materials
ArticleMechanical Properties of a Newly Additive
Manufactured Implant Material Based on Ti-42Nb
Christian Schulze 1, *, Markus Weinmann 2 , Christoph Schweigel 3 , Olaf Keßler 3
and Rainer Bader 1
1

2
3

*

Biomechanics and Implant Technology Research Laboratory (FORBIOMIT), Department of Orthopaedics,
University Medicine Rostock, Doberaner Straße 142, 18057 Rostock, Germany;rainer.bader@med.uni-rostock.de
H.C. Starck Tantalum and Niobium GmbH, Im Schleeke 78-91, 38642 Goslar, Germany;markus.weinmann@hcstarck.com
Chair of Material Science, Faculty of Mechanical Engineering and Marine Technology, University of Rostock,
Albert- Einstein- Str. 2, 18059 Rostock, Germany; christoph.schweigel@uni-rosotck.de (C.S.);
olaf.kessler@uni-rostock.de (O.K.)
Correspondence: christian_schulze@med.uni-rostock.de; Tel.: +49-381-494-9338

Received: 16 November 2017; Accepted: 9 January 2018; Published: 13 January 2018

Abstract: The application of Ti-6Al-4V alloy or commercially pure titanium for additive manufacturing
enables the fabrication of complex structural implants and patient-specific implant geometries.
However, the difference in Young’s modulus of α + β-phase Ti alloys compared to the humanbone promotes stress-shielding effects in the implant–bone interphase. The aim of the present
study is the mechanical characterization of a new pre-alloyed β-phase Ti-42Nb alloy for applicationin additive manufacturing. The present investigation focuses on the mechanical properties of
SLM-printed Ti-42Nb alloy in tensile and compression tests. In addition, the raw Ti-42Nb powder,
the microstructure of the specimens prior to and after compression tests, as well as the fracture
occurring in tensile tests are characterized by means of the SEM/EDX analysis. The Ti-42Nb raw
powder exhibits a dendrite-like Ti-structure, which is melted layer-by-layer into a microstructure witha very homogeneous distribution of Nb and Ti during the SLM process. Tensile tests display Young’s
modulus of 60.51 ± 3.92 GPa and an ultimate tensile strength of 683.17 ± 16.67 MPa, whereas, under acompressive load, a compressive strength of 1330.74 ± 53.45 MPa is observed. The combinationof high mechanical strength and low elastic modulus makes Ti-42Nb an interesting material for
orthopedic and dental implants. The spherical shape of the pre-alloyed material additionally allows
for application in metal 3D printing, enabling the fabrication of patient-specific structural implants.
Keywords: implant material; Ti-42Nb alloy; additive manufacturing; mechanical properties;
optimization

1. Introduction
The application of biomaterials, i.e., structural implants in the human body, is commonly indicatedto maintain or recover lost skeletal functionality caused by tumors [1,2], malformation, fractures [3],infections, and osteolysis [4,5]. Most of the biomaterials used, about 70%–80%, are pure metals or
metal alloys [6]. Titanium and its alloys, especially Ti-6Al-4V, are state-of-the-art implant materials [7]and are most frequently implanted [6]. Implants made of forged or additively manufactured Ti-6Al-4V
provide very good mechanical properties and stability [8]. Due to its good biocompatibility [9], i.e., cellcompatibility, which is most probably caused by a native oxidation layer on the metal surface [10,11],and its good osseointegration, Ti-6Al-4V is currently used for application in craniofacial [12],orthodontic [8,13–16], and orthopedic implants [8,15,17,18].
Materials 2018, 11, 124; doi:10.3390/ma11010124

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Titanium alloys usually possess higher corrosion resistance compared to commercially puretitanium [19]. Accordingly, the release of metal ions and their exposure to the human organism isreduced, resulting in fewer inflammations. Interestingly, Ti-42Nb exhibits improved cell tolerance
when compared to Ti-6Al-4V, displayed by a higher cell viability of both osteoblasts and fibroblasts [20].
Titanium-based implants in orthopedic surgery are often directly fixed in the host bone by press
fit [21–24] or screw fixation [13,24]. Thereby, the highly diverging elastic properties of the implantmaterial, e.g., cp-Ti or Ti-6Al-4V [25] and cortical bone [26–34], promote stress-shielding effects along
the implant–bone interphase [35,36]. As a consequence, the osseointegration of the implant, especiallybone ingrowth, may be affected [13]. In the worst cases, implant loosening [36], osteolysis [4,5],
and postoperative midterm migration of the implant occur [37].
In the past decade, additive manufacturing (AM), i.e., selective laser melting (SLM) or electron
beam melting (EBM) of titanium alloys, has become an important technology for fabricating implants
with complex [38] geometries that are otherwise difficult or even impossible to produce. Avoiding theconstraints related to traditional production methods, AM allows for the manufacturing of implantson the basis of 3D data obtainable by standard diagnostic methods, such as computer tomography(CT) or magnetic resonance imaging (MRI). These data can then be translated via standard tessellation
language (STL) into 3D print files [39–41].
SLM technology is applicable in various pure metals and alloys. However, the focus in recent
years has been on printing stainless steel [42–45], aluminum [46], and its alloys as well as titanium [47]and titanium-based alloys, e.g., Ti-6Al-4V [48,49]. There are several promising applications in fields
such as automotive, aerospace [50], medicine [51], etc., where SLM-printed parts have the potential toreplace those produced conventionally.
SLM or EBM is increasingly gaining popularity in biomedical and dental applications, especiallyfor realizing patient-specific implant designs [52]. A special focus is on the development of
biocompatible materials. This includes the ban of toxic, allergenic, or otherwise hazardous metals suchas Co, Ni, Cr, Al, V, etc., and adjusting mechanical properties to the requirements of the human bone,i.e., strength and elasticity.
Currently, the most promising materials are β-phase Ti alloys, which have been widely describedin the scientific literature as having an emphasis on cast and forged materials [53–56]. They provide
increased cell viability and improved mechanical compatibility with the cortical bone because of an
intrinsically reduced elastic modulus when compared to Ti-6Al-4V [19,57–59], which crystalizes as a
mixture of the α and β-phase.
Binary Ti/Nb alloys have been well examined and understood. Above a certain niobium content,
they entirely crystallize in the body-centered cubic β-phase [60]. Interestingly, the elastic modulus
of Ti-Nb is stoichiometry-dependent, with two minima of ca. 68 GPa at 15 wt % of niobium (thiscomposition crystallizes as a mixture of the α and β phase) and 62 GPa at 42 wt % of niobium [61].
Nevertheless, none of the literature discussed the AM or SLM of a spherical pre-alloyed Ti–Nb
powder with a niobium content of 42 wt % that provides a minimal Young’s modulus, as described by
Ozaki et al. [61] for Ti–Nb cast alloys. Some of the studies reported on in situ alloying of Ti and Nb withseveral content of niobium [57,59]. Prashanth et al. [25] described the mechanical properties of a porous
Ti-45Nb alloy manufactured by a classic sintering process and subsequent hot pressing. The aim of ourpresent study was the mechanical and microstructural characterization as well as the determinationof the printing parameters of the Ti-42Nb alloy that was applied in the SLM manufacturing processdescribed for the first time. Accordingly, we chose the latter composition (42 wt % of niobium) for
the development of SLM powders, including powder synthesis and full powder characterization.
The applicability of the newly produced spherical Ti-42Nb powders in AM was finally evaluatedby SLM and the microstructure as well as the mechanical properties of manufactured 3D specimens.
It could be shown that Ti-42Nb surpasses the mechanical properties of common standard implantmaterials, especially in terms of elasticity.

Materials 2018, 11, 124Materials 2018, 11, 124

2. Results
2.
Results
Materials 2018, 11, 124

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3 of 20

3 of 202.1.2.SLM
Powder Synthesis and Characterization
Results
Ti-42Nbobtained
by by
means
of electrodeinduction-melting
gas atomization(EIGA)
Ti-42Nb powders
powderswere
were
obtainedmeansof electrodeinduction-melting
gas atomization
2.1. SLM Powder Synthesis and Characterizationofpre-alloyedTi-42Nb Ti-42Nb
rods using
Arusinggas (4.6,
Munich,for atomization.
The raw(EIGA)of pre-alloyedrods
Ar Linde,gas (4.6,
Linde, Germany)
Munich, Germany)for atomization.
powders
were
obtainedby meansof< theelectrodeinduction-meltinggas
powderwasfractionizedbysieving,andthefraction
63 µm
was
used
furtherand
The rawTi-42Nb
powderwas fractionizedby sieving,andfraction< 63forμm
was investigationusedatomization
for further(EIGA)
ofpre-alloyedTi-42Nb
rods
using
Ar
gas
(4.6,
Linde,
Munich,Germany)foratomization.
processing.
Figure1 indicates thatthe 1particles
arethathighlysphericalareandhighlythat theirsurfacessmoothinvestigationand processing.
Figureindicates
the particles
spherical
andarethat
their
The raw
fractionizedby sieving,andthefraction< flow
63 μm
waspossessusedfurther
without
visiblecrackswas
or otherfaults. Accordingly,
theyfaults.
possess
ideal
properties
forforapplication
surfacesare powdersmooth
without
visiblecracksor other
Accordingly,
they
ideal
flow
investigation
andprocessing.
Figure1indicatesthattheparticles
arehighlyspherical
and
that
their
in
SLM. for application in SLM.
properties
surfaces are smooth without visible cracks or other faults. Accordingly, they possess ideal flow
properties for application in SLM.

