Evolution of helium bubbles in FeCoNiCr-based high-entropy alloys containing γ′ nanoprecipitates

A series of high-entropy alloys(HEAs) containing nanoprecipitates of varying sizes is successfully prepared by a non-consuming vacuum arc melting method.In order to study the irradiation evolution of helium bubbles in the FeCoNiCrbased HE As with γ' precipitates,these samples are irradiated by 100-keV helium ions with a fluence of 5 × 10^(20) ions/m^(2) at 293 K and 673 K,respectively.And the samples irradiated at room temperature are annealed at different temperatures to examine the diffusion behavior of helium bubbles.Transmission electron microscope(TEM) is employed to characterize the structural morphology of precipitated nanoparticles and the evolution of helium bubbles.Experimental results reveal that nanosized,spherical,dispersed,coherent,and ordered L1_(2)-type Ni_(3)Ti γ' precipitations are introduced into FeCoNiCr(Ni_(3)Ti)_(0.1) HEAs by means of ageing treatments at temperatures between 1073 K and 1123 K.Under the ageing treatment conditions adopted in this work,γ' nanoparticles are precipitated in FeCoNiCr(Ni_(3)Ti)_(0.1) HE As,with average diameters of 15.80 nm,37.09 nm,and 62.50 nm,respectively.The average sizes of helium bubbles observed in samples after 673-K irradiation are 1.46 nm,1.65 nm,and 1.58 nm,respectively.The improvement in the irradiation resistance of FeCoNiCr(Ni_(3)Ti)_(0.1) HEAs is evidenced by the diminution in bubbles size.Furthermore,the FeCoNiCr(Ni_(3)Ti)_(0.1) HEAs containing γ' precipitates of 15.8 nm exhibits the minimum size and density of helium bubbles,which can be ascribed to the considerable helium trapping effects of heterogeneous coherent phase boundaries.Subsequently,annealing experiments conducted after 293-K irradiation indicate that HEAs containing precipitated phases exhibits smaller apparent activation energy(E_(a)) for helium bubbles,resulting in larger helium bubble size.This study provides guidance for improving the irradiation resistance of L1_(2)-strengthened high-entropy alloy.

Atomistic study on the microscopic mechanism of grain boundary embrittlement induced by small dense helium bubbles in iron

The helium bubbles induced by 14 MeV neutron irradiation can cause intergranular fractures in reduced activation ferritic martensitic steel,which is a candidate structural material for fusion reactors.In order to elucidate the susceptibility of different grain boundaries(GBs)to helium-induced embrittlement,the tensile fracture processes of 10 types of GBs with and without helium bubbles in body-centered cubic(bcc)iron at the relevant service temperature of 600 K were investigated via molecular dynamics methods.The results indicate that in the absence of helium bubbles,the GBs studied here can be classified into two distinct categories:brittle GBs and ductile GBs.The atomic scale analysis shows that the plastic deformation of ductile GB at high temperatures originates from complex plastic deformation mechanisms,including the Bain/Burgers path phase transition and deformation twinning,in which the Bain path phase transition is the most dominant plastic deformation mechanism.However,the presence of helium bubbles severely inhibits the plastic deformation channels of the GBs,resulting in a significant decrease in elongation at fractures.For bubble-decorated GBs,the ultimate tensile strength increases with the increase in the misorientation angle.Interestingly,the coherent twin boundary∑3{112}was found to maintain relatively high fracture strength and maximum failure strain under the influence of helium bubbles.

Interactions between a central bubble and a surrounding bubble cluster

The interaction of multiple bubbles is a complex physical problem.A simplified case of multiple bubbles is studied theoretically with a bubble located at the center of a circular bubble cluster.All bubbles in the cluster are equally spaced and own the same initial conditions as the central bubble.The unified theory for bubble dynamics[35]is applied to model the interaction between the central bubble and the circular bubble cluster.To account for the effect of the propagation time of pressure waves,the emission source of the wave is obtained by interpolating the physical information on the time axis.An underwater explosion experiment with two bubbles of different scales is used to validate the theoretical model.The effect of the bubble cluster with a variation in scale on the pulsation characteristics of the central bubble is studied.

Theoretical estimation of sonochemical yield in bubble cluster in acoustic field

In order to learn more about the physical phenomena occurring in cloud cavitation,the nonlinear dynamics of a spherical cluster of cavitation bubbles and cavitation bubbles in cluster in an acoustic field excited by a square pressure wave are numerically investigated by considering viscosity,surface tension,and the weak compressibility of the liquid.The theoretical prediction of the yield of oxidants produced inside bubbles during the strong collapse stage of cavitation bubbles is also investigated.The effects of acoustic frequency,acoustic pressure amplitude,and the number of bubbles in cluster on bubble temperature and the quantity of oxidants produced inside bubbles are analyzed.The results show that the change of acoustic frequency,acoustic pressure amplitude,and the number of bubbles in cluster have an effect not only on temperature and the quantity of oxidants inside the bubble,but also on the degradation types of pollutants,which provides a guidance in improving the sonochemical degradation of organic pollutants.

Interaction of a bubble and a bubble cluster in an ultrasonic field

Using an appropriate approximation, we have formulated the interacting equation of multi-bubble motion for a system of a single bubble and a spherical bubble cluster. The behavior of the bubbles is observed in coupled and uncoupled states. The oscillation of bubbles inside the cluster is in a coupled state. The numerical simulation demonstrates that the secondary Bjerknes force can be influenced by the number density, initial radius, distance, driving frequency, and amplitude of ultrasound. However, if a bubble approaches a bubble cluster of the same initial radii, coupled oscillation would be induced and a repulsive force is evoked, which may be the reason why the bubble cluster can exist steadily. With the increment of the number density of the bubble cluster, a secondary Bjerknes force acting on the bubbles inside the cluster decreases due to the strong suppression of the coupled bubbles. It is shown that there may be an optimal number density for a bubble cluster which can generate an optimal cavitation effect in liquid for a stable driving ultrasound.

Molecular dynamics study of helium bubble pressure in titanium

In this paper, the pressure state of the helium bubble in titanium is simulated by a molecular dynamics （MD） method. First, the possible helium/vacancy ratio is determined according to therelation between the bubble pressure and helium/vacancy ratio; then the dependences of the helium bubble pressure on the bubble radius at different temperatures are studied. It is shown that the product of the bubble pressure and the radius is approximately a constant, a result justifying the pressure-radius relation predicted by thermodynamics-based theory for gas bubble. Furthermore, a state equation of the helium bubble is established based on the MD calculations. Comparison between the results obtained by the state equation and corresponding experimental data shows that the state equation can describe reasonably the state of helium bubble and thus could be used for Monte Carlo simulations of the evolution of helium bubble in metals.

Study of acoustic bubble cluster dynamics using a lattice Boltzmann model

The search for the development of a reliable mathematical model for understanding bubble dynamics behavior is an ongoing endeavor.A long list of complex phenomena underlies the physics of this problem.In the past decades,the lattice Boltzmann method has emerged as a promising tool to address such complexities.In this regard,we have applied a 121-velocity multiphase lattice Boltzmann model to an asymmetric cluster of bubbles in an acoustic field.A problem as a benchmark is studied to check the consistency and applicability of the model.The problem of interest is to study the deformation and coalescence phenomena in bubble cluster dynamics,as well as the screening effect on an acoustic multibubble medium.It has been observed that the LB model is able to simulate the combination of the three aforementioned phenomena for a bubble cluster as a whole and for every individual bubble in the cluster.

Reconstructing bubble profiles from gas-liquid two-phase flow data using agglomerative hierarchical clustering method

The knowledge of bubble profiles in gas-liquid two-phase flows is crucial for analyzing the kinetic processes such as heat and mass transfer, and this knowledge is contained in field data obtained by surface-resolved computational fluid dynamics (CFD) simulations. To obtain this information, an efficient bubble profile reconstruction method based on an improved agglomerative hierarchical clustering (AHC) algorithm is proposed in this paper. The reconstruction method is featured by the implementations of a binary space division preprocessing, which aims to reduce the computational complexity, an adaptive linkage criterion, which guarantees the applicability of the AHC algorithm when dealing with datasets involving either non-uniform or distorted grids, and a stepwise execution strategy, which enables the separation of attached bubbles. To illustrate and verify this method, it was applied to dealing with 3 datasets, 2 of them with pre-specified spherical bubbles and the other obtained by a surface-resolved CFD simulation. Application results indicate that the proposed method is effective even when the data include some non-uniform and distortion.

