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To investigate the influence of different rhenium contents on the helium desorption behavior in tungsten–rhenium alloys, pure tungsten and tungsten–rhenium alloys were irradiated with helium under the same conditions. All irradiated samples were characterized using TDS and DBS techniques. The results indicate that the addition of rhenium can reduce the total helium desorption quantity in tungsten–rhenium alloys and slightly accelerate the reduction in the concentration of vacancy-type defects accompanying helium dissociation. The desorption activation energy of helium is approximately 2 eV at the low-temperature peak (~785 K) and about 4 eV at the high-temperature peak (~1475 K). An increase in rhenium content causes the desorption peak to shift towards higher temperatures (>1473 K), which is attributed to the formation of the stable complex structures between rhenium and vacancies. Besides, the migration of He-vacancy complexes towards traps and dynamic annealing processes both lead to the recovery of vacancy-type defects, resulting in a decrease in the positron annihilation S parameters. [ABSTRACT FROM AUTHOR]

In this work, we studied the evolution of vacancy-like defects and the formation of brittle precipitates in a reduced-activation V−Cr−Mn medium-entropy alloy. The evolution of local electronic circumstances around Cr and Mn enrichments, the vacancy defects, and the CrMn3 precipitates were characterized by using scanning electron microscopy with energy-dispersive spectroscopy, X-ray diffraction, and positron annihilation spectroscopy. The microstructure measurements showed that the Mn and Cr enrichments in the as-cast sample significantly evolved with temperature, i.e., from 400 °C, the Cr/Mn-segregated regions gradually dissolved into the matrix and then disappeared, and from 900 °C to 1000 °C, they existed as CrMn3 precipitates. The crystallite size of the phase corresponding to CrMn3 precipitates was about 29.4 nm at 900 °C and 43.7 nm at 1000 °C. The positron annihilation lifetime results demonstrated that the vacancies mediated the migration of Cr and Mn, and Cr and Mn segregation finally led to the formation of CrMn3 precipitates. The coincidence Doppler broadening results showed that the characteristic peak moved to the low-momentum direction, due to an increase in the size of the vacancy defects at the interface and the formation of CrMn3 precipitates. [ABSTRACT FROM AUTHOR]

Here, the storage sites of hydrogen in FeCr alloy, namely (H, He)–V nano-clusters with open volume, have been investigated by first-principles calculations and positron annihilation spectroscopy. The positron lifetimes in the nano-clusters obtained by theoretical calculations and experiments were compared. These results suggest that positron is sensitive to the relative position of the gas atoms decorating the open volume; and a helium atom forms a more repulsive ion core than a hydrogen atom when it occupies the vacancy, resulting in a decrease in positron lifetime. Interpretation of the PAS data was successfully combined with theoretical calculations, and allowed us to determine the defect status in specimens after H/He irradiation with different implantation patterns. The speculated kinds of hydrogen storage sites formed after irradiation are summarized. For the He-ions pre-implanted situation, the helium-vacancy clusters or bubbles forming in advance may recombine with H atoms to form stable He–H–V complex. • Positron is sensitive to the relative position of the H atoms decorating the open volume. • Various point defect structures were modeled. • The lifetime values of positrons in various (H, He)-V complexes are given. • The kinds of defects formed after He/H sequential-ion irradiations are summarized. [ABSTRACT FROM AUTHOR]

An element analysis method, coincidence Doppler broadening spectroscopy of slow positron annihilation, was employed to detect helium in ion-irradiated Fe9Cr alloys. Spectra with higher peak to background ratio were recorded using a two-HPGe detector coincidence measuring system. It means that information in the high-momentum area of the spectra can be used to identify helium in metals. This identification is not entirely dependent on the helium concentration in the specimens, but is related to the structure and microscopic arrangement of atoms surrounding the positron annihilation site. The results of Doppler broadening spectroscopy and transmission electron microscopy show that vacancies and dislocations were formed in ion-irradiated specimens. Thermal helium desorption spectrometry was performed to obtain the types of He traps. [ABSTRACT FROM AUTHOR]

