Internal fatigue crack propagation in a Ti-6Al-4V alloy: An in situ study

Fatigue specimens of a Ti-6Al-4V alloy containing internal artificial defects with controlled and reproducible size and shape have been produced. These defects systematically led to the initiation of a fatigue crack which propagation has been monitored in situ by synchrotron X-ray tomography during R=0.1 uniaxial fatigue tests at 20 Hz. The crack growth curves of the internal cracks have been obtained for 6 samples. Ex situ fatigue tests have been performed on samples submitted to a supplementary heat treatment or containing a defect put into contact with air. The results obtained tend to support the fact that internal fatigue cracks grow from the notch in a vacuum environment. On the fracture surfaces of samples containing an artificial defect not connected to air, two regions have been observed. They correspond to the Rough Area and the Fish Eye regions observed for internal cracks initiated from natural defects. The transition between those two regions takes place when the plastic radius size is equivalent to the grain size

Consolidation rapide à haute température d'aciers renforcés par dispersion d'oxydes (ODS) : Procédé, microstructure, précipitation, propriétés mécaniques

This work aims to lighten the understanding of the behavior of a class of metallic materials called Oxide-Dispersion Strengthened (ODS) ferritic steels. ODS steels are produced by powder metallurgy with various steps including atomization, mechanical alloying and high-temperature consolidation. The consolidation involves the formation of nanoparticles in the steel and various evolutions of the microstructure of the material that are not fully understood. In this thesis, a novel consolidation technique assisted by electric field called "Spark Plasma Sintering" (SPS) or "Field-Assisted Sintering Technique" (FAST) was assessed. Excellent mechanical properties were obtained by SPS, comparable to those of conventional hot isostatic pressed (HIP) materials but with much shorter processing time. Also, a broad range of microstructures and thus of tensile strength and ductility were obtained by performing SPS on either milled or atomized powder at different temperatures. However, SPS consolidation failed to avoid heterogeneous microstructure composed of ultrafine-grained regions surrounded by micronic grains despite of the rapid consolidation kinetics. A multiscale characterization allowed to understand and model the evolution of this complex microstructure. An analytical evaluation of the contributing mechanisms can explain the appearance of the complex grain structure and its thermal stability during further heat treatments. Inhomogeneous distribution of plastic deformation in the powder is argued to be the major cause of heterogeneous recrystallization and further grain growth during hot consolidation. Even if increasing the solute content of yttrium, titanium and oxygen does not impede abnormal growth, it permits to control the fraction and the size of the retained ultrafine grains, which is a key-factor to tailor the mechanical properties. Since precipitation through grain boundary pinning plays a significant role on grain growth, a careful characterization of the precipitation state was performed on consolidated ODS steels. The experimental data obtained by transmission electron microscopy, small angle neutron scattering and atom-probe tomography evidenced the presence of dense and nanosized particles in SPS ODS steels, similarly to what is observed in conventional ODS steels. This is of great importance since it proves that the precipitation is very rapid and mainly occurs during the heating stage of the consolidation process. Using a thermodynamic model, the precipitation kinetics of Y2O3 and Y2Ti2O7 were successfully reproduced at various consolidation temperatures. Both experimental and numerical findings agree with the rapid precipitation of nanoparticles that are then extremely stable, even at high temperature. Consequently, this model can be an efficient tool to design ODS steelsby the optimization of the precipitation state.Ce travail vise à améliorer la compréhension de la microstructure d’aciers ferritiques appelés aciers ODS. Ils sont fabriqués par métallurgie des poudres, ce qui inclut le cobroyage d’une poudre ferritique avec une fine poudre d’oxydes, suivi d'une consolidation à haute température. La consolidation permet de former un matériau dense renforcé par des particules nanométriques qui sont responsables des bonnes propriétés mécaniques à haute température. Cependant, les procédés conventionnels, notamment la Compaction Isostatique à Chaud, provoquent des microstructures hétérogènes qui étaient jusqu’à ce jour mal comprises. Ainsi, la technique rapide de consolidation assistée par courant électrique appelée "Spark Plasma Sintering" (SPS), a été testée afin d’étudier la microstructure. Pour la première fois, on montre que d’excellentes propriétés mécaniques peuvent être obtenues par SPS, comparables à celles des matériaux ODS obtenus classiquement par Compaction Isostatique à Chaud, mais avec un temps de procédé largement réduit. Cependant, la consolidation par SPS échoue quand il s’agit d’obtenir une micro-structure ferritique homogène. En effet, malgré la cinétique rapide de consolidation, on obtient des grains dits ultrafins (D 10 μm). Une caractérisation microstructurale poussée a permis de comprendre l’évolution du matériau durant la consolidation. Un modèle d’évolution microstructurale a été proposé. Le calcul des pressions gouvernant la mobilité des interfaces souligne l’importance de la déformation plastique hétérogène issue du cobroyage des poudres. Par ailleurs, il est montré que la précipitation des particules d’oxydes ancre les joints de grains et stabilise la microstructure hétérogène, même à très haute température. On montre aussi qu’augmenter la teneur en renforts n’empêche pas la croissance anormale mais permet de contrôler la fraction et la taille de grains ultrafins, et donc les propriétés mécaniques des ODS. Parce que les particules jouent un rôle primordial dans la croissance des grains, une caractérisation fine de l’état de précipitation a été réalisée sur les matériaux consolidés par SPS. L’étude par Microscopie Electronique en Transmission, Diffusion des Neutrons et Sonde Atomique révèle une grande densité d’oxydes qui varient en taille et en composition chimique. Un modèle thermodynamique de type germination/croissance/coalescence a été développé pour simuler les cinétiques de précipitation des phases Y2O3 et Y2Ti2O7 durant les étapes de consolidation non isothermes. Tant les résultats expérimentaux que numériques démontrent la précipitation rapide des nano-particules qui sont ensuite extrêmement stables durant les recuits. Ce modèle permet de mieux comprendre la spécificité des microstructures et de la précipitation dans les ODS, de la formation rapide de particules nanométriques à la précipitation grossière d’oxydes de titane aux interfaces

