Comparative study of helium effects on EU-ODS EUROFER and EUROFER97 by nanoindentation and TEM M. Roldán a,⇑ , P. Fernández a , J. Rams b , D. Jiménez-Rey c , E. Materna-Morris d , M. Klimenkov d a National Fusion Laboratory – CIEMAT, Avda. Complutense, 40, 28040 Madrid, Spain b Departamento de Matemática Aplicada, Ciencia e Ingeniería de Materiales y Tecnología Electrónica, ESCET, Universidad Rey Juan Carlos, C/Tulipán s/n, Móstoles, Madrid 28933, Spain c Centre for Micro-Analysis of Materials (CMAM, UAM), C/Faraday 3, 28049 Madrid, Spain d Karlsruhe Institute of Technology (KIT), Institute for Advanced Materials (IAM-AWP), Hermann-von-Helmholtz-Platz, 1, 76344 Eggenstein-Leopoldshafen, Karlsruhe, Germany highlights  EU-ODS EUROFER has been studied before and after He implantation by nanoindentation.  Specimens implanted in stair-like proﬁle from 15 to 2 MeV (750 to 350 appm He).  Exhaustive comparison of nanoindentation results with EUROFER97 is presented.  TEM on EUROFER97 and EU-ODSEUROFER to correlate microstructure with hardness results. article info Article history: Received 17 October 2014 Accepted 16 February 2015 Available online 25 February 2015 abstract Helium effects on EU-ODS EUROFER were studied by means of nanoindentation and TEM. The results were compared with those of EUROFER97. Both steels were implanted in a stair-like proﬁle conﬁguration using energies from 2 MeV (maximum He content 750 appm He) to 15 MeV (minimum He 350 appm He) at room temperature. The nanoindentation tests on He implanted samples showed a hardness increase that depended on the He concentration. The maximum hardness increase observed at 5 mN was 21% in EU-ODS EUROFER and 41% in EUROFER97; it corresponded with the zone with the highest He concentration which was around 750 appm, according to MARLOWE simulation. In addition, FIB lamellae were prepared from EUROFER97 and EU-ODS EUROFER containing the aforementioned zones with maximum (750 appm) and minimum (300 appm) He. TEM investigations carried out showed small and homogeneously distributed He nanobubbles on both alloys in the zone corresponding with maximum He content. These microstructural features seem to be the cause of the hardness increase measured by nanoindentation. Ó 2015 Elsevier B.V. All rights reserved. 1. Introduction Reduced activation ferritic martensitic steels have been selected as baseline structural materials for the breeder blanket of the future fusion reactor [1]. Those materials present a good overall balance of mechanical properties, good corrosion resistance for LiPb breeder concept and enough compatibility with He cooled blanket design [2]. But regarding resistance to irradiation damage, there is not enough data to validate them. European candidates steels EUROFER97 and EU-ODS EUROFER have been characterized under ﬁssion neutron spectra, but only EUROFER97 has been studied at 300 °C and up to 80 dpa [3]. However under fusion neu- tron environment, together with irradiation damage (dpa), He atoms will be produced inside of the structural materials due to the transmutation reactions. For this reason it is critical to study the He effect on mechanical properties and microstructure, in spite that nowadays it is not still well known, due to the lack of an irra- diation facility which simulates it [4,5]. Several ways to produce He atoms inside of the material are available, such as neutron irradiation on B or Ni doped materials [6], or by using 54 Fe ions [7]. But those methods have important dis- advantages, such as composition changes, formation of new phases during irradiation, nuclear activation or high cost [8,9]. Helium ion implantation eliminates those effects although the implanted area http://dx.doi.org/10.1016/j.jnucmat.2015.02.025 0022-3115/Ó 2015 Elsevier B.V. All rights reserved. ⇑ Corresponding author. E-mail address: marcelo.roldan@ciemat.es (M. Roldán). Journal of Nuclear Materials 460 (2015) 226–234 Contents lists available at ScienceDirect Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat