Materials Research Letters

ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/tmrl20

Novel single crystalline-like non-equiatomic
TiZrHfNbTaMo bio-high entropy alloy (BioHEA)
developed by laser powder bed fusion
Ozkan Gokcekaya, Takuya Ishimoto, Yuki Nishikawa, Yong Seong Kim, Aira
Matsugaki, Ryosuke Ozasa, Markus Weinmann, Christoph Schnitter, Melanie
Stenzel, Hyoung Seop Kim, Yoshitsugu Miyabayashi & Takayoshi Nakano
To cite this article: Ozkan Gokcekaya, Takuya Ishimoto, Yuki Nishikawa, Yong Seong Kim, Aira
Matsugaki, Ryosuke Ozasa, Markus Weinmann, Christoph Schnitter, Melanie Stenzel, Hyoung
Seop Kim, Yoshitsugu Miyabayashi & Takayoshi Nakano (2023) Novel single crystalline-like nonequiatomic TiZrHfNbTaMo bio-high entropy alloy (BioHEA) developed by laser powder bed
fusion, Materials Research Letters, 11:4, 274-280, DOI: 10.1080/21663831.2022.2147406
To link to this article: https://doi.org/10.1080/21663831.2022.2147406© 2023 The Author(s). Published by Informa
UK Limited, trading as Taylor & Francis
Group.

Published online: 29 Nov 2022.

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MATER. RES. LETT.
2023, VOL. 11, NO. 4, 274–280
https://doi.org/10.1080/21663831.2022.2147406

ORIGINAL REPORTS

Novel single crystalline-like non-equiatomic TiZrHfNbTaMo bio-high entropy
alloy (BioHEA) developed by laser powder bed fusion
Ozkan Gokcekaya a,b , Takuya Ishimoto a,b,c , Yuki Nishikawaa , Yong Seong Kima , Aira Matsugaki a,b ,Ryosuke Ozasa a,b , Markus Weinmann d , Christoph Schnitter d , Melanie Stenzel d , Hyoung Seop Kim
Yoshitsugu Miyabayashi f and Takayoshi Nakano a,be,a Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, Osaka, Japan; b Anisotropic Design and
Additive Manufacturing Research Center, Osaka University, Osaka, Japan; c Aluninium Research Center, University of Toyama, Toyama, Japan;d TANIOBIS GmbH, Goslar, Germany; e Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang,South Korea; f Graduate School of Engineering, Osaka University, Osaka, Japan
ABSTRACTARTICLE HISTORYThis study developed a non-equiatomic Ti28.33 Zr28.33 Hf28.33 Nb6.74 Ta6.74 Mo1.55 super-solid solutionized BioHEA using laser powder bed fusion (LPBF) to reach the full potential as BioHEA. We succeededin significant suppression of elemental segregation, thus, resulting in a single crystalline-like texture
by activating layer-to-layer epitaxial growth. Relatively low Young’s modulus was achieved in the
single crystalline-like BioHEA. Moreover, LPBF-fabricated BioHEA exhibited significantly higher yieldstress (1355–1426 MPa) due to the effective solid solution hardening compared to as-cast counterpart with marked segregation (949 MPa), and good biocompatibility. This is first report achieving
BioHEA with low modulus, excellent strength-ductility balance, and good biocompatibility via LPBF.

Received 7 October 2022
KEYWORDS

Laser powder bed fusion;
high-entropy alloys; supersolid solution;
crystallographic texture;mechanical property

IMPACT STATEMENT

This study achieved super-solid-solutionized and single crystalline-like TiZrHfNbTaMo bio-highentropy alloy using laser powder bed fusion, resulting in low Young’s modulus along < 001 >oriented direction, excellent strength-ductility balance, and good cytocompatibility.

1. Introduction
Next-generation metallic biomaterials should exhibit
superior mechanical performance and biocompatibility
while allowing custom design for individual needs [1].
Thus, developing new metallic biomaterials with highstrength, low elastic modulus, and biocompatibility isimportant to fulfil the demands for biomedical applications.
Recently, a new class of structural and functional
materials called high-entropy alloys (HEAs) has emerged[2,3]. Our group proposed equiatomic TiNbTaZrMo
HEAs for biomedical applications, defined as BioHEAs[4], which have now expanded to diverse alloy systems using biocompatible elements [4–12]. However,
CONTACT Takayoshi Nakano
nakano@mat.eng.osaka-u.ac.jp
Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan

severe segregation and consequent phase separation are
inevitable because of the super-multicomponent natureof the HEA system when general solidification methods
such as casting are used. Severe segregation reduces lattice distortion that is the origin of the high functionalityof HEAs [13,14]. Therefore, the occurrence of segregation means that the materials do not perform as well as
HEAs.
To suppress elemental segregation and simultaneously achieve shape customisation, we proved the advantage of the laser powder bed fusion (LPBF) method.
The extremely rapid cooling ( ∼ 107 K/s) [15] achieved
during solidification in LPBF significantly inhibits diffusion and thus suppresses segregation, and promotes theDivision of Materials and Manufacturing Science, Graduate School of Engineering,© 2023 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use,
distribution, and reproduction in any medium, provided the original work is properly cited.

