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 AV I O R O F B I O M E D I C A L M A T E R I A L S 3 ( 2 0 1 0 ) 5 6 5 5 7 3available at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/jmbbm

Research paper

Mechanical properties of low modulus titanium alloysdesigned from the electronic approach

P. Laheurtea,, F. Primab, A. Eberhardtc, T. Gloriantd, M. Warye, E. Patoorc

aUniversit de Metz, FRE CNRS 3143 Laboratoire dEtude des Textures et Application aux Matriaux, Ile du Saulcy, F57012 Metz cedex,FrancebCHIMIE ParisTech, UMR CNRS 7045 Laboratoire de Physico-Chimie des Surface, 11 rue Pierre et Marie Curie, F-75231 Paris cedex 5, FrancecUniversit de Metz, FRE CNRS 3236 Laboratoire de Physique et Mcanique des Matriaux, Ile du Saulcy, F57012 Metz cedex, Franced INSA Rennes, UMR CNRS 6226 Sciences Chimiques de Rennes/Equipe Chimie-Mtallurgie, 20 avenue des Buttes de Cosmes CS 70839,35708 Rennes cedex, FranceeArts et Mtiers ParisTech 4, Rue Augustin Fresnel 57078 Metz cedex, France

A R T I C L E I N F O

Article history:

Received 18 March 2010

Received in revised form

30 June 2010

Accepted 1 July 2010

Published online 24 July 2010

Keywords:

Beta titanium alloys

Low modulus

Superelasticity

Stress-induced martensite

A B S T R A C T

Titanium alloys dedicated to biomedical applications may display both clinical and me-

chanical biocompatibility. Based on nontoxic elements such as Ti, Zr, Nb, Ta, they should

combine high mechanical resistance with a low elastic modulus close to the bone elas-

ticity (E = 20 GPa) to significantly improve bone remodelling and osseointegration pro-

cesses. These elastic properties can be reached using both lowering of the intrinsicmodulus

by specific chemical alloying and superelasticity effects associated with a stress-induced

phase transformation from the BCC metastable beta phase to the orthorhombic marten-

site. It is shown that the stability of the beta phase can be triggered using a chemical

formulation strategy based on the electronic design method initially developed by Mori-

naga. This method is based on the calculation of two electronic parameters respectively

called the bond order (Bo) and the d orbital level (Md) for each alloy. By this method, two

titanium alloys with various tantalum contents, Ti29Nb11Ta5Zr and Ti29Nb6Ta5Zr

(wt%) were prepared. In this paper, the effect of the tantalum content on the elastic modu-

lus/yield strength balance has been investigated and discussed regarding the deformation

modes. The martensitic transformation has been observed on Ti29Nb6Ta5Zr in

contrast to Ti29Nb11Ta5Zr highlighting the chemical influence of the Ta element on the

initial beta phase stability. A formulation strategy is discussed regarding the as-mentioned

electronic parameters. Respective influence of cold rolling and flash thermal treatments

(in the isothermal omega phase precipitation domain) on the tensile properties has been

investigated.c 2010 Elsevier Ltd. All rights reserved.d

Corresponding author. Tel.: +33 03 87 31 53 70.E-mail address: pascal.laheurte@univ-metz.fr (P. Laheurte).

1751-6161/$ - see front matter c 2010 Elsevier Ltd. All rights reservedoi:10.1016/j.jmbbm.2010.07.001.

www.sciencedirect.comwww.sciencedirect.comwww.sciencedirect.comhttp://www.elsevier.com/locate/jmbbmmailto:pascal.laheurte@univ-metz.frhttp://dx.doi.org/10.1016/j.jmbbm.2010.07.001

566 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 AV I O R O F B I O M E D I C A L M A T E R I A L S 3 ( 2 0 1 0 ) 5 6 5 5 7 31. Introduction

