He-vacancy Complexes Research Articles

The Influence of Rhenium Content on Helium Desorption Behavior in Tungsten-Rhenium Alloy.

To investigate the influence of different rhenium contents on the helium desorption behavior in tungsten-rhenium alloys, pure tungsten and tungsten-rhenium alloys were irradiated with helium under the same conditions. All irradiated samples were characterized using TDS and DBS techniques. The results indicate that the addition of rhenium can reduce the total helium desorption quantity in tungsten-rhenium alloys and slightly accelerate the reduction in the concentration of vacancy-type defects accompanying helium dissociation. The desorption activation energy of helium is approximately 2 eV at the low-temperature peak (~785 K) and about 4 eV at the high-temperature peak (~1475 K). An increase in rhenium content causes the desorption peak to shift towards higher temperatures (>1473 K), which is attributed to the formation of the stable complex structures between rhenium and vacancies. Besides, the migration of He-vacancy complexes towards traps and dynamic annealing processes both lead to the recovery of vacancy-type defects, resulting in a decrease in the positron annihilation S parameters.

Effect of pre-irradiation defects on helium trapping and diffusion in RAFM steel

In order to analyze the effect of pre-irradiation defects on helium trapping and diffusion behaviors in detail by slow positron beam, Si ion with high energies (300 keV and 6 MeV) and 100 eV helium plasma were sequentially introduced into reduced activation ferritic/martensitic steel. Massive microscopic defects were observed in Si ion irradiated specimens. Low energy helium (∼100 eV) atoms could diffuse into much deeper region in metals without displacement damage. In the diffusion process of helium atoms, they would be trapped by the Si-ion pre-irradiation defects. The formation of He-vacancy complexes would affect the annihilation of positrons with the electrons in pre-irradiation defects and decrease the S parameters. The shallow region defects induced by 300 keV irradiation have more obvious advantage of hindering the diffusion of helium over the deep defects than that in 6 MeV irradiation. Thus, the pre-introduced defects can trap effectively helium atoms and hinder their diffusion.

Helium irradiation-induced ultrahigh hardening in niobium

In this work, using in-situ uniaxial tensile and compressive testing, microscopy, and theoretical analysis, we study the mechanism underlying the ultrahigh irradiation hardening in niobium (Nb). We show that irradiated Nb pillar exhibits a more than two-fold increase in the yield stress. With in-situ mechanical testing, we observe that He bubbles in Nb promote dislocation nucleation and multiple slip systems. The Nb pillars with 1.2 nm He bubbles fail by bubble coalescence and form a faceted fracture surface. In contrast, the Nb pillars with 8 nm He bubbles fail by bubble elongation and fragmentation. A theoretical analysis of the hardening contribution based solely on the density and size of He bubbles finds that it is less than one third of the experimentally observed hardening. To explain the large gap between the model and the experiment, we propose that the ultrahigh irradiation hardening originates from a large quantity of atomic-size, undetectable He-vacancy (He-V) complexes. The implanted He ions only account for less than 50% percent in the visible He bubbles, while most of them bind to vacancies to form stable He-V complexes that are distributed throughout the lattice. The strong interaction between dislocations and high density of He-V complexes is the chief source for the remarkable irradiation hardening observed in Nb.

Influence of helium on the evolution of irradiation-induced defects in tungsten: An object kinetic Monte Carlo simulation* *Project supported by the Science Challenge Project (Grant No. TZ2018002), the National Natural Science Foundation of China (Grant No. 11905135), and the National MCF Energy R&D Program of China (Grant No. 2018YFE0308103).

Understanding the evolution of irradiation-induced defects is of critical importance for the performance estimation of nuclear materials under irradiation. Hereby, we systematically investigate the influence of He on the evolution of Frenkel pairs and collision cascades in tungsten (W) via using the object kinetic Monte Carlo (OKMC) method. Our findings suggest that the presence of He has significant effect on the evolution of irradiation-induced defects. On the one hand, the presence of He can facilitate the recombination of vacancies and self-interstitial atoms (SIAs) in W. This can be attributed to the formation of immobile He-SIA complexes, which increases the annihilation probability of vacancies and SIAs. On the other hand, due to the high stability and low mobility of He-vacancy complexes, the growth of large vacancy clusters in W is kinetically suppressed by He addition. Specially, in comparison with the injection of collision cascades and He in sequential way at 1223 K, the average sizes of surviving vacancy clusters in W via simultaneous way are smaller, which is in good agreement with previous experimental observations. These results advocate that the impurity with low concentration has significant effect on the evolution of irradiation-induced defects in materials, and contributes to our understanding of W performance under irradiation.

