Etude des mécanismes d'endommagement d'un système barrière thermique déposé sur un superalliage base nickel

Thermal barrier coatings (TBC) deposited on nickel superalloy are developed for high-pressure turbine of turbofan.TBC is a duplex coating composed of a ceramic top-coat designed for thermal insulation and an intermetallic compound which accommodate the difference of the thermal expansion coefficient between the superalloy and the the ceramic top-coat. The oxidation of the intermetallic compound lead to the formation of a protective alumina scale which prevent oxidation of the superalloy.This study enabled to improve the understanding of the damage mechanism of TBC systems for complex loadings close to in-service loadings. For this purpose, isothermal and cyclic oxidation tests but also thermomechanical fatigue were performed. Tests were stopped before spalling of the TBC in order to study damage mechanisms and, for some of them, kinetics of damage. Some of these tests were followed by compression tests in order to measure the residual spalling resistance, which is responsible of the life duration of the turbines.Thanks to the use of observations from the macroscopic scale to the transmission electron microscope scale, damage mechanisms as well as their consequences on the residual spalling resistance were discussed according to the loadings conditions (isothermal or cyclic with or without mechanical loadings) and according to the sulphur level in the superalloyLes systèmes barrières thermiques constitués d'un revêtement céramique et d'une sous-couche d'accrochage déposés sur un superalliage sont développés pour les aubes mobiles de turbine haute pression des turboréacteurs. La durée de vie de l'aube estcorrélée à l'écaillage de la barrière thermique.Cette thèse a permis d'améliorer la compréhension des mécanismes à l'origine de l'endommagement du système barrière thermique sous des sollicitations complexes, proches des sollicitations des aubes en conditions réelles. Dans ce but, des essais d'oxydation isotherme, d'oxydation anisotherme et de fatigue anisotherme ont été réalisés. Les essais ont été interrompus avant la durée de vie afin d'étudier les mécanismes, les cinétiques d'endommagement et de déterminer la résistance résiduelle à l'écaillage. Les différents mécanismes d'endommagement et leurs conséquences sur la résistance à l'écaillage ont été discutés en fonction de la nature des sollicitations thermiques et / ou mécaniques imposées et en fonction du taux de soufre dans le superalliage

Stress corrosion cracking of Ni-base alloys in pressurized water reactors: from the «Coriou effect» to the use of tracers

International audienceThis overview of stress corrosion cracking (SCC) of nickel base alloys in pressurized water reactors (PWRs) includes an historical perspective and assesses the latest developments on the investigations regarding the SCC mechanisms. The “Coriou effect” named the stress corrosion cracking of a nickel alloy (Alloy 600) in pure (or primary) water at high temperature. The SCC of Alloy 600 has been observed for the first time by Henry Coriou at the end the 1950s in pure water at 350°C. Even if these tests were more representative of water reactor conditions than those obtained in boiling magnesium chloride solutions where Alloy 600 is not susceptible to SCC, nevertheless, Alloy 600 has been used extensively in water reactors up to the 80s. Large researches and developments have been made and lead to the use of Alloy 800 or Alloy 690.In the second part of the presentation, updated knowledge of SCC in PWRs is summarized and focuses on the use of tracers to determine SCC mechanisms: (i) formation of oxide layers and intergranular oxidation are studied with the help of oxygen 18, (ii) using deuterated species (D2 versus D2O), the main hydrogen uptake by the alloy comes from the cathodic reaction on water molecules, (iii) in the oxide layer of Ni-base alloys, the transport of hydrogen and of oxygen is concomitant, as observed by comparing diffusion of D and 18O. These observations, together with some atomistic modelling, lead to a local intergranular approach where both oxidation and hydrogen may be involved in initiation and propagation of cracks in Ni-base alloys and under PWRs primary conditions (while impurities play a major role in secondary PWRs environments).In conclusion, the good behavior of Alloy 800 and of Alloy 690 is underlined and linked to the higher chromium content. The remaining question is to know if these alloys are SCC immune, or if it is only the initiation time which is longer, over decades in PWRs conditions

