Transmission electron microscopy studies of thin films of gallium arsenide have shown that the surface regions become non-crystalline (i.e. amorphous) after bombardment at 21 °C with Ne20 ions of energies between 40 and 100 keV. This effect occurs after ion doses of about 1 × 1015 ions cm−1 and the damaged material recrystallizes onto the underlying single crystal in an apparently epitaxial manner during post-bombardment annealing at temperatures between 270 °C and 300 °C. Annealing at temperatures up to ∼700°C in an inert atmosphere causes growth of dislocation loops which result from the initial damage structure. These dislocations have not been studied in detail due to the occurrence of complicating structure which results from the dissociation of GaAs at temperatures above ∼400 °C. GaAs which was bombarded with ∼3 × 1016 60 keV Ne20 ions cm−2 while held at different temperatures between 35° and ∼200 °C contained a dense network of dislocations and dislocation loops, but did not become amorphous. However, bombardment at 40 °C with 1017 60 keV Ne20 ions cm−2 did produce an amorphous surface phase.

The spatial ordering of small helium bubbles (diameter 1.5–2.0 nm) produced in the bcc metals V, W, Mo, Cr, Fe and Ta by ion implantation with 30–50 keV He+ is studied using transmission electron microscopy (TEM) and earlier studies are reappraised. Implantations are at temperatures in the range 300–773 K. Gas-bubble ordering onto {110} planes is observed in all the metals studied. Vor V, W and Mo, the bubbles order fully in three dimensions to form a bcc gas-bubble superlattice oriented parallel with the metal lattice. A similar conclusion is reached also for Cr, Fe and Ta although the study of these metals is less detailed. The bubble lattice parameters are found from bubble spacings determined from electron diffraction patterns. The results, confirmed from bright-field micrographs, are as follows: V, ∼ 3.9 nm at 300 K; W, ∼ 6.2 nm at 775 K; Mo, ∼ 6.2 nm at 675 K; Cr, ∼ 5.1 nm at 300 K; Fe, ∼ 6.0 nm at 300 K; and Ta, ∼ 6.4 nm at 575 K. The results suggest that ordered bcc bubble superlattices are a consistent feature in bcc metals following high-dose implantation with helium at temperatures ⋍0.2Tm, where Tm is the melting temperature of the metal.

Results are given of a transmission electron microscope (TEM) study of the damage structure of selected alloys following ion bombardment with 46 MeV nickel ions in the Harwell variable energy cyclotron (VEC) to simulate fusion reactor exposures up to 40 dpa (̃3 MW-year m-2) at temperatures from 475 to 650°C. The alloys were 316 L (UNS 31603), modified 316-Ti, 316-Nb, a ferritic alloy 1.4914, and the commercial alloy TENELON. The 316 L alloy is being considered for the Next European Torus (NET). At temperatures between 475 and 575°C swelling in the matrix and weld of 316 L was similar but at 625°C swelling in welds was ̃60% greater than in the 316 L matrix. The 316-Ti alloy showed only 2.5% swelling after 40 dpa at 625°C, but a maximum swelling of 10 to 14% was observed in the 316-Nb alloys. Swelling in TENELON was moderate up to 575°C but rose to 5% after 40 dpa at 625°C. Low swelling of 0.2% after 40 dpa was observed in the 1.4914 alloy. The results confirmed the superiority of the ferritic steel over the austenitic steels in respect of void-induced swelling.

IMPLANTATION of metals at temperatures near ambient with helium ions can result in the formation of an ordered array—a superlattice—of small (∼2 nm in diameter) helium bubbles1–11. The superlattice has the same symmetry as the atomic lattice of the metal but with a lattice spacing about 20 times greater. We now report the observation, in helium-implanted gold, of an ordered array—a 'macrolattice'—of artefacts ∼60 nm in diameter, which we believe also to be helium bubbles. Again, the macrolattice has the same symmetry as the gold lattice, but the lattice spacing is about 400 times larger. The coexistence in a single specimen of three separate lattices of very similar structure but which differ so markedly in scale provides a new perspective on ordering phenomena. In particular, an understanding of the processes that drive bubble ordering may provide more general insight into the development of microstructures in metals during ion irradiation12. Such information is of value for the production of wear- and corrosion-resistant surfaces by ion implantation13,14 and the development of materials with appropriate erosion and gas-loading properties for use in future nuclear-fusion reactors15,16.

