Solidification evolution and corrosion performance of CoCrFeNiBx eutectic high entropy alloy coatings

CoCrFeNiBx (x = 0.15, 0.30, 0.45, 0.60, 0.75) eutectic high entropy alloy (EHEA) coatings, abbreviated as B0.15-B0.75, were prepared by laser cladding. The inevitability of solid-state phase transition in B0.75 coating has been verified from the perspective of enthalpy. The solidification and crystallization process and laws of the CoCrFeNiBx coatings were obtained by observing the phase and microstructure, combining the DSC testing and analysis. When the mole ratio of B atoms x <= 0.45, the FCC1 phase solidifies into dendrites, and the eutectic structure composed of the coupling of FCC1 and FCC2 lamellae forms between dendrites. As the content of B increases, the proportion of eutectic structure rises steadily while the dendrites refine and decrease. When x = 0.60, the B content reaches the eutectic point, forming the eutectic structure with a feathery appearance. When x = 0.75, a strip-shaped boride structure will also precipitate after the formation of the metastable eutectic structure. The B element can promote the formation of eutectic structures, thereby improving the microhardness of the CoCrFeNiBx coatings from 267.0 HV0.2 to 555.4 HV0.2. However, as the B content rises, the corrosion current density of the CoCrFeNiBx coating in 3.5 % NaCl solution decreases from 1.08E-07 A/cm2 to 8.92E-06 A/cm2. The corrosion mechanism changes from pitting corrosion to intergranular corrosion. The high-phase interfacial energy of the eutectic structure exacerbates the galvanic corrosion of the coatings.