Insight into the kinetic stabilization of Al0.3CoCrFeNi high-entropy alloys

The AlxCoCrFeNi family of high entropy alloys (HEAs) has received considerable attention due to its promising thermal, mechanical, and corrosion-resistant properties which make it widely suited for aerospace and marine applications. While the formation of secondary phases has been studied at various annealing temperatures, the results have focused on the late stages of precipitation, highlighting the need for the analysis of the intermediate stage precipitation. Here we use in-situ heating in the transmission electron microscope (TEM) complemented by ex-situ characterization of bulk annealed specimens, thermodynamic calculations, and precipitation simulations to study the phase evolution of Al0.3CoCrFeNi. Due to the high density of nucleation sites in the thin film, in-situ TEM reveals the formation of an additional intermediate phase, Co-B2 at 550 degrees C, where hundreds of hours are predicted for this phase to be shown during ex-situ experiments. At higher annealing temperatures between 700 and 900 degrees C, in-situ TEM shows the formation of Cr-rich precipitates as the first intermediate phase, followed by NiAl precipitates that form co-precipitates. The formation of these precipitates occurs concurrently, contrary to the findings of previous studies. In conjunction with the in-situ and ex-situ TEM studies, thermodynamic calculations and precipitation simulations have been performed to predict the formation of these phases and are found to support the experimental results. The present work provides new insight into the microstructural evolution of HEAs and reveals the importance of intermediate stages of thermal evolution, enabling an enhanced predictive view of phase evaluation in this class of alloys.