A new complex-regular eutectic in the Fe₇(CoNiMn)_93-xB_x high entropy alloys

Phase constituents and solidification microstructures of as-cast Fe₇(CoNiMn)_93-xB_x (x = 9, 13) HEAs were investigated via electron backscattered diffraction and transmission electron microscopy. It was observed that the phase constituents contain an FCC1 solid solution and an M₂₃B₆ compound. The microstructure of hypoeutectic Fe₇(CoNiMn)₈₄B₉ high-entropy alloy consists of primary face-centered-cubic (FCC1) dendrite and trigonal-shaped complex-regular eutectic, and that of Fe₇(CoNiMn)₈₀B₁₃ eutectic high-entropy alloy is fully consisted of trigonal-shaped complex-regular eutectic. The formation of this rare complex-regular eutectic is attributed to the preferential growth of M₂₃B₆ in a trigonal-shaped trunk due to its crystallography anisotropy. In addition, the FCC1 eutectic phase has a cube-on-cube orientation relationship with the M₂₃B₆ phase where the same orientation but different lattice constants for them leads to the appearance of misfit dislocations at the interface, which enhances the strength of materials. The compression strength for Fe₇(CoNiMn)₈₀B₁₃ eutectic high-entropy alloy is higher than that of hypoeutectic Fe₇(CoNiMn)₈₄B₉ high-entropy alloy.