Development of ultra-fine grained W-(0.25-0.8)wt%TiC and its superior resistance to neutron and 3 MeV He-ion irradiations

W-(0.25-0.8)wt%TiC with equiaxed grain sizes of 50-200 nm and nearly full density of 99% was fabricated utilizing mechanical alloying (MA) in different gas atmospheres of H₂, Ar and N₂ and hot isostatic pressing. Microstructural and mechanical property examinations were conducted before and after irradiations with neutrons at 600 °C to 2 × 10²⁴ n/m² and 3 MeV He-ions at 550 °C to 2 × 10²³ He/m². It is found that TiC additions and MA atmospheres significantly affect grain refinement and baseline mechanical properties. The room-temperature fracture strength takes a maximum of 2 GPa for W-(0.25-0.5)%TiC with MA in H₂ (W-(0.25-0.5)TiC-H₂). At 1400-1700 °C superplastic behavior occurs for W-0.5TiC-H₂, but is suppressed for W-0.5TiC-Ar. No neutron irradiation hardening is recognized in W-0.5TiC-H₂ and W-0.5TiC-Ar. The critical fluence for surface exfoliation by He irradiation for W-0.3TiC-H₂ is more than 10 times as large as that for commercially available W materials. These results suggest that ultra-fine grained W-TiC is capable of improved performance as the spallation neutron source solid target.     

Surface Irradiation Damage of 316 Stainless Steel,Vanadium and Molybdenum by Helium Ion Bombardment

Molybdenum, V and 316 stainless steel were irradiated with 50~150 keV He ions at the temperatures between 413 and 1,298K for total doses ranging 1× 10²²~10ײ³ m^?2, and the characteristics of the surface damage were compared. Severe exfoliation was observed in all of these materials for the irradiation at 413±110 and 748±25K. The number of exfoliated skins was larger than that in literature, and increased nearly in proportion with the total dose. It increased in the order Mo<316SS<. When the dose was low, the amount of erosion increased rapidly with total dose, but tended to be saturated for higher doses than 3×10²² m^?2. It increased in the order Mo<V<316SS at 413±110K, while in the order 316SS<Mo<V at 748±25K. At higher temperatures than 923 K, blisters and porous surface were formed and the exfoliation of skins ceased. The amount of erosion increased with increasing incident ion energy in the energy range between 50 and 150 keV at 413±110K for a total dose of 1×10²² m^?2.     

The effect of displacement damage on deuterium retention in ITER-grade tungsten exposed to low-energy,high-flux pure and helium-seeded deuterium plasmas

Samples prepared from polycrystalline ITER-grade tungsten were damaged by irradiation with 20 MeV W ions at room temperature to a fluence of 1.4 × 10¹⁸ W/m². Due to the irradiation, displacement damage peaked near the end-of-range, 1.35 μm beneath the surface, at 0.89 displacements per atom. The damaged as well as undamaged W samples were then exposed to low-energy, high-flux (10²² D/m² s) pure D and helium-seeded D plasmas to an ion fluence of 3 × 10²⁶ D/m² at various temperatures. Trapping of deuterium was examined by the D(³He,p)⁴He nuclear reaction at ³He energies varied from 0.69 to 4.0 MeV allowing determination of the D concentration at depths up to 6 μm. It has been found that (i) addition of 10% helium ions into the D plasma at exposure temperatures of 440–650 K significantly reduces the D concentration at depths of 0.5–6 μm compared to that for the pure plasma exposure; (ii) generation of the W-ion-induced displacement damage significantly increases the D concentration at depths up to 2 μm (i.e., in the damage zone) under subsequent exposures to both pure D and D–He plasmas.     

Radiation damage and surface deformation effects on stainless steel produced by helium-ion bombardment

Radiation damage and surface deformation by neutrons to first-wall CTR materials were simulated by means of 3 MeV helium ions. The irradiation was performed at a CIP cyclotron, with beam intensities of 1–2 μA at room temperature. We have irradiated commercial Romanian, Soviet and Japanese stainless steels (W 4016, 12KH18N10T, W 4541) at doses between 2 × 10¹⁷ and 6 × 10¹⁸ ions per cm². The exfoliations were investigated by means of a TEMSCAN 200-CX electron microscope and a metallographic ORTHOPLAN POL LEITZ microscope. The main post-irradiation characteristics for each type of stainless steel (critical doses for exfoliation, dominant surface morphologies) are discussed. An irradiation facility for obtaining a homogeneous distribution of damage for 27 MeV helium ions (rotating energy degrader) is also presented.     

