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The interaction of carbon atoms with point defects and the core of edge and screw dislocations with Burgers vector a 0 / 2 ⟨ 111 ⟩ in W and a W-Re matrix is studied by means of ab initio calculations. The structure and energetics of the ground-state atomic configurations are presented and rationalized. It is found that di-vacancies, which are thermally unstable in pure W according to the state-of-the-art ab initio calculations, can nucleate at C and Re-C complexes, which fill the gap in the explanation of the emergence of nanovoids observed experimentally under irradiation. Also, on the basis of the recent experimental evidence and our calculations, the temperature ranges for the manifestation of the yield drop phenomenon, which is related to the obstruction of dislocation motion due to their decoration by impurities such as carbon, are revealed. [ABSTRACT FROM AUTHOR]

Issues of the development of the steel casting technology and structures for protecting the stream of liquid metal on the way from the ladle to the tundish are considered. The motion of steel is simulated mathematically. Considering features of the motion of liquid metal as a Newtonian fluid, initial and boundary conditions for the simulation are chosen. An analysis of the simulation results shows the need to modify the plastic refractories installed between the shroud and the equipment of the ladle nozzle. The designed structures allow rational supply of inert gas (argon) to the shroud. This reduces the saturation of liquid steel with nitrogen, which, in turn, reduces the adverse effect of nitrogen on the characteristics of concast billets. [ABSTRACT FROM AUTHOR]

This article presents the structure and evaluation of strength properties at room temperature (r.t.) and at 900°C of materials based on nickel aluminide obtained via electrospark sintering at 1100°C of industrial powders of two grades, PN85U15 and PN75U23V, separately and mixed preliminarily with each other in the ratio of 7:3. When powders of two types are mixed, regions with a typical banded pattern, resulting from the martensitic transformation and L 10 phase formation, predominate in the structure. X-ray phase analysis results of the material under study revealed the intermetallic phases Ni3Al and NiAl. This material has enhanced strength characteristics at r.t. (σu = 385 MPa and σizg = 925 MPa) with a characteristic transcrystalline fracture. At 900°C, its tensile strength is 120 MPa. [ABSTRACT FROM AUTHOR]

The selective sputtering of reduced-activation ferritic-martensitic steel Eurofer upon deuterium plasma irradiation with ion energy of 100 eV is investigated. The experiments are carried out at 550 K and the radiation-dose range of (3–70) × 1024 ion/m2, and in the temperature range 350–730 K at fixed a radiation dose of 3 × 1025 ion/m2. The sputtering-coefficient dependences and information about the complex relief formed on the surface of Eurofer steel under ion irradiation are obtained. The elemental composition of the surface layer is studied by energy-dispersive X-ray spectroscopy and Rutherford backscattering. Enrichment of the surface layer with tungsten is found, which increases with the fluence of plasma irradiation. At the maximum radiation dose, the concentration of tungsten on the surface, averaged over an area of the analyzing beam of 1.7 mm2, was increased by about 13 times and reached a value of about 6 at % (about 14 wt %). [ABSTRACT FROM AUTHOR]

In this work, an interatomic potential for the W-Re system is fitted and benchmarked against experimental and density functional theory (DFT) data, of which part are generated in this work. Having in mind studies related to the plasticity of W-Re alloys under irradiation, emphasis is put on fitting point-defect properties, elastic constants, and dislocation properties. The developed potential can reproduce the mechanisms responsible for the experimentally observed softening, i.e., decreasing shear moduli, decreasing Peierls barrier, and asymmetric screw dislocation core structure with increasing Re content in W-Re solid solutions. In addition, the potential predicts elastic constants in reasonable agreement with DFT data for the phases forming non-coherent precipitates (r- and v-phases) in W-Re alloys. In addition, the mechanical stability of the different experimentally observed phases is verified in the temperature range of interest (700-1500 K). As a conclusion, the presented potential provides an excellent tool to study plasticity in W-Re alloys at the atomic level. [ABSTRACT FROM AUTHOR]

