Concentration Of Vacancy-type Defects Research Articles

The Influence of Rhenium Content on Helium Desorption Behavior in Tungsten-Rhenium Alloy.

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.

Influence of Ge to the formation of defects in epitaxial Mg2Sn1−x Ge x thermoelectric thin films

Defect formation in epitaxial Mg2Sn1–x Ge x thermoelectric (TE) thin films grown via MBE was studied. We examined the defect formations and structures using cross-sectional transmission electron microscopy and positron annihilation spectroscopy. The defect formation tends to be influenced by Ge incorporation into the Mg2Sn matrix phase of epitaxial thin films. Mg vacancies (V Mg) were identified as point defects, primarily concentrated in the film’s mid-layer. In films with higher Ge composition, stacking faults were observed. The concentration of vacancy-type point defects decreased as the Ge concentration increased. This implies that the vacancy atoms, which would have otherwise been created by increasing chemical pressure due to the higher Ge content, might have played a role in the formation of stacking faults. The high concentration of vacancy-type defects resulted in the lowest thermal conductivity, demonstrating their significance as effective phonon scattering centers in epitaxial TE films.

Bridging the gap between theory and experiment in vacancy concentration, C/N/O diffusivity, and divacancy interaction in tungsten: Role of vacancy-C/N/O interaction

Light element solutes such as carbon (C), nitrogen (N), and oxygen (O) are impurities commonly seen in tungsten alloys, often exerting great impact on materials properties due to their strong interaction with defects. In this work, we conducted a comprehensive investigation into the atomistic properties of small vacancy-impurity defect complexes in tungsten using rigorous first-principles calculations. Through the integration of a statistical approach, we meticulously examined the concentration distributions of these complexes across various temperatures and impurity concentrations. Our findings reveal that the introduction of C/N/O impurities significantly augments the concentration of vacancy-type defects to a level well above the thermal equilibrium vacancy concentration in pure tungsten. Moreover, we conducted a thorough assessment of the diffusivity of C/N/O atoms, conclusively demonstrating that the inclusion of the vacancy trapping effect is paramount in comprehending their unexpectedly low diffusivity observed in experiments. Finally, in conjunction with an object kinetics Monte Carlo method, we revisited the experimental determination of the divacancy binding energy in tungsten. We showed that the presence of C/N/O impurities may have conceivably impacted the experimentally ascertained divacancy binding energy. This work provides accurate data on evaluating vacancy-impurity interaction in tungsten, unveils the pivotal role played by C/N/O in affecting vacancy concentration and divacancy binding energy, and sheds new insights toward understanding the diffusion behavior of impurities in tungsten.

New understanding of one-dimensional thermal glide of a nano-sized prismatic dislocation loop in bcc iron: an atomic scale study

Vacancy-type prismatic dislocation loops (PDLs) can be formed directly through cross-slip mechanisms of screw dislocations or via the accumulation and transformation of high-density vacancy-type defects. The concentration of vacancy-type defects in body-centered cubic (BCC) iron can be increased through plastic deformation and in the presence of hydrogen. Nano-sized PDLs can act as high-speed vacancy transport in BCC iron, causing embrittlement. Based on molecular dynamics simulations, this study comprehensively analyzes the one-dimensional diffusion of vacancy-type PDLs composed of 37–169 vacancies (V37-V169) at temperatures ranging from 100 to 1000 K. The activation energy of diffusion increased with decreasing PDL size for small loops (V37–V61) but remained almost constant for larger loops (>V91). This behavior was found to be associated with the tensile stress produced by the opposing edge dislocation segment in the loop where the distance is closer for a smaller PDL. That is, the tensile stress along the slip direction of edge dislocation causes the unstable stacking fault energy to increase, and the change of dislocation core structure further contributes to an increase in the activation energy for the diffusion of PDL. In contrast, an external compressive stress reduces the activation energy and pre-exponential factor of diffusion in small PDLs, as exemplified in V37 PDL.

