Evolution In Tungsten Research Articles

Twinning dominated microstructural evolution in tungsten under impact loading

Twinning dominated microstructural evolution in tungsten under impact loading

Simulation of defect evolution in tungsten during annealing by developing a vacancy and interstitial-type defect evolution model

In this work, a Vacancy and Interstitial-Type Defect Evolution (VITDE) model is developed to study the annealing and clustering behaviors of the irradiation-induced defect in tungsten (W). The post annealing of W-ion damaging at 300 K (called post-damaging annealing) and simultaneous annealing of W-ion damaging (called dynamic annealing) are simulated. In the post-damaging annealing case, the W is damaged by 20 MeV W-ion irradiation for 1h at 300 K, then the damaged W is annealed for 1h via increasing the temperature to 1000 K. In the dynamic annealing case, the defect evolution under simultaneous damaging and annealing is simulated using the same parameters as Case 1. The simulation results indicate the dynamic annealing case has lower induced defects concentration than the post-irradiation annealing case, which is in qualitative agreement with the experiment. The diffusion of interstitial, vacancy, di-interstitial and di-vacancy, and the cluster properties in the defect annealing are investigated and larger size Vn clusters are observed in dynamic annealing case than in post-damaging annealing case. The dependences of the defect evolution on material temperature (T) and grain size are revealed. By increasing the T, the defect concentration is decreased, which reproduces and explains the experimental result. The Vn trap amount with the reduction of grain size for two cases show a larger difference at T = 400 K, observing the Vn trap amount is unchanged for post-damaging annealing, whereas it is significantly increased by dynamic annealing.

Initial microstructure and temperature dependence of irradiation defects evolution in tungsten

Using the object kinetic Monte Carlo (OKMC) method and the sink strength model, we systematically investigated the influence of the initial microstructure and temperature on the aggregation, migration, removal, and resulting size distribution of defects emerging in tungsten (W) under neutron irradiation. It is found that the probability of defect absorption at grain boundaries is generally higher than that at dislocations. This suggests that the grain size (2 ∼ 20 μm) has a stronger effect on the evolution of irradiation defects compared to dislocation density (1010 m−2 ∼ 1013 m−2). As the grain size increases, the number of residual defects decreases but the size distribution of vacancies is basically unchanged. Besides initial microstructure, the evolution of radiation damage also depends on the irradiation temperature. The increase in temperature induces an initial increase in the number density of voids, and then followed by subsequent decreases, corresponding to the peak density at 800–1000 K. Furthermore, the defect evolution in irradiated W with different initial microstructures and temperatures is simulated, and the calculated results are compared with recent experiments. These results help us to understand the role of the initial microstructure of the material in the evolution of the irradiation defects.

TEM investigation of helium bubble evolution in tungsten and ZrC-strengthened tungsten at 800 and 1000°C under 40keV He+ irradiation

Helium-induced defect nucleation and accumulation in polycrystalline W and W0.5 wt%ZrC (W0.5ZrC) were studied in-situ using the transmission electron microscopy (TEM) combined with 40 keV He+ irradiation at 800 and 1000°С at the maximum damage level of 1 dpa. Radiation-induced dislocation loops were not observed in the current study. W0.5ZrC was found to be less susceptible to irradiation damage in terms of helium bubble formation and growth, especially at lower temperature (800 °C) when vacancies were less mobile. The ZrC particles present in the W matrix pin the forming helium bubbles via interaction between C atom and neighbouring W atom at vacancies. This reduces the capability of helium to trap a vacancy which is required to form the bubble core and, as a consequence, delays, the bubble nucleation.At 1000 °C, significant bubble growth occurred in both materials and all the present bubbles transitioned from spherical to faceted shape, whereas at 800 °C, the faceted helium bubble population was dominated in W.

