Fabrication and Mechanical Properties of Porous Ti-24Nb-4Zr-8Sn Alloy Devel­oped for Biomedical Application

Xuying Cheng, Shujun Li, Yulin Hao, Rui Yang

Shenyang National Laboratory for Materials Science, Institute of Metal Research ,Chinese Academy of Sciences, 72 Wenhua

Road ,Shenyang 110016 ,China

Metal-based porous materials are currently the preferred choice as bone/cartilage implants due to their idiographic porosity improving the bio­

compatibility of implants. Ti-24Nb-4Zr-8Sn (in weight percent and abbreviated as Ti2448) is a new beta-type titanium alloy developed for bio­

medical application. The alloy possesses low modulus, high strength.good corrosion resistance in the physiological environment and non-toxicity.

This study focuses on the fabrication and mechanical properties of porous Ti2448 alloy aimed for use as load-bearing implants. A bulk porous bi­

omedical Ti2448 alloy was prepared by powder-metallurgy technique including cold-roll forming and vacuum sintering. The effects of processing

parameters, such as rolling reduction, size and volume percentage of space-holder on porosity, average pore size, compressive strength and

Young's modulus of porous Ti2448 alloy were investigated. The results showed that porous Ti2448 alloys of 17. 6%-46. 8% in porosity, 166-

509 µ.min average pore diameter,30-325 MPa in compressive strength and 2. 3-27. 2 GPa in dynamic modulus can be fabricated by controlling

process parameters. Compared with porous CP Ti fabricated by the same method, porous Ti2448 alloy has much higher strength-to-modulus ratio

and better mechanical compatibility, which may greatly reduce stiffness mismatch between implant and bone after implantation.

Keywords: Biomedical titanium alloys, porous Ti-2448 alloy ,elastic modulus, rolling fanning ,mechanical properties

1. Introduction

Metal-based biomaterials have been extensively used to produce orthopedic implants due to their better balance between strength and toughness than ceramics and polymeric materials!). Since titanium and its alloys possess good mechanical properties and corrosion re­sistance, they have been used widely to produce the "load-bearing" implants for biomedical applications2

•3>.

However, several studies have demonstrated that one of the major drawbacks of implants made of bulk me­tallic materials is the elastic mismatch between the im­plant and its surrounding bone tissue, which leads to the stress shielding phenomena4

•6>. Together with the

bio-inert nature of their surface, it is difficult to get strong bonding between implant and bone tissue, which might result in the loosening of implant-bone interface. The introduction of the porous structure would be a valid way to reduce elastic modulus and provide neces­sary space to facilitate cell vascularization and new bone in-growthn. The porous materials can be fabrica­ted with many techniques such as solid-state foaming by superplastic expansion of argon-filled pores8> , rapid prototyping9> , self-propagating high-temperature syn­thesis10> , polymeric foam replication1

I> , selective elec­tron/laser beam melting12> and powder metallurgical process7 ·1 3> .

Ti-24Nb-4Zr-8Sn (Ti2448) is a new beta-type ti­tanium alloy developed for biomedical application with the desired biomechanical properties such as high strength, low elastic modulus, large recoverable strain and good corrosion resistance1us>. In this study, the

porous Ti2448 alloy with a controllable interconnected pore was fabricated by the powder metallurgy tech­nique including the cold rolling of the capsule powder

and vacuum sintering. The effect of processing parame-

ters including rolling reduction in thickness, size and volume percent of space-holder on porosity, average pore size, compressive properties and Young's modulus was investigated.

2. Materials and Methods

2. 1 Starting Materials The powders were prepared by the electrode in­

duction melting gas atomization ( EIGA) equipment. They were sieved in an Ar protection atmosphere to get powders with two diameter ranges of 180~300 µm and 50~80 µm. The particles of polymethyl methacry­

late (PMMA) were used as the space-holder material which is entirely spherical in shape with the diameters

of 150 ~ 500 µm. The PMMA was selected as the space-holder material due to its good mechanical strength and hardness to avoid falling to pieces during the cold rolling forming, and favorable chemical proper­ty to decompose completely at relatively low tempera­ture so as to avoid the reaction with the host powders.

