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

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.