MATER. RES. LETT.

formation of a solid solution [16], which could resultin an effective solid solution strengthening. Anotheradvantage of LPBF is that crystallographic texture control is enabled and a single crystalline-like microstructure
can be obtained [17,18]. For single crystalline-like texture formation, the implementation of epitaxial growth
between the laser scanning tracks and layers is essential [19]. Thus, if factors that inhibit epitaxial growth
are present, a single crystalline-like texture cannot berealised. β-type titanium-based materials with a BCC
structure show a crystallographic orientation-dependantvariation in Young’s modulus, with the lowest modulus along < 001 > [17,20]. Acquisition of single crystals is an important strategy [21] for achieving lowmodulus biomaterials [22]. However, inevitable severe
segregation nature due to the intrinsic fact that HEA
is an ultra-multicomponent alloy system could disturb
the formation of single crystalline-like texture. Elemental segregation results in a microstructure composed ofmulti-phases with differed melting point. The phaseswith high melting points can remain undissolved in themelt pool during remelting and acted as nucleation sites,
inhibiting epitaxial growth from the beneath layer andpreventing the formation of a single crystalline structure[23].
In this study, the principles of BioHEA creation thru
LPBF fabrication are (1) suppression of elemental segregation, (2) formation of single crystalline-like prominent texture, and (3) reduction in low Young’s modulus. To minimise the segregation tendency, the difference
between the liquidus and solidus temperatures (T) was
designed to be small. Segregation inhibition promotes
epitaxial growth and strong texture formation during
LPBF. The reduction in electron per atom ratio (e/a)
is expected to contribute to a reduced Young’s modulus along the crystallographic < 001 > direction, as
reported in β-Ti alloy system. The decrease in the shear
modulus for the 110 < 11̄0 > shear as a function of the
reduction in e/a leads to the decrease in Young’s modulus along < 001 > [20]. A low e/a which is equivalent toa low valence electron concentration (VEC) in this alloy
system is also beneficial to improve room-temperatureductility [24,25]. Under this line, we decided to fabricate
Ti28.33 Zr28.33 Hf28.33 Nb6.74 Ta6.74 Mo1.55 (at. %) BioHEA
with a mixing entropy of 1.5R (R is the gas constant)thru LPBF. For this BioHEA, T estimated by thermodynamic calculations using FactSage and SGTE2017 was 50
K, which was significantly lower than the ∼ 300 K of thepreviously reported Ti1.4 Zr1.4 Nb0.6 Ta0.6 Mo0.6 BioHEA
with severe elemental segregation [26]. Consequently, thedeviation of the distribution coefficients, which is definedas the ratio of the element’s concentration in the solidus
phase compared to that in the liquidus phase of each275

Table 1. Overall compositions (at. %) of powder, cast and LPBFfabricated specimens.
Powder
Cast
LPBF

Ti

Zr

Hf

Nb

Ta

Mo

28.1
27.7
28.7

28.0
28.7
27.9

28.5
28.6
28.3

6.9
6.8
6.9

6.8
6.7
6.6

1.7
1.5
1.7

element, from 1 decreased, which was expected to reduce
the tendency to segregate. VEC was reduced from 4.52
to 4.17, which is somewhat larger than that of Ti-15Mo5Zr-3Al (wt. %), ISO-approved low-modulus β-Ti alloy
for metallic implant materials (ISO 5832-14). To finetune these parameters, a nonequiatomic six-elemental
alloy system was created instead of equiatomic quintettecompositions that are commonly designed as HEAs.

