Effect of Prestrain on Martensitic Transformation in a Ti46.4Ni47.6Nb6.0Superelastic Alloy and its Application to Medical Stents

T. Takagi,1 Y. Sutou,2 R. Kainuma,1,3 K. Yamauchi,2 K. Ishida3

1 Management of Science and Technology, Graduate School of Engineering, Tohoku University, Aoba-yama 04,Sendai 980-8579, Japan

2 Tohoku University Biomedical Engineering Research Organization, 2–1 Seiryo-machi, Aoba-ku, Sendai 980-8575, Japan

3 Department of Materials Science, Graduate School of Engineering, Tohoku University, Aoba-yama 02,Sendai 980-8579, Japan

Received 2 March 2005; revised 26 March 2005; accepted 6 April 2005Published online 12 October 2005 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jbm.b.30415

Abstract: The effect of applied strain on martensitic transformation in a superelasticTi46.4Ni47.6Nb6.0 alloy at room temperature was investigated by tensile tests, differentialscanning calorimetry measurements, and X-ray diffraction. Reverse transformation starting(As) and finishing (Af) temperatures increased with the application of tensile-strain over 13%,the undeformed specimen showing As � �29.2°C and Af � 17.9°C, while the 13% pre-deformed alloy exhibited As � 37.1°C and Af � 40.2°C. Furthermore, the values of the As andAf for the predeformed alloy almost recovered to those of the undeformed alloy when heatedto about 42°C and then showed superelasticity again at room temperature. This characteristicis significant for application in sensors, actuators, and medical devices. Especially, medicalstents with such qualities show promise as a new class of self-expandable stents with bothexcellent mountability and deliverability. © 2005 Wiley Periodicals, Inc. J Biomed Mater Res Part B:Appl Biomater 76B: 179–183, 2006

Keywords: stent; shape memory alloy; Ti-Ni-Nb alloy; superelasticity; martensitic trans-formation

INTRODUCTION

Shape-memory alloys (SMAs) exhibiting unique propertiesof shape-memory effect (SME) and superelasticity (SE) havebeen used in a wide range of devices such as orthodontic archwire, eyeglass frames, and antennas for cellular phones.1,2

Recently, SMAs showing SE are drawing strong attention asmaterials for medical devices such as stents, guidewires forcatheters, and so forth.1,3–5

Stents are devices used as scaffolds or braces in biologicallumens, most commonly, diseased arteries.6 In recent years, twotypes of metallic stents have been utilized: balloon-expandable(BX) and self-expandable (SX) types.7 A comparison betweenthe BX stents and SX stents is exhibited in Table I.8 The mainmaterials of BX and SX stents are stainless steel and Ti–Nisuperelastic alloy, respectively. BX stents are superior in mount-ability and radical force because of their high Young’s modulus.However, they may fracture because of the accumulation of

plastic strain. On the other hand, Ti–Ni superelastic stents aremore flexible in native vessels and rarely injure the arteries, sowhen employed in carotid arteries near the surface of the body,they are able to sustain deformation by external pressure. How-ever, Ti–Ni SX stents have drawbacks with regard to deliver-ability and mountability.8,9 Although delivery sheaths with ahigher strength than the recovery force of the SX stents areneeded for delivery to a specific site, such a hard sheath hindersthe deliverability in narrow or complex vessels. It is also difficultto accurately position the SX stent at the desired site by pushingit out of the sheath. Hence, a new class of stents having theadvantages of both BX and SX stents is required.

Ti–Ni–Nb ternary SMAs are promising candidates for newSMA stents. It is known that the Ti–Ni–Nb alloy systemshows wide thermal and stress hysteresis on martensitic trans-formation,10–14 where the thermal and stress hystereses cor-respond to the shape memory effect (SME) and SE modes,15

respectively.In the present study, the effect of applied strain on the

martensitic transformation temperatures and SE properties inTi–Ni–Nb ternary alloys was examined. Based on the results,a promising Ti–Ni–Nb SX stent with both excellent deliver-ability and mountability is proposed, in which the hysteresis

Correspondence to: T. Takagi (e-mail: takamitsu.takagi@most.tohoku.ac.jp)Contract grant sponsor: Encouraging Development Strategic Research Centers

Program, the Special Coordination Funds for Promoting Science and Technology

© 2005 Wiley Periodicals, Inc.

