Helical dislocations and dislocation line decoration in ion-irradiated Fe-9Cr studied by scanning transmission electron microscopy

Fe-9Cr is a model alloy for studying irradiation effects that are relevant for potential applications of high-chromium ferritic/martensitic steels in nuclear energy devices. Here we report on scanning transmission electron microscopy (STEM) studies of the microstructure of Fe-9Cr irradiated with 8 MeV Fe3+ ions. Two samples were irradiated with different ion fluences resulting in peak values of displacement damage of 2 and 10 dpa, respectively. The spatial distribution of irradiation-induced dislocation loops was studied with special emphasis on the effects of pre-existing network dislocations.
The most striking feature of the irradiated microstructure is the presence of helical dislocations. From comparison of the irradiated layer with the dislocation arrangement in the non-irradiated substrate it is concluded that the helices were formed from straight pre-existing line dislocations that originally had a dominating screw component. Other types of dislocations observed in the material did not adopt a helical shape during irradiation. Decreasing the dose has a significant effect on the helical dislocations, the helices are less developed and have a smaller diameter. For both irradiation conditions, an inhomogeneous distribution of irradiation-induced dislocation loops is observed. A high number of loops is present close to the helical dislocations and also close to dislocations that have not adopted a helical shape. In areas away from the dislocations, the number of visible loops is very low.
The loop clustering close to helical dislocations resembles observations reported for neutron irradiated Fe-9Cr. Hence we conclude that ion irradiations can produce similar defect configurations like neutron irradiations when the arrangement of pre-existing dislocations is comparable.