Materials & Design 233 (2023) 112265

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Materials & Design
journal homepage: www.elsevier.com/locate/matdes

Laser beam powder bed fusion of novel biomedical titanium/niobium/
tantalum alloys: Powder synthesis, microstructure evolution andmechanical properties
Jan Johannsen a, Christian Lauhoff b, Melanie Stenzel c, Christoph Schnitter c,
Thomas Niendorf d, Markus Weinmann c, *aFraunhofer Einrichtung für Additive Produktionstechnologien, IAPT, Am Schleusengraben 14, 21029 Hamburg, Germany
RMIT Centre for Additive Manufacturing, School of Engineering, RMIT University, Melbourne, VIC 3000, AustraliacTaniobis GmbH, Im Schleeke 78-91, 38642 Goslar, GermanydUniversität Kassel, Institut für Werkstofftechnik, Metallische Werkstoffe, Mönchebergstr. 3, 34125 Kassel, Germanyb

A R T I C L E I N F O

A B S T R A C T

Keywords:
Electrode induction melting gas atomization(EIGA)
Laser beam powder bed fusion (PBF-LB/M)
Biomedical alloys
Pre-alloyed refractory metal powder

The synthesis of spherical titanium/niobium/tantalum (TNT) alloy powders, namely Ti-20Nb-6Ta, Ti-27Nb-6Ta,
Ti-35Nb-6Ta, and Ti-22Nb-19Ta (in wt-%) by electrode induction melting gas atomization is reported. The
powder materials are characterized in detail using X-ray diffraction and scanning electron microscopy. Their
processability via laser beam powder bed fusion (PBF-LB/M) is proven, and microstructure as well as mechanicalproperties of the additively manufactured specimens are assessed. All powders feature a dendrite-type
microstructure with Nb/Ta-rich dendritic and Ti-rich inter-dendritic phases. Crystal structures of the powders
are strongly composition-dependent. Nb- and Ta-rich Ti-35Nb-6Ta and Ti-22Nb-19Ta feature a body-centeredcubic lattice, whereas Ti-rich Ti-20Nb-6Ta and Ti-27Nb-6Ta powders are characterized by multi-phasemicrostructures, consisting of non-equilibrium martensitic phases. Processing by PBF-LB/M causes significant
changes in their microstructures: the dendrite-type morphologies vanish, and the formation of microstructureswith a homogeneous element distribution can be observed in all additively manufactured parts. Ultimate
tensile strength (UTS) as well as elongation at fracture are assessed by tensile testing. UTS values are found to be
in a range from 651 MPa (Ti-35Nb-6Ta) to 802 MPa (Ti-20Nb-6Ta); strain-to-failure is between 21.3 %(Ti-35Nb-6Ta) and 31.7 % (Ti-22Nb-19Ta). Ductile fracture behavior is seen for all TNT alloys investigated.

1. Introduction
Titanium and its alloys, such as Ti-6Al-4V and Ti-6Al-7Nb (if not
explicitly indicated, then all values are given in wt-%), are state-of-theart implant materials [1-7]. These materials feature good biocompati­
bility, i.e., cell tolerance and osseointegration. Ti-6Al-4V and Ti-6Al7Nb are preferred over commercially pure titanium (CP-Ti) due totheir higher strength and superior corrosion resistance [8,9]. The latter
results in a reduced release and exposure of metal ions into the humanorganism. Accordingly, implant-associated inflammations are likewise
reduced [10].
However, an detrimental issue of CP-Ti, Ti-6Al-4V and Ti-6Al-7Nb
for implant integration in the human body is their high Young’s
moduli ranging from 108 to 115 GPa. Highly diverging elastic propertiesof implant and cortical bone (Young’s modulus of 12 – 25 GPa) promotestress-shielding effects at the implant-bone interphase [11,12]. There­
fore, bone ingrowth, i.e., osseointegration of the implant, can be unfa­
vorably affected and, thus, implant loosening and/or osteolysis may
occur [13,14]. Moreover, the application of Ti-6Al-4V as an implantmaterial must be questioned due to toxicological concerns: vanadium
has been shown to be toxic [15,16] and aluminum is suspected to cause
Alzheimer’s disease [17-19]. Replacement of these elements in perma­
nent implants is consequently anticipated.
In the last two decades, extensive work has been carried out toexplore novel biocompatible Ti-based alloys, featuring mechanicalproperties being close to those of human bone [3,5,6,20,21]. Promising
new alloy systems are characterized by a favorable combination of highelasticity (a low Young’s modulus), high strength, good fatigue* Corresponding author.
E-mail address: markus.weinmann@taniobis.com (M. Weinmann).
https://doi.org/10.1016/j.matdes.2023.112265
Received 20 April 2023; Received in revised form 18 August 2023; Accepted 20 August 2023
Available online 23 August 2023
0264-1275/© 2023 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

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Materials & Design 233 (2023) 112265

