Helium Precipitation Research Articles

Molecular dynamics study of the role of symmetric tilt grain boundaries on the helium distribution in nickel

Helium impurities, from either direct implantation or transmutation reactions, have been associated with embrittlement in nickel-based alloys. Helium has very low solubility in nickel, and has been found to aggregate at lattice defects such as vacancies, dislocations, and grain boundaries. The retention and precipitation of helium in nickel-based alloys have deleterious effects on the material mechanical properties. However, the underlying mechanisms that lead to helium effects in the host metal are not fully understood. In the present work, we investigate the role of symmetric tilt grain boundary (STGB) structures on the distribution of helium in nickel using molecular dynamics simulations. We investigate the family of STGBs specific to the 〈110〉 tilt axis. The present results indicate that accumulation of helium at the grain boundary may be modulated by details of grain boundary geometry. A plausible correlation between the grain boundary energy and misorientation with the accumulation and mobility of helium is proposed. Small clusters with up to 6 helium atoms show significant interstitial mobility in the nickel bulk, but also become sites for nucleation and grow of more stable helium clusters. High-energy GBs are found mainly populated with small helium clusters. The high mobility of small clusters along the GBs indicates the role of these GBs as fast two-dimensional channels for diffusion. In contrast, the accumulation of helium in large helium clusters at low-energy STGB creates a favorable environment for the formation of large helium bubbles, indicating a potential role for low-energy STGB in promoting helium-induced GB embrittlement.

Helium migration and precipitation in irradiated and annealed copper boron alloy

Helium migration and precipitation in irradiated and annealed copper boron alloy

Variation in the surface morphology of polycrystalline UO2 powder induced by helium precipitation

This report addresses the precipitation of helium in polycrystalline UO2, which deforms the morphology of the grains and their surfaces. The formation of intragranular gas bubbles by the diffused helium can adversely affect the sintering process of ceramic-type nuclear fuel pellets. Helium was injected into pulverized UO2 particles at 1473 K by hot isostatic pressing (HIP). The specific surface area measured by volumetric gas adsorption instrument implied that small pores should exist on the as-helium-treated sample surface. Field-emission scanning electron microscopy observations showed that numerous shallow basins (approximately 500 nm in radius) with hexagonal fringe were formed on the surface. The basin resembles a ruptured blister whose lid has been forced open. SEM observations showed a uniform polygonal-shaped section of the gas bubble on the fracture surface; this implies that precipitated helium forms a negative crystal in the grain. These interesting results suggest the possibility that the rupture of the negative crystal formed in the vicinity of the surface is related to the formation of the basin with a hexagonal fringe.

Formation and Growth of Image Crystals by Helium Precipitation

We found that porous UO2 grain was formed by the precipitation of helium injected by HIP. Scanning electron microscopy analysis showed that polyhedral negative crystals were formed in the sample. The shape of the negative crystal changes dramatically with the conditions of helium precipitation. A truncated octahedron-type, an octa-triacontahedron-type, and a pentacontahedron-type negative crystal were observed. Our study implies that the shape of the negative crystal should change depending on the helium inner pressure enclosed in the negative crystal. It is difficult to arbitrarily control the shapes of these polyhedral negative crystals embedded in a solid medium. However, the shape of the negative crystal can easily be controlled by the helium injection method. In this article, we call the shape controlled negative crystal an image crystal. Here, we report a relationship between the surface energy and the shape and discuss the transformation mechanism of the image crystal. Our detailed observation indicates that the growth process of the image crystal can be explained by a step free energy model rather than an attachment energy. We could not find a cuboctahedron-type negative crystal of which the surface area is larger than that of a pentacontahedron-type negative crystal with the same volume.

