Assessment of phase stability of oxide particles in different types of 15Cr-ODS ferritic steels under 6.4 MeV Fe ion irradiation at 200 °C

Three types of 15Cr-ODS ferritic steels were irradiated with 6.4 MeV Fe³⁺ at 200 °C up to a nominal displacement damage of 10 dpa to assess the phase stability of different sorts of oxide particles under ion-irradiation. Before the irradiation, dominant oxide particles in (Y, Ti) ODS, (Y, Al) ODS and (Y, Zr) ODS steels were identified as Y-Ti-O, Y-Al-O, and Y-Zr-O types, respectively, and the difference in the average diameters and number densities of oxide particles among the three ODS ferritic steels was as small as within the factor of two. After the irradiation, the number densities and volume fractions of oxide particles in all the three ODS steels decreased with increasing dpa regardless of the different dispersion morphologies, chemical compositions and crystal structures of the particles, which was interpreted in terms of the dominant mechanism of ballistic dissolution with a moderate occurrence of radiation-enhanced diffusion (RED) and re-precipitation of the nano-clusters. Almost no significant difference in the phase stability of oxide particles was recognized among the three types of ODS ferritic steels. However, normalized volume fractions of Y-Zr-O particles were slightly bigger than those of Y-Ti-O and Y-Al-O at high damage levels. Moreover, Y-Al-O particles underwent the loss of moiré fringe because of the amorphization of oxide particles, while the δ-phase Y₄Zr₃O₁₂ in (Y, Zr) ODS still maintained crystalline structure even at ∼ 20 dpa/local. It is concluded that Y-Zr-O particles are more stable than Y-Al-O and possibly Y-Ti-O particles.