1. (a) SEM
SEM images
images at
at 100100×
1000×
Figure 1.
× and (b) 1000
×magnification
magnificationofof AMPERTECAMPERTEC MAP
MAP Ti-42Nb appliedcompression
test
andtensilemagnification
test specimen
specimenbymeans ofof SLM.
SLM.
for Figure
the production
ofimagescompression
test
means
1. (a) SEM
at 100×test
andand(b) tensile
1000×
ofby
AMPERTEC
MAP Ti-42Nb applied
for the production of compression test and tensile test specimen by means of SLM.

A representative
representative SEM
SEM image
image ofthe polishedpolished crossa Ti-42Nb
Ti-42Nb particle
particle isis displayed
displayed ininAof thecross section
section ofof aA indicating
representative
SEM image ofsegregation
the polishedintocrosssectionof Nb-richa Ti-42Nb
particledisplayed
is displayedinFigure2,adendrite-typeTi-richandregions,inred
Figure 2, indicating a dendrite-type segregation into Ti-rich and Nb-rich regions, displayed in red
2, indicating aPores
dendrite-type
segregation
into Ti-richandobserved.
Nb-rich regions,displayedchemical
in redandFigure
blue, respectively.
respectively.
or voids
voids within
within thethe particle
particleare notnot
The maximum
maximumandblue,
Pores orareobserved. The
chemicalandblue,respectively.
Pores
or
voids
withintheparticlearenotobserved.
The
maximumchemicalfluctuation within
inin
thetherange
Ti-30Nb(spot1) andTi-48Nb(spot(spot3). Thefluctuation
within thetheas-atomized
as-atomizedparticle
particleis isrange
Ti-30Nb(spot1) andTi-48Nb3).
fluctuation
within theas-atomized
particlecorresponding
is in the range Ti-30Nb(spot 1) and Ti-48Nb(spot3). Theoverallcomposition
(spot
5)
is
Ti-42.6Nb,tothewet-chemical
analysis
performed
Theoverall
overallcomposition
composition(spot(spot5)5)isisTi-42.6Nb,
Ti-42.6Nb,corresponding
correspondingtotothe
thewet-chemical
wet-chemicalanalysis
analysisperformed
performed
using ICP-OES.
ICP-OES.According
Accordingtoto
X-ray
diffraction
studies,thematerial
is entirely
β-phase,whereasusing
X-ray
diffraction
studies,thematerialisentirelyβ-phase,whereasthere
using
ICP-OES.
Accordingto X-rayofdiffraction
studies,thematerial
isfor
entirelyβ-phase,whereasthereisnoindication
forthepresence
α-,
α′-,
or
ω-phase,asreportedTi/Nballoyswithhigher0
is noindication
forthepresence
of
α-,
α
-,
or
ω-phase,asreportedforTi/Nballoyswithhigherthere is no indication for the presence of α-, α′-, or ω-phase, as reported for Ti/Nb alloys with higherTi
Ticontent[62].
content[62].
Ti content[62].

Figure2. Energy-dispersive
X-ray
spectrometer(EDX)
mappingof
55µµm
particle.
The
Figure2.
X-ray
spectrometer(EDX)
mappingof a of55 aaµm
particle.
The image
2. Energy-dispersive
Energy-dispersive
X-ray
spectrometer(EDX)
mapping
55Ti-42Nb
m Ti-42Nb
Ti-42Nb
particle.
The
image
displaysapartially
segregateddendrite-type
microstructure
with
Ti-rich
spots
(red;
1,
2)
and
displaysa partiallysegregateddendrite-type
microstructurewith Ti-rich
(red;
1, 2)
and1,Nb-rich
image displays
a partiallysegregateddendrite-type
microstructurewithspots
Ti-richspots(red;2) andNb-richspots
3,4). Thecomposition
overall composition(5) is Ti-42.6Nb
and only
negligiblydeviating
from
spots
(blue;
4).(blue;Theoverall(5) is Ti-42.6Nb
and only
negligiblydeviatingfromthe
Nb-richspots3,(blue;3, 4).
The overall composition(5) is Ti-42.6Nb
and only
negligiblydeviatingfromthe chemical
analysis
determinedbyOES
ICP (Ti-42.5Nb;
OES (Ti-42.5Nb;
data
not shown).
chemicalanalysis
determined
by
ICP
data
not
shown).
the chemical analysis determined by ICP OES (Ti-42.5Nb; data not shown).

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Materials 2018, 11, 124

4 of 202.2.
2.2. Additive
Additive Manufacturing
Manufacturing
Additiveof thethe specimens
specimens byby meansmeans ofof SLM
SLM waswas performed
performed using
using aa Trumpf
TrumpfAdditive manufacturing
manufacturing of
TruPrint
1000
printing
system,attheLaserzentrum
Hannover,
Germany,
applying
empirically
TruPrint 1000 printing system, at the Laserzentrum Hannover, Germany, applying empiricallydeterminedlaser power
power of
of 80
80 W,
W, aa hatch
hatch ofof 100100 µm,determined parameters:
parameters: aa laser
μm, andand aa powder
powder feed
feed rate
rate (layer(layerthickness)
of
50
µm.
The
scan
speed
was
400
mm/s.
Based
on
the
empiricallydetermined
process
thickness) of 50 μm. The scan speed was 400 mm/s. Based on the empirically determined process3
parameters,
parameters, thethe SLMSLM processprocess waswas carried
carried out
out withwith aa relatively
relativelylow
lowenergy
energydensity
densityof
of40
40J/mm
J/mm 3..
Ina
In thisthis process,process, thin
thin layers
layersofofTi-42Nb
Ti-42Nbpre-alloyed
pre-alloyedpowder
powderwere
werehomogeneously
homogeneouslydistributed
distributedonon
niobium
substrateplate
andconsolidated
inx/y
directionusing
thefocused
laser
beam.
Only
those
a niobium substrate plate and consolidated in x/y direction using the focused laser beam. Only thoseareas
areas thatthat were
were emblazed
emblazed were
were consolidated.
consolidated. The
The substrate
substrate plate
plate waswas again
again lowered,lowered, evenly
evenly coatedcoated
with
the
metal
powder
and
the
layer
consolidated
by
the
laser
beam.
Repetitionof
thisprocedurewith the metal powder and the layer consolidated by the laser beam. Repetition of this procedure
resultedformationof three-dimensional
structures,i.e., compression
test andtensileresulted ininthethelayer-wiselayer-wiseformationof three-dimensional
structures,i.e., compression
test
and
test
specimens.
A subsequent
thermalthermal
treatmentsuch as diffusion
annealingor hot isostatic
tensiletest specimens.
A subsequent
treatmentsuch as diffusion
annealingor hot pressing
isostatic
to
reducetointernal
porosity
not applied.
pressing
reducestresses
internalorstressesorwasporosity
was not applied.
Figure3displaysasetofcompression
test
printedon the above
Nb substrate
Figure 3 displays a set of compression specimens
test specimens
printedon thementioned
above mentioned
Nbplate.
Forplate.
practical
reasons, 1.5
mm thick
supportwere printed
The printed
substrate
For practicalreasons,
1.5 mmthick structures
support structures
were initially.
printed initially.
Thecylinders(without(without
support support
structures)
had dimensionsof d × l =of10.7
× 16mmmm.×After
removalprinted cylinders
structures)
had dimensions
d ×mm
l = 10.7
16 mm.
After
from
the
base
plate,
theyweremachined
to
d
×
l
=
6.9
mm
×
10.35
mm
(d:l
=
1.5)
for
furtherremoval from the base plate, they were machined to d × l = 6.9 mm × 10.35 mm (d:l = 1.5) for testing.
further
Tensilespecimenswith as-printed
dimensionsof d ×ofl d=×15
× ×7070mm
testing. test
Tensiletest specimens
with as-printed
dimensions
l = mm15 mm
mmwerewereobtained
obtained ininaccordancetotheremovalprocessofthecompression
test
specimens.
The
final
dimensions
accordance to the removal process of the compression test specimens. The final dimensions ofof thethetensiletensile test
test specimens
specimens inin thethe “as-machined”“as-machined” condition
condition areare shownshown inin chapter
chapter Materials
Materialsand
and Methods.
Methods..

testtest
specimensin theinas-built
condition
before separationfrom the fromsubstrate
Figure 3.
3. Compression
Compression
specimens
the as-built
condition
before separation
theplate.
substrate
plate.

