Nucleation Of Helium Bubbles Research Articles

High temperature helium embrittlement in austenitic stainless steels - correlations between microstructure and mechanical properties

The key for the understanding of the effect of helium on the high temperature mechanical properties of a certain material is its microstructure. This paper reports detailed microstructural investigations of different austenitic stainless steels after in-beam creep rupture tests using transmission electron microscopy. The microstructures of the different materials, characterized by precipitates, dislocations, grain size, and helium bubble sizes and densities, are compared and correlated to the degree of helium embrittlement measured by the change of creep rupture times and strains of in-beam tested specimens and of unimplanted controls. Different microstructures result in large differences in the resistance against helium embrittlement which are correlated to differences in helium bubble microstructure. Most important is the influence of precipitates on the nucleation of helium bubbles, especially in grain boundaries.

The influence of helium and temperature on the microstructure of a dual-ion-irradiated type-316 stainless steel

A commercial AISI 316-Type austenitic steel (FV555) in the solution-treated condition has been irradiated in the range 575–775°C to doses of 3 to 35 dpa in the ORNL dual beam (Ni + He) facility. Irradiation-induced cavity dislocation structure has been investigated by TEM after irradiations which encompassed helium/dpa ratios of 1:1 and 20:1. The evolution of the dislocations and cavity components were found to be closely linked and dependent upon He/dpa ratio. Low-dose structure were characterised by the initial formation of faulted dislocation loops followed by loop growth and formation of small cavities at loops and network dislocations. At higher doses the loops were often ‘petular’ in shape. Extensive nucleation of helium bubbles on loops was seen under a 20:1 He/dpa ratio. Bimodal cavity size distributions apparently consisting of helium bubble and voids were found at 575 and 675°C (30 dpa: 1 appm He/(dpa)) and at 625°C (20 appm He/dpa). The critical diameter for the apparent transition between bubbles and voids was − 6 nm. Intergranular brittleness of the samples was noted between 625 and 725°C even at low He/dpa ratios: this effect occurred when helium bubbles were visible in the boundaries and was also manifest when no bubbles could be resolved. The observations indicate a strong effect of helium on the development of microstructure and of its segregation to grain boundaries leading to subsequent embrittlement at the higher temperatures.

Effects of 600 MeV proton irradiation on nucleation and growth of precipitates and helium bubbles in a high-purity Al-Mg-Si alloy

Solution treated specimens of a high-purity Al-0.75%Mg-0.42%Si alloy were irradiated with 600 MeV protons at 150 and 240°C to a dose level of 0.47 and 0.55 dpa, respectively. Mg 2Si-type precipitates formed during irradiation at 150 and 240°C; at 240°C, however, a large number of precipitates seem to have dissolved during the later stages of irradiation. Thermally aged reference specimens have also been investigated. The needle-shaped precipitates in the aged and the irradiated specimens lie along the 〈100〉 matrix directions. At 150°C bubbles were observed only at grain boundaries whereas at 240°C bubbles were seen in the grain interior as well as at the grain boundaries. Long rows of bubbles were observed with the same orientation in the matrix as the precipitate needles. Grain boundary bubbles were found to grow faster in the Al-Mg-Si alloy than in high-purity aluminium.

Gas diffusion and temperature dependence of bubble nucleation during irradiation

The continuous production of gases at relatively high rates under fusion irradiation conditions may enhance the nucleation of cavities. This can cause dimensional changes and could induce embrittlement arising from gas accumulation on grain boundaries. Computer calculations have been made of the diatomic nucleation of helium bubbles, assuming helium to diffuse substitutionally, with radiation-enhanced diffusion at lower temperatures. The calculated temperature dependence of the bubble density shows excellent agreement with that observed in 600 MeV proton irradiations, including a reduction in activation energy below T m/2 . The coalescence of diatomic nuclei due to Brownian motion markedly improves the agreement and also provides a well-defined terminal density. Bubble nucleation by this mechanism is sufficiently fast to inhibit any appreciable initial loss of gas to grain boundaries during the nucleation period, provided that incubation effects do not occur.

Helium bubble growth in ferritic stainless steel

The nucleation and growth of helium bubbles in a 12% Cr ferritic stainless steel has been investigated by ion implantation and subsequent annealing using transmission electron microscopy. Bubble growth in the temperature range 600–850°C is slower than was anticipated, in part because, with a high bubble density, growth by vacancy collection is not diffusion-limited but is controlled by vacancy source or sink behaviour. Migration and coalescence is found to be the dominant growth mechanism even at temperatures in excess of 0.6 T m .