Research status and issues of tungsten plasma facing materials for ITER and beyond

This review summarizes surface morphology changes of tungsten caused by heat and particle loadings from edge plasmas, and their effects on enhanced erosion and material lifetime in ITER and beyond. Pulsed heat loadings by transients (disruption and ELM) are the largest concerns due to surface melting, cracking, and dust formation. Hydrogen induced blistering is unlikely to be an issue of ITER. Helium bombardment would cause surface morphology changes such as W fuzz, He holes, and nanometric bubble layers, which could lead to enhanced erosion (e.g. unipolar arcing of W fuzz). Particle loadings could enhance pulsed heat effects (cracking and erosion) due to surface layer embrittlement by nanometric bubbles and solute atoms. But pulsed heat loadings alleviate surfaces morphology changes in some cases (He holes by ELM-like heat pulses). Effects of extremely high fluence (∼10³⁰ m⁻²), mixed materials, and neutron irradiation are important issues to be pursued for ITER and beyond. In addition, surface refurbishment to prolong material lifetime is also an important issue.     

Deuterium and helium retentions of V–4Cr–4Ti alloy used as first wall of breeding blanket in a fusion reactor

The deuterium and helium retention properties of V–4Cr–4Ti alloy were investigated by thermal desorption spectroscopy (TDS). Ion energies of deuterium and helium were taken at 1.7 and 5 keV, respectively. The retained amount of deuterium in the sample irradiated at 380 K increased with the ion fluence and was not saturated to fluence of up to 1 × 10²³ D/m². For the irradiation at 773 K, 0.1% of implanted deuterium was retained at the highest fluence. For the helium ion irradiation at room temperature, three groups of desorption peaks appeared at around 500, 850, and 1200 K in the TDS spectrum. In the lower fluence region (<1 × 10²¹ He/m²), the retained helium desorbed mainly at around 1200 K. With increasing fluence, the amount desorbed at 500 K increased. Total amount of retained helium in the samples saturated at fluence up to 5 × 10²¹ He/m² and saturation level was 2.7 × 10²¹ He/m².     

Low-energy helium implantation of aluminum

A series of 20 keV He⁺ implantations was conducted on well-annealed MARZ grade aluminum at fluxes of 6 × 10¹⁴ and 8 × 10¹³He⁺/cm² sec. Three distinct, temperature dependent He release mechanisms were found by He re-emission measurements during implantation, and by subsequent SEM and TEM investigations. At 0.08 of the melting temperature (T_m) gas re-emission rose smoothly after a critical dose of 3 × 10¹⁷He⁺/cm², with extensive blistering. The intermediate temperature range (~0.3 T_m) was characterized by repeated flake exfoliation and bursts of He after a dose of 3 × 10¹⁷He⁺/cm². Rapid He evolution, with hole formation, was found above 0.7 T_m. No significant differences in either gas re-emission or surface deformation were found between the two fluxes employed.     

Tungsten Blister Formation Kinetic as a Function of Fluence,Ion Energy and Grain Orientation Dependence Under Hydrogen Plasma Environment

This work deals with the formation kinetic of tungsten (W) blisters under smooth plasma conditions, i.e. low hydrogen flux and energy in order to analyze the first stages of their formation. In addition, we focus on determining the W grain orientation where blisters grow preferentially. For this purpose, mirror-polished polycrystalline tungsten samples were exposed to hydrogen plasma under fixed hydrogen flux of 2.2 × 10²⁰ m^?2 s^?1, with a fluence in the range of?~?10²⁴ m^?2, ion energy of?~?20, 120 and 220 eV, and sample surface temperature of?~?500 K. The formation of blisters at the surface was investigated using SEM, AFM and EBSD to determine the size, the distribution and the orientation of grain where blisters are formed, respectively. The critical fluence for initiating blisters was established around 2.3 × 10²⁴ m^?2. The evolution of blister size distribution and density is discussed as function of fluence and ion energy. At lower ion energy, i.e. 20 eV, only nanoblisters (less than 150 nm) are observed whatever the fluence value (1.5 and 2.3 × 10²⁴ m^?2). At higher ion energy i.e. 120 and 220 eV, micrometric (~ few to tens of µm) blisters are observed and their density highly depends on fluence. We show that blisters can also be formed on (001) oriented grains contrarily to previous results from the literature where the (111) orientation seemed more favorable. Such information is of importance for tungsten based fusion tokamak operation and design.     