Metallic implants and osseointegrated prostheses are cur-rently made from CrCo alloys, stainless steels or conven-tional ( + ) titanium alloys such as TA6V ELI alloy. Thetitanium alloys aremainly used in the biomedical field thanksto their unique combination of mechanical properties andtheir superior biocompatibility. However, the potential toxiceffect of some chemical elements such as vanadium or alu-minium has been pointed out for a long time. Therefore, thisis a strong driving force for the development of a next gen-eration of alloys with improved compositions with respect tothe general biocompatibility criterion. One of the major keysfor successful applications is connected to the use of mate-rials with reduced modulus since long-term clinical investi-gations indicate that insufficient load transfer from artificialimplants to adjacent remodelling bone may result in boneresorption and potential loosening of the prosthetic device.This effect called stress shielding effect is a direct resultof the stiffness mismatch between implant material and sur-rounding natural bone (Meunier et al., 1990; Niinomi, 2008).With respect to this concept called isoelasticity, the betatitanium alloys display superior properties compared to stain-less steels and CoCr alloys with elastic modulus approach-ing the 6080 GPa range. However, these values are still 3or 4 times higher than the cortical elastic modulus (20 GPa).Additional decrease of the apparent elastic modulus can beachieved from the ability of these metastable titanium al-loys to undergo a stress-induced martensitic transformationduring deformation. This transformation, from the parent phase retained in a metastable state after water quench andthe orthorhombic martensite, results in an extrinsic lowpseudo-modulus that can be modulated through microstruc-tural control.

The ideal material should possess good strength, high fa-tigue resistance, and a low elastic modulus matching thebone elasticity. Considerable efforts have been devoted bymaterials engineers to enhance the yield strength and toreduce the modulus. However, for a long time, all thesecompositions have been formulated principally by trial anderror methods, with no physical background representing theoptimum choices. Therefore, to reduce the intrinsic modu-lus of Ti alloys, Morinaga et al. (1988) developed an inno-vative approach based on electronic design of alloys (calledthe d-electron alloy design method). They showed a re-lationship between some elastic properties of titanium al-loys and the value of two electronic parameters respectivelycalled the average bond order Bo which is a measure ofthe covalent bond strength between titanium and alloyingelements and Md, the average d orbital energy level of for-mulated titanium alloys, correlating with the average elec-tronegativity and the radius of elements. The Bo and Mdvalues calculated on conventional titanium alloys give aBoMd map (Fig. 1), where , + and -type titanium re-gions are clearly defined (Abdel-Hady et al., 2006; Kuradaet al., 1998). Calculations were made for alloying chemical el-ement, using a cluster basedmethod called DVX (Morinagaet al., 1988). Based on this formulation strategy and consider-ing only bio-inert alloying elements such as Nb, Ta or Zr, theyfinally developed an optimized quaternary beta titanium alloycalled TNTZ (TitaniumNiobiumTantalumZirconium) with

the nominal composition (wt%) of Ti29Nb13Ta4.6Zr (Ni-

inomi et al., 2007). Improvements were obvious both from the

biocompatibility and from the mechanical point of view since

elastic moduli of around 60 GPa were found. The electronic

design approach allows the comparison of titanium alloys

with very different chemical compositions. The interest of

this electronic approach is undeniable, showing reliable and

consistent experimental results for binary titanium systems

such as TiNb or TiTa (Abdel-Hady et al., 2006; Kurada et al.,

1998). However, we presently think that extension to multi-

elementary alloys actually rises open questions since elec-

tronic interactions between alloying elements are not taken

into account into the DVX- model (using a composite ap-

proach). As a result, the respective influence of each alloying

element on the mechanical behaviour remains unclear with

regard to multielementary (ternary or quaternary) systems.

On this basis, starting from the well-known TNTZ system

(Ti29Nb13Ta4.6Zr) (referred as TN13TZ in this paper), we

formulated modified TNTZ compositions Ti29Nb11Ta5Zr

(TN11TZ) and Ti29Nb6Ta5Zr (TN6TZ) with various tan-

talum contents to investigate and compare mechanical

properties such as elastic modulus, yield strength and stress-

induced martensitic transformation ability. The results are

discussed in relation to their respective position in the BoMdelectronic diagram and remaining questions are highlighted.

2. Experimental methods

Chemical formulation of the titanium alloys were performed

following the Morinaga model based on the cluster DVX

method. Electronic parameters Bo and Md for each alloy were

calculated from the following expressions: Md =

Xi(Md)iet Bo =

Xi(Bo)i where Xi is the molar fraction of the i

element and (Md)i, (Bo)i the numerical values of Md and Bofor each alloying element. The Ti29Nb11Ta5Zr (TN11TZ)

and Ti29Nb6Ta5Zr (TN6TZ) (wt%) alloys were prepared

using cold crucible levitation melting technique (CCLM). The

ingots were subsequently homogenized at 1223 K for 12 h

under inert argon atmosphere and then cold rolled with

controlled reduction in thickness of 1.90 (true deformation).

