Regular article The nanostructure and mechanical properties of nanocomposite Nb x -CoCrCuFeNi thin films B.R. Braeckman a , F. Misják b , G. Radnóczi b , M. Caplovicová c , Ph. Djemia d , F. Tétard d , L. Belliard e , D. Depla a, ⁎ a Department of Solid State Sciences, Ghent University, Krijgslaan 281 (S1), 9000 Gent, Belgium b Centre for Energy Research, Hungarian Academy of Sciences, H-1525 Budapest, P.O. Box 49, Hungary c TU Centre for Nanodiagnostics, University Science Park Bratislava Centre, Slovak University of Technology, Vazovova 5, 81243 Bratislava, Slovakia d Laboratoire des Sciences des Procédés et des Matériaux (LSPM)-UPR 3407 CNRS, Université Paris 13, Sorbonne Paris Cité, 99 Avenue J.B. Clément, 93430 Villetaneuse, France e UPMC, Institut des NanoSciences de Paris UMR 7588, 4 Place Jussieu, 75252 Paris Cedex 05, France abstract article info Article history: Received 16 May 2017 Accepted 24 June 2017 Available online xxxx The relation between the nanostructure and the mechanical properties of Nb x -CoCrCuFeNi high entropy alloy thin films was explored. With increasing Nb concentration (0 to 24 at.% Nb), a transition from a single phase face-centered cubic solid solution to an amorphous phase is observed. At intermediary Nb fractions (5 to 15 at.% Nb) a nanocomposite structure is formed that consists of nanosized crystallites embedded in an amor- phous matrix. The nanocomposite structure leads to an increase in hardness beyond the Hall-Petch breakdown. The shear and Young's moduli decrease with increasing Nb concentration, which is beneficial for the alloy ductility. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Sputtering Thin films Nanocomposites High-entropy alloys The mechanical strength of polycrystalline materials is mainly deter- mined by the difficulty for dislocations to propagate through the mate- rial. The addition of “obstacles” that hinder this propagation strength can be decreased. For example, the modification of the average grain size and size dispersion, the incorporation of extra solute elements in solid solutions, and the segregation of secondary phases at the grain boundaries are common techniques to strengthen metals and alloys. These strengthening principles were applied in the present study to a range of Nb x -CoCrCuFeNi high entropy alloys (HEA) thin films. HEAs are multi-component alloys that exhibit a high mixing entropy [1–2]. These alloys usually consist of at least five different metals in near- equiatomic proportions, and they introduce an exciting perspective to materials science. Ceramic HEA thin films (e.g. carbides [3] and nitrides [3,4–7] exhibit promising properties such as high thermal and chemical stability, and a high hardness. These properties originate from the strong covalent bonds between the metallic constituents and the non- metal (e.g. carbon or nitrogen) atoms. In the present study, the nanocrystalline Nb x -CoCrCuFeNi thin films were synthesized by sputtering compacted powder targets. The deposi- tion conditions have been previously described [8]. The samples exam- ined in the present study were deposited at a pressure-distance product of 3.6 Pa·cm. Transition electron microscopy (TEM) was used to study the film nanostructure. The elastic properties were determined with acoustic techniques, i.e. Brillouin light scattering (BLS), and picosecond ultrasonics (PU). The hardness was determined by nanoindentation. More information on these techniques and the corresponding measure- ment settings can be found in our previous work [9,10]. Fig. 1 shows the cross section bright field HR-TEM images, and the electron diffraction patterns of the deposited thin films. The electron patterns confirm previously published XRD results. The CoCrCuFeNi base alloy exhibits a single phase face centered cubic (FCC) solid solu- tion. With an increase in the Nb content, the diffraction rings are broad- ened, and the higher order diffraction rings eventually vanish. This agrees with the decrease in the intensity, and the increase in the FWHM of the XRD Bragg peaks of the FCC solid solution with increasing Nb concentration. In the XRD study, only one diffraction peak is ob- served for samples with Nb content of 9.6 at.% and higher. The bright field images reveal the presence of nanocrystallites in the film with 0, 5, 7, and 9.6 at.% Nb. The TEM images of the films with 15.2 and 24.3 at.% are basically amorphous. A thorough HREM investigation of the 15.2 at.% sample revealed nanocrystallites of 2–3 nm in size, and with a lattice spacing corresponding to the observed FCC phase in sam- ples with a lower Nb content. These crystallites are embedded into an amorphous matrix. It is also observed that the size and the number den- sity of the crystallites decrease for increasing Nb concentration. A com- parison between both grain sizes (as determined from HRTEM, and XRD) is shown in Fig. 1(f). For XRD the grain size was calculated with the Scherrer's equation, and a continuous decrease with increasing Nb concentration is observed. The HREM analysis shows more or less the same trend. The origin of this behavior can be understood from previous studies [9,11]. Indeed, HEAs of the type X-CoCrCuFeNi, with X a Scripta Materialia 139 (2017) 155–158 ⁎ Corresponding author. E-mail address: diederik.depla@ugent.be (D. Depla). http://dx.doi.org/10.1016/j.scriptamat.2017.06.046 1359-6462/© 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Contents lists available at ScienceDirect Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat