Deformation Mechanisms in the Ti-25Ta-24Nb Biomedical Titanium Alloy

E. Bertrand1, D. M Gordin1

, P. Castany1, T. Gloriant1

, D. Ioan2, D. Cojocaru3

, D. Raducanu3

1 > I NSA de Rennes /UMR CNRS 6226, 20 avenue des Buttes de Coesmes, 35043 Rennes cedex, F ranee

2> SC R&D Consulting and Services SRL, Tudor Arghezi 21, 020946 Bucharest ,Romania

J> POLITEHNICA University of Bucharest ,Spl. lndependentei 313 ,Sector 6, 060042 ,Bucharest,Romania

In this study.a new biocompatible metastable beta titanium alloy was elaborated by cold crucible levitation melting process. This alloy, Ti-25Ta-

24Nb (mass%) ,exhibits a low Young's modulus associated with a superelastic effect. The deformation mechanisms were investigated by elec­

tron backscattering diffraction and showed the existence of two twinning systems.

Keywords: Metastable beta tita11ium ,superelastic ef feet, twi1111i11g

I. Introduction

Biomedical applications require a choice of very well-tolerated materials by their environment. As a bone substitute, the mechanical properties of a biomaterial must be as close as possible of bone's ones, especially Young's modulusll. Decreasing Young's modulus of metallic implants minimizes bone atrophy due to stress shielding and increases the durability of the implant2>.

Titanium and titanium alloys have been widely used for a long time as biomaterials, due to their low density, interesting mechanical properties and biocom­patibility. Standard first generation biomaterials for im­plants were pure Ti and Ti-4Al-6V. Their high Young's modulus (respectively 105 and 114 GPa) affects the long-term performances of these implants; this high value is caused by the a or a+~ microstructure of these alloys. Moreover, Alions released by Ti-4Al-6V are known to be associated with long term health problems like neurological pathologies. Vanadium, such as its ox­ide V2 Os , are also known to be cytotoxic2>.

New generation titanium alloys for biomedical ap­plications are made with ~-stabilizing and highly bio­compatible elements, such as Nb, Mo and especially Ta. Moreover,~ and metastable~ titanium alloys pres­ent the advantage to have a lower Young' s Modulus than a or near-a alloys.

The apparent elastic modulus can be decreased in me­chanically unstable metastable ~ titanium alloys3

•1>. Stress

imrx:ised to such alloys is accommodated by normal elastic­ity and by the transformation of the~ phase of titanium in stress induced d' martensite. This phenomenon occurs when the Ms temperature of the d' martensitic transfor­mation is dose to the room temperature.

Morinaga et al. 5> have developed a predictive

method based on the average electronic parameters the alloy. The two electronic parameters respectively the aver­age bond order & and the average d orbital energy level Md. They are calculated using the following equations.

Ii5 = ~X,Bo,and Md= ~X,,Md, where X; is the atomic fraction, Bo, and Md; is the

value of Bo and Md of each additive element, respectively.

These parameters were calculated for numerous titanium alloys and a Bo- Md map was made to predict mechanical and structural properties6

•1> of design alloys

such as Young's modulus, plastic deformation mecha­nisms (twinning, slip) and phase stability. According to this method, superelastic titanium alloys are placed on the Ms= RT boundary on the Bo-Md map.

This paper focuses on the Ti - 25Ta - 24Nb (mass. % ) titanium alloy. The high tantalum content was chosen to improve the corrosion resistance8>. As shown on Figure 1, the calculation of Bo and Md pa­rameters place this alloy on the Ms = RT boundary. The objective of this paper is to characterize the de­formation mechanisms that occur in this alloy and toe­valuate the competition between them.

2. Experimentals

2. 1 Material Preparation Because both nitrogen and oxygen are highly reactive

with titanium, the synthesis and the heat treatments must be carried out under high vacuum or neutral atmosphere.

The adequate weight of pure metals were melted by Cold Crucible Levitation Melting (CCLM) in a CE­LES induction furnace under a pure Ar atmosphere, which was introduced after several cycles of high vacu­um pumping. This melting technique enables to have homogeneous alloys made from elements with very dif­ferent melting points (3017"C for Ta;2468 °C for Nb; 1668 °C for Ti) with a minimum contamination. The chemical composition was controlled by energy - dis­persive spectroscopy (EDS).

The ingots were solution treated under high vacu­um ( 10-1 mbar) at 950°C for 20 hours and water

quenched in order to homogenize the microstructure. They were then cold rolled (CR= 90 % ) . The samples were then machined, recrystallized at 850°C for 0. 5 hour and water quenched (see Figure 2 ). The aim of the quench is to retain a fully ~-phase structure, even

if the alloy is near-~· The recrystallisation treatment is a way to restore the grain structure after cold rolling

and to control the grain size.