Unveiling the radiation-induced defect production and damage evolution in tungsten using multi-energy Rutherford backscattering spectroscopy in channeling configuration

Radiation-induced defect production in tungsten was studied by a combination of experimental and simulation methods. The analysis of structural defects was performed using multi-energy Rutherford backscattering spectroscopy in channeling configuration (multi-energy C-RBS). To create different microstructures, (111) tungsten (W) single crystals were irradiated with W ions at two different doses (0.02 and 0.2 dpa) at 290 K. Detailed transmission electron microscopy (TEM) analysis of the samples revealed the presence of dislocation lines and loops of different sizes. The RBSADEC code was used to simulate the measured C-RBS spectra, recorded with four different He beam energies along the 〈111〉 direction. For the first time for tungsten, molecular dynamics (MD) simulations of overlapping cascades were used as input. The well-known method of randomly displaced atoms (RDA) was applied for comparison. RDA does not provide a satisfactory understanding of the nature of the induced defect structure. With MD, a very good agreement between the simulated and experimental spectra was obtained for the sample prepared at a lower dose, despite the fact that the absolute defect densities are two orders of magnitude higher than those found with TEM. A discrepancy is observed for the high-dose-irradiated sample, which is ascribed to the presence of extended defects such as dislocation lines, which are clearly observed by TEM, but cannot be formed in finite size MD cells. RBSADEC with MD cells as input can describe correctly the response of the RBS signal with analysing beam energy while RDA as input gives the wrong trend.

Evolution of Tungsten Fiber‐Reinforced Tungsten‐Remarks on Production and Joining

Material issues pose a significant challenge for the design of future fusion reactors. These issues require new advanced materials to be developed. W‐fiber‐reinforced W‐composite material (W f/W) incorporates extrinsic toughening mechanisms increasing the resistance against failure and thus granting steps toward application in a future fusion reactor. W f/W can be produced based on chemical vapor deposition or powder metallurgical routes. In this contribution, the efforts of upscaling the production of W f/W will be reviewed based on recent results. In addition, the activities related to enabling large‐scale production for new fusion applications are being studied. Herein, two main achievements are to be highlighted. First, an upscaled production is established to produce flat tile samples for joining tests on copper and steel, and second, a new method of joining W f/W on copper is established and tested under high heat‐flux conditions.

Dependence of tungsten crack behaviors on heat source parameters under transient heat flow

As a plasma facing material in TOKAMAK, tungsten may suffer plastic deformations, cracking, melting, even ablation induced by transient heat loads, such as Edge Localized Modes (ELMs). Focused on investigation of cracking behaviors of tungsten under transient heat flux and its dependence on pulse parameters, powder metallurgical pure tungsten samples have been tested with 1–100 pulses of intense pulse electron beams (IPEB) at 5 ms and 1 ms pulse duration. Three types of cracks were observed, including primary crack, secondary crack and blunt crack. The primary cracks can be assorted as branch, annular and network cracks according to their patterns. Branch cracks were formed in the irradiation area center, and annular cracks appeared surrounded the irradiation area. The width of annular cracks increases as the pulse number increasing. The initiation and evolution of annular cracks are mainly studies in this paper. Considering the ductile-to-brittle transition property, an ideal elasto-plastic mechanics model is adopted in the thermodynamic calculation model of tungsten under the IPEB irradiation. Irreversible plastic deformation plays an important role, which determined the tensile stress distribution during the temperature decreasing. The formation mechanism of annular crack and its dependence on the heat source parameters are discussed, based on the calculation results of temperature, stress distribution and evolution of tungsten under different pulse parameters with this model. It is found that the formation mechanism of annular crack depends on the high radial gradient of energy density E, and the radius of annular crack obviously depends on the energy density deposited per pulse and the heat source boundary gradient for large volume sample. The influence of pulse number on branch crack and annular crack is different. The widening of the annular crack with the increasing of pulse number is caused by the axial residual plastic strain after pulse irradiation.

Helium-induced morphology evolution in tungsten under thermal treatment

Surface and near-surface morphology evolution of helium-irradiated tungsten due to thermal cycling up to the ITER-relevant temperature of 1350 K was studied using electron microscopy and positron annihilation spectroscopy techniques. Holes at the surface and bubbles in the near-surface of recrystallized polycrystalline tungsten samples were created by 75 eV helium plasma irradiation with the fluence of 3 × 1023 He m−2 at the surface temperature of 1073 K. Subsequent annealing experiments were combined with a detailed electron microscopy analysis to investigate the shape and density changes of holes and helium bubbles with respect to grain orientation. We show that the initially circular holes and round bubbles became faceted upon heating with 1 K s−1 ramp up to 870 K. Annealing cycles up to 1350 K induced resulted in bubbles removal in the first 5 nm below the surface and surface smoothing. Electron energy loss spectroscopy measurements allowed estimation of helium gas density and pressure inside bubbles. Positron annihilation spectroscopy allowed to investigate the nature of defects and their evolution in the helium-irradiated tungsten with thermal cycling.