Figure 44 demonstrates
demonstrates SEM
SEM images
images ofof aa polishedpolished surface
surface ofan as-printed
as-printed tensile
Figureof antensile test
test specimen,
specimen,
a
BSE
image
as
well
as
an
EDX
mapping.
The
overview
image(a)pointstothethat thethea BSE image as well as an EDX mapping. The overview image (a) points to the fact
fact that
as-printed specimen
specimen isis homogeneously
homogeneously densified.
densified. Larger
voids, oror segregation
segregation ofof large-scalelarge-scaleas-printed
Larger pores,
pores, voids,
inhomogeneities
arenotvisible.
On
the
basis
of
image
(b),
the
relative
densityofthespecimencaninhomogeneities are not visible. On the basis of image (b), the relative density of the specimen can
opticallybeestimatedtobe>99.5%.
The
BSE
image
(c)
likewisedisplaysadensematrixthatisrarelyoptically be estimated to be >99.5%. The BSE image (c) likewise displays a dense matrix that isinterspersed
by tinyby
pores
in the micron
Asrange.
a main As
feature,grainboundaries
rarelyinterspersed
tinyhaving
pores diameter
having diameterin therange.
microna mainfeature,grain
arevisible(whitearrows).
The
segregation
in
the
upper
left
corner
of
the
BSE
can
boundaries are visible (white arrows). The segregation in the upper left corner of the BSEimage
image canundoubtedlybeattributedtoaTi-42Nb
particleortheresidueofsuchaparticle,
which
has
not
been
undoubtedly be attributed to a Ti-42Nb particle or the residue of such a particle, which has not been
melted byby thethelaserlaserbeam
beamduring
duringthe
theSLM
SLMprocess.
process.
This
is strongly
supported
EDX
mapping
meltedThis
is strongly
supported
by bythetheEDX
mapping
(d),
(d),
which
displaysthedendrite-type
structureofthespherical
SLM
powderparticles
shown
which displays the dendrite-type structure of the spherical SLM powder particles shown in Figurein
2.
Figure2.
However,suchinhomogeneities
areonlyextremely
rarelyobservedthroughout
theHowever, such inhomogeneities are only extremely rarely observed throughout the specimens.
specimens.
Most importantly, the entire microstructure of the applied material dramatically changed during
Mostprocessing.
importantly,
entirethat
microstructureof the microstructure
applied material
dramaticallythe SLM
It istheevident
the initial dendriticof theprecursor changed
powderduringthe
SLM
processing.
Itisevidentthattheinitialdendritic
microstructureoftheprecursor
completely vanishes during the SLM process, most probably to the extreme cooling rates between 104
powder completely vanishes during the SLM process, most probably to the extreme cooling rates

Materials 2018, 11, 124

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between 104 and 106 K/s [63]. Accordingly, the constituting elements Ti and Nb are dispersed
Materials 2018, 11, 124
5 of 20
homogeneously
on a sub-micron scale.
Materials 2018, 11, 124

5 of 20

6 K/s [63].
between104 and106 K/s [63].
Accordingly, elements
the constituting
elements
Ti and Nb
are dispersed on aand 10Accordingly,
the constituting
Ti and Nb
are dispersed
homogeneously
homogeneously
sub-micronscale. on a sub-micron scale.

Figure 4. SEM/EDX images of a polished surface of a Ti-42Nb tensile test specimen at (a) 10×; (b) 50×
Figure4. SEM/EDX
imagesa polishedsurface
surface ofof aa Ti-42Nb
Ti-42Nb tensile
at at(a)(a)
10×;
Figure4. SEM/EDX
imagesof ofa polishedtensiletest
testspecimen
specimen
10(b)
×;50×
(b) 50×and (c)1000×magnification
in in
BSE
mode;
(d)
The
EDX
mappingatat3000×
3000×
magnificationarises
and
(c)
1000×
magnification
BSE
mode;
(d)
The
EDX
mappingmagnificationarisesfromfromand (c) 1000× magnification in BSE mode; (d) The EDX mapping at 3000× magnification arises
fromthe cliptheimage.
thein
clip
inBSE
the BSE
image.
the clip in the BSE image.

While
Ti-richTi/Nballoysmay
composedof mixtures
While
Ti-richTi/Nballoysmayform
formcomplex
complex microstructures
microstructures composedof mixturesof α,ofα′,α, α′,While Ti-rich Ti/Nb alloys may form complex microstructures composed of mixtures of α, α0 ,α′′,ωand/orβphase[64–67],
X-ray
diffraction(XRD)
studiesof
SLM-printed
Ti-42Nb(Figure
5) 5)
α′′,
ω
and/orβphase[64–67],
X-ray
diffraction(XRD)
studiesof
SLM-printed
Ti-42Nb(Figureα00 , ωclearly
and/orβ phase[64–67],
X-ray
diffraction(XRD)
studiesof SLM-printed
Ti-42Nb (Figure
5)
indicatematerial
entirely
crystallizes
in
This
is in
with withclearly
indicate
thatthat
thethematerial
entirely
crystallizes
in aa bcc
bccβ-phase.
β-phase.
This
is accordancein accordanceclearly
indicatethatthematerial
entirely
crystallizesinabcc
β-phase.
Thisisinaccordancewithresults
obtained
by XRD
investigationsusing
usingcast
cast Ti-42Nb.
Ti-42Nb. Interestingly,
a comparisonof theresults
obtained
by XRD
investigations
Interestingly,
a comparisonof XRDthe XRD
results
obtained
by XRD investigations
using
Ti-42Nb. Ti-42Nb
Interestingly,
comparisonof thepatterns
of pre-alloyedprecursor powderandcastSLM-printed
displaya notably
different
line XRD
patterns of pre-alloyed precursor powder and SLM-printed Ti-42Nb display notably different line
widths.
The reflectionsof the printed
material
are markedly Ti-42Nb
broadeneddisplay
comparedto thoseof the line
patterns
of pre-alloyedprecursor
powderand SLM-printed
notablydifferent
widths. The reflections of the printed material are markedly broadened compared to those of thepowder,suggesting ofa significantly
microstructure,which
forms as compareda consequenceof theof thewidths.
The reflections
the printed finermaterial
are markedly
broadenedto thosepowder,suggestinga significantly
finer microstructure, which forms as a consequence of theextremely
fastcoolingduringSLM
processing.
powder, suggesting a significantly finer microstructure, which forms as a consequence of the extremely
extremely fast cooling during SLM processing.
fast cooling during SLM processing.

Figure 5. Results of the X-ray diffraction analysis performed for the Ti-42Nb precursor powder and the
SLM-printed Ti-42Nb bulk material.

Materials 2018, 11, 124

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FigureResultsof the X-ray diffraction analysis performed for the Ti-42Nb precursor powder and6 of 20
Materials
2018,5.11,
124
the SLM-printed Ti-42Nb bulk material.

