Evolution of the Structure and Phase Composition of a High-Entropic CoCrFeNiCu Alloy during Prolong Annealing

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Abstract—The thermal stability of a high-entropy alloy (HEA) CoCrFeNiCu was studied during long-term annealing for 204 days in the temperature range 873–1273 K. The alloy obtained by mechanochemical alloying of metal powders in a planetary mill during 120 min in an Ar environment is a substitutional solid solution based on a high-entropy phase with a face-centered cubic structure (FCC). Upon annealing, the initial FCC phase decomposes within 1–3 days into a copper-enriched FCC1 phase and a copper-depleted FCC2 phase with similar crystal lattice parameters of 3.60 and 3.57 Å, respectively. During the entire annealing time, the intensities of the diffraction lines of theFCC1 and FCC2 phases are redistributed, the dynamics and nature of which depends on the temperature and duration of the process. After the first three days and until the end of the annealing, the HEA retains the FCC structure of the substitutional solid solution, and the unit cell parameters of the formed FCC phases remain constant with an error of 0.1% up to 204 days. The five-component matrix formed after annealing has the chemical composition Co0.23Cr0.23Fe0.23Ni0.23Cu0.08 and is stable. Also, during the entire period of annealing, an increase in the grain size is observed: at first, when an active rearrangement of the structure occurs, the size increases rapidly, then, in the period from 1 to 30 days, grain growth is limited by the diffusion of components, and at very long time intervals, grain growth is even more inhibited.