Effect of dose rate on dislocation loop evolution in tungsten: Combination of defect generation rate and elastic interaction

Ion irradiation is often used to simulate neutron irradiation in reactor components, while the dose rate discrepancy makes it difficult to directly compare ion irradiation with neutron irradiation. A better understanding of dose rate effect is thus necessary to emulate neutron irradiation effect with ions. Here, based on in-situ 30 keV He+ irradiation in tungsten in TEM, we find that defect generation rate and elastic interaction are two key factors controlling dislocation loop evolution at different dose rates. By adjusting defect generation rate, the elastic interaction between the loops will play different roles. Finally, a schematical illustration is given to explain the different in loop density, size and spatial distribution caused by the combination of these two factors. Our results provide the underlying insights for understanding the dose rate effects of energetic ion irradiation.

A combined experimental and theoretical study of small and large vacancy clusters in tungsten

Tungsten is considered to be used in the future fusion reactors as plasma-facing material. In such extreme environments, defects are induced in materials that modify their macroscopic properties such as the mechanical ones. It is of paramount importance to be able to determine concentration and size of the vacancy defects, from the mono vacancy to the large cavities, to validate the models developed to predict the evolution of the microstructure of irradiated materials. Positrons are very useful non-destructive probes that can characterize vacancy-type defects in materials. We present a combined experimental and theoretical study on detecting and estimating the sizes of vacancy clusters that are invisible with electron microscopy in tungsten, using positron annihilation spectroscopy. We here model the positron annihilation in the tungsten lattice and in vacancy-type defects using state-of-the-art first principles methodology. The Doppler broadening spectra and positron lifetimes in tungsten are calculated with two-component density functional theory with local density approximation and weighted density approximation. Our calculations are in excellent agreement with our experimental results. We show that the sizes of vacancy clusters in tungsten can be well estimated by combining both positron lifetimes and Doppler broadening spectra. We also determine the limit of validity of the canonical calculation method, which here is shown to fail when the vacancy clusters grow beyond their nucleation stage. This work is a first step needed to better interpret the measured positron annihilation characteristics (Doppler and lifetime) in tungsten and then extract quantitative data on small vacancy defects required to improve the understanding of early-stage vacancy defect evolution in tungsten. The method used in this paper could be used to study other metallic materials.

A unified model for yield strength and plastic behavior of nanovoid evolution in tungsten based on molecular dynamics simulations

Void evolution is closely related to the damage and failure of materials under dynamic conditions, such as high strain rate. The rearrangement of atoms on the entire void surface results in microscopic plastic behaviors like dislocation emission and twinning on the surface and further macroscopic plastic flow nearby. In this work, molecular dynamics simulations are conducted to study the compression flow stress and the corresponding plastic behavior of nanovoid inside monocrystal tungsten as a function of crystallographic orientation, void size, and strain rate. Colorful plastic deformation mechanisms, like shear loops, prismatic dislocation loop, twinning, amorphization, etc., are observed at different strain rates and for compression along various crystallographic orientations. A Kelvin tetrakaidecahedron, formed before yielding, is utilized to determine the critical shear stress for initiation of plastic deformation on void surface within the framework of yield criterion and facture mechanics. A unified model is thus proposed based on the macroscopic hardening effect raised by plastic strain and strain rate, and the microscopic plastic behavior of dislocation emission related to the crystallographic orientation and nanovoid size. The newly proposed unified equation gives a good recount on the yield strength and plastic behaviors obtained from molecular dynamics simulations, but also shed lights on the future works of crack members, inclusions, voids, and porous materials at extremely high strain rate.

