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

On some advanced nickel-based superalloys for disk applications

International audienceRecent works performed at ONERA on nickel-based superalloys for disk applications are presented. In the first section, disk characteristics and metallurgical routes used to produce these specific aero-engine components are reviewed. Then, two alloy development programmes carried out to satisfy the requirements of the industrial partners are detailed. Finally, the results of studies aiming at identifying the complex relationships linking the microstructure and the creep behaviour of disk superalloys are described: i) influence of the strengthening gamma prime precipitation on the deformation micromechanisms, and ii) effect of the microstructure on the grain boundary sliding

γ′ Precipitation Study of a Co-Ni-Based Alloy

International audienceA Co-Ni-based alloy strengthened by γ′-(L12) precipitates was utilized to investigate the precipitation evolution after various cooling rates and several aging conditions. In this study, the precipitate size and volume fraction have been studied via scanning electron microscopy and transmission electron microscopy. The influence of the precipitation evolution was measured via microhardness tests. The cooling rate study shows a more sluggish γ′ precipitation reaction compared to that observed in a Ni-based superalloy. Following a rapid cooling rate, the application of appropriate double aging treatments allows for the increase of the γ′ volume fraction as well as the control of the size and distribution of the precipitates. The highest hardness values reach those measured on supersolvus cast and wrought Ni-based superalloys. The observed γ′ precipitation behavior should have implications for the production, the heat treatment, the welding, or the additive manufacturing of this new class of high-temperature materials

Development of a new fatigue and creep resistant PM nickel-base superalloy for disk applications

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

International audienceNickel-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 gamma 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

Analysis of high-temperature creep deformation in a polycrystalline nickel-base superalloy

International audienceA new quantitative method has been developed for the analysis of the high-temperature creep deformation of polycrystalline nickel-base superalloys. Local deformation was measured by means of microextensometry using ceramic square grids deposited on flat specimens thanks to an electron lithography technique. The efficiency of this method has been proved for tensile creep tests performed under vacuum at 750 degrees C and 525 MPa on the NR6 superalloy and for a wide range of creep strains. This study revealed localization of deformation at grain boundaries and slip bands within the grains. A specific analysis method based on image correlation was developed to quantify the local strain fields. Electron back-scatter diffraction analysis showed that grain boundary sliding is mainly concentrated at general grain boundaries

Quantitative evaluation of high temperature deformation mechanisms: a specific microgrid extensometry technique coupled with EBSD analysis

GRAIN-BOUNDARIES ; CREEP ; DISLOCATIONS ; POLYCRYSTAL ; OXIDATION ; ALUMINUM ; BEHAVIOR ; COPPER ; MOSI2International audienceA microgrid extensometry method has been developed and used to obtain information about intragranular and intergranular creep mechanisms. An oxide grid was deposited on a creep specimen using an electron lithography technique. This oxide grid offers high backscattered electron contrast and can withstand long duration creep tests under vacuum in the 700-850 A degrees C range without degradation. Specific methods were used to measure in-plane displacements at the grid nodes or at the grain boundaries using correlation of grid images taken before and after the creep test. The local strain and grain boundary sliding (GBS) data were then calculated. Combined information about grain boundary crystallography and GBS has been obtained by superimposing the electron backscattered diffraction (EBSD) map on the deformation maps. To illustrate the potential of this set of processes, two examples of application on a nickel-base disc superalloy are presented. The first one concerns the influence of the creep temperature on the local strain and the GBS. The second application quantitatively shows the influence of grain boundary character on GBS of this material