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[en] Ferritic-martensitic steels are excellent potential candidates for a use as structural materials in future fusion reactors. For this application, they will have to withstand high fluxes of 14 MeV neutrons that will create atomic displacement cascades and transmutation reactions which will produce large quantities of helium. In order to understand the basic mechanisms under irradiation with helium, we have studied the effects of helium and those of chromium. Pure iron and a Fe-5.4 wt.% Cr model alloy were irradiated at the JANNuS platform in dual-beam mode with Fe+ and He+ ions and in single-beam mode with Fe+ ions at 500 C. This platform enabled us to follow the evolution of damage up to low doses (1 dpa) and to characterize the microstructure at high doses (100 dpa). It also allowed us to observe in situ irradiation/implantation kinetic effects in a TEM coupled to two accelerators. The nature and distribution of irradiation defects was determined: they are essentially dislocation loops with a<100> Burgers vectors and cavities/bubbles. We have demonstrated that the co-implantation of helium and the addition of chromium both reduce the mobility of dislocation loops. The addition of chromium reduces swelling for all the irradiation conditions studied, with or without helium. In pure iron irradiated with helium, an original phenomenon was discovered for the first time: cavities/bubbles nucleate heterogeneously on the planes of the dislocation loops. We have also interpreted our experimental results in pure iron irradiated with helium using the cluster dynamics code CRESCENDO. (author)
[fr]Les aciers ferritiques/martensitiques au chrome sont d'excellents materiaux candidats pour les structures des futurs reacteurs de fusion. En conditions de service, ils devront resister a des flux intenses de neutrons de 14 MeV qui creeront des cascades de deplacements atomiques et des produits de transmutation tels que l'helium. Afin de mieux comprendre le comportement de ces materiaux sous irradiation en presence d'helium, nous avons etudie les effets de l'helium et ceux du chrome dans le cadre de ce travail de these. Du fer pur et un alliage modele Fe-5,4%pds Cr ont ainsi ete irradies dans la plateforme JANNuS a 500 C en bi-faisceau avec des ions Fe et He et en mono-faisceau avec des ions Fe+. L'utilisation de cette plateforme a permis de suivre l'evolution du dommage jusqu'a des doses faibles (1 dpa) et de caracteriser la microstructure apres irradiation a forte dose (100 dpa) Elle a egalement permis l'observation in situ dans un MET couple a deux accelerateurs des effets cinetiques d'implantation/irradiation. La nature et la repartition des defauts d'irradiation ont ete determines: ce sont essentiellement des boucles de dislocations de vecteur de Burgers de type a<100> et des cavites/bulles. Nous avons montre que la co-implantation d'helium et l'addition de chrome reduisaient la mobilite des boucles. Par ailleurs, avec ou sans helium, l'addition de chrome reduit le gonflement dans toutes les conditions etudiees. De plus, dans le fer pur irradie avec helium, un phenomene original de germination heterogene de cavites dans les plans des boucles a ete mis en evidence. Enfin, nous avons egalement utilise le code de dynamique d'amas CRESCENDO pour interpreter les resultats experimentaux dans le fer pur irradie avec helium
[en] The capabilities of the Jannus facility are presented, along with some recent representative experimental results. A brief review is also presented of supporting modelling efforts. (author)
[en] Highlights: • Investigation of He and dpa effects in EUROFER97 by irradiation with He/Fe-ions at 330 °C, 400 °C and 500 °C. • TEM analysis of the size, density and Burgers vector of dislocation loops as a function of irradiation temperature. • Determination of irradiation hardening by means of nanoindentation. • Evaluation of a kinetic reaction rate model for helium bubble nucleation and growth. - Abstract: Fusion like conditions for reduced activation ferritic/martensitic steels in the first wall are simulated with single Fe3+ and He+/Fe3+ dual ion beam irradiation of EUROFER97 at the Jannus laboratory, CEA Saclay, introducing a damage of 16 dpa and a helium content up to 260 appm. The samples are irradiated at temperatures of 330 °C, 400 °C and 500 °C. The quantitative determination of size distribution and density of dislocation loops is obtained using weak-beam dark-field imaging mode. Burgers vectors of are observed for the majority of dislocation loops at irradiation temperatures of 330 °C and 400 °C. At 500 °C no dislocation loops are found. The impact of single and dual ion beam irradiation on mechanical properties is determined by means of nanoindentation. An increase in nano-hardness of up to 35% due to irradiation was measured at samples irradiated at 400 °C. A kinetic rate model is applied for the description of nucleation and evolution of helium bubbles and compared with the experimental results. Evaluating the rate model with help of TEM-results for size and density of bubbles indicates the nucleation scheme as the main source for quantitative disagreement between the model and irradiation.
