Simplifying the microstructure and improving the corrosion resistance of the FeCoCrNiNbx high-entropy alloys via homogenizing annealing heat treatment

Abstract

FeCoCrNiNb_x high-entropy alloys were subjected to a homogenizing annealing heat treatment at 1200 ℃ for 4 h, which simplified the microstructure of the alloys and reduced the degree of compositional segregation. After heat treatment, FeCoCrNi alloy still maintained the single-phase FCC structure, and the grains grew into coarser columnar crystals reducing the number of grains; the intergranular eutectic FCC phase of FeCoCrNiNb_x (x=0.15, 0.33) alloys disappeared and the segregated Laves phase content in FeCoCrNiNb_x (x=0.15, 0.33, 0.5) alloys was significantly reduced. The special Ostwald ripening phenomenon was discovered in the FeCoCrNiNb_0.5 alloy. The homogenizing heat treatment drastically raised the work function (WF) of the alloys by nearly a factor of one, improved the homogeneity of WF distribution in each phase, and weakened the tendency of selective dissolution; moreover, it also improved the compactness of the alloys' passive films, significantly decreased the maintaining passivation current density of the passive films by more than one order of magnitude, enhanced the tendency of the alloys repassivation, and ultimately improved the corrosion resistance of the alloys in 3.5 wt.% NaCl solution. The thermodynamic calculation of alloy phase diagrams is of vital significance in guiding the design of alloy compositions, optimizing heat treatment processes, and improving alloy properties.