Growth Of Helium Bubbles Research Articles

Effect of Annealing Temperature on the Growth of Helium Bubbles in Silicon

Porous silicon has broad application prospects in the fields of Optics and catalysis. Manufacturing helium bubbles in silicon is one of the effective methods to prepare porous silicon. However, the research on the optimal parameters of this preparation technology and the growth mechanism of helium bubbles in silicon is not deep enough. In this paper, the experimental method of Ic + A is adopted. Firstly, 200 keV He ions (5 × 1016 ions / cm2) was implanted into monocrystalline silicon at room temperature, subsequent the effects of different annealing temperatures on the growth of helium bubbles in silicon were studied. It is found that after annealing at 400 °C and below, there are only some fine structures of interstitial particles in monocrystalline silicon samples. At the annealing temperature of 500 °C ~ 800 °C, two sizes of helium bubble structures appear in the defect layer, and the diameter of helium bubble increases with the increase of annealing temperature.

Influence of grain boundaries on the loop-punching mechanism and shape of helium bubbles in tungsten

In the present work, the growth of helium bubble at Σ3(112)[110], Σ5(013)[100] and Σ13(015)[100] three grain boundaries in tungsten is studied using molecular dynamics simulation. The simulation results show that interstitial dislocation loops are pushed out earlier for the case He bubble growing at the Σ3(112)[110] and later for the case bubble growing at the Σ5(013)[100], while no prismatic interstitial dislocation loop is pushed out for the case bubble growing at Σ13(015)[100] by the end of the simulation. This is because the larger the excess volume per area of grain boundary, the later the prismatic interstitial dislocation loop pushed out. The shapes of helium bubbles are monitored by measuring bubbles sizes along three dimensions during their growing process. Helium bubble grows into a shape slightly deviating from the sphere at Σ3(112)[110] twin boundary; while grows into ellipsoid at Σ5(013)[100] and Σ13(015)[100] grain boundaries. Based on a two-dimensional model, the shapes of helium bubbles are then investigated quantitatively in terms of wetting theory. The theoretical shapes of helium bubbles at the three grain boundaries are in agreement with those obtained by molecular dynamics simulation method, indicating that the growth of helium bubble at grain boundaries obeys the wetting theory.

Helium behavior and vacancy‐defect evolution in nickel‐base alloy by helium ion beam irradiation and annealing

Nickel‐base alloys have been selected as a potential metallic material for reactor internals in Generation IV reactors. In order to evaluate their helium embrittlement resistance, microstrain changes and evolution of vacancy‐type defects after Helium irradiation and postimplantation annealing have been studied by grazing incidence X‐ray diffraction (GIXRD) and slow positron doppler broadening spectroscopy (PDB). GIXRD results show that the increase in microstrain after helium irradiation is more pronounced at high doses. The observed S parameters of PDB increase with the irradiation doses, which indicates a significant number of vacancy‐type defects, possibly helium‐vacancy clusters. For postimplantation annealing specimens, the microstrain characterized by GIXRD has recovered. The surprise is the S parameters that significantly increase, revealing that helium atoms trapped at unstable helium‐vacancy clusters desorb during annealing, contributing to the growth of helium bubbles. These findings of microstrain mechanisms and vacancy‐type defects behaviors give us new insights into the helium irradiation evaluation.

On the helium bubble swelling in nano-oxide dispersion-strengthened steels

The development of structural materials resistant to harsh radiation environments requires an in-depth understanding of the early stage of the aging processes. In radiation environments with high transmutation helium production rates such as in fusion and spallation applications, even materials with otherwise acceptable radiation stability may suffer from radiation embrittlement related to helium bubble formation. While theoretical modeling of helium-assisted cavity nucleation in pure metals and simple alloys provides some useful guidelines at the atomic scale level, these, however, do not overlap with the size resolution of available experimental techniques. In this study, we employed slow positron beam spectroscopy to characterize the nucleation and growth of nano-scale helium bubbles in martensitic steels strengthened by thermodynamically stable nano-oxide dispersoids. In combination with transmission electron microscopy, we experimentally characterized the evolution of helium bubbles from small clusters of radiation-induced vacancies to large cavities well resolvable by TEM. Superior radiation resistance of oxide-dispersion strengthened steels dominates only in the early stages of bubble evolution, where positron lifetime measurements provide a missing piece of the microstructural puzzle conventionally constructed by TEM.

Helium-induced swelling and mechanical property degradation in ultrafine-grained W and W-Cu nanocomposites for fusion applications

Besides high dose radiation and extreme thermal loads, a major concern for materials deployed in novel nuclear fusion reactors is the formation and growth of helium bubbles. This work investigates the swelling and mechanical property degradation after helium implantation of ultrafine-grained W and nanocrystalline W-Cu, possible candidates for divertor and heat-sink materials in fusion reactors, respectively. It is found that ultrafine-grained W and single crystalline W experience similar volumetric swelling after helium implantation but show different blistering behavior. The W-Cu nanocomposite, however, shows a reduced swelling compared to a coarse-grained composite due to the effective annihilation of radiation-induced vacancies through interfaces. Furthermore, the helium-filled cavity structures lead to considerable softening of the composite.

