Helium-vacancy Clusters Research Articles

The ferrite/perovskite interface and helium partition in nano-structured ferritic alloys from the first-principles

Nano-oxides are largely responsible for the excellent mechanical properties and irradiation tolerance of nano-structured ferritic alloys (NFAs) for nuclear reactor applications. In this work, the roles of perovskite YTiO3 and its interface in trapping helium in NFAs were investigated from the first-principles. Similarly as other two Y-Ti-oxide phases (Y2TiO5 and Y2Ti2O7), bulk YTiO3 can trap insoluble helium at its interstitial sites, but with a lower trapping ability that is only comparable to matrix vacancies. The ferrite/YTiO3 interface phase diagram was constructed based on the experimental orientation relationship and the calculated interface formation energy, and the lowest-energy interface structure was predicted as the ns-Ti or the stoichiometric. Helium always prefers to consume individual interfacial vacancies and interstitial sites to the extent possible, before forming higher-order helium-vacancy clusters at the interface. Similarly as Y2Ti2O7, YTiO3 preferably traps helium at its interface, followed by its bulk interior and the ferritic matrix. However, in view of all the bulk and interface results, perovskite YTiO3 cannot compete with pyrochlore Y2Ti2O7 in trapping helium in NFAs.

Simulations of radiation damage accumulation in Fe-9Cr under pulsed irradiation conditions representative of inertial fusion energy

Structural materials for laser-based inertial fusion energy (IFE) reactor concepts are expected to operate under pulsed irradiation conditions, with cycles consisting of microsecond-long neutron bursts followed by inter-pulse periods of up to one second in duration. During each laser shot, irradiation damage is introduced at dose rates that are up to six orders of magnitude higher than those in their magnetic fusion energy (MFE) counterparts. Under certain conditions, the inter-pulse periods may last an amount of time sufficient to anneal much of the damage introduced during each shot. This phenomenon is highly temperature dependent, with pulsed heating directly linked to the pulsed damage, large surface temperature spikes may also occur. As such, this intermittent mode of operation has the potential to lead to fundamental differences in how irradiation damage accumulates in structural reactor materials. However, damage to structural materials under IFE conditions has received comparatively much less attention than in MFE, as pulsed conditions add yet an extra dimension to the already extremely challenging problem of microstructural evolution under fusion neutron irradiation in structural materials. In this work we use the stochastic cluster dynamics (SCD) method to simulate the evolution with time of defect cluster concentrations under IFE conditions. We consider the Laser Inertial Fusion Energy (LIFE) reactor concept as the representative IFE design for our study, for which detailed spectral information is available, including gas transmutant production. We simulate several pulse frequencies and three different temperatures, and compare the results with continuous irradiation cases under identical average dose rates. The simulations are run in Fe-9Cr system as a model alloy for reduced-activation ferritic/martensitic (RAFM) steels, which are the leading structural material candidates for first-wall structures in MFE and IFE devices. We find that, in practically all scenarios, pulsed irradiation restricts the formation of helium-vacancy clusters relative to the levels seen under equivalent steady irradiation conditions. As well, although self-interstitial atom clusters do accumulate under pulsed operation, their number densities remain up to an order of magnitude lower than in continuous irradiation conditions. Based on the SCD results, we provide a temperature-pulse rate map to identify regions where pulsed irradiation may lead to larger defect accumulation than under continuous irradiation.

The temperature effect on the formation and growth of helium-vacancy clusters in bcc iron: A comprehensive molecular dynamics simulation

