Evolution Of Helium Bubbles Research Articles

Helium bubble formation in nickel under in-situ krypton and helium ions dual-beam irradiation

Helium has profound effects on the microstructure and mechanical property of materials used in nuclear power systems. To understand the interaction between helium and irradiation-induced defects and its effect on bubble formation, single-beam and dual-beam irradiations on nickel were performed with 16 keV helium ions and 1 MeV krypton ions under in-situ TEM observation. For 1 MeV krypton single-beam irradiation, voids were observed at 600 °C but not at 500 °C and 700 °C. For helium single-beam irradiation at 500 °C, helium moved freely and formed bubble-loop complexes. For dual-beam irradiation at 500 °C, helium was trapped by vacancies and formed bubbles homogeneously. The structure of dislocation loops was also affected by the presence of helium. For krypton single-beam irradiation at 500 °C, the microstructure was {100} periodic wall of faulted and perfect dislocation loops. For dual-beam irradiation at 500 °C, periodic wall was not observed, and dislocation loops were predominantly perfect. For single-beam helium at 500 °C, dislocation loops were produced by athermal process of SIAs ejection or loop punching mechanisms. The interplay between helium and vacancies played an important role on the evolution of helium bubbles and defect microstructures.

Long-term helium bubble evolution in sequential He+ and H+ irradiated Li4SiO4

In present study, Li4SiO4 ceramics were sequentially irradiated at room temperature using 540 keV He+ and 250 keV H+ with ion fluences of 1 × 1016 ions/cm2 and 1 × 1017 ions/cm2, corresponding to 2.14 displacements per atom (dpa) and 21.4 dpa, respectively. The change of featured structures and the long-term evolution of helium bubbles in irradiated Li4SiO4 were investigated by grazing incidence X-ray diffraction (GIXRD), Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy and transmission electron microscope (TEM). For the as-irradiated Li4SiO4, the results revealed that the lattice parameter and the amorphous fraction increased and the crystallinity decreased with the increasing irradiation dose, and the anionic disordering was the mainly induced damage. When the irradiation dose reached to 21.4 dpa, high-density bubbles with a mean diameter of 1.6 nm were observed in Li4SiO4. The following 480-day aging at room temperature rendered bubble growth due to surface diffusion, which was demonstrated by the fact of an increase in bubble size (mean diameter of 18.4 nm) and a decrease in bubble density. The formation and growth of bubbles could benefit from the presence of amorphous structures and hydrogen atoms. Furthermore, the dynamic evolution of bubbles during annealing at 723 K was monitored by in-situ TEM, and the results indicated that the migration and coalescence mechanism dominated the growth of bubbles in Li4SiO4 under relatively low temperatures.

In-Situ TEM Annealing Observation of Helium Bubble Evolution in Pre-Irradiated FeCoNiCrTi0.2 Alloys.

The FeCoNiCrTi0.2 high-entropy alloys fabricated by vacuum arc melting method, and the annealed pristine material, are face centered cubic structures with coherent γ’ precipitation. Samples were irradiated with 50 keV He+ ions to a fluence of 2 × 1016 ions/cm2 at 723 K, and an in situ annealing experiment was carried out to monitor the evolution of helium bubbles during heating to 823 and 923 K. The pristine structure of FeCoNiCrTi0.2 samples and the evolution of helium bubbles during in situ annealing were both characterized by transmission electron microscopy. The annealing temperature and annealing time affect the process of helium bubbles evolution and formation. Meanwhile, the grain boundaries act as sinks to accumulate helium bubbles. However, the precipitation phase seems have few effects on the helium bubble evolution, which may be due to the coherent interface and same structure of γ’ precipitation and matrix.

Evolution of helium bubbles in nickel-based alloy by post-implantation annealing* *Project supported by the Special Funds for the Key Research and Development Program of the Ministry of Science and Technology of China (Grant Nos. 2017YFB0702201 and 2020YFB1901800) and the National Natural Science Foundation of China (Grant Nos. 11975135 and 12005017).

