. The cryostat for deformation of crystals at low temperatures // Journal of Physics: Conference Series. – 2012. – Vol. 400(5). – 052032. 3. Zhanturina N., Myasnikova L., Shunkeyev K., Maratova A., Popov A. Efficiency of H-center stabilization in alkali halide crystals at low-temperature uniaxial deformation // Low temperature physics. - 2020. - Vol.46. - P.1165-1169.

Resume : Understanding the radiation effects in wide-bandgap semiconductors upon swift heavy ion (SHI) bombardment is of high importance for the electronic device’s technology. On the one hand, since SHIs occur in cosmic radiation they can, e.g., affect the device's lifetime in space applications. On the other hand, they can be a powerful tool for processing devices, e.g., for the nanoengineering of semiconductors. Here we present our recent study on the radiation effects of Pb and Xe SHIs on InGaN/GaN layers. The influence of the SHI-bombardment has been investigated using Rutherford backscattering/channeling spectrometry (RBS/C) followed by Monte Carlo (MC) simulations using the updated McChasy code. The research was supported by Molecular Dynamics (MD) simulations. Post-irradiation damage has been investigated regarding different fluences of 1.2 GeV Pb ions and different energies of Xe ions that were tuned from ~80 to ~38 MeV by degrading a 90 MeV Xe beam using Al foils of different thicknesses. Mechanisms of the damage formation will be discussed and possible types of created defects will be indicated. Structure recrystallization and morphology of tracks left behind after passing the SHI will be presented, as visualized by MD simulations. Difficulties in the analysis of experimental data resulting from the different influence of point and extended defects on RBS/C spectra will be pointed out, explaining the necessary use of computational techniques. The model of dislocation loops implemented in the MC channeling simulation code (McChasy) and the way of quantification of their depth-distribution will be described. The possible influence of the SHIs on InGaN/GaN-related devices will also be concluded.

Resume : Nowadays GaN is well recognized as the material of interest for a wide range of electronic device applications. Understanding of the processes take place during irradiation with fast ions is extremely important to design technologically improved and radiation hard GaN devices. Co-implantation with different ions and/or ion energies is used in technology, for example, to produce deep and uniform dopant distributions. However, the resulting distribution of structural damage appear during such co-implantation can significantly differ from the sum of damage-depth distributions obtained by irradiation with each of the ions separately. Moreover, it strongly depends on the sequence with which such a co-implantation is performed. In this work investigation of ion-induced damage formation in GaN under sequential irradiation with ions of two energies at room temperature has been carried out. Silicon-doped n-GaN epitaxial layers grown by MOCVD on sapphire substrates were used in this study. Radiation damage was measured by RBS/C. First irradiation raises well known defect distribution consisting of surface and bulk maxima for all cases studied. Damage in the bulk exhibits saturation well below amorphization level. Second implantation changes obtained distributions different ways. It was found that the bulk disorder slightly decreases, that is, radiation-induced annealing takes place when the primary damage was created by other kinds of ions.. On the contrary, if the primary disorder is formed with fluorine ions, high energy ion irradiation leads to an increase in the bulk defect peak above the saturation level, up to the amorphization of the material. A noticeable effect of F on these phenomena begins at concentrations of implanted fluorine atoms ~ 5 × 10^21 cm ^-3. Possible mechanisms of the effects found will be addressed and discussed.

Resume : One of the most efficient channels for transferring a large portion of the energy of local electronic excitations to atomic degrees of freedom(the Auger cascade), which leads to Auger destruction of molecular objects, was studied in this work as applied to quasi-one-dimensional systems. A number of Auger destruction models were built, taking into account the following factors, manifested both partially and in combination: a) the physicochemical nature of the object, b) its conformation, c) its configuration, including fractal. A special sensitivity to these factors was revealed at the stage of competitive realization of either spred of a charged group of ions or electron flooding (neutralization) of Auger charges. It is shown that the combined effect of the listed factors increases the probability of Auger destruction by orders of magnitude. The developed models are used to interpret the following phenomena: increased radiation resistance of carbon chains in comparison with silicon ones; the prevalence of light elements over heavy ones in the composition of macromolecules of living nature; stability of monachiral structures in the conditions of the evolution of the Earth. Particular attention is paid to the assessment of the maximum possible reduction in the dose of X-rays when exposed to RNA and DNA of actual viruses in comparison with the basic quasi-linear molecules of a living organism. It is concluded that the processes of the studied type of Auger destruction belong to the category of complexity

