Wire of ultra-elastic nickel titanium alloy and formation method therefor

Abstract

To provide a good alloy, etc.SOLUTION: The present specification discloses a nickel titanium alloy whose mean particle size is 0.2-10 micron and whose recoverable distortion exceeds 9%. The alloy is formed using a method including subjecting the nickel titanium alloy to shape fixation heat treatment. The heat treatment includes applying heat at a temperature of 225-350°C for 20-240 minutes.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Provisional Application No. 62 / 907,500, filed September 27, 2019, the disclosure of which is incorporated herein by reference in its entirety.

[0002] Field The present disclosure relates generally to nickel-titanium alloys and methods of making same. [Background technology]

[0003] background There has been much interest in shape memory and superelastic alloys such as nickel titanium. This family of alloys is also known as Nitinol (Nickel Titanium Naval Weapons Laboratory). The key to harnessing the performance of Nitinol alloys is the phase transformation of their crystal structure, which transitions between the austenite and martensite phases. The austenite phase is commonly referred to as the high temperature phase, and the martensite phase is commonly referred to as the low temperature phase. The specific phase characteristics and the repeatable transformation from one phase to another are the mechanism for achieving Nitinol's unique superelastic and shape memory properties.

[0004] Nickel titanium alloys are frequently used in medical devices for diagnostics and therapy. For example, minimally invasive implant procedures for medical devices require devices that can be contracted to a smaller size for delivery to a target treatment site and then released and expanded to a functional configuration, taking advantage of the superelastic and / or shape memory properties of Nitinol. There is a need for Nitinol material property improvements and manufacturing methods to produce nickel titanium alloys with properties such as increased superelasticity beyond the currently known 6% superelasticity, particularly suitable for medical devices where a smaller contraction profile is preferred, while maintaining or improving the properties of the nickel titanium alloy. Summary of the Invention

[0005] Abstract Various disclosed concepts relate to nickel-titanium alloys. According to one example ("Example 1"), the alloy has an average grain size of 0.2 to 10 microns and a recoverable strain of greater than 9%.

[0006] According to Example 1 as well as another example ("Example 2"), the nickel titanium alloy has an upper plateau length of greater than 7%.

[0007] According to another example ("Example 3") in addition to Examples 1 or 2, the nickel-titanium alloy has a lower plateau stress of less than 250 MPa.

[0008] According to another example ("Example 4") in addition to any of the preceding examples, the nickel-titanium alloy has an upper plateau stress of less than 400 MPa.

[0009] According to another example ("Example 5") in addition to any of the preceding examples, the nickel titanium alloy has an elongation at break of greater than 15%.

[0010] According to another example ("Example 6") in addition to any of the preceding examples, the alloy is processed into an implantable medical device.

[0011] According to another example ("Example 7"), a nickel titanium alloy is formed using a method that includes providing a nickel titanium alloy having an average grain size of 0.2 to 10 microns and subjecting the nickel titanium alloy to a shape-setting heat treatment, the heat treatment including applying heat at a temperature of 225°C to 350°C for a time period of 20 minutes to 240 minutes.

[0012] According to Example 7 as well as another example ("Example 8"), the nickel titanium alloy has a recoverable strain of greater than 9%.

[0013] According to another example ("Example 9") in addition to Examples 7 or 8, the nickel titanium alloy has an upper plateau length of greater than 6%.

[0014] According to any one of Examples 7-9 as well as another example ("Example 10"), the nickel-titanium alloy has a lower plateau stress of less than 250 MPa.

[0015] According to any one of Examples 7-10 as well as another example ("Example 11"), the nickel-titanium alloy has an upper plateau stress of less than 400 MPa.

[0016] According to any one of Examples 7-11 as well as another example ("Example 12"), the nickel-titanium alloy has an elongation at break of greater than 15%.

[0017] According to any one of Examples 7-12, as well as another example ("Example 13"), the alloy is processed into an implantable medical device.

[0018] According to another example ("Example 14"), a method for forming a nickel-titanium alloy is disclosed. The method includes providing a nickel-titanium alloy in a 35% to 46% cold worked condition, subjecting the nickel-titanium alloy to a partial annealing heat treatment at a temperature between 550°C and 700°C for up to 60 minutes, and subjecting the nickel-titanium alloy to a heat treatment. The heat treatment includes applying heat at a temperature between 225°C and 350°C for a time period between 20 minutes and 240 minutes.

[0019] According to another example ("Example 15") in addition to Example 14, the nickel titanium alloy has a recoverable strain of greater than 9%.

[0020] According to another example ("Example 16") in addition to Examples 14 or 15, the nickel titanium alloy has an upper plateau length of greater than 6%.

[0021] According to any one of Examples 14-16 plus another example ("Example 17"), the nickel titanium alloy has a lower plateau stress of less than 250 MPa.

[0022] According to any one of Examples 14-17 plus another example ("Example 18"), the nickel-titanium alloy has an upper plateau stress of less than 400 MPa.

[0023] According to any one of Examples 14-18 as well as another example ("Example 19"), the nickel titanium alloy has an elongation at break of greater than 15%.

[0024] According to any one of Examples 14-19 as well as another Example ("Example 20"), the method further includes processing the nickel titanium alloy into an implantable medical device.

[0025] The foregoing examples are exemplary only and should not be read to limit or otherwise narrow the scope of any of the inventive concepts otherwise provided by this disclosure. While multiple examples have been disclosed, still other embodiments will become apparent to those skilled in the art from the following detailed description, which shows and describes illustrative examples. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not as restrictive in nature. [Brief description of the drawings]

[0026] BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings are included to provide a further understanding of the disclosure, and are incorporated in and constitute a part of this specification, illustrate embodiments and, together with the description, serve to explain the principles of the disclosure.

[0027] [Figure 1] FIG. 1 shows a flow diagram of the various steps involved in the processing (cold working, annealing and shape setting) of NiTi (nickel-titanium) alloys according to conventional processing methods.

[0028] [Diagram 2] FIG. 2 illustrates a flow diagram of the steps involved in a method of processing a NiTi (nickel-titanium) alloy (cold working, annealing, and shape setting) according to an embodiment disclosed herein.

[0029] [Diagram 3]FIG. 3 shows an example of the cross-sectional microstructure of a wire formed using the methods disclosed herein.

[0030] [Figure 4] FIG. 4 shows an example stress-strain curve for a wire formed in one embodiment according to the methods disclosed herein.

[0031] [Diagram 5] FIG. 5 shows examples of stress-strain curves having different physical properties formed using the processing methods disclosed herein and compared to typical stress-strain responses obtained using conventional processing methods.

[0032] [Figure 6] 6A-6D show examples of implantable medical devices that can be formed using wires as disclosed herein according to various embodiments.

[0033] Those skilled in the art will readily appreciate that the various aspects of the present disclosure may be implemented by any number of methods and devices configured to perform the intended functions. It should also be noted that the accompanying drawings referred to herein are not necessarily drawn to scale and may be exaggerated to illustrate various aspects of the present disclosure, and in that regard, the drawings should not be construed as limiting. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0034] Detailed Description Definitions and Terminology This disclosure is not intended to be read in a limiting manner, for example, the terms used in this application should be read broadly with respect to the meaning that terms of the art ascribe to such terms.

[0035] With respect to imprecision terms, the terms "about" and "approximately" may be used interchangeably to refer to measurements that include the stated measurement and that are reasonably close to the stated measurement. A measurement that is reasonably close to the stated measurement deviates from the stated measurement by a reasonably small amount, as would be understood and readily ascertained by one of ordinary skill in the relevant art. Such deviations may be due, for example, to measurement error or small adjustments made to optimize performance.