Nickel-based alloys have been considered as candidate structural materials used in generation IV nuclear reactors serving at high temperatures. In the present study, alloy 617 was irradiated with 180-keV helium ions to a fluence of 3.6 × 1017 ions/cm2 at room temperature. Throughout the cross-section transmission electron microscopy (TEM) image, numerous over-pressurized helium bubbles in spherical shape are observed with the actual concentration profile a little deeper than the SRIM predicted result. Post-implantation annealing was conducted at 700 °C for 2 h to investigate the bubble evolution. The long-range migration of helium bubbles occurred during the annealing process, which makes the bubbles of the peak region transform into a faceted shape as well. Then the coarsening mechanism of helium bubbles at different depths is discussed and related to the migration and coalescence (MC) mechanism. With the diffusion of nickel atoms slowed down by the alloy elements, the migration and coalescence of bubbles are suppressed in alloy 617, leading to a better helium irradiation resistance.

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.

In-situ TEM observation of the evolution of helium bubbles in Mo during He+ irradiation and post-irradiation annealing* *Project supported by the National Natural Science Foundation of China (Grant Nos. U1967211, U1832112, and 11975191).

The evolution of helium bubbles in purity Mo was investigated by in-situ transmission electron microscopy (TEM) during 30 keV He+ irradiation (at 673 K and 1173 K) and post-irradiation annealing (after 30 keV He+ irradiation with the fluence of 5.74 × 1016 He+/cm2 at 673 K). Both He+ irradiation and subsequently annealing induced the initiation, aggregation, and growth of helium bubbles. Temperature had a significant effect on the initiation and evolution of helium bubbles. The higher the irradiation temperature was, the larger the bubble size at the same irradiation fluence would be. At 1173 K irradiation, helium bubbles nucleated and grew preferentially at grain boundaries and showed super large size, which would induce the formation of microcracks. At the same time, the geometry of helium bubbles changed from sphericity to polyhedron. The polyhedral bubbles preferred to grow in the shape bounded by {100} planes. After statistical analysis of the characteristic parameters of helium bubbles, the functions between the average size, number density of helium bubbles, swelling rate and irradiation damage were obtained. Meanwhile, an empirical formula for calculating the size of helium bubbles during the annealing was also provided.

In-situ TEM observation of the evolution of helium bubbles & dislocation loops and their interaction in Pd during He+ irradiation

The microstructural evolution of purity Pd under 30 keV He+ irradiation at 573 K was investigated by in-situ transmission electron microscopy. The nucleation, growth, merging, annihilation, size change, number density variation, and types of dislocation loops were analyzed under the influence of irradiation fluence and sample thickness. Both perfect dislocation loops with b = 1/2 < 110> and faulted dislocation loops with b = 1/3 < 111> were formed. However, at low irradiation fluence, most of the loops were 1/3 < 111> loops. The thickness of TEM foil obviously affected the ratio of 1/3 < 111> loop variants, the size and number density of dislocation loops, and the characteristics of bubble-loop complexes. With the increase of irradiation fluence, the size of dislocation loops increased, but loop volume number density remained almost constant until dislocation loops merged and evolved into dislocation network. There was an obvious interaction between dislocation loops and bubbles, indicating that 1/3 < 111> loop was first formed at the initial stage of irradiation, and when the loop grew to a certain size, obvious helium bubbles appeared inside its region.

In-situ TEM investigation of nano-scale helium bubble evolution in tantalum-doped tungsten at 800°C

The aim of this work is to probe the helium-induced defect production and accumulation in 40 keV He+ irradiated polycrystalline W and its alternative alloy W-5wt.%Ta using transmission electron microscopy (TEM) combined with in-situ helium irradiation at 800°С. A maximum damage level of 1 dpa with a maximum He-to-dpa ratio of 5.5 at%/dpa has been reached in this work for both materials, which corresponds to an ion fluence of 7.33 × 1016 He+/cm2. The presence of radiation-induced dislocation loops was not observed at this temperature. The low density of the incipient bubbles in W has been already detected at 0.004 dpa, which corresponds to a fluence of 3.3 × 1014He+/cm2. The experiments conducted at 800°C have shown that the addition of 5wt.% of tantalum into tungsten may diminish the binding of He ions with vacancies into complexes, which serve as the core of the bubble, thus hindering helium bubble formation below 0.02 dpa and their further growth and population at higher damage levels. By exceeding the damage dose ≥0.3 dpa, a progressive transition from a spherical to a faceted shape of the bubbles has been observed in W but not in the W-5Ta alloy. At 1 dpa, >80% of the bubbles in W were of the faceted type with the facet planes of {110}.