For thermal treatments, the specimens were encapsulated in

quartz tubes under a partial pressure of high-purity argon.

The specimens were quenched into water by breaking the

quartz tubes. After the solution treatment (1173 K, 2 h), XRD

measurements were conducted at room temperature with Cu

Ka radiation. Tensile tests were carried out at a strain rate of

2.7 103 s1. The gage length of specimens was 30 mm and

an extensometer was used for all the mechanical testing. For

each tensile cycle, the recovered deformation, the apparent

elastic modulus, the incipient modulus and the critical phase

transformation stresses are measured (Fig. 2). Specimens for

TEM observation were prepared by a conventional twin-jet

polishing technique. TEM observations were conducted using

a JEOL 2000F instrument operated at 200 kV.

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 AV I O R O F B I O M E D I C A L M A T E R I A L S 3 ( 2 0 1 0 ) 5 6 5 5 7 3 567Fig. 1 Evolution of electronic parameters for TN13TZ, TN11TZ and TN6TZ as a function of the tantalum content.Fig. 2 Stress-induced transformation, recovereddeformation, and different definitions of Young modulus(Ea: apparent modulus; Ei: incipient modulus).

3. Results and discussion

3.1. Chemical formulation from d energy electron method

Ti29Nb11Ta5Zr (TN11TZ) Ti29Nb6Ta5Zr (TN6TZ) havebeen designed as modified TNTZ alloys from the initial Ni-inomi composition Ti29Nb13Ta4.6Zr. Electronic parame-ters such as Bo (Bond order), Md (d orbitals level of energy)and e/a (electron to atom ratio) are reported in Table 1 and itcan be seen from the Bo/Md map Fig. 1 that they belong to thesame group of alloys with neighbouring electronic parame-ters. The Bo/Md map connects directly with the relative chem-ical stability of the high temperature BCC phase and givesa sight on the theoretical as-quenched microstructure. Fromthese data, it is possible to reach information on the subse-quent deformation mode and the macroscopic mechanicalproperties since stress-induced phase transformation, me-chanical twinning or dislocations slip can occur as a functionof the chemical stability of beta phase (Morinaga et al., 1988).The figurative spots are spread along the tantalum alloyingvector with increasing Bo and e/a values when the tantalumcontent is increased (keeping the Md value quite constant).From a direct reading of the Bo/Md map, different sets of con-clusions can be deducted from the relative position of theTNTZ alloys on the map: (i) with respect to the Ms = RT (roomtemperature) dotted line, we are theoretically dealing witha series of metallic systems with quenched microstructuredisplaying coexistence of both and martensite. (ii) thehigher the tantalum content, the lower the Ms temperature,which is fully consistent with other investigations conductedby different authors (Buenconsejo et al., 2009; Kim et al., 2006;Miyazaki et al., 2006; Sakaguchi et al., 2005) (iii) increasing ofthe Ta content should result in decreasing the Young mod-ulus of the alloy. Hypothesis supported by the work of Songet al. (1999) on binary systems but not by the work of Taneet al. (2008) who showed theoretically that the Youngmoduluswas decreasing with e/a value for various binary titanium sys-tems. Moreover, some discrepancies arises from microstruc-tural investigations on TN13TZ (Niinomi et al., 2007) in whichno precipitation is observed after water quench. The hy-pothesis of an existing shift between the theoretical positionof the transformation lines on the BoMd map (extrapo-lated from transformation lines of various binary systems)and the experimental ones on multielementary systems canbe reasonably made. These shifts can result in unexpectedas-quenched microstructures and unpredictable subsequentmechanical behaviour. However, experimental evidence hasto be produced regarding the behaviour of the two neighbour-ing compositions we studied here.

568 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 AV I O R O F B I O M E D I C A L M A T E R I A L S 3 ( 2 0 1 0 ) 5 6 5 5 7 3Table 1 Electronic parameters calculated from theDVX method for TN13ZT, TN11ZT and TN6ZT alloys.

Alloy ref. Ta Content(wt%)

Bo Md e/a

TN13TZ 13 2.878 2.464 4.248TN11TZ 11 2.875 2.463 4.236TN6TZ 6 2.865 2.460 4.209

Table 2 EDX compositional analysis.

Alloys Elements wt% (normalised)

Ti Nb Ta Zr

Ti29Nb6Ta5Zr (TN6TZ) 55.70.7 29.50.7 6.10.7 5.10.6

Ti29Nb11Ta5Zr (TN11TZ) 54.90.5 28.40.6 11.30.5 4.90.3

3.2. Composition and microstructure

Since the capacity of beta titanium alloys to undergo marten-sitic transformation is closely connected to chemical compo-sition through Ms variations, EDX compositional analysis hasbeen performed on the TN11TZ and TN6TZ alloys to checkthe chemical homogeneity after a 12 h homogenization treat-ment of at 1123 K. Average chemical compositions are re-ported on Table 2.

After solution treatment (1173 K, 2 h), the samples arewater quenched and the microstructures are analysed usingoptical microscopy and X-ray diffraction traces. Optical mi-croscopy images are presented on Fig. 3. Optical microscopyrevealed that these microstructures display equiaxed grainswith an average diameter of approximately 50 m. Only grains were visible on these micrographs with no evidenceof precipitation in the beta matrix. A possible precipita-tion of nanosized athermal omega phase cannot be detectedwith optical microscopy due to the small size of these parti-cles. The absence of precipitation after the water quench-ing is confirmed by the X-ray diffraction profiles of TN6TZand TN11TZ alloys (Fig. 4) in which only diffractions peakscan be indexed. It can be noticed that the as-quenched struc-tures are single phased with no phase meaning that theMstemperature is below room temperature (however, XRD can-not detect a trace amount of phases). Surprisingly, these mi-crostructural results are consistent with the extensive workof Niinomi et al. (2007) on the TN13TZ system. Therefore, itsuggests that there is probably a discrepancy between the po-sition of the Ms + RT theoretical line and the experimentaltransformation lines of multielementary alloys on the BoMdelectronic map, since this series of quaternary alloys obvi-ously possessesmartensite transformations below room tem-perature. It can be reasonably concluded that the effects ofvarious beta stabilizers on as-quenched microstructures ob-tained in binary systems have to be extrapolated to morecomplex alloys with a lot of precaution. However, supplemen-tary information can be taken from the electron to atom (e/a)scale that has been built for titanium alloys. On Fig. 5, theas-quenched microstructures are reported as a function ofe/a values. We can see that the stability of the beta phaseincreases when the e/a ratio rises towards high values. Thestability limit of a fully phased titanium alloy has been cal-culated to be around 4.20. This can be reasonably connectedwith our results since the three different alloys possess e/avalues comprised between about 4.21 and 4.25. From thesevalues, prediction can be made that the TN6TZ system willpresent a Martensite Start temperature very close to roomtemperature (just below, actually) with a low mechanical sta-bility of phase upon deformation.

3.3. Mechanical properties of as-quenched TN6TZ andTN11TZ alloys

Mechanical characterization of the as-quenched alloys hasbeen performed using tensile tests at room temperature. Forlinear elastic materials the tensile Youngs modulus (Ei) is de-fined as the slope of linear elastic range before yielding. How-ever, for materials exhibiting nonlinear elastic behaviour, theabove definition no longer applies. Both incipient and appar-ent Youngs moduli can be adopted to characterize the elasticbehaviour of such materials, as illustrated in Fig. 2. The ap-parent Young modulus is a good measure of stiffness at largestrains and the incipient Young modulus is a more appropri-ate quantity to characterize the elastic compatibility with hu-man bone. The superelastic characterization of specimens isdescribed as two kinds of recovered strains respectively de-scribed as SE. (defined as recovered superelastic strain) andE (pure elastic recovered strain upon unloading).

