Materials Science and Engineering A 441 (2006) 112118Effect of Zr and Sn on Youngs modulus and superelasticityof TiNb-based alloysY.L. Hao, S.J. Li, S.Y. Sun, R. YangShenyang National Laboratory for Material Science, Institute of Metal Research, Chinese Academy of Sciences,72 Wenhua Road, Shenyang 110016, PR ChinaReceived 12 April 2006; received in revised form 9 July 2006; accepted 14 September 2006AbstractQuaternaryTi(2026)Nb(28)Zr(3.511.5)Sn(wt%)alloyswereinvestigatedtoevaluatetheeffectsofZrandSnonYoungsmodulusandsuperelasticityofTiNb-basedalloys.X-raydiffractionanalysisshowedthatsolution-treatedalloyshave+

, +,

+,

,ormicrostructures. Zr and Sn increase the lattice parameters of the phase; for orthorhombic

matensite, they increase the lattice parameter a butdecrease both b and c. The martensitic start temperature of the

is depressed by Zr and Sn additions, whereas the formation of athermal isdependent on Zr and Sn contents. Differential scanning calorimetry (DSC) measurements show that 1 wt% of Nb, Zr or Sn addition decreases themartensitic start temperature by 17.6, 41.2 or 40.9 K, respectively, due to their negative effect on lattice parameter ratios of the martensite (c/a andb/a). Tensile tests were used to evaluate Youngs modulus and superelasticity of the solution-treated alloys. Of the studied alloys Ti24Nb4Zr7.5Snwith single microstructure has the lowest Youngs modulus of 52 GPa and recoverable elastic strain of about 2% at room temperature after cyclicstrain. 2006 Elsevier B.V. All rights reserved.Keywords: Youngs modulus; Superelasticity; Biomedical titanium alloy; Martensitic transformation; Alloying effect1. IntroductionTitanium and its alloys have been widely used as biomedicalmaterialstoreplacedisfunctionedhardtissueinhumanbodyduetotheirlight weight, lowelasticmodulus, highstrengthand excellent biocompatibility and corrosion resistance. Duringthepastdecade, bothnear-typeand-typetitaniumalloyscomprising only non-toxic and non-allergic elements have beendeveloped in an effort to match high strength with low modulusso as to further ease stress shielding problem [1,2]. The addi-tion of high concentration of stabilizers to some of the newlydeveloped alloys resulted in

martensite with orthorhombicstructure under the condition of quenching [35]. The presenceof

martensite, however, reduceshardnessandtensileandfatiguestrengthsofthealloys[48]. Toavoidimpairmenttoservice life, therefore, alloys containing the

martensite aregenerally considered unsuitable for making implant devices forhard tissue replacement.Corresponding author. Tel.: +86 24 2397 1961; fax: +86 24 2390 2021.E-mail address: ylhao@imr.ac.cn (Y.L. Hao).On the other hand, the martensitic and reverse transforma-tions concerningthe

canbeexploitedtodevelopshapememory titanium alloys for bio-functional applications, as pio-neeredbyBaker andbyDueriget al. [911]. Their resultssuggested that shape memory effect is quite sensitive to heat-ing rate and the maximum recovered strain is about 3% underthe condition of salt bath heating with heating rate higher than10C/s. IncreasingconcernoverallergicandtoxiceffectsofNi ions released from TiNi led to recent research efforts towarddevelopingNi-free shape memoryor superelastic titaniumalloys[1219].In-typetitaniumalloys, athermal phaseis generallyformedinas-quenchedconditions[20]. Theinuenceofthephaseonshapememoryeffect andsuperelasticityoftita-nium alloys, however, is not well understood. Moffat and Lar-balestier [21] investigated systematically the competing forma-tionbetweenthe

martensiteandthephaseinquenchedbinary TiNb alloys. Their results suggested that the phaseprecipitateswouldnotfavourtheformationof

martensiteand would impair shape memory effect and super-elastic prop-erties of titaniumalloys. Recent experimental ndings [22,23] ofsuper-elastic deformation in the absence of

