Three-dimensional Vacancy Clusters Research Articles

Investigation of Sulphur Nanoclusters in Ageing Ni-S System after Irradiation and Thermal Treatment by Positron Annihilation

The pure Ni and Ni S ageing systems, thermally aged and also irradiated by 5 MeV electrons at 270 K with subsequent annealing, are studied by means of positron annihilation spectroscopy. The formation of ultra ne S-rich precipitates (clusters) is observed in Ni S during thermal ageing at about 650 K. It is assumed that these clusters do not contain vacancy-type defects and they are coherent to the Ni matrix. The positrons localize in sulphur clusters that is due to a nity-induced con nement. In irradiated Ni S system, the mobile vacancy-S atom complexes are formed at post-irradiation annealing above 350 K (stage III). These complexes form the three-dimensional vacancy clusters (nanovoids) decorated with S atoms. The radiation induced nucleation of S-rich clusters occurs when nanovoids dissociate.

Influence of impurities on the evolution of vacancy-type defects in neutron-irradiated nickel

In order to investigate the effect of impurities on vacancy defect evolution in nickel, specimens with high (5N) and technical (3N) purity were neutron-irradiated at ∼330K in the IVV-2M reactor (Russia) to fluencies in the range of 1×1021–1×1023n/m2 (E>0.1MeV) corresponding to displacement dose levels in the range of about 0.0001–0.01dpa and subsequently stepwise annealed to about 900K. The specimens of Ni with different purities were characterized both in as-irradiated state as well as after post-irradiation annealing by positron annihilation spectroscopy. The formation of three-dimensional vacancy clusters (3D-VCs) in cascades was observed under neutron irradiation. The density and size of 3D-VCs depended not only on dose level, but also on purity. The population of 3D-VCs in the technical Ni is lower than that in the high-purity Ni. 3D-VCs collapse into secondary-type clusters (stacking fault tetrahedra (SFTs) and vacancy loops) during stepwise annealing at 350–450K (stage III in Ni). The suppression of secondary cluster formation in 3N Ni is attributed to an effective vacancy interaction with impurity carbon atoms, which based on a relatively large vacancy–carbon atom binding energy (0.32–0.35eV). The trapping of vacancies released at the collapse of 3D-VCs by the interstitial impurity atoms dominates at low irradiation dose level (10−4dpa). Thus, we found that carbon impurity atoms have strong effects both on the primary vacancy-type defect aggregation and on the secondary vacancy clusters and complexes formation.

The effect of neutron irradiation dose on vacancy defect accumulation and annealing in pure nickel

In order to investigate the dose dependence of vacancy defect evolution in nickel, specimens of high-purity Ni were neutron-irradiated at ∼330 K in the IVV-2M reactor (Russia) to fluencies in the range of 1 × 10 21–1 × 10 23 n/m 2 ( E > 0.1 MeV) corresponding to displacement dose levels in the range of about 0.0001–0.01 dpa and subsequently stepwise annealed to about 900 K. Ni was characterized both in as-irradiated state as well as after post-irradiation annealing by positron annihilation spectroscopy. The formation of three-dimensional vacancy clusters (3D-VCs) in cascades was observed under neutron irradiation, the concentration of 3D-VCs increases with increasing dose level. 3D-VCs collapse into secondary-type clusters (stacking fault tetrahedra (SFTs), and vacancy loops) during stepwise annealing at 350–450 K. It is shown that the thermal stability of SFTs grow with increasing dose level, probably, it is due to growth of the average SFT size during annealing. The results of annealing experiments on electron-irradiated Ni at 300 K are indicated in the paper, for comparison. We also have briefly discussed the positron response to the SFT-like structures.

Radiation-induced defects and low-temperature hardening in the Fe–Cr alloy

Positron annihilation spectroscopy was used to study processes of accumulation and annealing of vacancy defects in the Fe-15.7 at % Cr alloy of commercial purity and the influence of these processes on the low-temperature hardening of the alloy upon electron irradiation to a damaging dose of 10−3 dpa in a temperature range of 270–370 K. Tensile tests were carried out both in the process of irradiation of the alloy and upon the subsequent annealing. It is shown that at the initial stages of irradiation the increase in the yield stress is caused by the accumulation of vacancy-carbon complexes. With an increase in the radiation dose, the growth of the yield stress is connected with an increase in the density of three-dimensional vacancy clusters. For the alloy irradiated at 370 K, the possibility of the contribution from a high-chromium phase, which is generated as a result of irradiation-accelerated α-α′ phase separation has also been considered.

