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In this work, the nano-indentation of plasma-exposed tungsten is simulated at room temperature and elevated temperature (300–700 K) by the recently developed crystal plasticity finite element model. A nonlinear function is applied to characterize the depth profile of plasma-induced dislocation density in the sub-surface region. The model parameters are calibrated by comparing the simulated results with corresponding experimental data at 300 K for both the force-depth and hardness-depth relationships. Furthermore, the mechanical responses of plasma-exposed tungsten are predicted at 500 K and 700 K in order to characterize the plasma effect at the fusion-relevant operational temperature. The dominant results and conclusions are that: (1) The heterogeneously distributed dislocations in the sub-surface region induced by the plasma exposure are responsible for the increase of hardness at 300 K. (2) The plasma-induced microstructural modification does not yield to considerable increase of hardness at operational temperature. (3) The expansion of the plastic zone in the sub-surface region is, to some extent, limited by the presence of plasma-induced dislocations. Whereas, the increase of temperature can effectively reduce this limitation. • Nano-indentation of plasma-exposed tungsten is simulated by CPFEM. • Numerical results can match well with corresponding experimental data. • Increase of hardness at 300 K comes from heterogeneously distributed dislocations. • Expansion of the plastic zone is limited by the plasma-induced dislocations [ABSTRACT FROM AUTHOR]

Abstract It is very well known that tungsten is intrinsically brittle at room temperature, and the characterization of its ductile properties by conventional mechanical tests is possible only above the ductile-to-brittle transition temperature (DBTT), i.e. above 500–700 K. However, the design of tungsten-based components often requires the knowledge of constitutive laws below the DBTT. Here, we carried out instrumented hardness measurements in the temperature range of 300–691 K by nano-indentation. The obtained results are used to extend a set of constitutive laws for the plastic deformation of tungsten, developed earlier on the basis of tensile data, which now covers the temperature range of 300–1273 K. The validation of the constitutive laws was realized by the crystal plasticity finite element method (CPFEM) model, which was applied to simulate the nano-indentation loading curves. The distribution of stress and strain under the indenter was also studied by the CPFEM to bring an insight on the extension of the plastic zone in the process of the indentation, which is of crucial importance when nano-indentation is used to resolve the microstructural features generated by e.g. irradiation by energetic particles, plasma exposure or thermo-mechanical treatment. [ABSTRACT FROM AUTHOR]

The interaction of dislocation pile-ups with several tilt grain boundaries (GB) is studied in copper by using a hybrid continuum-atomistic approach. The effects of temperature, pile-up intensity and GB structure on absorption and transmission of slip as a function of local stress state are explored. By considering several high-angle GBs with different misorientation angles, we demonstrate that GB atomic structure primarily defines its ability to accommodate incoming pile-up dislocations, thus limiting the direct transmission of pile-ups through the interface. [ABSTRACT FROM AUTHOR]

Microstructure induced by plastic deformation at 600 °C in as-fabricated and recrystallized tungsten grades of ITER specification is studied by means of transmission electron microscopy (TEM). Both grades are produced by AG Plansee and being used by fusion community as research material for plasma facing and high heat flux components. Reference microstructure was investigated by a combination of TEM, scanning electron microscopy and nano-indentation techniques. TEM was applied to investigate the necking region in the tensile samples deformed up to ~ 30% of stain. The deformation-induced microstructure was characterized and compared for the two grades in terms of the dislocation density, heterogeneity, observation of pile-ups and tangles specifically near high angle grain boundaries. The obtained evolution of the dislocation density was compared with the prediction of the previously developed thermo-mechanical model for the plastic deformation of polycrystalline tungsten and a good agreement was found without modification of the original parameter set. [ABSTRACT FROM AUTHOR]

We have performed a combined study using transmission electron microscopy (TEM), nuclear reaction analysis (NRA) and nano-indentation (NI) techniques to reveal the impact of high flux plasma exposure on the properties of a sub-surface region of the commercially available pure tungsten fabricated following the ITER specification. TEM examination revealed the formation of a dense dislocation network and dislocation tangles, resulting in a strong increase in the dislocation density by at least one order of magnitude as compared to the bulk density. The plasma-induced dislocation microstructure vanishes within a depth of about 10–15 μm from the top of the exposed surface. Surface hardness after the plasma exposure was characterized by NI and was found to increase significantly in the sub-surface region of 1.5–3 μm. That was attributed to the resistance of the plasma-induced dislocation networks and deuterium-induced defects, whose presence within a depth of ∼1 μm was unambiguously detected by the NRA measurements as well. [ABSTRACT FROM AUTHOR]

We have performed high flux plasma exposure of tungsten and subsequent microstructural characterization using transmission electron microscopy (TEM) techniques. The aim was to reveal the nanometric features in the sub-surface region as well as to compare the microstructural evolution in tungsten single crystal and ITER-relevant specification. In both types of samples, TEM examination revealed the formation of a dense dislocation network and dislocation tangles. The estimated dislocation density in the sub-surface region was of the order of 10 14 m −2 and it gradually decreased with a depth position of the examined sample. Besides individual dislocation lines, networks and tangles, the interstitial dislocation loops have been observed in all examined samples only after the exposure. Contrary to that, examination of the pristine single crystal W and backside of the plasma-exposed samples did not reveal the presence of dislocation loops and tangles. This clearly proves that high flux plasma exposure induces severe plastic deformation in the sub-surface region irrespective of the presence of initial dislocations and sub-grains, and the formation of dislocation tangles, networks and interstitial loops is a co-product of thermal stress and intensive plasma particles uptake. [ABSTRACT FROM AUTHOR]