2. Materials and methods
The mostly spherical BioHEA powders (Figure 1(a))
were prepared using electrode induction melting gas
atomisation of pre-alloyed rods (TANIOBIS, Germany).
The powder was sieved to obtain a compatible size distribution for LPBF, resulting in an average diameter of
31.5 μm. The chemical composition of the powder, measured using the inductively coupled plasma method, isshown in Table 1. The composition was approximately
the same as expected. The rest angle of the powder measured using a Revolution powder analyser (Mercury Scientific, USA) was 41.2°, which is close to that of commercial Ti-6Al-4V spherical powders (EOS, Germany)(39.2°) [27], indicating good flowability. The quality ofthe powders and processability of complex shapes by
LPBF were demonstrated by fabricating a jungle-gymshaped lattice structure, as shown in Figure 1(b).
To investigate the intrinsic microstructure, mechanical properties, and biocompatibility of the LPBFprocessed BioHEA, cubic samples with dimensions of
5 × 5 × 5 mm were fabricated using an LPBF apparatus(EOS M290, EOS) with a laser power, scan pitch, andlayer thickness of 240 W, 80, and 60 μm, respectively.
The laser scanning speed was varied at 1000, 1200, and1600 mm/s to change the crystallographic texture; they
were named V1000 , V1200 , and V1600 , respectively. The
fabrication was performed under an Ar atmosphere. The
scanning strategy adopted in this study was bidirectional(zigzag) scanning with a rotation of 90° between layers(Scan strategy XY), in which a cube texture with low
modulus < 001 > alined in the x-, y-, and z-directions
is expected to be obtained [28]. The relative densities ofthe fabricated specimens were determined using fieldemission scanning electron microscopy (FE-SEM; JIB4610F, JEOL, Japan). As a control, an ingot with the same
composition was prepared by arc melting.

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O. GOKCEKAYA ET AL.

Figure 1. Powder and LPBF-built Ti28.33 Zr28.33 Hf28.33 Nb6.74 Ta6.74 Mo1.55 BioHEA. (a) SEM image of gas-atomised powder and (b) appearance of LPBF-built jungle gym-shaped product.

The microstructures were analysed using an FE-SEM
equipped with energy-dispersive X-ray spectroscopy(EDS, Aztec 3.1, Oxford Instruments, UK) and an electron backscatter diffraction system (EBSD; NordlysMax3 ,Oxford Instruments). To quantitatively analyse theevolved texture, the degree p of 001 orientation along thebuild direction was calculated based on the Euler angles
obtained by EBSD using the following Equation (1):p001 = cos2 α001 ,(1)where α 001 is the angle between the observation direction and one of the equivalent orientations < 001 > . The
maximum value of p<001> is 1, which represents < 001 >perfectly oriented along the observation direction. To
evaluate the mechanical properties of the BioHEA, rectangular test pieces with dimensions of 2 × 2 × 5 mm wereprepared, where the 5 mm-length side was parallel tothe build direction. Compression tests (n = 3) were conducted using an Instron-type testing machine (AG-X,
Shimadzu, Japan) at a nominal strain rate of 1.67 × 10−4
s−1 at room temperature. A non-contact extensometer(TRViewX, Shimadzu) was used to detect the strain anddetermine the Young’s modulus.
A primary osteoblast culture experiment was performed according to a previously described protocol [8]to evaluate the biocompatibility of the BioHEA. Platesof 5 × 5 × 1 mm (n = 5) were prepared, and the surface for cell culture was mirror-polished. Specimens of
the same size as commercially pure Ti (CP-Ti), 316
L-type stainless steel (SUS316L), and arc-melted (ascast) BioHEA were also prepared for comparison. Celldensity was evaluated using Giemsa staining (FUJIFILM Wako Chemicals, Japan). Quantitative results are
expressed as mean ± standard deviation. Statistical significance was assessed using a two-tailed unpaired t-testor one-way analysis of variance (ANOVA) followed by

Tukey’s post-hoc test. A p-value < 0.05 was considered
statistically significant.

3. Results and discussion
All three samples prepared at different laser scanningspeeds exhibited high densities of 99.92%, 99.87%, and99.73% for specimens V1000 , V1200 , and V1600 , respectively. According to the X-ray diffraction analysis, only
BCC peaks were detected, and no peaks corresponding
to intermetallic compounds and/or BCC-based ordering
structures were observed in either the cast or LPBFfabricated specimens (data not shown). The overall compositions of the cast and LPBF-fabricated specimens arelisted in Table 1 and were approximately as designed.
Processing with LPBF caused little change in powder
composition. Figure 2(a,b) shows the SEM-BSE images
and the corresponding EDS maps of Ti, Zr, and Ta in the
cast specimen and LPBF-processed specimen (V1000 ). In
the cast specimen, the compositional differences between
the dendritic (bright contrast, indicated by ‘D’) and interdendritic (dark contrast) regions are obvious. The EDS
quantitative data in Figure 2(c) clearly indicate that Ti,
Zr, and Mo were enriched in the interdendritic region,and Hf, Nb, and Ta were enriched in the dendritic region
in the cast specimen. This segregation tendency agrees
well with the estimated distribution coefficients at theliquidus temperature using FactSage: 0.85, 0.93, 1.14,
1,13, 1.25, and 0.85 for Ti, Zr, Hf, Nb, Ta, and Mo,
respectively. Elemental segregation leads to low mixing entropy, in some cases below 1.5R, and causes thealloys to deviate from the definition of HEAs. In contrast, as expected, elemental segregation was largely suppressed in the LPBF-fabricated sample compared to thecast material, indicating the realisation of a super-solid
solution owing to ultrarapid cooling in LPBF.

