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Investigation of microstructures and hardening property of initial irradiated, post-irradiation annealed, and re-irradiated Chinese-type low-Cu (0.01 wt%) reactor pressure vessel (RPV) steel were conducted by slow positron beam (SPB), TEM, and nanoindentation. Results of the SPB measurements indicated that a large number density of open volume defects such as vacancies, vacancy clusters, vacancy-solute/H complexes, and dislocation loops were introduced in both initial-irradiated and re-irradiated specimens. The TEM results indicated that interstitial-type dislocation loops with a number density of ∼1022 m−3 were formed and the mean size (∼3 nm) of the dislocation loops was almost unchanged in both initial-irradiated and re-irradiated specimens. The open volume defects such as vacancy-type defects and dislocation loops were almost annealed out and some stable defects still existed in the post-irradiation annealed specimen. The nanoindentation results identified that the obvious hardening phenomena were found in the initial irradiated, post-irradiation annealed and re-irradiated specimens. The quantitative analysis suggested that irradiation hardening of Chinese-type low-Cu RPV steel were attributed to matrix defects including large-size vacancy clusters, vacancy-solute/H complexes, and dislocation loops produced during initial irradiation and re-irradiation. On the other hand, the existed stable defects such as solute-atom (Mn, Ni, and Si) complexes/precipitates, which cannot be distinguished by SPB or TEM, might be responsible for the rest fraction of irradiation hardening of highly initial irradiated and re-irradiated Chinese-type low-Cu RPV steel.

Owing to its excellent properties, high-entropy alloys (HEAs) have been regarded as a kind of promising candidate structural material for nuclear devices. For the safety concern, it is vital to figure out the compatibility of hydrogen isotopes with the HEAs. In this work, the hydrogen isotope permeation and retention behavior of a representative HEA, CoCrFeMnNi, has been mainly checked by a gas-driven permeation device and a thermal desorption spectra device. The hydrogen isotope permeation parameters have been obtained, which are quite similar to those of the 316L austenitic steels. The hydrogen isotope retention behavior has been compared with that for a reduced activation ferritic/martensitic steel, and the relatively high hydrogen isotope inventory in HEA has been explained by crystalline structure and high density of hydrogen trapping site.

To provide a basic understanding of the synergy effect of hydrogen and helium on structural materials for future advanced nuclear systems, the vacancy-type defects in SIMP steel induced by separate and sequential H and He implantation at room temperature was investigated using positron annihilation Doppler broadening spectroscopy (DBS) and transmission electron microscopy (TEM). The DBS results indicated that, when implanted by H and He separately, the ΔS parameter of the H-implanted sample was greater than that of He, although the damage level (that is, the displacement per atom) induced by H was lower than that induced by He. This could indicate that He is more easily trapped by vacancy than H and prefers to occupy the center of the vacancy. When sequentially co-implanted by He and H, whatever the implantation sequence, the ΔS parameter was lower than that of H-only implantation. In addition, the ΔS parameter of sequential He + H implantation was greater than that of H + He implantation when the dose of H was sufficiently high. Combined with the TEM results, the synergistic effects of He and H on vacancy-type defect evolution are discussed.

To simulate the uniform depth-distribution of damages introduced by neutrons, He-ions irradiations with different energies were respectively (W-1@16 keV, W-2@70 keV, W-3@200 keV) or successively (W-4@200-70-16 keV) conducted on the recrystallized tungsten. The damage defect distribution and helium behavior in tungsten were investigated by Positron Annihilation Spectroscopy (PAS). Results show that a sufficient number of small-size vacancy clusters containing low ratio of He atoms are produced in tungsten during a single-energy He-ions irradiation with a fluence below 1021 ions m−2 at room temperature (RT). But more irradiation dose will promote the saturation of the interstitial helium around the positron annihilation sites and accelerate the formation of helium-vacancy clusters. The fitted S profile of W-4 presents a platform in the depth range of 27–477 nm. Meanwhile, S-W line of it deviates from the line of W-1, 2 to W-3, and the (S, W) data form clusters in the depth range of 57–230 nm. These changes may be explained by a large quantity captures of injected He ions in defect sites and the formation of large-size H e n V m clusters with continuously increasing n/m ratio. Fortunately, successive He-ions irradiation with various energies can, to some extent, achieve a quasi-uniform damage distribution in a certain depth range in metals, which meets the expectation in this paper.