martensite is not0921-5093/$ see front matter 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.msea.2006.09.051Y.L. Hao et al. / Materials Science and Engineering A 441 (2006) 112118 113consistent with the above conclusion. Reversible to phasetransformation or dislocation-free deformation mechanismhavebeen suggested as possible origins of the superelastic behaviour[22,23].The objective of this study is to investigate the effect of Nb, Zrand Sn on phase formation, Youngs modulus and superelasticityin as-quenched titanium alloys, in the hope of reducing Youngsmodulus and improving super-elastic properties by alloying withZr and Sn.2. ExperimentalQuaternary TiNbZrSn alloys were melted in an arc melt-ing furnace using a tungsten electrode under argon protectionwith magnetic agitation. They were melted three times using aTiSn master alloy and pure Ti, Nb and Zr as raw materials. Thenominal chemical compositions of alloys are listed in Table 1(all in wt%). The 40 g buttons obtained were forged at 950Cto 10 mm diameter cylinders. These cylinders were encapsuledinevaluatedquartztubeswithpressureof102Paandweresolution-treated at 850C for 1 h before quenching into waterbybreakingthecapsules.Twobuttonswithdifferentcompo-sitions were analysed by wet chemical analysis to characterizethe differences between nominal and actual compositions, andtheresultsareshowninTable2.InterstitialcontentsofN,Hand O of both alloys were also listed in Table 2. The nominalcompositions shall be used to denote the alloys hereafter.Tensile testing was conducted at roomtemperature (21C) bycyclic loading at initial strain rate of 1 103s1using spec-imenswithagaugesectionof3 mmindiameterand15 mmlongbyMTS810MaterialTestSystem.Inordertoimprovetheaccuracyof measurement, tensileYoungs modulus andrecovered strains were determined from the stressstrain curvesrecordedbyusingastraingauge. TheVickershardnesswasmeasured under a load of 10 kg applied for 15 s along longitudi-nal direction of mechanically ground and polished specimens.Transformation temperature was measured by differential scan-ning calorimetry (DSC) at heating or cooling rate of 10C/minTable 1Nominal chemical compositions (wt%) and corresponding phase constitutionsof as quenched TiNbZrSn alloys determined by X-ray diffraction analysis20Nb 22Nb 24Nb 26Nb2Zr7.5Sn +

4Zr7.5Sn +

+

8Zr7.5Sn +4Zr3.5Sn

+

+ +

+

4Zr11.5Sn + + +Table 2Chemical analysis of TiNbZrSn alloys (wt%)Composition Nb Zr Sn O N HNominal 24 4 7.5Analysed 24.0 3.93 7.54 0.069 0.0076 0.0049Nominal 26 4 7.5Analysed 26.1 3.89 7.35 0.063 0.0079 0.0044usingPerkin-Elmer Pyris DiamondType calorimeter. Phase con-stitutions and their lattice parameters in as-quenched specimenswere determined by 2/ coupling method of X-ray diffractionanalysisalongthelongitudinal directionof specimensusingD/max2500PCRigakudiffractometer.Inordertoavoidarti-fact of stress-induced martensitic transformation, the specimenswere heavily etched in a water solution with 8 vol.% of HF toremove surface layer with internal stress introduced by grindingand polishing. In order to increase accuracy of lattice parame-ter measurement, a low scanning speed of 1/min was adoptedinthisstudy. Specimensfor optical microscopeobservationwere etched at the boiling temperature of a water solution with40 vol.% HCl. Transmission electron microscopy (TEM) speci-mens were prepared from mechanically-thinned plates by elec-tropolishing in a solution of 21%perchloric acid, 50%methanoland 29% n-butyl alcohol at about 40C. The thin foils wereexamined on a Philips EM420 transmission electron microscopeoperating at 100 kV.3. Results and discussions3.1. Microstructures and phase constitutions ofTiNbZrSn alloysFig. 1 shows three typical microstructures (opticalmicrographs) of as-quenchedquaternaryalloys, representedbyTi24Nb4Zr7.5Sn, Ti24Nb4Zr3.5SnandTi20Nb4Zr7.5Sn alloys. They appear to have single microstructure(Fig. 1(a)), ormatrixwithhighandlowamountofthe

martensite (Fig. 1(b and c)). The small dark spots in Fig. 1(a) areoften observed on metallographic samples of heavily deformed-phase titanium alloys and are probably dislocation etch pits.These metallographs also showthat the average grain size of the phase is about 80 m.Systematic X-ray diffraction analyses revealed that alloys ofthestudiedrangeofcompositionhave+,

+, +

,

, or microstructures (Table 1). It is clear from Table 1 that,for identical Zr and Sn contents, the

martensite is depressedwith the increase of Nb content. This is consistent with exper-imentalresultsforbinaryTiNballoys[21]. Asimilartrendwas also observed in the effects of Zr and Sn on the martensitictransformation, as demonstrated by X-ray diffraction proles ofTi24Nb-based alloys presented in Fig. 2: under the conditionof 7.5 Sn addition, the

martensite forms in the alloy with 2%Zr (Fig. 2(e)) but is suppressed in the alloys with 4 and 8% Zr(Fig. 2(c and d)). For 4Zr addition, the

martensite appears inalloy with low Sn addition (Fig. 2(b)) but disappears with theincrease of Sn contents (Fig. 2(a and d)). Clearly, the additionof Nb, Zr and Sn decreases the stability of the

martensite, asconrmed by the variation of lattice parameters with chemicalcompositions to be described in the next section.TheeffectsofZrandSnonphaseformationarecom-plicated. DatapresentedinTable1andFig. 2showthat, forTi24Nb7.5Sn base composition, the phase appears ifZradditionis8%but disappearsforlowerZrcontents(2%and4%);forTi24Nb4Zrbasecomposition, thephaseissuppressedfor 3.5%and7.5%Snadditionbut formsif Sn114 Y.L. Hao et al. / Materials Science and Engineering A 441 (2006) 112118Fig. 1. Typical optical microstructures of water quenched quaternaryTiNbZrSn alloys: (a) Ti24Nb4Zr7.5Sn, (b) Ti24Nb4Zr3.5Sn and (c)Ti20Nb4Zr7.5Sn alloys.content is increased to 11.5%. By contrast, in Ti20Nb4Zr andTi22Nb4Zrbasecompositions,thephaseappearsforSnadditionsof3.5%and11.5%butdisappearsforintermediateamount of Sn(e.g., 7.5%). Clearly, theeffectsof Zr and/orSn on the to transformation is complex compared to theirmonotonic inuence on the to

transformation.Competitionbetweenthephaseandthe

martensiteduringquenchinghasbeeninvestigatedinmetastabletypeFig. 2. X-ray diffraction proles of alloys based on Ti24Nb with: (a)4Zr11.5Sn, (b) 4Zr3.5Sn, (c) 8Zr7.5Sn, (d) 4Zr7.5Sn and (e) 2Zr7.5Snadditions.titanium alloys. Moffat and Larbalestier [21] conducted trans-missionelectronmicroscopy(TEM) analysis of alloys withdifferent Nb contents and concluded that the modes of phase decomposition to both the phase and the

marten-sitearemutuallyexclusive. Thephaseformsgenerallyinmetastabletypetitaniumalloysiftheformationofthe

martensiteissuppressedduringwaterquenchingfromabovethe transus [24]. In the present investigation, TEM analysis(Fig. 3) did not nd characteristic diffraction spots due to the phase in Ti24Nb4Zr7.5Sn alloy that features single phasemicrostructure (Figs. 1(a) and 2(d)). This suggests that appro-priate amounts of Zr and Sn have the advantage of suppressingthe formation of both the martensite and the phase when thealloy is cooled to room temperature.Theabsenceof (1 1 1) and(0 1 2) superlatticediffractionpeaks of the phase in X-ray diffraction proles of allstudiedalloyssuggeststhat thephaseisdisordered. Thisis consistent withthe results for Ti29Nb13Ta4.6Zr andTi39Nb13Ta4.6Zr alloys examined by TEM [24]. By con-trast, Banerjee et al. [25] recently reported that the metastable phase in Ti34Nb9Zr8Ta alloy is ordered.3.2. Effects of Zr and Sn on lattice parameters of the and

Lattice parameters of both the phase and the

marten-site determined by X-ray diffraction analysis are listed inY.L. Hao et al. / Materials Science and Engineering A 441 (2006) 112118 115Fig. 3. Bright eld TEM micrograph (a) and {1 1 0} selected area diffractionpattern (b) of Ti24Nb4Zr7.5Sn alloy.Table 3Lattice parameter (A) of phase in TiNbZrSn alloys20Nb 22Nb 24Nb 26Nb2Zr7.5Sn 3.2914Zr7.5Sn 3.295 3.299 3.3048Zr7.5Sn 3.3084Zr3.5Sn 3.2934Zr11.5Sn 3.297 3.299 3.302Tables 3 and 4, respectively. In agreement with the binary TiNballoys [21], the addition of Nb increases the lattice parameter ofthe phase. The data in Table 3 also showthat the lattice param-eter increases with Zr up to 8%content, whereas its increase withTable 4Lattice parameters (A) of the orthorhombic

