Helium Cavities Research Articles

Suppression of helium migration in arc-melted and 3D-printed CoCrFeMnNi high entropy alloy

We compare the evolution and migration of helium cavities in 3D-printed and arc-melted CoCrFeMnNi high entropy alloys (HEAs), and 304 stainless steel (SS), using transmission electron microscopy (TEM). The materials are subjected to Ni and He sequential implantation as well as subsequent thermal annealing. After irradiation at room temperature, all samples initially display similar He cavity sizes. Annealing up to 673 K has a minimal impact on cavity size. However, annealing at 873 K leads to significant cavity growth. Both HEA materials exhibit larger average cavity sizes compared to 304 SS, while a higher proportion of small cavities is found in the 304 SS. This indicates that Ostwald Ripening is more prevalent in 304 SS, confirmed by in-situ thermal annealing in TEM. Depth distribution analysis reveals distinct cavity belts after annealing: from the narrowest in 3D printed HEA to the broadest belts in 304 SS. These results indicate that the He concentration at the peak of irradiation damage decreases in the same order. We suggest that the long-range migration of He atoms is limited in both the Cantor and 3D-printed Cantor HEAs, potentially due to the complex diffusion paths of the He atoms.

Microstructure and Magnetism of Heavily Helium-Ion Irradiated Epitaxial Iron Films

This study reports on the microstructure and magnetism of pure iron irradiated with high doses of helium ions. Iron alloys are important structural materials used as components in fusion reactors, and a comprehensive database of their various properties has been developed. But little has been investigated on magnetic properties, in particular, the effects of high doses and helium cavities are lacking. Single-crystal iron films, with a thickness of 200 nm, were prepared using the ultra-high vacuum evaporation method. These films were then irradiated with 30 keV He+ ions at room temperature up to a dose of 18 dpa. X-ray diffraction measurements and cross-sectional transmission electron microscope observations revealed significant microstructural changes, including a large lattice expansion perpendicular to the film plane and the formation of high-density cavities after irradiation. However, the saturation magnetization and the shape of the magnetization curve showed almost no change, indicating the robustness of the magnetic properties of iron.

Visualization of Three-dimensional Helium Cavity Distribution in an Ion-irradiated Tungsten Heavy Alloy for Nuclear Fusion Materials.

Visualization of Three-dimensional Helium Cavity Distribution in an Ion-irradiated Tungsten Heavy Alloy for Nuclear Fusion Materials.

Behavior and properties of helium cavities in ductile-phase-toughened tungsten irradiated with He+ ions at an elevated temperature

Ductile-phase toughened tungsten (DPT W) composites have demonstrated a great potential for plasma-facing components in fusion power systems. These materials can retain the outstanding thermomechanical properties of W while significantly enhancing the fracture toughness. The DPT W composite in this study consists of W particles (88 wt.%) embedded in a ductile NiFeW (12 wt.%) solution matrix. Irradiation of the composite was performed with 90 keV He+ ions to 1.0 × 1017 He+/cm2 at 973 K. The He cavities formed in both W and NiFeW phases are examined using convergent-beam scanning transmission electron microscopy (CB-STEM) at an atomic-level resolution. Full-range depth profiles of the He cavity diameters and number densities are analyzed. The results show that the average cavity diameter in NiFeW is ∼2.7 times that in W, while the number density is an order of magnitude lower. A quantitative analysis of the He density and pressure in the cavities is also achieved using STEM electron energy loss spectroscopy (STEM-EELS) mapping. The He atoms in the cavities in NiFeW are found to be under-pressurized, while those in similar-sized cavities in W are not detectable. The He energy-loss shift exhibits a linear relationship with the He density in cavities in NiFeW, with the slope comparable to that for martensitic steel.

Grain boundary softening from stress assisted helium cavity coalescence in ultrafine-grained tungsten

The formation of helium cavities in coarse-grained materials produces hardening proportional to the number density and size of the cavities and due to the interaction of dislocations with intragranular helium defects. In nanostructured metals containing a high density of interfacial sinks, preferential cavity formation in the grain boundaries instead produces softening that is often attributed to enhanced interfacial plasticity. Employing two grades of ultrafine-grained tungsten, we explore this effect using targeted implantation studies to map cavity evolution as a function of the irradiation conditions and quantify its impact on the mechanical response through nanoindentation. Softening is reported at implantation temperatures above the threshold for preferential grain boundary cavity formation but at a sufficiently low fluence prior to the growth of intragranular cavities. Collective changes in the mean cavity size, density, and morphology beneath a residual impression on an implanted surface indicate that cavity coalescence accompanied the reduction in hardness. Complementary atomistic simulations demonstrate that, in tungsten grain structures exhibiting softening, grain boundary bubble coalescence is driven by stress concentrations that further act to localize strain in the grain boundaries through cooperative deformation processes involving local atomic shuffling and sliding, dislocation emission, and even the nucleation of unstable twinning events.

