Scripta Materialia 194 (2021) 113658

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Scripta Materialia
journal homepage: www.elsevier.com/locate/scriptamat

Development of TiNbTaZrMo bio-high entropy alloy (BioHEA)
super-solid solution by selective laser melting, and its improved
mechanical property and biocompatibility
Takuya Ishimoto a,b, Ryosuke Ozasa a,b, Kana Nakano a, Markus Weinmann c,
Christoph Schnitter c, Melanie Stenzel c, Aira Matsugaki a, Takeshi Nagase a,d,Tadaaki Matsuzaka a, Mitsuharu Todai e, Hyoung Seop Kim f, Takayoshi Nakano a,b,∗aDivision of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
Anisotropic Design & Additive Manufacturing Research Center, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japanc
TANIOBIS GmbH, 78-91 Im Schleeke, Goslar D-38642, GermanydResearch Center for Ultra-High Voltage Electron Microscopy, Osaka University, 7-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japane
Department of Environmental Materials Engineering, National Institute of Technology, Niihama College, 7-1 Yagumo-cho Niihama, Ehime 792-8580, Japanf
Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang 37673, South Koreaba r t i c l e

i n f oArticle history:
Received 11 November 2020
Revised 2 December 2020
Accepted 2 December 2020
Available online 13 December 2020
Keywords:
Selective laser melting
High entropy alloy
BioHEA
Cooling rate
Segregation suppressiona b s t r a c t
BioHEAs, speciﬁcally designed high entropy alloy (HEA) systems for biomedical applications, represent a
new era for biometals. However, recent challenges are (1) the poor shape customizability, and (2) the
inevitable severe segregation due to the intrinsic fact that HEA is an ultra-multicomponent alloy system.
To achieve shape customization and suppression of elemental segregation simultaneously, we used an
extremely high cooling rate (~107 K/s) of the selective laser melting (SLM) process. We, for the ﬁrst time,developed pre-alloyed Ti1.4 Nb0.6 Ta0.6 Zr1.4 Mo0.6 BioHEA powders and SLM-built parts with low porosity,
customizable shape, excellent yield stress, and good biocompatibility. The SLM-built specimens showed
drastically suppressed elemental segregation compared to the cast counterpart, representing realizationof a super-solid solution. As a result, the 0.2% proof stress reached 1690 ± 78 MPa, which is signiﬁcantly
higher than that of cast Ti1.4 Nb0.6 Ta0.6 Zr1.4 Mo0.6 (1140 MPa). The SLM-built Ti1.4 Nb0.6 Ta0.6 Zr1.4 Mo0.6 BioHEA
is promising as a next-generation metallic material for biomedical applications.
© 2020 The Authors. Published by Elsevier Ltd on behalf of Acta Materialia Inc.
This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

Manuscript
It is highly desirable that next-generation metallic biomaterials for orthopedic and dental implants exhibit superior mechanical performance and biocompatibility and shape customizability to
ﬁt individual patients’ skeletons [1]. In this study, to the best of
our knowledge, we ﬁrst attempted to create such excellent metallic biomaterials using an approach that combines a state-of-theart material and manufacturing technology—a high-entropy alloy(HEA) for biomedical applications, called BioHEA, and selective
laser melting (SLM) as a powder bed fusion-type additive manufacturing (AM) method.
∗
Corresponding author at: Division of Materials and Manufacturing Science,
Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 5650871, Japan.
E-mail address: nakano@mat.eng.osaka-u.ac.jp (T. Nakano).

