WO2011141553A2

WO2011141553A2 - High ductility superelastic material

Info

Publication number: WO2011141553A2
Application number: PCT/EP2011/057708
Authority: WO
Inventors: Benoît GRUEZ
Original assignee: Minitubes Sa
Priority date: 2010-05-12
Filing date: 2011-05-12
Publication date: 2011-11-17
Also published as: FR2960003A1; WO2011141553A3

Abstract

The invention concerns a superelastic shape memory alloy of Ni and Ti, having an elongation of at least 30%.Said alloy preferably has a UTS less than 100 MPa.

Description

HIGH DUCTILITY SUPERELASTIC MATERIAL

DESCRIPTION

TECHNICAL FIELD AND PRIOR ART The present invention concerns materials having high ductility and superelastic properties. Typical example for such materials are shape memory alloys, for example Ni-Ti alloys, which are used for many different applications.

There is a well known tradeoff between superelastic properties and ductility in such material. The material is usually processed to optimize the superelastic properties which induce an elongation in the 10-20% range, a limitation to device design.

One application of such material, and in particular alloys of Nickel and Titanium, concerns the realization of self expandable stents with design imitation due to low ductility.

There is therefore a need, for various applications, to find or make a material having increased ductility while maintaining superelastic properties .

SUMMARY OF THE INVENTION

The invention firstly concerns a material, having an elongation range of 30% to 60 % or of at least 30% or 40% or 50% or having a maximum value higher than 30% or 40% or 50% or 55% or 60% or between 30% and 40% or 50% or 55% or 60%. Preferably, said material has a UTS less than 1200 MPa or 1100 MPa. The invention further concerns a material, having a UTS less than 1200 MPa or 1100 MPa.

A material according to the invention can be a superelastic material, and more particulary a shape memory alloy.

The material can be a binary alloy of Ni and Ti or a ternary alloy comprising Ni, Ti and a further material, for example Cr or Cu or Fe or Nb or Pt .

A tube or a wire or a flat piece can be made of such an alloy material.

The invention also concerns a method to make an improved alloy material, in particular a superelastic material and notably a shape memory alloy, comprising grinding a surface of said material over about 1/10 mm, involving at least one sub-grinding step removing in a single step or pass a material thickness of at least 3/100 mm and preferably at least 5/100 mm.

A process according to the invention results in a material having high elongation (up to 30% or 40% or 50% or 60%) and preferably superelasticity or superelastic properties.

In a material and a process according to the invention, elongation is expanded from the traditional 10 to 20% range up to 30-60%, without compromising superelasticity, in particular in the case of a binary alloy of Ni and Ti or ternary alloy comprising Ni, Ti and an additive material for example selected from the above list.

In a process according to the invention high ductility material, in particular a tube a wire or tubes or wires, can be manufactured through a modification of grinding parameters, when compared with the standard process.

A process according to the invention can further comprise at least one further sub-grinding steps and/or at least one step of hot drawing said material and/or at least one step of annealing said material and/or at least one step of heat straightening said material and/or cooling said material during grinding and/or at least one step of cold drawing said material .

Devices may utilize the previously unreported mechanical properties of a material according to the invention, in particular by allowing high strain conditions without generating a material rupture .

For example a stent device, in particular of the self expandable type, can be made of a material according to the invention. But other devices can also be made from said material, for example part of a stent combined with a cardiac valve or a catheter or an endoscope shaft. More generally minimally invasive devices or parts of such devices can be made from a material according to the invention. Mechanical parts of devices, like spectacles sides, or a bridge for connecting spectacle lenses can also be made of, or comprise at least one part made of, a material according to the invention. BRIEF DESCRIPTION OF THE DRAWINGS

Figures 1A -IB is an example of a tube on which a process according to the invention was performed .

Figure 1C is a front view of a tube being processed;

Figure 2 is a set of stress-strain curves for samples subject to a standard process and samples subject to a process according to the invention ("high ductility") .

Figure 3 is a focus on the superlastic range of the curves shown on figure 2.

Figure 4A is a shape recovering curve per BFR method on samples subject to a standard process, with Af = 5°C and figure 4B is a shape recovering curve per BFR method on samples subject to a process according to the invention, with Af = -15°C.

Figure 5 is a machine for performing a process according to the invention.

Figure 6 are spectacles comprising parts made from a material according to the invention.

Figures 7A and 7B are stents structures made from a material according to the invention;

Figure 8 shows a cardiac valve combined with a stent according to the invention;

Figure 9 is an example of an endoscope comprising a shaft according to the invention;

Figure 10 is an example of a catheter comprising a shaft according to the invention.

Figure 11 is an illustration of a BFR test. DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

As illustrated on figures 1A and IB, a tube (or stick) 2 obtained by a process according to the invention has an external or outer diameter OD of, for example, 4.5mm and a wall thickness of, for example, T = 0.125mm. But a tube obtained by a process according to the invention could have other dimensions, in particular a larger outer diameter, for example up to 10 mm or 20 mm or 30 mm. A wire could also be made by a process according to the invention, having a diameter of for example up to 10 mm or 20 mm or 30 mm. A band of material or a flat piece of material could also be made based on a process according to the invention.

The process below is explained for making a tube like the one of figure 1A but can be readily adapted for making a wire or a flat piece of material.

Starting from hollow tubes 2 with a larger outer diameter ODa>OD and larger wall thickness Ta>T, steps of a standard process and a process according to the invention result in a tube having the above dimensions OD and T. A process according to the invention grants the material specific and unexpected properties. As explained below, a first lot of such tubes was subject to a standard process and a second lot was subject to a process according to the invention .

Such tube (or wire or flat piece) is made of a binary alloy of Ni and Ti in the present example, for example containing 50.8 at % Ni and 49.2 at % Ti (in the present description and in the claims all percentages are at % except where indicated) . Other compositions or alloy materials are concerned by the present invention, in particular other Ni-Ti compositions with a proportion of Ni comprised between 50% and 52 %, or between 50.5 % and 51.5 % and corresponding proportions of Ti comprised between 50% and 48%, or between 49.5 % and 48.5 %.

The invention is also applicable to a ternary composition of Ni, Ti and another material which can be selected from among Cr or Cu or Fe or Nb or Pt, in particular containing S ΟΓΠ.Θ % of one of these additives, for example between 1% and 10% or 15~6 or between 1% and 20% or 25%.

Following examples concern ternary alloys compositions :

- NiTiFe : 52.5 - 54.5 wt% Ni; 1-2 wt% Fe;

43.5 - 47.5 wt% Ti;

- NiTiCr: 54.8 - 56.8 wt% Ni; 0.1-0.4 wt% Cr; 42.8 - 45.1 wt% Ti.

- NiTiPt: 37-42 wt% Ni; 22 - 24 wt% Pt; 36 - 39 wt% Ti.

- NiTiNb: 46-48 at% Ni; 8-10 at% Nb; 42-46 at% Ti.

- NiTiCu: 39-41 at% Ni; 9-11 at% Cu; 48-52 at% Ti.

A comparative study of the properties of tubes made according to a standard process and tubes made according to a process of the invention will be presented below. Therefore steps for performing both the standard process and a process according to the invention are first given. Most steps are common to both processes, therefore steps which are specific to the invention are indicated as such.

The process involves the following steps:

1. Mandrel Hot Drawing, at a temperature of between 300°C and 500°C and hot working between 10% and 40%. At this first stage a oxide layer can appear on the surface of the material.

2. Annealing at a temperature of between 700°C and 800°C during 3 to 10 min.

Steps 2 and 3 are repeated several times, for example 15 times because drawing has to be performed very progressively. The material can be drawn up to 40% each time. Degreasing can be performed after each mandrel hot drawing and before each annealing step .

3. Mandrel Hot Drawing (it is the last one and sets the wall thickness T - see figure IB) , the conditions being those given above.

The above steps 1 - 3 result in a tube having the required thickness T but an outer diameter larger than required.

Further steps are performed:

- 4. Degreasing;

- 5. Annealing in the above conditions (see step 2 ) .

6. Cold Drawing at room temperature, comprising cold working of 5 to 15 % per step and an overall cold working of 35% to 45%.

Again, step 6 is repeated several times, for example 4 times because drawing has to be performed very progressively. 7. Heat Straightening at a temperature of between 470°C and 510°C during 5 to 20 min.

8. Grinding: The tubes resulting from the foregoing steps have an outer diameter which is still larger than OD. So a grinding step is performed at the end of the process to eliminate a certain thickness of material over the whole surface of the tube. It can comprise a coarse grinding sub-step and a fine grinding sub-step. During this step the tube is preferably cooled with a flow of cooling fluid, for example water or a mixture of water and soluble oil (oil is used against corrosion and for lubricating the surface and also for cooling purposes) .

At this stage, the initial lot of tubes resulting from the foregoing steps was split in two before the grinding operation, in order to compare sticks or samples subject to a standard process and those subject to a process according to the invention.

According to the usual practice and recommendations for performing a standard process, a total material thickness of less than one tenth of mm or about one tenth of mm is eliminated by grinding, with each of several grinding sub-steps removing a material thickness from 0.01 mm up to 0.02 mm, but not more.

According to the invention, a total material thickness of less than one tenth of mm or about one tenth of mm is eliminated from the whole outside surface of the tube by grinding, for example a thickness comprised between 0.05 mm or 0.08 mm and 0.12 mm or 0.14 mm, with at least one grinding step or one of several grinding sub-steps removing at least 0.03 mm or 0.04 mm or 0.05 mm material. This is in contradiction with the usual standard grinding step.

In both processes the removed thickness may include an oxide layer.

Figure 1C shows a front view of a tube 2 with an initial diameter OD_a, dotted lines representing intermediate diameters obtained during a process as disclosed above. The final diameter OD is the smallest one obtained at the end of the process.

The value of at least 0.03 mm material thickness removed during the coarse grinding step can vary depending on the tube or wire diameter or the thickness of the piece of material. But the material thickness to be removed during said step will not be less than 0.025 mm, in particular for a tube or a wire having an outside diameter of less than 5 mm.

After this coarse grinding step the tube has approximately the required OD and T values. It is followed by a final grinding step for smoothing the outer surface of each stick or tube after coarse grinding and removing a very thin film or layer of material, approximately 1 ym up to 0.01 mm thick.

In other words, a standard process differs from the process according to the invention at least by the coarse grinding step: a process according to the invention involves at least one elementary grinding sub-step removing material in one single pass over a thickness higher than in the standard process. It is possible to combine one or several grinding sub-step eliminating in one single pass a material thickness of more than 0.03 mm or more than 0.025 mm and one or several classical grinding sub-steps eliminating at each pass a material thickness of less than 0.03 mm or even less than 0.02 mm.

A process as disclosed above can be performed by a system comprising drawing machines and furnaces, adapted to the shape of the piece of material. A front view of a grinding machine is schematically illustrated on figure 5. It comprises a back up wheel 4 to maintain a tube 2 being processed. Said back up wheel also has an axis slightly inclined with respect to an axis perpendicular to figure 5 to move a tube 2 being processed forwardly. The grinding machine further comprises a rail 8 having a « V » profile, on which tubes 2 are pushed forward from the rear of the machine to between back up wheel 4 and abrasive wheel 6, on which surface an abrasive paper 12 is applied. The abrasive paper is brought by abrasive wheel into contact with the outside surface of piece 2. The combined rotating movement of both wheels makes the tube rotate around its axis and thus the whole outside surface of the piece is processed according to the invention. Reference 10 is a flow of cooling fluid. Other grinding machines are well known and adapted to grinding flat pieces of material.

Control or measurements were performed during some of the above steps.

