Helium Bubbles Research Articles

Cavitation on single electron bubbles in liquid helium at small negative pressures

Liquid helium under negative pressure represents a unique possibility for studying the macroscopic quantum nucleation phenomena in condensed media. We analyze the quantum cavitation rate of single electron bubbles at low temperatures down to absolute zero. The energy dissipation and sound emission processes result in the different temperature behavior of quantum cavitation rate in normal fluid $^{3}\mathrm{He}$ and superfluid $^{4}\mathrm{He}$ below the thermal-quantum crossover temperature. The position of the rapid nucleation line in the temperature-pressure phase diagram is discussed as well.

Helium Bubble Growth in He+ Ions Implanted 304L Stainless Steel Processed by Laser Powder Bed Fusion During Post-Irradiation Annealing at 600 °C

Helium Bubble Growth in He+ Ions Implanted 304L Stainless Steel Processed by Laser Powder Bed Fusion During Post-Irradiation Annealing at 600 °C

Microstructural Evolution and the Irradiation Sensitivity of Different Phases of a High Nb-Containing TiAl Alloy under He Ions Implantation at Room- and Elevated Temperatures

A high Nb-containing TiAl alloy Ti-45Al-8.5Nb-(W, B, Y) with nearly lamellar microstructure has been irradiated by 200 kV He2+ to a fluence of 1 × 1021 ions/m2 with a dose of about 1.1 dpa at 298 K and 773 K in this work. It is found that an amorphous layer formed on the surface, and no helium bubbles can be observed in the alloy after room temperature ion implantation. The surface roughness of the alloy increases significantly with the bombardment of helium ions, indicating that the ion implantation increases the surface defects. The high-temperature ion implantation leads to the phenomenon of blistering on the alloy surface, and helium bubbles are observed in both α2 and γ phases of the alloy irradiated at 773 K. The average size of the helium bubbles in the α2 (15~20nm) is larger than that in the γ (3~5 nm) phase, while the helium bubble density is opposite. Moreover, the growth mechanism of helium bubble is also investigated. By means of nanoindentation, an obvious irradiation hardening phenomenon is measured after the room temperature ion implantation. In addition, the irradiation sensitivity of different phases is also discussed in this work. The results show that the γ phase has the highest irradiation sensitivity, α2 phase second, β phase minimum. The results of this work, especially microstructure evolution and the evaluation of phase-related irradiation sensitivity during ion implantation, can be expected to provide experimental evidence for the applications of TiAl intermetallic compounds in the nuclear industries.

Evolution of helium bubbles in SLM 316L stainless steel irradiated with helium ions at different temperatures

The temperature dependence of helium bubble evolution in 316L stainless steel (SS) fabricated by selective laser melting (SLM) has been studied by performing 500 keV helium ion irradiation at temperatures between 350 and 900 °C. The number density of helium bubbles decreased steadily whereas the bubble diameter gradually increased with increasing temperature. The material swelling caused by helium bubbles had no obvious change at the temperature below 800 °C. However, at the maximum temperature of 900 °C, the swelling sharply increased to 0.17% mainly due to the rapidly growing bubble diameter. In particular, heterogeneous nucleation of bubbles occurred in the vicinity of dislocation structures at high temperatures of 700–900 °C, and the formed helium bubbles rapidly grew at the oxide-matrix interface at 900 °C. It was confirmed various intrinsic structures (dislocation structures or oxide particles) differently affected the bubble evolution process. The apparent activation energy (Eact) of helium bubbles determined by the Arrhenius model were used to elucidate the most probable helium bubble evolution mechanisms at various temperatures, as it is an experimentally observed activation energy, which contained information about the activity of helium atoms. It was found that the low-temperature regime with low apparent activation energies (ECbact=0.15eV,ERbact=0.04eV) and high-temperature regime with high apparent activation energies (ECbact=1.50eV,ERbact=0.58eV) favored different helium bubble evolution mechanisms. In the low-temperature regime, helium bubble evolution was controlled by the diffusion of helium atoms via a vacancy mechanism in the SLM 316L SS. On the other hand, the migration and coalescence of helium bubbles also occurred in the high-temperature regime which caused the apparent activation energy of density was higher than the theoretical value of helium atoms diffuse via a replacement mechanism.

