Experimental study of extruded powder metallurgy γ-TiAl based alloys

International audienceIn the context of research of lighter and/or stronger high temperature materials in order to improve aeroengines, the present work shows an example of a powder metallurgy (PM) manufacturing route for TiAl alloys that could be used for low pressure turbine (LPT) blades processing. In this frame, two different alloys, Ti-47Al-1Fe-1Zr-1Mo-0.2Si (at.%) FZM and Ti-43.5Al-4Nb-1Mo-0.1B (at.%) TNM-B1, were gas-atomized into powders, extruded, heat-treated and mechanically tested.Dans le contexte de la recherche de matériaux haute température plus légers et/ou plus résistants pour l'amélioration des performances des moteurs d'aéronefs, cet article présente un exemple de voie d'élaboration par métallurgie des poudres d'alliages base TiAl. Ces alliages sont potentiellement utilisables pour la fabrication d'aubes de turbine basse pression. Ainsi, l'alliage FZM de composition Ti-47Al-1Fe-1Zr-1Mo-0.2Si (% at.) et l'alliage TNM-B1 de composition Ti-43.5Al-4Nb-1Mo-0.1B (% at.) ont été atomisés par gaz, extrudés, traités thermiquement et testés mécaniquement

Numerical modelling of the microstructure effect on fatigue behaviour of Ni-base superalloys for turbine disk

Nickel-based alloy like N18 can present various types of precipitate distributions according to the applied heat treatment. A model involving a three scale homogenization procedure is developed to characterize the influence of this microstructure on fatigue life. The microstructural parameters are the size and the volume fraction of the secondary and tertiary precipitates of γ\u27 phase. Experimental results at 450 °C, specially designed to calibrate the model, allow to understand the role of tertiary precipitation. The first identification of the three scale homogenization model is shown

Kinetics pathway of precipitation in model Co-Al-W superalloy

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Influence of y' precipitate-size and distribution on LCF behavior of a PM disk superalloy

International audienceThe influence of γ' precipitate distribution on tensile and low cycle fatigue (LCF) behaviors of a powder metallurgy (PM) disk superalloy was investigated at 450°C. Four γ' particle distributions were obtained through various cooling paths and/or aging treatments in coarse grain size superalloy N18. The mechanical tests show that the main influence of the intragranular microstructure concerns the 0.2% yield stress (0.2%YS) and the ultimate tensile stress. Wide variations of the 0.2%YS affect the mean stress under non symmetrical loading but have only little effect on fatigue life, the lower the 0.2%YS, the longer the fatigue life. The fatigue life of N18 at 450°C is independent of the intragranular microstructure as long as the mean stress effect is correctly taken into account. As expected with the coarse grain size N18, no crack initiation at pores or inclusions was observed. A precipitation model was coupled with a critical resolved shear stress calculation providing 0.2%YS value for a given heat treatment sequence. Finally, this computation procedure was implemented in a numerical modeling of the LCF life of a disk taking into account the heat treatment applied to its wrought preform

Suivi de la cinétique associée à la phase gamma' dans le superalliage N18 en utilisant des mesures de résisitivité électrique in situ

International audienceIn nickel-based superalloys, temperatures related to the formation or the dissolution of the different types of γ' precipitates are important parameters for optimizing the mechanical properties of components but also for developing models which can reproduce the kinetics of their phase transformation. We showed that the electrical resistivity variations during heat treatment of the N18 superalloy was sufficient to monitor the kinetics related to secondary and tertiary γ' precipitates. In particular, the effects of the heating rate and the initial microstructure on the dissolution kinetics of the γ' phase were investigated. Experimental results were also compared to outputs of a precipitation model developed for the N18 alloy showing that in situ electrical resistivity measurements can be used for calibration and validation purposes

γ′ precipitation kinetics in the powder metallurgy superalloy N19 and influence of the precipitation latent heat