[0036] The term "nitinol" (NiTi) refers to a near-equiatomic alloy of nickel and titanium (which may contain ternary, quaternary or higher elements such as copper, chromium, iron, vanadium, cobalt or niobium). "Nitinol alloy", "NiTi alloy", "nickel titanium alloy" and / or "NiTi alloy material" may be used interchangeably throughout this disclosure to refer to NiTi that exhibits shape memory or superelastic behavior.

[0037] Additionally, although the term "wire" is referenced in several instances throughout this disclosure, it should be understood that other forms of metal, such as sheets and stent frames, may undergo the same processes as wire to achieve the same or similar mechanical properties as disclosed herein.

[0038] The term "cold working" refers to a process in which a metallic material is formed at a temperature below its recrystallization temperature, e.g., ambient temperature. During the cold drawing process of cold working, the wire is pulled through a lubricated die to reduce the diameter. The deformation associated with drawing increases stresses in the material that must ultimately be relieved by various methods of heat treatment or annealing at elevated temperatures to restore ductility so that the material can be cold worked to an even smaller diameter. Cold working is typically expressed as a percentage that represents the reduction in cross-sectional area during the final pass through the die. The initial preparation of the wire begins with rod stock, and then the cold working and annealing can be repeated one or more times. The physical properties of the wire are determined based on the cold working and annealing processes. Cold working includes squeezing, bending, drawing, and shearing performed on the wire, which plastically deforms the hardened workpiece in the wire section to change shape. Cold working is so named because the procedure is performed at a temperature below the recrystallization temperature of the alloy from which the wire is made.

[0039] The terms "annealing" or "annealing heat treatment" include either full annealing heat treatment or partial annealing heat treatment. Full annealing heat treatment can result in a material with random crystal orientation, and the various material or fiber "texture" created during cold wire drawing is largely removed during full annealing and recrystallization. During the partial annealing process, grain growth and recrystallization occur, but a certain amount of material texture developed during the cold wire drawing process is retained. Partial annealing is sometimes referred to as a cold work reduction annealing process.

[0040] Description of Various Embodiments 1 shows a conventional treatment process 100 by which products such as wire made from nickel-titanium alloys acquire their properties. Wire products made from shape memory materials are produced by forming a relatively thick, hot-worked rod stock from a melt process. The rod stock is then further processed into wire by drawing the rod stock into a fine diameter wire using a combination of cold working and annealing.

[0041] Conventional wire annealing typically results in grain growth. Typical annealing temperatures for Nitinol are above 550° C. for various time ranges.

[0042] In step 102 of process 100, the wire undergoes the cold working and annealing processes described above. Cold working includes, among other things, squeezing, bending, drawing, and shearing performed on the wire to cause the desired workpiece, which has hardened on portions of the wire, to undergo plastic deformation and change shape. Cold working is so named because the procedure is performed at a temperature below the recrystallization temperature of the alloy from which the wire is made. Dislocations formed during cold working accumulate and result in internal stress fields within the wire, thereby resulting in closed-loop superelasticity. However, if too much cold working is performed on the wire, the wire may suffer a reduction in the material involved in the transformation, such that much of the material forming the wire becomes fixed or locked internally. An intermediate annealing heat treatment helps restore the ductility of the wire, and alternating cold working and annealing can help reduce the bar to the desired wire size. In some instances, step 102 leaves the wire in a 35% to 46% cold worked condition.

[0043] In step 104, the wire then passes through a subsequent heat treatment step or process often referred to as "shape setting" or "shape setting heat treatment." This step forms the Nitinol wire (or in some examples, tube or sheet) into a specific geometry by clamping the material to a mandrel to constrain the material into a new shape prior to the shape setting heat treatment. If the desired resulting shape includes a complex geometry, an additional shape setting step 106 follows the heat treatment of step 104. For Nitinol and its alloys, typical shape setting temperatures can range from 400°C to 550°C with shape setting heat treatment times of up to 40 minutes.