In-situ TEM observation of migration behavior of helium bubbles in F82H

ABSTRACT Formation and migration behaviors of helium (He) bubbles in reduced activation ferritic/martensitic steel F82H have been investigated by in-situ electron microscope observation, where the investigation of bubble formation was done under 5 keV He+ ions irradiation at three different temperatures, and then the investigation of bubble migration was done in thermal annealing process at temperatures in a range from 573 to 1073 K for samples after the He+ ions irradiation. The formation of He bubbles much depends on temperature and fluence; many He bubbles with small size are observed at below 773 K, while a few He bubbles with large size are observed at 1073 K. At relatively high temperatures above 873 K, the Brownian type motions of He bubbles were clearly detected, in which the diffusion coefficient of bubbles strongly depends on temperature and bubble size. A relation between the bubble diffusion coefficient and the bubble diameter indicates that the observed bubbles in this study migrate by the surface diffusion mechanism. In construction of an approximate expression for the bubble diffusion coefficient, the obtained fitting curves based on an empirical equation under assumption of the surface diffusion nicely reproduce the experimental data.

Thermal conductivity of boron carbide under fast neutron irradiation

Due to the complex products and irradiation-induced defects, it is hard to understand and even predict the thermal conductivity variation of materials under fast neutron irradiation, such as the abrupt degradation of thermal conductivity of boron carbide (B4C) at the very beginning of the irradiation process. In this work, the contributions of various irradiation-induced defects in B4C primarily consisting of the substitutional defects, Frenkel defect pairs, and helium bubbles were re-evaluated separately and quantitatively in terms of the phonon scattering theory. A theoretical model with an overall consideration of the contributions of all these irradiation-induced defects was proposed without any adjustable parameters, and validated to predict the thermal conductivity variation under irradiation based on the experimental data of the unirradiated, irradiated, and annealed B4C samples. The predicted thermal conductivities by this model show a good agreement with the experimental data after irradiation. The calculation results and theoretical analysis in light of the experimental data demonstrate that the substitutional defects of boron atoms by lithium atoms, and the Frenkel defect pairs due to the collisions with the fast neutrons, rather than the helium bubbles with strain fields surrounding them, play determining roles in the abrupt degradation of thermal conductivity with burnup.

Deciphering the influence of high-flux helium plasma exposure on tungsten nanostructure growth using real-time, in situ spectroscopic ellipsometry

In situ analysis of surfaces during high-flux plasma exposure represents a long-standing challenge in the study of plasma–material interactions. While post-mortem microscopy can provide a detailed picture of structural and compositional changes, in situ techniques can capture the dynamic evolution of the surface. In this study, we demonstrate how spectroscopic ellipsometry can be applied to the real-time characterization of W nanostructure (also known as “fuzz”) growth during exposure to low temperature, high-flux He plasmas. Strikingly, over a wide range of sample temperatures and helium fluences, the measured ellipsometric parameters (Ψ,Δ) collapse onto a single curve that can be directly correlated with surface morphologies characterized by ex situ helium ion microscopy. The initial variation in the (Ψ,Δ) parameters appears to be governed by small changes in surface roughness (<50 nm) produced by helium bubble nucleation and growth, followed by the emergence of 50 nm diameter W tendrils. This basic behavior appears to be reproducible over a wide parameter space, indicating that the spectroscopic ellipsometry may be of general practical use as a diagnostic to study surface morphologies produced by high-flux He implantation in refractory metals. An advantage of the methods outlined here is that they are applicable at low incident ion energies, even below the sputtering threshold. As an example of this application, we apply in situ ellipsometry to examine how W fuzz growth is affected both by varying ion energy and the temperature of the surface.