The early growth process of helium blistering in tungsten and molybdenum: First-principles and statistical model calculations

Based on first-principles simulations combined with statistical physical model, we have performed systematically the investigations on the nucleation and the early growth process of helium (He) blistering in W and Mo. The thermodynamics and structural stability of He-vacancy complexes (HenVm) have been explored in detail. The energy-demand has been determined for He blistering nucleation, indicating that one mono-vacancy can capture up to eight He atoms. To trap more He atoms, the mono-vacancy must emit a Frenkel-pair to release the extensive stress in the vicinity of W/Mo lattice. In comparison with the HenV complex, the HenVm (e.g. HenV9) complex exhibits a relatively lower energy-demand for the nucleation and growth of He blistering. Based on statistical physical analysis, the chemical potential of He can significantly affect He blistering nucleation and growth. Two critical chemical potentials of He are found to be especially important. One indicates the transformation from a He atom to multiple He atoms trapped by one mono-vacancy, while the other illuminates He-caused superabundant vacancy formation which in turn attracts more He atoms to make the blistering grow even bigger, resulting into a cascading effect in both W and Mo.

Effects of helium on critical hydrogen concentration for bubble formation in molybdenum

Helium (He) has a synergistic effect on hydrogen (H) retention in metallic plasma-facing materials for nuclear fusion devices, which further makes a potential impact on hydrogen bubble formation in the materials. Using first-principles calculations, we investigated the interactions between H and He in molybdenum. The calculated results show that the binding energies of H are decreased at the pre-formed He-vacancy complex compared to those at the monovacancy without He. This indicates H trapping at the He-vacancy complex is energetically less favorable and the plasma-facing material will have less H retention. Most importantly, we revealed the critical characteristics of hydrogen concentration with the presence of He based on a thermodynamic model and the calculated energies of HmHeV complexes. The existence of He could apparently enhance the critical hydrogen concentration beyond which an initial stage of hydrogen bubble formation will occur. Our findings provide explicit evidences for the suppressing effect of helium on hydrogen bubble formation in molybdenum.

Characterisation of open volume defects in Fe–Cr and ODS Fe–Cr alloys after He+ and Fe+ ion irradiations

Different ODS RAF steels with nominal composition Fe–14Cr–2W–0.3-Ti–0.3Y2O3 (wt%) and a model Fe–14Cr (wt%) alloy have been subjected to single Fe+ or He+ ion irradiations to simulate the effect of a fusion environment at different doses and temperatures. The irradiation induced open volume defects have been characterised by positron annihilation spectroscopy and transmission electron microscopy. For low temperature Fe+ irradiations, positron annihilation spectroscopy results show a high concentration of irradiation induced vacancy clusters, much more evident in the model alloy as compared to the ODS steels. The extent of damage due to He+ irradiations at RT and high temperatures is considerably lower than for the Fe+ irradiations. Regardless of the temperature He+ irradiations introduced changes in the chemical environment of open-volume positron traps possibly due to the creation of He-vacancy complexes. Positron annihilation spectroscopy results also suggest that irradiation induced vacancy-type defects in ODS steels are not associated with nanoparticles but are mainly in the ferritic matrix. The presence of irradiation induced bubbles was studied after He+ irradiation at high temperatures. Small bubbles were observed, being smaller in the ODS steels as compared to the model alloy.

H dissolution and desorption in W (1 1 0) surface with the presence of He: A first-principles study

Experiments shew that the presence of helium (He) can suppress total hydrogen (H) retention/penetration and surface blistering, while increase tungsten (W) surface H retention. It is thus necessary to reveal the micro-mechanisms of the dissolution and desorption of H in the W surface with the presence of He to understand these two opposite phenomena systematically. It has already known that a surface tungsten vacancy (V) is easier to form than in the bulk and has a large trapping effect on H dissolving. So, the first-principles method is employed to study the effect of the single-vacancy and He-vacancy (HeV) complex on the dissolution and desorption behaviors of H in the W (1 1 0) surface. We found that there are more H capture sites around HeV complex than that around V, which should response for the increase of H retention near surface. In addition, both the trapping energies and desorption barriers of H around HeV complex are lower comparing to those around V, which makes it easier for H to desorb with the presence of He. Therefore, under the irradiation of low-energy plasma, the pre-exposure of He can increase both the trapping sites and desorption rate of H in W surface, which would suppress H penetration into bulk as well as surface blistering.

A single nano-void precisely positioned in SiO2/Si substrate by focused helium ion beam technique

We report on the formation of a single nano-void, precisely positioned in SiO2/Si, using He-ion beam technique. The in-plane position of the void can be achieved by the direct writing capability of the He-ion beam system. While the depth and size of void are able to be determined by the acceleration voltage and dosage of the He+-ions, respectively. The as-written sample shows an ellipsoidal shape amorphous region in Si. Thermal treatments aggregate He-vacancy complexes and He bubble defects to form a void in the central of the region. At the same time, recrystallization starts from the boundary between the amorphous and crystal Si. Through HAADF imaging, EDX scanning profile and SEM imaging, we show the existence of a nano-void at a predicted depth. For the sample annealed at 825 °C, we observed a single void formed by {111} facets.

Evaluation of defect formation in helium irradiated Y2O3 doped W-Ti alloys by positron annihilation and nanoindentation

Helium implanted tungsten-titanium ODS alloys are investigated using positron annihilation spectroscopy and nanoindentation. Titanium reduces the brittleness of the tungsten alloy, which is manufactured by mechanical alloying. The addition of Y2O3 nanoparticles increases the mechanical properties at elevated temperature and enhances irradiation resistance. Helium ion implantation was applied to simulate irradiation effects on these materials. The irradiation was performed using a 500 kV He ion implanter at fluences around 5 × 1015 cm−2 for a series of samples both at room temperature and at 600 °C. The microstructure and mechanical properties of the pristine and irradiated W-Ti-ODS alloy are compared with respect to the titanium and Y2O3 content. Radiation damage is studied by positron annihilation spectroscopy analyzing the lifetime and the Doppler broadening. Three types of helium-vacancy defects were detected after helium irradiation in the W-Ti-ODS alloy: small defects with high helium-to-vacancy ratio (low S parameter) for room temperature irradiation, larger open volume defects with low helium-to-vacancy ratio (high S parameter) at the surface and He-vacancy complexes pinned at nanoparticles deeper in the material for implantation at 600 °C. Defect induced hardness was studied by nanoindentation. A drastic hardness increase is observed after He ion irradiation both for room temperature and elevated irradiation temperature of 600 °C. The Ti alloyed tungsten-ODS is more affected by the hardness increase after irradiation compared to the pure W-ODS alloy.

The effect of He + irradiation on hardness and elastic modulus of Fe-Cr–40 wt.% TiB 2 composite rod designed for neutron absorbing

Abstract A composite rod, designed for the neutron absorption in nuclear research reactors or Nuclear Power Plants (NPPs), was synthesized by ball milling and subsequent Hot Isostatic Pressing (HIP-ing), similarly to the one reported in Ref. [3]. Its core consists of the oxide dispersion strengthened ferritic Fe-Cr matrix and 40 wt.% of TiB 2 reinforcement, surrounded by a Ti tubing/cladding with a functionally graded interface layer. The Specific Surface Area (SSA) for the ball-milled pre-alloyed powders increases from 0.64 to 2.92 m 2 /g and XRD shows that the crystallite/grain size of TiB 2 is ∼38 nm. Berkovich nanoindentation study after irradiation with 160 keV He + ions at a fluence up to 1 × 10 17 He + /cm 2 shows an initial hardening effect at fluences of 1 × 10 15 and 1 × 10 16 ions/cm 2 . Hardness and elastic modulus of the Fe-Cr -TiB 2 core rapidly drops when the fluence reaches 1 × 10 17 ions/cm 2 . The Ti cladding seems to be relatively impervious to increased radiation fluence since its hardness and elastic modulus change very slightly with increasing ion fluence. The observed changes in the mechanical properties are discussed in terms of vacancy/dislocation loops and He bubble formation in the irradiated microstructure. Although the He-vacancy complexes are widely regarded as being immobile, it is hypothesized here, based on the grazing incidence XRD (GI XRD), that interstitial helium diffuses outward through the boundaries of the (100) hcp-TiB 2 nanograins. As a result, the relaxation of compressive strains due to high concentration of vacancies in a nanograin crystalline lattice finally leads to the hcp-TiB 2 unit cell contraction.

