High entropy alloys (HEAs) are of considerable interest due to their superior properties. Since 2004, they are defined as quasi- or equimolar alloys and they consist, at least, of five elementary elements with an atomic percentage ranging from 5 to 35 at.% [1-3]. High Entropy Films (HEFs) have been also reported to have excellent properties such as good wear [4] and corrosion resistance [5] and excellent thermal stability [6]. Rare earths have excellent physical and chemical properties. They play an interesting role to improve the performance of the coatings. The effect of the addition of yttrium Y as a rare earth element, on the microstructure, the mechanical properties and on the thermal stability of TiTaZrHfW refractory film is studied. A series of (TiTaZrHfW)_100-xY_x films were synthesized using magnetron sputtering technique. Y content is varied by changing the discharge current applied to Y target. No change of the films microstructure is observed when Y is added with small amount, however with high Y content new phases are formed. The phase evolution is evaluated by calculating the thermodynamic criteria ΔHmix, ΔSmix, Ω and δ and the result are compared to the experimental analysis. Nanoindentation measurements indicate a degrading of mechanical properties when the film is doped with Y element. The highest values of hardness and Young’s modulus are obtained for TiTaZrHfW film at 9.69 GPa and 111.63 GPa respectively. Moreover, thermally stability of TiTaZrHfW film without and with Y element is investigated.

[1] Yeh,J.-W.; Chen, S.-K.; Lin, S.-J.; Gan, J.-Y.; Chin, T.-S.; Shun, T.-T.; Tsau,C.-H., Chang,S.-Y.; Nanostructured high-entropy alloys with multiple principal elements: novel alloy design concepts and outcomes, Adv. Eng. Mater., 6(5), 299-303 (2004)

[2] Cantor, I. Chang, P. Knight, A. Vincent, Microstructural development in equiatomic multicomponent alloys, Mater. Sci. Eng. A, 375, 213-218 (2004)

[3] W. Zhang, P.K. Liaw, Y. Zhang, Science and technology in high-entropy alloys, Sci. China Mater., 61(1), 2-22 (2018)

[4] Cheng, J. B.; Liang, X. B.; Xu, B. S., Effect of Nb addition on the structure and mechanical behaviors of CoCrCuFeNi high-entropy alloy coatings. Surf. Coat. Tech. 2014, 240, 184-190

[5] Hsueh, H.-T.; Shen, W.-J.; Tsai, M.-H.; Yeh, J.-W., Effect of nitrogen content and substrate bias on mechanical and corrosion properties of high-entropy films (AlCrSiTiZr)_{100− x}N_x. Surf. Coat. Tech. 2012, 206 (19-20), 4106-4112

[6] Sheng, W.; Yang, X.; Wang, C.; Zhang, Y., Nano-crystallization of high-entropy amorphous NbTiAlSiW_xN_y films prepared by magnetron sputtering. Entropy 2016, 18 (6), 226

Thin films of the high entropy alloy HfNbTiVZr have shown promising mechanical properties and interesting charge transfer effects, reducing atomic size mismatch and in turn the lattice distortion δ. Being able to tune the electronic structure is especially interesting for the design of multicomponent carbides, as their desirable properties are a result of their bond character. This, along with general properties of group 4-6 carbides, like ceramic hardness, high wear resistance and ultra-high temperature strength, motivates the investigation of the (HfNbTiVZr)C system.

This study focused primarily on multicomponent carbide (HfNbTiVZr)C thin films with varying carbon concentrations (0–44 at.%), synthesised by non-reactive DC magnetron sputtering. All carbide films exhibit a single solid solution phase with NaCl-type structure and a lattice parameter of approximately 4.53 Å. The hardness increases to 34 GPa compared to 10 GPa for the metallic films. Additionally, phase stability based on films deposited at elevated temperatures (300-700°C), compared with predictions made by CALPHAD methods, will be discussed.

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. Within this work, we report on the phase formation and thermal stability of high entropy metal-sublattice carbides to provide a further insight to a more extensive understanding of the high-entropy effect, according to which, based on the Gibbs-free energy, such materials should be stabilised in the high-temperature regime. Therefore, (Hf,Ta,Ti,V,Zr)C coatings were reactively and non-reactively sputtered from a single powder-metallurgically produced composite target (either metallic or consisting of the respective binary carbides). Reactively sputtered coatings were synthesised using an C₂H₂ – Ar mixture with different C₂H₂/(C₂H₂+Ar) ratios (f_C2H2). After deposition, the coatings were investigated in as-deposited state and after vacuum annealing between 800 and 1200°C. The structure and morphology, the chemical composition, the mechanical properties, and the thermal stability of the coatings were investigated by scanning electron microscopy, X-ray diffraction, and nanoindentation.

The non-reactively sputtered as well as reactively sputtered coatings with f_C2H2= 20 % show a single-phased face-centred cubic (fcc) structure. The hardness for the non-reactively sputtered HESCs is with ~41 GPa higher than that of the reactively sputtered one which exhibits a hardness of 35 GPa. This indicates that due to the use of C₂H₂ also regions of amorphous carbon form, which slightly weaken the coating already in the as-deposited state. After vacuum annealing up to 1200 °C the non-reactively sputtered coatings maintain a hardness of ~40 GPa indicating retarded softening mechanisms due to sluggish diffusion. Additionally, powdered free-standing coating material was investigated by XRD in as deposited state and after vacuum annealing up to 1300 °C. The results of these investigations show, independent of the synthesis route, no phase transformation within the investigated temperature range. This behaviour was also observed in previous studies on different material classes such as nitrides, borides, and oxides indicating a stabilisation due to the high-entropy metal sublattice.