Model calculation of positron states in tungsten containing hydrogen and helium

Tungsten is a candidate material for plasma-facing first wall of a fusion power plant. Understanding of defects, tritium and helium behaviour in plasma facing materials [PFM] is an important issue for fusion reactor from viewpoints of its mechanical properties under neutron irradiation. Experiments with high-Z materials show that erosion of these materials under normal operation condition is considerably lower than the plasma induced erosion of low-Z materials like carbon or beryllium. Quantitative understanding of the experimental results for defects in tungsten needs a comprehensive theory of electron-positron interaction. The properties of defects in tungsten containing hydrogen or helium atoms have been investigated by model positron lifetime quantum-mechanical calculations. The electron wave functions have been obtained in the local density approximation LDA to the density functional theory DFT. On the bases of calculated results, the behaviour of vacancies, empty nano-voids and nano-voids with hydrogen and helium were discussed. It was established that hydrogen and helium in larger three-dimensional vacancy clusters in W change the annihilation characteristics dramatically. The hydrogen and helium atoms are trapped by lattice vacancies. These results provide physical insight for positron interactions in tungsten defects and can be used for prediction of hydrogen-H or helium-He4 and (tritium-H3) generation for the design of fusion reactors.

The effects of fast neutron irradiation on oxygen in Czochralski silicon

The effects of fast neutron irradiation on oxygen atoms in Czochralski silicon (CZ-Si) are investigated systemically by using Fourier transform infrared (FTIR) spectrometer and positron annihilation technique (PAT). Through isochronal annealing, it is found that the trend of variation in interstitial oxygen concentration ([Oi]) in fast neutrons irradiated CZ-Si fluctuates largely with temperature increasing, especially between 500 and 700°C. After the CZ-Si is annealed at 600°C, the V4 appearing as three-dimensional vacancy clusters causes the formation of the molecule-like oxygen clusters, and more importantly these dimers with small binding energies (0.1–1.0eV) can diffuse into the Si lattices more easily than single oxygen atoms, thereby leading to the strong oxygen agglomerations. When the CZ-Si is annealed at temperature increasing up to 700°C, three-dimensional vacancy clusters disappear and the oxygen agglomerations decompose into single oxygen atoms (O) at interstitial sites. Results from FTIR spectrometer and PAT provide an insight into the nature of the [Oi] at temperatures between 500 and 700°C. It turns out that the large fluctuation of [Oi] after short-time annealing from 500 to 700°C results from the transformation of fast neutron irradiation defects.

The effect of deuterium and tritium on formation and annealing of vacancy‐type defects in deformed nickel

AbstractPure nickel and nickel loaded with deuterium or tritium were examined during deformation at 270–300 K to 3–45% and subsequent annealing up to 850 K using the measurements of the electrical resistivity and positron annihilation spectroscopy. Three‐dimensional vacancy clusters were formed during deformation in nickel loaded with the hydrogen isotopes unlike in pure nickel. Vacancy clusters decorated with deuterium or tritium atoms created at room temperature, i.e. below a temperature at which monovacancies, are mobile in nickel. Vacancy clusters decorated with deuterium atoms have two types of atomic configurations or degrees of decoration with the binding energy of 0.73 eV and 1.05 eV, respectively. Only one configuration having the binding energy of 1.05 eV was detected for vacancy clusters decorated with tritium atoms. Possible reasons for different behaviors of vacancy clusters in T‐ and D‐loaded nickel are also discussed. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Positron annihilation characteristics of ODS and non-ODS EUROFER isochronally annealed

