Electrochemical conversion of oxide spinels into high-entropy alloy

Article abstract of DOI:10.1016/j.jallcom.2018.10.171

Single-phase equiatomic CoCrFeNi high-entropy alloy has been synthesised by direct electrochemical reduction from oxide precursors according to the concept of the FFC-Cambridge process. Metal oxide mixtures were heat treated in air to provide a blend of spinels, which was then employed as the cathode in the electro-deoxidation process. Melts of eutectic CaCl₂-NaCl and pure CaCl₂ were used as the electrolytes. Varying the experimental conditions in terms of processing temperature and time yielded CoCrFeNi alloys of diverse morphologies, ranging from fine dispersed powders to solid three-dimensional objects, as they are of relevance for additive manufacturing and near-net-shape fabrication. Notably, all solid alloy samples exhibited a higher hardness than calculated by the rule of mixture, despite the presence of significant residual porosity. The results highlight the versatility of the FFC-Cambridge process for alloy preparation and suggest the possibility of economically viable large-scale manufacture of steel-type high-entropy alloys.

Full text of DOI:10.1016/j.jallcom.2018.10.171

Accepted Manuscript
Electrochemical conversion of oxide spinels into high-entropy alloy
Jagadeesh Sure, D. Sri Maha Vishnu, Carsten Schwandt
PII:
S0925-8388(18)33845-3
10.1016/j.jallcom.2018.10.171
JALCOM 47989
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 23 July 2018
Revised Date: 29 September 2018
Accepted Date: 13 October 2018
Please cite this article as: J. Sure, D. Sri Maha Vishnu, C. Schwandt, Electrochemical conversion
of oxide spinels into high-entropy alloy, Journal of Alloys and Compounds (2018), doi: https://
doi.org/10.1016/j.jallcom.2018.10.171.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Electrochemical Conversion of Oxide Spinels into High-Entropy Alloy
Jagadeesh Sure1,2,*, D. Sri Maha Vishnu1,2, Carsten Schwandt1,2
1Department of Materials Science and Metallurgy, University of Nizwa,
Birkat Al Mouz, 616 Nizwa, Sultanate of Oman
2Department of Materials Science and Metallurgy, University of Cambridge,
27 Charles Babbage Road, Cambridge CB3 0FS, United Kingdom
*Tel: +44 1223 331694; E-mail: jagadeesh.sure@unizwa.edu.om, js2195@cam.ac.uk
Abstract
Single-phase equiatomic CoCrFeNi high-entropy alloy has been synthesised by direct
electrochemical reduction from oxide precursors according to the concept of the FFC-
Cambridge process. Metal oxide mixtures were heat treated in air to provide a blend of
spinels, which were then employed as the cathode in the electro-deoxidation process. Melts
of eutectic CaCl2-NaCl and pure CaCl2 were used as the electrolytes. Varying the
experimental conditions in terms of processing temperature and time yielded CoCrFeNi
alloys of diverse morphologies, ranging from fine dispersed powders to solid three-
dimensional objects, as they are of relevance for additive manufacturing and near-net-
shape fabrication. Notably, all solid alloy samples exhibited a higher hardness than
calculated by the rule of mixture, despite the presence of significant residual porosity. The
results highlight the versatility of the FFC-Cambridge process for alloy preparation and
suggest the possibility of economically viable large-scale manufacture of steel-type high-
entropy alloys.
Keywords: High-entropy alloys, Metals and alloys, Chemical synthesis, Microstructure,
Scanning electron microscopy, Microhardness
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Introduction
A new class of equiatomic multi-component alloys was originally prepared by Cantor et al.
[1] and by Yeh et al. [2] in 2004. These alloys are composed of four or more metallic
elements and combine complex multi-element chemistry with simple single-phase
crystallography. They are now commonly termed as high-entropy alloys (HEAs), emphasising
the large contribution of the configurational entropy to their Gibbs free energy [2]. HEAs can
be divided into three categories based on the elements involved. The first category consists
of commodity metals like Fe, Co, Ni, Cr, Mn, Cu, Al and Si [3,4]; the second category involves
refractory metals like Ti, Zr, Hf, V, Nb, Ta, Mo, W and Cr [5-7]; and the third and less known
category centres on rare earth metals like Y, Gd, Tb, Dy and Ho [8]. Consequent of the
simple crystal structure, many HEAs possess unique properties, such as high hardness and
strength [9], as well as excellent wear resistance [10], oxidation resistance [11] and
corrosion resistance [3,4,12]. These properties can be tailored within considerable margins
by changing the composition of the HEAs.
HEAs can be synthesised via liquid, solid and gaseous processing routes [13-15]. The most
important methods used at present are based on arc or induction melting followed by
casting [16-19] or splat quenching [17]. Other methods rely on laser melting [19-22] or
electron beam melting [23-25], and on mechanical alloying followed by spark plasma
sintering [26]. Further options are thermal spraying [27] and magnetron sputtering [28].
Thin films can be created by electro-deposition from organic solvents [29,30], and single
crystals can be formed by Bridgman solidification [31]. All these methods are however
rather sophisticated, requiring advanced equipment and precise process control and mostly
multiple steps. In particular, no method is appropriate for the preparation of large
quantities of fine HEA powders as they are demanded in modern additive manufacturing
technologies such as 3D printing; and likewise, no method is suitable for the preparation of
porous three-dimensional near-net-shape HEA objects as they are required in a range of
structural and biomedical applications.
