74 research outputs found

    Near-infrared identification of the counterpart to X1908+075: a new OB-supergiant X-ray binary

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    We report the near-infrared (IR) identification of the likely counterpart to X1908+075, a highly-absorbed Galactic X-ray source recently suspected to belong to the rare class of OB supergiant-neutron star binary systems. Our JHKs-band imaging of the field reveals the existence within the X-ray error boxes of a near-IR source consistent with an early-type star lying at d=7 kpc and suffering A(V)=16 mag of extinction, the latter value being in good agreement with the hydrogen column density derived from a modelling of the X-ray spectrum. Our follow-up, near-IR spectroscopic observations confirm the nature of this candidate and lead to a late O-type supergiant classification, thereby supporting the identification of a new Galactic OB-supergiant X-ray binary.Comment: Accepted for publication in MNRAS, 7 pages, 3 figure

    Stress-assisted versus strain-induced martensites formed by cryogenic ultrasonic shot peening in austenitic stainless steels

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    International audienceIntroduction Severe plastic deformation (SPD) is used to create nanocrystalline metallic materials resulting in high strength but associated, generally and unfortunately, with a reduced ductility [1]. On one side, the cryogenic temperature that improves the grain refinement by preventing dynamic recrystallization or self-annealing, has been used during SPD processes such as equal channel angular extrusion (ECAE) or high pressure torsion (HPT), effectively producing significant extra grain refinement down to the nanometer scale [2-4]. On the other side, numerous research works have been done to improve the low ductility by creating multi-length scale structures [5] or grain size gradients [6]. In steels, other mechanisms can be active and lead to a significant improvement of the strength/ductility balance such as TRIP (Transformation Induce Plasticity) [7] or the TWIP (TWinning Induced Plasticity) [8] effects. In the case of the metastable austenitic stainless steel, the TRIP effect is produced through the martensitic phase transformation. The martensitic transformation requires an activation energy to be triggered which can be produced either thermally or by a mechanical loading. Two temperatures, the Ms and Md30, are used to evaluate the occurrence of the martensitic transformation. The Ms temperature represents the temperature at which the martensitic phase transformation can be triggered spontaneously without an external loading. By applying a loading, the transformation can take place at higher temperatures than Ms and the stress or strain required to activate the process will vary with the temperature [9]. The Md30 temperature, higher than the Ms, reflects the temperature at which a martensitic fraction of 50% can be formed under a true strain of 30 %. When the martensitic phase transformation is triggered slightly higher than the material Ms temperature, elastic stresses in the microstructure are enough to activate the transformation and the elastic energy induced in the material is enough to compensate the missing chemical driving force at this temperature [11]. On the other hand, when the deformation is applied close to the material Md30 temperature, the transformation will be mainly controlled by plastic deformation and the role of deformation defects will control the transformation process [10]. The so-formed martensites can then be considered as different and called Stress-Assisted Martensite (SAM) and Strain-Induced Martensite (SIM), respectively. On the other hand, TWIP can happen when Stacking Fault Energies (SFE) is in the range 18-45 m.Jm-2 for austenitic structures. Deformation twinning is especially promoted by high strain rate. The ' martensite can be produced at the intersection of mechanical twins as this volume is double-sheared, resulting in the nucleation of the phase:    (twins)  '. In the case of lower SFE (<18 mJm-2), martensitic transformation can involve the formation of a transient phase named -martensite. The formation of the -martensite is driven by the insertion of Shockley partial dislocations in every two successive {111} plans [13]. The face-centered cubic austenite is consequently transformed in the hexagonal close-packed -martensite as they share their same atomic packing factor. Thus, under increasing loading, the -martensite will act as a transient phase to produce the more stable ' martensite as follows:     '

    Nanomaterials by severe plastic deformation: review of historical developments and recent advances

