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    Hybrid effect of Sm and B4C on grain size stability and hardness of nanocrystalline CoCrFeNi high entropy alloy after isothermal annealing
    (Elsevier Science Sa, 2025) Tasdemir, Esma; Koc, Recep C.; Batibay, Ahmet B.; Kotan, Hasan
    In this study, a hybrid effect of Sm and B4C additions is explored to enhance the thermal stability of mechanically alloyed nanocrystalline CoCrFeNi HEA after exposures to long-term annealing at 700 degrees C and 900 degrees C. The structural and microstructural investigations were carried out using X-ray diffraction (XRD), focused ion beam microscopy (FIB), and scanning transmission electron microscopy (STEM) with energy dispersive spectroscopy (EDS). The results revealed that 7 nm grain size of as-milled CoCrFeNi HEA coarsened to 247.38 f 69 nm after 1 h of annealing at 900 degrees C. However, the hybrid addition of Sm and B4C significantly retarded grain growth, reducing the average grain size to 98.74 f 47 nm after 24 h of annealing at the same temperature. The resistance to grain growth is attributed to Sm and B4C additions and the formation of Sm-based oxide phases during annealing. The influence of microstructures on hardness was utilized to examine the mechanical changes as a function of annealing temperature and annealing time. Accordingly, the hardness of CoCrFeNi, measured as 517.8 f 25 HV after mechanical alloying, decreased significantly to 321 f 10 HV and 277 f 13 HV after 1 and 24 h of annealing at 900 degrees C, respectively. In contrast, the hybrid HEA maintained a hardness of 439.5 f 10 HV even after 24 h of annealing at 900 degrees C. Compared to the individual additions of Sm and B4C, the hybrid addition also proved more effective in preserving hardness at elevated temperatures over time. Overall, exploring the hybrid effect of alloying and second-phase addition provides a promising strategy for improving the grain size stability and the mechanical properties of nanocrystalline CoCrFeNi HEAs.
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    Influence of Isochronal and Isothermal Annealing on Microstructure and Hardness of Nanocrystalline CoCrFeNi-B4C High Entropy Alloy Composites
    (Springer, 2025) Koc, Recep C.; Polat, Gokhan; Batibay, Ahmet B.; Kotan, Hasan
    Adding nano-sized reinforcements to nanocrystalline alloys is an effective strategy for achieving stable microstructures with enhanced mechanical properties. This study investigates the effect of nano-sized B4C particles on the microstructural stability and mechanical properties of nanocrystalline CoCrFeNi high-entropy alloys (HEAs) synthesized by high-energy mechanical alloying. Comprehensive characterizations, including x-ray diffraction (XRD), focused ion beam (FIB) microscopy, and transmission electron microscopy (TEM) revealed that B4C addition improved microstructural stability resulting in an average grain size of 225 nm after annealing at 900 degrees C for 1 h. The in-situ formation of secondary phases, such as Cr7C3 and Cr2O3, during annealing process further contributed to the retardation of grain growth in CoCrFeNi HEA, improving the overall thermal stability. The mechanical properties, particularly hardness, were enhanced by the addition of B4C following mechanical alloying and annealing at 900 degrees C for up to 6 h. Beyond this duration, the hardness values plateaued, exhibiting minimal variations with B4C additions. Specifically, the as-milled hardness increased from 518 HV to 573 HV with incorporating 4 wt.% B4C. After annealing for longer than 6 h at 900 degrees C, the B4C containing HEA showed a hardness of 309 HV, compared to 278 HV for the HEA without B4C. Despite some reduction after prolonged annealing at elevated temperatures, this enhancement shows the alloy's potential to maintain mechanical performance under such conditions, while further optimization is needed to ensure durability over extended temperature exposures.
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    Remarkable thermal stability of nanocrystalline CoCrFeNi high entropy alloy achieved through the incorporation of rare-earth element samarium
    (Elsevier Sci Ltd, 2025) Kotan, Hasan; Koc, Recep C.; Batibay, Ahmet B.
    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.