Fabrication‐Driven Oxidation Pathways in Y/Hf‐Modified High‐Entropy Alloys: From Elemental Segregation to Oxide Homogeneity

This study elucidates the oxidation mechanisms governing Y/Hf‐doped AlCoCrFeNiTi high‐entropy alloys (HEAs) and reveals the pivotal role of fabrication processes in dictating high‐temperature oxidation behavior. We demonstrate that casting and spark plasma sintering (SPS) promote Y/Hf segregation, resulting in heterogeneous oxide formation and compromised scale stability, whereas filtered cathodic vacuum arc (FCVA) deposition yields grain‐boundary‐free coatings with amorphous microstructures that foster the development of continuous dense Al 2 O 3 films. Compared to conventional MCrAlY coatings, the FCVA‐fabricated HEAs exhibit a tenfold reduction in oxidation rate. Notably, Al depletion during oxidation induces a B2→FCC phase transition, accelerating Fe/Co/Ni/Cr outward diffusion and facilitating spinel formation. Although Y/Hf co‐doping effectively suppresses Al diffusion via the formation of large ionic clusters, the synergy is limited by atomic size mismatch. Our findings underscore that oxidation resistance is governed not merely by reactive element content but critically by their spatial distribution and the continuity of the protective oxide layer, offering a process‐informed pathway to optimize high‐temperature performance of HEA‐based bond coats.