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

Decorrelated movements of Shockley partial dislocations in the γ phase channels of nickel-based superalloys at intermediate temperature

International audienceIn nickel-based superalloys with high volume fraction of γ ' precipitates, dislocations have to experience high curvatures in order to enter narrow channels by glide in the {111} planes of the f.c.c. 冏 matrix. Observation of in situ dynamic sequences performed in a Transmission Electron Microscope on several industrial superalloys have shown the occurrence of decorrelated movements of Shockley partial dislocations, originating from perfect dislocation dissociation. By evaluating the effective stress acting on each one of these partial dislocations, as well as their respective flexibility, the possible occurrence of such movements for some particular dislocation characters and channel widths is accounted for. These movements can play an important role in the creep behaviour of these materials in the low deformation rate regime

A new TEM method for the caracterization of the tertiary γ' nano-precipitates in a PM disk superalloy. Influence of ageing.

International audienceAn original method to characterize the γ'-phase tertiary precipitates in a Ni-based superalloy manufactured by powder metallurgy is described. This investigation is made using post-mortem Transmission Electron Microscopy. It is based on the analysis of sheared areas within crept specimens, which allows the precipitates seen by the dislocations in their glide plane to be evidenced. The characteristics of these nano-precipitates, i.e. their size, their volume fraction and the channel width between them, are determined for two different heat treatments (HT). The results show a wide distribution of the microstructural parameters for a same HT, but only slight differences between the microstructures produced by the two different HT. This microstructural information allows understanding the wide variety of the deformation micromechanisms observed during creep at high temperature