World's first commercial CMOS Electron Backscatter Diffraction Detector: applications for a few ceramic materials

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Characterization and modeling of oxides precipitation in ferritic steels during fast non-isothermal consolidation

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Microstructure, texture and mechanical properties with raw surface states of Ti-6Al-4V parts built by L-PBF

International audienceDue to dramatic thermal gradients and cooling rates induced by Laser Powder Bed Fusion (L-PBF), Ti-6Al-4V as-built parts show a fine martensitic microstructure with high residual stresses that are detrimental for their mechanical behaviour. In order to optimize the properties and meet the product requirements, post-processing heat treatments are often compulsory. The present work first describes the microstructure (residual stresses, beta grains and laths size, crystallographic texture) and the mechanical behaviour (impact and tensile tests) of as-built parts with raw surface. Then, one discusses the effect of a stress-relieve sub-transus heat treatment on the strength/ductility compromise

Development of an austenitic/martensitic gradient steel by additive manufacturing

International audienceIn many applications, additive manufacturing techniques are rising to build, layerby-layer,component with complex shapes and specific microstructures. DirectMetal Deposit (DMD) process allows to tune the material's composition usingdifferent powder feeder. In this study, we focused on a gradient from a 316Laustenitic stainless steel to a 9Cr-1Mo martensitic steel. These steels are widelyused for nuclear applications, but due to their chemical composition differences,welding them is uneasy. In this study, microstructures obtained by Tungsten InertGas welding are compared with graded parts obtained by powder metallurgy, afterSpark Plasma Sintering or additive manufacturing with Direct Metal Depositprocess

Fast high-temperature consolidation a novel way to understand the microstructural heterogeneities in nano-reinforced ferritic steels

International audienceOxide-Dispersion Strengthened (ODS) ferritic steels were produced by powder metallurgy using a field-assisted sintering technique called spark plasma sintering (SPS). A multiscale characterization using electron microscopy combined with an in situ synchrotron X-Ray Diffraction allowed to understand the evolution of the heterogeneous microstructure. The influence of oxides precipitation was quantified by nanoscale observations to explain the microstructure heterogeneities in ferritic ODS alloys

Tailoring the microstructure and the mechanical properties of ultrafine grained high strength ferritic steels by powder metallurgy

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