Effect of a heterogeneous distribution of particles on the formation of banded grain structure in wrought Alloy 718

Alloy 718 is known to be sensitive to interdendritic segregation formed during ingot solidification. The occurrence of banded grain structures under heat treating conditions close to 1000 ° C related to interdendritic segregation is often reported. In order to have a better understanding of this microstructural evolution, an extensive experimental program has been carried out. Consequently, a model taking into account the selective dissolution of δ-phase (Ni3Nb) is proposed. A grain growth simulation by Monte-Carlo method is then used to illustrate the grain structure evolution in a banded particle distribution. By comparing experimental data and computer simulation, the relationship between the Monte-Carlo step and the real time is assessed and the range of parameters when heterogeneous microstructures appear is specified

First-principles study of diffusion and interactions of vacancies and hydrogen in hcp-titanium

We present a study of the stability of n-vacancies (Vn) and hydrogens in the hexagonal close-packed titanium system computed by means of first-principles calculations. In this work, performed by using the generalized gradient approximation of density functional theory, we focused on the formation energies and the processes of migration of these defects. In the first part, the calculated formation energy of the monovacancy presents a disagreement with experimental data, as already mentioned in the literature. The activation energy is underestimated by almost 20%. The stability of compact divacancies was then studied. We show that a divacancy is more stable than a monovacancy if their migration energies are of the same order of magnitude. We also predict that the migration process in the basal plane of the divacancy is controlled by an intermediate state corresponding to a body-centered triangle(BO site). The case of the trivacancies is finally considered from an energetic point of view. In the second part, the insertion of hydrogen and the processes of its migration are discussed. We obtain a satisfactory agreement with experimental measurements. The chemical nature of the interactions between hydrogen and titanium are discussed, and we show that the H-atom presents an anionic behavior in the metal. The trapping energy of hydrogen in a monovacancy as a function of the number of hydrogen atoms is finally presented

Characterizing precipitation defects in nickel based 718 alloy

In the present study we examine the crystallographic structure of the ϒ, ϒ'' and δ phases present in nickel base 718 alloy. The chemical ordering of Nb atoms and possible planar faults that may be observed in ϒ'' precipitates are detailed. High resolution transmission electron microscopy (HRTEM) observations of various faults are reported. The decomposition of a matrix dislocation to form a locked V shaped configuration is shown. The observation along [110] type direction allows to identify the type of defect, which is observed as a pure geometric stacking fault

Coopérer au sein d'un Lieu d'éducation Associé à l'Institut français de l'éducation et envisager la recherche-collaborative sur le terrain de l'enseignement supérieur.

International audienceAfin de poursuivre sa transformation pédagogique, Toulouse-INP est devenu en septembre 2018 un Lieu d'éducation Associé à l'Institut français de l'éducation (LéA). Le choix des conseillers pédagogiques de Toulouse-INP à s'engager et à coopérer avec des chercheurs de sciences de l'éducation dans le cadre de ce LéA, émane :- d'une part, du besoin d'accéder à une autre intelligibilité de la qualité des dispositifs de formation et d'accompagnement qu'ils proposent, que celle produite par les indicateurs quantitatifs classiquement mobilisés- d'autre part, du souhait de proposer aux enseignants et enseignants-chercheurs une autre modalité de développement professionnel, à savoir la recherche-collaborative. Celle-ci conduit pour ces derniers à envisager leur pratique comme un objet de recherche qu'ils acceptent de partager avec d'autres chercheurs. Le résultat visé est la production d'un savoir métissé pouvant être partagé tant par la communauté des praticiens que par la communauté d'experts.Après avoir décrit le contexte et la problématique qui ont amenés à la construction de cette coopération praticien/chercheurs, nous prendrons le temps de présenter le réseau des LéA, le projet et les recherches initiés et l'enjeu particulier que revêt ce LéA unique actuellement dans l'enseignement supérieur. Cela nous amènera à questionner la place de la recherche-collaborative dans l'enseignement supérieur, et l'intérêt que nous avons trouvé à nous y engager

Mechanical behavior of entangled fibers and entangled cross-linked fibers during compression

Entangled fibrous materials have been manufactured from different fibers: metallic fibers, glass fibers, and carbon fibers. Specimens have been produced with and without cross links between fibers. Cross-links have been achieved using epoxy spraying. The scope of this article is to analyze the mechanical behavior of these materials and to compare it with available models. The first part of this article deals with entangled fibrous materials without crosslink between fibers. Compression tests are detailed and test reproducibility is checked. In the second part, compression tests were performed on materials manufactured with cross-linked fibers. The specific mechanical behavior obtained is discussed

Characterization of the mechanical behaviour of both fusion zone and base metal of electron beam welded TA6V titanium alloy

