Size dependency in stacking fault-mediated ultrahard high-entropy alloy thin films

High-entropy alloy (HEA) thin films are known to exhibit excellent and tunable mechanical properties, making it an attractive candidate for various engineering applications. A series of nanocrystalline face-centered cubic (FCC) Al0·1CoCrFeNi HEA films with film thicknesses spanning from 250 to 1500 nm were prepared by magnetron co-sputtering in this work. The size-dependent microstructure and mechanical behaviors including the hardness, strain rate sensitivity and activation volume of stacking faulted HEA thin films were systematically investigated. Importantly, the stacking faulted HEA thin films achieved ultrahigh hardness of up to 9.9 GPa at a grain size of 9.4 nm, above which the hardness decreases with increasing grain size or film thickness. Furthermore, the strain rate sensitivity was found to continuously increase with decreasing the grain size or film thickness (which is opposite to the trend of activation volume). The thermally activated model was employed to account for the size and stacking fault dependence of mechanical responses. The findings not only provide insights into understanding the size-dependent mechanical behavior of FCC stacking faulted HEAs, but also offer some clues to achieve their optimized mechanical performance at small scales. [Display omitted] •A series of Al0.1CoCrFeNi HEA thin films with different thicknesses were synthesized by magnetron co-sputtering.•The Al0.1CoCrFeNi HEA thin films exhibit nanocrystalline FCC structure with ample stacking faults.•The stacking faulted HEA thin film achieved ultrahigh hardness of up to 9.9 GPa compared to other related FCC metals.•The Al0.1CoCrFeNi HEA thin films demonstrate strong size-dependent hardness, strain rate sensitivity and activation volume.