Corrosion sous contrainte assistée par l'irradiation en milieu primaire d'aciers inoxydables austénitiques

National audienceLes internes de cuves des REP sont soumis à divers modes de sollicitation, notamment de corrosion sous contrainte assistée par l'irradiation (IASCC). L'objectif de ce travail est d'étudier le couplage de plusieurs phénomènes physiques et chimiques interagissant en IASCC. En particulier, nous nous intéresserons au couplage microstructure (hétérogénéités chimique, cristallographique, nature des joints de grains...) / mécanismes de déformation / mécanismes d'irradiation / effet de l'environnement / champs mécaniques. Nous présenterons notamment la méthodologie et les premiers résultats concernant l'irradiation aux protons sur matériau massif et les essais de corrosion sous contrainte associés ; ainsi que la méthodologie concernant l'irradiation de lames minces qui sera effectuée sur la plate-forme JANNUS du CSNSM, où le matériau sera exposé simultanément à un faisceau d'irradiation et un faisceau d'implantation d'hélium

Comportement dissymétrique des dislocations entre traction et compression dans des superalliages base-nickel

The elementary shearing configurations of γ' precipitates in single crystals of Nibased superalloys have been studied after creep and dynamical experiments using TEM techniques. A change of configurations has been observed between tension and compression. This tension/ compression asymetry may be explained by considering the interplay between the crossing of the γ/γ' interface by elementary Shockley dislocations and the effect of the applied stress on the dissociation width of matrix dislocations. An original model of shear of precipitates by dissociated matrix dislocations is proposed and an understanding of the dependence of the shearing processes with the temperature is provided.La microscopie électronique en transmission a été utilisée pour étudier les configurations élémentaires de cisaillement des précipités γ' dans des superalliages base nickel après des essais de fluage et des essais dynamiques. Un changement de configuration a été observé entre les essais de traction et de compression. Cette assymétrie traction/compression peut être expliquée en considérant la résultante de deux effets: le franchissement de l'interface par des partielles de Shockley et l'effet de la contrainte appliquée sur la largeur de dissociation des dislocations de matrice. Un modèle original de cisaillement des précipités par des dislocations de matrice a été proposé ainsi qu'une explication de la dépendence vis à vis de la température des processus de cisaillement

Dramatic reduction of void swelling by helium in ion-irradiated high purity α-iron

Effect of helium on void swelling was studied in high-purity α-iron, irradiated using energetic self-ions to 157 displacements per atom (dpa) at 773 K, with and without helium co-implantation up to 17 atomic parts-per-million (appm) He/dpa. Helium is known to enhance cavity formation in metals in irradiation environments, leading to early void swelling onset. In this study, microstructure characterization by transmission electron microscopy revealed compelling evidence of dramatic swelling reduction by helium co-implantation, achieved primarily by cavity size reduction. A comprehensive understanding of helium induced cavity microstructure development is discussed using sink strength ratios of dislocations and cavities. IMPACT STATEMENT Reduction of void swelling by helium co-implantation is reported, highlighting that it’s not always true that swelling will be higher in metals when helium is present along with irradiation damage

A new type of 0 0 1 junction observed in a (B2) Fe-Al-Ni-B alloy by TEM in situ straining

International audienceSingle crystal foils of an FeAl alloy containing Ni and B are strained at elevated temperatures in an in situ dedicated transmission electron microscope (TEM). The observed characteristic "fork-shaped" dislocation configurations are shown to be a kind of "0 0 1 junction". This junction comes from a reaction between two 1 1 1 superdislocations gliding on different {1 1 2} planes; it results in the presence of a long and sessile edge 0 0 1 dislocation along the intersection of the glide planes. Several hypothetical core structures of such a junction are proposed and their possible effects on macroscopic mechanical properties of the alloy are discussed

Drastic influence of micro-alloying on Frank loop nature in Ni and Ni-based model alloys

International audienceNickel and its alloys are f.c.c. model materials to investigate the elementary mechanisms of radiation damage and solute effects. This paper focuses on the drastic influence of micro-alloying (0.4 wt.% Ti or Cr) on the nature of defects after ion irradiation. Ultra-high purity materials are used to avoid impurity effects. For the first time, (i) large stable intrinsic Frank loops are identified in nickel while in alloys extrinsic Frank loops are observed; (ii) an eradication mechanism of intrinsic Frank loops is clearly identified; (iii) the morphology of Frank loops is shown to be characteristic of their nature. IMPACT STATEMENT For the first time, a drastic influence of micro-alloying on the defect nature is shown in irradiated Ni systems. Minor addition of solutes modifies significantly elementary mechanisms of radiation damage

Helium causing disappearance of a/2<111> dislocation loops in binary Fe-Cr ferritic alloys