The FFC-Cambridge process [32] has established itself over recent years as a new
metallurgical route for the preparation of metals and alloys directly from the corresponding
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pure and mixed metal oxides. In this process, the metal oxide powder is made the cathode
versus an anode of typically carbon, and it is polarised such that the oxygen from the oxide
is expelled in the form of O2- ions into an electrolyte based on CaCl2 without the electrolyte
undergoing electrolysis. The method has been verified for the preparation of a wide range
of commodity and reactive metals, including Fe, Cr, Ti, Zr, Hf, Nb, Ta, Mo, W, as well as for
many alloys and intermetallics, including Ni-Ti, Ti-Fe, Ti-Mo, Ti-W, Ti-6Al-4V, Ni-Cr, CeNi5,
LaNi5, NdCo5. The scope and the flexibility of the FFC-Cambridge process regarding
composition and morphology of the oxide precursor and the resulting metal product has
been highlighted in several reviews [33-36]. Specifically with regard to HEAs, the present
authors have already demonstrated the suitability of the FFC-Cambridge process for the
preparation of two refractory HEAs in a recent study [37]. Therein, TiNbTaZr and TiNbTaZrHf
were chosen as the target materials, and it has been shown that these can be made with
uniform composition and homogeneous microstructure at a processing temperature as low
as 1173 K. This is because the alloy-forming process takes place entirely in the solid state
and does not require the metals to be taken into their liquid states. Similar findings and
conclusions have now also been reported for two other HEA systems [38,39].
Three-dimensional near-net-shape components made from metals and alloys are important
for many functional and structural applications. A classical powder metallurgical method to
manufacture these with dimensions as close as possible to the final specification consists in,
first, compacting a metal powder in moulds and, subsequently, sintering this to the desired
level of density and strength. This is well established in ferrous powder metallurgy [40,41].
The FFC-Cambridge process has now enabled a completely new route towards the
fabrication of near-net-shape metallic objects, in which an oxide powder, as opposed to a
metal powder, is compacted and sintered, and then converted electrochemically into the
corresponding metal body. The feasibility of this approach has already been reported for Zr-
and Ti-based materials [42, 43].
Additive manufacturing, including 3D printing, presents a more modern and currently
thriving approach towards the fabrication of three-dimensional components [44-48]. In this,
powders are successively printed into layers and heated with a focused high-energy heat
source after each printing step, which permits precise control of the preparation of even
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complex objects. Additive manufacturing requires little or no additional tooling and
produces low wastage. The feasibility of additive manufacturing with HEAs has recently
been demonstrated [19,23-25]. The metal powders used must be optimised for efficient
particle flow and layer packing to permit straightforward handling, and they must consist of
small uniform particles to allow for homogeneous melting. The FFC-Cambridge process is
known to be capable of producing such powders successfully. In fact, Metalysis, the UK-
based company tasked with the commercialisation of the process, now has the preparation
of metal powders for additive manufacturing as its primary business goal [49].
The objective of the present study has been to highlight the full versatility of the FFC-
Cambridge process in the preparation of HEAs with respect to processing conditions and
product morphology in terms of powders and three-dimensional objects. For this purpose,
face-centred cubic CoCrFeNi has been selected because it is the most important commodity-
metal, or steel-type, HEA system. It is attractive due to its high tensile ductility and fracture
toughness [23], its relatively large homogeneous composition range around the equiatomic
point [50], and the comparatively low price of the required oxide feedstock. It is also an
important platform for the development of more complex HEAs containing additional
elements such as Al [2], Cu [2], Mn [18], Ti [51], Mo and Nb [52,53].
It is noted that a range of empirical criteria have been established, aiming to distinguish
single-phase HEAs from other complex metallic systems and to predict their formation.
These criteria are based on mixing enthalpy, mixing entropy, atomic radius mismatch,
valence electron concentration (VEC), and electronegativity [54-56]. The present CoCrFeNi
HEA system fulfils all these rules as they are detailed in the Supplementary Material (Tables
S1 to S3). It should be considered though that these criteria are of an empirical nature, and
that in some complex HEA systems the VEC rule cannot predict the phase selection [9].
Experimental
Appropriate amounts of the oxide powders CoO, Cr2O3, Fe2O3 and NiO were combined so as
to provide a mixture with equal numbers of all metal atoms. 1 mass% of polyvinyl alcohol
and 0.5 mass% of polyethylene glycol were added as a binder/plasticiser system. The
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precursor powder was made into a slurry in isopropyl alcohol and thoroughly mixed. Dry
oxide powder was obtained by evaporation of the alcohol, and discs between 0.5 and 3 g
were made by uniaxial pressing. These were sintered at 1173 or 1373 K in air for 3 h, which
provided compact oxides bodies with open porosity of around 40% and sufficient
mechanical strength for further handling. The properties of the starting oxides are listed in
Table S4. Anhydrous CaCl2 was prepared from CaCl2·2H2O (Sigma Aldrich) by heating at 443
K in air for 48 h and subsequent drying at the same temperature in vacuum for 48 h. NaCl
(Alfa Aesar) was used as received.
The salt powder, either mixed CaCl2-NaCl of eutectic composition or pure CaCl2, was loaded
into an alumina crucible and placed inside an Ar-flushed Inconel reactor with provisions for
the insertion of electrodes. The set-up was heated to the processing temperature ranging
between 923 and 1173 K. First the molten salt was purified by pre-electrolysis, using a
cathode of a nickel coil and an anode of a high-density graphite rod at an applied constant
potential of 2.8 V until a small and stable background current was reached. Then HEAs were
synthesised by electro-deoxidation, using a sintered oxide disc as the cathode and the same
graphite rod as the anode at applied constant potentials of around 3.0 V for processing
times of 3, 10 and 15 h. After completion of electro-deoxidation, the electrodes were lifted
above the melt and the reactor was cooled. Photographs of a typical cathode assembly are
shown in Fig. S1. A list of experimental details is given in Table S5. The processed samples
were washed with distilled water until free from salt and then vacuum dried at room
temperature.