Irradiation damage effects on helium migration in sintered uranium dioxide

In this study, the effects of radiation on helium migration are investigated through the analysis of polycrystalline uranium dioxide samples irradiated at fluences up to 5 × 10 15 at. cm −2 with 8 MeV iodine ions. Following irradiation, samples are implanted with 500 keV 3He + ions at fluences in the range of 10 16 at. cm −2. Three nuclear reaction analysis (NRA) techniques are subsequently implemented using the 3He( 2H, 1H) 4He reaction. The influence of temperature using NRA was first studied based upon 3He depth profile changes and the on-line monitoring of helium release. The effect of the sample microstructure was also investigated at the grain scale by performing analyses of the helium spatial distribution with a nuclear microprobe. Neither substantial helium release nor depth profile changes are observed at temperatures below 900 °C in irradiated samples. Following annealing at temperatures above 1000 °C, a substantial proportion of the implanted helium is released from the samples. From this temperature upwards, the two dimensional He cartographies reveal that the gas has been preferentially released in the vicinity of grain boundaries. These results can be interpreted in the light of previous studies in terms of gas precipitation and re-solution. Helium precipitation is enhanced in irradiated samples up to 900 °C because of the presence of irradiation induced defects. At temperatures in excess of 1000 °C, the precipitated helium is partly returned to the matrix hence it is preferentially released in regions adjacent to grain boundaries, which appear to act as defect sinks.

Helium effusion, diffusion and precipitation as a probe of microstructure in hydrogenated amorphous silicon

The effusion of implanted helium was studied for doped and undoped radio frequency plasma-deposited a-Si:H films as well as for Si:H films deposited by other techniques. The results suggest that the observed helium diffusion and precipitation effects give information on the material microstructure.

ANFIBE: A Comprehensive Model for Swelling and Tritium Release from Neutron-Irradiated Beryllium—I: Theory and Model Capabilities

A mechanistic model for the description of helium swelling and tritium release in neutron-irradiated beryllium is presented. Initially aimed at predicting the mechanical stability and the tritium retention capacity of beryllium in a fusion reactor blanket, the ANFIBE code was finally extended to provide an exhaustive description of the properties of this material under fast neutron irradiation. In-solid diffusion and precipitation of helium and tritium, radiation re-solution, and bubble growth and coalescence in different structural domains of the material are taken into account and formulated in a compact rate equation system, enabling the evolution of swelling and release to be calculated under stationary and nonstationary irradiation and temperature conditions. A particular feature of the model is the treatment of the growth of gas bubbles and pores in the interactive compressive stress field created by the gas precipitated in cavities of different sizes and at different pressures, enabling a realistic and accurate calculation of the stress-sensitive intergranular-swelling components and of the related pore-venting effects. The salient physical hypotheses of the model are discussed, as well as the formalism adopted for the description of helium and tritium diffusion precipitation and swelling.

Helium 3 precipitation in AISI 316L stainless steel induced by radioactive decay of tritium: Microstructural study of helium bubble precipitation

This article deals with the study of the influence of thermomechanical heat treatments, aging conditions (temperature and time), and helium concentration on helium bubble precipitation in a 316L austenitic steel. Helium was generated by the radioactive decay of tritium (tritium trick). Helium bubbles impede the grain growth in 316L steel aged at 1373 K and also the recrystal-lization reaction at this temperature if cold working is performed prior to aging. Transmission electron microscopy (TEM) observations indicated a weak helium precipitation at 1073 and 1223 K, presumably due to the presence of trapping sites for tritium, and no bubble growth after aging up to 100 hours. Precipitation sites are mainly dislocations in the matrix at 1073 K and grain boundaries and individual dislocations in the matrix at 1223 K. The large bubble size (50 nm) observed at 1373 K, even for short aging times (0.083 hour), can partly be attributed to bubble dragging by dislocations toward the grain boundaries. Cold deformation prior to aging leads to a larger bubble size due to growth enhancement during recrystallization. Decreasing the helium content leads to a smaller helium bubble size and density. Tritium trapping at helium bubbles may favor helium 3 accumulation on defects such as grain boundaries, as observed by tritium autoradiography.

Helium 3 precipitation in AISI 316L stainless steel induced by radioactive decay of tritium: Growth mechanism of helium bubbles

The growth of helium bubbles in 316L stainless steel in which helium was generated from the tritium decay is examined using image analysis of transmission electron microscopy (TEM) micrographs. The influence of temperature (1073, 1223, and 1373 K), annealing time (0.083 to 1000 hours), cold deformation (92 pct) and helium content (35 and 3.7 appm) on the bubble’s density, volume fraction, and mean size is investigated. For the chosen conditions of helium precipitation and growth (high temperature and large annealing time), the experimental results suggest that the observed increase in the size of the large bubbles present after a 0.083-hour aging at 1373 K proceedsvia a facet limited migration and coalescence mechanism.