Resume : Gallium oxide nowadays is recognized as a promising semiconductor material for a number of important applications, such as high-power electronic devices and solar-blind ultraviolet photo-detectors. Besides its superior properties themselves, an additional driving force pushing interest to this material is the progress achieved in large diameter bulk β-Ga2O3 crystal growth. Ion implantation is a well established tool of semiconductor technology, likely to be utilized in Ga2O3-based device fabrication. However, ion bombardment is always accompanied by the formation of structural defects, which can dramatically affect the properties of semiconductor and device performance. Despite this, information on the processes of radiation damage formation and recovery in Ga2O3 is very scarce. In this contribution, we study structural disorder in Ga2O3 single crystals irradiated at room temperature with monatomic (P and F) and molecular (PFn, n=2,4) ions. Implantation-produced disorder was measured by RBS/C spectrometry. Results show that, at small doses of monatomic ions, the level of damage created is the greater, the heavier stopping ion is. Molecular ions produce more damage both near the surface and in the region between surface and bulk defect peaks. Note that, according to the binary collision approximation, amount of point defects generated during irradiation was kept constant for all ions used. Thus, strong effect of collision cascade density on irradiation defect formation is clearly observed. Interestingly, damage accumulation saturates at ~90% of amorphization level with ion dose increase. Similar behavior was observed in [1], whereas in [2] gallium oxide sample was shown to be fully amorphized by heavy Eu ion bombardment. The dose needed to reach heavily damaged state is estimated to be about 1 DPA. Hence, gallium oxide is not as radiation resistant as other ionic semiconductors such as GaN and ZnO. Effects of ion beam flux and possible mechanisms of damage formation will be discussed. Work was supported by Russian Science Foundation grant # 21-19-00196. [1] E. Wendler et al. Nucl. Instrum. Methods Phys. Res. B, vol. 379, p.85, 2016 [2] K. Lorenz et al., Proceedings of SPIE, vol. 8987, p.89870M, 2020

Resume : Since irradiations with heavy ions have the abilities to cause high damages at a lower cost and for shorter periods than neutron irradiations, they are used as part of developing fourth generation reactors as well as extending life of actual power stations [1]. Nevertheless, ion irradiations representability for neutron irradiations is frequently questioned for many reasons such as flux difference, charged particles and beam rastering. In order to move forward with this issue, our study will be based on comparing the effects caused by both neutron and ion irradiations on a candidate material for GenIV and Fusion reators. This candidate is an oxide dispersion strengthened steel MA957 [2] resistant under high temperature and high irradiation damages [3]. Here we will focus on a new phase precipitation called α’ phase (phase rich in Cr) (its formation, density and size ) for different irradiation conditions (flux, temperature, fluence) using Transmission Electron Microscopy (TEM), Energy-Filtered TEM (EFTEM) and Atom Probe Tomography (APT). [1] A. Gentils, C. Cabet, NIMB 447 (2019) 107; [2] J. Ribis et al., J. Mater. Res. 30 (2015) 2210; [3] J.L Seran, document technique DMN (2006) Keywords: ion irradiation, neutron irradiation, ODS steel, precipitation, TEM, APT