The sintered oxide precursors and the electro-deoxidised metallic products were analysed
by X-ray diffraction (Philips PW 1830) to identify their crystallographic phases. Powders and
fracture surfaces were characterised by scanning electron microscopy (SEM) and energy-
dispersive X-ray spectroscopy (EDX) (Nova-NanoSEM 450 with Bruker X Flash 6I100) to
assess their morphology and chemical composition. Densities and open porosities were
determined by Archimedes’ method using distilled water. Microhardness of the metallic
samples was measured along polished cross-sections with a Vickers indenter (Laizhou
Huayin 200HV-5) under an applied load of 200 g for a dwell time of 10 s. At least 10
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individual measurements were performed and averaged, and a metallurgical optical
microscopy (Olympus BHM) was used to visualise the indents.
Results and discussion
Figs. 1a-c present the X-ray diffractograms of the oxide powder mixture and of the oxide
discs sintered at 1173 and 1373 K in air. The pattern of the mixed powder confirms the
starting composition of CoO, Cr2O3, Fe2O3 and NiO (Fig. 1a), while the patterns of the
sintered oxide discs show the β-spinel phase plus NiO (Figs. 1b,c). This indicates that the
sintering of the above oxide mix in air causes the formation of spinels of type AB2O4. It is
known that in this crystallographic structure Co2+ and Ni2+ ions occupy the tetrahedral A
sites and Cr3+, Fe3+ and Co3+ occupy the octahedral B sites. The spinel units formed can thus
be of the types CoCr2O4, CoFe2O4, NiCr2O4, NiFe2O4 and NiCo2O4, all of which have virtually
indistinguishable X-ray diffractograms. The first four of the above spinels possess highly
negative Gibbs free energies of formation at the sintering temperatures used [57]. For
NiCo2O4 no thermodynamic data seem to exist, and so its formation from the binary oxides
at these temperatures has been proven experimentally, as shown in Fig. S2. Therefore, all of
the above five spinel types can occur in principle as long as the mass balance for the
equiatomic system is fulfilled, although their individual proportions remain unknown.
Fig. 1d presents low- and high-magnification SEM images of the cross-section of an oxide
spinel disc sintered at 1173 K. It exhibits particles of two different shapes, one being
relatively small and rather rectangular, the other being relatively large and more spherical.
Similarly, Fig. 1e presents SEM images of an oxide spinel disc sintered at 1373 K. The
microstructure is much coarser than that at lower temperature while two types of particles
are again clearly visible. Fig. 1f displays EDX area maps of an oxide spinel disc sintered at
1173 K. It reveals that the smaller particles are rich in Cr and Fe with lesser amounts of Co
and Ni, pointing to the possible coexistence of either CoCr2O4, CoFe2O4, NiCr2O4 and
NiFe2O4; or only CoCr2O4 and NiFe2O4; or only CoFe2O4 and NiCr2O4. The larger particles are
rich in Ni and Co, indicating NiCo2O4 and NiO, with some distinct particles of the latter
clearly seen. Higher magnification SEM images of the sintered oxide discs and their EDX
analyses are given in Figs. S3,S4.
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Fig. 2a presents the current versus time curves recorded during the electro-deoxidation of
oxide spinel discs, of mass 0.5 g and sintered at 1173 K, in the eutectic CaCl2-NaCl melt at
923, 1023, 1123 and 1173 K for 10 h. In each case, the current initially peaked, then
gradually declined, and finally reached a rather small and stable value. The early part of
each curve indicates directly the progress during electro-deoxidation, while the later part is
determined largely by the intrinsic electronic current through the melt [58]. The effect of
temperature on the magnitude of the current is minor. Similar current curves were obtained
in the electro-deoxidation of oxide spinel discs sintered at 1373 K (not shown).
The insets in Fig. 2a are photographs of the CoCrFeNi alloy samples reduced in the CaCl2-
NaCl melt from the oxide spinel discs sintered at 1173 K. At the electro-deoxidation
temperature of 923 K the reduced oxide disc was crushed entirely into black powder, at
1023 and 1123 K the discs were broken into several large fragments; and at 1173 K the
original shape of the disc was retained completely throughout the oxide-to-alloy conversion.
The reduced solid discs and fragments displayed a shiny grey lustre after polishing of the
surface, clearly indicating their metallic character. The densities and porosities of the
samples prepared at 1173, 1123 and 1023 K were, respectively, 3.67 g cm-3 and 45%, 3.42 g
cm-3 and 48%, and 3.17 g cm-3 and 55%. The density is lower than the value calculated by
the rule of mixture (Table S3) due to the presence of porosity in the alloy body. Overall the
visual examination demonstrates that a remarkable difference in product morphology can
be accomplished by simple changes in the experimental conditions applied in the electro-
deoxidation. The same observations were made for the CoCrFeNi alloy samples reduced in
the CaCl2-NaCl melt from the oxide spinel discs sintered at 1373 K. This signifies that the
morphology of the oxide precursor is not a key parameter for the control of the morphology
of the alloy product.
Figs. 2b-e present the X-ray diffractograms of the CoCrFeNi alloy samples reduced in the
CaCl2-NaCl melt at the four different electro-deoxidation temperatures. Single-phase fcc
crystal structures were obtained in all cases. The X-ray diffractograms of the electro-
deoxidised CoCrFeNi samples agree with those of arc-melted [18] and laser-melted [23]
samples of the same composition. The occurrence of the fcc phase for this type of HEA is in
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accordance with the VEC rule [56], which stipulates that alloys with VEC values in excess of
8, such as the present one with a VEC of 8.25, should exist in the fcc phase (Table S3). The
results of the present study are hence confirmation of the VEC rule as it continues to hold
despite the much lower temperature used in the electrochemical synthesis method
compared with melt-based methods.
Figs. 2f-i present low-magnification SEM images of cross-sections of the CoCrFeNi alloy
samples reduced in the CaCl2-NaCl melt. As the electro-deoxidation temperature increased,
the thickness of the cross-sections decreased and the extent of sample densification rose.