Kinetics of the growth of helium bubbles at the grain boundaries. formation of the spatially inhomogeneous distribution of helium bubbles near the grain boundaries

In the present paper the kinetics of nucleation and development of gas bubbles near and at the grain boundaries have been investigated. A self-consistent theoretical calculation of the diffusion growth of three ensembles of gas bubbles has been made: bubbles at the grain boundaries, bubbles on dislocations and bubbles in the grain interior. Emphasis is placed on investigating the influence of the spatially inhomogeneous distribution of the dislocation density on the kinetics of growth of ensembles of gas bubbles. The theory formulated makes it possible to explain the more rapid growth of bubbles at the grain boundaries in comparison with the growth of bubbles on dislocations and in the grain interior. We have obtained a spatially inhomogeneous distribution of gas bubbles near the grain boundaries. The basic results of the theory are compared with the results of an experiment on the kinetics of nucleation of helium bubbles under the conditions of annealing near and at the grain boundaries in polycrystal nickel irradiated with alpha particles.

Formation, coalescence AMD stability of helium bubbles in high purity aluminum and some dilute alloys

An electron microscopic study has been made on high purity aluminum and the dilute alloys which were irradiated with 20 keV helium ions. It was found that the nucleation of helium bubbles occurs easily below 673 K, and that the growth and the coalescence of high density bubbles are remarkable above 473 K. It is suggested that the diffusion of vacancies and helium-vacancy complexes control these processes of growth and coalescence. No significant effects of 1000 ppm Mg and Si atoms were observed in these processes. Thermal stability of helium bubbles was recognized to be size dependent. Annealing kinetics of the bubbles was developed by assuming emssion and absorption of helium, and the comparison with the experimental results suggests a large activation energy of about 2 eV for helium emission from the bubbles.

Helium irradiated nickel investigated by positron annihilation

Nickel specimens have been homogeneously implanted with helium ions at about 325 K to a total dose of 500 appm. Distinct defect structures are observed in the irradiated specimen during isochronal annealing. Positron lifetime and Doppler broadening techniques were applied to study the evolution of the defect configurations. The influence of helium on the positron annihilation characteristics is clearly visible. The nucleation of helium bubbles could be determined. At higher annealing temperatures two distinct positron bound states are found. For both states the temperature dependence of the annihilation characteristics is similar to that reported for voids. Clear differences in the annihilation parameters have been established for empty and helium-decorated vacancy clusters.

Nucleation of helium bubbles on dislocations, dislocation networks and dislocations in grain boundaries during 600 MeV proton irradiation of aluminium

High-purity aluminium (99.9999%) was irradiated with 600 MeV protons with a damage rate of 3.5 × 10 −6 dpa/s . Irradiation with 600 MeV protons produces helium and hydrogen at the rate of 140 and 615 appm per dpa, respectively. Specimens irradiated at temperatures in the range 116 to 318 °C to doses in the range 0.04 to 5 dpa were examined in a transmission electron microscope (TEM). The TEM investigation has shown that helium bubbles are formed on dislocations in the grains as well as dislocations in the grain boundaries. Dislocation nodal points whether present in dislocation walls or in grain boundaries are found to be the most favourable sites for bubble nucleation. The mean diameter of the bubbles on individual dislocation lines are found to be larger than those for the bubbles in the matrix. The bubble size and density on grain boundaries vary from boundary to boundary. The size of these bubbles on the boundaries is larger than or equal to the size of those in the matrix. It is suggested that helium atoms once arrived at a dislocation remain bound to the dislocation line but at the same time remain mobile within the dislocation core; the bubble nucleation behaviour in the core would thus be affected by the core structure of the different dislocations. An estimate of the effective helium diffusion in the dislocations relative to that in the lattice has been made on the basis of the measured bubble parameters and the width of the bubble-denuded zone along dislocation lines; the diffusion coefficient of helium in the dislocations is found to be about the same as that in the lattice.

Helium bubble nucleation at grain boundaries

Abstract Helium bubble nucleation has been studied at grain boundaries in a ternary austenitic steel after helium implantation in the temperature range 450–600°C. Transmission electron microscopy shows that all interfaces except the coherent twin are preferred nucleation sites for bubbles, and that the density of bubbles is greater on interfaces containing resolvable grain boundary dislocations (GBDs), A model has been developed which accounts for the bubble density as a function of GBD spacing. This model implies that there is a positive binding energy between He and GBDs until the dislocation spacing becomes very small. The diffusion coefficient of helium along GBDa is much lower than that arising from interstitial diffusion in the lattice. It appears that helium migrates in grain boundaries and along GBDs by a slow vacancy-type mechanism.

Vacancy trapping and helium decoration in Sb ion-implanted tungsten studied by Mössbauer spectroscopy

Mossbauer effect measurements have been performed using sources of119Sb implanted in W without and with post-implanted helium. Each of the sources was subjected to an isochronal annealing sequence in order to study vacancy trapping, helium decoration and recovery of damage. Four sites have been identified for Sb implanted in tungsten; one of these corresponds with substitutional Sb atoms, two others are assigned to Sb atoms associated with vacancies, while the last one can be either vacancy or impurity associated. The development of site occupation as a function of annealing temperature is in accordance with the one-interstitial model. Injection of 2·1016 He/cm2 leads to nucleation of helium bubbles. Helium atoms that are released from these bubbles at about 1300 K are retrapped by Sb atoms to form new bubbles.

