Helium Embrittlement Research Articles

Tensile and compressive response of tungsten g-TPMS lattice structures

The present study explores the mechanical behavior of tungsten (W) gyroid Triply Periodic Minimal Surface (g-TPMS) lattice structures through atomistic simulations, focusing on models with varying lattice thickness or simply volume fractions. It examines the mechanical behavior of these structures under both tensile and compressive loads. The research demonstrates that strength and ductility in g-TPMS can be adjusted through design, showcasing unique behaviors not found in bulk materials. A relationship between elastic modulus and volume fraction is established, providing coefficients for multi-scale modeling and simulation. W g-TPMS structures with higher volume fractions exhibit increased tensile strength, peaking at approximately 7.6 GPa for structures with a 59 % volume fraction. The study further reveals that temperature variations significantly affect the mechanical response, with tensile strength decreasing from 4.5 GPa at 300 K to below 2.0 GPa at 3000 K. Additionally, helium embrittlement is shown to markedly reduce tensile strength, with a 15 % helium interstitial concentration leading to a 60 % reduction in peak stress. These findings emphasize the potential of tailored W g-TPMS materials for advanced applications like nuclear fusion reactors.

Modeling the helium transport and premature intergranular fracture by dislocation motion in irradiated metals

Helium migration is an important mechanism of helium embrittlement in irradiated metallic materials, which can severely affect their service reliability. Despite many efforts to reveal the mechanism of helium migration during plastic deformation, a mechanistic understanding of helium transport through dislocation motion is still lacking. In this work, we developed a theoretical model within the coupled framework of crystal plasticity and helium diffusion, to account for the helium transport due to the bidirectional dislocation motion. Our simulation results show that, for austenitic stainless steel, such motion has a significant impact on helium migration, leading to an enriched helium concentration on the grain boundaries (GBs), and thus resulting in a higher risk of intergranular fracture. Besides, our results also indicate that temperature and irradiation defect affect the helium concentration on the GBs by regulating the intragranular helium distribution. The present study reveals the key role of dislocation motion in regulating helium migration, a firm step toward a more comprehensive understanding over the failure mechanisms of irradiated metallic material.

A DFT + U study on diffusion and aggregation behavior of He atoms in Li2TiO3

Future fusion power plants are designed based on the deuterium-tritium fusion reactions. Yet, as an element with a half-life of 12.5 years, tritium cannot be obtained in large quantities from nature. To meet future demand, an artificial nuclear reaction is needed to use tritium breeder materials to produce tritium. At present, Li2TiO3 is the most promising tritium breeder blanket material in fusion reactor. The existing experimental results show that helium irradiation will change the crystal structure of Li2TiO3 and cause embrittlement failure. In order to study the microscopic helium embrittlement mechanism of Li2TiO3, the diffusion and aggregation behavior of helium atoms in Li2TiO3 bulk was calculated by first principles. The reasons for the formation of the most stable interstitial configuration of helium atoms and helium clusters in Li2TiO3 bulk are explained by lattice vibrational theory and electronic structure analysis. At the same time, the relatively new research results in this work are mainly as follows: In Li2TiO3 bulk, the three spatial inversion symmetries Li vacancy helium atoms tend to diffuse from all Li vacancies to Li2 vacancies. Meanwhile, the Li2 vacancy is the most likely site for helium clusters to nucleate. And helium atoms and the helium clusters tend to arrange and grow along the plane of the Li layer.

Overview on performance degradation behavior of 20Cr25NiNb steel for gas cooled reactor cladding during service

In this paper, the performance degradation behavior of fuel cladding material 20Cr25NiNb for the British Advanced Gas Reactor (AGR) is reviewed in detail, which is a strong guideline for the material selection of supercritical carbon dioxide cooled reactors. The degradation behavior during in-core service mainly includes high-temperature creep, thermal aging and mechanical property degradation caused by neutron irradiation (fission gas products, helium embrittlement and irradiation sensitization) and CO2 (oxidation and carburizing). Long-term service in AGR leads to coarsening of the second phase and precipitation of harmful phases such as σ, leading to performance degradation of the cladding. A point that should require special attention is that intergranular stress corrosion cracking (IGSCC) and intergranular attack (IGA) problems occur during wet storage of spent fuel.

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.

