TITANIUM ALLOY

Abstract

The disclosure relates to a titanium alloy, in particular to be used for biocompatible implants, which contains no aluminum (Al), vanadium (V), cobalt (Co), chromium (Cr), nickel (Ni) and tin (Sn) and contains at least the following alloy components in wt % in addition to inevitable trace amounts of impurities contained in the alloy components or absorbed during the production: a) 0.2 to 1.5% oxygen (O), b) 0.1 to 1.5% iron (Fe), c) 0.01 to 2% carbon (C), d) the remainder being titanium (Ti).

Claims

1. A titanium alloy for use for biocompatible implants, which comprises, with exclusion of aluminum (Al), vanadium (V), cobalt (Co), chromium (Cr), nickel (Ni), and tin (Sn) as alloying elements, besides unavoidable trace amounts of impurities which are present in the alloying constituents or have been taken up during production, at least the following alloying constituents in wt %: a) 0.2 to 1.5% oxygen (O), b) 0.1 to 1.5% iron (Fe), c) 0.01 to 2% carbon (C), d) balance titanium (Ti).

2. The titanium alloy as claimed in claim 1, further comprising 0.4% oxygen (O).

3. The titanium alloy as claimed in claim 1, further comprising 0.5% iron (Fe).

4. The titanium alloy as claimed in claim 1, further comprising 0.08% carbon (C).

5. The titanium alloy as claimed in claim 1, further comprising less than 1% gold (Au).

6. The titanium alloy as claimed in claim 5, further comprising 0.1% gold (Au).

7. The titanium alloy as claimed in claim 1, further comprising less than 1% niobium (Nb).

8. The titanium alloy as claimed in claim 7, further comprising 0.1% niobium (Nb).

9. The titanium alloy as claimed in claim 1, further comprising less than 1% molybdenum (Mo).

10. The titanium alloy as claimed in claim 9, further comprising 0.1% molybdenum (Mo).

11. The titanium alloy as claimed in claim 1, further comprising less than 1% zirconium (Zr).

12. The titanium alloy as claimed in claim 11, further comprising 0.1% zirconium (Zr).

13. The titanium alloy as claimed in claim 1, further comprising less than 1.5%, nitrogen (N).

14. The titanium alloy as claimed in claim 13, further comprising 0.2% nitrogen (N).

15. The titanium alloy as claimed in claim 1, further comprising less than 0.5% silicon (Si).

16. The titanium alloy as claimed in claim 15, further comprising 0.05% silicon (Si).

17. The titanium alloy as claimed in claim 1, further comprising less than 0.2% hydrogen (H).

18. An implant, more particularly a prosthetic implant, at least partly comprising an alloy as claimed in claim 1.

19. The titanium alloy as claimed in claim 13, further comprising less than 0.4%, nitrogen (N).

Description

[0009] This object is achieved by means of a titanium alloy which, with exclusion of aluminum (Al), vanadium (V), nickel (Ni), chromium (Cr), cobalt (Co), and tin (Sn) as alloying elements, besides unavoidable trace amounts of impurities which are present in the alloying constituents or have been taken up during production, comprises the following alloying constituents in wt %: [0010] a) 0.2 to 1.5% oxygen (O), [0011] b) 0.1 to 1.5% iron (Fe), [0012] c) 0.01 to 2% carbon (C), [0013] d) balance titanium (Ti).

[0014] Although trace amounts of impurities can never be avoided, the increase in strength in the case of the pure titanium variety is achieved by using only alloying constituents that are already present in the human body.

[0015] Preferably only these constituents are used, and in that case, especially preferably, 0.4 wt % oxygen (O) and/or 0.5 wt % iron (Fe) and/or 0.08 wt % carbon (C).

[0016] In the titanium alloy of the invention, however, for the purpose of boosting strength, it is possible additionally to use alloying constituents which have no known adverse effects on the body, such as gold, molybdenum, niobium, silicon, and zirconium, for example.

[0017] The fraction of gold (Au) is preferably less than 1 wt % and especially preferably is 0.1 wt %.

[0018] Niobium (Nb) is preferably used additionally with a fraction of less than 1 wt %, and especially preferably with a fraction of 0.1 wt %.

[0019] Molybdenum (Mo) is preferably used additionally with a fraction of less than 1 wt % and especially with a fraction of 0.1 wt %.

