Cobalt alloy for medical implants and stent comprising the alloy

Abstract

An embodiment of the invention relates to a cobalt-based alloy, which due to the composition exhibits twinning as the dominating deformation mechanism: Cr: 13.0 to 30.0% by weight Mn: 2.0 to 10.0% by weight W: 2.0 to 18.0% by weight Fe: 5.0 to 15.0% by weight C: 0.002 to 0.5% by weight N: 0 to 0.2% by weight Si: 0 to 2.0% by weight Ni: 0 to 5.0% by weight
wherein the aforementioned alloying components and manufacturing-related impurities add up to 100% by weight, and the following restrictions according to formulas (1) and (2) apply to the contents of nitrogen and carbon, and the following restrictions according to formula (3) apply to the contents of oxygen, phosphorus and sulfur:
0.003%≦C+N≦0.5% weight  (1)
N/C(wt. %)≦1.00 for 0.07%<C<0.15% (weight)  (2)
O+P+S<0.10% weight  (3).

Claims

1. A cobalt alloy comprising a tensile strength of greater than 900 MPa and having the following composition: Cr: 13.0 to 30.0% by weight Mn: 2.0 to 10.0% by weight W: 2.0 to 18.0% by weight Fe: 5.0 to 15.0% by weight C: 0.002 to 0.5% by weight N: 0 to 0.2% by weight Si: 0 to 2.0% by weight Ni: 2.0 to 3.5% by weight Co: balance, wherein the aforementioned alloying components and manufacturing-related impurities add up to 100% by weight, and the following restrictions according to formulas (1) and (2) apply to the contents of nitrogen and carbon, and the following restrictions according to formula (3) apply to the contents of oxygen, phosphorus and sulfur:
0.003%≦C+N≦0.5% (by weight)  (1)
N/C(wt. %)≦1.00 for 0.07%<C<0.15% (by weight)  (2)
O+P+S<0.10% (by weight)  (3), wherein the alloy is free from Mo.

2. The alloy according to claim 1, wherein a content of Cr in the alloy is 17.0 to 25.0% by weight.

3. The alloy according to claim 1, wherein a content of Mn in the alloy is 4.0 to 10.0% by weight.

4. The alloy according to claim 1, wherein a content of W in the alloy is 9.0 to 18.0% by weight.

5. The alloy according to claim 1, wherein a content of Fe in the alloy is 9.5 to 10.5% by weight.

6. The alloy according to claim 1, wherein a content of Si in the alloy is less than 0.7% by weight.

7. The alloy according to claim 1, wherein a content of C in the alloy is 0.002 to 0.15% by weight.

8. The alloy according to claim 1, wherein a content of C in the alloy is 0.07 to 0.15% by weight and N/C (wt. %) ranges between 0.25 and 1.

9. The alloy according to claim 1, wherein a content of Ni in the alloy is 2.5 to 3.5% by weight.

10. The alloy according to claim 1, wherein a content of Ni in the alloy is 2.5 to 3.5% by weight, and the total weight % of C and N together is between 0.003 and 0.3.

11. The alloy according to claim 1, wherein the total weight % of C and N together is between 0.003 and 0.1.

12. The alloy according to claim 1 wherein Cr is present in a weight % of between 19.0 and 21.0.

13. The alloy according to claim 1 wherein W is present in a weight % of between 14.5% to 15.5%.

14. The alloy according to claim 1 wherein the Si is present in a weight % between 0.05 to 0.5%.

15. The alloy according to claim 1 wherein the alloy is free of Ti, Ta, Nb and Al.

16. The alloy according to claim 1 wherein the content of O, S and P in alloy is below 0.07% weight in total.

17. A cobalt alloy, having the following composition: Cr: 13.0 to 30.0% by weight Mn: 2.0 to 10.0% by weight W: 2.0 to 18.0% by weight Fe: 5.0 to 15.0% by weight C: 0.002 to 0.5% by weight N: 0 to 0.2% by weight Si: 0 to 2.0% by weight Ni: 2.0 to 3.5% by weight Co: balance, wherein the aforementioned alloying components and manufacturing-related impurities add up to 100% by weight, and the following restrictions according to formulas (1) and (2) apply to the contents of nitrogen and carbon, and the following restrictions according to formula (3) apply to the contents of oxygen, phosphorus and sulfur:
0.003%≦C+N≦0.5% (by weight)  (1)
N/C(wt. %)≦1.00 for 0.07%<C<0.15% (by weight)  (2)
O+P+S<0.10% (by weight)  (3), wherein the alloy is free from Mo; and wherein the alloy has a stacking fault energy between 20 and 30 mJ/m.sup.2.

