He-vacancy Clusters Research Articles

Microstructural modifications induced by He implantation at elevated temperature in AlN

The elastic strain and damage build-ups induced by 50 keV He implantations at elevated temperature in (0001)AlN were studied using a combination of X-ray diffraction, transmission electron microscopy and elastic recoil detection analysis experiments. No long-range migration of He atoms occurs up to 700°C. The point defect recombination is found to be enhanced with the increase of implantation temperature. When He concentration exceeds a determined temperature-dependent threshold, the stabilization of vacancies through the formation of He-vacancy clusters of critical size strongly enhances the interstitial concentration and elastic strain in proportion with the local He concentration. Above this threshold, for a critical He concentration that decreases from several at% at room temperature to a few tenths of at% at 700°C, bubbles form and induce a strong increase of disorder. Specific features observed at 700°C, including the systematic superimposition of bubble clusters with basal stacking faults, suggest the triggering between 550 and 700°C of a mechanism promoting disorder, which is discussed.

Study of irradiation-induced vacancy-type defects in tungsten using positron annihilation spectroscopy

Tungsten has been considered as a potential candidate material for the application of plasma-facing components in magnetic confinement fusion reactors. Fusion neutrons introduce severe displacement damage into the first-wall materials along with a high amount of helium which is detrimental to the desirable material properties. Ion beam implantation combined with positron annihilation spectroscopy is a well-adopted method to investigate the irradiation damage in materials without any induced radioactivity. In this study, two separate tungsten samples were individually irradiated with 9.1 MeV Au5+ ions to a dose of 2.5 × 1014 ions/cm2 and 130 keV He ions to a dose of 1 × 1016 ions/cm2 with the intention to investigate the evolution of vacancy-type defects in the absence and presence of helium. The irradiated samples were subjected to isochronal annealing and characterized using positron annihilation Doppler broadening spectroscopy. The evolution of vacancy-type defects during different stages of annealing was investigated using defect-sensitive positron S-parameter. An increase in S-parameter due to the presence of vacancy-type defects was observed upon irradiation in both samples. The S-parameter variations with respect to the temperature showed different stages of vacancy annealing corresponding to stage III from 473 K to 673 K where the migration and clustering of mono-vacancies occurs, stage IV from 673 K to 973 K due to the growth of nano-voids and stage V from 973 K to 1273 K due to the dissociation of nano-voids. Complete recovery of irradiation-induced defects occurred in both samples at 1273 K. Both the samples showed similar vacancy annealing stages and no signature on the formation and growth of He-vacancy clusters or He bubbles was observed in the present study.

The impact of vacancy formation energies on the nucleation and growth of helium (He) bubbles in low-activation multicomponent vanadium-based alloys

Generally, the complexity of concentrated solid solution alloys (CSAs) can effectively enhance their radiation tolerance. We studied the effect of helium (He) ion irradiation on body centered cubic (BCC) V, VCr, VCrFe and VCrFeMn alloys with the increase of complexity. The comparison shows the void swelling in terms of the size and density of bubbles in V, VCrFe and VCrFeMn decreases with the increase of the complexity after the same He ion irradiation. However, binary VCr alloy has the smallest void swelling although it has only two types of elements, that is less complexity, which is significantly different from VCrFe and VCrFeMn subjected to the same He ion irradiation. Meanwhile, the irradiated VCr lattice exhibits almost no shrinkage. First-principles calculations reveal that the high vacancy formation energy (VFE) of VCr could prevent He-vacancy clusters from absorbing vacancies to grow. The results suggest that the VFE could play a more important role in enhancing the radiation tolerance under He ion irradiation in addition to the compositional complexity.