Positron annihilation study of the reactor pressure vessel model steels irradiated in the high flux reactor

Twelve reactor pressure vessel (RPV) model steels with parametric variation of alloying elements Ni and Mn were characterized by means of positron annihilation spectroscopy before and after irradiation at the High Flux Reactor (HFR) to ∼0.18 dpa at an average temperature of 286 °C. Two independent positron annihilation lifetime spectrometers (PALS) and one coincidence Doppler broadening spectrometer (CDBS), used in the post-irradiation examination, provided a substantial amount of experimental data used to characterize the vacancy-type defects and their agglomerations in the studied materials. The obtained results show an apparent increase in the positron trapping at vacancy-type defects after irradiation, with the concentration of the radiation-induced vacancy-type defects increasing with the Ni and Mn content. The effect of Si and Cu was found to be less pronounced, and the concentration of vacancy-type defects seems to anti-correlate with their content. In addition to open-volume vacancy-type defects, the PALS technique enabled a quantitative estimation of the dislocation-type defects, where only a modest increase in density was observed after the irradiation at the given temperature.

Transition between a nano-sized prismatic dislocation loop and vacancy cluster in α-iron: An atomic scale study

Plastic deformation and the presence of hydrogen enhance the concentration of vacancy-type defects in α-iron, wherein monovacancies can accumulate as a planar vacancy cluster (VC). The planar VC can then be nucleated into a corresponding prismatic dislocation loop (PDL), which can also be directly generated through cross-slip mechanisms of a screw dislocation. Inversely, a PDL can be converted to a corresponding VC. High densities of VCs and PDLs can significantly influence the mechanical properties of iron-based materials. Here, a quantitative energy barrier is established for the transition between nano-sized 1/2 〈111〉 PDLs and VCs using the nudged elastic band method for different applied strains. It was concluded that a cluster size of more than 19 vacancies can easily nucleate into a PDL if a VC with planar configuration is formed in α-iron. Further, the nano-sized VC to PDL transition is enhanced under compressive strain while the nano-sized PDL to VC transition is enhanced under tensile strain. This result indicates that nano-sized VCs generated by tensile deformation are converted to a PDL after unloading and diffusing away. Inversely, nano-sized PDLs that form by deformation can easily diffuse to high tensile stress regions near stress singularities, such as crack tips and triple-junctions of grain boundaries, and then accumulate as high-density VCs.

Deuterium retention and desorption behavior of W-Ta-Cr-V high entropy alloy

Deuterium retention and desorption behavior of W-Ta-Cr-V films under deuterium plasma irradiation were studied. The W-Ta-Cr-V films were prepared on the molybdenum (Mo) substrate by magnetron sputtering. Subsequently, all films annealed at 1000 °C were exposed to deuterium plasma at a flux of 8.13 × 1019 D/m2·s for 9 h to achieve an irradiation fluence of 2.63 ×1024 D/m2. Electron backscattered diffraction (EBSD) was used to characterize the texture and average grain size of films. Doppler broadening spectrometry of positron annihilation (DBS-PA) was carried out with an energy-variable slow positron beam to detect the concentration of vacancy-type defects in the films. The results suggest that there are fewer vacancy-type defects inside W-Ta-Cr-V grains. Deuterium retention and the desorption behavior of W-Ta-Cr-V were studied by compared to pure tungsten films. In the 300–800 K temperature range, the peaks of thermal desorption spectroscopy (TDS) of W-Ta-Cr-V films are substantially consistent with that of the W films. Notably, in the range of 800 K-1000 K, W-Ta-Cr-V film has no thermal desorption peak. We attribute this to the large lattice distortion and low vacancy formation energy, which inhibit the growth of voids inside the grains of W-Ta-Cr-V films. The total deuterium retention of W-Ta-Cr-V films is compared with that of W films. It is measured by TDS that the deuterium retention of W-Ta-Cr-V films is nearly 5 times that of W films. Based on the analysis of the TDS spectra, the higher deuterium retention in W-Ta-Cr-V films is due to the more grain boundary defects.

