Nickel-titanium shape memory alloy microwire with high fatigue stability and preparation method thereof

Abstract

The invention provides a nickel-titanium shape memory alloy microwire with high fatigue stability and a preparation method thereof, and relates to the field of shape memory alloys. The method comprises the following steps: taking nickel and titanium as raw materials and carrying out medium-high frequency induction smelting ingot casting; carrying out homogenizing heat treatment and preheating treatment on the cast ingot, and rapidly cooling to obtain a sample cast ingot; forging and hot-rolling the sample cast ingot to a wire rod, and drawing to obtain a drawn nickel-titanium shape memory alloy microwire; the nickel-titanium shape memory alloy microwire is trained after being straightened, and the nickel-titanium shape memory alloy microwire with high fatigue stability is obtained; the nickel content of the microfilament is 49.5-50.5%, the size of precipitates in the microstructure of the microfilament does not exceed 10 microns, and the average area percentage does not exceed 2%. According to the prepared nickel-titanium shape memory alloy microwire with the high fatigue stability, the high fatigue stability is achieved by controlling the size and the area percentage of precipitates in a product.

Description

Technical Field

[0001] The invention relates to the technical field of shape memory alloys, and in particular to a nickel-titanium shape memory alloy microwire with high fatigue stability and a preparation method thereof. Background Art

[0002] Shape memory alloys have good biocompatibility and unique superelasticity, and have been widely used in the field of medical devices. Fatigue loads are critical to NiTi components in medical applications. The FDA requires that the fatigue life of intravascular stents exceed 400 million cycles, and the microstructure of NiTi components plays a major role in crack propagation; however, the existing reports on improving the high fatigue stability and extending the fatigue life of nickel-titanium shape memory alloys are mostly centered on material strength and fatigue load types, and few studies have been conducted on the effects of precipitates formed during material smelting and subsequent thermal mechanical processing on the fatigue stability of nickel-titanium shape memory alloys. Summary of the invention

[0003] The purpose of the present invention is to provide a nickel-titanium shape memory alloy microwire with high fatigue stability and a preparation method thereof. The scheme reduces the maximum size of precipitates in the product to below 10 μm and reduces the average area percentage of precipitates to less than 2% by controlling the process parameters for preparing the nickel-titanium shape memory alloy microwire, thereby obtaining a nickel-titanium shape memory alloy with high fatigue stability.

[0004] To achieve the above object, the present invention proposes the following technical solutions:

[0005] In a first aspect, a nickel-titanium shape memory alloy microwire with high fatigue stability is provided, wherein the size of precipitates in the microstructure of the nickel-titanium shape memory alloy microwire does not exceed 10 μm, and the average area percentage of the precipitates does not exceed 2%.

[0006] Furthermore, the precipitate in the microstructure of the nickel-titanium shape memory alloy microwire is Ti 2 Ni is in the form of short round rods and precipitates along the grain boundaries in the microstructure.

[0007] Furthermore, the nickel content of the nickel-titanium shape memory alloy microwire is 49.5-50.5%.

[0008] Furthermore, the fatigue life of the nickel-titanium shape memory alloy microwire is not less than 100,000 times, and the driving strain attenuation is less than 0.5%.

[0009] In the second aspect, a method for preparing a nickel-titanium shape memory alloy microwire with high fatigue stability is proposed, comprising the following steps:

[0010] (1) Using nickel and titanium as raw materials, medium and high frequency induction melting and ingot casting is adopted;

[0011] (2) performing homogenization heat treatment and preheating treatment on the ingot, and obtaining a sample ingot after rapid cooling;

[0012] (3) forging the sample ingot and hot rolling it into a round coil to obtain a nickel-titanium shape memory alloy material;

[0013] (4) drawing the coiled nickel-titanium shape memory alloy material to obtain drawn nickel-titanium shape memory alloy microwires;

[0014] (5) straightening the nickel-titanium shape memory alloy microwire to obtain a treated nickel-titanium shape memory alloy microwire;

[0015] (6) The straightened nickel-titanium shape memory alloy microwires are trained to obtain nickel-titanium shape memory alloy microwires with high fatigue stability.

[0016] Furthermore, the parameters of the homogenization heat treatment of the ingot in step (2) are: treatment temperature of 480-930°C, treatment time of 48h; the parameters of the preheat treatment are: treatment temperature of 430-480°C, holding time of 1h; the rate of rapid cooling after the preheat treatment is 15-35°C / min.