BioHEAs are speciﬁcally designed HEAs, composed of nonbiotoxic elements. For the ﬁrst time, our group proposed a novel
equiatomic Ti–Nb–Ta–Zr–Mo HEA and demonstrated its potential
as a new class of metallic biomaterials owing to its excellent
mechanical properties and biocompatibility compared to those ofpure Ti [2]. We then optimized the composition and developed
new alloy systems, resulting in Ti2.6 NbTaZrMo, Ti1.7 NbTaZrMo0.5 ,Ti1.5 NbTaZrMo0.5 , Ti1.4 Nb0.6 Ta0.6 Zr1.4 Mo0.6 [3–5], TiZrHfCr0.2 Mo,
and TiZrHfCo0.07 Cr0.07 Mo [6] with excellent biocompatibility. Many
researchers have developed new BioHEAs [7–9], and characterized their properties [7–11]. Research interest in BioHEAs is now
growing rapidly, and the issue to be overcome has been identiﬁed as the improvement of the integrity of BioHEAs. One reason
behind this drawback is the poor shape customizability and another is the remarkably high segregation tendency during solidiﬁcation [2,4,5,12,13]. Because HEAs are ultra-multicomponent alloy
systems, elemental segregation is highly likely. Extensive elemental

https://doi.org/10.1016/j.scriptamat.2020.113658
1359-6462/© 2020 The Authors. Published by Elsevier Ltd on behalf of Acta Materialia Inc. This is an open access article under the CC BY license(http://creativecommons.org/licenses/by/4.0/)

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Scripta Materialia 194 (2021) 113658

from the products. 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, Japan) wasused to detect the strain for the determination of Young’s modulus. To evaluate biocompatibility, the cell culture experiment was
performed according to a previously described protocol [6]. The
specimens were cut into plates of 5 mm × 5 mm × 1 mm. The
plate specimens of commercially pure Ti (CP-Ti), 316L-type stainless steel (SS316L), and arc-melted Ti1.4 Nb0.6 Ta0.6 Zr1.4 Mo0.6 alloy(as-cast) [4] were prepared as reference materials (n = 4–5). Primary osteoblasts were isolated from neonatal mice calvariae, diluted to 80 0 0 cells/cm2 , and cultured on the specimens for 24 h in
a 5% CO2 humidiﬁed atmosphere. Cell density was evaluated using Giemsa staining (FUJIFILM Wako Chemicals, Japan). Moreover,immunocytochemistry was evaluated [6]. The quantitative results
are expressed as the mean ± standard deviation. Statistical signiﬁcance was assessed using a one-way analysis of variance followed
by Tukey’s posthoc test. A P-value <0.05 was considered statistically signiﬁcant.
Fig. 1(a) shows the appearance and XRD pattern of the powders.
The XRD pattern indicated the BCC solid solution with a peak split.
SEM and EDS investigations represented a pore-free dendrite-type
microstructure (Supplementary Fig. 1S). While the dendrite phase(purple) was rich in Nb, Ta, and Mo compared with the overallcomposition, the inter-dendrite phase (green) was rich in Ti andZr.
Dense products with a porosity of less than 0.5% were obtained(Fig. 1(b)). The top surface of the product was overall ﬂat. There
was no lack of fusion type pores; therefore, the laser energy used
for fabrication was appropriate for this alloy. The XRD peaks of the
product were indexed as BCC structure (Fig. 1(b)). It should be emphasized that the peak separation was reduced, indicating a solid
BCC solution with suppressed elemental segregation. No peaks corresponding to intermetallic compounds and/or BCC-based ordering
structures were observed. Moreover, a jungle gym shape without
any cracks and deformation or beam fragmentation was successfully achieved via SLM (Fig. 1(c)).
The SLM-built Ti1.4 Nb0.6 Ta0.6 Zr1.4 Mo0.6 BioHEA product exhibited a unique solidiﬁcation microstructure, as shown in Fig. 2. Oneof the remarkable features was the formation of a ﬁne grain structure with the segregation suppression and a bimodal grain size distribution. Inverse pole ﬁgure (IPF) maps at low magniﬁcation takenin the xy-plane (Fig. 2(a1, a2)) demonstrated a stripe-like patternwith a relatively coarse grain region and a very ﬁne grain region.
From the IPF map in the yz-plane (Fig. 2(a3, a4)), it could be recognized that ﬁne grains were equiaxed and present along the meltpool edge. In the yz-plane, the melt pool transverse cross-section
formed by the X-scan with semi-elliptical shape and the longitudinal melt pool cross-section formed by the Y-scan running horizontally were stacked alternately along the z-direction. In the relatively coarse grain region, the grains elongated vertically in themelt pool longitudinal cross-section and elongated radially toward
the melt pool center in the transverse cross section. The overall
crystallographic texture can be described as a ﬁber texture where{100} preferentially orients in the z-direction (Fig. 2(a5)).
To analyze the microstructure of ﬁne grain region which is acharacteristic of this material, observations at high magniﬁcation
were conducted. From the backscattered electron (BSE) image(Fig. 2(b1)), ﬁne grains were observed at the bottom of the melt
pool. The average ﬁne grain size was 0.25 μm based on the highangle grain boundary (Fig. 2(b3)). The crystallographic texture of
the ﬁne grain region was almost random. Immediately beneaththe ﬁne grain region, there were elongated grains with {100}orients in the z-direction. The average dimensions of the elongatedgrains were 2.5 μm and 31 μm in width and length, respectively.