2.3. Mechanical
Mechanical Characterization
Characterization
2.3.
All tensile
tensile test
For thetheAlltest specimens
specimens (n
(n =
= 8)
8) showed
showed nearly
nearly equal
equal slope
slope inin thethe elastic
elastic regime.
regime. For
SLM-printed
Ti-42Nb
tensilespecimens,anaverageYoung’smodulus
E
=
60.51
±
3.92
GPa
was
SLM-printed Ti-42Nb tensile specimens, an average Young’s modulus E = 60.51 ± 3.92 GPa wascomputed. During
During thethe tensile
tensile tests,
tests, nono distinct
distinct yield
yield strength
strength could
could bebe observed.
Thus, thethe proof
proof
computed.
observed. Thus,
strengthat
0.2%
plastic
extensionσwas
ascertained
at674.08
±
24.77
MPa
from
the
stress-strain
0.2
strength at 0.2% plastic extension σ0.2 was ascertained at 674.08 ± 24.77 MPa from the stress-strain
curves. Due
Due toto thethe lacklack ofof distinct
distinct yield
yield strength,
strength, thethe point
point ofof thethe 0.2%
0.2% plastic
plastic extension
extension (elongation)(elongation)
curves.
−1−1
was chosen
for determination
determination ofof thethewaschosen toto switch
switch thethe strainstrainrate
ratefrom
fromėė1 1(0.00025(0.00025s s−1)) toto ėė22 (0.0067(0.0067 ss−1)) for
ultimateyieldstrength
(UTS)
in
the
plastic
regionofthematerial.
In
four
of
the
eight
tensilespecimens,
ultimate yield strength (UTS) in the plastic region of the material. In four of the eight tensile
the universal
testing
machinedetecteda rapiddecreasein stress
directly
after reaching
σ0.2 (Figure
6).
specimens,
theuniversal
testing
machinedetecteda rapiddecreasein stress
directly after
reachingThus,
the strain
rate waschangedfromnotė1 changed
to ė2 for thesenumbers(specimen
1; 3; 6; 7),σ0.2 (Figure6). Thus,
the notstrainrate wasfrom specimensė1 to ė2 for(specimen
these specimens
and
only
four
specimens(specimen
numbers
2;
4;
5;
8)
could
be
consideredforthedeterminationof thethenumbers 1; 3; 6; 7), and only four specimens (specimen numbers 2; 4; 5; 8) could be considered for
ultimate yield strength
UTS. Anultimateyieldstrengthof 683.17± 16.67
MPaof(n683.17= 4) without
distinct
determinationof the ultimate
yieldstrength
UTS.
An ultimate
yieldstrength± 16.67 MPa(ndensification
wasobserved
inthetensiletests.
For
specimens
testedwiththehigherstrain
rate
(
ė
)
in
2
= 4) without distinct densification was observed in the tensile tests. For specimens tested with thetheplastic
regionfracture
strengthof384.49±31.36,
MPawasdeterminedatanelongation
atfracturehigher strain rate (ė2) in the plastic region fracture strength of 384.49 ± 31.36, MPa was determined atof 11.65%± 2.03%,whereastested whereas
with lower
strain rate(ė1) showedfracture
strengthofanelongation
at fractureof specimens
11.65% ± 2.03%,
specimens
testedwith lower
strainrate (ė1)
435.29
±
58.22
MPa
for
elongation
atfractureof
15.37%±
2.00%.
Thereby,thelattershowedhighershowed fracture strength of 435.29 ± 58.22 MPa for elongation at fracture of 15.37% ± 2.00%. Thereby,
variance
stress in
the plastic
region.
thelatter of
showedhighervarianceof stress in the plastic region.

Figure 6.
thethetensiletest test
specimens
testedtested
with higher
strain rate
(blackFigure6. Stress-strain
Stress-straincurves
curvesof of
tensilespecimenswith higher
strainratedashed)(blackand withandlower
strainratestrain(red dotted)
the plastic
region.
Allregion.
the determinedstresses andstrainsand
aredashed)withlower
rate (redindotted)
in theplastic
All the determinedstressesengineering
stresses andstresses
strains. andSpecimenswith higher
in theplastic
strainsare engineeringstrains.tested
Specimens
testedstrain
with rate
higherstrainrateregion
in theshowed
plastic
higher showeddecreasehigher
in stress
with increasingstrain, whereas
the lowerstrainatrate,
fractures
regiondecreasein stress tensile
with increasing
tensileatstrain,
whereasthe thelower
strainoccurredathighertensilestrains.
rate, the fractures occurred at higher tensile strains.

All specimens
specimenstested
testeddisplayed
the typical
fracture appearanceof ductile
materials
displayed
the typical
fracture appearanceof ductile materials(cup-and-cone(cup-and-cone
fractures)(c.f. determination
Figure 7b). The
determinationof fracture
the necking
at the fracturespot
fractures) (c.f. Figure7b). Theof thenecking at the
spot revealed
nearly equal
revealednearly equal
necking behavior
for specimens,of whether
theyrate
weretestednecking behavior
for specimens,
independentof whetherindependent
they were tested
with strainė1 andė2
withstrain
rate
ė
1
and
ė
2
(44.17%±
4.52%)
orwithconstant
strainrate
ė1(42.59%±7.02%).
Oneofthe(44.17% ± 4.52%) or with constant strain rate ė1 (42.59% ± 7.02%). One of the eight specimens, i.e.,eight
specimens,
3, which
was tested
lower astrain(ė1), resulting
revealed ina lowershear
specimen
3, whichi.e.,
wasspecimen
tested withlower strain
rate (ė1with), revealed
shearrate
fracture
fracture
resultinginlowernecking(31.40%)than
specimens
withcup-and-coneshapedfracturesnecking (31.40%) than specimens with cup-and-cone shaped fractures (Figure 7c).
(Figure7c). 1, the mechanical properties determined in tensile tests of the SLM-printed Ti-42Nb are
In Table
In Table and1, themechanicalproperties
determinedin tensileof theSLM-printed
Ti-42Nb aresummarizedcompared
to Ti
alloys, commonly
employed
as tests
implant
material.
The comparisonofsummarized
andcomparedtoTialloys,commonly
employedasimplant
material.
The
comparison
the mechanical properties of the Ti-42Nb alloy under tensile load and the mechanical properties ofofthe mechanicalproperties
ofintheTi-42Nbhuman
cortical bone
is shown
Table
2. alloy under tensile load and the mechanical properties ofhuman cortical bone is shown in Table 2.

Materials 2018, 11, 124

7 of 20

Table 1. Mechanical properties of selected Ti-based implant materials determined in tensile tests.
Material

Manufacturing

Condition

E (GPa)σ0.2 (MPa)UTS/UCS (MPa)εb (%)

CP-Ti [8]Ti-6Al-4V [8]Ti-6Al-4V ELI [8]Ti-6Al-7Nb [8]annealed
mill annealed-

Grade 1–4-

102.7–104.1
110–114101–110114

170–485825–869
795–875
880–950

240–550
895–930
860–965
900–1050

15–24
6–10
10–15
8.1–15

Ti-6Al-4V [34]EBMas built
hippedwrought/annealed

118 ± 5117 ± 4104 ± 2830 ± 5795 ± 10790 ± 20915 ± 10870 ± 10870 ± 1013.2 ± 0.6
13.7 ± 1.0
18.1 ± 0.8

Ti-6Al-4V [17,68,69]

SLM

-

112

990–1330

1095–1407

2–5

Ti-45Nb [25] (compression test)

green compacted + sintered
hot-pressed
Hot-pressed + sintered-18.5 ± 2.5
64.4 ± 9.4
60.3 ± 8.9-

282 ± 26667 ± 56686 ± 64-

Ti-26Nb * [57] (* at %; compression test)

Ingot
SLM (in situ alloyed)-

69.9 ± 0.2
77 ± 1.4---

Ti-27.5Nb * [58] (* at %)CLAD® -Process

as built
Solution treated

70 ± 370 ± 4---

Ti-42Nb (present study)

SLM

CNC-machined60.51 ± 3.92

674.08 ± 24.77

683.17 ± 16.67

11.65 ± 2.03

Materials 2018, 11, 124Materials 2018, 11, 124

8 of 20
8 of 20

FigureFigure 7.
7. Exemplary
Exemplary depiction
depiction andand visualvisual comparison
comparison ofof thethe tensile
tensile test
test specimens
specimens according
according toto ISOISO
6892-1
before
(a)
and
after
(b,c)
tensiletests.
Seven
of
the
eight
fractured
specimens
markedly
displayed
6892-1 before (a) and after (b,c) tensile tests. Seven of the eight fractured specimens markedly
thetypical thenecking(b),
whereasoneof the specimens(c) showed(c)shearfracture.
displayed
typicalnecking(b),
whereasone of the specimens
showedshear fracture.
human
bones
of different
anatomic
regions
and andthe comparison
with
Table
Table 2.2.Mechanical
Mechanicalproperties
propertiesofof
human
bones
of different
anatomic
regions
the comparison
Ti-42Nb
specimens
mademade
SLM-printed.
with Ti-42Nb
specimens
SLM-printed.
Material

Test Method

Anatomic Region

Material

Test Method

Anatomic Region 13.6–16.8
E (GPa)Femur [26,27,70]

TensileCortical Bone

TensileCompression

Cortical BoneTi-42Nb(present study)

TensileCompression
Compression

E (GPa)Tibia [70]σ0.2 (MPa)