Dynamic interaction between the dislocations and cavities in tungsten during tensile test

The irradiation hardening factor of the cavities (helium bubbles) in tungsten, as the divertor candidate material, in nuclear fusion reactor, has been investigated through an in-situ TEM observation during tensile test. The obstacle barrier strength α has been calculated by the bow-out behavior of a dislocation based on the line tension model. The obstacle barrier strength α of the cavity with a diameter of 2 nm in W has been measured to be 0.53 ± 0.02. The fraction of cross-slip behavior except the obstacle-cutting mechanism owing to the dynamic interaction between a dislocation of screw type and a cavity has been about 11 %. The fraction of cross-slip behavior of the screw dislocation in W has been smaller than those of other BCC structure materials.

The microstructure evolution and irradiation hardening in 15Cr-ODS steel irradiated by helium ions

Oxide dispersion strengthened (ODS) steels with excellent irradiation swelling resistance and high temperature creep resistance are regarded as one of the priority candidate materials for the first wall/blanket materials in fusion reactors. Microstructure evolution and irradiation hardening in 15Cr-ODS steel irradiated by helium ions at R.T. and 400 °C respectively have been studied by transmission electron microscopy and nanoindentation technique. Three main oxides were identified, including high-density dispersed Y-Ti-O nanoclusters, tens of nanometers Y2TiO5 and a few hundred nanometers Y2Ti2O7 particles with the largest size but the least density. Helium bubbles were homogeneously distributed in the matrix in both irradiated specimens and the aggregation state of bubbles near GBs was related to the nature of GB. The average size of bubbles increased and the density decreased in the steel irradiated at elevated temperature. A defect-depleted zone was observed near a GB with a Y2TiO5 precipitate imbedded and a large helium bubble was present at the interface of the Y2TiO5 particle/GB. It suggests that the GBs combined with Y2TiO5 can enhance the aggregation of helium bubbles since GBs can act as defect sinks as well as diffusion channels and Y2TiO5 has a high ability of trapping helium atoms. Significant irradiation hardening was obtained at both irradiation temperatures. The relationship between microstructure and the irradiation hardening was discussed.

Observations of He platelets during He ion irradiation in 3C SiC

Polycrystalline 3C SiC was irradiated and observed in-situ via Transmission Electron Microscopy with a 20 keV He ion beam at 400, 800, 1000 and 1200 °C at the Microscopes and Ion Accelerators for Materials Investigations facility. During the 400, 800, 1000 and 1200 °C irradiations, black-spot damage was observed at 3.1, 1.1, 2.1 and 2.1 dpa respectively. Helium bubbles were observed after 6.3 dpa at 400 °C and 2.1 dpa at 800 °C, and He platelets were seen after 1.1 dpa at 800, 1000 and 1200 °C but not observed during the 400 °C irradiation. This work shows for the first time, the preferential nucleation of platelets within stacking faults in 3C SiC. The dependence of He platelet diameter with temperature and dose has also been observed.

Dopant binding with vacancies and helium in metal hydrides

Metal hydrides are crucial for the long-term storage of tritium but suffer degradation due to the buildup and release of helium decay products. Therefore, it is of interest to explore how dopants in these metal hydrides may impact helium bubble nucleation and distributions, as well as associated fracture and helium retention. Prior studies have focused on helium behavior in pure metal hydrides or with one or two types of impurities. Analytical models have also shown that the concentration of nucleated bubbles can impact the time to fracture of materials. This study utilizes high-throughput density functional theory calculations to identify the impact of transition metal substitutional dopants on helium binding energies in face-centered cubic metal hydrides, such as erbium hydride, holmium hydride, scandium hydride, titanium hydride, yttrium hydride, and zirconium hydride. This study also explores the impact of hydrogen vacancies on the binding energy of helium near the substitutional defects. Finally, this study presents an initial assessment of dopant stability and solubility limits at many temperatures and pressures. Several metals strongly bind to helium in these metal hydrides, making them promising for influencing helium nucleation and subsequent bubble growth. However, many of these potentially beneficial substitutional defects have low solubility limits. The calculations show that some strong binding dopants may be soluble in quantities that affect the bubble concentration and impact material performance.

