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Electrolytic hydrogen charging experiments on cold-deformed and well-annealed (annealing at 700 °C for 2 h) pure titanium samples were carried out, respectively. Positron annihilation spectroscopy and X-ray diffraction were used to characterize all experimental samples to explore the formation of vacancy defects and the storage form of hydrogen in pure titanium after charging. Results showed that hydrides formed in well-annealed samples after electrolytic hydrogen charging, but a new phase in the cold-deformed samples was not observed. The annealed samples formed vacancy-type defects in the process of electrolytic hydrogen charging, and the excess hydrogen atoms were easily trapped by vacancies to form a hydrogen vacancy complex (HmVn). The defects formed in the cold-deformed hindered the diffusion of hydrogen atoms and inhibited the formation of vacancies. Compared with the well-annealed electrolytic hydrogen charging samples, the S parameters of the deformed electrolytic hydrogen charging samples hardly changed. The coincidence Doppler broadening spectrum results showed that wide peaks related to hydrogen vacancy complexes were found in electrolytic hydrogen charging samples. The formation of hydride in titanium affected the positron annihilation environment in the low-momentum region. The hydride-related peak was observed only in the electrolytic hydrogen-charged samples after being well annealed. [ABSTRACT FROM AUTHOR]

The formation of mono-vacancy, vacancy clusters and hydrogen-vacancy complexes with 30 keV H ion-irradiated pure titanium at different doses and temperatures was measured using by Positron annihilation spectroscopy (PAS). Results show a large number of H m V n clusters and vacancy-like defects in the samples irradiated at for room temperature, and that the formation of H m V n (m > n) at the sample irradiated at a high dose inhibits the increase of the S parameter. At increased irradiation temperature, the shrinkage of vacancy clusters and the effective open volume of defects decrease the S parameters. The high-temperature irradiation results in decreased vacancy-type defect concentration, and some hydrogen atoms diffuse from the cascade region to the track region, forming a large number of hydrogen-vacancy complexes in the track region. The coincidence Doppler broadening spectroscopy, an element analysis method, used to detect hydrogen in the ion-irradiated pure titanium sample, and results show hydrogen-related peaks in the high-momentum region, which may be due to the information of positron annihilation in the covalent bond formed by the H and the Ti elements. The increased radiation dose and temperature contribute to the formation of the hydrogen vacancy-complex, and the positron annihilation in high-momentum regions easily obtain hydrogen-related information. • The increased radiation dose and temperature contribute to the formation of the hydrogen vacancy-complex (H m V n). • The formation of H m V n (m > n) at the sample irradiated at a high dose inhibits the increase of the S parameter. • Hydrogen-vacancy complexes (H m V n) formed as hydrogen atoms deposited in the material. [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]

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]

• High concentration of hydrogen induces martensitic transformation. • Defects induced by hydrogen-charged may be dislocations. • Hydrogen desorption peaks shift to higher temperature for annealing specimen. • Positron annihilation technique studies the evolution of hydrogen and defects. The effects of dislocations and hydrogen concentration on the hydrogen embrittlement (HE) in 316 austenitic stainless steels (316SS) were systematically investigated in the present study. The results revealed a decrease in tensile strength and ductility of 316SS after hydrogen charging. Furthermore, the severity of HE was dependent on the hydrogen concentration. The results of x-ray diffraction revealed the presence of martensite in the hydrogen-charged (50 mA /cm2) specimen annealed at 1000 ℃. This was attributed to the internal stress originating from hydrogen-induced lattice expansion. The analysis of the fracture surface of all the specimens revealed an increase in the brittleness with the absorption of hydrogen. An increase in the hydrogen concentration induced a transformation of the fracture mode i.e., from ductile dimple fracture to cleavage and intergranular fractures. The evolution of hydrogen in 316SS was associated with the defects induced by deformation. The shift in the major desorption peaks of the annealed specimen to the high-temperature region indicated the formation of effective hydrogen-trapping sites during hydrogen-charging. The specimens were subjected to positron lifetime spectroscopy and thermal desorption spectroscopy. The results for the raw specimen, charged at 20 mA /cm2, revealed the gradual escape of hydrogen atoms and recovery of vacancy defects with an increase in the annealing temperature. In addition, the hydrogen-induced and cold rolling-induced dislocations was preserved. [ABSTRACT FROM AUTHOR]