properties and sufficient biocompatibility. Binary, ternary as well as
higher-component systems and, recently, even Ti-based high-entropy
alloys have been introduced for use as metallic implant materials[22,23,24]. To prove biocompatibility, many efforts have been made inrecent studies to assess the interaction of human cells with these alloys.
From the results obtained it was concluded that bio-tolerant refractory
metals (RM) such as Zr, Hf, Nb and Ta seem to be very promising as
alloying elements.
Among the binary systems, Ti/Nb [25,26,27,28,29] and Ti/Ta[30,31,32,33] seem to be the most promising materials for application inorthopedic implants. Alloying Ti with either Nb or Ta leads to similar
characteristics since Nb and Ta are each β-phase stabilizing elements
featuring highest biocompatibility [34,35,36]. Conventionally pro­
cessed Ti-based alloys with low Nb or Ta content show a martensitic
α′-phase microstructure at room temperature (RT). With increasing Nb
or Ta contents, in turn, the formation of martensitic α″-, dual β + α″-, or
single β-phase alloys are observed [30]. The Young’s moduli areconcentration-dependent in both systems, featuring two minima foreach alloy, i.e., Ti-16Nb and Ti-42Nb as well as Ti-30Ta and Ti-70Ta,
respectively [37]. The lowest moduli are observed for body-centeredcubic (bcc) β-phase microstructures, i.e., for Nb- and Ta-rich alloys.
Combining Ti with both Nb and Ta, leads to ternary Ti/Nb/Ta alloys,
which have also attracted scientific attention. Even though some alloy
compositions have been proven to exhibit shape memory effect orsuperelasticity [38,39,40], the scientific focus is predominantly on
biomedical applications. Bertrand et al. investigated Ti-25Nb-25Ta
processed by cold crucible levitation melting [41,42]. Tensile tests forthis β-phase alloy revealed a very low Young’s modulus of about 55 GPa
as well as good ductility and strength, i.e., 20 % elongation at fracture
and about 530 MPa UTS. Biocompatibility, i.e., corrosion behavior andcytotoxicity of Ti-30Nb-18Ta (in the original publication referred to as
Ti77Nb17Ta6) and partially oxidized Ti-30Nb-18Ta(O) alloys were re­ported by Husein et al. and compared with Ti-6Al-4 V [43]. The arcmelted, solution-treated and finally 50 % cold-rolled material predom­
inantly consisted of the orthorhombic α″ martensitic phase embedded ina β-phase matrix. A comparison of the corrosion current density of Ti30Nb-18Ta, Ti-30Nb-18Ta(O) and Ti-6Al-4V clearly revealed an effi­
cient passivation of Ti-30Nb-18Ta due to the presence of Nb and Ta,
both forming chemically inert native oxide surfaces.
Powder metallurgical approaches for fabricating Nb-25Ti-xTa (x =10, 15, 20, 25, 35 at-%) alloys have been reported by Liu et al., [44].
Green compacts were sintered at 1600, 1700 and 1800 ◦ C in a vacuum(10− 3 Pa) for 2 h, followed by investigations on their relative densities,
mechanical strength and Young’s moduli. Interestingly, due to Ta par­ticle segregation dispersion strengthening of the materials was observedwith increasing Ta content.
The focus of the present study is on TNT alloys in the Ti-rich domain
for medical applications, i.e., dental and orthopedic implants with thehighest level of biocompatibility. To keep the density of the targeted
implants low, Nb (ρ = 8.57 g/cm3) was used as the main alloying con­stituent, whereas Ta (ρ = 16.65 g/cm3) was added in lower concentra­tion. Accordingly, Ti-xNb-6Ta (x = 20, 27, 35 wt-%) alloys were
fabricated and characterized in depth. For comparison, Ti-22Nb-19Ta
containing almost equal contents of Nb and Ta, was also produced andinvestigated.
TNT powder materials have been prepared from pre-alloyed elec­
trodes by electrode induction melting gas atomization (EIGA) [45,46].
The powders were chemically and structurally characterized in detailand conditioned for application in additive manufacturing (AM) tech­
nologies, i.e., for PBF-LB/M. In the biomedical sector, this technology
offers great potential to produce personalized orthopedic implants withhighest fitting accuracy [47,48]. The required patient-specific structural
data for creating a 3D model can easily be obtained from examinations
using computed tomography (CT) or magnetic resonance imaging (MRI)[49,50].

2. Materials and methods
2.1. Powder synthesis and refinement
Spherical titanium/niobium/tantalum (TNT) powders were obtained
from pre-alloyed electrodes by EIGA conducted under purified argon(4.6, Linde) atmosphere [51]. The nominal chemistry as well as chem­
ical compositions of the powder materials determined by inductively
coupled plasma optical emission spectroscopy (ICP-OES; Analytik Jena
PlasmaQuant PQ 9000/9100), carrier gas hot extraction (LECO TCH600/ONH-836) and/or combustion analysis (LECO CS-744) are listed
in Table 1.
As the as-atomized powders featured minor fractions of coarse and/or irregular-shaped particles, all powder materials were sieved using
150 µm meshes. In the following, the powders were transferred into an
air classifier for deagglomeration and removal of fine particles < 10 µm
to improve flow properties and avoid dusting during the manufacturing
process. From the remaining powders, the PBF-LB/M particle size frac­tion (10 – 63 µm) was extracted by ultrasonic sieving with 63 µm
meshes. Particle size distributions (PSD) were determined using a
Master Sizer 2000 (Malvern, Worcestershire, United Kingdom).
2.2. Laser beam powder bed fusion
PBF-LB/M was performed using a commercially available system (3D
Systems, DMP350 Flex) equipped with an 1 kW single-mode laser (YLR1000-WC-Y14 by IPG). Argon gas was used as shielding gas to prevent
oxidization during manufacturing. The oxygen concentration of theclosed build chamber was kept below 50 ppm during processing. To
evaluate suitable processing windows, cuboids of 10 × 10 × 11 mm3 insize were fabricated. A stripe based, bi-directional scanning strategy
with a stripe width of 5 mm (cf. Fig. 1) was employed, while the scan­ning direction was rotated by 67◦ from layer to layer. Heating of the baseplate was not applied, even though such heating has been numerously
reported to be advantageous for PBF-LB/M processing, especially whenless ductile Ti alloys and intermetallics are processed [52]. However, as
will be shown below, the TNT powder materials studied in the present
work show excellent processability at RT.
In order to elaborate adequate process parameter windows, laser
power (PL) and scan speed (vs) were varied between 135 and 280 W and
1050 – 1500 mm/s, respectively. The hatch distance (Hs) was kept
constant at 69 µm, while the powder layer thickness was set to 30 µm.
Resulting energy per unit area (EA) values were calculated by: [53]()
J
PL (W)
EA= mm
2mm
vS ( s ) • HS (mm)Following porosity analysis (details are given in Section 3.2), foreach TNT alloy the set of process parameters leading to the highest
density value was used to fabricate specimens for mechanical testing.
Stacks of flat tensile test specimens (blocks of 12 mm in height, Fig. 2)
were manufactured and cut into 2 mm thick slices using electricaldischarge machining (EDM).
2.3. Microstructural and mechanical characterization
For residual porosity analysis, the initially PBF-LB/M processed
Table 1
Chemical compositions of the TNT powders after gas atomization.