Accelerated fracture due to tritium and helium in 21-6-9 stainless steel

The effects of internal tritium and helium on the room-temperature tensile properties of a nitrogen-strengthened stainless steel, forged 21Cr-6Ni-9Mn (NITRONIC 40), were investigated by thermally charging tritium into tensile specimens and aging for selected times. The precipitation of helium as bubbles on dislocations greatly increased the yield strength, and as a consequence of dislocation pinning, the deformation mode changed from long-range dislocation motion to deformation twinning. The tensile specimens exhibited a 90 pct decrease in tensile ductility at 1438 appm 3 He, accompanied by a severe change in fracture mode from ductile rupture to a dominantly intergranular fracture mode. Grain-boundary facets showed multiple striations where deformation twins had intersected the boundaries. Twinning began immediately upon yielding, and at small strains, the microstructure evolved into a fully hardened state, normally observed at 40 pct or greater strain in unexposed or hydrogen-charged 21-6-9. Fracture occurs at low strains in tritium-charged and aged 21-6-9, in part, because helium bubble precipitation causes the deformed microstructure to evolve to a heavily deformation-twinned state at a lower strain. Helium bubble precipitation on the grain boundaries may have caused a further loss in ductility. Fracture appeared to nucleate at the intersection of deformation twins with the grain boundary.

Thermal release behavior of helium implanted into nickel

The thermal release behavior of helium implanted into nickel with 20 keV He + ions at fluences ranging from 1 × 10 17 to 5× 10 17 ions/cm 2 has been studied. The number of helium release peaks increased and the temperature at which the release began became lower with increasing fluence. The surfaces of nickel samples heated after ion implantation were also examined. The release peaks were associated with distinct surface damage features such as holes and blisters. The release peak at high temperatures appeared to be controlled by the thermal vacancy absorption of bubbles. The release peaks at lower temperatures was attributable to the interbubble fracture associated with helium precipitation into bubbles.

Tritium and helium effects on plastic deformation in AISI 316 stainless steel

Pronounced yield strength increases are observed in forged AISI 316 stainless steel when tritium is thermally charged into tensile specimens and allowed to decay to 3He. The yield strength increase persists following low temperature vacuum annealing to remove the tritium. Analysis of helium precipitation by thin foil transmission electron microscopy indicates that prior to either vacuum annealling or mechanical deformation much of the helium present in the material is not in the form of imageable bubbles. However, the apparent volume of precipitate bubbles and the amount of helium included in the bubbles both increase after annealing or deformation. Several simple strengthening models are evaluated considering electron microscopy data, tensile test data and activation volumes calculated from dynamic strain rate change tests. Prior to annealing, the observed strength increases agree poorly with the model predictions. In the vacuum-annealed case, the observed strength increase agrees best with a small-bubble Orowan mechanism prediction, but the strain-hardening and activation volume data support a bubble-shearing mechanism.

A TEM study of helium trapping in cold worked single crystalline molybdenum

With Transmission Electron Microscopy (TEM) the dislocation density in a molybdenum single crystal directly after cold work has been determined. Subsequently, the trapping and precipitation of helium at cold-work induced lattice defects (vacancies, dislocations) has been examined. From a comparison with Thermal Helium Desorption Spectrometry (THDS) data, it was found that the vacancies do not evolve to helium precipitates in a one to one ratio as expected. The observed results can be explained by considering the vacancy production during cold work to be strongly inhomogeneous in space. This is in agreement with resistance recovery data. No significant helium precipitation at dislocations has been observed.