First-principles investigation of helium dissolution and clustering at a tungsten (1 1 0) surface

Using a first-principles method, we have investigated dissolution, self-trapping and clustering of He at a W(110) surface. We found that the He atom is not energetically favorable at both the surface and the subsurface, but it becomes stable under the second atomic layer from the surface. The He is easier to be self-trapped to form an He cluster at the near surface in comparison with the bulk due to the larger self-trapping range and the stronger binding energy. With the formation of such He cluster, the vacancy and thus the He-vacancy complex are able to form at the near surface. The results will provide a useful reference for understanding formation of the He bubble at the W surface.

Investigation of vacancy-type defects in helium irradiated FeCrNi alloy by slow positron beam

The evolution of microstruture for Fe16.7Cr14.5Ni model alloy and 316 stainless steel irradiated with 140keV He ions were studied by Positron annihilation spectroscopy. The fluences were 1×1016 and 5×1016 Heions/cm2. The irradiation temperature was room temperature and 573K, respectively. The variation of S parameter-incident positron energy profile indicated that large amount of vacancy-type defects formed after He ion irradiation. Meanwhile, helium atoms deposited in bulk and certain amount of He-vacancy complexes were formed. The vacancy-type defects could be the major defects in track region and He-vacancy complexes would be the main defects in cascade region. The vacancy-type defects could migrate and aggregate to form vacancy clusters and even microvoids at elevated temperature irradiation. The diffusion mechanism of helium atoms might be changed at different irradiation temperature.

Helium bubble nucleation and growth in α-Fe: insights from first–principles simulations

We have carried out a first-principles study on the nucleation and early-stage growth of He bubbles in Fe. The energetics, atomic and electronic structure of He-vacancy complexes, involving both a monovacancy and a nine-vacancy cluster, are examined. Based on the energetics, we then perform thermodynamics analysis to gain deeper insights into He bubble nucleation and growth. We have determined the energy cost for the nucleation of He bubbles and found that up to eight He atoms can be trapped at a single vacancy. In order to capture more He atoms, the vacancy has to emit Frenkel pairs to release the substantial stress building on the surrounding Fe lattice. Compared to the monovacancy, the nine-vacancy cluster has a lower energy cost for He bubble nucleation and growth. He atoms at the vacancy repel the surrounding electronic charge and redistribute it on the neighboring Fe atoms. The thermodynamic analysis reveals that He chemical potential provides a driving force for He bubble nucleation and growth. There are two critical He chemical potentials that are of particular importance: one of them marks the transition from single He occupation to multiple He occupation at a monovacancy while the other sets off He-induced superabundant vacancy formation.

Diffusion of small He clusters in bulk and grain boundaries in α-Fe

The diffusion properties of He interstitials and He clusters in the bulk and grain boundaries (GBs) of α-Fe have been studied using molecular dynamics with a newly developed Fe–He potential. The low migration energy barrier for a single He interstitial in the bulk is consistent with that obtained using ab initio methods. Small He clusters can migrate at low temperatures, but at higher temperatures they will kick out a self-interstitial atom (SIA) and become trapped by the vacancy, forming an He-vacancy complex. It is of great interest to note that small Hen-vacancy clusters (n<5) in the bulk are able to absorb an SIA, and the clusters become mobile again. Trapping and de-trapping of He clusters by emitting and absorbing an SIA represent an important dynamic process that provides a mechanism for the diffusion of He clusters and the nucleation of He bubbles in bulk Fe, particularly under irradiation in which numerous SIAs and vacancies are constantly being produced. A single He interstitial can migrate one-dimensionally or two-dimensionally within GBs, depending on the GB structure. Small interstitial Hen clusters (n∼1–10) can easily kick out an SIA, and become trapped by the vacancy, while the SIA quickly diffuses away from the clusters, disappearing into the GB, such that de-trapping of the He clusters by absorbing an SIA is less likely to occur. This suggests that small He clusters may be treated as relatively immobile defects in GBs. The different behavior of He clusters in the bulk compared to their behavior in GBs may explain the different He bubble sizes experimentally observed in the bulk and in GBs in reduced activation ferritic/martensitic steels that have been simultaneously neutron irradiated and He implanted.

