Since their early development, High-Entropy Alloys have fueled the investigation of exotic metal combinations. Here, we present a strategy for the rational design of a library for multi-component alloys based on six hcp-structured metals. Seven five- and six-component equimolar alloys based on Co, Gd, Y, Sc, Ti and Zr were prepared via induction melting and characterized by PXRD, SEM–EDX and Vickers hardness. They all present ternary hexagonal phases (ScTiZr or GdScY) co-existing with one or more cubic phases and intermetallic compounds. Both ScTiZr and GdScY appear promising as the starting point for new single-phase High-Entropy Alloys families

A. Takeuchi

C-H. Tsau

D. B. Miracle

I. Toda-Caraballo

J. O. A. Paschoal

K. M. Youssef

K. V. Yusenko

L. Rogal

M. C. Gao

M. Feuerbacher

M. Hellenbrandt

N. Mattern

N. P. Lavery

S. G. R. Brown

S. Riva

S. Riva, 

U. Dahlborg

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Scandium-Based Hexagonal-Closed Packed Multi-Component Alloys

Nicholas Lavery

Steve Brown

Cronfa at Swansea University

 Cronfa -  Swansea University Open Access Repository   _____________________________________________________________   This is an author produced version of a paper published in:Physics of Metals and Metallography                                                Cronfa URL for this paper:http://cronfa.swan.ac.uk/Record/cronfa37646_____________________________________________________________ Paper:Riva, S., Lavery, N., Yusenko, K. & Brown, S. (2018).  Scandium-based hexagonally-closed packed multi-componentalloys. Physics of Metals and Metallography(8)       _____________________________________________________________  This item is brought to you by Swansea University. Any person downloading material is agreeing to abide by the termsof the repository licence. Copies of full text items may be used or reproduced in any format or medium, without priorpermission for personal research or study, educational or non-commercial purposes only. The copyright for any workremains with the original author unless otherwise specified. The full-text must not be sold in any format or mediumwithout the formal permission of the copyright holder. Permission for multiple reproductions should be obtained from the original author. Authors are personally responsible for adhering to copyright and publisher restrictions when uploading content to therepository. http://www.swansea.ac.uk/library/researchsupport/ris-support/  ФИЗИКА МЕТАЛЛОВ И МЕТАЛЛОВЕДЕНИЕ  1  НАЗВАНИЕ РАЗДЕЛА  УДК Scandium-based hexagonally-closed packed multi-component alloys Sephira Riva, Stephen G.R. Brown, Nicholas P. Lavery, and Kirill V. Yusenko College of Engineering, Swansea University, Bay Campus, Swansea SA1 8QQ, Wales, UK e-mail: k.yusenko@swansea.ac.uk Since their early development, High-Entropy Alloys have fueled the investigation of exotic metal combinations. Here, we present a strategy for rational design of a library for multi-component alloys based on six hcp-structured metals. Seven five- and six-component equimolar alloys based on Co, Gd, Y, Sc, Ti and Zr were prepared via induction melting and characterized by PXRD, SEM –EDX and Vicker’s hardness. They all present ternary hexagonal phases (ScTiZr or GdScY) co-existing with one or more cubic phases and intermetallic compounds. Both ScTiZr and GdScY appear promising as the starting point for new single-phase High-Entropy Alloys families. Keywords: High-entropy alloys; Hexagonal close-packed alloys; Scandium.  INTRODUCTION High-Entropy Alloys (HEAs) are often defined as multi-principal component alloys containing more than 5 elements in 5-35 at.