• 2060 • Proceedings of the 12'h World Conference on Titanium

2.96 ~------------------------i

2.94

2.92

2.9

2.88

Bo ( 60-70 )

2.86

2.84 slip

2.82

70-80 )

2.8

2.78 2.35 2.4

( 50-60 )

2.45

() Ti-25Ta-24Nb

2.5

®

QlTi-42Nb @Ti- IOTn @Ti-50Tn @Ti-80Tn @Ti-64Tn

2.55

( 44 )=" @

2.6

Figure I. Posi tion of T i-25Ta-24 b alloy on the Bo-Md map

TC'C)

950

850

20h

u 5 = a ~

" ~

0.5h

-------- ....

C R=90% u

t-c: OJ = a ~

<>

~

time

Figure 2. Thennomechanical protocol applied to Ti-25Ta-24Nb alloy

2. 2 Microstructural and Mechanical Characterization Methods The phase ana lysis was investigated by X-ray dif­

fraction (XRD) on a Philips PW 1830/ 00 diffractome­

ter with CuK. l radiation O. = l. 54060 A). A Leica op­tical microscope was used to characterize the micro­

structure. T he samples were mechanically polished on different SiC abrasive papers until grade 4000. The "mirror polished" state was obtained, using a colloida l silica suspension ( particles size : 50 nm) . The micro­

structure was r~vea led by chemical etching with a solu­

tion composed of 5 % HF, 5 % H N03 and 90 % HZO. Tensi le tests were carried out in loading/ unloa­

ding cycles or until rupture on an Instron 3369 tensile

tester. Cycl ic tensile tests were composed of increments of O. 5% fo llowed by stress release out until 5% . Strain was measured with a 10 mm extensometer and strain

rate was 10- • s 1• The tensi le direction was chosen

para llel to the rolling direction. Electron BackScattering Diffraction ( EBSD) ca r­

tographies were carried out on a JEOL JSM 4600 Scan­

ning Electron Microscope equipped with a TSL EBSD

system. The samples were prepared by mechanical

poli shing unt il reaching a "mirror poli shed" state. In ti­

tanium alloys, mechanica l po lishing is known to induce an important su rface strain that has deleterious effects

in EBSD. A chemical etching was used to remove the

surface strain.

3. Results and Discussion

3. I Microstructural Characterization The optica l images of the alloy afte r the recrystal­

lisation annea ling (Figure 3 (a)) show that the micro­

structure of thi s alloy is made of equiaxia l grains with

an average diameter of SOµm. The thermomechanica l

protocol enabled to retain~ phase of titanium. Loca lly, needles were observed and attributed to a small amount

of a" martensite. The small misorientation between

some grains made them difficu lt to revea l by the chemi­

ca l etching. The X-ray diffraction profi le <Figure 3(b)) shows

the presence of ~ peaks and other peaks that can be ex-

10. Biomedical and H ealthcare Applications • 2061

plained by the precipitation of a small quantity of a" phase. o nanometric w phase was detected.

As the microstructure is composed of ~ phase and a small quantity of a" martensite , the Ms temperature

is very close to the room temperature.

f3 a" ( 11 0)

30 40 50

f3 (200)

a " a

60

f3 (2 11 )

a

70 2 theta( degrees )

80 90

Figure 3. Optical micrography after recrystallization annealing

(a) and X- rays diffraction diagram (b)

3. 2 Mechanical Characterization Conventiona l and cyclic True stress-strain tensile

curves are plotted on Figure 4. Conventional tensile test clearly show a non linear elastic domain followed by a stress plateau that corresponds to a stress induced

martensitic transformation. The mechanical character­

istics are given in Table l,E is Young' s modulus, ac is

the critical stress for stress induced martensitic trans­

formation, ay is the yielding stress , a max is the maximum

resistance and A is strain at rupture. They are compa­

rable to those found for metastable beta titanium al­loys4·9> .

This specific elastic behavior led us to perform cy­

clic tensile tests. The release of the stress shows an im­

portant recoverable elastic domain that is caused by the

reversion of the stress induced mar tensite. The recoverable strain was separated into a part

due to the normal elastici ty and the rest due to the re­

verse phase transformat ion4'10' . The maximum recovera­

ble deformation is obtained for 1. 5% strain: 0. 6% due to

normal elasticity and 0. 9 % due to the superelastic effect.

