CN115261656A - Preparation method of low-cost element mixed porous NiTi shape memory alloy through vacuum high-temperature sintering and aging treatment - Google Patents

Abstract

The invention discloses a preparation method of a low-cost element mixed porous NiTi shape memory alloy by vacuum high-temperature sintering and aging treatment, comprising the following steps: mixing Ni-containing powder, Ti-containing powder and NaCl powder, and the obtained mixed powder is cooled After molding, desalination is carried out, and the obtained green body is sintered to obtain a sintered sample. The sintered sample is vacuum-sealed with a quartz tube, solution-treated at 900-1100°C in a tube furnace muffle furnace, then quenched, and then heated in Ar Aging at 350‑500℃ in air flow. The invention reduces the oxygen content of the EP NiTi alloy to 0.22-0.36 wt.% through high vacuum sintering, overcomes the problems of deflagration reaction and liquid phase loss, and increases the strength of the EP porous NiTi alloy skeleton matrix to 200 MPa·cm ³ ·g ^-1 Above, when the porosity is higher than 35%, the 8% compression recovery rate is still higher than 95%.

Description

Low-cost element-mixed porous NiTi form by vacuum high-temperature sintering and aging treatment The preparation method of shape memory alloy

technical field

The invention belongs to the technical field of shape memory materials, and in particular relates to a method for preparing a low-cost element mixed porous NiTi shape memory alloy which improves specific strength and stress cycle stability through vacuum high-temperature sintering and aging treatment.

Background technique

On the one hand, porous NiTi alloy has the superelasticity and shape memory effect of the matrix material, and at the same time has the functional properties of most porous materials. Therefore, it has attracted extensive attention in many fields such as energy absorption, hydrogen isotope separation, and material lightweight. development of bone implant materials. Human bones have a recoverable strain of about 2%. The superelasticity of NiTi alloy is conducive to the strain matching of implants. At the same time, compared with structural materials such as Ti alloy and Co alloy, it can improve long-term fatigue under complex and variable external loads. reliability. The controllable porous structure effectively reduces the apparent modulus of the implant, reduces the stress shielding phenomenon, and the connected pore structure is conducive to tissue growth and nutrient delivery.

Over the past 20 years, self-propagating high-temperature synthesis (SHS), microwave sintering (MWS), spark plasma sintering (SPS), hot isostatic pressing (HIP), vacuum sintering (VS), and metal injection molding (MIM) have been gradually developed. , additive manufacturing (AM) and other powder metallurgy techniques are used as the main preparation methods for porous NiTi alloys. Christian Greiner et al (2005) used HIP to trap gas, followed by high temperature expansion to obtain a porous NiTi alloy with a porosity of 16%. When the compressive strain reached 11%, the strength of the alloy was 850MPa, and about 6% was recovered after stress unloading8 strain. A.Bansiddhi et al (2008) used NaCl as the pore-forming agent, and prepared a porous NiTi alloy with 34% porosity by using pre-alloyed NiTi powder after hot isostatic pressing at 1065 °C and then performing high-temperature homogeneous sintering at 1250 °C , The average pore size is about 151 μm, no fracture occurs after 48% compressive strain, the compressive strength is about 1060MPa, and about 6% of the strain is recovered after 8% compressive strain and then unloading.

In contrast, the low-cost elemental NiTi porous material (EP) has poor performance. W.Sirikul et al prepared a porous NiTi alloy with a porosity of 32%-58% by self-propagating high-temperature synthesis after Ni and Ti powders were pressed into shape, with an average pore diameter of about 337-497μm and a maximum compressive strength of 344MPa; By mixing Ni, Ti and NH ₄ HCO ₃ powders, a porous NiTi alloy with a porosity of 21%-48% was prepared by using uncoated HIP (CF-HIP), with an average pore diameter ranging from about 50-800 μm. % and 21% of the samples fully recovered when strained to 4.2% and 4.7%. Xu et al (2015) used Ni and Ti powders as raw materials to prepare porous NiTi alloys with different porosity by microwave sintering and space occupation method. When the porosity increased from 22% to 62%, the ultimate compressive strength of porous NiTi increased from 880MPa dropped to 69MPa, and the maximum compressive strain also dropped from 25% to about 10%, and the shape recovery ability dropped from 4.56% to 2.28% at a strain of 5%. Khashayar Khanlari et al prepared a porous NiTi alloy with a porosity of 32-36% by using Ni and Ti powders by pressing and sintering (1050°C, different speeds). The porosity is about 49%-71%, the compressive strength is 500-820MPa, and the maximum compressive strain range is about 9%-16%. The samples can all recover completely after a strain of 2.1%. Other similar work has been reported.

