Environmental Resistance of High Entropy Alloys: Impact of Downstream Hydrogen Combustion on Oxidation Resistance

Abstract This study explores the environmental resistance of Al-CoCrFeNi-based high entropy alloys (HEAs) placed downstream from a combustor using hydrogen fuel. High entropy alloys are interesting candidates for gas turbines due to their severely mismatched lattice structure leading to sluggish diffusion, which is theorized to provide superior environmental resistance. Three HEAs with different doping elements, namely, Al6Co21Cr21Fe21Ni30, Al4Co21Cr21Fe21Ni30Ti2, and Al4Co20Cr20Fe20Mo1Ni30Ti2 (at %), were compared against Hastelloy X in terms of oxidation and corrosion. Samples were impinged by the flame within a demonstrator hydrogen combustor cell, which was run for a total of 10 h. The combustor cell burned a 100% hydrogen flame using a combination of premixed/micromixed fuel injection to provide a stable flame with a calculated adiabatic temperature of 1800 K. In situ imaging and temperature measurements continuously monitored the samples and combustion conditions within the chamber. Ex situ scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses revealed the depth of oxidation, oxide species, and damage caused by the high-temperature flame. The results highlight the consequences of burning pure hydrogen on the metals downstream of a hydrogen combustor. Aluminum stimulated the formation of a vapor-resistant Al2O3 scale, leading to sluggish oxidation kinetics. Ti increased the oxidation rate by favorizing the formation of Fe-rich oxides, and Mo reduced the oxide thickness but induced significant formation of subscale pores.