Superelastic nickel-titanium alloy wire and method of forming same

Abstract

Disclosed herein is a nickel-titanium alloy having an average grain size of 0.2 to 10 microns and a recoverable strain greater than 9%, the alloy being formed using a method that includes 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.

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 FIELD OF THE DISCLOSURE The present disclosure relates generally to nickel-titanium alloys and methods for making same. [Background technology]

[0003] background There has been considerable 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 crystalline 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 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 treatments. For example, minimally invasive implantation procedures for medical devices require devices that can be contracted to a smaller size for delivery to a target treatment site, 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 improved Nitinol material properties and manufacturing methods to produce nickel-titanium alloys with properties such as increased superelasticity beyond the currently known 6% superelasticity while maintaining or improving the properties of nickel-titanium alloys, particularly suitable for medical devices where a smaller contraction profile is preferred. 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 Example 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 fabricated into an implantable medical device.

[0011] According to another example ("Example 7"), a nickel-titanium alloy is formed using a method comprising 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 comprising applying heat at a temperature of 225°C to 350°C for a time period of 20 to 240 minutes.

[0012] In addition to Example 7, according to 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 Example 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 plus 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 plus 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 of 550°C to 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 of 225°C to 350°C for a time period of 20 minutes to 240 minutes.

[0019] According to Example 14 as well as another example ("Example 15"), 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 and 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 are 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. [Brief explanation of the drawings]

[0026] BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are included to provide a further understanding of the present disclosure, and which are incorporated in and constitute a part of this specification, illustrate embodiments and, together with the description, serve to explain the principles of the present 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] [Figure 2] FIG. 2 shows a flow diagram of the steps involved in a method of processing (cold working, annealing, and shape setting) a NiTi (nickel titanium) alloy according to an embodiment disclosed herein.

[0029] [Figure 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] [Figure 5] FIG. 5 shows examples of stress-strain curves with 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 apparatus 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 INVENTION

[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 meanings ascribed to such terms by terms of the art.

[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 include 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 those 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 its diameter. The deformation associated with drawing increases stresses in the material, which must ultimately be relieved by various methods, such as heat treatment at high temperatures or annealing, to restore ductility, allowing the material to be cold worked to an even smaller diameter. Cold working is typically expressed as a percentage representing the reduction in cross-sectional area upon the final pass through the die. Initial preparation of the wire begins with rod stock, and then 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 and changes its 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 a full annealing heat treatment or a partial annealing heat treatment. A 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 illustrates 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 manufactured by forming relatively thick, hot-worked rod stock from a melting process. The rod stock is then further processed into wire by drawing the rod stock into a thin 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 the wire, causing the desired workpiece, which has hardened over a portion of the wire, to undergo plastic deformation and change shape. Cold working is so called because the procedure is performed at temperatures below the recrystallization temperature of the alloy from which the wire is made. Dislocations formed during cold working accumulate, creating an internal stress field within the wire, thereby producing closed-loop superelasticity. However, if too much cold working is performed on the wire, the wire can suffer from a loss of material participating 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 wire's ductility, and alternating cold working and annealing can help reduce the bar to the desired wire size. In some instances, step 102 results in a wire that is 35% to 46% cold worked.

[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 instances, a tube or sheet) into a specific geometry by clamping the material onto 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 process 200, a wire undergoes the above-described cold working and annealing processes described in step 102. Cold working includes, among other actions, squeezing, bending, drawing, and shearing the wire, causing the desired hardened structure in the wire to undergo plastic deformation and thus achieve a shape change. The cold working is performed at a temperature below the recrystallization temperature of the alloy. Dislocations formed during cold working accumulate, creating an internal stress field within the wire, thereby producing closed-loop superelasticity. However, if too much cold working is performed on the wire, the wire can suffer from 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 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 undergoes 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 but below the melting temperature of the alloy, maintained at that temperature for a period of time, and then cooled, thereby partially relieving 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, thereby reducing internal stress and enabling closed-loop superelasticity. Meanwhile, precipitates formed by subsequent heat treatment of the wire enable a coherent stress field within the wire, but precipitate formation 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 so 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, wherein 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 length of time 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 transforms 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] FIG. 3 shows an enlarged 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 its length, and image 302 is a view of the wire cut transversely relative to its length. 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, increasing the temperature of the annealing heat treatment process increases the grain size.

