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Scripta Materialia 82 (2014) 29–32

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Changeable Young’s modulus with large elongation-to-failurein b-type titanium alloys for spinal fixation applications

Huihong Liu,a Mitsuo Niinomi,b,⇑ Masaaki Nakai,b Junko Hiedab and Ken Chob

aDepartment of Materials Science, Graduate School of Engineering, Tohoku University, Sendai 980-8579, JapanbInstitute for Materials Research, Tohoku University, Sendai 980-8577, Japan

Received 29 January 2014; revised 13 March 2014; accepted 15 March 2014Available online 21 March 2014

Materials used for implant rods in spinal fixation systems must show changeable Young’s moduli, high strength and large elon-gation-to-failure. A d-electron design method was used to determine the chemical composition of Ti–10Cr, which showed all theseproperties and significant work-hardening characteristics owing to multiple plastic deformation mechanisms, such as deformation-induced x-phase transformation, {332}<113> mechanical twinning and dislocation gliding. Therefore, Ti–10Cr exhibits greatpotential for use in spinal fixation applications.� 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Titanium alloys; Changeable Young’s modulus; Phase transformation; Plasticity; Twinning

A low Young’s modulus, which can preventstress shielding [1], is one of the most essential propertiesfor ensuring metallic biomaterial mechanical biocom-patibility. Most previous research has focused on lower-ing the Young’s moduli of metallic biomaterials anddeveloping many low-Young’s-modulus b-titanium al-loys, the moduli of which (�40–60 GPa) [2–4] are closeto that of bone (�10–30 GPa) [5], for biomedical appli-cations. However, surgeons must manipulate and bendspinal fixation implant rods to conform to spinal curva-ture within a limited space inside the body [6,7]. Thus,implant rods must show sufficiently low springback,which depends on the material strength and Young’smodulus, so surgeons can easily manipulate them. Fortwo identically strong implant rods, the higher-Young’s-modulus one exhibits lower springback. Thus,surgeons prefer high-Young’s-modulus rods to suppressspringback; however, low-Young’s-modulus rods aredesirable for patients to prevent postoperative stressshielding. Hence, several b-titanium alloys, such asTi–30Zr–3Cr–3Mo [8] and Ti–12Cr [9], have recentlybeen developed to satisfy these conflicting requirements.The deformed parts of these alloys show high Young’s

http://dx.doi.org/10.1016/j.scriptamat.2014.03.0141359-6462/� 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights

⇑Corresponding author. Tel.: +81 22 215 2574; fax: +81 22 2152553; e-mail: niinomi@imr.tohoku.ac.jp

moduli through deformation-induced x-phase transfor-mation, while the non-deformed parts show lowones—a novel property termed “changeable Young’smodulus.” However, most such alloys show onlymoderate elongation-to-failure. For example, Ti–30Zr–3Cr–3Mo and Ti–12Cr exhibit �15% [8] and �13% [9]elongations, respectively, although higher plasticity isdesirable for their practical use. Therefore, alloys show-ing changeable Young’s moduli, good plasticity and suf-ficiently high strength must be developed for spinalfixation applications.

The deformation-induced x phase reportedly formswhile cold working b-Ti–Cr alloys containing8–11.5 mass% Cr [10,11]; thus, they can show change-able Young’s moduli. Further, b-Ti–Cr alloys reportedlyshow {332}<1 13> mechanical twinning during defor-mation [10]. Twinning-induced plasticity has recentlybeen used to improve the mechanical properties of steels[12] and titanium alloys [13]. Moreover, several studieshave reported that activating {332}<1 13> twinning intitanium alloys can significantly increase the work-hardening rate [14]. Therefore, Ti–Cr alloys canprobably show good plasticity thorough high deforma-tion-twinning-induced work hardening. The “d-electronalloy design” method, in which the bond order (Bo) andd-orbital energy (Md) electronic parameters of alloyatoms are mapped, has recently been used to develop

reserved.

30 H. Liu et al. / Scripta Materialia 82 (2014) 29–32

a strategy for formulating titanium alloys showing spe-cific improved properties [15,3]. In the present study,the chemical composition of a titanium alloy was de-signed based on this theoretical formulation, and the al-loy showed a changeable Young’s modulus and asuperior combination of good plasticity and sufficientlyhigh strength.

