Polycrystalline Tungsten Research Articles

Morphology and crystalline evolution of tungsten induced by low-energy helium ions irradiation

The responses of polycrystalline tungsten to low energy helium ions irradiation were investigated. SEM and AFM measurements revel that nano-scale helium bubbles formed on the surface of tungsten at earlier stage. The size and number of the formed helium bubbles increase with the increase of helium fluence until they break. Mass loss and sputtering yields analysis indicated that the surface spalling caused by the damage of helium bubbles is the main format of tungsten damage. Helium atom is shown to be not energetically favorable to dissolve, self-trap and cluster at (101) surface in comparison to (111) and (001) surface.

Hydrogen blister formation in single crystal and polycrystalline tungsten irradiated by MeV protons

Single and polycrystalline tungsten samples were irradiated with 2.2 MeV protons at Soreq Applied Research Accelerator Facility (SARAF). Hydrogen blisters were obtained for both single crystal and polycrystalline samples, elucidating the role of grain boundaries in blister formation. The effect of temperature and flux on the critical formation dose for blisters and on their dimensions was studied. It was found that for single crystals, the critical formation dose is one order of magnitude higher than for polycrystalline tungsten at high temperature irradiation conditions. Upon reducing the irradiation temperature to ambient, the critical dose for formation of blisters in single crystals was reduced by a factor of three while in polycrystalline tungsten there was no significant change with temperature, thus indicating the role of grain boundaries in blister formation. Larger blisters were obtained in single crystals than in polycrystalline tungsten at ambient temperature conditions, identifying the grain boundaries as a preferential additional hydrogen trap. The height to area ratio of the blisters is found to be strongly temperature dependent and only weakly dependent on irradiation flux for both single and polycrystalline samples.

Detection and plasma assisted degradation of dye on reusable gold coated tungsten nanofuzz array surface-enhanced Raman scattering substrate

A novel gold coated tungsten nanofuzz array (TNFA) substrate has been prepared by irradiating polycrystalline tungsten using radio frequency plasma, followed by gold sputter coating. The surface morphology of the tungsten can be modified by tuning the irradiation parameters. The surface-enhanced Raman scattering (SERS) effect of the as prepared substrates has been tested using different molecule probes, namely, Rhodamine B (RHB), brilliant blue (X-BR) and reactive brilliant red (X-3B). Results show that the TNFA substrate exhibit the highest dye detection sensitivity (as low as <10 ppb) and excellent repeatability. A simple room temperature atmospheric pressure plasma treatment can be used to clean the substrate for re-use purpose through complete oxidization/degradation of the dye species. Systematic study reveals that the SERS enhancement effect is strongly dependent on the substrate surface morphologies and the laser excitation wavelength used. In addition, the SERS activity achieved by our novel substrate can be applied to a variety of dye molecules, demonstrating its potential to be generalized for a wider range of applications.

Surface degeneration of tungsten wires irradiated with low-energy (12–220 eV) helium ions

In order to investigate the evolutionary process of deformed tungsten on first wall and divertor plate, polycrystalline tungsten (W) wires are utilized as irradiation samples exposed to the low-energy, high-flux (∼1022 ions/m2·s) helium (He) ions. The irradiation experiments are performed by using a high-density plasma source system. The energy of He+ changes from 12 to 220 eV with an ion fluence of 1.0 × 1025/m2. Surface degeneration of these samples is observed by using a scanning electron microscopy. Results show that surface microstructures of irradiated W wires are strongly dependent on the energy of He+. Increasing the energy of He+ from 30 to 220 eV leads to an increase in the sputtering yield of W wires from 1.40 × 10−3 to 4.72 × 10−2 W/He+. As a result of He+ irradiation, loose structures and holes are formed in the surface layer of the W wires. The He+ -induced surface degenerations of W wires are discussed based on the experimental observation. As the He+ energy increases, the sputtering yield rapidly increases, and serious surface etching occurs.

