Irradiation

Résumé

L'irradiation désigne l'exposition, volontaire ou accidentelle, d'un organisme, d'une substance, d'un corps, à des rayonnements. Ce terme est en particulier utilisé lorsque l'on considère l'exposition à des rayonnements ionisants. Unités d'irradiation et d'activité Rayonnement ionisant L'irradiation et la radioactivité s'expriment dans des unités spécifiques (sievert (symbole : Sv), becquerel (Bq)) : Gray L'unité SI utilisée pour mesurer une irradiation physique est le gray (Gy). Le gray mesure une énergie fournie par unité de masse, indépendamment de ses effets biologiques. La température augmente pour des puissances de l'ordre du kilogray, et le mégagray est l'ordre de grandeur de ce qu'il faut pour cuire un rôti dans un four à micro-ondes. Les irradiations rencontrées en radiobiologie correspondent à des énergies par unité de masse beaucoup plus faibles, insuffisantes pour chauffer la matière exposée. On utilise généralement le gray pour exprimer des ir

Source officielle

https://fr.wikipedia.org/wiki/Irradiation

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Ferritic/martensitic steels are candidate materials for fusion reactor structural components, liquid metal containers of spallation neutron sources, and accelerator driven systems, with good radiation resistance and thermo-mechanical properties. However, embrittlement resulting from the combined effects of radiation induced displacement damage (measured in dpa = displacement per atom) and transmutation products, especially helium gas, is one of the key issues. Four different steels were selected for mechanical and microstructural studies to understand the mechanisms of embrittlement induced by the combined effects of displacement damage and helium after irradiation in SINQ, the Swiss spallation source. The irradiations conditions were in the range: 10.7 ¿ 20.4 dpa with 850-1750 appm He at 160-300 °C. The evolution of the mechanical properties after irradiation was investigated by tensile and hardness tests. Radiation-induced defect clusters and helium bubbles were quantified by transmission electron microscopy (TEM). Emphasis was put on the deformation mechanisms under the different observed fracture mode, (ductile, quasi-cleavage and intergranular), whose occurrence depends on the irradiation conditions (dpa, He content and irradiation temperature). The tensile stress-strain curves and the scanning electron microscopy images of fracture surfaces showed distinct fracture mechanisms under different irradiation and test conditions. The tensile tests showed a yielding stress increase and loss of ductility of irradiated specimens. Hardness was measured on the specimens before tensile testing. The hardness results demonstrated an increasing trend with irradiation dose and helium content. TEM observations were done for all irradiated fractured specimens. Small defect clusters were observed in the 12.3 dpa specimen, but large defect clusters with loop-shape were very few. In the specimens of 17.2, 17.7 and 20.4 dpa, many large dislocation loops were detected besides small clusters. In addition, helium bubbles were observed in all specimens. The average size of defect clusters increased from 4.2 nm to 11.8 nm with dose increasing from 12.3 dpa to 20.4 dpa, whereas the number density did not change significantly. Meanwhile, the average size of visible helium bubbles increased from 1.03 nm to 1.93 nm. The microstructures in deformed area of irradiated specimens were observed and defect free channels with {110} and {112} slip planes were found in some specimens, indicating plastic flow localization. The average width of the channels is about 100 nm. Regarding the brittle samples, the TEM-lamella were extracted directly below intergranular fracture surfaces or cleavage surfaces by focused ion beam. Strikingly, deformation twinning was observed as the main feature in three irradiated specimens at high dose. Only twins with {112} planes were observed in all of these samples. The average thickness of twins is about 34 nm. Twins started from a fracture surface became gradually thinner with distance away from the fracture surface and stopped in the matrix finally. Features such as twin-precipitates interaction, twin-grain boundary and/or lath boundary interaction were observed. Twinning bands were seen to be arrested by grain boundaries or large precipitates, but could penetrate martensitic lath boundaries. Unlike the case of defect free channels, inside twins small defect-clusters, dislocation loops and dense small helium bubbles were observed.

