Annealing behavior of hardening and ductility loss of a 16Cr–4Al ODS ferritic steel irradiated with high energy Ne ions

Abstract Annealing behavior of the hardening and ductility loss of an oxide-dispersion-strengthened (ODS) ferritic steel (16Cr–4Al) irradiated with high-energy heavy ions is studied. 20Ne ions with 123.4 MeV supplied by a cyclotron was used to produce a quasi-uniform atomic region of atomic displacement damage and Ne concentration from surface to a depth of 33 μm in the specimens. Two atomic displacement levels (0.16dpa, 0.70dpa) were approached at about 220 K. The irradiated specimens were subsequently thermally annealed at room temperature, 473 K and 673 K, respectively. Hardness and ductility of the specimens were investigated with nano-indentation technique and small-punch test. The data of the nano hardness were fitted by Nix-Gao model to obtain the bulk equivalent hardness values. The irradiated specimens show observable hardening, which recedes monotonously with the increase of annealing temperature. The Arrhenius plots show a good linearity in the entire temperature range investigated, with an apparent activation energy of 0.13 ± 0.01 eV for both the two damage levels. Assuming that the hardening is caused by the self-interstitial-atom (SIA) clusters initially produced by the cascade damage, an activation energy of 0.70 ± 0.05 eV is deduced for the migration and coalescence process of the SIA-clusters. The small punch test (SPT) of the high-dose specimens shows that a minor ductility loss occurred under conditions as-irradiated or subsequently thermally annealed at 473 K, while a remarkable increase of the ductility loss was observed after the thermal annealing at 673 K, which coincides with the formation of nano-scale gas bubbles in high density in the specimen. It is indicated that the formation of bubbles has minor on the irradiation hardening. A comparison of three ODS ferritic steels Ne-ion-irradiated to 0.7 dpa/260 appm(Ne) shows that the ductility loss decreases with the increase of the sink strength of the oxide dispersoids/ferritic substrate interfaces.