Influence of grain size on deuterium transport and retention in self-damaged tungsten

The influence of grain size on deuterium transport and retention in tungsten was studied. For this purpose an experiment was carried out on three polycrystalline tungsten samples with different grain sizes and a single crystal sample with surface orientation <100>. In order to increase deuterium retention and hence the sensitivity for detection, samples were first damaged by high energy W ions. After damaging, the samples were exposed to a flux of deuterium atoms at 600 K for 70 h. During the exposure the depth profile of the retained deuterium was measured by Nuclear Reaction Analysis using a 3He ion beam. After the exposure the samples were also analysed by Thermal Desorption Spectroscopy. A clear difference in the time dependence of deuterium uptake was noticed between different samples. The experimental results were modeled using a rate-equation model. The influence of different grain size was modeled by changing the effective height of the potential barrier for deuterium atoms to enter into the bulk. We managed to successfully describe the transport of deuterium into the bulk of tungsten by reducing the potential barrier for samples with smaller grain sizes while the barrier for the sample with larger grain size was close to the value for the damaged single crystal sample.

Formation of bubbles and blisters in hydrogen ion implanted polycrystalline tungsten

ABSTRACTTungsten (W) has been regarded as one of the most promising plasma facing materials (PFMs) in fusion reactors. The formation of bubbles and blisters during hydrogen (H) irradiation will affect the properties of W. The dependence of implantation conditions, such as fluence and energy, is therefore of great interest. In this work, polycrystalline tungsten samples were separated into two groups for study. The thick samples were implanted by 18 keV H3+ ions to fluences of 1 × 1018, 1 × 1019 and 1 × 1020 H+/cm2, respectively. Another thick sample was also implanted by 80 keV H2+ ions to a fluence of 2 × 1017 H+/cm2 for comparison. Moreover, the thin samples were implanted by 18 keV H3+ ions to fluences of 9.38 × 1016, 1.88 × 1017 and 5.63 × 1017 H+/cm2, respectively. Focused ion beam (FIB) combined with scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used for micro-structure analysis, while time-of-flight ion mass spectrometry (ToF-SIMS) was used to characterize the H depth profile. It is indicated that bubbles and blisters could form successively with increasing H+ fluence. H bubbles are formed at a fluence of ∼5.63 × 1017 H+/cm2, and H blisters are formed at ∼1 × 1019 H+/cm2 for 18 keV H3+ implantation. On the other hand, 80 keV H2+ ions can create more trapping sites in a shallow projected range, and thus enhancing the blisters formation with a relatively lower fluence of 2 × 1017 H+/cm2. The crack-like microstructures beneath the blisters are also observed and prefer to form on the deep side of the implanted range.

Hydrogen embrittlement of tungsten induced by deuterium plasma: Insights from nanoindentation tests

Abstract

Experimental and theoretical studies on the elasticity of tungsten to 13 GPa

Compressional (VP) and shear wave (VS) velocities of polycrystalline tungsten have been measured up to ∼13 GPa at room temperature using ultrasonic interferometry in a multi-anvil apparatus. Using finite strain equation of state approaches, the elastic bulk and shear moduli and their pressure dependences are derived yielding KS0=325.9±4.8 GPa, G0=164.1±2.5 GPa, KS0′=3.65±0.05, and G0′=1.28±0.02. On the basis of the current experimental data, the high-pressure behavior of Young's modulus, Poisson's ratio, and ductility/brittleness for tungsten are also investigated. Complementary to the experimental data, the single crystal elastic constants, as well as the elastic anisotropy of tungsten are computed using density functional theory (DFT). The Voigt-Reuss-Hill average of the bulk and shear moduli calculated using the single crystal elastic constants from DFT are found comparable to the current experimental results within about 5%. The present study offers a dataset for the elasticity of polycrystalline bcc tungsten to a maximum pressure more than 25-fold higher than other previous ultrasonic studies, which can further our understanding about the elastic, mechanical, and electronic properties of tungsten under extreme conditions as well as thermodynamic modelling of its alloys.