18. A stent comprising a filigree support structure made entirely or partially of a cobalt alloy comprising a tensile strength of greater than 900 MPa and having the following composition: Cr: 13.0 to 30.0% by weight Mn: 2.0 to 10.0% by weight W: 2.0 to 18.0% by weight Fe: 5.0 to 15.0% by weight C: 0.002 to 0.5% by weight N: 0 to 0.2% by weight Si: 0 to 2.0% by weight Ni: 0 to 3.5% by weight Co: balance, wherein the aforementioned alloying components and manufacturing-related impurities add up to 100% by weight, and the following restrictions according to formulas (1) and (2) apply to the contents of nitrogen and carbon, and the following restrictions according to formula (3) apply to the contents of oxygen, phosphorus and sulfur:
0.003%≦C+N≦0.5% (wt.)  (1)
N/C(wt. %)≦1.00 for 0.07%<C<0.15% (wt.)  (2)
O+P+S<0.10% (wt.)  (3) wherein the alloy is free from Mo.

19. A stent as defined by claim 18, wherein the stent further includes a generally tubular shaped base body that is made entirely of the alloy.

Description

DETAILED DESCRIPTION

(1) Prior to the present invention, the problems described above within the present art had not been solved. The alloy according to the invention solves many of these problems by alloying the cobalt base with matching contents of corresponding suitable alloying elements, which stabilize the austenite, have high solubility and lower the stacking fault energy (for example W, Mo) or raise it (for example C, N, Ni). Some alloy embodiments achieve a stacking fault energy between 15 and 50 mJ/m.sup.2, and some others between 20 and 30 mJ/m.sup.2.

(2) Elements used in cobalt alloys can be divided into two groups in terms of the effect thereof on the transition temperature from fcc to hexagonal close-packed (hcp), which is to say the effect thereof on the stabilization of the austenitic state. For example, Al, B, C, Cu, Fe, Mn, Nb, Ni, Sn, Ti and Zr lower the transition temperature from fcc to hcp and stabilize the austenitic state, while Sb, As, Cr, Ge, Ir, Mo, Os, Pt, Re, Rh, Ru, Si, Ta and W raise the transition temperature.

(3) The elements C, N, Mn and optionally Ni are thus used to stabilize the austenitic state. In at least some embodiments, one or more of these is present, and in some embodiments each is present.

(4) One embodiment of an alloy according to the invention contains C and N in total up to a maximum of 0.5% by weight and at least 0.003% by weight. The sum preferably ranges between 0.003 and 0.3% by weight, and more particularly between 0.003 and 0.1% by weight. Other concentrations may be used in other embodiments.

(5) Carbon has an austenite-stabilizing effect and moreover is extremely effective at raising the stacking fault energy. Carbon additionally increases the strength by forming carbides. The carbon content should therefore not drop below 0.002% by weight in many embodiments. On the other hand, care must be taken to ensure that higher carbon contents do not lead to embrittlement and/or a decrease in the corrosion resistance due to excess carbide formation. It has been discovered that a useful balance between these competing parameters can be achieved by maintaining the carbon content below 0.5% by weight. The carbon content in many embodiments of the alloy is thus 0.002 to 0.5% by weight, preferably 0.002% to 0.15% by weight, and still more preferably 0.002 to 0.07% by weight. Other concentrations may be used in other embodiments.

(6) Similarly, nitrogen also stabilizes the austenite, raises the stacking fault energy and increases the strength as well as hardness by forming nitride. In addition, nitrogen can prevent the formation of carbides if higher carbon contents are present. Depending on the carbon content, the content of nitrogen in many embodiments of the alloy can thus amount up to 0.2% by weight, preferably 0.15% by weight, and still more preferably 0.05% by weight. Nitrogen should be added at a ratio of 0.25 to 1, especially if the carbon content is between 0.07 and 0.15% by weight. Other concentrations may be used in other embodiments.

(7) Nickel raises the stacking fault energy and thus is favorable in terms of forming the desired TWIP effect. For biocompatibility reasons, a content of Ni in many embodiments of the alloy must not exceed 5.0% by weight and preferably ranges between 2.0 and 5.0% by weight, and more particularly between 2.5 and 3.5% by weight. Other concentrations may be used in other embodiments.

