Nanocrystalline NiTi shape memory alloy and preparation method thereof
By forming a gradient structure and an oxide ceramic layer on a nanocrystalline NiTi shape memory alloy substrate, the problem of poor wear performance of nanocrystalline NiTi shape memory alloy was solved, achieving a balance between high wear resistance and super elasticity in the material.
Patent Information
- Application Number
- CN202511390848.4
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-09-26
- Publication Date
- 2026-02-06
AI Technical Summary
Existing nanocrystalline NiTi shape memory alloys have poor wear resistance, making it difficult to meet the requirements for long-term service. Furthermore, existing methods sacrifice the material's superelasticity while improving wear resistance.
A gradient structure transitioning from an amorphous surface to a nanocrystalline interior is formed on the alloy substrate, and an oxide ceramic layer is formed on it. Through mechanical surface treatment, an oxide ceramic layer is formed that is metallurgically bonded to the alloy substrate, thereby improving surface hardness and resisting ploughing and adhesion during the friction process.
It significantly improves the surface hardness and wear resistance of the material while maintaining its superelasticity, reducing the wear rate by an order of magnitude.
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Figure CN121472622A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of metal material processing technology, and in particular to a nanocrystalline NiTi shape memory alloy and its preparation method. Background Technology
[0002] In recent years, nanocrystalline NiTi shape memory alloys have attracted considerable attention in aerospace and solid-state refrigeration fields due to their excellent superelasticity, fatigue resistance, and elastic thermal effect. Compared with other traditional materials, NiTi shape memory alloys exhibit superior wear resistance; however, due to their unique application requirements, which necessitate undergoing tens of millions of repeated deformations, their wear resistance is insufficient to meet the demands of long-term service.
[0003] Currently, a common method to improve wear resistance is grain refinement, which increases material hardness and reduces wear rate. However, this method reduces the hyperelasticity required for practical applications, making it difficult to achieve practical value. Therefore, some researchers have innovatively proposed constructing gradient nanostructures to improve the surface hardness and wear resistance of materials without sacrificing their inherent hyperelasticity. However, this method has limited effectiveness in improving the wear resistance of nanocrystalline NiTi shape memory alloys, thus limiting the application of this alloy.
[0004] Therefore, the existing technology still needs further research and improvement. Summary of the Invention
[0005] To address the aforementioned technical problems, this invention proposes a nanocrystalline NiTi shape memory alloy and its preparation method, aiming to solve the problem of poor wear performance of existing nanocrystalline NiTi shape memory alloys. Specifically:
[0006] In a first aspect, embodiments of the present invention provide a nanocrystalline NiTi shape memory alloy, comprising:
[0007] Alloy substrate;
[0008] A gradient structure layer is formed on the alloy substrate; the gradient structure in the gradient structure layer is a structure that transitions from an amorphous state on the surface to a nanocrystalline state on the inside.
[0009] An oxide ceramic layer is formed on the gradient structure layer.
[0010] The following are preferred technical solutions of the present invention, but are not intended to limit the technical solutions provided by the present invention. The purpose and beneficial effects of the present invention can be better achieved and realized through the following preferred technical solutions.
[0011] As a preferred technical solution, in the nanocrystalline NiTi shape memory alloy, the thickness of the oxide ceramic layer is 0.5 to 50 μm.
[0012] As a preferred technical solution, in the nanocrystalline NiTi shape memory alloy, the thickness of the gradient structure layer is 50-250 μm.
[0013] As a preferred technical solution, in the nanocrystalline NiTi shape memory alloy, the thickness of the amorphous surface layer is 1–10 μm.
[0014] As a preferred technical solution, the nanocrystalline NiTi shape memory alloy, wherein the oxide ceramic layer comprises: titanium dioxide, nickel oxide, zirconium oxide, iron oxide, and titanium oxide.
[0015] In a second aspect, a method for preparing the nanocrystalline NiTi shape memory alloy according to claim 1, comprising: fixing an alloy substrate in a sealed ball milling device, continuously impacting the surface of the substrate alloy with grinding balls in the ball milling device, and obtaining the nanocrystalline NiTi shape memory alloy after impact treatment.
[0016] As a preferred technical solution, in the preparation method of the nanocrystalline NiTi shape memory alloy, the diameter of the grinding ball is 1-3 mm, and the impact treatment time is 0.5-5 hours.
[0017] As a preferred technical solution, in the method for preparing the nanocrystalline NiTi shape memory alloy, the rotation speed of the ball milling equipment is 100-1000 rpm.
