Structure, corrosion resistance and nanomechanical properties of CoCrFeNiX (X=Nb,Mo,B,Si) high entropy alloys

Abstract

In this work, the four high entropy CoCrFeNiX alloys (X=Nb,Mo,B,Si) were prepared by induction melting to comparatively analyze their structure, nanomechanical properties, and corrosion resistance. The CoCrFeNiNb and CoCrFeNiMo alloys were composed of FCC solid solution and intermetallic phases (TM)₂Nb and Cr-Mo-TM. In the case of the CoCrFeNiB alloy, a complex phase structure was revealed, consisting of FCC solid solution and three types of borides. In turn, the addition of Si substantially altered the phase composition of the CoCrFeNi alloy, resulting in the formation of two intermetallic phases. The corrosion behaviour of the alloys was studied in 3.5 and 5% NaCl solutions. The highest corrosion resistance was characteristic for the CoCrFeNiSi alloy, which showed the most uniform chemical element distribution, showing the lowest corrosion current density and the highest positive corrosion potential values in both environments used. For measurements in a 5% NaCl solution, i_corr and E_corr were equal to 0.24 μA/cm² and -0.136 V. The least favourable corrosion parameters were recorded for the CoCrFeNiMo alloy. The results of EIS measurements confirmed the high protective abilities of the passive film formed on the CoCrFeNiSi alloy surface. The SKPFM map of the Volta potential differences showed that the Cr-Mo-TM phase with a high molybdenum content was less noble than the FCC solid solution. Similarly, the Ni-rich FCC phase was more noble compared to the (TM)₂Nb phase with a high niobium content for the CoCrFeNiNb alloy. The highest strength properties were shown by the alloys with the addition of metalloids. For the CoCrFeNiSi alloy, the highest nanohardness value was obtained (above 15 GPa), while the CoCrFeNiB showed the highest Young's modulus (above 275 GPa).