Dose Rate Effects on Dislocation Loop Evolution in Tungsten: Combination of Defect Generation Rate and Elastic Interaction

Dose Rate Effects on Dislocation Loop Evolution in Tungsten: Combination of Defect Generation Rate and Elastic Interaction

Defect evolution in tungsten exposed to helium plasma and deuterium plasmas studied by slow positron beam

• W samples have been exposed to He plasma and D plasmas. • DBS-PA technique has been used to characterize the defects in W. • Low energy plasma exposure could create defects in W. • D plasmas exposure could reduce the detectable defects in He plasma exposed W. Comparison of defect formation in tungsten samples exposed only to a helium plasma and those exposed to both helium plasma and subsequent deuterium plasmas is performed via Doppler broadening spectroscopy in the positron annihilation technique (DBS-PA). The S parameters of tungsten samples after helium plasma exposure increased with the increase of the incident fluence, which indicates that low energy helium plasma could induce the formation of defects such as nano-sized helium bubbles and dislocations in tungsten. The S parameters of samples exposed to helium incident fluence of 7.9 × 10 21 He/m 2 enhanced after deuterium plasmas exposure, indicating that low energy deuterium plasmas could form defects in tungsten too. Unexpectedly, deuterium plasmas exposure did not significantly influence the S parameters of the tungsten sample with helium incident fluence of 3.9 × 10 22 He/m 2 , and reduced the S parameters of the tungsten sample with helium incident fluence of 7.9 × 10 22 He/m 2 . These variations of S parameters could be associated with the occupation of nano-sized helium bubbles by deuterium atoms. This indicates possible competition between the increased density of vacancy-type defects by deuterium plasmas exposure and the filling of nano-sized helium bubbles by deuterium atoms, reducing detectability of these defects.

Molecular dynamics simulations of high-dose damage production and defect evolution in tungsten

Tungsten has been chosen as the plasma-facing wall material in fusion reactors, due to its high density and melting point. The wall material will not only be sputtered at the surface, but also damaged deep inside the material by energetic particles. We investigate the high-dose damage production and accumulation by computational means using molecular dynamics. We observe that the choice of interatomic potential drastically affects the evolution. The structure and stability of the obtained defect configurations are validated using a quantum-accurate Gaussian approximation potential.

Anisotropic grain boundary area and energy distributions in tungsten

Describing microstructure evolution in tungsten requires a quantitative description of the anisotropic grain boundary energy. We present a grain boundary energy function for tungsten that specifies the energy of an arbitrary boundary given its five macroscopic crystallographic parameters. A comparison of measured grain boundary areas and the grain boundary energies given by the function at the Σ11, Σ17b, and Σ33a misorientations, which are problematic to determine by measurement or atomistic calculations, reveals inverse correlations that are similar to what have been observed in other metals.

Effect of rhenium on defects evolution behavior in tungsten under irradiation

The influence of the transmutation element rhenium (Re) on defect evolution in tungsten (W) during irradiations with heavy ions and deuterium (D) plasma was investigated. Rolled W and W–5Re alloy (Re concentration 5 wt.%) were irradiated with 500 keV iron ions to 0.06 and 0.6 dpa (displacement per atom), and deuterium plasma at 38 eV/D to a fluence of 2.2 × 1025 D m−2 at ∼373 K. The results of Doppler broadening spectroscopy by the positron annihilation technique indicate that more or larger vacancy-type defects were produced in W than in the W–5Re during Fe ion irradiation, suggesting the important role of Re on inhibiting the migration and/or clustering of vacancies. The much smaller and shallower blistering in W–5Re than in W after the same D plasma exposure points to the pinning effects on dislocations by Re. The total retained D amounts in W–5Re and W materials are comparable, but with very different concentration profile at corresponding depth. This is explained by the blister formation exerting influence on the D inward diffusion. Demonstrating the important role of Re on the defect evolution in W under irradiation, the present work provides an enhanced understanding on the possible effects of neutron irradiation on the performance of plasma-facing materials in future fusion devices.

Suppressing/enhancing effect of rhenium on helium clusters evolution in tungsten: Dependence on rhenium distribution

