Abstract The microstructure and microchemistry of neutron-irradiated Type 304L and 316Ti stainless steels was characterized by means of several techniques of transmission electron microscopy. Both alloys were irradiated in the two experimental reactors with a dose rate of about 10x10−7 dpa/s, BOR-60 and PHENIX. Irradiation was carried out up to 91-120 dpa at 330°C for the first reactor and 39 dpa and 380-394°C for the second reactor. Different types of defects, including Frank loops, dislocation networks, cavities and precipitates were studied as a function of dose and temperature. With a tendency to evolve toward a saturation level, the Frank loops were larger and less numerous with increasing temperature. The distance between dislocations was slightly larger at high than at low temperature. Cavities were detected at high temperature. The swelling was found to be compatible with the cavity fraction. Ni and Si were found to be enriched at the cavity surfaces. Numerous Ni-Si-rich clusters were detected in the matrix and regularly in association with cavities whereas Cr-rich clusters and Ti-rich ones were found in the matrix. At least, based on the reflections, these different clusters correspond to Cr-rich carbides, Ti-rich carbides and G and γ’ phase particles. Depending on the dose and temperature, they could coexist or not. An enrichment of Ni and a depletion of Cr were observed in grain boundaries. BCC-Ferrite/martensite was also observed with non-negligible coverage on grain boundaries. Microstructural differences in the two steel grades were also discussed.

Austenitic steel was the reference cladding material in sodium cooled fast neutron nuclear reactors. These alloys exhibit good high temperature mechanical properties and they can withstand very high irradiation doses. To further increase the performance of these alloys, two distinct ways of improvement are possible: oxides dispersion strengthening or ultra fine grain microstructure. In order to achieve the desired enhanced material, powder metallurgy and in particular mechanical alloying route is promising. However, the high ductility of austenitic steel significantly reduces the powder production yield and worsens the supersaturation of oxides during mechanical alloying. To overcome this issue, multiple batches were produced using high-energy horizontal attritor at times ranging from 5 to 40 hours and consolidated by Spark Plasma Sintering (SPS). This paper tackles the characterization of powders after mechanical alloying and SPS compacts.

ODS steels are serious candidates for fuel cladding in 4th generation nuclear fast fission and fusion reactors. Meanwhile nano-precipitation is beneficial, the presence of a coarse precipitation (mostly carbides) usually known to deteriorate mechanical properties, could be detrimental.\n In this study, two ODS ferritic steels with different carbon contents (200 ppm and 700 ppm) have been elaborated by powder metallurgy and consolidated by Hot Isostatic Pressing, in order to evaluate C-rich precipitates influence on microstructural and mechanical properties.\n Nano-precipitates analysis revealed that nano-oxides size and density were similar in low and high carbon content materials. Microstructure examination showed a bimodal grain size for both materials, with equivalent mean size. Coarse precipitation of Cr23C6 only occurred in the higher C content material with an elongated shape, located at grain boundaries. Impact toughness properties are highly degraded when C content is increased, as C-rich precipitates act as initiation sites of failure.

ODS steels are candidate materials for the future generation of nuclear power plants. Ferritic / Martensitic (F/M) ODS steels display better formability thanks to high temperature austenitic transformation. The precipitation kinetics of a F/M Fe-9Cr ODS steel during powder consolidation up to 1100 degrees C has been characterized by in-situ Small Angle X-ray Scattering (SAXS). The influence of the matrix phase transformation has been established, showing an increase of the growth rate of the nano-oxides in austenite, leading to nano-oxides similar to 2x larger in diameter than in Fe-14Cr ferritic ODS grades at the end of the thermal treatment. These results are further supported by local atom probe tomography (APT) performed across grains showing contrasted microstructure and composition. Anomalous SAXS as well as comparison between APT and SAXS provide evidence that the nano-oxides stabilize with a Y2Ti2O7 or Y2TiO5 stoichiometry around 1100 degrees C.