2

Nominalwt-%

Ti-20Nb-6Ta
Ti-27Nb-6Ta
Ti-35Nb-6Ta
Ti-22Nb-19Ta

Ti
73.75
66.98
58.87
59.02

ppm
Nb
19.98
26.94
34.45
21.79

Ta
5.96
6.02
5.98
18.58

C
73
51
33
32

H
<45
<10
<10
13

N
74
206
147
93

O
2556
2278
1791
2409

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Materials & Design 233 (2023) 112265

Fig. 1. Schematic illustrating the scan strategy used for PBF-LB/M processing: a bi-directional stripe scanning strategy for the core and a one vector width contouring
strategy. From layer to layer the scanning direction was rotated by 67◦ counterclockwise.

Fig. 2. Schematic showing the tensile specimen geometry with a gauge length (L0) of 16 mm. A stack of flat tensile specimens was manufactured according to thefinal contour (in the X-Y plane) and cut into 2 mm thick slices.

s− 1 [55]. Strains were calculated based on the crosshead displacement
and the gauge length of the specimens.

cuboids were cut parallel and perpendicular to the build direction (BD)using EDM, followed by mechanical grinding and polishing. Optical
micrographs of the surfaces recorded using light microscopy (LM, VHX5000 by KEYENCE) were converted to black and white, enabling the
calculation of the porosity from the number of black and white pixels.
For initial microstructure analysis via LM, the specimens were addi­
tionally etched for 45 s using a solution of ammonium persulphate andsodium fluoride in deionized water (TitanEtch by Titan Tech Europe).
Moreover, scanning electron microscopy (SEM) including energy
dispersive X-ray spectroscopy (EDS) and electron backscatter diffraction(EBSD) were employed. EDS analysis was conducted on both polished
powder particle cross-sections as well as additively manufactured
specimens to assess the local and global (average) chemical compositionof the TNT powders and the bulk materials after processing, respec­tively. For EBSD, specimens were mechanically ground and electropolished for 35 s at 30 V and 1.1 A using A3 electrolyte (Struers).
Employing a Panalytical X-ray diffractometer (Almelo, Overijssel,
Netherlands; X’Pert MPD-PRO, Cu Kα radiation), X-ray diffraction on the
TNT powders and as-built specimens was conducted with Cu Kα radia­
tion in a 2θ range between 10 and 80◦ to assess the phase composition.
Indexing of the diffractograms was performed using the ICDD pdf-2
database [54].
To investigate the mechanical properties, quasi-static uniaxial tensile
tests were conducted on a ZwickRoell AllroundLine 10kN testing ma­chine at RT in displacement control with a nominal strain rate of 0.0067

3. Results and discussion
3.1. Powder synthesis and characterization
The PSDs of all TNT alloy powders following sifting and subsequent
sieving (cf. Section 2.1) with 63 µm meshes are shown in Fig. 3. All
powders feature a monomodal distribution. While for the Ti-22Nb-19Ta
powder a small number of particles with diameters in the range of 5 – 12
µm is present, particles smaller than 15 µm are not found in the otherpowder materials. In case of Ti-22Nb-19Ta, however, the minor 5 – 12µm fraction observed has no obvious detrimental effect on the flow
properties of the powder material, as determined by Hall Flow testingaccording to ASTM B213-20 [56]. The upper particle size was limited by
the 63 µm mesh width and, thus, D95 values are between 63 and 69 µm
for the different powders.
Fig. 4 displays SEM results of the powder materials. The secondary
electron (SE) images proof that all TNT alloys are obtained as highly
spherical particles, featuring smooth surfaces without visible cracks. Ti20Nb-6Ta (Fig. 4a) and Ti-27Nb-6Ta (Fig. 4b) particles are fully deag­
glomerated, whereas minor agglomeration is observed for the other two
alloy powders. However, the marginal degree of agglomeration did notcause any issues during PBF-LB/M processing. Structures of highest
3

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Materials & Design 233 (2023) 112265

Fig. 3. Particle size distribution of the TNT alloys after sifting and sieving.

Fig. 4. SEM images of the TNT alloy powders after sifting and sieving. White arrows in (c) and (d) highlight agglomerations found in case of the Ti-35Nb-6Ta and Ti22Nb-19Ta powders.

density could be obtained for each material as shown below (cf. Fig. 7
and Table 3).
SEM/EDS results of polished cross-sections are provided in Fig. 5,
showing particles with a dense, pore-free morphology. Quantitative
results of the local and the overall chemistry are listed in Table 2. The
TNT powder particles exhibit a dendrite-type microstructure with
noticeable chemical fluctuations between dendritic and inter-dendritic
regions. The overall chemical composition of the particles (areas 5 inFig. 5 and Table 2) is in good agreement with data experimentally
determined by ICP-OES (cf. Table 1). Only the Ti values are determinedslightly too high in the EDS analysis. Accordingly, Nb and Ta values are
generally found to be slightly too low, except for Ti-35Nb-6Ta, in which
an overall content of 6.5 wt-% of Ta is determined by EDS. For example,
EDS of Ti-20Nb-6Ta (entire particle, area 5) reveals an overall compo­
sition of Ti-17.4Nb-5.3Ta. The deviations from the data experimentally
determined by ICP-OES are: Ti + 3.3 wt-%, Nb − 2.6 wt-%, Ta − 0.7 wt%.
Assessment of the local chemistry within the individual specimens(cf. spots 1 – 4 in Fig. 5 and Table 2) reveals different chemical com­positions for the dendritic and inter-dendritic regions. For example,
4