%. While this composition-based definition gives no indication on the structure or scope of the alloy, the search for HEAs coincides in most cases with the search for multicomponent single-phase solid solutions [1, 2]. Over 99 % of reported HEA consist of cubic (bcc or fcc) single or duplet co-existing phases. On the other hand, hcp-structured HEAs are still an unexplored field. This is partly because previous research has tackled only a small portion of the composition space, focusing mostly on some transition or refractory metals [3]. The preparation of hcp-structured HEAs has been mostly driven by uncommon elements, especially those belonging to the lanthanides series. Dy20Gd20Lu20Tb20Y20 and Dy20Gb20Lu20Tb20Tm20 HEA consist of an hcp alloy plus an unknown phase [4]. Two-phase hcp structures were reported in the six-component Hf20La20Sc20Ti20Y20Zr20 alloy [5]; one of these was isolated as the equiatomic Hf25Sc25Ti25Zr25 [6]. Exceptions are the Co25Fe25Ni25Ti25 alloy [7], which is a mixture of hcp and fcc phases, and the predicted Co25Os25Re25Ru25 alloy [2]. So far, the only single phase hcp HEAs are AlLi0.5MgScTi1.5, produced via mechanical milling followed by high temperature annealing [8], and Dy20Gd20Ho20Tb20Y20, synthesized via arc melting from pure metals [9]. Phase pure three- and four-component alloys have also been reported (MoRhRu and MoPdRhRu); but, using the composition definition, strictly speaking they cannot be considered as HEAs [10]. The idea of combining hcp-structured elements to investigate the formation of hcp alloys in multi-principal component alloys may seem naïve, but the State of the Art of hcp HEA highlights that even pan-hexagonal metallic compositions usually form poly-phase mixtures of hcp-alloys or intermetallic compounds. Furthermore, as in the case of AlLi0.5MgScTi1.5, melting temperatures of individual metals are sometimes so different that mechanical alloying substitutes more traditional synthesis routes.  The aim of this study was to prepare a combinatorial library of multi-principal component alloys containing only high-temperature melting hcp-structured metals through induction melting and then identify hexagonally-structured single-phase solid solutions. The chosen metals, combined in equal atomic concentrations, were Co, Gd, Sc, Ti, Y and Zr. EXPERIMENTAL The target alloys were prepared using induction melting. Co, Ti and Zr were taken as powders or rods, while Gd, Sc and Y as pieces. All metals were purchased from Goodfellows at ≥ 99% purity, mixed, placed in a BN crucible (Kennametal) and melted using an induction coil in a glove-box operated under Ar pressure (scrubbed by catalysts to O2 levels below 500 ppm) to protect metals from high-temperature oxidation. In all cases, complete melting was achieved above 1600 °C. After 5 minutes at the melting temperature the samples were cooled down naturally to room temperature (2 minutes). All as-cast materials were annealed in a dynamic vacuum (10-2 Pa) at 900 °C for 12 h, followed by 2 hours of natural cooling to room temperature.   2 For characterization, all samples were mounted in carbonized resin, polished using MetaDiTM Supreme Polycrystalline Diamond Suspension (1 μm) and chemically etched with a solution of HNO3 5 vol.% in ethanol. Morphology and elemental composition were analyzed using a Hitachi S-4800 Field Emission scanning-electron microscope (SEM) equipped with energy dispersive X-ray (EDX) analyzer. The average elemental composition was obtained from 0.5×0.5 mm maps. Vickers hardness was investigated on a WilsonR VH3100 Automatic Knoop/Vickers Hardness tester: 25 individual points under a 9.81 N (1 kg) testing load were measured to get statistically significant results. Powder X-ray diffraction patterns were collected from the powdered samples. Powders were fixed between two Kapton foils and measured at room temperature in transmission mode at the ID06A beam-line at the European Synchrotron Radiation Facility (position sensitive detector, λ = 0.22542 Å). Phase identification was performed via automated and manual search-match through the using the Inorganic Crystal Structure Database [11].   RESULTS AND DISCUSSION  Seven High-Entropy Alloys containing 5 or 6 hcp-structured elements were prepared by induction melting (Table 1). All samples are multiphase (Figure 1) and contain at least one hcp phase (Figure 2). Due to the large difference in atomic radius and electronegativity among the elements, only a few of them form solid solutions while several form stable intermetallic compounds [12-14].  