The investigation of distribution on size and concentration of helium bubbles in Y-bearing ODS steel using by SAXS and GIXRD

The nucleation and growth of the helium bubble in the structure material usually were subjected to temperature in a thermodynamic environment. In this study, Y-bearing oxide dispersed strengthened (ODS) ferritic/martensitic steels were used as the research material. The Y-bearing ODS steel samples with a thickness of 30 microns were irradiated successively by helium ions with three energies of 300 keV, 400 keV and 550 keV at room temperature and 450°C respectively. Firstly, the evolution of defects caused by H e + irradiation is analyzed by using slow positron beam Doppler broadening spectroscopy (DBS) in positron annihilation spectroscopy technology (PAS). The formation and growth of helium bubbles are deduced from the changes in the concentration of vacancy-type defects. The increase in the size of the helium bubbles at high temperatures leads to a certain distortion of the crystal lattice, which is also verified by analyzing the grazing incidence X-ray diffraction (GIXRD) patterns. The characterization of radius and concentration of helium bubbles in samples irradiated at room temperature and 450°C have been studied by Small Angle X-ray Scattering (SAXS) and Transmission Electron Microscopy (TEM). The results showed that the intensity (I) ~ q-vector (q) curves of samples vary with irradiation temperature over ranges of 0.13 ≤ q ≤ 4.6 nm −1 , which is attributed to the growth of helium bubbles at different irradiation temperature. It is found that the average size of helium bubbles under room temperature irradiation is about 1.78 nm, and the size distribution is narrow, while at the irradiation temperature of 450°C, the size of helium bubbles increases to 2.9 nm, and the size distribution is wider. This result is also verified from the TEM analysis. Besides, the shape of helium bubbles was assumed roughly and tends to spherical with higher temperatures.

Comparison of Hydrogen Thermal Desorption Analysis Curves of Electron-Irradiated F82H and Creep-Ruptured Pure Fe Obtained by Experiments and Simulations

Herein, we compared thermal desorption analysis (TDA) curves obtained by conducting experiments and simulations. In addition, we discussed the validation of our simulations and trapping sites of hydrogen atoms. In as-received F82H, when the samples contained solute atoms, grain boundaries, dislocations, and precipitates, the experimental curve corresponded to the simulated curve. In positron annihilation lifetime (PAL) measurements, di-vacancies were detected in the electron-irradiated F82H. When we changed the growth and the concentration of vacancy-type defects during temperature increase using the rate theory, the simulation results agreed with experiment results. In creep-ruptured Fe, only dislocations were detected by the PAL measurements. However, the existence of a type of defect, which was related to grain boundaries, must be assumed to fit the simulation curve to the experimental one. In the next step, the diffusion of hydrogen atoms on grain boundaries should be added to simulation program.

Investigation of the interaction between hydrogen and irradiation defects in titanium by using positron annihilation spectroscopy

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 HmVn clusters and vacancy-like defects in the samples irradiated at for room temperature, and that the formation of HmVn (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.

Evolution of Defects in CVD-W Irradiated by H/He Neutral Beam Using Positron Annihilation Spectroscopy