This study aims to investigate the effects of temperature and dose on the vacancy-defect evolution in 304L stainless steel irradiated by the triple beam of Ni, H, and He ions below 673 K, which is relevant to water-cooled nuclear energy facilities. The irradiation experiments were performed simultaneously using 150-MeV Ni ions whose energy was degraded by passing through a tantalum foil of 8.5-μm thick, 100-keV H ions, and 200-keV He ions at 473, 523, 573, 623, and 673 K, respectively. The irradiation-induced microstructures in the steel were probed by the slow positron beam technique. The results show that the S parameter of the irradiated sample decreases as the temperature increases, and the S parameter of the high-dose irradiated sample decreases further. We explained the results by analyzing the structure of the defects in the irradiated samples at various temperatures and the effects of the annealing effect. In addition, the analysis of the S W plots reveals the transition of the helium-vacancy cluster to the overpressured HemVn (m > n) cluster under the influence of temperature and irradiation dose.

In recent years, high entropy alloys (HEAs) have attracted significant attention due to their excellent physical, chemical, mechanical properties, and good irradiation resistance, thus are considered as potential candidates for fission and fusion structural applications. CoCrFeMnNi HEA was irradiated by high intensity pulsed ion beam (HIPIB) to investigate the effects of thermal shock irradiation on their microstructure, surface morphology and mechanical properties. It was found that CoCrFeMnNi HEA maintained the face-centered cubic single-phase structure after HIPIB irradiation. The ion beam effect of irradiation produced numerous defects such as vacancies and stacking faults within the range of carbon ions. While the thermal effect reduced vacancy concentration beyond the ion range by promoting the recombination of vacancies with interstitials, and decreased the nano-hardness of CoCrFeMnNi HEA. The thermal effect and shock wave effect promoted the migration of vacancies and formed defects such as stacking faults etc. far beyond the ion range in CoCrFeMnNi HEA. Because of the very compositional complexity, the high-level chemical disorder and local lattice distortion of CoCrFeMnNi HEA, its lattice parameter was almost unchanged after HIPIB irradiation. Even if the temperature reached the melting point of CoCrFeMnNi HEA, there was no crack on the surface after surface remelting and rapid cooling. CoCrFeMnNi HEA showed good thermal stability and thermal shock irradiation resistance.

The irradiation resistance of CLF-1 steel was investigated by using He ion irradiation with an energy of 300 keV. After the irradiation, the structure of CLF-1 steel mainly remained martensitic structure, while the lattice distortion and grain refinement occurred in the irradiation damaged layer. As irradiation fluence increased, the number of vacancies in CLF-1 steel increased, and the vacancies combined with the He atoms to form a bubble layer with the thickness of about 300 nm which located at the end of the He ions range. In the bubble layer, local amorphous region formed and the carbide particles showed a tendency to decompose. When the irradiation fluence was up to 1 × 1018ions/cm2, extensive blisters and a small amount of peelings appeared on the surface of CLF-1 steel. Due to grain refinement and dispersion of small carbide particles, the hardness of CLF-1 steel increased after the irradiation.

In this work, we utilized spherical nanoindentation to investigate the mechanical property changes of helium (He) implanted tungsten (W) at different damage doses. According to the indentation stress-strain curve, two dose dependent phenomena were revealed: one is an increase in yield stress with increasing damage dose, and the other is enhanced softening following yield. Experimental measurements such as transmission electron microscopy (TEM) and positron annihilation spectroscopy (PAS) techniques, as well as molecular dynamics (MD) simulation were combined to explore the intrinsic mechanism of the dose dependent phenomena. This study proposed an efficient and promising way to measure the damage of the plasma facing materials and established an effective correlation between the mechanical property changes of He implanted W and its microstructure evolutions.

