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J. Phys. IV France
Volume 12, Numéro 8, September 2002
Page(s) III - VI
Structural Materials for Hybrid Systems: A Challenge in Metallurgy
D. Gorse, J.-L. Boutard
J. Phys. IV France
12 (2002) Pr8-III


D. Gorse and J.-L. Boutard

The Symposium on "Structural materials for Hybrid systems: A challenge in metallurgy" was held during the "Journées d'Automne 2001" of the "Société Française de Métallurgie et de Matériaux", Paris from October 29 through 31, 2001. The editors of this volume, D. Gorse and J.-L. Boutard, would like to acknowledge GDR GEDEON, CEA - Direction de l'énergie Nucléaire, CNRS - Département des Sciences Chimiques, et Centre d'études de Chimie Métallurgique for sponsoring this symposium.

This symposium was divided into three sessions dealing with i) thermodynamics, intergranular penetration and liquid metal embrittlement, ii) irradiation effects in structural materials and iii) compatibility of structural materials with lead alloys in relation with R& D studies for MEGAPIE. The intent of this symposium was to provide a forum for discussing the most recent results obtained in the frame of the materials research program of the "Groupement De Recherche (GDR) GEDEON". Special emphasis was given to all factors susceptible to affect the durability of structural materials for spallation targets, like irradiation effects under proton and neutron mixed spectrum, Liquid Metal Corrosion (LMC) and Embrittlement (LME).

The material research program of GEDEON is a joint CEA-CNRS venture. In 1997, the GDR GEDEON gave opportunity to metallurgists and nuclear physicists of both organizations to collaborate for validating the concept of Pb-Bi spallation targets as a key component for Accelerator-Driven Systems (ADS). Historically, since 1995, GDR GEDEON has promoted ADS, also called Hybrid System, as an option for waste management. The starting point of the material program was the 1st GEDEON Workshop on "Materials For Hybrid Systems" held in Paris in 1997, where reference materials of the 9-12 Cr martensitic steels series were selected: EM10 (9Cr-1Mo) and the modified 9Cr-1Mo, T91 (9Cr-1Mo-V-Nb). However, besides their specific concern for ADS, our studies are also of interest for the next generation of LM spallation targets in EU, U.S.A. and Japan.

These proceedings contain manuscripts from 90% of the presented papers. The organizers would like to thank all their Colleagues who presented papers, contributed with manuscripts and attended the sessions at the symposium. For sake of clarity, this volume is divided into five sections: 1) general R& D for spallation targets, 2) irradiation effects in liquid metal spallation targets, 3) oxygen control: thermodynamics and monitoring, 4) resistance to liquid metal corrosion and embrittlement of structural materials for spallation targets and 5) basic studies of intergranular penetration and liquid metal embrittlement.

Section 1 begins with a description of the spallation neutron source facility SINQ and of ongoing R& D programs at PSI (Switzerland), including MEGAPIE, the joint initiative by six European research institutions and JAERI (Japan), DOE (USA) and KAERI (Korea) to design, build, operate and assess the performance of a liquid lead-bismuth spallation target for 1MW of beam power (G. Bauer et al.). The materials aspects related to the MEGAPIE target and to the LiSoR (Liquid Solid Reactions under irradiation) experiment are reviewed by T. Auger et al. The advantages and drawbacks of solid tungsten spallation targets, compared to liquid Pb-Bi eutectic spallation targets are examined by R. Enderlé et al., presenting the CEA point of view.

Section 2 is dedicated to irradiation effects in Liquid Metal (LM) spallation targets structure, a crucial problem for the feasibility of ADS. P. Jung is pointing out the specificity of the irradiation conditions in LM targets by comparison with fast neutron fission and fusion reactors, and the metallurgical consequences like irradiation and helium-induced embrittlement. The author emphasizes the importance of spallation residues whose deleterious effects on in-service properties of target container and window are largely unknown. Until recently, say 1997, only predictions of spallation residues production were available. This motivated the GEDEON workshop on "Nuclear Data for materials studies in the frame of hybrid systems" held in 1999 in Vichy (France). The paper presented by C. Villagrasa et al. highlights the progress achieved during the past five years, promoted by GDR GEDEON. The reverse kinematics experiments performed during that period allowed to obtaining reliable values for production cross sections of residual nuclei produced by spallation reactions. The authors also mention improvements on the computation codes. The case of the target window, thin target simply made of iron from the nuclear physics point of view, is thoroughly investigated. The authors also treat the production of residues in heavy metals, like lead.

The experimental data reported below results from the cooperation of CEA, CNRS and Forschungszentrum Jülich within the program SPIRE of the 5th European Framework Program.

From the metallurgists point of view, O. Danylova et al. reported an interesting attempt to simulate by doping production of three spallation residues, namely phosphorus, sulfur and titanium, certainly detrimental regarding the mechanical properties (fracture toughness...) of the 9% Cr martensitic steels pre-selected for the target window and container. The authors describe the inherent difficulties encountered for obtaining the microstructure and modified steel composition expected under irradiation.