[0044] FIG. 2 illustrates a processing process 200 as disclosed herein. In step 202 of the process 200, the wire undergoes the above-mentioned cold working and annealing process described in step 102. Cold working includes squeezing, bending, drawing, and shearing, among other actions, performed on the wire to cause the desired fabrication, which has hardened in the wire portion, to undergo plastic deformation, thus achieving a shape change. The cold working is performed at a temperature below the recrystallization temperature of the alloy. Dislocations formed during cold working accumulate and result in an internal stress field within the wire, thereby creating closed-loop superelasticity. However, if too much cold working is performed on the wire, the wire may suffer a reduction in the material involved in the transformation, such that much of the material forming the wire becomes fixed or locked internally. An intermediate annealing heat treatment helps restore ductility to the wire, and alternating cold working and annealing can be used to help reduce the bar to the desired wire size. In some examples, step 202 cold works the wire to between 35% and 40%, between 40% and 42%, between 42% and 44%, between 44% and 46%, or any combination thereof.

[0045] In step 204, the wire is subjected to a cold work reduction annealing process in which the wire is heated, for example, via a fluidized bath, salt pot, or air furnace, to a temperature above the recrystallization temperature and below the melting temperature of the alloy, maintained at that temperature for a period of time, and then cooled, thereby partially relieving the stresses imposed on the wire by the cold work process, i.e., reducing the cold work. In some examples, the temperature of the cold work reduction annealing process is between 550°C and 700°C. In some examples, the temperature ranges from 550°C to 600°C, 600°C to 650°C, 650°C to 700°C, or any combination thereof. In some examples, the cold work reduction annealing process lasts for a period of time, for example, up to 30 minutes, up to 45 minutes, or up to 60 minutes. In some examples, the amount of cold work reduced by the cold work reduction annealing process of step 204 is greater than 5%, greater than 7%, greater than 10%, greater than 12%, greater than 15%, greater than 20%, or any value therebetween.

[0046] The cold work reduction annealing process from step 204 results in annihilation of dislocations, which can reduce internal stresses and enable closed-loop superelasticity. On the other hand, precipitates formed by subjecting the wire to subsequent heat treatments enable a coherent stress field within the wire, but the formation of precipitates alone cannot generate closed-loop superelasticity. Therefore, a sufficient cold work process must be combined with the precipitation to induce closed-loop superelasticity. Step 204 can be performed subsequent to step 202 such that the wire is received in a form with substantially less cold work.

[0047] Alternatively, the wire can be processed to have less cold work (i.e., less reduction in cross-sectional area in the final step), as indicated by step 206, where the wire is provided with a low cold work percentage. In some examples, a low cold work percentage is defined as less than 35%, less than 30%, less than 25%, less than 20%, or any range therebetween. In some examples, the wire after step 204 and the wire provided in step 206 both have an average grain size in the range of 0.2-0.5 microns, 0.5-1 microns, 1-2 microns, 2-4 microns, 4-6 microns, 6-8 microns, 8-10 microns, or any combination thereof.

[0048] The wire then undergoes a low temperature heat treatment process according to step 208, where the temperature of the heat treatment process is lower than the conventional shape setting heat treatment temperature as described in step 104. For example, the shape setting heat treatment process of step 104 is in a temperature range of 400° C. to 530° C., while the temperature of the low temperature shape setting heat treatment process of step 208 is in a range of 225° C. to 350° C. In some examples, the heat treatment temperature is in a range of 225° C. to 270° C., 270° C. to 300° C., 300° C. to 320° C., 320° C. to 350° C., or any combination thereof.

[0049] The time that each process is performed also varies. In some examples, the shape-setting heat treatment process of step 104 lasts between 5 and 40 minutes. In comparison, the low-temperature shape-setting heat treatment process of step 208 in some examples takes longer, ranging from 20 to 30 minutes, 30 to 40 minutes, 40 to 60 minutes, 60 to 120 minutes, 120 to 200 minutes, more than 200 minutes, or any combination thereof. In some examples, the heat treatment process of step 208 lasts less than 60 minutes, while in other examples, the heat treatment process of step 208 lasts up to 240 minutes.