In-situ transmission electron microscopy observation of the helium bubble evolution in pre-irradiated fluorapatite during annealing

The polycrystalline fluorapatite Ca10(PO4)6F2 ceramic synthesized by a standard solid-state sintering method was pre-irradiated with 80 keV He+ ions to a fluence of 5 × 1016 ions/cm2 at room temperature. After that, an in-situ annealing experiment was performed inside a transmission electron microscope to monitor the evolution of helium bubbles during heating to 723 and 823 K. Initially, no helium bubble formation was observed in the damage layers of the pre-irradiated samples. However, as the temperature increased, helium bubbles first became visible and then began coarsening, ultimately reaching an asymptotic radius during annealing. The migration and coalescence of helium bubbles in the fluorapatite matrix was complete at a temperature of 823 K, and its likely mechanism involved the existence of two different types of coalescing bubbles.

Improvement of helium swelling resistance of nickel-based alloy via proper SiCNP dispersion

Ni-Mo-SiCNP alloy is a promising candidate as a structural material for building high-temperature molten salt reactors in the future. The irradiation damage resistance of this alloy is a key factor in determining its application prospects. In this study, Ni-Mo-SiCNP alloy samples containing different dispersion amounts (0.5, 1.5, and 2.5 wt.%) of SiCNP were irradiated with 1 MeV He+ ions at a dose of 3 × 1016 ion/cm2 at 750 °C. The evolution of helium bubbles was observed using transmission electron microscopy. The results show that with an increase in the dispersion amount of SiCNP, the number density of helium bubbles increases whereas the mean size decreases gradually. It is shown that a proper dispersion amount of SiCNP can effectively improve the helium swelling resistance of nickel-based alloys. Moreover, the related mechanism of the suppressed swelling was revealed.

In-situ TEM observation of the evolution of dislocation loops and helium bubbles in a pre helium irradiated FeCrAl alloy during annealing

FeCrAl alloys are regarded as excellent potential candidates of fuel claddings in nuclear reactors. To further understand the evolution of helium bubbles and dislocation loops during the annealing process, He+-irradiated TEM samples with a normal chemical composition of Fe-13.2Cr-5.6Al-2Mo-2ZrC (wt.%) were annealed and observed by in-situ transmission electron microscopy. The dislocation density decreased with the increase of annealing temperature, but the loop size was opposite. No obvious bubbles were observed at the annealing temperature below 772 K, even if the annealing time was prolonged. The average diameters of helium bubbles in the sample annealed at 772, 872 and 972 K for 60 min were 1.4, 2.1 and 3.1 nm, respectively, which indicated that bubble sizes increased with the rise of annealing temperature. When the temperature was increased to 1072 K, the bubbles grew rapidly while their density decreased significantly. The dynamic process, during which two or more bubbles merged into one large-sized bubble, was observed. The migration and coalescence of small bubbles should be attributed to the control of surface diffusion mechanism.

Cr-coated Zr-4 alloy prepared by electroplating and its in situ He+ irradiation behavior

With the aim of improving the oxidation resistance of zirconium alloy claddings in light water reactors and enhancing the deposition efficiency and economics of surface coating, electroplating was used to deposit a Cr coating with a rate exceeding 12 μm/h on the surface of thermally treated Zr-4 alloy predeposited with an intermediate transition nickel layer. The evolution of helium bubbles in the cross-section of the Cr-coated Zr-4 alloy during 30 keV He+ irradiation at 400 °C was in situ analyzed at a newly constructed facility named the Xiamen Multiple Ion Beam In situ Analysis TEM Facility, which is the only running in situ irradiation facility at China. The thermal treatment induced chemical reactions between nickel and zirconium to form a multilayer structure (Ni5Zr, Ni7Zr2, Ni3Zr, Ni10Zr7, NiZr and NiZr2 in sequence) with metallurgical bonding. Meanwhile, the Cr coating combined with the Ni layer in a lattice-matching mode. No defects could be found at the interface between each layer. The different phases and interfaces showed different resistance abilities towards He+ irradiation, where the size and distribution characteristics of bubbles were different. A bubble-free zone could be observed adjacent to the interface between Cr and Ni layers; however, bubbles mainly aggregated at the interface between Ni and Ni5Zr layers. With the increase in helium concentration, both the size and density of bubbles increased in the Cr coating. Helium atoms precipitated preferentially along Cr grain boundaries to form bubbles and then further gathered to form short bubble microcracks. The results were discussed in terms of the energetics of the material system and the surface diffusion mechanism.

