# Patent

## Title
Method for imparting improved fatigue strength to wire made of shape memory alloys, and medical devices made from such wire

## Abstract

Wire products, such as round and flat wire, strands, cables, and tubing, are made from a shape memory material in which inherent defects within the material are isolated from the bulk material phase of the material within one or more stabilized material phases, such that the wire product demonstrates improved fatigue resistance. In one application, a method of mechanical conditioning in accordance with the present disclosure isolates inherent defects in nickel-titanium or NiTi materials in fields of a secondary material phase that are resistant to crack initiation and/or propagation, such as a martensite phase, while the remainder of the surrounding defect-free material remains in a primary or parent material phase, such as an austenite phase, whereby the overall superelastic nature of the material is preserved.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This is a divisional application of U.S. patent application Ser. No. 12/610,297 filed on Oct. 31, 2009, entitled METHOD FOR IMPARTING IMPROVED FATIGUE STRENTH TO WIRE MADE OF SHAPE MEMORY ALLOYS, AND MEDICAL DEVICES MADE FROM SUCH WIRE, which claims the benefit under Title 35, U.S.C. §119(e) of of U.S. Provisional Patent Application Ser. No. 61/110,084, filed on Oct.31, 2008, and entitled NANOGRAIN DAMAGE RESISTANT WIRE, U.S. Provisional Patent Application Ser. No. 61/179,558, filed on May 19, 2009, and entitled NANOGRAIN DAMAGE RESISTANT WIRE, and U.S. Provisional Patent Application Ser. No. 61/228,677 filed on Jul. 27, 2009 and entitled NANOGRAIN DAMAGE RESISTANT WIRE. The entire disclosures of each of the foregoing applications are hereby expressly incorporated herein by reference.


BACKGROUND
1. Technical Field
The present disclosure relates to fatigue damage resistant wire and, in particular, relates to a method of manufacturing wire made of a shape memory alloy, which demonstrates improved fatigue strength properties, as well as medical devices made with such wire.
2. Description of the Related Art
Shape memory materials are materials that “remember” their original shape, and which, after being deformed, return to that shape either spontaneously or by applying heat to raise their temperature above a processing and material related threshold known as the transformation temperature. Heating to recover shape is commonly referred to in the art as “shape memory”, whereas spontaneous recovery is commonly referred to as pseudoelasticity. Pseudoelasticity, sometimes called superelasticity, is a reversible response to an applied stress, caused by a phase transformation between the austenite or parent phase and the martensite or daughter phase of a crystal. It is exhibited in shape memory alloys. Pseudoelasticity and shape memory both arise from the reversible motion of domain boundaries during the phase transformation, rather than just bond stretching or the introduction of defects in the crystal lattice. A pseudoelastic material may return to its previous shape after the removal of even relatively high applied strains by heating. For example, even if the secondary or daughter domain boundaries do become pinned, for example due to dislocations associated with plasticity, they may be reverted to the primary or parent phase by stresses generated through heating. Examples of shape memory materials include iron-chrome-nickel, iron-manganese, iron-palladium, iron-platinum, iron-nickel-cobalt-titanium, iron-nickel-cobalt-tantalum-aluminum-boron, copper-zinc-aluminum, copper-zinc-aluminum-nickel, copper-aluminum-nickel, and nickel-titanium alloys. Shape memory materials can also be alloyed with other materials including zinc, copper, gold, and iron.
Shape memory materials are presently used in a variety of applications. For example, a variety of military, medical, safety and robotic applications for shape memory materials are known. Medical grade shape memory materials are used for orthodontic wires, guide wires to guide catheters through blood vessels, surgical anchoring devices and stent applications, for example. One shape memory material in wide use, particularly in medical device applications, is a nickel-titanium shape memory material known as “Nitinol”.
Many medical grade shape memory wire products are made of biocompatible implant grade materials including “NiTi” materials. As used herein, “nickel-titanium material”, “nickel-titanium shape memory material” and “NiTi” refer to the family of nickel-titanium shape memory materials including Nitinol (an approximately equiatomic nickel-titanium, binary shape memory material) as well as alloys including nickel and titanium as primary constituents but which also include one or more additional elements as secondary constituents, such as Nitinol tertiary or quaternary alloys (Nitinol with additive metals such as chromium, tantalum, palladium, platinum, iron, cobalt, tungsten, iridium and gold).
Significant research has been dedicated to understanding how NiTi behaves in the body from the viewpoint of biological host response, but much less has been published that quantitatively correlates structure with mechanical properties.
More particularly, the fatigue properties of NiTi material have been the subject of recent research. The fatigue crack propagation behavior of Nitinol was studied in detail by McKelvey and Ritchie, as published in Fatigue-Crack Growth Behavior in the Superelastic and Shape-Memory Alloy Nitinol, Metallurgical and Materials Transactions, 32A, 2001, pgs. 731-743. McKelvey et al. observed that the crack growth propagation rate and ΔKth, which denotes the stress-intensity fatigue threshold in a given fatigue-crack growth scenario, were different for equivalent composition at martensite-stable and austenite-stable temperatures where the crack growth rate was generally lower at martensite-stable temperatures. They also observed that, under plane strain conditions, the heavily slipped material near the crack tip at superelastic regime temperatures remained austenitic, presumably inhibited from undergoing volume contractile, stress-induced phase transformation by the triaxial stress state, while plane stress conditions generally resulted in stress-induced martensite near the crack tip.
Wire products made of shape memory materials are manufactured by forming a relatively thick piece of hot-worked rod stock from a melt process. The rod stock is then further processed into wires by drawing the rod stock down to a thin diameter wire. During a drawing process, often referred to as a “cold working” process, a wire is pulled through a lubricated die to reduce its diameter. The deformation associated with wire drawing increases the stress in the material, and the stress eventually must be relieved by various methods of heat treatment or annealing at elevated temperatures to restore ductility, thus enabling the material to be further cold worked to a smaller diameter. Conventional wire annealing typically results in grain growth with a concomitant random crystal orientation, and the various material or fiber “textures” that are generated during cold wire drawing are mostly eliminated during conventional annealing and recrystallization. These iterative processes of cold working and annealing may be repeated several times before a wire of a desired diameter is produced and processing is completed.
Wire materials manufactured by the above processes typically contain microstructural defects, such as pores, inclusions, interstitials, and dislocations. An inclusion comprises a phase which possesses distinct properties from the primary material matrix and is divided from the matrix by a phase boundary. Inclusions may result from oxide or other metallic or non-metallic precipitate formation during primary melting or other high temperature treatment and may include carbides, nitrides, silicides, oxides or other types of particles. Inclusions may also arise from contamination of the primary melt materials or from the mold which contains the molten ingot. In the case of an interstitial, an atom occupies a site in the crystal structure at which there usually is not an atom. The atom may be a part of its host material, such as a base metal or alloying metal, or it may be an impurity. A dislocation is a linear defect around which some of the atoms of the crystal lattice are misaligned and appear as either edge dislocations or screw dislocations. Edge dislocations are caused by the termination of a plane of atoms in the middle of a crystal, while a screw dislocation comprises an internal structure in which a helical path is traced around the linear defect or dislocation line by the atomic planes of atoms in the crystal lattice. Mixed dislocations, combining aspects of screw and edge dislocations, may also occur.
Internal or external defects, such as inclusions, pores, or defects induced during wire processing may weaken the host material at the site of the defect, potentially resulting in failure of a material at the site of that defect. This weakening may be particularly acute where the defect is relatively large and/or of significantly disparate stiffness compared with adjacent dimensions of the material (such as for fine or small diameter wire). Failure of shape memory wires is more likely to occur at the site of the defect. Since inherent defects cannot be completely eliminated from the wire material, management of inherent defects and mitigation of their negative impact on wire properties is desirable.
One previously proposed solution to the problem of inherent defects has been to treat selected regions of a wire that are expected to be subjected to high strain by converting the bulk material in such regions to a different phase than the remainder of the bulk material of the wire. For example, under predetermined operating conditions, such as a predetermined operation temperature, the high strain wire regions are stabilized in a martensite phase while the lesser strain regions remain in an austenite phase. This method is therefore directed to treating predetermined regions of a wire to convert the bulk material in the regions to a more stable phase regardless of the presence, number, and location of any defects in the bulk material.
However, it may not always be possible or practical to predict what regions of a continuous wire will be subjected to high strains when portions of the wire are later incorporated into a medical device. It may also be desirable to leave defect-free portions of wire unaffected by mitigation efforts and, therefore, available to meet other design considerations. For example, a disadvantage of the above process is that for wire made of shape memory material, the regions that are stabilized in the martensite phase will lose the superelastic characteristic.
Although wires made in accordance with foregoing processes may demonstrate excellent fatigue strength, further improvements in fatigue strength are desired, particularly with reference to fatigue damage that propagates from defects.
What is needed is a method of manufacturing a wire that demonstrates improved fatigue strength, and medical devices that include such wire.
SUMMARY
The present disclosure relates to wire products, and medical devices including wire products, such as round and flat wire, strands, cables, coils, and tubing, made from a shape memory material or alloy. Defects within the material are isolated from a primary, or parent, material phase within one or more areas of stabilized secondary, or daughter, material phases that are resistant to failure, such that the wire product demonstrates improved fatigue strength. In one application, a method of mechanical conditioning in accordance with the present disclosure isolates defects in nickel-titanium or NiTi shape memory materials in localized areas or fields of a secondary material phase that are resistant to crack initiation and/or propagation, such as a martensite phase, while the remainder of the surrounding defect-free material remains in a primary material phase, such as an austenite phase, whereby the overall superelastic and/or nature of the material is preserved.
Wire products manufactured in accordance with the present disclosure maintain good mechanical properties in addition to improved fatigue performance Increases in the strain fatigue limit for both high cycle and low cycle fatigue are observed, while shape memory or superelastic characteristics are preserved.
As discussed below and shown in the Working Examples, the amount of secondary phase material formed about the defects during the mechanical conditioning process is sufficient to either completely isolate the defects or at least partially isolate high stress concentrator areas about the defects in order to the improve fatigue strength of the material and yet, when the bulk of the material reverts back to the primary phase after the mechanical conditioning, the overall amount of remaining secondary phase material that is formed about the defects is not sufficient compromise the shape memory or superelastic characteristic of the material as a whole. In this respect, the amount of mechanical conditioning may be specifically tailored to achieve a desired balance between fatigue strength and material elasticity.
In one form thereof, the present invention provides a medical device including a wire made of a nickel-titanium shape memory material, the wire having a fatigue endurance exceeding 0.95% strain amplitude at greater than 106 cycles.
In other embodiments, the medical device may include a wire having a fatigue endurance exceeding 1.1% strain amplitude at greater than 106 cycles, or a fatigue endurance exceeding 1.1% strain amplitude at greater than 109 cycles. In a further embodiment, the medical device may include a wire having a residual strain of less than 0.25% after being subjected to engineering strain of at least 9.5%.
In another form thereof, the present invention provides a medical device including a wire product made of a shape memory material, the shape memory material having a plurality of defects, the wire product substantially comprised of the shape memory material in a primary phase and including portions of the shape memory material comprising a secondary phase at localized regions disposed proximate respective defects, with at least some of the secondary phase portions separated by the primary phase.
The shape memory material may be a nickel-titanium shape memory material, in which the primary phase is an austenite phase, and the secondary phase portions comprise a martensite phase. The secondary phase portions may together comprise less than 15% of the shape memory material, by volume.
In a further embodiment, the shape memory material may be a nickel-titanium shape memory material, with the wire product having a fatigue endurance exceeding 0.95% strain amplitude at greater than 106 cycles, a fatigue endurance exceeding 1.1% strain amplitude at greater than 106 cycles, or a fatigue endurance exceeding 1.1% strain amplitude at greater than 109 cycles. The wire may also have a residual strain of less than 0.25% after being subjected to engineering strain of at least 9.5%. The wire product may be selected from the group consisting of wire having a circular cross-section, wire having a non-circular cross-section, cable, coil, and tubing.
In a further form thereof, the present invention provides a method, including the steps of: providing a wire product made of a shape-set, shape memory material; mechanically conditioning the wire product by: applying an engineering stress between 700 MPa and 1600 MPa; and releasing the applied engineering stress; and incorporating the wire product into a medical device. The mechanical conditioning step may occur either prior to or after the incorporation step.
In another embodiment, the mechanical conditioning step includes: applying an engineering stress between 900 MPa and 1450 MPa; and releasing the applied engineering stress. In a further embodiment, the mechanical conditioning step includes: applying an engineering stress between 1100 MPa and 1350 MPa; and releasing the applied engineering stress. The method may further include the repeating the mechanically conditioning step at least once.
In one embodiment, the shape memory material may be a nickel-titanium shape memory material, and the mechanical conditioning step may be conducted below a martensite deformation temperature (Md) of the nickel-titanium shape memory material. The mechanical conditioning step may further include: applying the first force to the wire product in an environment having a temperature T, wherein

T=Af±50° C.,

wherein Af is the austenite transformation finish temperature of the nickel-titanium shape memory material. The wire product may be selected from the group consisting of wire having a circular cross-section, wire having a non-circular cross-section, cable, coil, and tubing.




BRIEF DESCRIPTION OF THE DRAWINGS
The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention itself will be better understood by reference to the following descriptions of embodiments of the invention taken in conjunction with the accompanying drawings, wherein:
FIG. 1 is a schematic view of a portion of wire having an equiaxed grain structure;
FIG. 2 is a schematic view of the portion of wire of FIG. 1 having an elongated grain structure after cold work conditioning;
FIG. 3 is a schematic view of the portion of wire of FIG. 2 having an equiaxed grain structure with smaller grains than the equiaxed grain structure of the wire in FIG. 1 after a shape set annealing process;
FIG. 4 is a schematic view illustrating an exemplary drawing process using a lubricated die;
FIG. 5 is a depiction of the processing step of conditioning a wire using a mechanical conditioning method in accordance with the present disclosure;
FIG. 6 is a depiction of the processing step of releasing the tension in the wire of FIG. 6;
FIG. 7(a) is a view of a portion of a wire having internal and external defects;
FIG. 7(b) is an fragmentary view of a defect in the wire of FIG. 7(a);
FIG. 8 is a stress-strain curve for a mechanical conditioning process in accordance with the present disclosure;
FIG. 9 is a view of a portion of a wire having internal and external defects substantially surrounded by dislocation-stabilized secondary phase;
FIG. 10(a) is a fragmentary view of a defect in the wire of FIG. 9 substantially surrounded by dislocation-stabilized secondary phase;
FIG. 10(b) is a view of a portion of a wire having internal and external defects substantially surrounded by dislocation-stabilized secondary phase;
FIG. 10(c) is a view of a portion of a wire having internal and external defects substantially surrounded by dislocation-stabilized secondary phase;
FIG. 11 is a graphical strain-life representation of rotary beam fatigue data generated in accordance with Example 1 under the following test conditions: R=−1, T=298 K, f=60 s−1; environment: quiescent air, N=5 at each strain level;
FIG. 12(a) is a secondary electron (SE) image of a 100 nm deep cue mark for optical determination of defect zone;
FIG. 12(b) is an image of a transversely oriented, 10×3×0.5 μm (T×R×L) FIB-milled sharp defect;
FIG. 12(c) is an image that provides an overall view of a sharp defect zone;
FIG. 12(d) is an optical photograph of 150 μm diameter NiTi wires with cue marks evident near centerline;
FIG. 12(e) is a transverse SEM micrograph of a failed fatigue fracture specimen showing the FIB-milled sharp defect (FSD) depth corresponding to FIG. 12(b);
FIG. 13(a) shows graphical representations of cyclic tensile data for samples including overload conditioning cycle, non-conditioned, and conditioned samples, with varying test temperatures;
FIG. 13(b) is an enlarged insets showing a loading region of the graph shown in FIG. 13(a);
FIG. 13(c) is an enlarged insets showing an unloading region of the graph shown in FIG. 13(a);
FIG. 14 is a graphical representation of rotary bend fatigue data for conditioned (C) and non-conditioned (NC) samples under the following test conditions: T=300 K, rate=60 s−1, R=−1, with a maximum stress error=3% and a maximum cycle count error=0.5%;
FIG. 15 is a graphical representation of single test level (1% alternating engineering strain) data for FIB-sharp defect (FSD) and FSD-conditioned (FSD-C) samples, with the extension bars in the inset representing the data spread for n=3 samples;
FIG. 16 is a bright field TEM (BF-TEM) image of an FSD crack root after mechanical conditioning, with the insets showing selected area electron diffraction patterns (SADP) for regions within (left) and outside of (right) the structurally distinct zone demarcated by a dashed line and extending approximately 0.5 μm from the crack tip;
FIG. 17 is a graphical representation of crack growth rate data inferred from high resolution scanning electron microscopy of ductile striation spacing observations and estimated stress intensity at probable crack front location based on a semi-elliptical crack in an infinite rod;
FIG. 18(a) is a graph showing cycles to failure for five sets of wire samples, where a sample from each set of wires has been mechanically conditioned with a given level of engineering stress, and where the wires were tested at a 1.25% strain level;
FIG. 18(b) is a graph showing cycles to failure for the five sets of wire samples shown in FIG. 18(a), where a sample from each set of wires has been mechanically conditioned with a given level of engineering stress, and where the wires were tested at a 1.1% strain level;
FIG. 18(c) is a graph showing cycles to failure for the five sets of wire samples shown in FIG. 18(a), where a sample from each set of wires has been mechanically conditioned with a given level of engineering stress, and where the wires were tested at a 0.95% strain level;
FIG. 18(d) is a graph showing cycles to failure for the five sets of wire samples shown in FIG. 18(a), where a sample from each set of wires has been mechanically conditioned with a given level of engineering stress, and where the wires were tested at a 0.80% strain level;
FIG. 19(a) is a stress-strain curve for five wire samples, where each wire sample was loaded using the conditioning regime indicated by the legend at the right of the figure and described in Table 2;
FIG. 19(b) is a stress-strain curve for five wire samples, where each wire sample was loaded using the conditioning regime indicated by the legend at the right of the figure and described in Table 2;
FIG. 19(c) is a stress-strain curve for five wire samples, where each wire sample was loaded using the conditioning regime indicated by the legend at the right of the figure and described in Table 2;
FIG. 19(d) is a stress-strain curve for five wire samples, where each wire sample was loaded using the conditioning regime indicated by the legend at the right of the figure and described in Table 2;
FIG. 19(e) is a stress-strain curve for five wire samples, where each wire sample was loaded using the conditioning regime indicated by the legend at the right of the figure and described in Table 2;
FIG. 20 is a graph showing the percentage of isothermally non-recoverable strain in various wire materials as a function of a mechanical conditioning parameter;
FIG. 21(a) is a section view of Drawn Filled Tubing (DFT®) wire manufactured in accordance with an embodiment of the present disclosure (DFT® is a registered trademark of Fort Wayne Metals Research Products Corporation of Fort Wayne, Ind.);
FIG. 21(b) is a cross sectional view taken along line 18B-18B of FIG. 18(a);
FIG. 22(a) is an elevation view of a braided tissue scaffold or stent including a wire made in accordance with the present process; and
FIG. 22(b) is an elevation view of a knitted tissue scaffold or stent including a wire made in accordance with the present process.



Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate preferred embodiments of the invention and such exemplifications are not to be construed as limiting the scope of the invention in any manner.
DETAILED DESCRIPTION
The present disclosure relates to wire products, and medical devices including wire products, such as round and flat wire, strands, cables, coils, and tubing, made from a shape memory material or alloy. Defects within the material are isolated from a primary, or parent, material phase within one or more stabilized secondary, or daughter, material phases that are resistant to failure, such that the wire product demonstrates improved fatigue strength. In one application, a method of mechanical conditioning in accordance with the present disclosure isolates defects in nickel-titanium or NiTi shape memory materials in localized areas or fields of a secondary material phase that are resistant to crack initiation and/or propagation, such as a martensite phase, while the remainder of the surrounding defect-free material remains in a primary material phase, such as an austenite phase, whereby the overall superelastic nature of the material is preserved.
As used herein, a “defect” refers to material defects such as crack-like defects, inclusions, dislocations, and other non-uniformities, as well as any other internal or external defects or stress risers present in a material, as well as melt intrinsic and extrinsic defects such as inclusions, porosity, voids and oxide precipitate formation after melting.
Exemplary manufacturing processes by which wires may be made in accordance with the present disclosure are set forth in Section I below, and general descriptions of the resulting physical characteristics of wires made in accordance with the present process are set forth in Section II below. Working Examples are set forth in Section III below. Applications using wires made in accordance with the present disclosure are set forth in Section IV below.
Several suitable shape memory materials may be used for forming wire products according to the present disclosure. As used herein, “shape memory material” encompasses medical grade shape memory materials including nickel-titanium or NiTi (defined above), as well as medical grade shape memory alloys including beta titanium alloys (such as Beta C that comprise primarily the beta phase at room temperature), and any other medical grade shape memory alloys exhibiting similar superelastic and/or shape memory characteristics such as tantalum-titanium, titanium-niobium, and iron-nickel-cobalt alloys. Additionally, as used herein, “shape memory material” also encompasses non-medical grade shape memory alloys such as iron-chrome-nickel, iron-manganese, iron-palladium, iron-platinum, iron-nickel-cobalt-titanium, iron-nickel-cobalt-tantalum-aluminum-boron, copper-zinc-aluminum, copper-zinc-aluminum-nickel, copper-aluminum-nickel, and nickel-titanium alloys.
Moreover, it is contemplated that various shape-memory materials having either a one-way memory effect or a two-way memory effect, and other related materials, may be subjected to the present mechanical conditioning process to achieve enhanced physical characteristics identified in the discussion below and in the corresponding Working Examples.
As discussed in detail in Section IV below, fatigue damage resistant shape memory wire made in accordance with the present disclosure may be used in medical devices such as, for example, implantable cardiac pacing, shocking and/or sensing leads, implantable neurological stimulating and/or sensing leads, wire-based stents, blood filter devices, or any other medical device application in which high fatigue strength and/or a shape memory or superelastic characteristic is desired. Wire products produced in accordance with the present disclosure may also be used in non-medical device applications in which high fatigue strength and/or a shape memory or superelastic characteristic is desired.
As used herein, “wire” or “wire product” encompasses continuous wire and wire products, such as wire having a round cross section and wire having a non-round cross section, including flat wire, as well as other wire-based products such as strands, cables, coil, and tubing.
I. Description of the Present Manufacturing Process
1. Wire Preparation
Prior to the mechanical conditioning process of the present disclosure, discussed below, wire made of a shape memory material is subjected to cold work prior to undergoing a shape set annealing process. The shape setting step imparts the primary shape memory and/or superelastic characteristics of the material prior to mechanical conditioning.
Initial preparation of a wire may involve first forming a piece of rod stock, for example, based on conventional melt processing techniques, followed by one or more iterations of conventional cold working and annealing. Referring to FIG. 1, a schematic or exaggerated view of a portion of wire 10 manufactured in accordance with conventional cold working and annealing techniques is shown. Wire 10 has been subjected to one or more, perhaps several or a very large number of, iterations of conventional cold working and annealing, as described above, to form an equiaxed crystal structure within the material of wire 10. Representative equiaxed crystals are depicted in wire 10 at 12. As used herein, “equiaxed” refers to a crystal structure in which the individual crystals 12 have axes that are approximately the same length, such that the crystals 12 collectively have a large number of slip planes, leading to high strength and ductility. However, it is not necessary that the grain structure be equiaxed. The grain structure may, for example, contain deformed grains that have been recovered to the B2 cubic austenite phase through the high temperature shape setting process described below.
Referring now to FIG. 2, prior to the shape-set anneal, wire 10 may optionally subjected to further cold work in the form of a cold work conditioning step if a nanograin microstructure is desired. As used herein, “cold work conditioning” means imparting a relatively large amount of cold work to a material, such as by wire drawing, swaging, or otherwise forming.
Referring to FIG. 4, the cold work conditioning step is performed by drawing wire 10 through a lubricated die 18 (FIG. 4) having a an output diameter D2, which is less than diameter D1 of the undrawn wire 10 shown in FIG. 2. In one exemplary embodiment, the cold work conditioning step by which the diameter of wire 10 is reduced from D1 to D2 is performed in a single draw and, in another embodiment, the cold work conditioning step by which the diameter of wire 10 is reduced from D1 to D2 is performed in multiple draws which are performed sequentially without any annealing step therebetween.
Further discussion of exemplary cold work conditioning processes are presented in U.S. patent application Ser. No. 12/563,062, entitled FATIGUE DAMAGE RESISTANT WIRE AND METHOD OF PRODUCTION THEREOF, filed Sep. 18, 2009, assigned to the present assignee, the disclosure of which is hereby expressly incorporated by reference herein in its entirety. The foregoing reference also discloses methods of limited annealing following the cold work conditioning to create a nanograin microstructure, which may optionally be applied to wires prior to subjecting same to the mechanical conditioning process in accordance with the present disclosure, discussed below.
Regardless of the amount of cold work imparted to the wire and/or whether cold work conditioning is used, once drawn to the desired size, and as shown in FIG. 5, wire 10 undergoes a shape setting annealing process in which it is continuously annealed under constant tension sufficient to hold the wire in a substantially linear configuration during the shape set annealing process. Shape set annealing typically occurs at a temperature between 300° C. (673 K) and 600° C. (1073 K), where the temperature is sufficiently high to restore a majority of the wire material to the primary or austenite phase. The shape setting annealing process may result in the formation of a new crystallographic structure, which may comprise nano-scale equiaxed crystals 16 in wire 10, as shown in FIG. 3. However, as noted above, an equiaxed or nanograin crystal microstructure is not required for the mechanical conditioning process described below.
2. Mechanical Conditioning
In accordance with the present disclosure, wire made of a shape memory material may be subjected to a mechanical conditioning process to improve its resistance to fatigue damage. In the present embodiment, mechanical conditioning is performed by application of a force to the wire in an environment having a temperature range within approximately 50° C. of the austenite transformation finish temperature (Af), i.e., T=Af±50° C.
Referring now to FIGS. 5 and 6, in the mechanical conditioning process, shape memory wire, shown in FIG. 5, in the heat set annealed condition is secured at a first end. Controlled engineering strain is applied in the direction of arrow F to a finished engineering strain, εc, of 0.08 to 0.14 units, wherein:




  
    
      ɛ
      c
    
    =
    
      
        Δ
        ⁢
        
          
        
        ⁢
        L
      
      
        L
        o
      
    
  
  ,




with Lo being the initial length, and ΔL being the length increase imparted by strain.

The range for εc may be from as little as 0.06, 0.08, or 0.10 units to as much as 0.13, 0.14 0.16 units, or within any range encompassed by the foregoing values, to provide the increased benefits to the shape memory wire discussed herein. It is thought that if the end point is lower than 0.08, the mechanical conditioning may not impart the desired physical properties to the wire, as any dislocation or stress-induced secondary phase, such as martensite that is formed, may revert back to the primary or parent phase, such as austenite. In contrast, if the end point is in excess of 0.14, the wire may potentially be strained by the mechanical conditioning beyond its elastic deformation range to the extent that an undesirably large amount of plastic deformation may result.
An alternative characterization of the mechanical conditioning process of the present disclosure may be expressed as the application of engineering stress to the material. For an application of force in a tensile test as described herein, engineering stress is calculated using the following equation:




  
    σ
    e
  
  =
  
    P
    
      A
      0
    
  




where σe is the engineering stress, P is the force applied, and A0 is the cross-sectional area of the material before application of force. In engineering stress terms, the range for σe may be from as little as 700 MPa, 900 MPa or 1100 MPa to as much as 1350 MPa, 1450 MPa or 1600 MPa, or within any range encompassed by the foregoing values, to provide the increased benefits to the shape memory wire discussed herein.

However, it is contemplated that force may be applied to the wire or wire product using alternate stress loading regimes, such as via methods other than a tensile test, that may be more appropriate to the geometry of a particular wire product.
The temperature should be maintained below the martensitic deformation temperature, Md, during the application of force to the wire in the mechanical conditioning process. If the temperature exceeds the martensitic deformation temperature, the bulk of the shape memory wire material will not transform to martensite upon loading, because any plastic deformation will occur in the austenite phase, and therefore the localized phase transformation mechanism would not occur. The entirety of the wire material will remain in a plastically deformed austenite state, with the austenite or primary phase containing significant plasticity and little retained martensite phase.
Referring to FIG. 6, once the desired stress or strain is applied to the wire, the force is removed and the sample is allowed to freely recover and, if the temperature is below the austenitic finish temperature, or if further recovery of the bulk material is required, heat Q is applied to drive the temperature, T to sufficiently greater than the austenitic finish temperature (i.e., T>Af) for recovery of the bulk material, leaving the material in a state wherein defects are still isolated by a dislocation stabilized secondary phase as discussed below.
The steps of applying a controlled engineering stress or strain and subsequently removing the force to allow the sample to freely recover may repeated as few as 1, 2 or 3 times or as many as 6, 8 or 10 times, for example, to increase the amount of dislocation stabilized secondary phase within the wire material, as discussed in detail below. Thus, after one or more applications of load cycles and the attendant recovery of applied strain εc, the length of wire 10 is greater than its original length Lo. More particularly, the length of wire 10 after load conditioning is Lo+ps, where ps is the permanent set or isothermally non-recoverable strain resulting from plastic, pseudoplastic and other deformation mechanisms, as shown in FIG. 6. Some of this isothermally non-recoverable strain can be recovered in the bulk material by slight heating of the material as discussed above.
As discussed in more detail below in Section III, this isothermally non-recoverable strain is indicative of the amount or volume of the wire material that has been converted from the primary phase to the secondary phase and, upon recovery of the wire material, remains stabilized in the secondary material phase. These localized areas of secondary phase material isolate defects and inhibit crack propagation in the primary phase material.
For example, as calculated in Examples 3-7 below, this isothermally non-recoverable strain may be calculated by measuring the difference in wire length after load removal in a tensile test. Known tensile test devices (including the test device used for the present Working Examples) collect wire length data as the test is conducted. This data, not presented herein, is used to generate the permanent set data presented in the tables. This non-recoverable length, with the original length subtracted therefrom, gives a positive value where isothermally non-recoverable deformation has occurred (i.e., “permanent set”). This difference can then be divided by the original length, the product of which is a strain value representing the isothermally non-recoverable strain. This amount arises from a residual volume of altered material within the wire which has accommodated a given amount of strain not recovered upon load removal. The observed isothermally non-recoverable strain may be divided by the load plateau strain length, which is associated with the forward transformation from parent austenite phase to secondary, stress-induced, martensite phase, thereby providing a quantitative indication of the volume fraction of altered material within the wire.
This volume fraction, referred to as “max. volume martensite %” in Tables 3-7 below is calculated using the following formula:




  
    V
    m
  
  =
  
    
      I
      ⁢
      
        
      
      ⁢
      N
      ⁢
      
        
      
      ⁢
      R
      ⁢
      
        
      
      ⁢
      S
    
    
      L
      ⁢
      
        
      
      ⁢
      P
      ⁢
      
        
      
      ⁢
      S
      ⁢
      
        
      
      ⁢
      L
    
  




wherein VM is the maximum volume fraction of secondary phase, INRS is the isothermally non-recoverable strain and LPSL is the loading plateau strain length.

