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Thin films of the CrMnFeCoNi high entropy alloy were deposited by magnetron sputtering from a sintered equimolar target. The substrate temperature and bias were varied during deposition, and the structure, morphology and elemental distribution were studied in detail. All films formed phase mixtures of multiple crystal structures. This contrasts with studies on the bulk alloy, where it typically forms a single phase with a simple cubic closed packed (ccp) structure, with other phases precipitating only after long annealing times. For higher substrate temperatures, we observed a mixture of phases with ccp and bcc (body centered cubic) structures, and the intermetallic phases o-phase and L1(0), the first three being the predicted equilibrium phases at the deposition temperature. For room temperature depositions, we found evidence of very limited diffusion of metal atoms during the deposition. These films formed a mixture of a ccp and the intermetallic chi-phase. Two mechanisms can be distinguished that govern the phase formation at lower and higher temperatures. From the present results and comparisons with the literature, we also discuss why the small grain size, the low process temperature, and the fast surface diffusion during synthesis causes magnetron sputtering to yield different results compared to bulk synthesis from the melt. These principles explain why it is easier to form the equilibrium phases by sputtering, and why a single ccp phase should not be expected as a rule for this deposition method. Following the thermodynamic principles of high entropy alloys, this may also be the case in other high entropy alloy systems.

This study explores the influence of a 316L stainless steel substrate on the magnetron sputtering of the Cantor alloy CoCrFeMnNi at different substrate bias. The study was carried out on a polycrystalline 316L substrate where the growth behavior of the coating could be investigated on grains with different orientations. By combining electron backscatter diffraction (EBSD) before and after deposition and characterization of the same area, it was possible to determine growth behaviour and surface morphologies on individual substrate grains. No strong influence of the substrate was observed at a floating bias. At a bias of -100V, however, the coating was strongly influenced by the orientation of the individual substrate grains.  Epitaxial coating grains with a smooth surface were observed on the [102]-oriented grains while a more columnar growth was observed on [111]-oriented grains.  Furthermore, a small difference in growth rate was observed on different substrate orientations. The growth behaviour could be related to differences in surface energies and diffusion rates on different surface orientations.

This study provides principles for designing new corrosion resistant high entropy alloys. The theoretical framework is a percolation model developed by Newman and Sieradzki that predicts the ability of an alloy to passivate, i.e., to form a protective surface oxide, based on its composition. Here, their model is applied to more complex materials than previously, namely amorphous CrFeNiTa and CrFeNiW alloys. Furthermore, the model describes a more complex passivation process: reforming the oxide layer above the transpassive potential of Cr. The model is used to predict the lowest concentration of Ta or W required to extend the passive region, yielding 11–14 at% Ta and 14–17 at% W. For CrFeNiTa, experiments reveal a threshold value of 13–15 at% Ta, which agrees with the prediction. For CrFeNiW, the experimentally determined threshold value is 37–45 at% W, far above the predicted value. Further investigations explore why the percolation model fails to describe the CrFeNiW system; key factors are the higher nobility and the pH sensitivity of W. These results demonstrate some limitations of the percolation model and offer complementary passivation criteria, while providing a design route for combining the properties of the 3d transition metal and refractory metal groups.

The properties of high entropy alloys (HEAs) are strongly affected by the addition of carbon past the solubility limit. Despite this is the local chemistry in these meta-stable materials not well-characterized. To better understand how carbon affects the elemental distribution of alloys whose constituent elements have widely varying carbon affinities, this paper studies amorphous sputter-deposited coatings of CoCrFeMnNi with concentrations of up to 11\% carbon. Hard x-ray photoelectron spectroscopy (HAXPES), near-edge x-ray absorption fine structure (NEXAFS), and transmission electron microscopy (TEM) were used to determine how each metallic element is affected by the presence of carbon. As-deposited samples are also compared to annealed samples to study the thermal stability and the Calphad method was used to predict the thermodynamic equilibrium state. All five component metals had weak interaction with carbon, including Ni which had a less metallic character in the carbon-containing samples. While elemental segregation is expected at all temperatures at thermodynamic equilibrium, carbon did not promote segregation in the as-deposited samples. During annealing, however, the elements rearranged and formed a mixture of alloy phases and crystalline Cr-rich carbides. Rearrangement of the elements also occurred in the surface oxide, where Mn became dominant. The combination of techniques to characterize HEAs revealed promising trends for future research into these important materials.

This study explores carbon addition as a materials design approach for simultaneously improving the hardness, crack resistance, and corrosion resistance of high entropy thin films. CoCrFeMnNi was selected as a starting point, due to its high concentration of weak carbide formers. The suppression of carbides is crucial to the approach, as carbide formation can decrease both ductility and corrosion resistance. Films with 0, 6, and 11 at.% C were deposited by magnetron co-sputtering, using a graphite target and a sintered compound target. The samples with 0 at.% C crystallized with a mixture of a cubic closed packed (ccp) phase and the intermetallic χ-phase. With 6 and 11 at.% C, the films were amorphous and homogenous down to the nm-scale. The hardness of the films increased from 8 GPa in the carbon-free film to 16 GPa in the film with 11 at.% C. Furthermore, the carbon significantly improved the crack resistance as shown in fragmentation tests, where the crack density was strongly reduced. The changes in mechanical properties were primarily attributed to the shift from crystalline to amorphous. Lastly, the carbon improved the corrosion resistance by a progressive lowering of the corrosion current and the passive current with increasing carbon concentration.

The corrosion resistances of near equimolar CoCrFeMnNi magnetron-sputtered thin films with different carbon concentrations were examined in 0.05 M HCl and 0.05 M H2SO4. Polarization curves were recorded with different scan rates with and without reducing the native oxide. The results showed that the carbon concentration and the experimental conditions affected the electrochemical behaviour mainly in the Cr transpassive region. At potentials above 850 mV, the carbon-containing films were more corrosion resistant in 0.05 M HCl than in 0.05 M H2SO4 due to a lower carbon oxidation rate in 0.05 M HCl, facilitating the formation of a Mn-rich oxide layer. (C)& nbsp;2021 The Author(s). Published by Elsevier Ltd.& nbsp;

The synthesis of metal/carbon nanocomposites through magnetron sputter deposition has been explored for a large number of metals and a few alloys. In this study, the concept is expanded to high entropy alloys, which are systems where many metals are mixed with near-equimolar ratios. Two alloy systems were compared to study the effect of the composition in a complex alloy system; the equimolar CoCrFeMnNi and the near-equimolar Cr26Fe27Ni27Ta20. The two systems had different average carbon affinities. Carbon was added through co-sputtering to concentrations of around 20-50 at%. Thermodynamic calculations using the CALPHAD method predicted a separation into multiple alloy phases, Cr-rich carbides, Ta carbide, and, at higher concentrations, graphite. In the lower carbon concentration range, both material systems formed, instead, single-phase amorphous alloys. At higher carbon concentrations, a phase mixture with a metallic phase and a phase consisting of sp2- and sp3-hybridized carbon. This can be described as an alloy/amorphous carbon nanocomposite. The mechanical properties were investigated with multiple methods, including tensile tests on polyimide strips. The results were compared to a reported result from the literature where the formation of a nanotubular FeCrNi/a-C:H nanocomposite improved both hardness and toughness. In the present case, the microstructure was more disordered, and the nanocomposites had both lower hardness and lower crack resistance than the amorphous alloys. Lastly, electrochemical tests showed that the free carbon neither degraded nor largely improved the corrosion resistance in 0.05 H2SO4 at potentials up to 0.7 V vs Ag/AgCl