The present paper focuses on the microstructural changes in Ti64 alloy after irradiation. The titanium alloys allow a significant decrease in activation and mass with a similar mechanical strength and a similar corrosion resistance than 304 stainless steels, widely used for core internal structures of Pressurized Water Reactors. The use of titanium alloys could be therefore a significant improvement for reactor exploitation and decommissioning. The nature, size and density of the radiation-induced defects are likely to be important parameters governing the in-reactor behavior of the material. The aim of the study was to devise new achievements in thin foil preparation and Transmission Electron Microscopy (TEM) imaging in order to accurately identify and quantify the Ti64 irradiation defects, at temperatures representative of irradiation temperatures in a reactor. The method to image and count the precipitates was developed with samples irradiated by ions at the temperature of 600 & DEG;C. Indeed, the microstructure of the sample irradiated at 600 & DEG;C was characterized by a low density of tangled dislocations, and big precipitates, that were easily evidenced. Then the process was applied in samples irradiated with ions at 430 & DEG;C and 300 & DEG;C characterized by a microstructure of tangled dislocations, loops and small precipitates. In 430 & DEG;C irradiation condition, the measurement in dark field with a B similar to[11-23](alpha) orientation and with a high magnification micrograph provided similar results than Atom Probe Tomography (APT) analyses. In irradiation condition at 300 & DEG;C, the precipitates were imaged by TEM as soon as the dose of 1 dpa. However they were not observed at the dose of 0.4 dpa. The precipitates were so small and their density was so high that the measurement by TEM was very imprecise and APT analyses were needed. The results of this microstructural analysis allowed a discussion on the nucleation mechanism of the irradiation defects in Ti64 alloy.

Oxide Dispersion Strengthened (ODS) steels are studied for their application in fission and fusion nuclear power plants. These steels are processed by powder metallurgy involving a high energy milling of Fe-Cr with oxides powders, followed by outgassing and hot consolidation. The temperature increase during these last two stages triggers the precipitation of a fine dispersion of nano-oxides. The outgassing stage is generally conducted at temperature between 300 and 800 degrees C for several hours, which can already trigger the formation of nano-oxides, and thus may have an influence on the final nano-oxide characteristics. In this work we assess the nano-oxides size by Small Angle X-Ray Scattering (SAXS) and get insight on the nano-oxides chemical and structural prop-erties by comparison between SAXS and Atom Probe Tomography (APT) during a thermal treatment at 700 degrees C for 5 h, simulating the outgassing stage. We show that despite that the nano-oxides slightly grow during the 700 degrees C heating, the final properties of the nano-oxides in term of size and number density are extremely close to those where no 700 degrees C isothermal was applied. Moreover, the growth rate of the nano-oxides is surprisingly higher at 700 degrees C than 1100 degrees C. APT and SAXS comparison shows that the nano-oxides do not stabilize in terms of stoi-chiometry and structure in the studied conditions, and that a small quantity of Al remain in the precipitates during this 700 degrees C isothermal treatment. Both of these factors could influence the nano-oxides kinetics at low temperature.

Ferritic Oxides Dispersion Strengthened (ODS) steels are of great interest for nuclear fission and fusion power plants. The nano-oxides embedded into the matrix provide the main contribution to the ODS steel strength. Understanding of the precipitation mechanism of ODS steels is thus critical for optimizing the fabrication process, involving Mechanical Alloying (MA) of Fe-14Cr, Y2O3 and TiH2 powders. In this study, results from small-angle X-ray and neutron scattering, atom probe tomography and electron microscopy have been combined to investigate the nano-oxides evolution throughout the whole consolidation thermal treatment until 1100 °C. After MA clusters are observed, composed of Y, O and Ti. During heating these clusters grow and new ones nucleate, together with a sequential enrichment in Ti (from as-MA to 700 °C) and Y (between 900 and 1100 °C). A small quantity of Al is also found in the nano-oxides between 700 and 1100 °C. At 1100 °C the nano-oxides are found to be mainly Y2Ti2O7 and subsequently progressively transform to Y2TiO5 during isothermal holding. Nano-oxides display however an unchanged extremely low coarsening rate, demonstrating the outstanding stability of both Y2Ti2O7 and Y2TiO5 at 1100 °C.

