The effect of alloying elements on the vacancy defect evolution in electron-irradiated austenitic Fe-Ni alloys studied by positron annihilation

The vacancy defect evolution under electron irradiation in austenitic Fe-34.2 wt% Ni alloys containing oversized (aluminum) and undersized (silicon) alloying elements was investigated by positron annihilation spectroscopy at temperatures between 300 and 573 K. It is found that the accumulation of vacancy defects is considerably suppressed in the silicon-doped alloy. This effect is observed at all the irradiation temperatures. The obtained results provide evidence that the silicon-doped alloy forms stable low-mobility clusters involving several Si and interstitial atoms, which are centers of the enhanced recombination of migrating vacancies. The clusters of Si-interstitial atoms also modify the annealing of vacancy defects in the Fe-Ni-Si alloy. The interaction between small vacancy agglomerates and solute Al atoms is observed in the Fe-Ni-Al alloy under irradiation at 300-423 K.     

Characterization of defect accumulation in neutron-irradiated Mo by positron annihilation spectroscopy

Positron annihilation lifetime spectroscopy measurements were performed on neutron-irradiated low carbon arc cast Mo. Irradiation took place in the high flux isotope reactor, Oak Ridge National Laboratory, at a temperature of 80 ± 10 °C. Neutron fluences ranged from 2 × 10²¹ to 8 × 10²⁴ n/m² (E > 0.1 MeV), corresponding to displacement damage levels in the range from 7.2 × 10⁻⁵ to 2.8 × 10⁻¹ displacements per atom (dpa). A high density of submicroscopic cavities was observed in the neutron-irradiated Mo and their size distributions were estimated. Cavities were detected even at a very low-dose of ∼10⁻⁴ dpa. The average size of the cavities did not change significantly with dose, in contrast to neutron-irradiated bcc Fe where cavity sizes increased with increasing dose. It is suggested that the in-cascade vacancy clustering may be significant in neutron-irradiated Mo, as predicted by molecular dynamics simulations.     

Investigation by slow positron beam of defects in CLAM steel induced by helium and hydrogen implantation

Hydrogen and helium ion beams delivering different doses are used in the ion implantation, at room temperature, of China Low Activation Martensitic (CLAM) steel and the induced defects studied by Doppler broadening of gamma-rays generated in positron annihilation. Defect profiles are analysed in terms of conventional S and W parameters, measures of relative contributions of low and high-momentum electrons in the annihilation peak, as functions of incident positron energies E up to 30 keV. The behaviours of the S-E, W-E and S-W plots under different implantation doses indicate clearly that the induced defect size has obvious variation with depth, taking values that interpolate between surface and bulk values, and depend mainly on helium ion fluences. The S-W plot indicates that two types of defects have formed after ion implantation.     

Microdefects and 3d electrons in ordered B2-FeAl alloys investigated by positron annihilation techniques

Microdefects and 3d electrons in B2-FeAl alloys with different chemical composition, single crystal of Fe and cold-rolled Fe has been studied by positron lifetime and coincidence Doppler broadening spectroscopy. The coincidence Doppler broadening spectrum of the single crystal of Fe shows the highest 3d electron signal in the spectra of all tested samples. The 3d electron signal in the spectrum of Fe₅₀Al₅₀ alloy is much lower than that of the cold-rolled Fe. This indicates that some of the 3d electrons of Fe atoms and 3p electrons of Al atoms in B2-FeAl alloy are localized to form strong covalent bonds, thus decreasing the probability of positron annihilation with 3d electrons of Fe atoms. With the increase of Al content in B2-FeAl alloys, the 3d electron signal in the spectrum of the alloy decreases, while the open volume of defect increases.     

