Effect of aging treatment on microstructure and properties of high-entropy Cu 0.5CoCrFeNi alloy

Abstract

The microstructure and properties of Cu 0.5CoCrFeNi high-entropy alloys with as-cast structure and heat treated structures were investigated. The as-cast alloy specimens were firstly heated at 1050 °C for a holding time of 1 h. Serial aging heat treatment processes were performed at 350 °C, 500 °C, 650 °C, 800 °C, 950 °C, 1100 °C, 1250 °C and 1350 °C with a holding time of 24 h at each temperature. The microstructures, chemical composition, and precipitate phase of alloys with as-cast and various aging heat treated specimens characterized analyses were performed by scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results show that FCC phase structures remain unchanged after aging the Cu 0.5CoCrFeNi alloy of the as-cast specimens that had been heated to 1350 °C. The microstructure of the alloy specimens consisted of FCC matrix, Cu-rich phase, and Cr-rich phase. This Cr-rich phase precipitates to FCC matrix after being aged at 1100–1350 °C. The hardness of the Cu 0.5CoCrFeNi alloy was unchanged for the specimens after various heat treatments. The corrosion resistance of the specimens was evaluated by potentiodynamic polarization in immersion tests. The as-cast specimen and those that had undergone aging heat treatments from 350 to 950 °C were seriously corroded in 3.5% NaCl solution due to segregation of the Cu-rich phase precipitate formed in the FCC matrix. Cl − ions preferentially attacked the Cu-rich phase which was a sensitive zone exhibiting an appreciable potential difference with the consequent galvanic action. The specimens that were heat treated at 1100–1350 °C showed the best corrosion properties, because the Cu-rich phase was dissolved into the FCC matrix at elevated temperatures.