Figs. 2j-m show high-magnification SEM images of the same cross-sections. With increasing
temperature, the particle size increased and the microstructure became coarser. The
sample reduced at 923 K consisted of spherical particles of average diameter 250 nm, and
many such particles were grouped together to form globular agglomerates of 10 μm in size
(Fig. 2j). The sample reduced at 1023 K contained nodular particles of diameters around 1
μm, and these were combined into bigger aggregates. This indicates the onset of particle
coarsening as a result of in-situ sintering and enabled the formation of macroscopic solid
pieces (Fig. 2k). At 1123 K, particles showed some elongation due to neck formation from
further sintering and they had diameters around 1.7 µm (Fig. 2l). At 1173 K, particles were
strongly interconnected across the entire sample volume and they grew to around 2 μm
(Fig. 2m). This compact microstructure endowed the material with the strength necessary to
maintain its three-dimensional shape.
Figs. 2n-q display EDX elemental area maps for Co, Cr, Fe and Ni of the CoCrFeNi alloy
samples reduced in the CaCl2-NaCl melt. The distribution of each element was rather
uniform within the particles of the powder samples and throughout the cross-sections of
the solid samples. Quantitative analysis showed that local deviations from the expected
equiatomic composition were typically less than 3% and never exceed 5% for both powders
and discs. Achieving this high level of homogeneity may have been facilitated by the rather
similar atomic radii of the four constituents. Further EDX information of is given in Table S6.
Fig. 3a presents the current versus time curves acquired during the electro-deoxidation of
oxide spinel discs, of mass 0.5 g and sintered at 1173 K, in the pure CaCl2 melt at 1173 K for
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3 and 10 h. The curves show the same features as discussed for the experiments in the
CaCl2-NaCl melt and the current at the various stages was of a similar magnitude. The insets
in Fig. 3a are photographs of the CoCrFeNi alloy samples reduced in the CaCl2 melt. The
samples were in the form of discs, demonstrating that the geometry of the solid oxide
precursor had been retained and, in particular, that solid alloy bodies could be achieved
within only 3 h. The densities and porosities of the samples prepared at 1173 K for 10 and 3
h were, respectively, 4.25 g cm-3 and 38%, and 3.94 g cm-3 and 44%. Figs. 3b,c show the X-
ray diffractograms of the two samples. These confirm again the single-phase fcc structure.
Figs. 3d,e present low-magnification SEM images of cross-sections of the CoCrFeNi alloy
samples reduced in the pure CaCl2 melt. At longer electro-deoxidation time, the thickness of
the cross-section was lower and the extent of densification was higher. Figs. 3f,g show high-
magnification SEM images of the same cross-sections. These reveal a relatively coarse
microstructure with the individual particles closely joined together along their interfaces.
They had diameters of around 1.2 μm after 3 h and of around 3.5 μm after 10 h, indicating
progressive in-situ sintering at this temperature. These findings signify that melt
composition and processing time have an impact on the morphology of the alloy product.
Figs. 3h,i display EDX elemental area maps of the CoCrFeNi alloy samples reduced in the
CaCl2 melt. All four elements were distributed uniformly. Further EDX information is given in
Table S6.
Fig. 4 presents SEM images of the CoCrFeNi HEA powder synthesised in this study. As
discussed, the as-prepared material consists of fine particles lumped together into large
agglomerates (Fig. 4a). Importantly, the morphology changes greatly through
ultrasonication, as the lumps disperse into a fine mesh of individual submicron particles
(Figs. 4b,c). This morphology is very similar to that of the metal powders, with different
compositions, prepared at Metalysis [49], and would hence suggest the potential
applicability of steel-type HEAs in additive manufacturing. Notably, an EDX line scan across
several particles showed that the composition within the sample is so homogeneous that
transitions between individual particles cannot be distinguished by elemental analysis (Fig.
S5).
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Fig. 5 presents images of a variety of centimetre-sized free-standing CoCrFeNi HEA objects
prepared in this study. Fig. 5a shows that an HEA sample, which has been reduced at 1173
K, is sufficiently strong to be cut into various shapes without breakage despite its significant
porosity. Fig. 5b shows a boat-shaped sample, which has been created by carving a groove
into the cross-section of a segmented disc, along with its coarse microstructure. Both these
findings underscore the excellent machinability of the three-dimensional objects made. Fig.
5c shows a strip-type sample, which has been cut and ground with SiC, along with the
resulting polishing lines at its surface. Such treatment leads to a densification of the surface
film and a local microstructure resembling that of HEA samples prepared by melt-based
techniques. Fig. 5d shows the surface elemental composition of the polished alloy, and
indicates that all four elements are uniformly distributed at the surface. Fig. 5e shows a
photograph of the polished cross-section of a sample strip mounted for microhardness
testing, along with the corresponding light optical micrograph of a Vickers indentation. It is
seen that the electrochemically prepared samples tolerate the indentation in a similar
fashion to arc-melted samples. This observation reflects their high strength.
Fig. 6 compiles the microhardness values of the various solid CoCrFeNi HEA samples
prepared. The average microhardness of the samples prepared in the CaCl2-NaCl melt at
1023, 1123 and 1173 K was, respectively, 87.8 HV (861 MPa), 93.4 HV (916 MPa) and 106.5
HV (1044 MPa), and that of the samples made in the pure CaCl2 melt at 1173 K for 3 and 10
h was, respectively, 110.8 HV (1087 MPa) and 147.1 (1443 MPa). As expected, hardness is
dependent on the processing conditions, because they determine the microstructure of the
alloy product. More specifically, there is an inverse correlation between hardness and
porosity. All hardness values measured for the electrochemically prepared CoCrFeNi HEA
samples were higher than the value of 85.3 HV (837 MPa) calculated by the rule of mixture
(Table S3). The values measured for the samples made at 1173 K were furthermore
comparable with those reported for dense arc-melted CoCrFeNi HEA at 118 HV (1157 MPa)
[18,23] and 135 HV (1324 MPa) [52]. The high hardness of these alloys results from the
homogeneous composition of the four elements in the single-phase solid solution and the
ensuing solid-solution strengthening.