The fusion zone of an electron beam welded Ti-6Al-4V alloy presents a a' martensitic structure which leads to a change of mechanical properties. Starting from two manufacturing processing routes for the alloy (1) a b processing followed by the weld (which will be considered as the reference microstructures), (2) an a+b processing followed by welding and a post weld heat treatment (PWHT), the microstructure can be adjusted to avoid local difference of strength, fatigue properties and impact toughness. This results from the optimisation of the process and of the PWHT. The present work investigates the mechanical behaviour and the damage mechanism of both base metal and fusion zone in regards to the microstructure and to the heat treatment parameters

Le projet ACTIFS face à l’altérité des étudiants et des établissements

La méthode « Peer instruction » (Mazur 1997) consiste à poser des questions à l’auditoire, qui répond à l’aide d’un dispositif de vote. En fonction du taux de bonnes réponses, l’enseignant invite les étudiants à argumenter leur réponse avec leurs voisins, puis à voter à nouveau avant de commenter les résultats. Le projet ACTIFS vise à développer cette méthode sur l’ensemble des 15 formations d’ingénieurs de Midi-Pyrénées, et a touché environ 120 enseignants, 7000 étudiants. La diversité de cet environnement est donc conséquente et a été largement prise en compte dans ce projet. Nous examinons dans cette étude le type et l’impact des altérités impliquées par ce projet et nous présentons un premier bilan en termes de ressenti par les étudiants et les enseignants

Mechanical and thermo-physical properties of plasma-sprayed thermal barrier coatings: a literature survey

Atmospheric plasma-sprayed thermal barrier coatings (APS TBCs) have been studied from an extensive review of the dedicated literature. A large number of data have been collected and compared, versus deposition parameters and/or measurement methods, and a comparison was made between two different microstructures: standard APS coatings and segmented coatings. Discussion is focused on the large scattering of results reported in the literature even for a given fabrication procedure. This scattering strongly depends on the methods of measurement as expected, but also—for a given method—on the specific conditions implemented for the considered experimental investigation. Despite the important scattering, general trends for the correlation of properties to microstructure and process parameters can be derived. The failure modes of TBC systems were approached through the evolution of cracking and spalling at various life fractions

Characterization of the mechanical properties of thermal barrier coatings by 3 points bending tests and modified small punch tests

Issu de : ICMCTF 2017 - 44th International Conference on Metallurgical Coatings and Thin Films, San Diego, UNITED STATES, April 24-28, 2017International audienceThe Atmospheric Plasma Sprayed Thermal Barrier Coatings (APS TBCs) is commonly used to insulate hot sections in gas turbines. Cyclic oxidation failure usually results from the spallation of the ceramic top coat. In order to predict such spalling phenomena, understanding the mechanisms for cracks initiation and propagation in thermal barriers is a major issue for engine-makers. Failure of the TBC is strongly related to the thermal and mechanical properties of each component of the multi-materials system (substrate, bond coat and ceramic) but also to the response of the TBC as a whole. The purpose of the present work is to assess the mechanical behavior of a complete TBC using comparative experimental (uniaxial and biaxial loading) and Digital Images Correlation (DIC) analysis approaches for classical TBC microstructures obtained through APS coatings.The experimental characterization of the mechanical behavior of the TBC systems was studied on as deposited specimens. Three Points Bending (3PB) tests were performed at room temperature, with the ceramic coating either under tension or compression. Additionally, in situ observations during 3PB tests by a camera, associated to a DIC analysis, allow determining the evolution of the strain field of surface sample correlated with the damage evolution. Location of crack initiation and crack propagation paths up to macroscopic failure were investigated. These tests highlighted the strong differences in mechanical behavior and fracture mode depending on the tension or compression stress state in ceramic coating. Small Punch Tests (SPT) were also performed at room temperature using both geometries (tension/compression). This allows pointing out the similarities of failure modes between uniaxial solicitation and biaxial flexure. Tests performed at 850 °C in the SPT ring show that when temperature varies, different mechanical properties can be observed

Study of crack propagation mechanisms during Charpy impact toughness tests on both equiaxed and lamellar microstructures of Ti–6Al–4V titanium alloy

The impact toughness of two highly textured rolled plates of Ti–6Al–4V alloy with an α equiaxed and an α lamellar microstructures has been investigated. The results show a strong anisotropy of the fracture energy for both materials and underline that a coincidence of the prismatic planes with the shear bands at the notch tip is favorable for higher fracture energies. Moreover, it is pointed out, as it was already done by previous studies, that the α lamellar microstructure presents higher fracture energy than the α equiaxed one. Thanks to electron back scattering diffraction, and tensile tests, local microstructure heterogeneities, called macrozones, have been observed and characterized. Their size depends on microstructure element and is larger for α lamellar microstructure than for the α equiaxed. High strain is localized on the macrozones favorably oriented for prismatic slip with respect to the direction of impact and leads to a particular dimple free zone on the fracture surface. The contribution of these macrozones in the fracture behavior, and more precisely on the crack propagation rate was evaluated; thus the effects of the macroscopic texture and of the microstructure element on the impact toughness are discussed separately