NiTiNb shape memory alloy and preparation method thereof

Abstract

This invention provides a NiTiNb shape memory alloy and its preparation method. The preparation method specifically includes the following steps: (1) uniformly mixing Ni source, Ti source and Nb source to obtain mixed powder; (2) cold molding the mixed powder to obtain alloy green blank; (3) sintering the green blank to obtain the NiTiNb shape memory alloy. The preparation method of this invention, by introducing Nb source powder for sintering and controlling the sintering temperature parameters, can prepare a NiTiNb alloy with uniform composition and high density, increasing the strength of the NiTiNb alloy to over 1600 MPa, increasing the density of the alloy to 92%, the compressive strength to over 2000 MPa, the fracture strain to over 30%, and the tensile recovery rate to over 50% (8%), significantly improving the mechanical properties of the alloy.

Description

Technical Field

[0001] This invention belongs to the field of shape memory materials technology, and particularly relates to a NiTiNb shape memory alloy and its preparation method. Background Technology

[0002] NiTi and NiTi-based shape memory alloys have become the most important shape memory alloys due to their good fatigue resistance, excellent shape memory effect, superelasticity, and good biocompatibility. After phase transformation with binary NiTi alloys, the addition of Nb results in ternary NiTiNb alloys exhibiting greater phase transformation hysteresis, higher strength, and higher plasticity. Furthermore, the complex oxide surface enhances the alloy's corrosion resistance and biocompatibility. Therefore, NiTiNb alloys have very broad prospects in engineering and biomedical applications.

[0003] However, there are still many difficulties and problems in the preparation methods of NiTiNb alloys. The smelting process for preparing NiTiNb alloys presents numerous challenges, such as inhomogeneous composition and coarse grain size, necessitating further post-processing to ensure the quality and performance of the finished product meet requirements. Due to the significant differences in melting points between Ni (1726K), Ti (1941K), and Nb (2741K), and the chemical reactions at high temperatures, compositional segregation easily occurs during smelting, which is detrimental to maintaining good mechanical properties. Heat treatment is necessary to optimize its microstructure and properties. Furthermore, smelting only yields finished products with a single geometric shape; to meet practical applications, forming processes (forging, hot drawing, hot drawing, etc.) are required.

[0004] Powder metallurgy is one of the main methods for preparing NiTiNb alloys. Powder metallurgy technologies, including compression molding, additive manufacturing, and injection molding, are an important class of preparation methods for NiTiNb alloys. They can improve material utilization, reduce processing volume, and even construct gradient and porous structures that are difficult to achieve ingot metallurgy. Among these methods, vacuum reaction sintering using elemental powders as raw materials has the advantages of flexible composition design, low cost, and simple process. However, due to the challenges of sintering, alloys obtained by this method have uncontrollable porosity and undesirable phases, resulting in poor strength (tensile strength of 131~282 MPa), which is significantly lower than the strength of alloys prepared by pre-alloyed powder sintering and casting metallurgy (600~930 MPa).

[0005] The preparation of NiTiNb alloys is mainly constrained by two major challenges. First, there is the contamination from oxygen impurities. Elemental powders using hydrogenated dehydrogenated Ti powder as raw materials have a higher initial oxygen content, and new oxygen impurities are easily introduced during sintering. Increased oxygen content leads to the formation of more brittle second phases, such as Ti₂Ni / (Ti,Nb)₂Ni. While these second phases can improve the alloy's hardness in some cases, they reduce its plasticity and toughness. Therefore, improving the purity of raw materials and avoiding impurity contamination during preparation has always been an important measure for the preparation of high-performance NiTi-based alloys. Secondly, the difference in diffusion rates between elemental powders and the complex and intense eutectic liquid phase reaction pose difficulties in the preparation of NiTi-based alloys. Due to the large difference in interdiffusion rates between elements, Kirkendall pores are formed, which adversely affect the alloy's properties. Simply increasing the sintering temperature is not only insufficient to eliminate Kirkendall pores but also poses the problem of liquid phase loss. During the elemental reaction sintering process of binary NiTi alloys, the formation energies of Ti2Ni, Ni3Ti, and NiTi intermediate phases are similar, and these intermediate phases all appear around 720℃. Therefore, with the increase of sintering temperature, liquid phases generated by two eutectic reactions, β-Ti(Ni) + Ti2Ni → L and TiNi + TiNi3 → L, exist at 942℃ and 1118℃, respectively. The distribution and volume fraction of these non-uniform eutectic liquid phases are very sensitive to the purity, size, and process conditions of the raw material powder. If not properly controlled, excessively high liquid phase volume fractions will lead to evaporation and loss, resulting in large-scale pores and compositional inhomogeneity, affecting the dimensional accuracy and performance of the alloy. Furthermore, after the addition of Nb, the eutectic reaction between the NiTi and Nb phases (at 1150℃) introduces an additional liquid phase. Therefore, compared to binary NiTi alloys, the mixed liquid phase in ternary NiTiNb alloys is further affected by Nb during reaction sintering, making its properties more complex and more difficult to control. Impurity contamination and the generation and loss of mixed eutectic liquid phases are the main challenges faced in the preparation of high-performance NiTiNb alloys. Therefore, developing a new preparation process to improve the current poor performance of all NiTiNb alloys is of great significance to this technical field. Summary of the Invention

