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

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 °C, 650 °C, 950 °C, and 1250 °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 °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 °C. The microstructure of the Cu-rich phase showed spinodal decomposition with an increase in the annealing temperature from 350 °C to 1250 °C. Differential scanning calorimetry (DSC) analysis revealed that in the as-cast high-entropy FeCoNiCrCu0.5 alloy specimen annealed below 1300 °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.

► 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 °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 ̊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.