Mechanism of Hydrogen-induced defects and cracking in Ti and Ti–Mo alloy was systematically analyzed from the perspective of microstructure. Results show that more hydrogen atoms can be dissolved in Ti–Mo alloy due to the existence of β phase, and the precipitation amount of hydride is reduced under the same hydrogen charging conditions. The analysis of positron annihilation results show that the solid solution of Mo element reduces the hydrogen induced defect concentration in the alloy and inhibit the combination of hydrogen and defects. At the same time, Mo atom can affect the electronic structure of Titanium hydrides, and then reduce the bonding between titanium atoms and hydrogen atoms, thereby reducing the formation of brittle titanium hydride. The reduction of hydride and hydrogen induced defect concentration significantly decrease the hardening rate of Ti–Mo alloy, thereby effectively reducing the hydrogen embrittlement sensitivity of titanium and inhibiting its hydrogen absorption cracking tendency. • The formation of β phase in Ti–Mo alloy improves the solubility of hydrogen, thereby reducing the precipitation of hydride. • The solid solution of Mo can affect the electronic structure of titanium hydride and weaken the bonding between Ti–H. • The solid solution of Mo can reduce the concentration of hydrogen-induced defects in Ti–Mo alloy. • Reducing the concentration of hydride and hydrogen induced defect can inhibit hydrogen absorption cracking tendency. [ABSTRACT FROM AUTHOR]

The atomic-scale defects such as (deuterium, helium)-vacancy clusters in nuclear energy materials are one of the causes for the deterioration of the macroscopic properties of materials. Unfortunately, they cannot be observed by transmission electron microscopy (TEM) before they grow to the nanometer scale. Positron annihilation spectroscopy (PAS) has been proven to be sensitive to open-volume defects, and could characterize the evolution of the size and concentration of the vacancy-like nanoclusters. We have investigated the effects of He-D interaction on the formation of nanoscale cavities in Fe9Cr alloys by PAS and TEM. The results show that small-sized bubbles are formed in the specimen irradiated with 5 × 1016 He+/cm2, and the subsequent implanted D-ions contribute to the growth of these helium bubbles. The most likely reason is that helium bubbles previously formed in the sample captured deuterium injected later, causing bubbles to grow. In the lower dose He-irradiated samples, a large number of small dislocations and vacancies are generated and form helium-vacancy clusters with the helium atoms. [ABSTRACT FROM AUTHOR]

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 Cu n V m complexes (n > m) dominated the microstructure in the track region. Vacancy clusters and Cu n V m complexes (n < m) were the main defect types in the damage region. There finding were supported by the results of the performed theoretical calculations. The coincidence Doppler broadening showed that the Cu precipitates formed easily at a low irradiation dose, and that the irradiation-induced Cu precipitation was enhanced by the increase of the irradiation dose in the track region. We showed that the Cu precipitates finally reach a steady-state morphology under multiple active processes. [ABSTRACT FROM AUTHOR]

To investigate the thermal evolution of vacancy-type defects and the relationship between the evolution of vacancy-type defects and the formation of fuzz structure on the surface of W-Ni-Fe heavy alloys irradiated with He ions, isochronal annealing treatments at temperatures from 100 °C to 1000 °C were conducted on the irradiated 97W-2Ni-Fe alloy. The evolution of microdefects was characterized by positron annihilation spectroscopy (PAS). Thermal desorption spectroscopy (TDS), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) were used to supplement the positron annihilation method. A large number of vacancy-type defects were produced in the 97W-2Ni-Fe alloy after irradiation at room temperature and isochronal annealing, and the main defect types of positron capture changed at three stages: RT-600 °C, 600–800 °C, and 800–1000 °C. SEM found that a large number of nanoscale protrusions appeared on the surface of samples annealed at 800 °C, and these nanoscale protrusions were seeds of fuzz structures. The formation of fuzz structure is closely related to the movement of vacancy-type defects by PAS, TDS, SEM, and other characterization methods. In addition, we found that the high density Ni and Fe in the matrix phase of the 97W-2Ni-Fe alloy as He capture points would accelerate the nucleation and growth of He bubbles and promote the growth of the fuzz structure. [ABSTRACT FROM AUTHOR]

Hydrogen embrittlement (HE) can lead to the unintentional fracture of large engineering materials, resulting in severe economic losses and safety hazards. It is critical to explore economical and scalable solutions to overcome the challenges of HE. In this work, the HE resistances of four face-centered cubic (FCC) concentrated solid solution alloys (CSAs), including NiFe 20 , NiCoCr, NiCoCrFe, NiCoCrFeMn, and pure Ni were comprehensively investigated and compared by using TDS, EBSD and TEM. It was found that Fe and Cr element can effectively inhibit the solubility of deuterium in the CSAs and alleviate the HE behaviors. In contrast, Mn element significantly increases the solubility of deuterium and caused brittle fractures. Tensile experiments were performed to compare the HE sensitivity of CSAs. The tensile strength of pure Ni and NiFe 20 alloys decreased after deuterium charged, whereas the tensile strength of NiCoCr, NiCoCrFe and NiCoCrFeMn alloys increased. Based on the experimental results, it is deduced that deuterium atoms promote dislocation accumulation and twin growth by reducing the stacking fault energy, which eventually leads to an increase in yield strength and a decrease in plasticity. In addition, the accumulation of deuterium atoms near grain boundaries and cracks during deformation leads to dislocation pinning and stress concentration, which ultimately promotes crack nucleation and propagation. The HE resistances of CSAs are strongly influenced by the deuterium concentration, which depends on the effects of constituent elements and deuterium charging time on the deuterium concentration. Collectively, the specific alloying elements play more important role for the HE resistances of CSAs than the number of elements in the current study. These results provide valuable insights for further understanding and design higher HE resistance CSAs in the future. [Display omitted] ● The addition of Fe and Cr elements to CSAs effectively inhibits deuterium dissolution and enhances the resistance to HE. ● The addition of the Mn element to CSAs significantly promoted deuterium dissolution and caused HE fractures. ● Deuterium atoms reduce the SFE and promote dislocation propagation and twin growth. ● The accumulation of localized stresses induced by deuterium promote the initiation and propagation of cracks. ● Higher deuterium concentrations and deuterium charging times are more likely to trigger HE. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Long-term exposure of structural materials to extreme hydrogen-enriched environments will lead to hydrogen embrittlement, and thus search and design of new alloys with economic and safety benefits against hydrogen embrittlement have become a top priority for the development of many engineering fields. In this work, the hydrogen embrittlement behavior of a new cobalt-free high-entropy alloy (HEA) Fe 27 Ni 28 Mn 27 Cr 18 was systematically studied and compared with a typical quaternary HEA Fe 25 Co 25 Cr 25 Ni 25. Numerous vacancies are produced in the process of electrochemical deuterium charging and the corresponding concentration is related with the solubility of deuterium atoms. Fe 27 Ni 28 Mn 27 Cr 18 has much higher concentrations of deuterium and vacancies than Fe 25 Co 25 Cr 25 Ni 25 , which is attributed to the Mn induced severe lattice distortion and results in different variations of mechanical properties. The ultimate tensile strength (UTS) of Fe 27 Ni 28 Mn 27 Cr 18 increases from 353.73 MPa to 589.50 MPa after electrochemical hydrogen charging while the elongation still remains at 74.73%, and its deformation mechanism is dominated by dislocation proliferation. In contrast, the UTS of hydrogen charged Fe 25 Co 25 Cr 25 Ni 25 decreases from 552.79 MPa to 501.53 MPa and the corresponding elongation also decreases from 74.65% to 67.24%, and the plastic deformation mechanism is dominated by twinning. This work demonstrates an economic cobalt-free HEA with great resistance to hydrogen embrittlement, which would enhance the applications of HEAs in hydrogen-enriched environments. • The severe lattice distortion in Fe 27 Ni 28 Mn 27 Cr 18 promotes the dissolution of deuterium atoms. • Deuterium charging leads to a decrease in the plasticity of the sample and an increase in the yield strength. • Fe 27 Ni 28 Mn 27 Cr 18 alloy has high plasticity and low hydrogen embrittlement susceptibility. [ABSTRACT FROM AUTHOR]

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 W x Re (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 m 0 c 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. [ABSTRACT FROM AUTHOR]