Structural Evolution and Tensile Response of CrCoNi Medium-Entropy Alloy Thin Films

Medium/high entropy alloys (M/HEAs) have prospered in the field of metallurgy since their first discovery, as different combinations of principal elements resulted in unforeseen, yet impressive material properties. Face-centered cubic (fcc) CrCoNi MEA in particular has been highlighted for its superb work-hardening and thermal stability, but its practical use in structural applications is currently limited due to the low yield strength.<br/>In this respect, CrCoNi MEAs with micro/nano-scale grain size and thickness have garnered scientific interest, because these dimensions effectively impede dislocation movement and strengthen the alloy. CrCoNi thin film in particular has demonstrated potential as a superb coating material, ascribed to its high corrosion resistance and hardness. However, the mechanical behavior of M/HEA thin films was mainly investigated by hardness measurement or micropillar compression, which may overlook the adverse effects of nano-scale defects.<br/>Therefore, this study examines the tensile response of submicron-thick CrCoNi films. Sputter-deposited CrCoNi thin films were shaped into freestanding dog-bone-shaped samples via photolithography-based microfabrication. Tensile tests were conducted with a custom-built micro-mechanical tester at a constant strain rate. From transmission electron microscopy (TEM), we discovered that as-deposited films comprise nanocrystalline columnar grains with high-density twin boundaries, stacking faults, and hexagonally-close packed (hcp) phases. Annealing the films at 600 and 700 °C results in detwinning and de-faulting, grain growth, and Cr-rich M₂₃C₆ carbide precipitation. All films tested in this study possess high tensile strengths – 2.4 GPa in the as-deposited samples, and 1.8 GPa in the samples annealed at 700 °C.<br/>Understanding the characteristics of equiatomic CrCoNi thin films also establishes a foundation for investigating non-equiatomic CrCoNi films. Delving into non-equiatomic variants of M/HEAs can pinpoint the effect of each principal element and potentially enhance important mechanical properties. This process could be time-consuming with bulk-scale characterization, but small-scale characterization can significantly accelerate sample fabrication and mechanical testing. This study also includes high-throughput characterization of Ni-rich CrCoNi thin film with a compositional gradient via the membrane deflection experiment (MDE), offering an overview of the film’s tensile behavior at different chemical compositions.