EPFL2015

Chargement

APT/TEM characterization of the element segregation in ferritic/martensitic steels after irradiation in SINQ

Lijuan Cui

Ferritic /martensitic (FM) steels are among the most important materials in various nuclear applications. As radiation-induced segregation (RIS) is one of the major factors for embrittlement of nuclear materials, thereofre it is essential to study the RIS in FM steels. A comprehensive study has been performed on F82H, Eurofer 97, and ODS Eurofer steels after irradiation at SINQ by combining APT and TEM techniques. The study reveals that after irradiation, the RIS at low-angle GBs in F82H is less pronounced as compared to the random high-angle GBs, while in ODS Eurofer the RIS is similar. It was also found that the element segregation extent is not influenced by the GB CSL structures. The results confirmed that, as reported in literature, the small interstitial elements, such as C, N, P, exhibit GB segregation behaviour. Whereas, the GB segregation of substitutional elements depends on their atomic size. The undersized elements are enriched, and the oversized elements V and W are generally depleted at GBs. In some special cases, oversized elements are also enriched at GBs after irradiation, which may be caused by the âsolute drag effectâ of O or N. For the elements with giant sizes (with a volume over 2 times of Fe) such as Ca, Sc and K, their RIS behaviour was not known in literature, and in this work they showed strong enrichment at GBs. Further, this study also revealed that the segregation of these elements at interfaces between phases of chromium carbides, vanadium nitrides, and the matrix is essentially similar to that at GBs, except for Ti. The radiation-induced clustering of solutes was also studied. Two kinds of clusters were investigated in this study, including oxide clusters in ODS Eurofer and nano-clusters (N-clusters) with Si, Mn, Ti, Sc, Ca and K. In F82H, with increasing irradiation temperature, the N-clusters are changed from dilute, nonuniform-size and -shape to more uniform-size, compacted, and sphericial shape. Comparing with that of F82H, N-clusters have a lower number density and bigger size in Eurofer 97 irradiated under the same condition ~20 dpa/ ~300 Â°C. In ODS Eurofer, N-clusters were only observed in specimen irradiated at 300 Â°C. Some oxide clusters in the specimen irradiated at 480 Â°C contain a high portion of Si, Mn, Ti, Sc, Ca, and K. The number density of oxide clusters increases and their size decreases with irradiation temperature. Last but not least, the concentrations of spallation transmutants were quantified in detail. The results agree well with that of the neutronic calculations. This comprehensive study of irradiation effects on GB segregation and solute clustering revealed a number of novel features in FM steels. The investigation on the transmutation elements not only greatly contributes to spallation materials technology, but also broadens the basic understanding of atomic size effect on GB segregation. The comparison between the RIS behaviors in F82H / Eurofer 97 and ODS Eurofer demonstrated that the oxide clusters or oxygen could promote the segregation of some elements such as V, W and Ti, which is of significance for the application of ODS steels in intensive irradiation environments.

EPFL2021

Chargement

Tensile properties and microstructure of 590 MeV proton-irradiated pure Fe and a Fe-Cr alloy

Polycrystalline pure Fe and Fe-12Cr alloy have been proton-irradiated to doses of 10(-3)-3 x 10(-1) dpa at room temperature and 523 K. Samples were mechanically tested in tension and transmission electron microscopy (TEM) was performed on the as-irradiated material as well as on the irradiated and deformed material. At room temperature, pure Fe presents an initial yield point, that increases with the dose, followed by a yield region, indicative of a localized deformation mode. The Fe-12Cr alloy shows the same behavior, but the irradiation amplifies the effects previously mentioned. No yield point or yield region appears after irradiation at 523 K. TEM observations of the irradiated material showed a high density of small defect clusters which increases with the dose. Defect-free channels are observed in Fe-12Cr irradiated and deformed at room temperature. Results of the present investigation are compared with neutron irradiation of similar materials. (C) 2000 Elsevier Science B.V. All rights reserved.

2000

Chargement

EPFL2015

APT/TEM characterization of the element segregation in ferritic/martensitic steels after irradiation in SINQ

Lijuan Cui

EPFL2021