Role of Ni on the helium diffusion, stability and energetics of vacancy–type clusters in Fe–6.25Cr–3.13Ni (at.%) ternary alloys: From first-principles

Ab initio methods based on DFT are utilized to study the role of Ni addition on the interstitial helium diffusion, energetics and stability of HenVm (n = 0–6, m = 0–4) clusters in austenitic Fe–6.25Cr–3.13Ni (at.%) ternary alloys. The limit vacancy concentration considered in our simulation will be as close to the real irradiation conditions as possible. The presence of Ni obviously hinders the long-range migration of an individual helium between tetrahedral and octahedral interstitials, or between tetrahedral gaps, but greatly promotes the movement between octahedral gaps, and the travel between octahedral (tetrahedral) gaps cannot cross the tetrahedral (octahedral) interstitials. The addition of Ni enhances the attractions between interstitial helium, drives the short-range migration and tends to form small helium bubbles; on the other hand, it seriously hinders the trapping of helium by multiple-vacancies, and reduces the risk of helium bubble nucleation. In addition, the effects of Ni on the stability and energetics of helium vacancy complexes are evaluated, and it is confirmed that Ni improves the stability of the cluster models and enhances the irradiation expansion resistance of the alloy configurations, although it is not conducive to the formation of HenVm clusters. Our findings provide a theoretical reference for understanding the thermal helium desorption spectra and helium-induced embrittlement or hardening under irradiation conditions.

Collapse of helium-filled voids in extreme deformation: Dislocation mechanisms

The mechanisms responsible for the collapse of helium-filled bubbles during the passage of shock waves in monocrystalline copper are revealed. Both internal pressure (caused by pre-existing helium atoms) and bubble size are varied in molecular dynamics simulations to understand the atomistic scale deformation as they are subjected to shock compression at pressures of 48, 123, and 170 GPa, corresponding to particle velocities of 1.0, 2.0, and 2.5 km/s. Both empty and helium filled bubbles serve as dislocation sources, generating intense, localized plastic regions. There are distinct differences in the collapse of empty voids compared to He-filled bubbles, the former requiring less stress and generating a greater density of dislocations for a given shock strength. A generalized model for dislocation emission is proposed, where the inclusion of shear stress generated by the helium bubble increases the critical stress to generate dislocations at the defect surface, demonstrating the change in plastic deformation.

Gordon Coupler with Inductive or Capacitive Iris for Small EPR Resonators for Aqueous Samples.

The Gordon coupler was introduced for use in EPR experiments at liquid helium temperatures. It provides an evanescent wave incident on the iris of a microwave resonator. Match of power incident on the coupler to the resonator is obtained by variation of the amplitude of an evanescent wave that arises from displacement of a dielectric wedge in a tapered waveguide. Reduced microphonics from helium bubbling was reported. The Gordon coupler was subsequently extended from cavity resonators to loop-gap resonators, initially at helium temperatures but later for aqueous samples. Plastics with low dielectric constants, usually Teflon, were used. Here, we extend the Gordon coupler for application in X-band five-loop-four-gap resonators using fused quartz, sapphire, or rutile dielectrics, noting that the size of the coupler can then be commensurate with dimensions of dielectric loop-gap resonators as well as dielectric tube resonators. Finite element modeling of electromagnetic fields has been carried out, and use of a capacitive iris that interfaces with the Gordon coupler reduces pulling of the resonant frequency when matching the resonator.

