Observation of a partially-ordered void lattice in aluminium irradiated with 400 keV Al+ ions

Transmission electron microscopy observations of voids formed in aluminium during irradiation at 50°C and 75°C with 400 keV Al⁺ ions, have shown that partially-ordered void arrays are often present. These arrays occur in high-purity annealed aluminium, which has been implanted with 10^?4 atom/atom helium before ion irradiation. The void concentration is found to be $\text{～3 × 10}{}^{16}\text{/ cm}{}^3$, and the void lattice parameter ～ 700 Å. The ratio of void lattice parameter to void radius is ～ 12. Ordered void lattices have been observed frequently in irradiated body-centred cubic metals but the only previous observation for a face-centred cubic metal was in nickel. Theoretical predictions of void lattices in metals are discussed and related to the observations reported herein.     

Suppression of sputtering of nickel by coverage with self-sustaining thin segregated carbon layers

Sputtering of two-layered films composed of nickel (~5000 Å) and nickel carbide (~1500 Å) at 600° C by 5 keV Ar⁺ bombardment on the nickel side has been studied using Rutherford backscattering of 1.3 MeV H⁺ ions. It is found that the removal rate of nickel atoms from specimens is dependent on ion current density and that the removal rate of nickel atoms is very much smaller than that of carbon atoms when the ion current density is low. During ion bombardments at a low current density carbon segregation by a thickness of nearly two monolayers is observed at the nickel surface. Thus suppression of the removal rate of nickel atoms is ascribed to coverage of the nickel surface with segregated carbon atoms which are continuously supplied by diffusion through the nickel film from the carbide layer.     

Momentum transfer and damage energy gradient effects on helium distributions in type 304 stainless steel

The elastic scattering of 18 MeV α particles and transmission electron microscopy were used to study the effects of atomic displacement damage on preinjected helium distributions in type 304 stainless steel at 500°C. Helium implantation was conducted at 25°C or 500°C. The target foils were bombarded in either the solution annealed or 60% cold worked condition. Damage energy profiles were varied by changing the bombarding ion species (oxygen or hydrogen) or incident ion energy. Theoretical calculations of helium transport by atomic collisions were made for comparison with the experiment. For all of the experimental conditions, helium was shown to be very effectively trapped. No significant changes in helium concentration profiles or long-range migration (&> 100 nm) were induced by the superposition of highly nonuniform atomic displacement profiles at damage levels up to ~20 dpa. Most of the helium could be accounted for by the appearance of small near-equilibrium bubbles.     

Gonflement du nickel irradie par des ions Ni+ de moyenne energie

After briefly summarising the advantages and drawbacks of bombardment with heavy ions, we describe the experimental apparatus we used for studying swelling in nickel. Next we discuss the problems connected with the determination of the number of atoms displaced during ion-bombardment.We studied the influence of the following parameters on the swelling of nickel: temperature, gas content, fluence, ion energy. Swelling varied with temperature and increases to a maximum at 620°C for 500 keV nickel ions. Prior helium-loading of the samples (10ppm) reduces swelling and the mean void size and increases the number of cavities. Swelling grows linearly with fluence up to 25 displacements per atom. The number and size of voids grows rapidly in the early stages of irradiation.We explain how helium reduces swelling in our experiments, and we determine how far the fluences calculated for ion-bombardments need to be corrected to allow valid comparisons with in-pile results.     

Stability of defects created by high fluence helium implantation in silicon

Stability of extended defects created by high fluence helium implantation (50 keV, 5 × 10¹⁶ cm⁻²) from room temperature to 800 °C has been studied using transmission electron microscopy. Our results clearly show that the cavities behave as good sinks for interstitial type defects generated during ion implantation, leading in some cases to the cavity dissolution. A three-dimensional “phase diagram” related to the formation and evolution of interstitial-type defects is also proposed. It is plotted in terms of quantity of damage, annealing time and implantation temperature.     

