Evolution of interfacial dislocation network during anisothermal high-temperature creep of a nickel-based superalloy

The effect of thermal cycling creep on the dislocation networks at the γ/γ′ interfaces in the MC2 superalloy is investigated. Tensile creep tests were performed under thermal cycling and isothermal conditions at low stress (80 MPa) and high temperature (1150 °C). In these conditions γ′ rafts may dissolve and reprecipitate during thermal cycling creep. The difference between the effects of isothermal and thermal cycling conditions on the γ/γ′ interface dislocation networks, characterized by transmission electron microscopy, is exposed, as well as their evolution during the cycle

Determination of precipitate strength in aluminium alloy 6056-T6 from transmission electron microscopy in situ straining data

International audienceThe transmission electron microscopy in-situ straining technique is employed to measure the breaking angles of strengthening precipitates in aluminium alloy 6056-T6 as they are sheared by dislocations. The experimental determination of the character of bowed dislocation segments when dislocations are pinned on precipitates allows us to calculate the corresponding line tensions. From this, the maximum forces F-m that precipitates can sustain before being sheared by dislocations are deduced. It is suggested that F-m may be regarded as a quantitative parameter which includes the effects of the various strengthening mechanisms operative in precipitation-hardened alloys. An attempt is made to relate the maximum force calculated from in-situ straining data to the macroscopic yield strength of the material

Influence du vieillissement sur le comportement en fluage d'un superalliage pour disques d'une turbine

Le projet national Avion de Transport Supersonique du Futur prévoit la création d un successeur au Concorde avec une durée de vie quatre fois plus longue et un rendement du moteur amélioré. Dans ce cadre, un nouveau superalliage pour disques de turbine appelé NR3, développé par l ONERA, a été élaboré à la SNECMA par métallurgie des poudres afin de résister aux nouvelles conditions d utilisation correspondant à une déformation en fluage à 700C sous 300 MPa pendant 20000 heures. Un maintien de longue durée sous température de ce type de matériau biphasé entraîne le vieillissement de sa microstructure de précipitation et donc l effondrement de ses propriétés mécaniques. L objectif de ce travail est d étudier, après différents traitements thermiques, l influence du vieillissement sur la microstructure de précipitation bimodale retenue et d en déduire la répercussion sur la tenue en fluage à partir de l étude des micro-mécanismes de déformation. Pour cela, plusieurs techniques expérimentales ont été utilisées : essais de fluage, microscopie électronique en transmission (MET) post mortem et in situ et spectroscopie EDX. Ce travail a permis de montrer que le vieillissement affecte uniquement la population de petits précipités appelés tertiaires qui tend à disparaître. L observation par MET et la caractérisation des micro-mécanismes responsables de la déformation ont permis de comprendre le rôle essentiel de ces précipités. Ils contrôlent la tenue en fluage par leur taille, la distance qui les sépare, leur chimie et la qualité de leur ordreThe purpose of the national project called Future Supersonic Transport Airplane concerns the creation of a successor for Concorde able to have a life time four times longer and an increase of the aeroengine output. A new powder metallurgy superalloy called NR3, developed at ONERA, was then elaborated at SNECMA to be used for turbine disks in order to resist the new real working conditions. They are defined by a creep deformation near 700C under 300 MPa during 20.000 hours and the long time maintenance, at such a temperature, of this two-phase alloy, inducing a microstructural change negative for the mechanical properties must be understood. The aim of our work is to study, after different thermal treatments, the evolution of the deformation micro-mechanisms during this microstructural ageing, in order to deduce its impact on the creep resistance of the alloy. Several different experimental techniques were used at different scales: macroscopic creep tests, post mortem and in situ transmission electron microscopy (TEM) and EDX spectroscopy. Finally, a particular emphasis is made on the decrease of the population of the smallest tertiary precipitates during ageing. Their important role is revealed by TEM observations through the evolution of the micro-mechanisms responsible for the deformation. It is clearly demonstrated that their dimension, the channel width between them, their chemistry and the quality of their order control at this temperature the variation of the creep resistance of the alloyTOULOUSE-INSA (315552106) / SudocPARIS12-THIAIS CNRS ICMPE (940732301) / SudocSudocFranceF

Etude des micromécanismes de déformation d'un alliage Ti-6Al-4V par déformation MET in situ (influence d'un traitement de surface)