Conclusions
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CoCrFeNi high-entropy alloy has been synthesised by means of electro-deoxidation of a mix
of oxide spinels in molten eutectic CaCl2-NaCl over a temperature range from 973 to 1173 K
and in molten pure CaCl2 at 1173 K. In all cases, XRD and SEM/EDX analyses revealed a
single-phase fcc crystal structure with uniform chemical composition and no visible
segregation within the microstructure. Depending on processing conditions, either alloy
powders or three-dimensional alloy bodies were obtained. The CoCrFeNi powders contained
a fine dispersion of submicron-sized particles with little variation in particles size. The
CoCrFeNi discs were centimetre-sized and had a lower density than calculated by the rule of
mixture, due to residual porosity, and a higher microhardness than calculated, due to solid
solution strengthening within the chemically homogeneous single phase. With a view to the
practical use of the alloy objects, it is important to note that their porosity can be lowered
to a desired level by conventional metallurgical downstream processing. For example, a
two-dimensional system can be cold or hot rolled, and a three-dimensional system can be
vacuum sintered. This may be necessary for applications requiring high strength.
Overall, the present results highlight the versatility of the FFC-Cambridge process in terms of
accomplishing alloy products of different morphologies and bring out its potential to play a
role in the further development of advanced manufacturing methods. The key process
parameters have been identified as the operating temperature, electrolyte composition,
and processing time. The study moreover serves as the basis for the synthesis of CoCrFeNi-
based high-entropy alloys with five or more elements, as they may be required in electronic,
magnetic, thermo-electric, superconductive, and biomedical applications. The study can also
be extended towards the synthesis of high-entropy alloys from high-entropy oxides that
have recently entered the research arena.
Acknowledgements
This study was funded through the Research Chair Grant Program of The Research Council
of the Sultanate of Oman. The authors acknowledge Prof. R.V. Kumar, Head of Materials
Chemistry Group, Department of Materials Science and Metallurgy, University of
Cambridge, for his support of this work.
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References
[1] B. Cantor, I.T.H. Chang, P. Knight, A.J.B. Vincent, Microstructural development in
equiatomic multicomponent alloys, Mater. Sci. Eng. A 375-377 (2004) 213-218.
[2] J.-W. Yeh, S.-K. Chen, S.-J. Lin, J.-Y. Gan, T.-S. Chin, T.-T. Shun, C.-H. Tsau, S.-Y. Chang,
Nanostructured high-entropy alloys with multiple principal elements: novel alloy
design concepts and outcomes, Adv. Eng. Mater. 6 (2004) 299-303.
[3] Y.-J. Hsu, W.-C. Chiang, J.-K. Wu, Corrosion behaviour of FeCoNiCrCux high-entropy alloys
in 3.5% sodium chloride solution, Mater. Chem. Phys. 92 (2005) 112-117.
[4] Y.Y. Chen, U.T. Hong, H.C. Shih, J.W. Yeh, T. Duval, Electrochemical kinetics of the high
entropy alloys in aqueous environments – a comparison with type 304 stainless steel,
Corros. Sci. 47 (2005) 2679-2699.
[5] O.N. Senkov, G.B. Wilks, D.B. Miracle, C.P. Chuang, P.K. Liaw, Refractory high-entropy
alloys, Intermetallics 18 (2010) 1758-1765.
[6] O.N. Senkov, G.B. Wilks, J.M. Scott, D.B. Miracle, Mechanical properties of
Nb25Mo25Ta25W25 and V20Nb20Mo20Ta20W20 refractory high entropy alloys,
Intermetallics 19 (2011) 698-706.
[7] O.N. Senkov, J.M. Scott, S.V. Senkova, D.B. Miracle, C.F. Woodward, Microstructure and
room temperature properties of a high-entropy TaNbHfZrTi alloy, J. Alloys Compd. 509
(2011) 6043-6048.
[8] M. Feuerbacher, M. Heidelmann, C. Thomas, Hexagonal high-entropy alloys, Mater. Res.
Lett. 3 (2015) 1-6.
[9] Z.G. Zhu, K.H. Ma, Q. Wang, C.H. Shek, Compositional dependence of phase formation
and mechanical properties in three CoCrFeNi-(Mn/Al/Cu) high entropy alloys,
Intermetallics 79 (2016) 1-11.
[10] J.-M. Wu, S.-J. Lin, J.-W. Yeh, S.-K. Chen, Y.-S. Huang, H.-C. Chen, Adhesive wear
behavior of AlxCoCrCuFeNi high-entropy alloys as a function of aluminum content,
Wear 261 (2006) 513-519.
[11] G.R. Holcomb, J. Tylczak, C. Carney, Oxidation of CoCrFeMnNi high entropy alloys, JOM
67 (2015) 2326-2339.
[12] B.L. Zhang, Y. Zhang, S.M. Guo, A thermodynamic study of corrosion behaviors for
CoCrFeNi-based high-entropy alloys, J. Mater. Sci. 53 (2018) 14729-14738.
12

ACCEPTED MANUSCRIPT
[13] Y. Zhang, T.T. Zuo, Z. Tang, M.C. Gao, K.A. Dahmen, P.K. Liaw, Z.P. Lu, Microstructures
and properties of high-entropy alloys, Prog. Mater. Sci. 61 (2014) 1-93.
[14] B.S. Murthy, J.-W. Yeh, S. Ranganathan, High-Entropy Alloys, first ed., Butterworth-
Heinemann, London, UK, 2014.
