Formation of strong and ductile FeNiCoCrB network-structured high-entropy alloys by fluxing

A series of [(FeNiCo)_0.85 Cr_0.15 ]_100-xB_x (x = 12, 15, 17) high-entropy alloys with network-like microstructures (N-HEAs) and a wavelength of 3-5 μm was prepared using the fluxing method. The novel N-HEAs exhibited higher strength and ductility compared with samples obtained by suction casting. Neutron diffraction and scanning electron microscopy measurements showed that the network-like structure contained soft face-centered cubic (FCC) and hard tetragonal Cr₂B-type sub-networks. The network-like structure was formed during the solidification of the molten alloy from a deeply undercooled state, achieved by removing impurities and most metallic oxides through B₂O₃ fluxing. The mechanical properties could be tuned by modifying the composition to change the volume fractions of the different sub-networks. When x decreased from 17 to 12, the compressive yield strength decreased from 1.6 to 1.1 GPa, while the compressive strain increased from ~20% to ~70%. The N-HEA samples with x = 12 and 15 also exhibited a good tensile ductility of 19% and 14%, respectively. In situ synchrotron X-ray diffraction results revealed an inhomogeneous deformation behavior, i.e., the soft FCC phase yielded prior to the hard Cr₂ B-type phase, which bore more stress in the initial stage of the plastic deformation. In the later stage of the plastic deformation, the ductility of the sample was provided by the FCC phase, together with some contributions from the Cr₂ B-type phase.