MATER. RES. LETT.

277

Figure 2. Compositional distribution analyses. BSE images and EDS maps for Ti, Zr, and Ta of (a) LPBF-built and (b) cast
Ti28.33 Zr28.33 Hf28.33 Nb6.74 Ta6.74 Mo1.55 BioHEA specimen. (c) Concentrations of constituent elements in the dendrite and inter-dendrite
regions of the cast specimen and LPBF specimen determined by EDS. ∗: P < 0.05.

Figure 3(a) presents the inverse pole figure maps takenin the x-, y-, and z-axes, representing the developed distinctive crystallographic textures depending on the scan
speed. Figure 3(b) shows the corresponding 001 pole figures, and the orientation degree of < 001 > along thebuild (z-) direction is shown in Figure 3(c). Specimen
V1000 showed an intense single crystalline-like crystallographic texture where < 001 > was strongly oriented
along the x-, y-, and z-directions (the so-called cube texture), which is a typical texture that evolves under the
Scan strategy XY adopted in this study [28]. The crystallographic orientation was inherited across multiple layers
along the z-axis (Figure 3(a1)); the layer thickness was
60 μm. This indicates that epitaxial growth at the meltpool boundaries plays an important role in the intense
crystallographic textures. Once the cube texture is stabilised, epitaxial growth facilitates the inheritance of thecrystal orientation in both laser scans in the x- and ydirections. Consequently, grain boundaries with large
misorientations were eliminated, resulting in a single
crystalline-like texture. In other words, the reduction in
grain boundary energy would be the driving force behindthe stabilisation of the cube texture. In our previous
study, a bimodal microstructure was developed in whichfine equiaxed grains formed along the melt pool bottomthat intercepted epitaxial growth between the melt pooltracks and between the layers; hence, single crystalline
formation was not achieved [12]. This phenomenon was
attributed to a difference in the melting point based onelemental segregation, which was a challenge to overcome. Ti28.33 Zr28.33 Hf28.33 Nb6.74 Ta6.74 Mo1.55 BioHEA
in this study successfully achieved a single crystalline-liketexture by markedly suppressing segregation.
With increasing laser scanning speed, the texture
intensity significantly decreased, and a polycrystalline
structure with an almost random orientation was
obtained in specimen V1600 . This is closely related tothe solidification condition expressed by the relationship between the thermal gradient (G) and migration
rate of the solid–liquid interface (R). It has been reported
that R increases with an increase in scanning speed,
whereas G changes insignificantly [18,29]. Polycrystalline formation at high scanning speeds could be dueto the increased degree of compositional undercooling
owing to the decrease in G/R, and this trend of texturechange is consistent with previous reports for ordinary
alloys [18].
Figure 4(a) shows the yield stress of the cast and LPBFfabricated BioHEAs accompanied by fracture strain. The
cast specimens did not fracture under compression.

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O. GOKCEKAYA ET AL.

Figure 3. Crystallographic texture of the LPBF-built specimens. (a) Inverse pole figure maps taken in the orthogonal three planes and(b) the corresponding 001 pole figures. (c) Quantified degree of < 001 > crystallographic orientation along z-axis. ∗: P < 0.05.

Figure 4. Mechanical properties of the BioHEA: (a) Yield stress and fracture strain and (b) Young’s modulus of the cast and LPBF-built
Ti28.33 Zr28.33 Hf28.33 Nb6.74 Ta6.74 Mo1.55 BioHEA specimen; ∗: P < 0.05 by Tukey’s test; a: P < 0.05 vs cast; b: P < 0.05 vs V1600 by t-test.