Yttrium oxide dispersion strengthened (ODS) and non-ODS EUROFER produced by mechanical alloying and hot isostatic pressing have been subjected to isochronal annealing up to 1523K, and the evolution of the open-volume defects and their thermal stability have been investigated using positron lifetime and coincidence Doppler broadening (CDB) techniques. Transmission electron microscopy (TEM) observations have also been performed on the studied samples to verify the characteristics of the surviving defects after annealing at 1523K. The CDB spectra of ODS EUROFER exhibit a characteristic signature that is attributed to positron annihilation in Ar-decorated cavities at the oxide particle/matrix interfaces. The variation of the positron annihilation parameters with the annealing temperature shows three stages: up to 623K, between 823 and 1323K, and above 1323K. Three-dimensional vacancy clusters, or voids, are detected in either materials in as-HIPed condition and after annealing at T⩽623K. In the temperature range 823–1323K, these voids’ growth and nucleation and the growth of other new species of voids take place. Above 1323K, some unstable cavities start to anneal out, and cavities associated to oxide particles and other small precipitates survive to annealing at 1523K. The TEM observations and the positron annihilation results indicate that these cavities should be decorated with Ar atoms absorbed during the mechanical alloying process.

Positron lifetime calculations of defects in chromium containing hydrogen or helium

The characteristics of defects in chromium containing hydrogen or helium atoms have been investigated by positron lifetime quantum calculations. On the basis of calculated results, the behaviors of empty nano-voids and nano-voids with hydrogen or helium were discussed. It was found that hydrogen and helium in larger three-dimensional vacancy clusters change the annihilation characteristics dramatically. The hydrogen and helium atoms are trapped by lattice vacancies. Helium bound with vacancies can form larger size cluster in chromium. These results provide physical insight for positron interactions with defects in chromium and can be used for prediction of hydrogen or helium generation for the design of fission and fusion reactors. The positron lifetime calculations were performed by the standard DFT density functional theory method. The electron wave functions have been obtained in the local density approximation LDA to the DFT.

Slow positron beam study of corrosion-related defects in pure iron

Corrosion-related defects of pure iron were investigated by measuring Doppler broadening energy spectra (DBES) of positron annihilation and positron annihilation lifetime (PAL). Defect profiles of the S-parameter from DBES as a function of positron incident energy up to 30 keV (i.e. ∼1 μm depth) were analyzed. The DBES data show that S-parameter increases as a function of positron incident energy (mean depth) after corrosion, and the increase in the S-parameter is larger near the surface than in the bulk due to corrosion. Furthermore, information on defect size from PAL data as a function of positron incident energy up to 10 keV (i.e. ∼0.2 μm depth) was analyzed. In the two-state trapping model, the lifetime τ 2 = 500 ps is ascribed to annihilation of positrons in voids with a size of the order of nanometer. τ 1, which decreases with depth from the surface to the bulk, is ascribed to the annihilation of positrons in dislocations and three-dimensional vacancy clusters. The corroded samples show a significant increase in τ 1 and the intensity I 2, and near the surface the corroded iron introduces both voids and large-size three-dimensional vacancy clusters. The size of vacancy clusters decreases with depth.

Investigation of lattice defects in the early stage of fatigue in iron by positron annihilation techniques

Positron annihilation lifetime (PAL) and Doppler broadening (DB) techniques have been performed to identify defects, such as vacancy clusters and dislocations, and to study their distributions on the sample in the early stage of fatigue cycle (0–10% of cycles to failure) in pure Fe. From two-lifetime component analysis, the component of the longer lifetime of about 130 ps, corresponding to a mono-vacancy on edge dislocation, has been observed from 0.01% Nf fatigue. However, the long lifetime component of about 200 ps appears in 10% Nf fatigue at a certain position of the sample. Vacancy clusters have possibly been formed to some extent at this fatigue life. The results suggest that the generation of micro-cracks during fatigue test is related to the formation of three-dimensional vacancy clusters.