martensite inTiNbZrSnalloysa b c c/a b/a24Nb-2Zr7.5Sn 3.119 4.868 4.703 1.508 1.56120Nb4Zr7.5Sn 3.089 4.942 4.709 1.524 1.60022Nb4Zr7.5Sn 3.123 4.892 4.691 1.502 1.56820Nb4Zr3.5Sn 3.050 4.981 4.702 1.542 1.63322Nb4Zr3.5Sn 3.074 4.947 4.692 1.526 1.60924Nb4Zr3.5Sn 3.096 4.928 4.674 1.510 1.59226Nb4Zr3.5Sn 3.134 4.888 4.664 1.488 1.560Table 5Lattice strains of to

transformation in TiNbZrSn alloys1 1 1/0 0 1 (%)0 0 1/0 1 0 (%)1 1 0/1 0 0 (%)24Nb-2Zr7.5Sn 1.1 4.6 5.222Nb4Zr7.5Sn 0.64 5.0 5.226Nb4Zr3.5Sn 0.35 5.9 6.024Nb4Zr7.5Sna0.16 3.8 4.3aThelatticeparametersofthe

martensiteofthisalloyareextrapolatedfrom Ti(20, 22)Nb4Zr7.5Sn alloys.Sn is obvious up to 7.5% but then remains almost constant withfurther increase up to 11.5%.Astothe

martensite, Table4showsthat Nbtendstoincreasetheabut decreasebandcoftheorthorhombiclat-tice;thelatticeparameterratiosofb/aandc/adecreasewiththe increase of Nb content as a result. Similar tendency is alsoobserved for Zr and Sn from Table 4. For binary TiNb alloys,previous investigations showed that phase transformation starttemperatureofthe

martensitedecreaseswiththedecreaseof lattice parameter ratios of b/a and c/a, as reviewed in [21].Thus, the depressing effects of Zr and Sn on the

martensiteformationcanberelatedtotheireffectsonlatticeparameterratios.Latticestrainsaccompanyingtheto

phasetransfor-mationinquaternaryTiNbZrSnalloysareestimatedandpresented in Table 5. A dependence on chemical compositionis clearly seen and the minimum lattice strains occur at chemi-cal compositions close to Ti24Nb4Zr7.5Sn. This quaternaryalloyhasmuchreducedlatticestrainscomparedwithternaryTiNbSn alloys reported in [25].3.3. Transformation temperatures of TiNbZrSn alloysThetemperaturesfortheto

andreversephasetrans-formations in quaternary alloys were measured by DSC methodbetween 150C at cooling and heating rates of 10C/min. Insharp contrast with NiTi shape memory alloy, phase transfor-mation peaks of the studied alloys are quite weak; only thosein several compositions can be distinguished and the results areshown in Table 6. The peak heights of the phase transformationin the alloys listed in Table 6 are generally lower than 0.02 W/gand even difcult to distinguish. Fig. 4 gives an example of DSCmeasurement of Ti24Nb4Zr7.5Sn alloy. The peak heights ofthe studied alloys are also lower than those reported for ternaryTiNbSn alloys [13].Inordertoexaminetheoriginofweakphasetransforma-tion peaks of DSCmeasurements, polished Ti24Nb4Zr7.5SnTable 6Temperatures of

phase transformation in TiNbZrSn alloysMs (C) Mf (C) As (C) Af (C)26Nb4Zr7.5Sn 77.5 78.824Nb4Zr7.5Sn 50.1 53.2 44.2 42.424Nb4Zr3.5Sn 113.7 112.524Nb2Zr7.5Sn 32.3 30.2116 Y.L. Hao et al. / Materials Science and Engineering A 441 (2006) 112118Fig. 4. DSC curves of water quenched Ti24Nb4Zr7.5Sn alloy.specimens were subjected to in situ optical microstructure obser-vationduringcoolingandheatingbetween20and 150C.The results show that the transformation from the matrix to