16.2–23.83

Femur[26,27,70]
13.6–16.8
Tibia
osteons[71]
22.5–25.8
Femur
[70]
17.6
Tibia
[70]
16.2–23.83
Tibia [70]28

UTS/UCS(σc 35 ) (MPa)UTS/UCS(σc 35)

σ0.2 (MPa)--

Tibia osteons [71]22.5–25.8-

Tibia [70]

28

-

Femur
[70]

60.51 ± 3.92 674.08 ± 24.77- 17.6 831.58 ± 30.11-

εf (%)

εf (%)

68–141(MPa)
84–157

1.07–2.83
1.56–3.09- 68–141194
84–157195

1.07–2.83--

683.17 ± 16.67
1330.74 ±194
53.45

195

1.56–3.09--

11.65 ± 2.03---

All Ti-42Nb specimens
for compression
tests(n ±=3.92
17, specimen
numbers
9; 10;
12–26)11.65
showedTi-42Nb
Tensile60.51
674.08 ± 24.77
683.17± 16.67± 2.03similarplastic material
behavior in thetest.± One
was identified(present study)
Compression- uniaxial compression
831.5830.11 specimen
1330.74 ±53.45- asaberration (specimen 11) and was excluded from the calculation of the mean values and standard
deviationsof the investigatedparameters.
Alltests
specimens
compressed
until
load
All Ti-42Nb
specimens forcompression(n = 17,werespecimennumbers
9; the10; maximum
12–26) showed
of
101.9
kN
was
applied,resulting
in
about
65%
compressionofthespecimens;
thereby,nofractures
similar plastic material behavior in the uniaxial compression test. One specimen was identified as
could
be observed(Figure8). Thediameterof themaximumload wasaberration(specimen
11) andwasmeanexcluded
fromenlargement
the calculation
ofspecimen
the meanatvalues
and standard
4.37
±
0.34
mm.
deviations of the investigated parameters. All specimens were compressed until the maximum load
Under
the lack ofofa the
distinct
yield limit
for whichreasonof 101.9
kN compressive
was applied, loading,resultingTi-42Nb
in aboutrevealed
65% compression
specimens;
thereby,no fractures
theoffsetyield
strengthat
0.2%
compressionσ=831.58
±
30.11
MPa
and
offsetyield
strengthc
0.2
could be observed (Figure 8). The mean diameter enlargement of the specimen at maximum load
at2%4.37compressionwas± 0.34 mm.σc 2 = 921.36 ± 31.07 MPa was determined. The compression yield strength σc 35= 1330.74
53.45 MPa wasdetermined
the predefinedcompressionof 35%.
compression
Under±compressive
loading,
Ti-42Nbatrevealed
the lack of
a distinct yield
limitThefor whichreasonmoduluswasnotdetermined
due
to
the
fact
that
the
compression
tests
were
not
carriedout
withthe offset yield strength at 0.2% compression σc 0.2 = 831.58 ± 30.11 MPa and offset yield strengthan
atadditional
externalσcextensometer.
Table 2 shows the determined mechanical properties of Ti-42Nb and
2% compression
2 = 921.36 ± 31.07 MPa was determined. The compression yield strength σ c 35 =themechanicalcortical boneatfoundin the literature.
1330.74± 53.45properties
MPa wasofdetermined
the predefinedcompression of 35%. The compression
The
SEM
analysisofthefracturesurfacesofspecimen2 (testedė1notto ėcarried
specimen2 ) and out
modulus was not determined due to the fact that thecompression
testswith
were
with an3(Figure9) external
revealedextensometer.
two differentTable
fracturemechanisms.
As discussed,
showedaadditional
2 shows
the determinedmechanicalspecimen
properties2 ofTi-42Nb
cup-and-cone
fracture,
whereasas
anexception,
specimen3
faileddue
toshear
fracture.
The
shear
and the mechanical properties of cortical bone found in the literature.
fracture
arise
from internal
imperfections;however,
there could
currentlyinternal
imperfections
ThemaySEM
analysisof the fracturesurfacesof specimen
2 (testedwith ėno
1 to
ė2) andspecimen 3
bedeterminedin the microstructure
the tensile
test specimens,
excluding thesegregation
shown in(Figure9) revealed
two different offracture
mechanisms.
As discussed,
specimen
2 showedaFigure4d.
Thefracturesurfaceofspecimen2
displayedacomb-likestructure
with
combs
in
different
cup-and-cone fracture, whereas as an exception, specimen 3 failed due to shear fracture. The shear
sizes.
These
clearlyinternal
indicate imperfections;an entirely ductile
material
behavior
SLM-printed
fracturemayfeaturesarise fromhowever,
there
couldofcurrently
no Ti-42Nb
internal
specimens.
In
contrast,
theSEM
investigations
intothefracturesurfaceofspecimen3
revealed
circularimperfections be determined in the microstructure of the tensile test specimens, excluding
thesegregation shown in Figure 4d. The fracture surface of specimen 2 displayed a comb-like structurewith combs in different sizes. These features clearly indicate an entirely ductile material behavior ofMaterials
Materials 2018,2018, 11,
11, 124
124

99 of
of 20
20

Materials 2018, 11, 124
9 of 20
SLM-printed
SLM-printed Ti-42Nb
Ti-42Nb specimens.
specimens. In
In contrast,
contrast, thethe SEMSEM investigations
investigations into
into thethe fracture
fracture surface
surface ofofspecimen
specimen 33 revealed
revealed circular
circular voids
voids with
with fracture
fracture lines
lines highlighted
highlighted byby thethe white
white arrows
arrows inin Figure
Figure 9d.
9d.
These
spots
are
µµm
However,thosesamples
with
Thesewithspots
are about
about
30
m in
in diameter.
diameter.
However,asinobserved
observedforthosesampleswith30typical
typicalvoids
fracturelines30highlighted
by the whitearrowsas
Figure 9d.for
These
spots
are aboutµm in
cup-and-cone
fracture,acomb-typestructure
with
very
small
combs
can
be
observed
betweenthese
cup-and-cone
fracture,a comb-type
with very
combs
can be observed
thesediameter.
However,
as observed
for structure
those samples
with small
typicalcup-and-cone
fracture,between
a comb-type
circularspots.
circularspots.
structure with very small combs can be observed between these circular spots.

Figurecurvesof 17
compression
test specimens.
The moreormoreless identicalFigure8.
(a)Stress-strain
curvesof17
compression
test
The
or
Figure 8.
8.(a)(a)Stress-strain
Stress-strain
curvesofanalyzed
17 analyzed
analyzed
compression
test specimens.
specimens.
The
moreor lesslessslopesofthestress-strain
curvesofallsamplessuggestaconsistent
SLM-process,independentofidentical
identical slopes
slopes ofof thethe stress-strainstress-strain curves
curves ofof all
all samples
samples suggest
suggest aa consistentconsistent SLM-process,
SLM-process,thepositionofthespecimensonthebuilding
platform.
The
inserts
correspondtocompression
test
independent
independent ofof thethe position
position ofof thethe specimens
specimens onon thethe buildingbuilding platform.
platform. The
The inserts
inserts correspondcorrespond totospecimens before(b) and after(c) applicationof a(c)compressive
load
of 101.9 kN. load of 101.9 kN.
compression
test
specimens
before
(b)
and
after
applicationofacompressive
compression test specimens before (b) and after (c) application of a compressive load of 101.9 kN.

Figure9.
Scanning
electronmicroscope
images
of
the
after
of
Figure 9.
9.Scanning
Scanningelectron
electronmicroscope
imagesof fracture
the fracture
fracture
spots
after tensile
tensiletestingof thetheFiguremicroscope
imagesof thespots spots
after tensile
testing testingof
the Ti-42Nb
Ti-42Nb
tensilespecimens
at
a
100×
(a,b)
and
500×
(c,d)
magnificationforspecimen
2
(a,c)
and
Ti-42Nb
tensile specimens
at aand100×
and
500× (c,d) magnification
2 (a,c)andtensilespecimens
at a 100× (a,b)
500(a,b)× (c,d)magnification
for specimen 2 for(a,c)specimen
and specimen
3 (b,d).
specimen
33 (b,d).
The
specimen
22 (c)
comb
typicalfor
specimen(b,d).
Theoffracture
fracturesurface ofof tensile
specimen(c) displays
displays aatypical
comb structure
structure
typicalforThe
fracturesurfacetensile surface
specimen
2tensile(c) displays
a combstructurefor ductile
material
ductilematerial
failure.
Voids
thearrowsin
33 (d)
ductile Voids
material
failure.
Voids
with circular
circularlines
markedbyspecimen
the white
white
arrowsin specimen
specimen(d)failure.
with
circularlines withmarkedby thelines
whitemarked
arrows byin
3 (d)indicate
the presenceofindicatethepresenceofdefective
structuresintheprintedspecimen.
indicatethepresenceofdefective
structuresintheprintedspecimen.
defective structures in the printed specimen.