A Ti–10 mass% Cr alloy with a Bo and Md of �2.789and �2.357, respectively, was designed based on thephase stability diagram presented in the literature[15,3]. This alloy, which simultaneously exists in theb+x and twin regions in the Bo–Md diagram, is a goodcandidate for achieving a changeable Young’s modulus,a large elongation to failure and a high work-hardeningrate owing to expected deformation-induced x-phasetransformation and mechanical twinning.

A Ti–10Cr ingot was prepared using a levitationmelting under a high-purity argon atmosphere. The ana-lyzed chemical composition of the alloy was approxi-mately the nominal chemical composition, showing alow oxygen concentration of �0.06 mass%. The ingotwas homogenized at 1373 K for 21.6 ks under an argonatmosphere and was subsequently quenched with icewater, hot forged, hot rolled into a 70%-reduced plateat 1273 K under an argon atmosphere, and air cooled.The hot-rolled plate was then solution-treated at1123 K for 3.6 ks under vacuum, after which it wasquenched with ice water. A 10%-reduced cold rollingwas performed on the solution-treated specimen insteadof bending in order to evaluate the Young’s modulus ofthe deformed specimen. The solution-treated and cold-rolled specimens are labeled as “ST” and “CR”,respectively.

The specimen microstructures were examined using anX-ray diffraction (XRD), an optical microscopy (OM),an electron backscattered diffraction (EBSD) and a trans-mission electron microscopy (TEM). Mirror-polishedspecimens were etched using an aqueous solution consist-ing of hydrofluoric and nitric acids in preparation for OMand EBSD. TEM was conducted at 200 kV acceleration.

For Young’s modulus measurement, the ST and CRspecimens were polished using up to 2400-grit SiCwaterproof papers along their longitudinal direction,parallel to the rolling direction, and tested by the free

Figure 1. (a–c) Optical micrographs of Ti–10Cr-ST, Ti–10Cr-TENS and Ti–1boundary map showing boundaries misoriented �50.5� to the matrix aroundmarked in (d) relative to the first point. (For interpretation of the references tothis article.)

resonance method. An INSTRON� type tester was usedat a crosshead speed of 8.33 � 10�6-ms�1 to determinethe room-temperature tensile strength of the ST speci-men, and an interrupted tensile test was performed whileunloading the specimen under �0.1 strain. The micro-structures of the tensile and interrupted tensile speci-mens (TENS and 10TENS, respectively) wereexamined. Three specimens were used for each Young’smodulus measurement and tensile test to minimizeexperimental error.

Figure 1 shows the optical micrographs of theTi–10Cr-ST, Ti–10Cr-TENS and Ti–10Cr-CR speci-mens, and the EBSD analysis for the Ti–10Cr-TENSone. The optical micrograph and XRD profile (notshown) for Ti–10Cr-ST suggest that its microstructureis composed of equiaxed single-b-phase grains. The opti-cal micrographs of Ti–10Cr-TENS and Ti–10Cr-CRshow complex straight-band networks inside the grains.The XRD profiles for Ti–10Cr-TENS and Ti–10Cr-CR(not shown) only exhibit b-phase-associated peaks. TheEBSD grain boundary map (Fig. 1e) and the plot ofmisorientation vs. distance (Fig. 1f) measured alongthe arrow marked in the orientation map for Ti–10Cr-TENS (Fig. 1d) suggest that the misorientation betweenthe band structures and matrix is �50.5� around the<1 10>b direction. The bands were thus identified as{332}<1 13> mechanical twins [16]. As shown inFigure 1d, grain 1 (G1) contains a greater volume oftwins than grain 2 (G2). The grain crystallographic ori-entations were normal direction ðNDÞ==½ �13 �13 22� andtransverse direction (TD)//[14 8 13] for G1 andND==½�1 �8 29� and TD==½45 �2 1� for G2. The tensiledirections of both grains were parallel to the TDs.According to the Schmid law, the ð3�23Þ[131] andð33�2Þ½113� systems, the Schmid factors of which were0.497 and 0.366, respectively, are the twinning systemsshowing the two highest Schmid factors among the 12possible {332}<1 13> twinning systems in G1, whileG2 did not show any {33 2}<11 3> twinning systemswith a Schmid factor of >0.300. The experimentallyobserved substantial ð3�23Þ½131� twins activated in G1,marked by the dotted circle, obey the Schmid lawtheoretical prediction; therefore, {332}<1 13> twinningis activated more easily in G1 than in G2 because of the

0Cr-CR, respectively; (d) orientation map for Ti–10Cr-TENS; (e) grainthe <110>b direction (green lines); (f) misorientations along the arrowcolour in this figure legend, the reader is referred to the web version of

Figure 3. (a) Young’s moduli of Ti–10Cr-ST, Ti–10Cr-CR and TNTZ[22]; (b) true tensile stress–strain curves for Ti–10Cr-ST and TNTZ(black), and the corresponding work-hardening rates, dr/de (red).(For interpretation of the references to colour in this figure legend, thereader is referred to the web version of this article.)

