Method for observing self-adaptive morphology of martensite in aging Ni-rich nickel-titanium alloy

Abstract

The invention relates to an observation method for martensite self-adaptive morphology in an aging Ni-rich component nickel-titanium alloy, which comprises the following steps: removing a surface oxide layer from a blocky Ni-rich component nickel-titanium alloy subjected to aging treatment, then putting the blocky Ni-rich component nickel-titanium alloy into a nitric acid methanol polishing solution for electrolytic polishing, and soaking a sample into absolute ethyl alcohol for ultrasonic cleaning and drying after electrolytic polishing; if it is found through the process that martensite transformation in the sample alloy is incomplete, the martensite self-adaptive morphology of the Ni-rich nickel-titanium alloy subjected to aging treatment can be observed under a scanning electron microscope after low-temperature treatment of a differential scanning calorimeter. According to the method, efficient and high-quality characterization of the structure of the low-temperature martensite phase of the blocky aged nickel-titanium alloy can be achieved, and meanwhile the problems that it is difficult to obtain a high-quality metallographic phase sample from an alloy containing a high-density coherent precipitated phase and a large number of twin crystal structures, and it is difficult to directly observe the low-temperature structure through a conventional characterization means are solved.

Description

Technical Field

[0001] The invention relates to the technical field of material surface treatment, in particular to a method for observing the adaptive morphology of martensite in an aging Ni-rich nickel-titanium alloy. Background Art

[0002] Nickel-titanium shape memory alloys are widely used in various fields due to their unique shape memory effect, superelasticity and biocompatibility. In particular, applications based on their shape memory effect have a history of nearly half a century. After aging treatment, the Ni-rich nickel-titanium alloy will produce Ni4Ti3 precipitation phase in the matrix, giving the alloy a two-way shape memory effect. At this time, the shape memory performance of the alloy will be controlled by the coherent stress field generated by the Ni4Ti3 precipitation phase inside it, as well as the internal martensitic phase transformation and the resulting martensitic microstructure.

[0003] When the nickel-titanium shape memory alloy undergoes a martensitic phase transformation, the martensitic variants will spontaneously form a specific morphology in order to reduce the elastic strain energy during the phase transformation and offset the macroscopic shape change of the sample. This morphology is called martensitic adaptive morphology. In the martensitic nickel-titanium alloy containing Ni4Ti3 precipitation phase, due to the presence of Ni4Ti3 precipitation phase, the volume and morphology of the martensitic adaptive morphology formed will change compared to the nickel-titanium alloy without precipitation phase. Therefore, it is urgent to find a better surface treatment method to observe it completely and clearly. Traditional chemical corrosion is difficult to observe clearly, and the electrolytic double-spraying sample preparation has a small sample size and a thin film effect, which leads to an incomplete presentation of the martensitic adaptive morphology. In addition, the operation is complicated and the cost is high.

[0004] Although the electrolytic polishing method can overcome the film effect caused by too thin samples, the traditional electrolytic polishing method generally optimizes the processing effect from parameters such as voltage, current, electrolyte type and concentration, and is suitable for samples of specific sizes. The nickel-titanium two-way shape memory alloy, which is currently being studied more, has filaments, rods, blocks, etc. depending on the usage scenario. It is difficult to obtain samples of exactly the same size during the experiment, especially for samples that have been constrained and aged. The alloy is curved after being constrained. After mechanical polishing to remove the oxide scale, the size of samples cut from different positions will be quite different. Therefore, the impact of different sample surface areas must be considered during the electrolysis process.

[0005] In addition, different aging methods will also bring different phase transition temperatures. For NiTi alloys whose martensitic phase transition temperature is lower than the electrolysis temperature, further cooling is required after electrolysis to allow sufficient martensitic phase transition to occur inside before the complete martensitic adaptive morphology can be observed under SEM. However, conventional cooling methods (such as water cooling, liquid nitrogen cooling, etc.) have uncontrollable cooling rates, which may lead to unbalanced cooling of the overall sample; on the other hand, too fast cooling can easily cause sample deformation or cracking, and even adsorb some pollutants, thereby affecting the subsequent characterization of the sample. In addition, some low-temperature characterization methods, such as in-situ scanning electron microscopy and cryo-electron microscopy, are costly and have high requirements on sample quality, and are not suitable for conventional characterization. Therefore, there is an urgent need to develop an observation method for the adaptive morphology of martensitic in aged nickel-titanium alloys. Summary of the invention

[0006] In view of the problems existing in the prior art, the purpose of the present invention is to provide a method for observing the adaptive morphology of martensite in aged Ni-rich nickel-titanium alloy, which can realize large-area direct observation of the adaptive morphology of martensite through conventional characterization means and is not affected by the thin film effect.

