Low Young’s Modulus TiNbO with High Strength and Good Plasticity

Qiang Li1,+, Dong Ma1, Junjie Li2, Mitsuo Niinomi1,3,4,+, Masaaki Nakai5, Yuichiro Koizumi3,Daixiu Wei3, Tomoyuki Kakeshita4, Takayoshi Nakano4, Akihiko Chiba3, Kai Zhou1 and Deng Pan1,6

1School of Mechanical Engineering, University of Shanghai for Science and Technology, Shanghai 200093, P. R. China2INL - International Iberian Nanotechnology Laboratory, Av. Mestre José Veiga s/n, 4715-330 Braga, Portugal3Institute for Materials Research, Tohoku University, Sendai 980-5377, Japan4Department of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, Osaka University,Suita 565-0871, Japan5Department of Mechanical Engineering, Faculty of Science and Engineering, Kindai University, Higashiosaka 577-8502, Japan6Research Center for Advanced Metallic Materials, Yangtze Delta Region Institute of Tsinghua University,705 Yatai Road, Jiaxing, Zhejiang, China

Oxygen was added to Ti38Nb (mass%) alloys to improve their mechanical properties. Ti38NbxO (x = 0.13, 0.24, 0.46, mass%) alloyswere prepared by arc melting, and subsequently subjected to homogenization, hot rolling, and solution treatment. It was found that addingoxygen suppresses the martensite transformation and exhibits strong solution strengthening effect. Single ¢ phase is obtained in Ti38Nb0.24O,whereas Ti38Nb0.13O is composed of both ¡AA and ¢ phases. Both alloys exhibit double yielding phenomena during tension, indicating astress-induced martensitic transformation. Ti38Nb0.46O exhibits a non-linear deformation, a low Young’s modulus of 62GPa, high tensilestrength up to 780MPa, and elongation around 23%, which are promising characteristics for biomedical applications.[doi:10.2320/matertrans.M2018021]

(Received January 18, 2018; Accepted March 7, 2018; Published April 25, 2018)

Keywords: Ti alloys, oxygen, biomaterials, mechanical property

1. Introduction

Titanium and its alloys are widely used in biomedicalfields owing to their excellent mechanical properties andgood biocompatibility.1) ¢-Ti alloys that are composed ofnon-toxic elements have been widely investigated due to theirlow Young’s modulus for avoiding stress shielding effects ofbio-implants.2) The strength of ¢-Ti alloys usually need tobe improved by solution strengthening of alloying elements(Zr, Ta, N and O), or precipitation strengthening by ¡ or ½phases.36) In ¢-Ti alloys, oxygen is an interstitial elementwhich shows solution strengthening. High strength andhigh plasticity have been obtained in Ti36Nb2Ta3Zr0.3O.7,8) It’s reported that oxygen increases the strength andsuper-elasticity of Ti22Nb and Ti26Nb (both in mol%)alloys.911) Ti29Nb13Zr4.6Ta (TNTZ) alloys with oxygenin mass% of 0.13, 0.33, and 0.70 have shown abnormaldeformation behaviour in previous studies, and with anincrease in oxygen content, elongation was shown to initiallydecrease and then increase owing to the change ofdeformation mechanism.12,13) Metastable Ti38Nb (mass%,24% in mol%, abbreviated as TN in this study) alloy withe/a of 4.24, Bo of 2.864, and Md of 2.441 shows both ¡AA and¢ phases after solution treatment, and exhibits both a lowYoung’s modulus and low strength.14) In this study, oxygenis added to Ti38Nb alloy in order to obtain high strengthwith low Young’s modulus and good plasticity maintaining,which are desirable properties for biomedical applications.

2. Experimental Procedures

Ingots of Ti38Nb-based alloys with oxygen mass percentof 0.13, 0.24, and 0.46 (abbreviated as TN0.13O, TN

0.24O, and TN0.46O hereafter) were arc-melted in a water-sealed copper crucible in Ar atmosphere. The ingots werehomogenized, and then hot-rolled to a thickness of 2.2mmwith a reduction of 80%. These sheets were solutionized at50K above the ¢ transition temperature for 3.6 ks followedby quenching in water. Microstructures were observed byoptical microscopy. Phase constitutions were determined bya Bruker D8 X-ray diffractometer with Cu-K¡ radiation ata voltage of 40 kV and current of 40mA. Young’s moduliwere measured via the free response method. Tensilemeasurements were performed on specimens with 12mmgauge length at a strain rate of 6.94 © 10¹4 s¹1 by using anInstron-type testing machine at room temperature.

3. Results and Discussion

3.1 Microstructure and phase constitutionThe XRD patterns of the Ti38Nb alloys are presented

in Fig. 1. The TN0.13O alloy exhibits both the ¢ and ¡AAphases (Fig. 1(a)), and both the TN0.24O and TN

30 40 50 60 70 80

(c)

(b)

(a)

Inte

nsity

, I/ a

.u.

Diffraction angle, 2θ/ °

α″

β

Fig. 1 XRD patterns of solutionized (a) Ti38Nb0.13O, (b) Ti38Nb0.24O, and (c) Ti38Nb0.46O.+Corresponding author, E-mail: jxli@tju.edu.cn; niinomi@imr.tohoku.ac.jp

Materials Transactions, Vol. 59, No. 5 (2018) pp. 858 to 860©2018 The Japan Institute of Metals and Materials EXPRESS RAPID PUBLICATION

https://doi.org/10.2320/matertrans.M2018021

0.46O alloys exhibit only the ¢ phase (Fig. 1(b) and 1(c),respectively). The microstructures of the alloys can be seenfrom the optical micrographs, as shown in Fig. 2. Grain sizesover 200 µm are observed in the alloys owing to the hotrolling process. By comparison, alloys that are cold rolledand subsequently solutionized typically exhibit grain sizesof 2040 µm.14) Furthermore, the ¡AA phase of the TN0.13Oalloy shows an acicular structure dispersed in equiaxed ¢grains (Fig. 2(a)). Although oxygen is an “¡ stabilizer”which raises the ¢ transus,15) it effectively suppresses ¡AAtransformation by decreasing the martensite starting temper-ature (Ms). Only equiaxed ¢ grains are observed in the TN0.24 and TN0.46 samples (Fig. 2(b) and 2(c)).

3.2 Mechanical propertiesFigure 3(a) shows the stress-strain curves of the alloys

obtained by tensile tests at room temperature. Figure 3(b)shows the Young’s modulus (E) obtained by the freeresponse method, as well as the 0.2% proof strength (·0.2),

tensile strength (·b), and elongation (¾l), all measured by thetensile test.

Both the TN0.13O and TN0.24O alloys show a doubleyielding phenomenon owing to stress-induced martensitictransformation (SIMT), indicating an unstable ¢ phase inthe two alloys. TN0.46O shows non-linear deformationbehaviour without significant yielding during tension. It hasbeen reported that non-linear elastic behaviour are due tothe formation of nano-domain lattice distortion induced byinterstitial oxygen atoms.10,11) The ·0.2 in TN0.13O and TN0.24O is related to the first yield stress, which corresponds tothe stress for SIMT and is affected by the Ms temperature.Oxygen reduces the Ms temperature and thereby increases·0.2.15) The disappearance of double yielding phenomenon inTN0.46O indicates a more stable ¢ phase in the alloy andfurther increases the ·0.2. Non-linear deformation behaviourand strain hardening make ·b much higher than ·0.2 in TN0.46O. The ·b of the alloys are significantly enhanced byadding oxygen, owing to its great solution strengthening.TN0.46O exhibits high ·0.2 and ·b, values of 570MPaand 780MPa, respectively. The Young’s moduli are slightlyincreased by increasing oxygen content, but a low value(62GPa) is observed in the TN0.46O alloy. TN0.46Oalloy shows large elongation around 25% although oxygendecreases elongation.

3.3 DiscussionIt has been reported that TNTZxO (x = 0.13, 0.33, 0.70)

alloys exhibit an abnormal deformation behaviour.12,13) Theirstrength monotonically increases, while elongation initiallydecreases and then increases with increasing oxygen. In thisstudy, the Ti38NbxO (x = 0.13, 0.24, 0.46) alloys withsimpler chemical composition exhibit the same increasingtrend in strength. TN0.24O and TN0.46O show similarstrength with TNTZ0.13 and TNTZ0.33, respectively.However, TN0.46O exhibits much higher elongationthan TNTZ0.33. Increasing oxygen content monotonicallyincreases the Young’s modulus, thus, TNTZ0.77 alloyshows a high Young’s modulus (75GPa). In the Ti38NbxO alloys, x = 0.46 is found to be a suitable amount ofoxygen to maintain a lower Young’s modulus.

Comparing the oxygen added TN and TNTZ alloys,differences in both the Young’s modulus and elongation canbe attributed to the influence of oxygen content on ¢ phasestability and solution-strengthening effects. TNTZ and TNhave similar Moeq values, but the Zr in TNTZ acts as a¢ stabilizer to further decrease the Ms temperature. Forcomparison, microstructures of TN0.24O and TN0.46Owere measured by XRD and optical microscopy and are

Fig. 2 Optical morphologies of solutionized (a) Ti38Nb0.13O, (b) Ti38Nb0.24O, and (c) Ti38Nb0.46O.

Fig. 3 Comparison of (a) tensile stress-strain curves at room temperatureand (b) mechanical properties of Ti38Nb-(0.13, 0.24, 0.46) O alloys.

Low Young’s Modulus TiNbO with High Strength and Good Plasticity 859

given in Fig. 4. Multiple deformation bands and residual ¡AAphase are both observed in TN0.24O and TNTZ0.13O,which suggests a similar deformation mechanism betweenthe two alloys. It is worth noting that more deformation bandsare found in TN0.46O than in TNTZ0.33O, indicatinga more unstable ¢ phase. The well-developed deformationbands in TN0.46O indicate that twinning is also a primarydeformation mechanism, which is why TN0.46O exhibits alarge elongation. However, oxygen supplies strong solutionstrengthening which makes TN0.46O exhibit similar

strength with TNTN0.33. The strain hardening observedin TN0.46O may be attributed to both the deformation bandand the influence of oxygen on slip. Further studies will becarried out to illustrate the reason for the good compre-hensive mechanical properties of O added Ti38Nb alloys.

4. Conclusion

In Ti38Nb alloys, oxygen increases the ¢ phase stabilityand suppresses the ¡AA phase martensite transformation inboth quenching and tension. The 0.2% proof strength isincreased upon increasing oxygen content owing to thedecreasing Ms temperature, and the tensile strength improvesby solution strengthening. A high elongation above 20% isobserved, even at higher oxygen content (x = 0.46). Due toits suitable ¢ phase stability and solution strengthening,Ti38Nb0.46O alloy exhibits high strength (780MPa), lowYoung’s modulus (62GPa), and an elongation higher than20%, which is desirable for biomedical applications.

Acknowledgement

This work was sponsored by the Natural ScienceFoundation of Shanghai (No. 15ZR1428400), ShanghaiKey Technology Support Program (No. 16060502400),and National Natural Science Foundation of China (No.61504080).

REFERENCES

1) Q. Chen and G.A. Thouas: Mater. Sci. Eng. R 87 (2015) 157.2) M.A.H. Gepreel and M. Niinomi: J. Biomed. Mater. Res. A 20 (2013)

407415.3) M. Niinomi, M. Nakai and J. Hieda: Acta Biomater. 8 (2012) 3888

3903.4) Q. Li, J.J. Li, G.H. Ma, X.Y. Liu and D. Pan: Mater. Des. 111 (2016)

421428.5) J. Coakley, K.M. Rahman, V.A. Vorontso, M. Ohnuma and D. Dye:

Mater. Sci. Eng. A 655 (2016) 399407.6) J. Stráský, P. Harcuba, K. Václavová, K. Horváth and M. Landa:

J Mech Behav Biomed. 71 (2017) 329336.7) T. Saito et al.: Science 300 (2003) 464467.8) M. Tane, T. Nakano, S. Kuramoto, M. Hara, M. Niinomi, N. Takesue,

T. Yano and H. Nakajima: Acta Mater. 59 (2011) 69756988.9) J.I. Kim, H.Y. Kim, H. Hosoda and S. Miyazaki: Mater. Trans. 46

(2005) 852857.10) M. Tahara, H.Y. Kim, T. Inamura, H. Hosoda and S. Miyazaki: Acta

Mater. 59 (2011) 62086218.11) H.Y. Kim and S. Miyazaki: Mater. Trans. 56 (2015) 625634.12) F. Geng, M. Niinomi and M. Nakai: Mater. Sci. Eng. A 528 (2011)

54355445.13) H. Liu, M. Niinomi, M. Nakai, X. Cong, K. Cho, C.J. Boehlert and V.

Khademi: Metall. Mater. Trans. A 48 (2017) 139149.14) Q. Li, M. Niinomi, M. Nakai, Z.D. Cui, S.L. Zhu and X.J. Yang: Mater.

Sci. Eng. A 536 (2012) 197206.15) M. Abdel-Hady, K. Hinoshita and M. Morinaga: Scr. Mater. 55 (2006)

477480.

Fig. 4 (a) XRD patterns and optical morphologies of (b) Ti38Nb0.24Oand (c) Ti38Nb0.46O alloys after tensile tests.

Q. Li et al.860

https://doi.org/10.1016/j.mser.2014.10.001https://doi.org/10.1016/j.actbio.2012.06.037https://doi.org/10.1016/j.actbio.2012.06.037https://doi.org/10.1016/j.matdes.2016.09.026https://doi.org/10.1016/j.matdes.2016.09.026https://doi.org/10.1016/j.msea.2015.12.024https://doi.org/10.1016/j.jmbbm.2017.03.026https://doi.org/10.1126/science.1081957https://doi.org/10.1016/j.actamat.2011.07.050https://doi.org/10.2320/matertrans.46.852https://doi.org/10.2320/matertrans.46.852https://doi.org/10.1016/j.actamat.2011.06.015https://doi.org/10.1016/j.actamat.2011.06.015https://doi.org/10.2320/matertrans.M2014454https://doi.org/10.1016/j.msea.2011.03.064https://doi.org/10.1016/j.msea.2011.03.064https://doi.org/10.1007/s11661-016-3836-5https://doi.org/10.1016/j.msea.2011.12.103https://doi.org/10.1016/j.msea.2011.12.103https://doi.org/10.1016/j.scriptamat.2006.04.022https://doi.org/10.1016/j.scriptamat.2006.04.022