High-entropy alloy (HEA) thin films based on the equimolar Al-Cr-Nb-Ta-Ti system were deposited using magnetron sputtering. Without substrate heating, the films exhibited an amorphous structure with an Al0.12Cr0.21Nb0.23Ta0.28Ti0.16 composition. Partial crystallization was induced by increasing the substrate temperature, improving the hardness from 11.0 GPa to 14.0 GPa at 600 °C, while the composition is almost unchanged. Further vacuum annealing resulted in exceptionally high hardness reaching 19.1 GPa. X-ray diffraction revealed the formation of a body-centered cubic (bcc) solid-solution phase, along with peaks attributed to either a hexagonally close-packed (hcp) phase and Laves-type phases. After extended vacuum annealing at 800 °C, films deposited at lower substrate temperatures predominantly exhibited a bcc structure, whereas those synthesized at higher temperatures showed stronger contributions of hcp and Laves-type phases. The oxidation behavior of the films during annealing in ambient air was consistent with previously studied high-entropy (Al,Cr,Nb,Ta,Ti)-based carbide and nitride thin films, primarily leading to the formation of a rutile-structured oxide phase, with minor contributions from other oxide phases. Oxide scale breakdown occurred at 900 °C but was significantly improved by the addition of Si (~13 at.%). Cross-sectional imaging revealed porosity and blister formation at higher temperature as oxidation accelerates, primarily due to selective oxidation, the formation of Ta and Nb pentoxide phases, associated stresses, and oxide volatility. Elemental mapping of the oxide cross-sections further highlighted the critical role of Cr in the oxidation process.
Astecker et al. (Tue,) studied this question.