Study of Helium Swelling and Embrittlement Mechanisms in SiC Ceramics

This work is devoted to the study of the radiation damage kinetics and subsequent embrittlement of the near-surface layer of SiC ceramics subjected to irradiation with low-energy He2+ ions. Interest in these types of ceramics is due to their great prospects for use as structural materials for nuclear power, as well as for use in the creation of protective structures for long-term storage of spent nuclear fuel. During the study, the dependences of changes in the structural, mechanical, strength, and morphological characteristics of SiC ceramics depending on irradiation fluence were obtained. It has been established that the greatest changes in the strength properties are associated with the dominance of the crystal lattice swelling effect in the structure due to an increase in the concentration of implanted helium, and its further agglomeration with the formation of vacancy complexes of the He-V type. A model for changing the structural properties of ceramics irradiated with low-energy He2+ ions based on the change in the contributions of the dislocation density concentration, anisotropic distortion of the crystal lattice, and the effect of swelling as a result of implantation is proposed.

MXene‐Sponge Based High‐Performance Piezoresistive Sensor for Wearable Biomonitoring and Real‐Time Tactile Sensing (Small Methods 2/2022)

Flexible Piezoresistive Pressure Sensor In article number 2101051, Ye, Xie, Su, Chen and co-workers were inspired by Helium embrittlement during nuclear fusion processes, to utilize Helium plasma irradiation to yield a nano-fuzz-shaped molybdenum electrode for a flexible piezoresistive pressure sensor, which was demonstrated to measure human radial pulse, detect the tapping, stamping, and perform object identification, revealing great feasibility in wearable biomonitoring and healthcare assessment.

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.

Synergistic and separate effects of plasma and transient heat loads on the microstructure and physical properties of ITER-grade tungsten

Once ITER commences full power operation, the ITER divertor will be exposed to high thermal and particle loads. Tungsten was chosen as plasma facing material for the ITER divertor. It is, therefore, of the utmost importance to understand the behavior of ITER-grade tungsten under conditions similar to those it will have to withstand inside the reactor. In this study, ITER-grade tungsten samples were exposed to stationary D/He(6%) plasma and ELM-like transient heat loads in the linear plasma device PSI-2. The effects of each kind of load was first studied separately, and the synergistic effects obtained when exposed to both loads simultaneously were then analyzed. Additionally, the hardness of a recrystallized tungsten sample after exposure to simultaneous loads was tested via nanoindentation. The results indicate that hydrogen and helium embrittlement worsens the cracking behavior of the material when exposed to the simultaneous loads compared to only heat loads. Additionally, bubbles of up to 1 μm are formed under the surface due to the synergistic effects at the highest heat load. The nanoindentation tests showed that plasma and heat loads increase the hardness of the material by 39%, but only plasma loads appeared to have no effect on it.

Atomistic simulations to study the effects of helium bubbles on crack tip behavior in single crystal Ni

Safety of nuclear reactors is an issue that constantly hovers in the mind of the designers. Helium embrittlement in nickel-based alloys poses challenges hitherto unfathomed in the nuclear industry. Experimental and theoretical studies have revealed that helium bubbles have a detrimental effect on mechanical properties of nickel. However, the exact mechanism governing helium-induced embrittlement has not been properly explored. In the present work, the effect of various configurations of helium bubbles on the mechanical properties of nickel crystal has been investigated using atomistic simulations. The effect of various orientations of the crack plane and direction in the face centered cubic crystal of Ni was studied in conjunction with helium bubbles. In the three orientations of the crack, deformation was governed by Shockley and stair rod type dislocation in two of the orientations, while twinning and Lomer Cottrell locks govern the deformation in the third orientation. Despite the fact that helium bubbles reduce the strength of a single crystal of Ni containing a centrally embedded crack, an increase in the number of helium atoms has a negligible effect of the toughness of the material. Switching between the spatial coordinates of a helium bubble in front of the crack tip significantly affects the strength of the single crystal Ni. The results indicate that consideration of the geometry of the crystal along with modulation of helium clusters is an effective means to understand various defects in Ni crystals.

Investigation of irradiated metal of WWER-type reactor internals after 45 years of operation. Part 4. Mechanical properties and fracture mechanisms

Experimental data are obtained for austenitic chromium-nickel steel of 18Cr–10Ni–Ti grade(analog of AISI 321 steel) irradiated with mixed neutron spectrum to various damage doses at temperature near 300 o C. Metal for investigations has been taken from decommissioned components of internals of WWER type reactor. On the basis of the tests of standard cylindrical specimens the strengthand the plasticity characteristics are determined over wide temperature range. The fracture mechanisms are studied by SEM examination. It is revealed that the fracture strain decreases with decrease of temperature and the transition occurs from transgranular ductile to intergranular brittle fracture. The performed review of other published works has shown that this phenomenonmay be observed for various austenitic steels irradiated with mixed neutron spectrum and for austenitic and ferritic steels with implanted helium. This phenomenon is termed as low temperature helium embrittlement (LTHE).