[0006] The technical problem to be solved by this invention is to address the issues of impurity contamination and the generation and loss of mixed eutectic liquid phase during the preparation of NiTiNb alloys, overcoming the shortcomings and defects mentioned in the background art, and providing a NiTiNb shape memory alloy and its preparation method.

[0007] To solve the above-mentioned technical problems, the technical solution proposed by this invention is as follows:

[0008] A method for preparing a NiTiNb shape memory alloy specifically includes the following steps:

[0009] (1) The Ni source, Ti source and Nb source are uniformly mixed to obtain a mixed powder;

[0010] (2) The mixed powder is cold-pressed to form an alloy green blank;

[0011] (3) The alloy green billet is sintered with the following parameters: at a vacuum degree of 10 -3 ~10 -5 Under Pa conditions, the temperature is first increased to 580-620℃ at 5-10℃ / min and held for 0.5-2h, then increased to 680-720℃ at 1-2℃ / min and held for 2-4h, and then increased to 1130-1150℃ at 1-2℃ / min and held for 1-6h to obtain a sintered sample, which is the NiTiNb shape memory alloy.

[0012] Compared to smelting processes that require subsequent heat treatment, powder pressing reaction sintering not only allows for direct control of the material's microstructure and properties by adjusting sintering process parameters, resulting in high-performance parts with uniform composition that can meet various engineering requirements, but also addresses the issue of carbon and oxygen impurities leading to the formation of coarse Ni4Ti3 precipitates during the sintering preparation of binary elemental powder NiTi alloys or porous NiTi alloys. These coarse precipitates negatively impact the alloy's mechanical properties, necessitating subsequent heat treatment to eliminate them.

[0013] Through the sintering mechanism of NiTiNb shape memory alloys, we can find that although the appearance of complex eutectic liquid phases increases the difficulty of alloy preparation, if the liquid phase is reasonably controlled and utilized, element diffusion can be promoted, the alloy densification process can be accelerated, the coarse precipitates formed during sintering can be eliminated, and the alloy performance can be significantly improved.

[0014] Since the formation of Ti2Ni and Ni3Ti during sintering is unavoidable, this application initially sets a holding plateau at 600℃ during the initial heating phase. This significantly eliminates β-Ti, which weakens the eutectic melting (β-Ti + Ti2Ni → L) during further heating and reduces large-scale porosity defects caused by self-propagating deflagration. The subsequent holding plateau at 700℃-1150℃ is primarily to ensure that the eutectic reactions involving Ni3Ti and Nb (Ni3Ti + NiTi → L, Nb + NiTi → L) can occur without generating excessive liquid phase due to temperature increases, thus preventing loss due to excessively high local liquid phase volume fractions.

[0015] Since the formation of Ti2Ni and Ni3Ti during sintering is uncontrollable, the eutectic liquid phase generated by them is difficult to control and is prone to loss, resulting in porosity. However, the liquid phase generated by introducing Nb to participate in the NiTi-Nb eutectic reaction can be controlled by adjusting the parameters of Nb, thereby achieving the goal of controllable liquid phase sintering and achieving the effects of composition homogenization and matrix densification.

[0016] During solid-state diffusion, differences in element diffusion rates between powder particles make Kirkendal porosity unavoidable. The presence of a large amount of transient, unpredictable impurity eutectic liquid phase in binary Ni-Ti alloys not only hinders sintering densification but also increases the risk of macroscopic defects. By controlling the eutectic liquid phase of NiTi-Nb, the liquid phase can fill some of the pores under capillary action. More importantly, the liquid phase can significantly promote element diffusion, weaken the partial diffusion effect in solid-state diffusion, and promote densification.

[0017] Preferably, the particle size of the Nb powder is -900 to -500 mesh.

