Ultrahigh specific hardness of Co-Ni-V-Al medium entropy alloy thin films

Abstract

The high-performance alloy films deposited at low-cost substrates could be a cost-effective way to incorporate these materials of superior properties in harsh environment. Here, we report successful synthesis of body-centered cubic (BCC) dominated dual-phase Co-Ni-V-Al medium-entropy alloy (MEA) thin films prepared by magnetron sputtering at 298 K and 573 K, respectively, different from the equilibrium solidification products of face-centered cubic (FCC) matrix and B2 nanoprecipitates with 6% volume fraction. The 573 K-film exhibits an average hardness and compressional yield strength of 16.8 GPa and 5.3 GPa, and its specific hardness (hardness/density) is 2.249 GPa·cm3·g−1, to be the highest value among all reported high entropy alloys (HEAs). In addition, the films with a thickness of 950 ± 50 nm have an average particle size and surface roughness of 45.7 ± 9.8 nm and 1.53 ± 0.05 nm for 298 K-films and 46.5 ± 10.4 nm and 1.46 ± 0.05 nm for 573 K-films, respectively. The high hardness and yield strength of the 573 K-films are attributed to the tighter bonding of particles, the smaller grain size of both FCC and BCC phase, and collaborative deformation between BCC and FCC phase, suggesting that these HEA films could be beneficial for light-weight and high-strength material industrial applications.