Temperature dependence of strain rate sensitivity, indentation size effects and pile-up in polycrystalline tungsten from 25 to 950 °C

Elevated temperature nanoindentation measurements were performed on polycrystalline tungsten to 950 °C under high vacuum conditions with very low thermal drift. The temperature dependence of the hardness, elastic modulus, strain rate sensitivity, activation volume and the indentation size effect in hardness were studied. More significant time-dependent deformation was observed from 850 °C. Strain rate sensitivity determined by analysis of indentation creep data increased with temperature. Activation volume reached a peak of ~50 b3 at 750–800 °C. Decreasing activation volume >800 °C was a consequence of the increased strain rate sensitivity. For a bcc metal lattice resistance depends on T/Tc (where Tc, the critical temperature, at which flow stress becomes insensitive to temperature, is 527 °C for W); size effects would be expected scale with this relative temperature. Stronger indentation size effects in hardness were found at elevated temperatures. The influence of the time-dependent deformation on the unloading data was accounted for by a viscoelastic compliance correction. After correction the elastic moduli were to within ~1% of literature values at 750–800 °C and to within 6% at 950 °C. These small remaining differences are consistent with AFM measurements which show that pile-up is significant in these high temperature indentations.

Deuterium retention behavior in simultaneously He+–D2+ implanted tungsten

Poly-crystalline tungsten (W) samples were simultaneously irradiated with Helium (He) and Deuterium (D) ions using the triple-ion implantation device. He effect on D retention and transportation was studied using different combination of ion energies and He/D flux ratios in the simultaneous implantation. The experimental results show that D trapping at dislocation loops is significantly reduced in the case of 3 keV He+–3 keV D2+at He/D flux ratios over 0.6. D trapping by stronger trapping sites such as vacancies and vacancy clusters showed less dependence on the flux ratio. On the contrary, the D retention increases at each He/D flux ratio in the case of 3 keV He+–1 keV D2+compared to only D2+ implantation even the He/D flux ratio reaches a value of 1.0. TEM observations confirmed that dense dislocation loops are formed rather than He bubbles, which is responsible for the enhanced D retention in W.

Tungsten Surface Morphology Control and Its Thermal Fatigue Behavior Under Cyclic Heat Loading

Thermal fatigue behavior under various cyclic heat loading is an important concern for tungsten as armor material in fusion devices. Surface morphology controlling experiments have been performed on polycrystalline tungsten using an ECR plasma system and cyclic heat loading tests have been conducted upon the polished and modified samples using an electron beam apparatus. The results indicate that the surface topography has little effect on the characteristics of damage caused by the cycle plastic deformation during the cyclic heat loading. The micro-cracks and extrusions form in some grains after suffered 300 cyclic heat loading, which are aligned in different directions for varying grains. In addition, the modified specimens with both different triangular pyramid and homogeneous nanostructures were fabricated by dry etching under different conditions. The cross-section of damage regions were analyzed and a set of schematic diagram was presented to explain the mechanism for the formation of micro-cracks and extrusions under cyclic heat loading.

Experimental determination of the 16O(3He,p0)18F differential cross section

The differential cross section for the 16O(3He,p0)18F nuclear reaction is measured for a reaction angle of 135° in the laboratory frame for 3He energies between 1800 keV and 6100 keV. SiO2 thin films thermally grown on crystalline Si were used as reference samples. Their oxygen content was quantified by 3He Rutherford backscattering. The p0 cross section shows several resonances with the largest reaching 6.2 mbarn/sr at 4400 keV. At 2390 keV the measured cross section value coincides nicely with the measurement of Lennard et al. (1989). As an example, the oxygen areal density of the native oxygen coverage of polycrystalline tungsten was determined to be (11 ± 1) × 1015 O/cm2.

Stress-driven grain boundary movement during nanoindentation in tungsten at room temperature

Nanoindentations were performed in the vicinity of grain boundaries (GBs) in polycrystalline tungsten at room temperature, observing in some cases a secondary pop-in (known as GB pop-in) in the load–displacement curve. The dislocation microstructure in the plastic zone of the residual impression was analysed using sequential polishing, electron channelling contrast imaging (ECCI) and electron backscatter diffraction (EBSD) analysis on and below the surface. For some indentations, the interaction of the dislocations within the plastic zone and the GB leads to a localized GB movement on or below the surface. The occurrence and magnitude of GB movement are found to be strongly influenced by misorientation between the adjacent grains, the orientation of the indenter, as well as the applied load and the distance to the GB. The results show that the localized GB movement under an inhomogeneous stress field at room temperature is a possible deformation mechanism for tungsten.