[0018] The particle diameter of Nb powder has a certain impact on the final sintering effect. In engineering practice, it has been found that larger Nb particles (such as -325 mesh) cannot be uniformly distributed within the Ni-Ti powder interstices, thus having a limited effect on the densification of Ni-Ti during sintering. Furthermore, after sintering, large Nb phase particles remain in the matrix, causing matrix softening. Only the addition of Nb particles with appropriate particle size and volume fraction can help Ni-Ti powder further eliminate residual porosity, increase sintering density, and enhance mechanical properties.

[0019] Preferably, the atomic ratio of Ni to Ti in the Ni source and Ti source is 50.0~51.0:49.0~50.0, and more preferably 50.5:49.5.

[0020] Preferably, the amount of Nb source added is 1% to 15% of the atomic percentage of Nb in the mixed powder, and more preferably any one of 3%, 9% or 12%.

[0021] At room temperature, the maximum solid solubility of Nb in the B2 NiTi lattice is 5 at.%. When the Nb content exceeds 5 at.%, the excess Nb exists as β-Nb with a BCC structure. With increasing Nb content, the microstructure of the alloy changes in the following order: "B2 → B2+ eutectic → eutectic → β-Nb+ eutectic". Different Nb contents cause significant changes in the microstructure, affecting the mechanical properties and phase transformation behavior of the NiTiNb alloy. For example, the soft and tough β-Nb phase can participate in alloy deformation, improving the alloy's mechanical properties. Nb dissolution in the NiTi lattice produces severe lattice distortion, which is beneficial for increasing the phase transformation thermal hysteresis width. This application selects an appropriate amount of Nb source to control the eutectic liquid phase of NiTi-Nb, thereby achieving controllable liquid phase sintering and achieving the effects of compositional homogenization and matrix densification.

[0022] Preferably, the Ni source is carbonyl Ni powder, the Ti source is hydrogenated dehydrogenated Ti powder, and the Nb source is Nb powder.

[0023] Preferably, the particle size of the hydrogenated dehydrogenated Ti powder is -325 mesh to -200 mesh, and more preferably -325 mesh.

[0024] Preferably, the particle size of the carbonyl Ni powder is -2000 mesh to -800 mesh, and more preferably -1250 mesh.

[0025] Preferably, the pressure of the cold molding process is 350-550 MPa.

[0026] Under the same technical concept, the present invention also provides a NiTiNb shape memory alloy, which is prepared by the above-described preparation method.

[0027] Preferably, the NiTiNb shape memory alloy has a tensile fracture stress of 263-709 MPa, a relative density of 84-99%, an elongation of 3.5-12.6%, a martensitic transformation peak temperature of -17 to -59℃, an austenitic transformation peak temperature of 1.3 to 37℃, an oxygen content of no more than 0.22 wt.%, and a carbon content of no more than 0.08 wt.%.

[0028] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0029] (1) The preparation method of the present invention introduces Nb source powder sintering and controls the sintering temperature parameters to prepare NiTiNb alloy with uniform composition and high density. Based on the analysis of the NiTiNb alloy sintering mechanism, the sintering scheme of this application mainly focuses on optimizing the suitability of Nb addition amount, adjusting the volume fraction of NiTi-Nb eutectic liquid phase, and promoting homogenization and densification. In this case, the control of sintering process parameters is crucial. The present application adopts a reasonable oxygen control strategy in the preparation scheme. By optimizing the sintering process, the eutectic liquid phase of the NiTiNb alloy is well controlled. The eutectic liquid phase is used to promote composition homogenization and matrix densification, which greatly improves the mechanical properties and shape memory effect of the elemental powder NiTiNb alloy.

[0030] (2) This invention uses a total of 10 -3 ~10 -5 Sintering NiTiNb alloys using a vacuum system ensures an oxygen content no higher than 0.22 wt.%, with strict control of oxygen flow during the reaction process. This significantly reduces impurity phase formation and guarantees product purity. The resulting NiTiNb alloys exhibit tensile fracture stress strengths of 263-709 MPa, relative densities of 84-99%, elongation of 3.5-12.6%, martensitic transformation peak temperatures of -17 to -59℃ and austenitic transformation peak temperatures of 1.3 to 37℃, with oxygen and carbon contents not exceeding 0.22 wt.% and 0.08 wt.%, respectively, significantly improving the alloy's mechanical properties.

[0031] (3) The preparation method of the present invention is simple and easy to implement. It only requires temperature-controlled sintering and does not require subsequent aging heat treatment or other steps. Moreover, the sintering temperature is within a reasonable range, which has high industrial value. Attached Figure Description

[0032] 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 some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0033] Figure 1 These are scanning electron microscope images of NiTiNb alloys in Examples 1-3: showing the pore morphology and phase composition of NiTiNb alloys with different Nb contents;

[0034] Figure 2The EBSD inverse pole figures for Examples 1-3 are as follows: when the Nb addition amount is 3, 9, and 12 at.%, the maximum texture index is 1.17, 1.29, and 1.38, respectively, showing an upward trend but without any obvious dominant orientation; the average grain size is 10, 13, and 15 μm, respectively, showing an upward trend.

[0035] Figure 3 The X-ray diffraction analysis of Examples 1-3 reflects the basic phase composition;

[0036] Figure 4 The DSC curves of Examples 1-3 reflect the martensitic phase transformation temperature of the NiTiNb alloy;

[0037] Figure 5 These are the tensile stress-strain curves of Examples 1-3;

[0038] Figure 6 The fracture surface morphology of the tensile fracture surface of the NiTiNb alloy in Examples 1-3 is shown in the scanning electron microscope.

[0039] Figure 7 The images show the transmission electron microscope morphology of the NiTiNb alloys after tensile fracture in Examples 1-3. Detailed Implementation

[0040] To facilitate understanding of the present invention, the present invention will be described more fully and in detail below with reference to the accompanying drawings and preferred embodiments, but the scope of protection of the present invention is not limited to the following specific embodiments.

[0041] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0042] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this invention can be purchased from the market or prepared by existing methods.

[0043] Example 1:

[0044] A method for preparing a NiTiNb shape memory alloy includes the following steps:

[0045] (1) Hydrogenated dehydrogenated Ti powder, carbonyl Ni powder, and Nb powder are uniformly mixed to obtain a mixed powder. The ratio of the amount of hydrogenated dehydrogenated Ti powder to carbonyl Ni powder is: hydrogenated dehydrogenated Ti powder (-325 mesh, 49.5 at.%) to carbonyl Ni powder (-1250 mesh, 50.5 at.%). The amount of Nb powder added is 3 at.% (900 mesh, 3 at.%) of the total amount of NiTiNb shape memory alloy raw materials. The theoretical composition of the mixed powder is Ni49 Ti 48 Nb3;

[0046] (2) The mixed powder is cold-pressed into shape at 400MPa;

[0047] (3) The green body pressed in step (2) is sintered with the following sintering parameters: at a vacuum degree of 10 -4 Under Pa conditions, the temperature was increased to 600℃ at 5℃ / min and held for 0.5h, then increased to 700℃ at 1℃ / min and held for 2h, and then increased to 1150℃ at 1℃ / min and held for 2h to obtain NiTiNb shape memory alloy.

[0048] The performance of the obtained NiTiNb shape memory alloy was tested according to ASTM E8-04. The oxygen content of the NiTiNb shape memory alloy was 0.18 wt.%, the density was about 84%, the average carbon content was about 0.07 wt.%, the tensile strength was 263 MPa, the elongation was 3.5%, the peak temperature of martensitic transformation was -17℃, and the peak temperature of austenitic transformation was 37℃.

[0049] Example 2:

[0050] A method for preparing NiTiNb shape memory alloy, the formula and steps of which are exactly the same as those in Example 1, the only difference being that the amount of Nb powder added is 9 at.% (-900 mesh, 9 at.%) of the total amount of NiTiNb shape memory alloy raw materials.

[0051] The obtained NiTiNb shape memory alloy was subjected to performance tests. The NiTiNb shape memory alloy had an oxygen content of 0.20 wt.%, a density of about 96%, an average carbon content of about 0.06 wt.%, a tensile strength of 499 MPa, an elongation of 5.7%, a martensitic transformation peak temperature of -64℃, and an austenitic transformation peak temperature of -0.8℃.

[0052] Example 3:

[0053] A method for preparing NiTiNb shape memory alloy, the formula and steps of which are exactly the same as those in Example 1, the only difference being that the amount of Nb powder added is 12 at.% (-900 mesh, 12 at.%) of the total amount of NiTiNb shape memory alloy raw materials.

[0054] The obtained NiTiNb shape memory alloy has an oxygen content of 0.21 wt.%, a density of about 99%, an average carbon content of about 0.06 wt.%, a tensile strength of 709 MPa, an elongation of 12.6%, a martensitic transformation peak temperature of -59℃, and an austenitic transformation peak temperature of 1.3℃.