In all cases, production of hydrogen and helium is expected very high, ~90 000 appm/year for hydrogen and ~5000 appm/year for helium, corresponding to a damage level of ~100 dpa/year in the target window. These estimated values are just mentioned to show the importance of the metallurgical problem to be faced. These data motivated studies of helium implantation effects by J. Henry et al. in two 9Cr-1Mo (EM10) and modified 9Cr-1Mo-V-Nb (T91) pre-selected martensitic steels. Homogeneous implantation of 5000 appm He in T91 and EM10 thin foils followed by TEM examination and tensile testing revealed an important hardening and embrittlement at 250 °C. At 500 °C both materials exhibit far less hardening and retain significant ductility. In the latter case, those findings were correlated with the appearance of helium bubbles detected in Transmission Electron Microscopy (TEM) examination. At 250 °C, no He bubble is detected by TEM. Conversely, Small Angle Neutron Scattering (SANS) allows to identify a high density of objects ~1-2 nm, most probably tiny He bubbles. The low temperature He embrittlement is most probably due to the conjunction of grain hardening and decrease of the cohesive energy of prior austenitic grain boundaries due to He segregation.

Besides hydrogen and helium, significant quantities of other spallation residues will be also accumulated in the target window during one year of operation. About 1000 appm of calcium form after one year of irradiation with 1 GeV protons. This exemplary insoluble spallation residue could also contribute to embrittle the window. G. Amiri et al. studied the implantation of both calcium and sulfur (up to 1-2 at% corresponding to a 20 dpa damage level) at 500 °C, assumed to be the highest temperature of the target window in service conditions. P. Pareige et al. detected the formation of Ca nanoclusters using the 3D atomic probe. Neither calcium sulfides nor carbides were detectable whatever the temperature (300 °C or 500 °C) and damage levels. E. Cadel, P. Pareige and M.O. Ruault showed how powerful is the 3D atom probe to characterize both chemical and structural evolution of irradiated steel specimens, at a nanometric scale allowing for interpretation of radiation damages "from first principles".

Section 3 is dedicated to oxygen control, including thermodynamic aspects and monitoring. The GDR GEDEON material program promoted the development of oxygen sensors in liquid metals. V. Ghetta et al. studied carefully the possibilities, limitations of existing sensors and proposed new developments allowing to interpret e.m.f measurements at temperatures well below 450 °C. Intercomparisons between oxygen sensors, using various static facilities (BIP, JACOMEX and COLIMESTA) were carried out by J.-L. Courouau et al. This study is now going on including our EU partners in the closely related TECLA program granted by the 5th framework program. C. Lesueur et al. developed original methods for measuring wettability, permitting to determine the stability of native oxides (on Al, Fe, Ni and also T91 steel) in contact with molten lead. A. Maitre et al. presented a preliminary thermodynamic study of the quinary Bi-Fe-Hg-O-Pb system, beginning with the Bi-Hg-Pb system. No solid ternary phase was found in Pb-Bi eutectic in the temperature range expected for the spallation target. Then the authors focused on the possible formation of oxygenated compounds of low melting point either at the T91 steel/Pb or at the T91 steel/Pb-Bi eutectic interface.

Section 4 concerns the resistance to LMC and LME of structural materials pre-selected for the spallation target of ADS, with special emphasis on the liquid Pb-Bi eutectic target concept. The compatibility of 9% Cr martensitic steels (like EM10 or T91...) with liquid Pb and Pb-Bi eutectic was examined. Qualification of the T91 steel/LBE system is required to build the liquid Pb-Bi target in PSI in 2005 and for further applications. A comparative study of the long-term corrosion behaviour of various martensitic and austenitic steels in forced circulation loop was carried out by F. Balbaud-Celerier et al. allowing to vary the temperature and oxygen content in LBE covering the range 10-8wt% -10-6wt.% . This work results from a fruitful collaboration with IPPE (Obninsk, Russia). The two following papers of Section 4 deal with the tensile behaviour of T91 steel in contact with lead and its alloys. The most severe metallurgical (J.-B. Vogt et al.) and environmental (J.-L. Pastol et al.) conditions are considered in order to estimate the risk of LME for the target window in service. J.-B. Vogt et al. observed a transgranular (TG) brittle fracture after hardening heat treatment of T91 steel after tensile testing of notched specimens at 350 °C in oxygen-saturated lead. Note that cases of TG-LME are rather scarcely observed. On the other hand, J.-L. Pastol et al. obtained an embrittling effect on notched specimens by optimizing the environmental parameters (in LBE at 350 °C under He-4% H2 cover gas...) and tensile testing conditions. F. Gamaoun et al. showed that a few days exposure of T91 steel in lead under either reducing or oxidizing conditions gives rise to a significant porosity in the steel. This could justify the increased stress relaxation kinetics of specimens submitted to 4-point bending tests under the same conditions.

