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Short Communication
β-Type Zr –Nb–Ti biomedical materials with high
plasticity and low modulus for hard
tissue replacements
Li Nie, Yongzhong Zhann, Tong Hu, Xiaoxian Chen, Chenghui Wang
College of Materials Science and Engineering, Guangxi University, Nanning, Guangxi 530004, PR China
a r t i c l e i n f o
Article history:
Received 13 July 2013
Accepted 15 August 2013
Available online 28 August 2013
Keywords:
Zr –Nb–Ti alloys
β-Type
Biomedical materials
Mechanical behavior
a b s t r a c t
In order to develop new biomedical materials for hard tissue replacements, Zr –20Nb–xTi
(x¼0, 3, 7, 11 and 15) alloys with required properties were designed and prepared by using
the vacuum arc melting method for the rst time. Phase analysis and microstructural
observation showed that all the as cast samples consisted of equiaxed β-Zr phase. The
mechanical properties and fracture behaviors of the Zr –20Nb–xTi alloys have been
analyzed. It is found that these alloys exhibit high plasticity, moderate compressive
strength (1044–1325 MPa) and yield stress (854–1080 MPa), high elastic energy (12–20 MJ/m3)
and low Young's modulus (28–31 GPa). This good combination of mechanical properties
makes them potential biomedical materials for hard tissue replacement.
& 2013 Elsevier Ltd. All rights reserved.
1. Introduction
Currently, development of biological hard tissue replace-
ments (HTR) has attracted increasing attention. However,
only part of the investigated materials may be applied for
HTR, as a number of conditions should be satised. Firstly,
the implant materials must possess excellent biocompatibil-
ity without adverse reaction with human body. Secondly, it
should have excellent corrosion resistance in body
uid, highmechanical strength and fatigue resistance. Finally, good
wear resistance and low elastic modulus (close to that of a
human bone (15–30 GPa)) are required, in order to transfer
adequate mechanical stress to the surrounding bones (Guo
et al., 2010; Li et al., 2011).
Ti–6Al–4V alloy was one of the earliest Ti-based biomater-
ials introduced in implantable components and devices.
However, the modulus of Ti–6Al–4V reaches about 110 GPa,
which is substantially higher than that of natural human
bone (10–30 GPa) (Niinomi, 1998). As a result, the human body
may be seriously damaged under external applied load due to
the elastic mismatch. Therefore, biomaterials with low elastic
modulus are attracting the attention of researchers. It is
found that new type biomedical Ti alloys with single β-Ti
phase can effectively reduce the elastic modulus to be about
60–80 GPa (Nag et al., 2007; Niinomi, 2004), which is closer to
the human bone. However, to develop suitable HTR materials,the elastic modulus should be further reduced without
deterioration of the other properties. Consequently, develop-
ment of the other metal-based biocompatible materials is an
important solution.
Zr is an important alloying element for Ti-based alloys to
improve mechanical properties. More importantly, similar to
Ti, Zr is a favorable non-toxic metal with good biocompat-
ibility with organism. Zr and its alloys are known as excellent
1751-6161/$ - see front matter &
2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.jmbbm.2013.08.019
nCorresponding author. Tel.: þ86 771 3272311; fax:þ86 771 3233530.E-mail address: zyzmatres@yahoo.com.cn (Y. Zhan).
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bioactive metallic materials because a bone-like apatite layer
may be formed on their surfaces in living body. In addition, Zr
alloys exhibit high mechanical strength, fracture toughness
and corrosion resistance (Inoue, 2000; Niinomi, 2003; Okazaki
et al., 1998; Wen et al., 2006), therefore they are potential
biomaterials for hard tissue replacements. However, up to
now very few report can be found on the Zr-based biomedical
materials. Microstructure and magnetic susceptibility of as-
cast Zr –Mo alloy have been investigated, and it was reported
that the alloys are useful for medical devices applied under
magnetic resonance imaging (Suyalatu et al., 2010). Li et al.
(2011) have developed Zr –Si biomaterials with high strength
and low elastic modulus (25.08–29.63 GPa), which satises
well the requirements of HTR. Zr –Nb alloys were previously
used as nuclear materials, thus the investigations were
mainly focused on microstructural stability and corrosion
behaviors under various conditions (Arima et al., 2005;
Aurelio et al., 2001; Benites et al., 2000; Dey et al., 1995;
Jeong et al., 2002; Kim et al., 2008). However, as biomedical
materials, the Zr –Nb alloys have gradually attracted the
attention of some researchers (Kondo et al., 2011; Akahori
et al., 2011a, 2011b). For instance, recently the mechanical
properties and biocompatibilities of the Zr –Nb alloys have
been experimentally studied (Akahori et al., 2011a, 2011b).