Materials 2018, 11, 124

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Materials 2018, 11, 124

10 of 20

To
To getget insights
insights into
into thethe microstructure
microstructure ofofTi-42Nb
Ti-42Nbspecimens
specimensunder
undercompression,
compression,SEM/EDX
SEM/EDX
investigations
wereperformedonthepolished
surfacesofcompression
specimens.
investigations were performed on the polished surfaces of compression testtest
specimens.
SEM SEM(BSE(BSEmode)/EDX
investigationsofthetestedcompression
specimens
displayedaformationofmode)/EDX investigations of the tested compression specimens displayed a formation of structures
structures
that
are
not
completely
understoodyet(Figure10).
Compression
specimens
showedthat are not completely understood yet (Figure 10). Compression specimens showed mostly dense
mostlydense morphology.
specimen
10 verysmall
pores
with diameterin the sub-micron
morphology.
However, inHowever,specimenin10
very small
pores
with
diameterin the sub-micron
range
range
be observed,
whereasin compression
specimen
11, greaterµm werecould could
be observed,
whereasin compression
specimen
11, greater
porespores
up to up
2 µto
m 2werefoundfound
with
with
recurrent
patterns.
Alongencounteredstructures
mayfore.g.,e.g.,grain
grain boundaries
boundaries orrecurrent
patterns.
Along
the theencounteredstructures
thatthatmaybebeforor
slidingbands,
slightdemixing
isobserved
with
a
local
enrichmentofTi(indicatedinredcolor)
andsliding bands, slight demixing is observed with a local enrichment of Ti (indicated in red color) andNb
The chemicalchemical composition
composition ofof Ti
Ti andNb measured
measured with
EDX fromfrom measuring
Nb (blue(blue color).
color). The
and Nbwith EDX
measuring spots
spots 11
to3
(Figure
10)
is
given
in
Table
3.
to 3 (Figure 10) is given in Table 3.

10. Scanning
Figure 10.
Scanning electron
electron microscope
microscope images
images (a,b; BSE
BSE mode)mode) and energy dispersive X-ray
spectrometer (EDX)(EDX) mapping
mapping (c,d)(c,d)ofofpolishedpolishedcross-sections
cross-sectionsofof
Ti-42Nbcompression
specimens
Ti-42Nbcompression
specimens
10
10
(a,c)
and
3 (b,d)aftercompression
compressiontests
testsatat3000×
3000×magnification.
magnification.The
Theimages
images(a,b)(a,b) display
display structures
(a,c)
and
3 (b,d)after
thatslidingbands,
respectively,
indicatedbythat may
may bebeidentified,
identified,forfore.g.,e.g.,grain
grainboundary
boundaryformation
formationandandslidingbands,
respectively,
indicated
white
arrows.
TheTheencounteredstructuresizes
are arein therange
of few
microns.
For For
specimen
10 (a),
by whitearrows.
encounteredstructuresizes
in therange
of few
microns.
specimen10
only
a fewa very
voids within the sub-micron
range arerange
observed,
whereas awhereas
recurrenta(a), only
few smallvery small
voidsdiameterwith diameterin the sub-micron
are observed,
patternof voids(up
2 µm)
is foundforisspecimen
11specimen(b). The images
display (c)a partiallyrecurrent
pattern
ofto
voids
(up
to 2 µ m)
found for11 (b). (c,d)
The imagesand (d) de-mixed
display amicrostructurewith
Ti(red)andNb-richspots(blue).
A
spatialseparation
of
Ti
and
Nb
along
partially de-mixed microstructure with Ti- (red) and Nb-rich spots (blue). A spatial separation oftheTiencounteredis ascertainedin bothsamples. in both samples.
and Nb alongstructures
the encounteredstructures
is ascertained

Apart from the encountered structures with slight enrichment of Ti and Nb, a microstructure withvery homogeneous distribution of Nb and Ti could be observed, whereas there was no indication of
the presence of dendrite-type structures, as observed in the powder. EDX scans over the entire particle
area (spot 3) exactly mirrored the results of the element analysis.

Materials 2018, 11, 124

11 of 20

Table 3. Chemical composition of compression test specimens 10 and 11 at different locations
determined by SEM/EDX.
Specimen 10

Specimen 11

EDX Measuring
Spot

Ti (wt %)Nb (wt %)Ti (wt %)Nb (wt %)

1
2
3

55.7
59.1
57.7

44.3
39.9
42.3

56.3
58.1
57.5

43.7
41.9
42.5

3. Discussion
The difference in elastic modulus between implant material and host bone leading to bone loss
due to stress shielding has been extensively discussed in the literature [72–75]. Ti alloys are widely usedimplant materials with sufficient biocompatibility. However, α- and α + β-type Ti alloys, which arecommonly applied in AM, possess relatively high elastic moduli [74] when compared to entirely β-type
Ti alloys. Accordingly, they may cause stress-shielding and associated implant loosening [55,76,77].
Accordingly, we describe the synthesis and SLM technology of spherical Ti-42Nb with the goal toproduce 3D bulk material with an elastic modulus much closer to that of the human bone than standard
Ti alloys. Test specimens manufactured by SLM (as-built) were used for tensile and compression
tests. The material characterization was supported by SEM/EDX analysis of the newly produced alloy
powder. SEM revealed a significant change in the microstructure of the powder after the laser melting
process. Furthermore, the characterization of mechanical properties showed decreased elastic modulus
and good ductility when compared to the commonly employed implant materials [8,34].
The main limitation of the present study is the inherent simplification of the loads applied to thematerials in mechanical testing in comparison to combined loads in vivo appearing due to complex
force situation [78]. Thereby, bending and torsional loads are predominantly [78,79] applied in vivo toorthopedic implants, e.g., the femoral component of total hip endoprostheses. However, torsion andbending test were not conducted to characterize the mechanical properties of Ti-42Nb under such loads.
Furthermore, the material behavior under cyclic loading and the fatigue resistance have not been
investigated so far. In the tensile test set up applied within this study in four of eight specimens theUTS was determined at a strain rate of ė1 = 0.00025 s−1 . In general, the UTS should be determined witha strain rate of ė2 = 0.0067 s−1 [80] due to omitted change from strain rate ė1 to ė2 . This certainly enables
the assessment of the UTS under different load rate conditions. In contrast, the compression test was
conducted without an external extensometer. Thus, the validity of the measured force-displacement
behavior in the elastic region of the material is narrowed; due to this, the compressive modulus was
not computed. All specimens were tested in the as-machined condition. Additional heat treatment was
not performed due to the fact that the mechanical characterization of the Ti-42Nb alloy newly appliedin additive manufacturing should be conducted in the initial condition (“as machined”). Nevertheless,
it is expected that a heat treatment of the printed specimens, such as hot isostatic pressing (HIP) and/or
diffusion annealing, will lead to reduced internal stresses. However, either of these treatments will
release a coarser-grained microstructure when compared to the as-printed state and most probably
result in increased elongation at fracture and a decrease in UTS. The analysis of the microstructure
by means of SEM and EDX could only be conducted at two of the tensile test specimens and only
three of the compression test specimens. The segregation that indicates the dendrite-like structureof the Ti-42Nb precursor powder in Figure 4d was the only observed particle residue in all analyzed
specimens independent from the direction of the polished cross section.
The dendrite-like microstructure in the precursor Ti-42Nb particles, which was observed in
EDX investigations, was not maintained during the SLM processing. Against this, the printed
parts displayed a microstructure with a very homogeneous distribution of niobium and titanium.
This pointed to the fact that the melt generated by the laser rapidly cooled down under the appliedfabrication conditions; literature values reported cooling rates during SLM processes of about