Deformation mechanism of neutron irradiated Al-B based on SEM-DIC

The electron backscatter diffraction (EBSD) characterization of irradiated Al-B shows that there is a high concentration defect region around the borides. Nanoscale speckle particles were successfully prepared on the surface of Al-B before and after irradiation, and then the mesoscale strain during in-situ deformation was obtained by digital image correlation (DIC) technique. The results shows that slip band bypass such an area through cross slips with slip band deflection. Transmission electron microscopy (TEM) indicates that abundant helium bubbles exist in the deflected slip band area pinning the dislocations.

Atomistic Study on the Energetic and Mechanical Behaviors of Helium Bubble Nucleation and Growth in RAFM Steel

Helium produced by neutron irradiation is a crucial inducement to bring about the property of deterioration of structural materials served in a fusion reactor. To investigate the nucleation and growth behavior of helium bubbles in reduced activation ferritic/martensitic steels, which comprise one of the most promising candidate structural materials, the Molecular Statics method and the Metropolis Monte Carlo algorithm are combined to investigate the energetic and mechanical behaviors of HenVm clusters in α-Fe. The simulation results show that the vacancy and helium atom binding energy are inclined to reach a saturation state, i.e., 4.0 eV for the vacancy and 2.4 eV for the helium atom; however, the binding energy of self-interstitial atoms decreases to minus values at high helium-to-vacancy (He/V) ratios. The crossover of the binding energy curve of the helium and vacancy indicates that the equilibrium He/V ratio is 1.68 during the nucleation of helium bubbles. Meanwhile, the dissociation energy analysis indicates that the stable He/V ratio of the clusters is 1.3 at high temperatures. Moreover, the pressure analysis of the HenVm clusters indicates that the He/V ratio corresponding to their mechanical equilibrium state varies from 0.50 to 0.65 at 0 K. Furthermore, the analysis combined with the relevant experimental data of helium density in helium bubbles indicates that the actual He/V ratio of helium bubbles in the served materials is closely relevant to the irradiation condition, such as helium production rate, temperature, etc. The investigation results in this paper contribute to elucidate the microscopic process of helium bubble nucleation and growth and provides the energetic and mechanical parameters of small-sized helium bubbles with different sizes for large-scale simulation studies.

Effects of helium bubbles on deformation of aluminum matrix

Helium bubble influence on the deformation of aluminum is investigated via simultaneous electron backscatter diffraction and digital image correlation (DIC) during in situ tensile and compressive tests . The majority of helium is generated through the transmutation reaction 10 B(n, α ) 7 Li under neutron irradiation , which is carried out at 40 ∘ C to ∼ 0.3 dpa. The mesoscale deformation behaviors of aluminum matrix with and without helium bubbles are compared, and the interactions between helium bubbles and slip bands are analyzed. Helium bubbles along with other types of irradiation-induced defects (like dislocation loops and black dots) harden significantly the aluminum matrix around second-phase AlB 2 particles. Slip bands cannot penetrate the areas with high-density irradiation-induced defects. They bypass such areas through cross slip, exhibiting slip band deflection. Transmission electron microscopy on irradiated materials after deformation reveals abundant helium bubbles, dislocations, dislocation loops and black dots around AlB 2 particles, due to complex interactions between irradiation-induced defects with dislocations. Helium bubbles together with other types of irradiation-induced defects have pinning effects on dislocations, and pressurized helium bubbles may also punch out dislocation loops. The multiscale analyses demonstrate the helium bubble-dominated hardening.

