Solid-state decomposition following partitionless solidification in dendrite arms of rapidly solidified CoCrCuFeNi0.8 high-entropy alloy

Partitionless solidification is a relatively understudied phenomenon in high-entropy alloys. In this study, a two-phase FCC CoCrCuFeNi0.8 high-entropy alloy was subjected to containerless rapid solidification in a drop-tube. The resultant droplets solidified at liquid phase cooling rates between 600 K s−1 and 60,000 K s−1. In a substantial portion of solidified droplets, the cores of solidified CoCrFeNi-rich primary dendrites were found to contain Cu-rich spherical dispersoids of similar composition to the Cu-rich interdendritic phase formed from rejected copper during solidification. The average composition of the decomposed regions of these dendrites matches the composition of the starting alloy and does not vary with droplet cooling rate. We reason that droplets featuring these dispersoids were undercooled below the T0 temperature when in freefall in the liquid state. This caused the first dendrites to form via partitionless solidification. These regions became supersaturated as the droplet cooled further and likely underwent spinodal decomposition in the solid state. We also use CALPHAD modelling to estimate the solid phase spinode in the CoCrCuFeNi0.8 alloy, arguing that the required precursor to spinodal decomposition is likely a dendrite core that has become supersaturated with copper after partitionless solidification. The consistency in average composition of the decomposed dendrites at every cooling rate provides substantial evidence that this high-entropy alloy underwent undercooling mediated partitionless solidification rather than severe kinetic solute trapping. Furthermore, this work provides an analysis of conditions which may favour partitionless solidification in high-entropy alloys.