Materials 2018, 11, 124

12 of 20106 K/s [63]. Accordingly, fast temperature dissipation during the laser melting process resulted
in a mostly homogenous distribution of Ti and Nb on a sub-micrometer scale apart from the structuresthat may be identified, e.g., grain boundaries and sliding bands, respectively. An enrichment of Ti andNb appeared solely in these structures.
The density of 99.5%, which was estimated visually from the cross section of the Ti-42Nb
specimens made by SLM, is in agreement with the observations of Yasa et al. [81] for SLM-manufactured
Ti-6Al-4V specimens. The solid material structure was indicated by the presence of only a very small
amount of round-shaped weak features within the x–y plane of the as-printed tensile specimens.
No large pores were observed, as reported by Vandebrouke et al. [52], for low energy input by the laser
beam. Nonetheless, laser molten material contained fewer pores and showed finer microstructures
when compared to cast materials [52].
All raw cylinders had been printed with the same height, and only the position on thesubstrate plate (initial x-y-plane) varied. In the polished cross section (x-y-plane) of the printed
bulk material cylinders, a homogenous distribution of niobium and titanium on a sub-micron scale
was determined using EDX-analysis. In a preliminary investigation, electron backscatter diffraction(EBSD) of SLM-printed Ti-42Nb was used to determine the predominant grain orientation paralleland perpendicular to the building direction (pictures are not shown). The results revealed (i) the
existence of very fine microstructures with (ii) the orientation of elongated grains in z, i.e., in thebuilding direction.
All specimens were machined along the building direction out of the raw cylinders to matchthe specimen dimension claimed by the DIN 50106 standard (tensile specimens) [82] and DIN 50125
standard (compression specimens) [83]. Both the standards demand a polished surface of the effective
measure length l0 ; due to this recommendation, we decided to machine the specimens out of theprinted bulk material. Parameters such as surface roughness, building direction as well as global
position of the printed part in the powder bed and the influence of these parameters on the mechanicalproperties should be investigated in further studies.
Wysocki et al. [84] reported that the UTS of SLM-printed CP Ti varied with the position of theprinted part on the building platform, whereas the porosity changed with the height of the printed
parts. It was furthermore reported that the building direction of the parts had an influence on theelongation at fracture under tensile loading [84]. The graduated porosity along the building direction
and the different elongation in both cross sections had an effect on the necking of the sample [84].
It was observed that tensile test specimen 3 fractured at 45◦ in relation to the load direction
and circular voids with fracture lines at the fracture surface. The reason for the formation of thesefeatures is not understood at the moment. Most probably they arise from pores introduced during theSLM process. Several reasons may be attributed to their formation, as reported by Zhang et al. [85],for example, increased solubility of gas in the melt, trapped gas bubbles in the melt pool, or insufficient
powder bed density (“voids” in the powder bed). The other seven among eight samples showed
typical cup-and-cone fractures, and the typical comb-type structure could be observed. In contrast,
no fractures were observed during the compression tests of the β-type Ti-42Nb alloy. Nevertheless,
Chlebus et al. [86] described the occurrence of shear fracture in compression tests of α + β-type
Ti-6Al-7Nb alloy.
Mechanical testing of the SLM-manufactured β-type Ti-42Nb alloy confirmed the findings of
Ozaki et al. [61] that the Young’s modulus is about 62 GPa at a concentration of 42 wt % of niobium,
which was determined for cast material. Furthermore the prediction of a distinct lower modulus for
β-stabilized Ti alloys [6] compared to α-type Ti and α + β-type Ti alloys [6] could be approved in
the present study. However, even though the UTS of SLM-printed Ti-42Nb is markedly higher thanthat of CP Ti, it is distinctly reduced about 71% when compared to Ti-6Al-4V. Both Young’s modulus
and the strength of SLM-printed Ti-42Nb compare well to those of conventionally manufactured, i.e.,
forged Ti-42Nb [61]. SLM-printed Ti-6Al-4V, which has been developed as a high-strength alloy for
application in aerospace has a markedly higher ultimate strength (1075 MPa in the building direction)

Materials 2018, 11, 124

13 of 20and a about 80% higher elastic modulus (115–119 MPa in the building direction) [38]. Eventually,an additional heat treatment may optimize the UTS [87] of the Ti-42Nb alloy, but this will be partof follow-up studies. However, this issue is discussed controversially. For example, Leuders et al.
showed that heat treatment considerably above the β-transus of Ti-6Al-4V increased the elongation at
fracture with a coincident decrease of UTS [49].
The Young’s modulus of SLM-printed Ti-42Nb determined under tensile load is 2.5 to 4.4 times
higher than that of the cortical bone of the femur [26,27,70] and the tibia [70]. A high difference in theelastic modulus between the implant and the bone is associated with insufficient load transfer [53]leading to eventual bone resorption and loosening of the implant [53]. Nonetheless, the observed
modulus of the β-type Ti-42Nb is only about half the modulus provided by Ti-6Al-4V. Such an increasein elasticity is expected to prevent stress-shielding, which in the worst case is associated with bonemass loss and implant loosening. Furthermore, adjusting the intrinsic properties of a material is not
the only approach to reduce the stiffness of implant structures. Structural behavior and stiffness of
the implants can be influenced by the grade of porosity and the geometry of the applied structuralelements [88–90]. Disadvantage of porous structures is the reduced fatigue resistance when comparedto solid materials. Apart from this due to the high stiffness of commonly used metallic implantmaterials the size of the unit cells and the resulting porosity relative to the diameter of the struts haveto be enlarged to match the mechanical and structural properties of human bone. This may affect the
ingrowth of the bone cells into porous structures if the unit cell size exceeds more than 600 µm [91,92].
With a SLM-printed Ti-42Nb material that provides about half the stiffness of the commonly usedbone implant materials based on Ti alloys the unit cell size can be better adjusted to the mechanicalproperties of human bone.
4. Materials and Methods
4.1. Powder Synthesis
Ti-42Nb spherical powder (Trade name AMPERTEC MAP Ti-42Nb) is a newly developed
commercial product distributed by H.C. Starck. It is obtained by electrode induction-melting gas
atomization (EIGA) of Ti-42Nb rods using purified Ar (4.6, Linde). The SLM fraction is refined byultrasonic sieving through 63 µm stainless steel meshes and subsequent sifting to remove particles
with diameter < 5 µm. AMPERTEC MAP Ti-42Nb is almost 100% spherical; satellites or irregular
non-spherical particles are rarely observed (Error! Reference source not found.). The particle surfaces
are smooth without visible cracks or other faults. This is confirmed by PSD (Master Sizer 2000 (Malvern,
Worcestershire, UK); not shown), which displays a monomodal distribution with D(10) = 28.20 µm,
D(50) = 44.80 µm and D(90) = 69.61 µm.
4.2. Specimen Manufacturing
3D-manufacturing by means of SLM was performed using a Trumpf TruPrint 1000 printing
system (TRUMPF GmbH + Co. KG, Ditzingen, Germany; 200 W fibre laser; wavelength ~1070 nm) at
Laserzentrum Hannover (LZH), Germany. Parameters were determined empirically under variationof laser power (40–120 W) and scan speed (80–500 mm/s). The hatch width was 100 µm and thepowder feed rate was 50 µm/layer. Based on microstructural investigations of polished cross-sectionsof 4 × 4 × 4 mm3 test cubes, a laser power of 80 W and a scan speed of 400 mm/s were chosen
for printing specimens applied in compression and tensile strength tests. The specimens were layer
wise printed with a square scanning pattern. Post heat treatments such as diffusion annealing to
reduce internal stresses or hot isostatic pressing to remove closed pores were not applied. The sizesof the rods printed for compression and tensile strength testing are provided in Table 4. The raw
cylinders were subsequently CNC-machined to match the specimen dimensions according to DIN
50125 specimen shape B [83] for tensile test and according to DIN 50106 [82] for the compression test
(Table 4; Figure 11).

Materials 2018, 11, 124

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according to DIN 50125 specimen shape B [83] for tensile test and according to DIN 50106 [82] for the
Materials 2018, 11, 124
14 of 20compression test (Table 4; Figure 11).
Materials
2018, 11,124 50125 specimen shape B [83] for tensile test and according to DIN 50106 [82] for14
of 20
accordingto DINthe

Table 4. Specimen
dimension
to DIN 50106 (compression specimens) and DIN 50125
compression
test (Table
4; Figureaccording
11).
(tensile specimens).

Table
4. 4.Specimen(compressionspecimens)
specimens)andandDIN
DIN
50125
Table
Specimendimension
dimension according
according toto DINDIN 50106
50106 (compression
50125
As
PrintedAs
Machined
As
Measured(tensile
specimens).
(tensile specimens).