Oxide Dispersion Strengthened (ODS) steels are candidate materials for both fission and fusion nuclear reactors. In this study, the microstructure of ferritic (Fe-14Cr-1W) and ferritic / martensitic (Fe-9Cr-1W) ODS steels has been characterized after two different processing routes: Hot Extrusion (HE) and Hot Isostatic Pressing (HIP). Transmission Kikuchi Diffraction (TKD) revealed the presence of Ti-rich precipitates on every specimen. They are identified by X-Ray Diffraction (XRD) as Ti(C,N) and Ti-O (Ti2O3 or TiO2). Intergranular M7C3 and M23C6 have been observed on most Fe-14Cr ODS steels except one, where no Cr-carbides have been found. In Fe-9Cr ODS, intergranular Cr-rich M23C6 carbides have been found. The lack of M7C3 on Fe-9Cr ODS is possibly linked to the matrix phase transformation, not occurring in the Fe-14Cr ODS. Cr-carbides display highly elongated shapes for the Fe-14Cr HE specimens that could be detrimental to the mechanical behavior of the material.

Oxide Dispersion Strengthened (ODS) steels are candidates for nuclear applications. ODS are produced by mechanical alloying of Fe-14Cr, Y2O3 and TiH2 powders and consolidation at 1100°C, resulting in finely dispersed nano-oxides. Their precipitation kinetics has been quantitatively determined by in-situ Small Angle X-ray Scattering during continuous heating up to 1100°C. Clusters, found in the as-milled state start growing at 450°C until 1100°C, while almost no coarsening was recorded during subsequent isothermal annealing. The nano-oxides resulting from these in-situ experiments were found to be representative of those in materials processed by hot isostatic pressing and hot extrusion.

Ion irradiations have been performed at 450 degrees C on 304 and 304L austenitic stainless steels representative of PWR internals structure up to high doses (close to 100 dpa). TEM and APT have been carried out on irradiated samples. Depending on the dose, Frank loops, dislocation network, cavities, precipitates and segregations have been observed in both 304 steels grades. Their evolution with dose, and in particular for cavities and corresponding swelling, is described and potential microstructural differences between materials are highlighted. It appears that swelling is limited even up to high doses in both materials due to only slight evolution of cavity density and size from intermediate to high doses. As the irradiation temperature has been chosen to simulate PWR microstructure taking into account a temperature/flux shift effect, the microstructural results are discussed and compared with those of PWR. (C) 2019 Elsevier B.V. All rights reserved.

The effect of strengthened elements (Y2O3 and TiO2) on the microstructure has been investigated in Fe-14Cr-1 W based ferritic steels. Titanium content and/or volume fraction of yttria range between similar to 0 up to 0.3 wt%. The volume fraction of nanoparticles is shown to control the grain size whereas titanium content has a significant influence on the chemical homogeneity. Martensite phase was obtained on a Fe-14Cr-1 W based ferritic steel after consolidation by Hot Isostatic Pressing because of a higher content of carbon. A relationship between martensite phase and chemical heterogeneities was evidenced showing a chemical partitioning phenomenon. This phenomenon was also observed on ODS ferritic steels containing a low content of titanium (<= 0.05 wt%) however microstructural parameters, such as size and volume fraction of nano-precipitates and grain size, are shown to delay the formation of martensite during cooling by decreasing the value of critical cooling rate. Thermodynamic calculations indicate that the minimal titanium content to get a microstructure with 100% of ferrite is about 0.09 wt%.

Ion irradiations on the Ti6-4 titanium alloy were conducted at the JANNUS French platform in two different conditions of temperatures, doses and fluxes, to simulate neutron irradiation damage. Quantification of defects and chemical microanalyses were carried out thanks to Transmission Electron Microscopy and Atom Probe Tomography. -type loops and radiation induced precipitates (a vanadium-rich β BCC phase) were observed for both irradiation temperatures. During an irradiation at 300 °C, there was no notable influence of the dose and flux for the considered doses and fluxes ranges on the -type defects. The influence of raising the irradiation temperature up to 430 °C was a lowering of their density and an increase of their mean diameter for both defects. In addition, a lower flux seemed to enhance this temperature effect. These phenomena were very significant for precipitates whereas it appeared very modest for -type loops. The probable mechanism to explain the distribution of vanadium-rich β precipitates inside the α phase is the heterogeneous nucleation. The nucleation is dominated by the Radiation Induced Segregation (RIS) phenomenon at 430 °C and could be dominated by the mechanism of vanadium-rich clusters formation by ballistic effects in the cascades at 300 °C.