[0017] Furthermore, the nickel content of the nickel-titanium shape memory alloy microwires obtained in step (5) is 49.5-50.5%, and the size of the precipitates in the microstructure thereof does not exceed 10 μm, and the average area percentage does not exceed 2%.

[0018] Furthermore, the precipitate in the microstructure of the nickel-titanium shape memory alloy microwire obtained in step (5) is Ti 2 Ni is in the form of short round rods and precipitates along the grain boundaries in the microstructure.

[0019] Furthermore, the specific process of step (1) using nickel and titanium as raw materials to melt and cast ingots by medium and high frequency induction is as follows:

[0020] Using nickel with a purity of not less than 99.9% and titanium with a purity of not less than 99.9% as raw materials, in a vacuum degree higher than 8×10 -2 The ingot was obtained by several remelting steps under Pa.

[0021] Thirdly, the application of the above-mentioned nickel-titanium shape memory alloy microwires with high fatigue stability in the field of medical devices is proposed.

[0022] It can be seen from the above technical solutions that the technical solutions of the present invention have the following beneficial effects:

[0023] The invention discloses a nickel-titanium shape memory alloy microwire with high fatigue stability and a preparation method thereof, wherein the method comprises: using nickel and titanium as raw materials to perform ingot melting by medium and high frequency induction; wherein the purity of nickel is not less than 99.9%, the purity of titanium is not less than 99.9%, and the vacuum degree of the medium and high frequency induction melting is higher than 8×10 -2 Pa; homogenizing heat treatment and preheating treatment are performed on the ingot, and a sample ingot is obtained after rapid cooling; the sample ingot is forged and hot-rolled into a coil and then drawn to obtain the drawn nickel-titanium shape memory alloy microwire; the nickel-titanium shape memory alloy microwire is straightened and then trained to obtain a nickel-titanium shape memory alloy microwire with high fatigue stability; the nickel content of the microwire is 49.5-50.5%, and the size of the precipitates in its microstructure does not exceed 10μm, and the average area percentage does not exceed 2%. The nickel-titanium shape memory alloy microwire with high fatigue stability prepared by the present invention achieves a fatigue life of not less than 100,000 times and a driving strain attenuation of less than 0.7% by controlling the size and area percentage of the precipitates in the product.

[0024] When the nickel-titanium shape memory alloy microwire prepared in the present invention is used as a driving microwire, it maintains output performance in the target sub-cycle application, achieves the control accuracy required by the application, and does not suffer sudden fatigue fracture. That is, the nickel-titanium shape memory alloy microwire of this scheme has high fatigue stability and can pass the application test.

[0025] It should be appreciated that all combinations of the foregoing concepts, as well as additional concepts described in greater detail below, may be considered to be part of the inventive subject matter of the present disclosure, provided such concepts are not mutually inconsistent.

[0026] The foregoing and other aspects, embodiments and features of the present invention can be more fully understood from the following description in conjunction with the accompanying drawings. Other additional aspects of the present invention, such as the features and / or beneficial effects of the exemplary embodiments, will be apparent from the following description or learned from the practice of the specific embodiments according to the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The accompanying drawings are not drawn to scale according to actual reference objects. In the accompanying drawings, each identical or nearly identical component shown in various figures may be represented by the same reference numeral. For the sake of clarity, not every component is labeled in each figure. Now, embodiments of various aspects of the present invention will be described by way of example and with reference to the accompanying drawings, in which:

[0028] Figure 1 Scanning electron microscope image of the sample ingot obtained after heat treatment in Example 1 of the present invention Figure 1 ;

[0029] Figure 2 This is a fatigue test curve of nickel-titanium shape memory alloy microwires prepared in Example 1 of the present invention;

[0030] Figure 3 This is a scanning electron microscope image of the fracture of the nickel-titanium shape memory alloy microwire prepared in Example 1 of the present invention;

[0031] Figure 4(a) is a scanning electron microscope image of the sample ingot obtained after heat treatment in Example 1 of the present invention. Figure 2 ;

[0032] FIG4( b ) is a scanning electron microscope image of the nickel-titanium shape memory alloy material obtained by coiling in Example 1 of the present invention;

[0033] FIG4( c ) is a scanning electron microscope image of the nickel-titanium shape memory alloy microwire prepared in Example 1 of the present invention;

[0034] Figure 5 This is a metallographic diagram of the sample ingot obtained in Comparative Example 2 of the present invention;

[0035] Figure 6 This is a metallographic diagram of the ingot obtained after high-frequency induction melting in Comparative Example 3 of the present invention. DETAILED DESCRIPTION

[0036] In order to make the purpose, technical solution and advantages of the embodiment of the present invention clearer, the technical solution of the embodiment of the present invention will be clearly and completely described below in conjunction with the drawings of the embodiment of the present invention. Obviously, the described embodiment is a part of the embodiment of the present invention, not all of the embodiments. Based on the described embodiment of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention. Unless otherwise defined, the technical terms or scientific terms used herein should be the common meaning understood by people with general skills in the field to which the present invention belongs.

[0037] The words "first", "second" and similar words used in the patent application specification and claims of the present invention do not indicate any order, quantity or importance, but are only used to distinguish different components. Similarly, unless the context clearly indicates otherwise, the singular forms of "a", "an" or "the" and other similar words do not indicate a quantitative limitation, but indicate the existence of at least one. Words such as "include" or "comprise" mean that the elements or objects appearing before "include" or "comprise" cover the features, wholes, steps, operations, elements and / or components listed after "include" or "comprise", and do not exclude the existence or addition of one or more other features, wholes, steps, operations, elements, components and / or their collections. "Up", "down", "left", "right" and the like are only used to indicate relative positional relationships. When the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0038] Based on the existing technologies, most of the solutions for improving the high fatigue stability and extending the fatigue life of nickel-titanium shape memory alloys are based on improvements in material strength and fatigue load types, and few studies have been conducted on the effects of precipitates formed during material smelting and subsequent thermal mechanical processing on the fatigue stability of nickel-titanium shape memory alloys. Therefore, the present invention aims to propose a nickel-titanium shape memory alloy microwire with high fatigue stability and a preparation method thereof, and to improve the fatigue stability of the prepared nickel-titanium shape memory alloy microwire by studying the size and percentage of precipitates produced during the material smelting process and the heat treatment process.

[0039] Specifically, the method for preparing nickel-titanium shape memory alloy microwires with high fatigue stability disclosed in the present scheme includes the following steps: 1) using nickel and titanium as raw materials to adopt medium and high frequency induction melting to cast ingots; 2) performing homogenization heat treatment and preheat treatment on the ingots, and obtaining sample ingots after rapid cooling; 3) forging and hot rolling the sample ingots into coils to obtain nickel-titanium shape memory alloy materials; 4) drawing the coiled nickel-titanium shape memory alloy materials to obtain drawn nickel-titanium shape memory alloy microwires; 5) straightening the nickel-titanium shape memory alloy microwires to obtain treated nickel-titanium shape memory alloy microwires; 6) training the straightened nickel-titanium shape memory alloy microwires to obtain nickel-titanium shape memory alloy microwires with high fatigue stability.

[0040] The specific process of step 1) using nickel and titanium as raw materials to melt ingots by medium and high frequency induction is as follows: using nickel with a purity of not less than 99.9% and titanium with a purity of not less than 99.9% as raw materials, in a vacuum higher than 8×10 -2 Pa, and then the ingot is obtained by remelting several times; the parameters of the homogenization heat treatment of the ingot in step 2) are: the treatment temperature is 480-930℃, and the treatment time is 48h; the parameters of the preheat treatment are: the treatment temperature is 430-480℃, and the holding time is 1h; the rate of rapid cooling after the preheat treatment is 15-35℃ / min. The nickel content of the nickel-titanium shape memory alloy microwire finally obtained is 49.5-50.5%, and the size of the precipitates in its microstructure does not exceed 10μm, and the average area percentage does not exceed 2%; the precipitates are analyzed by elemental analysis, and the precipitates are Ti 2 Ni is in the form of short round rods in the microstructure and precipitates along the grain boundaries in the microstructure; the ingot is obtained by multiple remelting in high vacuum by medium and high frequency induction melting technology, and the internal microstructure is observed by scanning electron microscope after the ingot is sampled. The precipitate Ti 2 Ni mainly precipitates along the grain boundaries; fracture studies of the final microwires revealed that the microwires 2 Cracks initiate near the Ni precipitation sites and lead to microwire breakage under cyclic loading.

