Direct Observation of Chemical Short-Range Order in a CrMnFeCoNi High-Entropy Alloy Studied with Scanning Tunneling Microscopy

High-entropy materials are a family of materials described by their large amount of configurational entropy. High-entropy alloys (HEAs) typically contain five or more equimolar elements that are randomly distributed through a crystalline lattice. The elemental configuration and ratios can be adjusted to easily tune mechanical properties, such as strength, hardness, and ductility, in addition to chemical properties, including electrocatalytic performance. Moreover, some HEAs exhibit chemical short-range order (CSRO) that can have great impacts on these properties. CSRO has been studied mainly with electron-diffraction through scanning transmission electron microscopy (STEM). However, hwo to distinguish in-plane and out of plane directional effects is still unclear. Hence, it has been heavily discussed that extra care is needed to reveal CSRO from the electron-diffraction measurements. In the present work, we use scanning tunneling microscopy (STM) to study the surface of a CrMnFeCoNi HEA system, and directly observe two types of CSRO domains and the domain boundaries deriving from the ordered crystalline FCC structure. The CSRO domains are, respectively, accompanied by quasi-long-range orders. Using density functional theory (DFT) calculated density of states (DOS), the apparent height of atoms in topographic STM images can be assigned with the five constituent elements, revealing detailed CSRO structures. The results strongly suggest that using STM to study HEAs provides unambiguous and vital information about the CSRO.