[0050] In some instances, the wire also undergoes a shape-setting process during the heat treatment of step 208, which converts the Nitinol wire (or in some instances, tube or sheet) into a particular geometry by constraining the material in its new shape by fastening the material to a mandrel prior to the heat treatment. In some instances, an additional shape-setting process similar to step 106 (not shown in FIG. 2) described above follows the heat treatment of step 208 if the desired resulting shape includes a complex geometry.

[0051] 3 shows a close-up of a cross-section of a wire processed using process 200 described above. For example, image 300 is a view of the wire cut longitudinally relative to the length of the wire, and image 302 is a view of the wire cut transversely relative to the length of the wire. In some examples, the wire shown in image 300 or 302 exhibits an average grain size of 0.2 to 10 microns and an austenitic finish transformation temperature of -100°C to 50°C. In some examples, the grain size increases as the temperature of the annealing heat treatment process increases.

[0052] During each of the above steps in the process 200, the physical properties of the wire undergo changes, including modulus of elasticity, permanent set, recoverable strain, plateau stress, as well as other properties discussed further herein. These properties are measured using a test method known as ASTM F2516 Standard Test Method for Tensile Testing of Nickel-Titanium Superelastic Materials. In the ASTM F2516 method, a sample of Nitinol wire is loaded by fastening both ends of the wire in a fixture of a measuring device. After the wire is securely fastened to the measuring device, the ends of the wire are pulled away from each other at a specified rate until the wire reaches a specified strain percentage. In some examples, this specified strain percentage is 6% of the length of the wire. The direction of movement of the ends of the wire is then reversed and the ends are moved toward each other until the load falls below a predetermined limit. Finally, the ends of the wire are again pulled away until the wire breaks. The measurements taken at each step are used to create a stress-strain curve that represents the physical properties of the Nitinol being tested. An example of such a stress-strain curve is shown in FIG. 4, as explained below.

[0053] 4 shows an example of a stress-strain curve 400 illustrating the closed-loop superelasticity of a Nitinol wire. The stress-strain curve 400 includes some of the properties of Nitinol wire that can be determined using the ASTM F2516 method. When an initial strain is applied to the wire by pulling both ends in opposite directions, the wire undergoes a strain-induced deformation, E, which defines the ratio of tensile stress to tensile strain. AThe wire undergoes a linear elastic (non-permanent) deformation 402 with a gradient in the modulus of elasticity (i.e., Young's modulus or elasticity). The tensile stress then experiences a flat region or plateau 404 where the wire undergoes tensile strain without any additional stress being applied to the wire. Per ASTM F2516 method specifications, the upper plateau stress (UPS) is defined as the stress measured at this plateau 404, specifically at 3% strain when the tensile stress is initially applied. Next, per ASTM F2516 method specifications, the tensile stress is applied until the strain reaches the applied strain 405, which per ASTM F2516 specifications is 6%, which is the initial load strain or recoverable (i.e., reversible) strain for testing Nitinol. The direction is then reversed during the unloading process 406, and the ends of the wire approach each other. The wire goes through another plateau 408 in the process and the lower plateau stress (LPS) is measured according to the ASTM F2516 method specifications at 2.5% strain after the wire reaches an initial 6% strain. The difference between the UPS 404 and LPS 408 values ​​is defined as the mechanical hysteresis. Note that for different values ​​of applied strain 405, different amounts of residual elongation 411 are observed. A recoverable strain is one where little or no residual elongation 411 is observed after strain 405 is applied (e.g., less than 0.5%).