In situ TEM observation of the evolution of helium bubbles in Hastelloy N alloy during annealing

In situ transmission electron microscopy (TEM) was used to capture the dynamic evolution of helium bubbles in Hastelloy N alloy during annealing at 400 °C, 500 °C and 600 °C. The TEM results showed that intragranular helium bubbles hardly coarsened after annealing at 400 °C and 500 °C for 60 min. Although the formation of a new helium bubble was observed at the grain boundary for the sample annealed at 500 °C, these intergranular helium bubbles did not coalesce due to the low surface diffusion, the distance between helium bubbles and the misorientation of two grains. The high internal pressure of the helium bubbles calculated using the Carnahan–Starling hard sphere model illustrated that the migration and coalescence (MC) mechanism was more apparent in helium bubbles coarsening for the sample annealed at 600 °C. However, the MC mechanism was restricted by chromium atoms and dislocation lines, which caused the emergence of the suppressed MC mechanism.

Revealing the Dynamics of Helium Bubbles Using In Situ Techniques

As one of the major irradiation defects, helium bubbles have a marked influence on the microstructures and mechanical properties of metals. In recent decades, many experiments and simulations have focused on helium bubbles to reveal their nucleation and growth mechanisms, dynamic evolution under stimulations, and their effects on mechanical properties. With the quick development of various in situ techniques, the abundant dynamic features of helium bubbles have been revealed. In this review, we briefly explore the related researches on the dynamic evolution of helium bubbles under simulated service conditions, such as at high temperatures, under irradiation, and upon mechanical loading. We also discuss the challenges and opportunities in revealing the dynamics of helium bubbles using in situ technologies. This short review intends to advance our understanding of the failure mechanisms of helium-irradiated metals and the basic properties of irradiation-induced helium bubbles.

Microstructural and elemental evolution of polycrystalline α-SiC irradiated with ultra-high-fluence helium ions before and after annealing

Helium atoms are the main reaction products that affect the properties of SiC, which is an important structural material used in the nuclear industry. Understanding the behaviour of helium in SiC is important for predicting the lifetimes of SiC in nuclear power systems. Unfortunately, the behaviour of SiC toward ultra-high concentrations (near 50 at.%) of helium have not been investigated extensively. In this study, polycrystalline α-SiC was irradiated with 300 keV He2+ ions at a fluence of 7.3 × 1017 He2+/cm2 at room temperature, followed by annealing at 900 ℃ and 1200 ℃, with microstructural and elemental evolution investigated using transmission electron microscopy, energy-dispersive X-ray and Raman spectroscopies. The evolution of helium bubbles, as well as C and Si clusters were investigated by molecular dynamics simulations. Significant volume swelling occurred during helium irradiation, and the material recrystallised after annealing at 900 ℃. Stacking faults and dislocation loops were observed in the recrystallised and crystalline layers, respectively. This work is expected to provide insight into the damage mechanism of SiC exposed to ultra-high-fluence He ions and will provide references for the application of SiC in future advanced nuclear energy systems.