One of the key problems for the application of nuclear fusion energy is to select the suitable plasma facing materials (PFMs). Among the W-based materials, CVD-W exhibits some unique advantages. In order to estimate the performance of CVD-W under the fusion environment, the vacancy-type defects and their evolution are investigated by the Doppler-broadening slow positron beam analysis (DB-SPBA) combined with SEM (scanning electron microscope). There are two kinds of neutral beam irradiation, the pure H neutral beam and the H + 6 at.% He neutral beam irradiation, which are performed at the neutral beam facility GLADIS (IPP, Germany). The surface temperatures of CVD-W irradiated by H (H + 6 at.% He) are 850 and 1000 (700 and 800 °C). By comparing the samples under different conditions, the defect evolution of CVD-W is obtained. As for the pure H neutral beam irradiated samples, the DB-SPBA results demonstrate that the CVD-W sample at the surface temperature of 1000 °C, compared to the 850 °C sample, shows a decrease in S parameters, which is due to the reduction of vacancy-type defect concentration. The defect damage layer in 1000 °C sample is narrower than that of 850 °C sample and the defect type tends to be consistent in 1000 °C sample. The SEM results suggest that the surface damage of the 1000 °C sample was recovered to some extent. As for the H + 6 at.% He neutral beam irradiated samples, compared with the CVD-W sample at the surface temperature of 700 °C, the 800 °C sample shows an increased S parameters, which can be attributed to the volume increase of vacancy-type defect. The defect damage layer in the 800 °C sample is wider than that of the 700 °C sample. Both the H + 6 at.% He irradiated samples show complex defect types. The surface of the 800 °C sample exhibits more dense pinhole damage structures compared to that of the 700 °C sample.

Hydrogen Desorption Spectra from Excess Vacancy-Type Defects Enhanced by Hydrogen in Tempered Martensitic Steel Showing Quasi-cleavage Fracture

An attempt was made to separate and identify hydrogen peaks desorbed from plastic-strained, hydrogen-enhanced lattice defects from among various trapping sites in tempered martensitic steel showing quasi-cleavage fracture using thermal desorption spectroscopy from a low temperature (L-TDS) and positron annihilation spectroscopy (PAS). The amount of the lattice defects beneath the quasi-cleavage fracture surface was measured by L-TDS. The L-TDS results made it possible to separate two peaks, namely that of the original desorption and also that of new desorption from the steel specimens due to the application of plastic strain in the presence of hydrogen. The PAS results revealed that the new desorption obtained by L-TDS corresponded to vacancy-type defects. Hydrogen and plastic strain noticeably enhanced lattice defects formed within 1.5 mm from the fracture surface, where the average concentration of vacancy-type defects reached approximately 10−5 order in terms of atomic ratio. These results indicate that the accumulation of excess vacancy-type defects enhanced by hydrogen in the local region can lead to nanovoid nucleation and coalescence in plastic deformation, resulting in quasi-cleavage fracture of tempered martensitic steel.

Study of damage in binary Fe85Cr15 alloys irradiated by ions and the effect of an external magnetic field during irradiation

Understanding radiation-induced damage in iron-chromium alloys is critical for the use of advanced steels in future fusion reactors. However, the role of the strong magnetic fields present in such magnetically confined plasma devices on material damage, though considered important, has not been thoroughly explored to date. In this work, irradiation experiments of Fe85Cr15 alloy (15% Cr content) by heavy (Fe+) and light (He+) ions have been carried out in order to characterize the damage and, additionally, the influence of an external magnetic field (B = 0.4 T) has been taken into account. The analysis of the data has been done with different techniques, the distribution of the Cr and/or vacancies around probe nuclei has been explored by Mössbauer spectroscopy and the vacancy profile by Slow Positron Annihilation Spectroscopy (SPAS). Comparison between samples irradiated by He+ and Fe+, in absence of B, reveals significant differences in the average hyperfine magnetic field (<ΔH> = 0.8 T). This can be attributed to changes in the local environment around the probe nuclei (57Fe) where either vacancies or the Cr distribution play a role. From depth-profiling by SPAS in Fe+ irradiated samples, a large concentration of vacancy-type defects is found with a profile that extends deeper than that predicted by computer simulations. Furthermore, sequential irradiation by Fe+ and He+ ions indicates that the He atoms tend to fill the vacancy clusters just in the depth where the He+ is implanted. On the other hand, small but significant differences in vacancy-type defect profiles are found between the samples irradiated both in the presence and absence of a magnetic field (B = 0.4 T) both independently of single and sequential irradiation. Detailed Mössbauer spectroscopy and SPAS analysis seem to point out that external magnetic fields could be a non-negligible parameter in the material damage due to ion irradiation.