[0054] The wire is then stressed to 7 MPa, as specified by the ASTM F2516 method. In the absence of other stresses, the wire is allowed to return to its initial length. In many Nitinol wires, the length of the wire does not change or changes very little, e.g., less than 0.5% change in length before and after reaching an initial strain of 6%, but in some cases the resulting length of the wire is longer than the initial length. In these cases, the wire is said to have undergone a permanent deformation 410 with a residual elongation 411 determined by comparing the resulting length of the wire to the initial length. Finally, both ends of the wire are pulled in opposite directions again, and the wire undergoes elastic deformation 402 and an upper plateau 404. However, this time the wire is pulled beyond a strain of 6%, undergoing a second elastic loading region 412, which may involve plastic (permanent) deformation. The wire then breaks or fractures, thereby representing the elongation at break 413 of the wire. In some instances, the elongation at break 413 is a strain of 15% or more. For wires tested in the manner described above, the upper plateau length (UPL) 414 is defined as the total strain from the start of loading of the wire (i.e., 0% strain) to the end of the upper plateau 404 just before the second loading region 412. In some instances, the UPL 414 is greater than 7% strain, greater than 8% strain, or greater than 9% strain. In some instances, the UPS 404 is less than 400 MPa and the LPS 408 is less than 250 MPa.

[0055] Examples of methods used to process Nitinol wire are disclosed below, as well as the resulting properties of each wire prepared using the methods. Although these examples describe the use of Nitinol wire, they are also applicable to other forms of material, such as tubes or sheets. These methods are illustrative and not limiting, and further uses will be recognized by those skilled in the art.

[0056] Example 1: Nitinol Wire (e.g., ASTM F2516 compliant superelastic Nitinol wire with a wire diameter of 0.0206 inches or 0.523 mm) shows an example of a typical Nitinol wire that has been heat treated at 450° C.-500° C. for a time period of 15-20 minutes. The heat treatment relieves some of the cold work and also causes precipitates to form within the wire, which aids in the process of precipitation hardening within the Nitinol wire, increasing its yield strength.

[0057] Example 2: A Nitinol wire is subjected to a cold working process as described in step 202 of process 200 above to reduce the diameter of the wire. Specifically, the Nitinol wire is subjected to a drawing process at room temperature (approximately 25°C), which is much lower than the recrystallization temperature of the wire (550°C). Next, in step 204, the wire is subjected to an annealing process in which the wire is heated to a temperature above the recrystallization temperature of the alloy (550°C) but below the melting temperature (1310°C) and maintained at that temperature for a period of time to partially relieve the stresses associated with the cold working. In this example, the wire was subjected to an annealing process at 670°C for 10 minutes. Then, in the next step 106, the partially annealed wire is subjected to a low temperature heat treatment in which the wire is heated to a temperature below the recrystallization temperature (320°C in this example) for 40 minutes.

[0058] FIG. 5 compares different closed loop stress-strain curves 500 and 502 resulting from processing the same NiTi alloy material according to Examples 1 and 2. Specifically, stress-strain curve 500 is the curve of the wire processed in Example 1, and stress-strain curve 502 is the curve of the wire processed in Example 2. As can be seen from FIG. 5, curve 502 has a longer upper plateau length at 9.2%, while curve 500 has a plateau length at 6%. Furthermore, UPS 404 and LPS 408 are also lower in curve 502 than curve 500. Furthermore, the elongation at break of curve 500 is 11% when the wire of Example 1 breaks under strain. On the other hand, curve 502 can withstand a strain higher than 11% without breaking, thereby indicating the ductility of the wire of Example 2.

[0059] In some instances, the resulting wire has a recoverable strain of greater than 8%, greater than 9%, or greater than 10%. In some instances, the NiTi alloy is processed into a wire structure, such as a wound device. In some instances, the wound device has a cover attached thereto to form a covered stent with a wound NiTi tube. Other suitable structures using the NiTi alloy are also envisioned, for example for medical purposes.

[0060] 6A-6D show examples of various implantable medical devices that may be made using wires as disclosed herein. The wires may be processed into any one or more of these medical devices using any suitable method known in the art. The examples are merely illustrative and are not intended to be exhaustive, and the shapes and configurations of the medical devices are not limited to those shown in the figures.

[0061] 6A shows an example of a stent graft 600 that includes one or more wires or stents 601 and one or more membrane-like materials or grafts 602. The wires or stents 601 may be spirally formed around the graft 602, although any suitable configuration may be envisioned.