Although there is no unified evaluation standard, and the test methods for bearing failure and recoverable strain are not all consistent, it is still not difficult to find that the EPporous NiTi alloy matrix with similar porosity has more small hole defects in the matrix skeleton, and the compressive strength, plasticity and durability The recovery strains are much lower than those of PPporous NiTi alloy. The reason why the performance of EP matrix is poor, on the one hand, is because of the high impurity content of the raw materials, and the Ti ₄ Ni ₂ O stabilized by oxygen is an important source of cracks in mechanical deformation and high cycle fatigue; on the other hand, it is due to the complex Mesophase evolution process and heterogeneous reaction liquid phase.

Porous NiTi alloys are more prone to failure during stress cycling than dense NiTi SMAs. The main causes of failure include: local plastic deformation caused by stress concentration; generation of dislocations accompanying phase transformation during stress-induced martensitic transformation or martensitic detwinning. Studies have shown that uniformly distributed Ni ₄ Ti ₃ nano-precipitates can effectively inhibit the movement of dislocations and enhance the resistance of the matrix to plastic deformation during loading. Heat treatment is one of the important methods to improve the matrix strength and functional stability of NiTi SMAs. Chen et al heat treatment introduced Ni ₄ Ti ₃ nanophase phase into rolled NiTi alloy, combined with precipitation strengthening and fine grain strengthening, the stress of the alloy was significantly improved. cycle stability. Wang et al introduced dislocation networks inside the grains through repeated phase transformations in rolled NiTi, followed by low-temperature aging treatment to obtain uniformly distributed Ni4Ti3, which improved the functional stability of NiTi alloys with micron-scale grain sizes. Lu et al conducted heat treatment on SLM NiTi alloy and found that the spherical Ni ₄ Ti ₃ nano-precipitation is more helpful to the stress cycle stability of the alloy. Therefore, strengthening the skeletal matrix of EPporous NiTi alloy by nanoprecipitation can further improve the stress cycle stability.

For orthopedic implants, higher strength means higher connection reliability, and better superelasticity means better service durability. Therefore, a new preparation process was developed to improve the current situation of poor performance of all EP porous NiTi alloys (the strength of the matrix skeleton, that is, the specific strength does not exceed 200MPa cm ³ g ^-1 , and the compressive superelasticity is less than 5%) , is of great significance to this technical field.

Contents of the invention

The technical problem to be solved by the present invention is that, aiming at the current situation of extremely poor performance of all EP porous NiTi alloys (the strength of the matrix skeleton, that is, the specific strength does not exceed 200MPa·cm ³ ·g ^-1 , and the compressive superelasticity is lower than 5%), it is proposed A method for preparing a low-cost element-mixed porous NiTi shape memory alloy by vacuum high-temperature sintering and aging treatment.

In order to solve the problems of the technologies described above, the technical solution proposed by the present invention is:

A method for preparing a low-cost element-mixed porous NiTi shape memory alloy through vacuum high-temperature sintering and aging treatment, comprising the following steps:

(1) uniformly mixing Ni-containing powder, Ti-containing powder and NaCl powder to obtain mixed powder;

(2) The mixed powder is cold-formed, and the compacted green body is desalted in deionized water;

(3) The desalted green body is sintered, and the sintering parameters are as follows: under a vacuum degree of 10 ^-3 ~ 10 ^-5 Pa, first raise the temperature to 600°C at 5-20°C/min for 0.5-2h, and then 1-2°C/min to 700°C for 2-4h, then 1-2°C/min to 1050°C for 1-4h, then 1-2°C/min to 1120°C for 2-4h, finally Raise the temperature at 1-2°C/min to 1220-1250°C for 6-10 hours to obtain a sintered sample;

(4) The sintered sample is vacuum-sealed with a quartz tube, and solid solution treatment is carried out in a tube furnace. The temperature of the solid solution treatment is 900-1100 ° C, and the time is 1-2 h, then quenched in ice water, and then heated in Ar Perform aging treatment at 350-500° C. in air flow, and the aging time is 0.5-2 h, and obtain the low-cost element-mixed porous NiTi shape memory alloy.

Different from the current prior art, the present invention proposes to adjust the porosity through the addition and removal of NaCl, homogenize sintering at high temperature (1250°C) under high vacuum (10 ^-4 Pa), and obtain a compressive property comparable to PP with superelasticity. EP porous NiTi alloy. Furthermore, the precipitation of Ni ₄ Ti ₃ precipitates through aging regulation, which effectively enhances the stress cycle stability.

In the above preparation method, preferably, the Ni-containing powder is carbonyl Ni powder, and the Ti-containing powder is hydrodehydrogenated Ti powder.

Preferably, the particle size of the hydrodehydrogenation Ti powder is -325 mesh to -200 mesh, more preferably -325 mesh.

