Nucleation Of He Bubbles Research Articles

Radiation-enhanced precipitation and the impact on He bubble formation in V-Ti-based refractory alloys containing interstitial impurities

V-Ti-based refractory high entropy alloys (RHEAs), e.g., VTiTa and VTiTaNb, have been found with outstanding mechanical properties and irradiation resistance, granting them application potential in nuclear engineering. Nonetheless, similar to conventional V-Ti-based alloys such as V-4Ti-4Cr, these alloys are sensitive to C/N/O interstitial impurities. Under irradiation, such impurities may promote the formation of precipitates which may affect the defect evolution, especially at the appearance of helium. Previous studies have observed different helium behavior in the above alloys, including the heterogeneous distribution of helium bubbles in VTiTa, but the origin was not clear. In this work, detailed microstructural characterization is performed to analyze the irradiation induced precipitates and their impact on helium bubble formation in these alloys. Under helium ion irradiation at 700°C, the TiN precipitates with the NaCl-type structure are formed in all the three alloys. Nonetheless, the interfaces between precipitates and matrix in VTiTa and VTiTaNb are semi-coherent while those in V-4Ti-4Cr are coherent, due to the various lattice parameters of the alloys. Moreover, severe lattice distortion is also found inside the precipitates in the two RHEAs. Consequently, significant heterogeneous nucleation of He bubbles is observed in these two RHEAs but not in V-4Ti-4Cr. Among the three alloys, the growth of precipitates is the fastest in VTiTa, causing the formation of bubble clusters. Furthermore, the growth direction as well as the shape of precipitates affects the shape of bubbles, leading to the {001} cuboids shape in V-4Ti-4Cr and the truncated {110} dodecahedra shape in V-Ti-Ta and V-Ti-Ta-Nb.

Effect of H and Pd atoms on the migration of He atoms in 3C–SiC

3C–SiC is an important structural material for tri-structural isotropic coated fuel particles in high-temperature gas reactors. It serves as a vital pressure vessel for gas fission products and a shielding layer for metal fission products. However, the accumulation of interstitial He atoms can lead to the formation of He bubbles, which threaten the safe service of materials. A study of how interstitial atoms affect the migration of He atoms can help us understand the nucleation of He bubbles in SiC cladding materials. The effects of H and Pd interstitials on the solution energy and migration behavior of He atoms were investigated. It was found that H favored He atoms to approach it, while Pd has the opposite effect, causing He atoms to move away from it. Under the synergistic effect of H and Pd atoms, He atoms tend to move away from the Pd interstitial, similar to the effect of the Pd interstitial alone. Furthermore, the external H interstitial strengthens the effect of Pd atoms on He atoms, drastically reducing the migration energy of He atoms away from Pd.

DFT study on the nucleation of He bubbles in Pd: Effect of H and self-interstitial atoms

The nucleation process of helium (He) clusters in the palladium (Pd) system is investigated via Ab initio calculations in this work. When there are five He atoms around a lattice Pd atom, the self-trapping behavior of He atoms in a pure Pd system can be observed. A Pd-Pd split interstitial along the <0 0 1> direction near the He cluster is found when the number of interstitial He atoms reaches six. Interstitial H atoms are considered to have little effect on the nucleation of He clusters due to the negligible binding energy between H and He atoms. Besides, the interstitial H atom near He clusters is knocked out by the displaced Pd atom, which further demonstrates that the influence of low concentration of H atoms on the nucleation of He clusters can be ignored. However, self-interstitial atoms (SIAs) can promote the aggregation of He atoms because of the mutual attraction between SIAs and He atoms. The existence of SIAs increases the local pressure around the cluster so that only four He atoms are needed to emit a lattice Pd atom, thereby promoting the nucleation of He clusters.

