Ion-irradiation hardening accompanied by irradiation-induced dissolution of oxides in FeCr(Y, Ti)-ODS ferritic steel Peng Song a, * , Jin Gao b , Kiyohiro Yabuuchi b , Akihiko Kimura b a Graduate School of Energy Science, Kyoto University, Yoshida-honmachi, Sakyou-ku, Kyoto, 606-8501, Japan b Institute of Advanced Energy, Kyoto University, Gokasho, Uji, Kyoto, 611-0011, Japan article info Article history: Received 20 June 2018 Received in revised form 6 September 2018 Accepted 6 September 2018 Available online 7 September 2018 Keywords: Phase stability Oxide particles Ballistic dissolution Hardening abstract Irradiation effects on hardness and phase stability were investigated for an FeCr(Y, Ti)-ODS ferritic steel strengthened by Y-Ti-O nano-particles after irradiation with 6.4 MeV Fe 3þ at room temperature (RT) up to nominal damages of 2, 10 and 50 dpa. With increasing local displacement damage up to ~20 dpa, nano-sized oxide particles slightly shrank, while the corresponding number density drastically decreased by almost two orders of magnitude compared to that of before irradiation. It is considered that ballistic dissolution should be responsible for such reductions in the particle size and number density. Dislocation loops consisting of 1/2<111> type (>80%) and <100> type were observed under weak beam dark field (WBDF) imaging condition in the specimen irradiated to the nominal damage of 50 dpa. The average size and number density of all the dislocation loops were 2.8 ± 0.7 nm and (4.1 ± 0.7)  10 22 m 3 , respectively, at the local damage of ~72 dpa. Although the oxide particles were almost completely dissolved, nano- indentation hardness measurements revealed that the hardening went up continuously with increasing displacement damage and was estimated to be 1.63 ± 0.39 GPa by the Nix-Gao model at the nominal damage of 50 dpa. The irradiation hardening accompanied by the dissolution of oxide particles was interpreted in terms of loss of oxide particles, solid solution hardening and formation of fine dislocation loops. The contribution of dislocation loops observed by transmission electron microscopy (TEM) to the hardening was insufficient to overcome the loss of strengthening by dissolution, suggesting the importance of solid solution hardening and the larger strength factor of dislocation loops as a hardening contributor. © 2018 Elsevier B.V. All rights reserved. 1. Introduction Because of the prominent high-temperature performance, oxide dispersion strengthened (ODS) steels have been developed as candidate for structural materials for the blanket components of fusion reactors [1] and the fuel claddings of fast breeder reactors [2]. The oxide particles dispersed in the steel matrix can impede dislocation motion as well as suppress recovery and grain growth to a great extent with maintaining the superior tensile and creep strength of the steels at elevated temperatures [3]. Furthermore, the oxide/matrix (O/M) interfaces provide numerous sinks for point defect recombination and helium trapping, which contributes much to excellent radiation tolerance of ODS steels [4e6]. Because of the significance in the roles of oxide particles, both the thermal stability and radiation tolerance of dispersed particles are required for ensuring the property integrity of the ODS steels. Yttria (Y 2 O 3 ) is generally selected as the dispersed oxide parti- cles for strengthening because it is one of the most thermody- namically stable oxides [7]. Meanwhile, the Ti addition to ODS steels effectively refined oxide particles by the formation of ultra- fine Y-Ti-O complex oxides, which improved drastically the high temperature strength of steels [8]. Accordingly, a great number of studies were done on oxide particles in various (Y, Ti)-added ODS steels, such as dispersion morphology, crystallographic structure and chemical compositions, as well as interface coherency and orientation relationship with the matrix [9e12]. Sakasegawa et al. [9] analyzed the chemical compositions of small particles in MA957 ODS alloy by energy dispersive X-ray spectroscopy (EDS), revealing that the ones with diameters ranging from 2 to 15 nm were non- * Corresponding author. Graduate School of Energy Science, Kyoto University, Gokasho, Uji, Kyoto, 611-0011, Japan. E-mail addresses: p-song@iae.kyoto-u.ac.jp, songpeng1123@gmail.com (P. Song). Contents lists available at ScienceDirect Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat https://doi.org/10.1016/j.jnucmat.2018.09.007 0022-3115/© 2018 Elsevier B.V. All rights reserved. Journal of Nuclear Materials 511 (2018) 200e211