Hydrogen behavior and its related property degradation have been long-standing problems for structural materials used in hydrogen energy. The hydrogen atoms can easily interact with vacancy defects, forming hydrogen vacancy complexes which play an important role in the hydrogen-induced structural damage. However, the interaction mechanisms and its evolutions are still unclear. In this work, the hydrogen behavior and the interaction between hydrogen with defects in pure titanium implanted by 30 keV and 50 keV hydrogen ions were studied by positron annihilation spectroscopy. The implantation doses were 5 × 1016 H/cm2, 1 × 1017 H/cm2 and 5 × 1017 H/cm2, respectively. The results show that the structural damage of pure titanium is positively correlated with the ion implantation energy. For the implantation of 50 keV hydrogen ions, a large number of hydrogen atoms are deposited in the samples. With the increase of implantation dose, the formation of hydrogen vacancy complexes (H m V n) reduces the effective open-volume of defects and changes the structural features of defects in implanted samples, thus suppressing the formation of vacancy defects and causing the damage range shifting from the peak damage (PD) region to the near surface (NS) region. Eventually, the movement of hydrogen atoms intensifies, and the "hydrogen peak" becomes more obvious. The chemical information related to deposited hydrogen atoms can be easily identified in the processing and analysis of positron annihilation results. • The formation rate of vacancy type defects increases in direct proportion to the implantation energy. • Implantation dose promoted the formation of hydrogen vacancy complex. • The increase of the proportion of hydrogen vacancy complex leads to the inhibition of vacancy formation. • With the increase of hydrogen concentration, the "hydrogen peak" signal is more obvious. [ABSTRACT FROM AUTHOR]

• Two types of titanium hydrides form after electrochemical hydrogen charging. • The formation of hydride leads to lattice expansion. • Hydrogen content affects the concentration and type of hydrogen induced defects. • The titanium hydride and hydrogen-vacancy complex were identified by CDB spectrum. The influence of hydrogen on defect structure in pure titanium was systemically investigated using X-ray diffraction (XRD) and positron annihilation spectroscopy (PAS). The results showed that hydrogen damage was related to hydrogen concentration, and PAS provided a new idea for the study in the formation of hydride and open volume defects. XRD results showed that two types of titanium hydrides were observed in hydrogen-charged samples, i.e., δ-TiH x and ε-TiH 2 , and the degree of lattice expansion and the content of hydride increased with hydrogen content. The variation of the S parameter and W parameter indicated that a great deal of vacancy-type defects formed during electrochemical hydrogen charging, and hydrogen atoms could combine with vacancy to form a certain amount of hydrogen-vacancy complexes (H n V m), meanwhile, the hydrogen damage varied significantly with depth. The effective open volume of vacancy-type defect increased with hydrogen charging concentration, and hydrogen-vacancy cluster transformed into H n V m (n < m) cluster, vacancy defects were gradually dominated. In addition, According to the different annihilation environment of positron in titanium hydrides and hydrogen-vacancy complexes, the existence of titanium hydrides and the existing form of hydrogen in defect could be identified in Doppler broadening spectroscopy (CDB). This provided a new way to identify the formation of hydrides and to detect the existing form of low concentration hydrogen in samples. [ABSTRACT FROM AUTHOR]

[Display omitted] • High density dislocations inhibit hydrogen atoms diffusion. • Different types of hydrides were identified by CDB spectrum and theoretical calculations. • Hydrogen concentration decreases with increasing dislocation density. This work aims to elucidate the interaction between hydrogen atoms and pre-introduced dislocations in commercially pure titanium (CP-Ti). Positron annihilation spectroscopy supplemented by the first-principle calculations was utilized to reveal the effects of dislocation densities on the formation of hydride types and the concentration of hydrogen-induced defects in CP-Ti. Results show that, in the hydrogen-charged sample with 40 % deformation, hydrogen-induced damage gradually decreases when the hydrogen enters the sample at a depth of about 350 nm. This finding suggests that the high-density dislocations present in deformed specimens effectively trap hydrogen atoms and inhibit their diffusion, which ultimately mitigate damage within the samples. γ-TiH and δ-TiH 2 hydrides are observed in the hydrogen-charged sample. The thermal desorption spectroscopy results show that the desorption amount of deuterium decreases from 1.14 × 1018 D/cm2 to 5.10 × 1017 D/cm2 with an increase in dislocation density because dislocations inhibit the diffusion of deuterium. In samples with higher deformation, the high dislocation density inhibits the formation of δ-TiH 2 hydrides and promotes the formation of γ-TiH hydrides, which significantly reduce the hardening of samples. These results provide insight into the interaction of hydrogen with pre-introduced defects and provide strong theoretical support for the development and improvement of more damage-resistant materials. [ABSTRACT FROM AUTHOR]