Abstract text (incl. figure legends and references)

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.

Figs. 1a and b display STEM images of a large ferrite grain, which were acquired along the [001] zone axis, Fig. 1c contains the damage profiles calculated by SRIM. In the depth range of the highest concentration of implanted ions, a dark band is visible similar to the defect-rich bands observed in previous studies [1]. 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 (examples are labelled H1 and H2). These helices are aligned at an angle of about 45° or -45° with respect to the [100] direction. No helices are present in the non-irradiated substrate. Here we observe a network of curved dislocation segments as well as nearly straight dislocation lines (S1 and S2), the latter showing the same alignment as the helices. The higher magnified image in Fig. 1b reveals a large number of small dislocation loops appearing as black dots, most of them located close to the helices.

For Burgers vector analysis, STEM images were acquired under two-beam conditions with different diffraction vectors g as displayed in Fig. 2. The helix H1 and the straight dislocation S1, both aligned at 45°, are visible with g = -110 but invisible with g = 110. To fulfill the g · b = 0 invisibility criterion, the Burgers vector b must be ±½[-111] or ±½[1-11]. In contrast, H2 and S2, aligned at -45°, must have a Burgers vector of ±½[11-1] or ±½[111]. The straight dislocations as well as the helices are aligned nearly parallel to the projection of their Burgers vectors (indicated by the blue and red arrows), which means that the straight dislocations have a dominating screw component and that the helices are formed from this type of pre-existing dislocations.

The presence of helical dislocations and the accumulation of loops close to them resembles observations reported for neutron-irradiated Fe-9Cr [2]. 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.

[1] K. Vogel et al., NME 27 (2021) 101007

[2] J.C. Haley et al., Acta Mater. 181 (2019) 173

[3] This work has received funding from the Euratom research and training programme 2014-2018 under grant agreement No. 755039 (M4F project).

Fig. 1: (a), (b) STEM images acquired along the [001] zone axis, (c) damage profiles calculated by SRIM.

Fig. 2: STEM images acquired with different diffraction vectors g as indicated by the white arrows.