International audienceSingle and dual-beam self-ion irradiations were performed at 500°C on ultra-high purity Fe14%Cr alloy to ∼0.33 displacements-per-atom (dpa), and 0 or 3030 atomic-parts-per-million (appm) helium/dpa, respectively. Using transmission electron microscopy, we reveal that helium can drastically modify the dislocation loop Burgers vector in Fe-Cr alloys. Helium co-implantation caused complete disappearance of a/2 type dislocation loops, and the microstructure consisted of only a loops. Conversely, a/2 type loops were predominant without He co-implantation. The total loop density remained largely unaffected. The results strikingly contrast literature asserting that helium stabilizes a/2 type loops in bcc Fe alloys, based on low temperature irradiations. Collectively analyzing the results with literature suggest that the small positive interaction between helium and self-interstitial atoms (SIA) in Fe predicted by atomistic simulations maybe insufficient to holistically explain the dislocation loop microstructure development in presence of helium. Helium-SIA positive binding inadvertently implies elevated a/2 loop fraction and higher loop densities that the present results contradict. Helium induced high cavity density causing a preferential loss of highly glissile clusters, leaving the matrix saturated with type clusters is proposed as a potential mechanism. Further, the in-situ irradiations combined with Burgers vector analysis strengthened the evidence of Cr-induced dislocation loop mobility reduction that appears to stabilize the a/2 type loops and causes higher loop densities in Fe-Cr alloys

Helium causing disappearance of a/2<111> dislocation loops in binary Fe-Cr ferritic alloys

International audienceSingle and dual-beam self-ion irradiations were performed at 500°C on ultra-high purity Fe14%Cr alloy to ∼0.33 displacements-per-atom (dpa), and 0 or 3030 atomic-parts-per-million (appm) helium/dpa, respectively. Using transmission electron microscopy, we reveal that helium can drastically modify the dislocation loop Burgers vector in Fe-Cr alloys. Helium co-implantation caused complete disappearance of a/2 type dislocation loops, and the microstructure consisted of only a loops. Conversely, a/2 type loops were predominant without He co-implantation. The total loop density remained largely unaffected. The results strikingly contrast literature asserting that helium stabilizes a/2 type loops in bcc Fe alloys, based on low temperature irradiations. Collectively analyzing the results with literature suggest that the small positive interaction between helium and self-interstitial atoms (SIA) in Fe predicted by atomistic simulations maybe insufficient to holistically explain the dislocation loop microstructure development in presence of helium. Helium-SIA positive binding inadvertently implies elevated a/2 loop fraction and higher loop densities that the present results contradict. Helium induced high cavity density causing a preferential loss of highly glissile clusters, leaving the matrix saturated with type clusters is proposed as a potential mechanism. Further, the in-situ irradiations combined with Burgers vector analysis strengthened the evidence of Cr-induced dislocation loop mobility reduction that appears to stabilize the a/2 type loops and causes higher loop densities in Fe-Cr alloys

Helium causing disappearance of a/2<111> dislocation loops in binary Fe-Cr ferritic alloys

International audienceSingle and dual-beam self-ion irradiations were performed at 500°C on ultra-high purity Fe14%Cr alloy to ∼0.33 displacements-per-atom (dpa), and 0 or 3030 atomic-parts-per-million (appm) helium/dpa, respectively. Using transmission electron microscopy, we reveal that helium can drastically modify the dislocation loop Burgers vector in Fe-Cr alloys. Helium co-implantation caused complete disappearance of a/2 type dislocation loops, and the microstructure consisted of only a loops. Conversely, a/2 type loops were predominant without He co-implantation. The total loop density remained largely unaffected. The results strikingly contrast literature asserting that helium stabilizes a/2 type loops in bcc Fe alloys, based on low temperature irradiations. Collectively analyzing the results with literature suggest that the small positive interaction between helium and self-interstitial atoms (SIA) in Fe predicted by atomistic simulations maybe insufficient to holistically explain the dislocation loop microstructure development in presence of helium. Helium-SIA positive binding inadvertently implies elevated a/2 loop fraction and higher loop densities that the present results contradict. Helium induced high cavity density causing a preferential loss of highly glissile clusters, leaving the matrix saturated with type clusters is proposed as a potential mechanism. Further, the in-situ irradiations combined with Burgers vector analysis strengthened the evidence of Cr-induced dislocation loop mobility reduction that appears to stabilize the a/2 type loops and causes higher loop densities in Fe-Cr alloys

Inversion of dislocation loop nature driven by cluster migration in self-ion irradiated nickel

International audienceNickel is widely used as fcc model material to obtain insight into fundamental mechanisms of radiation damage. This work presents irradiation of high-purity nickel using self-ions at 450°C and fine analysis of dislocation loops in specimens prepared by Focus Ion Beam using Transmission Electron Microscopy. For the first time to our knowledge, a drastic change of both loop nature (vacancy-type in irradiated zones v.s. interstitial-type in unirradiated zones) and loop Burgers vector (1/3 and 1/2 loops v.s. only 1/2 loops) is identified along the implantation direction in irradiated nickel. This change may be attributed to the formation of interstitial crowdions and their long-range 1D migration. A defect-free layer related to the annihilation of vacancy defects by injected atoms is detected. It provides an easy way to identify the injection peak in self-ion irradiated specimens and solid validation for damage calculations