The irradiation effects on the nanoindentation hardness and helium bubbles evolution mechanism of Ni-based alloy

High-dose neutron irradiation could produce various kinds of microstructural defects, and the radiation resistance of nickel alloy is put forward for higher requirements under the high temperature. To simulate the (n, α) reaction, helium ions with an energy of 540 keV were implanted into nickel-based alloy 617 at 700 °C. The microstructure and mechanical properties were established by comparing the nanoindentation hardness before and after irradiation. For the low dose irradiation, the microstrain induced by vacancy-type defects like He-vacancy clusters and helium bubbles is slight, which would not result in a large increase of hardness. The hardening effect of high-temperature irradiation was much lower than that of room-temperature irradiation due to the recovery of matrix defects. The actual distribution of helium bubbles observed by Transmission Electron Microscope (TEM) was comparable to the SRIM simulation result. The helium bubbles were spherical near the surface of the alloy and polyhedral deeper in the matrix. Moreover, chromium carbide had a strong trapping effect on helium bubbles. The shapes and distribution of the helium bubbles were analyzed to evaluate irradiation effects on nickel-based alloys.

Helium interaction with solutes and impurities in neutron-irradiated nanostructured ferritic alloys: A first principles study

Density functional theory (DFT) calculations are performed to explore the binding between He and alloying solutes, impurities, and transmutation products expected in neutron irradiated nanostructured ferritic alloys (NFAs), here 14YWT is taken as an example. Elements that exhibit significant binding (attraction) with an interstitial He are: Y (binding energy = 0.46 eV), Mg (0.32), O (0.33), Ti (0.16), and C (0.15). Those that provide significant binding to a substitutional He are: O (1.44), Y (1.24), N (0.73), H (0.56), Mg (0.52), Ti (0.34), Si (0.34), C (0.33), Al (0.32), Ni (0.26), Ta (0.23), and Mn (0.16). The presence of these elements in Fe matrix could reduce the transport of He towards oxide particles, dislocations, and internal boundaries, and could promote He bubble nucleation in the matrix. For convenience, we compile existing binding energy data of He with Hen and HenV (He-vacancy) clusters. Dissociation pathway analysis reveals that, in general, the most likely dissociation of a HenV cluster is by a sequential emission of individual He atoms. Furthermore, larger bubbles are more prone to dissociation than smaller ones. In addition, we estimate the binding energy (segregation energy) of He in bulk Y2Ti2O7 (YTO) single crystal, YTO/Fe interface, and YTO particle embedded in Fe, with respect to interstitial He in Fe, from existing formation energies of He in these structures. We also compile available data of He binding with Fe self-interstitial atom (SIA), SIA clusters, and edge and screw dislocations. Note that given the absence of DFT data, the binding with SIA clusters and dislocations are gathered from simulations with empirical potentials. The data presented in this paper is important to inform multiscale simulations of He bubble accumulation.

Effect of He on the irradiation resistance of equiatomic CoCrFeMnNi high-entropy alloy

Recent studies have shown that equiatomic CoCrFeMnNi high-entropy alloy (HEA) has excellent mechanical properties and irradiation resistance. However, upon neutron irradiation, He is always generated as a byproduct. In this study, the irradiation resistance of thin-film CoCrFeMnNi HEA samples irradiated by He ions at 773 K was compared with that of stainless steel 304 (SS304), which as the same crystal structure. Although the formation of He bubbles was observed during irradiation in both alloys, the He bubble density of the CoCrFeMnNi HEA was lower than that of SS304 under the same exposure conditions. The simulation of the effect of He on the formation of vacancy clusters in the CoCrFeMnNi HEA was carried out based on first-principles calculations. The results show that the He atom significantly improved the stability of the vacancy clusters in the CoCrFeMnNi HEA. Thus, even in the CoCrFeMnNi HEA, irradiation formed stable He-vacancy clusters that decreased the irradiation resistance compared with irradiation without He.