The volume fraction sets an upper limit on the amount of wire material that has been converted to the secondary phase from the primary phase and remains stable at the given test temperature after load removal. That is to say, the total volume of material represented by the non-recoverable strain comprises secondary phase material, and may also comprise other non-primary phase material arising from plastically deformed primary phase material or other deformation phenomena.
It is counter-intuitive that the application of stress to a wire product at a level sufficient to initiate plastic deformation according to the present mechanical conditioning process could be beneficial for the use of that wire product in a medical device that utilizes the shape memory or superelastic characteristic of the wire product. One of ordinary skill in the art would consider a wire product made of a shape memory material that has been subjected to a stress level sufficient to induce any amount of plastic deformation to be compromised in its shape memory or superelastic characteristic and therefore unsuitable for use in a medical device in which this characteristic is desired.
II. Description of Material Properties of Wire Products Made in Accordance with the Present Manufacturing Process
Wire products made of shape memory materials or alloys that have been subjected to the mechanical conditioning process of the present disclosure exhibit several novel physical characteristics and/or novel combinations of physical characteristics, including the following:
1. Isolation of Defects
Referring to FIGS. 7(a)-(b), shape memory wire 10 may have one or more defects, such as internal defects 28 and/or external defects 30. These defects may include extrinsic defects and/or intrinsic defects such as inclusions or porosity as discussed above, for example.
These defects are isolated in localized fields or areas of secondary phase material by subjecting the wire to mechanical conditioning, as exemplified by the curve shown in FIG. 8. As discussed above, this may be accomplished by applying an engineering stress (and concomitant engineering strain) so that at least some parts of wire 10 experience plastic deformation. In an exemplary embodiment of the present process, however, nearly all of the strain may be recovered upon unloading (FIG. 8).
Referring now to FIGS. 9-10(c), mechanical conditioning results in areas of dislocation stabilized B19′, R, and/or martensite, shown as secondary phase areas 26 in FIG. 10, forming proximate defects 28 in wire 10. The formation of the secondary phase areas around and/or adjacent defects 28 during mechanical conditioning helps to retard fatigue crack growth in subsequent cyclic loading in a direction emanating from defects 28, as it is known that cracks propagate more slowly in B19′, R, and/or martensite than in austenite. However, the bulk of wire 10, where defects are not present, reverts back to the austenite phase after mechanical conditioning, such that the overall wire still exhibits its shape memory or superelastic characteristic while at the same time having an enhanced degree of fatigue strength due to the isolation of defects within the secondary phase material.
Referring still to FIGS. 9-10(c), wires 10, 10′ and 10′′ are shown after mechanical conditioning. As a result of mechanical conditioning in accordance with the present process, areas of dislocation stabilized secondary material phase 26 formed proximate material defects stabilize the defects. That is to say, while the bulk of the wire material reverts back to the primary phase from the secondary phase, localized areas of secondary phase material remain formed proximate material defects. This stabilization of the secondary phase areas 26 is advantageous in that it helps to retard fatigue crack growth in subsequent cyclic loading, for example, as a crack generally propagates more slowly in the secondary (i.e., martensite) phase than in the primary (i.e., austenite) phase.
Stabilization of secondary phase areas 26 is, at least in part, due to plastic deformation 27 comprising dislocations and/or dislocation networks. This plastic deformation acts to stabilize secondary phase 26 after removal of the conditioning mechanical conditioning stress or strain (when primary phase returns to the bulk of the wire material) and during subsequent service. The defect-free portions of the wire material may have less plastic deformation, or may have substantially no plastic deformation. Therefore, this defect-free material will revert back to the primary phase more readily and completely than the localized secondary phase areas near the defects, which have experienced plastic deformation.
As shown in FIGS. 10(a)-(c), the shape, size and/or spatial configuration of secondary phase 26 varies depending upon the characteristics of the defect proximate the localized secondary phase field. In general, the secondary phase field will form around the highest stress areas of the defect, and may not form at lower stress areas. This is because plastic deformation occurs most readily at the site of stress concentrators during the mechanical conditioning process; the dislocation stabilized secondary phase areas, which include some plastic deformation 27, will form at these stress concentration points even though the primary phase portions of the wire are still within a relatively elastic or pseudoelastic (where pseudoelastic is defined as elasticity associated with primary to secondary phase transformation) deformation range. Primary phase material remains present between any pair of defects that are sufficiently far apart, such that their isolation fields to not overlap.
For example, wire 10 shown in FIG. 10(a) has a secondary phase area 26 extending around substantially the entirety of defect 28. The geometry of defect 28, as well as the direction application of force F, determines the overall shape of secondary phase area 26.
As shown in FIG. 10(b), wire 10′ has defects 28′ with stabilized secondary phase areas 26′ at the highest stress concentration areas created by the application of force F. Plastic deformation 27 also occurs within secondary phase areas 26′ as discussed above. Some of secondary phase areas 26′ are adjacent one another and have overlapping boundaries, so that a defect 28′ and another nearby defect 28′ will be influence one another.
Similarly, wire 10′′ shown in FIG. 10(c) has multiple defects 28′′ with stress fields 26′′ and plastic deformation 27. Again, the stress fields 28′′ form at the highest stress concentration points, which are a function of the geometry of defects 28′′ and the direction of application of force F (shown as a longitudinal force along the axis of wire 10′′) as well as the temperature of the material during force application.
In this manner, shape memory material wire subjected to the present mechanical conditioning process exhibits an enhanced fatigue life and fatigue strain threshold. Moreover, a shape memory wire made in accordance with the present process retains overall material properties consistent with wire in the austenitic phase, while exhibiting inhibition of crack propagation at defect sites consistent with the martensitic phase.
2. Increased High-Cycle Fatigue Resistance
As a result of the isolation of defects and/or defect boundaries (i.e. the sites along the defect/primary phase boundary most susceptible to stress concentration and crack propagation) in a secondary phase area or field, mechanical conditioning increases the fatigue life and fatigue-strain threshold of the shape memory wire. As discussed in Section III, wire conditioned in accordance with an embodiment of the present disclosure exhibited a gain in the fatigue strain limit at 100 million (108) cycles of greater than 25% (FIG. 11). Also, as shown in FIG. 14, conditioned wire demonstrated an upward strain shift of greater than 20% at a 10 million (107) cycle life (i.e., 1.1% engineering strain versus 0.9% engineering strain). Further, eight samples of this conditioned material survived more than 109 cycles and were still running at the time conclusion of Example I discussed below.
3. Increased Damage Tolerance and Low-Cycle Fatigue Resistance
As discussed in Section III at Example 2, wire conditioning in accordance with an embodiment of the present disclosure demonstrated increased tolerance of damage to the wire. Three specimens with focused ion beam (FIB)-milled sharp defects were tested at an alternating strain of 1% in the conditioned and non-conditioned states. As shown in FIG. 15, Conditioned samples demonstrated a 50% increase in damage tolerance compared with the non-conditioned samples.
4. High Recoverable Strain, Low Residual Strain
As shown in FIGS. 13(a)-(c), greater than 8% recoverable engineering strain was observed with zero residual strain and good plateau stresses at body temperature (i.e., 310 K) after mechanical conditioning.
This recoverable strain renders wire made in accordance with the present disclosure particularly suitable for certain medical device applications. As mentioned above, this is a counter-intuitive result. Typically, wire made of a shape memory or superelastic material which has been subjected to forces sufficient to cause any plastic deformation in the wire material would be considered “damaged”, and therefore unsuitable for use in any medical device application. The surprising result of the present process is that, under proper mechanical conditioning parameters as discussed herein, wire subjected to such forces is actually superior for medical device applications.
III. EXAMPLES
The following non-limiting Examples illustrate various features and characteristics of the present invention, which are not to be construed as limited thereto.
The examples offer analysis of the effect of mechanical conditioning in fine (such as less than 250 μm diameter) Nitinol wire, and particularly of the effect of stress riser or defect isolation.
Example 1
Fatigue Resistant Nitinol Intermetallic Wire
Nanocrystalline, nominally Ti-56 wt. % Ni Nitinol wire (“NiTi wire”) was manufactured to create a superelastic, precipitate free wire with a median grain size of 50 nm. An exemplary process for creating such a wire is described in U.S. patent application Ser. No. 12/563,062, entitled FATIGUE DAMAGE RESISTANT WIRE AND METHOD OF PRODUCTION THEREOF, filed Sep. 18, 2009, the disclosure of which is hereby expressly incorporated by reference herein in its entirety.
The resulting wire was drawn through diamond drawing dies beginning at a diameter of 0.230 mm and ending at a diameter of 0.177 mm to yield a retained cold work level of about 40% cold work. The wire was then continuously annealed at a temperature of 773K-873K for less than 60 seconds, to yield a 50 nm grain size as verified by TEM electron microscopy scanning. Specifically, field emission scanning electron microscopy or transmission electron microscopy (TEM) is used to gather an image containing, for example, several hundred crystals or grains exhibiting strong grain boundary contrast. Next, the image is converted to a binary format suitable for particle measurement. Resolvable grains are modeled with ellipsoids and subsequently measured digitally yielding statistics regarding the crystal or grain size, such as the average size, maximum size, and minimum size. The resulting average crystal size is taken to be the average crystal size for the material from which the sample was taken. Grain size verification is discussed in detail in U.S. patent application Ser. No. 12/563,062, entitled FATIGUE DAMAGE RESISTANT WIRE AND METHOD OF PRODUCTION THEREOF, filed Sep. 18, 2009, incorporated by reference above.
After annealing, the wire of the present Example was subjected to cyclic tensile testing and was determined to exhibit pseudoelasticity out to greater than 10% engineering strain. A first wire sample was preserved at this point in the process.
The remainder of the wire was then subjected to mechanical conditioning, as described in detail above, by loading the wire to about 12% axial engineering strain (i.e., 0.12 units), completely releasing the load, reloading to about 12% engineering strain, and once again completely releasing the load. A second wire sample was preserved after this point in the process.
The first and second wire samples were subjected to rotary beam fatigue testing. Referring now to FIG. 11, the first, non-mechanically conditioned sample generated data curve 100 exhibiting a 100 M cycle engineering strain limit of about 0.85% at N=10 data points, shown by right most data point 100′ of the curve 100. The second, mechanically conditioned sample generated data curve 102 exhibiting a 100 M cycle engineering strain limit of about 1.1% at N=10 data points, exemplifying a greater than 25% gain in the fatigue strain limit at 100 M cycles, shown by right most data point 102′ of curve 102.
Example 2
Mechanical Conditioning of Superelastic NiTi Wire for Improved Fatigue Resistance
In this example, the effects of mechanical overload conditioning of superelastic wire and the possibility of increased fatigue damage resistance associated with near-defect, plasticity-locked phase transformation were investigated. In thin wires, where plane stress dominates, it is expected that sufficient loading will result in phase transformation near the largest or shape-conducive crack-like defects, such as constituent inclusion particles, before conversion of the bulk, defect-free material.
1. Experimental Technique
Samples for this Example were subjected to a total engineering strain departure, measured by crosshead extension, of about 11.5%. Conditioning was applied by approaching the martensitic yield point at 295 K using strain-rate-controlled loading in order induce some dislocation locking of stress-transformed material in the vicinity of stress concentrators. Referring now to FIG. 8, the conditioning cycle comprises a strain-controlled ramp to a stress level of 1240 MPa engineering stress, resulting in an engineering strain of about 11.5%, followed by a 3 second hold, and finishing with a strain-controlled ramp to zero load.
In order to prepare samples for this Example, Nitinol wire with an ingot austenite start temperature, AS, of 243 K, having Ti-56 wt. % Ni was repetitively drawn and annealed from a diameter of 2 mm to a diameter of 177 μm in accordance with the process described above. At this stage, wires were continuously annealed at 770 to 800 K. Final cold working was completed using diamond dies to draw round wire with a diameter of 150 μm prior to continuous, reel-to-reel annealing at 750 to 780 K under constant engineering stress for less than 60 seconds to effect linear shape setting. The final wire comprised a room-temperature-superelastic Nitinol wire with an active austenitic finish temperature, Af, of 280 K and an approximately 120 nm thick, dark brown oxide layer similar to that disclosed in an article by the present inventor entitled “Structure-Property Relationships in Conventional and Nanocrystalline NiTi Intermetallic Alloy Wire” and published in the Journal of Materials Engineering and Performance, February 2008, ISSN 1544-1024, pgs. 1-6, the entire disclosure of which is hereby expressly incorporated by reference herein.
Focused ion beam (FIB)-milled sharp defects 202 (“FSD”) were then milled into material 200 of each sample in order to act as preferential sites of incipient fatigue crack formation and to facilitate user-defined damage localization and monitoring. An FEI dual-beam (Nova 200 NanoLab) focused ion beam (FIB) with in situ scanning electron microscopy (SEM) was used to simultaneously monitor samples by SEM during the FIB milling process. A 30 keV Ga+ ion beam was used to precisely mill transverse defects into wire specimens at a 0.50 nA beam current. Defects 202 were of consistent dimension measuring 10 μm transverse length by 3 μm radial depth by 0.5 μm axial surface width, an example of which is shown in FIGS. 12(a)-(e). Cue lines 204 (FIG. 12(a)) were milled into the oxide surface at a depth of about 50 nm on either side of each sharp defect in order to enhance optical detection for accurate placement in fatigue test gages after removal from the SEM chamber. As shown in FIGS. 12(a)-(e), the cue lines were of sufficient depth to create a visually detectible gradient associated with the reduced oxide thickness, while shallow enough to minimize undesirable mechanical impact.
Electron microscopy of fracture surfaces was carried out using a Hitachi S4800 field emission SEM (FE-SEM) operated at 10 to 20 kV. Transmission electron microscopy samples were extracted and prepared using the FIB/SEM dual beam equipment previously mentioned with an in situ sample manipulator for thin foil removal and transport to TEM grids. Additional details regarding this method can be found in the article by the present inventor, which is incorporated by reference herein above, namely the article entitled “Structure-Property Relationships in Conventional and Nanocrystalline NiTi Intermetallic Alloy Wire” and published in the Journal of Materials Engineering and Performance, February 2008, ISSN 1544-1024, pgs. 1-6. TEM imaging and diffraction experiments were carried out on a 200 kV machine equipped with a LaB6 emitter (Tecnai 20, FEI Company, Oregon).
Cyclic and monotonic uniaxial tensile properties were measured at an ambient temperature of 295 K at a strain rate of 10−3 s−1 using an Instron Model 5565 Tensile Test Bench equipped with pneumatic, smooth face grips. Six hundred grit emery-cloth was used to reduce grip-specimen interface slip. Elevated temperature testing was completed on an equivalent tensile bench fitted with an environmental chamber capable of maintaining a temperature of 310±0.5 K.
Fatigue behavior was characterized using rotary beam fatigue test equipment manufactured by Positool, Inc., at a test rate of 60 s−1 in ambient 298 K air. The test rate was chosen at a rate significantly higher than physiological loading frequencies to promote expediency. Data have recently been presented by Robertson and Ritchie, in an article entitled “In vitro fatigue-crack growth and fracture toughness behavior of thin-walled superelastic Nitinol tube for endovascular stents: A basis for defining the effect of crack-like defects”, published in Biomaterials 28, 2007, pgs. 700-709, suggesting that high rate testing may well estimate in vivo fatigue failure lifetimes.
As shown in FIGS. 13(a)-13(c), specimens from each group, non-conditioned (NC) and conditioned (C), were tested at alternating engineering strain (1⁄2 peak-to-peak amplitude) levels ranging from 0.8 to 1.6% to a maximum of about 109 cycles or 200 days test time. Further, specimens with FSD 202 were tested at 1% engineering strain before and after conditioning. The FSD zone was located at the apex of the fatigue bend by optical positioning using the cue marks 204 as guides.
2. Results
The resulting tensile data is shown graphically in FIGS. 13(a)-(c). As noted above, the conditioning cycle comprised a strain-controlled ramp to a stress level of 1240 MPa engineering stress followed by a 3 second hold, finishing with a strain-controlled ramp to zero load. This conditioning cycle generated data curve 220, shown in FIGS. 13(a)-(c). The total engineering strain departure for this cycle, measured by crosshead extension, was 11.5%. Conditioning initially resulted in 0.3% residual strain comprising both plastic and pseudo-plastic strain contributions.
The martensite to austenite reversion plateau stress associated with unloading was significantly reduced during unloading from the conditioning cycle, but was observed to elevate in subsequent testing to 8% engineering strain. Some of this effect can be accounted for by strain rate differences: the conditioning cycle was run at a significantly higher strain rate than the 8% test cycles. High strain rates can cause heating during loading and cooling during unloading resulting in increasing stress hysteresis. Further testing of a C sample at body temperature (310 K) generated data curve 226 depicted on FIGS. 13(a)-(c) by circular marks 226′, showing elevation of the unloading plateau stress to levels greater than an NC sample at 295 K, shown as data curve 222 with square-shaped marks 222. This result was consistent with known test temperature-plateau stress relationships.
The conditioned sample at 295K, shown as curve 224 on FIGS. 13(a)-(c) with triangle-shaped marks 224′, exhibited a downward shift in the unloading plateau stress. This can be attributed to plastic deformation, some of which may be directly beneficial to resistance against subsequent fatigue crack growth. The lack of significant shift in the strain length of the plateaus indicates that plastic deformation to the overall microstructure was minimal during overload conditioning.
FIG. 14 illustrates the observed differences in fatigue performance for conditioned (C) wire specimens, shown as data curve 230 with triangular-shaped marks 230′, versus non-conditioned (NC) wire specimens, shown as data curve 232 with triangular-shaped marks 232′. The conditioning resulted in an upward strain shift of greater than 20% at the 107 cycle life (i.e., 1.1% engineering strain versus 0.9% engineering strain). Eight samples of the conditioned material survived more than 109 cycles and were still running at the time conclusion of the experiment.
As shown in FIG. 15, three FSD specimens in each of the NC and C states were tested at an alternating strain of 1%. In this case, the FSD-C group generated data bar graph 242, showing an average of 21,228 cycles to failure with a margin of error indicated at the top of the bar. The FSD-NC group generated data bar graph 240, showing an average of 14,196 cycles to failure with a margin of error indicated at the top of the bar. Thus, the conditioned wire samples outperformed the non-conditioned samples by 50%. All FSD samples failed considerably before the non-FSD samples; this is attributable to the geometry of the FIB-milled defects, which were purposefully milled larger and sharper than the 2-6 μm inclusion particles typically found at fatigue failure sites in this grade of Nitinol wire in order to direct site-specific, locatable failure for study.
A microstructurally distinct region resulting from the mechanical conditioning was found within an approximately 500 nm radius of the approximately 10 nm width FSD crack root. FIG. 16 shows the results of TEM work performed to help elucidate mechanisms giving rise to mechanical property changes associated with the mechanical conditioning. The selected area diffraction patterns 250, 252 in FIG. 16, outside of and within the distinct FSD concentration zone respectively, reveal significant differences in contributing bright field contrast signal. A typical polycrystalline, B2 pattern 250 was observed at approximately 1 μm from the crack tip, while the selected area diffraction pattern adjacent to the root, shown as 252, revealed what appears to be superimposed diffuse rings, B2 polycrystalline reflections, as well as some evidence of 1⁄2(110) reflections associated with the B19′ martensitic phase. Also evident is a significant increase in dislocation density and associated contrast.
The diffuse (110) rings observed within the FSD zone, shown in the left and upper-right insets of FIG. 16, may be related to partial amorphization and/or due to (110) reflection splitting and the presence of 1⁄2(110) reflections associated with a mixed B2-B19′ structure.
The presence of plasticity in the FSD region may also contribute to increased fatigue performance due to residual stresses which offset the effective crack-opening stress intensity range. The reduction in the operating stress intensity range can serve to increase the effective ΔKth fatigue threshold (defined above) thereby elevating the strain load level required to initiate or maintain crack growth. The mixed microstructure may also promote crack front tortuosity thereby precluding crack arrest associated with roughness-induced closure at near-threshold crack growth conditions.
The narrowest ductile striations were observed in conditioned samples near the FSD incipient crack front. High resolution SEM (HR-SEM) analysis of fatigue failure sites in NC and C specimens was completed and the stress intensity was estimated based on the probable crack front location at the examined site using assumptions of a semi-elliptical crack in an infinite cylinder. Referring to FIG. 17, a crack growth rate plot as a function of the estimated stress intensity factor (not taking into account crack closure effects) is shown, with square-shaped marks 260 indicating data on non-conditioned material and triangle-shaped marks 262 indicating conditioned material. The difference between the two data sets may suggest martensitic growth rate inhibition.
In this example, it has been demonstrated that mechanical conditioning of superelastic NiTi wire results in improved fatigue performance, while maintaining good mechanical properties. In addition, greater than 8% recoverable engineering strain was observed with zero residual strain and good plateau stresses at body temperature after mechanical conditioning. Further, an increase in the strain fatigue limit of greater than 20% at 107 cycles is observed in conditioned versus non-conditioned wire with an observed increase in low cycle life of 50%. The tensile overload conditioning treatment also resulted in a mixed-phase microstructure in the vicinity of stress concentrators that comprises increased dislocation density and possible plasticity-induced or roughness-induced crack closure.
Introduction to Examples 3-7
For examples 3-7, various wire materials were tested in a similar manner for comparison to one another. FIGS. 18(a)-(d) show results for wire materials from each of Examples 3-7, with each Figure representing a different strain condition (as indicated on each respective Figure). Table 1, below, indexes the Example materials:







TABLE 1







Index of Example Materials for Examples 3-6



















Austenitic




Nominal
Actual
Shape set
Surface
Finish



Nitinol
Composition
Diameter
temperature
Finish
Temperature


Example
type
(weight %)
(mm)
(K)
Description
Af (K)





3
NiTi #1
56Ni—Ti
0.151
740-800
Oxide
280


4
NiTi #2
55.8Ni—0.25Cr—Ti
0.269
740-800
Oxide
283


5
NiTi #4
55.8Ni—Ti
0.302
740-800
Oxide
298


6
NiTi #1
56Ni—Ti
0.076
740-800
Etched
288


7
NiTi #1
56Ni—Ti
0.638
740-800
Polished
291









FIGS. 19(a)-(e) also show results for wire materials from each of Examples 3-7, with each Figure representing a stress-strain curve for a corresponding Example. Table 2, below, indexes the mechanical conditioning regimes applied to each wire product shown in FIGS. 19(a)-(e). The “regime” number corresponds to a given curve on each figure, as indicated by the corresponding number on the legend at the right side of each of FIGS. 19(a)-(e), where, the digit preceding the decimal point in X.X refers to the Example (e.g. 1, 2, 3, 4, and 5 correspond to Examples 3, 4, 5, 6, and 7 respectively), and the digit following the decimal point corresponds to regimes 1, 3, 4, 6, and 7 given below in Table 2.







TABLE 2







Index of Conditioning Regimes for Examples 3-6









Regime (See

Hold stress


also legends at
Ramp rate
(MPa) (See also


right of FIGS.
(engineering
X-axes in FIGS.


19(a)-(e))
strain/min)
18(a)-(d))












1
0.10
700


3
0.10
1100


4
0.10
1240


6
0.10
1400


7
0.10
1500









Results shown in Tables 3-7 show data used to create FIGS. 19(a)-19(e) respectively. Thus, Tables 3-7 correspond with Examples 3-7, respectively.
Example 3
Mechanical Conditioning of Superelastic NiTi Wire for Improved Fatigue Resistance
In this example, the effects of mechanical overload conditioning of superelastic wire and the possibility of increased fatigue damage resistance associated with near-defect, plasticity-locked phase transformation were further investigated over a broader range of loads as compared to Example 2.
1. Experimental Technique
Samples for this Example were subjected to a total engineering strain departure, measured by crosshead extension, ranging from about 8% to 12.5% Conditioning was applied by approaching the martensitic yield point at 295 K using strain-rate-controlled loading in order induce some dislocation locking of stress-transformed material in the vicinity of stress concentrators. Referring now to FIG. 19(a), the conditioning cycle comprises a strain-controlled ramp to five stress levels of 700, 1100, 1240, 1400, and 1500 MPa engineering stress, resulting in an engineering strain of about 8.3%, 9.8%, 10.3%, 11.1% and 12.2% respectively, followed by a 3 second hold, finishing with a strain-controlled ramp to zero load.
In order to prepare samples for this Example, Nitinol wire with an ingot austenite start temperature, AS, of 243 K, having Ti-56 wt. % Ni was repetitively drawn and annealed from a diameter of 2 mm to a diameter of 201 μm in accordance with the process described above. At this stage, wires were continuously annealed at 950 to 1000 K. Final cold working was completed using diamond dies to draw round wire with a diameter of 151 μm prior to continuous, reel-to-reel annealing at 750 to 780 K under constant engineering stress for 40 to 80 seconds to effect linear shape setting. The final wire comprised a room-temperature-superelastic Nitinol wire with an active austenitic finish temperature, Af, of 280 K and an approximately 120 nm thick, dark brown oxide layer similar to that disclosed in an article by the present inventor entitled “Structure-Property Relationships in Conventional and Nanocrystalline NiTi Intermetallic Alloy Wire” and published in the Journal of Materials Engineering and Performance, February 2008, ISSN 1544-1024, pgs. 1-6, which is incorporated by reference above.
Cyclic and monotonic uniaxial tensile properties were measured at an ambient temperature of 295 K at a strain rate of 10−3 s−1 using an Instron Model 5565 Tensile Test Bench equipped with pneumatic, smooth face grips. Six hundred grit emery-cloth was used to reduce grip-specimen interface slip.
Fatigue behavior was characterized using rotary beam fatigue test equipment manufactured by Positool, Inc., at a test rate of 60 s−1 in ambient 298 K air. The test rate was chosen at a rate significantly higher than physiological loading frequencies to promote expediency.
As shown in FIGS. 18(a)-(d), three specimens from each group at each conditioning cycle, ranging from 0 MPa which indicates non-conditioned wire to 1500 MPa which indicates the maximum conditioning load used, were tested at alternating engineering strain (1⁄2 peak-to-peak amplitude) levels ranging from 0.80 to 1.25% strain to a maximum of about 106 cycles. The total samples tested for each conditioning load regime for each sample was 12 resulting in a total of 72 fatigue samples tested for this portion of the study. Samples which did not fracture after 106 cycles were stopped and recorded.
2. Results
The resulting tensile data is shown graphically in FIGS. 18(a)-(d) as line 1 on each curve. As noted above, the conditioning cycle comprised a strain-controlled ramp to a stress level of 0, 700, 1100, 1240, 1400 or 1500 MPa engineering stress, as indicated along the horizontal axis of the plot, followed by a 3 second hold, finishing with a strain-controlled ramp to zero load.
This conditioning cycle generated data curves 1.1, 1.3, 1.4, 1.6 and 1.7 shown in FIG. 19(a). The total engineering strain departure for respective cycles, measured by crosshead extension, was about 0%, 8.3%, 9.8%, 10.3%, 11.1% and 12.2% for increasing load levels respectively. Conditioning resulted in residual strains (i.e. isothermally non-recoverable strains) of respectively 0%, 0.19%, 0.20%, 0.24%, 0.17%, ad 7.3% respectively.
FIGS. 18 (a) to (d) illustrate the observed differences in fatigue performance for non-conditioned (e.g. 0 load level on x-axis) and conditioned wire specimens (700, 1100, 1240, 1400, and 1500 MPa on the x-axis) The conditioning resulted in an upward cycle life shift of at least 55% and 3000% at the 1.25% and 0.95% alternating strain test levels respectively at a conditioning load level of 1240 MPa. An overall upward trend in lifetime for a given test strain level was observed for increasing conditioning load through 1500 MPa. Most samples of the material conditioned at greater than 1240 MPa survived more than 106 cycles and were still running at the time of conclusion of the experiment for test strain levels below 0.95%.
FIG. 19(a) illustrates the observed tensile behavior during load conditioning of each sample. In each case, an upper bound of the maximum volume of retained martensite was calculated as described above based on the ratio of isothermally non-recoverable strain to the strain length of the load plateau. FIG. 20 illustrates the positive correlation between isothermally non-recoverable strain and conditioning load. Non-recoverable strain was less than 0.17% for all samples conditioned below 1400 MPa resulting in a max. volume of retained martensite estimate of 3.7% for the same samples loaded below 1400 MPa.
In this Example, it has been demonstrated that mechanical conditioning of superelastic NiTi wire results in improved fatigue performance, while maintaining good mechanical properties. In addition, less than 3.7% of the matrix was left in the martensite phase after load removal for conditioning below 1400 MPa with a concomitant maximum isothermally non-recoverable strain of 0.17%. Further, an increase in the strain fatigue life of greater than 3000% at 106 cycles is observed in wire conditioned at 1240 MPa versus non-conditioned wire while maintaining good elastic properties suitable for said medical device applications.
Example 4
Mechanical Conditioning of Superelastic NiTi Wire for Improved Fatigue Resistance
In this Example, the effects of mechanical overload conditioning of superelastic wire and the possibility of increased fatigue damage resistance associated with near-defect, plasticity-locked phase transformation were further investigated over a broader range of loads as compared to Example 2 using a high strength chromium doped tertiary Nitinol compound.
1. Experimental Technique
Samples for this Example were subjected to a total engineering strain departure, measured by crosshead extension, ranging from about 7.7% to 13.1% Conditioning was applied by approaching the martensitic yield point at 295 K using strain-rate-controlled loading in order induce some dislocation locking of stress-transformed material in the vicinity of stress concentrators. Referring now to FIG. 19(b), the conditioning cycle comprises a strain-controlled ramp to five stress levels of 0, 700, 1100, 1240, 1400, and 1500 MPa engineering stress, resulting in an engineering strain of about 0%, 7.7%, 9.6%, 10.2%, 11.3%, and 13.1% respectively, followed by a 3 second hold, finishing with a strain-controlled ramp to zero load.
In order to prepare samples for this Example, Nitinol wire with an ingot austenite start temperature, AS, of about 235 K, having Ti-55.8 wt. % Ni-0.25 wt % Cr was repetitively drawn and annealed from a diameter of 2 mm to a diameter of 361 μm in accordance with the process described above. At this stage, wires were continuously annealed at 950 to 1000 K. Final cold working was completed using diamond dies to draw round wire with a diameter of 269 μm prior to continuous, reel-to-reel annealing at 750 to 780 K under constant engineering stress for 40 to 80 seconds to effect linear shape setting. The final wire comprised a room-temperature-superelastic Nitinol wire with an active austenitic finish temperature, Af, of 283 K and an approximately 120 nm thick, dark brown oxide layer similar to that disclosed in an article by the present inventor entitled “Structure-Property Relationships in Conventional and Nanocrystalline NiTi Intermetallic Alloy Wire” and published in the Journal of Materials Engineering and Performance, February 2008, ISSN 1544-1024, pgs. 1-6, which is incorporated by reference above.
Cyclic and monotonic uniaxial tensile properties were measured at an ambient temperature of 295 K at a strain rate of 10−3 s−1 using an Instron Model 5565 Tensile Test Bench equipped with pneumatic, smooth face grips. Six hundred grit emery-cloth was used to reduce grip-specimen interface slip.
Fatigue behavior was characterized using rotary beam fatigue test equipment manufactured by Positool, Inc., at a test rate of 60 s−1 in ambient 298 K air. The test rate was chosen at a rate significantly higher than physiological loading frequencies to promote expediency.
As shown in FIGS. 18 (a) to (d), three specimens from each group at each conditioning cycle, ranging from 0 MPa which indicates non-conditioned wire to 1500 MPa which indicates the maximum conditioning load used, were tested at alternating engineering strain (1⁄2 peak-to-peak amplitude) levels ranging from 0.80 to 1.25% strain to a maximum of about 106 cycles. The total samples tested for each conditioning load regime for each sample was 12 resulting in a total of 72 fatigue samples tested for this portion of the study. Samples which did not fracture after 106 cycles were stopped and recorded.
2. Results
The resulting tensile data is shown graphically in FIG. 19(b). As noted above, the conditioning cycle comprised a strain-controlled ramp to a stress level of 0, 700, 1100, 1240, 1400 or 1500 MPa engineering stress, as indicated along the horizontal axis of the plot, followed by a 3 second hold, finishing with a strain-controlled ramp to zero load. This conditioning cycle generated data curves 2.1, 2.3, 2.4, 2.6 and 2.7 shown in FIG. 19(b). The total engineering strain departure for respective cycles, measured by crosshead extension, was about 0%, 7.7%, 9.6%, 10.2%, 11.3%, and 13.1% for increasing load levels respectively. Conditioning resulted in residual strains (i.e. isothermally non-recoverable strains) of respectively 0%, 0.07%, 0.09%, 0.35%, 0.18%, and 2.23% respectively.
FIGS. 18 (a)-(d) illustrate the observed differences in fatigue performance for non-conditioned (e.g. 0 load level on x-axis) and conditioned wire specimens (700, 1100, 1240, 1400, and 1500 MPa on the x-axis) The conditioning resulted in an upward cycle life shift of at least 46% and 2500% at the 1.25% and 0.95% alternating strain test levels respectively at a conditioning load level of 1240 MPa. An overall upward trend in lifetime for a given test strain level was observed for increasing conditioning load through 1500 MPa. Most samples of the material conditioned at greater than 1240 MPa survived more than 106 cycles and were still running at the time of conclusion of the experiment for test strain levels below 0.95%.
FIG. 19(b) illustrates the observed tensile behavior during load conditioning of each sample. In each case, an upper bound of the maximum volume of retained martensite was calculated as described above based on the ratio of isothermally non-recoverable strain to the strain length of the load plateau. FIG. 20 illustrates the positive correlation between isothermally non-recoverable strain and conditioning load. Non-recoverable strain was less than 0.35% for all samples conditioned below 1400 MPa resulting in a max volume of retained martensite estimate of 5.9% for the same samples loaded below 1400 MPa.
In this Example, it has been demonstrated that mechanical conditioning of superelastic NiTi wire results in improved fatigue performance, while maintaining good mechanical properties. In addition, less than 5.9% of the matrix was left in the martensite phase after load removal for conditioning below 1400 MPa with a concomitant maximum isothermally non-recoverable strain of 0.35%. Further, an increase in the strain fatigue life of greater than 2500% at 106 cycles is observed in wire conditioned at 1240 MPa versus non-conditioned wire while maintaining good elastic properties suitable for said medical device applications.
Example 5
Mechanical Conditioning of Superelastic NiTi Wire for Improved Fatigue Resistance
In this Example, the effects of mechanical overload conditioning of superelastic wire and the possibility of increased fatigue damage resistance associated with near-defect, plasticity-locked phase transformation were further investigated over a broader range of loads as compared to Example 2 using a Nitinol with warmer transformation temperatures as compared to Examples 2 and 3.
1. Experimental Technique
Samples for this Example were subjected to a total engineering strain departure, measured by crosshead extension, ranging from about 8.3% to 12.8% Conditioning was applied by approaching the martensitic yield point at 295 K using strain-rate-controlled loading in order induce some dislocation locking of stress-transformed material in the vicinity of stress concentrators. Referring now to FIG. 19 (c), the conditioning cycle comprises a strain-controlled ramp to five stress levels of 0, 700, 1100, 1240, and 1400 MPa engineering stress, resulting in an engineering strain of about 0%, 8.3%, 10%, 10.8%, and 12.8% respectively, followed by a 3 second hold, finishing with a strain-controlled ramp to zero load.
In order to prepare samples for this Example, Nitinol wire with an ingot austenite start temperature, AS, of about 255 K, having Ti-55.8 wt. % Ni was repetitively drawn and annealed from a diameter of 2 mm to a diameter of 380 μm in accordance with the process described above. At this stage, wires were continuously annealed at 950 to 1000 K. Final cold working was completed using diamond dies to draw round wire with a diameter of 302 μm prior to continuous, reel-to-reel annealing at 750 to 780 K under constant engineering stress for 40 to 80 seconds to effect linear shape setting. The final wire comprised a room-temperature-superelastic Nitinol wire with an active austenitic finish temperature, Af, of 298 K and an approximately 120 nm thick, dark brown oxide layer similar to that disclosed in an article by the present inventor entitled “Structure-Property Relationships in Conventional and Nanocrystalline NiTi Intermetallic Alloy Wire” and published in the Journal of Materials Engineering and Performance, February 2008, ISSN 1544-1024, pgs. 1-6, which is incorporated by reference above.
Cyclic and monotonic uniaxial tensile properties were measured at an ambient temperature of 295 K at a strain rate of 10−3 s−1 using an Instron Model 5565 Tensile Test Bench equipped with pneumatic, smooth face grips. Six hundred grit emery-cloth was used to reduce grip-specimen interface slip.
Fatigue behavior was characterized using rotary beam fatigue test equipment manufactured by Positool, Inc., at a test rate of 60 s−1 in ambient 298 K air. The test rate was chosen at a rate significantly higher than physiological loading frequencies to promote expediency.
As shown in FIGS. 18 (a) to (d), three specimens from each group at each conditioning cycle, ranging from 0 MPa which indicates non-conditioned wire to 1400 MPa which indicates the maximum conditioning load used, were tested at alternating engineering strain (1⁄2 peak-to-peak amplitude) levels ranging from 0.80 to 1.25% strain to a maximum of about 106 cycles. The total samples tested for each conditioning load regime for each sample was 12 resulting in a total of 60 fatigue samples tested for this portion of the study. Samples which did not fracture after 106 cycles were stopped and recorded.
2. Results
The resulting tensile data is shown graphically in FIG. 19 (c). As noted above, the conditioning cycle comprised a strain-controlled ramp to a stress level of 0, 700, 1100, 1240, or 1400 MPa engineering stress, as indicated along the horizontal axis of the plot, followed by a 3 second hold, finishing with a strain-controlled ramp to zero load. This conditioning cycle generated curves 3.1, 3.3, 3.4, and 3.6 shown in FIG. 19(c). The total engineering strain departure for respective cycles, measured by crosshead extension, was about 0%, 8.3%, 10%, 10.8%, and 12.8% for increasing load levels respectively. Conditioning resulted in residual strains (i.e. isothermally non-recoverable strains) of respectively 0%, 0.03%, 0.13%, 1.1%, and 7.31% respectively.
FIGS. 18 (a) to (d) illustrate the observed differences in fatigue performance for non-conditioned (e.g. 0 load level on x-axis) and conditioned wire specimens (700, 1100, 1240, and 1400 MPa on the x-axis) The conditioning resulted in an upward cycle life shift of at least 116% and 2400% at the 1.25% and 0.95% alternating strain test levels respectively at a conditioning load level of 1240 MPa. An overall upward trend in lifetime for a given test strain level was observed for increasing conditioning load through 1400 MPa. Most samples of the material conditioned at greater than 1240 MPa survived more than 106 cycles and were still running at the time of conclusion of the experiment for test strain levels below 0.95%.
FIG. 19(c) illustrates the observed tensile behavior during load conditioning of each sample. In each case, an upper bound of the maximum volume of retained martensite was calculated as described above based on the ratio of isothermally non-recoverable strain to the strain length of the load plateau. FIG. 20 illustrates the positive correlation between isothermally non-recoverable strain and conditioning load. Non-recoverable strain was less than about 1% for all samples conditioned below 1240 MPa resulting in a max. volume of retained martensite estimate of about 17% for the same samples loaded below 1240 MPa.
In this Example, it has been demonstrated that mechanical conditioning of superelastic NiTi wire results in improved fatigue performance, while maintaining good mechanical properties. In addition, less than about 17% of the matrix was left in the martensite phase after load removal for conditioning below 1240 MPa with a concomitant maximum isothermally non-recoverable strain of about 1%. Further, an increase in the strain fatigue life of greater than 2400% at 106 cycles is observed in wire conditioned at 1240 MPa versus non-conditioned wire while maintaining good elastic properties suitable for said medical device applications.
Example 6
Mechanical Conditioning of Superelastic NiTi Wire with an Etched Surface Finish for Improved Fatigue Resistance
In this Example, the effects of mechanical overload conditioning of superelastic wire and the possibility of increased fatigue damage resistance associated with near-defect, plasticity-locked phase transformation were further investigated over a broader range of loads as compared to Example 2 and in a finer diameter using a Nitinol with an etched surface finish comprising a substantially oxide-free surface.
1. Experimental Technique
Samples for this Example were subjected to a total engineering strain departure, measured by crosshead extension, ranging from about 7.8 to 11.9% Conditioning was applied by approaching the martensitic yield point at 295 K using strain-rate-controlled loading in order induce some dislocation locking of stress-transformed material in the vicinity of stress concentrators. Referring now to FIG. 19 (d), the conditioning cycle comprises a strain-controlled ramp to five stress levels of 0, 700, 1100, 1240, 1400 and 1500 MPa engineering stress, resulting in an engineering strain of about 0%, 7.8%, 9.5%, 10%, 10.8%, and 11.9% respectively, followed by a 3 second hold, finishing with a strain-controlled ramp to zero load.
In order to prepare samples for this Example, Nitinol wire with an ingot austenite start temperature, AS, of about 246 K, having Ti-56 wt. % Ni was repetitively drawn and annealed from a diameter of 2 mm to a diameter of 102 μm in accordance with the process described above. At this stage, wires were continuously annealed at 950 to 1000 K. Final cold working was completed using diamond dies to draw round wire with a diameter of 76 μm prior to continuous, reel-to-reel annealing at 750 to 780 K under constant engineering stress for 40 to 80 seconds to effect linear shape setting. The final wire comprised a room-temperature-superelastic Nitinol wire with an active austenitic finish temperature, Af, of 288 K and an etched, substantially oxide-free surface finish.
Cyclic and monotonic uniaxial tensile properties were measured at an ambient temperature of 295 K at a strain rate of 10−3 s−1 using an Instron Model 5565 Tensile Test Bench equipped with pneumatic, smooth face grips. Six hundred grit emery-cloth was used to reduce grip-specimen interface slip.
Fatigue behavior was characterized using rotary beam fatigue test equipment manufactured by Positool, Inc., at a test rate of 60 s−1 in ambient 298 K air. The test rate was chosen at a rate significantly higher than physiological loading frequencies to promote expediency.
As shown in FIGS. 18 (a) to (d), three specimens from each group at each conditioning cycle, ranging from 0 MPa which indicates non-conditioned wire to 1500 MPa which indicates the maximum conditioning load used, were tested at alternating engineering strain (1⁄2 peak-to-peak amplitude) levels ranging from 0.80 to 1.25% strain to a maximum of about 106 cycles. The total samples tested for each conditioning load regime for each sample was 12 resulting in a total of 72 fatigue samples tested for this portion of the study. Samples which did not fracture after 106 cycles were stopped and recorded.
2. Results
The resulting tensile data is shown graphically in FIG. 19 (d). As noted above, the conditioning cycle comprised a strain-controlled ramp to a stress level of 0, 700, 1100, 1240, 1400 or 1500 MPa engineering stress, as indicated along the horizontal axis of the plot, followed by a 3 second hold, finishing with a strain-controlled ramp to zero load. This conditioning cycle generated curves 4.1, 4.3, 4.4, 4.6 and 4.7 shown in FIG. 19(d). The total engineering strain departure for respective cycles, measured by crosshead extension, was about 0%, 7.8%, 9.5%, 10%, 10.8%, and 11.9% for increasing load levels respectively. Conditioning resulted in residual strains (i.e. isothermally non-recoverable strains) of respectively 0%, 0.09%, 0.14%, 0.29%, 0.26% and 7.2% respectively.
FIGS. 18 (a) to (d) illustrate the observed differences in fatigue performance for non-conditioned (e.g. 0 load level on x-axis) and conditioned wire specimens (700, 1100, 1240, 1400 and 1500 MPa on the x-axis) The conditioning resulted in an upward cycle life shift of at least 7.2% and 2600% at the 1.25% and 0.95% alternating strain test levels respectively at a conditioning load level of 1240 MPa. An overall upward trend in lifetime for a given test strain level was observed for increasing conditioning load through 1500 MPa. Most samples of the material conditioned at greater than 1240 MPa survived more than 106 cycles and were still running at the time of conclusion of the experiment for test strain levels below 0.95%.
FIG. 19(d) illustrates the observed tensile behavior during load conditioning of each sample. In each case, an upper bound of the maximum volume of retained martensite was calculated as described above based on the ratio of isothermally non-recoverable strain to the strain length of the load plateau. FIG. 20 illustrates the positive correlation between isothermally non-recoverable strain and conditioning load. Non-recoverable strain was less than about 0.29% for all samples conditioned below 1240 MPa resulting in a max. volume of retained martensite estimate of about 4.7% for the same samples loaded below 1400 MPa.
In this Example, it has been demonstrated that mechanical conditioning of superelastic NiTi wire results in improved fatigue performance, while maintaining good mechanical properties. In addition, less than about 4.7% of the matrix was left in the martensite phase after load removal for conditioning below 1400 MPa with a concomitant maximum isothermally non-recoverable strain of about 0.29%. Further, an increase in the strain fatigue life of greater than 2600% at 106 cycles is observed in wire conditioned at 1240 MPa versus non-conditioned wire while maintaining good elastic properties suitable for said medical device applications.
Example 7
Mechanical Conditioning of Superelastic NiTi Wire with a Polished Surface Finish for Improved Fatigue Resistance
In this Example, the effects of mechanical overload conditioning of superelastic wire and the possibility of increased fatigue damage resistance associated with near-defect, plasticity-locked phase transformation were further investigated over a broader range of loads as compared to Example 2 and in a larger diameter using a Nitinol with an etched and mechanically polished surface finish comprising a substantially oxide-free surface.
1. Experimental Technique
Samples for this Example were subjected to a total engineering strain departure, measured by crosshead extension, ranging from about 7.5 to 13.1% Conditioning was applied by approaching the martensitic yield point at 295 K using strain-rate-controlled loading in order induce some dislocation locking of stress-transformed material in the vicinity of stress concentrators. Referring now to FIG. 19 (d), the conditioning cycle comprises a strain-controlled ramp to five stress levels of 0, 700, 1100, 1240, 1400 and 1500 MPa engineering stress, resulting in an engineering strain of about 0%, 7.5%, 9.6%, 10.3%, 11.6%, and 13.1% respectively, followed by a 3 second hold, finishing with a strain-controlled ramp to zero load.
In order to prepare samples for this Example, Nitinol wire with an ingot austenite start temperature, AS, of about 249 K, having Ti-56 wt. % Ni was repetitively drawn and annealed from a diameter of 2 mm to a diameter of 813 μm in accordance with the process described above. At this stage, wires were continuously annealed at 950 to 1000 K. Final cold working was completed using diamond dies to draw round wire with a diameter of 638 μm prior to continuous, reel-to-reel annealing at 750 to 780 K under constant engineering stress for 60 to 150 seconds to effect linear shape setting. The final wire comprised a room-temperature-superelastic Nitinol wire with an active austenitic finish temperature, Af, of 291 K and an etched, substantially oxide-free and mechanically polished surface finish.
Cyclic and monotonic uniaxial tensile properties were measured at an ambient temperature of 295 K at a strain rate of 10−3 s−1 using an Instron Model 5565 Tensile Test Bench equipped with pneumatic, smooth face grips. Six hundred grit emery-cloth was used to reduce grip-specimen interface slip.
Fatigue behavior was characterized using rotary beam fatigue test equipment manufactured by Positool, Inc., at a test rate of 60 s−1 in ambient 298 K air. The test rate was chosen at a rate significantly higher than physiological loading frequencies to promote expediency.
As shown in FIGS. 18 (a) to (d), three specimens from each group at each conditioning cycle, ranging from 0 MPa which indicates non-conditioned wire to 1500 MPa which indicates the maximum conditioning load used, were tested at alternating engineering strain (1⁄2 peak-to-peak amplitude) levels ranging from 0.80 to 1.25% strain to a maximum of about 106 cycles. The total samples tested for each conditioning load regime for each sample was 12 resulting in a total of 72 fatigue samples tested for this portion of the study. Samples which did not fracture after 106 cycles were stopped and recorded.
2. Results
The resulting tensile data is shown graphically in FIG. 19 (e). As noted above, the conditioning cycle comprised a strain-controlled ramp to a stress level of 0, 700, 1100, 1240, 1400 or 1500 MPa engineering stress, as indicated along the horizontal axis of the plot, followed by a 3 second hold, finishing with a strain-controlled ramp to zero load. This conditioning cycle generated data curves 5.1, 5.3, 5.4, 5.6 and 5.7 shown in FIG. 19(e). The total engineering strain departure for respective cycles, measured by crosshead extension, was about 0%, 7.5%, 9.6%, 10.3%, 11.6%, and 13.1% for increasing load levels respectively. Conditioning resulted in residual strains (i.e. isothermally non-recoverable strains) of respectively 0%, 0.21%, 0.40%, 0.61%, 2.17%, and 7.84% respectively.
FIGS. 18 (a) to (d) illustrate the observed differences in fatigue performance for non-conditioned (e.g. 0 load level on x-axis) and conditioned wire specimens (700, 1100, 1240, 1400 and 1500 MPa on the x-axis) The conditioning resulted in an upward cycle life shift of at least 54% at the 0.95% alternating strain test levels respectively at a conditioning load level of 1240 MPa. An overall upward trend in lifetime for a given test strain level was observed for increasing conditioning load through 1500 MPa. Most samples of the material conditioned at greater than 1240 MPa survived more than 106 cycles and were still running at the time of conclusion of the experiment for test strain levels below 0.80%.
FIG. 19(e) illustrates the observed tensile behavior during load conditioning of each sample. In each case, an upper bound of the maximum volume of retained martensite was calculated as described above based on the ratio of isothermally non-recoverable strain to the strain length of the load plateau. FIG. 20 illustrates the positive correlation between isothermally non-recoverable strain and conditioning load. Non-recoverable strain was less than about 0.61% for all samples conditioned below 1240 MPa resulting in a max. volume of retained martensite estimate of about 11% for the same samples loaded below 1240 MPa.
In this Example, it has been demonstrated that mechanical conditioning of superelastic NiTi wire results in improved fatigue performance, while maintaining good mechanical properties. In addition, less than about 11% of the matrix was left in the martensite phase after load removal for conditioning below 1240 MPa with a concomitant maximum isothermally non-recoverable strain of about 0.61%. Further, an increase in the strain fatigue life of greater than 54% at 106 cycles is observed in wire conditioned at 1240 MPa versus non-conditioned wire while maintaining good elastic properties suitable for said medical device applications.







TABLE 3







Tensile Data for Various Wire Samples, see Examples 3-6, FIG. 19(a)









Strain level


















1.1
Engr.
1.3
Engr.
1.4
Engr.
1.6
Engr.
1.7
Engr.



Engr.
Stress
Engr.
Stress
Engr.
Stress
Engr.
Stress
Engr.
Stress



Strain
(MPa)
Strain
(MPa)
Strain
(MPa)
Strain
(MPa)
Strain
(MPa)






















0
0
0
0
0
0
0
0
0
0



0.0019
106.7
0.0020
108.5
0.0009
51.2
0.0016
86.8
0.0010
55.8



0.0154
655.3
0.0132
601.7
0.0047
295.3
0.0053
315.4
0.0048
287.4



0.0157
614.5
0.0163
680.2
0.0134
626.4
0.0085
431.0
0.0096
461.7



0.0201
644.5
0.0180
591.1
0.0170
686.4
0.0139
630.5
0.0135
614.8



0.0523
643.1
0.0195
635.8
0.0209
612.7
0.0189
642.5
0.0160
663.5



0.0796
639.8
0.0513
636.2
0.0305
636.1
0.0462
657.7
0.0393
621.6



0.0817
699.7
0.0747
638.4
0.0701
634.9
0.0612
631.2
0.0579
624.2



0.0821
716.8
0.0784
635.4
0.0753
643.3
0.0766
642.1
0.0787
631.7



0.0826
700.5
0.0797
669.7
0.0793
690.4
0.0793
665.2
0.0804
680.6



0.0746
431.8
0.0822
739.3
0.0826
755.0
0.0824
748.1
0.0856
809.0



0.0718
360.0
0.0899
933.9
0.0866
854.4
0.0872
870.6
0.0948
1036.8



0.0688
293.9
0.0968
1100.3
0.1006
1189.9
0.0953
1082.6
0.1083
1341.4



0.0646
222.5
0.0974
1116.1
0.1012
1201.9
0.1075
1351.4
0.1209
1500.8



0.0633
206.7
0.0976
1114.3
0.1019
1218.4
0.1082
1353.9
0.1223
1508.2



0.0618
190.8
0.0969
1092.6
0.1026
1234.8
0.1105
1400.7
0.1202
1427.7



0.0593
171.7
0.0921
891.0
0.1030
1245.6
0.1113
1406.2
0.1184
1353.7



0.0558
160.0
0.0801
460.0
0.1033
1243.3
0.1084
1286.7
0.1096
1012.9



0.0526
168.1
0.0774
381.8
0.1012
1157.5
0.1069
1225.5
0.0940
463.0



0.0461
160.4
0.0742
303.2
0.0985
1043.9
0.1007
970.9
0.0798
100.3



0.0393
178.4
0.0705
225.0
0.0887
662.5
0.0788
240.8
0.0734
4.9



0.0288
170.2
0.0667
165.6
0.0791
358.0
0.0725
116.4
0.0729
0.1



0.0233
169.8
0.0615
113.5
0.0745
246.1
0.0640
33.1
0.0727
0.0



0.0113
167.4
0.0584
106.0
0.0691
149.6
0.0590
39.1



0.0063
169.4
0.0540
111.2
0.0651
103.4
0.0511
33.0



0.0019
0.0
0.0426
113.7
0.0585
84.8
0.0440
35.5





0.0309
112.0
0.0520
91.9
0.0352
38.7





0.0074
113.2
0.0435
93.6
0.0265
35.2





0.0044
92.9
0.0335
95.8
0.0152
67.2





0.0030
53.3
0.0262
93.2
0.0115
56.6





0.0020
0.0
0.0174
81.9
0.0048
25.7







0.0122
99.3
0.0017
0.0







0.0089
111.8







0.0062
76.2







0.0029
17.0







0.0024
0.0


load plateau
0.0663

0.0690

0.0658

0.0654

0.0652


length (strain) →


unload plateau
0.0627

0.0648

0.0627

0.0623

0.0071


length (strain) →


permanent set
0.0019

0.0020

0.0024

0.0017

0.0727


(strain) →


max volume fraction
2.9%

2.8%

3.7%

2.6%

100.0%


martensite (%) →
















TABLE 4







Tensile Data for Various Wire Samples, see Examples 3-6, FIG. 19(b)









Strain level


















2.1
Engr.
2.3
Engr.
2.4
Engr.
2.6
Engr.
2.7
Engr.



Engr.
Stress
Engr.
Stress
Engr.
Stress
Engr.
Stress
Engr.
Stress



Strain
(MPa)
Strain
(MPa)
Strain
(MPa)
Strain
(MPa)
Strain
(MPa)






















0
0
0
0
0
0
0
0
0
0



0.0052
313.2
0.0009
49.5
0.0009
51.0
0.0007
37.9
0.0008
44.9



0.0107
540.7
0.0047
278.5
0.0047
277.4
0.0044
268.7
0.0046
274.8



0.0148
647.1
0.0092
470.9
0.0093
470.0
0.0090
464.1
0.0092
469.4



0.0152
645.2
0.0165
668.0
0.0166
668.5
0.0163
671.1
0.0164
670.8



0.0159
650.8
0.0205
641.1
0.0205
634.1
0.0203
632.2
0.0204
638.9



0.0164
654.0
0.0253
640.1
0.0253
627.5
0.0251
647.9
0.0252
647.7



0.0170
657.3
0.0584
649.5
0.0584
644.5
0.0582
624.2
0.0583
641.6



0.0245
650.4
0.0694
646.7
0.0695
641.9
0.0692
642.2
0.0694
644.0



0.0684
641.0
0.0728
655.8
0.0728
658.1
0.0726
666.4
0.0727
656.6



0.0734
637.0
0.0757
702.2
0.0757
701.7
0.0755
701.9
0.0756
701.7



0.0743
652.5
0.0782
749.1
0.0782
743.7
0.0780
747.0
0.0781
746.1



0.0749
666.0
0.0901
1006.6
0.0901
1000.8
0.0899
1007.8
0.0900
999.6



0.0763
697.5
0.0928
1059.7
0.0941
1088.6
0.0938
1097.3
0.0939
1084.7



0.0767
706.1
0.0932
1061.0
0.0970
1149.8
0.0967
1159.7
0.0969
1145.1



0.0767
696.1
0.0936
1070.4
0.0992
1189.1
0.1003
1230.4
0.1004
1213.8



0.0653
386.6
0.0944
1087.2
0.1000
1195.9
0.1059
1326.9
0.1060
1310.6



0.0603
304.9
0.0951
1103.2
0.1007
1210.8
0.1084
1345.7
0.1114
1384.3



0.0553
260.5
0.0955
1109.7
0.1014
1226.5
0.1091
1352.0
0.1169
1440.2



0.0506
244.2
0.0955
1104.3
0.1021
1239.8
0.1099
1363.8
0.1199
1442.0



0.0446
249.3
0.0953
1098.4
0.1022
1240.6
0.1102
1369.1
0.1202
1438.8



0.0361
260.2
0.0919
961.0
0.1022
1236.0
0.1106
1377.1
0.1219
1454.3



0.0183
258.0
0.0877
800.9
0.1022
1231.9
0.1112
1385.9
0.1239
1475.3



0.0158
267.6
0.0867
762.9
0.1016
1210.2
0.1113
1388.1
0.1260
1488.8



0.0086
232.7
0.0827
626.9
0.0976
1045.7
0.1118
1396.0
0.1279
1497.3



0.0071
218.2
0.0792
518.0
0.0941
905.5
0.1123
1401.5
0.1309
1499.0



0.0053
192.6
0.0759
428.1
0.0908
780.5
0.1128
1399.3
0.1287
1417.2



0.0028
113.0
0.0707
313.8
0.0856
601.6
0.1129
1397.9
0.1252
1281.9



0.0008
22.5
0.0671
255.5
0.0820
493.4
0.1106
1307.1
0.1162
952.8



0.0007
0.0
0.0634
210.8
0.0783
393.2
0.1087
1230.3
0.1114
787.8





0.0588
185.1
0.0737
291.6
0.1041
1047.3
0.1060
614.2





0.0540
179.0
0.0689
212.5
0.0993
864.9
0.0943
298.8





0.0517
180.0
0.0666
184.7
0.0970
781.4
0.0908
222.1





0.0469
186.4
0.0618
148.3
0.0922
617.6
0.0864
141.9





0.0323
190.5
0.0472
152.9
0.0851
407.8
0.0820
78.7





0.0205
191.7
0.0354
166.8
0.0777
237.4
0.0779
35.3





0.0096
180.8
0.0245
165.2
0.0735
169.3
0.0750
14.6





0.0069
148.3
0.0218
154.2
0.0679
108.1
0.0708
3.0





0.0038
55.4
0.0187
152.5
0.0618
81.8
0.0660
10.7





0.0017
0.2
0.0166
155.5
0.0568
91.8
0.0627
14.5





0.0009
0.0
0.0158
163.8
0.0541
94.1
0.0506
16.1







0.0110
152.6
0.0508
101.1
0.0335
14.0







0.0064
104.3
0.0324
101.4
0.0260
5.4







0.0049
55.6
0.0154
98.9
0.0231
1.2







0.0039
13.1
0.0076
49.0
0.0223
0.0







0.0035
0.0
0.0039
0.2









0.0018
0.0


load plateau
0.0601

0.0592

0.0592

0.0592

0.0592


length (strain)


unload plateau
0.0596

0.0662

0.0654

0.0717

0.0485


length (strain)


permanent set
0.0007

0.0009

0.0035

0.0018

0.0223


(strain)


max vol. fraction
1.2%

1.5%

5.9%

3.1%

37.6%


martensite (%)
















TABLE 5







Tensile Data for Various Wire Samples, see Examples 3-6, FIG. 19(c)









Strain level
















3.1
Engr.
3.3
Engr.
3.4
Engr.
3.6
Engr.



Engr.
Stress
Engr.
Stress
Engr.
Stress
Engr.
Stress



Strain
(MPa)
Strain
(MPa)
Strain
(MPa)
Strain
(MPa)




















0
0
0
0
0
0
0
0



0.0003
16.0
0.0009
52.2
0.0007
37.7
0.0006
32.1



0.0048
237.0
0.0047
230.8
0.0044
239.5
0.0043
232.3



0.0105
450.1
0.0095
406.1
0.0092
420.0
0.0091
410.1



0.0138
550.1
0.0122
509.7
0.0119
525.8
0.0118
512.5



0.0173
544.5
0.0151
556.9
0.0149
570.7
0.0147
563.6



0.0358
527.9
0.0305
529.6
0.0303
539.6
0.0302
550.0



0.0618
529.2
0.0522
523.1
0.0519
538.8
0.0518
513.0



0.0733
549.4
0.0618
523.1
0.0615
543.5
0.0614
530.4



0.0748
571.5
0.0630
510.2
0.0628
534.2
0.0627
539.5



0.0785
631.8
0.0662
525.3
0.0659
548.4
0.0658
522.6



0.0802
661.4
0.0687
529.5
0.0684
552.7
0.0683
507.3



0.0807
664.3
0.0716
543.2
0.0713
567.4
0.0712
536.6



0.0813
673.7
0.0755
590.1
0.0753
609.2
0.0752
591.3



0.0820
686.2
0.0797
668.3
0.0794
670.0
0.0793
663.0



0.0827
700.6
0.0841
767.6
0.0838
761.7
0.0837
758.2



0.0832
709.6
0.0901
908.9
0.0899
899.3
0.0897
896.7



0.0831
701.3
0.0969
1060.2
0.0976
1069.0
0.0975
1068.7



0.0801
591.9
0.0973
1059.3
0.1001
1118.4
0.1000
1118.5



0.0746
411.1
0.0979
1070.4
0.1043
1193.2
0.1045
1200.1



0.0711
316.3
0.0983
1078.4
0.1046
1184.5
0.1075
1244.7



0.0666
219.2
0.0988
1089.9
0.1051
1191.0
0.1112
1293.0



0.0601
128.9
0.0994
1103.9
0.1059
1204.6
0.1166
1346.2



0.0536
100.1
0.0998
1107.3
0.1066
1218.6
0.1186
1337.7



0.0444
100.8
0.0992
1078.3
0.1076
1236.3
0.1197
1345.4



0.0424
102.4
0.0975
1004.8
0.1077
1238.6
0.1199
1348.3



0.0371
108.2
0.0931
821.3
0.1079
1235.3
0.1205
1355.8



0.0306
107.0
0.0877
611.1
0.1080
1232.6
0.1212
1364.3



0.0249
107.5
0.0829
445.7
0.1034
1040.9
0.1218
1370.6



0.0209
111.3
0.0796
343.7
0.1001
903.4
0.1223
1374.5



0.0159
109.8
0.0754
234.2
0.0959
738.9
0.1228
1378.9



0.0101
100.7
0.0706
134.6
0.0911
563.4
0.1235
1383.3



0.0059
78.9
0.0671
80.0
0.0876
445.1
0.1239
1386.1



0.0031
43.4
0.0648
53.3
0.0853
374.5
0.1242
1387.8



0.0021
17.8
0.0640
45.1
0.0845
350.1
0.1244
1388.4



0.0006
2.2
0.0627
34.6
0.0832
314.7
0.1245
1389.3



0.0003
0.0
0.0596
17.8
0.0801
233.3
0.1249
1391.5





0.0567
12.8
0.0772
167.0
0.1253
1393.4





0.0531
15.1
0.0736
100.4
0.1258
1395.6





0.0510
15.6
0.0716
68.2
0.1261
1396.7





0.0485
18.0
0.0691
36.9
0.1264
1398.2





0.0458
22.0
0.0664
11.4
0.1267
1399.6





0.0446
23.7
0.0651
2.6
0.1269
1400.2





0.0433
24.1
0.0639
2.0
0.1271
1400.6





0.0333
26.7
0.0539
2.0
0.1281
1399.7





0.0306
26.5
0.0511
2.0
0.1256
1303.2





0.0279
27.4
0.0484
2.0
0.1229
1197.3





0.0217
23.0
0.0422
2.0
0.1166
960.1





0.0167
22.8
0.0372
13.7
0.1116
779.5





0.0140
18.3
0.0345
17.6
0.1089
686.3





0.0102
9.8
0.0307
19.4
0.1052
563.2





0.0063
0.4
0.0268
19.6
0.1012
441.7





0.0019
0.5
0.0224
18.1
0.0968
318.9





0.0013
0.0
0.0180
14.7
0.0924
208.9







0.0141
8.3
0.0885
121.9







0.0116
3.2
0.0860
73.7







0.0109
0.0
0.0837
34.4









0.0806
1.2









0.0731
0.0


load plateau
0.0664

0.0646

0.0633

0.0675


length (strain)


unload plateau
0.0598

0.0658

0.0606

0.0075


length (strain)


permanent set
0.0003

0.0013

0.0109

0.0731


(strain)


max volume fraction
0.4%

1.9%

17.3%

100.0%


martensite (%)
















TABLE 6







Tensile Data for Various Wire Samples, see Examples 3-6, FIG. 19(d)









Strain level


















4.1
Engr.
4.3
Engr.
4.4
Engr.
4.6
Engr.
4.7
Engr.



Engr.
Stress
Engr.
Stress
Engr.
Stress
Engr.
Stress
Engr.
Stress



Strain
(MPa)
Strain
(MPa)
Strain
(MPa)
Strain
(MPa)
Strain
(MPa)






















0
0
0
0
0
0
0
0
0
0



0.0063
344.2
0.0037
195.8
0.0029
158.3
0.0024
127.7
0.0022
119.3



0.0108
520.9
0.0059
276.6
0.0050
239.7
0.0046
229.8
0.0043
211.8



0.0133
606.8
0.0076
335.0
0.0066
296.5
0.0063
294.0
0.0059
270.9



0.0158
644.6
0.0098
410.4
0.0087
372.3
0.0085
367.3
0.0080
341.4



0.0188
613.0
0.0119
498.4
0.0108
461.2
0.0106
448.9
0.0101
426.9



0.0218
635.5
0.0139
574.4
0.0127
534.1
0.0126
529.7
0.0120
503.2



0.0258
633.3
0.0161
625.0
0.0148
576.3
0.0148
596.8
0.0140
556.0



0.0308
623.0
0.0171
627.8
0.0158
585.0
0.0158
619.5
0.0151
569.8



0.0358
637.5
0.0206
631.5
0.0191
582.2
0.0193
630.6
0.0184
576.1



0.0403
638.8
0.0249
596.2
0.0233
556.1
0.0236
596.0
0.0226
539.8



0.0485
606.7
0.0293
591.9
0.0275
561.4
0.0280
634.6
0.0268
544.2



0.0668
614.5
0.0332
631.1
0.0312
581.7
0.0319
621.6
0.0305
578.1



0.0733
626.7
0.0403
622.8
0.0381
586.4
0.0390
628.7
0.0374
572.2



0.0745
662.3
0.0561
614.6
0.0533
559.5
0.0548
622.2
0.0526
564.7



0.0750
671.1
0.0618
621.3
0.0587
560.3
0.0605
615.9
0.0580
571.1



0.0756
687.8
0.0633
629.5
0.0602
580.9
0.0620
627.9
0.0595
572.1



0.0762
700.6
0.0661
599.2
0.0629
571.0
0.0648
608.4
0.0622
575.9



0.0767
711.5
0.0704
633.3
0.0670
575.6
0.0691
617.3
0.0663
573.1



0.0771
720.3
0.0741
633.6
0.0706
570.7
0.0728
623.8
0.0699
579.1



0.0776
729.8
0.0778
696.1
0.0741
619.8
0.0765
658.5
0.0734
595.5



0.0767
702.1
0.0808
771.2
0.0770
693.4
0.0795
735.8
0.0763
669.4



0.0740
605.1
0.0847
872.9
0.0808
788.7
0.0834
834.0
0.0801
760.8



0.0700
483.3
0.0891
988.3
0.0850
892.8
0.0878
949.9
0.0843
863.6



0.0662
393.5
0.0915
1054.0
0.0873
948.0
0.0902
1012.6
0.0865
927.3



0.0615
306.7
0.0921
1062.2
0.0906
1042.0
0.0936
1104.8
0.0899
1014.4



0.0572
254.7
0.0926
1072.8
0.0937
1120.0
0.0969
1190.8
0.0930
1093.3



0.0537
231.9
0.0931
1088.1
0.0967
1190.4
0.1010
1290.1
0.0970
1193.3



0.0502
231.4
0.0936
1100.9
0.0972
1193.8
0.1042
1337.2
0.1005
1275.6



0.0452
231.0
0.0940
1111.5
0.0976
1200.4
0.1046
1350.1
0.1034
1335.9



0.0405
235.6
0.0943
1121.8
0.0980
1210.2
0.1050
1355.4
0.1063
1387.3



0.0392
232.5
0.0948
1132.9
0.0985
1223.1
0.1055
1366.6
0.1105
1447.9



0.0372
229.3
0.0945
1124.8
0.0990
1238.3
0.1061
1380.9
0.1120
1440.1



0.0347
228.3
0.0934
1071.7
0.0991
1239.9
0.1062
1382.7
0.1122
1439.3



0.0317
227.7
0.0917
997.2
0.0993
1247.0
0.1065
1389.6
0.1124
1442.2



0.0297
228.9
0.0895
906.9
0.0996
1251.1
0.1068
1394.0
0.1127
1443.3



0.0255
230.4
0.0869
802.3
0.0998
1257.5
0.1071
1400.9
0.1130
1448.4



0.0235
231.8
0.0852
736.5
0.1000
1260.7
0.1073
1406.4
0.1132
1451.1



0.0205
223.7
0.0815
606.5
0.0995
1240.7
0.1077
1414.0
0.1137
1457.8



0.0182
225.8
0.0798
549.2
0.0979
1160.1
0.1079
1419.0
0.1139
1459.9



0.0157
227.6
0.0772
472.3
0.0954
1044.6
0.1082
1421.9
0.1143
1467.0



0.0130
225.4
0.0752
417.1
0.0935
961.4
0.1083
1423.1
0.1145
1469.6



0.0092
222.3
0.0731
363.5
0.0914
874.3
0.1069
1357.3
0.1148
1474.9



0.0072
222.4
0.0707
310.6
0.0891
779.1
0.1045
1251.5
0.1151
1478.9



0.0020
150.0
0.0674
250.9
0.0860
659.2
0.1012
1110.7
0.1155
1484.5



0.0010
75.0
0.0657
225.5
0.0843
597.0
0.0995
1036.4
0.1157
1486.3



0.0009
0.0
0.0629
193.1
0.0816
502.9
0.0967
919.4
0.1161
1490.9





0.0607
172.9
0.0795
436.5
0.0945
834.3
0.1164
1492.7





0.0579
160.3
0.0768
357.3
0.0917
725.4
0.1167
1496.4





0.0542
154.2
0.0733
264.8
0.0880
593.6
0.1172
1501.1





0.0494
161.9
0.0687
174.8
0.0832
439.3
0.1178
1506.8





0.0458
159.4
0.0652
121.4
0.0796
335.0
0.1182
1510.6





0.0427
155.4
0.0622
91.0
0.0765
261.1
0.1186
1511.7





0.0395
158.5
0.0591
73.2
0.0733
197.1
0.1187
1478.2





0.0375
157.3
0.0572
71.6
0.0713
163.8
0.1168
1425.1





0.0341
161.1
0.0539
69.4
0.0679
118.9
0.1134
1282.1





0.0302
166.9
0.0502
67.5
0.0640
85.0
0.1097
1123.1





0.0269
165.2
0.0470
71.3
0.0607
70.6
0.1066
994.2





0.0215
164.2
0.0418
77.9
0.0553
77.5
0.1014
788.7





0.0172
165.1
0.0377
75.9
0.0510
80.4
0.0972
634.1





0.0130
164.5
0.0337
75.8
0.0468
79.0
0.0932
495.0





0.0083
160.4
0.0291
80.2
0.0421
84.5
0.0887
350.3





0.0042
139.0
0.0252
77.5
0.0380
85.2
0.0847
241.7





0.0014
0.0
0.0229
76.3
0.0356
84.8
0.0824
183.3







0.0208
74.3
0.0334
83.8
0.0803
136.0







0.0181
76.3
0.0306
79.3
0.0776
83.7







0.0145
72.2
0.0269
80.6
0.0741
25.2







0.0133
69.9
0.0256
75.6
0.0728
8.4







0.0124
66.8
0.0247
72.9
0.0720
0.0







0.0058
71.1
0.0178
79.5







0.0029
0.0
0.0148
74.8









0.0059
69.5









0.0035
29.8









0.0026
0.0


load plateau
0.0617

0.0639

0.0615

0.0639

0.0623


length (strain)


unload plateau
0.0563

0.0643

0.0623

0.0652

0.0021


length (strain)


permanent set
0.0009

0.0014

0.0029

0.0026

0.0720


(strain)


max volume fraction
1.5%

2.1%

4.7%

4.1%

100.0%


martensite (%)
















TABLE 7







Tensile Data for Various Wire Samples, see Examples 3-6, FIG. 19(e)









Strain level


















5.1
Engr.
5.3
Engr.
5.4
Engr.
5.6
Engr.
5.7
Engr.



Engr.
Stress
Engr.
Stress
Engr.
Stress
Engr.
Stress
Engr.
Stress



Strain
(MPa)
Strain
(MPa)
Strain
(MPa)
Strain
(MPa)
Strain
(MPa)






