Graphite surface erosion by 100 keV helium and hydrogen bombardment

Surface erosion in pyrolytic graphite by 100 keV ⁴He⁺ and 200 keV H⁺₂ ion bombardment has been observed by scanning electron microscopy. The particle fluence ranged from 1 × 10¹⁷ to 5 × 10¹⁸ particles/cm². Although the surface is eroded at 1 × 10¹⁷ particles/cm² in helium bombardment, it is not eroded so heavily even at 5 × 10¹⁷ particles/cm² in hydrogen bombardment. In helium bombardment flaking is significantly observed at 1 × 10¹⁸ particles/cm², and a cone structure appears at 5 × 10¹⁸ particles/cm², which is produced after the first cover flakes off completely. In hydrogen bombardment at 1 × 10¹⁸ particles/cm², many circular blisters are formed which are sputtered off at 5 × 10¹⁸ particles/cm². The surface roughness of the target also affects the erosion.     

Microscopic radiation damage to air-dried human blood cells caused by 1.7-MeV 1,2,3H and 4He beams

Monocellular layers of air-dried human blood on glass microscope slides were bombarded by 1.7 MeV ^1,2,3H or ⁴He beams at ambient room temperature at one-third atmosphere pressure of helium gas, with particle fluences up to 3×10¹⁷ cm^?2. Air-dried blood cells were studied because such specimens are readily available in hospitals and because air-drying is not expected to alter elemental composition or distribution. Observations of microscopic radiation damage to these specimens indicate approximate upper limits of tolerable fluence prior to cell destruction and therefore indicate where the determination of elemental concentrations in air-dried cells by the technique of charged particle-induced X-ray fluorescence may encounter problems. Characteristic P, S, Cl, and Ca X-ray yields were measured from multicellular layers of air-dried blood during proton bombardment, and neutron emission was measured from triton-bombarded cells fixed in deuterated formaldehye. These measurements are discussed with reference to disruption of cell structure expected from particle irradiation.     

Depth distribution and migration of helium in vanadium at elevated temperatures

The depth profiles and retention behavior of ⁴He implanted at 80 keV into vanadium have been investigated in cold rolled bulk and foil samples. The specimens were implanted at temperatures of 100, 400, and 800° C and at fluences up to 1 × 10¹⁸He/cm². Helium depth distributions were found to be Gaussian for the 100° C implants but exhibited a double peak appearance for some of the highest fluence 400° C and 800° C implants. In polished samples blistering and flaking occurred at both 100° C and 400° C implant temperatures for fluences greater than 5 × 10¹⁷He/cm², but little of the implanted helium appeared to have been released at a result of the exfoliation. Conversely, most of the implanted helium was released from 800 C implant areas though no significant blistering or surface perforation was observed. Blistering was observed to occur only on polished samples, through the release behavior was similar to that of the unpolished specimens on which no blistering was observed at any temperature or fluence. The surface condition of samples polished by different techniques was found to exert an influence on the temperature dependence of release and on the character of the blister topography.     

Ion beam studies of H and He in metals

The use of ion beams to study hydrogen and helium in metals is demonstrated. The ³He (d,p)⁴He nuclear reaction previously has been used together with ion channeling to determine the lattice locations of ion-implanted D and ³He in tungsten. Preliminary results applying these techniques to helium bubble and blister formation in tungsten are also presented and show that changes attributed to helium bubble formation are observed in tungsten at a He fluence as low as 6 × 10¹⁶ He/cm². The retention of ion-implanted deuterium in W, Au, and Pd surfaces is shown to be greatly enhanced by prior He ion-induced lattice damage. The amount of the damage trapping is also found to depend on whether the metal is in single crystal or polycrystalline form.     