NiCoCr-based medium-entropy alloys with superior resistance to radiation hardening and helium cavity growth

In this work, three NiCoCr-based medium-entropy alloys (MEAs), i.e., NiCoCr, NiCoCrW0.1 and NiCoCrTi0.1, were irradiated with helium (He) ions at 400 ℃ and 700 ℃ to study the radiation hardening and He-induced cavity growth behavior. The typical quaternary FeCoNiCr alloy was irradiated similarly and evaluated for comparison. Using transmission electron microscopy and nanoindentation measurements, He-induced microstructural evolution and mechanical changes were systematically investigated. Here we show that mitigated radiation hardening at 400 ℃ and cavity growth behavior at 700 ℃ were exhibited in the NiCoCr-based MEAs, in contrast to significant radiation hardening and He cavity growth of FeCoNiCr. Among the studied alloys, NiCoCrW0.1 displayed the best radiation hardening resistance, while NiCoCrTi0.1 presented the strongest tolerance in terms of He cavity growth. The underlying reasons were carefully analyzed based on the observed microstructures. Our results demonstrate the great potential of NiCoCr-based MEAs to function as critical nuclear structural components at relatively low and high temperatures.

Behavior of helium cavities in ion-irradiated W-Ni-Fe ductile-phase toughened tungsten

This study reports on the distribution of helium (He) cavities in a hot-rolled W-Ni-Fe ductile-phase toughened tungsten (DPT W) composite irradiated to a dose and a helium concentration that are comparable to those in the material after 5-year irradiation in a conceptual fusion power plant. The DPT W sample consists of W particles embedded in a ductile-phase NiFeW matrix with a nominal composition of 90W-7Ni-3Fe by weight. It was hot-rolled to a thickness reduction by 87% (87R DPT W). Sequential and individual irradiations of the material with 1.2 MeV Ni+ ions to a fluence of 2.15 × 1016 Ni+/cm2 and 90 keV He+ ions to 6.5 × 1015 He+/cm2 was performed at 973 K. Larger He cavities with a lower number density are observed in NiFeW than W. Helium cavities are aggregated preferentially along the NiFeW/W interphase boundary. This behavior is not observed along the W/W grain boundary under the same irradiation conditions. The average corrected cavity diameter or volume appears to be smaller in the sequentially irradiated sample than He+ ion irradiated sample. There is no evidence for formation of visible voids or Ni precipitates in W phase irradiated with Ni+ ions only. Diffusion, clustering and trapping of vacancies and He atoms during He+ ion irradiation at 973 K may be responsible for the formation of the He cavities.

Effects of helium cavity size and morphology on the strength of pure titanium

High-purity α-titanium was implanted with helium to observe cavity morphology – size, number density, and form (i.e., bubbles vs. voids) – effects on materials strength. Increasing implantation temperatures lead to an Arrhenius-type (i.e., exponential) increase in cavity size, a transition from spherical bubbles to faceted voids, and a marked increase in strength (∼20–50%). Implanted samples deformed uniformly in contrast to nano-compression of unimplanted or alloyed titanium, but pillars with the largest faceted voids developed a type of bulging localized deformation. Affected voids were repeatedly sheared, becoming rough and shrunken. Dislocation-cavity interactions and the role of helium content within cavities are considered.

Effects of temperature on helium cavity evolution in single-phase concentrated solid-solution alloys

Single-phase concentrated solid-solution alloys (SP-CSSAs) have shown great potential as structural materials in advanced reactor systems. Up to date, nevertheless, the synergistic effect of compositional complexity and irradiation temperature on helium (He) irradiation-induced cavity formation has not been systematically studied yet. In this work, pure Ni and three prototype face-centered-cubic SP-CSSAs, i.e., NiCo, NiCoCr and NiCoFeCrMn, were irradiated with He + ions at 673, 773, 873 and 973 K. The evolution of He-induced cavities were investigated in detail with transmission electron microscopy . It was found that growth of He cavities was suppressed with increasing compositional complexity at 673 and 773 K. Due to the lower melting temperature, NiCoFeCrMn exhibited the strongest temperature susceptibility in terms of He cavity growth with the increasing temperature. Interestingly, the He cavity growth tended to be enhanced with increasing compositional complexity when the radiation temperature increaseed to 973 K, which can be attributed to the prominent chemical-biased vacancy migration at high temperatures.