Mécanisme de la corrosion sous contrainte de l'alliage 600 en milieu REP : apport à la compréhension des mécanismes

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Assessment of TBC oxidation-induced degradation using compression tests

International audienceCompression tests at room temperature are used to estimate the critical strain to spallation of EBPVD thermal barrier coating deposited on Ni based single crystals for blades used in aero-engines. The observation of fracture surfaces allows for investigating the location of delamination events leading to spallation, the eventual porosity at bond coat-alumina interface and kinetics of damage evolution. The effect of isothermal oxidation at 1,100 °C is presented for standard or low sulfur AM1 superalloy, and coating process variants. The degradation is shown to depend on the thermal-mechanical loading and varies from isothermal oxidation, cyclic oxidation and thermal-mechanical fatigue with hold time. These observations are consistent with the damage observed in blades in engine tests or in service. These tests are a useful complement to standard cyclic oxidation tests to identify engineering lifetime models

Advanced TEM characterization of stress corrosion cracking of Alloy 600 in pressurized water reactor primary water environment

International audienceAdvanced transmission electron microscopy techniques were carried out in order to investigate stress corrosion cracking in Alloy 600 U-bend samples exposed in simulated PWR primary water at 330 °C. Using high-resolution imaging and fine-probe chemical analysis methods, ultrafine size oxides present inside cracks and intergranular attacks were nanoscale characterized. Results revealed predominance of Cr2O3 oxide and Ni-rich metal zones at the majority of encountered crack tip areas and at leading edge of intergranular attacks. However, NiO-structure oxide was predominant far from crack tip zones and within cracks propagating along twin boundaries and inside grains. These observations permit to suggest a mechanism for intergranular stress corrosion cracking of Alloy 600 in PWR primary water. Indeed, the results suggest that stress corrosion cracking is depending on chromium oxide growth in the grain boundary. Oxide growth seems to be dependent on oxygen diffusion in porous oxide and chromium diffusion in strained alloy and in grain boundary beyond crack tip. Strain could promote transport kinetic and oxide formation by increasing defaults rate like dislocations

Effect of aerated transients on oxidation and SCC of stainless steels in PWR primary water

International audienc

Effect of aerated transients on oxidation and SCC of stainless steels in PWR primary water

International audienc

The role of intergranular chromium carbides on intergranular oxidation of nickel based alloys in pressurized water reactors primary water

International audienceAlloy 600 is used in pressurized water reactors (PWRs) but is susceptible to primary water stress corrosion cracking (PWSCC). Intergranular chromium carbides have been found beneficial to reduce PWSCC. Focussed ion beam coupled with scanning electron microscopy (FIB/SEM) 3D tomography has been used to reconstruct the morphology of grain boundary oxide penetrations and their interaction with intergranular Cr carbides in Alloy 600 subjected to a PWR environment. In presence of intergranular Cr carbides, the intergranular oxide penetrations are less deep but larger than without carbide. However, the intergranular oxide volumes normalized by the grain boundary length for both samples are similar, which suggest that intergranular oxidation growth rate is not affected by carbides. Analytical transmission electron microscopy (TEM) shows that the intergranular oxide consists mainly in a spinel-type oxide containing nickel and chromium, except in the vicinity of Cr carbides where Cr 2 O 3 was evidenced. The formation of chromium oxide may explain the lower intergranular oxide depth observed in grain boundaries containing Cr carbides

SCC crack growth rate of alloy 82 in PWR primary water conditions - effect of a thermal treatment

International audienceStress corrosion cracking (SCC) of wrought alloy 600 and parent weld metals (alloys 182/82) is a significant cause of failure in the pressurized water reactors (PWR). Only a small number of welds fabricated from Alloy 82 is affected by PWSCC. Most of these welds were not thermally heat treated unlike the industrial practice in France. This paper describes constant load crack growth rate (CGR) tests on alloy 82 with and without post weld heat treatment. Metallurgical examination of alloy 82 was carried out using among others Electron. Backscattering Diffraction and Transmission Electron Microscopy. The heat treatment seems to be highly beneficial by decreasing the CGR. This result can be explained by the effect of thermal treatment on the precipitation in alloy 82