Resume : Nuclear fusion is a promising option for providing clean energy, and it may be able to fulfill the increasing energy demands worldwide. However, for nuclear fusion to become a reality some weak points need to be overcome. One of them is the development of plasma fusion materials (PFMs) able to withstand the harsh conditions taking place in these kind of reactors. W has been proposed at the best option for PFMs applications. However, studies based on the combined effects of thermal load and atomistic damage (both Frenkel pair production and implantation of light species, namely, He, and H isotopes) evidence that coarse-grained W is not an appropriate option [1–3]. Therefore, alternatives to this material need to be identified. Some of these new approaches are based on the use of nanostructured materials in which the grain size has been reduced down to the nanometric scale. In this work we investigate the role of grain boundaries (GBs) on the radiation-induced damage (vacancies) and H behavior in irradiated and annealed nanostructured W by combining experiments and multiscale (DFT and OKMC) computer simulations. We do it at temperatures ranging from RT to 573 K (below and above the activation temperature of vacancy migration, respectively). To this aim, commercial coarse grained (CGW) and nanostructured W (NW) samples were sequentially irradiated with C (650 keV) and H (150 keV) ions at RT and at a fluence of 5x1020 m-2. After irradiation, a first set of samples was annealed at 473 K and a second set at 573 K. Morphological and microstructural studies evidence that all samples remain stable after irradiation and annealing. In all cases, the vacancy concentration calculated for the nanostructured W samples is notably higher than that for monocrystalline W, showing that the GBs significantly influence the number of vacancies after irradiation by acting as sinks for (highly mobile) self-interstitial atoms. For all samples annealed at T ≤ 473 K, the vacancy concentration is constant. However, for nanostructured samples annealed at T = 573 K it decreases slightly. Agreement between experimental and simulated H depth profiles can be achieved only if the H atoms in the simulation are in the interior of the grains, disregarding those at GBs. Thus, the experimental and simulated data agree only if GBs are considered as effective diffusion channels for H. Indeed, at T ≤ 473 K, the GBs reduce by up to ∼50% the H retained fraction, which indicates that, below this temperature, H is released from the samples only via GBs. Experimental results support a picture in which retained H atoms are accumulated in vacancies located in the interior of the grains, forming HnVm clusters (n ⩽ 5, m ⩾ 1). At T ≤ 475 K, H is mainly in monovacancies and GBs favor the formation of low-H/V clusters, whereas at T =573 K, GBs favor H trapping in vacancy clusters [4,5]. All these results imply that nanostructuration shifts the H fluence threshold for blistering to higher values. REFERENCES [1] R. Gonzalez-Arrabal et al., Matter and Radiation at Extremes. 5 (2020) 055201. [2] A. Rivera et al., Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms. 303 (2013) 81–83. [3] T.J. Renk et al., Fusion Science and Technology. 61 (2012) 57–80. [4] G. Valles et al., Acta Materialia. 122 (2017) 277–286. [5] M. Panizo-Laiz et al., Nucl. Fusion. 59 (2019) 086055.

Resume : Uranium dioxide is the most widely used fuel material. In reactors, 235U fission causes the formation of various defects and gaseous fission products that can interact with each other leading to structural damage and significant evolution of the fuel properties. In order to better understand and quantify the effect of such damage on the structural evolution of the fuel, separated effect experiments are performed based on heavy ions implantations in UO2. In the present study, we deal with the results of experiments performed on Au 4 MeV irradiated samples in order to identify vacancy defects and characterize their thermal evolution. These implanted samples are investigated using Positron Annihilation Spectroscopy (PAS). PAS is a non-destructive method allowing to study open volume defects in solids near the surface (from nanometer scale up to 160 µm) using two properties of the positron. First, positron as the anti-particle of the electron can annihilate with it leading to 511 keV gamma rays emission. Secondly, positrons can be trapped in defects. It is complementary to TEM, RBS and DRX measurements. In this way, two types of annihilation characteristics are investigated for this study using two positron annihilation techniques allowing us to characterize defects in materials: Doppler broadening of the annihilation radiation (DB) and positron Lifetime spectroscopies. The first technique allows to measure the electron momentum distribution. The second gives the positron lifetime which is inversely proportional to the electron density probed by the particle. In this work, Doppler broadening spectroscopy (DBS) is performed in the first 500 nm under the surface using a slow mono-energetic positron beam. A predominance of defects associated with the uranium vacancy of VU nVO (n=0-2) type was detected in these irradiated samples. Experimental results suggest also the trapping of positrons in other vacancy clusters and in negative ions. In order to provide new elements for the identification of the nature of these vacancy defects, the DBS measurements have been carried out as a function of the sample temperature between 50 and 400 K. These results were simulated using various positron trapping models in order to evaluate the proportion and nature of the detected vacancy clusters. Using fast positrons, some positron lifetime measurements have been performed as a function of the temperature to provide additional information on the defects nature.

