Microstructure, thermal expansion, and high-temperature oxidation behavior of spark plasma sintered AlCoCrSiNi high entropy alloy

In this investigation, an equiatomic AlCoCrSiNi High Entropy Alloy (HEA) was synthesized via mechanical alloying of elemental powders, followed by consolidation using spark plasma sintering (SPS) at 900 °C. The ball milling of 30 h results in the formation of HEA, showing a body-centered cubic solid solution phase as the major phase. Upon SPS, a fully dense alloy to near theoretical density is achieved and the formation of σ-phase, Cr-oxide, and Cr5Si3 alongside the parent solid solution phase is noticed. Our investigation revealed that the coefficient of thermal expansion of the SPSed AlCoCrSiNi HEA at 800°C is significantly lower than that of Inconel 718, Hastelloy X, stainless steels, and most transition metal-based HEAs. The oxidation study was performed in static air at three temperatures (800, 900, and 1100 °C) with isothermal exposure (up to 100 h). At 800 and 900 °C for initial hours of isothermal holding duration, Cr-oxide (forms during SPS) along with minor amount of Al2O3 is present. When the holding duration increases to 100 h, Al2O3 forms as a protective oxide layer. The predominant presence of Al2O3 can be noticed in the case of oxidation test at 900°C. The oxidation at 800 °C having power law exponent n = 4.59 is an indication of the protective oxide, while the value of n = 1.72 at 900 °C suggests the formation of semi-protective layer. Notably, a higher oxidation temperature (1100 °C) led to a substantial mass loss and subsequent failure of the Al2O3 oxide layer, which can be attributed to the volatilization of Cr-oxide.