0
0
0
0
0
0
0
0
0
0



0.0001
2.5
0.0002
11.8
0.0001
5.8
0.0002
11.1
0.0002
13.5



0.0036
197.0
0.0033
182.0
0.0032
174.2
0.0033
191.0
0.0034
193.3



0.0068
321.2
0.0060
293.0
0.0059
283.2
0.0060
305.5
0.0061
307.4



0.0092
420.8
0.0081
372.7
0.0080
362.4
0.0081
384.4
0.0082
387.0



0.0113
518.3
0.0100
450.1
0.0099
439.8
0.0100
460.9
0.0100
462.1



0.0137
602.3
0.0121
539.4
0.0120
529.9
0.0121
549.4
0.0121
550.0



0.0149
606.7
0.0131
577.0
0.0130
567.9
0.0131
588.0
0.0132
588.3



0.0187
609.1
0.0165
633.9
0.0164
628.5
0.0165
639.8
0.0165
634.2



0.0235
624.1
0.0206
623.3
0.0205
637.6
0.0206
646.8
0.0207
639.2



0.0283
620.0
0.0248
628.6
0.0247
620.0
0.0248
625.2
0.0248
631.3



0.0329
628.2
0.0288
620.0
0.0286
624.8
0.0287
606.2
0.0288
619.6



0.0458
614.3
0.0400
606.6
0.0399
622.1
0.0400
615.0
0.0400
615.0



0.0537
612.7
0.0469
615.0
0.0468
615.0
0.0469
620.9
0.0469
624.5



0.0647
636.0
0.0565
620.0
0.0564
620.8
0.0565
615.0
0.0565
610.8



0.0674
656.6
0.0588
626.8
0.0586
603.6
0.0587
626.8
0.0588
611.5



0.0684
646.8
0.0623
610.0
0.0622
620.0
0.0623
634.8
0.0623
636.4



0.0694
637.6
0.0688
646.3
0.0686
640.2
0.0687
662.6
0.0688
655.3



0.0703
640.1
0.0748
705.0
0.0747
701.0
0.0748
719.4
0.0748
715.1



0.0707
642.7
0.0777
749.6
0.0776
740.1
0.0777
755.5
0.0777
752.3



0.0712
646.6
0.0808
805.1
0.0807
795.0
0.0808
805.0
0.0809
805.5



0.0717
651.1
0.0840
868.1
0.0839
858.0
0.0840
866.6
0.0840
867.0



0.0723
658.7
0.0879
952.9
0.0878
941.8
0.0879
947.4
0.0880
947.7



0.0730
671.6
0.0925
1052.9
0.0924
1040.2
0.0925
1041.5
0.0925
1043.2



0.0737
682.3
0.0934
1060.4
0.0966
1128.3
0.0967
1125.7
0.0967
1127.9



0.0742
691.1
0.0939
1067.1
0.0999
1195.1
0.1004
1196.8
0.1005
1199.4



0.0748
701.8
0.0944
1077.3
0.1004
1187.8
0.1044
1265.3
0.1044
1268.1



0.0751
703.7
0.0949
1087.6
0.1009
1194.8
0.1081
1324.6
0.1082
1325.4



0.0753
704.0
0.0956
1100.8
0.1015
1205.8
0.1109
1346.4
0.1130
1386.1



0.0754
703.6
0.0958
1103.6
0.1021
1217.1
0.1115
1347.3
0.1175
1432.4



0.0735
637.3
0.0959
1101.9
0.1024
1223.2
0.1118
1350.7
0.1196
1448.7



0.0701
525.6
0.0960
1104.0
0.1028
1230.1
0.1122
1356.5
0.1215
1450.5



0.0679
461.5
0.0960
1102.9
0.1030
1234.8
0.1124
1358.8
0.1217
1440.6



0.0653
391.9
0.0961
1103.0
0.1031
1235.8
0.1127
1363.5
0.1221
1438.2



0.0627
331.9
0.0940
1019.0
0.1032
1233.0
0.1130
1367.1
0.1224
1439.4



0.0603
285.9
0.0919
933.0
0.1033
1234.5
0.1133
1372.6
0.1226
1441.9



0.0567
231.8
0.0888
809.1
0.1034
1234.1
0.1137
1379.2
0.1231
1446.7



0.0538
200.7
0.0863
714.9
0.1034
1234.0
0.1141
1383.4
0.1234
1450.9



0.0507
182.2
0.0836
618.5
0.1009
1131.5
0.1144
1388.9
0.1237
1455.6



0.0473
177.7
0.0807
522.3
0.0980
1009.8
0.1148
1393.5
0.1241
1458.7



0.0445
174.5
0.0782
446.6
0.0955
908.5
0.1151
1399.1
0.1245
1463.5



0.0409
177.4
0.0751
361.8
0.0924
786.7
0.1154
1401.2
0.1249
1467.6



0.0387
171.2
0.0732
316.3
0.0905
716.6
0.1155
1399.1
0.1251
1470.9



0.0351
164.4
0.0701
250.1
0.0874
605.7
0.1157
1400.1
0.1256
1474.3



0.0315
169.3
0.0669
195.9
0.0843
502.9
0.1158
1399.5
0.1260
1479.3



0.0277
164.7
0.0636
151.1
0.0809
403.7
0.1146
1351.2
0.1264
1483.6



0.0248
165.0
0.0611
125.9
0.0784
336.9
0.1121
1247.1
0.1267
1485.3



0.0215
162.7
0.0582
105.2
0.0755
267.7
0.1092
1127.3
0.1271
1489.0



0.0181
153.3
0.0553
100.4
0.0726
208.4
0.1063
1009.9
0.1275
1491.0



0.0152
139.7
0.0528
95.4
0.0701
165.6
0.1038
911.7
0.1279
1493.8



0.0119
123.9
0.0499
106.2
0.0672
125.2
0.1009
800.8
0.1282
1494.9



0.0083
98.7
0.0467
102.2
0.0641
92.7
0.0978
687.0
0.1287
1498.3



0.0042
28.0
0.0432
93.8
0.0605
70.8
0.0942
565.7
0.1291
1501.2



0.0021
0.0
0.0413
104.6
0.0586
68.3
0.0923
505.0
0.1294
1502.6





0.0392
96.9
0.0566
64.6
0.0903
441.0
0.1296
1503.6





0.0374
96.6
0.0547
67.4
0.0884
386.6
0.1298
1503.4





0.0344
94.9
0.0518
62.4
0.0855
308.1
0.1301
1503.0





0.0315
105.2
0.0489
69.0
0.0826
237.9
0.1304
1503.2





0.0286
103.0
0.0459
72.7
0.0796
176.4
0.1307
1503.1





0.0257
91.7
0.0430
67.4
0.0767
124.0
0.1289
1432.0





0.0228
89.8
0.0401
59.2
0.0738
81.0
0.1259
1317.7





0.0192
96.3
0.0366
59.5
0.0703
41.4
0.1224
1180.7





0.0165
85.6
0.0339
63.0
0.0676
20.2
0.1197
1077.6





0.0130
68.3
0.0303
67.8
0.0640
3.5
0.1161
946.0





0.0094
48.8
0.0268
60.4
0.0605
5.2
0.1126
818.7





0.0074
28.9
0.0247
59.9
0.0584
2.3
0.1105
746.4





0.0059
9.8
0.0232
64.1
0.0569
0.6
0.1091
696.9





0.0040
0.0
0.0214
62.4
0.0551
5.8
0.1072
634.9







0.0189
59.6
0.0526
3.6
0.1047
555.1







0.0153
46.2
0.0490
12.5
0.1024
485.2







0.0116
27.2
0.0453
14.5
0.0982
366.4







0.0061
0.0
0.0398
9.6
0.0934
244.1









0.0342
14.2
0.0897
160.5









0.0305
13.6
0.0861
92.3









0.0244
11.7
0.0828
38.4









0.0217
0.0
0.0805
7.3











0.0784
0.0


load plateau
0.0617

0.0617

0.0556

0.0556

0.0617


length (strain)


unload plateau
0.0546

0.0596

0.0610

0.0485

0.0021


length (strain)


permanent set
0.0021

0.0040

0.0061

0.0217

0.0784


(strain)


max volume fraction
3.3%

6.5%

11.1%

39.0%

100.0%


martensite (%)









IV. Applications
Wires made in accordance with the present disclosure are susceptible of a variety of applications including, but not limited to the applications detailed below. Exemplary applications of wires in accordance with the present disclosure are set forth below, and shown generally in FIGS. 18(a)-19(b).
In some cases, a wire may have no remaining biased curvature, such as in a percutaneous transluminal coronary angioplasty (PTCA), steerable, and torque whip free guidewire application, or for a torque transmission wire for coronary plaque removal, for example.
Wire products used for medical devices as discussed herein will typically be subjected to mechanical conditioning in accordance with the present disclosure prior to integration into a medical device. However, it is contemplated that wire products may alternatively be installed into, or at least partially configured as, a medical device prior to subjecting the wire product to mechanical conditioning, followed by conducting the mechanical conditioning on the wire product after same is installed into, or at least partially configured as, a medical device, in order to impart benefits as disclosed herein.
A. DFT® and Other Composite Wire Materials
Wires disclosed herein may be used for composite wire products, such as shown in FIGS. 21(a)-(b). Composite wire 300 includes an outer shell 302 made of a first material, and a core 304 comprising at least one core segment of a second material, and optionally, additional core segments of third or more materials. Outer shell 302 may be made of a wire in accordance with the present disclosure, and core 304 may have a variety of desired properties, such as resistance, radiopacity, or any other property.
Thus, composite wire 300 may confer the benefits of load-conditioned and therefore fatigue damage resistant outer shell, such as fatigue strength, low or zero permanent set, etc, as described above by applying a suitable conditioning load to the wire product prior to installation within a medical device and/or configuration as a medical device, while also having other properties associated with the second material comprising core 304. An exemplary composite wire product is DFT®, available from Fort Wayne Metals Research Products Corp. of Fort Wayne, Ind.
B. Shape Memory Devices
1. Wire-Based Stents
Referring to FIG. 22(a), a tissue scaffold or vessel stent device 370 is shown which is made from one or more wires 372 made in accordance with the present process, which are braided, knitted, or otherwise formed together to produce the generally cylindrical cross-sectional shape of device 370.
Referring to and FIG. 22(b), a tissue scaffold or vessel stent device 370′ is shown which is made from one or more wires 372′ made in accordance with the present process, which are knitted together to form the generally cylindrical cross-sectional shape of device 370′.
Upon release from the delivery catheter, stents move to some degree, dependent on the relative vessel and device compliance, with the artery due to fluctuations in blood pressure, arterial vessel smooth muscle contraction and dilation, and due to general anatomical movement. Such mechanical displacement results in cyclic straining of wires 372, 372′ comprising the structure of stent 370, 370′ structure.
Non bioerodable tissue scaffolds or stents are generally implanted permanently, and therefore should be able to withstand millions of mechanical load cycles without losing structural integrity due to mechanical fatigue.
Stents 370, 370′, which are constructed from wires 372, 372′ made in accordance with the present process, possess a high degree of resistance to fatigue damage and thus offer optimized performance as compared to conventional stents made with wires having lower fatigue strength.
2. Blood Filters
Referring still to FIGS. 22(a)-(b), devices 370, 370′ may also take the form of a blood filter which is made from one or more wires 372, 372′ made in accordance with the present process, which are braided, knitted, or otherwise formed together to produce the generally cylindrical cross-sectional shape of devices 370, 370′. In this respect, many blood filter devices may be similar to braided, knitted, or laser-cut stents, and many interior vena cava (IVC) filters may be shaped as umbrella-shaped devices. In use, the superelastic characteristic of the device is utilized in that the device is inserted into a blood vessel via catheter in a collapsed condition and is deployed by expansion into the blood vessel, where the device captures and/or redirects larger blood clots from critical anatomical organs or regions. In use, particularly in permanent or non-retrievable devices, the device may be subjected to repeated, movements such that high fatigue strength is desired.
While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.

## Claims

1. A method, comprising the steps of:
providing a wire product made of a shape-set, nickel-titanium shape memory material, a majority of the shape memory material in an austenitic phase at a human body temperature;
mechanically conditioning the wire product by:
applying an engineering stress between 700 MPa and 1600 MPa; and releasing the applied engineering stress such that said wire product has a fatigue endurance such that said wire survives 106 cycles at a 1.90% peak-to-peak strain amplitude; and

incorporating the wire product into a medical device.

2. The method of claim 1, wherein said mechanical conditioning step occurs prior to said incorporation step.

3. The method of claim 1, wherein said mechanical conditioning step occurs after said incorporation step.

4. The method of claim 1, wherein said mechanical conditioning step comprises:
applying an engineering stress between 900 MPa and 1450 MPa;
and releasing the applied engineering stress.

5. The method of claim 1, wherein said mechanical conditioning step comprises:
applying an engineering stress between 1100 MPa and 1350 MPa;
and releasing the applied engineering stress.

6. The method of claim 1, further comprising repeating said mechanically conditioning step at least once.

7. The method of claim 1, wherein said mechanical conditioning step is conducted below a martensite deformation temperature (Md) of the nickel-titanium shape memory material.

8. The method of claim 1, wherein said mechanical conditioning step further comprises:
applying the stress to the wire product in an environment having a temperature T, wherein


T=Af±50° C.,


wherein Af is the austenite transformation finish temperature of the nickel-titanium shape memory material.

9. The method of claim 1, wherein the wire product is selected from the group consisting of wire having a circular cross-section, wire having a non-circular cross-section, cable, coil, and tubing.

10. A method of conditioning a wire made of a shape-set, nickel-titanium shape memory material, a majority of the shape memory material in an austenitic phase at a human body temperature, the method including the steps of:
applying an engineering stress to the wire between 700 MPa and 1600 MPa; and releasing the applied engineering stress such that an isothermally non-recoverable strain

of the wire is less than 1% and such that said wire has a fatigue endurance such that said wire survives 106 cycles at a 1.90% peak-to-peak strain amplitude.

11. The method of claim 10, wherein said steps of applying an engineering stress and releasing the applied engineering stress are conducted below a martensite deformation temperature (Md) of the nickel-titanium shape memory material, and in an environment having a temperature T, wherein


T=Af±50° C.

12. The method of claim 10, wherein said step of applying an engineering stress comprising applying an engineering stress to the entirety of the wire.

13. The method of claim 10, wherein the wire has an active austenitic finish temperature (Ar) of between 280K and 298K.

14. The method of claim 10, wherein the applied engineering stress comprises an engineering stress between 900 MPa and 1450 MPa.

15. The method of claim 10, wherein the applied engineering stress comprises an engineering stress between 1100 MPa and 1350 MPa.

16. The method of claim 10, further comprising the step of repeating said steps of applying an engineering stress and releasing the applied engineering stress at least once.

17. The method of claim 10, wherein a length oftime elapses between said step of applying an engineering stress and said step of releasing the applied engineering stress, the length of time selected to produce said isothermally non-recoverable strain of the wire ofless than 1%.

18. The method of claim 17, wherein:
said step of applying an engineering stress to the wire comprises raising the stress level via a strain-controlled ramp up to between 700 MPa and 1400 MPa, and then holding the wire at said stress level the length of time, and
said step of releasing the applied engineering stress comprises a strain-controlled ramp down to zero engineering stress.

19. The method of claim 10, wherein said step of releasing the applied engineering stress is performed such that said isothermally non-recoverable strain of the wire is less than 0.25%.

20. The method of claim 10, wherein the wire is subjected to engineering strain of between 8% and 14% during the step of applying an engineering stress.

21. The method of claim 10, wherein said step of applying an engineering stress to the wire comprises applying a tensile force.