segregation would lead to a heterogeneous elemental distributionthat could reduce sophisticated properties peculiar to BioHEAs.
SLM is a suitable method for simultaneously achieving shapecustomization and suppressing segregation during solidiﬁcation.
Regarding elemental segregation during solidiﬁcation, the coolingrate is considered to be the most important factor. Cooling rates oftypical solidiﬁcation processes, such as casting, centrifugal metallic mold casting, arc-melting, and single-roller melt spinning, arereported to be < 100 K/s [14], in the order of 100 K/s [15], approximately 20 0 0 K/s [3], and 105 K/s [16], respectively. In contrast, the cooling rate in SLM process has been estimated to be
105 –107 K/s [17,18]. We hypothesized that this ultrahigh coolingrate during SLM prevents extensive elemental segregation, leading to high-performance BioHEA parts. However, AM fabrication of
TiNbTaZrMo BioHEA has not yet been attempted due to the diﬃculty of fabricating pre-alloyed powders with a targeted composition. The constituent elements in BioHEAs developed to date weresimilar to those in refractory HEAs (RHEAs) [19], as producing prealloyed powders was extremely diﬃcult because of the high melting temperatures of the constituent elements.
Here, we report the production of BioHEA parts via the SLM
method using pre-alloyed BioHEA powders. Furthermore, the formation of characteristic solidiﬁcation microstructures in BioHEA
products through the segregation suppression during solidiﬁcation,and the simultaneous achievement of superior mechanical properties and biocompatibility are discussed. We demonstrate the potential of additively manufactured BioHEA for biomedical applications.
The Ti1.4 Nb0.6 Ta0.6 Zr1.4 Mo0.6 BioHEA, which was modiﬁed fromequiatomic TiNbTaZrMo to reduce the melting point and improve
the room-temperature ductility [4], was used. The empirical alloyparameters [20,21] of mixing entropy, mixing enthalpy, difference
in atomic radii, -parameter, and average valence electron concentration in Ti1.4 Nb0.6 Ta0.6 Zr1.4 Mo0.6 were 1.52R, where R is the gas
constant, –0.75 kJ/mol, 6.15, 38.0, and 4.52, respectively. The alloyparameters indicate a high solid solution formation tendency witha body-centered cubic (BCC) structure in Ti1.4 Nb0.6 Ta0.6 Zr1.4 Mo0.6
BioHEA. The distribution coeﬃcients of each element at the liquidus temperature estimated by thermodynamic calculations using
FactSage and SGTE2017 were 0.72, 1.18, 1.54, 0.80, and 1.41 for Ti,
Nb, Ta, Zr, and Mo, respectively, which deviate signiﬁcantly from 1
and indicate a strong tendency for two-phase separation via segregation, as reported previously [4].
Ti1.4 Nb0.6 Ta0.6 Zr1.4 Mo0.6 BioHEA spherical powders were obtained by electrode induction melting gas atomization of prealloyed rods (Fig. 1(a)). Details of the applied process will be published separately. The as-obtained powders were conditioned bysieving to 10–63 μm (D50: 26.8 μm).
Products with dimensions of 5 mm (depth) × 5 mm(length) × <10 mm (height) were manufactured in an Ar atmosphere using an SLM apparatus (EOS M290, EOS, Germany) witha laser power, scanning speed, hatch distance, and layer thicknessof 360 W, 1200 mm/s, 80 μm, and 60 μm, respectively. Here, thebuild direction was deﬁned as the z-direction, and the two laser
scanning directions were the x- and y-directions. To demonstrate
the shape customizability, we also attempted to fabricate a jungle
gym-structured BioHEA with a side of 10 mm and a beam width of
0.7 mm. The specimen’s constituent phases, microstructure, composition distribution, and crystallographic orientation distribution
were investigated by X-ray diffraction (XRD) (X’Pert PRO, PANalytical, Netherland), energy dispersive X-ray spectroscopy (EDS) (Astec
3.1, Oxford Instruments, UK), and electron backscatter diffraction(EBSD) (NordlysMax3 , Oxford Instruments, UK) with ﬁeld-emission
scanning electron microscopy (FE-SEM) (JIB-4610F, JEOL, Japan). To
evaluate the mechanical properties, rectangular specimens with dimensions of approximately 2 mm × 2 mm × 5 mm were cut
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Scripta Materialia 194 (2021) 113658