Preferably, the atomic ratio of Ni and Ti in the mixed powder is (50.0-51.0):(49.0-50.0), more preferably 50.5:49.5.

Preferably, the added amount of the NaCl powder is 10%-50% of the volume fraction of the mixed powder, more preferably 15%, 25% and 40%.

Preferably, the particle size of the NaCl powder is -325 mesh to -200 mesh, more preferably -200 mesh.

Preferably, the mixed powder is cold molded and pressed at 350-550 MPa (preferably 400 MPa), and the pressed green body is desalted in constant temperature deionized water at 37-60°C (preferably 45°C).

Preferably, the deionized water is replaced every 1-12 hours during the desalination, and the deionization time is 1-7 days.

Preferably, the sintering is performed at a vacuum degree of 10 ^-4 Pa.

Preferably, the sintering parameters are as follows: first, the temperature is raised to 600°C at 5°C/min for 0.5h, then the temperature is raised to 700°C at 1°C/min for 2h, and then the temperature is raised to 1050°C for 2h at 1°C/min, and then Raise the temperature at 1°C/min to 1120°C for 2 hours, and finally raise the temperature to 1240°C for 8 hours at 1°C/min.

Preferably, the temperature of the solution treatment is 1000° C., and the time is 1 h; the temperature of the aging treatment is 450° C., and the time is 0.51 h.

Compared with prior art, the beneficial effect of the present invention is:

1. The preparation method of the present invention aims at the current situation of extremely poor performance of all EP porous NiTi alloys (the strength of the matrix skeleton, that is, the specific strength does not exceed 200MPa·cm ³ ·g ^-1 , and the compression superelasticity is lower than 5%), a new method is proposed. The material preparation idea solves the problems of low-temperature deflagration reaction and high-temperature liquid phase loss, so that the strength of the EP porous NiTi alloy skeleton matrix is increased to more than 200MPa·cm ³ ·g ^-1 , and the tensile recovery rate of 8% is higher than 95%.

2. In the present invention, the porous EPNiTi alloy is sintered with a full-range 10 ^-4 Pa vacuum system, and the oxygen content of the EP porous NiTi alloy is reduced to 0.22-0.36wt.%.

3. Through process optimization, simultaneously solve the problem of deflagration reaction in the low temperature zone (700°C) and liquid phase loss in the high temperature zone (1250°C), and use the inevitable process liquid phase and high temperature sintering to promote the homogenization of elements. Compared with the existing technology, Significantly improve the engineering performance of EP porous NiTi.

4. Through detailed process control, the EPNiTi alloy with uniform composition is obtained, the compressive strength of the matrix skeleton is increased to more than 1200MPa, the fracture strain is higher than 30%, and the tensile recovery rate of 8% has reached more than 99%; hydrogenated dehydrogenated titanium at low cost Powder is used as raw material to prepare NiTi alloy, and there is no other scheme that can make the related performance of NiTi alloy exceed the test effect declared by the present invention.

5. Through solid solution and aging treatment, the ability of plastic deformation of porous NiTi matrix in the process of cyclic stress loading-unloading is strengthened, and it can guarantee no more than 0.8% residual in the process of cyclic loading-unloading for more than 60 times under 8% strain strain, showing good compressive stress loading-unloading cycle stability.

Description of drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings that need to be used in the description of the embodiments or the prior art. Obviously, the accompanying drawings in the following description are For some embodiments of the present invention, those skilled in the art can also obtain other drawings based on these drawings without creative work.

Fig. 1 is the topography diagram of preferred raw material powder; wherein: (a) Ni powder; (b) Ti powder; (c) NaCl powder.

Fig. 2 is the microscopic morphology and the element distribution spectrum (distribution of Ni and Ti element) of sintered state porous NiTi surface; Wherein: (a1-c1) the addition of NaCl powder is 15%; (a2-c2) the addition of NaCl powder The amount is 25%; the addition amount of (a3-c3)NaCl powder is 45%.

Fig. 3 is the pore size distribution diagram of porous NiTi alloy; Wherein: (a) the addition amount of NaCl powder is 15%; (b) the addition amount of NaCl powder is 25%; (c) the addition amount of NaCl powder is 45%.

Figure 4 is the X-ray diffraction pattern of the EP porous NiTi alloy after 1250°C high-temperature homogenization sintering and solution aging treatment at room temperature, and (b) is a partially enlarged view of (a).