The influence of rhenium (Re) on helium (He) evolution in tungsten (W) with different Re distribution is investigated using the first-principles calculations in combination with thermodynamic models. It is found that the He behaviors in W-Re system are closely related to the Re distribution. For the dispersed distribution of Re in W, it is found that a single Re atom only has slight effect on the behaviors of a He atom due to the weak interaction of Re-He1. Interestingly, with the increasing of He numbers, the binding energy between a single Re and He clusters increases rapidly from 0.03 eV for Re-He1 to 0.72 eV for Re-He4. Such high binding energy indicates the strong attraction of Re with He clusters in W. This can be rationalized by the large lattice distortion induced by He clusters, which reduces the electron density of He-occupied position and leads to the transition of neighboring Re/W atom from a substitutional site to an interstitial site. Because of the strong attraction of Re with He clusters, the Re addition effectively reduce the mobility of He clusters in W. As for the aggregated Re distribution, the He trapping capability of vacancy is weakened by Re clusters. These results are well consistent with the experimental observed suppressing effects of Re on the He-induced morphology in W. Further, based on the He behaviors in W-Re σ phase, we suppose that He will segregate from bulk W to Re-rich precipitates, thus enhancing the formation of He clusters. Consequently, our calculations suggest that the distribution of Re plays a key role on the He behaviors in W, which provides a good reference for the design and development of W-based alloys.

Displacement cascade evolution in tungsten with pre-existing helium and hydrogen clusters: a molecular dynamics study

Abstract The effects of radiation damage on bcc tungsten with preexisting helium and hydrogen clusters have been investigated in a high-energy environment via a comprehensive molecular dynamics simulation study. This research determines the interactions of displacement cascades with helium and hydrogen clusters integrated into a tungsten crystal generating point defect statistics. Helium or hydrogen clusters of atoms~0.1% of the total number of atoms have been randomly distributed within the simulation model and primary knock-on-atom (PKA) energies of 2.5, 5, 7.5 and 10 keV have been used to generate displacement cascades. The simulations quantify the extent of radiation damage during a simulated irradiation cycle using the Wigner-Seitz point defect identification technique. The generated point defects in crystals with and without pre-existing helium/hydrogen defects exhibit a power relationship with applied PKA energy. The point defects are classified by their atom type, defect type, and distribution within the irradiated model. The presence of pre-existing helium and hydrogen clusters significantly increases the defects (5 - 15 times versus pure tungsten models). The vacancy composition is primarily tungsten (e. g., ~70% at 2.5 keV) in models with pre-existing helium, but the interstitials are primarily He (e. g., ~89% at 10 keV). On the other hand, models with pre-existing hydrogen have a vacancy composition that is primarily tungsten (more than 90% irrespective of PKA energy), and the interstitial composition is more balanced between tungsten (average 46%) and hydrogen (average 54%) interstitials across the PKA range. The distribution of the atoms reveals that the tungsten point defects prefer to reside close to the position of cascade initiation, but helium or hydrogen defects reside close to the positions where clusters are built.

Effects of cascade-induced dislocation structures on the long-term microstructural evolution in tungsten

In recent years, a number of systematic investigations of high-energy collision cascades in tungsten employing advanced defect analysis tools have shown that interstitial clusters can form complex non-planar dislocation structures. These structures are sessile in nature and may potentially have a strong impact on the long-term evolution of the radiation microstructure. To clarify this aspect, we selected several representative primary damage states of cascades debris and performed annealing simulations using molecular dynamics (MD). We found that immobile complexes of non-planar dislocation structures (CDS) evolve into glissile and planar shaped 1/2〈1 1 1〉 loops with an activation energy of ~1.5 eV. The CDS objects were implemented in an object kinetic Monte Carlo (OKMC) model accounting for the event of transformation into 1-D migrating loops, following the MD data. OKMC was then used to investigate the impact of the transformation event (and the associated activation energy) on the evolution of the microstructure.

Object kinetic Monte Carlo model for neutron and ion irradiation in tungsten: Impact of transmutation and carbon impurities

We upgrade our object kinetic Monte Carlo (OKMC) model, aimed at describing the microstructural evolution in tungsten (W) under neutron and ion irradiation. Two main improvements are proposed based on recently published atomistic data: (a) interstitial carbon impurities, and their interaction with radiation-induced defects (point defect clusters and loops), are more accurately parameterized thanks to ab initio findings; (b) W transmutation to rhenium (Re) upon neutron irradiation, impacting the diffusivity of radiation defects, is included, also relying on recent atomistic data. These essential amendments highly improve the portability of our OKMC model, providing a description for the formation of SIA-type loops under different irradiation conditions. The model is applied to simulate neutron and ion irradiation in pure W samples, in a wide range of fluxes and temperatures. We demonstrate that it performs a realistic prediction of the population of TEM-visible voids and loops, as compared to experimental evidence. The impact of the transmutation of W to Re, and of carbon trapping, is assessed.