179

of the martensitic transformation is precisely controlled bypredeformation.

EXPERIMENTAL PROCEDURES

In this study, a Ti46.4Ni47.6Nb6.0 30% cold-drawn wire with adiameter of about 1 mm, supplied by NEC/TOKIN Corpora-tion, was used. The wire for tensile-testing was 80 mm inlength and aged at 400°C for 3.6 ks in an evacuated quartztube to obtain SE at room temperature, and then subjected toair-cooling.

Tensile tests were carried out with a Tensilon machine,ORIENTEC RTA-1T type, at room temperature with a tensilestrain rate of 4.17 � 10�2 s�1 and a gauge length of 50 mm.The martensitic transformation temperatures were determinedby differential scanning calorimetry (DSC), Seiko Instru-ments DSC6200 type. The crystal structure was identified byX-ray diffraction (XRD), Philips X’Pert type, employing theCu K� radiation, and the diffraction intensity was recorded inthe range of 20–100° (2�) with a scanning rate of 0.02° s�1.

RESULTS AND DISCUSSION

Figure 1 shows the stress–strain curves of the Ti46.4Ni47.6Nb6.0

wire resulting from cyclic tensile deformation at 20°C afterheat-treatment at 400°C for 1 h. The wire was loaded in tensileup to a strain of � t

l (2%) and then unloaded at the first cyclewhere � r

l is the residual strain. It was then reloaded up to a strainof � t

2 (4%), unloaded in the second cycle, and so forth. This agedalloy, which contained a small number of �-Nb particles in thematrix, as will be mentioned later by X-ray analysis, exhibitedSE because of the growth of stress-induced martensite (SIM).The obtained maximum recovery strain, including the SE andelastic strains, was about 8% in the fifth cycle, where the residualstrain was around 2%. It should be noted that the stress hyster-esis of about 400 MPa in the SE is larger than that of Ti–Nibinary alloys, which is around 200 MPa. These phenomenaindicate that the SIM is retained at room temperature because ofthe increment of the reverse transformation temperature.12

Figure 2 shows the DSC curves for (a) the wire specimenwithout predeformation, (b) the 1st heating of the tensile-deformed wire up to a strain of 13%, and (c) the 1st coolingand the 2nd heating after (b).

Figure 1. Cyclic stress–strain curves at 20°C obtained from a wirespecimen aged at 400°C for 3.6 ks.

Figure 2. DSC curves of Ti46.4Ni47.6Nb6.0 alloy (a) undeformed, (b) the1st heating for the wire tensile-deformed up to a strain of 13%, and (c)the 1st cooling and 2nd heating curves after the 1st heating (b).

TABLE I. Comparisons of Balon-Expandable and Self-Expanding Stents

Characteristics Ballon-Expandable Self-Expandable

Common materials Stainless steel NitinolDeployment By ballon inflation By retracting sheath and self-expandingAdvantages � Rigidity � Flexibility

� Mountability � Deliverability in veinDisadvantages � Deliverability � Mountability