During step 8 (both coarse and fine grinding steps) measurements were performed for each stick or tube: UPS (upper plateau stress, see figure 2A) , UTS (ultimate tensile stress, see figure 2A) , and A% (strain when breaking, roughly at UTS) . These values can be obtained by tensile testing as explained below.

Tubes from both lots (the one lot resulting from the standard process, the other one from a process according to the invention) were characterized by tensile testing according to the ASTM E8-09 standard, bend and free recovery (BFR) test according to the ASTM F2082 (2006) standard. The principle of the BFR test is illustrated on figure 11: a sample of material to be tested is bent and then relaxes, thereby raising up a bar 31 on a distance d according to the temperature of the sample.

Comparative results are presented on figures 2 and 3.

Curves I and II are results from tensile testing on a material resulting from a standard process .

Curves III and IV are results from tensile testing on a material resulting from a process according to the invention.

In the plastic portion of tensile curves III and IV ("high ductility" material) , elongation up to 40% or even 50% or even 55% or 60 % can be observed, coupled with a drop in the UTS, typically from 1600 MPa to 950 MPa.

In the superelastic cycle up to 8% strain, the tensile curves III and IV exhibit a longer plateau at UPS than curves I and II with similar UPS values and unrecoverable strain (strain for zero stress) .

It is apparent from these results that a material having high ductility without compromising superelasticity is obtained with a process according to the invention, with elongation up to 30% or 40 ~6 or e en 55 % or 60 % before breaking.

A material obtained with a process according to the invention has a UTS less than 1200 MPA, or even less than 1100 MPa or less than 1000 MPa and possibly higher than 800 MPa or 900 MPa.

BFR testing was also performed, the results of which are illustrated on figures 4A and 4B.

Figure 4A is an Af curve per BFR method on samples subject to a standard process, from which it appears that an Af (austenite finish) value of 5°C is obtained .

Figure 4B is an Af curve per BFR method on samples subject to a process according to the invention, from which it appears that an Af (austenite finish) value of - 15°C is obtained.

It therefore seems that there is a shift of the Af temperature to lower values when performing a process according to the invention.

The BFR testing shows significantly colder transformation temperatures for the "high ductility" process despite similar plateau stress values (UPS and LPS) and the same level of cold work, in apparent contradiction with the well known Clausius- Clapeyron equation .

The shape recovery seems faster on the "high ductility" material as shown by the slope of line T, higher on figure 4B than on figure 4A. As explained above measurements of various parameters have been performed during several steps of the above processes according to the invention.

No sample taken after the heat straightening operation (step 8) show low UTS tensile curve although low UTS curves are found on sticks after the coarse grinding operations.

After having a look at the actual quantities of material removed during the coarse grinding operation, especially on the sticks with a low UTS, it appears that high quantities of material removed in one single pass (or in at least one pass of several passes) lead to the mechanical behavior evidenced, namely superelasticity, with an elongation range of 30% to 60 %, and a high ductility, with a UTS less than 1200 MPa or even 1100 MPa.

It also seems that in a preferable embodiment of a process according to the invention the flow of cooling fluid during the grinding step is reduced compared to usual flows. Tests have been carried out with a flow of 3.4 1/mn and the with a flow of 0.8 1/mn; tubes obtained with the lower flow had improved characteristics in terms of high ductility.

Some experiments have been designed based on the teaching of the invention.

The quantity of material removed during 1 grinding pass was, respectively, 0.05 mm, 0.03 mm and 0.01 mm, and 3 grinding passes were carried out to remove a total thickness of, respectively, 0.15 mm, 0.09 mm and 0.03 mm. To interpret results we only focus on the UTS value after the grinding operation. Following results were obtained:

for a material thickness removal of 0.05 mm per grinding pass, following values of UTS were obtained: 779 MpPa, 820 MPa, 820 MPa and 772 MPa,

for a material thickness removal of 0.03 mm per grinding pass, following values of UTS were obtained: 1144 MPa, 1116 MPa,

- for a material thickness removal of 0.01 mm per grinding pass, significantly higher values of UTS were obtained: 1488, 1447, 1454, 1454, 1426, 1433, 1419, 1463, 1481, 1433, 1460, 1440 MPa.