The tensile stressstrain curves on as-quenched TN6TZand T11TZ are presented on Fig. 6 and can be compared withTN13TZ. The tensile stressstrain curves TN6TZ et TN11TZexhibit superelasticity behaviour (noted SE). Therefore,both TN6TZ and TN11TZ undergo a stress-induced phasetransformation during the mechanical tests incontrast to TN13TZ, where no could be detected and whichexhibits a classical elasto-plastic behaviour. The occurrenceof stress-induced precipitation has been confirmed onX-ray diffraction traces (Fig. 4) made on deformed TN6TZsamples where small peaks can be detected aftermechanical testing. One can notice that the critical stressto trigger the martensitic transformation is decreased whentantalum content is lower: 250 MPa for the TN11TZ alloyand 170 MPa for TN6TZ with a larger martensitic plateau forthe TN6TZ. These results are fully consistent with the factthat Ta element induces a decrease of the Martensite Starttemperature (about 30 K/at.%) fromMiyazaki et al. (2006) workindependently from the number of alloying elements in thematerial. For the TN13TZ, the Ms temperature is apparentlytoo low below the room temperature. As a consequence,plastic deformation occurs before the critical stress formartensitic transformation can be reached. One interestingfeature is the evolution of the incipient modulus a functionof the Ta content. During the first loading cycle, TN11TZ andTN6TZ both display a linear elastic behaviour correspondingto the elastic deformation of single phase and the elasticmodulus have been measured to be respectively 50 GPa and43 Gpa. Comparison with Niinomis work is interesting sincethe modulus on TN13TZ has been measured at 60 GPa inthe as-quenched state (Niinomi et al., 2007). This comparisonsuggests that, in this series of titanium alloys, Ta elementcould contribute to increase the intrinsic modulus. This is notin accordance with some results on binary TiTa alloys whereTa is shown to result in Young modulus decrease. Sakaguchi

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 AV I O R O F B I O M E D I C A L M A T E R I A L S 3 ( 2 0 1 0 ) 5 6 5 5 7 3 569b a

Fig. 3 Optical images of as-quenched microstructures after solution treatment (1173 K, 2 h): (a) TN6TZ, (b) TN11TZ.Fig. 4 X-ray diffraction profiles of TN6TZ and TN11TZ alloy solutions treated at 1173 K for 1.8 ks followed by waterquenching and TN6TZ alloy solution treated at 1173 K for 1.8 ks followed by water quenching and deformed by tensile test(10%).Fig. 5 Expected structures of Ti alloys after water quench with respect to electron to atom ratio scale.et al. (2005) shows the elastic modulus of Ti30NbXTa5Zralloys as a function of their Ta content. The lowest elasticmodulus of 67 GPa is observed in 10Ta. The elastic modulusdecreases when the Ta content increases up to 10 mass%, andit increases when the Ta content increases over 10 mass%.This suggests as well that additional decrease of the moduluscan be reached on the TNTZ series by minor modificationsof chemical compositions. However, TN13TZ has beendeveloped to optimize the strength/modulus balance and wecan clearly see that the composition modification has beendetrimental to the ultimate tensile strength (UTS) level ofthe as-quenched samples. The UTS value is progressivelydecreased from 600 (TN13TZ) to 570 and 400 MPa when Tacontent is lowered. This clearly highlights a possible solutionstrengthening effect of the Ta element.

Cyclic loadingunloading during tensile test causes grad-ual decrease of the incipient modulus and apparent moduluswith increasing tensile strain For example, for the TN11TZthe incipient modulus at the sixth loading is 30 GPa (Fig. 7);this is half the incipient modulus obtained during the firstloading. Cyclic loadingunloading causes gradual increase ofthe recovery strain and gradual decrease of stress transforma-

570 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 AV I O R O F B I O M E D I C A L M A T E R I A L S 3 ( 2 0 1 0 ) 5 6 5 5 7 3Fig. 6 Stressstrain curves of TN6TZ, TN11TZ andTN13TZ, during tensile tests on the as-quenched specimen.

tion due to the presence of martensite. The solution-treated(ST + WQ) of TN6TZ revealed recovered elastic deformation(recov.) of 1.5% (max) at room temperature owing to a smalltransformation stress and a low slip stress. As to TN11TZ,the recovery strain (2.2% max) and the critical stress for slipare higher (Fig. 8). The critical transformation stress SIM de-crease with increasing the number of dformation cycle. Aninternal stress field assisting the martensitic transformationis formed by the accumulation of dislocations introduced dur-ing cyclic loading. TN11TZ SIM for is smaller than that ofTN6TZ. The difference can be explained by lower Ms, belowroom temperature. SIM is a measure of energy dissipationand it is caused by interfacial friction and creation or rear-rangement of defects during the martensitic transformation.The effect of defect generation and rearrangement is high inthe first cycle, and less significant with each additional cycle.

3.4. Strategies to improve the strength/modulus balance

We have seen that both the modulus and the tensile strengthwere reduced on the modified TNTZ alloys. One of the ideasarising from this first set of results could be to optimize thisbalance: keeping the advantageous low modulus propertiesbut with increased tensile strength. Several possibilities areoffered by thermomechanical treatments. Improved strengthcould be obtained by a prior heavy deformation sequence(cold rolling treatment) or by prior flash (brief) thermaltreatment at low aging temperature (typically 573 K) to favorthe nucleation of nanostructured isothermal phase that canpotentially act as powerful barriers for dislocations slip.

