Mechanical properties of biomedical titanium alloys

Mechanical properties of biomedical titanium alloys

Materials Science and Engineering A243 (1998) 231 – 236 Mechanical properties of biomedical titanium alloys Mitsuo Niinomi * Department of Production...

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Materials Science and Engineering A243 (1998) 231 – 236

Mechanical properties of biomedical titanium alloys Mitsuo Niinomi * Department of Production Systems Engineering, Toyohashi Uni6ersity of Technology, 1 -1 Hibarigaoka, Tempaku-cho, Toyohashi 441, Japan

Abstract Titanium alloys are expected to be much more widely used for implant materials in the medical and dental fields because of their superior biocompatibility, corrosion resistance and specific strength compared with other metallic implant materials. Pure titanium and Ti–6Al–4V, in particular, Ti–6Al–4V ELI have been, however, mainly used for implant materials among various titanium alloys to date. V free alloys like Ti–6Al–7Nb and Ti – 5Al – 2.5Fe have been recently developed for biomedical use. More recently V and Al free alloys have been developed. Titanium alloys composed of non-toxic elements like Nb, Ta, Zr and so on with lower modulus have been started to be developed mainly in the USA. The b type alloys are now the main target for medical materials. The mechanical properties of the titanium alloys developed for implant materials to date are described in this paper. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Biomedical titanium alloys; Mechanical properties; Fracture characteristics; Fatigue characteristics

1. Introduction Pure titanium and titanium alloys are now the most attractive metallic materials for biomedical applications. Ti–6Al–4V has been a main biomedical titanium alloy for a long period. New types of alloys like Ti –6Al–7Nb [1] and Ti – 5Al – 2.5Fe [2] have been, however, recently developed because of the problem of toxicity of elements in the Ti – 6Al – 4V alloy and the development of the required performance of the alloy. Biomedical titanium alloys with much greater biocompatibility have been proposed and are currently under development [3]. They are mainly b type alloys composed of non-toxic elements. The b type alloys have greater biocompatibility because their moduli are much less than those of a + b type alloys like Ti–6Al–4V and so on. They are also able to gain greater strength and toughness balance compared with a +b type alloys. These titanium alloys are mainly used for substituting materials for hard tissues. Fracture of the alloys is, therefore, one of the big problems for their reliable use in the body. The fracture characteristics of the alloys are affected by changes in microstructure. * Tel.: +81 532 446706; fax: [email protected]

+81 532 446690; e-mail:

0921-5093/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S 0 9 2 1 - 5 0 9 3 ( 9 7 ) 0 0 8 0 6 - X

Therefore, their fracture characteristics, including tensile and fatigue characteristics should be clearly understood with respect to microstructures. The fracture characteristics in the simulated body environment should also be identified because the alloys are used as biomedical materials. The effect of living body environment on the mechanical properties is also very important to understand. The mechanical properties such as tensile characteristics, fracture toughness, fatigue characteristics and so on of various biomedical titanium alloys developed to date will be described in this paper in as much detail as possible. The effects of simulated body environment and living body environment on the mechanical properties will be also described in the paper.

2. Titanium alloys for implant and dental materials Titanium alloys developed for implant materials to date are listed in Table 1 [1–3,5–13]. Commercial pure titanium, Ti–6Al–4V and Ti–6Al–4V ELI have basically been developed for structural materials although they are still widely used as representative titanium alloys for implant materials. More recently, V free a+ b type alloys such as Ti–6Al–7Nb [1] and Ti–5Al–2.5Fe [2] have appeared as implant materials.

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In addition, V and Al free a +b type alloys composed of non-toxic elements like Ti – 15Sn – 4Nb–2Ta– 0.2Pd and Ti– 15Zr – 4Nb – 4Ta – 0.2Pd have been developed [4]. Low modulus alloys are nowadays desired because the moduli of alloys are required to be much more similar to that of bone. The b type alloys have been, therefore, developed or are developing mainly in the USA [3]. They are composed of nontoxic elements like Nb, Ta, Zr and so on. Pure titanium and Ti – 6Al – 4V type alloys are also the main implant materials in the dental field. The titanium alloys for dental implant materials are, however, the same as those for surgical implant materials. The alloys for other dental usage like crown, clasp and so on have somewhat different compositions compared with those for surgical implant materials except for Ti–6Al – 4V and Ti – 6Al – 7Nb as listed in Table 2 [14]. They are in general processed by casting.

The tensile properties of dental casting titanium alloys are listed in Table 2. The elongation of the alloys are fairly lower than those of wrought or forged implant alloys as shown in Table 3 and Fig. 1.

4. Modulus The moduli of elasticity of biomedical titanium alloys are shown in Fig. 2 although their values have been already shown in Table 3 [3,5,6,10,11,13]. The moduli of other metallic biomaterials like stainless steel and Co type alloy are round 206 and 240 GPa, respectively [6]. They are much greater than that of bone whose modulus of elasticity is generally between 17 and 28 GPa [6]. The moduli of elasticity of biomedical titanium alloys are much smaller than those of other metallic biomaterials. The moduli of recently developed b type alloys are between 55 to 85 GPa. They are much smaller than that of a and a+ b type biomedical titanium alloys. They are however greater than that of bone.