Fig. 1. Powder and SLM-built Ti1.4 Nb0.6 Ta0.6 Zr1.4 Mo0.6 BioHEA, and their XRD patterns. (a) SEM image and XRD pattern of gas-atomized powder, (b) appearance and crosssectional optical microscope image, and XRD pattern of SLM-built cube specimen, and (c) appearance of SLM-built jungle gym-shaped product.

Fig. 2. Microstructural and crystallographic analyses of SLM-built Ti1.4 Nb0.6 Ta0.6 Zr1.4 Mo0.6 BioHEA specimen. (a) IPF maps taken in the xy- and yz-planes, and pole ﬁgure
representing a ﬁber texture in which {100} preferentially orients in z-direction. The black line that is seen in the IPF maps indicates not the crack but the part where the
Kikuchi line could not be analyzed in EBSD analysis at low magniﬁcation. The very ﬁne grains shown in (b3) are present in this portion. (b) BSE image and the corresponding
IPF map, and magniﬁed IPF map in the squared part in (b1). The very ﬁne grains with random crystallographic orientation are present at the melt pool bottom and relatively
large elongated grains occupies other regions in the pelt pool, resulting in characteristic bimodal microstructure.

The length was much shorter than the 60-μm layer thickness.
The interface (broken lines in Fig. 2(b1, b2)) between the ﬁne
equiaxed grain region and the underlying elongated grain region
corresponded to the melt pool boundary and was smooth, andno crystallographic orientation continuity was present (Fig. 2(b2,b3)). The microstructure conformation inside a melt pool (ﬁneequiaxed grains on the bottom and elongated grains in the centerof the melt pool) is opposite to what is explained based on theconventional solidiﬁcation theory [22,23], in which the balance
between the thermal gradient (G) and the migration velocity ofthe solid/liquid interface (R) determines the microstructure type.

At the melt pool bottom, a higher G/R is observed due to the large
G and small R. As the solidiﬁcation progresses toward the centerof the melt pool, G decreases and R increases; thus, G/R decreases[24,25]. As a result, equiaxed grains normally appear in the meltpool center with low G/R [18]. The equiaxed grains are eventually
re-melted by the melting of the next layer, enabling epitaxial
growth at the melt pool boundary. In contrast, equiaxed grains
without preferential orientation at the bottom of the melt poolobserved in this study hindered epitaxial crystal growth along
the z-direction. The bimodal solidiﬁcation structure could not beeffectively explained only by the consideration of G/R. The possible

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Scripta Materialia 194 (2021) 113658