[0041] The nickel-titanium shape memory alloy microwire with high fatigue stability and the preparation method thereof disclosed in the present invention will be further specifically introduced below in conjunction with the specific embodiments shown in the accompanying drawings.

[0042] Example 1

[0043] Using 99.9% pure nickel and 99.9% pure titanium as raw materials, medium and high frequency induction melting is used to cast 1kg ingots with a vacuum degree of 10 -3 Pa, cooling rate 30℃ / min; the nickel content (atomic percentage) in the ingot is 49.8%; the ingot is subjected to homogenization heat treatment at a temperature of 850℃ for 48h, cooled to 450℃ with the furnace for preheating treatment of aging for 1h, and then rapidly cooled to room temperature at a cooling rate of 35℃ / min to obtain a sample ingot; the sample ingot is sampled, metallographic observation is made, and the size and area percentage of the precipitates are analyzed and counted; the sample ingot is forged and hot-rolled into a coil to obtain a nickel-titanium shape memory alloy material; Optionally, the process parameters of forging are: hot forging temperature 750-1100°C, heat preservation 3-72h; the process parameters of hot rolling are: rolling temperature 800°C-1050°C; optionally, it can also be cold rolled into a round coil after forging, and the process parameters of cold rolling can be selected as: multiple annealing treatments and at least one vacuum solution treatment during the cold rolling process; the temperature of the annealing treatment is 750°C-660°C, and the heat preservation is 20-30min; the temperature of the vacuum solution treatment is 850-900°C, Keep warm for 1 to 2 hours; draw the coiled nickel-titanium shape memory alloy material to obtain a nickel-titanium shape memory alloy microwire with a diameter of 25 μm after drawing; wherein the drawing process can be performed multiple times, and the drawing and the annealing process after drawing can be performed according to the existing technology; straighten the nickel-titanium shape memory alloy microwire to obtain the treated nickel-titanium shape memory alloy microwire; wherein the process parameters of the straightening treatment are: straightening temperature 420°C, heat treatment time 1min, straightening tension 100MPa; train the nickel-titanium shape memory alloy microwire after straightening to obtain a nickel-titanium shape memory alloy microwire with high fatigue stability; wherein the training standard is: apply a load of 400Mpa to the shape memory alloy microwire, the training current waveform is a triangular wave, so that it undergoes 80 cycles of repeated phase transformation between martensite and austenite, the training current duty cycle is 50%, the training current amplitude is 10mA, the training current power-on time is 1s, the training current cooling time is 9s, and the training frequency is 0.1Hz.

[0044] The metallographic images of the above ingot sampling test are as follows: Figure 1 As shown, the maximum size of the precipitate is 5.8 μm and the average area percentage is 0.8%. The fatigue test of the nickel-titanium shape memory alloy microwire with high fatigue stability prepared in Example 1 shows that the fatigue life of the microwire exceeds 400,000 times and the driving strain decay is less than 0.7% under a load of 600 MPa and a driving strain of more than 2%. Figure 2 The end surface of the broken microwire after the fatigue test was observed by scanning electron microscope. The results are shown in Figure 3 As shown in the figure, the precipitate can be clearly observed at the fracture. The element analysis shows that the precipitate is Ti 2 You.

[0045] Further research was conducted to investigate the effect of subsequent heat treatment of the ingot on its precipitates. Figure 4(a) to Figure 4(c) As shown: The SEM image of the ingot sample obtained by medium-high frequency induction melting clearly shows the short round rod-shaped Ti 2 Ni is mainly precipitated along the grain boundaries and is densely distributed; after the ingot is forged and rolled into an 8mm round plate, Ti 2 The Ni distribution is relatively diffuse, and the area and size are decreasing; after being drawn and annealed to a roundness of 0.1 mm, the precipitate shape becomes thin and slender; that is, the distribution of precipitates is reduced from the raw material ratio of medium and high frequency induction melting as the source, and the subsequent thermal mechanical processing process makes the precipitates slender; compared with the spherical short round rod-shaped precipitates, the thin and moderately long Ti 2 Ni has a greater influence on fatigue properties, therefore, the present invention also focuses on exploring the influence of smelting and heat treatment processes on the shape, size and proportion of precipitates.