The effect of tempering temperatures on the surface swelling of helium-ion-implanted Ti/Ta-added reduced-activation ferritic/martensitic steel

The swelling of a newly developed Ti/Ta-added reduced-activation ferritic/martensitic (RAFM) steel is evaluated using helium ion implantation. He ions are injected into sample surfaces covered with a transmission electron microscopy (TEM) grid; subsequent post-implantation annealing (PIA) causes surface mounds to form where He ions are injected. The heights of surface mounds on Ti/Ta-RAFM steels tempered at three different temperatures are found to increase with increasing tempering temperature. TEM observation reveals that helium bubbles formed along lath/block boundaries, dislocations, and precipitate-matrix interfaces. The increased step height of the Ti/Ta-RAFM steel tempered at higher temperature can be attributed to increases in the block and precipitate sizes and a decrease in dislocation density. The mechanism of helium bubble evolution in RAFM steel after helium implantation at room temperature and subsequent PIA is illustrated schematically. The swelling of Ti/Ta-RAFM steel is compared to that of a reference steel with a composition similar to that of Eurofer97.

Investigating Helium Bubble Nucleation and Growth through Simultaneous In-Situ Cryogenic, Ion Implantation, and Environmental Transmission Electron Microscopy.

Palladium can readily dissociate molecular hydrogen at its surface, and rapidly accept it onto the octahedral sites of its face-centered cubic crystal structure. This can include radioactive tritium. As tritium β-decays with a half-life of 12.3 years, He-3 is generated in the metal lattice, causing significant degradation of the material. Helium bubble evolution at high concentrations can result in blister formation or exfoliation and must therefore be well understood to predict the longevity of materials that absorb tritium. A hydrogen over-pressure must be applied to palladium hydride to prevent hydrogen from desorbing from the metal, making it difficult to study tritium in palladium by methods that involve vacuum, such as electron microscopy. Recent improvements in in-situ ion implantation Transmission Electron Microscopy (TEM) allow for the direct observation of He bubble nucleation and growth in materials. In this work, we present results from preliminary experiments using the new ion implantation Environmental TEM (ETEM) at the University of Huddersfield to observe He bubble nucleation and growth, in-situ, in palladium at cryogenic temperatures in a hydrogen environment. After the initial nucleation phase, bubble diameter remained constant throughout the implantation, but bubble density increased with implantation time. β-phase palladium hydride was not observed to form during the experiments, likely indicating that the cryogenic implantation temperature played a dominating role in the bubble nucleation and growth behavior.

Phase-field modeling of helium bubble evolution in nickel-graphene nanocomposite

Metal–graphene (Gr) nanocomposites are regarded as potential anti-irradiation advanced materials for Gen-IV nuclear reactors. In this work, a phase-field model was proposed to simulate helium (He) bubble evolution of nickel–Gr (Ni/Gr) nanocomposite during constant irradiation at 800 K. Ni/Gr nanocomposite presented an earlier onset time of He bubble nucleation but eventually formed bubbles with smaller size and lower density than those of pure Ni. The early He bubbles nucleated along Ni/Gr interfaces, while a He depletion zone formed nearby consequently, which confirmed that Ni/Gr interfaces inhibit He bubble growth. The effects of vacancies and He atoms on the He bubble growth rate were also quantitatively analyzed. The amount of irradiation-induced vacancies acted as an effective rate-controlling factor that determined the bubble growth rate with respect to the He atoms.

Corrosion-driven outward migration and growth of helium bubbles in a nickel-based alloy in high-temperature molten salt environment

High-temperature helium ion irradiation was conducted on a nickel-based alloy at 650 °C. A unique outward migration of helium bubbles was observed in the irradiated alloy under molten salt environment, distinguished from the evolution of helium bubbles induced by room-temperature irradiation, which is related to the deficit of vacancies in the high-temperature irradiated alloy and slow growth rate of the bubbles. Moreover, it was elucidated that corrosion induced vacancies rather than high-temperature annealing effect of molten salt is responsible for the evolution of helium bubbles formed by high-temperature helium ion irradiation.

Evolution of helium bubbles induced by stress concentration in He-irradiated aluminium

By employing an in-situ tension experiment in a transmission electron microscope, a stress concentration is demonstrated as a significant contributor to evolution of helium bubbles in a non-uniform stress field. In the stress concentration, a bubble growth occurs through bubble migration and coalescence driven by stress gradients as well as a dislocation slip-induced vacancy absorption upon straining. A bubble-shape evolution occurs after a coalescence of bubbles with similar size. As the bubbles grow to critical sizes, gradual elongation and disappearance of bubbles are found with further straining.