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 degrees C, 500 degrees C, 650 degrees C, 800 degrees C, and 950 degrees 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 degrees 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 degrees 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. (C) 2010 Elsevier Ltd. All rights reserved.