H. Liu et al. / Scripta Materialia 82 (2014) 29–32 31

favorable crystal orientation with regards to the tensiledirection. The average twin area fraction for Ti–10Cr-TENS, which is approximately the volume fraction,was �52%, as determined by analyzing the opticalmicrographs with an image analysis software program(Image-J). The EBSD analysis also clearly revealed the{33 2}<1 13> twins in Ti–10Cr-CR (not shown).

Figure 2 shows the TEM images and selected-areaelectron diffraction (SAED) patterns for Ti–10Cr-STand Ti–10Cr-TENS. The SAED pattern for Ti–10Cr-ST shows weak spots and athermal-x-phase-related cir-cular diffuse streaks, suggesting a small amount of theathermal x phase had formed during quenching. Thecorresponding dark-field (DF) image of the weak x spotrevealed athermal x nanoparticles uniformly distributedthroughout the matrix. The bright-field image of Ti–10Cr-TENS shows a {332}<1 13> twin (Fig. 2c).Although the two variant x spots in the SAED for thematrix (Fig. 2d) were more intense than that in theSAED for the Ti–10Cr-ST, one of the variants was sig-nificantly favored, indicating that Ti–10Cr-TENS hadundergone deformation-induced x-phase transforma-tion. The corresponding DF image of one favored xspot shows that specifically aligned deformation-in-duced x particles were well dispersed/randomly distrib-uted throughout the matrix. The SAED for the twin alsoclearly showed x spots associated with one favored var-iant. The corresponding DF image of one favored xspot showed that the deformation-induced x phasehad formed along the twin boundary and was well dis-persed/randomly distributed throughout the twin. Thisresult is consistent with that of the previous study [17]reporting that b-Ti–Mo alloys could undergo deforma-tion-induced x-phase transformation along twin bound-aries, where compressive stress accumulates. Severalother positions containing the {332}<1 13> twin inTi–10Cr-TENS were analyzed by TEM and showed re-sults similar to that above.

Figure 3 shows the results of the mechanical evalua-tion of the alloy. Ti–10Cr-ST showed a Young’s modu-lus of �85 GPa, which is lower than those of thecommonly used SUS 316L stainless steel, commerciallypure a-titanium and (a+b)-Ti–6Al–4V ELI, which are�200, �105 and �110 GPa, respectively [18,19].Ti–10Cr-CR showed a Young’s modulus of �96 GPa.TEM images (not shown) of the Ti–10Cr-CR revealedthat the deformation-induced x phase, which can in-

Figure 2. (a) SAED pattern for Ti–10Cr-ST; (b) DF image of x spot labeledSAED pattern for the matrix (M) marked in (c); (e) DF image of x spot markand twin (T) marked in (c), respectively; (h) DF image of x spot labeled bdirection.

crease the Young’s moduli of titanium alloys [8,9], hadformed; hence, the increased Young’s modulus is attrib-utable to the deformation-induced x-phase transforma-tion. Using an XRD or a TEM to accurately estimatethe volume fraction of deformation-induced x-phaseparticles is difficult because the size and volume fractionof the particles are so small; hence, it was theoreticallyestimated based on data from the literature [20,21].The x phase reportedly shows a Young’s modulus of�165 GPa [20], as calculated using the mixture law. ItsYoung’s modulus was used to theoretically predict itsvolume fraction [21]; therefore, the volume fraction, f,of the deformation-induced x phase formed in Ti–10Cr-CR can be estimated using the following mix-ture-law-based equation:

Ec ¼ Esð1� f Þ þ Exf ð1Þwhere Ec, Es and Ex represent the Young’s moduli ofTi–10Cr-CR, Ti–10Cr-ST and the x phase, respectively.Hence, the volume fraction of the deformation-inducedx phase in Ti–10Cr-CR was estimated to be �14%.b-Ti–29Nb–13Ta–4.6Zr (TNTZ) [22] is a biomedicalalloy which does not undergo any deformation-inducedx-phase transformation during CR and shows similarYoung’s moduli in both the ST and CR conditions(TNTZ-ST and TNTZ-CR, respectively). Figure 3bexhibits the true tensile stress–strain curves forTi–10Cr-ST and TNTZ and the correspondingwork-hardening rates. The true pre-necking stress forTi–10Cr-ST was �1150 MPa, nearly twice that ofTNTZ. Moreover, Ti–10Cr-ST exhibits �30% uniformelongation, much higher than those of TNTZ and con-ventional biomedical titanium alloys, which show

by a dotted ellipse in (a); (c) bright-field image of Ti–10Cr-TENS; (d)ed by a dotted circle in (d); (f, g) SAED patterns for the boundary (B)

y a dotted circle in (g). The beam direction is parallel to the [110] b

Figure 4. (a) SAED pattern for the Ti–10Cr-10TENS matrix; (b) one-dimensional intensity profiles generated along the ½�f�ff��b direction ofSAEDs of Ti–10Cr-ST, Ti–10Cr-10TENS and Ti–10Cr-TENS.

32 H. Liu et al. / Scripta Materialia 82 (2014) 29–32

�10–20% elongation [23,24]. The corresponding work-hardening rates for Ti–10Cr and TNTZ are plotted asred curves. The work-hardening rate for Ti–10Cr shar-ply decreases from an extremely high value to�2000 MPa from the elastic limit to e � 0.1 (stage 1)and subsequently smoothly decreases to �1200 MPa un-der strain up to �0.25 (stage 2). TNTZ exhibits a muchlower work-hardening rate than Ti–10Cr in the entireplastic deformation region.

Figure 4 shows the SAED for Ti–10Cr unloaded ate � 0.1 under tension (Ti–10Cr-10TENS) and the one-dimensional intensity profiles generated along the½�1�11�b direction of the SAEDs for Ti–10Cr-ST, Ti–10Cr-10TENS and Ti–10Cr-TENS. The dotted lines inFigures 2a, 4a and 2d correspond to the analyzed posi-tions. The peaks corresponding to the x phase rapidlyintensified from Ti–10Cr-ST to Ti–10Cr-10TENS, thenonly slightly broadened from Ti–10Cr-10TENS to Ti–10Cr-TENS, indicating that the Ti–10Cr-ST had rapidlyundergone deformation-induced x-phase transforma-tion up to a tensile deformation of e � 0.1, which thenslowed up to the tensile fracture point. Based on theaforementioned discussion, we speculate that theremarkable work hardening of Ti–10Cr during stage 1is mainly attributable to significant dynamic second-phase strengthening due to deformation-induced x-phase transformation and to dynamic microstructurerefinement, the so-called “dynamic Hall–Petch effect”,due to twin formation and twin–twin intersection. Thephase transformations slow and the dislocation–secondphase, dislocation–twin and dislocation–dislocationinteractions also affect the work hardening with increas-ing strain (stage 2), as reported in the literature [25]. Thedeformation-induced x phase is believed to accommo-date deformation, relieving the stress concentratingwhile it forms and helping pin gliding dislocations as dis-persed obstacles, thereby contributing to plasticity andsignificant work hardening. Furthermore, substantialtwinning can significantly refine grains, increasing thenumber of pathways and decreasing the effective glidedistances for dislocation slip, thereby preventing stressfrom concentrating locally, improving the elongationto failure and increasing the work-hardening rate.

In summary, the “d-electron-design”-fabricated Ti–10Cr exhibited a changeable Young’s modulus and an

excellent combination of high strength, large elongationto failure and significant work hardening owing to com-plex plastic deformation mechanisms, including defor-mation-induced x-phase transformation, {332}<1 13>mechanical twinning and dislocation gliding. Thus, itexhibits great potential for use in spinal fixationapplications.

This work was supported in part by the Indus-trial Technology Research Grant Program in 2009 fromthe New Energy and Industrial Technology Develop-ment Organization (NEDO), Japan and Grant-in-Aidfor Scientific Research (A) and Young Scientists (A)from the Japan Society for the Promotion of Science(JSPS), Japan.

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