Resume : Nanostructured materials with multiple grain boundaries showing diverse orientations have shown a great ability to reduce the damage that will be produced by irradiation in a future nuclear fusion reactor [1, 2], being nanostructured tungsten a specially prominent candidate material [3]. Among the diversity of deleterious effects produced by the impact of high energetic particles on first wall or plasma facing materials, the formation and aggregation of vacancies and interstitials and the ubiquitous presence of hydrogen and helium that would eventually end up in bubble formation, in obvious detriment of the performance and operational life of such structural materials, play a major role. We have performed ab initio simulations focused on completing a thorough energetic and mobility analysis of the behavior of diverse defects, namely self-interstitial atoms (SIA) and light impurity atoms (LIA), as H and He, that would appear as a consequence of the impinging radiation in a W110/W112 grain boundary. The aim is to shed light on the experimental controversy about the combined synergistic effect of the dominant retention of H close to He clusters [4], vs. the reduced/supressed blistering caused by He [5], whereby nanobubble formation near surfaces leads to reduced H retention [6]. Helium tends to be strongly repelled in the vicinity of a SIA. Hydrogen is only slightly repelled by the SIA, both at the GB and in the bulk. When H and He are present in the GB, they will not interact with each other whenever the distance between them is large enough. When we analyse the three different defects altogether, we may claim that the independent character of H with respect to He is proven for the former moves further away from the pair formed by the SIA and the He. This is in partial contradiction to what is found for bulk W. Additionally, we have assessed defect mobility to find that, as expected, SIAs can move quite easily along the interfacial grooves, so to recombine with the vacancies present and self-heal damage at the GB in the process. Bulk formation energies have been found to be at least three times greater than at the GB, meaning that defects would be strongly attracted by the GB, that would be capable to act as a defect sink and positively contribute to self-heal the material. References [1] I. Beyerlein et al, Prog. Mater. Sci. 74 (2015) 125. [2] X. Zhang, et al, Prog. Mater. Sci. 96 (2018) 217. [3] R. González-Arrabal et al, J. Nucl. Mater. 453 (2014) 287. [4] S. Markelj, et al, Nucl. Fusion 57 (2017) 064002. [5] Y. Ueda, et al, J. Nucl. Mater. 386 (2009) 725. [6] M. Miyamoto, et al, J. Nucl. Mater. 463 (2015) 333.

Resume : The structural materials in energy applications are mostly polycrystalline, and hence, during the modelling, it is crucial to describe the properties in the presence of the grain boundaries (GBs). The alterations of chemical composition, local symmetry for the atoms, and the strain state may occurs at boundary region. Each of these factors have a contribution to the behaviour of point defects (PDs) such as interstitial atoms or vacancies which play an important role in numerous properties. As a consequence, a proper survey of the performance of the structural materials, such as Fe-Cr alloys, require a precise knowledge of PDs-GBs interactions. In this study, we investigate how the GB presence affects the distribution and behaviour of alloying elements and point defects in bcc Fe-Cr random alloy. We are using the spin-polarized density functional theory (DFT) calculations, molecular dynamics and DFT-based Monte Carlo simulations in order to explore energy landscape of GBs cohesion and segregation tendency.