Ion beam induced defects in solids studied by optical techniques

Optical methods can provide important insights into the mechanisms and consequences of ion beam interactions with solids. This is illustrated by four distinctly different systems.X- and Y-cut LiNbO₃ crystals implanted with 8 MeV Au³⁺ ions with a fluence of 1 × 10¹⁷ ions/cm² result in gold nanoparticle formation during high temperature annealing. Optical extinction curves simulated by the Mie theory provide the average nanoparticle sizes. TEM studies are in reasonable agreement and confirm a near-spherical nanoparticle shape but with surface facets. Large temperature differences in the nanoparticle creation in the X- and Y-cut crystals are explained by recrystallisation of the initially amorphised regions so as to recreate the prior crystal structure and to result in anisotropic diffusion of the implanted gold.Defect formation in alkali halides using ion beam irradiation has provided new information. Radiation-hard CsI crystals bombarded with 1 MeV protons at 300 K successfully produce F-type centres and V-centres having the structure as identified by optical absorption and Raman studies. The results are discussed in relation to the formation of interstitial iodine aggregates of various types in alkali iodides. Depth profiling of and aggregates created in RbI bombarded with 13.6 MeV/A argon ions at 300 K is discussed.The recrystallisation of an amorphous silicon layer created in crystalline silicon bombarded with 100 keV carbon ions with a fluence of 5 × 10¹⁷ ions/cm² during subsequent high temperature annealing is studied by Raman and Brillouin light scattering.Irradiation of tin-doped indium oxide (ITO) films with 1 MeV protons with fluences from 1 × 10¹⁵ to 250 × 10¹⁵ ions/cm⁻² induces visible darkening over a broad spectral region that shows three stages of development. This is attributed to the formation of defect clusters by a model of defect growth and also high fluence optical absorption studies. X-ray diffraction studies show evidence of a strained lattice after the proton bombardment and recovery after long period storage. The effects are attributed to the annealing of the defects produced.     

Irradiation-induced precipitation modelling of ferritic steels

In high strength low alloy (HSLA) steels typically used in reactor pressure vessels (RPV), irradiation-induced microstructure changes affect the performance of the components. One such change is precipitation hardening due to the formation of solute clusters and/or precipitates which form as a result of irradiation-enhanced solute diffusion and thermodynamic stability changes. The other is irradiation-enhanced tempering which is a result of carbide coarsening due to irradiation-enhanced carbon diffusion. Both effects have been studied using a recently developed Monte Carlo based precipitation kinetics simulation technique and modelling results are compared with experimental measurements. Good agreements have been achieved.     

Pressure of stable He-vacancy complex in bcc iron: Molecular dynamics simulations

Molecular dynamic simulation was employed to study the stable state of He-vacancy (He-V) complex in bcc iron. The pressure of He-V complex was calculated using the concept of atomic-level stress. In the case of no initial vacancies introduced in the simulation box, self-interstitial atoms (SIAs) are emitted by the small He cluster. As the number of the He cluster is above a critical value, interstitial-type dislocation loops (I-loop) will be generated. After the interstitial-type defects (SIA or I-loop) were created, it is found that the ratio of He atoms to athermal vacancies keeps nearly constant in the He-V complex.     

Irradiation induced dislocation loop and its influence on the hardening behavior of Fe-Cr alloys by an Fe ion irradiation

Nano indentation analysis and transmission electron microscopy observation were performed to investigate a microstructural evolution and its influence on the hardening behavior in Fe-Cr alloys after an irradiation with 8 MeV Fe⁴⁺ ions at room temperature. Nano indentation analysis shows that an irradiation induced hardening is generated more considerably in the Fe-15Cr alloy than in the Fe-5Cr alloy by the ion irradiation. TEM observation reveals a significant population of the a₀<1 0 0> dislocation loops in the Fe-15Cr alloy and an agglomeration of the 1/2a₀<1 1 1> dislocation loops in the Fe-5Cr alloy. The results indicate that the a₀<1 0 0> dislocation loops will act as stronger obstacles to a dislocation motion than 1/2a₀<1 1 1> dislocation loops.     

Molecular model for annihilation rates in positron complexes

The molecular approach for positron interaction with atoms is developed further. Potential energy curves for positron motion are obtained. Two procedures accounting for the nonadiabatic effective positron mass are introduced for calculating annihilation rate constants. The first one takes the bound-state energy eigenvalue as an input parameter. The second is a self-contained and self-consistent procedure. The methods are tested with quite different states of the small complexes HPs, e⁺He (electronic triplet) and e⁺Be (electronic singlet and triplet). For states yielding the positronium cluster, the annihilation rates are quite stable, irrespective of the accuracy in binding energies. For the e⁺Be states, annihilation rates are larger and more consistent with qualitative predictions than previously reported ones.     

Mechanical properties of Eurofer 97 in Pb-16Li and irradiation effect

To be used in a fusion reactor, structural materials, and in particular steels, has to be selected and optimised in their composition to achieve a reduction in the long-term radioactive waste. A reduction in the long-term radioactive inventory could be reached substituting elements like molybdenum, niobium and nickel with other ones like tantalum and tungsten which have the same functions as alloying elements and, if irradiated, do not produce long lived radioisotopes. The martensitic steel belonging to the family of 8-9% Cr Eurofer 97 is considered the reference structural steel for fusion application. However, only few information are available about its mechanical properties in the liquid eutectic alloy Pb-16%Li. Particularly, the problem of liquid metal embrittlement (LME) has not been studied in detail and the effect of neutron irradiation on LME has not been investigated at all so far. This work presents the results obtained irradiating tensile specimens of Eurofer 97 up to 5.9 dpa in lead lithium. Tensile tests of samples have been performed out of pile in the same alloy at the same temperature at which irradiation was carried out.     

