4.08.8.1 Simulated Irradiation

Testing that involves the simulated irradiation of 9Cr-ODS steel was conducted by Allen et al. at the

Environmental and Molecular Science Laboratory at Pacific Northwest National Laboratory, using 5 MeV Ni ions at 500, 600, and 700 °C with a damage rate of 1.4 x 10~3dpas~ The results regarding measured particle size distribution as a function of dose are plotted in Figure 38 for irradiation at 500, 600, and 700 °C.60 Due to TEM’s limited resolution of the images, particles smaller than 2 nm were not detected. At all temperatures, the size of the oxide particles decreases as the dose increases. At higher tempera­tures (600-700 °C), the average size appears to reach a value of ^5 nm. At all three temperatures, the density increases as the radiation dose increases. The decrease in size takes place faster at 600 and 700 °C than at 500 °C, indicating that the reduction in size is not strictly a ballistic effect and that a diffusion-based mechanism is also involved in the dissolution.

Allen extensively reviewed previous papers that presented different approaches to the irradiation of ODS ferritic-martensitic steels that employed various ion beams, electrons, and neutrons; the results are summarized in Table 3.61 A great many findings asserted that oxide particles are stable under radiation. However, as shown in Table 4, the dissolution of oxide particles at higher temperatures and doses has been reported in other studies. Dubuisson62 and Monnet63 reported that small oxides dissolved under radiation at higher temperatures and doses, but did not dissolve at a lower irradiation dose. Their data will be dis­cussed in detail in the following section. In material irradiated in the JOYO fast reactor at temperatures 450-561 °C to doses of 21 dpa, Yamashita found that small particles disappear and average particles increase slightly in size with increasing temperature or dose.64 Monnet supplemented neutron radiation studies with the electron irradiation of yttrium oxides and magnesium oxides in the EM10 alloy at tempera­tures between 300 and 550 °C, and to doses of 100 dpa. In these studies, the yttrium oxides were stable at 400 °C when irradiated with 1.0 MeV electrons, but dissolved under 1.2 MeV electron irradiation.

Allen59 pointed out that the displacement energy for Y and O in yttrium oxide is 57 eV65,66 while that for iron is 40 eV. Assuming similar displacement energies in the Y-Ti-O oxide, the radiation-induced vacancy concentration should be larger in the metal matrix, providing a driving force for a net vacancy flux to the precipitate. This could drive the precipi­tate mass loss if vacancy absorption frees a precipitate atom. From a comparison between electron irradia­tion (Frenkel pairs) and ion irradiation (displacement cascades), Monnet63 also concluded that the ballistic ejection of atoms alone cannot be responsible for the loss of diameter in oxide particles. Free point defects and their diffusion-based mechanism are therefore of major importance and play a dominant role in the dissolution of oxide particles.