[0046] Table 1 Relationship between nickel content, precipitate size and average area percentage

[0047] Serial number Nickel content (atomic percentage) % Maximum size of precipitate (μm) Average area percentage (%) Fatigue life (10,000 times) Example 1 49.8 5.8 0.8 43 Example 2 49.7 8.7 1 11 Example 3 50.4 7.2 1.5 15 Example 4 49.5 9.8 1.7 10 Comparative Example 1 49.3 12.6 4.2 5 Comparative Example 2 50.6 13.5 5.4 2

[0048] The difference between Examples 2-4 and Comparative Examples 1-2 and Example 1 is only the nickel content. The effect of nickel content on the precipitates in the ingot is studied, and the results are shown in Table 1. A slight change in nickel content will have a great impact on the phase transition temperature and the precipitate content. Too low a nickel content will lead to an increase in the precipitate content. Specifically, when the nickel atomic percentage is 49.5-50.5%, the maximum size of the precipitate is in the range of 5-10 μm, which is much smaller than the maximum precipitate size of 39 μm specified in ASTM F 2063-05 standard (nickel-titanium shape memory alloy for medical devices and surgical implants), and the average area percentage is 2.8%; combined with the example 4 (a), Ti 2 Ni precipitates along the grain boundaries and is less distributed. As shown in Table 1, when the atomic percentage of nickel is 50.6%, the maximum size of the precipitate reaches 13.5 μm. The reason is that too much nickel content will sharply reduce the phase transition temperature and increase the precipitate. Figure 5 As shown, the surface morphology of the ingot of Comparative Example 2 was observed under a scanning electron microscope after sampling at the same sampling position. 2 Ni precipitates along the grain boundaries, and the precipitation is significantly increased compared with Example 1.

[0049] Based on the above examples 1-4 and comparative examples 1-2, when the nickel content is 49.5-50.5%, the medium-high frequency induction melting technology is used in a vacuum greater than 8×10 -2 When smelted under Pa conditions, although the nickel content affects the area percentage and maximum size of the precipitates, the fatigue life of the product nickel-titanium shape memory alloy is greater than 100,000 times, that is, it has a high fatigue life.

[0050] Table 2 Relationship between homogenization heat treatment parameters and precipitate size and average area percentage

[0051] Serial number Processing temperature / ℃ Maximum size of precipitate (μm) Average area percentage (%) Fatigue life (10,000 times) Example 1 850 5.8 0.8 43 Example 5 520 8.9 1.68 11 Example 6 650 7.7 1.2 25 Comparative Example 3 0 15.1 2.34 8

[0052] The difference between Examples 5-6 and Example 1 is that the temperature of the homogenization heat treatment is adjusted, and the difference between Comparative Example 3 and Example 1 is that the homogenization heat treatment is not performed, and the preheating treatment and rapid cooling process remain unchanged. The results are shown in Table 2. Figure 1 As shown in Figure 2, the surface morphology of the ingot after homogenization heat treatment is similar to Figure 6 By comparing the surface morphology without homogenization heat treatment, it can be seen that Figure 1 The shape of the precipitates gradually becomes round and fat, and the distribution of the precipitates on the grain boundaries becomes more discontinuous; it is known that the continuous precipitates in the ingot will be drawn more slenderly during the subsequent processing to coiling and wire drawing and other thermomechanical processing, and the final wire is easy to break during fatigue testing, that is, Figure 6 The slender and continuous precipitates shown in the figure are more harmful to the fatigue performance of the wire. Homogenization heat treatment can make the internal composition of the ingot more uniform, so that the precipitates formed during smelting and subsequent cooling can be dissolved into the matrix at a higher temperature, which reduces the distribution of precipitates to a certain extent, and makes the precipitates grow slightly from slender to round, and then change to thinner and moderately long precipitates after thermomechanical processing, reducing the influence of stress concentration on fatigue performance caused by the precipitates of the smelted ingot directly changing to the tip of the slender precipitates after thermomechanical processing.

[0053] Table 3 Relationship between cooling rate, precipitate size and average area percentage after preheating

[0054]

[0055] The difference between Example 7-8 and Example 1 is only that the rapid cooling rate after preheating is different. The effect of cooling rate on precipitates in the ingot is studied, and the results are shown in Table 3. As the cooling rate increases, the maximum size and area percentage of the precipitate decrease. When the cooling rate is 5°C / min, the precipitate size and area percentage are at a higher value. When the cooling rate is not less than 25°C / min, the precipitate size and area percentage are at a lower value. Among them, the purpose of preheating is to induce the precipitation of small-scale precipitates, strengthen the reinforced material through precipitation, and improve fatigue strength and fatigue stability. The temperature of preheating is controlled to be 450°C unchanged in the scheme. In specific implementation, the above effect can be achieved by keeping the temperature within the set range.