[0052] During each of the above steps in process 200, the wire undergoes changes in physical properties, including modulus of elasticity, permanent set, recoverable strain, plateau stress, and 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 Nitinol wire sample is loaded by securing both ends of the wire in fixtures on a measuring device. After the wire is securely fastened to the measuring device, the ends 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 wire length. The direction of movement of the wire ends is then reversed, and the ends are moved toward each other until the load falls below a predetermined limit. Finally, the ends are again pulled away until the wire breaks. The measurements taken at each step are used to generate 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] FIG. 4 shows an example 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 linear elastic (non-permanent) deformation 402 with a slope of the EA (i.e., Young's Modulus or Elastic Modulus), which defines the ratio of tensile stress to tensile strain. 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. According to the 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, a tensile stress is applied until the strain reaches an applied strain 405, which is 6% per ASTM F2516 specification and is the initial load strain or recoverable (i.e., reversible) strain for testing Nitinol. The direction is then reversed during an unloading process 406, bringing the ends of the wire closer together. The wire undergoes another plateau 408 in this process, and the lower plateau stress (LPS) is measured at 2.5% strain per ASTM F2516 specification after the wire reaches the initial 6% strain. The difference between the UPS 404 and LPS 408 values ​​is defined as the mechanical hysteresis. Note that different amounts of residual elongation 411 are observed for different values ​​of applied strain 405. A recoverable strain is a strain at which little or no residual elongation 411 is observed after strain 405 is applied. (0 (less than 0.5%).

[0054] The wire is then subjected to a stress of 7 MPa, as specified in the ASTM F2516 method. In the absence of other stresses, the wire is allowed to return to its original length. For many Nitinol wires, the wire length remains unchanged or changes very little—for example, 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 its 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 wire length to its initial length. Finally, the ends of the wire are again pulled in opposite directions, and the wire undergoes elastic deformation 402 and an upper plateau 404. However, this time, the wire is pulled beyond 6% strain and experiences a second elastic loading region 412, which may involve plastic (permanent) deformation. The wire then breaks or fractures, thereby representing the wire's elongation at break 413. In some instances, the elongation at break 413 is 15% or greater. For wires tested in the above manner, the upper plateau length (UPL) 414 is defined as the total strain from the start of wire loading (i.e., 0% strain) to the end of the upper plateau 404 just before the second loading region 412. In some examples, the UPL 414 is greater than 7% strain, greater than 8% strain, or greater than 9% strain. In some examples, 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, as well as the resulting properties of each wire prepared using the methods, are disclosed below. While 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 F 0206 inch or 0.523 mm wire diameter) 2063Compliant Superelastic Nitinol Wire) shows an example of a typical Nitinol wire that has been heat treated at 450°C-500°C for 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: Nitinol wire undergoes a cold working process as described in step 202 of process 200 above to reduce the diameter of the wire. Specifically, the Nitinol wire undergoes 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 undergoes 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 held at that temperature for a period of time to partially relieve stresses associated with the cold working. In this example, the wire underwent an annealing process at 670°C for 10 minutes. Then, in the next step 106, the partially annealed wire undergoes 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 for the wire processed in Example 1, and stress-strain curve 502 is for 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 in curve 500. Furthermore, the elongation at break for curve 500 is 11%, where the wire of Example 1 breaks under strain. On the other hand, curve 502 can withstand a strain higher than 11% without fracture, thereby demonstrating 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 to it to form a covered stent with a wound NiTi tube. Other suitable structures using NiTi alloys are also envisioned, for example, for medical purposes.

[0060] 6A-6D show examples of various implantable medical devices that can be made using wires as disclosed herein. The wires can 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 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 helically wound around the grafts 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 or may not be occluded to control fluid flow therethrough. Wire frame 621 can be attached to or mounted with eyelets 623, and sealing members 622 can be attached to wire frame 621. Wire frame 621 can have any suitable configuration as known in the art.

[0064] 6D shows an example of a heart valve or prosthesis retention element 630 that includes 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 have 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 (approximately 37°C). An implantable medical device is defined as a device intended to remain in the body for a period of 24 hours or more.

[0066] Shape memory alloy articles can be fabricated from materials in a variety of shapes, such as wire, in a variety of cross-sectional shapes, including round, oval, square, rectangular, etc. Alternatively, the articles can be fabricated by machining a precursor form, such as a sheet, tube, or rod, or by electrical discharge machining (EDM), laser cutting, chemical milling, etc.