First-principles study of helium-vacancy complexes in Be12Ti

First-principles calculations have been conducted to study the stability of He-vacancy clusters and the trapping behavior of vacancies for He atoms in Be12Ti. We considered the various metastable microstructures of four types of He-vacancy clusters, and the most stable configurations were determined by the calculations of solution energy. Some He-vacancy clusters in Be12Ti have specific geometrical shape, and grow along specific crystal plane. For example, three types of He2V clusters (two He atoms trapped by vacancy) form dumbbell-shaped structure, three He atoms of He3V clusters (three He atoms trapped by vacancy) are located in the specific crystal plane. The solution energies of four types of clusters increase with the increase of number of He atoms, and the increasing rate is basically identical. The solution energies of HenVBe1 clusters are always the largest, and the solution energies of the HenVTi clusters are always the smallest, which mainly attribute to the larger vacancy volume and perfect spatial symmetry of Ti vacancies. The binding energies of a single He atom with HenV (n=0-3) clusters are all positive, and the segregation of He atoms on the inner surface of the vacancy is closer to the center of the vacancy, indicating that the He-vacancy has a stronger trapping ability for He atoms. The present research results provide a theoretical foundation of vacancy trapping mechanism for He atoms in Be12Ti, which is extremely important for the nucleation and growth mechanism of He bubbles in Be12Ti.

Ab initio study of the stability of H-He clusters at lattice defects in tungsten

The interaction of a H-He pair embedded in various types of lattice defects in the bcc tungsten, such as vacancies, tilt grain boundaries, a core of screw and edge dislocations has been studied using ab initio calculations. It is shown that H-vacancy and He-vacancy clusters are weaker traps for He and H single atoms, respectively, as compared to a single vacancy. H or He atom, solely, is strongly attracted to the studied grain boundary interfaces, while the mutual H-He interaction is negligible (close to the interaction in vacuum) if both atoms are placed in the adjacent trapping sites located on the grain boundary. The H-He pair placed in the core of the screw or edge dislocation exhibits the binding energy of ~0.2–0.3 eV, which is close to the H-He attraction in the bulk W. The ground-state configurations for H-He dimers located in various lattice defects in W were rationalized on the basis of atomic structure visualizations, bond analysis and charge density distributions. The obtained data are discussed in the frame of the thermal desorption spectroscopy results offering a better understanding of the detrapping stages of He and H isotopes under mixed plasma exposure conditions.

Microscopic Properties of Point Defect and its Cluster in Delta-Phase Plutonium: A Molecular Dynamics Study

Self-irradiation effect induced by Pu α decay is an important influencing factor for long-term storage of Pu-based materials. In order to reveal the collision displacement cascade for uranium recoil nucleus induced by alpha decay in delta-phase plutonium at atomic level, we review the recent progress in self-irradiation of metallic plutonium and its alloys. We perform a molecular dynamics (MD) calculation on basis of modified embedded atom (MEAM) interatomic potentials, and obtain the minimum of displacement threshold energy (MDTE) for {1 1 1} lattice direction and microscopic evolution of He self-interstitial cluster. These findings are in agreement with previous experimental and theoretical results, and can be viewed as an essential input parameter for mesoscopic simulation to obtain the evolution of microscopic configuration at longer time and space, and might be also helpful for understanding the nucleation and growth mechanisms for vacancy and/or self-interstitial and its clusters, He-vacancy cluster and He bubbles in delta-phase plutonium and its alloys.

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.

A molecular dynamics simulation of energetics and diffusion of point defects in a Au–Ag alloy

For revealing an aging mechanism for self-irradiation in a Pu–Ga alloy, we carried out a molecular dynamics (MD) simulation on a substitutional material, i.e., Au–Ag alloy. In this work, we estimate physical and microscopic properties of the Au–Ag alloy containing various point defects using a MD method, in particular, formation energy for point defects, migration energy for point defects diffusion into interstitial sites, and diffusion coefficient for the Au–Ag alloy containing point defects, such as vacancy, He atom and He-vacancy (He-V) cluster. The results indicate that volumetric heat capacity and linear expansion coefficient would decrease due to the various point defects, and He atom has the most remarkable influence on the physical properties of the Au–Ag alloy for point defects considered in this work. The formation energy of Au and Ag self-interstitial atom indicates that Octa1 is the most stable site, and structural stability of octahedral (Octa) interstitial sites for the He atom obeys \(\hbox {Octa1}> \hbox {Octa2}> \hbox {Octa4} > \hbox {Octa3}\). For the \(\hbox {He}_{n}\hbox {V}_{m}\) cluster, the formation energy of the defect structure is most stable at \(n = m\). The diffusion coefficient of the He-V cluster is relatively smaller, showing that vacancy defects would further decrease atomic diffusion. An influence of various point defects on the diffusion velocity in the Au–Ag alloy obeys the He-V \(\hbox {cluster}> \hbox {He}> \hbox {vacancy}> \hbox {Ag} > \hbox {Au}\).