Stable isotopes used in the definition of corrosion mechanisms

International audienceThe ability of secondary-ion mass spectrometry (SIMS) to separate isotopes and to analyze thin layers by sputtering provides unique tools for studying corrosion mechanisms. The methodology with oxygen 18 has been tested with success at the beginning of the 70s for the oxidation of tantalum (Ta) by water in two steps: first oxidation by H216O and subsequent one by H218O [1]. Nevertheless, very few corrosion studies have taken the advantage of isotope substitution for the investigation of corrosion mechanisms. The objective of the paper is to show that the use of stable isotopes has been a major step in the understanding and the modeling of several corrosion phenomena. The first illustrations will be linked to the localization of the anodic and cathodic reactions on archeological analogues. Then the use of isotopic tracers will be shown in more complex environments.In liquid lead-bismuth, sequential experiments with dissolved 18O and 16O have been performed to determine the mechanisms of growth of the duplex structure oxide layer: It was found that the magnetite layer grows at the Pb–Bi/oxide interface whereas the Fe–Cr spinel layer grows at the metal/oxide interface. The modeling of the growth mechanisms of the duplex layer lead to the evaluation of corrosion damages, in accordance with available data. It should be underlined that the same type of growth of duplex layer has been observed in supercritical water with the subsequent used of H216O followed by an exposure to H218O.Stress corrosion cracking of Alloy 600 in water at 300-350°C has been investigated with Alloy 600 (nickel base alloy with 15% Cr and more than 72% Ni) samples exposed to heavy water with dissolved hydrogen (D2O / H2, diss) or to natural water with dissolved deuterium (H2O / D2, diss). SIMS analysis of D (2H) and 16O were done to determine the deuterium concentration profiles together with the oxide film thickness on the alloy surface. Almost no deuterium is observed for samples exposed in the H2O/D2 environment and only in the oxide layer, whereas the intensity of the deuterium profile is much larger in D2O/H2 with deuterium observed not only in the oxide layer but also in the alloy. Clearly, the main source of hydrogen is the cathodic reaction (water dissociation). Two mechanisms may be proposed for modelling the hydrogen transport associated with the oxide growth during alloy passivation: (i) diffusion of hydrogen as an interstitial proton through the oxide lattice, or (ii) diffusion as a hydroxide ion towards the oxide in the anionic sub-lattice. The latter hypothesis implies the oxygen and hydrogen diffusivities through the oxide layer to be the same. To check which hypothesis is correct, Alloy 600 specimens have been exposed in PWR primary conditions using 2H and 18O as markers. The values obtained for diffusion coefficient of 2H and 18O are very close (around 5 10-17 cm2/s) which supports the idea of a hydrogen transport mechanism through the oxide layer as hydroxide ions. The strong correlation between hydrogen absorption and oxidation occurs not only for the formation of the oxide layer on the surface of the alloy, but also during intergranular oxidation of grain boundaries. The question here is to assess whether the oxide grown at the grain boundaries in the case of intergranular corrosion would act as a barrier to hydrogen arrival to the oxide/crack tip or not. After a primary oxidation in nominal primary water followed by a short period under the same conditions but with D and 18O isotopes, deuterium and oxygen 18 are found at the tip of the intergranular oxidation, even for short exposure times. The results lead to the conclusion that oxygen and hydrogen transport in the oxidized grain boundary are not the rate-controlling step for SCC initiation in PWR nominal conditions. To check if chromium diffusion is the limiting step, diffusion experiments with 54Cr in Ni-Cr alloys have been performed.The conclusive remarks will include some recommendations and some interests for the use of radioactive tracers to determine corrosion kinetics