Tungsten Blister Formation Kinetic as a Function of Fluence, Ion Energy and Grain Orientation Dependence Under Hydrogen Plasma Environment

This work deals with the formation kinetic of tungsten (W) blisters under smooth plasma conditions, i.e. low hydrogen flux and energy in order to analyze the first stages of their formation. In addition, we focus on determining the W grain orientation where blisters grow preferentially. For this purpose, mirror-polished polycrystalline tungsten samples were exposed to hydrogen plasma under fixed hydrogen flux of 2.2 × 1020 m−2 s−1, with a fluence in the range of ~ 1024 m−2, ion energy of ~ 20, 120 and 220 eV, and sample surface temperature of ~ 500 K. The formation of blisters at the surface was investigated using SEM, AFM and EBSD to determine the size, the distribution and the orientation of grain where blisters are formed, respectively. The critical fluence for initiating blisters was established around 2.3 × 1024 m−2. The evolution of blister size distribution and density is discussed as function of fluence and ion energy. At lower ion energy, i.e. 20 eV, only nanoblisters (less than 150 nm) are observed whatever the fluence value (1.5 and 2.3 × 1024 m−2). At higher ion energy i.e. 120 and 220 eV, micrometric (~ few to tens of µm) blisters are observed and their density highly depends on fluence. We show that blisters can also be formed on (001) oriented grains contrarily to previous results from the literature where the (111) orientation seemed more favorable. Such information is of importance for tungsten based fusion tokamak operation and design.

Deuterium retention in dense and disordered nanostructured tungsten coatings

A systematic and attentive comparison of the deuterium (D) retention in tungsten (W) coatings with different nano-crystalline structures after the plasma exposure in comparison with polycrystalline tungsten (PCW) is presented. While a wide database is available for PCW, only a few data about the D retention in coatings with different structures exist. The D retention in W coatings produced by three different deposition techniques on different substrates was studied with respect to the influence of (a) coating crystallite size, (b) coating thickness, (c) specimen temperature during D plasma exposure, (d) presence of argon (Ar) used as working gas during the coating deposition and (e) substrate material. It is shown that the variation of the processing parameters, such as temperature, deposition rate, Ar implantation, etc. even within one deposition method results in different grain size distributions and structure of coating and has a significant effect on the D retention. It is revealed that the substrate material and the presence of Ar in a coating play a minor role in the D retention in the coating. It is shown that both the D concentration and the D retention in coatings drastically increase with decreasing the grain size. Consequently, in the case of using of W coating as a protective layer of a structural material, a compromise in the development of nanostructured tungsten films is necessary to keep the hydrogen isotope concentration at an acceptable level.

Fracture in annealed and severely deformed tungsten

Bulk tungsten normally undergoes brittle fracture at ambient temperatures and has a brittle-to-ductile transition in the range 200–300 °C. This limits the use of tungsten for a host of applications. In general, the fracture mode of tungsten at ambient temperature is intergranular whereas at high temperatures it undergoes transgranular fracture. In this work the focus is on the influences of microstructure and temperature on three-point bend fracture of polycrystalline pure tungsten. The samples were processed by equal channel angular extrusion (ECAE) through a 90° die angle via route A in order to produce an elongated microstructure. The mechanical behavior of both unprocessed and processed materials was then characterized by three-point bending at temperatures ranging from −45 °C to 425 °C. The results show that a single ECAE extrusion (strain ~ 1.15) reduces the flexural toughness of the material and increases the brittle-to-ductile transition temperature, while two and four extrusions dramatically increase the flexural toughness with little effect on the transition temperature compared to that of the unprocessed material. The flexural toughness of the material subjected to four extrusions (strains exceeding 4.5) is more than 50 times greater than that of the unprocessed material at ambient temperature. This is mainly due to microstructural changes that increase the resistance to intergranular fracture, enhance plastic dissipation, and activate relatively high fracture toughness crack systems for transgranular fracture. The results show that substantial elongation of grains by deformation processing at a temperature near the brittle-to-ductile transition temperature is an effective method for improving the ambient temperature ductility and toughness of bulk polycrystalline tungsten.