In-situ TEM observation of the evolution of helium bubbles in Mo during He^(+)irradiation and post-irradiation annealing

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 5.74×10^(16)He^(+)/cm^(2)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.

Statistical analysis of helium bubbles in transmission electron microscopy images based on machine learning method

Helium bubbles,which are typical radiation microstructures observed in metals or alloys,are usually investigated using transmission electron microscopy(TEM).However,the investigation requires human inputs to locate and mark the bubbles in the acquired TEM images,rendering this task laborious and prone to error.In this paper,a machine learning method capable of automatically identifying and analyzing TEM images of helium bubbles is proposed,thereby improving the efficiency and reliability of the investigation.In the proposed technique,helium bubble clusters are first determined via the densitybased spatial clustering of applications with noise algorithm after removing the background and noise pixels.For each helium bubble cluster,the number of helium bubbles is determined based on the cluster size depending on the specific image resolution.Finally,the helium bubble clusters are analyzed using a Gaussian mixture model,yielding the location and size information on the helium bubbles.In contrast to other approaches that require training using numerous annotated images to establish an accurate classifier,the parameters used in the established model are determined using a small number of TEM images.The results of the model formulated according to the proposed approach achieved a higher F1 score validated through some helium bubble images manually marked.Furthermore,the established model can identify bubble-like objects that humans cannot facilely identify.This computationally efficient method achieves object recognition for material structure identification that may be advantageous to scientific work.

A numerical model for cloud cavitation based on bubble cluster

The cavitation cloud of different internal structures results in different collapse pressures owing to the interaction among bubbles. The internal structure of cloud cavitation is required to accurately predict collapse pressure. A cavitation model was developed through dimensional analysis and direct numerical simulation of collapse of bubble cluster. Bubble number density was included in proposed model to characterize the internal structure of bubble cloud. Implemented on flows over a projectile, the proposed model predicts a higher collapse pressure compared with Singhal model. Results indicate that the collapse pressure of detached cavitation cloud is affected by bubble number density.

Spall in Aluminium with Helium Bubbles under Laser Shock Loading

The spallation behaviors of Al＋0.2wt% ^10B targets and neutron irradiated Al＋0.2wt% ^10B targets with 5nm radius helium bubble subjected to direct laser ablation are presented. It is found that the spall strength increases significantly with the tensile strain rate, and the helium bubble or boron inclusions in aluminum reduces the spall strength of materials by 34%. However, slight difference is observed in the spall strength of unirradiated samples compared with the irradiated sample with helium bubbles.

Atomic origins of solid helium bubbles in tungsten

Solid helium bubbles were directly observed in the helium ion implanted tungsten（W）, by different transmission electron microscopy（TEM） techniques at room temperature. The diameters of these solid helium bubbles range from1 nm to 8 nm in diameter with the mean bubble size about 3 nm. The selected area electron diffraction（SAED） and fast Fourier transform（FFT） images revealed that solid helium bubbles possess body-centered cubic（bcc） structure with a lattice constant of 0.447 nm. High-angle annular dark-field scanning transmission electron microscopy（HAADF-STEM）images further confirmed the existence of helium bubble in tungsten. The present findings provide an atomic level view of the microstructure evolution of helium in the materials, and revealed the existence of solid helium bubbles in materials.

Evolution of helium bubbles in nickel-based alloy by post-implantation annealing

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 3.6×10^(17) ions/cm^(2) 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℃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.

Helium bubble formation and evolution in NiMo-Y_(2)O_(3)alloy under He ion irradiation

We report helium ion irradiation experiments for a new type of dispersion-strengthened NiMo-Y_(2)O_(3)alloy with three different irradiation doses and varying irradiation dose rates at 750℃to evaluate its helium-induced damage behavior.Transmission electron microscopy was used to reveal the evolution of helium bubbles after irradiation.The experimental results show that with increasing ion dose,the number density of helium bubbles increases continuously.However,the mean size of helium bubbles first increases and then decreases,mainly due to the varied ion dose rates.The volume fractions of helium bubbles in the three investigated samples after irradiation are 0.15%,0.32%,and 0.27%,which are lower than that of the Hastelloy N alloy(0.58%)after similar irradiation conditions.This indicates that the NiMo-Y_(2)O_(3)alloy exhibits better helium-induced-swelling resistance than the Hastelloy N alloy,highlighting its potential applicability to MSRs,from the perspective of irradiation performance.