Characterization of proton-irradiated Chinese A508-3-type reactor pressure vessel steel by slow positron beam, TEM, and nanoindentation

Evolution of matrix defects and hardening of Chinese A508-3-type reactor pressure vessel steel irradiated to the doses of 0.05 dpa, 0.1 dpa, and 0.2 dpa by 110-keV proton at room temperature were investigated by slow positron beam, TEM, and nanoindentation. Slow positron beam results showed that proton irradiation-induced vacancy-type defects were observed and only one type of vacancy-type defects was characterized. Size and concentration of vacancy-type defects increased with the dose. TEM images revealed that dislocation loops were produced in the proton-irradiated samples. The mean size of the dislocation loops was unchanged but the number density of the dislocation loops was increased with the dose. An obvious hardening phenomenon of proton irradiated samples was detected by nanoindentation test. The irradiation hardening was mainly attributed to irradiation-induced matrix defects such as vacancy clusters, H-vacancy complexes, and dislocation loops.

Microstructure evolution and magnetic properties of Eu doped CuFeO2 multiferroic ceramics studied by positron annihilation

A series of multiferroic ceramics CuFe1-xEuxO2 (x = 0–0.10) are prepared by traditional solid-state reaction. The effects of Eu doping on the microstructure, vacancy-type defects, and magnetic properties of CuFeO2 ceramics are investigated systematically by means of X-ray diffraction, Raman spectroscopy, scanning electron microscope, positron annihilation lifetime and physical property measurement system. The results show that no phase transition occurs in the entire range of doping content (x = 0–0.10), but the single phase structure is damaged by high Eu content (x = 0.04–0.10). Positron annihilation measurements indicate that the local electron density and the vacancy-type defect concentration increase gradually with the increase in Eu content from 0 to 0.08. Furthermore, abnormal changes in lifetime parameters can be found in x = 0.10 sample induced by the existence of impurity phase in the system. The magnetic measurements reveal that all the samples exhibit two successive magnetic transitions at T = 15 and 11 K. In x = 0.02 sample, the coexistence of ferromagnetism and antiferromagnetism can be found, and a maximum saturation magnetization of 11.548 emu/g at 5 kOe is achieved. The possible reasons for the above observations are discussed in detail.

Microstructure, defect structure and hydrogen trapping in zirconium alloy Zr-1Nb treated by plasma immersion Ti ion implantation and deposition

The effect of low energy plasma immersion ion implantation and deposition of titanium on microstructure, defect structure and hydrogen trapping in zirconium alloy Zr-1Nb was studied. Defect structure and distribution were analyzed by Doppler broadening using slow positron beam. The surface microstructure after modification is represented by nanostructured Ti grains with random orientation. The gradient distribution of titanium as well as vacancy type defects were analyzed. The concentration of vacancy type defects is rising with increasing bias voltage. Gas-phase hydrogenation of the Ti-modified Zr-1Nb alloy was performed at 400 °C for 60 min. The strong interaction of hydrogen with vacancy type defects was demonstrated. Two different changes in the defect structure after hydrogenation were observed: when a titanium film is formed on the surface (after deposition at 500 V) hydrogen trapping occurs with the formation of titanium hydride phases, while in the implanted layer (deposition at 1000 and 1500 V) hydrogen is trapped due to interaction with vacancy type defects. The physical basis of Ti diffusion and its influence on the evolution of defect structure after surface modification and hydrogenation were discussed.