[0018] As a preferred technical solution, in the preparation method of the nanocrystalline NiTi shape memory alloy, the mechanical work generated by continuous impact is converted into heat energy and continuously accumulated, and the surface temperature of the alloy substrate is 50-500℃.
[0019] Beneficial effects: Compared with the prior art, the embodiments of the present invention have the following advantages:
[0020] The oxide ceramic layer in the nanocrystalline NiTi shape memory alloy provided by this invention is bonded to the alloy substrate. Its ultra-high hardness and chemical stability allow it to directly resist ploughing and adhesion during friction, forming the first line of defense against wear. The gradient structure, transitioning from an amorphous surface to a nanocrystalline interior, significantly improves surface hardness and mitigates the abrupt performance changes between the surface and substrate through its gradient distribution characteristics. Attached Figure Description
[0021] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.
[0022] Figure 1 The transmission electron microscope image (a) and SAED pattern (b) of the nanocrystalline NiTi shape memory alloy provided in Example 1 are shown. The original sample has a single-phase austenitic (B2) structure.
[0023] Figure 2 The microstructure of the NiTi alloy cross section before and after mechanical surface treatment.
[0024] Figure 3 a) shows the surface hardness of the NiTi alloy before and after mechanical surface treatment, and b) shows the curve of the cross-sectional hardness of the NiTi alloy after mechanical surface treatment as a function of the depth from the surface.
[0025] Figure 4 a) shows the friction and wear curves of NiTi alloy before and after mechanical surface treatment; b) shows the 3D contour map of the wear track before and after mechanical surface treatment; c) shows the cross-sectional data of the wear track; d) shows the comparison of wear rate before and after mechanical surface treatment.
[0026] Figure 5 This is a schematic diagram of the fabrication process for nanocrystalline NiTi shape memory alloy. Detailed Implementation
[0027] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.
[0028] This invention utilizes SMAT (Mechanical Surface Treatment) technology to form a gradient structure on the surface of an alloy substrate, transitioning from an amorphous surface to a nanocrystalline interior. This structure not only significantly improves surface hardness through grain refinement but also mitigates the abrupt performance changes between the surface and the substrate through its gradient distribution characteristics. It provides a mechanically well-matched transition zone for the subsequent adhesion of the ceramic layer. Simultaneously, the heat generated during the SMAT process forms an oxide ceramic layer bonded to the alloy substrate. This ceramic layer, with its ultra-high hardness and chemical stability, directly resists ploughing and adhesion during friction, becoming the first line of defense against wear.
[0029] In this invention, mechanical surface treatment refers to the continuous impact of high-energy hard microspheres on the alloy surface. On one hand, under the gradient plastic deformation (the deformation is greatest at the surface and gradually decreases with depth), the original grains of the alloy surface are violently broken and refined, ultimately forming a gradient structure that transitions from an amorphous surface to a nanocrystalline interior. This structure not only significantly improves surface hardness through grain refinement but also mitigates the abrupt performance change between the surface and the substrate by utilizing the gradient distribution characteristics. On the other hand, in the closed environment of the sealed ball mill jar, the large amount of mechanical work generated by high-speed impact is converted into heat energy and continuously accumulates, promoting the diffusion and chemical reaction between atoms on the microsphere surface and atoms on the highly reactive NiTi alloy surface, forming an oxide ceramic layer (mainly composed of ZrO2, FeO, etc.) that is metallurgically bonded to the substrate.
[0030] The technical solutions provided by the present invention will be further explained and illustrated below through specific embodiments.
[0031] Example 1
[0032] Polycrystalline superelastic Ni with an original thickness of 1.5 mm was used. 50.8 -Ti 49.2 (Atomic percentage) Alloy sheet. The initial sheet was homogenized at 800℃ for 2 hours, followed by water quenching; then the sheet was sandwiched between stainless steel sheets and repeatedly cold-rolled to a thickness of 0.6 mm (thickness reduction of 60%). The cold-rolled NiTi alloy sheet was then heat-treated at 500℃ for 8 minutes, followed by water quenching. The sample was machined into 10×10×0.6 mm pieces by wire cutting. 3 Small samples were then mechanically polished to obtain a mirror-like surface.
[0033] The sample was fixed on the top of the ball mill jar, and 300 zirconia balls with a diameter of 3 mm were loaded into the jar and sealed. The ball mill was operated at a vibration frequency of 30 W for 5 hours with an impact distance of 50 mm to obtain modified NiTi alloy sheets. The entire preparation process can be found in [reference needed]. Figure 5As shown in the figure. The thickness of the oxide ceramic layer is approximately 3 μm, the thickness of the gradient structure layer is approximately 50 μm, and the thickness of the surface amorphous layer is 8 μm.
