He-vacancy Complexes Research Articles

Modeling the Interaction of Helium with Dislocations and Grain Boundaries in Alpha-Iron

Abstract Helium is a ubiquitous impurity in nuclear materials that can have significant deleterious effects on mechanical properties, including deformation and fracture. To determine ways to mitigate the effects of helium it is necessary to understand the behavior of helium with respect to its interaction with various microstructural features. Toward that end, we have employed molecular statics, molecular dynamics, and the dimer method of potential surface mapping to study the fate of helium in the vicinity of dislocations and grain boundaries in alpha-iron. Even at very low temperatures interstitial helium atoms can migrate to dislocations and grain boundaries, where they are strongly bound. The binding energies of helium to these microstructural features relative to the perfect crystal and the migration energies of helium diffusing within them have a strong correlation to the excess atomic volume that exists in these extended defects. Helium atom migration energies within the dislocations and grain boundaries studied are in the range of 0.4–0.5 eV. Helium “kick out” mechanisms have been identified within dislocations and grain boundaries by which interstitial helium atoms replace an Fe lattice atom, creating a stable He-vacancy complex that may be a nucleation site for an He bubble.

The Recovery of Gas-Vacancy-Complexes in Fe Irradiated with High Energy H or He Ions

Isochronal annealing experiments for Fe irradiated with H or He ions were carried out to study interactions between gas atoms and vacancy clusters. The accelerating energy of H and He ions was 1.0 MeV and 3.3 MeV, respectively. The total dose was 1:0 10 21 H ions/m 2 (0.2 dpa) and 9:6 10 19 He ions/m 2 (0.3 dpa), and the irradiation temperature was 300C. Positron annihilation lifetime and coincidence Doppler broadening (CDB) measurements were carried out after irradiation and annealing for 1 hour up to the complete recovery temperature. Microvoids were detected after H or He ion irradiation. The characteristic difference was mainly shown in the recovery of vacancy type defects. The recovery of vacancy type defects introduced by H ion irradiation finished completely at 450C, while that by He ion irradiation finished at 1000C. It suggests that H-vacancy complexes were less stable than He-vacancy complexes. In addition, vacancy clusters formed by He ion irradiation grew after annealing at 600C. This was caused by the emission of He atoms from small He-vacancy clusters and the coalescence between free vacancies and vacancy clusters, and by the migration and coalescence of He bubbles.

Helium implanted ZrHf as studied by time differential perturbed angular correlation and positron lifetime measurements

Time differential perturbed angular correlation (TDPAC) at the 482 keV state of 181Ta and positron lifetime measurements were performed on homogeneously helium implanted ZrHf samples. TDPAC on the as-implanted sample shows the association of 0.12±0.02 fraction of probe nuclei with stacking faults experiencing a local fcc environment. The dissociation of this defect complex from probe nuclei is observed following the annealing treatment at 423 K. TDPAC measurements lead to the conclusion that Hf impurities do not bind He-vacancy complexes or helium bubbles in ZrHf. A recovery stage associated with helium migration is identified from the variation of the relative width of the Lorentzian distribution of the quadrupole frequency in TDPAC measurements. This correlates well with the variation of the helium associated positron lifetime component. The nucleation and growth stages of He bubbles in the sample have been identified by positron lifetime measurements as a function of the annealing temperature on the irradiated sample. Based on the analysis of positron lifetime measurements, helium bubble parameters such as bubble radius and bubble concentration have been extracted in the growth stage, leading to a quantitative understanding of the bubble growth in ZrHf.

Dislocation loops and their depth profiles in He+ and D+ ion irradiated nickel

Abstract Effects of implanted deuterium and helium on the formation of dislocation loops in Ni have been systematically investigated in terms of the irradiation temperature and dose by means of transmission electron microscopy. Significant enhancement of loop formation is observed only for He+ irradiation. The number density of loops at 200° C for He+ irradiation is about one order of magnitude higher than that of D+ irradiation and the difference increases at higher temperatures. Significant decrease in the density occurs at about 300° C for D+ irradiation, but it appears at about 600° C for He+ irradiation. Below 200° C, continuous nucleation of loops is observed only for He+ irradiation. The change in the loop depth distribution suggests that the nucleation of loops for He+ irradiation is enhanced by some defects with a low mobility such as small He-vacancy complexes. The significant decrease in the loop density above 600° C is explained by the decrease in the concentration of the small He-vacancy complexes due to their absorption by large cavities or their escape to the specimen surface.