First-principles calculations of positron lifetimes of lattice defects induced by hydrogen absorption

Positron lifetime spectroscopy is a sensitive tool for the study of vacancy-type defects in solids. Our positron lifetime measurements have revealed that huge numbers of excess vacancies are formed in addition to dislocations during the first hydrogen absorption process in some hydrogen storage materials. In this work, we have performed first-principles calculations of positron lifetimes of vacancies as well as the bulk state in LaNi 5–H and Pd–H systems in order to identify lattice defects formed during hydrogen absorption and during isochronal annealing after hydrogen desorption. By comparison between the experimental and theoretical positron lifetimes, one of the defect components formed during hydrogen absorption can be ascribed to the annihilation at the vacancy clusters composed of two or three vacancies in both LaNi 5–H and Pd–H systems. During isochronal annealing after hydrogen desorption in LaNi 5, the positron lifetime of the vacancy-cluster component in LaNi 5 increases from 190 to over 400 ps, which indicates the formation of three-dimensional vacancy clusters. On the other hand, the vacancy-cluster component in Pd decreases from 200 to 160 ps during isochronal annealing. This suggests that not three-dimensional but two-dimensional vacancy clusters are formed in the recovery process of vacancies in Pd.

Positron Annihilation Study in the Early Stage of Fatigue in Type 304 Stainless Steel

Positron annihilation lifetime and Doppler broadening measurements have been performed to study the early stage of fatigue in type 304 stainless steel at various positions of samples at room temperature. Both the average lifetime of positron and the S parameter have increased with increasing number of fatigue cycles at each position. From two-lifetime component analysis, the component of the longer lifetime of about 170 ps, corresponding to single or di-vacancies, has been observed from 0.01% Nf fatigue, and it has remained stable until 10% Nf of the fatigue life. However, the lifetime has increased rapidly to about 230 ps toward a fatigue life of about 20% Nf at a certain position of the sample. Vacancy clusters have possibly been formed to some extent at this fatigue life. Moreover, the fatigue failure has occurred at almost the same position as the one where the longer positron lifetime was observed. These results suggest that the generation of micro cracks during fatigue test is related to the formation of three-dimensional vacancy clusters. Detailed measurements of positron lifetime and Doppler broadening, have also been carried out for small areas of fractured samples. From these experiments, it has been found that stress concentration occurred at a central position of a sample during fatigue and that this can be probed by the increase of the positron lifetime.

The interaction of deuterium and tritium with radiation and other defects in austenitic steel and nickel

The methods of residual resistivity, annihilation of positrons, and nuclear reactions in nickel and austenitic steels were used to examine the interaction of deuterium and tritium with defects produced by electron- neutron- or ion-irradiation and plastic deformation. It was found that vacancies, dislocations and three-dimensional vacancy clusters captured hydrogen in nickel. Vacancy clusters were formed in the presence of deuterium and tritium at room temperature, when single vacancies were immobile. The type of the hydrogen isotope determined configurations of those clusters. Hydrogen atoms interacted with interstitial atoms at 150 K in nickel and at 220 K in steel. Deuterium segregation occurred both in nickel and austenitic steel under irradiation with deuterium ions, but the segregation exhibited a different behavior. A possible scheme of formation and evolution of deuterium segregation in the studied materials was proposed.

Damage structure due to displacement cascades in 14 MeV neutron-irradiated metals studied by positron lifetime and TEM techniques

Transmission electron microscopy (TEM) and positron lifetime measurements were performed for Au, Cu, Ni, Fe, SUS316 and Fe-9Cr irradiated with 14 MeV D-T neutrons to (2.9–12.0) × 10 19 nm −2 at 333 and 423 K. TEM observations for the specimens irradiated at 333 K showed visible defect clusters in Au, Cu and Ni. There were no visible defect clusters in Fe, SUS316 and Fe-9Cr. Positron lifetime analysis and the trapping model analysis suggested an important role of invisible defect clusters during cascade defect formation. In Fe, the depleted zone from a single cascade event rarely collapses to a dislocation loop, and preferentially forms a three-dimensional vacancy cluster. Three-dimensional vacancy clusters are formed in Cu, Ni, Fe, SUS316 and Fe-9Cr. The size of these clusters is very small (di- or tri-vacancies) in SUS316 and Fe-9Cr because of the trapping of vacancies by impurities or solute atoms.