[0007] In order to achieve the above object, the present invention adopts the following technical solution: The method for observing the adaptive morphology of martensite in aged Ni-rich nickel-titanium alloy is characterized by comprising the following steps: S1, removing the surface oxide layer of the aged block Ni-rich nickel-titanium alloy and cleaning and drying it; S2, placing the bulk nickel-titanium alloy treated in S1 into an electrolytic cell containing nitric acid methanol polishing solution for electrolytic polishing, and after electrolytic polishing, immersing the sample in anhydrous ethanol for ultrasonic cleaning and drying; S3. The nickel-titanium alloy treated with S2 is placed under a scanning electron microscope to observe the martensite adaptive morphology of the Ni-rich nickel-titanium alloy treated with aging.

[0008] Furthermore, the method for removing the surface oxide layer in S1 is mechanical polishing. The total area of ​​the blocky martensitic nickel-titanium alloy after mechanical polishing is more than three times larger than the clamping part during the electrolytic polishing process, and the thickness is greater than 0.4±0.1 mm to prevent electrical breakdown during the electrolysis process.

[0009] Furthermore, the nitric acid-methanol polishing liquid is prepared according to a volume ratio of concentrated nitric acid: methanol = 1.0±0.5: 3±0.5.

[0010] Furthermore, when preparing the polishing liquid, concentrated nitric acid is added to methanol, and an ice water bath is used for cooling the whole process.

[0011] Furthermore, the electrolytic cell is placed in a foam insulation tank, and liquid nitrogen is added in small amounts multiple times to control the temperature of the electrolyte at -15°C to -5°C.

[0012] Furthermore, the current density of electrolytic polishing was 0.3 A / cm 2 ~1.0A / cm 2 , temperature is -15℃~-5℃, time is 45±5s.

[0013] Furthermore, the distance between the cathode and anode of the electrolytic cell is controlled between 4 cm and 8 cm.

[0014] Furthermore, during the electrolysis process, the polishing surface of the blocky martensitic nickel-titanium alloy is kept parallel to the cathode, so that the polishing surface is polished evenly.

[0015] Furthermore, if the martensite transformation of the nickel-titanium alloy is incomplete after the treatment of S2, the nickel-titanium alloy is firstly controlled to be cooled by a differential scanning calorimeter to complete the martensite transformation in the nickel-titanium alloy, and then S3 is performed.

[0016] Furthermore, the nickel-titanium alloy was controllably cooled using a differential scanning calorimeter by first maintaining the temperature at 25°C for 10 minutes, then slowly cooling at a rate of 5 to 10°C / min to -50°C to -100°C, and maintaining the temperature for 10 minutes.

[0017] In general, the present invention has the following advantages: 1. A method for preparing SEM samples of martensitic nickel-titanium alloy after aging treatment is provided, which can realize large-area observation of the adaptive morphology of martensite and is not affected by the film effect.

[0018] During the martensitic phase transformation of martensitic nickel-titanium alloy, one or more martensitic adaptive morphologies will be generated in the matrix. Previous studies generally used transmission electron microscopy for observation, but transmission electrolysis has many restrictions on the size of the sample, and the preparation process is complicated and time-consuming. The most important thing is that the sample is too thin and there is a film effect, which prevents the derivation of martensitic variants, so that only limited variants can be produced inside the alloy, and the complete martensitic adaptive morphology cannot be observed. The SEM sample prepared by this method, on the one hand, can maintain a thickness of more than 0.4±0.1mm, there will be no limitation of the film effect, and no electrical breakdown will occur during the electrolysis process; on the other hand, the observation area is large, and all possible martensitic adaptive morphologies can be fully observed, which is statistically significant.