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    International audienceSevere plastic deformation (SPD) is effective in producing bulk ultrafine-grained and nanostructured materials with large densities of lattice defects. This field, also known as NanoSPD, experienced a significant progress within the past two decades. Beside classic SPD methods such as high-pressure torsion, equal-channel angular pressing, accumulative roll-bonding, twist extrusion, and multi-directional forging, various continuous techniques were introduced to produce upscaled samples. Moreover, numerous alloys, glasses, semiconductors, ceramics, polymers, and their composites were processed. The SPD methods were used to synthesize new materials or to stabilize metastable phases with advanced mechanical and functional properties. High strength combined with high ductility, low/room-temperature superplasticity, creep resistance, hydrogen storage, photocatalytic hydrogen production, photocatalytic CO2 conversion, superconductivity, thermoelectric performance, radiation resistance, corrosion resistance, and biocompatibility are some highlighted properties of SPD-processed materials. This article reviews recent advances in the NanoSPD field and provides a brief history regarding its progress from the ancient times to modernity

    Microstructure evolution of ODS Fe-40Al intermetallics prepared by spark plasma sintering

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    Hétérogénéité de la déformation plastique des monocristaux de nickel déformés à froid par ECAE

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    L'extrusion angulaire à aire transversale égale a attiré une grande attention dans les dernières années principalement grâce au potentiel de cette technique d'atteindre des très grands taux de déformation avec le but final d'obtenir des matériaux massifs ayant des microstructures à grains très fins. Le procédé consiste dans le passage d'un échantillon à travers deux canaux ayant la même section transverse formant un angle psi, qui est souvent de 90 et parfois de 120. Ceci impose un très grand taux de déformation au matériau métallique dans un passe et le procédé peut être répété afin de diminuer la taille des grains jusqu'à une valeur caractéristique du matériau déformé. Il a été montré avant que l'ECAE est un procédé hétérogène mais il reste encore beaucoup de questions à répondre sur la compréhension des phénomènes liées à la déformation par ECAE ainsi qu'en ce qui concerne l'ampleur de son hétérogénéité. Deux monocristaux en nickel très pur ayant des différentes orientations initiales ont été déformés par ECAE et étudiées à l'aide de plusieurs techniques. L'évolution de la texture et de la microstructure a été analysée en utilisant la diffraction des neutrons et des rayons X, le MEB, le MET, l'imagerie d'orientation ainsi que la microscopie optique. Ce travail a révélé la présence d'une importante hétérogénéité sur l'hauteur des deux échantillons et on a attribué cette hétérogénéité au changement continu du mode de déformation. Il a été aussi montré que l'orientation initiale a une très importante influence sur les mécanismes d'affinage de la microstructure par la formation des hétérogénéités de déformation comme les bandes de déformation et les bandes de cisaillementThe processing of metals through Equal Channel Angular Extrusion (ECAE) has attracted much attention within the last decade primarily because this technique has shown the potential to achieve very large strains with the final aim of obtaining ultra-fine grain microstructures in bulk polycrystalline materials. The process involves passing a billet through two channels of equal area of cross-section inclined at an angle psi, generally 90 or sometimes 120. This imparts large deformation to the material in one pass and the process can be repeated to refine the grain size down to a value characteristic of the material. It has been shown previously that ECAE is a heterogeneous process but there is still an important lack of understanding of the phenomena involved in ECAE deformation as well as regarding the extent of this its heterogeneity. Two high purity nickel single crystals of different initial orientation were deformed by ECAE and analyzed by different techniques. Texture and microstructure evolution was examined using neutron and local X-ray diffraction, SEM, TEM, orientation imaging microscopy and optical microscopy. This work has revealed an important heterogeneity along the height of both samples due to the continuous change of the deformation mode from simple shear on top towards a complex shear deformation mode at the center at which an important friction induced deformation is added at the lower part of the samples. The initial orientation has been observed to have a great influence on the microstructure refining mechanisms by the formation of deformation heterogeneities like deformation bands and shear bandsMETZ-SCD (574632105) / SudocSudocFranceF

    Microstructure evolution of ODS Fe-40Al intermetallics prepared by spark plasma sintering

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    X1908+075: A Late O-Type Supergiant with a Neutron Star Companion

