Role of yttrium addition and annealing temperature on thermal stability and hardness of nanocrystalline CoCrFeNi high entropy alloy

CoCrFeNi high entropy alloys (HEAs) with yttrium (Y) additions (1 and 4 at. ) were nanostructured by mechanical alloying process and annealed at various temperatures between 500 degrees C and 1100 degrees C. The structure, grain growth, and hardness were studied as a function of solute addition and annealing temperature using X-ray diffraction (XRD), focused ion beam (FIB), and scanning transmission electron microscope (S/TEM) techniques, and hardness test. The thermo-physical calculations were utilized to discuss the phase evolution after mechanical alloying and annealing with respect to added solutes. The results showed that Y additions did not affect the main crystal structure of the base CoCrFeNi HEA as the solid solution with a single face-centered cubic (fcc) crystal structure was maintained even after 1 h annealing at 1100 degrees C. The as-milled nanocrystalline grain size of CoCrFeNi HEA yielded extensive grain growth with the temperature exposures reaching 291 nm and 1.4 mu m after annealing at 900 degrees C and 1100 degrees C, respectively. However, Y additions retarded the grain growth and decreased the average grain size upon annealing as compared to the base HEA. That is, 1 and 4 at. Y additions stabilized the grain size around 88 nm and 95 nm (both determined by TEM) after annealing at 900 degrees C and 1100 degrees C, respectively. Accordingly, the as-milled hardness of CoCrFeNi HEA dropped from 475 HV to 220 HV after annealing at 1100 degrees C, while the reduction in hardness was relatively gradual with Y additions and retained around 435 HV with 4 at. Y addition even after annealing at 1100 degrees C. Such thermal stability may facilitate the use of HEAs at high temperatures and enable the consolidation routes of powders into dense nanocrystalline compact HEAs.