Bubble generation can significantly degrade the performance of fusion reactor structural materials. In this work, the temperature effects on the initial clustering of helium (He) atoms, the formation of helium-vacancy clusters (HenVm) and the subsequent growth of the clusters into bubbles in bcc iron under different conditions were simulated by molecular dynamics. The results show that He behaviors are primarily dominated by coalescence at temperature range from 400 K to 500 K, which leads to a preference for He to bind together. Conversely, when temperature is higher than 500 K, dissociation become more prominent, causing large He clusters to be hard to form. Under the conditions of increasing He concentration and irradiation dose, the higher the temperature, the higher the density of HenVm clusters at He concentration around 100 appm. However, the temperature dependence reversed at high He concentration. The detailed analysis of formation cases revealed the percentage of nucleation sites for He bubbles at defects during He segregation. In addition, there is no significant difference among the He/V ratio of HenVm clusters formed at vacancies, interstitial dislocation loops (IDLs) and interstitial sites during the cluster growth. However, the He/V ratio of He bubbles growing on edge dislocation (ED) is overall lower than those growing at other formation sites, since the growth of HenVm clusters on ED requires a lower pressure to kick out self-interstitial atoms (SIAs) or SIA clusters. The average He/V ratio of He bubbles in bulk decreases as temperature rises from 400 K to 800 K, while that of He bubbles growing on ED is independent of temperature.

Atomic-scale study of He ion irradiation-induced clustering in α-Zirconium

Zircaloy-4 alloy specimens consisting of α-Zr grains were irradiated on a tandem accelerator with 5 MeV He ions to a fluence of 5 × 1021 ions m − 2 at different temperatures. Defect clusters and He bubbles were observed in the TEM micrographs of the irradiated microstructure. The small defect clusters and He bubbles were found to exhibit spherical shapes whereas the large defect clusters showed plate-like shapes. Molecular dynamics simulations and crystallography analyses revealed that the defect clusters are mainly composed of Zr interstitials. During He-ion irradiation, the clustering of Zr dumbbell interstitials results in the formation of small and spherical defect clusters. With further irradiation, the Zr interstitials prefer to occupy the octahedral sites that align along the {12¯14} planes with other site interstitials, forming cluster sections. These initial planar clusters then grow by extending along the <101¯0> directions and by forming more cluster sections along the <12¯16> directions, leading to large plate-like defect clusters on the {1¯21¯1}planes. He bubbles nucleate by the clustering of He atoms and vacancies that are formed by kicking out the lattice atoms, and then grow by the migration and coalescence of helium-vacancy clusters. Small He atoms have many interstitial sites to occupy in the α-Zr HCP crystal, allowing He bubbles to grow to spherical shapes. In addition, grain boundaries induce the accumulation of primary point defects and He atoms and high irradiation temperature accelerates diffusion, and hence larger defect clusters and He bubbles form in the grain boundary regions and at higher irradiation temperature.

Helium bubble facetation in tungsten thin films

Helium bubbles naturally form in irradiated materials and their shape can potentially have a significant impact on mechanical property degradation. Experimental observations of helium bubble facetation are compared to theoretical equilibrium shapes based on surface energy, known as Wulff constructions, obtained from density functional theory (DFT). The shape and orientation of implanted He bubbles match very well with those determined from a thermodynamic analysis, with discrepancies attributed to kinetic factors, such as the anisotropic absorption of mobile helium-vacancy clusters and increased barrier to nucleate facets from the density of helium contained within the bubbles.

Effects of He-D Interaction on Irradiation-Induced Swelling in Fe9Cr Alloys.

The atomic-scale defects such as (deuterium, helium)-vacancy clusters in nuclear energy materials are one of the causes for the deterioration of the macroscopic properties of materials. Unfortunately, they cannot be observed by transmission electron microscopy (TEM) before they grow to the nanometer scale. Positron annihilation spectroscopy (PAS) has been proven to be sensitive to open-volume defects, and could characterize the evolution of the size and concentration of the vacancy-like nanoclusters. We have investigated the effects of He-D interaction on the formation of nanoscale cavities in Fe9Cr alloys by PAS and TEM. The results show that small-sized bubbles are formed in the specimen irradiated with 5 × 1016 He+/cm2, and the subsequent implanted D-ions contribute to the growth of these helium bubbles. The most likely reason is that helium bubbles previously formed in the sample captured deuterium injected later, causing bubbles to grow. In the lower dose He-irradiated samples, a large number of small dislocations and vacancies are generated and form helium-vacancy clusters with the helium atoms.