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Materials & Design 233 (2023) 112265

Fig. 5. SEM/EDS cross-section mappings of the TNT alloy powder particles. The chemical composition was determined locally in dendritic (spots 1, 2) and interdendritic regions (spots 3, 4), as well as globally (area 5). Quantitative results are provided in Table 2.

processes. During solidification which takes place at the solid/liquidinterface, preferential crystallization of niobium (2477 ◦ C) and tantalum(3017 ◦ C) occurs due to their significantly higher melting points
compared to titanium (1668 ◦ C). As a result, niobium and tantalum areenriched in the dendritic structures which initially form, while titanium
accumulates in the inter-dendritic domains. Such observations have also
been reported for other high-alloyed materials with significant differ­ences in the melting points of the constituents, e.g., gas-atomized Ti42Nb [57], Al-modified Ti-Ta alloy [58] or low-density Al-steels [59].
Against this, the microstructures of the additively manufactured TNT
specimens, which are discussed below, do not comprise dendritic fea­tures. Due to the very high cooling rates of 1 – 40 K/µs in the PBF-LB/M
process, a homogeneous distribution of all constituting elements isfinally obtained [60].
One of the main motivations for varying the chemistry in the present
study is to assess the influence of the chemical composition on the phaseevolution and mechanical properties of bulk structures fabricated fromthe powder feedstock. It is well known that Ti-based alloys can exist indifferent phases and solidification microstructures as well as phasecompositions [61,62]. Ti-rich alloys typically reveal a hexagonal closepacked (hcp) structure, also known as the α-phase, or mixtures of hcp
and bcc (β-phase) phases, also referred to as α + β. Heating of α-phasealloys leads to a reversible phase transformation into the hightemperature bcc β-modification, which transforms back into the orig­
inal hcp α-phase upon cooling [63]. The addition of so-called β-stabi­lizing elements lowers the transformation temperature. Alloys with highcontents of β-stabilizers can retain in the β-phase modification evendown to RT and below. The α- and β-phase can be converted into one
another by diffusionless transformations [64,65]. The orthorhombic
ω-phase and the non-equilibrium martensitic α’- (hexagonal) andα″-phase (orthorhombic) are to be named in this regard [66,67,68].
Fig. 6a depicts the XRD diffractograms of the TNT powder materials
after sifting and sieving, clearly revealing the influence of the chemistryon the phase evolution. In line with pure Nb, whose XRD pattern isplotted as a reference, the alloys with high content of Nb and Ta, i.e., Ti22Nb-19Ta and Ti-35Nb-6Ta, only reveal reflections attributed to thebcc β-phase. A reduction of the Nb/Ta content leads to significant
changes in the phase constitution. Accordingly, the diffractograms

Table 2
Quantitative EDS results revealing the chemical compositions (in wt-%) of the
TNT alloy powders. Spots 1 – 4 and area 5 investigated are marked in Fig. 5. ICPOES data are listed for comparison.
NominalTi-20Nb-6Ta
Ti-27Nb-6Ta
Ti-35Nb-6Ta
Ti-22Nb-19Ta

Ti
Nb
Ta
Ti
Nb
Ta
Ti
Nb
Ta
Ti
Nb
Ta

1

2

3

4

5

ICP-OES

74.5
19.6
5.9
65.1
29.2
5.8
60.9
32.0
7.0
57.2
20.5
22.3

77.0
17.9
5.1
66.8
27.5
5.6
61.9
31.9
6.5
57.3
20.3
22.4

85.0
13.2
1.9
77.0
21.0
2.1
73.2
24.6
2.2
70.6
16.9
12.5

81.4
16.0
2.7
74.8
21.9
3.3
71.6
25.0
1.1
69.6
17.1
13.3

77.3
17.4
5.3
68.9
25.5
5.6
62.0
31.6
6.4
61.9
19.2
18.8

74.0
20.0
6.0
66.5
26.8
6.1
59.0
35.0
6.0
59.0
22.0
19.0

Table 3
Process parameters and resulting energy per unit area (EA) found to lead to thehighest density in the TNT alloys investigated.
TNT alloy

Laser energy

Scan speed

EA value

Relative density

Ti-20Nb-6Ta
Ti-27Nb-6Ta
Ti-35Nb-6Ta
Ti-22Nb-19Ta

170 W
170 W
170 W
230 W

1250 mm/s
1350 mm/s
1500 mm/s
1500 mm/s

1.97 J/mm2
1.83 J/mm2
1.64 J/mm2
2.22 J/mm2

99.96 ± 0.01 %99.97 ± 0.01 %99.97 ± 0.01 %99.97 ± 0.03 %while spots 1 and 2 in Ti-20Nb-6Ta (Fig. 5a) reveal a composition of Ti19.6Nb-5.9Ta and Ti-17.9Nb-5.1Ta for the dendritic phase, the com­positions of spots 3 and 4, both located in the inter-dendritic area, are Ti13.2Nb-1.9Ta and Ti-16.0Nb-2.7Ta, respectively. Similar results areobserved for the other TNT alloys. Table 2 points out that the differencesin the contents of Ti, Nb and Ta are up to about 10 wt-%, 6 wt-% and 4
wt-%, respectively.
The occurrence of dendrite-type solidification microstructures isthought to be related to the characteristic cooling rates during powder
synthesis. In contrast to PBF-LB/M processes, the inherent cooling rates
during EIGA are not sufficiently high to fully prevent segregation
5