[0056] The nickel-titanium shape memory alloy microwire with high fatigue stability prepared in the above embodiment is used in the field of medical devices, such as being made into a drug delivery flow control drive wire. Since the nickel-titanium shape memory alloy microwire has a fatigue life of more than 400,000 times and a drive strain attenuation of less than 0.7% under a fatigue test load of 600 MPa, the control accuracy of the flow control drive wire is significantly improved during application, and the service life is more than doubled compared with the existing technology.

[0057] Although the present invention has been disclosed as above with preferred embodiments, it is not intended to limit the present invention. A person with ordinary knowledge in the technical field to which the present invention belongs may make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, the protection scope of the present invention shall be determined by the definition of the claims.

Claims

1. A nickel-titanium shape memory alloy microwire with high fatigue stability, It is characterized in that The size of the precipitates in the microstructure of the nickel-titanium shape memory alloy microwire does not exceed 10 μm, and the average area percentage of the precipitates does not exceed 2%.

2. The nickel-titanium shape memory alloy microwire with high fatigue stability according to claim 1, It is characterized in that The precipitate in the microstructure of the nickel-titanium shape memory alloy microwire is Ti 2 Ni is in the form of short round rods and precipitates along the grain boundaries in the microstructure.

3. The nickel-titanium shape memory alloy microwire with high fatigue stability according to claim 1, It is characterized in that The nickel content of the nickel-titanium shape memory alloy microwire is 49.5-50.5%.

4. The nickel-titanium shape memory alloy microwire with high fatigue stability according to claim 1, It is characterized in that The fatigue life of the nickel-titanium shape memory alloy microwire is not less than 100,000 times, and the driving strain attenuation is less than 0.7%.

5. A method for preparing nickel-titanium shape memory alloy microwires with high fatigue stability, It is characterized in that The steps include: (1) Using nickel and titanium as raw materials, medium and high frequency induction melting and ingot casting is adopted; (2) performing homogenization heat treatment and preheating treatment on the ingot, and obtaining a sample ingot after rapid cooling; (3) forging the sample ingot and hot rolling it into a round coil to obtain a nickel-titanium shape memory alloy material; (4) drawing the coiled nickel-titanium shape memory alloy material to obtain drawn nickel-titanium shape memory alloy microwires; (5) straightening the nickel-titanium shape memory alloy microwire to obtain a treated nickel-titanium shape memory alloy microwire; (6) The straightened nickel-titanium shape memory alloy microwires are trained to obtain nickel-titanium shape memory alloy microwires with high fatigue stability.

6. The method for preparing nickel-titanium shape memory alloy microwires with high fatigue stability according to claim 5, It is characterized in that The parameters of the homogenization heat treatment of the ingot in step (2) are: treatment temperature of 480-930°C, treatment time of 48h; the parameters of the preheat treatment are: treatment temperature of 430-480°C, holding time of 1h; the rate of rapid cooling after the preheat treatment is 15-35°C / min.

7. The method for preparing nickel-titanium shape memory alloy microwires with high fatigue stability according to claim 5, It is characterized in that The nickel content of the nickel-titanium shape memory alloy microwires obtained in step (6) is 49.5-50.5%, and the size of the precipitates in the microstructure thereof does not exceed 10 μm, and the average area percentage does not exceed 2%.

8. The method for preparing nickel-titanium shape memory alloy microwires with high fatigue stability according to claim 6, It is characterized in that The precipitate in the microstructure of the nickel-titanium shape memory alloy microwire obtained in step (5) is Ti 2 Ni is in the form of short round rods and precipitates along the grain boundaries in the microstructure.

9. The method for preparing nickel-titanium shape memory alloy microwires with high fatigue stability according to claim 6, It is characterized in that The specific process of step (1) using nickel and titanium as raw materials to melt ingots by medium and high frequency induction is as follows: Using nickel with a purity of not less than 99.9% and titanium with a purity of not less than 99.9% as raw materials, in a vacuum degree higher than 8×10 -2 Pa to obtain an ingot.

10. Application of the nickel-titanium shape memory alloy microwire with high fatigue stability as claimed in any one of claims 1 to 4 in the field of medical devices.

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