Large helium-vacancy clusters coalescence during helium bubble evolution under cascade in iron with edge dislocation: A MD simulation

The nucleation and growth of helium bubbles under the mutual effect of cascade and dislocation in bcc Fe was investigated by molecular dynamic simulations. The helium atoms along with primary knock-on atoms (PKA) were gradually added in the simulation system to imitate the real neutron irradiation condition. Several coalescence events of large helium-vacancy (He-V) clusters during the molecular dynamic simulation in bcc Fe were discovered. The detailed analysis of these coalescence events revealed that in the bulk region, if two helium-vacancy clusters are close enough, they are inclined to combine into one cluster by kicking out host atoms between the two clusters. However, two close helium-vacancy clusters near the dislocation line will not merge by this kick-out mechanism. Instead, it’s found that they could be promoted to combine together by cascade, even if the two clusters locate on the edge of the cascade thermal spike. The mutual influence of dislocation and cascade will lead to the formation of low content and large size helium-vacancy clusters around the dislocation line.

A Slow Positron Beam Study of Helium Clustering in Oxide Dispersion Strengthened Steels

The present work provides an innovative approach to the experimental study of the radiation tolerance of oxide dispersion strengthened (ODS) steel comparing to its non-ODS variant. Inhomogeneous radiation damage, introduced by helium irradiation, was probed in a depth profile manner by a variable energy positron beam. Although numerous studies reporting the use of slow positron beam (SPB) experiments on ion-implanted materials have been published in the literature, only a few of them address the continuous distribution of microstructural damage introduced by ion beam. In this paper, we use SPB to probe a wide range of radiation damage in a single sample of each material. By evaluating positron diffusion in different parts of the Bragg peak, we were able to track the evolution of radiation-induced damage from a high density of small defects such as helium-vacancy clusters to large bubbles grown by coalescence. Combining with the transmission electron microscopy results and slow positron Doppler broadening spectroscopy, we offer new insight into different stages of microstructural bubble evolution in steels exposed to harsh radiation conditions. The obtained results show that the higher density of vacancy-type defects in the oxide dispersion strengthened steel, which act as recombination centres, suppress the formation of large defect agglomerations. While the TEM provides an indication of this effect in high displacement damage and helium production rates, positron annihilation spectroscopy elucidates the origin of this behaviour, improving the understanding of the fundamental mechanism of helium embrittlement.

Recent Progress on the Positron Annihilation Experiments on Irradiated Ferritic/Martensitic Steels Containing Helium

The well-known and often acceptable radiation tolerance of ferritic/martensitic (f/m) steels can be severely diminished when neutron irradiation is accompanied by the production of helium. The presence of helium in the irradiated materials changes the kinetics of the nucleation, recombination, and clustering of the radiation-induced defects. High production rates of helium may lead to a non-negligible volumetric bubble swelling at relatively low temperatures. Extrapolation of the knowledge gained from neutron irradiation experiments to fusion or spallation environments is additionally complicated due to the unknown and comprehensive effects of dpa rate, temperature, the presence of sinks in the crystal lattice and others. To improve the understanding of the microstructure and irradiation parameters effects, close attention must be paid to the early stages of the radiation damage. It is expected that the pre-existing vacancy-type defects, attributed to lattice distortion at the grain/subgrain boundaries and oxide-matrix interfaces, are effective sinks for primary defects and helium, i.e. they control the formation and growth of helium-vacancy agglomerations. This early-stage radiation damage, however, cannot be captured by conventional transmission electron microscopy, and thus other experimental techniques are called for. One of the most perspective experimental approaches to investigate small vacancy-type defects, with a high sensitivity to confined helium, is to utilize positron annihilation spectroscopy (PAS). In particular, two spectroscopy techniques, positron annihilation lifetime spectroscopy (PALS) and Doppler broadening spectroscopy (DBS) of the annihilation line, can be beneficially used for the characterization of helium-vacancy clusters. This paper reviews the recent positron annihilation spectroscopy characterization of various irradiation experiments involving helium. Mainly two types of irradiation experiments are addressed, helium implantation and spallation neutron source irradiation experiments. Discussion is aimed at the potential of PAS in the early-stage formation of helium bubbles and the investigation of the effects of irradiation parameters in defect production and accumulation.

