Transmission Electron Microscopy Examination (TEM) and Atom Probe Tomography (APT) Analysis of Ion irradiated Alloy 718.
Résumé
Transmission Electron Microscopy Examination (TEM) and Atom Probe Tomography (APT) Analysis of Ion irradiated Alloy 718.
Sylvie Doriot 1*, Isabelle Mouton1, Joël Malaplate1, Benoît Arnal1, Philippe Bossis2, Antoine Ambard3, Didier Bardel4, Florent Bourlier4
1 Université Paris-Saclay, CEA, Service de Recherches Métallurgiques Appliquées, 91191 Gif-sur-Yvette, France
2 Université Paris-Saclay, CEA, Département des Matériaux pour le Nucléaire, 91191 Gif-sur-Yvette, France
3 Electricité de France, R&D Division, Materials and Mechanics of Components, Les Renardières, 77818 Moret sur Loing Cedex, France
4 Framatome, Fuel Business Unit, 2 rue du Professeur Jean Bernard, 69007 Lyon, France
* Main Author, E-mail: sylvie.doriot@cea.fr
Keywords: Alloy 718, irradiation, APT, MET
Introduction
Precipitation-hardening nickel base Alloy 718 is used in Pressurized Water Reactor (PWR) fuel as-semblies because of its high strength, irradiation swelling resistance and corrosion resistance. As these properties are strongly dependent on the microstructure [1], Framatome is continuously as-sessing the material performance in service. More specifically, it is of interest to identify how the g'and g'' phases evolve under irradiation. This study thus presents TEM and APT characterizations that were carried out on one 718 Alloy before and after ion irradiation at the temperature of 350°C and at two different doses.
Materials and experimental methods
Table 1 shows the composition of the studied Alloy 718. The heat treatment was carried out by Framatome. It was the classical two-steps heat treatment [2]: solutioning at high temperature (~1000°C) followed by age-hardening at lower temperature (between 600°C and 800°C).
For irradiation, disk specimens with a diameter of 3 mm and a thickness of 100 µm were irradiated using Ni3+ ions with an energy of 6.5 MeV at the JANNUS CEA platform, up to respectively ~ 0.5 and 10 dpa. Thanks to a specific preparation method, an electropolished thin foil located up to about 200 nm from the irradiated surface was observed by TEM. The object distributions were counted by im-age analysis, using the Noesis Visilog software™. In addition to the TEM observations, APT analyses were carried out in order to obtain chemical and morphological information about the precipitates. APT needle tips were prepared by the focused ion beam (FIB) lift-out technique in a FEI Helios Nanolab 650 dual beam FIB and were analyzed using a LEAP 4000X HR instrument, both at CEA.
Results
TEM observations in the as-received material showed delta phases and a fine and intensive distribution of g' and g'' precipitates (Figure 1a). This fine precipitation appeared as alternate g'/g'' welded platelets as seen by APT analyses, Figure 2a, in agreement with the literature [3].
After irradiation, Frank loops and dislocation network were observed as soon as a dose of 0.5 dpa was reached. As previously seen in the literature [4], the spots corresponding to the g'' phase disap-peared as the spots corresponding to the g' phase remained. The observed particles presented the global size and shape of the former g'/g'' welded platelets (APT) and was enlighten by dark fields on g' spots (TEM). Thus the alternate g'/g'' welded platelets seemed to merge into g' ovoid shaped particles containing variable Al, Nb and Ti amounts. As measured by APT, the outer part of the particles contained a lower amount of Al and a higher amount of Nb than the core and came probably from the former g'' particle.
After irradiation at the dose of 10 dpa, Frank loops doubled their diameter with about the same num-ber density. The spots corresponding to the g' particles disappeared in agreement with the literature [4] at this high dose. Nevertheless, APT analyses detected particles with similar shape, size and den-sity but with a dilute concentration of Al, Nb and Ti, compared to the particles observed after irradia-tion at 0.5 dpa. Diffraction diagrams presented additional tiny spots situated under the matrix spots (arrowed in Figure 1b). Dark fields on these tiny spots provided micrographs showing highly broken down objects with a global shape and size similar to the former ovoid g' particles (Figure 1b). Thus the ovoid g' particles seemed to transform into highly broken down cfc particles with a slightly higher crystal parameter than the matrix and with the same crystalline orientation but with a higher Ti, Al and Nb content.
Conclusion
The present results prompt to reconsider the usually assumed interpretation of the gradual disappearance of the g'' and g' spots in the TEM diffraction patterns during ion irradiation, which would consist in a loss of ordering of the precipitates followed by a dissolution of the g'' precipitates and then by a dissolution of the g' particles [4]. In our study, we observed that, at 0.5 dpa, the g'/g'' welded platelets seem to merge into g' ovoid shaped particles, that further transform, at 10 dpa, into highly broken down cfc particles.
Acknowledgments
This work has been funded by the project ASSEMBLAGE from the French nuclear tripartite institute I3P CEA-EDF-Framatome. It received assistance from the Agence Nationale de la Recherche pro-gram GENESIS referenced as ANR-11-EQPX-0020. In addition, the authors would like to thank warmly the staff of JANNUS facility for their assistance during ion irradiation.
References
1) M.G. Burke, T.R. Mager, M.T. Miglin and J.L. Nelson, The effect of thermal treatment on SCC of alloy 718: a structure properties study, Superalloys 718, 625, 706 and Various Derivatives, Edited by E.A. Loria The Minerals,Metals & Materials Society, (1994) 763-773.
2)RCC-C 2016, afcen, p. 141
3)B.H. Sencer, G.M. Bond, F.A. Garner, M.L. Hamilton, S.A. Maloy, W.F. Sommer, Journal of Nuclear Materials 296 (2001) 145-154.
4)S. Jin, F. Luo, S. Mab, J. Chen, T. Li, R. Tang, L. Guo, Nuclear Instruments and Methods in Physics Re-search