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International audience; The dislocation densities were measured on the same samples using transmission electron microscopy (TEM), scanning electron microscopy (electron channeling contrast imaging (ECCI) and high-angular-resolution-electron backscattered diffraction (HR-EBSD)), and X-ray diffraction (XRD). Notably, these different methods do not observe the same types of dislocations, i.e., statistically stored dislocations (SSDs) and/or geometrically necessary dislocations (GNDs). ECCI and TEM imaging are direct-measurement techniques, whereas HR-EBSD and XRD are indirect methods. Therefore, a quantitative comparison of the measurements obtained using these four techniques on undeformed and deformed duplex steels is proposed. For low deformation, where the dislocation density is quite small (1 − 5 × 10$^{13}$ m$^{−2}$), imaging methods are rather performant, whereas XRD measurements suffer from high uncertainty levels. HR-EBSD measurements show results that are in good agreement with the other methods for these deformation levels. For higher deformation levels (with dislocation densities above 1 − 3 × 10$^{14}$ m$^{−2}$), imaging methods are no longer relevant because of the increasing uncertainty arising from local contrast variation and overlapping of dislocations. The different results obtained highlight the necessity of taking a step back on each method used. Correctly defining what is to be measured (SSDs or GNDs), in which condition (solid material or thin plate) as well as the parameters (pixel size, area, etc.) and their bias is essential, especially if the objective is to use the measurement in a micromechanical model.

International audience; In hypothetical accidental conditions, zirconium-based nuclear fuel claddings can absorb high hydrogen contents (up to several thousand wppm) and be exposed to high temperatures (βZr phase temperature range) before being cooled. This paper thoroughly investigates the microstructural and microchemical evolutions that take place in such conditions. Two zirconium-based alloys and unalloyed zirconium were pre-charged with hydrogen at various contents up to 3300 wppm and heat-treated at 1000 or 850 °C. Neutron diffraction analyses were performed in situ upon slow cooling from 700 °C and at room temperature. In the materials containing 3300 wppm of hydrogen, βZr progressively transforms into αZr during slow cooling then extensively transforms into αZr and δZrH2-x hydrides precipitate via a eutectoid reaction. Thermodynamic predictions at equilibrium are in good agreement with the experimental results. However, depending on the cooling scenario and the average hydrogen content, the precipitation of γZrH hydrides, potentially metastable, is evidenced below 350 °C and a significant amount of hydrogen can remain in solid solution in αZr. These metallurgical evolutions and the evolution of the different phase lattice parameters are strongly influenced by the partitioning of oxygen and hydrogen (revealed by electron probe and elastic recoil detection microanalyses) that occurs during the βZr to αZr transformation and hydride precipitation.

International audience; To improve the safety of nuclear power plants, a Cr protective layer is deposited on zirconium alloys to enhance oxidation resistance of the nuclear fuel cladding during both in-service and hypothetical accidental transients at High Temperature (HT) in Light Water Reactors. The formation of the Cr2O3 film on the coating surface considerably helps in reducing the oxidation kinetics of the zirconium alloy, especially during hypothetic Loss of Coolant Accident (LOCA). However, if the Cr coating is successful to increase the oxidation resistance at HT of the zirconium substrate, for in-service conditions, under neutron irradiation, Cr desquamation has to be avoided to guarantee a safe use of the Cr-coated zirconium alloys. Therefore, the adhesion properties have to be maintained despite the structural defects created by sustained neutron irradiation in the reactor environment. This paper proposes to study the behavior of the Zircaloy-Cr interface of a first generation Cr-coated material during a specific in situ ion irradiation. As deposited, the Cr-coated sample presents a f.c.c. C15 Laves-type intermetallic phase at the interface with off-stoichiometric composition. After irradiation and for the specific conditions applied, this interfacial phase has significantly dissolved. Energy Dispersion Spectroscopy revealed that the dissolution was accompanied by a counterintuitive “sharpening” effect.

International audience; 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.

International audience; A new β-metastable Ti-alloy is designed with the aim to obtain a TWIP alloy but positioned at the limit between the TRIP/TWIP and the TWIP dominated regime. The designed alloy exhibits a large ductility combined with an elevated and stable work-hardening rate. Deformation occurring by formation and multiplication of {332} twins is evidenced and followed by in-situ electron microscopy, and no primary stress induced martensite is observed. Since microstructural investigations of the deformation mechanisms show a highly heterogeneous deformation, the reason of the large ductility is then investigated. The spatial strain distribution is characterized by micro-scale digital image correlation, and the highly deformed regions are found to stand at the crossover between twins, or at the intersection between deformation twins and grain boundaries. Detailed electron back-scattered imaging in such regions of interest finally allowed to evidence the formation of thin needles of stress induced martensite. The latter is thus interpreted as an accommodation mechanism, relaxing the local high strain fields, which ensures a large and stable plastic deformation of this newly designed Ti-alloy.

International audience; 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. Graphical abstract Main matter Ferritic Oxide Dispersion Strengthened (ODS) steels were developed for applications where a good resistance to harsh environment is required such as generation IV nuclear fission [1] or nuclear fusion power plants [2]. Their Fe-Cr ferritic matrix gives them a good swelling resistance under irradiation [3,4] as well as a good corrosion resistance [5,6]. Nano-precipitates of Y-Ti-O lead to high yield strength and high improvement of thermal creep resistance [7-10] as these nano-oxides act as strong pinning points for both grain boundaries and dislocations, even at high temperatures [7-9,11-13].