[0019] 2. Current density is used as one of the electrolytic polishing optimization parameters, which fully considers the influence of sample surface area on the polishing effect.

[0020] The nickel-titanium shape memory alloy containing Ni4Ti3 precipitate phase obtained by constraint treatment is curved, and it is difficult to obtain completely consistent block samples using a wire cutting machine. At this time, if the voltage, current and other parameters are used for optimization, it is necessary to optimize the parameters multiple times for different samples, which is time-consuming and laborious. However, by using the current density parameter for optimization, the influence of the sample surface area on the polishing effect is fully considered, and a single parameter optimization can be applied to all samples.

[0021] 3. For samples with too low phase transition temperature, DSC (differential scanning calorimeter) is used for controllable cooling.

[0022] For cryogenic tissues, it is very difficult to characterize them at room temperature, and the cost of using technologies such as cryo-electron microscopy is too high. At this time, the method of cooling first and then observing is generally adopted. Although conventional cooling methods (such as water cooling, liquid nitrogen cooling, etc.) are simple to operate, the cooling process is uncontrollable and easy to contaminate the sample surface. Using DSC to cool the sample, on the one hand, controllable cooling can be achieved by setting the program, and on the other hand, nitrogen protection during the cooling process can effectively isolate possible contamination and ensure the cleanliness of the sample observation surface, effectively solving the problem that cryogenic tissues are difficult to directly observe through conventional characterization methods. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a BSE-SEM image of the SEM sample of the blocky martensitic nickel-titanium alloy after electrolytic polishing in Example 1 of the present invention.

[0024] Figure 2 This is a BSE-SEM image of the SEM sample of the blocky martensitic nickel-titanium alloy after electrolytic double spraying treatment in Example 1 of the present invention.

[0025] Figure 3 This is a BSE-SEM image of the SEM sample of the blocky martensitic nickel-titanium alloy after chemical corrosion treatment in Example 1 of the present invention.

[0026] Figure 4 This is a BSE-SEM image of the SEM sample of the blocky martensitic nickel-titanium alloy after electrolytic polishing in Example 2 of the present invention.

[0027] Figure 5 This is a BSE-SEM image of a bulk martensitic nickel-titanium alloy SEM sample after DSC low-temperature treatment in Example 2 of the present invention.

[0028] Figure 6 This is a BSE-SEM image of a SEM sample of a blocky martensitic nickel-titanium alloy after liquid nitrogen cooling treatment in Example 2 of the present invention. DETAILED DESCRIPTION

[0029] The present invention will be described in further detail below.

[0030] Example 1 The nickel block and sponge titanium are weighed according to an atomic ratio of 50.6:49.4, and then placed together in a vacuum arc melting furnace for smelting. After the smelting is completed, the molten alloy ingot is quickly sucked into a water-cooled copper mold and quickly cooled into a strip.

[0031] The prepared martensitic nickel-titanium alloy strips were solution treated, and then the alloy strips were bent and placed into a constrained mold for constrained aging treatment at a temperature of 450°C for 5 hours. After the aging treatment, they were immediately water quenched.

[0032] Use a wire cutting machine to cut the martensitic nickel-titanium alloy strips into blocks of 10mm×8mm×0.7mm. After cutting, the block martensitic nickel-titanium alloy should be polished off the surface oxide layer by mechanical polishing. During mechanical polishing, 400#, 800#, 1500# and 3000# sandpapers are used for polishing in sequence. When changing sandpapers of different mesh sizes, the sample should be rotated 90°. After sandpaper polishing, 0.3μm alumina polishing liquid is used to polish the sample on the polishing cloth to make the sample surface present a smooth mirror surface. Finally, the sample is immersed in anhydrous ethanol and ultrasonically cleaned for 5 minutes.