A study of the irradiation behaviour of Zr3Al

Zr₃Al is an ordered f.c.c. alloy of the L1₂ type. In this investigation, the stability of the ordered phase when subjected to Ar⁺ ion bombardment was explored using transmission electron microscopy. At low ion fluences (up to 10¹² ions/cm²) individual damaged regions were observed. Imaging with fundamental reflections revealed those regions having a spherically symmetrical strain field whereas imaging with superlattice reflections also revealed disordered regions. At ion fluences above 10¹² ions/cm² overlap of the damage regions occurred thus resulting in a complex damage configuration. In the fluence regime 5 × 10¹⁴ to 1 × 10¹⁶ ions/cm², complete disordering of Zr₃Al initially occurred and this was followed by complete transformation to the amorphous state. The fluence required to reach a particular disordered state and degree of amorphicity was dependent upon the amount of annealing which occurred within the defect cascade at the temperature of the bombardment.     

Surface damage of metallic glasses by argon ion bombardment

The surface damage of metallic glasses Fe₄₀Ni₄₀P₁₄B₆, Fe₄₀Ni₃₈Mo₄B₁₈, Fe₄₀Ni₄₀B₂₀, Fe₈₀B₂₀ and Ni₆₄Zr₃₆ under argon ion bombardment at room temperature has been investigated. Blister formation was observed in the dose range of 1 × 10¹⁷ to 1 × 10¹⁸ ions/cm². At higher doses, blisters disappeared with concurrent roughening of the bombarded surface. Erosional features like cones and pyramids are not observed. Argon induced blisters also disappeared on room temperature aging without any further bombardment after blister formation. Post-irradiation annealing at high temperatures (673 and 873 K) resulted in blister formation, severe surface exfoliation and pin-hole formation. The precipitation of the implanted argon into bubbles is also observed.     

Damage evolution of yttria-stabilized zirconia induced by He irradiation

The study presents an investigation of damage evolution of yttria-stabilized zirconia (YSZ) induced by irradiation of 100 keV He ions at room temperature as a function of fluence. Transmission electron microscopy (TEM), X-ray diffraction (XRD) and atomic force microscopy (AFM) were used in order to study the nature and evolution of structural damage at different levels. Our study shows that various kinds of defects are formed with the increasing fluence. Firstly, at low fluences, from 1 × 10¹⁶ to 4 × 10¹⁶ cm^?2, of which maximum values of displacement per atom (dpa) range from 0.29 to 1.17, an elastic strain which is attributed to the accumulation of irradiation-induced discrete point defects, is presented. Secondly, in the intermediate fluences ranging from 8 × 10¹⁶ to 1 × 10¹⁷ cm^?2 with corresponding dpa varying from 2.33 to 2.91, a large drop of elastic strain occurs accompanied by presence of an intensive damage region, which is comprised by large and interacted defect clusters. Thirdly, at the two high fluences of 2 × 10¹⁷ and 4 × 10¹⁷ cm^?2, of which dpa are 5.83 and 11.65 respectively, a great amount of ribbon-like He bubbles with granular structure and cracks are presented at the depth of maximum concentration of deposited He atoms. The structural damage evolution and the mechanism of formation of He bubbles are discussed.     

Recovery behavior in neutron irradiated β-sic

Reaction sintered β-SiC specimens were neutron-irradiated in the fast breeder reactors, JOYO, Rapsodie and Phenix, at temperatures around 430 to 550°C to a fluence from 4.0 × 10²³to 1.0 × 10²⁷ n/m² (E > 0.1 MeV). A change in the specimen length was accurately measured using a conventional micrometer after isochronal annealing at high temperatures. The specimens irradiated to a neutron fluence of about 10²⁵ n/m² showed a larger dimensional change on anneals than those irradiated to a fluence of 10²⁶ n/m². A change in lattice parameters by annealing was also measured. It showed that the temperature dependence is nearby identical to that of the macroscopic length change. The transmission electron microscopic study of the neutron-irradiated β-SiC specimens showed the formation of irradiation induced defects, considered to be dislocation loops. An increase in the neutron fluence resulted in the growth of the dislocation loops. The dislocation loops in strongly irradiated β-SiC interacted with each other, forming the heavily disturbed dislocation structure. The effect of the neutron fluence on the microstructure and dimendional change was discussed.     