Reduced activation (with rapid activity decay) ferritic–martensitic steels are promising structural materials for fusion power engineering. This article discusses the capability of using them as a plasma-facing material in areas of relatively low energy load. Samples of Rusfer, Eurofer, CLAM, and F82H steels were irradiated with pulsed flows of deuterium plasma typical for peripheral plasma disruptions. Two variants of load were selected: 0.3 MJ/m2, 1 ms, i.e., the conditions under which the surfaces of these steels do not melt, and 0.6 MJ/m2, 1 ms, i.e., the conditions under which the surface layer melts. The samples were irradiated by 1–25 pulses during each load. The surface-layer modification was analyzed. It was established that if upon irradiation the surface layer of the samples melts, then already after one pulse the surface layer of Rusfer, CLAM, and F82H samples becomes wavy, and the surface of Eurofer steel remains flat until irradiation with 25 pulses with the following parameters: 0.6 MJ/m2, 1 ms. This can be related to the different compositions of the steels. The calculations demonstrate that the coefficient of surface tension in the molten layer of Eurofer steel is higher than that of Rusfer steel at the same temperature. Steel with a higher coefficient of surface tension keeps its shape better under the action of external forces. [ABSTRACT FROM AUTHOR]

The effect of severe plastic deformation on the deuterium retention in tungsten exposed to highflux low-energy plasma (flux ∼ 1024D/m2/s, energy ∼ 50 eV, and fluence up to 3 × 1026D/m2) at the plasma generator Pilot-PSI was studied by thermal desorption spectroscopy and scanning electron microscopy. The desorption spectra in both reference and plastically deformed samples were deconvolved into three contributions attributed to the detrapping from dislocations, deuteriumvacancy clusters, and pores, respectively. The plastically induced deformation, resulting in high dislocation density, does not change the positions of the three peaks, but alters their amplitudes as compared to the reference material. The appearance of blisters detected by scanning electron microscopy and the desorption peak attributed to the release from pores (i.e., deuterium bubbles) were suppressed in the plastically deformed samples but only up to a certain fluence. Beyond 5 × 1025D/m2, the release from the bubbles in the deformed material is essentially higher than in the reference material. Based on the presented results, we suggest that a dense dislocation network increases the incubation dose needed for the appearance of blisters, associated with deuterium bubbles, by offering numerous nucleation sites for deuterium clusters eventually transforming into deuterium-vacancy clusters by punching out jogs on dislocation lines. [ABSTRACT FROM AUTHOR]

The paper presents the results obtained for assessing oil absorption of the microrelief of heavily loaded surface layers. The studies have shown that it is preferable to form a friction surface by means of grinding, since such a surface is characterized by a higher level of oil absorption, which promotes a better involvement of the lubricant. An analytical model has been developed, which makes it possible to determine the thickness of the lubricating layer taking into account the oil absorption level of the contacting surfaces. It should be noted that as the load acting on the contact of the surfaces in a friction assembly increases, the thickness of the lubricating film decreases, and in the range of low load values, the intensity of its changes is more significant than it is in the zone of increased load. It has been shown that increasing velocity from 0.5 to 2.0 m/s leads to an average 2.8-fold increase in the thickness of the lubricating film, whereas changing relative the oil absorption level from 0.4 to 0.6 results in an almost 1.6-fold increase in the thickness of the lubricating film. Thus, the parameter of relative oil absorption by rough contacting surfaces exerts the greatest effect on the change in the thickness of the lubricating film in friction assemblies. [ABSTRACT FROM AUTHOR]

The development of appropriate materials for fusion reactors that can sustain high neutron fluence at elevated temperatures remains a great challenge. Tungsten is one of the promising candidate materials for plasma-facing components of future fusion reactors, due to several favorable properties as for example a high melting point, a high sputtering resistivity, and a low coefficient of thermal expansion. The microstructural details of a tungsten sample with a 1.25 dpa (displacements per atom) damage dose after neutron irradiation at 800 °C were examined by transmission electron microscopy. Three types of radiation-induced defects were observed, analyzed and characterized: (1) voids with sizes ranging from 10 to 65 nm, (2) dislocation loops with a size of up to 10 nm and (3) W–Re–Os containing σ- and χ-type precipitates. The distribution of voids as well as the nature of the occurring dislocation loops were studied in detail. In addition, nano-chemical analyses revealed that the σ- and χ-type precipitates, which are sometimes attached to voids, are surrounded by a solid solution cloud enriched with Re. For the first time the crystallographic orientation relationship of the σ- and χ-phases to the W-matrix was specified. Furthermore, electron energy-loss spectroscopy could not unambiguously verify the presence of He within individual voids. [ABSTRACT FROM AUTHOR]