martensiteisquitelimitedwhenthespecimeniscooledto 80C(Fig. 5), 30Clower thanthemartensiticn-ishtemperaturemeasuredbyDSC(Table6). This suggeststhat themartensitictransformationisnot fullythermoelasticFig. 5. In situ optical microstructure observation of Ti24Nb4Zr7.5Sn alloywater quenched to 20C (a) and 80C (b).in the studied TiNbZrSn quaternary alloys, in contrast withfully thermoelastic transformations during cooling and heatingobserved in TiNbSn ternary alloys [13]. The in situ opticalmicrostructure observation also shows that the transformationstart temperature is about 10C, higher than the DSC mea-sured value of about 50C, suggesting internal stress inducedby surface polishing has signicantly increased the martensitictransformation temperature.ComparingthedataofMstemperatureshowninTable6,it canbe estimatedthat 1 wt%of Nb, Zr or SndecreasesMstemperaturebyabout 17.6, 41.2or40.9C, respectively.The estimation was made using actual chemical compositionsobtainedbychemicalanalysisinsteadofnominaloneslistedin Table 1. It also should be noted that the inuence of inter-stitial elementssuchasoxygenwasassumedconstant intheabove calculations. Based on these data, Ms temperatures of thealloys not listed in Table 6 can be roughly estimated: the phasetransformation temperatures of Ti(2026)Nb4Zr11.5Sn andTi24Nb8Zr7.5Sn alloys are lower than 150C, while thoseofTi(20, 22)Nb4Zr3.5Snalloysarehigherthan+150C.They are all out of the temperature range we set for DSC study.Theeffectsof NbandSnonmartensiticstart (Ms) tem-perature have been investigated in ternary TiNbSn alloys byHanada and coworkers [13,26]. Inconsistent results, with 1%Sndecreasing Ms by about 78C [13] or 52C [26], were reported.Both reported data were much higher than the estimation of thepresent study (40.9C). In addition, the Ms decrease due to Nbin ternary alloys [26] is about twice that in the studied quaternaryalloys. One possible reason of the above quantitative differenceis related to chemical composition. The following experimen-tal results tend to support such speculation: Sn was reported tohave quite weak effect on martensitic transformation in binaryTiSn but the effect is strong in ternary TiNbSn alloys [26];additional evidence is provided by small addition of Pd in TiNballoys that weakens the effect of Nb on martensitic transforma-tion and results in trivial change of transformation temperatureeven when Nb content is increased from30 to 40 wt%[19]. Howthe interaction of these alloying elements inuences the marten-sitic transformation temperature is open for further theoreticalinvestigation.Table 6 shows that the differences of Msand Mfas well asAs and Af are just about 2C, much lower than those for ternaryTiNbSn alloys [13,26]. Furthermore, the difference betweenAfand Msis about 7.7C in Ti24Nb4Zr7.5Sn alloy, muchlower than TiNbSn ternary alloys. Takahashi et al. [13] andNittaetal. [26]reportedthatthecorrespondingdifferenceisabout25and90CinTi25.4Nb9.9SnandTi27Nb8.1Snalloys, respectively. This small difference in transformation tem-peratures in Ti24Nb4Zr7.5Sn alloy can be attributed to thelowlatticestrainsof to

phasetransformation, asmen-tioned in Section 3.2. It was noted earlier that the martensiticand reversible transformations are difcult to complete in thequaternary alloys during cooling and heating (see Fig. 5). This isconsistent with the small differences of start and nish transfor-mation temperatures, Ms and Mf as well as As and Af. However,it cannot explain the small difference between both start tem-peratures, As and Ms.Y.L. Hao et al. / Materials Science and Engineering A 441 (2006) 112118 117Fig. 6. Tensile Youngs modulus (a) and Vickers hardness (b) of water quenchedTiNbZrSn alloys.3.4. Youngs modulus and superelasticity of TiNbZrSnalloysTensile Youngs moduli of the studied alloys are plotted inFig.6(a).TheYoungsmodulusofas-quenchedalloysvarieswith chemical composition and a minimum of about 52 GPa isobtained in Ti24Nb4Zr7.5Sn alloy. Examining phase consti-tutions of the alloys given in Table 1, it can be seen that the alloyscontaining the phase generally have high Youngs modulus.This is because the phase has higher Youngs modulus thanthe and the

phases [46]. The data in Fig. 6(a) also showthatYoungsmoduliofthealloyscontainingthephaseareonly slightly higher than those without the phase. This sug-gests that the volume fractions of the phase are quite low inthese alloys. The measurements of Vickers hardness presentedin Fig. 6(b) also support this deduction.Superelasticity of water quenched TiNbZrSn alloyswas evaluated by cyclic deformation at initial strain rateof 1 103s1at roomtemperature (21C). Three typi-cal stressstrain curves, demonstrated by Ti24Nb-2Zr7.5Sn,Ti24Nb4Zr7.5Sn and Ti24Nb4Zr11.5Sn alloys, areshowninFig. 7. Themartensiticstart (Ms) temperaturefortheabovethreealloysdecreasesinturn.Althoughthetensiletest temperature is about 9C lower than the martensite nish(Mf) temperature of Ti24Nb2Zr7.5Sn (Table 6), the par-entphasedidnotfullytransformtothe

