Effect of annealing treatment on microstructure and properties of high-entropy FeCoNiCrCu0.5 alloy

Highlights: → Microstructural, thermal and corrosion property evolution of high entropy FeCoNiCrCu0.5 alloy at annealing treatment were studied. → Microstructure with annealing process induces a spinodal decomposition reaction which the amount of Cu-rich phase in the annealing microstructure changed; this phase transformed into a Cr-rich phase with an increase in annealing temperature from 350 to 1250 deg. C microstructure show BEI image in the SEM, and TEM. → The thermal property analysis revealed that in the as-cast high-entropy FeCoNiCrCu0.5 alloy specimen annealed below 1300 deg. C, the Cu-rich (Cu-(Ni, Co, Cr, Fe)) phase precipitated in the matrix by spinodal decomposition. → The corrosion potential and pitting potential of the FeCoNiCrCu0.5 alloy when immersed in a 3.5 wt% NaCl solution in the main corrosion mechanism were studied. → Cl- ions preferentially attacked the Cu-rich phase. - Abstract: This study examined the microstructure and electrochemical corrosion behaviour of high-entropy FeCoNiCrCu0.5 alloys annealed at various temperatures. The alloy microstructures were characterized and analyzed chemically by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Further, the effects of annealing temperatures of 350 deg. C, 650 deg. C, 950 deg. C, and 1250 deg. C with a holding time of 24 h at each temperature on the alloy microstructure and properties were investigated. XRD spectra of the as-cast specimens and those heated to 1250 deg. C showed a face-centred cubic (FCC) solid-solution phase. All specimens contained a matrix, which included a Cu-depleted phase, a Cr-rich phase (second structure), and a Cu-rich phase (third structure). The Cr-rich phase precipitated in the matrix after annealing at 1250 deg. C. The microstructure of the Cu-rich phase showed spinodal decomposition with an increase in the annealing temperature from 350 deg. C to 1250 deg. C. Differential scanning calorimetry (DSC) analysis revealed that in the as-cast high-entropy FeCoNiCrCu0.5 alloy specimen annealed below 1300 deg. C, the Cu-rich (Cu-(Ni, Co, Cr, Fe)) phases precipitated in the matrix by spinodal decomposition. The electrochemical corrosion behaviours of the as-cast and annealed specimens were evaluated by potentiodynamic polarization performed in immersion tests. The as-cast and annealed specimens were severely corroded in 3.5% NaCl solution; the main corrosion mechanism was the precipitation of the Cu-rich phase in the matrix. The Cu-rich phase was susceptible to corrosion, and its potential differed considerably from that of the matrix. The Cl- ions preferentially attacked this susceptible area (Cu-rich phase). This preferential attack was attributed to the fact that the presence of copper in the alloy degraded the corrosion resistance, thereby leading to corrosion by pitting.