High fluence He irradiation of materials using Helium Ion Microscopy

I will present some recent results on the high fluence irradiation of metals using gas field ion source (GFIS) based
helium ion microscope (HIM)1 .
High entropy alloys (HEAs) are a relatively new class of metal alloys composed of several principal elements, usually
at (near) equiatomic ratios. Here, 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 HIM at temperatures between room temperature and 500 ∘ C. The
irradiation fluence was varied between 6 × 1017 ions cm−2 to 1 × 1020 ions cm−2 . High-resolution images of the irradiated
areas were taken with the same HIM. Selected irradiated areas were additionally studied by transmission electron
microscopy (TEM) in combination with energy dispersive X-ray spectroscopy (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 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. Scanning TEM-based EDXS of these structures indicates a He-induced
change in composition.
In the second part I want to present an intriguing observation shedding light on the fundamental processes related
to interstitial diffusion during irradiation. I will show how epitaxial growth of tin extrusions on tin-oxide-covered tin
spheres can be induced and simultaneously observed by implanting helium using a HIM2 . Calculations of collision
cascades based on the binary collision approximation (BCA) and 3D-lattice-kinetic Monte Carlo (3D-lkMC) simulations
show that the implanted helium will occupy vacancy sites, leading to a tin interstitial excess. Sputtering and phase
separation of the tin oxide skin, which is impermeable for tin atoms, create holes and will allow the epitaxial overgrowth
to start. Simultaneously, helium accumulates inside the irradiated spheres. Fitting the simulations to the experimentally
observed morphology allows us to estimate the tin to tin-oxide interface energy to be 1.98 J m−2 .
Both approach have in common that they employ spatially resolved irradiation and in-situ observation of defect
diffusion-driven effects to improve the understanding of the formation mechanism of ion induced structures.
Financial support by the COST Action CA19140 is acknowledged. http://www.fit4nano.eu/