3. Tensile properties The tensile properties of titanium implant materials developed to date are listed in Table 3 [1 – 13]. The data of tensile yield stress and elongation in Table 3 are plotted in Fig. 1 with those of structural a, a+ b and b type titanium alloys. The data of the structural titanium alloys are shown as a scatter band in Fig. 1. The yield strength of biomedical titanium alloys are distributed nearly between 500 and 1000 MPa. The yield strength of pure titanium designated by a type biomedical titanium alloys is slightly lower than that of the structural one. The elongation of biomedical titanium alloys is distributed between  10 and 20%. Table 1 Titanium alloys for biomedical applications 1. Pure titanium (ASTM F67): Grade 1, 2, 3 and 4 2. Ti – 6Al – 4V ELI (Wrought: ASTM F136 and forged: ASTM F620): a+b type 3. Ti – 6Al – 4V (Casting: F1108): a+b type 4. Ti – 6Al – 7Nb (ASTM F1295): a+b type (Switzerland)a 5. Ti – 5Al – 2.5Fe (ISO/DIS 5832 – 10): b rich a+b type (Germany)a 6. Ti – 5Al – 3Mo – 4Zr: a+b type (Japan)a 7. Ti – 15Sn – 4Nb – 2Ta–0.2Pd: a+b type (Japan)a 8. Ti – 15Zr – 4Nb – 2Ta–0.2Pd: a+b type (Japan)a a

9. Ti–13Nb–13Zr: near b type (USA), low modulusa 10. Ti–12Mo–6Zr–2Fe: b type (USA), low modulusa 11. Ti–15Mo: b type (USA), low modulusa 12. Ti–16Nb–10Hf: b type (USA), low modulusa 13. Ti–15Mo–5Zr–3Al: b type (Japan), low modulus 14. Ti–15Mo–3Nb: b type (USA), low modulusa 15. Ti–35.3Nb–5.1Ta– 7.1Zr: b type (USA), low modulusa 16. Ti–29Nb–13Ta–4.6Zr: b type (Japan), low modulusa

Developed for biomedical applications.

5. Fatigue strength Fatigue strength of biomedical titanium alloy at 107 cycles are shown in Fig. 3 [8,10,13] with those of other metallic biomedical materials such as stainless steels; AISI 316 LVM and SUS 316L and Co type alloys; Co–Cr–Mo and Co–Ni–Cr–Mo. Fatigue strength of biomedical titanium alloys listed in Table 3 is from 265 to 816 MPa.

6. Fracture toughness Fracture toughness of biomedical titanium alloys are shown in Fig. 4 [4,6,13]. The fracture toughness of b type medical titanium alloys are similar to those of a+ b type ones.

7. Fatigue behavior in simulated body environment

7.1. Fatigue strength S–N curves of Ti–6Al–4V ELI with equiaxed and Widmansta¨tten a structure and annealed SUS 316L in air and Ringer’s solution obtained from rotating bending fatigue tests have been reported [15]. The rotating bending fatigue strength of Ti–6Al–4V ELI in air and Ringer’s solution are equivalent. The fatigue strength of SUS 316L is degraded in Ringer’s solution at relatively greater number of cycles to failure comparing with that in air. The concentration of oxygen in body liquid or muscle tissue except blood is rather

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Table 2 Titanium alloys for dental applications and their mechanical properties Alloy

Process

Tensile strength (Mpa)

Yield strength (Mpa)

Elongation (%)

Vickers hardness (Hv)

1. 2. 3. 4. 5. 6. 7.

Casting Casting Casting Casting Superplastic forming Casting Casting

874 880 703 976 954 933 470

669 659 — 847 729 817 —

6 5 2.1 5.1 10 7.1 8

318 261 — — 346

Ti – 20Cr – 0.2Si Ti – 25Pd – 5Cr Ti – 13Cu – 4.5Ni Ti – 6Al – 4V Ti – 6Al – 4V Ti – 6Al – 7Nb Ti – Ni

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Table 3 Mechanical properties of titanium alloys for biomedical applications Alloy

1. Pure Ti grade 1 2. Pure Ti grade2 3. Pure Ti grade 3 4. Pure Ti grade 4 5. Ti – 6Al – 4V ELI (mill Annealed) 6. Ti – 6Al – 4V (annealed) 7. Ti – 6Al – 7Nb 8. Ti – 5Al – 2.5Fe 9. Ti – 5Al – 1.5B

Tensile strength (UTS) (Mpa)

Yield strength (sy)

Elongation (%)

RA (%)

Modulus (GPa)