In this paper, the Zr –20Nb–xTi alloys are designed, with
Ti content in the range from 0 to 15.0 at%. The object is to
develop potential biomedical materials with appropriate
mechanical properties (i.e. low Young's modulus, favorable
ductility, and moderate strength) for hard tissue replace-
ments. The phase composition, microstructure and mechan-
ical properties including compressive stress, Young's
modulus and elastic energy of the as-cast Zr –20Nb–xTi (x¼
0, 3, 7, 11 and 15) alloys have been investigated.
2. Experimental procedure
The Zr –20Nb–xTi alloys with nominal composition (in atomic
fraction) of Zr –20Nb, Zr –20Nb–3Ti, Zr –20Nb–7Ti, Zr –20Nb–11Ti
and Zr –20Nb–15Ti have been melted by using the WK-II type
non-consumable vacuum arc melting furnace. The raw
materials were pure sponge zirconium, pure niobium and
pure sponge titanium with purity all higher than 99.9 wt%. In
order to ensure adequate mixing and reaction of the samples
in molten state, the melting temperature was controlled to be
as high as 2500 1C by setting the current intensity. The
reaction and melting time for each sample was kept for
80 s. The molten samples were then cooled down directly in
the water-cooled copper melting pots (by running water at
room-temperature) with cooling time is about 500 s. In order
to ensure chemical homogeneity of each sample, all the
ingots was turned over and remelted for six times.
Samples for optical and secondary electron microscopy
were cut by electric discharge machining (EDM) from the
ingots. They were mounted and then mechanically polished
with SiC paper and Al2O3 particles with water. The polished
samples were etched in an erodent with composition of
HF (aqueous solution): HNO3 (aqueous solution): water ¼1:2:6
(ratio by volume). The weight percents of the HF aqueous
solution and the HNO3 aqueous solution were 40% and 65%,
respectively.
Phases identication were carried out via X-ray diffraction
(XRD) using Rigaku D/Max 2500 V diffractometer with Cu Ka
radiation and graphite monochromator operated at 40 kV and
200 mA. The microstructures were determined from DMM-
660 type optical microscopy and Hitachi S-3400N scanning
electron microscope (SEM) equipped with energy dispersive
(EDX) analysis. The dimensions of the compression specimen
were 5 mm5 mm10 mm. Three samples were tested for
each alloy to get the average values. Compression test was
conducted at room temperature in air at an initial strain rate
of 1 mm/min by using the Instron 8801 axial servohydraulic
dynamic testing system, to determine the mechanical prop-
erties including ultimate compressive strength, fracture
strain and Young's modulus etc.
3. Results and discussion
3.1. Component selection principle
In this work, there are two objects for the design of these
biological HTR materials. Firstly, the elastic modulus should
be effectively reduced to comply with that of the human
bones (10–30 GPa). This measure can reduce the effects of
stress shielding between the implant material and bones.
Secondly, the strengths (including the yield strength and the
compressive strength) of the HTR material should be moder-
ately improve to withstand external force. This measure
helps to prolong the working life of the HTR components. It
should be pointed out that as the hard tissue is mainly
affected by compression stress, so the compressive properties
are principally considered in this work. At the same time,
excellent ductility is also required because it is important for
processing the HTR materials into components with various
shapes.
It is known that for the two crystal structures of Zr (i.e.
close-packed hexagonal α-Zr and body-centered cubic β-Zr),
the elastic modulus of the β-Zr (about 60 GPa) is much lower
than that of the α-Zr (about 100 GPa) (Cai et al., 2009; Kondo
et al., 2011). As a result, in order to effectively reduce the
elastic modulus of the zirconium alloy, β-Zr should be
obtained at room temperature. However, since the β-Zr
belongs to high-temperature phase, the so-called β phase
stabilizing elements like Mo, Nb, Ta, etc. should be added in
the alloys to effectively decrease the β-Zr and α-Zr phase
change transition temperature. Considering the future prac-
tical applications, the expensive raw material Ta is excluded.
In this work, we mainly want to improve the strength of the
Zr alloy through the solid solution strengthening of alloying
elements. For this strengthening mechanism, the distortion
energy caused by lattice distortion can hinder the movement
of dislocations and thus improve the strength. Larger lattice
distortion results in more signicant strengthening effect, but
also decreases the plasticity. According to the Zr –Nb and the
Zr –Mo binary phase diagrams (Abriata and Bolcich, 1982;
Zinkevich and Mattern, 2002), the elements Nb and Mo can
form innite and limited solid solutions in Zr, respectively.