The corrosion behavior of Zr-4 and N5 alloy specimens corroded in 0.01 mol/L LiOH aqueous solution at 360 °C/18.6 MPa and in super heated steam at 400 °C/10.3 MPa for 1, 3 and 14 days were investigated by slow positron beam based Doppler broadening spectroscopy. Results showed that there was an evident interfacial layer with pre-existed vacancies and voids in uncorroded Zr-4 specimens, while in uncorroded N5 specimen, the interfacial defect layer can not be identified or a thin interfacial layer was only contained. When the specimens were corroded in super heated steam at 400 °C/10.3 MPa for a few days, the existence of the interface layer in the Zr-4 specimen would delay the diffusion rate of the oxygen atoms and decelerated the oxidation rate of the corrosion process. However, at very early stage of the corrosion, as Zr-4 and N5 specimens were corrded in 0.01 mol/L LiOH aqueous solution at 360 °C/18.6 MPa, the effect of Li+ accelerated the diffusion rate of the oxygen atoms, while the effect of the interface defect layer became a minor effect.

Surface blistering and deuterium (D) retention of heavy ion pre-irradiated (1 dpa) tungsten (W) exposed to low-energy (40 eV) and high-flux (1–2 × 1022 D/m2s) D plasma has been investigated with low fluence of 0.1 × 1027 D/m2 and a high fluence of 2.2 × 1027 D/m2. Surface morphology observations show that a large number of blisters are formed on the undamaged W after low-fluence exposure while the area density of the blister will have significantly increased and blister bursting will be triggered in the case of high-fluence exposure. In contrast, the heavy ion pre-irradiation noticeably reduces the area density of blisters for both low- and high-fluence exposures. The thermal desorption spectroscopy (TDS) shows that the total D retention in the pre-damaged W is greater than that of the undamaged sample, and this trend is more significant with increasing fluence/duration of D plasma exposure. The results of positron annihilation Doppler broadening spectrometry (PA-DBS) and TDS indicate that a large number of vacancy-type defects, especially those with a higher trapping energy of D, are induced by the heavy ion pre-irradiation. The increasing defects/D-trap sites may result in two outcomes, the D concentration will disperse at each blister nucleation site and D inward diffusion is enhanced and, therefore, lead to the mitigation of D-induced blistering and increase the D retention. In addition, combined with the enhanced D inward diffusion caused by increasing exposure duration, the D retention is therefore much higher in the pre-damaged W in the case of high-fluence exposure.

The formation of helium bubble precursors, i.e., helium-vacancy complexes, was investigated for Fe9Cr alloy, which was uniformly irradiated by using 100 keV helium ions with fluences up to 5x10(16) ions/cm(2) at RT, 523, 623, 723, and 873 K. Helium-irradiation-induced microstructures in the alloy were probed by positron annihilation technique. The results show that the ratio of helium atom to vacancy (m/n) in the irradiation induced HemVn clusters is affected by the irradiation temperature. Irradiated at room temperature, there is a coexistence of large amounts of HemV1 and mono-vacancies in the sample. However, the overpressured HemVn (m > n) clusters or helium bubbles are easily formed by the helium-filled vacancy clusters (HemV1 and HemVn (m approximate to n)) absorbing helium atoms when irradiated at 523 K and 823 K. The results also show that void swelling of the alloy is the largest under 723 K irradiation. (C) 2018 Elsevier B.V. All rights reserved.