[0062] 6B shows an example of an embolic filter 610 that includes wires or struts 611 and one or more membrane-like materials or filter elements 612 attached to a central support member 613. The struts 611 can be straight, curved, or have any other suitable shape and configuration.

[0063] 6C shows an example of a septal occluder or cardiac sealing device 620 that includes one or more wires forming a wire frame 621, one or more membrane-like materials or sealing members 622, and one or more eyelets 623 that may be occluded or unoccluded to control fluid flow therethrough. The wire frame 621 may be attached to or mounted with the eyelets 623, and the seal members 622 may be attached to the wire frame 621. The wire frame 621 may be of any suitable configuration as known in the art.

[0064] 6D shows an example of a heart valve or prosthesis retention element 630 including one or more wires forming a valve frame 631 and one or more membrane-like materials or covers 632. The covers 632 can be attached to or implemented with the valve frame 631. The valve frame 631 can be of any suitable configuration as known in the art.

[0065] The embodiments and methods disclosed herein can be used to enable a variety of shape memory articles. The shape memory articles are preferably medical devices, more preferably implantable medical devices. The implantable medical devices are most preferably Nitinol shape memory alloy devices, most particularly those that are superelastic at normal body temperature (about 37° C.). An implantable medical device is defined as a device that is intended to remain in the body for a period of time of 24 hours or more.

[0066] Shape memory alloy articles can be manufactured from materials in various shapes, such as wires, in various cross-sectional shapes including round, oval, square, rectangular, etc. Alternatively, the articles can be manufactured by machining precursor forms, such as sheets, tubes or rods, or by electrical discharge machining (EDM), laser cutting, chemical milling, etc.

[0067] The invention of this application has been described above both generally and with reference to specific embodiments. It will be apparent to those skilled in the art that various changes and modifications can be made in the embodiments without departing from the scope of the present disclosure. Therefore, the embodiments are intended to cover the modifications and modifications of the present invention provided they come within the scope of the appended claims and their equivalents.

Claims

1. A wire comprising a nickel-titanium alloy having an average grain size of 0.2 to 10 microns and a recoverable strain of greater than 9% in length, said wire undergoing a residual elongation of less than 0.5% in response to the imposition of said recoverable strain, said alloy being medical grade, ASTM compliant Nitinol.

2. The wire described in claim 1, wherein the nickel-titanium alloy has an upper plateau length of greater than 7%.

3. The wire of claim 1, wherein the nickel-titanium alloy has a lower plateau stress (LPS) of less than 250 MPa, the LPS being measured in response to imposing the recoverable strain on the wire.

4. A wire as described in claim 1, wherein the nickel-titanium alloy has an elongation at break of greater than 15% in length.

5. A wire according to any one of claims 1 to 4, which is further processed into an implantable medical device.

6. A method for forming a nickel-titanium alloy, comprising: providing a nickel-titanium alloy having an average grain size of 0.2 to 10 microns in a 35% to 46% cold worked condition; Imposing a partial annealing heat treatment at a temperature of 550°C to 700°C for a maximum of 60 minutes; and subjecting the nickel-titanium alloy to a heat treatment such that the nickel-titanium alloy has a recoverable strain in length of greater than 9% and experiences a retained elongation of less than 0.5% in response to application of the recoverable strain; The method wherein the heat treatment comprises applying heat at a temperature between 225°C and 350°C for a time period between 20 minutes and 240 minutes, and the alloy is medical grade, ASTM compliant Nitinol.

7. The method of claim 6, wherein the nickel-titanium alloy has an upper plateau length of greater than 6%.

8. The method of claim 6, wherein the nickel-titanium alloy has a lower plateau stress of less than 250 MPa.

9. The method of claim 6, wherein the nickel-titanium alloy has an elongation at break in length of greater than 15%.

10. The method of any one of claims 6 to 9, further comprising processing the nickel-titanium alloy into an implantable medical device.

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