Hierarchical FeCoNiCr high entropy alloy thin films with combined high strength and excellent corrosion resistance

A two-step temperature-controlled magnetron sputtering procedure was adopted to obtain a bilayer hierarchical Fe25.2Co25.2Ni27.4Cr22.2 high entropy alloy (HEA) thin film consisting of the external nano-lamellar layer and internal equiaxed zone, which displayed combined advantages of high hardness, good modulus and excellent corrosion resistance. The roles of heterogeneous nanostructure on the mechanical and corrosion behavior were revealed by nano-indentation and Scanning Kelvin probe (SKP) methods. Compared to the substrate 304 ASS, the hierarchical HEA film not only shows much higher nano-hardness and modulus values, but also possesses a higher electron transfer resistance (12158 vs 4567 Ω.cm2) and lower overall corrosion rate (6.28*10−4 vs 6.84*10−3 mm·year−1). Moreover, the outer nano-lamellar layer shows higher nano-hardness (∼8.1 GPa) but lower surface potential values (∼-261 mV) than those of the inter equiaxed zone (∼5.9 GPa and ∼−150 mV). The former arises from the significant grain refinement strengthening effect, while the latter is attributed to an obvious increase in the high energy and chemically active areas such as grain boundaries and triple junctions. This work demonstrates the capability of hierarchy to adjust the comprehensive performance of multi-functional films, which possess superior mechanical property and comparable corrosion performance than those of the as-cast bulk HEA alloys.