The Fe1.0Cu alloy was irradiated by 2 MeV Fe ions to a peak dose of 0.06dpa, 0.45dpa and 1.35dpa, respectively. The irradiation evolution of Cu precipitates in Fe1.0Cu alloy was studied at various depths by positron annihilation spectroscopy. The values of the S parameters directly indicate the formation of vacancy clusters. The S-W relationship showed that CunVm complexes (n > m) dominated the microstructure in the track region. Vacancy clusters and CunVm complexes (n

Transmutation helium may causes serious embrittlement which is considered to be due to helium from clustering as a bubble in materials. Suppression of transmutation helium can be achieved by introducing trapping sites such as dislocations and impurities in materials. Here, effects of intentionally-induced dislocations and hydrogen on helium migrate and release behaviors were investigated using thermal desorption spectrometry (TDS) technique applied to well-annealed and cold-worked Fe9Cr alloys irradiated by energetic helium/hydrogen ions. Synchronous desorption of helium and hydrogen was observed, and the microstructure states during helium release at different temperatures were analyzed. High thermally stable He n D type complexes formed in cold-worked specimens, resulting in the retardation of helium migration and release. The existence of hydrogen will strongly affect the thermal helium desorption which could be reflected in the TDS spectrum. It was confirmed that hydrogen retained in the specimens can result in obvious delay of helium desorption.

To investigate the influence of transmutation rhenium on the irradiation-induced defects in tungsten, H+ and He+ irradiation of 50 keV with a fixed fluence of 1 × 1016 atoms·cm−2 were conducted on WxRe (x =0, 3, 5 and 25 wt.%) alloys, respectively. Doppler Broadening Spectroscopy and Coincidence Doppler Broadening Spectroscopy both based on slow positron beam were employed in the current work to characterize the depth distribution and the chemical surroundings of the irradiation-induced defects. The results showed that under 723K irradiation, Re addition suppressed the accumulation of vacancy-type defects compared with pure W. This suppression was enhanced by increasing Re content in W. An obvious Re-related peak from (7-28) × 10−3 m0c was observed in CDB spectra of well-annealed W-Re alloys. The irradiation effect led to the height deviation of Re-related peak. The decrease of the Re-related peak could be attributed to the formation of irradiation-induced defects in irradiated samples, and the reduction of positron annihilation fraction with core electrons of Re.

The spatial relationship between helium bubbles and dislocation loops was observed in detail by TEM in RAFM steel irradiated with 100 keV helium ion to a fluence of 1 × 1021 He+ • m−2 at 723 K. Two families of dislocation loops were observed, with b = ± and b = ± , and habit planes of (010) and (100), respectively. The striking observation is that all of the loops were decorated with helium bubbles. Meanwhile, bubbles are determined to be located inside the loop plane through the investigation using different g-vectors under zone axes of [111], [001] and [011].

The effects of dislocations created by deformation on helium retention in pure iron, including the helium atoms diffusion along the dislocation line and desorption from dislocation trapping sites, were investigated. The dislocation defect was introduced in specimens by cold-rolling, and then 5 keV helium ions were implanted into the deformed specimens. Slow positron beam technology and thermal desorption spectroscopy were used to investigate the evolution of dislocation defects and the desorption behavior of helium atoms under influence of dislocation. The behaviors of S-E, W-E and S-W plots indicate clearly that lots of helium atoms remain in the deformed specimen and helium atoms combining with dislocation change the distribution of electron density. The helium desorption plot indicates that dislocation accelerates helium desorption at 293 K-600 K and facilitates helium dissociation from He n V m ( n / m = 1.8) cluster.

Understanding the hydrogen isotope (H) permeation behavior in neutron irradiated tungsten (W) is highly desired for the accurate prediction of H behavior in W first walls in a fusion DEMO reactor. Contrary studies have been published where some indicated a relatively higher H permeability in the irradiated W whereas others showed a lower H permeability. In these studies the radiation defects were produced with ions (as surrogates for neutrons) and were concentrated in a shallow layer (

This paper investigated oxide film of P92 steel in 300 °C/10 MPa water at different corrosion time. The surface morphology, phase structure and micro-defects were investigated by scanning electron microscope, X-ray diffraction and positron annihilation spectroscopy. The results reveal the formation of double oxide film, the outer oxide film was mainly composed of magnetite and the inner oxide film was composed of magnetite and FeCr spinel. The positron annihilation lifetimes spectra shows that the average size of defects increases with the increase of corrosion time. Doppler broadening spectroscopy of slow positron beam indicates that the outer oxide film structure was relatively dense in the initial stage of corrosion. The inner oxide film was characterized with relative loose structure due to the increase of defects size and density.

