High Temperature Oxidation of Y doped Equiatomic AlCrFeNi Medium Entropy Alloy

Abstract

High temperature oxidation (HTO) of Y-doped (0.08 at.%) equiatomic AlCrFeNi alloy produced by vacuum arc melting was studied at 1100 °C for 168h in dry air. As-cast alloys consisted of disordered Fe-Cr rich A2 and Ni-Al rich B2 phase with the additional Y-rich precipitates rich in Ni and Al resembling B2 phase. Alloys possessed a columnar-dendritic microstructure in which dendritic regions contained weave-like morphology (?120 nm), while interdendritic regions contained relatively coarse structures. Y-rich coarse precipitates were mostly found to segregate into interdendritic regions. After HTO tests the only oxide phase found was ?-Al2O3 and lower mass gains compared to undoped material were recorded (lean? 0.95 mg.cm-2, Y-doped 0.83 mg.cm-2). Two distinct regions were observed based on top-view investigations I) Y-rich regions coupled with smooth Al2O3, II) Y-poor regions containing whiskers and smooth Al2O3. In both cases, wrinkling of Al2O3 scales was not observed. No oxide spallation was observed except at the edges. Additional stress formation on the edges coupled with the high strength of the alloy is assumed to result in oxide spallation. Compact Al2O3 scales exhibiting planar metal-oxide interface without wrinkles were observed by cross-sectional analysis. Y/Al-rich precipitates were found within the oxide scale and within the alloy (internal oxidation). Exposing samples also resulted in coarsening of A2 and B2 phases yet the alloy experienced only a 10% reduction in Vickers microhardness values (398 ± 6.8 HV). More studies on optimization of reactive element doping as well as mechanical properties are needed for further improvement of HTO performance.