To examine temperature-induced effects in melted compositions and equilibrated systems, as-cast and thermally annealed samples (900 °C, 12 hours in dynamic vacuum 10-2 Pa) were investigated. All samples appear to be multi-phase both before and after annealing (see Figure 1), with annealing affecting microstructure and mechanical properties of the alloys, but having low impact on element distributions. Elemental distribution maps obtained by EDX analysis suggest the following phases and elemental separation in the samples. For Co20Gd20Sc20Ti20Y20 (figure 1a) Co, Sc and Ti segregate from Gd and Y. Co20Gd20Sc20Ti20Zr20 (figure 1b) displays a Co—Ti—Zr matrix and Gd-rich areas; scandium is homogeneously distributed. In Co20Gd20Sc20Y20Zr20 (figure 1c), Co—Sc—Zr regions segregate from the matrix, surrounded by Gd and Y.   Fig. 1. SEM images of the as-cast (orange, left) and annealed (red, right) alloys. Element distributions of different areas as from EDX maps are highlighted.   3  For Co20Gd20Ti20Y20Zr20 (figure 1d), Y is homogeneously dispersed. After annealing Gd-Y and Co—Ti—Y—Zr areas are formed. In the case of the Co20Sc20Ti20Y20Zr20 (figure 1e) alloy, areas rich in Sc, Ti and Zr dissolve during heat treatment, forming a matrix in which Sc—Y-rich structures grow. Gd20Sc20Ti20Y20Zr20 (figure 1f) presents a Gd—Y matrix and Sc—Ti—Zr alloy. Finally, the six-elements Co16Gd16Sc16Ti16Y16Zr16 (figure 1g) alloy displays two main areas: Co—Sc—Ti—Zr and Gd—Sc—Y. In the six-component alloy the two phases ScTiZr and GdYSc co-exist and scandium is homogeneously dispersed in the matrix. We can therefore deduce that the formation of a scandium-containing single-phase solid solution is deeply affected by the balance between atomic size and chemical similarity. If the former prevails, scandium mixes preferably with Ti and Zr. If chemical similarity between scandium and rare-earth metals dominates, Sc dissolves in Y and Gd. The formation of the ScTiZr solid solution is consistent to the reported Hf25Sc25Ti25Zr25 single-phase hcp alloy [6], whereas, to the authors’ knowledge, the GdYSc solid solution has never been used as the starting point for the development of a multi-component single-phase system. To do so, computational modelling based on binary and ternary phase diagrams will be an essential requirement [15,16]. Nevertheless, knowledge about hcp-metals ternary phase diagrams is still fragmented: the intrinsic complexity of some of these binary phase diagrams, which contain a huge variety of intermetallic compounds, poses a challenge to the exploration of the hyper-dimensional composition space. To the authors knowledge, out of all possible combinations with Co, Gd, Sc, Ti, Y and Zr, only an isothermal section (T =773 K) of the Co-Gd-Ti phase diagram has been investigated [17]. Yttrium disperses in the matrix homogeneously only in the absence of scandium. This is an unexpected result, because yttrium and scandium are often considered chemically interchangeable and because the binary Y-Sc phase diagram displays complete miscibility in both solid and liquid states. This suggests that the addition of Y to ScTiZr would not lead to a single-phase four-component alloy, but rather to the segregation of Sc—Y-rich areas in a ScTiZr matrix. The PXRD patterns of the as-melted and annealed alloys are very complex and show several crystalline phases (Figure 2 and Table 2). The broad diffraction Table 1. Parameters of elements adopting hcp-structure used to produce the HEA library [14]. Element Atomic number Atomic radius, Å Lattice constant, Å Pauling electronegativity Tm, °C a c Cobalt 27 1.25 2.5061 4.0695 1.7 1495 Gadolinium 64 1.79 3.6330 5.7739 1.2 1312 Scandium 21 1.64 3.2899 5.2529 1.3 1539 Titanium 22 1.46 2.9509 4.6826 1.5 1670 Yttrium 39 1.81 3.6474 5.3706 1.2 1526 Zirconium 40 1.60 3.2312 5.1477 1.4 1850   Fig.2 Powder X-Ray Diffraction patterns ( = 0.22542 Å) of the as-cast (black) and annealed (red) alloys. Lines corresponding to primary and secondary phases are marked.   4 lines characteristic for hcp-structured alloys might mask several phases with close cell parameters [18].  