Evolution mechanisms of irradiation-induced helium bubbles, C15 clusters and dislocation loops in ferrite/martensite steels: A cluster dynamics modeling study

• Evolution of helium bubbles , C15 clusters and dislocation loops is studied by a cluster dynamics model. • The model considers the formation of C15 clusters and followed transformation into loops. • Irradiation temperature and dose effects on the microstructural evolution are analyzed. • Helium bubble evolution upon irradiation is governed in sequence by three phases. • Loop growth driven by long-term defect migration follows a power law. Using a cluster dynamics model, we studied the evolution of helium bubbles, C15 clusters and dislocation loops produced by Fe 3+ /He + dual ion beam irradiation in reduced activation ferrite/martensite Eurofer97 steels. The model reasonably reproduced size distributions of experimentally detected helium bubbles and dislocation loops. We show that bubble evolution upon irradiation was governed by three phases in sequence: bubble nucleation dominated phase controlled by irradiation cascade, coupled phase of bubble nucleation and growth, and bubble growth dominated phase. As a first attempt, the model considers the formation of widely reported C15 clusters and develops a scheme for C15 clusters transformation into loops. We show that the loop growth driven by long-term defect migration follows a power law f(n) ~ a/n S . The increase of irradiation temperature from 603 K to 673 K promoted the growth of C15 clusters resulting in pronounced loop evolution. While at 773 K, the high flux of vacancies and low fluxes of single, di- and tri-interstitials suppressed loop nucleation and growth, leading to no loop formation confirmed by experiment. The effectiveness of the model developed in this study demonstrates its potential to be used in similar irradiation cases.

Highly stable nanocrystalline oxide dispersion strengthened alloys with outstanding helium bubble suppression

Oxide dispersion strengthened (ODS) alloys containing high-density nano-oxides show excellent mechanical properties and irradiation tolerance. However, ODS alloys are often ultrafine-grained materials since the grain size of matrix is located between 100 and 1000 nm. Here, nanocrystalline and ultrafine-grained ODS alloys with different grain sizes (50 nm and 300 nm) were prepared by mechanical alloying and high-pressure consolidation techniques, which show exceptional thermal stability. To understand their irradiation response, the two alloys were irradiated with 400 keV helium ions to a fluence of 1 × 1017He+ /cm2 at 350 ℃, 400 ℃ and 450 ℃. The results show that the size and volume fraction of helium bubbles in nanocrystalline ODS alloys increase when the irradiation temperature increases from 350 ℃ to 450 ℃. Grain boundaries (GBs) play a key role in bubble formation and evolution. We identified the bubble denuded zone (BDZ) near the GBs, but the width of the denuded zone narrowed and finally vanished with increasing helium concentration. Nanocrystalline ODS alloy with 50 nm grains (Y50) can disperse helium into much finer bubbles than ultrafine-grained ODS alloy with 300 nm grains (Y300) because of the extremely high density of GBs and oxide-matrix boundaries. To our best knowledge, Y50 alloy presents the best helium bubble suppression ability among all studied ODS and nanostructured ferritic alloys (NFA), which makes it a promising candidate material for using in fission and fusion nuclear reactors.

Effect of helium bubbles on the irradiation hardening of GH3535 welded joints at 650°C

Irradiation damage due to helium embrittlement impacts the safety and integrity of welded structural materials in power reactors. To understand this helium effect, in this study, GH3535 welded joints were irradiated at 650°C with 500 keV He ions at various doses to investigate the effects of He bubbles on the weld, heat-affected zone (HAZ), and base metal. The three regions exhibited different hardening behaviors. The hardness of the weld and HAZ after irradiation increased proportionately with the ion dose. The hardness of the base metal gradually saturates at a dose of 2 × 1016 ions/cm2. At the highest dose (1 × 1017 ions/cm2), the degree of hardening in the three regions was as follows: weld (92.9%) > HAZ (91.3%) > base metal (72.7%). The microstructures of the samples revealed that the mean diameters of the He bubbles in the weld, HAZ, and base metal were approximately 2.74, 2.80, and 2.36 nm, respectively, with corresponding number densities of 15.4 × 1023, 10.8 × 1023, and 9.69 × 1023 m−3. Furthermore, the yield strength increment was calculated using the dispersed barrier hardening model, which suggested that the helium bubbles played an important role in the different hardening behavior of the welded joint. The nucleation and growth of helium bubbles was influenced by the structures of the intrinsic dislocations in the weld, HAZ, and base metal.