&lt;i&gt;Ab Initio&lt;/i&gt; Study of Helium in Tantalum: Interaction, Migration, and Clustering with Helium and Vacancies

Ab initio calculations based on the Density Function Theory (DFT) have been performed to study the interaction between helium and helium, helium and vacancy, migration of helium, and the stability of small helium-vacancy clusters in tantalum. The following results are found: (I) The tetrahedral interstitial helium atoms have weak interactions in tantalum, suggesting that no stable covalent bond is formed between this two helium atoms; (II) The stability of small helium-vacancy clusters is investigated. The interstitial helium atom and vacancy to the clusters are found to be positive in almost all case, i.e., all interactions are attractive; (III) The activation energies for a substitutional helium atom migration by the dissociation or vacancy mechanisms are estimated under the irradiation condition.

Integrated modeling of helium-vacancy clustering in Eurofer97 steel upon He+/Fe3+ dual-beam irradiation

By developing an integrated rate theory model, we studied helium-vacancy (He-V) clustering in ferritic/martensitic Eurofer97 steels under He+/Fe3+ dual-beam irradiation. Different from the known single peak concentration profile of total atomic He, the concentration of ‘solute’ He atom (interstitial He) increases and decreases twice during irradiation, showing two peaks as a function of irradiatiom time (dose). The second increase in concentration was caused by the operation of the kick-out mechanism aided by irradiation-induced self-interstitials. We show that the kicked-out interstitial He enhances the growth of He-V clusters and leads to the shift of cluster pressurization states. He-V clustering direction is also governed by the kicked-out interstitial He.

Sensitivity of positrons at hydrogen storage sites in FeCr alloy containing vacancy and helium atom

Here, the storage sites of hydrogen in FeCr alloy, namely (H, He)–V nano-clusters with open volume, have been investigated by first-principles calculations and positron annihilation spectroscopy. The positron lifetimes in the nano-clusters obtained by theoretical calculations and experiments were compared. These results suggest that positron is sensitive to the relative position of the gas atoms decorating the open volume; and a helium atom forms a more repulsive ion core than a hydrogen atom when it occupies the vacancy, resulting in a decrease in positron lifetime. Interpretation of the PAS data was successfully combined with theoretical calculations, and allowed us to determine the defect status in specimens after H/He irradiation with different implantation patterns. The speculated kinds of hydrogen storage sites formed after irradiation are summarized. For the He-ions pre-implanted situation, the helium-vacancy clusters or bubbles forming in advance may recombine with H atoms to form stable He–H–V complex.

Spatially dependent kinetics of helium in tungsten under fusion conditions

Tungsten is the prime candidate material for divertor applications in future nuclear reactors (e.g. ITER and DEMO). In the present work, a spatially dependent cluster dynamics model is developed to investigate and understand the microstructure evolution of tungsten under low energy helium implantation and neutron irradiation varying over bulk length scales of millimetres and irradiation time scales of hours. The diffusion of helium, helium clusters and their trapping at neutron induced defects is simulated along the tungsten monoblock depth. The temperature gradient resulting from a steady state heat load of 10 MWm−2 along the monoblock depth is considered and its influence on the evolution of defects is discussed. The trapping of helium at vacancies and the associated formation of helium-vacancy clusters is found to be pronounced in the sub-surface layers. A significant influence of helium detrapping from grain boundaries and dislocations, along with its resolution from clusters, on the helium diffusion length scales is observed. Additionally, the effect of helium cluster mobility is investigated and overall lower retention in the monoblock bulk is observed through significant release of helium at the surface.