Damage microstructure evolution of helium ion irradiated SiC under fusion relevant temperatures

In-situ transmission electron microscopy (TEM) with ion irradiation has been used to study the damage microstructure evolution of He ion irradiated 4-H SiC at nuclear fusion relevant temperatures. The SiC samples were irradiated with 20 keV He ions at 25, 400, 800 and 1200 °C to a dose of 5.0 displacements per atom (DPA). At 25 °C, the material fully amorphises at 1.5 DPA and no He bubble nucleation occurs up to the doses studied. At 400 and 800 °C, He bubble nucleation occurs and the material remains crystalline. Bubble nucleation occurs at 2.0 DPA at 400 °C but occurs at only 0.5 DPA at 1200 °C. This is attributed the He atoms de-trapping from vacancies and migrating interstitially to larger He-vacancy clusters at higher temperatures, leading to faster nucleation of observable He bubbles. Helium platelets form at an irradiation temperature of 1200 °C at 0.5 DPA showing a preference for nucleation between the {0001} basal planes.

DFT study of hydrogen and helium defects at the (112̄1) twin boundary in hcp scandium

We have investigated the grain boundary energy of ([Formula: see text]) twin boundaries, the formation energies of hydrogen (H) and helium (He) defects in tetrahedral (T) and octahedral (O) interstitial sites at the ([Formula: see text]) twin boundary in hcp scandium (Sc) by first-principles calculations based on density functional theory. It is found that the formation energies of the tetrahedral and octahedral interstices H, and tetrahedral interstice He increase significantly towards the ([Formula: see text]) twin boundary plane, while the formation energy of the octahedral interstice He atom near the ([Formula: see text]) twin boundary plane decreases. To analyze these results, we present the electronic densities of states (DOSs) of H, He and their nearest-neighbor Sc (NN-Sc) atoms in several tetrahedral and octahedral configurations. We have also calculated the formation energies of He-vacancy clusters (He[Formula: see text]V) in the Sc grain boundary, which indicates the stabilities of He[Formula: see text]V clusters depend on the variations of the relaxed vacancy volume near the ([Formula: see text]) twin boundary plane.

Temperature dependence of underdense nanostructure formation in tungsten under helium irradiation

Recently, tungsten has been found to form a highly underdense nanostructured morphology (“W fuzz”) when bombarded by an intense flux of He ions, but only in the temperature window 900–2000 K. Using object kinetic Monte Carlo simulations (pseudo-3D simulations) parameterized from first principles, we show that this temperature dependence can be understood based on He and point defect clustering, cluster growth, and detrapping reactions. At low temperatures (<900 K), fuzz does not grow because almost all He is trapped in very small He-vacancy clusters. At high temperatures (>2300 K), all He is detrapped from clusters, preventing the formation of the large clusters that lead to fuzz growth in the intermediate temperature range.

Development of a pair potential for Ni–He

An interatomic potential for helium in bulk nickel was developed by fitting first-principles calculation results. This two-body Ni–He potential can give a good description of relative stability and migration of interstitial and substitutional He defects in bulk nickel. The zero-temperature binding energies of additional He to existing He-vacancy clusters and interstitial He clusters were studied and found in agreement with first-principles calculations.