Comparison between Subsequent Irradiation and Co-Irradiation into SIMP Steel

A modern Chinese ferritic/martensitic steel SIMP, is a new perspective nuclear structural material for the spallation target in accelerator driven sub-critical system. In this work, aimed at exploring the radiation resistance properties of this material, we investigate the differences between simultaneous Fe and He ions irradiation and He implantation of SIMP steel pre-irradiated by Fe self-ions. The irradiations were performed at 300 °C. The radiation-induced hardening was evaluated by nano-indentation, while the lattice disorder was investigated by transmission electron microscopy. Clear differences were found in the material microstructure after the two kinds of the ion irradiation performed. Helium cavities were observed in the co-irradiated SIMP steel, but not the case of He implantation with Fe pre-irradiation. In the same time, the size and density of Frank loops were different in the two different irradiation conditions. The reason for the different observed lattice disorders is discussed.

An immersed boundary method for fluid–structure interaction with compressible multiphase flows

This paper presents a two-dimensional immersed boundary method for fluid–structure interaction with compressible multiphase flows involving large structure deformations. This method involves three important parts: flow solver, structure solver and fluid–structure interaction coupling. In the flow solver, the compressible multiphase Navier–Stokes equations for ideal gases are solved by a finite difference method based on a staggered Cartesian mesh, where a fifth-order accuracy Weighted Essentially Non-Oscillation (WENO) scheme is used to handle spatial discretization of the convective term, a fourth-order central difference scheme is employed to discretize the viscous term, the third-order TVD Runge–Kutta scheme is used to discretize the temporal term, and the level-set method is adopted to capture the multi-material interface. In this work, the structure considered is a geometrically non-linear beam which is solved by using a finite element method based on the absolute nodal coordinate formulation (ANCF). The fluid dynamics and the structure motion are coupled in a partitioned iterative manner with a feedback penalty immersed boundary method where the flow dynamics is defined on a fixed Lagrangian grid and the structure dynamics is described on a global coordinate. We perform several validation cases (including fluid over a cylinder, structure dynamics, flow induced vibration of a flexible plate, deformation of a flexible panel induced by shock waves in a shock tube, an inclined flexible plate in a hypersonic flow, and shock-induced collapse of a cylindrical helium cavity in the air), and compare the results with experimental and other numerical data. The present results agree well with the published data and the current experiment. Finally, we further demonstrate the versatility of the present method by applying it to a flexible plate interacting with multiphase flows.

Irradiation behaviour of α2 and γ phases in He ion implanted titanium aluminide alloy

A Ti–45Al–2Nb–2Mn + 0.8 vol.% TiB2 (at.%) alloy with fully lamellar microstructure consisting of hexagonal-close-packed (hcp) α2 and face-centred-tetragonal (fct) γ phases was irradiated by implanting helium ions to different fluences. Microstructural examination showed that helium cavities are formed in both the α2 and γ phases after He-ion irradiation. However, the helium cavities and their size change with fluence are much larger in the α2 phase than those in the γ phase, indicating that the γ phase exhibits better tolerance to the He-ion irradiation than the α2 phase. Since α2 and γ phases have different crystal structures, they possess differences in helium solubility and interstitial migration. These differences are responsible for the variation in radiation damage behaviour between the two phases.

Atomistic study of the thermodynamic equilibrium of nano-sized helium cavities in βSiC

The estimation of the number of inert gas atoms contained at equilibrium in microscale bubbles in a solid usually relies on a well-known formula equilibrating the internal pressure of He to the surface energy of the bubble. This approach evidences a strong variation with temperature of He content for a given bubble. At the opposite, at the Angstrom scale, ab initio calculations for He contained in vacancy assemblies neglect temperature effects. In this work, empirical potential molecular dynamics simulations are used to study, in the case of helium inserted in cubic silicon carbide, the variation of the He content of sub-nanoscale cavities with temperature. To do so free energy for He atoms inserted in cavities made of a few vacancies (up to 29) are calculated. One then evidences the existence of a sub-surface segregation in interstitial sites close to the surface of the cavity. The variation of the He content with temperature is observed to be negligible at the nanoscale, thus validating the ab initio approach.

The influence of the chemical compression on the electric properties of molecular systems within the supermolecular approximation: the LiH molecule as a case study

The influence of the chemical compression on the electric properties of the model system is investigated. Two types of chemical environments are applied to exert pressure on the LiH molecule: nanotube-like and fullerene-like. The effects of two shapes of helium cavities onto the electric properties of the model system are discussed. The obtained results are compared to the data generated for the model spherical and cylindrical potentials by other authors.

Helium cavity formation research on oxide dispersed strengthening (ODS) ferritic steels utilizing dual-ion irradiation facility

A dual-ion irradiation method is one of the most popular techniques for the research of microstructural evolution of materials under fusion environmental conditions. The application of oxide dispersion strengthened (ODS) ferritic steels is anticipated for realization of high temperature operation of fusion reactors above 600 °C. In this research, the investigation of the microstructural evolution of high corrosion resistant ODS steels was performed using the ion irradiation methods. Dual ion-irradiation experiments were performed at the DuET facility, Kyoto University using 6.4 MeV Fe self-ion and 1 MeV helium ion. The irradiation temperatures were 300 and 500 °C. The dose, dose rate, and helium over dpa ratio were 20 dpa, 5 × 10 −4 dpa/s, and 15 appm/dpa, respectively. The post-irradiation experiments were performed using SEM, TEM and the nano-indentation methods. Significant microstructural evolution of grains and oxides were not observed after the single-ion irradiation at 20 dpa, 500 °C. Fine helium cavities were observed in the dual-ion irradiated 19Cr–2W–4Al ODS steel. Fine cavities of approximately several nanometers diameter were densely and uniformly formed in dual-ion irradiated range.

