Influence of crystal structure on helium-induced tendril formation in an FeCoCrNiV high-entropy alloy

High-entropy alloys (HEAs) are a relatively new class of metal alloys
composed of several principal elements, usually at (near) equiatomic
ratios. Our goal is to understand how such a multicomponent alloy
behaves under irradiation. The FeCoCrNiV HEA exhibits both a face-
centred cubic (fcc) and a body-centred tetragonal (bct) phase, thus
allowing us to specifically study the influence of crystalline structure
at very similar chemical composition. We irradiated both phases with
a focussed He beam provided by a helium ion microscope (HIM) at
temperatures between room temperature and 500∘ C. The irradiation
fluence was varied between 6 × 1017 ions/cm2 and 1 × 1020 ions/cm2 .
High-resolution images of the irradiated areas were taken with the same
HIM. Selected irradiated areas were additionally studied by TEM in
combination with EDXS. Under irradiation, pores start to be generated
in the material with pore sizes differing significantly between the two
phases. At higher fluences and above a critical temperature, a tendril
structure forms in both phases. We found that the critical tempera-
ture depends on the phase and is lower for fcc. TEM images reveal
that the tendrils span the whole depth of the irradiated area, and are
accompanied by bubbles of various sizes. Scanning TEM-based EDXS
of these structures indicates a He-induced change in composition.