Dissolved organic carbon (DOC) plays diverse roles in carbon biogeochemical cycles. Here, we explored the link between DOC and pCO2 using high-performance size-exclusion chromatography (HPSEC) with UV254 detection and excitation emission matrix (EEM) fluorescence spectroscopy to determine the molecular weight distribution (MW) and the spectral characteristics of DOC, respectively. The relationship between DOC and pCO2 was investigated in the Poyang Lake wetlands and their adjacent aquatic systems. The results indicated significant spatial variation in the DOC concentrations, MW distributions, and pCO2. The DOC concentration was higher in the wetlands than in the rivers and lakes. pCO2 was high in wetlands in which the dominant vegetation was Phragmites australis, whereas it was low in wetlands in which Carex tristachya was the dominant species. DOC was divided into five fractions according to MW, as follows: super-low MW (SLMW, <1 kDa); low MW (LMW, 1–2.5 kDa); intermediate MW (IMW, 2.5–3.5 kDa); high MW (HMW, 3.5–6 kDa); and super-high MW (SMW, > 40 kDa). Rivers contained high proportions of HMW and extremely low amounts of SLMW, whereas wetlands had relatively high proportions of SLMW. The proportion of SMW (SMWp) was particularly high in wetlands. We found that pCO2 significantly positively correlated with the proportion of IMW, and significantly negatively correlated with SMWp. These data improve our understanding of the MW of bioavailable DOC and its conversion to CO2. The present results demonstrate that both the content and characteristics of DOC significantly affect pCO2. pCO2 and DOC must be studied further to help understanding the role of the wetland on the regional CO2 budget.

This paper surveys and discusses the fundamental aspects of employing ion beams for simulating radiation damage phenomena, particularly those effects which will influence the performance of materials in controlled thermonuclear fusion devices. A wide spectrum of ion beams having a range of energies has been progressively developed and deployed over the past thirty years for fission- and fusion-reactor materials studies. Some of the ways in which ion beams have been used to study fusion materials problems are considered, e.g. microstructural stability, cavity formation and growth, helium and hydrogen gas behaviour, plasma-materials interactions such as sputtering and blistering and the effects of damage and transmutation products on mechanical properties. A large body of data has been obtained and the greater control of experimental parameters that can be obtained using ion beams has enabled a detailed understanding to be obtained of fundamental damage processes and material behaviour under simulated fusion-reactor conditions. The main deficiency in understanding still lies in the inter-correlation of damage between ion and neutron conditions. It is concluded that the technique is still a useful complement to fission-reactor irradiation methods and is likely to have a continuing role to play, particularly in the plasma-materials interaction area, until such time as high-fluence 14 MeV neutron irradiation facilities become available.

Five martensitic, nominally 9 and 11% Cr-W-V-Mn-Ta stainless steels which have been developed as low-activation alloys for fusion-reactor structural applications have been irradiated with 52 MeV Cr6+ ions to 20 dpa at 475°C in the Harwell Variable Energy Cyclotron (VEC). Four of the alloys contained additions of 0.1 wt% Ta and these had been shown in prior tests to have mechanical properties comparable with the conventional FV 448 alloy. Examinations by TEM showed that irradiation-induced precipitates were present on a fine-scale in all of the alloys. These comprised Cr-rich lath-like defects in the 9Cr, Ta-free alloy; small Cr-rich particles in the 9Cr-3W-0.1Ta alloy and Cr-rich planar precipitates in the remaining alloys. Little or no irradiation-induced cavitation was observed. The other important irradiation-induced response was in the dislocation structure in the Ta-containing alloys which comprised an extensive rafted array of elongated a <100> type dislocation loops having major axes aligned in <100> directions. A significant fraction of the presumed a <100> loops contained stacking-fault fringes and analysis suggested that these were Cr2N or Fe4N nitride phase which it is known can form on {001} habit planes. Such nitrides are observed frequently under thermal-annealing conditions in ferritic steels, but less frequently under irradiation. Their formation in relation to the void swelling resistance of ferritic-martensitic alloys is discussed.