Besides the R& D research related to the spallation target for ADS, a non-negligible part of the GEDEON material research program was dedicated to intergranular penetration and LME. Some exemplary LME studies are reported in Section 5 of this volume, illustrating various aspects of this very old, tricky and exotic problem in metallurgy. One can find in this volume studies involving various Solid Metal/Liquid Metal (SM/LM) couples, going from high purity metal couples (Ni/Pb, Ni/Bi...) to industrial metallic alloys put in contact with liquid metals or alloys of varying purity. LME is a multi scale phenomenon that cannot be explained on the sole basis of either thermodynamics, metallurgical or kinetics criteria. In this section, the role of the stress-strain state is emphasized. The importance of the microstructure of the grains boundaries (facetting...) on LM penetration is also pointed out. This aspect of the problem can now be dealt with thanks to Atomic Force Microscopy (AFM), High Resolution Transmission Electron Microscopy (HRTEM), Electron Energy Loss Spectroscopy (EELS) and Auger Electron Spectroscopy (AES) techniques. Crack propagation kinetics can now be followed at ESRF (Grenoble) using synchrotron radiation imaging techniques. It is now possible to follow the propagation, structure and chemistry of the embrittling phase in the crack, at microscopic and at nanometric scale. It is also possible, in principle, to take into account the role of defects in grain-boundaries (GB) on the penetration of the embrittling species and conversely to investigate the role of the defects produced by wetting on the GB microstructure. We do hope that these techniques open a new and attractive field of research to improve the understanding of LME.

The HRTEM study carried out by A. Charai et al. enlightens the importance of crystallography on wetting. Two exemplary couples are thoroughly investigated: the Mo/Ni couple giving rise to an interfacial nanometer thick interface for one type of Mo bicrystal wetted by nickel and the Ni/Pb couple pointing out the role of the GB plane on wetting.

Y. Brechet et al. investigated not only the microstructure effects on "LM" penetration into cracks (cracks kinetics and morphologies), but also the plasticity effects ahead of the crack tip, studying a variant of the mythic Al/Ga couple, say the 7010 Aluminum alloy in contact with Gallium, allowing for modifications of the precipitation state at the GBs by applying various heat treatments. Y. Brechet et al. conclude in favour of a step by step penetration process, quasi at atomic scale and give an estimate of crack propagation velocity, of the order of a few micrometers per second, in good agreement with experimental observations. The author stresses the importance of defects in the GBs (precipitates...) at different scales which can explain discrepancies between the experimental crack velocity and the one calculated using a continuum elastic model. The paper presented by W. Ludwig et al. concentrates on two synchrotron radiation imaging techniques (X-ray micro-radiography and micro-fluorescence) available at ESRF (Grenoble) in order to characterize the penetration kinetics of embrittling species in the crack. Beside the Al/Ga couple, it is shown that those techniques are suitable to follow intergranular propagation of cracks in the Ni/Bi system, even when the embrittling layer becomes nanometric.

Structuration of liquid Al in contact with Cu(100) and Cu(111) surfaces of a bulk Cu crystal was studied by P. Geysermans et al. using Molecular Dynamics simulations. The authors showed an epitaxial relationship, whatever the density of the solid and contacting liquid phases. This feature is of importance to understand the structure of the wetting layers in GBs. J. Bernardini et al. emphasize the role of macroscopic defects like facets in GBs on "LM" penetration and propose a model allowing to describe the real shape of the cracks they observed with Ni annealed in contact with pure lead, inconsistent with Mullins grooving theory. Cu/Bi, Cu/LBE (Lead-Bismuth Eutectic) and Ni/LBE systems were studied by K. Wolski et al. using AES semi-quantitative analysis of fractured specimens under vacuum within an Auger Electron spectrometer, enlightening the embrittling role of the nanometer-thick wetting layer in the crack tip area.

In summary, during the past five years close cooperation between researchers within the material research program of GEDEON co-sponsored by the 5th EU Framework program allowed to make significant progress. Research dedicated to radiation damage in structural materials for ADS and spallation targets was very active and a number of impressive results were obtained. The low temperature He embrittlement is certainly a matter of concern. The controlling mechanisms and their dependence with temperature, He and dpa production rate has to be clarified. Likewise, compatibility studies of structural materials with lead and lead alloys, particularly LBE, gave rise to a number of remarkable results. One can mention here specifically the progress made with oxygen sensors. Significant is also the renewal of interest for LME during the past five years. Promising results on LME and intergranular penetration are already obtained and should be obtained in this attractive field of research. Of course, a great deal of work remains to be done in order to validate the concept of Pb-Bi spallation target for ADS. Especially, the effect of spallation elements production in structural materials and in LBE has to be assessed as well as LMC and LME under irradiation.

© EDP Sciences 2002