[0020] Zirconium (Zr) is preferably used with a fraction of less than 1 wt % and especially with a fraction of 0.1 wt %.

[0021] Nitrogen (N) may additionally be used with a fraction of less than 1.5 wt % and especially with a fraction of less than 0.4 wt %, and in that case especially with a fraction of 0.2 wt %.

[0022] Silicon (Si) is preferably used with a fraction of less than 0.5 wt % and especially with a fraction of 0.05 wt %.

[0023] Hydrogen (H) as well can be used with a fraction of preferably less than 0.2 wt % in the alloy of the invention.

[0024] The elements may be present cumulatively in the alloy. Individual elements, however, may also be entirely absent, according to the strength required in the specific application scenario. Fundamentally, however, it is necessary to rule out the use of the elements aluminum (Al), vanadium (V), and tin (Sn), although here of course it cannot be out of the question for these elements, as impurities in other alloying constituents, to be unavoidable and therefore to have to be tolerated as trace element.

[0025] The alloy of the invention can be used preferably for an “intelligent implant with interlocking technology” of the kind described, for example, in EP 1 211 993 B1 or EP 1 143 867 B1. The alloy may also be used for producing material for bone screws and bone nails. The alloy, however, is not intended to be confined necessarily to the field of biocompatible implants, but may instead be employed wherever its strength is sufficient for the desired application.

[0026] A suitable starting material is CP titanium grade 4, whose approved maximum levels of accompanying elements are laid down in the specification ASTM F-67, which is valid for medical engineering. The table below shows one possible composition of the titanium alloy of the invention, the FIGURES being in wt %:

TABLE-US-00001 Material Ti O Fe C Au Nb Mo Zr N Si CP Ti balance 0.4 0.5 0.08 grade 4.sup.+ Gold-titanium balance 0.4 0.5 0.08 0.1 grade 4.sup.+ Gold-titanium balance 0.4 0.5 0.08 0.1 0.1 grade 4.sup.+ Nb Gold-titanium balance 0.4 0.5 0.08 0.1 0.1 0.1 variant 1 Gold-titanium balance 0.4 0.5 0.08 0.1 0.1 0.1 0.1 variant 2 Gold-titanium balance 0.4 0.5 0.08 0.1 0.1 0.1 0.1 0.2 variant 3 Gold-titanium balance 0.4 0.5 0.08 0.1 0.1 0.1 0.1 0.2 0.05 variant 4

[0027] To estimate the appropriate amounts of alloying elements for the planned use scenario, different binary alloys were investigated first of all. As well as the microstructure and the hardness, analysis focused in particular on the impact strength at room temperature of Ti—O (0.2 to 1.5), Ti—Fe (0.2 to 1.5), and Ti—Nb (0.1 to 2) (FIGURES in wt %).

[0028] It then emerged that the addition of gold produces, on the one hand, a further solid solution strengthening of the material and, on the other hand, caused a surprisingly grain-refining effect by precipitation of additional particles in the micrometer range, primarily at the grain boundaries. The reason that this is surprising is that the binary Ti—Au phase diagram predicts something different. This effect is probably due to the low solubility of gold in titanium.

[0029] Niobium likewise results in a slight additional solid solution strengthening.

[0030] On account of possible adverse effects on the human body, the first three alloys in the table above are preferred, despite the fact that the strength is somewhat lower than that of the gold-titanium variants 1 to 4.

[0031] The invention resorts to alloying elements which have hitherto been used only rarely, if at all, for titanium alloys. The success which occurred was therefore not predictable. Instead it is necessary to employ all mechanisms which may lead to strengthening, such as solid solution hardening, fine grain hardening or deformation strengthening, for example.

[0032] Alloy production on the laboratory scale took place in a plasma electric arc furnace, with trouble-free melting and casting. This was followed by solution annealing under inert gas (Ar 99.998), microstructure analysis, and a hardness test for estimation of the mechanical properties. For the alloy CP—Ti grade 4.sup.+, deformation tests (static: degree of deformation=0.9; dynamic: degree of deformation=0.3) were conducted, and showed that the titanium material of the invention is amenable to hot deformation, this being a precondition for its technical use. On account of the degree of deformation of around 0.3 in the dynamic deformation test, which is inherent in the instrumentation, it was not possible to achieve fine grain by recrystallization annealing. On the basis of additional solid solution strengthening and possibly by the formation of a two-phase titanium alloy, however, the strength of the further solution variants described above ought in any case to be greater than the strength of CP—Ti grade 4.sup.+.