Helium desorption behavior and growth mechanism of helium bubbles in FeCrNi film

Abstract He-charged FeCrNi films were prepared by radio frequency (RF) magnetron sputtering technique, and helium desorption behavior, growth process and evolution mechanism of helium bubbles in FeCrNi film were studied by thermal desorption spectroscopy (TDS) spectra combined with microstructure analysis under different annealing treatments. Three different He desorption peaks were observed in TDS spectra, which can be related to the release of interstitial helium atoms from the near-surface regions, helium-vacancy clusters with different sizes in FeCrNi film. TEM analysis showed that helium bubbles can be uniformly distributed in He-charged FeCrNi nanocrystalline films. Under annealing treatments at different temperatures, the size of helium bubbles obviously increases with increasing annealing temperature, corresponding to the significantly decrease in the number density of helium bubbles in FeCrNi films, and the growth mechanism of helium bubbles was suggested to originate from the migration and coalescence (MC) mechanism.

Influence of nanochannel structure on helium-vacancy cluster evolution and helium retention

In the fusion reactors, plasma facing materials (PFMs) will be bombarded by large amounts of energetic particles, including helium (He) atoms. These metal-insoluble He atoms are prone to self-trapping or are trapped by vacancies, dislocation cores or grain boundaries, aggregating to form high concentration He bubbles which leads to serious degradation of the PFMs’ properties. Studying the initial stages of He-defect interactions becomes increasingly important because it not only helps us understand the formation mechanism of He bubbles but also provides insight to help design new irradiation-resistant PFMs. In this work, we used the positron annihilation Doppler broadening spectroscopy (DBS) and secondary ion mass spectrometry (SIMS) to explore the evolutions of the interactions between vacancy-type defects and He and the retention of He in the nanochannel W film irradiated with 190 keV He+ ions to different fluences under different temperatures. It is found that the presence of a nanochannel structure accelerates the release of He from the film even at low irradiation fluences, and the release of He is significantly enhanced at higher fluences, thus inhibiting or delaying the formation of large He-vacancy clusters in the nanochannel W film. Both irradiation fluence and temperature have significant influence on the formation and evolution of He-vacancy clusters, and the evolution of these microstructure also causes hardness changes.

Modeling the effects of helium-vacancy clusters on the stress-strain response of a grain boundary in iron by a mechanistic finite element approach informed by molecular dynamics data

The effects of helium (He)-vacancy clusters on the stress-strain behavior of polycrystalline iron (α-Fe) are investigated by a mechanistic finite element (FE) approach using a continuum damage mechanics (CDM) description of the material behavior informed by molecular dynamics (MD) data. First, MD analyses of a single crystal (loading normal to {332} plane) and a bicrystal system containing a Σ11<110>{332} grain boundary (GB) were performed to compute the uniaxial tensile response of an Fe single crystal and a system with a GB. MD results were then used in FE analyses of the same systems to identify parameters for the CDM constitutive relations for the crystal and the traction-separation law for the GB depicted by cohesive elements. Next, a 3D FE model of an α-Fe bicrystal system with an imperfect GB subjected to uniaxial tensile loading was developed. This model includes an equivalent hollow sphere under internal pressure in the middle of the GB to model the effects of pressurized He bubbles at 5 K, room temperature (RT) and 600 K on stress, strain and damage distributions. The radius of the equivalent sphere was determined assuming the presence of two vacancies in the system. Finally, MD stress/strain data of the same bicrystal system with He-vacancy clusters were compared to the corresponding FE results to validate this mechanistic approach that appears to be efficient in terms of computational time. FE model predictions of system strength and fracture strain are in fairly good agreement with the MD results at all three temperatures. Our results show that small and highly pressurized He-vacancy clusters reduce GB strength and fracture strain more significantly at 5 K than at RT and 600 K.

