THERMAL STABILITY AND EXTRA-STRENGTH OF AN ULTRAFINE GRAINED STAINLESS STEEL PRODUCED BY HIGH PRESSURE TORSION

International audienceInvestigations of an ultrafine-grained (UFG) Cr-Ni austenitic stainless steel produced by high pressure torsion (HPT) at room and elevated (400 C) temperatures followed by series of annealing up to 700 C are reported. The grain size of the alloy processed at room temperature (55 nm) was found to be about twice lower than the grain size of the alloy (90 nm) processed at elevated temperature. Besides, both as-processed states demonstrated a very high value of microhardness (~590 Hv) , while the steel in initial quenched state had the microhardness about 155 Hv. It is shown that the hardness of the steel in both UFG states does not decrease with annealing up to 650 C, and even a certain increase in hardness was observed for the steel produced at room temperature. At higher temperature (700 C), the recrystallization starts, and precipitation was observed

Etude des effets d'irradiations et de la nanostructuration dans des aciers austénitiques inoxydables

Internal structures of pressurized water reactors, made of 304 and 316 austenitic stainless steels, undergo a high neutron flux. A degradation of macroscopic properties (hardening, loss of corrosion resistance), caused by this irradiation, involves cracking of bolts of internal structures by irradiation assisted stress corrosion cracking. Modification of macroscopic properties is attributed to an evolution of microstructure under irradiation: formation of point defect clusters (Frank loops, cavities and/or gas bubbles), radiation induced segregation at grain boundaries. However, only few studies concern neutron irradiation effects on solute distribution into grains in these materials. So, the first aim of this work is to observe, at the atomic scale, solute distribution after neutron irradiation by tomographic atom probe. Ni-Si clusters are thus revealed. Then, thanks to model ion irradiations and with the help of transmission electron microscopy and cluster dynamics modelling, some information about mechanism of solute cluster formation has been provided. Heterogeneous radiation-induced precipitation seems to be the more plausible assumption. The second aim is to elaborate an ultrafine grained austenitic steel. The larger the grain boundary surface is, the larger the elimination of point defects, responsible of microstructure evolution under irradiation, is. Microstructure of this material has been characterized after thermal ageing and after irradiation. Results show a limitation of intra-granular damage.Les structures internes des réacteurs à eau pressurisée, en aciers austénitiques inoxydables 304 et 316, sont soumises à un fort flux de neutrons. Cette irradiation engendre une dégradation des propriétés macroscopiques (durcissement, perte de la résistance à la corrosion...) entraînant la fissuration de certaines vis des structures internes par un phénomène complexe de corrosion sous contrainte assistée par l'irradiation. La modification des propriétés macroscopiques est attribuée à un changement de la microstructure sous irradiation : formation d'amas de défauts ponctuels (boucles de Frank, cavités et/ou bulles de gaz), ségrégation induite aux joints de grains. Cependant, peu d'études traitent de l'effet de l'irradiation neutronique sur la répartition des solutés au sein des grains de ces matériaux. Le premier objectif de ces travaux est donc d'observer, à l'échelle atomique, la répartition des solutés après irradiation aux neutrons, par sonde atomique tomographique. La présence d'amas Ni-Si est ainsi mise en évidence. Puis, grâce à des irradiations modèles aux ions et avec l'apport de la microscopie électronique en transmission et d'un modèle de dynamique d'amas, des informations sont apportées concernant le mécanisme de formation des amas de solutés. L'hypothèse de la précipitation hétérogène induite semble la plus plausible. Le second objectif est d'élaborer un acier austénitique à grains ultrafins. L'augmentation de la surface de joints de grains permet alors une plus grande élimination des défauts ponctuels responsables de l'évolution de la microstructure sous irradiation. La microstructure de ce matériau est caractérisée après recuits et irradiation. Les résultats indiquent une limitation du dommage intra-granulaire dans ces matériaux

Etude des effets d'irradiations et de la nanostructuration dans des aciers austénitiques inoxydables

Internal structures of pressurized water reactors, made of 304 and 316 austenitic stainless steels, undergo a high neutron flux. A degradation of macroscopic properties (hardening, loss of corrosion resistance), caused by this irradiation, involves cracking of bolts of internal structures by irradiation assisted stress corrosion cracking. Modification of macroscopic properties is attributed to an evolution of microstructure under irradiation: formation of point defect clusters (Frank loops, cavities and/or gas bubbles), radiation induced segregation at grain boundaries. However, only few studies concern neutron irradiation effects on solute distribution into grains in these materials. So, the first aim of this work is to observe, at the atomic scale, solute distribution after neutron irradiation by tomographic atom probe. Ni-Si clusters are thus revealed. Then, thanks to model ion irradiations and with the help of transmission electron microscopy and cluster dynamics modelling, some information about mechanism of solute cluster formation has been provided. Heterogeneous radiation-induced precipitation seems to be the more plausible assumption. The second aim is to elaborate an ultrafine grained austenitic steel. The larger the grain boundary surface is, the larger the elimination of point defects, responsible of microstructure evolution under irradiation, is. Microstructure of this material has been characterized after thermal ageing and after irradiation. Results show a limitation of intra-granular damage.Les structures internes des réacteurs à eau pressurisée, en aciers austénitiques inoxydables 304 et 316, sont soumises à un fort flux de neutrons. Cette irradiation engendre une dégradation des propriétés macroscopiques (durcissement, perte de la résistance à la corrosion...) entraînant la fissuration de certaines vis des structures internes par un phénomène complexe de corrosion sous contrainte assistée par l'irradiation. La modification des propriétés macroscopiques est attribuée à un changement de la microstructure sous irradiation : formation d'amas de défauts ponctuels (boucles de Frank, cavités et/ou bulles de gaz), ségrégation induite aux joints de grains. Cependant, peu d'études traitent de l'effet de l'irradiation neutronique sur la répartition des solutés au sein des grains de ces matériaux. Le premier objectif de ces travaux est donc d'observer, à l'échelle atomique, la répartition des solutés après irradiation aux neutrons, par sonde atomique tomographique. La présence d'amas Ni-Si est ainsi mise en évidence. Puis, grâce à des irradiations modèles aux ions et avec l'apport de la microscopie électronique en transmission et d'un modèle de dynamique d'amas, des informations sont apportées concernant le mécanisme de formation des amas de solutés. L'hypothèse de la précipitation hétérogène induite semble la plus plausible. Le second objectif est d'élaborer un acier austénitique à grains ultrafins. L'augmentation de la surface de joints de grains permet alors une plus grande élimination des défauts ponctuels responsables de l'évolution de la microstructure sous irradiation. La microstructure de ce matériau est caractérisée après recuits et irradiation. Les résultats indiquent une limitation du dommage intra-granulaire dans ces matériaux

Understanding silicon-rich phase precipitation under irradiation in austenitic stainless steels

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Experimental atomic scale investigation of irradiation effects in CW 316SS and UFG-CW 316SS

International audienceMaterials of the core internals of pressurized water reactor (austenitic stainless steels) are subject to neutron irradiation. To understand the ageing mechanisms associated with irradiation and propose life predictions of components or develop new materials, irradiation damage needs to be experimentally investigated. Atomic scale investigation of a neutron-irradiated CW316 SS with the laser pulsed atom probe gives a detailed description of the solute segregation in the austenitic grains. In order to understand the mechanism of solute segregation detected in the neutron-irradiated materials, ion irradiations were performed. These latest irradiations were realized on a CW 316SS as well as on a nanostructured CW 316SS. The study of irradiation effects in a nanograin material allows first, to easily analyse grain boundary segregation and second, to test the behaviour under irradiation of a new nanostructured material. The three aspects of this atomic scale investigation (neutron irradiation effect, model ion irradiation, new nanostructured CW 316 SS) are tackled in this paper

Experimental atomic scale investigation of irradiation effects in CW 316SS and UFG-CW 316SS

International audienceMaterials of the core internals of pressurized water reactor (austenitic stainless steels) are subject to neutron irradiation. To understand the ageing mechanisms associated with irradiation and propose life predictions of components or develop new materials, irradiation damage needs to be experimentally investigated. Atomic scale investigation of a neutron-irradiated CW316 SS with the laser pulsed atom probe gives a detailed description of the solute segregation in the austenitic grains. In order to understand the mechanism of solute segregation detected in the neutron-irradiated materials, ion irradiations were performed. These latest irradiations were realized on a CW 316SS as well as on a nanostructured CW 316SS. The study of irradiation effects in a nanograin material allows first, to easily analyse grain boundary segregation and second, to test the behaviour under irradiation of a new nanostructured material. The three aspects of this atomic scale investigation (neutron irradiation effect, model ion irradiation, new nanostructured CW 316 SS) are tackled in this paper

Understanding silicon-rich phase precipitation under irradiation in austenitic stainless steels

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Where Does the FIB Sputtered Matter Accumulate in the SEM Chamber ?

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Where Does the FIB Sputtered Matter Accumulate in the SEM Chamber ?

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Effects of friction stir welding and post-weld annealing on nanostructured ferritic alloy

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