Studies have been made of Nimonic PE16 alloy to determine the void swelling under selected temperature-cycle conditions during 46 MeV Ni6+ irradiation to 60 dpa. Six different starting conditions were investigated: 1090° C and 1035° C solution-treated (ST); ST plus 710° C and 750° C aged (STA) and ST + 16% cold-work plus aged (STCWA). All samples were pre-injected with 20 appm He at 525° C and subsequently irradiated in 5 dpa increments after selected temperature changes of ± 25 ° C from a base temperature of 525° C. Two cyclic patterns were explored but in both cases the total void swelling was < 0.25% for all material conditions except 1035° C ST where swelling of 0.4% was observed. The differences in swelling conditions were small and no adverse swelling response was apparent under the cyclic conditions employed. For temperature reductions of 50° C from 550° C and increases of 50° C from 500° C the swelling was reduced with respect to 525°C isothermal values. This was attributed to a large reduction in void concentration, possibly arising from cavity shrinkage by interstitial absorption.

One cast of FV448 and two of FI ferritic steel in the nitrided condition have been irradiated to 100 dpa with 52 MeV Cr6+ ions in the Harwell VEC following pre-injection with 100 appm He to stimulate cavity nucleation. Un-nitrided samples of the same steels have also been irradiated as controls. Irradiations were carried out at 425, 475 and 525 °C, and the pre- and post-irradiation microstructures were characterised using Transmission Electron Microscopy (TEM).

Three austenitic (316 L, 316-Ti, 316-Nb); four high-nickel (IN 625, IN 706, PE 16, Fe-25Ni-8Cr) and four ferritic (CRM 12, FV 448, FV 607, FI) alloys have been irradiated with 46 MeV Ni or 20 MeV Cr ions in the Harwell VEC to simulated fusion-reactor doses up to 110 dpa ( ~ 10 MW-yr m−2) at temperatures from 425 to 625°C. Gas production rates appropriate to fusion were obtained from a mixed beam of He + H2 in the ratio 1:4 He:H with gas/dpa ratios of 13 appm He/dpa and 52 appm H/dpa. The 316 alloys showed irradiation-induced precipitation and swelling as high as 40% in ST 316-Ti after 110 dpa at 625°C. Low swelling (e.g. < 2% at 110 dpa) was observed in the high-nickel alloys. The ferritic/martensitic alloys showed negligible swelling (e.g. < 0.2% in FV 607 after 100 dpa at 475°C). The results demonstrate the high swelling behaviour of 316 alloys and the better swelling resistance of high-nickel and ferritic alloys under simulated fusion conditions.

Although the temperature window of helium ion irradiation for gas bubble superlattice (GBS) formation was found to be in the range of approximately 0.15–0.35 melting point in literature, the thermal stability of He GBS has not been fully investigated. This work reports the experiment using an in-situ heating holder in a transmission electron microscope (TEM). A 3.0 mm TEM disc sample of Mo (99.95% pure) was irradiated with 40 keV He ions at 300 °C to a fluence of 1.0E+17 ions/cm2, corresponding to a peak He concentration of approximately 10 at.%, in order to introduce He GBS. In-situ heating was conducted with a ramp rate of ∼25 °C/min, hold time of ∼30 min, and temperature step of ∼100 °C up to 850 °C (0.39Tm homologous temperature). The result shows good thermal stability of He GBS in Mo with no noticeable change on GBS lattice constant and ordering. The implication of this unique and stable ordered microstructure on mechanistic understanding of GBS and its advanced application are discussed.