Release processes of He implanted in Cu and Ni studied by a monoenergetic positron beam

Variable-energy positron-beam studies have been carried out on 70-keV He-implanted Cu and Ni specimens. Doppler broadening profiles of the positron annihilation were measured at a number of different incident positron energies. Observed drastic change of a lineshape parameter induced by He implantation was attributed to the positron annihilation in He-vacancy complexes. From isochronal annealing experiments, it was found that He atoms implanted into the Ni specimen deformed prior to the He implantation were trapped by vacancy-type defects and could not grow up to stable He bubbles. Just after a recovery of such complexes, He atoms were released and diffused back toward the surface of the specimen. As a result, the re-emission of He for the deformed Ni occurred at lower temperature than that for the annealed Ni.

Temperature dependence of4He/3He Exchange in Ni

Abstract The release of 4He implanted to saturation with an energy of 15 keV in polycrystalline Ni by subsequent 3He implantation with equal energy has been measured between 295 and 1053 K. The fraction of 4He that can be exchanged is shown to decrease with increasing temperature up to T≲800 K. At T>800 K, however, it increases drastically. The results can be interpreted by assuming He trapping in two subsystems: gas bubbles and He-vacancy complexes. At sufficiently high density of the complexes He atoms can move along interlinked chains of complexes in “relay-race” type of motion in which each individual He-atom moves only one step and an atom from the complex nearest to the surface is released into the vacuum. At T > 900 K all complexes should be thermally dissociated. In this case bubble migration may be the mechanism responsible for He re-emission.

Microstructural evolution in high energy helium implanted nickel—I. Room temperature ( t − 100° c) implantation

Nickel foils of about 12 μm thickness have been homogeneously implanted with 0 to 6 MeV He ions at room temperature. For helium concentrations ranging between 0.1 and 5 at.% the implantation induced changes of the microstructure have been investigated by measurements of the electrical resistivity, the length and the lattice parameter and by transmission electron microscopy (TEM). Beginning from the lowest implantation dose clustering of irradiation induced vacancies and self-interstitial atoms leads to the formation of stacking fault tetrahedra (SFT), dislocation loops and of a dislocation network. In addition a large number of small He-vacancy complexes is present that are invisible in the TEM at low He concentrations, c He , and can be identified as small cavities for c He ≳ 0.7 at.%. The He-vacancy complexes are characterized by a He-density that is rather low compared to the density necessary to activate the loop punching mechanism for bubble growth: i.e. c He is about one half of the vacancy concentration c V for c He ≲ 0.8 at.%. At higher implantation doses the ratio c He / c V increases slowly up to a value of 1.4. Hence, the limiting pressure for loop punching may be obtained only for c He ≈ 8 at.%. A “model” is proposed that describes this microstructural evolution by the continuous nucleation of stable He-vacancy complexes as a result of high energy collision cascades. This model directly explains the low average helium density observed during the early stages as well as the coexistence of small cavities and SFT's. A saturation of the bubble density at high doses and the slow increase of the He density in bubbles might be understood by cascade overlap.

Microstructural evolution in high energy helium implanted nickel—II. Thermal annealing after room temperature implantation

The microstructural evolution during annealing of thin nickel foils (≈ 12 μm) that had been homogeneously implanted with helium up to concentrations of 1 at.% has been investigated by measurements of the electrical resistivity, by differential dilatometry and transmission electron microscopy (TEM). The most characteristic changes of the microstructure are: (i) for temperatures below annealing stage V of radiation damage annealing ( T </ 650 K) major changes are observed only by a decrease of the electrical resistivity that reveals recombination reactions of small clusters of radiation defects; (ii) by annealing through the temperature range of stage V ( T </ 790 K) a rearrangement and growth of the He-vacancy complexes into bubbles is observed; (iii) at higher temperatures ( T < 1070 K) bubble growth is observed by TEM and differential dilatometry directly shows that thermal vacancies are absorbed by He-bubbles. The average bubble pressure tends to approach the thermal equilibrium pressure.