A phase field modeling based study of microstructure evolution and its influence on thermal conductivity in polycrystalline tungsten under irradiation

In this work, we study effects of neutron irradiation and elastic tensile loading on microstructure evolution in polycrystalline tungsten by using a phase field modeling based approach. The proposed model involves the interaction of point defects with grain boundaries (GBs), thermal resistance of GBs, and voids. The void evolution analysis shows that the elastic energy field affects the shape and growth rate of voids. The overall trend of porosity evolution during irradiation follows the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation. Voids nucleation and growth can reduce both elastic strain energy and chemical free energy in the system. In addition, increase of porosity could significantly degrade the effective thermal conductivity of analyzed tungsten polycrystalline microstructures. The observations comply with the predictions from the theoretical models.

Incipient plasticity in tungsten during nanoindentation: Dependence on surface roughness, probe radius and crystal orientation

The influence of crystallographic orientation, contact size and surface roughness effects on incipient plasticity in tungsten were investigated by nanoindentation with indenters with a range of end radius (150, 350, 720 and 2800 nm) in single crystal samples with the (100) and (111) orientations. Results for the single crystals were compared to those for a reference polycrystalline tungsten sample tested under the same conditions. Surface roughness measurements showed that the Ra surface roughness was around 2, 4, and 6 nm for the (100), (111) and polycrystalline samples respectively. A strong size effect was observed, with the stress for incipient plasticity increasing as the indenter radius decreased. The maximum shear stress approached the theoretical shear strength when W(100) was indented using the tip with the smallest radius. The higher roughness and greater dislocation density on the W(111) and polycrystalline samples contributed to yield occurring at lower stresses.

Effects of baking in deuterium atmosphere on tritium removal from tungsten

Tritium-containing polycrystalline tungsten, which was pre-irradiated with helium prior to tritium exposure, was isothermally heated under a vacuum or deuterium gas in order to investigate the effect of the deuterium on tritium removal from the tungsten. It is found that in the case of 3 h of baking at temperatures varying from 423 K and 573 K, tritium retention after baking in deuterium gas became smaller than that in a vacuum. At the baking temperature of 573 K, tritium removal was hastened in the case of baking in deuterium atmosphere. The results indicate that the baking in deuterium atmosphere is effective to remove tritium from tungsten. The acceleration of diffusion of tritium in bulk and recombination of tritium with deuterium at the surface might be possible mechanisms.

Surface damages of polycrystalline W and La2O3-doped W induced by high-flux He plasma irradiation

In this study, polycrystalline tungsten (W) and three oxide dispersed strengthened W with 0.1 vol %, 1.0 vol % and 5.0 vol % lanthanum trioxide (La2O3) were irradiated with low-energy (200 eV) and high-flux (5.8 × 1021 or 1.4 × 1022 ions/m2⋅s) He+ ions at elevated temperature. After He+ irradiation at a fluence of 3.0 × 1025/m2, their surface damages were observed by scanning electron microscopy, energy dispersive spectroscopy, scanning electron microscopy-electron backscatter diffraction, and conductive atomic force microscopy. Micron-sized holes were formed on the surface of W alloys after He+ irradiation at 1100 K. Analysis shows that the La2O3 grains doped in W were sputtered preferentially by the high-flux He+ ions when compared with the W grains. For irradiation at 1550 K, W nano-fuzz was formed at the surfaces of both polycrystalline W and La2O3-doped W. The thickness of the fuzz layers formed at the surface of La2O3-doped W is 40% lower than the one of polycrystalline W. The presence of La2O3 could suppress the diffusion and coalescence of He atoms inside W, which plays an important role in the growth of nanostructures fuzz.