Helium Bubbles in Fe: Equilibrium Configurations and Modification by Radiation

ABSTRACTWe have examined the properties of helium bubbles in Fe using two different Fe-He potentials. The atomic configurations and formation energies of different He-vacancy complexes are determined and their stability in the region of nearby collision cascades is investigated. The results show that the optimal He to Fe vacancy ratio increases from about 1:1 for approximately 5 vacancies up to about 4:1 for 36 vacancies. Collision cascades initiated near the complex show that Fe vacancies produced by the cascades readily become part of the He-vacancy complexes. The energy barrier for an isolated He interstitial to diffuse was found to be 0.06 eV. Thus a possible mechanism for He bubble growth would be the addition of vacancies during a radiation event followed by the subsequent accumulation of mobile He interstitials produced by the corresponding nuclear reaction.

Effect of vacancy on the dissolution and diffusion properties of hydrogen and helium in molybdenum

We present first-principles study of the stability and diffusion properties of H and He in Mo. The results show that diatomic H–H and He–He prefer 〈100〉 and 〈111〉 dumbbell configurations in single vacancy, respectively. The occupation and migration of H and He nearby a vacancy (or He-vacancy complex) are very different from that in bulk: both H and He are more preferable to occupy the tetrahedral interstitial sites closer to the vacancy (or He-vacancy complex), and the diffusion barriers of H and He into the vacancy (or He-vacancy complex) are slightly reduced but the diffusion barriers out of the defect are severely increased, especially for He. Besides, the presence of single He at tetrahedral interstitial site reduces the energy required for its nearby vacancy formation more considerably than that of H (the produced vacancy traps the H or He), contributing to the accumulation and different disposition depth of H and He in Mo.

Influence of carbon on the kinetics of He migration and clustering inα-Fefrom first principles

Density functional theory (DFT) calculations have been performed to study the interaction of carbon with He-vacancy complexes in $\ensuremath{\alpha}\text{-Fe}$. Using the DFT predictions, a rate theory model that accounts for the evolution of carbon, helium, and defects created during irradiation has been developed to explore the influence of carbon on the kinetics of He diffusion and clustering after implantation in $\ensuremath{\alpha}\text{-Fe}$. This DFT-based rate theory model predicts that carbon not only influences vacancy $(V)$ migration but also He desorption, enhancing He mobility in particular for low $V/\text{C}$ ratios. The reason for this behavior is mainly the formation of $V\text{C}$ and $V{\text{C}}_{2}$ complexes, which significantly reduces the mobility of vacancies with respect to pure Fe, inhibiting the formation of higher order clusters, i.e., ${\text{He}}_{n}{V}_{m}$, and increasing thus the number of He at substitutional positions at room temperature. Assuming reasonable values of carbon concentration, we successfully reproduce and interpret existing desorption experimental results, where all the energetic parameters for the relevant reactions were obtained from first-principles calculations. In addition, our study provides a detailed explanation of the various He migration mechanisms that prevail under the considered experimental conditions.

Influence of Self-Interstitial Mobility on He-Vacancy Cluster Nucleation and Growth in Nickel

The presence of impurities could affect the results observed in pure Ni. In particular, impurities could interact with self-interstitial atoms, which are highly mobile, effectively reducing their mobility. In this work we study the influence of the mobility of self-interstitials on He desorption. The nucleation of He-vacancy complexes is studied depending on the mobility of these self-interstitials in terms of He to vacancy content as well as concentration of these complexes.

Optical properties revealing annealing behavior of high-temperature He-implantation induced defects in silicon carbide

6H-SiC single crystal specimens were implanted at 600 K with 100 KeV He ions to three successively increasing fluences and subsequently annealed at different temperatures ranging from 600℃ to 1200℃ in vacuum. After the annealing, the samples were investigated by using Raman scattering spectroscopy and photoluminescence spectrometry，respectively. Both of the two methods showed that the damage induced by helium-ion-implantation in the lattice is closely related to the dose. The thermal annealing brings about recovery of the damage，and different levels of damage require different annealing temperature to recover efficiently. It is indicated that different annealing stages involve different mechanisms，corresponding to recombination of point defects，formation of He-vacancy complexes，and nucleation and coarsening of bubbles，respectively. The experimental results indicate that high temperature implantation is an effective way to avoid amorphization of the implanted layer due to damage accumulation. Helium implantation can be used to introduce buried nanoscale cavities as the nucleation site for the buried oxide in a well defined region proposed for an alternative and more economical method of manufacturing SiC-on-Insulator （SiCOI）.