� Potential of crush by the charge of plastic strain � Necessity of bulky sheath at the deliveryCommon uses Coronary arteries, renal arteries, iliac arteries Carotid arteries, fernoropopliteal arteries, some

usage in iliac arteries

180 TAKAGI ET AL.

The DSC curves show the exothermic and endothermicpeaks caused by the forward and reverse martensitic trans-formations, respectively, where the forward transformationstarting (Ms) and finishing (Mf) temperatures, and the reversetransformation starting (As) and finishing (Af) temperaturesare defined as the temperatures at which the extrapolationlines of those peaks and the baseline cross, and the Mp and Ap

are defined as the temperatures at each peak. The martensitictransformation temperatures Ms, Mf, As, and Af for unde-formed wire are �50.6, �96.4, �29.2, and 17.9°C, respec-tively. On the other hand, the reverse transformation temper-atures As and Af in the 1st heating cycle, which are those forwire loaded up to 13% and then unloaded, are higher thanthose of undeformed wire and are 37.1 and 40.2°C, respec-tively. Those in the 2nd heating cycle are �23.6 and 22.1°C,respectively. It is noted that reverse transformation tempera-ture interval (TTI) (Af–As) in the 1st heating for predeformedwire is considerably smaller than that for the undeformedwire. Furthermore, it can be seen that the martensitic trans-formation temperatures in the 1st cooling and 2nd heating forpredeformed wire are almost the same as those for the unde-formed wire. The results of DSC measurements for the wirespredeformed to various strains are listed in Table II.

The variation of the reverse transformation temperatureswith the applied strain is shown in Figure 3. As and Af in the1st heating drastically increase when a strain of over 13% isapplied, while a prestrain less than about 8% does not bringabout an increase of the As and Af. Such increment of As andAf with TTI is caused by the relaxation of inner strain energystored by the predeformation.16 Further deformation over15% causes an increment of the TTIs in the 1st heating andan increase of As and Af in the 2nd heating, compared withthose of the 13% predeformed wire. This can be explained bythe introduction of plastic strain in the Ti–Ni matrix. It issuggested that in the deformation process after the SIMtransformation mode indicated by the plateau region in thestress–strain curve, �-Nb particles in the matrix preferentiallyundergo plastic deformation because of the yield stress lowerthan that of the matrix.16 However, further applied strain over15% may introduce plastic strain not only in the �-Nb par-ticles, but also in the Ti–Ni matrix. The sharp peak in thereverse transformation of the predeformed Ti–Ni–Nb alloyshas also been reported by He et al.,17,18 although their alloysdid not show SE.

The phase conditions were identified by XRD at 20°C. Figure4 shows the X-ray profiles for the specimens (a) heat-treated at

Figure 3. Dependence on the amount of prestrain at reverse trans-formation temperatures.

Figure 4. X-ray profiles of Ti46.4Ni47.6Nb6.0 at 20°C. (a) Heat-treatedat 400°C, (b) predeformed 13%, and (c) heated up to 50°C afterpredeformation.

TABLE II. Transformation Temperatures (°C) of the Ti46.4Ni47.6Nb6.0 Alloy

Degree of appliedstrain Ms Ms (2nd) Mf Mf (2nd) As As (1st) As (2nd) Af Af(1

st) Af(2nd)

Non �50.6 / �96.4 / �29.2 / / 17.9 / /8% / �50.4 / �95.3 / / �30.3 / / 17.413% / �43.5 / - / 37.1 �23.6 / 40.2 22.115% / �38.9 / - / 63.5 �22.9 / 65.7 26.018% / �21.4 / �84.2 / 78.2 �5.5 / 84.4 40.4

181EFFECT OF APPLIED STRAIN ON MARTENSITIC TRANSFORMATION IN A SUPERELASTIC ALLOY

400°C, (b) predeformed 13%, and (c) heated up to 50°C afterpredeformation. The specimen aged at 400°C consists of parentB2, �-Nb, and P-phases, as shown in Figure 4(a), where “P”means the (Ni, Nb)3Ti precipitation phase with h.c.p. structure.19

By the predeformation of 13%, the B2 phase transforms to themartensite M phase [Figure 4(b)] and the martensite phasetransforms the B2 phase again by heating to 50°C above Af (1st)as shown in [Figure 4(c)]. It is interesting to note that the peakwidth of (110) �-Nb in Figure 4(a) increases with the predefor-mation as shown in Figures 4(b,c), because of the plastic defor-mation of the Nb particles.