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Materials & Design 233 (2023) 112265

Fig. 6. XRD patterns of the TNT alloy powders after sifting and sieving (a) and after PBF-LB/M (b). For comparison, a characteristic pattern of pure Nb powder isplotted in (a). Rhombs in the diffractograms of Ti-20Nb-6Ta powders indicate reflections related to the orthorhombic α″ martensite, while diffraction peaks marked
with squares (Ti-27Nb-6Ta) cannot be assigned unequivocally. Black-framed squares indicate the theoretical peak positions of the hcp α-phase being absent in any ofthe materials.

Fig. 7. Optically determined relative densities of the PBF-LB/M processed TNT alloys in relation to the energy per unit area (EA). The insets depict optical mi­crographs of Ti-35Nb-6Ta processed with different EA values that are characterized by lack-of-fusion porosity, gas porosity and keyholing, respectively.

obtained for Ti-27Nb-6Ta and Ti-20Nb-6Ta indicate the formation of
structures with reduced symmetry. For Ti-20Nb-6Ta, the reflections canbe assigned to the orthorhombic α″-phase, indicating a fully martensitic
microstructure. In case of Ti-27Nb-6Ta, however, a reliable assignmentof the indentified reflections is challenging. The section highlighted inFig. 6a (2θ range from 32 to 44◦ ) displays two high-intensity peaks at
38.8 and 39.3◦ . In addition, low-intensity peaks are observed at 34.0,
37.0 and 41.7◦ . The peak positions of the dominant reflection at 38.8◦ aswell as of those at higher angles (56.0 and 70.4◦ ) indicate that the Ti27Nb-6Ta powder features a bcc β-phase component in addition toanother phase that cannot be assigned unequivocally. The theoreticalpeak positions of the hcp α-phase are also provided in Fig. 6 (dashedlines, open squares; intensities exceeding the background level are notseen at these positions). A more detailed phase analysis will be subject of
future work.
Remarkably, processing by PBF-LB/M (cf. section 3.2) has only aminor influence on the appearance of the XRD patterns and, thus, on thephase constitution. The patterns of the AM processed samples appear tobe very similar to those of the powders, especially with respect to thelocation of the reflections. An obvious difference, however, is theappearance of reflections in the XRD patterns of the as-built materials,
particularly the 200 and 211 β reflections, which are clearly broadened.
This is most likely an effect of the prevailing microstructure and the
crystallite sizes, respectively, which are smaller due to the rapid coolingin PBF-LB/M. This aspect is discussed in section 3.5 in more detail.
3.2. Laser beam powder bed fusion (PBF-LB/M)
Following processing, the process parameter windows were assessedby optical density measurements conducted on the PBF-LB/M manu­
factured cuboids. Depending on the energy per unit area (EA), significant
differences in the degree of residual porosity are found. In the inset of
Fig. 7, three representative micrographs of Ti-35Nb-6Ta are displayed,showing aspherical lack-of-fusion porosity (EA = 1.23 J/mm2), minor
residual porosity (EA = 1.83 J/mm2), and keyholing (EA = 3.06 J/mm2).
Finally, for all TNT alloys investigated, high density parts with relative
densities > 99.9 % were obtained in the process window of 1.5 J/mm2 <EA < 2.5 J/mm2. Table 3 summarizes the sets of process parameters and6

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Materials & Design 233 (2023) 112265

the resulting EA values, leading to the highest density value in each TNT
alloy. These parameters were used to manufacture the flat tensile
specimens for mechanical characterization (cf. Chapter 2 and section
3.4).

formation. Differences can be seen in the microstructure between the Tirichest alloy (Ti-20Nb-6Ta, Fig. 8a) and the three others. The micro­
graph of Ti-20Nb-6Ta recorded at higher magnification (Fig. 8e) illus­
trates three main characteristic features: (1) elongated, columnar grainsoriented along BD (grain boundaries are highlighted by white dottedlines), (2) platelet-like structures featuring an angle of 45◦ with respect
to BD (marked by white arrows), and (3) the melt pool boundaries(marked by white triangles). The depth of the melt tracks is about 160µm. The width of the columnar grains is up to 120 µm, whereas theirlong axis can be in the millimeter range. Obviously, these grains
comprise both the platelet-like structures and the melt pool boundaries.
It must be noted at this point that the platelet-like structures are notvisible in the micrographs of Ti-27Nb-6Ta, Ti-35Nb-6Ta, and Ti-22Nb19Ta. Furthermore, the tendency to form a highly anisotropic micro­
structure, i.e., extensive formation of columnar grains, seems todecrease with decreasing Ti content, as can be deduced from Fig. 8a-d.
Fig. 9 presents SEM/EDS results for the TNT alloys after PBF-LB/M
processing. It has to be noted that the platelet-like structures observedin Ti-20Nb-6Ta (Fig. 9a) via LM cannot be resolved in the corresponding
EDS mapping. However, the EDS results give a clear indication for minor
segregations along the melt pool boundaries into Ti- and Nb/Ta-richdomains. The local chemical compositions of specific spots, such as