Depth synergistic effect of irradiation damage on tungsten irradiated by He-ions with various energies

To simulate the uniform depth-distribution of damages introduced by neutrons, He-ions irradiations with different energies were respectively (W-1@16 keV, W-2@70 keV, W-3@200 keV) or successively (W-4@200-70-16 keV) conducted on the recrystallized tungsten. The damage defect distribution and helium behavior in tungsten were investigated by Positron Annihilation Spectroscopy (PAS). Results show that a sufficient number of small-size vacancy clusters containing low ratio of He atoms are produced in tungsten during a single-energy He-ions irradiation with a fluence below 1021 ions m−2 at room temperature (RT). But more irradiation dose will promote the saturation of the interstitial helium around the positron annihilation sites and accelerate the formation of helium-vacancy clusters. The fitted S profile of W-4 presents a platform in the depth range of 27–477 nm. Meanwhile, S-W line of it deviates from the line of W-1, 2 to W-3, and the (S, W) data form clusters in the depth range of 57–230 nm. These changes may be explained by a large quantity captures of injected He ions in defect sites and the formation of large-size HenVm clusters with continuously increasing n/m ratio. Fortunately, successive He-ions irradiation with various energies can, to some extent, achieve a quasi-uniform damage distribution in a certain depth range in metals, which meets the expectation in this paper.

Thermal Annealing Effect on Structure and Properties of Tungsten Surface Irradiated with High Fluence and Low Energy Alpha-Particles

The paper presents research into the surface morphology, helium cluster formation and mechanical properties of high-purity tungsten after the high fluence and low-energy helium ion implantation and subsequent annealing. Investigations are based on scanning electron and atomic force microscopy observations and thermal desorption measurements. Because of the local stresses appearing within the energy loss straggling of helium ions, atoms migrate at distances significantly increasing the ion projective range, and create mobile coalescence of helium atoms and immobile helium-vacancy clusters. The latter form helium bubbles in the energy loss straggling which cause the surface blistering. Local stresses induced by helium coalescence and helium-vacancy clusters beyond the energy loss straggling result in the material strengthening or hardness increase. Subsequent 600°С annealing provides the size growth of blisters on the metal surface irradiated with helium ions and high helium desorption due to the migration of mobile coalescence of helium atoms. With the increasing annealing temperature from 600 to 1000°С, the layer corresponding to the projective range of helium ions fully fractures due to the interstitial helium atoms releasing from immobile helium-vacancy clusters. After 1000°С annealing, the tungsten hardness returns to its initial value.

Effect of temperature and dose on vacancy-defect evolution in 304L stainless steel irradiated by triple ion beam

This study aims to investigate the effects of temperature and dose on the vacancy-defect evolution in 304L stainless steel irradiated by the triple beam of Ni, H, and He ions below 673 K, which is relevant to water-cooled nuclear energy facilities. The irradiation experiments were performed simultaneously using 150-MeV Ni ions whose energy was degraded by passing through a tantalum foil of 8.5-μm thick, 100-keV H ions, and 200-keV He ions at 473, 523, 573, 623, and 673 K, respectively. The irradiation-induced microstructures in the steel were probed by the slow positron beam technique. The results show that the S parameter of the irradiated sample decreases as the temperature increases, and the S parameter of the high-dose irradiated sample decreases further. We explained the results by analyzing the structure of the defects in the irradiated samples at various temperatures and the effects of the annealing effect. In addition, the analysis of the SW plots reveals the transition of the helium-vacancy cluster to the overpressured HemVn (m > n) cluster under the influence of temperature and irradiation dose.

Computational simulation of He bubble evolution in fcc Cu with Σ3 twin boundary using object kinetic Monte Carlo method

Nano-twin copper has excellent irradiation tolerance as the grain boundaries (GB) can capture and contain defects. However, the mechanism of long-term interaction between the GB and defects remains unclear, because of the extremely large computational cost of molecular dynamics simulations. In this article, a new simulation code based on the object kinetic Monte Carlo (OKMC) method is considered for investigation of the long-term evolution of the helium-vacancy (He-V) clusters in Σ3 nano-twin Cu. The results of this study confirm that the influence of the GB can only be observed when the evolution time is sufficiently long, and obvious defect enrichment effect near the GB can only be observed if the temperature exceeds 500 K. The increasing temperature can obviously decrease the He-V cluster concentration and, also, greatly increase the He pressure in the clusters, which will make them more active and unstable. The findings of this study can potentially explain the difference between experimental and computational results for He bubble evolution in nano-twin materials, and also indicates that the OKMC method is a promising candidate for long-term simulation of large-scale defects.