Evolution of microstructure, hardness, and corrosion properties of high-entropy Al0.5CoCrFeNi alloy

In this study, we investigate the microstructure, hardness, and corrosion properties of as-cast Al0.5CoCrFeNi alloy as well as Al0.5CoCrFeNi alloys aged at temperatures of 350 °C, 500 °C, 650 °C, 800 °C, and 950 °C for 24 h. The microstructures of the various specimens are investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron probe X-ray microanalysis (EPMA). The results show that the microstructure of as-cast Al0.5CoCrFeNi comprises an FCC solid solution matrix and droplet-shaped phases (Al–Ni rich phases). At aging temperatures of between 350 and 950 °C, the alloy microstructure comprises an FCC + BCC solid solution with a matrix, droplet-shaped phases (Al–Ni rich phase), wall-shaped phases, and needle-shaped phases (Al–(Ni, Co, Cr, Fe) phase). The aging process induces a spinodal decomposition reaction which reduces the amount of the Al–Ni rich phase in the aged microstructure and increases the amount of the Al–(Ni, Co, Cr, Fe) phase. The hardness of the Al0.5CoCrFeNi alloy increases after aging. The optimal hardness is obtained at aging temperatures in the range 350–800 °C, and the hardening effect decreases at higher temperatures. Both the as-cast and aged specimens are considerably corroded when immersed in a 3.5% NaCl solution because of the segregation of the Al–Ni rich phase precipitate formed in the FCC matrix. Cl− ions preferentially attack the Al–Ni rich phase, which is a sensitive zone exhibiting an appreciable potential difference, with consequent galvanic action.

Research highlights
► Phase formation and microstructural evolution of high entropy Al0.5CoCrFeNi alloy were studied.
► First matrix and droplet-shaped structures (Al-Ni rich phase) are formed in as- Al0.5CoCrFeNi, secondly when aging temperatures of between 350 and 950 °C has the form the matrix, needle-shaped structures (Al-(Ni, Co, Cr, Fe) phase), wall-shaped structures, and droplet-shaped structures (Al-Ni rich phase).
► Microstructure with aging process induces a spinodal decomposition reaction which reduces the amount of Al-Ni phase in the aged microstructure and increases the amount of Al-(Ni, Co, Cr, Fe) phase show BEI image in the SEM.
► The corrosion potential and pitting potential of the Al0.5CoCrFeNi alloy when immersed in a 3.5 wt% NaCl solution are significantly lower than those of AISI 304L stainless steel.
► Cl- ions preferentially attacked the Al-Ni rich phase and the Al-(Ni, Co, Cr, Fe) phase.