Corrosion behaviour of Nano-lamellar AlCrCoFeNi2.1 Eutectic High Entropy Alloy

R, Navneeth and Malladi, Sairam K (2019) Corrosion behaviour of Nano-lamellar AlCrCoFeNi2.1 Eutectic High Entropy Alloy. Masters thesis, Indian institute of technology Hyderabad.

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Abstract

High entropy alloys (HEAs) are an emerging class of alloys which have received massive research attention due to their excellent mechanical properties and are prime candidates for advanced structural applications. As a result, research in the field of HEAs has primarily focused on improving the mechanical and structural properties, while the electrochemical corrosion behaviour has seldom been studied. Recent developments in the area of HEAs has been the emergence of Eutectic High Entropy Alloys (EHEAs) (such as AlCoCrFeNi2.1) primarily developed to achieve a combination of high strength and ductility. Recent studies have shown that through a special thermomechanical processing route of cryo-rolling and annealing, AlCoCrFeNi2.1 achieves an extraordinary combination of strength and ductility and thus has a huge potential for commercialization. Nevertheless, for a commercial alloy, it is essential to have an in-depth understanding of the corrosion and degradation behaviour. Moreover, the nano-lamellar morphology makes this system highly prone to a localized corrosion attack. This study investigates the corrosion behaviour of AlCoCrFeNi2.1 EHEAs using immersion tests, electrochemical methods and a correlation between the microstructure and corrosion properties is carried out through scanning electron microscopy. From these studies, it is observed that the corrosion rate from immersion tests as well as electrochemical tests is of the order of 0.02 mm/y, which is considered as acceptable corrosion rate for metallic alloys. However, the microscopic investigations show that these alloys are prone to localized attack. Although this alloy tends to passivate, the passive layer on prolonged exposure to NaCl solution (over 15 days) is not adherent. Through polarisation experiments, the corrosion potential, corrosion current, passivation potential and passive layer resistance were determined and corresponding SEM-EDS analysis has been carried out. From the compositional analysis using EDS, it is observed that there is a preferential dissolution of Al over other alloying elements. In short, although AlCoCrFeNi2.1 EHEA shows a relatively low rate of dissolution/corrosion, it is prone to localized corrosion attack. Further research should focus on improving the resistance against localized corrosion using alloy and microstructural design pathways.

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