Effect of multiple Ti doping on helium behavior in ZrCo

Helium can cause segregation of solute atoms in the material, and in turn, the segregated solute atoms will have a great influence on the behavior of helium. Studying the behavior of helium in tritium storage materials is very important for the design of materials with strong helium retention capacity. In this paper, the first-principles method based on density functional theory is used to study the segregation of doped atoms Ti caused by defects (He atoms) in ZrCo system, and the effect of segregated Ti atoms on He behavior. The effect of segregated Ti atoms on the stability of He atoms was explored. The CI-NEB method was used to simulate the variance of the trapping for He atoms by vacancy in the pure and doped systems. Finally, it was found that the number of He atoms trapped by the vacancy is reduced due to the Ti cage. In addition, the Ti cage has a certain pinning effect on He atoms. In short, the segregated Ti inhibits the migration of He and hinders the flow of He to the grain boundaries, thereby suppressing the helium embrittlement effect.

Ab initio study of the effects of helium on the mechanical properties of different erbium hydrides

Although rare-earth metals have increasingly received attention for use in the storage and transportation of the tritium used in nuclear fusion reactions, they still face great challenges, such as the effect of helium on the mechanical properties of different erbium hydrides. In this work, first principles are used to study the mechanical properties (elastic constants, Young’s modulus, transverse shear modulus and bulk modulus) of different erbium hydrides exposed to helium. The Young’s modulus, the transverse shear modulus and the bulk modulus are given based on the elastic constants calculated according to first principles. It is found that the mechanical properties of all three erbium hydrides decrease in the presence of helium, and the decline of the mechanical properties of ErH3 is the most serious. To explain the reason for the decrease in the mechanical properties, the densities of the states of erbium hydrides are calculated. During the calculations, helium embrittlement is not found and the ductility of the erbium hydrides improves following the production of helium at the helium concentrations considered in this work.

A Slow Positron Beam Study of Helium Clustering in Oxide Dispersion Strengthened Steels

The present work provides an innovative approach to the experimental study of the radiation tolerance of oxide dispersion strengthened (ODS) steel comparing to its non-ODS variant. Inhomogeneous radiation damage, introduced by helium irradiation, was probed in a depth profile manner by a variable energy positron beam. Although numerous studies reporting the use of slow positron beam (SPB) experiments on ion-implanted materials have been published in the literature, only a few of them address the continuous distribution of microstructural damage introduced by ion beam. In this paper, we use SPB to probe a wide range of radiation damage in a single sample of each material. By evaluating positron diffusion in different parts of the Bragg peak, we were able to track the evolution of radiation-induced damage from a high density of small defects such as helium-vacancy clusters to large bubbles grown by coalescence. Combining with the transmission electron microscopy results and slow positron Doppler broadening spectroscopy, we offer new insight into different stages of microstructural bubble evolution in steels exposed to harsh radiation conditions. The obtained results show that the higher density of vacancy-type defects in the oxide dispersion strengthened steel, which act as recombination centres, suppress the formation of large defect agglomerations. While the TEM provides an indication of this effect in high displacement damage and helium production rates, positron annihilation spectroscopy elucidates the origin of this behaviour, improving the understanding of the fundamental mechanism of helium embrittlement.

Mechanisms of plastic deformation and fracture of austenitic chromium-nickel steel irradiated during 45 years in WWER-440

The test results of tensile round bars from austenitic stainless steel of 18Cr-10Ni-Ti grade (Russian analog of AISI 321 steel) irradiated during 45 years in Pressure Water Reactor of WWER-440 type with various damage doses are represented over wide temperature range. The standard mechanical properties including the fracture stress and the fracture strain are determined, and the fracture mechanisms are studied. A phenomenon of decrease in the fracture strain with decrease of temperature is revealed that is accompanied by transition to intergranular brittle fracture. This phenomenon is termed as low temperature helium embrittlement (LTHE). On the basis of the analysis of other published works it has been concluded that LTHE is sufficiently common phenomenon that is revealed for various austenitic steels irradiated with mixed neutron spectrum.It is shown that the strain-induced martensite and low cohesive strength of grain boundary (GB) due to the helium accumulation on GBs are two necessary factors which cause LTHE of the irradiated austenitic steels. Mechanism of microcrack nucleation on weakened GB is proposed that takes into account the morphology of the strain-induced martensite formed near GB.Some features of plastic deformation over the temperature range of LTHE are revealed and explained as follows: (i) a running neck and large yield drop; and (ii) a difference in the true stress-strain curves for the cross-section with the originally arisen neck and for other cross-sections of tensile round bar over initial plastic strain range.

