Microstructure characterization of ODS-RAFM steels
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1125-R05-03
Microstructure characterization of ODS-RAFM steels R. Mateus1, P.A. Carvalho1,2, D. Nunes1,2, L.C. Alves3, N. Franco3, J.B. Correia4, H. Fernandes1, C. Silva1, R.Lindau5, E. Alves3 1
Associação Euratom/IST, Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal 2 ICEMS, Departamento de Engenharia de Materiais, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal 3 ITN, Instituto Tecnológico e Nuclear, Estrada Nacional 10, 2686-953 Sacavém, Portugal 4 LNEG, Departamento de Materiais e Tecnologias de Produção, Estrada do Paço do Lumiar, 1649-038 Lisboa, Portugal 5 FZK, Institut für Materialforchung I, PO Box 3640, 76021, Karlsruhe, Germany ABSTRACT Results of the microstructural characterization of four different RAFM ODS Eurofer 97 batches are presented and discussed. Analyses and observations were performed by nuclear microprobe and scanning and transmission electron microscopy. X-ray elemental distribution maps obtained with proton beam scans showed homogeneous composition within the proton beam spatial resolution and, in particular, pointed to a uniform distribution of ODS (yttria) nanoparticles in the Eurofer 97 matrix. This was confirmed by transmission electron microscopy. Scanning electron microscopy coupled with energy dispersive spectroscopy made evident the presence of chromium carbide precipitation. Precipitates occurred preferentially along grain boundaries (GB) in three of the batches and presented a discrete distribution in the other, as a result of different thermo-mechanical routes. Additional electron backscattered diffraction experiments revealed the crystalline textures in the ferritic polycrystalline structure of the ODS steel samples. INTRODUCTION An imperative property of materials suitable for fusion applications is low neutron activation. In order to comply with this requisite, reduced activation ferritic/martensitic (RAFM) Eurofer 97 steel has been developed from the conventional 9Cr-1Mo composition [1]. The substitution of high activation elements (Mo and Nb) by low activation chemically equivalent elements (W, V and Ta) resulted in a material that additionally evidences high swelling and creep resistances, as well as suitable tensile strength up to a temperature of 550 ºC [2-4]. The subsequently developed oxide dispersion strengthened (ODS) Eurofer 97 represents an increase in operation temperature to 650 ºC [5]. Furthermore, nanoparticle dispersions minimize radiation induced defects and improve the thermal stability of the material [6,7]. One of the most promising ODS oxides for fusion applications is Y2O3 which should be homogeneously distributed over the matrix for an optimum effect [5,8]. Yet, ODS Eurofer 97 presents still an unsatisfactory impact resistance and a to high ductile-to-brittle transition temperature [5]. Recent investigations indicate that these shortcomings may be obviated by suitable thermo-mechanical processing routes [9].
“First generation ODS Eurofer 97 steels” have been previou
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