Effect of Fe Concentration on the High Temperature Oxidation Behavior of Fex(CrAlNi)100−x Medium Entropy Alloys

In this study, effect of Fe concentration on the high temperature oxidation and microstructural stability of Fe_x(CrAlNi)_100−x alloys (x = 25, 35, 45, 55, 65) at 1100 °C up to 168 h was investigated in air. Increasing Fe concentration decreased the molar fraction of B2 phase in as-cast alloys. However, microhardness values experienced only a 10% reduction (Fe₂₅: 517.7 ± 19 HV, Fe₆₅: 470.6 ± 22 HV) due to well-distributed B2 precipitates. After the exposures, coarsening of B2 precipitates was observed in all alloys, leading to a microhardness reduction of 20–25% after 168 h. Single-phase α-Al₂O₃ scales were formed on Fe₂₅–Fe₅₅ alloys. However, increasing Fe concentration resulted in deeper depletion zones due to reduced molar fraction and Al concentration of B2 phase. Moreover, Fe₆₅ alloy failed to develop a protective α-Al₂O₃ scale due to decreased molar phase fraction and Al concentration of B2 precipitates, along with the low Cr concentration of the A2 phase. Additionally, α-Al₂O₃ scales were highly wrinkled due to the absence of reactive elements. Absence of reactive elements also resulted in oxide spallation and seemed to intensify with the increasing Fe concentration. Possible reasons for the increased oxide spallation with the increasing Fe concentration are discussed. Nevertheless, Fe₂₅–Fe₅₅ alloys displayed oxidation properties comparable to those of lean FeCrAl alloys while also possessing enhanced mechanical properties due to B2 reinforcement.