Oxidation behaviour of an AlCrFeNiSi-based high-entropy alloy bond coat designed with the CALPHAD approach

In this study, equilibrium and non-equilibrium (Scheil) solidification phase diagrams were simulated to identify an optimised AlxCrFeNiSiy high-entropy alloy (HEA) composition aimed at promoting simple solid-solution phases while minimising undesirable intermetallics. The selected composition was synthesised into powder via gas atomisation and subsequently deposited on additively manufactured Ni-based superalloy substrates using the high-velocity oxygen fuel (HVOF) process. A thermal barrier coating (TBC) system incorporating the Al21CrFeNiSi8 HEA as a bond coat and yttria stabilised zirconia (YSZ) top coat was produced. The microstructures of the coatings were analysed and phase composition was compared with simulation predictions. Additionally, the isothermal oxidation resistance of the coating was evaluated at 1100 °C, resulting in the growth of a thin Al2O3 thermally grown oxide layer. While the oxidation rate coefficient was low, oxidation-induced phase transformations caused spalling after extended oxidation. The study provides valuable insights into the strengths and weaknesses of the CALPHAD approach, the HEAs performance and its potential for next-generation oxidation resistant TBCs.