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 degrees 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 similar to 1.35 +/- 0.5 mu m and similar to 4.5 +/- 1.1 mu m after 1 and 24 h annealing at 1100 degrees 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 degrees C (T/T-m = 0.74) with Sm addition. Furthermore, while large grains (similar to 1.5 mu m) appeared after 24 h of annealing at 1100 degrees 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 degrees C. These findings highlight the potential for optimizing the thermal and mechanical performance of CoCrFeNi HEAs in various applications.