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.

Cross－sectional TEM Study of Helium Bubble Nucleation in 316L SS Implanted at 400℃ and 550℃

Ｃｒｏｓｓ－ｓｅｃｔｉｏｎａｌ ＴＥＭ Ｓｔｕｄｙ ｏｆ Ｈｅｌｉｕｍ Ｂｕｂｂｌｅ Ｎｕｃｌｅａｔｉｏｎ ｉｎ ３１６Ｌ ＳＳ Ｉｍｐｌａｎｔｅｄ ａｔ ４００℃ ａｎｄ ５５０℃￥ＺｈａｎｇＣｈｏｎｇｈｏｎｇ；ＣｈｅｎＫｅｑｉｎ；ＷａｎｇＹｉｎｓ...

Effect of Dislocations on Helium Bubble Nucleation in 316L SS During Hot－implantation at 550℃

Ｅｆｆｅｃｔ ｏｆ Ｄｉｓｌｏｃａｔｉｏｎｓ ｏｎ Ｈｅｌｉｕｍ Ｂｕｂｂｌｅ Ｎｕｃｌｅａｔｉｏｎ ｉｎ ３１６Ｌ ＳＳ Ｄｕｒｉｎｇ Ｈｏｔ－ｉｍｐｌａｎｔａｔｉｏｎ ａｔ ５５０℃￥ＺｈａｎｇＣｈｏｎｇｈｏｎｇ；ＣｈｅｎＫｅｑｉｎ；ＷａｎｇＹｉ...

TEM Characterization of Helium Bubbles in T91 and MNHS Steels Implanted with 200 keV He Ions at Different Temperatures

Modified novel high silicon steel （MNHS, a newly developed reduced-activation martensitic alloy） and commercial alloy Tgl are implanted with 200 keV He2＋ ions to a dose of 5 × 1020 ions/m2 at 300, 450 and 560~C. Transmission electron microscopy （TEM） is used to characterize the size and morphology of He bubbles. With the increase of the implantation temperature, TEM observations indicate that bubbles increase in size and the proportion of ＇brick shaped＇ cuboid bubbles increases while the proportion of polyhedral bubbles decreases in both the steel samples. For the samples implanted at the same temperature, the average size of He bubbles in MNHS is smaller than that in T91. This might be due to the abundance of boundaries and precipitates in MNHS, which provide additional sites for the trapping of He atoms, thus reduce the susceptibility of MNHS to He embrittlement.

Numerical study of the acoustic spectrum of bubble clusters

This study delved into the acoustic spectrum of bubble clusters,each consisting of 352 vapor bubbles across volume fractions ranging from 0.005%to 40%.The clusters,organized in five distinct layers,were modeled using the volume of fluid(VOF)method to capture the bubble interfaces,and the Ffowcs Williams-Hawkings(FW-H)methodology to compute the far-field acoustic pressure from bubble collapse.Further analysis revealed distinct sound pressure behaviors across different volume fractions:For 25%–40%,time-domain analysis shows that the peak acoustic pressure pulses from the two innermost layers of bubbles are significantly higher than those from the outer layers.In the frequency domain,the octave decay rate of the acoustic pressure levels is relatively low,around−3dB/octave.For 0.5%–25%,four acoustic pressure pulses with similar widths and peak values were observed in the time domain.In the frequency domain,there are three distinct peaks in sound pressure levels(SPL),directly linked to the difference in collapse times of bubbles within the cluster,and the octave decay rate accelerates as the volume fraction decreases,stabilizing at−6dB/octave when the volume fraction is reduced to 17.5%.For 0.005%–0.5%,as the volume fraction decreases from 0.5%to 0.1%,the number of acoustic pressure pulses significantly reduces.Below 0.1%volume fraction,only a single wider pulse is observed.In the frequency domain,the octave decay rate gradually increases with decreasing volume fraction,significantly exceeding−10dB/octave when it drops below 0.1%,reaching up to−11.7dB/octave.