[0033] The surface area of ​​the measured sample is 257.84mm 2 , thickness is 0.42mm, at 0.7A / cm 2 The current density is used as an indicator to calculate that the current during electrolytic polishing should be set to 1.80A. Take 400ml of nitric acid methanol electrolyte with a volume ratio of 1:3 and place it in the electrolytic cell. Then slowly add liquid nitrogen to the electrolyte and stir continuously until the temperature of the electrolyte drops to -10±2℃. The block martensitic nickel-titanium alloy is clamped on the anode connected to the positive pole of the power supply, the cathode is a platinum sheet, turn on the switch of the electrolytic polishing corrosion instrument, set the current to 1.80A, and the polishing time to 45s. After polishing, quickly take out the sample and immerse it in anhydrous ethanol for ultrasonic cleaning for 5 minutes to prepare the martensitic nickel-titanium alloy SEM sample containing the precipitated phase.

[0034] like Figure 1 As shown, the SEM sample of the blocky martensitic nickel-titanium alloy containing the precipitated phase prepared by electrolytic polishing of the present invention has uniform corrosion as a whole and obvious morphology contrast. The obvious triangular adaptive morphology can be clearly seen from the BSE-SEM image.

[0035] Figure 2 The BSE-SEM images of the samples after the same aging treatment and then the samples made by electrolytic double spraying are shown in Figure 2. Figure 2 We can only observe grain boundaries and lamellar martensite, but no obvious martensite adaptive morphology can be observed.

[0036] Figure 3 BSE-SEM images of samples after the same aging treatment and then chemical etching. The etching solution prepared according to the volume ratio of HF:HNO3:H2O=1:4:5 was selected. Figure 3 Clear grain boundaries can be seen in the sample, but it is difficult to observe finer martensitic structures. In contrast, the sample after electrolytic polishing of the present invention is more suitable for observing the martensitic adaptive morphology existing in the martensitic nickel-titanium alloy containing precipitated phase under a scanning electron microscope.

[0037] Example 2 The nickel block and sponge titanium are weighed according to an atomic ratio of 50.6:49.4, and then placed together in a vacuum arc melting furnace for smelting. After the smelting is completed, the molten alloy ingot is quickly sucked into a water-cooled copper mold and quickly cooled into a strip.

[0038] The prepared martensitic nickel-titanium alloy strips were solution treated, and then the alloy strips were bent and placed into a constrained mold for constrained aging treatment at a temperature of 500°C for 5 hours. After the aging treatment, they were immediately water quenched.

[0039] Use a wire cutting machine to cut the martensitic nickel-titanium alloy strips into blocks of 10mm×8mm×0.7mm. After cutting, the block martensitic nickel-titanium alloy should be polished off the surface oxide layer by mechanical polishing. During mechanical polishing, 400#, 800#, 1500# and 3000# sandpapers are used for polishing in sequence. When changing sandpapers of different mesh sizes, the sample should be rotated 90°. After sandpaper polishing, 0.3μm alumina polishing liquid is used to polish the sample on the polishing cloth to make the sample surface present a smooth mirror surface. Finally, the sample is immersed in anhydrous ethanol and ultrasonically cleaned for 5 minutes.

[0040] The surface area of ​​the measured sample is 207.14mm 2 , thickness is 0.39mm, at 0.7A / cm 2 The current density is used as an indicator to calculate that the current during electrolytic polishing should be set to 1.45A. Take 400ml of nitric acid methanol electrolyte with a volume ratio of 1:3 and place it in the electrolytic cell. Then slowly add liquid nitrogen to the electrolyte and stir continuously until the temperature of the electrolyte drops to -10±2℃. Clamp the block martensitic nickel-titanium alloy on the anode connected to the positive pole of the power supply, and the cathode is a platinum sheet. Turn on the switch of the electrolytic polishing corrosion instrument, set the current to 1.45A, and the polishing time to 45s. After polishing, quickly remove the sample and immerse it in anhydrous ethanol for ultrasonic cleaning for 5 minutes.

[0041] like Figure 4As shown in the figure, the SEM sample of the blocky martensitic nickel-titanium alloy containing the precipitated phase prepared by the above process did not observe obvious martensitic adaptive morphology under scanning electrolysis. This is because the martensitic phase transformation temperature and the reverse phase transformation temperature of the sample itself are both low, resulting in incomplete martensitic transformation in the matrix. Therefore, further low-temperature treatment with DSC is required in the future.