Fig. 3. Compositional distribution analyses of SLM-built Ti1.4 Nb0.6 Ta0.6 Zr1.4 Mo0.6 BioHEA specimen. (a–c) BSE images, (d–f) EDS maps for Zr, and (g–i) EDS maps for Ta. The
difference in concentration was slight, indicating that the elemental segregation was considerably suppressed via SLM. The EDS images are displayed with high contrast for
the visibility of a slight difference in concentration.

mechanism for the formation of this unique bimodal structure isdiscussed below.
Fig. 3 shows the BSE images and EDS compositional distribution
maps taken around the ﬁne grain region to investigate the segregation tendency in the SLM products. As expected, elemental segregation was largely suppressed in the SLM-built specimen compared to the cast material prepared by arc melting (Supplementary
Fig. S2). SLM-built specimens also showed dendritic solidiﬁcation
structures, but their size was quite small, and the composition distribution was almost uniform. In the SLM-built specimen, Ti andZr tended to be slightly enriched in the inter-dendrite region, andNb, Ta, and Mo in the dendrite region; this is a similar tendency
to that in the cast material (Supplementary Fig. S2) [4] and thepowder (Supplementary Fig. S1). The EDS quantitative data (Supplementary Table S1) clearly indicated that, as expected, the SLM
process effectively suppressed elemental segregation and realizeda super-solid solution owing to ultrahigh cooling rate.
The ﬁne dendrite structure formation in the SLM process resulted from the increased nucleation frequency at the bottom of
the melt pool. Under high temperature by laser irradiation, thepreviously solidiﬁed part is re-melted, and the dendrite tip canbe detached and then transported into the melt, acting as a favorable nucleation site [26]. This is one possible condition for theﬁne grain formation at the bottom of the melt pool.
The epitaxial growth at the melt pool boundary plays a crucial role in crystallographic texture formation. The implementationof epitaxial growth can result in a highly textured microstructure[27–30]. By applying scan strategy XY to metallic materials withcubic crystal structures in which <100> is an easy growth direction [31], a cube texture where {100} is aligned in the x-, y-, andz-directions is obtained [32–35]. Once the cube texture is stabilized, it is easy to inherit a crystallographic orientation of both Xand Y-scans through epitaxial growth. Therefore, grain boundaries
with a large angle difference are culled, and a single crystallinelike structure is obtained. Namely, the decrease in grain boundary
energy becomes the driving force for stabilizing the cube texture.
In this study, the formation of ﬁne grains became an obstacle toepitaxial growth. Only the grains with {100} approximately parallel to the heat ﬂow direction selectively survived and grew in theheat ﬂow direction (z-direction), resulting in the ﬁber texture evolution with {100} orientation in the z-direction.
The SLM-built Ti1.4 Nb0.6 Ta0.6 Zr1.4 Mo0.6 BioHEA showed excellent mechanical properties. The typical true stress-true plasticstrain curve of the specimen is shown in Supplementary Fig. S3.
The 0.2% proof stress reached 1690 ± 78 MPa, which is signiﬁcantly higher than that of cast Ti1.4 Nb0.6 Ta0.6 Zr1.4 Mo0.6 (1140 MPa)[4]. True strain at break was 1.32 ± 0.19%, indicating moderateroom-temperature ductility. This remarkable increase in the proof
stress should be due to the combined effect of grain boundary
strengthening owing to grain reﬁnement and enhanced solid solution strengthening [36] achieved by suppressing elemental segregation and thus realizing super-solid solution. These beneﬁts are
drawn by the rapid SLM cooling rates. The Young’s modulus wasmeasured to be 140 ± 9 GPa, an intermediate value of frequently
used metallic biomaterials, such as CP-Ti, Ti–6Al–4V (110 GPa), andSS316L (~200 GPa) [37]. We have succeeded in imparting excellent
mechanical properties to a BioHEA as a metallic biomaterial using
the rapid solidiﬁcation feature that is unique to SLM.
Fig.
4
shows
the
biocompatibilityofSLM-built
Ti1.4 Nb0.6 Ta0.6 Zr1.4 Mo0.6 , together with as-cast counterparts,