Corrosion characteristics of Hastelloy N alloy after He+ ion irradiation

With the goal of understanding the invalidation problem of irradiated Hastelloy N alloy under the condition of intense irradiation and severe corrosion, the corrosion behavior of the alloy after He⁺ ion irradiation was investigated in molten fluoride salt at 700 °C for 500 h. The virgin samples were irradiated by 4.5 MeV He⁺ ions at room temperature. First, the virgin and irradiated samples were studied using positron annihilation lifetime spectroscopy (PALS) to analyze the influence of irradiation dose on the vacancies. The PALS results showed that He⁺ ion irradiation changed the size and concentration of the vacancies which seriously affected the corrosion resistance of the alloy. Second, the corroded samples were analyzed using synchrotron radiation micro-focused X-ray fluorescence, which indicated that the corrosion was mainly due to the dealloying of alloying element Cr in the matrix. Results from weight-loss measurement showed that the corrosion generally correlated with the irradiation dose of the alloy.     

Irradiation damage in proton irradiated palladium-iron solid solutions

Transmission electron microscopy was used to investigate the irradiation damage, in particular irradiation induced precipitation (IIP), in Pd-base alloys containing 2, 8, 12 and 18 at % Fe. The specimens were irradiated mainly using 400 keV protons at a current density of 0.16 μA/mm² over the temperature range 110 to 750°C. A few samples containing 2 and 8% Fe were also irradiated using 3 MeV NiP⁺ ions. The irradiation microstructure of the proton irradiated alloys consists of dislocation loops over the temperature range 110 to 550°C and voids up to 650°C in all the alloys. IIP of Pd₃Fe was observed only in the Pd-18% Fe alloy between 110 and 500°C, irradiated to a dose of 0.9 dpa. Pd₃Fe was associated with dislocation loops, voids and grain boundaries. IIP was not observed in the Pd-2,8 and 12% Fe alloys proton irradiated to the same dose, nor to a higher dose of 1.5 dpa. It was also not observed in the 2 and 8% Fe alloys irradiated at 600 and 700°C by 3 MeV Ni⁺ ions.The absence of IIP in the more dilute alloys is attributed to the fast back diffusion of Fe atoms, which is due to the high mobility of vacancies in these alloys. This causes the Fe concentration at the sinks to remain below the solubility limit. Therefore, even though Fe is an undersized solute, the size effect alone is not sufficient for the production of IIP at point defect sinks in most Pd-Fe alloys. It is proposed that IIP can occur only when the alloy concentration is high enough to minimize the rate of back diffusion, which depends not only on the vacancy mobility but also on the concentration gradient near point defect sinks.     

Helium release in uranium dioxide in relation to grain boundaries and free surfaces

Nuclear reaction analyses (NRA) based on the ³He(²H,⁴He)¹H reaction were previously performed to follow the evolution of implanted ³He in polycrystalline UO₂ samples. Experimental results pointed to an enhancement above 800 °C of the diffusion coefficient of helium over several microns in the vicinity of the grain boundaries, with respect to the diffusion coefficient within the grain. This was ascribed to the fact that grain boundaries are probably defect sinks which locally modify the defect concentrations.This study aims at demonstrating the particular effect of grain boundaries on helium migration. To this end, ³He implanted polycrystalline UO₂ samples were cracked then annealed at 900 °C. Helium migration in the vicinity of the grain boundaries and near the crack was investigated by means of NRA microanalyses. Helium depletion extends over far larger distances in the vicinity of the grain boundaries than near the crack. Experimental evidence has been collected of the particular effect of grain boundaries on helium migration, which do not act as free surfaces at which helium atoms are simply released.     

