Characterisation of Phase Separation in Drop-Tube-Processed Rapidly Solidified CoCrCuFeNi0.8 High-Entropy Alloy

We investigate the impact of cooling rate on a CoCrCuFeNi0.8 high-entropy alloy with a predicted metastable miscibility gap. Rapid solidification via drop-tube processing simulates a containerless, low-gravity solidification environment. Droplets were produced with diameters ranging from 850+ µm to 38 µm, with calculated liquid phase cooling rates of between 600 and 60,000 K s−1. Contrary to studies on similar alloys with a reported metastable miscibility gap and similar investigations on binary alloys known to undergo metastable liquid phase separation, almost no core–shell microstructures were observed in the droplets, likely due to a heavily unbalanced volume fraction ratio between the two phases formed from the parent liquid. Instead, drop-tube processing yielded myriad structures, the occurrences of which vary heavily with cooling rate. At cooling rates of 600 K s−1, a solid-state decomposition reaction begins to become noticeable, populating dendrites with copper-rich dispersions after solidification. The prevalence of these structures increases with increasing cooling rate, occurring in above 95% of droplets once cooling rate exceeds 20,000 K s−1. Occurrence rate of dispersions attributed to liquid phase separation peaks at 8% of droplets at intermediate cooling rates between 5000 and 12,000 K s−1. Spontaneous grain refinement has a maximum prevalence between 1000 and 5000 K s−1. This study begins to show how cooling rate and undercooling can be used to tailor microstructures in HEAs and highlights drastic differences in obtainable microstructures compared to those found in binary and ternary immiscible alloys.