Synergistic interplay between H and He in molybdenum: A first-principles study

The synergistic interplay between the hydrogen (H) and the helium (He) in molybdenum (Mo) has been investigated by using the first-principles. In perfect Mo, both H and He prefer to occupy the tetrahedral interstitial sites (t-sites) where the equilibrium distance between H and He is 2.74 Å. Both energetics and kinetics show that an interstitial He can bind up to three H atoms to form one H3–He cluster. With the presence of vacancy in Mo, He will occupy the vacancy center to form a He-vacancy (He–V) cluster; while a large amount of H atoms stay around He in vacancy. If the concentration of H is much higher than that of vacancies, one He–V cluster can capture as many as 12 H atoms, which is much larger than the H atoms trapped by an empty vacancy. The reason can be attributed to the electronic density redistribution of vacancy vicinity induced by He, which make capturing surface area of H expand outward from 19.00 Å2 (without He) to 32.96 Å2 (with He). The increasing multiple of H trapping area is 1.73, in nearly agreement with 1.71 that is the increasing multiples of H number captured by vacancy from 7 H to 12 H.

Modeling of helium bubble nucleation and growth in austenitic stainless steels using an Object Kinetic Monte Carlo method

Implantation of 10keV helium in 316L steel thin foils was performed in JANNuS-Orsay facility and modeled using a multiscale approach. Density Functional Theory (DFT) atomistic calculations [1] were used to obtain the properties of He and He-vacancy clusters, and the Binary Collision Approximation based code MARLOWE was applied to determine the damage and He-ion depth profiles as in [2,3]. The processes involved in the homogeneous He bubble nucleation and growth were defined and implemented in the Object Kinetic Monte Carlo code LAKIMOCA [4]. In particular as the He to dpa ratio was high, self-trapping of He clusters and the trap mutation of He-vacancy clusters had to be taken into account. With this multiscale approach, the formation of bubbles was modeled up to nanometer-scale size, where bubbles can be observed by Transmission Electron Microscopy. Their densities and sizes were studied as functions of fluence (up to 5×1019He/m2) at two temperatures (473 and 723K) and for different sample thicknesses (25–250nm). It appears that the damage is not only due to the collision cascades but is also strongly controlled by the He accumulation in pressurized bubbles. Comparison with experimental data is discussed and sensible agreement is achieved.

TDPAC studies of interaction between He and A = 140 elements in Fe

Room-temperature time-differential perturbed-angular-correlation (TDPAC) spectra of 140Ce in an Fe foil and in a He-doped Fe foil, unannealed and annealed in vacuum at various temperatures, were taken in order to see whether Ce and He form in Fe a complex that has a definite geometrical structure, as suggested in first-principles density functional theory calculations. Our TDPAC results do not indicate the existence of such a complex in Fe. We also discuss briefly He-vacancy clusters formed in the bombardments of He and 140Cs (a precursor of 140Ce).

He reemission implanted in metals

Helium (He) reemission of Al, Ni and Mo under energetic He implantation (10–30keV) in wide temperature range is studied to understand behavior of implanted He in correlation with structure changes.The reemission behavior is categorized into 4 different temperature ranges with the normalized temperature (Tm) to the melting point of each metal. At elevated temperatures (well above ∼0.6Tm), interstitial He atoms and/or He-vacancy (ies) clusters can migrate remaining no structure change and showing smooth reemission without any burst. Between ∼0.25 and 0.6Tm, He reemission always accompanies significant structure modification. For ∼04–0.6Tm, implanted He coalesce to make bubbles and the bubbles can move to the surface. Bubble migration accompanies materials flow to the surface resulting in fuzz surface or columnar structure, depending on implantation flux. Slower bubble motion at ∼0.25–0.4 prohibits the material migration. Instead the bubbles coalesce to grow large and multi-layered blistering appears as periodic reemission behavior.Below ∼0.25Tm, He migration is too slow for bubbles to grow large, but bubble density increases up to a certain fluence, where neighboring bubbles start to coalesce. Accordingly, He release is mostly caused by mechanical failure or blister rapture. With increasing fluence, all defects (bubbles and dislocation loops) tangle or inter connected with neighboring defects and accordingly He migration to the surface along the tangled or connected defects is enhanced, resulting 100% reemission easily without making multilayered blistering.