The retention of helium and deuterium in tungsten irradiated with a high-intensity plasma flux (~1024 m−2 s−1) using either helium or deuterium ions is studied. Irradiation is carried out at a temperature of 480 K. Recrystallized and plastically deformed samples are used to detect the effect of the initial microstructure on plasma confinement and release. Thermal-desorption spectroscopy is performed to reveal the stages of release and for quantitative determination of the retention integral for both types of samples. A comparison of the recrystallized and plastically deformed samples is performed to determine the effect of plastic deformation (expressed in a high dislocation density and dislocation networks) on the confinement and release of plasma components. It is demonstrated that plastic deformation causes a significant decrease in helium retention, and, conversely, an increase in the capture of deuterium. [ABSTRACT FROM AUTHOR]

The high-flux deuterium plasma impinging on a divertor degrades the long-term thermo-mechanical performance of its tungsten plasma-facing components. A prime actor in this is hydrogen embrittlement, a degradation phenomenon that involves the interactions between hydrogen and dislocations, the primary carriers of plasticity. Measuring such nanoscale interactions is still very challenging, which limits our understanding. Here, we demonstrate an experimental approach that combines thermal desorption spectroscopy (TDS) and nanoindentation, allowing to investigate the effect of hydrogen on the dislocation mobility in tungsten. Dislocation mobility was found to be reduced after deuterium injection, which is manifested as a 'pop-in' in the indentation stress-strain curve, with an average activation stress for dislocation mobility that was more than doubled. All experimental results can be confidently explained, in conjunction with experimental and numerical literature findings, by the simultaneous activation of three mechanisms responsible for dislocation pinning: (i) hydrogen trapping at pre-existing dislocations, (ii) hydrogen-induced vacancies, and (iii) stabilization of vacancies by hydrogen, contributing respectively 38%, 52%, and 34% to the extra activation stress. These mechanisms are considered to be essential for the proper understanding and modeling of hydrogen embrittlement in tungsten. [ABSTRACT FROM AUTHOR]

Molecular dynamics (MD) simulations were carried out to study the interaction between nanometric Cr precipitates and a 1/2 <111>{110} edge dislocation (ED) in pure Fe and Fe-9 at. % Cr (Fe-9Cr) random alloy. The aim of this work is to estimate the variation in the pinning strength of the Cr precipitate as a function of temperature, its chemical composition and the matrix composition in which the precipitate is embedded. The dislocation was observed to shear Cr precipitates rather than by-pass via the formation of the Orowan loop, even though a pronounced screw dipole was emerged in the reactions with the precipitates of size larger than 4.5 nm. The screw arms of the formed dipole were not observed to climb thus no point defects were left inside the sheared precipitates, irrespective of simulation temperature. Both Cr solution and Cr precipitates, embedded in the Fe-9Cr matrix, were seen to contribute to the flow stress. The decrease in the flow stress with temperature in the alloy containing Cr precipitates is, therefore, related to the simultaneous change in the matrix friction stress, precipitate resistance, and dislocation flexibility. Critical stress estimated from MD simulations was seen to have a strong dependence on the precipitate composition. If the latter decreases from 95% down to 80%, the corresponding critical stress decreases almost as twice. The results presented here suggest a significant contribution to the flow stress due to the α-α′ separation, at least for EDs. The obtained data can be used to validate and to parameterize dislocation dynamics models, where the temperature dependence of the obstacle strength is an essential input data. [ABSTRACT FROM AUTHOR]

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 motion with a migration energy of tens of meV. The presence of 10% Cr in the Fe matrix, however, retards their mobility. We discuss possible reasons for the decreased diffusivity of these He clusters in the Fe¿Cr alloy. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This work is devoted to the study of a nonthreshold method for detecting acoustic emission impulses. As a method of impulse detection, a statistical ranking algorithm based on comparing the ranks of samples in two neighboring sliding windows is considered. To select the setting parameters of the algorithm and verify its reliability, the modal analysis method is used. The modal analysis is a mathematically exact method that leads to a closed integral equation for the elastic fields in the waveguide and which naturally includes the problem of the selectivity of the mode, offering an idea of the physics of waveguide behavior. The non-threshold algorithm developed in the framework of this paper was tested using 10000 model signals. The results of the research showed its high reliability and accuracy. A numerical experiment based on model signals showed that using the proposed nonparametric rank detection algorithm, in comparison with the threshold method, the location error has decreased by 1.5-2 times. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The effect of externally applied stress on the dislocation bias factor (BF) in bcc iron has been studied using a combination of atomistic static calculations and finite element integration. Three kinds of dislocations were considered, namely,a0/2〈1 1 1〉{1 1 0} screw,a0/2〈1 1 1〉{1 1 0} edge anda0〈1 0 0〉{0 0 1} edge dislocations. The computations reveal that the isotropic crystal expansion leads to an increasing or constant dislocation bias, depending on the Burgers vector and type of dislocation. On the other hand, compressive stress reduces the dislocation bias for all the dislocations studied. Variation of the dislocation BF depending on dislocation type and Burgers vector is discussed by analysing the modification of the interaction energy landscape and the capture efficiency values for the vacancy and self-interstitial atom. [ABSTRACT FROM AUTHOR]