Effect of helium irradiation on deuterium permeation behavior in tungsten

In this study, we measured deuterium (D) gas-driven permeation through tungsten (W) foils that had been pre-damaged by helium ions (He+). The goal of this work was to determine how ion-induced damage affects hydrogen isotope permeation. At 873 K, the D permeability for W irradiated by 3.0 keV He+ was approximately one order of magnitude lower than that for un-damaged W. This difference diminished with increasing temperature. Even after heating to 1173 K, the permeability returned to less than half of the value measured for un-damaged W. We propose that this is due to nucleation of He bubbles near the surface which potentially serve as a barrier to diffusion deeper into the bulk. Exposure at higher temperatures shows that the D permeability and diffusion coefficients return to levels observed for undamaged material. It is possible that these effects are linked to annealing of defects introduced by ion damage, and whether the defects are stabilized by the presence of trapped He.

Diffusion and transformation kinetics of small helium clusters in bulk tungsten

The production of energy through nuclear fusion poses serious challenges related to the stability and performance of materials in extreme conditions. In particular, the constant bombardment of the walls of the reactor with high doses of He ions is known to lead to deleterous changes in their microstructures. These changes follow from the aggregation of He into bubbles that can grow and blister, potentially leading to the contamination of the plasma, or to the degradation of their mechanical properties. We computationally study the behavior of small clusters of He atoms in W in conditions relevant to fusion energy production. Using a wide range of techniques, we investigate the thermodynamics of the clusters and their kinetics in terms of diffusivity, growth, and breakup, as well as mutation into nano-bubbles. Our study provides the essential ingredients to model the early stages of He exposure leading up to the nucleation of He bubbles.

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

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

An atomistic modeling of He bubble stability at grain boundaries in alpha-Fe

Development of He bubbles at grain boundaries (GBs) may lead materials to serious embrittlement, but the nucleation mechanism of such bubbles is not well understood. In the present paper, we analyzed the stability of various He–vacancy clusters, which are precursors of He bubbles, at several kinds of GBs of α-Fe using a set of empirical potentials. The results clearly indicated the preferred nucleation of He bubbles at GBs. We also found that the dissociation energy of vacancies from He–vacancy clusters at GBs is generally less than that for the intragranular counterparts, and that the He-to-vacancy ratio in equilibrium for various GB cases becomes larger in comparison with the intragranular case. The results obtained here are invaluable in establishing a kinetic model of He bubble nucleation at GBs.

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

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

Fluence-dependent radiation damage in helium (He) ion-irradiated Cu/V multilayers

We have explored the capacity of Cu/V interfaces to absorb helium ion radiation-induced defects spanning a peak damage range of 0.6–18 displacements per atom (dpa). The study provides evidence of alleviated nucleation of He bubbles in the multilayer films from Cu/V 50 nm to Cu/V 2.5 nm. Layer interfaces are retained in all irradiated specimens. Peak bubble density increases monotonically with fluence, and is lower in multilayers with smaller individual layer thickness. Radiation hardening decreases with decreasing layer thickness and appears to reach saturation upon peak radiation damage of 6 dpa. Size- and fluence-dependent radiation damage in multilayers is discussed.

Nucleation of He bubbles in amorphous FeBSi alloy irradiated with He ions

It is interesting to investigate the formation of He bubbles in amorphous alloys because point defects do not exist in amorphous materials. In the present study, the microstructural evolution of amorphous Fe79B16Si5 alloy, either irradiated with 5 keV He+ ions or implanted with 150 eV He+ ions without causing displacement damage, and then annealed at a high temperature, was investigated using transmission electron microscopy (TEM). Vacancy-type defects were formed in the amorphous alloy after irradiation with 5 keV He+ ions, and He bubbles formed during annealing the irradiated samples at high temperature. On the other hand, for samples implanted with 150 eV He+ ions, although He atoms are also trapped in the free volume, no He bubbles were observed during annealing the samples even up to 873 K. In conclusion, the formation of He bubbles is related to the formation and migration of vacancy-type defects even in amorphous alloys.