Fe–0.6%Cu alloy was irradiated with H ions to 0.1 dpa, and then annealed for 30 min from 150 °C to 500 °C. We focused the evolution of Cu precipitates in irradiated Fe–0.6%Cu alloy after the isochronal annealing from the perspective of positron annihilation. The ΔW parameters after thermal annealing (400 °C and 500 °C) were much larger than that induced by 0.1 dpa H irradiation. Annealing could promote the aggregation of the Cu-vacancy complexes, and form the Cu cluster–vacancies complexes. When the vacancy-like defects recovered around 500 °C, it meant the formation and growing of the defect-free Cu precipitates.

In order to study the fundamental processes of helium retention and thermal desorption from the structural material of future fusion reactors, thermal desorption measurements were performed to investigate helium trapping from defects in binary Fe9Cr model alloys irradiated by 3 keV and 0.2 key He ions. Interstitial type dislocation loops, vacancies and vacancy clusters were produced by irradiation with 3 key helium ions, which acted as the sink trapped the helium atoms. Helium thermal desorption peaks from dislocations, helium-vacancies were obtained by thermal desorption spectroscopy at similar to 540 degrees C, in the range from 205 degrees C to 478 degrees C, respectively. Simple first order dissociation kinetics are used to estimate the activation energies associated with the desorption groups. A sharp desorption peak was observed at similar to 865 degrees C due to the BCC-FCC phase transformation for specimens under all examined implantation conditions. (C) 2015 Elsevier B.V. All rights reserved.

Evolution of vacancy-type defects has been investigated in undamaged and copper ion pre-damaged tungsten exposed to low-energy and high-flux helium plasma (60 eV, 1 × 1022 He/m2s). The results measured by Doppler broadening positron annihilation spectroscopy (DB-PAS) indicate that helium-vacancy complexes generate due to intense self-trapping in the undamaged tungsten after helium plasma exposure. In contrast, the occupation of pre-existing vacancies and vacancy clusters, caused by the presence helium atoms, plays a dominant role in the pre-damaged tungsten, but the density of vacancy-type defects in the pre-damaged case is still higher than that in the undamaged case. This means that the dominating process of helium-vacancy complexes formation in tungsten changes from self-trapping to vacancy-trapping in the case of high vacancy density. Meanwhile, the elastic recoil detection analysis (ERDA) and transmission electron microscopy (TEM) results reveal that these pre-existing defects can increase helium retention and helium nano-bubble size. However, it is surprising that in the case of pre-damaged sample, the density/volume of vacancy-type defects also decreased even outside of the helium distribution depth. We attribute this phenomenon to the interstitial dislocation loops punched by helium clusters preferentially diffusing into the pre-damage regions of tungsten far beyond the helium distribution depth, resulting in the significant recombination with vacancies or vacancy clusters. This intrinsic mechanism is further verified by TEM observations and molecular dynamics (MD) simulations.

In the fusion reactors, plasma facing materials (PFMs) will be bombarded by large amounts of energetic particles, including helium (He) atoms. These metal-insoluble He atoms are prone to self-trapping or are trapped by vacancies, dislocation cores or grain boundaries, aggregating to form high concentration He bubbles which leads to serious degradation of the PFMs’ properties. Studying the initial stages of He-defect interactions becomes increasingly important because it not only helps us understand the formation mechanism of He bubbles but also provides insight to help design new irradiation-resistant PFMs. In this work, we used the positron annihilation Doppler broadening spectroscopy (DBS) and secondary ion mass spectrometry (SIMS) to explore the evolutions of the interactions between vacancy-type defects and He and the retention of He in the nanochannel W film irradiated with 190 keV He+ ions to different fluences under different temperatures. It is found that the presence of a nanochannel structure accelerates the release of He from the film even at low irradiation fluences, and the release of He is significantly enhanced at higher fluences, thus inhibiting or delaying the formation of large He-vacancy clusters in the nanochannel W film. Both irradiation fluence and temperature have significant influence on the formation and evolution of He-vacancy clusters, and the evolution of these microstructure also causes hardness changes.