Combinant une faible densité à d'excellentes propriétés mécaniques, le Ti-6Al-4V est l'alliage de titane le plus utilisé en aéronautique. Malgré cela, les mécanismes élémentaires de déformation du Ti-6Al-4V, responsables de ses propriétés mécaniques, ne sont pas encore totalement connus. L'objectif principal de ce travail est donc d'apporter une meilleure compréhension de ces micromécanismes. Cet alliage présentant d'assez mauvaises propriétés de surface, notamment au niveau de l'usure mécanique, nous nous sommes aussi intéressés aux modifications apportées par un traitement de surface protecteur (nitruration). Pour mener cette étude à bien, nous avons réalisé un grand nombre d'essais de traction in situ dans un microscope électronique en transmission (MET). Nous avons ainsi déterminé que la résistance de cet alliage est essentiellement due à la structure de cœur des dislocations vis de vecteur de Burgers de type a et que l'ordre à courte distance (OCD) a une influence notable dans la déformation des nodules. Le rôle des différents types d'interface dans la déformation a aussi été clairement défini. Concernant l'alliage traité en surface, les observations post mortem d'échantillons pré-déformés montrent que la couche nano-cristallisée en surface est une barrière à la propagation des dislocations et que, dans la couche de diffusion, l'OCD évolue vers la formation de nano-précipités ordonnés à longue distance conduisant à un mouvement plus collectif des dislocations. Enfin, nous avons réalisé pour la première fois un essai de traction MET in situ sur un échantillon préparé en section transverse grâce à la mise au point d'une méthode originale utilisant un FIBTi-6Al-4V is the most used titanium alloy in aeronautics because of its low density combined with excellent mechanical properties. However, the elementary mechanisms of deformation responsible for mechanical properties of Ti-6Al-4V alloys are not yet well understood. To provide a better knowledge of these micromechanisms is the main purpose of this work. As this alloy has relatively bad surface properties, particularly regarding mechanical wear, we are also interested in modifications induced by a protective surface treatment (nitridation). Numerous in situ transmission electron microscope (TEM) tensile experiments have been performed during this study. They have allowed us to determine that the strength is mainly due to the core structure of screw dislocations with a-type Burgers vectors and that short range order (SRO) has a non negligible effect in the deformation of nodules. The role of the different interfaces has been also clearly enlightened. Concerning the surface treated alloy, post mortem observations of pre-deformed samples show that the nano-crytallised layer at the surface is a barrier for the propagation of dislocations. In the diffusion layer SRO evolves to the formation of long range ordered nano-precipitates leading to a more localised deformation. Finally, an in situ TEM tensile experiment has been performed for the first time on a cross-section sample prepared with an original way using a FIBTOULOUSE-INSA (315552106) / SudocSudocFranceF

ÉTUDE DES DÉFAUTS D'EMPILEMENT DANS LES ALLIAGES CUBIQUES A FACES CENTRÉES DU NICKEL

Une étude radiocristallographique (Méthode de Paterson à - 150 °C) et la détermination de la fréquence des macles de recuit ont permis de sélectionner quelques alliages c. f. c. du nickel dont l'énergie de défaut pouvait être nettement inférieure à celle du nickel pur. D'après le rayon de courbure des partielles des nœuds élargis mesuré au microscope électronique, on trouve une énergie de défaut intrinsèque de 80 erg/cm2 environ pour Ni-39 % Cr ou Ni-17 % Mo-5 % Fe, alors que cette énergie n'est que de 20 erg/cm2 pour un acier inoxydable. Cette étude souligne l'importance du choix et du nombre des nœuds (isolés ou en réseau), de la formule et des constantes élastiques utilisées, et surtout du traitement mécanique et thermique qui a formé les nœuds dissociés.X-ray studies (Paterson's method at - 150 °C) and determinations of the frequency of annealing twins have permitted the selection of a few f. c. c. nickel base alloys, the stacking fault energy of which might be considerably lower than that of. Curvature radii of extended node partials measured in an electron microscope, give as intrinsic fault energy about 80 erg/cm2 for Ni-39 % Cr or Ni-17 % Mo-5 % Fe, whilst this energy is only 20 erg/cm2 for a stainless steel. The study points out the importance of selection and number of nodes (isolated or in a network), of formula and elastic constants, and especially of thermal and mechanical treatment which has produced the extended nodes

TEM quantitative characterization of short-range order and its effects on the deformation micromechanims in a Ti-6Al-4V alloy

International audienceLocal order is evidenced in nodules of the duplex microstructure of a Ti-6Al-4V alloy using in situ straining experiments in a transmission electron microscope (TEM). This local order is identified to be short range order (SRO) because of the absence of superlattice diffraction spots, which are associated with alpha(2) (Ti3Al) precipitates and because of the formation of single pairs of mobile dislocations, which are a signature of SRO. The strengthening effect of this SRO is quantitatively evaluated. Qualitatively, the presence of SRO inhibits strongly the cross-slip in nodules in comparison with dislocations gliding in lamellar colonies where no SRO is present. The well-known strengthening effect of the core structure of dislocation in Ti-alloy is revisited here in the presence of SRO to determine its possible influence

Direct measurement of the variation the energy of a dislocation locked in specific orientations

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Quantitative TEM study of the precipitation microstructure in aluminium alloy Al(MgSiCu) 6056 T6

International audienceThe precipitate microstructure in the last-generation aluminium alloy 6056 T6 [AlMgSiCu] is investigated using three complementary techniques of transmission electron microscopy (TEM) with a special focus on the density and volume fraction of strengthening particles. High-resolution TEM allows the identification of the precipitates and the measurement of the precipitate sizes to be performed. Conventional TEM is used to evaluate the number of precipitates in the investigated area as well as their distribution in the matrix. In situ TEM straining, via the analysis of the dislocation slip traces, permits to determine precisely the thickness and the volume of the foil in the region where the precipitates are analysed. Taking into account the shape and the dimensions of precipitates with respect to the foil thickness, a novel methodology for measuring the volume density and the volume fraction of precipitates is proposed

Transmission electron microscopy study of precipitate morphology and precipitate overcoming processes in aluminum alloy 6056 T6

11th International Conference on the Strength of Materials (ICSMA-11), PRAGUE, CZECH REPUBLIC, AUG 25-29, 1997International audienceHigh resolution electron microscopy and in situ straining experiments are performed on aluminum alloy 6056 T6 to charaterize the morphology of precipitates and the processes of precipitate overcoming by the dislocations. Two types of precipitates (needles and laths) are identified. They are found to be sheared by the dislocations and the maximum force precipitates can sustain before being sheared is calculated