First-principles study of hydrogen in perfect tungsten crystal

Tungsten-based materials are used as the first wall materials in ITER. Hydrogen impurities were introduced via bombarding with the reaction plasma, which are important for the behavior and stability of the tungsten wall. Using the first-principles density functional theory and planewave pseudopotential technique, we have simulated the behaviors of hydrogen atoms inside the perfect tungsten bcc lattice. The binding energies for different interstitial sites were compared to determine the optimal trapping site for the hydrogen atom inside the tungsten lattice. The diffusion barriers for hydrogen atom between nearby trapping sites and the interaction between two interstitial hydrogen atoms were also calculated. The implication of our theoretical results on the hydrogen diffusion and accumulation behavior was discussed.     

Luminescence studies on gamma irradiated RbCl:Sm and RbBr:Smcrystals

This paper presents and discusses the results obtained on the thermoluminescence (TL) and other optical studies of gamma irradiated RbCl:Sm³⁺ and RbBr:Sm³⁺ crystals. Samarium when doped into the RbCl and RbBr is found to enter the host lattice in its trivalent state and act as electron trap during gamma irradiation, there by partially converting itself to Sm²⁺. The photoluminescence (PL) spectra of both RbCl and RbBr crystals doped with Sm exhibit the strong red/orange emissions of Sm corresponding to ⁴G_5/2 → ⁶H_9/2 (red) and ⁴G_5/2 → ⁶H_7/2 (orange) transitions. Z₃ centers are observed in RbBr on F bleaching subsequent to gamma irradiation and a TL glow peak attributable to the same has been identified. The thermoluminescence (TL) glow curve explains the defect annihilation process to be due to the mobilization of two different kinds of traps created as a result of exposure to high energy irradiation in both the cases. Spectral distribution under the thermoluminescence emission (TLE) and optically stimulated emission(OSL), support the idea that defect annihilation process to be due to thermal release of F electron in RbCl:Sm³⁺ and in RbBr:Sm³⁺ crystals. Both Sm³⁺ and Sm²⁺ emissions were observed in the thermoluminescence emission of the crystals.     

Neutron irradiation-induced dimensional changes in MEMS glass substrates

A study is made of radiation-induced expansion/compaction in Pyrex^® (Corning 7740) and Hoya SD-2^® glasses, which are used as substrates for MEMS devices. Glass samples were irradiated with a neutron fluence composed primarily of thermal neutrons, and a flotation technique was employed to measure the resulting density changes in the glass. Transport of Ions in Matter (TRIM) calculations were performed to relate fast (∼1 MeV) neutron atomic displacement damage to that of boron thermal neutron capture events, and measured density changes in the glass samples were thus proportionally attributed to thermal and fast neutron fluences. Pyrex was shown to compact at a rate of (in Δρ/ρ per n/cm²) 8.14 × 10⁻²⁰ (thermal) and 1.79 × 10⁻²⁰ (fast). The corresponding results for Hoya SD-2 were 2.21 × 10⁻²¹ and 1.71 × 10⁻²¹, respectively. On a displacement per atom (dpa) basis, the compaction of the Pyrex was an order of magnitude greater than that of the Hoya SD-2. Our results are the first reported measurement of irridiation-induced densification in Hoya SD-2. The compaction of Pyrex agreed with a previous study. Hoya SD-2 is of considerable importance to MEMS, owing to its close thermal expansivity match to silicon from 25 to 500°C.     

Positron lifetime calculations of defects in fusion irradiated beryllium

Numerical quantum-mechanical positron lifetime calculations were performed for mono-vacancies, di-vacancies, tri-vacancies and small nano-voids containing helium and hydrogen in neutron irradiated beryllium. Helium and hydrogen atoms in the sample after the irradiation are considered as atoms forming interstitial O-type loops. Spherical clusters of vacancies are included in the calculations as a reference. It was found that the presence of He and H atoms significantly changes the positron lifetime in irradiated beryllium. A correlation between the positron lifetime and mutual position of vacancies in nano-voids and interstitial loops was established.     