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

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.

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.

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.

Positron beam studies on the irradiation response of FeCrCoNi upon He ion implantation

Positron beam-based Doppler broadening results on He ion-implanted FeCrCoNi show distinct changes in defect-sensitive S-parameter corresponding to the formation of helium-vacancy complexes, nucleation/ growth of helium bubbles, and finally the helium bubble rupture, with isochronal annealing. Data analyses of the positron beam-based ratio curves provide clear information on the kinetics of bubble growth and rupture. The difference data in S-parameter (ΔS), which corresponds to the bubble growth, displays a maximum at 1123 K, with annealing temperature. Comparing the ΔS value to that reported for nickel, it is concluded that FeCrCoNi has higher bubble growth resistance than nickel.

In-situ TEM observation of microstructure evolution in Fe9Cr1.5W0.4Si alloy during He+ irradiation and post-implantation annealing

The evolution of helium bubbles in a newly developed Fe9Cr1.5W0.4Si alloy that is an important candidate used as fuel claddings in the Lead-cooled fast reactors was in-situ investigated by transmission electron microscopy during 30 keV He+ irradiation at 500 °C and subsequently annealed at 600 °C. Meanwhile, the evolution of dislocation loops was in-situ researched in the initial stage of irradiation. Two mechanisms including the absorption of irradiation defects and the combination of dislocation loops induced the loop growth. Both He+ irradiation and subsequently annealing induced the initiation, migration, aggregation, and growth of helium bubbles, and changed their shape from spherical to preferential faceted. But it was more obvious under the annealing condition. Helium bubbles would preferentially nucleate and grow at dislocation lines and grain boundaries and had large size. The bubble lines formed at dislocation line during helium irradiation, and annealing made them more obvious and larger. A denuded zone with few helium bubbles appeared near grain boundary, but its width decreased with the increase of helium fluence and annealing time. Meanwhile, annealing would lead to the appearance of super large helium bubbles at grain boundaries. The irradiation hardening and swelling were increased with the increase of helium fluence. The corresponding mechanism was analyzed based on in-situ experimental observation.

Molecular dynamics simulation study of helium bubble growth on W/Ta semi-coherent interface

Molecular dynamics simulations were performed to investigate the nucleation and growth of helium bubble on W/Ta semi-coherent interface. It is found that misfit dislocation has a significant impact on the nucleation and growth of helium bubble. He bubble energetically prefers to nucleate at the intersections of the misfit dislocations or on some misfit dislocations. During the bubble growing process, metal interstitial atoms pushed out by the bubble are directly absorbed into nearby misfit dislocations, leading them to climb towards the Ta side; in turn, the climbed misfit dislocations induce He bubble moving away from the bimetal interface and into Ta side. Finally, the misfit dislocation effect on the bubble growth was further discussed in terms of wetting theory.

Influence of exposure conditions on helium transport and bubble growth in tungsten

Helium diffusion, clustering and bubble nucleation and growth is modelled using the finite element method. The existing model from Faney et al. (Model Simul Mater Sci Eng 22:065010, 2018; Nucl Fusion 55:013014, 2015) is implemented with FEniCS and simplified in order to greatly reduce the number of equations. A parametric study is performed to investigate the influence of exposure conditions on helium inventory, bubbles density and size. Temperature is varied from 120 K to 1200 K and the implanted flux of 100 eV He is varied from 10^{17},{text{m}^{-2}, text{s}^{-1}} to 5 times 10^{21}, {text{m}^{-2}, text{s}^{-1}}. Bubble mean size increases as a power law of time whereas the bubble density reaches a maximum. The maximum He content in bubbles was approximately 4 times 10^{8} He at 5 times 10^{21},{text{m}^{-2}, text{s}^{-1}}. After 1 h of exposure, the helium inventory varies from 5 times 10^{16} ,{text{m}^{-2}} at low flux and high temperature to 10^{25} ,{text{m}^{-2}} at high flux and low temperature. The bubbles inventory varies from 5 times 10^{12} bubbles m^{-2} to 2 times 10^{19} bubbles m^{-2}. Comparison with experimental measurements is performed. The bubble density simulated by the model is in quantitative agreement with experiments.

Study on Helium Bubble Coalescence in the Slip Plane of Titanium

Helium bubble growth and coalescence in the slip plane as well as the influence on substrate were studied using the molecular dynamics method. In the slip plane, the helium bubbles grow first along the slip plane and then grow toward the side which is short one atomic layer in the form of a hexagonal structure at low temperature. The growth rates of helium bubbles are related to the addition rate of helium atoms and their surrounding environments. After coalescence, the coalesced helium bubble grows first toward the side that is short one atomic layer. Then it grows along the slip plane with a velocity less than the growth rate before coalescence. Helium bubble growth and coalescence in the slip plane have significant influence on the substrate. During the process, the preexisting slipping metal atoms are pushed back to the normal lattice sites, and the crystal structure of the metal is recovered around the helium bubbles. The recovered area changes with the number of helium atoms in the bubble and the temperature of the substrate. The simulation results indicate that the preexisting grain boundary is beneficial for enhancing the helium damage resistance of metal.

