Tuning nanostructure and mechanical property of Fe-Co-Ni-Cr-Mn high-entropy alloy thin films by substrate temperature

Improving the mechanical property of high-entropy alloys (HEAs) with face-centered cubic (fcc) structure is critical for their potential applications. In this study, the effects of substrate temperature, T-sub, on nanometer-sized structure and mechanical property of fcc-structured Fe-Co-Ni-Cr-Mn HEA thin films prepared by magnetron sputtering were systematically investigated. All films grow in a nanocolumnar manner and display a single fcc-structured solid solution phase with (111) preferential orientation in addition to the reduced amorphous phase fraction and increased average grain size with increasing T-sub. The top-surface cauliflower-like hierarchical microstructure with some voided boundaries for 293 K-film gradually disappears with increasing T-sub up to 573 K. For higher T-sub (673 K-773 K), the obvious grain growth occurs with significantly increased column size and roughness. The variations in normal hardness, H, and Young's modulus, E-s, obtained from nanoindentation tests on the surface of Fe-Co-Ni-Cr-Mn HEA thin films experienced a process of first increasing and then decreasing with T-sub,T- with a maximum at T-sub = 573 K. The former increase is due to enhanced interfacial adhesion and the later decrease is ascribed to the weakened grain boundary strengthening caused by grain growth. The 573 K-film with a nanocomposite structure including hybrid nanocrystalline and amorphous phase exhibits the highest H (similar to 10.0 GPa) and E-s (similar to 185.5 GPa) and is the strongest in terms of hardness among reported Fe-Co-Ni-Cr-Mn HEAs without post-treatment. In-situ micro-tensile tests alone the surface of Fe-Co-Ni-Cr-Mn HEA thin films uncovered the higher lateral yield strength (similar to 1.1 GPa), fracture strength (similar to 1.4 GPa), and Young's modulus (similar to 64.3 GPa) for 573 K-film than those of 293 K- and 773 K-films as well, consistent with nanoindentation results. Anisotropic mechanical responses normal to and along the surface were clearly found in Fe-Co-Ni-Cr-Mn HEA thin films, which are caused by the nanocolumnar morphology and the weakened columnar boundaries adhesion compared to the column itself. (C) 2021 Elsevier Ltd. All rights reserved.