The effect of compositional complexity on the atomic structure of grain boundaries in high entropy alloys
Extensive research has been focusing on face-centered cubic (FCC) high entropy alloys (HEAs) to establish the underlying mechanisms for their outstanding mechanical properties, for instance, an impressive combination of strength and ductility at cryogenic temperatures. One possibility suggested is that these new types of alloys show stronger Hall-Petch strengthening, where the grain size has a stronger impact on their yield strength. The origin of this grain boundary strengthening in HEAs seems to be originating from the different atomic radii of the supersaturated solid solution inducing high lattice strains. Hence, resolving the impact of compositional complexity on the atomic structure of grain boundaries in HEAs is crucial to understand their role in the strengthening mechanisms.
In this project, we use epitaxially-grown FeCrCoNi HEA thin films with a strong  texture to obtain well-defined tilt grain boundaries (see Figure 1a). By using a plan view lift-out technique, it is possible to extract specific  tilt grain boundaries for atomic resolution observations using aberration-corrected scanning transmission electron microscopy. In parts, the grain boundary core structure appears to be similar to that observed in conventional FCC alloys, such as copper [1,2], however, nanometer sized regions with a strong disordering o the atomic structure are observed (see Figure 1b). High resolution electron energy-loss spectroscopy (EELS) suggests that these regions are rich in chromium (Cr).
In future work, a more detailed analysis will be conducted to link the atomic structure of the grain boundaries to the local composition utilizing EELS or energy-dispersive X-ray spectroscopy. In situ TEM tensile testing is also planned to directly observe dislocation-grain boundary interactions to explore the impact of grain boundary structure and composition on the properties of individual grain boundaries in HEAs.