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A nanostructured oxide‐dispersion‐strengthened (ODS) CoCrFeMnNi high‐entropy alloy (HEA) is synthesized by a powder metallurgy process. The thermal stability, including the grain size and crystal structure of the HEA matrix and oxide dispersions, is carefully investigated by X‐ray diffraction (XRD) and electron microscopy characterizations after annealing at 900 °C. The limited grain growth may be attributed to Zener pinning of yttria dispersions that impede the grain boundary mobility and diffusivity. The high hardness is caused by both the fine grain size and yttria dispersions, which are also retained after annealing at 900 °C. Herein, it is implied that the combination of ODS and HEA concepts may provide a new design strategy for the development of thermally stable nanostructured alloys for extreme environments.
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Abstract (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C high‐entropy ceramics (HEC) with a submicron grain size of 400 to 600 nm were fabricated by spark plasma sintering using a two‐step sintering process. Both X‐ray and neutron diffractions confirmed the formation of single‐phase with rock salt structure in the as‐fabricated (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C samples. The effect of submicron grain size on the thermal stability and mechanical properties of HEC was investigated. The grain growth kinetics in the fine‐grained HEC was small at 1300 and 1600°C, suggesting high thermal stability that was possibly related to the compositional complexity and sluggish diffusion in HEC. Compared to the coarse‐grain HEC with a grain size of 16.5 µm, the bending strength and fracture toughness of fine‐grained HEC were 25% and 20% higher respectively. The improvement of mechanical properties in fine‐grained HEC may be attributed to micromechanistic mechanisms such as crack deflection.