Investigation of helium and deuteron irradiated stainless steel (and nickel) by positron annihilation

Abstract Positron annihilation studies have been carried out in austenitic stainless steel (SS 316), irradiated with high energy helium and deuteron ions from a cyclotron. Defect states have been deduced from simultaneous measurements of positron lifetime and Doppler lineshape in post-irradiation anneal up to 1473°K. Clear differences in the annealing characteristics are observed beyond 673°K between the helium- and deuteron-irradiated specimens. Evidence is presented for helium-vacancy interaction and nucleation of helium bubbles. Results on helium in nickel are also discussed for comparison. Helium agglomeration to microbubbles, which are well below the resolution of electron microscopy, is detected around 800°K. In the growth stage of bubbles at higher temperatures, two distinct positron bound states develop both in SS 316 and nickel. Helium retention in large bubbles is found to be stable even at 1473°K in SS 316.

On the low-temperature nucleation and growth of bubbles by helium bombardment of metals

A model for the nucleation and growth of helium bubbles at low temperatures is described. The model characteristics are based on recently published results of helium desorption experiments and on atomistic calculations, which indicate that mass transport is achieved by punching out and migration of self interstitials. Simulations are performed up to a helium content of 30 at % resulting in the prediction of the bubble density, bubble volume, fraction of helium in bubbles and point defect concentration. The results show good agreement with experiment and a possible mechanism of blistering is discussed.

Self-trapping of helium in metals

Atomistic calculations are presented which demonstrate that helium atoms in a metal lattice are able to cluster with each other, producing vacancies and nearby self-interstitial defects. Even a small number of helium atoms is found to be sufficient to create these large distortions. As few as five interstitial helium atoms can spontaneously produce a lattice vacancy and nearby self-interstitial. An eight-helium-atom cluster gives rise to two such defects, and 16 helium atoms to more than five self-interstitial vacancy pairs. It was noted that the self-interstitials prefer to agglomerate on the same "side" of the helium cluster rather than to spread themselves out uniformly. The binding energy of each additional helium atom to these clusters increases with helium concentration and the trap is apparently unsaturable. A rate theory using these atomistic binding energies has been used to calculate the kinetics of helium-bubble nucleation and growth. The results are consistent with measurements of the properties of helium resulting from tritium decay.

The effect of helium on the strength and microstructure of niobium

The effect of helium, from tritium decay, on the mechanical properties of single crystal noibium were determined. The yield stress in compression of helium charged niobium increased at all test temperatures. Annealing decreased the yield stress. Transmission electron microscopy was used to examine the as charged samples and to analyze the effects of annealing on nucleation and growth of helium bubbles. The results support the following conclusions: 1) Helium exists in niobium as clusters at ambient temperatures, 2) these clusters punch out prismatic dislocation loops, 3) the increase in strength due to the presence of helium can be explained by a cluster and bubble shearing model.

The atomistics of helium bubble nucleation in B.C.C. metals

physica status solidi (a)Volume 63, Issue 2 p. K183-K188 Short Note The atomistics of helium bubble nucleation in B.C.C. metals L. M. Caspers, L. M. Caspers Reactor Physics Group, Physics Department, Technological University, Interuniversity Reactor Institute, Delft Search for more papers by this authorM. R. Ypma, M. R. Ypma Reactor Physics Group, Physics Department, Technological University, Interuniversity Reactor Institute, Delft Search for more papers by this authorA. van Veen, A. van Veen Reactor Physics Group, Physics Department, Technological University, Interuniversity Reactor Institute, Delft Search for more papers by this authorG. J. Vander Kolk, G. J. Vander Kolk Reactor Physics Group, Physics Department, Technological University, Interuniversity Reactor Institute, Delft Search for more papers by this author L. M. Caspers, L. M. Caspers Reactor Physics Group, Physics Department, Technological University, Interuniversity Reactor Institute, Delft Search for more papers by this authorM. R. Ypma, M. R. Ypma Reactor Physics Group, Physics Department, Technological University, Interuniversity Reactor Institute, Delft Search for more papers by this authorA. van Veen, A. van Veen Reactor Physics Group, Physics Department, Technological University, Interuniversity Reactor Institute, Delft Search for more papers by this authorG. J. Vander Kolk, G. J. Vander Kolk Reactor Physics Group, Physics Department, Technological University, Interuniversity Reactor Institute, Delft Search for more papers by this author First published: 16 February 1981 https://doi.org/10.1002/pssa.2210630260Citations: 23 Mekelweg 15, 2629 JB Delft, Netherlands. AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Citing Literature Volume63, Issue216 February 1981Pages K183-K188 RelatedInformation