[en] The Reduced Activation Ferritic/Martensitic (RAFM) steels are promising structural materials for the first wall and blanket components of future fusion reactors. To obtain further insight into the temperature dependence of helium effects induced by 14 MeV energy neutrons under fusion like conditions, EUROFER97 was exposed to He+/Fe3+ dual-beam ion irradiation at the JANNUS laboratory at Saclay. The implantation was carried out at temperatures of 330 °C, 400 °C and 500 °C and induced a damage and helium concentration up to 26 dpa and 450 appm He, respectively. TEM microstructure analysis indicates a spatially homogeneous distribution of helium bubbles at 330 °C and 400 °C whereas a coexistence of homogeneous and heterogeneous nucleation of bubbles is found at 500 °C. An increasing mean bubble diameter and decreasing concentration of bubbles with rising irradiation temperature, as predicted by numerical results of a kinetic rate model for diffusion governed homogeneous nucleation of helium bubbles, are mostly confirmed by the irradiation experiment. Furthermore, within the rate model two approaches for the determination of the thermodynamic properties of helium filled voids in α-iron are applied. With respect to the final bubble size distribution, the commonly used surface energy of a void in the iron matrix is compared to the “variable gap model” of , J. Nucl. Mater. 418 (2011), which includes additionally the interaction between the helium atoms themselves, the energy at the helium-iron interface and the elastic deformation of the iron matrix. - Highlights: • Investigation of He and dpa effects in EUROFER97 by irradiation with He/Fe-ions at 330 °C, 400 °C and 500 °C. • TEM analysis of the size distribution of helium bubbles as a function of irradiation temperature. • Modeling of helium bubble formation with a kinetic rate model. • Influence of two different thermodynamic descriptions of helium bubbles on the rate model. • Comparison between experimental and numerical results.
[en] In this paper, the behavior of helium and its influence on the microstructure in polycrystalline titanium nitride (TiN) is studied by ion implantation technique, coupled with post-implantation annealing experiments between 1273 and 1873 K. The samples were implanted at room temperature with helium up to ∼2.2, 0.25 and 0.07 at. % at the implantation peak, respectively. Helium induced bubble microstructure was characterized using transmission electron microscopy (TEM) which revealed the effect of annealing temperature and helium concentration on the evolution of the bubbles in the material. TEM analysis also revealed that no amorphisation occurred up to the maximum dose level of 0.8 dpa at room temperature, corresponding to the highest implantation level. The technique of nuclear depth profiling using nuclear reaction analysis (NRA) revealed the helium diffusion and retention behavior in the material at different annealing temperatures and helium concentration levels. A sharp increase in helium loss from TiN occurred after annealing beyond 1373 K for the highest helium level, whereas no loss occurred for the lowest implanted helium level. The effective activation energies of helium diffusion and release were estimated utilizing Fick's law of diffusion and 1st order kinetic law, respectively. A novel method of estimating pressure inside bubbles is discussed by utilizing the high-density equation of state, incorporating the experimentally determined helium density, deduced from coupling the result of helium concentration measurement from NRA and vacancy concentration measurement from the experimentally determined size and number density of bubbles from TEM. These results reveal that such kind of calculation technique can be utilized for bubbles formed at higher annealing temperatures.
[en] Quantitative depth profiling measurements of implanted light elements is an important issue for electronics and nuclear applications. Conventional elastic recoil detection analysis (ERDA) has been improved by using heavy ions as incident particles for quantitatively profiling helium in materials. A new system has been implemented on the triple beam irradiation platform JANNUS at Saclay devoted to carry out HI-ERDA measurements. This device is dedicated to helium depth profiling using a 15 MeV "1"6O"5"+ incident ion beam. Capabilities of the technique (quantitative analysis, resolution and limit of detection) were tested on samples of known composition. For the first time, "4He depth profiles in pure α-iron, as-implanted and annealed, are obtained. HI-ERDA measurements have shown that helium release in pure α-iron can be described by a succession of two steps, the first having a slow kinetics below 700 °C and the second with a fast kinetics above 700 °C.
[en] Reduced activation high-chromium ferritic/martensitic steels are candidate materials for Generation IV fission and fusion reactors. To gain knowledge about the radiation resistance of these steels in such environments, the first step is to study the Fe–Cr matrix of this material. For that purpose and to understand ballistic damage by neutrons, self-ion irradiations, with and without simultaneous He injection, were performed on a series of high-purity Fe–Cr binary alloys at 773 K. Transmission electron microscopy (TEM) analysis revealed “displacement fringe contrast” inside the dislocation loops. This was attributed to the presence of Cr-enriched zones on their habit plane, which is a defect-free region for body-centered cubic Fe-based alloys. A plausible mechanism is discussed to explain the phenomenon, the first step of which would be the radiation-induced segregation of Cr atoms on the dislocation loop core. Energy-dispersive X-ray spectroscopy in scanning TEM mode and atom probe tomography (APT) gave a coherent quantitative estimate of the Cr concentration in these enriched areas. APT study showed that the enrichment was heterogeneous on the loop plane. Upon in situ annealing up to 900 K, the loops and the fringes disappeared completely, without leaving a secondary-phase particle, such as carbide, at their position. Fringes were present until the loop disappeared