Microstructure evolution and degradation mechanisms of reactor internal steel irradiated with heavy ions

Structure evolution and degradation mechanisms during irradiation of 18Cr-10Ni-Ti steel (material of VVER-1000 reactor internals are investigated). Using accelerator irradiations with Cr³⁺ and Ar⁺ ions allowed studying effects of dose rate, different initial structure state and implanted ions on features of structure evolution and main mechanisms of degradation including low temperature swelling and embrittlement of the 18Cr-10Ni-Ti steel. It is shown that differences in dose rate at most irradiation temperatures mainly exert their influence on the duration of the swelling transient regime. Calculations of possible transmutation products during irradiation of this steel in a VVER-1000 spectrum were performed. It is shown that gaseous atoms (He and H), which are generated simultaneously with radiation defects, stabilize the elements of radiation microstructure and influence the swelling. The nature of deformation under different temperatures of irradiation and of mechanical testing is investigated. It is shown that the temperature sensitivity of swelling behaviour in the investigated steel, with different initial structures can be connected with the dynamic behaviour of point defect sinks.     

Positron annihilation study of neutron irradiated model alloys and of a reactor pressure vessel steel

The hardening and embrittlement of reactor pressure vessel steels are of great concern in the actual nuclear power plant life assessment. This embrittlement is caused by irradiation-induced damage, and positron annihilation spectroscopy has been shown to be a suitable method for analysing most of these defects. In this paper, this technique (both positron annihilation lifetime spectroscopy and coincidence Doppler broadening) has been used to investigate neutron irradiated model alloys, with increasing chemical complexity and a reactor pressure vessel steel. It is found that the clustering of copper takes place at the very early stages of irradiation using coincidence Doppler broadening, when this element is present in the alloy. On the other hand, considerations based on positron annihilation spectroscopy analyses suggest that the main objects causing hardening are most probably self-interstitial clusters decorated with manganese in Cu-free alloys. In low-Cu reactor pressure vessel steels and in (Fe, Mn, Ni, Cu) alloys, the main effect is still due to Cu-rich precipitates at low doses, but the role of manganese-related features becomes pre-dominant at high doses.     

Effect of oxygen on gamma irradiation of polypropylenes studied by positron annihilation spectroscopy

The aim of this paper is to study the effect of oxygen on the microstructure of PP irradiated by gamma ray. The released gases from the sample in irradiation process were measured by gas chromatography. The results showed that the hydrogen release from the irradiated PP is independent on the irradiation atmosphere. It is only dependent on the irradiation dose that could be interpreted by a simple model. The microstructure of PP samples irradiated under different atmospheres is studied by positron annihilation spectroscopy. It is found that there are two sub-peaks in free volume distribution of PP irradiated under helium, while only one peak in the distribution of PP irradiated under air. It is the direct evidence to the argument that the new voids are formed in the irradiation process under inert gas and the oxygen may induce the voids to merge together. The influence of oxygen on the crystalline of PP irradiated by gamma is also discussed.     

Microstructure evolution of Ge implanted silicon oxide thin films upon annealing treatments

The structural evolution of silicon oxide films with Ge⁺ implantation was traced with a positron beam equipped with positron annihilation Doppler broadening and lifetime spectrometers. Results indicate that the film structure change as a function of the annealing temperature could be divided into four stages: (I) T < 300 °C; (II) 300 °C ? T ? 500 °C; (III) 600 °C ? T ? 800 °C; (IV) T ? 900 °C. In comparison with stage I, the increased positron annihilation Doppler broadening S values during stage II is ascribed to the annealing out of point defects and coalescence of intrinsic open volumes in silicon oxides. The obtained long positron lifetime and high S values without much fluctuation in stage III suggest a rather stable film structure. Further annealing above 900 °C brings about dramatic change of the film structure with Ge precipitation. Positron annihilation spectroscopy is thereby a sensitive probe for the diagnosis of microstructure variation of silicon oxide thin films with nano-precipitation.     

Dependence of resonant positron-molecule annihilation on molecular temperature

Measurements of positron-on-molecule annihilation as a function of positron energy have established that positrons can bind to hydrocarbon molecules via vibrational Feshbach resonances. The magnitudes of these resonances, which grow rapidly with increasing molecular size, are still not well understood. In this paper, the role of molecular temperature is investigated using a specially designed temperature-regulated annihilation cell. Only relatively small changes in annihilation rate are observed when the molecular temperature is varied by a factor of two, from 300 K down to approximately half that value for the alkane molecules pentane and heptane. These results place important constraints on theories of the annihilation rates and positron-molecule binding energies.     

Atomistic simulations of nanometric dislocation loops in bcc tungsten

Small dislocation loops formed from self-interstitial atoms (SIAs) are commonly found in irradiated metals. These defects significantly influence the mechanical properties of the materials. Atomistic simulations are used to describe nanometric circular dislocation loops with Burger’s vectors , and in bcc tungsten. Particular attention is paid to the habit plane of the loop. Two different embedded atom model (EAM) potentials are used. The energetics and geometry of the loops are studied as a function of their size.