3.3. Microstructure of PBF-LB/M processed parts
As detailed above, the TNT powder particles feature localizedchemical fluctuations, i.e., dendritic micro-segregations after EIGA
processing (Fig. 5). Using the PBF-LB/M technique, metallic materials
are melted by a laser beam and rapid solidification occurs due to thesmall melt pool dimensions in combination with the high thermal con­ductivity of the alloys fabricated. As mentioned above, cooling rates of 1
– 40 K/µs and thermal gradients in the range of 5 – 20 K/µm have beenreported [60], eventually promoting the formation of metastable phases
in PBF-LB/M processed structures. These differ significantly from thoseobserved in the initial powder feedstocks or structural features deter­
mined in conventionally cast components [69].
The morphological and microstructural properties of the PBF-LB/M
processed TNT alloys were examined by LM and SEM including EDS
and EBSD. The optical micrographs shown in Fig. 8 give clear evidence
that the chemistry has a significant impact on the structure/phaseFig. 8. Optical micrographs of the PBF-LB/M processed TNT alloys in the as-built condition: Ti-20Nb-6Ta (a), Ti-27Nb-6Ta (b), Ti-35Nb-6Ta (c), and Ti-22Nb-19Ta(d). Platelet-like structures, elongated (columnar) grains and melt pool boundaries are highlighted in the high-magnification image in (e), by white arrows, dottedlines, and white triangles, respectively.
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Materials & Design 233 (2023) 112265

Fig. 9. EDS mappings of the PBF-LB/M processed TNT alloys in the as-built condition: Ti-20Nb-6Ta (a), Ti-27Nb-6Ta (b), Ti-35Nb-6Ta (c), and Ti-22Nb-19Ta (d).
Arrows highlight minor chemical segregations along the melt pool boundaries. Chemical compositions of selected spots of Ti-20Nb-6Ta and Ti-22Nb-19Ta areprovided in Table 4.

grain boundaries, as well as the overall composition of two specimens,
namely Ti-20Nb-6Ta and Ti-22Nb-19Ta, are exemplarily given in
Table 4. Although the chemical variations are small, it is obvious that Nb
and Ta are enriched at the outer (colder) part of the melt tracks, while Ti
is preferentially found in the inner regions (in BD). For example, in caseof Ti-20Nb-6Ta the average Ti content (Fig. 9a, spot 1) is about 76.1 %.
In contrast, the corresponding Ti values close to the melt pool boundary
are 79.0 % in the inner part (Fig. 9a, spot 3) and 74.7 % in the outer part(Fig. 9a, spot 4). Despite these minor chemical fluctuations, it must beemphasized that the EDS results display a very homogeneous element
distribution for the additively manufactured TNT materials. This is inclear contrast to the dendrite-type microstructures observed on the same
scale in the initial feedstock powder material (Fig. 5). Here it is impor­tant to note, that investigations of powder and additively manufactured
bulk were performed using the same SEM equipment and almost similar
experimental conditions.
To assess the crystallographic characteristics of each condition, thePBF-LB/M processed TNT alloys were analyzed using EBSD. Fig. 10shows inverse pole figure (IPF) mappings recorded in the X-Y plane (i.e.,

perpendicular to BD).
Before EBSD analysis, the Ti-rich samples, i.e., Ti-20Nb-6Ta and Ti27Nb-6Ta, were heat treated at 850 ◦ C for 21 h under argon atmo­
sphere to dissolve potential non-equilibrium martensitic phases. Even­
tually, this procedure allowed to investigate the (prior)-β-grain
structure. EBSD data were additionally post-processed using OIM
Analysis software (EDAX) and the grain dilation routine. In the IPF
mappings, the β-phase was indexed using a cubic crystal structure(Im3m) and a lattice parameter of a = 0.3285 nm. As can be deduced
from the EBSD results, all TNT alloys are characterized by a similarsolidification behavior during PBF-LB/M processing:(1) Due to the bi-directional scanning strategy, a checkerboard-likegrain arrangement is formed within the X-Y plane, indicating a pro­nounced impact of epitaxy. Epitaxial solidification with elongated,columnar grains is also seen in the optical micrographs recorded parallelto BD (Fig. 8).
(2) The columnar grains feature a preferred crystallographic orien­
tation, i.e., grains mostly oriented in 〈001〉 direction with respect to BD
as well as in 〈101〉 and 〈111〉 direction with respect to the loading di­rection (LD) of the tensile specimens and the transversal direction (TD).
In a process parameter window being favorable for epitaxy, metallic
materials, which solidify in a cubic phase, are known to solidify pref­
erentially with the 〈001〉 direction alongside the direction of the
maximum thermal gradient [70,71,72]. Due the maximum thermal
gradient being mostly aligned in BD, the evolution of strongly 〈001〉textured, columnar-grained microstructures alongside BD was reported
for various cubic materials processed by different AM techniques[73,74,75]. In the present work, differences in terms of the intensity of
texture are obvious among the TNT alloys investigated. These differ­ences can be caused by differences in the process parameters employed(EA values) and the chemistry. In case of higher contents of Nb and Ta,
constitutional supercooling is thought to hamper epitaxial growth andTable 4
Chemical compositions (in wt-%) of PBF-LB/M processed Ti-20Nb-6Ta and Ti22Nb-19Ta determined from EDS measurements shown in Fig. 9. The spots∑
used for assessment are marked with 1 – 4 in Fig. 9. The symbol
corresponds
to the EDS analysis of the entire region depicted.
Ti-20Nb-6Ta

Ti-22Nb-19Ta

Spot

Ti

Nb

Ta

Ti

Nb

Ta

1
2
3
4
∑

75.7
76.9
79.0
74.7
76.1

18.1
18.2
16.2
19.2
17.9

6.2
5.0
4.8
6.2
6.1

57.2
57.1
63.8
66.9
61.3

22.4
22.6
19.6
21.1
21.5

20.4
20.3
16.6
12.0
17.3

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Materials & Design 233 (2023) 112265