Fast and simultaneous determination of tin and mercury species using SPME, multicapillary gas chromatography and MIP-AES detection

A procedure for the simultaneous and fast determination of methylmercury (MeHg+), inorganic mercury (Hg2+) and mono-, di- and tributyltin species (MBT, DBT, TBT) in water samples, as ethylated derivatives using a multicapillary GC column and MIP-AES detection, has been developed. The influences of the auxiliary gas pressure and the helium cavity vent flow in the MIP-AES response to mercury and tin species were evaluated and adjusted to obtain optimal detection conditions for both groups of organometallic compounds in the same chromatographic run. The proposed method was applied to the fast and simultaneous determination of mercury and tin species in water samples after a concentration step using solid-phase microextraction fibres.

Role of α2/γ and γ/γ phase boundaries in cavity formation in a TiAl intermetallic compound irradiated with He-ions

A Ti–48at.%Al intermetallic compound has been irradiated with 200 keV He-ions at 623 and 773 K. The helium cavity density decreases with decreasing α2 and γ lamella width. A plot of the cavity density and lamella width reveals a linear relationship after irradiation to 15 dpa. Cavity density in regions with 300 nm wide lamella is about half of that in large γ grains of the specimen. The α2/γ lamellar boundaries supply a preferential nucleation site for cavities. Although the cavities are also formed on the γ/γ lamellar boundaries, the nucleation is limited to misfit dislocations on the boundaries. Defect-free zones are in the regions of about 50 nm width immediately adjacent to the lamellar boundaries at 773 K. These results suggest that the lamellar boundaries are effective sinks for radiation defects and contribute to the suppression of radiation-induced defect cluster development in TiAl alloys.

Cavities in helium implanted and annealed silicon characterized by spectroscopic ellipsometry

The formation of helium induced cavities in silicon during short-time annealing is analyzed by spectroscopic ellipsometry. Specimens implanted with 40 keV He+ ions to a dose of 5×1016 cm−2 are heat treated at 800 °C for times of 1–1200 s by rapid thermal annealing. Spectroscopic ellipsometry is employed to obtain quantitative information on the cavity volume depth profiles. A newly developed formula is used to model the optical multilayer depth profiles. The cavity volume is found to increase during annealing for about 300 s and to decrease for longer annealing times. Over this characteristic time a marked change in the He loss occurs, which has been reported only recently. Swelling of the helium implanted and annealed silicon is analyzed using an atomic force microscope. Step heights are consistent with the cavity volume per unit area obtained from spectroscopic ellipsometry data analysis. The number density of cavities after annealing for 600 s is calculated to be ≈1.16±0.27×1017 cm−3 and is found to be largely independent of depth in the central part of the cavity layer.

Thermal behavior of hydrogen in helium-implanted high-purity SUS316L

We have studied the effects of helium incorporation on trap-sites of hydrogen in high-purity stainless steel SUS316L. We implanted 10 and 30 keV He ions into high-purity SUS316L samples with several doses ranging from 3 × 10 15 to 1 × 10 17/cm 2 and then 30 keV hydrogen ions with a dose of 1 × 10 17/cm 2 at a temperature of 300 K, and then observed depth profiles and thermal behavior of hydrogen in the samples by means of the elastic recoil detection (ERD) method. It was found that hydrogen implanted into the high-purity SUS316L is chemically absorbed in helium cavities.

Helium damage in metal-tritium systems.

The time evolution of Debye-Scherrer lines was investigated in metal-tritium-helium systems by neutron-scattering techniques. Polycrystalline samples of ${\mathrm{TaT}}_{\mathit{x}}$, ${\mathrm{YT}}_{\mathit{x}}$, and ${\mathrm{ScT}}_{\mathit{x}}$ were measured over a period of three years. The results show that helium damage is governed by the behavior of self-interstitial atoms and dislocation loops created by helium clustering and bubble formation. The self-interstitials and loops induce a lattice expansion in the early stages of helium formation. For higher helium concentrations the self-interstitials and loops produced are completely incorporated into an evolving dislocation network. In hexagonal rare-earth systems a preferential condensation of loops into a dislocation network lying in basal planes is observed. Additional small-angle-scattering experiments show that platelike helium cavities are formed in the hexagonal systems.