[0033] The table below shows an example of alloy production:

TABLE-US-00002 wt % 300 g Master 300 g wt % Element [target] [target] alloys [actual] [actual] Titanium 99.02 300.000 CP titanium 300.003 99.02 (−0.48) grade 4 Oxygen 0.40 1.206 TiO.sub.2 0.301 0.40 (+0.10) (+0.301)  99.98% Iron 0.50 1.507  99.98% 0.936 0.50 (+0.31) (+0.935) Carbon 0.08 0.241 99.995% 0.211 0.08 (+0.07) (+0.211) Total 100.00 301.447 301.451 100.00

[0034] As a master alloy, CP—Ti grade 4 from Daido Steel (FJ2-FJ3, Heat No. TN831G) was used as rod material in a diameter of 8 mm. The chemical composition was taken from the corresponding analytical certificate. To increase the oxygen and carbon contents, corresponding powders (TiO.sub.2 and graphite) were weighed out and, in order to avoid blowing losses, were packed into a titanium foil which was placed between titanium rods. The titanium content of the titanium foil was 99.6% and was therefore somewhat above the master alloy used. The resultant slight deviations in chemical composition were disregarded. Since the weight of the titanium foil was only 2.22 g in the context of a total weight of 301.45 g, and since the chemical composition of the foil corresponded approximately to that of the CP—Ti grade 4 used, the disregard appears to be acceptable. Iron was added in granular form.

[0035] The table below shows the measured hardnesses (method: Vickers HV10/15) and the tensile strengths estimated from them. Shown for comparison are the alloys Ti—Al6-V4, Ti—Al6-V4 ELI, and also the metastable β-titanium alloy Ti—Mo15, in the solution-annealed and quenched state (LG) and also in the precipitation-hardened state (AG).

TABLE-US-00003 Material HV10/15 Rm/MPa CP-Ti grade 4 221  570 CP-Ti grade 4.sup.+ 274  760 Gold-titanium grade 4.sup.+ 295  840 Gold-titanium grade 4.sup.+ Nb 300  860 Ti-A16-V4 290-340*.sup.) 820-1000*.sup.) Ti-A16-V4 ELI 285-330*.sup.) 800-960*.sup.) Ti-Mo15 LG 215  550 Ti-Mo15 AG 429 1320 *.sup.)according to microstructure condition

[0036] The hardness of the inventive pure titanium variants CP—Ti grade and gold-titanium grade 4.sup.+ is higher by approximately 20% than the hardness of the hardest pure titanium variety CP—Ti grade 4 and only approximately 10% below or at the lower limit of the hardness of the titanium alloys which have primarily been used to date, namely Ti—Al6-V4 and Ti—Al6-V4 ELI.

[0037] The table below shows the effect of the deformation temperature (deformation method: rotary kneading, degree of deformation 0.3) and of subsequent recrystallization annealing on the hardness of the CP—Ti grade 4.sup.+ material. Five impressions were made (Pos. 1 to 5).

TABLE-US-00004 Material Pos. 1 Pos. 2 Pos. 3 Pos. 4 Pos. 5 HV10/15 CP-Ti grade 4.sup.+ 271 276 282 279 275 276 ± 4 600° C. CP-Ti grade 4.sup.+ 262 265 258 262 260 261 ± 3 600° C. RK CP-Ti grade 4.sup.+ 265 267 263 263 263 264 ± 2 800° C. CP-Ti grade 4.sup.+ 248 257 258 263 262 258 ± 6 800° C. RK CP-Ti grade 4.sup.+ 272 270 270 276 279 273 ± 4 900° C. CP-Ti grade 4.sup.+ 262 262 265 254 281 265 ± 9 900° C. RK

[0038] The single FIGURE shows a flow point diagram. The plot is of the quasi-static flow curves of the CP—Ti grade 4.sup.+ alloy as a function of the temperature. When a degree of deformation of 0.9 was reached, the test was discontinued. The samples did not fracture. From the flow point diagram it is clearly evident that the CP—Ti grade 4.sup.+ alloy investigated is forgeable.