Defect and Diffusion Forum Vols. 323-325

Abstract: The interaction of water molecules on a nickel surface was studied using ReaxFF (reactive force field) molecular dynamics. This approach was originally developed by van Duin et al. to study the hydrocarbon chemistry and the catalytic properties of organic compounds. To our knowledge, this method has not been used to study the corrosion processes of nickel exposed to water, which is what we set out to achieve in the present investigation. To do so, calculations were first performed using ReaxFF in order to reproduce certain well-known properties of pure nickel and nickel-water systems. This allowed us to study the adsorption of a single water molecule interacting with an optimized nickel surface. We also investigated the interaction of 405 molecules of water (ρ=0.99 g.cm^-3) on the (100), (110) and (111) surfaces of a single crystal of nickel at 300 K. The results show that a water bilayer is adsorbed on nickel surfaces: the first water layer is directly bonded to the surface, whereas the molecules in the first and second layers are held together by hydrogen bonds.

Abstract: In this work we investigated the structural re-organization of thin nanocrystalline Pt films in the temperature range between 250 °C and 400 °C by in-situ XRD, GIXRD and XRR synchrotron experiments. A re-orientation of (111) atomic planes and a relaxation of residual stress occurs. After heating up, Bragg peak fringes can be observed in the diffractograms. They are a direct proof that the Pt films are built of (111) columnar grains which essentially reach the whole film thickness of about 40 nm. During isothermal annealing a relaxation of the dispersion parameter of the atomic planes takes place which is associated with an activation energy of (0.4 ± 0.1) eV.

Abstract: A method of grain boundary diffusion characterization has been suggested based on combined analysis of radiotracer serial-sectioning technique and Mössbauer spectroscopy data and a specified model of grain boundary diffusion. Its application for determination of parameters of Co grain boundary diffusion in W is demonstrated.

Abstract: Grain boundary (GB) diffusion of Ag in Cu-based alloys with Fe (0, 0.14, 0.29, 0.43, 0.55 and 0.99 at. % Fe) was investigated. The experiments were performed at 600, 650 and 700 °C, which corresponded to B-regime. It was proved by the presence of diffusion wedges. The triple product of silver GB diffusion was obtained by measuring of angle at the top of isoconcentration profiles which was detected by electrochemical etching. It was shown that iron decreased the triple product at higher temperatures and didnt affect significantly at lower temperatures. SEM investigation showed the presence of small (10-100 nm) precipitates on the copper grain boundaries. Analysis of particles composition by Auger electron spectroscopy indicated higher concentration of iron comparing with the alloy composition.

Abstract: Effect of microalloying of Al by Ce on Cu grain boundary diffusion (GBD) in Al and other results of GBD study in metals with different purity were analyzed. It is shown that in dependence on the chemical composition of the matrix the triple product of GB diffusion can be higher or less than that for pure metals. The main concept used for description of the results is local ordering of the atoms (including atomic complex formation and appearance of «poisonous» places around atoms of the alloying element). The increasing of atom mobility at GB is discussed also in terms of solidus temperature changes.

Abstract: The temperature dependence of the bulk diffusion coefficient of Fe in Cu is determined by EDX in the temperature range from 923 to 1273 K, , m²/s. These results are different from that obtained earlier by radiotracer technique: activation energy is less by 30 kJ/mol and pre-exponential factor is 50 times smaller. Deviations from ideality of investigated solutions do not explain the differences; consequently, the thermodynamical factor would not responsible for such an effect. Fast grain boundary diffusion of Fe in Cu was not observed in the temperature range from 823 to 1073 K.

Abstract: Grain boundary liquid grooving process takes place during the contact of solid metal phase with the metal melt. The liquid bismuth network formation along grain boundaries (GB) and triple junctions (TJ) was investigated in copper polycrystalline samples. The experimental observation in situ technique of Bi penetration through the Cu plate was used. Microscopic images of the liquid channels network were obtained. The temperature dependencies of GB and TJ effective penetration depths were determined. The effect of the GB and TJ diffusion on the liquid channels growth mechanism was discussed.

Abstract: High helium contents will be generated within minor actinide doped uranium dioxide blankets which could be used in fourth generation reactors. In this framework, it is essential to improve our understanding of the type of damage which a pellet could incur as a result of extensive helium build-up. This paper is an attempt at tackling this issue. Sintered uranium dioxide disks have been implanted with helium ions then annealed at various temperatures. Above a concentration of 0.4 at.% and above 1000°C, optical images of the sample surface revealed swollen grains and extensive areas which have exfoliated. Nuclear reaction microanalyses and atomic force microscopy observations were performed to demonstrate that helium has substantially precipitated within the swollen grains. Massive precipitation of the gas leads under these conditions to sample surface blistering which appears to precede flaking. Deuterium ion irradiations have also been performed at ambient and a direct flaking of the sample surface was observed, but for this phenomenon to be observed required much higher doses than in the He study, indicating that temperature could be an essential ingredient for gas to migrate and cause extensive precipitation. Such phenomena could possibly lead to degradation of the fuel.

Abstract: Transmission electron microscopy (TEM) characterizations were carried out on a set of UO₂ thin foils previously implanted at room temperature with 400 keV Xe²⁺ and 250 keV Kr²⁺ ions at the fluence 7.10¹⁵ at.cm^-2 (equivalent to 1 at.%/at. UO₂). The experiment was devoted to the study of the evolution of the fission gases bubbles populations with increasing temperature. Annealings were performed in the laboratory furnace at 600°C, 800°C, 1000°C for 12h, 1400°C for 4h and 1500°C for 2h under Ar-5%H₂ atmosphere. For each annealing condition and for as-implanted specimens the bubble population has been characterized in size and number density. A comparison between Xe and Kr has been done that showed a similar behaviour. Globally, from the as-implanted sample to the 1500°C annealed, the bubbles growth phenomenon and the microstructure evolution with temperature was put in relieve.

Abstract: The presence of iodine in UO₂ has a great impact on nuclear fuel behaviour, both during-in reactor operation or under long-term repository conditions. Here the volume diffusion coefficient of iodine is studied using a methodology which involves ion implantation, annealing under different oxygen potentials and SIMS (Secondary Ion Mass Spectroscopy) for the concentration profile characterization of samples. The changes in the initial concentration profile induced by annealing are interpreted using Ficks second law to determine the diffusion coefficient. The first part of this paper is devoted to the description of the applied methodology. The iodine diffusion coefficients in the bulk are shown to depend upon the annealing temperature and oxygen potential. In smaller grained polycrystalline samples, the SIMS signal is averaged out over several grains and therefore, the analysis of the depth profile changes in samples due to annealing may be efficiently compared to depth profiles in as-implanted samples. By contrast, in Cr-doped UO₂ samples in which the grain size is larger than the zone analysed by SIMS, a particular methodology has to be developed. To this end, large grained material was examined using EBSD (Electron Back Scattering Diffraction) in order to determine the relative crystalline orientations of the grains. Following iodine implantations, various grains with different orientations were studied using SIMS. An attempt is presented at correlating the different sputtering rates with the various grain orientations.