Mechanical behaviour versus structure of Al63.6Cu24.0Fe12.4

A series of polyquasicrystalline ingots with the nominal composition Al63.6Cu24.0Fe12.4 has been prepared by conventional drop casting and subsequent annealing at 1088 K for 24 h. Compression specimens prepared from the different ingots have been deformed at temperatures ranging from 750 to 1050 K. The details of the structure and microstructure of the as-cast, annealed and deformed materials have been investigated by optical, scanning and transmission electron microscopy. Whatever their initial structure (i.e. pure icosahedral, modulated or relevant to approximant phases) and their defect microstructure (dislocations and crystalline platelets), all the specimens, which have been compression tested, exhibit similar mechanical behaviours.

Modelling oriented investigations of primary radiation damage in ultra high purity Fe and FeCr alloys

Anna Prokhodtseva

In the quest of the structural materials for the future fusion reactor, it has been shown that ferritic/martensitic (F/M) steels are very promising candidates, with a good radiation resistance in terms of damage accumulation in the microstructure relative to for example austenitic steels. However, our ability to predict their response to irradiation in such harsh conditions is still not satisfactory. In this view, there is a critical need for information on the primary damage occuring in this materials class, as input to the multiscale modelling of the irradiation effects, including the impact of He and H. Despite numerous studies in the last 50 years, there is still lack of knowledge in many areas of the irradiation response of the microstructure of ferritic steels. This is due to the complexity of these materials, for they contain numerous alloying elements, grains boundaries and precipitates. The strategy nowadays to identify basic mechanisms of primary damage is to investigate so-called model alloys of those, which allows parametric studies in simplified structures. In this work ultra high purity Fe and Fe(Cr) model alloys were investigated in a transmission electron microscope (TEM) under ion irradiation in an attempt to better understand the fundamental mechanisms of radiation damage starting from the lowest doses, and the dependence on Cr content. Attention is also paid to the effects of He and H. Radiation induced dislocation loops and cavities were quantified. This study provides data for validation of the modelling efforts. Single, dual and triple beam ion irradiations were performed in JANNuS facility located on two sites, in Orsay and in Saclay in France. In Orsay, electron transparent thin foils of UHP Fe, Fe -5, -10 and -14Cr were irradiated in situ in TEM as a single beam experiment with 500 keV Fe+ ions and as a dual beam experiment with 500 keV Fe+ and 10 keV He+ ion beams, to a dose of 1 dpa with and without 1000 appm/dpa He at room and liquid nitrogen temperatures in order to observe the very first defects, desirably before their thermal evolution. Effects of dose rate on the produced damage were assessed in UHP Fe. The impact of He and Cr on defect accumulation was scrutinized in terms of the number density, size and Burgers vector of the visible defects. Special care was taken in the analysis of the TEM micrographs, for which new techniques were developed, with a particular one to determine the Burgers vector of a dense dispersion of fine nanometric defects. Emphasis was put on the identification of the loops Burgers vector, which is considered to be either a0〈100〉 or 1/2 a0〈111〉 in ferritic materials. 2 In order to study the effect of the free surfaces of the electron transparent thin specimens on the irradiation induced microstructure, the results obtained for TEM thin foils were compared to bulk samples that were irradiated ex situ in Saclay in single beam experiments with 24 MeV Fe8+ ions, and dual beam ones with 24 MeV Fe8+ to 1 dpa and 2 MeV He+ ions 1000 appm/dpa He at RT. To study synergistic effects of He and H a triple beam bulk irradiation with 24 MeV Fe8+, 2 MeV He+ and 0.6 MeV H+ ions, to 1 dpa, 1000 appm/dpa He and 4000 appm/dpa H at room temperature was preformed. From the bulk specimens thin lamellae were extracted by focused ion beam, which allowed the TEM observation of the damage through the whole implanted range of about 3.5 μm. In this way, the effect of irradiation with and without He and synergistic effects of simultaneous He and H implantation on the produced defects were assessed. In addition, radiation induced hardening was assessed by nano-indentation and related to the radiation induced microstructure. The analysis shows that in thin foils of UHP Fe the defect population is dominated by a0〈100〉 defects, while in presence of He it is dominated by 1/2 a0〈111〉 loops after irradiation to 1 dpa. It indicates that helium stabilizes mobile 1/2 a0〈111〉 loops, which in thin foils otherwise escape to 2 the free surfaces. Irradiation to the lowest dose of 0.05 dpa resulted in the production of either increased ratio of 1/2 a0〈111〉 loops, compared to the highest dose, or in the entire population of 2 1/2 a0〈111〉 loops in all materials in single and dual beam. In bulk UHP Fe and Fr(Cr) samples 2 after single and dual beam irradiation mainly 1/2 a0〈111〉 loops and few a0〈100〉 loops were 2 observed, but of smaller size than in thin foils, emphasizing the effects of free surfaces on the type of produced loops. It is thus inferred that 1/2 a0〈111〉 loops dominate the early loop 2 2 population and visible a0〈100〉 loops observed in UHP Fe and Fe(Cr) thin foils stem from addition and/or absorption reactions between 1/2 a0〈111〉 loops. It follows that these reactions 2 are reduced by the presence of He, which may eventually impede the formation of a0〈100〉 loops, leaving a loop population dominated by 1/2 a0〈111〉 loops. The same type of reactions 2 are valid for Fe(Cr) alloys, but the reduction in mobility of 1/2 a0〈111〉 loops by Cr in the single 2 beam case and the synergistic effects of He and Cr after dual beam irradiation result in a mixed population of 1/2 a0〈111〉 and a0〈100〉 type loops. This confirms the idea that the formation of 2 visible a0〈100〉 loops is promoted by the presence of free surfaces. Irradiation of UHP Fe thin foil at liquid nitrogen temperature confirmed the assumption of initial 1/2 a0〈111〉 loops. These then escape to the free surfaces after warming to RT, the more so 2 in the thinnest regions of the sample leaving a loop population dominated by a0〈100〉 loops. In single beam irradiated UHP Fe no dependence of the total density of defects or their Burgers vector on the rate of irradiation was observed. However, in the presence of He, only 1/2 a0〈111〉 2 loops formed under higher dose/implantation rates, while at the lowest dose rate results were similar to the single beam. This observation is attributed to the local increase of the effective He/defect ratio in the case of the high dose rate due to the increased temporal overlap of displacement cascades. Regarding mechanical properties, it appears that hardness of the as received materials increases with increasing Cr content. Following irradiation there is a significant hardening, which is not monotonous with Cr content, and which increases for all materials when irradiated together with He, and the more so when irradiated simultaneously with He and H. Hardening relates well to the observed microstructure and is the largest for Fe-5Cr.