The mechanical behavior of a 15 µm-thick chromium coating deposited on a zirconium alloy substrate using a particular physical vapor deposition process is studied at room temperature using several experimental techniques at different scales: biaxial tests (internal pressure + axial tension) with several stress biaxiality ratios on outer-coated tubes; in situ tensile tests in a scanning electron microscope (SEM) on coated sheet samples. The cracking and plasticity of the coating are studied using acoustic emission, digital image correlation and SEM observations. The results show that the first channeling cracks in the coating initiate at 0.3-0.4% macroscopic strain, when the substrate starts to yield. Under further loading, the crack density increases first rapidly then more slowly, before reaching saturation. Plastic strain of the non-cracked coating regions is observed, in addition to the increase of crack opening. No interface decohesion of the coating is observed. The cracks, both transgranular and intergranular, do not penetrate significantly into the substrate.

Strain-transformable Ti-based alloys are known to display a superior combination of strength, ductility and strain-hardening and attracted considerable interest on recent years. They generally still display, however, a limited yield strength that can be possibly overcome by further precipitation strengthening of the developed systems. In that work, we developed a design strategy to reach a forged dual-phase (α+β) microstructure with TRIP/TWIP properties in a Ti–10V–2Fe–3Al alloy. The results showed an excellent combination of mechanical properties due to the strain-transformable deformed β-matrix. The investigation on the deformation mechanisms in the Ti–10V–2Fe–3Al alloy was accurately performed by means of both in-situ synchrotron XRD, mechanical testing followed by SEM/EBSD mapping and “post mortem” TEM microstructural analyses. Combined Twinning Induced Plasticity (TWIP) and Transformation Induced Plasticity (TRIP) effects were shown to be intensively activated in the alloy. The particular role of stain-induced martensite α″, acting as a relaxation mechanism at the α∕β interfaces, as well as the strong interactions between mechanical twins and primary α nodules were particularly highlighted.

International audience; In-situ time-resolved Synchrotron X-ray diffraction analyses were performed on zirconium alloy (Zircaloy-4) sheet samples, during their heating, isothermal oxidation at 700, 800 and 900 degrees C under a flowing mixture of He and O-2 and cooling. The oxide growth and the evolution of the oxide structure as a function of time and temperature were studied with suitable time resolution. Oxide layer thicknesses of approximately 10 mu m were formed during the experiments. The incident X-rays penetrated the whole oxide thickness. The samples were examined after the experiments by field emission gun scanning electron microscopy, electron backscatter diffraction and electron-probe microanalysis. The results showed that the oxide contains a mixture of monoclinic and tetragonal zirconia evolving during heating, oxidation and cooling. The average volume fraction of tetragonal zirconia decreases during oxidation. This fraction is larger at 900 degrees C than at 700 and 800 degrees C. For oxide layers thinner than approximately 5 mu m, this fraction is larger at 800 degrees C than at 700 degrees C, but it is rather equivalent for both temperatures when the oxide thickness ranges between 5 and 8 mu m. Some of the tetragonal zirconia crystals transforms into the monoclinic phase during cooling after oxidation. This fraction of transformed tetragonal zirconia is larger after oxidation at 900 degrees C than after oxidation at 700 and 800 degrees C. It is suggested that these evolutions of the oxide crystallographic structure are related to micro-stresses and to temperature dependences of the critical size of zirconia crystals below which tetragonal zirconia is stabilized.

Aiming at increasing the yield strength of transformation and twinning induced plasticity (TRIP and TWIP) titanium alloys while maintaining a good work-hardening, a dual-phase α/β alloy is designed and studied. The composition Ti – 7 Cr – 1.5 Sn (wt.%) is proposed, based on an approach coupling Calphad calculations and classical Bo-Md design tool used in Ti-alloys. Its microstructure is made of 20% of α precipitates in a β matrix, the matrix having optimal Bo and Md parameters for deformation twinning and martensitic transformation. The alloy indeed displays a yield strength of 760 MPa, about 200 MPa above that of a Ti – 8.5Cr – 1.5Sn (wt.%) single β phase TRIP/TWIP alloy, combined with good work-hardening and ductility. In situ synchrotron X ray diffraction and post-mortem electron back-scattered analyses are performed to characterize the deformation mechanisms. They evidence that the TRIP and TWIP mechanisms are successfully obtained in the material, validating the design strategy. The interaction of the precipitates with the {332} β twins is analyzed, evidencing that the precipitates are sheared when hit by a twin, and therefore do not hinder the propagation of the twins. The nature of the interaction is discussed, as well as the impact of the precipitates on the mechanical properties.

Synchrotron X-ray diffraction is a powerful tool to analyse the mechanical behavior of multiphase materials due to its selectivity. Simultaneous stress analysis of both phases of a W/Cu thin multilayer has been performed during a continuous biaxial loading on DiffAbs beamline at SOLEIL synchrotron (France). The use of a 2D detector with a large sample-detector distance is shown to give relatively accurate applied stress analysis even if only a small part of the usual ψ range of the sin2ψ method is considered. The results show the failure of the thin film multilayer while the W components are still under a strong compressive stress state of-3 GPa. It is concluded that the mechanical behavior is in fact mainly governed by the residual stress state.