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.

The investigation of distribution on size and concentration of helium bubbles in Y-bearing ODS steel using by SAXS and GIXRD

The nucleation and growth of the helium bubble in the structure material usually were subjected to temperature in a thermodynamic environment. In this study, Y-bearing oxide dispersed strengthened (ODS) ferritic/martensitic steels were used as the research material. The Y-bearing ODS steel samples with a thickness of 30 microns were irradiated successively by helium ions with three energies of 300 keV, 400 keV and 550 keV at room temperature and 450°C respectively. Firstly, the evolution of defects caused by H e + irradiation is analyzed by using slow positron beam Doppler broadening spectroscopy (DBS) in positron annihilation spectroscopy technology (PAS). The formation and growth of helium bubbles are deduced from the changes in the concentration of vacancy-type defects. The increase in the size of the helium bubbles at high temperatures leads to a certain distortion of the crystal lattice, which is also verified by analyzing the grazing incidence X-ray diffraction (GIXRD) patterns. The characterization of radius and concentration of helium bubbles in samples irradiated at room temperature and 450°C have been studied by Small Angle X-ray Scattering (SAXS) and Transmission Electron Microscopy (TEM). The results showed that the intensity (I) ~ q-vector (q) curves of samples vary with irradiation temperature over ranges of 0.13 ≤ q ≤ 4.6 nm −1 , which is attributed to the growth of helium bubbles at different irradiation temperature. It is found that the average size of helium bubbles under room temperature irradiation is about 1.78 nm, and the size distribution is narrow, while at the irradiation temperature of 450°C, the size of helium bubbles increases to 2.9 nm, and the size distribution is wider. This result is also verified from the TEM analysis. Besides, the shape of helium bubbles was assumed roughly and tends to spherical with higher temperatures.

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.

Molecular dynamics study on the origin of fuzz structure on tungsten surface

Molecular dynamics was used to investigate the migration of He–V cluster and the irradiation of He on W(110) surface. Our research shows that the He-V complex can still move in the microsecond scale and tend to migrate to the surface. The formation of tungsten surface fuzz comes from two synergistic factors. On the one hand, the self-interstitial dislocation loop produced during the growth of helium bubbles annihilate at the surface and form adsorbed atoms, which leads to the formation of protrusions on the surface and increases the surface area; On the other hand, the enlarged surface attracts helium bubble and accelerates the coalescence and rupture of helium bubbles near the surface, resulting in formation of holes on the surface, further roughening and enlarging the surface. With the increase of irradiation fluence, the height of protrusions continues to increase, eventually forming fuzz. This formation mechanism is considered reasonable due to it can explain many experimental phenomena.

Enabling simulations of helium bubble nucleation and growth: A strategy for interatomic potentials

Helium bubbles are a severe form of radiation damage that has been frequently observed. It would be possible to understand the complex processes that cause bubble formation if suitable interatomic potentials were available to enable molecular dynamics simulations. In this paper, Pd-H-He embedded-atom method potentials based on both Daw-Baskes and Finnis-Sinclair formalisms have been developed to enable modeling of He bubbles formed by the radioactive decay of tritium in Pd. Our potentials incorporate helium into an existing Pd-H potential while addressing two challenging paradoxes: (a) Interstitial He atoms can dramatically lower their energies by forming dimers and larger clusters in Pd but are only bound by weak van der Waals forces in the gas phase. (b) He atoms diffuse readily in Pd yet significantly distort the Pd lattice with large volume expansions. We demonstrate that both of our potentials reproduce density functional theory results for (b). However, the Daw-Baskes formalism fails to resolve paradox (a) because it cannot reproduce the experimental helium equation of state. We resolved this problem through a modification of the Finnis-Sinclair formalism in which a (fictitious) negative embedding charge density is produced by Pd at the He binding sites. In addition to molecular statics validation of static properties, molecular dynamics simulation tests establish that our Finnis-Sinclair potential leads to the nucleation of helium bubbles from an initial random distribution of helium interstitial atoms.

Study on Helium Bubble Coalescence in Titanium with Dislocations

Dislocation and grain boundary have great influence on helium behavior in materials. In this paper, the helium bubble coalescence in titanium with dislocations was simulated using molecular dynamics method. The results show that, when the second helium bubble nucleates near the slip plane, it grows toward the first helium bubble which lies at the dislocation core till they coalesce with each other. However, it is not easy for the coalescence to occur if the two helium bubbles lie in different atomic layers in (001) plane. If the second helium bubble is nucleated on the side of the slip plane with full atomic layers, the second helium bubble growth could lead to the movement of the first helium bubble toward the other sides of the slip plane. The growth rate and direction of the second helium bubble are closely related to the pressure around it.