Figure 5 shows the stress–strain curves at room tempera-ture for the Ti46.4Ni47.6Nb6.0 wire (a) after loading up to thestrain of 13% and then (b) heated up to 42°C, which is higherthan Af (1st). It can be confirmed that the wire after loadingup to 13% is still in the martensite phase state at roomtemperature and that the wire does not show any SE. On theother hand, the wire reverse-transformed by heating to 42°CFigure 5. Stress–strain curves at room temperature for the

Ti46.4Ni47.6Nb6.0 wire (a) loaded up to the strain of 13% and thenunloaded (b) after reverse-transformation.

Figure 6. (a) The conventional stenting method with the Ti–Ni SX stent. (b) The concept of stentingwith the present Ti46.4Ni47.6Nb6.0 alloy

182 TAKAGI ET AL.

shows SE, where the stress induced transformation stress,which means that the plateau stress in the loading in Figure 5decreases because of the predeformation. This is due to thetraining effect.20

From these results, the Ti46.4Ni47.6Nb6.0 alloy is expected tobe a promising material for application to SX stents to achievehigh deliverability and mountability. Figures 6(a,b) demonstratethe conventional stenting method of the Ti–Ni SX stent and theconcept of the developed stenting by the Ti46.4Ni47.6Nb6.0 alloy.The Ti–Ni SX stent automatically expands when the stent ispushed out of delivery sheath, as shown in Figure 6(a). Thepositioning of such a stent to a desired site is difficult and theblood vessels are easily injured by the rough expanding. On theother hand, at a body temperature of around 37°C, the Ti–Ni–Nbstent with the desired applied prestrain maintains a martensitestate and possesses high conformability. Therefore, safe deliveryand mountability are expected to be obtained as well as in theBX stents. When the stent is heated up to a temperature above itsAf (1st) temperature after expansion with a balloon, the stentrecovers the SE property at body temperature, where expansionof the balloon is not always necessary because the Ti–Ni–Nbstent thermally self-expands by heating. The presentTi46.4Ni47.6Nb6.0 alloy with an Af (1st) of 40.2°C after a prestrainof 13% may be one of the most suitable materials to obtain suchunique characteristics, since the SE can be reattained by heatingup to 42°C, a temperature safe for the normal organism. Aspossible, methods to heat the stents pouring of a warm physio-logical salt solution into the balloon and the application of a highfrequency magnetic field to the stent21 are suggested.

The present Ti–Ni–Nb SX stents show great promise as anew type of SX stent having high deliverability and mount-ability. However, further investigations on methods of appli-cation of prestrain to the stent, stent design, and biocompat-ibility are required.

CONCLUSIONS

The SME and SE properties of a Ti46.4Ni47.6Nb6.0 alloy heat-treated at 400°C were investigated by tensile tests, DSC mea-surements, and XRD. The following conclusions were obtained:

1. As and Af in the 1st heating drastically increased by theapplication of strain over 13%, while a prestrain less than8% did not bring about an increase of the As and Af inTi46.4Ni47.6Nb6.0 alloy wire.

2. The Ti46.4Ni47.6Nb6.0 wire after 13% predeformationshowed As � 37.1°C and Af � 40.2°C in the 1st heating,and the reverse transformation temperature could be con-trolled by the amount of prestrain.

3. This wire showed SE after 13% predeformation followedby the reverse transformation.

4. The Ti46.4Ni47.6Nb6.0 alloy is a promising material for newSX stents possessing excellent mountability and deliver-ability.

The authors express their thanks to NEC/TOKIN Corporationand its staff for preparing the Ti46.4Ni47.6Nb6.0 alloy wire, as well asfor their helpful advice.

REFERENCES

1. Otsuka K, Ren X. Recent developments in the research of shapememory alloys. Intermetallics 1999;7:511–528.

2. Melton KN. General application of SMA’s and smart materials.In: Otsuka K, Wayman CM, editors. Shape Memory Materials.Cambridge: Cambridge University Press; 1998. p 220–239.