(8) Iron likewise stabilizes the austenite in cobalt starting at a content of approximately 5% by weight. Iron may be replaced completely or partially with manganese for further solid solution hardening. A content of Fe in many alloy embodiments is 5.0 to 15.0% by weight, and more particularly 9.5 to 10.5% by weight. Other concentrations may be used in other embodiments.

(9) A content of Mn in many embodiments of the alloy is preferably 4.0 to 10.0% by weight, and more particularly 4.5 to 5.5% by weight. Other concentrations may be used in other embodiments.

(10) Moreover, it is preferred if a content of Cr in the example alloy is 17.0 to 25.0% by weight, and more particularly 19.0 to 21.0% by weight. Chromium in solid solution increases the tensile strength. However, chromium also plays a key role in the corrosion and oxidation resistance. A high content of chromium means high corrosion resistance. The alloys according to the invention thus have high resistance to local corrosion, referred to as pitting.

(11) Tungsten and molybdenum have approximately the same effect. Both elements increase the strength and improve the corrosion protection, but also counteract the stabilization of the austenite and additionally lower the stacking fault energy. In addition to solid solution hardening and the increase in friction resistance due to tungsten, the increased material density of tungsten especially when used as a vascular support is advantageous, because good radiopacity is achieved. It is therefore particularly preferred if a content of W in the alloy is 9.0 to 18.0% by weight, and more particularly 14.5 to 15.5% by weight. Other concentrations may be used in other embodiments.

(12) Molybdenum counteracts the stabilization of the austenite and, together with cobalt, forms brittle intermetallic phases, which lower the ductility. Many embodiments of the alloy are therefore free of molybdenum.

(13) A content of Si in the example alloy is preferably less than 0.7% by weight, and more particularly the content is 0.05 to 0.5% by weight. Other concentrations may be used in other embodiments.

(14) Many alloy embodiments are additionally largely free of Ti, Ta, Nb and Al, and at least some are completely free of these materials.

(15) Impurities, notably O, S and P, may reduce the ductility, both in the form of oxides, sulfides and phosphides (Fe.sub.3P) and in solid solution. The content in each case should therefore be below 0.5% by weight. In total, the content of O, S and P should therefore be below 0.10% by weight, and more preferably below 0.07% by weight.

(16) The invention further relates to the use of the aforementioned cobalt alloy for producing a stent.

(17) The alloys can be produced analogously to the customary production methods for cobalt-based alloys.

(18) It will also be appreciated that embodiments of the invention include stents and other implants. Stents of the invention may include features such as a generally tubular base body. Stents embodiments may include a filigree supporting structure comprising metal struts, which is initially present in compressed form for introduction in the body and is expanded at the site of the application. The stents have a peripheral tubular or cylindrical wall with sufficient load-bearing capacity to hold the constricted vessel open to the desired extent, and a tubular base body through which blood continues to flow without impairment. The peripheral wall is generally formed by a lattice-like supporting structure, which allows the stent to be introduced in a compressed state, in which it has a small outside diameter, all the way to the stenosis to be treated in the particular vessel and to be expanded there, for example by way of a balloon catheter, so far until the vessel has the desired, enlarged inside diameter. One or more coatings may be provided, for example, to carry and elute a drug. Some or all metal elements of stents of the invention may be made of an alloy of the invention. Various features and elements of such stents are generally known (other than the alloy construction), and need not be discussed or illustrated herein for sake of brevity.

Exemplary Embodiment 1

(19)
Co-20Cr-15W-10Fe-5Mn-3Ni-0.05C

(20) Mechanical properties in the recrystallized state after annealing at approximately 1200° C. with subsequent water cooling:
Rp0.2=500-550 MPa
Rm=1000-1050 MPa
A>60%

Exemplary Embodiment 2

(21)
Co-22Cr-14W-8Fe-8Mn-0.15Si-0.15N-0.07C

(22) Mechanical properties in the recrystallized state after annealing at approximately 1200° C. with subsequent water cooling:
Rp0.2=520-570 MPa
Rm=1100-1150 MPa
A>50%

(23) It will be apparent to those skilled in the art that numerous modifications and variations of the described examples and embodiments are possible in light of the above teaching. The disclosed examples and embodiments are presented for purposes of illustration only. Other alternate embodiments may include some or all of the features disclosed herein. Therefore, it is the intent to cover all such modifications and alternate embodiments as may come within the true scope of this invention.