The helium abundance of Saturn from Voyager measurements

The helium abundance in the atmosphere of Saturn is obtained from Voyager measurements by two methods. The first method combines infrared spectra and vertical profiles derived from radio occultation measurements and yields a hydrogen mole fraction of q = 0.963 + or - 0.024 corresponding to a helium mass fraction of Y = 0.06 + or - 0.05. The estimated errors are primarily due to uncertainties in the radio occultation profile and in the abundance of methane, which contributes significantly to the mean molecular weight. The second method is based on the direct inversion of infrared spectra and yields values consistent with those from the first method; however, examination of the sensitivities of the two methods indicates that in the Saturnian case the first approach provides more accurate results. Comparison of the helium abundance of Saturn with that of Jupiter and the sun suggests that helium precipitation is significant in Saturn but may not have begun in Jupiter.

Nucleation of helium precipitates in nickel observed by HDS

Thermal Helium Desorption Spectrometry (HDS) has been used to study the room temperature nucleation of helium precipitates at point defects in Ni(110), notably HeV defects at depth ~ 20 nm below the crystal surface. Helium is injected into the crystal by 50 eV He ion-irradiation which causes no atomic displacements. It has been observed that He nV defects with occupation from n = 2 He to n = 4 He bind helium equally strongly,but weaker than for HeV. For n⩾5 He the binding increases rapidly. The observed behaviour is attributed to helium induced trap mutation and agrees qualitatively with results of atomistic calculations in nickel for this case. Helium precipitation at near surface trapping sites is held responsible for the observed increase of helium release temperatures with helium dose when an undamaged crystal is irradiated. Preliminary TEM observations of Ni specimens irradiated with 50 times higher helium doses than the maximum dose used in the HDS experiments indicated planar clustering of the helium.

Precipitation of helium at implanted substitutional silver atoms in tungsten

Precipitation of helium at implanted substitutional silver atoms in tungsten

Direct evidence for spontaneous precipitation of helium in metals

Using transmission electron microscopy, we have observed helium bubbles and interstitial dislocation loops in gold implanted with helium at below the displacement threshold energy. These results indicate that helium clusters may spontaneously form Frenkel-pair defects in metals.

Room temperature precipitation of helium in molybdenum observed by tem and THDS; Helium platelet formation

A combination of two well established techniques, thermal helium desorption spectrometry (THDS) and transmission electron microscopy (TEM), has led to a method of obtaining useful information on the nucleation of helium clusters in molybdenum at room temperature, and on the cluster growth processes. Although previous knowledge of helium behaviour in metals indicated that helium gas aggregates would be three-dimensional, in fact platelets or discs of helium were discovered. Other surprising observations included the increase of platelet thickness by the mechanism of loop punching, and the transformation of individual platelets into tight clusters of small bubbles rather than a single bubble. This paper describes the results in detail and includes details of the THDS results and helium filling techniques, and some theoretical aspects of the platelet nucleation process.

Auroral helium precipitation

Several measurements of the flux and charge state of auroral helium precipitation have been made and reported between 1967 and 1972. The primary purpose of these experiments has been to deduce the source location of auroral ions by comparison of auroral helium populations with those measured in the ionosphere and the solar wind. Accordingly, results of these experiments are reviewed and summarized, along with measurements of solar wind and ionospheric helium abundances. It is concluded that the available data suggest a solar wind origin of auroral ions and in addition place certain constraints on the acceleration mechanism. The problem of modification of the incident charge state of a helium beam by atmospheric charge exchange is discussed, and it is shown that auroral helium measurements should be made at altitudes above 400 km to insure that the measurements are a true indication of the original beam charge state composition. Attempts to measure optical emissions of auroral helium are also reviewed, with the conclusion that definitive reproducible detection of such emissions has not been accomplished to date.

Diffusion et precipitation de l'helium dans l'oxyde de beryllium irradie et recuit

With the help of several experimental methods we have shown that it is possible to determine from moment to moment the balance of helium in irradiated and annealed BeO. The specific heat at very low temperatures (near the λ-point of helium) establishes the mass of helium which has been precipitated; chromatography gives the amount of helium desorbed during the anneal and electron microscopy determines the radius and mean separation of helium bubbles in BeO. In this way the process of diffusion and precipitation of helium in BeO can be followed accurately. Precipitation of helium begins during irradiation; bubble nucléation begins from 600 °C and peaks at 1200 °C. The diffusion coefficient of helium in BeO satisfies the relation: D = 2.5 × 10 −3 exp (− A/ kT), where A = 3.45 eV between 1080 and 1400 °C.