Void swelling in pure iron after sequential self-ion-irradiation at different energies: Effect of injected interstitials and additional damage in regions containing pre-existing voids

The use of ion irradiation to add displacement dose onto previously neutron-irradiated material is becoming popular, especially since ion irradiation precedes on neutron-typical microstructures and microchemical distributions that would not be produced with ion irradiation alone. One issue to be addressed in these "neutronpreconditioned" specimens concerns the consequences of ion-injected interstitials on preexisting neutronproduced voids, especially since voids have been observed to disappear over a significant portion of the ion range in some recent preconditioning experiments. This situation can be explored using innovative ion simulation, thereby avoiding working with radioactive material. Two self -ion irradiation series of pure single-crystal iron were performed at 475 degrees C. The first series used 5 MeV Fe 2+ , reaching 50, 100 and 150 peak dpa, producing voids to -2 mu m depth. The second series used 2.5 MeV Fe 2+ to introduce additional 50 peak dpa onto each of the three first-step specimens with the ion range reaching -1.2 mu m. Importantly, 1.2 x 10 -3 peak dpa/s was used for both ions to minimize the influence of dpa rate. The major objective was to measure the influence of injected interstitials from the 2.5 MeV Fe 2+ irradiation on previously existing voids. Another objective was to observe the behavior of voids exposed to additional dose and time, focusing not only on injected interstitials, but also on the effects of coalescence, ripening, and aging. There were significant changes in depth distribution of swelling following the second irradiation, especially void density. Whereas injected interstitials are well-known to suppress void nucleation at the end of ion range, it was shown in this study that injected interstitials are important in another manner, reducing growth of previously existing voids, producing a 'notch" in both the swelling vs. depth profile and void size profile. At depths where the injected atoms for both ion energies are not significant, swelling proceeds at -0.2 %/dpa.