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    X1908 + 075 is a highly-absorbed Galactic X-ray source likely made up of a pulsar accreting wind material from a massive companion. We have used near-IR photometric data complemented by follow-up spectroscopy to identify the likely counterpart to this X-ray source and to assign a spectral type O7.5 9.5 If to the primary. Further details can be found in Morel and Grosdidier (2005)

    Influence de traitements thermomécaniques sur les textures, microstructures et propriétés élastiques d'un alliage Fe-40at.%AI à grains fins (procédés d'élaboration & traitements de recristallisation statique)

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    L'objectif de cette thèse est d'étudier les effets de traitements thermomécaniques sur les textures, les microstructures et les propriétés élastiques d'un alliage Fe-40%at.Al ODS à grains fins obtenu par consolidation de poudre broyée. La microstructure a été caractérisée par MEB et MET. Les textures ont été mesurées par diffraction des rayons X et par MEB couplé à un système EBSD. L'analyse de la formation des textures a été complétée par des simulations à l'aide du modèle de Taylor et l'anisotropie des propriétés élastiques a été caractérisée en utilisant le modèle de Hill à partir des mesures des textures. L'effet de plusieurs modes de consolidation des poudres à 1100ʿC est étudié tels que l'extrusion, la compression axiale et le martelage rotatif. Le meilleur module d'Young est obtenu pour une barre extrudée puis comprimée axialement de 50%, avec une texture mixte et . La composante est issue de l'extrusion et la composante de la compression et de la recristallisation dynamique partielle. La microstructure reste fine grâce à l'action des particules d'oxydes qui limitent la recristallisation dynamique. L'influence d'un traitement de recristallisation statique est aussi étudiée. L'écrouissage critique du matériau est de 8% pour un traitement thermique à 1000ʿC. La texture et la microstructure d'une barre initialement extrudée sont significativement modifiées après compression à froid de 10% suivie de traitements thermiques. Une étude de cinétique de recristallisation a permis de constater que les deux étapes de la recristallisation - la recristallisation primaire et la croissance des grains - conduisent à une anisotropie de la microstructure avec de larges grains allongés dans l'axe de la barre. Cette morphologie allongée est fortement liée à l'alignement des particules d'oxydes selon l'axe de la barre. Un mécanisme de croissance anormale conduit à une texture modérée qui agit favorablement sur le module d'Young. Après compression de 16%, le mécanisme de recristallisation est différent. La microstructure constituée de grains plus petits reste équiaxe. La texture est faible et montre des composantes proches des composantes de déformation. La croissance des grains est normale mais assez lente à cause de la présence des particules d'oxydes. D'une manière générale, la recristallisation statique à l'issue de la compression conduit à une amélioration du module d'Young.This thesis has been carried out to study the thermomechanical treatment effects on textures, microstructures and elastic properties of a fine grain ODS Fe-40at.%Al alloy obtained by consolidation of milled powder. Microstructure has been characterised by SEM and TEM. Textures have been measured by X-Ray Diffraction and by SEM associated with an EBSD system. The texture analysis formation has been completed by simulations using the Taylor model and the anisotropy of elastic properties has been characterised using the Hill model from the measured textures. The effect of several consolidation routes such as extrusion, axial compression and swaging at 1100ʿC is studied. The best Young modulus is obtained for an extruded bar axially compressed by 50% which has a double and fibre texture. The component results from the extrusion and the component from the axial compression and the partial dynamic recrystallization. The microstructure remains fine due to the action of the oxides particles which limit the dynamic recrystallization. The influence of a static recrystallization treatment is also studied. The critical strain to initiate recrystallization is 8% for 1000ʿC annealing treatments. The texture and microstructure of the extruded bar are significantly modified after 10% cold axial compression followed by annealing. A recrystallization kinetic study has allowed to notice that the two steps of the recrystallization - primary recrystallization and grain growth - lead to an anisotropy of the microstructure with large grains elongated in the bar axis. This elongated morphology is strongly connected with the oxides particles alignment in the bar axis...METZ-SCD (574632105) / SudocSudocFranceF
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