Blistering of niobium due to 0.5 to 9 keV helium and hydrogen bombardment

Blistering of well-annealed niobium single crystals due to 0.5 to 9 keV helium and hydrogen ion bombardment at temperatures between ?110°C and 1000°C has been investigated by Rutherford backscattering (RIBS) in double alignment and with a scanning electron microscope (SEM). For He bombardment blistering was observed by RIBS in the temperature range investigated for all energies above 1 keV. The critical dose at which blisters first appear is about 1 to 2 × 10¹⁷ incident He ions per cm². It increases slightly with increasing ion energy and with decreasing target temperature. Blisters of 500 to 5000 Å in diameter were found. The depth at which the blisters develop increases from ≈ 180 Å for 1 keV to 1100 Å for 9 keV He ions. It is a factor of ≈ 3 larger than the theoretical mean range of the ions in amorphous material. Above ≈ 600°C grain boundaries develop extending also into the unbombarded region. For hydrogen bombardment no blistering could be observed at room temperature up to doses of 2 × 10¹⁹ ions per cm².     

Investigation of damage in Pt,Al, and Al-0.1 at%Mg produced by d-Be and fission-reactor neutrons using positron-annihilation

Specimens of Al, Al-Mg, and Pt were irradiated using the U.C. Davis d-Be neutron source which has a spectrum peaking at ～14 MeV. Lifetime and Doppler-broadening measurements have been made on these specimens in the as-irradiated state, for three fluences, and for subsequent anneals to 500°C. Saturation of positron trapping at the higher fluences was observed in the Pt specimen, with the average trap size increasing after annealing through stage III (vacancy migration). Surviving traps after the 500°C anneal are presumed to be voids. The Al and Al-Mg showed some trapping introduced by the room-temperature irradiation, but complete recovery is seen at 500°C. These trapping sites are assumed to be small vacancy clusters or small dislocation loops. Some loops in very low concentration were observed near grain boundaries in the as-irradiated specimens of Al by TEM.     

Void-swelling in 20% cold-worked FV548 austenitic stainless steel irradiated with 22 MeV C2+ ions or 46.5 MeV Ni6+ ions

A study has been made of the void-swelling behaviour of 20% cold-worked FV548 steel irradiated with 22 MeV C²⁺ or 46.5 MeV Ni⁶⁺ ions in the Harwell Variable Energy Cyclotron after room-temperature pre-injection with 10 ppm helium. The dose dependence of void-swelling under 46.5 MeV Ni⁶⁺ irradiation at 500 and 600°C, and the temperature dependence of void-swelling of specimens irradiated with 22 MeV C²⁺ ions in the range 500–650°C have been established. In addition the temperature dependence of void-swelling in specimens aged for 1000 h at 650° C before irradiation has also been studied. Subsidiary annealing experiments have demonstrated the high recovery resistance of the 20% cold-worked structure in FV548, compared with types 316 and 321 steels.The void-swelling behaviour is interpreted in terms of the balance between the dislocation and NbC point defect sink strengths and the observed resistance to recovery of the cold-worked structure in FV548.     

Microstructural examination of several commercial alloys neutron irradiated to 100 dpa

Fusion reactor design to an integrated first wall loading of 40 MW/y/m² or 500 dpa, produces extreme demands on structural materials. It is very difficult to conceive of material behavior at such high damage level. In an effort to assess the consequences of such damage, a series of commercial alloy specimens have been examined following irradiation with fast neutrons to doses on the order of 100 dpa (20% of goal fluence) at 400°C, 425°C or 540°C. The alloys were iron or nickel based and included ferritic (martensitic) and austenitic alloys. Examinations incorporated transmission electron microscopy and analytical electron microscopy of irradiated specimens. Microstructural examination confirm that ferritic alloys are very low swelling, but precipitate development can be very complex. Austenitic alloys can be very high swelling but no clearly defined microstructural differences were found between two alloys of similar composition but very different swelling response or between alloys of very different composition but with similar levels of swelling. Differences are ascribed to differences in the onset of swelling.     