We have monitored the thermal evolution of the proton irradiated structure of W-5 wt% Ta alloy by in-situ annealing in a transmission electron microscope at fusion reactor temperatures of 500-1300 °C. The interstitial-type a/2<111> dislocation loops emit self-interstitial atoms and glide to the free sample surface during the early stages of annealing. The resultant vacancy excess in the matrix originates vacancy-type a/2<111> dislocation loops that grow by loop and vacancy absorption in the temperature range of 600-900 °C. Voids form at 1000 °C, by either vacancy absorption or loop collapse, and grow progressively up to 1300 °C. Tantalum delays void formation by a vacancy-solute trapping mechanism. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The interaction of interstitial hydrogen (H) with dislocations and point defects in tungsten (W) is studied via numerical simulation within the framework of classical molecular dynamics (MD). Two alternative models are considered to describe the interatomic interactions: the embedded-atom method (EAM) and the bond-saturation model (the bond-order potential (BOP)). The calculated results are compared with data obtained via ab initio quantum-mechanical simulation. The potential developed recently within the framework of the EAM model demonstrated better agreement with the ab initio results than the BOP one. Molecular- statics calculations showed that hydrogen atoms are attracted by the dislocation core in both cases of screw and edge dislocations. The classical MD simulation of hydrogen diffusion in the vicinity of the edge dislocation demonstrated one-dimensional migration along the dislocation line. [ABSTRACT FROM AUTHOR]

Atomistic calculations are performed to investigate plastic slip in the <1 1 1>{3 2 1} system in body-centred cubic iron. Several modern interatomic potentials, developed over the last decade, are applied to compute the stacking faultγ-line energy in the {3 2 1} plane and the results are compared with theab initioprediction. The applied potentials have shown strong deviations, but several potentials acquired good qualitative agreement with theab initiodata. Depending on the applied potential, the lowest value of the Peierls stress for the edge dislocation (ED) is 50 MPa (Ackland and Bacon from 1997) and the highest is 550 MPa (Dudarev and Derlet from 2005), while for the screw dislocation it is much higher, in the range 1–2 GPa. At finite temperature, however, the flow stress of the ED is found to decrease exponentially reaching a negligible value at about 200 K, irrespective of the applied potential. On the basis of the data obtained using Ackland–Mendelev potential from 2004, we conclude that the slip resistance of the <1 1 1>{3 2 1} system is in between the resistance of the <1 1 1>{1 1 0} and <1 1 1>{1 1 2} slip systems. [ABSTRACT FROM AUTHOR]

A kinetic model of hydrogen capture and accumulation in tungsten under low-energy bombardment with hydrogen atoms in the 'subthreshold' mode is proposed in this paper. Primary defects do not form in the bombarded material in this mode. This model takes into account the physical mechanisms for hydrogen bubble nucleation and growth at dislocations, i.e., the presence of 'traps' in the dislocation core and dislocation jog formation due to interstitial tungsten-atom emission during the bubble growth process. Features of the dynamics of bubble growth as a function of the temperature and the hydrogen flux are studied. It is shown that there is a 'threshold' temperature that is dependent on the flux of implanted particles and above which the process of hydrogen-bubble formation is suppressed significantly. The obtained estimates of the 'threshold' temperature (600-700 K for a flux in the range of 10-10 m s) agree with existing published experimental data. [ABSTRACT FROM AUTHOR]

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 FeNiCr 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. [ABSTRACT FROM AUTHOR]

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

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-Ni-Cr 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-Ni-Cr) 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. [ABSTRACT FROM AUTHOR]

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 10 m). [ABSTRACT FROM AUTHOR]

A method for the recognition of dispersion curves on the spectrogram of acoustic-emission signals has been developed based on the Hough transform. The method was successfully tested in an experiment using a Hsu-Nielsen source. It has been determined that the suggested method can determine the distance from the source of a signal, even when the signal arrives only at one acoustic emission transducer. This enables one to perform AE testing only with one-sided access to any dimensional object or when there is a sufficiently large distance between transducers, when an AE signal reaches only one of them. [ABSTRACT FROM AUTHOR]