2. 2 Preparation of Porous Ti2448 Alloy The capsules filled with Ti2448 alloy powders

(180~300 µm) were cold rolled with the rolling reduc­

tion of 20 % , 30 % , and 40 % in thickness respectively and then sintered in a vacuum of 1 X 10-3 Pa without the applied external pressure at a temperature of 1423 K for 4 h. To increase the porosity, the PMMA parti­cles were added in some samples. Since the previous studies have shown that the size of the metal powder should be smaller than that of the space-holder, Ti2448 alloy powders with smaller particle size ( 50 ~ 80 µm)

were mixed with the PMMA particles of 30%, 40%

and 50 % in volume fractions and the binding admix­

ture. The rolling reduction was controlled to 40%.

• 2190 • Proceedings of the 12'h World Conference on Titanium

These capsules containing space-holder were first heat­ed at temperature 673K for 4h to remove the PMMA and then sintered at 1423 K for 4 h.

2. 3 Sample Characterization The density of the porous materia ls was estimated

from the weight and the apparent volume of the speci­men. The porosity was calculated using the following equation :

p = (1 - pl p,) * 100% where p and p, are density of porous materials and dense materials , respectively. Microstructures of Ti2448 powders and the porous Ti2448 samples were observed by scanning electron microscope ( SEM) . Op­tical microscopy (QM) was used to determine the pore size distribution and mean pore size measured by quan­titative image analyses using the commercially available software Sisc-Ias8. The compression test of porous Ti2448 samples were carried out using cylindrical shaped samples with the diameter of 3 mm and length of 6 mm at room temperature under a s train rate of 10- 4 s- 1. Young' s modulus of porous samples was

measured by the resonance method at ambient temper­ature using a rectangular shaped sample with the di­mensions of 60 X 15 X 3 mm3

• The formula to ca lculate the modulus of a rectangular shaped sample is:

fR = 1. 028 X 106 X ( T/ L2 )(£ / p) 112

in which f R is the resonant frequency ( Hz ) , T the thickness of the sample , L the length of the sample (mm) ,Ethe modulus of the material (GPa) and p the density of the material (g/ cm3

) .

3. Results and Discussions

Figure 1 shows SEM micrographs of Ti2448 alloy powders produced by the EIGA process with the diam­eter ranges of 180~300 µm ( Figure l(a)) and 50~80 µm (Figure 1 ( b)) . Powder particles produced by the

gas atomization are almost spheroid, with a small frac­tion of ellipsoid, only occasionally with adhered satel­lites16> .

The characteristic parameters of pores of sintered porous Ti2448 alloys were summarized in T able 1. For

Figure 1. SEM micrographs of the Ti2448 alloy powder fabrica­

ted by the EIGA process with diameter ranges of 180-300 µ.m

(a) and 50-80 µ.m ( b)

the porous alloys free of the space-holder, both porosity

and average pore size decrease with the increase of the rolling reduction. Under the conditions of 30 vol. % PMMA particles and 40 % thickness reduction, the PMMA particle sizes have positive contribution on the average pore sizes but slight effect on the porosities. Under the conditions of similar PMMA particle size range and rolling reduction, the increase of PMMA par­

ticle volume results in higher porosities but a little effect on the average pore sizes. In summary, the por­ous alloys free of the space-holder have the total poros­ity and the average pore size in the ranges of 17. 6 % ~ 32. 4% and 166~271 µm respectively while the addi­tion of the space-holder increases them to 33. 6% ~

46. 8 % and 345 ~ 509 µm. It is clear that the addition of space-holder favors the increase of porosity and the governing of average pore size.

Table 1. The porosity and average pore size of porous Ti2448 alloys fab ricated by va rious processing

Order Volume percent Particle size Rolling

of PMMA ( Vol. % ) of PMMA (µ. m) red uction ( % )

0 20

2 0 30

3 0 40

4 30 150-225 40

5 30 225-300 40

6 30 300-500 40

7 40 300-500 40

8 50 300-500 40

Mechanical properties of the porous alloys were measured by compression test and the nominal stress-