Studies on Isotope Separation of Lithium by Electromigration in Fused Lithium Bromide and Potassium Bromide Mixture, (II)

Accurate knowledge on the salt composition in the anode compartment is indispensable when ⁶Li is to be highly enriched by electromigration in fused LiBr-KBr mixture. A study was made on the dependence on temperature shown by the salt composition in the anode compartment. It was observed that sustained electromigration led to a salt composition in the anode compartment that was determined by the prevailing temperature. The composition was observed for various temperatures between 380° and 740°C: In terms of the ratio Li/K in chemical equivalent, the values were 1.31 at 380°C, 1.29 at 420°C, 1.31 at 460°C, 1.41 at 500°C, 1.76 at 540°C, 1.82 at 580°C, 1.95 at 620°C, 2.16 at 680°C and 2.34 at 740°C. These results can be explained by assuming that the fused LiBr-KBr mixture is a system composed of two simple salts and their eutectic, and that at temperatures below 550°C, which is the melting point of LiBr, LiBr and KBr are dissolved in fused eutectic, while KBr is dissolved in the fused eutectic and LiBr at temperatures between 550° and 738°C, which latter is the melting point of KBr.     

Positron annihilation studies on alpha-irradiated and deformed niobium

Positron lifetime and annihilation lineshape measurements have been made to study the defect kinetics and He-vacancy interactions in alpha-irradiated Nb. A comparative study on plastically deformed Nb has also been performed. The isochronal annealing results are discussed with a brief review of previous investigations. Dislocation/vacancy loops annealing occurs above 700°C. Positron trapping rate at He-vacancy clusters is seen to increase with the addition of He atoms to the clusters. Results favour the idea of He-bubble growth by an addition of He atoms or vacancies at intermediate temperatures (350–750°C) and by bubble migration and coalescence at high temperatures (800–900 ° C). Annealing out of He-bubbles/He-vacancy complexes is seen above 900°C.     

Correlation of the wetting behaviour of sodium on nimonic PE16 with the surface O/Cr ratio determined by Auger spectroscopy

The contact angle of liquid sodium on Nimonic PE16 was measured by the sessile drop technique, as a function of temperature of the substrate. Prior surface treatments given to the PE16 included mechanical polishing, chemical polishing, and oxidation at 700°C; the effect of subsequent argon-ion bombardment was also studied. Surface analyses by Auger spectroscopy were performed on duplicate specimens after each of the above treatments, and again after ion bombardment. Although on any of the surfaces at any stage there were between six and nine elements present, the only obvious correlation was that between the low-temperature (160°C) contact angle and the oxygen/chromium atomic ratio at the surface. Up to about 260°C the sodium was apparently inert and non-wetting but beyond that temperature the contact angle dropped rapidly until at about 500°C irreversible wetting occurred. At the same time the correlation with the oxygen/chromium surface atomic ratio disappeared.     

Formation of Helium Porosity in Different Materials during Post-Radiation Annealing

The characterisitics of the development of helium porosity in bcc and fcc alloys and structural steel after irradiation with 40-keV He⁺ up to dose 5⋅10²⁰ m⁻² at 20°C and subsequent annealing at 650°C for 1 h and 5 h are studied by transmission electron microscopy. It it found that under these conditions smaller bubbles with high density are formed in bcc than in fcc materials. It is shown that for an annealing time of 5 h higher porosity is formed in all materials, except nickel, than with 1 h annealing. This is due to the inflow of thermal vacancies from the free surface. The data obtained are discussed from the standpoint of the formation of various helium-containing complexes, their thermal stability, and the diffusion mobility of the matrix atoms. __________ Translated from Atomnaya Energiya, Vol. 99, No. 2, pp. 115–120, August 2005.     