The degradation of austenitic stainless steels in a radiation environment is a known problem for the in-core components of nuclear light water reactors. For a better understanding of the prevailing mechanisms responsible for the materials' degradation, large-scale atomistic simulations are desirable. In this framework and as a follow-up on Bonny et al (2011 Modelling Simul. Mater. Sci. Eng.19 085008), we developed an embedded atom method type interatomic potential for the ternary FeNiCr system to model the production and evolution of radiation defects. Special attention has been drawn to the Fe10Ni20Cr alloy, whose properties were ensured to be close to those of 316L austenitic stainless steels. The potential is extensively benchmarked against density functional theory calculations and the potential developed in our earlier work. As a first validation, the potential is used in AKMC simulations to simulate thermal annealing experiments in order to determine the self-diffusion coefficients of the components in FeNiCr alloys around the Fe10Ni20Cr composition. The results from these simulations are consistent with experiments, i.e., DCr > DNi > DFe. [ABSTRACT FROM AUTHOR]

Samples of tungsten and tungsten-tantalum alloy (with 5 mass per cent of Ta) were exposed to high-flux deuterium plasma at different fluences. The surface modification was studied with scanning electron microscopy, and deuterium retention was measured by thermal desorption spectroscopy (TDS). In the high fluence range of ~3.5 x 1026-1027 m², multiple large-size blisters are formed on theWsurface, while blisters on the W-Ta surface are considerably smaller in size and number. Deuterium retention in this fluence rangewas found to be systematically higher in W than in W-Ta. Correlation between the evolution of the blistering patterns and the TDS spectra as a function of fluence suggests that trapping in the sub-surface cavities associated with blisters is the predominant trapping mechanism in tungsten in the case of high fluence exposures. We attribute the lower retention in W-Ta under the investigated conditions to the weaker blistering. [ABSTRACT FROM AUTHOR]

We present an Fe-Cr interatomic potential to model high-Cr ferritic alloys. The potential is fitted to thermodynamic and point-defect properties obtained from density functional theory (DFT) calculations and experiments. The developed potential is also benchmarked against other potentials available in literature. It shows particularly good agreement with the DFT obtained mixing enthalpy of the random alloy, the formation energy of intermetallics and experimental excess vibrational entropy and phase diagram. In addition, DFT calculated point-defect properties, both interstitial and substitutional, are well reproduced, as is the screw dislocation core structure. As a first validation of the potential, we study the precipitation hardening of Fe-Cr alloys via static simulations of the interaction between Cr precipitates and screw dislocations. It is concluded that the description of the dislocation core modification near a precipitate might have a significant influence on the interaction mechanisms observed in dynamic simulations. [ABSTRACT FROM AUTHOR]

A Hough transform-based method is developed for the extraction of dispersion curves from spectrograms. This method has been successfully tested using experimental data from both an acoustic emission (AE) signal simulator, and actual AE sources. The method determines the distance to the signal source with an accuracy from 1 to 10% even when the signal arrives only to one sensor. [ABSTRACT FROM AUTHOR]

High chromium ferritic-martensitic steels are commonly used for industrial applications requiring high strength at elevated temperature. Such steels typically contain 9-14 at.% Cr and a few percents of minor alloying elements. In recent studies it has been shown that the Cr solubility limit of the standard Fe-Cr phase diagram in that composition range is significantly underestimated. For the purpose of more reliable engineering and out of physical considerations, we reparameterize the Gibbs free energy so that the correct Cr solubility at low temperature (<700 K) is reproduced, while leaving the rest of the phase diagram changed only slightly. The mixing enthalpy and heat capacity resulting from the new parameterization are also compared with experiments and found to be in good agreement. [ABSTRACT FROM AUTHOR]

Interstitial dislocation loops with Burgers vector of [image omitted] 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 [image omitted] edge dislocation was reported earlier 1. Results are presented of a molecular dynamics study of interactions between a [image omitted] screw dislocation and [image omitted] 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 [image omitted] 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. [ABSTRACT FROM AUTHOR]

The results of molecular dynamics simulations of the diffusion of self-interstitial atom clusters in Fe-Cr alloys of different Cr content are presented. It is shown that, with increasing Cr concentration, the cluster diffusivity first decreases and then increases, in accordance with the predictions of a model developed recently and based on molecular static calculations. The minimum diffusivity is found at about 10 at% Cr for small clusters and it shifts towards lower concentration with increasing cluster size. The migration energy of SIA clusters is found to lie in between the binding energy of a Cr atom with a crowdion and half of it. This indicates that the mechanism of cluster migration is via the movement of individual crowdions from one Cr atom to another. The values obtained statically are much higher and are argued to be more reliable due to better sampling of different configurations in a bigger simulation box. [ABSTRACT FROM AUTHOR]