Microstructural evolution in high energy helium implanted nickel and copper

Thin nickel and copper foils (− 12 μm thick) have been homogeneously implanted with 0–6 MeV α-particles yielding helium concentrations between 0.1 and 5 at.%. These foils have been investigated by measurements of the length and the lattice parameter and by transmission electron microscopy (TEM). Starting at the lowest implantation dose clustering of irradiation induced vacancies and interstitial atoms leads to the formation of stacking fault tetrahedra (SFT) and dislocation loops that eventually interact to form a dislocation network. In addition a large number of small vacancy-helium complexes is present that are characterized by a rather low average He density, i.e. the concentration of He atoms CH, is about one half of the vacancy concentration cv. At higher implantation doses ( c He ≥ 0.6 at.%) c He / c V increases slightly up to a value of 1 and small cavities become visible in the TEM. A “model” is proposed that describes this microstructural evolution by the continuous nucleation of stable He-vacancy (He-V) complexes as a result of high energy collision cascades. This model directly explains the low average helium density ( c He / c V

Influence of He-vacancy complexes on the thermal behavior of nitrogen precipitates in Fe

The thermal evolution of Fe 2 N-type nitride precipitates in Fe metal obtained for N 2 + implantation is shown to be altered by post-bombardment ofa-particles at fluences of 1015 cm −2 . These effects as detected using 57 Fe Conversion Electron Mössbauer Spectroscopy are observed to depend on the type of nitride precipitate present when the a particles are implanted. In the two cases studied intermediate nitrogen rich phases (ε-Fe 3 N are detected in addition to the normal terminal phase γ′'-Fe 4 N. The terminal phase is also restored in the implanted zone at temperatures higher than normal when Fe 2 N-type precipitates arc present during a bombardment. § On leave at the Centre de Recherches Nucléaires, Strasbourg, France.

Defect studies in alpha-irradiated and deformed niobium by positron annihilation

The positron annihilation technique has been used to study the recovery of defects in alpha-irradiated Nb. A comparative study on plastically deformed Nb has also been performed. An antirecovery behaviour of Doppler broadening lineshape parameter, S , is observed only in alpha-irradiated specimens between 350–600°C. This has been identified to arise from the presence of He and to the formation of He-vacancy complexes. A further sharp rise in S occurring in both irradiated and deformed specimens at about 600°C is presumed to be due to the release of vacancies from dissociating vacancy-impurity complexes. Above 650°C, S decreased towards the reference value. In irradiated specimens this recovery is seen till 950°C in two stages, i.e. rearrangement in the configuration and dissociation of He-vacancy complexes, respectively.

Nitrogen trapping in helium-implanted α-Fe

Abstract Annealing behaviour of nitrogen in helium-implanted α-Fe has been studied using the (p, p) resonance broadening technique to probe the depth profiles of nitrogen. By comparing the measured N profiles with the calculated vacancy and depth profiles of He it was observed that migrating nitrogen first occupies the He damaged sites and then the sites corresponding to the He range distribution. The calculated and experimental profiles were in excellent agreement. As a by-product of the trapping procedure the activation energies of the nitrogen dissociation from He-vacancy complexes and pure α-Fe and the modal ranges of the 5–60 keV He in α-Fe were extracted.

Isochronal annealing studies on alpha-irradiated nickel

Abstract Isochronal annealing studies on alpha-irradiated nickel specimens have been performed via the positron annihilation technique using both the Doppler broadening and life-time measurement methods. A two-state trapping model favours the presence of dislocation loops and vacancy clusters like microvoids in the irradiated specimens. The decrease of the positron parameters between 400 and 650 K has been associated with He trapping by vacancy-type defects and He-vacancy complex formation. Above 650 K stabilization of this complex has been observed. The results agree with the model where the He atoms, though initially at interstitial sites, quickly interact with vacancy-type defects and relax to substitutional positions. Present observations favour the idea that a He atom in a metal vacancy (substitutional) acts as a trapping site for positrons.