4

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Scripta Materialia 194 (2021) 113658

Fig. 4. Biocompatibility analysis of SLM-built Ti1.4 Nb0.6 Ta0.6 Zr1.4 Mo0.6 BioHEA specimen. (a) Giemsa staining images of osteoblasts cultured on SLM-built specimen and reference materials of CP-Ti, SS316L, and cast BioHEA counterpart. (b) Quantitative analysis of cell density. ∗ : P < 0.05. (c) Fluorescent images of osteoblast adhesion on SLM-built
BioHEA, CP-Ti, SS316L, and cast BioHEA. (d) Localization of osteoblast adhesion spots on the cast BioHEA specimen with extensive elemental segregation and analyzed cellarea for cast and SLM-built BioHEAs. On cast BioHEA, cell ﬁlopodia preferentially (rather restrictedly) locates on the inter-dendrite (ID) region with enriched Ti and Zr,
resulting in inhibited cell spreading. ∗ : P < 0.05.

CP-Ti, and SS316L. Fig. 4(b) shows the osteoblast density adhered
to the specimens evaluated by Giemsa staining images (Fig. 4(a)).
The cell density on the SLM-built specimen was comparable tothat on the CP-Ti and cast counterpart, and was signiﬁcantly
higher than that on SS316L. Moreover, a cell morphology, which
directly inﬂuences the following cellular events involving tissue ororgan construction, provided characteristic properties among thealloys examined. Fig. 4(c) shows ﬂuorescent images of cytoskeletalcomponents and focal adhesions of osteoblasts adhered to thespecimens. The osteoblasts on the SLM-built specimen showed awidespread morphology with a dense network of actin ﬁbers, andthose on CP-Ti and cast specimens. Moreover, the uniform distribution of the composition in SLM-built specimens exhibited distinct
advantages for cell spreading, reﬂecting the cells’ osteogenic
ability [38]. The elemental segregation in cast material restricted

the ﬁlopodia distribution in osteoblasts by holding the adhesion
spots in Ti- and Zr-enriched inter-dendrite regions, resulting in
the reduced cell spreading area on cast material (Fig. 4(d)). Tiand Zr-enrichment likely induced the extracellular matrix proteinsatisﬁed microenvironment to develop cell adhesion molecules
involving integrin. The SLM-built Ti1.4 Nb0.6 Ta0.6 Zr1.4 Mo0.6 was
found to be much more advantageous than SS316L for bone tissuecompatibility, and the biocompatibility was comparable to that of
CP-Ti, the most common metals used in biomedical applications.
In conclusion, the extremely high cooling rate in the SLM process effectively suppressed the elemental segregation even for
the ultra-multi-component HEA system, which was not achieved
via conventional solidiﬁcation processes. The SLM-built specimens
showed superior yield strength and comparable biocompatibility tocast counterparts with custom-made shape capability. The SLM5

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Scripta Materialia 194 (2021) 113658

built Ti1.4 Nb0.6 Ta0.6 Zr1.4 Mo0.6 BioHEA is a promising near-future
metallic biomaterial.
This work was supported by Grants-in-Aid for Scientiﬁc Research from the Japan Society for the Promotion of Science (JSPS)[grant number JP18H05254]. This work was also partly supportedby the Cross-Ministerial Strategic Innovation Promotion Program(SIP) – Materials Integration for Revolutionary Design System ofStructural Materials – Domain C1: “Development of Additive Manufacturing Process for Ni-based Alloy” from the Japan Science andTechnology Agency (JST).

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Declaration of Competing Interest
The authors declare that they have no known competing ﬁnancial interests or personal relationships that could have appeared toinﬂuence the work reported in this paper.
Supplementary materials
Supplementary material associated with this article can befound, in the online version, at doi:10.1016/j.scriptamat.2020.
113658.
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