Vacancy trapping meachanism of multiple hydrogen/helium atoms in fcc-Fe: A first-principles study

Using first-principles calculations, we studied the dissolution and diffusion behaviors of H/He atom in perfect fcc-Fe and vacancy capture multiple H/He atoms meachanism in fcc-Fe. The calculation results show that H/He atom prefers to occupy octahedral site, and they diffused along octahedral site-tetrahedral site-octahedral site in perfect fcc-Fe. Inside vacancy, H atom prefers to occupy octahedral site, while He atom prefers to occupy the vacancy center. The exist of H/He atom would decrease the vacancy formation energy. According to capture energies, a vacancy can trap five H atoms in fcc-Fe, but more He atoms can be accommodated even when the number of He atoms reaches sixteen. Therefore, the result provides an important theoretical basis to research the mechanism of hydrogen and helium embrittlement.

Release of helium-related clusters through a nickel–graphene interface: An atomistic study

Nickel–graphene nanolayers with high-density interfaces are expected to have excellent resistance to helium (He) embrittlement and proposed as candidate materials for molten salt reactor systems. However, He irradiation effects on nickel–graphene nanolayers remains poorly understood at present. In this work, the influence of a nickel–graphene interface (NGI) on the nucleation and growth of He-related clusters was studied by using atomistic simulations. The NGI reduces formation energies and diffusion energy barriers for He-related clusters. The reduction makes He-related clusters easily be trapped by the interface, thus leading to significant segregation. Consequently, He concentration in the bulk is considerably reduced, and the nucleation and growth rates of He-related clusters in the bulk are delayed. Owing to the high mobility of He-related clusters at the NGI, these clusters easily coalesce to form larger clusters than those in the bulk. A reasonable design of nanolayers may promote He releasing from materials. Results of the current study can provide fundamental support for the service life assessment of nickel–graphene nanolayers in extreme environments.

Interaction between recrystallization and helium behavior in cold-rolled nickel

Introducing a high density of defect sinks is a promising approach to relieve helium embrittlement in structural alloys for future fusion energy applications. However, the interaction between preset defect evolution and helium behavior at elevated temperatures, which could lead to dramatic changes of microstructure and properties of alloys, is still not well understood. In this contribution, cold-rolled nickel subjected to He+ irradiation at 500 °C was investigated to uncover the interaction process and the consequent microstructure evolution. Our experimental results revealed that during the initial period partial recrystallization and grain growth were unaffected by He+ irradiation, leading to the formation of coarse grains. With the increasing of helium concentration, helium bubbles retarded dislocation rearrangement and grain boundary migration in the remaining recrystallization areas, and meanwhile the stabilized grain boundaries facilitated helium bubble coarsening and bubble-to-void transformation. Such interaction ultimately led to the retention of fine grains with severe swelling and hardening.

Helium bubble nucleation at grain boundaries and its influence on intergranular fracture

ABSTRACTAn in-depth understanding of the formation of intergranular helium bubbles and its relation to embrittlement is an important issue in the nuclear industry. In this paper, a thermodynamic model is developed to analyze the nucleation of intergranular helium bubbles. Microstructural observation using scanning electron microscopy and electron backscatter diffraction gives a detailed description for the relation between the bubble formation and the grain-boundary (GB) misorientation in helium-implanted nickel and Inconel X750. The theoretical and the experimental results confirm that the nucleation of intergranular helium bubbles is GB structure-dependent, the helium-to-vacancy ratio plays an important role in the bubble precipitation, and the interfacial tension of bubbles cannot be approximated to be the interfacial energy. The bubble-induced intergranular embrittlement in a polycrystal is modelled. The GB misorientation distribution, the intergranular bubble nucleation and growth and the GB connectivity are the key factors affecting the GB fracture toughness. The hoop ductility of the cladding tubes containing helium is analyzed. The hoop stress-induced increase in the GB energy promotes the precipitation of bubbles at the radial GBs and lead to the loss of tube ductility. Based on this work, the complicated correlation among the intergranular helium bubbles, the GB structure, the helium concentration, the applied stress and the helium embrittlement is clarified.

Cracking behavior of helium-irradiated small-volume copper

Radiation-induced grain-boundary helium bubbles are the cause of the significant helium embrittlement. However, the influence of grain-interior helium bubbles on the cracking behavior of metals has not been explored. Here, we investigate the cracking propensity of helium-irradiated small-volume Cu single-crystals, and compare with those of Cu single-crystals and Cu bicrystals. We found that the irradiation-induced helium bubbles only slightly increased the cracking propensity of the single-crystal Cu, while the cracking resistance of the helium-irradiated Cu single-crystals was superior to that of the Cu bicrystals, both with and without helium bubbles.