Abstract
Compositionally complex alloys (CCAs), also termed as high entropy alloys (HEAs) or multi-principal element alloys (MPEAs), are being considered as a potential solution for many energy-related applications comprising extreme environments and temperatures. Herein, a review of the pertinent literature is performed in conjunction with original works characterising the oxidation behaviour of two diverse Al-containing alloys; namely a lightweight (5.06 g/cm3) single-phase AlTiVCr CCA and a multiple-phase Al0.9FeCrCoNi CCA (6.9 g/cm3). The thermogravimetric results obtained during oxidation of the alloys at 700 and 900 °C revealed that both alloys tended to obey the desired parabolic rate law. Post-exposure analysis by means of electron microscopy indicated that while the oxide scale formed on the AlTiVCr is adherent to the substrate, the scale developed on the Al0.9FeCrCoNi displays a notable spalling propensity. This study highlights the need for tailoring the protective properties of the oxide scale formed on the surface of the CCAs.
Highlights
High-temperature alloys and superalloys are essential for many critical applications involving corrosive chemical and electrochemical reactions including jet engines, land-based gas turbines, materials processing, as well as a wide range of emerging technologies such as concentrated solar thermal power ( known as concentrated solar plants (CSPs)), biomass gasification and molten oxide electrolysis (MOE)[1]
Of the two family of high-temperature materials, i.e., alumina-formers and chromia-formers, alloys forming protective Al2O3 scales such as iron (Fe)-based (FeCrAls)[1], nickel (Ni)-based (NiCrAls)[3], cobalt (Co)based (CoCrAls)[4] and nickel aluminates (NiAl)-type materials[5] are considered for applications with operating temperatures >900 °C. This is because α-Al2O3 scale possess several characteristics that are highly desired for the high-temperature oxidation performance of alloys including: (i) they grow very slowly, (ii) thermodynamic stability, and (iii) chemically inert[1,2]
Compositionally complex alloys (CCAs) are currently being considered as promising candidate materials for hightemperature applications owing to their high melting points, hightemperature strength, wear resistance and microstructural stability[6]
Summary
High-temperature alloys and superalloys are essential for many critical applications involving corrosive chemical and electrochemical reactions including jet engines, land-based gas turbines, materials processing, as well as a wide range of emerging technologies such as concentrated solar thermal power ( known as concentrated solar plants (CSPs)), biomass gasification and molten oxide electrolysis (MOE)[1]. Of the two family of high-temperature materials, i.e., alumina-formers and chromia-formers, alloys forming protective Al2O3 scales such as iron (Fe)-based (FeCrAls)[1], nickel (Ni)-based (NiCrAls)[3], cobalt (Co)based (CoCrAls)[4] and nickel aluminates (NiAl)-type materials[5] are considered for applications with operating temperatures >900 °C. Compositionally complex alloys (CCAs) are currently being considered as promising candidate materials for hightemperature applications owing to their high melting points, hightemperature strength (creep resistance), wear resistance and microstructural stability[6] These alloys, by definition, consist of multiple elements in equimolar or near-equimolar ratios and tend to form metallic solid solutions (FCC, BCC, HCP, or a mixture of these)[6,7]. In the context of high-temperature oxidation, to date, studies on AlxFeCrCoNi-based CCAs are limited (see e.g., ref. 17), and for the AlTiVCr CCA, no prior studies exist
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