A series of Fe-Cr alloys, with 3 at.% - 18 at.% Cr, was irradiated in the Advanced Test Reactor (ATR) at the Idaho
National Labs (INL), USA, up to a dose of ~6.7 dpa at a temperature of ~456 ◦C. Transmission electron microscopy
(TEM) samples were extracted using a focused ion beam (FIB) instrument, and the resulting microstructural
defects, such as voids, dislocation loops, network dislocations, Cr rich precipitates, etc., were
characterized using a TEM. It was found that the size and number density of these defects varied widely over the
different alloys with varying Cr content. As expected, there were no Cr rich precipitates in samples with Cr up to
9 %, and they started appearing only in samples with 12 at.% Cr and above. The particle size decreased from
about 15 nm at 12 % Cr to 8 nm at 18 % Cr, while the number density increased from ~7e20 /m3 to 6e22 /m3 for
the same Cr contents. Grain boundary segregation of Cr, along with a precipitate free zone, was observed in the
cases where a boundary was present in the sample. Large voids (>1–2 nm) were almost invisible in the Fe-3at.%
Cr sample, while the average void size remained almost constant between 15 and 18 nm for samples with 6–15
at.% Cr and increased slightly at 18 at.% Cr to ~22 nm. Fe-3 %Cr showed a high density of small voids(<2 nm),
estimated to be about 1e23/m3. The number density of large voids increased from ~0 at 3 % Cr to a peak of
~6.4e20 /m3 at 12 % Cr, then decreased to about 1.1e20 /m3 at 18 % Cr. The dislocation loops, which appeared
in linear arrays in the Fe-9 %Cr sample, were analysed in detail using both invisibility criteria and image simulations
using the Oxford University TEMACI software, and it was found that they are most likely to be ½ 〈111〉
type loops on {100} planes. These loops seem shaped like sections of helices in some places and are most likely
formed by loops near screw dislocations climbing into a helix and then collapsing into a loop array. Similar
dislocation analysis was performed in the other samples as well wherever feasible, and it was found that there is a
mixture of ½ 〈111〉 and [100] dislocation loops.