Therefore "high ductility" processed tubing is favorably compared with "regular process" tubing from the same batch of raw material.

The behavior resulting from a process according to the invention that leads to a low UTS and an important elongation at break is mainly characterized by a low modulus of the stress/strain curve after the superelastic plateau. In addition to UTS, the existence of this behavior based on this apparent modulus is documented as a result of the experiment (to avoid any confusion with a low UTS that would come from a high modulus but an early break) .

A material according to the invention is applicable to making stents but also other kinds of medical devices. An example of stent 35 having a braided or interwoven construction is illustrated on figure 7A. Another example of stent structure 35' is illustrated on figure 7B. In particular tubes according to the invention can be advantageously employed for making a stent 40 comprising a cardiac valve 42 (see structure illustrated on figure 8 where the stent and the valve are both seen in cross section) . A cardiac valve can be mounted on a support having another shape than a stent but also made of a material according to the invention.

A material according to the invention can also be used for making an endoscope shaft 50. The structure of an endoscope is shown on figure 9, comprising a handle 54 connected to the shaft 50 according to the invention. One sees on another side of the handle an eyepieces 55 and an opening 52 for example for a medical instrument. A wire 58 is for connecting a display 59. The shaft can further contain fibers or wires not shown on the figure.

A material according to the invention can also be used for making a catheter shaft 60. The structure of a catheter is shown on figure 10, comprising a handle 64 connected to the shaft 60 according to the invention. The shaft can further contain fibers or wires not shown on the figure.

Spectacles parts, like sides 20 of spectacles (figure 6) or a spectacle bridge 22 for connecting spectacle lenses 24, 26 can also be made from a material according to the invention. Spectacles having at least such sides, and/or a bridge according to the invention can therefore be made. Spectacles comprising lenses, sides according to the invention and a bridge according to the invention are illustrated on figure 7.

Claims

1. A superelastic shape memory alloy of Ni and Ti, having an elongation of at least 30%.

2. A shape memory alloy according to claim 1, said alloy having a UTS less than 1200 MPa.

3. A shape memory alloy according to claim 1 or 2, said alloy being a binary alloy of Ni and Ti, comprising between 50 % and 52 % Ni and between 50% and 48 % Ti.

4. A shape memory alloy according to claims 1 to 3, said alloy being a ternary alloy of Ni, Ti, and a further material, selected from among Cr or Cu or Fe or Nb or Pt .

5. A method to make an improved shape memory alloy material according to claims 1 to 4, comprising grinding a surface of said material, involving at least one sub-grinding steps removing in a single step a material thickness of least 3/100 mm.

6. A method according to claim 5, said surface being grinded over a total thickness comprised between 0.05 mm and 0.14 mm.

7. A method according to claim 5 or 6, further comprising at least one step of hot drawing said alloy.

8. A method according to any of claims 5 to

7, further comprising at least one step of annealing said alloy.

9. A method according to any of claims 5 to

8, further comprising at least one step of heat straightening said alloy.

10. A method according to any of claims 5 to 9, further comprising cooling said alloy during grinding .

11. A method according to any of claims 5 to 10, further comprising at least one cold drawing step.

12. A method according to any of claims 5 to 11, said alloy having the shape of a tube or a wire or a flat piece.

13. A stent device made of a material according to any of claims 1 to 4.

14. A cardiac valve (42) in a support or a stent (40) made of a material according to any of claims 1 to 4.

15. An endoscope or catheter shaft (50, 60) made of a material according to any of claims 1 to 4.

16. A spectacles side (20), or a bridge (22) for connecting spectacle lenses (24, 26), made of a alloy according to any of claims 1 to 4.

17. Spectacles having at least sides (20), and/or a bridge (22) according to claim 16.