Light ion bombardment sputtering,stress buildup,and enhanced surface contamination

Several effects of high fluence light ion bombardment which are relevant to fusion reactor inner wall problems are under investigation. The impurity loading and lateral stress from high fluence ion bombardments can alter sputtering yields markedly. This is demonstrated with sputtering measurements of 45 keV Kr incident upon Au. Sputtering yields for 150 keV⁴ He onto Au are presented for two different background hydrocarbon partial pressures during bombardment. It is shown that there is a polymerized hydrocarbon buildup on the surface for hydrocarbon partial pressures greater than $\text{1 × 10}{}^{\mathrm{?10}}\text{mm Hg}$ even for ion current densities ranging as high as 4 μA/cm²; true sputtering of the Au has been observed only for lower hydrocarbon partial pressures. Additional effects of oxidizing background gases on sputtering measurements of reactive metals are discussed.It is concluded that the background gas partial pressures, temperatures, and particle fluxes in a fusion device and in simulation experiments must be well defined before sputtering effects can be understood.     

Irradiation Effects on Corrosion Resistance and Microstructure of Zircaloy-4

In order to investigate irradiation effects on nodular corrosion resistance of Zircaloy-4, an out-of-pile corrosion test was conducted using Zircaloy-4 specimens cut from the channel box of a fuel assembly irradiated in the BWR (Monticello reactor) up to the neutron fluence of 1.53×10²⁶ n/m² (E>1MeV). The corrosion test was carried out in high pressure steam of 10.3 MPa at 783 K for 24 h. No nodules appeared on the specimens cut from welded areas of the channel box and nodular corrosion resistance tended to be better with increasing neutron fluence. Microstructural evolution in the form of irradiation-induced release of Fe atoms from Zr(Fe, Cr)2 type precipitates was detected by an analytical electron microscope. It was found that the higher the concentration of dissolved Fe and Cr in the grains of Zircaloy-4, the better the nodular corrosion resistance.     

Modification of Ti6Al4V Corrosion Property by Nitrogen Ion Implantation

In this project the effect of nitrogen ion implantation on the corrosion resistance of Ti6Al4V alloy is studied. Nitrogen ions are implanted with four fluences from 8 × 10¹⁷ to 5 × 10¹⁸ ions/cm² in the Ti6Al4V substrates. The effect of ion implantation on surface morphology and roughness is investigated with Atomic Force Microscopy. X-ray diffraction is used for phase analysis of the samples and as a result Ti₂N(105) in the fluence of 1 × 10¹⁸ ions/cm² and TiN in fluence of 2 × 10¹⁸ ions/cm² are observed. For corrosion resistance analysis in one molar sulfuric acid a potansiostate device is used and the best corrosion resistance is obtained for the fluence 2 × 10¹⁸ ions/cm². Moreover corrosion resistance variation is explained by microstructure considerations.     

Void Formation in Magnesium Aluminate Spinel Heavily Irradiated with Fast Neutrons

Single crystals of magnesium aluminate spinel (MgAl₂O₄) were heavily irradiated to fission neutron fluences from ?1 × 10²⁶ to ?2×10²⁷ n/m² (E>0.1 MeV) at 658 and 1,023 K in FFTF (Fast Flux Test Facility) to investigate their microstructural evolution under heavy neutron irradiation and to confirm their previously observed resistance to void swelling. At 658 K no voids were observed up to 2.29 × 10²⁷ n/m², but 1/4[110] interstitial loops were formed. These loops changed their habit planes from (111) to (110) with increasing neutron fluence. At 1,023 K the 1/4[110] type of interstitial loops grew to form stacking fault netuorks composed of stacking faults on each of the six equivalent (110) planes. Tiny voids were also observed to form preferentially on or near stacking faults after 1.37×10²⁷ n/m². A limited number of very small cavities, contributing to a volumetric swelling of only 0.07%, were also observed in the crystal matrix after 2.17×10²⁷ n/m². The possible mechanisms of suppression of void formation in MgAl₂O₄ are discussed.