Microstructural Characterization and Hardness Study of Nanostructured CoCrFeNi High Entropy Alloys with Dual Effect of Y and Nano-Sized Y2O3 Additions

The motivation of this work is to increase the grain size stability of nanocrystalline CoCrFeNi high entropy alloys (HEA) with Y and Y2O3 additions to strike a balance between thermodynamic and kinetic stabilizations. The nanocrystalline HEAs prepared by mechanical alloying were annealed at different temperatures and characterized by X-ray diffraction, focused ion beam microscopy and micro-hardness test. The results revealed that as-milled nanostructured grain size yielded grain growth upon annealing reaching to 350 nm and 1.3 mu m after annealing at 900 degrees C and 1100 degrees C, respectively, while the addition of Y and Y2O3 appeared to stabilize the grain size in the nano-range after annealing at the same temperatures. Consequently, while the as-milled hardness of CoCrFeNi HEA dropped from 475 HV to around 200 HV after annealing at 1100 degrees C, the elevated hardness of 430 HV was retained with Y and Y2O3 additions after annealing at the same temperature.