The effect of cooling rate and content of niobium on the structure, wear and corrosion resistance of CoCrFeNiNbx high entropy alloys

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Abstract In this work, CoCrFeNiNbx (x = 0.25, 0.45 and 0.65) high entropy alloys were prepared by two different methods to determine the effect of cooling rate and the niobium content on the structure and properties of ingots and plates. The structure was investigated extensively using X-ray diffraction, scanning electron microscopy, and Mössbauer spectroscopy. The results confirmed the dual-phase structure, consisting of the FCC solid solution and the Laves phase. The increase in niobium content changed the microstructure from hypoeutectic (x = 0.25 and 0.45) to hypereutectic (x = 0.65). The high cooling rate during solidification from the liquid state enabled the formation of ultrafine eutectic structures with an average lamellae thickness of only 130 ± 9 nm in the CoCrFeNiNb0.65 plate. The corrosion behaviour of the alloys was studied in solutions of 3.5% NaCl and 3.5% NaCl + H3BO3. The beneficial effect of increasing the niobium content in as-cast CoCrFeNiNbx alloys on the corrosion resistance was confirmed in both environments. Furthermore, the alloys solidified with a higher cooling rate exhibited a lower corrosion susceptibility in the 3.5% NaCl solution. The results of the EIS study indicated that a higher content of niobium contributed to the formation of a more stable and compact passivation layer. The hardness of the CoCrFeNiNbx alloys increased with a higher niobium content, achieving the highest value of 669 HV1 for the CoCrFeNiNb0.65 plate. The increase in the cooling rate positively affected the tribological properties of the CoCrFeNiNbx alloys, contributing to the decrease in the friction coefficient for the CoCrFeNiNb0.25 and CoCrFeNiNb0.45 plates.

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