The effect of annealing on VmHn complexes and Cu precipitate behaviours in hydrogen ion irradiated Fe and Fe-0.3%Cu alloys was investigated by positron annihilation spectroscopy using a slow positron beam. The results of S parameters indicated that the room temperature irradiation was benefit for the formation of the VmHn complex compared to the elevated temperature irradiation. The S-W results confirmed the formation of Cu precipitates in Fe-0.3%Cu even at the irradiation dose of 0.1 dpa. The formation of the evident S value peaks in the damage region after annealing treatment suggested that the VmFn complexes were broken and a larger of hydrogen atoms were escaping. The residual vacancy defects would migrate towards both the surface region and the opposite direction with the increasing annealing temperature. (C) 2015 Elsevier B.V. All rights reserved.

The evolution of microstruture for Fe16.7Cr14.5Ni model alloy and 316 stainless steel irradiated with 140 keV He ions were studied by Positron annihilation spectroscopy. The fluences were 1 x 10(16) and 5 x 10(16) He ions/cm(2). The irradiation temperature was room temperature and 573 K, respectively. The variation of S parameter-incident positron energy profile indicated that large amount of vacancy-type defects formed after He ion irradiation. Meanwhile, helium atoms deposited in bulk and certain amount of He-vacancy complexes were formed. The vacancy-type defects could be the major defects in track region and He-vacancy complexes would be the main defects in cascade region. The vacancy-type defects could migrate and aggregate to form vacancy clusters and even microvoids at elevated temperature irradiation. The diffusion mechanism of helium atoms might be changed at different irradiation temperature. (C) 2014 Elsevier B.V. All rights reserved.

The microstructure of fatigued and unfatigued Ni in the presence or absence of neutron irradiation was investigated using positron annihilation lifetime measurements and transmission electron microscopy (TEM). In irradiated unfatigued Ni, vacancy clusters were directly formed from cascades at doses of less than 10 −4 dpa, and started to grow at higher doses. In irradiated fatigued samples, vacancy clusters were directly formed from cascades at 1.5 × 10 −4 dpa. In unirradiated fatigued samples, single vacancies were produced. In all of the fatigued samples, dislocations introduced by the deformation acted as sinks and suppressed the growth of vacancy clusters. Following annealing at 373 K, vacancy clusters were observed in irradiated fatigued Ni, but vacancies were trapped at dislocations in unirradiated fatigued Ni. No change in dislocation structure was brought about by annealing and no vacancy clusters could be observed by TEM.

Vacancy-type defects induced by the co-implantation of He and H ions in China low activation martensitic (CLAM) steel at room temperature were investigated by variable-energy position beam Doppler broadening spectra (DBS). The co-implantation occurred in two patterns. In one pattern, He ions were implanted before H ions; the other pattern was of the opposite sequence. Both He- and H-vacancy complexes were formed for pre-implanted He and H at different fluences. He-H-vacancy complexes were formed for pre-implanted He, which caused the S parameter of the pre-implanted He to be smaller than that of H at the lowest fluence. The defect density increased with increased fluence for the two implanted patterns, and the difference between the S parameters of pre-implanted H and He decreased with increased fluence. Nanovoids containing a few He atoms were the dominating defects at high implanted fluences for the two kinds of implantation. (c) 2012 Elsevier B.V. All rights reserved.