The LPBF specimens exhibited a fracture strain of
approximately 30%, indicating high ductility. The yieldstress of the cast specimen reached 949 MPa, while LPBF
processing led to a significant improvement in the yieldstress, reaching 1355–1426 MPa, 1.4–1.5 times greaterthan that of the cast specimen. The higher yield stressin specimen V1600 than in V1000 and V1200 is becauseof the grain boundaries (Hall-Petch effect). However,the strengthening effect of the LPBF fabrication itself
is much greater than that, which could be due to theenhanced solid solution strengthening in the heavily distorted lattice [30], effectively achieved by suppressing elemental segregation, thus realising a super-solid solution.
These benefits were obtained by LPBF-specific ultrarapid
cooling.
Figure 4(b) shows the Young’s modulus of the LPBFprocessed BioHEA as a function of the < 001 > orientation degree. The lowest Young’s modulus (88.6 GPa)
was observed for specimen V1000 , which had the highest degree of < 001 > orientation and the most prominent single crystalline-like texture. Young’s modulus

increased as the degree of the < 001 > orientation
decreased. This indicates that this BioHEA exhibits
anisotropy in Young’s modulus depending on the crystallographic direction, as in Ti-15Mo-5Zr-3Al [20] andTi-29Nb-Ta-Zr [22], which exhibit an unstable β-phase.
The modulus of the LPBF-fabricated BioHEA was lower
than those of CP-Ti ( ∼ 110 GPa) and SUS316L ( ∼ 200
GPa). Further reduction of e/a toward four may increase
BCC instability, and further lower Young’s modulus, as
observed in Ti-based alloys. However, the relationshipbetween BCC phase stability and elastic modulus for
multi-component BioHEAs needs to be clarified in the
future to create highly functional metal biomaterials fororthopaedic applications.
Figure 5 shows the biocompatibility of the LPBFfabricated BioHEA together with that of the cast specimen, CP-Ti, and SUS316L. Osteoblast density (Figure5(a)) adhered to the specimens was evaluated using
Giemsa staining (Figure 5(b)). The cell density of the
LPBF-fabricated BioHEA was comparable to that of thecast material and CP-Ti and was significantly higher

MATER. RES. LETT.

279

Figure 5. Biocompatibility of LPBF-fabricated Ti28.33 Zr28.33 Hf28.33 Nb6.74 Ta6.74 Mo1.55 BioHEA in comparison with cast specimen, CP-Ti,
and SUS316L. (a) Quantitative analysis of cell density; ∗: P < 0.05. (b) Giemsa and (c) fluorescent staining images of osteoblasts adhesionon the specimens.

than that of SUS316L. Figure 5(c) shows fluorescentimages of cytoskeletal components and focal adhesionof osteoblasts adhered to the specimens. Osteoblasts on
SUS316L were observed to shrink in size with poor
cytoskeletal organisation. In contrast, osteoblasts on CPTi, cast BioHEA, and LPBF-processed BioHEA exhibiteda widespread morphology with a dense network of actin
fibres. Furthermore, compositional segregation in BioHEA has been shown to reduce the cell spreading area[12], which is essential for osteogenic differentiation [31].

4. Conclusion
In conclusion, our strategy, that is, the combinationof (a) compositional design that reduces the difference between liquidus and solidus temperatures andvalence electron concentration (e/a), and (b) LPBF technology that enables segregation suppression (formationof super-solid solution) owing to ultra-rapid solidification and crystallographic texture control, which cannot be achieved by other methods, resulted in thecreation of a promising BioHEA with a low modulus, excellent strength-ductility balance, and good cytocompatibility, all of which are essential for biomaterials. In addition, the LPBF offers the added value ofpatient-specific shape customisability. Thus, the LPBFfabricated TiZrHfNbTaMo BioHEA has great potential
as a next-generation metallic biomaterial with multiple
advantages.

Disclosure statement
No potential conflict of interest was reported by the author(s).

Funding
This work was supported by CREST-Nanomechanics: Elucidation of macroscale mechanical properties based on understanding nanoscale dynamics for innovative mechanical materials[grant number JPMJCR2194] from the Japan Science and Technology Agency (JST).

Data availability statement
Data will be made available on request.

ORCID
Ozkan Gokcekaya http://orcid.org/0000-0002-9258-858X
Takuya Ishimoto http://orcid.org/0000-0003-0081-0591
Aira Matsugaki http://orcid.org/0000-0002-8427-7124
Ryosuke Ozasa http://orcid.org/0000-0002-2773-072X
Markus Weinmann http://orcid.org/0000-0002-9029-7535

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O. GOKCEKAYA ET AL.

Christoph Schnitter http://orcid.org/0000-0001-5488-1737
Melanie Stenzel http://orcid.org/0000-0002-6357-1391
Hyoung Seop Kim http://orcid.org/0000-0002-3155-583X
http://orcid.org/0000-0002-9134Yoshitsugu Miyabayashi
9149
Takayoshi Nakano http://orcid.org/0000-0001-8052-1698

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