martensitebeforetesting. This is evidenced by the double yielding phenomenonin the stressstrain curve (Fig. 7(a)) due to the stress-assistedmartensitic transformation. X-ray analysis (Table 1) that reveals+

microstructureofTi24Nb2Zr7.5Snsupportsaboveargument. As mentioned in Section 3.3, the martensitic trans-formation in the studied quaternary alloys appears much moresluggishthanisusual for thermoelasticmartensiteanddoesnot go to completion. The double yielding phenomenon is alsoobserved for Ti24Nb4Zr7.5Sn alloy (Fig. 7(b)) but not forTi24Nb4Zr11.5Sn alloy (Fig. 7(c)). In this case, the phaseis fully stabilised with respect to the martensitic transformationFig.7. Stressstraincurvesat21CofwaterquenchedTi24Nb2Zr7.5Sn(a), Ti24Nb4Zr7.5Sn (b) and Ti24Nb4Zr11.5Sn (c).118 Y.L. Hao et al. / Materials Science and Engineering A 441 (2006) 112118because its Ms temperature is too low as a result of its high Sncontent.Since elastic strain recorded by a tensile testing machine canbe twice as high as its true value [13], the stressstrain curvesshown in Fig. 7 were recorded by using a strain gauge to ensureaccuratemeasurement of elasticstrain. It canbeseenfromFig. 7(b) that, for Ti24Nb4Zr7.5Sn alloy, elastic strains of1%, 2% and 3% are totally, near totally and partially recovered,respectively, after unloading. The superelasticity of this alloy isslightly lower than ternary Ti25.4Nb9.9Sn alloy [13]. Fromthe view point of thermoelastic transformation, the above dif-ference can be explained by the difference between the testingtemperature and the austenitic nish (Af) temperature. For theternary alloy, the testing temperature is 30C higher than theAf temperature, a generally accepted temperature difference forachieving signicant superelasticity. On the other hand, the tem-perature difference for the quaternary alloy, about 63C, is toohightoobtaingoodsuperelasticity. The above argument suggeststhat increasingAftemperaturebydecreasingthecontentsofNb, Zr and/or Sn would improve superelasticity. However, addi-tional experiments yield contrary results. Quaternary alloys withincreased Sn content were found to possess improved supere-lasticity and a maximum recoverable tensile strain of 3.3% wasobtained in an alloy with 7.9 wt% Sn (compared to 7.5 wt% Sninthestudiedalloy)[27].Furthermore,stressstraincurveofas hot-rolled 7.9Sn alloy exhibits non-linear elastic deformationbehaviour without double yielding, substantially different fromthe curve shown in Fig. 7(b).It should be noted that there exist several reports of supere-lasticity that are not related to the

martensitic transforma-tion[19,22,23].Furtherinvestigationtorevealotherkindsofmechanism is therefore of crucial importance for improving thesuperelasticpropertiesoftitaniumalloys(e.g., bycombiningtwo or more mechanisms).4. ConclusionsThe effects of Zr andSncontents onYoungs modulusand superelasticity of water quenched quaternary TiNbZrSnalloys were investigated in this study. The main results are sum-marized below:(1) Both Zr and Sn tend to increase the lattice parameter of the phase; they increase the lattice parameter a but decreaseb and c of the

martensite.(2) The martensitic start temperature decreases by about 17.6,41.2and40.9Cduetotheadditionof1 wt%ofNb,ZrandSn,respectively.ZrandSnarethereforeeffectiveinsuppressingthe

martensitictransformation, ascanbeexplained by their effects on the lattice parameter ratios ofc/a and b/a.(3) The lattice strains of forming the orthorhombic