3.4.1. Heavy cold rolling

Both types of samples (solution-treated samples) have beencold rolled with a very high rolling rate of 160%. Microstruc-tural investigations have been carried out using TEM analysis.The cold rolled microstructure (Fig. 9) has been shown to bevery perturbed with a very high density of dislocations and ahigh volume fraction of stress-induced .

Tensile curves of heavily cold rolled TN6TZ and TN11TZsamples are reported on Fig. 10. Pseudoelastic behaviour in-volving nonlinear elasticity can be clearly seen from these55

50

45

40

35

30

25

20

150 2 4 6 8 0 2 4 6 8

Plastic strain (%) Tensile deformation (%)

10 12

40.0

35.0

30.0

25.0

20.0

15.0

App

aren

t mod

ulus

(G

Pa)

inci

pien

t mod

ulus

(G

Pa)

TN11TZ/ST 900C+WQ

TN6TZ/ST 900+WQTN11TZ-ST 900C+WQ

TN6TZ-ST 900+WQ

Fig. 7 Evolution of incipient modulus and apparent modulus as a function of cyclic loadingunloading tensile test on theas-quenched specimen.2.5

2

1.5

1

0.5

0

Rec

over

y st

rain

(%

)

0 2 4 8 10 12 14 0 2 4 86 6

Tensile strain (%) Plastic strain (%)

TN11TZ/ST 900C+WQ

TN6TZ/ST 900C+WQ

TN11TZ/ST 900C+WQ

TN6TZ/ST 900C+WQ

Crit

ical

tran

sfor

mat

ion

stre

ss

(MP

a)

270

220

170

120

70

20

Fig. 8 Dependence of recovery strain and critical transformation stress obtained by cyclic loadingunloading tensile teston the as-quenched specimen.

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 AV I O R O F B I O M E D I C A L M A T E R I A L S 3 ( 2 0 1 0 ) 5 6 5 5 7 3 571Fig. 9 TN6TZ after cold rolling treatment (160%)a dark field TEM image and the corresponding selected area diffractionpattern.1000

900

800

700

600

500

400

300

200

100

00 0.5 1 1.5 2 2.5 3 3.5 4 4.5

Strain (%)

Str

ess

(MP

a)

Fig. 10 Stressstrain curves of TN6TZ and TN11TZ alloys as cold rolled.curves but no additional martensitic transformation appar-ently occurs. This pseudoelastic behaviour probably origi-nates from the deformation of an initial dual microstructure + resulting in variant reorientations and/or inter-faces sliding between laths of martensite. In addition, thedeformation defects caused during cold rolling increase theyield strength of phase, which also enhances the resis-tance of a martensite transformation. As a result, much moretransforming driving force is needed to induce the marten-site phase during tensile processing at room temperature.The decrease of the apparent modulus with the increasedeformation is in agreement with the evolution observedby Matsumoto et al. (2007), allotted to the formation of a(200) [010] texture of the

phase and to the crystallo-graphic anisotropy of the phase. The incipient modulus ofthe cold rolled alloy measured from the stressstrain curve isof 50 GPa (true = 1.60) and 42 GPa (true = 2.60) for TN6TZand 65 GPa (true = 1.60) for TN11TZ; the maximum recoveredstrain is of 2%.

3.4.2. Flash treatment in the isothermal omega temperaturedomainMetastable Ti alloys are typically known to precipitate ad-ditional phases ( and phases) during thermomechanicaltreatments. The morphology, size and distribution of theseprecipitates determine in large part the mechanical proper-ties of the alloy. It is known that precipitates can be formedduring quenching by a diffusionless martensitic mechanism(ath) and during aging by a diffusion controlled process(iso). Previous work on the iso precipitation has shown thatnucleation density of phase was depending mainly on thetemperature and growth mechanisms were driven by exodif-fusion of beta stabilizer elements and an enrichment of betamatrix in beta stabilizers. The isothermal omega phase rangeof existence is usually situated between 473 and 673 K de-pending on the chemical stability of the parent phase. Theusual drawback associated with omega precipitation is con-nected to the coherency strains at the / interface. This canresult in severe embrittlement if the iso volume fraction ishigh (Laheurte et al., 2005). The other drawback, in the frameof this study, is the potential chemical stabilization of the betaphase during omega growth, that can result in the severe de-crease of the Ms temperature and a subsequent suppressionof the superelastic effect. Is has been previously shown thata flash treatment (short duration treatment) could result in adense precipitation of iso particles with a limitation of finalvolume fraction of the omega phase (by limitation of growth)and no subsequent embrittlement (Prima et al., 2000a,b). Athermal aging of 15 min at 573 K has been carried out onas-quenched TN6TZ samples. Comparative tensile tests arereported on Fig. 11. The tensile curves show that the nano-precipitation of isothermal omega phase results in a largeincrease of the tensile strength but with a similar level of su-perelastic effect and an improved elastic recovery of almost