3. Miyazaki S. Medical and dental applications of shape memoryalloys. In: Otsuka K, Wayman CM, editors. Shape Memory Ma-terials. Cambridge: Cambridge University Press; 1998. p 267–281.

4. Duerig TW, Pelton AR, Stockel D. An overview of nitinolmedical applications. Mater Sci Eng A 1999;273–275:149–160.

5. Stice J. The use of superelasticity in guidewires and arthro-scopic instruction. In: Duerig TW, Melton KN, Stockel D,Wayman CM, editors. Engineering Aspects of Shape MemoryAlloys. London: Butterworth-Heinemann; 1990. p 483–487.

6. Duerig TW, Pelton AR. An overview of superelastic stentdesign. Mater Sci Forum 2002;394/395:1–8.

7. Duerig TW, Pelton AR, Stockel D. The utility of superelasticityin medicine. Biomed Mater Eng 1996;6:255–266.

8. Stenting in Peripheral Vascular Disease. Medtech Insight 2004;6:246–254.

9. Nobuyoshi M. IVR: interventional radiology (in Japanese).1998;13:53–56.

10. Melton KN, Simpson J, Duerig TW. A new hysteresis NiTibased shape memory alloy and its applications. Proceedings ofInternational Conference on Martensitic Transformations (ICO-MAT-86), Japan, 1986. p 1053–1058.

11. Melton KN, Proft JL, Duerig TW. Wide hysteresis Shape Mem-ory Alloys based on the Ti-Ni-Nb System. Proceedings of MRSInternational Meeting on Advanced Materials, 1988.

12. Zhao LC, Duerig TW, Justi S, Melton KN, Proft JL, Yu W,Wayman CM. The study of niobium-rich precipitations in aTi-Ni-Nb shape memory alloy. Scripta Metall Mater 1990;24:221–226.

13. Zhang CS, Zhao LC, Duerig TW, Wayman CM. Effects of defor-mation on the transformation hysteresis and shape memory effectin a Ni47Ti44Nb9. Scripta Metall Mater 1990;24:1807–1812.

14. He XM, Rong LJ, Yan DS, Li YY. TiNiNb wide hysteresisshape memory alloy with low niobium content. Mater Sci EngA 2004;371:193–197.

15. Piao M, Miyazaki S, Otsuka. K. Characteristics of deformationand transformation in Ti44Ni47Nb9 shape memory alloy. MaterTrans JIM 1992;33:346–353.

16. Piao M, Otsuka K, Miyazaki S, Horikawa H. Mechanism of theAs temperature increase by pre-deformation in thermoelasticalloys. Mater Trans JIM 1993;34:919–929.

17. He XM, Rong LJ. DSC analysis of reverse martensitic trans-formation in deformed Ti-Ni-Nb shape memory alloy. ScriptaMater 2004;51:7–11.

18. He XM, Rong LJ, Yan DS, Jiang ZM, Li YY. Study of theNi41.3Ti38.7Nb20 wide transformation hysteresis shape-memoryalloy. Metall Mater Trans A 2004;35:2783–2788.

19. Di J, Wenxi L, Zhizhong D, Shenjun Y, Qi Z, Defa W, DaozhiL. Precipitations in a Ni-Ti-Nb alloy during annealing. ProgrNat Sci 1999;9:216–222.

20. Humbeek JV, Stalmans R. Training procedures. In: Otsuka K,Wayman CM, editors. Shape Memory Materials. Cambridge:Cambridge University Press; 1998. p 161–162.

21. Stauffer PR, Cetas TC, Fletcher AM, DeYoung DW, DewhirstMW, Oleson JR, Roemer RB. Observations on the use offerromagnetic implants for inducing hyperthermia. IEEE TransBiomed Eng 1984;31:76–90.

183EFFECT OF APPLIED STRAIN ON MARTENSITIC TRANSFORMATION IN A SUPERELASTIC ALLOY