Fig. 10. SEM/EBSD inverse pole figure (IPF)
mappings of the PBF-LB/M processed TNT alloys.
The mappings recorded perpendicular to thebuilding direction (BD) are plotted with respect
to the loading direction (LD) of the tensile spec­
imens (cf. Fig. 2) used for mechanical character­ization (a,d,g,j), the transversal direction (TD) (b,e,h,k) and BD (c,f,i,l). The Ti-rich alloys, i.e., Ti20Nb-6Ta and Ti-27Nb-6Ta, were heat treated
before characterization to stabilize the bcc
β-grain (solidification) structure for direct com­
parison of the four conditions. The reference co­ordinate system and the color-coded standard
triangle are shown in the lower left.

promotes grain refinement [76]. A follow-up study is already in prog­ress, detailing the influence of the scanning strategy and the processparameters on microstructural features and mechanical properties of
PBF-LB/M processed Ti-27Nb-6Ta.
3.4. Mechanical properties
Fig. 11 shows representative tensile stress–strain curves for the PBFLB/M processed TNT alloys in the as-built condition, i.e., without con­ducting a post-process heat treatment. The stress–strain curves of Ti20Nb-6Ta, Ti-27Nb-6Ta and Ti-35Nb-6Ta are characterized by similar
characteristics: the elastic regime is followed by a stress plateau,
exhibiting plastic deformation at constant stress, and finally a ductile
fracture behavior. The most important characteristic values obtained
under monotonic tensile loading, i.e., UTS and elongation at failure, aregiven in Table 5.
UTS values for Ti-20Nb-6Ta, Ti-27Nb-6Ta, and Ti-35Nb-6Ta are 802
MPa, 739 MPa, and 651 MPa, respectively. In line with these values, it
can be concluded that a decreasing Ti content (or accordingly an
increasing Nb content) reduces the strengths of the TNT materials. Thistendency, in turn, results from the different composition-dependent
microstructures of the alloys. XRD investigations depicted in Fig. 6
clearly show that Nb/Ta-rich alloys only contain the bcc β-phase,whereas the Ti-rich alloys Ti-20Nb-6Ta and Ti-27Nb-6Ta feature

Fig. 11. Representative tensile stress–strain curves of the PBF-LB/M processed
TNT alloys in the as-built condition.

structures with reduced symmetry, e.g., the orthorhombic α″-phase. For
Ti-20Nb-6Ta, a fully martensitic microstructure, and for Ti-27Nb-6Ta a
multi-phase material state consisting of at least the α″- and β-phase areseen. Furthermore, the formation of reinforcing platelet-type structures
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Materials & Design 233 (2023) 112265

seen in the micrographs obtained by LM. Plate-like structures are foundin the entire volume probed. Even if not directly proven by the micro­
graphs shown, these structures are attributed to a preferred selection ofmartensite variants. XRD analysis only revealed the presence of ortho­rhombic α″-phase, i.e., martensite. This is absolutely in line with manyother Ti-base alloys, where the non-equilibrium martensite dominates
the as-built microstructure. Its presence is rationalized by rapid cooling
during PBF-LB/M. The preferred alignment of the plate-like structures incase of the Ti-20Nb-6Ta alloy can be rationalized based on the strong
texture of the parent β-phase. EBSD revealed a preferred orientation not
only for BD, but also for the other two directions assessed. Consequently,
all columnar grains are oriented very similar. Due to the characteristicorientation relationships between the parent phase and the relatedmartensite, traces of martensitic features must be very similar in every
grain. Indeed, minor variations of the angle of traces can be seen in
Fig. 8e when a grain boundary is crossed.
Looking at the monotonic properties characterized by tensile tests,
the performance is good in all cases, i.e., high UTS values and highelongation at fracture are seen. The stress–strain response is obviously
affected by the alloying concept, i.e., the only condition featuring a highTa content (Ti-22Nb-19Ta) is characterized by significant hardening
during the test. As the focus of the present work was on the conditions
mainly alloyed by Nb, these conditions will be assessed in more depth.
Looking at the results obtained for Ti-20Nb-6Ta, Ti-27Nb-6Ta and Ti35Nb-6Ta, a decrease of UTS with increasing Nb content is obvious.
The general characteristics of the stress–strain response indeed are very
similar, dominated by the plateau-like behavior upon reaching the yield
point. Derived from microstructure analysis employing LM and XRD, themain difference in these three alloys is the fraction of martensitic phases.
Ti-35Nb-6Ta only shows peaks of the β-phase, Ti-20Nb-6Ta is fullymartensitic, and Ti-27Nb-6Ta is a condition being mainly composed ofbcc β-phase, however, containing most probably a nano-scale second
phase (this phase not being indexed successfully so far). As the UTS of
the respective alloys directly follows the trend derived from phaseanalysis, it is thought that martensite provides the most effective
contribution to strengthening.

Table 5Characteristic values obtained under monotonic loading extracted from thetensile tests shown in Fig. 11.
TNT alloy

Ultimate tensile strength (UTS)