The effect of a temperature change on void-swelling in electron-irradiated stainless steel type 316

The void-swelling behaviour of stainless steel type 316 has been investigated in electron-irradiation experiments to doses of 40 dpa, involving a temperature change between 475°C and 575°C, or vice versa, after 20 dpa. In the high/low temperature cycle the swelling was commutative. In the low/high temperature cycle the swelling rate, even at 575°C, was characteristic of 475°C. The dominant point-defect sinks are the dislocations and the effects can be understood in terms of changes in the dislocation density. A low/high temperature cycle is beneficial as long as the dislocations dominate the voids as point-defect sinks and thermal vacancies are unimportant. Implications for the operation of fast breeder reactors are discussed.     

Dancoff Factor in Arrayed-Type Cladded Fuel Lattice

The initial-stage sintering mechanism of hyperstoichiometric urania prepared by sol-gel process was determined in relation to temperature during constant rate heating (CRH). The urania powder used in this experiment was prepared by crushing in Ar atmosphere the micro- spheric gel of UO₂ obtained by sol-gel process, and reducing the resulting powder by heating in H₂ for 1 hr at 500°C. The results obtained from densification measurements indicated that the initial-stage sintering proceeded in two phases governed by different shrinkage mechanisms, as follows.

1. The sintering up to 675°C would be due to a mechanism such as rearrangement of grains and/or plastic flow.

2. Sintering from 750° to 800°C was interpreted as being controlled by uranium volume diffusion.

The estimated diffusion coefficient D = 1.42×10^?6 exp(-52,500/RT) cm²/sec. This value agreed in order of magnitude with the uranium diffusion coefficients measured by other workers for hyperstoichiometric urania.     

Light particle irradiation effects in Si nanocrystals

Si nanocrystals, formed by Si ion implantation into SiO₂ layers and subsequent annealing at 1150°C, were irradiated at room temperature either with He⁺ions at energies of 30 or 130 keV, or with 400 keV electrons. Transmission electron microscopy (TEM) and photoluminescence (PL) studies were performed. TEM experiments revealed that the Si nanocrystals were ultimately amorphized (for example at ion doses ∼10¹⁶ He cm⁻²) and could not be recrystallized by annealing up to 775°C. This contrasts with previous results on bulk Si, in which electron- and very light ion-irradiation never led to amorphization. Visible photoluminescence, usually ascribed to quantum-size effects in the Si nanocrystals, was found to decrease and vanish after He⁺ ion doses as low as 3 × 10¹²–3 × 10¹³ He cm⁻² (which produce about 1 displacement per nanocrystal). This PL decrease is due to defect-induced non-radiative recombination centers, possibly situated at the Si nanocrystal/SiO₂ interface, and the pre-irradiation PL is restored by a 600°C anneal.     

Nickel-ion bombardment of annealed and cold-worked type 316 stainless steel

Bombardment with high doses of 5 MeV nickel ions has produced swellings as high as 90% and 60%, respectively, in annealed and 20% cold-rolled Type 316 steels. The steels contained 15 ppm of cyclotron-injected helium. Swellings were determined by both transmission electron microscopy and by a step-height method that measures the total swelling integrated along the ion path. The swelling in annealed Type 316 has a pronounced peak in the vicinity of 625°C, which is about 155°C higher than the peak swelling temperature in-reactor. The magnitudes of the swelling, void densities and void sizes produced in annealed Type 316 by nickel ions and in-reactor at the respective peak swelling temperatures are similar and it is concluded that the nickel ion bombardments provide an acceptable simulation of in-reactor behavior. Using the high dose ion results to guide extrapolation of presently available EBR-II data to higher fluences leads to the prediction that the swelling of annealed Type 316 steel at the peak swelling temperature will reach 40% at $\text{2 × 10}{}^{\mathrm{p23}}\text{n/cm}{}^2\text{(E > 0.1 MeV)}$ in EBR-II core, and 70% at $\text{3 × 10}{}^{23}\text{n/cm}{}^2$. These fluences in EBR-II correspond to 155 and 230 dpa respectively. Twenty percent reduction by cold-rolling reduces the ion produced swelling by 35% at 625°C and by 50% at 575°C.