Remarkable thermal stability of nanocrystalline CoCrFeNi high entropy alloy achieved through the incorporation of rare-earth element samarium.

High entropy alloys (HEAs) with nanocrystalline grain sizes have received significant interest in recent years; however, their microstructural integrity is compromised by a tendency for grain growth due to their high-volume fraction of grain boundaries. Here, nanocrystalline CoCrFeNi with Sm addition was synthesized through mechanical alloying, followed by annealing at temperatures up to 1100 °C and for durations of up to 24 h. The results have revealed that the 16 ± 6 nm as-milled grain size of CoCrFeNi experienced grain coarsening during the annealing process, reaching ∼1.35 ± 0.5 μm and ∼4.5 ± 1.1 μm after 1 and 24 h annealing at 1100 °C, respectively. This indicates that the nanocrystalline microstructure of CoCrFeNi lacks thermal stability at elevated temperatures. The average grain size was maintained at 110 nm after 1 h annealing at 1100 °C (T/T m = 0.74) with Sm addition. Furthermore, while large grains (∼1.5 μm) appeared after 24 h of annealing at 1100 °C, pockets of nano-sized grains were still present in the microstructure. The resistance to grain growth is ascribed to the presence of rare earth element, Sm, and the formation of Sm-based additional mixed oxide phases (Sm/Cr-O). Consequently, 517.8 ± 25 HV as-milled hardness of CoCrFeNi decreased dramatically to 221.5 ± 11 HV due to extensive grain growth but remained elevated at 442.5 ± 15 HV (84 % of as-milled hardness) with Sm addition after annealing at 1100 °C. These findings highlight the potential for optimizing the thermal and mechanical performance of CoCrFeNi HEAs in various applications. [Display omitted] • The grain size of CoCrFeNi, which was 16 ± 6 nm before milling, grew to 1.35 ± 0.5 μm after annealing at 1100 °C. • With the addition of Sm, the average grain size was maintained at 110 ± 53 nm at the same temperature. • Grain growth was retarded by Sm-based mixed oxide phases formed during the process. • The in-situ formed second phases along with the achieved smaller grain size increased the hardness. • The hardness was retained at around 442.5 ± 15 HV (84 % of as-milled hardness) after 1 h annealing at 1100 °C with Sm. [ABSTRACT FROM AUTHOR]