Spinodal decomposition, ordering, and precipitation transformation in CoCrFeNiAl HEAs under GPa pressure

Abstract

Pressure is a critical component of preparation processes regardless of its magnitude, whether it is a few megapascals, such as in metal squeeze casting, or thousands of megapascals, such as in the experimental preparation of amorphous materials, quasicrystals and nanocrystals. Spinodal decomposition and ordering are two common forms of unstable phase transformation in solid solutions. Reasonable regulation of these two transformations can effectively control microstructures and improve properties. Therefore, the study of solid solution instability under the effect of ultrahigh pressure is of great significance for regulating the properties of high-entropy alloys (HEAs). From this perspective, CoCrFeNiAl HEAs were prepared under pressures of ambient pressure, 4 GPa and 7 GPa. Both solid spinodal decomposition and ordering of the BCC structure were detected when CoCrFeNiAl HEAs are solidified at ambient pressure. Nevertheless, after high-pressure solidification, liquid‒liquid phase spinodal decomposition occurred. One phase was rich in (Fe, Cr), and the other phase was rich in (Ni, Al). Both phases experienced solid spinodal decomposition during cooling. Discontinuous precipitates were generated in the (Ni, Al)-rich solid phase, each with a size of approximately 1.4 μm. Furthermore, a graphic thermodynamic model was established to reveal the solidification behavior. After solidification at 7 GPa, the corrosion rate of the alloy decreased by 98.3 %; that is, the high pressure greatly improved the corrosion resistance of the alloy. This improvement was attributed to the influences of high pressures on the dynamics of spinodal decomposition processes.