-
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:
[email protected] (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:[email protected]://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.
Buenconsejo, P.J.S., Kim, H.Y., Hosoda, H., Miyazaki, S.,
2009.Shape memory behavior of TiTa and its potential as
hightemperature shape memory alloy. Acta Mater. 57, 10681077.
Kim, H.Y., Hashimoto, S., Kim, J.I., Imamura, T., Hosoda,
H.,Miyazaki, S., 2006. Effect of Ta addition on shape
memorybehavior of Ti22Nb alloy. Mater. Sci. Eng. A 417, 120128.
Kurada, D., Niinomi, M., Morinaga, M., Kato, Y., Yashiro, T.,
1998.Design and mechanical properties of new beta titanium
alloysfor implant materials. Mater. Sci. Eng. A 243, 244249.
Laheurte, P., Eberhardt, A., Philippe, M.J., 2005. Influence of
themicrostructure on the pseudoelasticity of a metastable
betatitanium alloy. Mater. Sci. Eng. A 396, 223230.
Matsumoto, H., Watanabe, S., Hanada, S., 2007.
Microstructuresand mechanical properties of metastable beta TiNbSn
alloyscold rolled and heat treated. J. Alloys Compd. 439,
146155.
Meunier, A., Christel, P., Sedel, L., Witvoet, j., Blanquaert,
D., 1990.The influence of the Youngs modulus of the material of
animplanted femoral stem on the stress loading in the upperfemur.
Int. Orthopaedics. 14, 6773.
Miyazaki, S., Kim, H.Y., Hosoda, H., 2006. Development
andcharacterization of Ni-free Ti-base shape memory andsuperelastic
alloys. Mater. Sci. Eng. A 438440, 1824.
-
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
573Morinaga, M., Yukawa, N., Maya, T., Sone, K., Adachi, H.,
1988.Theoretical design of titanium alloys. In: Proc. the 6th
WorldConference on Titanium, Cannes, France, pp. 16011606.
Niinomi, M., Akahori, T., Katsura, S., Yamauchi, K., Ogawa,
M.,2007. Mechanical characteristics and microstructure of drawnwire
of Ti29Nb13Ta4.6Zr for biomedical applications. Mater.Sci. Eng., C.
27, 154161.
Niinomi, M., 2008. Mechanical biocompatibilities of
titaniumalloys for biomedical applications. J. Mech. Behav.
Biomed.Mater. I 3042.
Prima, F., Vermaut, P., Ansel, D., Debuigne, J., 2000a. Omega
precip-itation in a beta-metastable titanium alloy: resistometric
as-pects. Mater. Trans. JIM 41, 10921097.
Prima, F., Debuigne, J., Boliveau, M., Ansel, D., 2000b. Control
ofalpha-phase volume fraction precipitated in a beta-titaniumalloy:
development of an experimental method. J. Mater. Sci.Lett. 19,
22192221.
Sakaguchi, N., Niinomi, M., Akahori, T., Takeda, J., Toda,
H.,2005. Relationships between tensile deformation behavior
andmicrostructure in TiNbTaZr system alloys. Mater. Sci. Eng.C 25,
363369.
Song, Y., Xu, D.S., Yang, R., Li, D., Wu, W.T., Guo, Z.X.,
1999.Theoretical study of the effects of alloying elements on
thestrength andmodulus of -type bio-titanium alloys. Mater.
Sci.Eng. A 260, 269274.
Tane, M., Akita, S., Nakano, T., Hagibara, K., Umakoshi,
Y.,Niinomi, M., Nakajiima, H., 2008. Peculiar elastic behavior
ofTiNbTaZr single crystals. Acta Mater. 56, 2856.
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