Phase evolution and thermal stability of mechanically alloyed CoCrCuFeNi high entropy alloy

Abstract

A CoCrCuFeNi high entropy alloy (HEA) prepared by mechanical alloying (MA) exhibits evolution of two phases, a BCC phase (a = 0.287 ± 0.002 nm) and a small amount of FCC phase (a = 0.362 ± 0.002 nm) after 65 h of milling. From the scanning electron micrographs, flaky nature of milled powders and a wide range of particle size (5 to 10 μm) can be seen. The nanostructure of the crystallites evolved in the 65 h milled powder was analyzed using XRD and selected area electron diffraction techniques. The stability of the 65 h milled powder is studied through in situ high-temperature X-ray diffraction (HT-XRD) over a range of temperatures. The as-milled powder is thermally stable up to 623 K, and then the precipitation of the tetragonal (Cr-Co/Fe) based sigma (σ) phase (a = 0.845 ± 0.002 nm, c = 0.454 ± 0.002 nm) occurs. On annealing at 623 K, precipitation of the σ phase started, while on annealing at 1073 K the BCC phase disappears associated with a decrease in the amount of σ phase. Eventually, the FCC phase (a = 0.362 ± 0.002 nm) appeared to be the stable phase along with a negligible amount of σ phase.