Results are given of a TEM study of the void swelling behaviour of CW AISI 321 (En58B), CW FV548 stainless steel and STA Nimonic PE16 alloy during 46 MeV nickel ion irradiation under a near-linear rising temperature ramp. The dose range for the ramp was 0–75 dpa and four temperature ranges were investigated using 50 ° C intervals from 500–550 ° C, 525–575 ° C, 575–625 ° C and 625–675 ° C. Swelling in CW 321 was found to be no greater than that observed after isothermal irradiation to 75 dpa at temperatures corresponding to the end-of-ramp temperatures. This was not the case for CW FV548 where the swelling for the temperature ranges of 525–575 ° C and 575–625 ° C was slightly greater than that observed under isothermal irradiation to 75 dpa. Swelling in STA PE16 in the lower temperature ranges of 500–550 ° C and 525–575°C was much higher than corresponding isothermal values but in the 600–700 ° C temperature range the swelling was close to isothermal values. The results on PE16 indicate that a steady increase in irradiation temperature from just below or just above the low temperature cut-off into the void swelling region produces a significant increase in swelling with respect to that expected under isothermal conditions. Possible interpretations of the observed swelling behaviour in these alloys are discussed.

A simulation of neutron-induced void swelling in the Sandvik 12R72HV alloy has been carried out using 46 MeV nickel-ion irradiation in the Harwell Variable-Energy Cyclotron to doses of 50, 70 and 120 dpa at temperatures of 525, 575, 625 and 675°C. The alloy was irradiated primarily in two starting conditions: ST and STCWA, although some irradiations to 50 dpa only were made on STA material. Examinations using TEM showed that voids were formed over the whole temperature range with maximum swelling occurring in the range 575–625°C. For a dose of 120 dpa at 575°C swelling of ~14% in STCWA and ~9.5% in ST alloy was observed. Radiation-induced recrystallisation was observed at 625 and 675°C but not at the lower temperatures. Irradiation-induced precipitates were observed at all temperatures and comprised of fine-scale distributions at the lower temperatures with coarser precipitates occurring at 625 and 675°C. The phases observed comprised of M23C6; M6C and Si-rich G-phase which was attached to voids. The observations suggest that the 12R72HV alloy will exhibit a medium-to-high swelling response during in-reactor irradiation to 120 dpa.

In order to test their void swelling behaviour under irradiation, several alloys based on the solid-solution nickel alloy Incoloy DS (18Cr-38Ni-Fe) with additions of 0.05, 0.43, 0.92 and 2.24 wt% Si have been studied using 4 MeV helium and 46 MeV nickel ion irradiation in the Harwell VEC. For irradiations of 60 dpa with 10 appm He the void swelling decreased from ~ 0.9% to negligible levels with increasing silicon content. After irradiation to 90 dpa following injection with 10 appm He the 2.24% Si alloy showed <1.0% swelling at an apparent peak swelling temperature of 625° C. This alloy was subsequently irradiated to check the swelling response with concentrations of helium and hydrogen appropriate to fusion-reactor conditions. Following irradiation to 60 dpa after 1000 appm He injection the swelling peak was shifted to 575° C where a swelling maximum of 4% was observed. At 625° C with 1000 appm He alone, swelling was 2.0% compared with 1.2% in samples injected with 1000 appm He +1000 appm H. This small reduction in swelling was associated with a higher cavity (bubble) concentration in the hydrogen implanted sample. Fine-scale precipitation of Ni3Si(γ'), η-carbide and G-phase was observed after irradiation together with helium bubbles attached to the η- and G-phase precipitates. The precipitation and void swelling was significantly greater in irradiated samples containing 1000 appm He than in those with 10 appm and irradiated to 90 dpa. It is concluded that although the Incoloy DS alloy possibly has a potential for fission-reactor core applications it has little to commend it for fusion-reactor use where the high swelling response, microstructural instability and likely long-term induced activation arising from the higher nickel content are clearly undesirable factors.

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.

Microstructural changes induced in metals by ion bombardment have important implications for technology (see, for example, refs 1–3). Helium irradiation can result in the formation of small (∼2nm diameter) helium bubbles in high concentration (∼1025 m–3), ordered on a superlattice. Current theories are all directed towards explaining the formation of a superlattice having the same alignment as the crystal lattice of the metal—the matrix or m orientation. In the face-centred-cubic metal copper, although there was evidence in previous work that some domains4 in the bubble array were in orientations other than m (ref. 5), it may have been assumed that the proportion of such domains was small. Here we report new results that show that a high proportion of the ordered bubble array is in domains that have orientations different from m. We propose that a new mechanism, based on the spatial characteristics of the stress field around an overpressurized bubble, must play a dominant role in the later stages of bubble ordering.