Therefore, Nb element may result in smaller lattice distortion
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and obtain a good combination of ductility and strength, as
compared with Mo. Accordingly, we select Nb as the major β-
Zr-stabilizing element for the designed alloys. In order to
decrease the β-α phase transition temperature as far as
possible and avoid the generation of coarse grains, the
content of Nb is designed to be 20 at% (which is slightly
higher than the segregation reaction transition component
point, but slightly lower than that of the eutectic reaction
(Abriata and Bolcich, 1982)). To further improve the mechan-
ical properties, Ti is chosen as the third alloying element for
the Zr –Nb alloys. According to the Zr –Ti binary phase diagram
(Murray, 1981), these two elements can form an innite solid
solution. Ti is conducive to the formation of metastable β-Zr
alloys at room temperature. Ti content is designed to be in
the range from 0 to 15 at% to investigate the inuence of Ti
content on the mechanical properties of the Zr –20Nb alloy.
3.2. Phase identi cation and microstructure
XRD patterns of the as-cast Zr –20Nb–xTi (x¼0, 3, 7, 11 and 15)
alloys are shown in Fig. 1. It is clearly indicated that the solid
state phase composition of all the samples is β-type Zr. As
Ti and Zr can dissolve completely with each other at any
temperature (Murray, 1981), the effect of Ti element on the
phase composition of Zr alloys should be weak. Nb belongs to
4d transition metallic element, which is effective to stabilize
the β-Zr phase. In addition, it is noted that Zr and Nb can
completely dissolve with each other in the β phase region
(Abriata and Bolcich, 1982). The room-temperature phase
composition at is closely related to the cooling rate after
the alloys have been solidied from high-temperature liquid
(Li et al., 2012). In this work, all the alloys were produced in
the water-cooled copper crucible by using arc melting. The
cooling rate may be too fast for the small button samples to
reach equilibrium state. In other words there is no suf cient
time for the solid phase transition (β-Zr -α-Zr). Moreover, due
to the similarity of the crystal structure and lattice parameter
between Zr and Nb elements, they may completely dissolve
in each other and then result in ideal undercooling. This
helps to form the metastable β phase at high temperatures.
Hence, the high-temperature phase β-Zr is kept at room-
temperature in the as-cast samples. In addition, it is found
that with the increase of Ti content, the intensities of XRD
peaks change accordingly, which indicates that the crystal-
line process has been affected.
Typical secondary electron micrographs of the as-cast Zr –
20Nb–xTi (x¼0, 3, 7, 11 and 15) biomedical alloys are dis-
played in Fig. 2. As both Nb and Ti can dissolve in the Zr
matrix, equiaxed β-Zr crystal grain can be clearly observed.
The grain boundaries of the β-Zr crystal grain are also seen
clearly. In addition, compared with the Zr –20Nb sample,
addition of Ti element makes the grain boundary closer and
renes the microstructure. This may be due to the fact that Ti
element restrains the precipitation of Nb from the grain
boundary and then reduces the thickness of grain boundary.
3.3. Mechanical properties
Fig. 3 shows the room-temperature compressive stress–strain
curves of the β-type Zr –20Nb–xTi (x¼0, 3, 7, 11 and 15)
biomedical alloys. It is found that Ti element has an impor-
tant impact on the mechanical properties. It is noted that the
slope of elastic stage changes slightly with the increasing of
Ti content. This indicates that Ti element has little effect on
the stiffness of Zr –20Nb alloy. As we know, the stress–strain
curves can describe the hardening growth trend in the
process of plastic deformation. According to Fig. 3, it is found
that all the samples do not exhibit signicant hardening
effect with the increase of pressure. This means that the
Zr –20Nb–xTi (x¼0, 3, 7, 11 and 15) alloys are suitable to be
processed into different shapes. Table 1 lists the values of
Young's modulus, compressive strength, yield strength, elas-
tic energy and plastic strain of these biomedical alloys. It is
found that all the samples exhibit high (and also close) plastic
strains, which should be related to the mechanisms of
various solid solution types. Since both Nb and Ti can
completely dissolved in the β-Zr matrix, they may limitedly
enhance the strength and maintain relatively high ductility.
