ICMCTF 2023 Session F2-ThA: High Entropy and Other Multi-principal-element Materials

Based on a multi-principal-element-design concept, high-entropy alloys (HEAs) feature enhanced mechanical strength, excellent thermal stability, and superior wear and corrosion resistance, making them promising candidates for applications in harsh environments. Moreover, the microstructure and properties of HEAs can be fine-tuned by changing their composition or by alloying them with additional elements, boosting their application potential even further. Therefore, HEA-based protective films with suitably designed compositions could improve the wear and corrosion resistance of low-cost substrate materials under various service conditions. In this work, (CoCrFeNi)_100−xMo_x HEA thin films were deposited on Si and AISI 304 stainless steel substrates by high-power impulse magnetron sputtering (HiPIMS) using pure Mo and equiatomic CoCrFeNi targets. The effect of Mo content on the microstructure, mechanical properties, and corrosion and tribocorrosion behavior of the films was investigated by adjusting the Mo target-to-substrate distance. Higher amounts of Mo favored the formation of a dense microstructure, resulting in higher hardness values. The potentiodynamic polarization curves revealed that the corrosion resistance of the films in 3.5 wt.% NaCl solution is not significantly affected by Mo concentration. Differences were found, however, in terms of tribocorrosion behavior. With increasing Mo content up to 12 at.%, the wear rate of the films gradually decreased. Above this value, further Mo addition resulted in increased material loss. The obtained results aim to contribute to a comprehensive understanding of the corrosion and tribocorrosion behavior of HEA thin films in artificial seawater to broaden their application in marine engineering.

Multicomponent materials have opened a vast landscape for complex compositions and interesting properties. They exhibit complex bonding, due to their diverse chemical environment, as a result of the extensive alloying, typically involving 5 or more elements. Ab-initio calculations, using density functional theory (DFT) and X-ray photoelectron spectroscopy (XPS), were used to investigate the electronic structure of multicomponent thin films based on the HfNbTiVZr system. The charge transfer was evaluated theoretically using relaxed, non-relaxed, as well as elemental reference structures using a fixed sphere size model to quantify the charge transfer. High-resolution XPS spectra were obtained from HfNbTiVZr and (HfNbTiVZr)C magnetron sputtered thin films.

The HfNbTiVZr alloy shows core level binding energy shifts and peak broadening, compared with metal references, which are in good agreement with shifts calculated from DFT simulations, where charge transfer between the metal atoms is observed. The charge transfer follows the general electronegativity trend, and results in a reduced atomic size mismatch and lattice distortion δ, compared with estimates based on tabulated atomic radii. Similarly, the (HfNbTiVZr)C carbide films exhibit core level binding energy shifts and broadening effects, as a result of their complex chemical environment.

This study demonstrates the importance of chemical bonding and environment when discussing multicomponent materials. Moving beyond the concept of an ideal solid solution and considering the bonding in more detail opens up a deeper understanding and the possibility of tuning the electronic structure. This would be especially interesting for the design of multicomponent materials, as many desirable properties are a result of their bond character.

High-entropy alloys (HEAs) and high-entropy metal-sublattice ceramics (HESCs) have recently gained particular attraction in the field of materials research due to their promising properties, such as high hardness, high strength, and thermal stability. Especially oxides are of great interest as they can serve as oxidation protection coatings at high temperatures. A very common oxidation-protection coating is α-Al₂O₃, usually synthesised by chemical vapour deposition as very high temperatures are required to form the stable α form. Another interesting oxide for use at high temperatures is HfO₂ as it exhibits the highest melting point within the group of oxides besides Thorium oxide. As HfO₂ crystallises, similar to ZrO₂, in three different crystal structures, the desired cubic high-temperature modification is hard to stabilise. Within this work we report on the phase formation and mechanical properties of (Gd,Hf,Sc,Ti,Zr) oxide coatings. High-entropy metal sublattice oxides were synthesised using reactive magnetron sputtering utilising a single equiatomic target consisting of Gd, Hf, Sc, Ti, and Zr. For synthesis of oxide coatings different O₂/(Ar+O₂) flow ratios were used to investigate the influence of reactive gas flow on the structure and mechanical properties. X-ray diffraction analysis show that the coatings crystallise in a single-phase fcc structure. Nanoindentation measurements revealed hardness values of ~20 GPa and an indentation modulus of ~ 250 GPa. Furthermore, transmission electron microscopy investigations were done to get more detailed information about the growth morphology and crystal structure of the oxide coatings. To investigate the structural stability at elevated temperatures, vacuum annealing treatments and subsequent XRD and nanoindentation measurements were done.