The effect of severe plastic deformation on the deuterium retention in tungsten exposed to high-flux low-energy plasma (flux ∼10 24 m −2 s −1 , energy ∼50 eV and fluence up to 5 × 10 25 D/m 2 ) was studied experimentally in a wide temperature range (460–1000 K) relevant for application in ITER. The desorption spectra in both reference and plastically-deformed samples were deconvoluted into three contributions associated with the detrapping from dislocations, deuterium-vacancy clusters and pores. As the exposure temperature increases, the positions of the release peaks in the plastically-deformed material remain in the same temperature range but the peak amplitudes are altered as compared to the reference material. The desorption peak attributed to the release from pores (i.e. cavities and bubbles) was suppressed in the plastically deformed samples for the low-temperature exposures, but became dominant for exposures above 700 K. The observed strong modulation of the deuterium storage in “shallow” and “deep” traps, as well as the reduction of the integral retention above 700 K, suggest that the dislocation network changes its role from “trapping sites” to “diffusion channels” above a certain temperature. The major experimental observations of the present work are in line with recent computational assessment based on atomistic and mean field theory calculations available in literature. [ABSTRACT FROM AUTHOR]

Surface hardness of tungsten after high flux deuterium plasma exposure has been characterized by nanoindentation. The effect of plasma exposure was rationalized on the basis of available theoretical models. Resistance to plastic penetration is enhanced within the 100 nm sub-surface region, attributed to the pinning of geometrically necessary dislocations on nanometric deuterium cavities – signature of plasma-induced defects and deuterium retention. Sub-surface extension of thereby registered plasma-induced damage is in excellent agreement with the results of alternative measurements. The study demonstrates suitability of nano-indentation to probe the impact of deposition of plasma-induced defects in tungsten on near surface plasticity under ITER-relevant plasma exposure conditions. [ABSTRACT FROM AUTHOR]

Abstract: The behavior of crack propagation and blunting at different strain rates, temperature and crack geometry in BCC-Fe, W, and FCC-Ni, Cu has been studied by molecular dynamics under load mode I. Large scale atomistic simulations coupled with a structural analysis have been performed to reveal conditions at which dislocations can be emitted from a semi-infinite crack. In FCC metals the crack tip is blunted by the plastic deformation due to the nucleation and emission of dislocations. These processes, however, occur in a different manner in Ni and Cu. It can be concluded that the formation of stacking faults and twins at a crack tip is particularly important for brittle cleavage. In BCC crystals the crack was found to open without plastic deformation, however, the strain work hardening has a significant dependence on temperature. The critical stress intensity factor for brittle cleavage or dislocation emission was calculated from the stress tensor measured on atoms located at the crack tip. The obtained results are discussed and compared with predictions of elasticity theory. [Copyright &y& Elsevier]

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]

Abstract: The internal friction spectra in cold-worked pure iron and iron–chromium alloys are measured in the inverted torsion pendulum, operating at 1.8Hz and in the temperature range from 100K to 600K. The addition of chromium, in combination with a plastic deformation, activates the relaxation process producing an internal friction peak at about 210K. The peak intensity exhibits a strong increase by increasing the concentration of solute atoms. On the basis of the estimated activation energy, eV, and atomistic simulations, this mode is assigned to the single-kink relaxation process corresponding to the recombination of geometrical kinks formed as a result of Cr-dislocation core interaction in the screw dislocations. [Copyright &y& Elsevier]

Knowledge of mechanical properties of the tungsten surface region is extremely important for its application as first wall materials in plasma-facing components for nuclear fusion devices (e.g. ITER). Since tungsten is intrinsically brittle at room temperature, characterization of its ductile properties is possible only above the so-called ductile-to-brittle transition temperature (DBTT), which is above 500–700 K. This is why the development and qualification of instrumented hardness measurements at elevated temperature is an important task to enable the characterization of tungsten properties after exposure to heat shocks, plasma beam and ion irradiation, which all together mimic the actual operation conditions of nuclear fusion. We have performed nanoindentation measurements on tungsten in the constant stiffness mode using Bruker stage developed for high temperature operation with oxygen protective environment. Commercially pure tungsten of ITER specification is studied in the as-produced and as-recrystallized conditions to deduce the impact of the texture and forging on the hardness. The obtained results are analysed by means of crystal plasticity finite element method (CPFEM) model to subtract the constitutive laws for the elasto-plastic deformation and derive the strengthening term attributed to the contribution coming from statistically stored dislocations and grain boundaries. • Hardness measured up to 600C agrees with earlier published results. • Temperature induced softening is observed. • FEM simulations agree well with nanoindentation. [ABSTRACT FROM AUTHOR]