In this work, the maximum compression distance was set to
be 4 mm. However, all the samples were not crushed in this
compression distance. Compression cracks of the tested
samples are shown in Fig. 4. It is observed that the angles
between the compression direction and cracks are about 451.
According to the width of the cracks, it is inferred that the
plasticity is relatively good Ti content ranges from 7% to 11%.
In this work, low Young's modulus is a highlight for the
Zr –20Nb–xTi system. As listed in Table 1, the Young's
modulus ranges from 28 to 31 GPa, which meets the require-
ment of HTR for human body (15–30 GPa) well. Therefore, the
present material system is expected to solve the problem
caused by the mismatch of elastic properties between the
implant materials and human bones. It should be pointed out
that the solid solution strengthening is signicantly affected
by Ti element. As the Young's modulus is dened to be the
ratio of the normal stress and strain in the elastic deforma-
tion stage, both the deformation resistance and the Young's
modulus are increased due to more Ti addition. Accordingly,
the compressive strength and the yield strength increase
with increasing Ti content. The Zr –20Nb–15Ti sample has the
highest compressive strength (1307718 MPa) and yield
strength (1071710 MPa), which could be considered as rela-
tively high values for the biological hard tissue substitute
Fig. 1 – XRD patterns of the as-cast Zr –20Nb–xTi ( x¼0, 3, 7, 11
and 15) alloys.
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where δ e is the elastic energy, εe is the elastic strain, s y is the
yield strength and E is Young's modulus (Zhan et al., 2012).
According to Eq. (1), the elastic energies of the β-type
Zr –20Nb–xTi (x¼0, 3, 7, 11 and 15) alloys are calculated and
listed in Table 1. It is indicated that the elastic energy is
sensitive with the variation of Ti content. The elastic energy
of the Zr –20Nb–xTi alloy ranges from 12 MJ/m3 to 20 MJ/m3,
which is higher than most of the reported bio-Ti-base alloys
(i.e. 1–5 MJ/m3) (Niinomi, 1998; Zhan et al., 2012). This result
means that the present material system can withstand
greater elastic deformation.
4. Conclusions
In order to develop novel biomedical HTR materials with
required properties, the β-type Zr –20Nb–xTi (x¼0, 3, 7, 11 and
15) alloys were designed and fabricated for the rst time. The
effects of Ti element on the microstructure and mechanical
Table 1 – The mechanical properties of the β-type Zr –20Nb–xTi ( x¼0, 3, 7, 11 and 15) biomedical alloys.
Alloys Young's modulus
(GPa)
Compressive strength
(MPa)
Yield strength
(MPa)
Elastic energy (MJ/
m3)
Plastic strain
(%)
Zr –20Nb 28.97570.085 104672 85773 12.7370.01 37.8470.07
Zr –20Nb–3Ti 29.68570.465 111079 89077 13.7070.03 37.6270.13
Zr –20Nb–7Ti 29.02070.94 1145715 91075 13.6570.01 38.0270.06
Zr –20Nb–11Ti
29.30570.045 1204719 104377 18.5670.54 38.1070.02
Zr –20Nb–
15Ti 29.91570.535 1307718 1071710 19.1570.08 37.6970.01
Fig. 4 – SEM images of the crack of the compressively tested samples: (a) Zr –20Nb, (b) Zr –20Nb–3Ti, (c) Zr –20Nb–7Ti, (d) Zr –
20Nb–11Ti, and (e) Zr –20Nb–15Ti.
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properties of the binary Zr –20Nb alloy were investigated. The
following results are got:
(1) All the as cast Zr –20Nb–xTi samples contain only one
phase (β-Zr). The size of the equiaxed β-Zr grains is greatly
affected by the Ti content. The grain size and grain
boundary are rened with more Ti addition.
(2) The Zr –20Nb–xTi alloys exhibit a good combination of
mechanical properties including nice plasticity, moderate
compressive strength (1044–1325 MPa) and yield stress
(854–1080 MPa), high elastic energy (12–20 MJ/m3) and
low Young's modulus (28–31 GPa). As it meets most of
the required properties for hard tissue replacements,
these novel biomaterials are considered to be potential
candidate for future applications.
Acknowledgments
This research work is jointly supported by the National
Natural Science Foundation of China (51161002) and the
Program for New Century Excellent Talents in University of
China (NCET-12-0650).
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Fig. 5 – Illustration of elastic energy in a stress–strain plot.
j o u r n a l o f t h e m e c h a n i c a l b e h a v i o r o f b i o m e d i c a l m a t e r i a l s 2 9 ( 2 0 1 4 ) 1 – 66
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