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Specimens of an oxide dispersion strengthened (ODS) ferritic steel (15Cr-4Al-0.6Zr-0.1Ti) are implanted with multiple-energy He ions at room temperature to create a damage plateau of 0.4 dpa for the average (corresponding to an He concentration of about 7000appm) from the near surface to a depth about 1 μm.The specimen is subsequently thermally annealed at 800℃ for 1 h in a vacuum so that simple defects can be formed in the as-implanted state that has undergone significant recombination,meanwhile helium bubbles at nano-scale are formed.Hardness of the specimens are tested with the nano-indentation technique.A hardening by 25％ is observed.Microstructures of the specimen after irradiation/annealing are investigated with transmission electron microscopy.Helium bubbles are generally located at dislocations and grain boundaries.Using the dispersed barrier strength model,the strength factor of helium bubbles in the ODS ferritic steel is estimated to be between 0.1 and 0.26,which is close to that of helium bubbles in austenitic steels.