[0042] Place the sample on the sample rod of the DSC and an empty crucible on the reference rod. Set the cooling program as follows: first cool to 30°C, keep warm for 10 minutes, then reduce the temperature to -80°C at a cooling rate of 5°C / min, take out the sample after keeping warm for 10 minutes, and the processing is complete.

[0043] like Figure 5 As shown, the SEM sample of the blocky martensitic nickel-titanium alloy containing the precipitated phase after electrolytic polishing and DSC low-temperature treatment can be observed under the scanning electron microscope to have obvious V-shaped and triangular morphologies. The size of the triangular morphology is relatively small, which is caused by the different temperatures during the stress constraint treatment of the sample.

[0044] Figure 6 For samples cooled by liquid nitrogen, Figure 6 In the figure, a partially incomplete triangular morphology can be observed. At the same time, many holes can also be found. These are caused by the uneven cooling of the entire sample during the cooling process. Therefore, conventional cooling methods are not suitable for the observation of the adaptive morphology of martensite in aged nickel-titanium alloys.

[0045] The present invention can achieve efficient and high-quality characterization of the microstructure of the low-temperature martensitic phase of bulk aged nickel-titanium alloy, while solving the problems of difficulty in obtaining high-quality metallographic samples of alloys containing high-density coherent precipitate phases and a large number of twin structures, and difficulty in directly observing low-temperature structures through conventional characterization methods, providing an innovative optimization solution for the microstructural characterization of martensitic nickel-titanium alloys and alloys with similar structural characteristics.

[0046] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be equivalent replacement methods and are included in the protection scope of the present invention.

Claims

1. A method for observing the adaptive morphology of martensite in aged Ni-rich nickel-titanium alloy, characterized by: The following steps are included: S1, removing the surface oxide layer of the aged block Ni-rich nickel-titanium alloy and cleaning and drying it; S2, placing the bulk nickel-titanium alloy treated in S1 into an electrolytic cell containing nitric acid methanol polishing solution for electrolytic polishing, and after electrolytic polishing, immersing the sample in anhydrous ethanol for ultrasonic cleaning and drying; S3. The nickel-titanium alloy treated with S2 is placed under a scanning electron microscope to observe the martensite adaptive morphology of the Ni-rich nickel-titanium alloy treated with aging.

2. The observation method according to claim 1, characterized in that: The method for removing the surface oxide layer in S1 is mechanical polishing. The total area of ​​the blocky martensitic nickel-titanium alloy after mechanical polishing is more than three times larger than the clamped part during the electrolytic polishing process, and the thickness is greater than 0.4±0.1 mm.

3. The observation method according to claim 1, characterized in that: The nitric acid-methanol polishing solution is prepared according to the volume ratio of concentrated nitric acid: methanol = 1.0±0.5:3±0.

5.

4. The observation method according to claim 1, characterized in that: The polishing solution was prepared by adding concentrated nitric acid to methanol, and an ice water bath was used for cooling throughout the process.

5. The observation method according to claim 1, characterized in that: The electrolytic cell is placed in a foam insulation tank, and liquid nitrogen is added in small amounts multiple times to control the temperature of the electrolyte at -15°C to -5°C.

6. The observation method according to claim 1, characterized in that: The current density of electrolytic polishing is 0.3A / cm 2 ~1.0A / cm 2 , temperature is -15℃~-5℃, time is 45±5s.

7. The observation method according to claim 1, characterized in that: The distance between the cathode and anode of the electrolytic cell is controlled between 4 cm and 8 cm.

8. The observation method according to claim 1, characterized in that: During the electrolysis process, the polished surface of the blocky martensitic nickel-titanium alloy is kept parallel to and opposite to the cathode.

9. The observation method according to claim 1, characterized in that: If the martensite transformation of the nickel-titanium alloy is incomplete after the S2 treatment, the nickel-titanium alloy is firstly subjected to controllable cooling using a differential scanning calorimeter to complete the martensite transformation in the nickel-titanium alloy, and then S3 is performed.

10. The observation method according to claim 9, characterized in that: The method of controllably cooling the nickel-titanium alloy using a differential scanning calorimeter is to first maintain the temperature at 25°C for 10 minutes, then slowly cool it at a rate of 5 to 10°C / min to -50°C to -100°C, and maintain it for 10 minutes.

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