EPFL2013

Submicro- and Nano-Structured Porous Materials for Production of High-Intensity Exotic Radioactive Ion Beams

Sandrina Fernandes

ISOLDE, the CERN Isotope Separator On-line DEvice is a unique source of low energy beams of radioactive isotopes - atomic nuclei that have too many or too few neutrons to be stable. The facility is like a small 'chemical factory', giving the possibility of changing one element to another, by selecting the atomic mass of the required isotope beam in the mass separator, rather as the 'alchemists' once imagined. It produces a total of more than 1000 different isotopes from helium to radium, with half-lives down to milliseconds, by impinging a 1.4 GeV proton beam from the Proton Synchrotron Booster (PSB) onto special targets, yielding a wide variety of atomic fragments. Different components then extract the nuclei and separate them according to mass. The post-accelerator REX (Radioactive beam EXperiment) at ISOLDE accelerates the radioactive beams up to 3 MeV/u for many experiments. A wide international user radioactive ion beam (RIB) community investigates fundamental aspects of nuclear physics, particle and atomic physics, solid-state physics, materials science, astrophysics, biophysics and medicine. However, there are still a number of radioactive isotopes which are not accessible for experimental physics, either because it has been impossible to produce the element of interest, too low yields (number of isotopes per second measured in the beam line) are obtained or due to the higher post-acceleration energies required for the user's experiments. The aim of this thesis was to synthesize oxide and carbide porous materials, to be used as thick targets to increase the intensity of exotic RIBs. This was done by the evaluation of their radioactive isotope release properties and of their stability under irradiation conditions at high temperatures. This thesis is focused on the study of the chemical and physical processes occurring in the target system where the isotopes are produced, before these enter the ion-source system, are mass-separated and are sent to the user beam lines. The products formed in nuclear reactions between a proton beam and the target must, at a first step, diffuse from the interior of the target out to the surface and evaporate from it. Minimum delay times for the diffusion process can be realized by achieving the shortest diffusion lengths of highly-permeable, low-density, open-structure targets and by operating at the highest possible temperature so that release times are minimized and commensurate with the half-life of the isotopes of interest. Whereas until now target materials with micro-scale structures have been synthesized and used for RIB production, new porous submicro- and nano-structured materials offer the advantage of having lower diffusion activation energy of atoms and thus larger diffusion coefficient than the corresponding bulk counterpart, thanks to the increase of surface to volume ratio of these materials. The potential of this phenomenon for isotope mass separation online (ISOL) targets and many other applications is an evident drop of temperature to maintain a fast diffusion process, and higher release efficiency for the production of intense exotic isotope beams. In Chapter 1 the ISOL technique and the RIB production issues are introduced, followed by a review of recent progresses on target materials to increase yield production in the principal existing and next-generation ISOL and In-Flight facilities worldwide. Chapter 2 gives the theoretical fundamentals of diffusion and effusion in solid materials, and addresses specifically diffusion, effusion and release properties of ISOL targets. The study presented in Chapter 3 consists in the evaluation of release properties in different polycrystalline silicon carbide samples. The diffusion coefficients for Be, Na, Mg and F isotopes were extracted from off-line release measurements done at ISOLDE. The full characterization of the release time structure inherent to the physical and chemical processes of the ISOL RIB production on a submicrometric SiC target material (target SiC #334) used at ISOLDE, was made. This last target development resulted in improvement of magnesium short-lived isotopes yields. This was the first demonstration of the successful use of a submicrometric porous material as an ISOL target material, which delivered unprecedented 8x104 21Mg ions per µC. In Chapter 4, the synthesis of porous submicro- and nano-structured alumina to be used as an ISOL target, was followed by materials characterization in terms of microstructure and related with selected isotope release properties. Whenever possible, the release mechanisms were explained or proposed. The approach is relevant for the study of microstructure effects in other solid porous oxide and