SpecimenTensileSpecimen
Specimend (mm)

h (mm)

d0 (mm)As Printed
As Printed

h0 (mm)As Machined
As Machinedd0 (mm)h0 (mm)As Measured
As Measured

15.0
5.98 ± 0.01
60.06
± 0.05d(mm) d (mm)h70.0(mm) h (mm)d0 6.0(mm)
h060.00(mm)
h0 h(mm)
d0 (mm)h0 (mm) d0 (mm)d0 (mm)0 (mm)(Specimen 1–8)
TensileTensile(Specimen 1–8)
15.0
70.0 6.0
6.0 60.0060.00 5.985.98± 0.01 60.06
60.06± 0.05Compression
15.0
70.0
± 0.01± 0.05
10.7
16.0
0.02
± 0.03(Specimen(Specimen
1–8)
Compression
9–26)
10.7
16.0 6.9
6.9 10.3510.35 6.89
6.89± ±
0.02 10.33
10.33 ±
0.03
(Specimen 9–26)
Compression
10.7
16.0
6.9
10.35
6.89 ± 0.02
10.33 ± 0.03
(Specimen 9–26)

Figure 11. (a)(a) Tensile test specimen after CNC machining the as-built specimens to the dimension
50106
standard;(b)aftercompression
test specimen
afterspecimens
separation
substrate
claimed
by
the
DIN
50106
standard;(b)CNC
compression
test
after separation
fromtheFigureby
11.the(a)DIN
Tensiletest
specimenmachining
the specimen
as-built
tofromthe thedimension
plateand
CNC-machining
tothedimensions
claimed
by
the
DIN
50125
standard.
The
surfacesubstrate
plate
and
CNC-machining
tothedimensions
claimed
by
the
DIN
50125
standard.
The
claimed by the DIN 50106 standard; (b) compression test specimen after separation from thequalityof thetensileandcompression
specimens
is a resultof theprocess
and andwasnotsurfacequalityof theand compression
specimens
isclaimeda result
ofmachining
the machining
processwassubstrate
plate
andtensile
CNC-machiningtothe dimensions
by
DIN 50125
standard.
The
additionallypolished.
surfacequalityof the tensile and compression specimens is a result of the machining process and wasnotadditionallypolished.
not additionally polished.

4.3. Mechanical Test Setup
4.3. Mechanical Test Setup
The mechanicalmechanical properties
properties ofof thethe Ti-42Nb alloy were determined in tensile and compression
The mechanicalproperties[80]of thealloy
were determinedin tests
tensileandperformed
compression
tests according
to EN ISO6892-1
andTi-42Nb
DIN 50106
standard[83]. Tensile
were
on a
Tensiletests
accordingtoEN
ISO6892-1
[80]
and
DIN
50106
standard[83].
Tensiletests
were
performed
onauniversal testing machine Z050THW (Zwick GmbH & Co. KG,
KG, Ulm,
Ulm, Germany) and the compression
universal testing machine Z050THW (Zwick GmbH & Co. KG, Ulm, Germany) and the compression
tests were
wereconductedconductedonona servo-hydraulictesting
machineInstron8502 (Instron
Deutschlanda servo-hydraulictesting
machineInstron8502 (Instron
DeutschlandGmbH,tests were conducted on a servo-hydraulic testing machine Instron 8502 (Instron DeutschlandGmbH,
Pfungstadt,
Germany)atroom
temperature(Figure12).
Pfungstadt, Germany) at room temperature (Figure 12).
GmbH, Pfungstadt, Germany) at room temperature (Figure 12).

Figure 12. (a) Test setup for the characterization of the mechanical properties under tensile (a) andcompressive load (b). (a) In the tensile tests, the specimen elongation was recorded by a video
extensometer; (b) Highly polished compression plates applied uniaxial compression until the maximumof 101.9 kN compressive load was reached.

Materials 2018, 11, 124

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Uniaxial tensile test of the Ti-42Nb alloy was carried out until the fracture of the specimens (n = 8,
specimen 1–8) occurred; meanwhile, the specimen elongation was recorded by the optical extensometer
videoXtens (Zwick GmbH & Co. KG, Ulm, Germany). The parameters, Young’s Modulus E and proof
strength at 0.2% plastic extension σ0.2 , were determined at a strain rate of ė1 = 0.00025 s−1 based on theelongation measured with the extensometer. For determination of the ultimate tensile strength (UTS),
the strain rate was switched from ė1 to ė2 = 0.0067 s−1 after the plastic extension at 0.2% was reached.
Furthermore, the uniaxial compression tests for the Ti-42Nb specimens (n = 18, specimen 9–26) werecarried out at a cross speed of 30 MPa/s until the predefined maximum load of 101.9 kN was reached.
During the compression test, the reaction force was recorded, and the shortening of the compression
specimens were determined by a displacement of the compression stamp. The compression test was
conducted without an external extensometer. The compressive load was applied by highly polishedcompression plates on the specimens. Subsequently the compressive strength at 35% strain σc 35
and the offset yield strength at 0.2% strain σc 0.2 and 2% strain σc 2 were computed based on thecompression-stress compression-strain relation. Due to the lack of cracks or fractures of the specimens
during the compression test, the compressive strength was analyzed at 35% compressive strain. All thedetermined stresses and strains are engineering stresses and strains.
4.4. Chemical and Microstructural Characterization
For investigation of the structural characteristics of the raw Ti-42Nb powder material and the SLM
manufactured test specimens, the scanning electron microscopy (SEM; Jeol JSM 6490 LV, Akishima,
Tokyo, Japan) in combination with energy dispersive X-ray (EDX; Bruker Quantax EDS with SDD
Detector (Bruker, Berlin, Germany), resolution 130 eV; recording time 200 s, ca. 1.5 Kcps, excitation
voltage 28 KV; quantification standardless via ZAF routine) was performed. The chemical composition(Ti- and Nb-content in wt %) of the powder was evaluated on a polished 55 µm AMPERTEC MAP
Ti-42Nb particle at five measuring spots. The characterization of the chemical composition of thecompression specimens was conducted for specimens 10 and 11 after the compressive load wasapplied. Therefore, compression specimen 10 was polished perpendicular to the loading direction andcompression specimen 11 was polished to the cross section in the direction of the applied load. For both
specimens, the local chemical composition was measured at spots 1 and 2. The averaged chemicalcomposition was determined at spot 3. Furthermore, the fracture surfaces of the tensile specimen 2 and3 were exemplarily analyzed by SEM investigation. The fracture of the SLM-manufactured Ti-42Nb
alloy was characterized and assessed with regard to the pores and fracture behavior. In addition,
X-ray diffraction studies were performed using a PANalytical X-ray diffractometer (Almelo, Overijssel,
Netherlands) (X’Pert MPD-PRO, Cu Kα radiation, 2Θ = 10–80◦ )5. Conclusions
Ti-42Nb has been developed as an alternative implant material. The focus was on lowering theYoung’s modulus when compared to standard implant materials to improve the elastic compatibility
with human bone. Another objective was to develop Ti-42Nb with spherical morphology to allow
for a fabrication of patient-specific implants using additive manufacturing, i.e., powder bed-based
3D printing processes. Ti-42Nb spherical particles were obtained by electrode induction-melting gas
atomization (EIGA) of pre-alloyed Ti-42Nb rods and successfully 3D printed using selective laser
melting (SLM). As-printed bulk materials possessed densities > 99.5% and microstructures withextremely homogeneous element distribution of Ti and Nb. Tensile strength test revealed significantly
improved elasticity of SLM-printed Ti-42Nb (ca. 60 GPa in tension) when compared to CoCr (235 GPa),
CP Ti (ca. 105 GPa) or Ti-6Al-4V (ca. 110 GPa). The proof strength at 0.2% plastic extension of
Ti-42Nb was determined at ca. 675 MPa, which is lower than that of Ti-6Al-4V (790–870 MPa) but
significantly higher than that of CP Ti. In conjunction with its excellent biocompatibility publishedrecently, Ti-42Nb seems to be a very promising material for application in orthopedic and dental
implants in the near future.

Materials 2018, 11, 124

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Acknowledgments: The authors thank Mr. Holger Brauns (Fraunhofer IGP Rostock) for performing the tensile
test of the additive manufactured specimens. We thank Mr. Frank Richter (Institute of Physics, University of
Rostock) for machining the test specimens. Ms. Yvonne Wessarges and Dr. Joerg Hermsdorf (Laser Zentrum
Hannover, LZH) are acknowledged for SLM-printing compression and tensile test specimens.
Author Contributions: Christian Schulze and Rainer Bader designed the study and the mechanical experiments;
Christian Schulze and Christoph Schweigel performed the experiments; Christian Schulze, Christoph Schweigel,and Olaf Keßler analyzed the data; Markus Weinmann contributed to the metallurgical and microstructural
analysis tools and is responsible for the design and development of the test materials (Ti-42Nb precursor powder
and SLM-printed specimens) and manufacturing technologies; Christian Schulze and Markus Weinmann wrote
the manuscript; all authors proofed the manuscript.
Conflicts of Interest: The authors declare no conflict of interest.

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