572 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 AV I O R O F B I O M E D I C A L M A T E R I A L S 3 ( 2 0 1 0 ) 5 6 5 5 7 3600

500

400

300

200

100

0

Strain (%)

Str

ess

(MP

a)0 1 2 3 4 5

STA573K/15min

ST(1173K-1h)+WQ

Fig. 11 Cyclic stressstrain curves of TN6TZ water quenched and subsequently heat treated at 300 C 15 min.3% of strain. The other interesting feature is the conserva-tion of a very low incipient modulus of about 40 GPa. Theseevolutions are due to a combination of effects. The plastic de-formation is retarded because of interactions between dislo-cations and the nanoscaled omega particles dispersed in thebeta matrix. The precipitation of phase affects the distribu-tion of the solute content in phase. The stability of phaseis strengthened because of the slight enrichment of stabiliz-ing elements Nb, Ta, and Zr during the flash heat treatment.As a result, much more transforming driving force is neededto induce the martensite phase during tensile processing atroom temperature. A compromise has to be found concern-ing flash treatment because if the treatment is over timed,the chemical stabilization of beta phase becomes too high andthe martensitic transformation is suppressed. Additional re-search is ongoing on the ideal flash treatment (temperatureand time) resulting in the good compromise of low modulus,improved tensile strength and superelastic effect.

4. Summary and perspectives

Two Ni-free Ti alloys with modified TNTZ compositionswere designed using the d-electron alloy design methoddeveloped by Morinaga et al. and compared with the originalTN13TZ alloys. The following conclusions can be drawn fromthis study. From the obtained results it was highlightedthat lowering the Ta content was resulting in an additionaldecrease of the intrinsic modulus from 60 GPa for TN13TZto 43 for TN6TZ accompanied by a decrease of the ultimatetensile strength. From the microstructural investigations,some discrepancy raised between the theoretical positionof the Martensitic Start (Ms) line and the experimentalone, resulting in a relative lack of prediction concerningas-quenched microstructures. It suggested that extensionof the electronic design approach from binary systems tomultielementary alloys has to be considered with someprecaution. Finally some simple strategies such as heavycold rolling or flash thermal treatment have been proposedto optimize the strength/modulus balance of the developedalloys. Bo and Md parameters cannot be regarded as soleindicators of phase stability when designing low modulusTi alloys. The fact that Youngs modulus decreases with thedecreasing Bo is consistent with the definition of Bo whichcorresponds to the covalent bonding strength between Tiand an alloying element, since Youngs modulus decreaseswith decreasing bonding strength between atoms. Incipientmodulus of Ti at room temperature can be decreased toabout 30 GPa. This value is close to Youngs modulus ofbone. The observed low young modulus is associated withthe feature in the premartensite transformation. The alloyswe studied are deformed at a low stress by stress-inducedmartensitic transformation. High strength as well as lowYoungs modulus are needed for the applications.

Acknowledgement

This work was supported by the National Research Agency(No. ANR-08MAPR-0017)

R E F E R E N C E S

Abdel-Hady, M., Henoshita, K., Morinaga, M., 2006. Generalapproach to phase stability and elastic properties of beta-typeTi-alloys using electronic parameters. Scr. Mater. 55, 477480.

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Mechanical properties of low modulus titanium alloys designed from the electronic approachIntroductionExperimental methodsResults and discussionChemical formulation from d energy electron methodComposition and microstructureMechanical properties of as-quenched TN6TZ and TN11TZ alloysStrategies to improve the strength/modulus balanceHeavy cold rollingFlash treatment in the isothermal omega temperature domain

Summary and perspectivesAcknowledgementReferences