In the radiation damage process of austenitic stainless steels, there exists an incubation period before steady-state void growth, and the defect formation behaviors during that period strongly depend on alloy composition. Using the technique of positron annihilation lifetime measurement, the evolution of defect clusters during the incubation period in neutron, electron, and H-ion irradiations was studied for a variety of austenitic stainless steels including commercial and model alloys. The lifetime measurements indicated that in fission neutron irradiation to 0.2 dpa at 363 K, single vacancies were predominantly formed in the commercial alloys, SUS316L and Ti added, modified SUS316, while large voids were formed in Ni and Fe–Cr–Ni. After neutron irradiation at 573 K, stacking fault tetrahedra and/or precipitates were detected in the commercial alloys, while large voids were detected in the model alloys. In the 30 MeV electron irradiation to a dose of 0.012 dpa, the effect of alloying elements on lifetime data was less significant at 353 K, but a significant difference was found between model alloys and commercial alloys at 573 K. The H-ion irradiation at 2 MeV was also performed at room temperature. Defect evolution during the incubation period is discussed on the basis of the neutron, electron and H-ion irradiation results.

The defect structures in neutron-irradiated austenitic stainless steel, Ti-modified 316SS, were studied by using positron annihilation spectroscopy and transmission electron microscopy. By low dose irradiation of the order of 10−3 dpa, mono-vacancies and small microvoids were detected below 423 K by positron annihilation lifetime measurement. With increasing irradiation temperature, the positron lifetime was found to decrease, indicating no microvoid formation. Irradiation at doses above 4.2 dpa and temperatures above 673 K led to the formation of microvoids. These results indicated that microvoid formation below 423 K occurs even at low dose without incubation. Above 673 K, during the incubation period, irradiation damage changes the microstructure leading to vacancy clustering to form voids.

Helium atoms in metals have a strong tendency to accumulate at defects. Unfortunately, the experimental identification of the trapping sites of helium atoms is difficult. In this paper, the effects of dislocations on the trapping and desorption of helium atoms in nickel and iron were studied by thermal desorption spectroscopy. Defects were introduced in well-annealed high-purity metals by cold rolling. Positron annihilation spectroscopy was carried out to identify defects by measuring the positron lifetime before annihilation with various defects. The trapping and desorption of helium atoms at dislocations were investigated by implantation with low-energy He + ions, both without irradiation damage (0.1 keV or 0.15 keV) and with irradiation damage (5.0 keV).

Helium atoms, introduced into materials by helium plasma or generated by the (n, alpha) nuclear reaction, have a strong tendency to accumulate at trapping sites such as vacancy clusters and dislocations. In this paper, the effects of dislocations, single vacancies and vacancy clusters on the retention and desorption of helium atoms in nickel were studied. Low energy (0.1-0.15 keV) helium atoms were implanted in nickel with vacancies or dislocations without causing any displacement damage. He atoms, interstitial-type dislocation loops, and vacancy clusters were also introduced with irradiation damage by 5.0 keV helium ions. Helium thermal desorption peaks from dislocations, helium-vacancy clusters and helium bubbles were obtained by thermal desorption spectroscopy at 940 K, in the range from 900 to 1370 K, and at 1500 K, respectively. In addition, a thermally quasi-stable state was found for helium-vacancy clusters. (C) 2011 Elsevier B.V. All rights reserved.

It is well known that the minor elements Si and Sn can suppress the formation of voids in Ni alloys. In the present study, to investigate the effects of Si and Sn on the retention of helium in Ni alloys, Ni, Ni–Si, and Ni–Sn alloys were irradiated by 5 keV He ions at 723 K. Thermal desorption spectroscopy (TDS) was performed at up to 1520 K, and microstructural observations were carried out to identify the helium trapping sites during the TDS analysis. Two peaks, at 1350 and 1457 K, appeared in the TDS spectrum of Ni. On the basis of the microstructural observations, the former peak was attributed to the release of trapped helium from small cavities and the latter to its release from large cavities. Small-cavity helium trapping sites were also found in the Ni–Si and Ni–Sn alloys, but no large cavities were observed in these alloys. In addition, it was found that the oversized element Sn could trap He atoms in the Ni–Sn alloy.