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This study investigates the microstructural evolution of the CoCrFeNi system after incorporating Gallium (Ga) at varying concentrations (0, 15, and 20 at.%). The systems were synthesized by Vacuum Arc Melting (VAM) and characterized through X-ray Diffraction diffraction (XRD) and Scanning Electron Microscopy (SEM/EDS). Findings showed that the CoCrFeNi medium medium-entropy alloy stabilizes in a single-phase Face-Centered Cubic (FCC) structure. Upon the addition of 15 at.% Ga a dendritic morphology with a transition towards a duplex FCC + BCC microstructure was induced, a trend which was further solified in the equiatomic FeCoNiCrGa system. In this case the proportion of the Ga-rich BCC phase was increased from 18–22% to 31–34% for the Ga15 and Ga20 systems respectively. A combined approach of Electrochemical Frequency Modulation (EFM), Cyclic Potentiodynamic Polarization (CPP), and Electrochemical Impedance Spectroscopy (EIS) was selected for studying the electrochemical corrosion behavior of the produced systems. EFM results indicated a progressive deterioration of corrosion resistance when increasing Ga concentration (Icorr: 4.142, 5.619 and 10.01 μA/cm2, and Rp: 12,035, 10,736 and 7254 Ω for the Ga0, Ga15 and Ga20 alloys respectively). Surface inhomogeneity, rapid passivation, and diffusion-controlled processes caused deviations from the ideal causality factors’ values. CPP measurements revealed increasing corrosion current densities with Ga addition within the Tafel region (2.81 × 10−7, 3.72 × 10−7 and 5.11 × 10−7A/cm2 for the Ga0, Ga15 and Ga20 alloys respectively). All alloys showed positive hysteresis loops and an absence of repassivation, indicating susceptibility to pitting corrosion. Nevertheless, detailed analysis of the forward polarization region highlighted a more complex aspect. Reverse polarization scans confirmed stable pit growth in all alloys, with the absence of a repassivation tendency. EIS tests, performed after the completion of CPP measurements, further clarified the corrosion mechanisms. Equivalent circuit modeling revealed that although Ga-containing alloys exhibited relatively improved film characteristics in the forward polarization stage, the charge transfer resistance (Rct) was highest for the CoCrFeNi alloy, followed by Ga15 and Ga20 (22,620, 11,380, 10,060 Ω respectively). The overall impedance ranking (Ga0 > Ga15 > Ga20, i.e., 27,139 > 20,279.5 > 16,341 ohms respectively) showed that, despite microstructural and entropic effects enhancing certain passivation aspects, the reduced Cr content highly impacted long-term corrosion resistance. This holistic electrochemical approach showcases the complex interactions between compositional alterations, phase structure, grain refinement, passive film chemistry, and diffusion trends in establishing the corrosion performance of Ga-modified CoCrFeNi HEAs.
Georgatis et al. (Sat,) studied this question.