Vacancy-type defects in bulk GaN grown by the Na-flux method probed using positron annihilation

Defects in bulk GaN grown by the Na-flux method have been studied using a positron annihilation technique. Pyramidal bulk samples showed striation and inhomogeneous color distributions. Measurements of the Doppler broadening spectra of the annihilation radiation and lifetime spectra of positrons revealed that the concentration of vacancy-type defects increased with decreasing transparency of the samples. The major defect species was identified as a Ga vacancy coupled with nitrogen vacancies. A correlation between the oxygen incorporation and the introduction of such vacancies was observed. For c-plane GaN grown by a coalescence growth method, the concentration of vacancy-type defects was close to or under the detection limit of positron annihilation technique (≤1015cm−3), suggesting that high-quality bulk GaN can be fabricated using this method.

Evolution of vacancy-type defects and hardening behaviors of T91 induced by 1.625 MeV Fe-ions at different temperatures

Irradiation temperature is one of the most important parameters which determine the evolution of structural materials’ properties. In this study, the evolution of vacancy-type defects and hardening behaviors of T91 steel induced by Fe-ion irradiation at different temperatures were investigated. The 1.625MeV Fe ions were implanted in T91 to the fluence of 2.5×1015ions/cm2 at temperatures from room temperature (RT) to 550°C. After irradiation, the samples were analyzed by positron annihilation spectroscopy (PAS) and nano-indention technology (NIT). It was found that the sample irradiated at RT possessed the largest concentration of vacancy-type defects. At higher temperatures, the variation of vacancy-type defects concentrations were in a trend of Cv550°C<Cv300°C<Cv450°C. All the samples were hardened to different extents after irradiation. The hardness values increased initially and decreased afterward with the increasing temperatures, and the peak hardening is present at 300°C.

Modulation of the Dielectric Properties of CaCu&lt;sub&gt;3&lt;/sub&gt;Ti&lt;sub&gt;4&lt;/sub&gt;O&lt;sub&gt;12&lt;/sub&gt; System with Gd-Substitution

To specify the effects of modulating mechanism of Gd substitution on the dielectric properties for Ca1-xGdxCu3Ti4O12 system, the XRD, SEM and positron positron annihilation technique have been implemented. The results display that both grain size and vacancy-type defect concentration play important roles in controlling the dielectric properties of CaCu3Ti4O12. The dielectric properties are improved by adding an appropriate amount of Gd additives (x=0.01) but weakened by higher Gd doping content since this is closely related to the grain size and concentration of vacancy-type defect in the samples.

Gamma irradiation effect on optical and dielectric properties of potassium dihydrogen phosphate crystals

The effect of Co60 gamma-ray irradiation on potassium dihydrogen phosphate crystals is investigated at doses ranging from 1kGy to 100kGy with different diagnostics, including UV–Vis absorption spectroscopy, fluorescence spectroscopy, DC electrical conductivity, positron annihilation lifetime spectroscopy and Doppler-broadening spectroscopy. The optical absorption spectra show a wide absorption band between 250 and 400nm after γ-irradiation and its intensity increases with the increasing irradiation dose. The simulation of radiation defects show that this absorption is assigned to the formation of substitutional impurity defects due to Al, Mg ions substituting for K ions. The fluorescence peak at 355nm blue shifts after irradiation. The fluorescence intensity is observed to increase with the increasing irradiation dose. The positron annihilation lifetime spectroscopy is used to probe the evolution of vacancy-type defects in potassium dihydrogen phosphate crystal. The variation of size and concentration of vacancy-type defects with the different irradiation dose is investigated. The Doppler-broadening spectroscopy gives direct evidence of the formation of irradiation-induced defects. The dc electrical conductivity of γ-irradiated potassium dihydrogen phosphate crystals increases with the increasing irradiation dose when the dose is less than 10kGy, whereas keeps constant at high irradiation dose of 100kGy. The increase of electrical conductivity is associated with the increase of the proton defect concentration in the crystal. A possible explanation about the change of proton defect concentration with irradiation dose is presented.