3. 3 Deformation Mechanisms

The microstructural changes induced by the tensile

Table I. Mechanical properties of Ti-25Ta-24 b

Mechanical I E(GPa)

properties I 48

Superelasti c

properties( E= l. 5% )

( a ) 700

600

~ 500 ::if '-' 400 "' "' "' ~ 300

" :::> r= 200

100

a,C MPa) a, ( MPa) a~(MPa)

259 496 616

E,es( % ) E,es( % ) E,es( % )

0. 2 0. 3 0. 9

A %

18

e,( % )

0. 6

O+-~~~-.-~~~-.-~~~,......,~~~....-,

( b )

600

500 ,.... ci". 400 ::if

~ 300 ~ ~ 200 = r=

100

0 5 10 15 20

True Strain ( % )

5

True Strain (%)

Figure 4. True s tress-st rain conventional and cyclic tensile curve

test were first observed by optical microscopy. Figure 5 (a) shows that most of the grains are crossed by wide

deformation bands. EBSD showed that mother crystal and the deform­

ation bands have specific misorientations typical from twinning (see Figure 5 ( b )) . The twinning systems

were then characterized from the existence of poles be­longing to the parent crystal and the twin and Coinci­

dence Site. Lattice (CSL) analysis. l 3 CSL boundaries were attributed to { 112}<111> twinning system where­

as lll CSL boundaries were attributed to (332}<113>. { 112} < 111> is a wel l known twinning system in

bee structures11.

12>, { 332} < 113> have been observed

in some metastable a lloys subject to a martensitic transforma tion11 ' 13

'·

The coexistence of these two twinning systems in­

volves that their Cri tical Resolved Shear Stress (CRSS) are equivalent for this alloy. 14 '

As tensile tests show the high ductility of thi s al­loy, twinning cannot be the only plastic deformation

twinning, and sli p certainly occurs. Moreover , some of the grains were not deformed by twinning and are most

probably only deformed by slip.

• 2062 • Proceedings of the 12'h World Conference on Titanium

Boundary level: 15 ° 25.00µm =50steos I PF[OO I]

Figure S. Optical micrograph (a) and EBSD map (b) after ten­

sile test

4. Conclusion and Perspectives

A tantalum-rich biocompatible metastable beta ti­tanium alloy was synthesized by Cold Crucible Levita­tion melting. This alloy processes a low Young's mod­ulus associated with a superelastic effect caused by a reversible stress induced martensitic transformation.

Massive twinning occurred in this alloy and was

characterized by EBSD. The important proportion of twinned grains is explained by the coexistence of two twinning systems for this alloy.

As slip cannot be observed by EBSD, further in­vestigation by Transmission Electron Microscopy will be performed.

Acknowledgements Research founded in the frame of a Eureka/ MNT

ERA-Net European consortium. Project. NanoBioAll. Advanced Metallic Biomaterials, Nano-Structured, for Implantable Medical Devices (E! 4482).

REFERENCES 1) Williams, D. F. , Biomaterials 2008; 29: pp. 2941. 2) Geetha, M , A. K. Singh, R Asokamani, and A. K. Gogia, Pro­

gress in Materials Science 2009; 54: pp. 397. 3) Kim, H. Y. , T. Sasaki, K. Okutsu, J. I. Kim, T. lnamura, H.

Hosoda, and S. Miyazaki, Acta Materialia 2006; 54: pp. 423. 4) Kim, H. Y. , Y. Ikehara, J. I. Kim, H. Hosoda, and S. Miyazaki,

Acta Materialia 2006;54 : pp. 2419. 5) Morinaga, M , N. Yakawa, T. Maya, K. Sone, and H. Adachi,

Proc. Theoretical design of titanium alloys. the 6th World Con­

ference on Titanium. 1988. Cannes, France. pp. 1601-1606. 6) Kuroda,D. ,M. Niinomi,M Morinaga, Y. Kato, and T. Yashiro,

Materials Science and Engineering A 1998;243: pp. 244. 7) Abdel-Hady,M ,K. Hinoshita,and M Morinaga,Scripta Materi­

alia 2006;55: pp. 477. 8) Vasilescu, E. , P. Drob, C. Vasilescu, S. L Drob, E. Bertrand, D. M

Gordin, and T. Gloriant, Materials and Corrosion; 61: pp. 94 7. 9) Kim, H. Y. ,S. Hashimoto,]. I. Kim, T. lnamura, H. Hosoda, and

S. Miyazaki, Materials Science and Engineering: A 2006; 417: pp. 120.

JO) Grosdidier, T. and M J. Philippe, Materials Science and Engi­

neering A 2000;291: pp. 218. 11) Richman, R H. , The diversity of twinning in body centered

cubic structures, (Deformation twinning, Ne-cu York , 1964. 12) Christian, J. W., Deformation Twinning, (The Theory of

Transformations in Metals and Alloys,Oxford,2002) pp. 859. 13) Oka, M and Y. Taniguchi, MetallurgicalTransactions 1979;

lOA: pp. 651-653.

14) Bertrand,E. ,P. Castany, I. Peron, and T. Gloriant, Scripta Ma­terialia, in press 2011.