Depth distribution of irradiation-induced dislocation loops in an Fe-9Cr model alloy irradiated with Fe ions: The effect of ion energy

Abstract

• Cross-sectional STEM was applied to characterize Fe-9Cr irradiated with Fe ions of 1 and 5 MeV. • The arrangement and size of dislocation loops depends on the distance from the irradiated surface. • Irradiated microstructures correlate with the profiles of displacement damage and injected interstitials. • For 5 MeV ions, decoration of dislocations with loops was found at depths smaller than 0.8 µm. • The arrangement of loops in this region resembles the case of neutron irradiation; no such region forms for 1 MeV ions. It is generally accepted that the microstructure of ion-irradiated Fe-based alloys does not only depend on the local level of displacement damage and the initial microstructure. Other factors such as the vicinity of a surface and the injected ions also play a role and may give rise to peculiar depth dependencies of the irradiated microstructure. Some investigators reported a band-like appearance indicating depth ranges of relatively uniform microstructure clearly distinguished from other ranges. Clarification is important for at least two purposes: first, to identify a depth range suitable for gaining meaningful information about the behaviour of materials exposed to neutron irradiation and, second, to correctly interpret results obtained by methods, such as nanoindentation, that integrate over extended depth ranges. A variation of the ion energy is expected to gain additional insight. In this work, two samples of Fe-9%Cr were irradiated at 300 °C with Fe 2+ ions, one sample using 1 MeV ions and another sample using 5 MeV ions. Calculations using the binary collision code SRIM indicate displacement damage peaks at depths of 0.3 and 1.3 µm for ion energies of 1 and 5 MeV, respectively. The depth distribution of irradiation-induced dislocation loops was studied by cross-sectional scanning transmission electron microscopy (STEM). Loops visible in the STEM images were found to be arranged within two bands with the positions of these bands depending on the profiles of displacement damage and injected interstitials. The first and second band exhibit noticeably different number densities and mean sizes of the loops. For the 5 MeV irradiation, an extended range between the sample surface and the first band was observed, where decoration of preexisting line dislocations with loops is dominant. This microstructure resembles cases reported for neutron irradiation. For the 1 MeV irradiation, such a range does not exist. Estimates characterizing the loop size and number density in the distinct depth ranges are provided.