Sintering Porosi ty of Average pore size

condition porous Ti2448 ( % ) of porous Ti2448 (µ.m)

l l 50"C for 4h 32. 4 27 1

l 150"C for 4 h 26. 8 191

11 50"C for 4 h 17. 6 166

l l 50"C fo r 4h 36. 8 345

11 50"C for 4 h 34 . 2 407

l 150"C for 4h 33. 6 475

11 50"C fo r 4h 4 1. 7 503

1150"C for 4h 46. 8 509

nominal strain curves of porous Ti2448 alloys prepared

without space-holder were shown in Figure 2. It can be

10. Biomedical and Healthcare Applications • 2191 •

seen that there is an almost linear increase in stress in the first stage and then a plateau region with a nearly constant flow stress before the collapse of the tested specimens. The reason for this behavior is probably cells collapse as a result of cell walls yielding17

'18>. The

compressive fracture surface of the porous alloy was observed by SEM. For example , Figure 3 shows that the porous materials sintered with 50 ~ 80 µm alloy powders and 30 vol. % space-holder have different par­ticle sizes. It is clear that the fracture surfaces are rough due to the porous structure of the materials. There exist two kinds of pores. The larger pores have sizes being similar to those of the decomposed space­holder while the smaller pores relate with the sizes of the alloy powders. The variations of the compressive strength with porosity and average pore size are plotted in Figure 4 and 5 , respectively. It can be seen that the compressive strength has negative dependence on the porosity and the average pore size. The porous Ti2448 alloys of 30 ~ 325 MPa in compressive strength were successfully fabricated in this work , which is higher than the human cortical bone ( 20~200 MPa) 19>. Addi­tionally, the porosities of the fabricated porous Ti2448 alloys are in the range of 17. 6 % ~ 46. 8 % , which is close to the optimal porosity of implant materials for ingrowths of new-bone tissues (20%~50% ) 20> .

350 Ti2448/ 1423K 4h

300

~ 250 rf ~ 200 ';;;'

"' e 150 cii

100

50

2 4

a

6 8 10 12 14 Strain(%)

Figure 2. Compressive stress-nominal strain curves of the

porous T i2448 alloy with different porosities

The elastic moduli of the porous Ti2448 alloys were measured by the resonant method and the results are plotted in Figure 4 and 5. It is clear that Young 's modulus decreases with the increase of the porosity but has little relation to the average pore size. They have elastic moduli in the range of 2. 3~27. 2 GPa , being al­most the same as those of the human cortical bone (2~20 GPa) 19>. Compared with porous CP-Ti fabrica­ted by the same method [ 4] , the porous Ti2448 alloy has much higher strength-to-modulus ratio and better biomechanical compatibility.

4. Conclusions

In this study, the porous structures of the Ti2448 alloy with porosities ranging from 17. 6% to 46. 8%

Figure 3. SEM micrographs of the compressive fractu re surface

of the porous Ti2448 alloy sintered with 30 vol. % PMMA

space-holder having particle sizes of 300-500 µm (a) , 225-300 µm (b) and 150-225 µm (c)

"'300 0..

~

~ 250 ~

C/l

§! 200 ·v; "' e E" 150 0 u

- (165.8µm ) Ti2448/1423K 4h

~(190.lµm) ~(270.5µm)

-+- Compress ive Strength ---­o-­-o- Dynamic Modu lus

30

I 00 ;-~--.-~~-.-~--.-~~-.-~--.-~--15

I 0 15 20 25 30 35 40 Porosity/%

Figure 4. Variations of the compressive strength and dynamic

Young's modulus with volume percentage of porosities

were fabri cated successfully by the powder metallurgy technique. The pore characteristics can be adjusted by the control of processing parameters including the roll­

ing reduction in thickness , size of space-holder and its

volume percentage. The compressive strength decrea-

• 2192 • Proceedings of the 12'h World Conference on Titanium

ses with the increase of the porosities and the average pore size while Young's modulus has a strong depend­ence on the porosities but a weak relation to the aver­age pore sizes. The fabricated porous structures of Ti2448 alloy have compressive strengths in the ranges of 30 - 325 MPa and Young' s moduli of 2. 3 - 27. 2 GPa,which is in good matches with the biomechanical

properties of the cortical bone.

"' 225 Q..

~ ~ 200 Oil c ~ r/) 175 .. > -~ 150 ~ Q.

§ 125 u

Ti2448+30vol.% PMMN40%/1423K 4h

~ 36.8~·~----<>-

-e- Compressive Strength -o- Dynamic Modulus

100-i-------.-----.---~-.------ 0 300 350 400 450 500

Average Pore Diameter/µm

Figure 5. Compressive strength and dynamic Young's

modulus as a function of average pore size

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