Type of alloy

240 345 450 550 860–965

170 275 380 485 795 – 875

24 20 18 15 10 – 15

30 30 30 25 25 – 47

102.7 102.7 103.4 104.1 101 – 110

a a a a a+b

895–930 900–1050 1020 925–1080

825 – 869 880 – 950 895 820 – 930

6 – 10 8.1 – 15 15 15 – 17.0

20 – 25 25 – 45 35 36 – 45

110 – 114 114 112 110

a+b a+b a+b a+b

10. Ti – 15Sn – 4Nb – 2Ta–0.2Pd (Annealed) (Aged)

860 1109

790 1020

21 10

64 39

89 103

11. Ti – 15Zr – 4Nb – 4Ta–0.2Pd (Annealed) (Aged)

715 919

693 806

28 18

67 72

94 99

836 – 908 100 – 1060

10 – 16 18 – 22

27 – 53 64 – 73

79 – 84 74 – 85

b b

544 736

21 10

82

78 81

b b

a+b

12. Ti – 13Nb – 13Zr (aged) 973–1037 13. TMZF (Ti – 12Mo–6Zr–2Fe) 1060–1100 (annealed) 14. Ti – 15Mo (annealed) 874 15. Tiadyne 1610 (aged) 851 16. Ti – 15Mo – 5Zr – 3Al (ST) (aged) 17. 21RX (annealed) (Ti– 15Mo – 2.8Nb – 0.2Si) 18. Ti – 35.3Nb – 5.1Ta–7.1Zr 19. Ti – 29Nb – 13Ta – 4.6Zr (aged)

b 852 1060–1100

838 1000 – 1060

25 18 – 22

48 64 – 73

80

979–999

945 – 987

16 – 18

60

83

b

596.7 911

547.1 864

19.0 13.2

68.0

55.0 80

b b

small. The fatigue strength of Ti – 5Al – 2.5Fe in Ringer’s solution with introducing N2 gas, by which the oxygen concentration in Ringer’s solution can be lowered, has been studied. The results are shown in Fig. 5 [15], with the data in the ordinary Ringer’s solution. The fatigue strength of Ti – 5Al–2.5Fe beyond 106 cycles is smaller in the Ringer’s solution with lower oxygen content than in the ordinary Ringer’s solution. On the other hand, the fatigue

strength of Ti–6Al–4V has been reported not to be degraded in living rabbit body [16] where oxygen concentration can be considered to be rather small compared with that in the air. These different trends will come from the difference in the stress conditions of fatigue testing methods. Ti–5Al–2.5Fe was fatigued under rotating bending conditions while Ti– 6Al–4V was fatigued under tension-tension conditions.

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men in the Ringer’s solution is longer (a greater number of cycles).

7.2. Fatigue crack propagation beha6ior The fatigue crack propagation rate of titanium alloys is apparently affected by simulated body environment [16,17]. The fatigue crack propagation rate of pure titanium in 0.9% NaCl solution is greater than that in dry air [16]. The same trend has been reported for T–L direction of Ti–6Al–4V rolled plate in 3% NaCl solution [17]. Fig. 1. Relationship between yield strength, sy, and elongation of biomedical titanium alloys shown with the data of structural titanium alloys.

8. Mechanical properties after implanted in living body

Titanium alloys have greater corrosion resistance because the titanium oxide film formed on the surface of alloys acts as an electrochemically passive film and inhibits negative ions from invading the matrix of the alloys. The fracture of the passive film is highly possible when the bending stress is loaded on the specimen, even if the specimen itself is not fractured. The fracture of the specimen by corrosion fatigue will be accelerated in the Ringer’s solution with lower oxygen content, where N2 gas is introduced because the fractured oxide film will not reform sufficiently. The corrosion fatigue will occur easily in such an environment when the immersing time of the speci-

The Vickers hardness changes on the specimen surfaces of implant materials Ti–5Al–2.5Fe and Ti– 6Al–4V ELI with equiaxed a structure and SUS 316L before and after implanting into the paravertebral muscle of living rabbit for about 11 months are shown in Fig. 6 [15]. The Vickers hardness of both titanium alloys are not changed before and after implanting. The Vickers hardness of SUS 316L after implanting is however increased compared with that before implanting. The surface hardened area thickness was found to be around 80 mm. The fracture toughness of Ti–5Al–2.5Fe and Ti– 6Al–4V ELI has been reported to be unchanged before and after implanting into living rabbits [15].

Fig. 2. Comparison of modulus of elasticity of biomedical titanium alloy.

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Fig. 3. Fatigue strength at 107 cycles of biomedical titanium alloy. Data without designation of rotating bending are those obtained from uniaxial fatigue tests.

Fig. 4. Fracture toughness of biomedical titanium alloy. KQ means invalid fracture toughness.

9. Summary The tensile strength of biomedical titanium alloys developed to date lies between 500 and 1000 MPa. The elongation lies between 10 and 20%. The moduli of elasticity of the low modulus b type biomedical titanium alloys developed to date is between 55 and 85 MPa.The rotating bending fatigue strength of the biomedical titanium alloy is degraded in simulated

body environment with rather lower oxygen concentration. The crack propagation rate of biomedical titanium alloys is grater in simulated body environments in air.The Vickers hardness of the specimen surfaces and fracture toughness of biomedical titanium alloys are not changed before and after implanting into the living rabbit while the Vickers hardness of the specimen surface of SUS 316L after implanting is greater than that before implanting.

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Fig. 5. S – N curves of Ti–5Al–2.5Fe with equiaxed a structure in air, Ringer’s solution and Ringer’s solution with N2 gas.

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