[15] M.C. Gao, J.-W. Yeh, P.K. Liaw, Y. Zhang (Eds.), High-Entropy Alloys – Fundamentals and
Applications, Springer International Publishing AG, Cham, Switzerland, 2016.
[16] T.-T. Shun, Y.-C. Du, Age hardening of the Al0.3CoCrFeNiC0.1 high entropy alloy, J. Alloys
Compd. 478 (2009) 269-272.
[17] S. Singh, N. Wanderka, B.S. Murty, U. Glatzel, J. Banhart, Decomposition in multi-
component AlCoCrCuFeNi high entropy alloy, Acta Mater. 59 (2011) 182-190.
[18] Z. Wu, H. Bei, F. Otto, G.M. Pharr, E.P. George, Recovery, recrystallization, grain growth
and phase stability of a family of FCC-structured multi-component equiatomic solid
solution alloys, Intermetallics 46 (2014) 131-140.
[19] J. Joseph, T. Jarvis, X.H. Wu, N. Stanford, P. Hodgson, D.M. Fabijanic, Comparative study
of the microstructures and mechanical properties of direct laser fabricated and arc-
melted AlxCoCrFeNi high entropy alloys, Mater. Sci. Eng. A 633 (2015) 184-193.
[20] H. Zhang, Y. Pan, Y.-Z. He, Synthesis and characterization of FeCoNiCrCu high-entropy
alloy coating by laser cladding, Mater. Des. 32 (2011) 1910-1915.
[21] H. Zhang, Y. Pan, Y.-Z. He, Effects of annealing on the microstructure and properties of
6FeNiCoCrAlTiSi high-entropy alloy coating prepared by laser cladding, J. Therm.
Spray. Technol. 20 (2011) 1049-1055.
[22] Z.G. Zhu, Q.B. Nguyen, F.L. Ng, X.H. An, X.Z. Liao, P.K. Liaw, S.M.L. Nai, J. Wei,
Hierarchical microstructure and strengthening mechanisms of a CoCrFeNiMn high
entropy alloy additively manufactured by selective laser melting, Scr. Mater. 154
(2018) 20-24.
[23] Y. Brif, M. Thomas, I. Todd, The use of high-entropy alloys in additive manufacturing,
Scr. Mater. 99 (2015) 93-96.
[24] T. Fujieda, H. Shiratori, K. Kuwabara, T. Kato, K. Yamanaka, Y. Koizumi, A. Chiba, First
demonstration of promising selective electron beam melting method for utilizing high-
entropy alloys as engineering materials, Mater. Lett. 159 (2015) 12-15.
[25] H. Shiratori, T. Fujieda, K. Yamanaka, Y. Koizumi, K. Kuwabara, T. Kato, A. Chiba,
Relationship between the microstructure and mechanical properties of an equiatomic
13

ACCEPTED MANUSCRIPT
AlCoCrFeNi high-entropy alloy fabricated by selective electron beam melting, Mater.
Sci. Eng. A 656 (2016) 39-46.
[26] S. Praveen, B.S. Murty, R.S. Kottada, Alloying behavior in multi-component AlCoCrCuFe
and NiCoCrCuFe high entropy alloys, Mater. Sci. Eng. A 534 (2012) 83-89.
[27] A.S.M. Ang, C.C. Berndt, M.L. Sesso, A. Anupam, S. Praveen, R.S. Kottada, B.S. Murty,
Plasma-sprayed high entropy alloys: microstructure and properties of AlCoCrFeNi
and MnCoCrFeNi, Metall. Mater. Trans. A 46 (2015) 791-800.
[28] Z.N. An, H.L. Jia, Y.Y. Wu, P.D. Rack, A.D. Patchen, Y.Z. Liu, Y. Ren, N. Li, P.K. Liaw, Solid-
solution CrCoCuFeNi high-entropy alloy thin films synthesized by sputter deposition,
Mater. Res. Lett. 3 (2015) 203-209.
[29] C.-Z. Yao, P. Zhang, M. Liu, G.-R. Li, J.-Q. Ye, P. Liu, Y.-X. Tong, Electrochemical
preparation and magnetic study of Bi-Fe-Co-Ni-Mn high entropy alloy, Electrochim.
Acta 53 (2008) 8359-8365.
[30] V. Soare, M. Burada, I. Constantin, D. Mitrica, V. Badilita, A. Caragea, M. Tarcolea,
Electrochemical deposition and microstructural characterization of AlCrFeMnNi,
AlCrCuFeMnNi high entropy alloy thin films, Appl. Surf. Sci. 358 (2015) 533-539.
[31] M. Feuerbacher, E. Würtz, A. Kovács, C. Thomas, Single-crystal growth of a FeCoCrMnAl
high-entropy alloy, Mater. Res. Lett. 5 (2017) 128-134.
[32] G.Z. Chen, D.J Fray, T.W. Farthing, Direct electrochemical reduction of titanium dioxide
to titanium in molten calcium chloride, Nature 407 (2000) 361-364.
[33] D.J. Fray, Novel methods for the production of titanium, Int. Mater. Rev. 53 (2008) 317-
325.
[34] D.H. Wang, X.B. Jin, G.Z. Chen, Solid state reactions: an electrochemical approach in
molten salts, Annu. Rep. Prog. Chem. Sect. C 104 (2008) 189-234.
[35] K.S. Mohandas, Direct electrochemical conversion of metal oxides to metal by molten
salt electrolysis: a review, Miner. Proc. Extract. Metall. 122 (2013) 195-212.
[36] D.J. Fray, C. Schwandt, Aspects of the application of electrochemistry to the extraction
of titanium and its applications, Mater. Trans. 58 (2017) 306-312.
