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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: [email protected] (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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162.

Fig. 5 – Illustration of elastic energy in a stress–strain plot.

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