In all cases an hcp-structured alloy has been detected as the main phase, with two or more cubic phases (Au3Cu and B2 structure types) as minor admixtures. The PXRD profile for Co16Gd16Sc16Ti16Y16Zr16 (Figure 2g) also shows lines corresponding to the CoSc intermetallic phase (Pm m, a = 3.435). For Co20Gd20Sc20Y20Zr20 heat treatment results in the dissolution of the secondary hcp phase (probably pure Gd, which goes to enrich the GdYSc phase), which could explain its low decrease in hardness in comparison with other alloys. Other lines in the profiles are too broad to be indexed. As far as mechanical properties are concerned (Table 2), Vicker’s hardness values refer to the alloys overall compositions and microstructures. Such values are lower than those found in the literature for known HEAs, and generally drop after heat treatment. Exceptions are the Co20Sc20Ti20Y20Zr20 alloy, which shows remarkably high Vicker’s hardness after annealing (825(47) HV), and the Co20Gd20Ti20Y20Zr20 alloy, which displays a 50 % hardness enhancement. The increase in hardness might be due to the presence of the same intermetallic, of crystal structure Im m (a = 3.44(3)). All alloys present high brittleness and crack formation under relatively low load.  CONCLUSIONS The design of a combinatorial library of multi-principal elements alloys from hcp metals highlights that the formation of single-phase solid solutions, achievable as an fcc or bcc phase when only cubic metals are involved, is hindered by the formation of intermetallics in the case of hcp metals. The creation of a single-phase hcp-structured HEA from hcp-structured metals appears to be impossible. Although multicomponent hcp alloys are formed, cubic intermetallics as secondary phases are also present.  The possibility of using three-component scandium- or yttrium-containing alloys as the starting point for High-Entropy Alloys is discussed. The formation of the ScTiZr solid solution is consistent with the reported Hf25Sc25Ti25Zr25 single-phase hcp alloy [6], while the GdYSc solid solution shows the potential to become the basis for the development of a multi-component single-phase hcp alloying system. This suggests that phase diagrams are not enough to predict the behaviour of chemically similar metals in complex systems, electronic density and atomic size should also be considered for the design of new multi-principal alloy combinations.  Table 2. Phase composition from synchrotron PXRD profiles and Vicker’s hardness values (1 HV load) before and after annealing. Alloy composition Main phase (P63/mmc) Cell parameters, Å Secondary phase(s) cell parameters, Å Vicker’s Hardness, HV   As melted Annealed Co20Gd20Sc20Ti20Y20 a = 3.62(2); c = 5.70(3) unindexed phase(s) 218(28)1 186(29) Co20Gd20Sc20Ti20Zr20 a = 3.61(2); c = 5.71(3) P63/mmc (a = 3.03; c = 4.80) Pm m (a = 3.96) 345(35) 318(145)2 Co20Gd20Sc20Y20Zr20 a = 3.62(3); c = 5.71(1) Im m (a = 3.20(1)) P63/mmc (a = 3.05(2); c = 5.00(3)) 257(15) 233(41) Co20Gd20Ti20Y20Zr20 a = 3.65(7); c = 5.76(4) Pm m (a = 3.94(5)) Im m (a = 3.44(3)) 251(72) 429(85) Co20Sc20Ti20Y20Zr20 a = 3.62(9); c = 5.71(2) P63/mmc (a = 3.02(2); c = 4.90(3)) Pm m (a = 3.97(1)) Im m (a = 3.44(3)) 358(23) 825(47) Gd20Sc20Ti20Y20Zr20 a = 3.59(3); c = 5.66(3) Pm m (a = 4.15(3)) 289(22) 174(22) Co16Gd16Sc16Ti16Y16Zr16 a = 3.62(2); c = 5.70(3) Pm m (a = 3.97) CoSc Pm m (a = 3.435(1)) 269(31)1 188(38) 1 load of 0.2 HV; 2 load of 0.3 HV   5 REFERENCES  1. Yeh J-W. Overview of High-Entropy Alloys. 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Dahlborg U., Cornide J, Calvo-Dahlborg M., Hansen T.C., Fitch A., Leong Z., Chambreland S, Goodall R. Structure of some CoCrFeNi and CoCrFeNiPd multicomponent HEA alloys by diffraction techniques // J All Compd 2016. V. 681. P. 330-341.  ACKNOWLEDGEMENTS  The Authors gratefully acknowledge the financial support provided by the Welsh Government and Higher Education Funding Council for Wales through the Sêr Cymru National Research Network in Advanced Engineering and Materials and by the European Space Agency (contract number 4000111643/NL/PA). KVY is grateful to the EPSRC Impact Acceleration Account for financial support. The authors thank the Materials Advanced Characterisation Centre (MACH1) at Swansea University and the European Synchrotron Radiation Facility (Grenoble, France) for providing measurement time and technical support. 

Scandium-based hexagonally-closed packed multi-component alloys

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Scandium-based hexagonally-closed packed multi-component alloys

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Cronfa at Swansea University