[0034] Example 2
[0035] Polycrystalline superelastic Ni with an original thickness of 1.5 mm was used. 50.8 -Ti 49.2 (Atomic percentage) Alloy sheet. The initial sheet was homogenized at 800℃ for 2 hours, followed by water quenching; then the sheet was sandwiched between stainless steel sheets and repeatedly cold-rolled to a thickness of 0.6 mm (thickness reduction of 60%). The cold-rolled NiTi alloy sheet was then heat-treated at 400℃ for 8 minutes, followed by water quenching. The sample was machined into 10×10×0.6 mm pieces by wire cutting. 3 Small samples were then mechanically polished to obtain a mirror-like surface.
[0036] The sample was fixed on the top of a ball mill jar, and 300 zirconia microspheres with a diameter of 2 mm were loaded into the jar and sealed. The ball mill was operated at a vibration frequency of 90 W for 4 hours with an impact distance of 60 mm to obtain modified NiTi alloy sheets. The thickness of the oxide ceramic layer was approximately 4 μm, the thickness of the gradient structure layer was approximately 110 μm, and the thickness of the surface amorphous layer was 5 μm.
[0037] Example 3
[0038] A polycrystalline superelastic Ni50.8-Ti49.2 (atomic percentage) alloy sheet with an initial thickness of 2 mm was used. The initial sheet was homogenized at 850℃ for 1.5 h, followed by water quenching. The sheet was then sandwiched between stainless steel sheets and repeatedly cold-rolled to a thickness of 0.8 mm (thickness reduction of 60%). The cold-rolled NiTi alloy sheet was then heat-treated at 500℃ for 10 min, followed by water quenching. The samples were machined into 10×10×0.6 mm pieces by wire cutting. 3 Small samples were then mechanically polished to obtain a mirror-like surface.
[0039] The sample was fixed on the top of a ball mill jar, and 300 zirconia microspheres with a diameter of 2 mm were loaded into the jar and sealed. The ball mill was operated at a vibration frequency of 100 W for 3 hours with an impact distance of 60 mm to obtain modified NiTi alloy sheets. The thickness of the oxide ceramic layer was approximately 5 μm, the thickness of the gradient structure layer was approximately 150 μm, and the thickness of the surface amorphous layer was 10 μm.
[0040] Example 4
[0041] A polycrystalline superelastic Ni50.8-Ti49.2 (atomic percentage) alloy sheet with an initial thickness of 2 mm was used. The initial sheet was homogenized at 850℃ for 1.5 h, followed by water quenching. The sheet was then sandwiched between stainless steel sheets and repeatedly cold-rolled to a thickness of 0.8 mm (thickness reduction of 60%). The cold-rolled NiTi alloy sheet was then heat-treated at 500℃ for 10 min, followed by water quenching. The samples were machined into 10×10×0.6 mm pieces by wire cutting. 3 Small samples were then mechanically polished to obtain a mirror-like surface.
[0042] The sample was fixed on the top of the ball mill jar, and 300 stainless steel balls with a diameter of 3 mm were loaded into the jar and sealed. The ball mill was operated at a vibration frequency of 120 W for 5 hours with an impact distance of 100 mm to obtain modified NiTi alloy flakes. The thickness of the oxide ceramic layer was approximately 5 μm, the thickness of the gradient structure layer was approximately 250 μm, and the thickness of the surface amorphous layer was 10 μm.
[0043] The modified NiTi alloy sheet samples prepared above were subjected to reciprocating (5 mm stroke) sliding friction and wear tests on an Anton Paar TRB3 tribometer. A ball-disc dry sliding wear test was used, with Φ6 mm silicon nitride ceramic balls as the grinding material. The wear test was conducted in an atmospheric environment at room temperature (approximately 25°C) (relative humidity approximately 70%), with a friction sliding speed of 0.1 cm / s, a load of 5 N, and a sliding distance of 10 m (1000 cycles). After the experiment, the friction coefficient curve was recorded; and the wear trajectory morphology was measured using a 3D profilometer. The wear rate was calculated based on the wear volume.
[0044] Figure 1 The transmission electron microscopy (TEM) characterization results of a polycrystalline superelastic NiTi alloy sheet with an original thickness of 1.5 mm after homogenization treatment at 800 °C for 2 h, cold rolling, and heat treatment at 500 °C for 8 min are shown. Figure 1 Image a) is a typical bright-field TEM image. Statistical analysis of the grain size based on this image indicates that the average grain size of the sample is approximately 100 nm. Figure 1 In Figure b), the corresponding SAED pattern shows only diffraction rings belonging to the B2 phase. Comprehensive analysis indicates that the original sample was a single-phase B2 state with a grain size of approximately 100 nm.
[0045] Figure 2 The TEM image shows a cross-section of the NiTi alloy after SMAT. It can be clearly seen that the NiTi alloy after SMAT forms a structure of "surface oxide ceramic layer → subsurface completely amorphous structure → gradient amorphous-nanocrystalline structure → matrix".
[0046] Figure 3 To characterize the surface microhardness and cross-sectional nanohardness of the samples after SMAT, Figure 3 In Figure a), the surface microhardness of the samples before and after SMAT is compared. The hardness of the untreated sample is 289.73 HV, while the hardness of the sample after SMAT is 509.76 HV, indicating that SMAT treatment significantly improves the hardness of the samples. Figure 3 Figure b) shows the variation curve of nanohardness of the sample cross section from the surface to the depth direction after SMAT treatment. It can be seen that the surface hardness of the sample after SMAT treatment is significantly improved compared to the whole, and the hardness decreases significantly along the depth direction until it reaches 50 μm and stabilizes at about 4.5 GPa (the hardness of NiTi alloy amorphous). The SMAT effect range is more than about 200 μm. Therefore, it can be concluded that the overall hardness of the SMAT-treated sample is improved, especially the surface, and it has a significant ability to resist friction and wear.
[0047] Figure 4 In figure a), the friction coefficient changes with the number of sliding revolutions. The black curve represents the untreated sample, and the red curve represents the SMAT sample. It can be seen that the friction coefficient of the untreated sample fluctuates relatively more, while the friction coefficient of the SMAT sample is relatively stable and slightly lower. The SEM images in the inset show the wear marks of both samples, which are circular areas with a radius of 2 mm. It is clear that the SMAT wear track is narrower. Figure 4 (b) and Figure 4 c) shows the three-dimensional morphology of the wear trajectory and the two-dimensional cross-sectional depth distribution of the wear trajectory, respectively. It can be clearly seen that the wear depth of the untreated sample fluctuates greatly, while the wear depth of the SMAT-treated sample is relatively stable. Figure 4 (d) shows the wear rate comparison of the samples before and after SMAT. The wear rate of the untreated sample was 8.32 × 10⁻⁶. - 5 mm 3 / (N·m), the wear rate of the sample after SMAT was 5.95×10. -6 mm 3 The wear resistance (N·m) decreased by one order of magnitude. This indicates that SMAT significantly improves the wear resistance of the material.
[0048] It should be understood that the present invention is not limited to the precise structure described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The scope of the invention is limited only by the appended claims.
[0049] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.
Claims
1. A nanocrystalline NiTi shape memory alloy, characterized in that, include: Alloy substrate; A gradient structure layer is formed on the alloy substrate; The gradient structure in the gradient structure layer is a structure that transitions from an amorphous state on the surface to a nanocrystalline state on the inside. An oxide ceramic layer is formed on the gradient structure layer.
2. The nanocrystalline NiTi shape memory alloy according to claim 1, characterized in that, The thickness of the oxide ceramic layer is 0.5–50 μm.
3. The nanocrystalline NiTi shape memory alloy according to claim 1, characterized in that, The thickness of the gradient structure layer is 50–250 μm.
4. The nanocrystalline NiTi shape memory alloy according to claim 1, characterized in that, The thickness of the amorphous surface layer is 1–10 μm.
5. The nanocrystalline NiTi shape memory alloy according to claim 1, characterized in that, The oxide ceramic layer comprises titanium dioxide, nickel oxide, zirconium oxide, iron oxide, and titanium dioxide.
6. A method for preparing the nanocrystalline NiTi shape memory alloy according to claim 1, characterized in that, include: The alloy substrate is fixed in a sealed ball mill, and the surface of the substrate alloy is continuously impacted by the grinding balls in the ball mill to obtain the nanocrystalline NiTi shape memory alloy.
7. The method for preparing the nanocrystalline NiTi shape memory alloy according to claim 6, characterized in that, The diameter of the grinding ball is 1-3 mm, and the impact treatment time is 0.5-5 hours.
8. The method for preparing nanocrystalline NiTi shape memory alloy according to claim 6, characterized in that, The rotational speed of the ball mill is 100 to 1000 rpm.
9. The method for preparing nanocrystalline NiTi shape memory alloy according to claim 5, characterized in that, The mechanical work generated by continuous impact is converted into heat energy and accumulates continuously, and the surface temperature of the alloy substrate is 50 to 500°C.