Helical dislocations in ion-irradiated Fe-9Cr studied by scanning transmission electron microscopy

Fe-9Cr is a model alloy for studying irradiation effects relevant for potential applications of high-chromium ferritic/martensitic steels in nuclear energy devices. Ion irradiation is a tool extensively employed with the aim to emulate the neutron damage characteristic for irradiation environments in fission or fusion reactors. Here we report on STEM studies of the microstructure of ion-irradiated Fe-9Cr with special emphasis on the effects of pre-existing dislocations.
Irradiations with 8 MeV Fe3+ ions were carried out at the 3 MV tandetron accelerator at the Ion Beam Center at HZDR. Profiles of displacement damage and implanted ions were calculated using the binary collision code SRIM. Cross-sectional TEM specimens were prepared by focused ion beam lift-out technique using a Thermo Fisher Helios 5CX. The microstructure was studied in a Talos F200X scanning transmission electron microscope.
The most striking feature of the irradiated microstructure in the range of high displacement damage, but low concentration of implanted ions, is the presence of helical dislocations. From the results of the Burgers vector analysis we conclude, that the helices were formed from pre-existing straight line dislocations with a dominating screw component. After transformation into the helical shape, the dislocations contain screw, mixed and edge segments. The STEM images also reveal large numbers of small dislocation loops mainly located close to the helices.
The presence of helical dislocations and the accumulation of loops close to them resembles observations reported for neutron-irradiated Fe-9Cr. Hence we conclude that - in the depth range of low implanted ion concentration - ion irradiation can produce similar defect configurations like neutron irradiation if the arrangement of pre-existing dislocations is comparable.