Elongation at failure

Ti-20Nb-6 Ta
Ti-27Nb-6Ta
Ti-35Nb-6Ta
Ti-22Nb-19Ta(802 ± 3.0) MPa(739 ± 3.0) MPa(651 ± 1.2) MPa(717 ± 7.2) MPa(24.2 ± 1.0) %(26.0 ± 1.3) %(21.3 ± 1.2) %(31.7 ± 2.2) %in Ti-20Nb-6Ta observed by LM (Fig. 8e) potentially contributes toincreasing strength. Despite the differences in strength, the elongation at
failure is quite similar for these alloys. Even though likewise charac­terized by a ductile fracture, Ti-22Nb-19Ta features a different stress–­strain behavior. Following the onset of plastic deformation, thestress–strain curve reveals pronounced strain hardening. Failure occurs
at about 31.7 % strain.
3.5. Process-microstructure-property relationships
The results presented and discussed in the sections before are thebasis for an in-depth assessment of the TNT alloys studied in present
work. In direct comparison to the powders processed by EIGA, the mi­
crostructures established by PBF-LB/M are distinctly different for all
four alloys investigated here. This can be rationalized, as alreadyhighlighted before, by an extremely rapid cooling and solidification,
both being characteristic for the PBF-LB/M process. Thus, nonequilibrium microstructures form, which may be rearranged and/or
modified by a subsequent heat treatment. However, as the processing
conditions in the additive manufacturing of all TNT alloys were very
similar, differences observed in terms of microstructure evolution andmechanical properties can be attributed to the differences in chemicalcomposition of the TNT alloys. The small differences in the laser energy
input EA applied during PBF-LB/M of the powders, probably only had anegligible influence on the results obtained.
If the alloying contents of Nb and Ta are relatively high, a β-phasemicrostructure is established upon PBF-LB/M. The solidification struc­ture is characterized by relatively fine grains as evidenced by LM, EBSD,
and XRD (Fig. 6b). Due to the high alloying contents, epitaxial growth ofcolumnar β-phase grains is hampered. Instead, constitutional super­cooling promotes fragmentation of the microstructure. As is highlighted
by EBSD results and the checkerboard pattern seen, respectively,
microstructure evolution seems to be close to the equiaxed-columnar
transition. Thus, it is expected that major variations in the overall pro­cessing strategy will lead to pronounced differences in terms of micro­
structure evolution. Current work on this specific topic is underway andwill be subject of follow-up studies.
For the two compositions with the highest Ti contents, significantly
different phase compositions are observed. This is most obvious for the
Ti-20Nb-6Ta alloy, whereas the Ti-27Nb-6Ta alloy seems to mark atransition region. In case of the latter, LM seems to reveal a β-phasemicrostructure, however, XRD analysis led to the conclusion that asecond phase is present. As this second phase cannot be indexed reliably
and is not visible in the micrographs, it is assumed that a nano-scaled
phase formed, this phase being highly distorted. It is well-known thatsuch phases cannot be reliably probed by lab-scale XRD, and thatsynchrotron-based assessment and/or transition electron microscopy(TEM) are needed to further characterize this condition.
The Ti-20Nb-6Ta alloy again can be fully assessed based on the re­sults presented. EBSD analysis as well as LM suggest an epitaxial solid­
ification, large columnar β-phase grains are visible. Texture intensities
are highest and a preferred orientation of 〈0 0 1〉||BD, being character­istic for many alloys processed by AM, is seen. It should be emphasizedthat EBSD data for this condition stem from heat treated material, wherethe β-phase had been stabilized to allow for assessment of the solidifi­
cation structure. In the as-built condition, a very specific pattern can be

4. Conclusion
The microstructure and mechanical properties of novel Ti/Nb/Ta(TNT) powders and specimens processed by laser powder bed fusion,
PBF-LB/M have been investigated. From the findings presented thefollowing conclusions can be drawn:
1. TNT powders with spherical morphology are obtained by electrodeinduction melting gas atomization (EIGA). The phase composition of
the as-atomized powders depends strongly on their chemicalcomposition. In full agreement with conventionally processed Tibased alloys, the fraction of β-stabilizers is of utmost importancefor the prevailing microstructure. While an orthorhombic structure(α″) is observed in Ti-20Nb-6Ta, Ti-27Nb-6Ta features a multi-phasemicrostructure, at least consisting of a martensitic phase and the bcc
β-phase. For an in-depth phase identification, future work is cruciallyneeded. In turn, a further reduction of the Ti content, i.e., an increasein the Nb/Ta content, results in the formation of β-phase micro­
structures in Ti-35Nb-6Ta and Ti-22Nb-19Ta.
2. The spherical morphology of the TNT alloy particles makes thempromising candidates for application in additive manufacturing
processes. A wide process window for PBF-LB/M is revealed in thepresent study, yielding parts with > 99.96% relative density. While
the powder materials feature a dendrite-type microstructure withslight local chemical fluctuations, the additively manufactured parts
are characterized by a homogeneous element distribution.
3. Microstructure characterization after PBF-LB/M processing indicatessignificant differences in the microstructural evolution of the alloys.
While all materials exhibit characteristic melt pool features, i.e., melt
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Materials & Design 233 (2023) 112265

tracks with depths of around 160 µm and elongated columnar grainsoriented along the build direction (BD), platelet-like structures areobserved solely in Ti-20Nb-6Ta.
4. The chemistry of the TNT alloys does not only influence the micro­
structural features, but likewise the mechanical properties. Ultimate
tensile strength (UTS) clearly increases with increasing Ti content,
which can be attributed to increasing volume fractions of theorthorhombic α″-phase, i.e., martensite. In contrast, the Nb/Ta-richTi-35Nb-6Ta and Ti-22Nb-19Ta alloys featuring single bcc β-phasemicrostructures do not contain reinforcing structural features and,thus, exhibit the lowest strengths of the TNT materials investigated.
5. The TNT materials examined were specifically developed for AM in
the biomedical sector, e.g., for orthopedic and dental implants.
Ongoing research is currently in progress, including mechanicaltesting under cyclic loading as well as biomedical investigations toelaborate the material interactions with human cells, i.e., osseoin­tegration, vascularization and possible inflammatory reactions.

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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 to influence
the work reported in this paper.
Data availability
Data will be made available on request.
Acknowledgments
The authors are grateful for financial support by the Federal Ministryof Education and Research of Germany within the research project
HYTIMOX (support code 03XP0279C).
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