The N18 superalloy, powder metallurgy processed, is used for high-pressure turbine disks of the Snecma M88 engine and has been extensively studied. The size distribution of its γ′ precipitates was first modelled in the late 1980's using a binary alloy model [1]. The precipitation model has been recently revised, upgraded in a pseudo-binary alloy model and used in a multi-scale mechanical model for the calculation of the fatigue life in low cycle fatigue conditions [2]. We here present how the new calibration of the pseudo-binary model for the N19 superalloy has been accelerated through the use of the Thermo-Calc® software, its TCNI5 thermodynamic database and its MOBNI2 mobility database. On a more fundamental point of view, the influence of the γ > γ′ latent heat of precipitation on the microstructural parameters of quenched samples of N18 and N19 has been studied using this same model. We conclude on the very importance of taking into account the latent heat in a coupled thermal-microstructural model for correct numerical simulations of the precipitation process at each point of a superalloy sample or part

Influence de la microstruture sur le glissement intergranulaire lors du fluage d'un superalliage pour disques

L'objectif de cette thèse est de mettre en évidence l'influence de la microstructure initiale sur le glissement intergranulaire lors du fluage à haute température d'un superalliage polycristallin à base de nickel. Dans ce but, plusieurs microstructures sont obtenues à partir de la microstructure de référence de l'alliage NR6, par application de traitements thermiques spécifiques. L'influence des paramètres microstructuraux sur les déformations locales est ensuite étudiée à l'aide d'une technique de microextensométrie couplée à une analyse par diffraction des électrons rétrodiffusés. Il est ainsi possible de relier microstructure, déformations locales et comportement macroscopique en fluage. Pour la microstructure de référence de l'alliage NR6, la déformation opère principalement par cisaillement des phases g et g'. Ce mécanisme est favorable au glissement intergranulaire. L'absence de précipités tertiaires de phase g' favorise le contournement des précipités secondaires par les dislocations. Ceci permet de réduire le glissement intergranulaire mais est également néfaste pour la résistance à la déformation de l'alliage. La présence de joints de grains dentelés augmente la résistance au glissement intergranulaire mais diminue la résistance à la déformation intragranulaire en favorisant le contournement des précipités. Ainsi la résistance globale à la déformation n'est pas affectée. Enfin, l'augmentation de la taille de grains n'a d'influence ni sur les mécanismes de déformation mis en jeu ni sur l'amplitude du glissement. Cependant, la fraction moins élevée de joints de grains induit une diminution de la contribution du glissement intergranulaire à la déformation globale.The aim of this study is to highlight the influence of initial microstructure on grain boundary sliding during high-temperature creep of a polycrystalline nickel-based superalloy. To reach this goal, several microstructures are produced from the reference microstructure of NR6 alloy by adequate heat treatments. The influence of microstructural parameters on local deformations is then studied thanks to a microextensometry technique coupled with an electron back-scattered diffraction analysis. It thereby enables linking microstructure, local deformations and macroscopic creep behaviour. In the case of NR6 alloy reference microstructure, deformation occurs mainly by g and g' phases cutting by dislocations. This mechanism is grain boundary sliding-favourable. The absence of tertiary g' phase precipitates promotes secondary precipitates bypassing by dislocations. This results in a reduction of grain boundary sliding but is also harmful to the alloy creep resistance. Grain boundary serration improves grain boundary sliding resistance but diminishes intragranular deformation resistance by favouring precipitate bypassing. Then global deformation resistance is not changed. Finally, grain size increase has influence neither on activated deformation mechanisms nor on sliding amplitude. However, the decrease of grain boundary fraction leads to a reduction of grain boundary sliding contribution to overall strain.SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF

Powering reversible actuators using forward osmosis membranes: feasibility study and modeling

The aim was to determine the impact of membrane properties and operating conditions upon predicted performance of osmotically driven actuators. An actuator fitted with a forward osmosis membrane was studied, and significantly we examined the reversibility of the actuation process. It was discovered that the actuation-retraction cycle could be repeated for over 60 cycles before salt concentration became similar on both sides of the membrane. The cycle length and number of operative cycles were shown to be a dependent on membrane properties. It was demonstrated that issue of long actuation time can be addressed using membranes with high water permeability