[37] J. Sure, D. Sri Maha Vishnu, C. Schwandt, Direct electrochemical synthesis of high-
entropy alloys from metal oxides, Appl. Mater. Today 9 (2017) 111-121.
[38] M.B.D. Ellis, G.R. Doughty, Solid state manufacture of high entropy alloys – preliminary
studies, MRS Advances, 2 (2017) 1375-1380.
14

ACCEPTED MANUSCRIPT
[39] B. Wang, J. Huang, J.H. Fan, Y.P. Dou, H. Zhu, D.H. Wang, Preparation of FeCoNiCrMn
high entropy alloy by electrochemical reduction of solid oxides in molten salt and its
corrosion behavior in aqueous solution, J. Electrochem. Soc. 164 (2017) E575-E579.
[40] K.S. Narasimhan, Recent advances in ferrous powder metallurgy, Adv. Perform. Mater.
3 (1996) 7-27.
[41] K.S. Narasimhan, Sintering of powder mixtures and the growth of ferrous powder
metallurgy, Mater. Chem. Phys. 67 (2001) 56-65.
[42] J.J. Peng, K. Jiang, W. Xiao, D.H. Wang, X.B. Jin, G.Z. Chen, Electrochemical conversion of
oxide precursors to consolidated Zr and Zr-2.5Nb tubes, Chem. Mater. 20 (2008) 7274-
7280.
[43] D. Hu, W. Xiao, G.Z. Chen, Near-net-shape production of hollow titanium alloy
components via electrochemical reduction of metal oxide precursors in molten salts,
Metall. Mater. Trans. B 44 (2013) 272-282.
[44] L.E. Murr, S.M. Gaytan, D.A. Ramirez, E. Martinez, J. Hernandez, K.N. Amato, P.W.
Shindo, F.R. Medina, R.B. Wicker, Metal fabrication by additive manufacturing using
laser and electron beam melting technologies, J. Mater. Sci. Technol. 28 (2012) 1-14.
[45] W.E. Frazier, Metal additive manufacturing: a review, J. Mater. Eng. Perform. 23 (2014)
1917-1928.
[46] X.B. Gong, T. Anderson, K. Chou, Review on powder-based electron beam additive
manufacturing technology, Manuf. Rev. 1 (2014) 2 (12 pages).
[47] S.K. Everton, M. Hirsch, P. Stravroulakis, R.K. Leach, A.T. Clare, Review of in-situ process
monitoring and in-situ metrology for metal additive manufacturing, Mater. Des. 95
(2016) 431-445.
[48] M. Vaezi, H. Seitz, S.F. Yang, A review on 3D micro-additive manufacturing technologies,
Int. J. Adv. Manuf. Technol. 67 (2013) 1721-1754.
[49] Metalysis, Rotherham, UK, www.metalysis.com (last accessed 28/09/2018).
[50] F. He, Z.J. Wang, Q.F. Wu, S.Z. Niu, J.J. Li, J.C. Wang, C.T. Liu, Solid solution island of the
Co-Cr-Fe-Ni high entropy alloy system, Scr. Mater. 131 (2017) 42-46.
[51] F. He, Z.J. Wang, Y.Y. Li, Q.F. Wu, J.J. Li, J.C. Wang, C.T. Liu, Kinetic ways of tailoring
phases in high entropy alloys, Sci. Rep. 6 (2016) 34628 (9 pages).
[52] T.-T. Shun, L.-Y. Chang, M.-H. Shiu, Microstructure and mechanical properties of
multiprincipal component CoCrFeNiMox alloys, Mater. Charact. 70 (2012) 63-67.
15

ACCEPTED MANUSCRIPT
[53] S. Praveen, B.S. Murty, R.S. Kottada, Effect of molybdenum and niobium on the phase
formation and hardness of nanocrystalline CoCrFeNi high entropy alloys, J. Nanosci.
Nanotechnol. 14 (2014) 8106-8109.
[54] X. Yang, Y. Zhang, Prediction of high-entropy stabilized solid-solution in multi-
component alloys, Mater. Chem. Phys. 132 (2012) 233-238.
[55] S. Guo, C.T. Liu, Phase stability in high entropy alloys: formation of solid-solution phase
or amorphous phase, Prog. Nat. Sci.: Mater. Int. 21 (2011) 433-446.
[56] S. Guo, C. Ng, J. Lu, C.T. Liu, Effect of valence electron concentration on stability of fcc
or bcc phase in high entropy alloys, J. Appl. Phys. 109 (2011) 103505 (5 pages).
[57] A. Roine, HSC Chemistry, version 5.1, Outokumpu Research Oy, Pori, Finland, 2002.
[58] C. Schwandt, Understanding the electro-deoxidation of titanium dioxide to titanium
metal via the FFC-Cambridge process, Miner. Proc. Extract. Metall. 122 (2013) 213-
218.
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List of Figures
1 Cr2O3 [38-1479]
Mixed powder at RT
4
2 Fe2O3 [33-0664]
3 CoO [43-1004]
4 NiO [47-1049]
(d)
(a)
4
4
3
1
2
3
1
2
4
3
1
2
2
1
10 µm
2
4
50 µm
50 µm
1
2
3
1
1
1
4
1
3
2
1
2
2
2
2
2 2
1
1
1
1
2
1
1
2
(e)
2
1
1
10
20
30
40
50
60
70
80
90
2theta (degree)
1 CoCr2O4 [22-1084]
2 NiFe2O4 [86-2267]
3 CoFe2O4 [22-1086]
4 NiCr2O4 [88-0108]
5 NiCo2O4 [73-1702]
Sintered at 1173 K
S
10 µm
3µm
S-spinel
S
(b)
6 NiO [47-1049]
S
(f)
S
S
S
6
6
S
S
6
S
3µm
S
6
6
S
S
S
S
10
20
30
40
50
60
70
80
90
2theta (degree)
1 CoCr2O4 [22-1084]
2 NiFe2O4 [86-2267]
Sintered at 1373 K
S
S-spinel
3 CoFe2O4 [22-1086]
4 NiCr2O4 [88-0108]
5 NiCo2O4 [73-1702]
6 NiO [47-1049]
S
(c)
3µm
3µm
3µm
3µm
S
S
S
6
6
S
S
S
S
S
6
6
S
6
S
S
S
S
S
10
20
30
40
50
60
70
80
90
2theta (degree)
Fig. 1 X-ray diffractograms of (a) mixed oxide powder before sintering; (b) mixed oxide disc
after sintering at 1173 K in air for 3 h; (c) mixed oxide disc after sintering at 1373 K in air for
3 h. Insets of (a), (b) and (c) show the powder and disc samples, respectively. Low- and
high-magnification SEM images of oxide discs sintered at (d) 1173 K and (e) 1373 K. (f) EDX
elemental maps of the sample sintered at 1173 K.