[0067] The invention of this application has been described above both generically 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. Thus, the embodiments are intended to cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents. (Aspect) (Aspect 1) A nickel-titanium alloy having an average grain size of 0.2 to 10 microns and a recoverable strain greater than 9%. (Aspect 2) 2. The nickel titanium alloy of embodiment 1, wherein the nickel titanium alloy has an upper plateau length of greater than 7%. (Aspect 3) 3. The nickel-titanium alloy of claim 1 or 2, wherein the nickel-titanium alloy has a lower plateau stress of less than 250 MPa. (Aspect 4) 10. The nickel titanium alloy of any one of the previous embodiments, wherein the nickel titanium alloy has an upper plateau stress of less than 400 MPa. (Aspect 5) 10. The nickel-titanium alloy of any one of the preceding embodiments, wherein the nickel-titanium alloy has an elongation at break of greater than 15%. (Aspect 6) 2. The nickel titanium alloy of any one of the preceding embodiments, further processed into an implantable medical device. (Aspect 7) Providing a nickel-titanium alloy having an average grain size of 0.2 to 10 microns; and 1. A nickel-titanium alloy formed using a method comprising subjecting the nickel-titanium alloy to a shape-setting heat treatment, The heat treatment includes applying heat at a temperature of 225°C to 350°C for a time period of 20 minutes to 240 minutes. (Aspect 8) 8. The nickel-titanium alloy of embodiment 7, wherein the nickel-titanium alloy has a recoverable strain of greater than 9%. (Aspect 9) 9. The nickel titanium alloy of embodiment 7 or 8, wherein the nickel titanium alloy has an upper plateau length of greater than 6%. (Aspect 10) 10. The nickel-titanium alloy of any one of embodiments 7-9, wherein the nickel-titanium alloy has a lower plateau stress of less than 250 MPa. (Aspect 11) 11. The nickel-titanium alloy of any one of aspects 7 to 10, wherein the nickel-titanium alloy has an upper plateau stress of less than 400 MPa. (Aspect 12) 12. The nickel-titanium alloy of any one of aspects 7 to 11, wherein the nickel-titanium alloy has an elongation at break of greater than 15%. (Aspect 13) 13. The nickel titanium alloy of any one of embodiments 7-12, further processed into an implantable medical device. (Aspect 14) 1. A method for forming a nickel-titanium alloy, comprising: Providing nickel-titanium alloys 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; The heat treatment comprises applying heat at a temperature of 225°C to 350°C for a time period of 20 minutes to 240 minutes. (Aspect 15) 15. The method of embodiment 14, wherein the nickel-titanium alloy has a recoverable strain greater than 9%. (Aspect 16) 16. The method of claim 14 or 15, wherein the nickel-titanium alloy has an upper plateau length of greater than 6%. (Aspect 17) 17. The method of any one of embodiments 14-16, wherein the nickel-titanium alloy has a lower plateau stress of less than 250 MPa. (Aspect 18) 18. The method of any one of aspects 14-17, wherein the nickel-titanium alloy has an upper plateau stress of less than 400 MPa. (Aspect 19) 19. The method of any one of embodiments 14-18, wherein the nickel-titanium alloy has an elongation at break of greater than 15%. (Aspect 20) 20. The method of any one of embodiments 14-19, further comprising processing the nickel titanium alloy into an implantable medical device.

Claims

1. 1. A nickel-titanium alloy having an average grain size of 0.2 to 10 microns, a recoverable strain in length greater than 9%, and an upper plateau stress (UPS) of less than 400 MPa, said UPS being measured in response to imposing said recoverable strain on a wire, wherein the alloy is medical grade, ASTM compliant Nitinol.

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

3. 3. The nickel-titanium alloy of claim 1 or 2, wherein the nickel-titanium alloy has a lower plateau stress (LPS) of less than 250 MPa, the LPS measured in response to imposing the recoverable strain on the wire.

4. The nickel titanium alloy of any one of claims 1 to 3, wherein the nickel titanium alloy has an elongation at break in length of greater than 15%.

5. The nickel titanium alloy of any one of claims 1 to 4, further processed into an implantable medical device.

6. providing a nickel-titanium alloy having an average grain size of 0.2 to 10 microns; and 1. A method of producing a nickel titanium alloy formed using a method comprising subjecting the nickel titanium alloy to a shape-setting heat treatment to form the nickel titanium alloy having a recoverable strain in length of greater than 9% and an upper plateau stress (UPS) of less than 400 MPa; the UPS is measured in response to imposing the recoverable strain on the wire; The method of manufacturing a nickel-titanium alloy, 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 for producing a nickel-titanium alloy according to any one of claims 6 to 7, wherein the nickel-titanium alloy has a lower plateau stress of less than 250 MPa.

9. The method of any one of claims 6 to 8, wherein the nickel titanium alloy has an elongation at break in length of greater than 15%.

10. The method for producing the nickel-titanium alloy of any one of claims 6 to 9, which is further processed into an implantable medical device.

11. 1. A method for forming a nickel-titanium alloy, comprising: Providing the nickel-titanium alloy 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 an upper plateau stress (UPS) of less than 400 MPa; the UPS is measured in response to imposing the recoverable strain on the wire; the heat treatment comprises applying heat at a temperature of 225°C to 350°C for a time period of 20 minutes to 240 minutes; The nickel titanium alloy has an average grain size of 0.2 to 10 microns and is medical grade, ASTM compliant Nitinol.

12. The method of claim 11 , wherein the nickel-titanium alloy has an upper plateau length greater than 6%.

13. The method of any one of claims 11 to 12, wherein the nickel-titanium alloy has a lower plateau stress of less than 250 MPa.

14. The method of any one of claims 11 to 13, wherein the nickel titanium alloy has an elongation at break greater than 15% in length.

15. The method of any one of claims 11 to 14, further comprising processing the nickel titanium alloy into an implantable medical device.

Images