carbide targets, for optimization of their release performance. Chapter 5 shows the results of the TARPIPE experiment, set up within the design study task of the foreseen ISOL-type European facility called EURISOL, to investigate proton irradiation damage on typical and novel ISOL target materials under continuous proton irradiation at high temperature (> 1273 K). Materials such as SiC, Al2O3, Y2O3, C and Ta in a variety of forms were irradiated at the 72 MeV proton irradiation station of injector 1 at PSI (Paul Scherrer Institute). The effects of radiation damage on these materials were evaluated from microstructure and chemical analyses before and after irradiation by visual inspection, gamma spectroscopy, scanning electronic microscopy and electron dispersive X-ray analysis on a case-by-case basis. The moderate structure evolution of the porous oxide and carbide samples tested confirms that these materials are good candidates for high-power solid targets to be used at the existing and next generation high-power ISOL facilities, such as TRIUMF (TRI-University Meson Facility) and EURISOL (EURopean Isotope Separator On-line). In Chapter 6, the development and testing of a composite high-power alumina target for neon isotope production associated to a cold FEBIAD ion-source exposed to unprecedented proton beam intensities is reported. A 0.5 GeV proton beam from TRIUMF was used up to 25 µA, a factor 10 with respect to the present oxide targets in operation. The yields obtained were compared to the ones delivered by using a 70 µA proton beam in SiC targets at this facility. The target performance in terms of diffusion properties was evaluated. A general conclusion and discussion are given in Chapter 7. The release studies obtained with irradiation and ion implantation techniques on prospective SiC and Al2O3 matrices resulted in the identification of alternative target matrices for fast radioisotope release; this was confirmed experimentally from Na and Mg elements release efficiency determined online for a SiC target. A high-power oxide/metal composite target made of micrometric structural length scale Al2O3 pellets and Nb foils was operated online at unprecedented beam power intensities. Composite targets made of Nb foils and submicro- and nano-structured porous Al2O3 are expected to further increase yields of exotic isotopes. This ISOL target concept can be applied in the future for many other refractory insulating ceramics by using gradient microstructures and/or different ceramic/metallic combinations. Special care has to be taken, in order to minimize the stress induced by a mismatch of the coefficient of thermal expansion in the different ceramic layers and/or the underlying metallic substrate. Micro- and nano-technology fabrication techniques can be used to tailor pore and grain size distribution in ceramic and metallic complex-shaped composite materials and to shape these in compacts, monoliths and fenestrated structures. A balance between large surface area and large macropore volume for optimum target radioisotope release performance must be achieved. The resulting target material must in addition be stable at high temperatures, have good thermo-mechanical resistance and high thermal conductivity under irradiation for long irradiation time. The understanding of isotope release mechanisms in different solid materials, including the porous and dense fast-ion conductors will contribute to the production of exotic RIB at unprecedented high-intensity.

EPFL2010

Durability Of Field Concrete Made Of Portland Cement And Supplementary Cementitious Materials Under Several European Exposure Conditions

Aude Chabrelie, Emmanuel Gallucci, Karen Scrivener

Deterioration of cementitious materials by sulfate ions is a concern for concrete in contact with ground water (and in a lesser extent sea water) in many parts of Europe, and is an important issue for underground construction identified as a major area for progress in the construction industry. Sulfate resistance relates closely to testing and standardization and therefore to prescriptive approaches. The lack of feed back oil the durability of structures made of blended cements makes it difficult to fit these new concretes with existing test methods and performance criteria. This paper concerns the microstructural Study of field samples and structures made of blended concretes available across Europe. This concerns several materials exposed to various climate regimes, ranging from Southern to Northern Europe (Spain, Germany, United Kingdom, Denmark and Norway). Complementary techniques Such as SEM, micro-XR-F, XR-D and PIXE are used to evaluate the microstructural performance and stability of the phase assemblage of those blended concretes in the case of sulflate (and chloride in some instances) ingress, compare to pure Portland concrete.