[0055] Comparative Example 1

[0056] A method for preparing a NiTiNb shape memory alloy, the formula and steps of which are exactly the same as those in Example 1, the only difference being: in step (3), the desalted green blank is sintered, and the sintering parameters are as follows: at a vacuum degree of 10 -4 Under Pa, the temperature is increased to 600℃ at 5℃ / min and held for 0.5h, then increased to 700℃ at 1℃ / min and held for 2h, and then increased to 1200℃ at 1℃ / min and held for 2h.

[0057] The resulting NiTiNb shape memory alloy has an oxygen content of 0.22 wt.% and a density of approximately 68.4%. Due to the excessive volume fraction of the liquid phase, the liquid phase evaporates and is lost, resulting in compositional segregation and large-size pore defects.

[0058] Comparative Example 2

[0059] A method for preparing a NiTiNb shape memory alloy, the formula and steps of which are exactly the same as those in Example 2, the only difference being: in step (3), the desalted green blank is sintered, and the sintering parameters are as follows: at a vacuum degree of 10 -4 Under Pa, the temperature is increased to 600℃ at 5℃ / min and held for 0.5h, then increased to 700℃ at 1℃ / min and held for 2h, and then increased to 1200℃ at 1℃ / min and held for 2h.

[0060] The resulting NiTiNb shape memory alloy has an oxygen content of 0.22 wt.% and a density of approximately 65.9%. Due to the excessive volume fraction of the liquid phase, the liquid phase evaporates and is lost, resulting in compositional segregation and large-size pore defects.

[0061] Comparative Example 3

[0062] A method for preparing a NiTiNb shape memory alloy, the formula and steps of which are exactly the same as those in Example 3, the only difference being: in step (3), the desalted green blank is sintered, and the sintering parameters are as follows: at a vacuum degree of 10 -4 Under Pa, the temperature is increased to 600℃ at 5℃ / min and held for 0.5h, then increased to 700℃ at 1℃ / min and held for 2h, and then increased to 1200℃ at 1℃ / min and held for 2h.

[0063] The resulting NiTiNb shape memory alloy has an oxygen content of 0.22 wt.% and a density of approximately 79.9%. Due to the excessive volume fraction of the liquid phase, the liquid phase evaporates and is lost, resulting in compositional segregation and large-size pore defects.

Claims

1. A method for preparing a NiTiNb shape memory alloy, characterized in that, The NiTiNb shape memory alloy has a relative density of 96%-99%, and its preparation method specifically includes the following steps: (1) The Ni source, Ti source and Nb source are uniformly mixed to obtain a mixed powder; the Nb source is Nb powder with a particle size of -900 mesh to -500 mesh; the amount of Nb source added is 9% to 15% of the atomic percentage of Nb in the mixed powder; the atomic ratio of Ni to Ti in the Ni source and Ti source is 50.0 to 51.0: 49.0 to 50.0; (2) The mixed powder is cold-pressed to form an alloy green blank; (3) The alloy green billet is sintered with the following parameters: at a vacuum degree of 10 -5 ~10 -3 Under Pa conditions, the temperature is first increased to 580-620℃ at 5-10℃ / min and held for 0.5-2h, then increased to 680-720℃ at 1-2℃ / min and held for 2-4h, and then increased to 1130-1150℃ at 1-2℃ / min and held for 1-6h to obtain a sintered sample, which is the NiTiNb shape memory alloy.

2. The preparation method according to claim 1, characterized in that, The Ni source is carbonyl Ni powder, and the Ti source is hydrogenated dehydrogenated Ti powder.

3. The preparation method according to claim 2, characterized in that, The particle size of the hydrogenated dehydrogenated Ti powder is -325 mesh to -200 mesh.

4. The preparation method according to claim 2, characterized in that, The carbonyl Ni powder has a particle size of -2000 mesh to -800 mesh.

5. The preparation method according to claim 1, characterized in that, The pressure for cold molding is 350~550MPa.

6. A NiTiNb shape memory alloy, characterized in that, The NiTiNb shape memory alloy is prepared by the preparation method according to any one of claims 1-5.

7. The NiTiNb shape memory alloy as described in claim 6, characterized in that, The NiTiNb shape memory alloy has a tensile fracture stress of 263-709 MPa, a relative density of 96%-99%, an elongation of 3.5-12.6%, a martensitic transformation peak temperature of -17 to -59℃, an austenitic transformation peak temperature of 1.3 to 37℃, an oxygen content of no more than 0.22 wt.%, and a carbon content of no more than 0.08 wt.%.

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