1

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2.5
2.0
1.5
1.0
0.5
0.0
(111)
1123 K
FCC
923 K
FCC
(111)
CaCl2-NaCl at 923 K
(d)
(a)
CaCl2-NaCl at 1023 K
CaCl2-NaCl at 1123 K
CaCl2-NaCl at 1173 K
(b)
(200)
(220)
(200)
(220)
10
20
30
40
50
60
70
80
90
10
20
30
40
50
60
70
80
90
2theta (degree)
2theta (degree)
1173 K
FCC
(111)
(111)
1023 K
FCC
(c)
(e)
923 K
1023 K
1123 K 1173 K
(200)
(200)
(220)
(220)
0
2
4
6
8
10
10
20
30
40
50
60
70
80
90
10
20
30
40
50
60
70
80
90
2theta (degree)
2theta (degree)
Time (h)
(f)
(j)
(h)
(l)
(i)
(g)
1 mm
1 mm
1 mm
1 mm
(m)
(k)
10 µm
10 µm
10 µm
10 µm
923 K
1023 K
1123 K
1173 K
(p)
2µm
(n)
2µm
(q)
2µm
(o)
2µm
Fig. 2 (a) Current versus time curves during electro-deoxidation of 0.5 g oxide spinel discs
in CaCl2-NaCl melt at 923, 1023, 1123 and 1173 K for 10 h. Inset of (a) shows the reduced
samples for the four electro-deoxidation temperatures. X-ray diffractograms of the
samples obtained at (b) 923 K, (c) 1023 K, (d) 1123 K and (e) 1173 K. (f) to (i) low-
magnification SEM images of reduced powder or cross-section of reduced discs; (j) to (m)
high-magnification SEM images of the same samples; (n) to (q) EDX elemental maps of the
samples.
2

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3.0
2.5
2.0
1.5
1.0
0.5
0.0
(111)
CaCl2 at 1173 K for 3 h
CaCl2 at 1173 K for 10 h
1173 K for 3 h
FCC
(a)
(b)
(200)
50
(220)
10
20
30
40
(111)
60
70
80
90
1173 K for 10 h
FCC
(c)
3 h 10 h
(200)
50
(220)
10
20
30
40
60
70
80
90
0
2
4
6
8
10
2theta (degree)
Time (h)
(e)
(d)
1 mm
1 mm
(g)
(f)
10 µm
10 µm
(i)
(h)
2µm
5µm
Fig. 3 (a) Current versus time curves during electro-deoxidation of 0.5 g oxide spinel discs
in CaCl2 melt at 1173 K for 3 and 10 h. Inset of (a) shows the reduced samples for both
durations. X-ray diffractograms of the samples obtained at 1173 K for (b) 3 h and (c) 10 h.
(d) and (e) low-magnification SEM images of cross-section of reduced discs; (f) and (g) high-
magnification SEM images of the same samples; (h) and (i) EDX elemental maps of the
samples.
3

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(a)
(b)
50 µm
50 µm
(c)
2 µm
Fig. 4 Low-magnification SEM images of reduced powder obtained by electro-deoxidation
of an oxide spinel disc in CaCl2-NaCl melt at 923 K for 10 h, (a) in the as-reduced state and
(b) in the ultrasonicated state. (c) High-magnification SEM image of the reduced powder
after ultrasonication.
4

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(e)
(a)
(b)
(c)
(d)
Fig. 5 (a) Photograph of reduced disc obtained by electro-deoxidation of a 3 g oxide spinel
disc in CaCl2 melt at 1173 K for 15 h. Inset of (a) shows discs cut in different ways. (b) Boat-
shaped sample prepared by cutting and grooving, along with corresponding cross-section.
(c) Strip-shaped sample prepared by cutting, along with corresponding surface
morphology, and (d) EDX elemental maps of the strip surface. (e) Cross-section of strip
moulded for microhardness measurement, along with light optical microscopic image of
Vickers indentation.
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200
150
100
50
CaCl2-NaCl melt
CaCl2 melt
10 h
3 h
10 h
10 h
10 h
0
1023 K
1123 K
1173 K
1173 K
1173 K
Melt temperature (K)
Fig. 6 Microhardness of reduced fragments or discs obtained by electro-deoxidation of
oxide spinel discs in CaCl2-NaCl and in CaCl2 melts at different temperatures and for
different durations.
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Highlights
•
•
•
•
•
Low-temperature synthesis of single-phase CoCrFeNi high-entropy alloy at 923 to
1173 K
Direct conversion of metal oxide mixture into CoCrFeNi alloy via electrochemical
deoxidation
Preparation of both CoCrFeNi crystalline fine powders and three-dimensional solid
objects
Control of CoCrFeNi alloy morphology via processing temperature and other process
parameters
Evaluation of effect of process parameters on microhardness of CoCrFeNi alloy