Cavity formation at indium impurities in bcc metals

The defect formation in the bcc metals W and Mo above annealing stage III and the influence of rare gases on this process were investigated by means of the perturbed angular correlation technique using111In as radioactive probe. In both metals a relatively high electric field gradient (EFG) could be observed at the indium site, characterized by the quadrupole interaction frequencies υQ=263 MHz, ν=0 and υQ=220 MHz, ν≈0.15 for W and Mo, respectively. The observations are assigned to the growth of threedimensional vacancy clusters at the probe atoms with the indium atoms situated in the inner surface of this cavities, thus experiencing the corresponding surface EFG.

Annealing behaviour of α-induced defects in tantalum studied by positron annihilation

Abstract Positron lifetime and Doppler broadening measurements have been carried out to study the isochronal annealing behaviour of defects induced in tantalum irradiated by 32 MeV α-particles. The vacancy migration stage is shifted towards higher temperatures (100°C) and the presence of hydrogen as a common contaminant in this metal is inferred to be the cause behind this observation. Below 300°C, processes such as dissociation of hydrogen-vacancy complexes, vacancy migration leading to the formation of three-dimensional vacancy clusters, and dislocation relaxation forming dislocation or vacancy loops take place. Above this temperature, multiple vacancy-helium complexes are formed. This is soon followed by the release of vacancies from the dissociating vacancy-impurity complexes. Annealing of the defects takes place above 650°C. The effects of isochronal annealing on the positron annihilation parameters demonstrate that helium, trapped inside the metal during the irradiation, plays a significant role in ...

Positron lifetime and perturbed angular correlation measurements of the annealing of quenched Al and dilute AlIn alloys

Positron lifetime and perturbed gamma gamma angular correlation (PAC) measurements are used to compare the annealing of pure Al and dilute AlIn alloys, quenched from 650 K. After quenching, the formation of vacancy clusters as well as single vacancy-In atom and divacancy-In atom complexes is observed. During quenching of all samples the vacancy-vacancy interactions dominate the vacancy-In atom interactions. The single vacancy-In atom pairs disappear around 250 K because they either dissociate or migrate as a unit; in contrast, the divacancy-In atom pairs are converted into larger clusters at around 250 K. The presence of 22 p.p.m. In stabilises the three-dimensional vacancy clusters, so they do not disappear between 270 and 300 K, as observed by positron annihilation for pure Al, but persist up to 450 K. For this alloy, In-rich vacancy clusters are seen by PAC around 300 K, and continuous precipitation of In atoms is observed by positron lifetime measurements from 300 to 450 K.

Radiation damage and annealing studies of ion bombarded cobalt

The effects of implantation-induced radiation damage on the thermal oxidation of cobalt have been studied. Bombardment by both Co+ self-ions and by Xe+ has been studied as a function of ion dose, energy and annealing temperature. A major increase in oxidation was observed for doses of >1016 Co+ cm−2 in agreement with previous studies on Al. The oxidation behaviour as a function of annealing temperature was markedly different for Co+ and Xe+ bombarded samples. For Co+ bombarded samples, damage anneals rapidly in the temperature range 20–300°C due to thermally assisted repair of point defects and vacancy clusters. However, for Xe+ bombardment, it is proposed that the higher annealing temperatures required for damage repair arise due to the stabilisation of three-dimensional vacancy clusters by the oversized Xe atoms. The increase in oxidation after annealing in the temperature range 300–500°C is thought to be due to vacancy release mechanisms which may affect oxide nucleation.

Vacancy recovery in irradiated niobium

Positron lifetime and annihilation lineshape measurements are used to study defect recovery in pure and H-doped Nb after low-temperature electron and neutron irradiation. In neutron-irradiated specimens correlated vacancy migration and the concomitant vacancy clustering within the collision cascades are observed around 160K. However, in electron-irradiated Nb, no sign of vacancy annealing is seen until above 380K where the vacancies agglomerate strongly. According to earlier ideas this is interpreted as being due to hydrogen impurities bound at vacancies. Vacancy migration and clustering are possible only after the dissociation of the vacancy-hydrogen pairs at 380K. The results also indicate that hydrogen impurities play an important role in the stability of small three-dimensional vacancy clusters in Nb.