Microstructural Evolution from Dendrites to Core-Shell Equiaxed Grain Morphology for CoCrFeNiVx High-Entropy Alloys in Metallic Casting Mold

The CoCrFeNiVx (x = 0, 0.25, 0.5, 0.7, 0.8, 0.9, and 1.0) high-entropy alloys (HEAs) were fabricated by the copper mold casting process. The microstructure, phase constitution, and mechanical properties were investigated by using X-ray diffraction, scanning electron microscopy, transmission electron microscopy analyses and compressive testing. It revealed that, when x <= 0.25, the alloys solidified into a single fcc phase. When 0.5 <= x <= 0.8, the alloys solidified into a dendritic structure of the fcc phase with the formation of the sigma phase in the interdendrite region. Interestingly, when x exceeded 0.9, the alloys presented a typical core-shell equiaxed grain morphology. The core region consisting of a mixture of fcc + sigma phases was surrounded by the shell of the single sigma phase and the interdendrite region solidified into the single fcc phase. The dual-phase eutectiod structure in the core region of the equiaxed grain might be formed from the decomposition of the unidentified metastable phase. As the V fraction increased, the compressive yield strength of the CoCrFeNiVx alloys gradually increased from 164 MPa (x = 0) to 458 MPa (x = 0.8), and then sharply increased to 722 MPa (x = 0.9) and 1493 MPa (x = 1.0).