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. It has previously been reported that some HEAs dis-play superior radiation damage resistance, with composition and microstructure being cited as con-tributing factors, though the precise mechanism is still unknown. To study the influence of the crys-tal structure on the response to radiation, we have chosen FeCoCrNiV as a model system. While FeCoCrNi has a face-centred cubic (fcc) structure, adding V leads to a structural transformation to-wards a body-centred tetragonal (bct) structure with both phases present at near-stochiometric composition.
The as-cast sample was characterised by energy-dispersive X-ray spectroscopy (EDXS) and electron backscatter diffraction (EBSD) in a scanning electron microscope (SEM) confirming the presence of both phases. Irradiations were performed with a focussed He beam provided by a helium ion mi-croscope (HIM) at temperatures between room temperature and 500°C. The irradiation fluence was varied between 6x10^17 ions/cm² and 1x10^20 ions/cm². High-resolution images of the irradiated areas were taken with the same HIM, and an example is shown in Fig. 1a. Selected irradiated areas were additionally studied by transmission electron microscopy (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 as exemplary shown for the bct phase in Fig. 1a forms in both phases. We have found that the critical temperature 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 as shown in Fig. 1b for the bct phase. Scanning TEM-based EDXS of these structures indicates a He-induced change in composition.