Post-irradiation deformation behavior of solution-annealed (SA) and cold-worked (CW) 316 austenitic stainless steel irradiated to doses from 9 to 39 dpa is examined as a function of strain rate and irradiation conditions (neutron spectrum, temperature). Tensile properties are found to be significantly higher for lower irradiation temperature and for CW material, for similar irradiation levels. The effect of strain rate on tensile properties is shown to be weak in the range [10−8s−1; 10−4s−1]. TEM investigations after deformation for levels of plastic strain of about 1% show on SA 316 the presence of deformation bands corresponding to one or even a mixture of twins, extended stacking faults, α′-martensite islands and ε-martensite nanobands. Bundles of crisscrossing bands, found to be a composite of overlapping stacking faults, nanotwins and ε-martensite nanolayers, are observed at TEM foils edges near the grain boundaries with α′-martensite islands decorating these edges. Except observation of a slight decrease of the number of deformation bands in the specimen deformed at slower strain rate, no qualitative microstructural differences appear between specimens tested at slow and fast strain rates.

Deformation and damage mechanisms of Fe-14Cr based oxide dispersion strengthened (ODS) steels have been investigated through a multi scale approach on model materials elaborated by powder metallurgy Uniaxial tensile behavior was studied from room temperature to 800 degrees C at several strain rates. Furthermore, the plasticity of Fe-14Cr ODS steels has been analyzed at the grain scale by in situ transmission electron microscopy (TEM) straining experiments between room temperature and 650 degrees C, evidencing a clear evolution with temperature of the dislocation motion. The evolution of yield stress with temperature has been separated into three domains, which can be explained by changes of deformation mechanisms. At low temperatures, the hardening is associated to the pining of dislocations on nano-oxides whereas dislocations motion is thermally activated at higher temperatures. M high temperatures, a competition between intra- and inter-granular mechanisms is observed. The transition in damage mechanism, related to the change of deformation mode, explains the observed reduction of ductility.

The effect of injected interstitials on loop and cavity microstructures is investigated experimentally and numerically for 304L austenitic stainless steel irradiated at 450 degrees C with 10 MeV Fe5+ ions up to about 100 dpa. A cluster dynamics model is parametrized on experimental results obtained by transmission electron microscopy (TEM) in a region where injected interstitials can be safely neglected. It is then used to model the damage profile and study the impact of self-ion injection. Results are compared to TEM observations on cross-sections of specimens. It is shown that injected interstitials have a significant effect on cavity density and mean size, even in the sink-dominated regime. To quantitatively match the experimental data in the self-ions injected area, a variation of some parameters is necessary. We propose that the fraction of freely migrating species may vary as a function of depth. Finally, we show that simple rate theory considerations do not seem to be valid for these experimental conditions. (C) 2017 Elsevier B.V. All rights reserved.

The effect of microstructural parameters on the microstructure, tensile properties from room temperature to 800 degrees C and creep properties at 650 degrees C has been investigated in a 14%Cr oxide dispersion strengthened (ODS) steel. Combining a control of oxide addition, consolidation route and thermontechanical processing, the different parameters of the microstructure have been varied systematically, namely the size and volume fraction of oxide nano-precipitates, the grain size and the dislocation density.The volume fraction of nano-precipitates is shown to influence the tensile strength throughout the temperature range, whereas a change in precipitate size influences only the low temperature behavior. The effect of grain size is shown to be similar to that of precipitation strengthening. A recrystallized microstructure is shown to improve the ductility of the ODS steel while not degrading the strength or creep resistance. The material processed by hot extrusion presents improved strength and reduced creep rate as compared to that consolidated by hot isostatic pressing. This difference is attributed to the longrange internal stress resulting from the high dislocation density, stabilized by the oxide nano precipitates.A strength model is presented, and validated on every material using the quantitative microstructural parameters obtained by combination of electron back-scattered diffraction, transmission electron microscopy and small-angle X-ray scattering. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.