The opportunistic pathogen Pseudomonas aeruginosa utilizes a cell density-dependent signalling phenomenon known as quorum sensing (QS) to regulate several virulence factors needed for infection. Acylated homoserine lactones, or autoinducers, are the primary signal molecules that mediate QS in P. aeruginosa. The autoinducer N-3O-dodecanoyl-homoserine lactone (3O-C12) exerts effects on mammalian cells, including upregulation of pro-inflammatory mediators and induction of apoptosis. However, the mechanism(s) by which 3O-C12 affects mammalian cell responses is unknown. Here we report that 3O-C12 induces apoptosis and modulates the expression of immune mediators in murine fibroblasts and human vascular endothelial cells (HUVEC). The effects of 3O-C12 were accompanied by increases in cytosolic calcium levels that were mobilized from intracellular stores in the endoplasmic reticulum (ER). Calcium release was blocked by an inhibitor of phospholipase C, suggesting that release occurred through inositol triphosphate (IP3) receptors in the ER. Apoptosis, but not immunodulatory gene activation, was blocked when 3O-C12-exposed cells were co-incubated with inhibitors of calcium signalling. This study indicates that 3O-C12 can activate at least two independent signal transduction pathways in mammalian cells, one that involves increases in intracellular calcium levels and leads to apoptosis, and a second pathway that results in modulation of the inflammatory response. [ABSTRACT FROM AUTHOR]

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, &nvsim;1&ThickSpace;nm, attractive interaction between Cr atoms and crowdions. The minimum diffusivity is realized for &nvsim;11&ThickSpace;at.% Cr, where all crowdions constituting the cluster interact with Cr atoms, but the interaction fields of different Cr atoms do not overlap. [ABSTRACT FROM AUTHOR]

In metallurgical applications, precipitation strengthening is of great technological importance to engineer materials with the required strength. While precipitation hardening is essential for many applications involving operation at elevated temperatures, its subsequent embrittlement can be a showstopper for the overall performance of a component. In the nuclear industry, irradiation-induced/enhanced precipitation and the resulting embrittlement often limit the lifetime of components. In fusion applications, tungsten (W) based alloys are known to harden and embrittle as a result of irradiation-assisted transmutation to rhenium (Re) and its subsequent precipitation into non-coherent precipitates. Hence, a fundamental understanding of the interaction of dislocations with non-coherent precipitates is of great interest. In the present work, the interaction of dislocations with non-coherent Re-rich σ, χ and hcp phase precipitates embedded in a bcc W matrix is assessed. Large-scale atomistic simulations are performed to clarify the interaction mechanisms and derive the obstacle strength of the precipitates in the quasi-static limit. Thereby the impact of precipitate shape, size, interspacing and composition is assessed. Based on those results, an analytical model to predict precipitation hardening of σ, χ and hcp phase particles in bcc W is proposed and compared to available experimental data from mechanical tests on irradiated materials. [ABSTRACT FROM AUTHOR]

Tungsten and tungsten-based alloys are the primary candidate materials for plasma facing components in fusion reactors. The exposure to high-energy radiation, however, severely degrades the performance and lifetime limits of the in-vessel components. In an effort to better understand the mechanisms driving the materials' degradation at the atomic level, large-scale atomistic simulations are performed to complement experimental investigations. At the core of such simulations lies the interatomic potential, on which all subsequent results hinge. In this work we review 19 central force many-body potentials and benchmark their performance against experiments and density functional theory (DFT) calculations. As basic features we consider the relative lattice stability, elastic constants and point-defect properties. In addition, we also investigate extended lattice defects, namely: free surfaces, symmetric tilt grain boundaries, the 1/2〈1 1 1〉{1 1 0} and 1/2〈1 1 1〉 {1 1 2} stacking fault energy profiles and the 1/2〈1 1 1〉 screw dislocation core. We also provide the Peierls stress for the 1/2〈1 1 1〉 edge and screw dislocations as well as the glide path of the latter at zero Kelvin. The presented results serve as an initial guide and reference list for both the modelling of atomically-driven phenomena in bcc tungsten, and the further development of its potentials. [ABSTRACT FROM AUTHOR]