marten-site from the bcc phase are dependent on chemical com-position. The minimum is obtained in Ti24Nb4Zr7.5Snamong the studied alloys. A small temperature hysteresis,AfMs, of about 7.7C, is detected in this alloy.(4) The formation of the athermal is dependent on both Zrand Sn contents in Ti(2026)Nb-based alloys.(5) Ti24Nb4Zr7.5Sn alloy with single microstructure hasa minimumYoungs modulus of 52 GPa and recovered elas-tic strain larger than 2% after cyclic deformation at roomtemperature.AcknowledgementThe work was partly supported by the NSFC (grants50471074 and 30471754) and the Chinese MoST (grantTG2000067105).References[1]M. Long, H.J. Rack, Biomaterials 19 (1998) 16211639.[2]M. Niinomi, Metall. Mater. Trans. A 33A (2002) 477486.[3]X. Tang, T. Ahmed, H.J. Rack, J. Mater. Sci. 35 (2000) 18051811.[4]Y.L. Hao, M. Niinomi, D. Kuroda, K. Fukunaga, Y.L. Zhou, R. Yang, A.Suzuki, Metall. Mater. Trans. A 33A (2002) 31373144.[5]Y.L. Hao, M. Niinomi, D. Kuroda, K. Fukunaga, Y.L. Zhou, R. Yang, A.Suzuki, Metall. Mater. Trans. A 34 (2003) 10071012.[6]W.F. Ho, C.P. Ju, J.H. Chern Lin, Biomaterials 20 (1999) 21152122.[7]D.J. Lin, J.H. Chern Lin, C.P. Ju, Biomaterials 23 (2002) 17231730.[8]S.J. Li, M. Niinomi, T. Akahori, T. Kasuga, R. Yang, Y.L. Hao, Biomaterials25 (2004) 33693378.[9]C. Baker, J. Metal. Sci. 5 (1971) 92100.[10]T.W. Duerig, D.F. Richter, J. Albrecht, Scripta Metall. 16 (1982) 957961.[11]T.W. Duerig, D.F. Richter, J. Albrecht, Acta Metall. 30 (1982) 21612172.[12]T. Grosdidier, M.J. Philippe, Mater. Sci. Eng. A 291 (2000) 218223.[13]E. Takahashi, T. Sakurai, S. Watanabe, N. Masahashi, S. Hanada, Mater.Trans. 43 (2002) 29782983.[14]H.Y. Kim, H. Satoru, J.I. Kim, H. Hosoda, S. Miyazaki, Mater. Trans. 45(2004) 24432448.[15]T. Maeshima, M. Nishida, Mater. Trans. 45 (2004) 10961100.[16]T. Maeshima, M. Nishida, Mater. Trans. 45 (2004) 11011105.[17]Y. Fukui, T. Inamura, H. Hosoda, K. Wakashima, Mater. Trans. 45 (2004)10771082.[18]T. Inamura, Y. Fukui, H. Hosoda, K. Wakashima, S. Miyazaki, Mater. Sci.Forum 475479 (2005) 23232327.[19]D.H. Ping, Y. Mitarai, F.X. Yin, Scripta Mater. 52 (2005) 12871291.[20]E.W. Collings, Physical Metallurgy of TitaniumAlloys, ASM, Metals Park,OH, 1984.[21]D.L. Moffat, D.C. Larbalestier, Metall. Trans. A 19 (1988) 16771686.[22]T. Zhou, M. Aindow, S.P. Alpay, M.J. Blackburn, M.H. Wu, Scripta Mater.50 (2004) 343348.[23]T. Saito, T. Furuta, J.H. Hwang, S. Kuramoto, K. Nishino, N. Suzuki, R.Chen, A. Yamada, K. Ito, Y. Seno, T. Nonaka, H. Ikehata, N. Nagasako, C.Iwamoto, Y. Ikuhara, T. Sakuma, Science 300 (2003) 464467.[24]S.J. Li, Y.L. Hao, R. Yang, Y.Y. Cui, M. Niinomi, Mater. Trans. 43 (2002)29642969.[25]R. Banerjee, S. Nag, J. Stechschulte, H.L. Fraser, Biomaterials 25 (2004)34133419.[26]K. Nitta, S. Watanabe, N. Masahashi, H. Hosoda, S. Hanada, E.M. Taleff,D.R. Lesure, in:M. Niinomi, T. Okabe, E.M. Taleff, D.R. Lesure, H.e.Lippard(Eds.), Structural Materials for the 21st Century, TMS, Warrendale,PA, 2001, pp. 2534.[27]Y.L. Hao, S.J. Li, S.Y. Sun, C.Y. Zheng, Q.M. Hu, R. Yang, Appl. Phys.Lett. 87 (2005) 091906.