Grain size stabilization of oxide dispersion strengthened CoCrFeNi-Y2O3 high entropy alloys synthesized by mechanical alloying

Highlights: • CoCrFeNi high entropy alloys (HEAs) with 1 and 4 wt% Y₂O₃ additions were nanostructured by mechanical alloying. • The as-milled and annealed HEAs possessed a microstructure of solid solution with fcc crystal structure. • The annealing of initially nanocrystalline CoCrFeNi HEA yielded grain growth reaching ~1.45 µm after annealing at 1100 °C. • The grain size stabilization was retained with Y₂O₃ additions through the secondary oxide particles. • The higher hardness values were reached with increased Y₂O₃ contents upon annealing. -- Abstract: Nanocrystalline CoCrFeNi high entropy alloys (HEAs) with 1 and 4 wt% nanosized Y₂O₃ were synthesized by high energy mechanical alloying and subjected to annealing treatments at different temperatures up to 1100 °C. X-ray diffraction (XRD), focused ion beam microscopy (FIB), and transmission electron microscopy (TEM) were used to investigate the microstructures of as-milled and annealed HEAs as a function of annealing temperature and Y₂O₃ content. The results have shown that the as-milled HEAs were solid solutions with face-centered cubic (fcc) crystal structure, which remained stable even after annealing at 1100 °C. The as-milled nanocrystalline CoCrFeNi HEA revealed grain growth upon annealing, reaching 293 nm and 1.45 µm after annealing at 900 and 1100 °C, respectively. This suggests that the nanocrystalline microstructure of CoCrFeNi is not thermally stable at high temperatures. The grain size stability was found to reach around 72 nm with nanosized Y₂O₃ particles after annealing at 1100 °C. Accordingly, 477 ± 20 HV as-milled hardness of CoCrFeNi was dramatically reduced to 220 ± 14 HV after annealing at 1100 °C due to severe grain coarsening but retained around 450 ± 23 HV with 4 wt% Y₂O₃ addition. The correlation between microstructure and hardness was utilized to evaluate the mechanical properties.