Microstructure characterization of CoCrFeNiMnPd_x eutectic high-entropy alloys

A series of CoCrFeMnNiPd_x (x = 0.2–2 at.%) eutectic high entropy alloys (EHEAs) were prepared and analyzed to show the alloying effect of Pd on the microstructure and phase constituents. It was found that the microstructure as the increase of x changes from the divorced eutectic to the coarse granular divorced eutectic around which are the fine lamellar dendrite eutectic and then to the seaweed dendrite eutectic. Hitherto, such seaweed eutectic dendrite was rarely reported in either high entropy alloys or conventional alloys. Specifically, a multimodal grain size distribution including several hundred microns for the eutectic-dendrite, several microns for the coarse granular eutectic, several hundred nanometers for the fine lamellar eutectics and the lath-shaped microstructures in the intermetallic phases, and even several nanometers for the nano-twins was observed. Since the CoCrFeNiMnPd_x EHEAs obey all the design rules of HEAs with a single solid-solution phase but exhibits a two-phased microstructure, mere consideration of average physical properties of constituent elements was concluded to be not sufficient to the design of HEAs. The present work is not only enrichment of current EHEA systems but also of importance to the design of HEAs.