Figure 5 is a TEM image of the morphology and distribution of Ni ₄ Ti ₃ precipitates in sintered EP porous NiTi; among them: (a ₁ -a ₃ ) samples with 15% NaCl addition; (b ₁ -b ₃ ) 25% Samples with NaCl addition; (c ₁ -c ₃ ) samples with 40% NaCl addition; (a ₄ , b ₄ and c ₄ ) are selected area electron diffraction patterns, respectively. Among them, with the increase of NaCl addition, the physical properties of Ni ₄ Ti ₃ precipitates changed significantly.

Figure 6 is the TEM image of EP porous NiTi alloy with 15% NaCl addition after aging treatment; among them: (a) and (c) the morphology and distribution of Ni ₄ Ti ₃ precipitates; (b) the selected area electron diffraction pattern; (d) is the high-resolution image of the local area in (c); (g) is the Fourier transform image of (d); (e) is the high-resolution image of the R phase; (f) is the Fourier image of (e) The leaf transformation diagram, the arrows in the figure indicate the diffraction spots of the R phase.

Fig. 7 is a DCS curve diagram of phase transition temperature change; where: (a) sintered sample; (b) aged sample.

Figure 8 is a graph of mechanical properties at room temperature; where: (a) compressive stress-strain curve; (b) compressive specific strength comparison graph; (c) compressive loading-unloading curve of the sintered sample at 8% strain, unloading at 120°C (d) The compression loading-unloading curve of the aged sample at 8% strain, and the sample was kept at 120°C for 0.5 hours after unloading; (e) Comparison with other porous NiTi alloys on compression recovery ability.

Figure 9 shows the effect of porosity on the Young's modulus of the sintered and aged samples.

Figure 10 is the stress-strain curves of cyclic compression loading-unloading of the sintered sample and the aged sample.

Figure 11 shows the variation of the residual strain of EP porous NiTi with the number of compression cycles.

Figure 12 shows the effect of porosity on yield strength (a) and critical stress for martensitic transformation (b).

Detailed ways

In order to facilitate the understanding of the present invention, the present invention will be described in more detail below in conjunction with the accompanying drawings and preferred embodiments, but the protection scope of the present invention is not limited to the following specific embodiments.

Unless otherwise defined, all technical terms used hereinafter have the same meanings as commonly understood by those skilled in the art. The terminology used herein is only for the purpose of describing specific embodiments, and is not intended to limit the protection scope of the present invention.

Unless otherwise specified, various raw materials, reagents, instruments and equipment used in the present invention can be purchased from the market or prepared by existing methods.

Example 1:

A method for preparing a low-cost element-mixed porous NiTi shape memory alloy through vacuum high-temperature sintering and aging treatment, comprising the following steps:

(1) Hydrodehydrogenation Ti powder (-325 order, 49.5at.%), carbonyl Ni powder and NaCl powder (-200 order, 50.5at.%) are uniformly mixed to obtain mixed powder; the addition amount of NaCl powder is 15% of the volume fraction of the mixed powder;

(2) The mixed powder is cold-formed at 400 MPa, and the pressed green body is desalted in deionized water at a constant temperature of 45 ° C, and the deionized water is replaced every 12 hours, and the desalination time is 3 days;

(3) Sinter the desalted green body, and the sintering parameters are as follows: under a vacuum degree of 10 ^-4 Pa, raise the temperature to 600°C at 5°C/min and keep it for 0.5h, then raise the temperature to 700°C at 1°C/min Keep warm for 2 hours, then raise the temperature at 1°C/min to 1050°C for 2 hours, then raise the temperature at 1°C/min to 1120°C for 2 hours, and finally raise the temperature to 1240°C at 1°C/min for 8 hours.

The oxygen content of the obtained EPNiTi alloy is 0.23wt.%, the porosity is about 14.0±0.52%, the average pore diameter is about 60±15μm, the compressive strength exceeds 1600MPa, the specific strength exceeds 289MPa·cm ³ ·g ^-1 , and the ultimate fracture strain exceeds 28%. 7.99% recovery after 8% strain compression. The sample did not break after loading and unloading 60 times under 8% compressive strain, and the final residual strain was 1.40%.

Example 2:

A method for preparing a low-cost element-mixed porous NiTi shape memory alloy through vacuum high-temperature sintering and aging treatment, comprising the following steps:

(1) Hydrodehydrogenation Ti powder (-325 order, 49.5at.%), carbonyl Ni powder and NaCl powder (-200 order, 50.5at.%) are uniformly mixed to obtain mixed powder; the addition amount of NaCl powder is 25% of the volume fraction of the mixed powder;

(2) The mixed powder is cold-formed at 400 MPa, and the pressed green body is desalted in deionized water at a constant temperature of 45 ° C, and the deionized water is replaced every 12 hours, and the desalination time is 3 days;

(3) Sinter the desalted green body, and the sintering parameters are as follows: under a vacuum degree of 10 ^-4 Pa, raise the temperature to 600°C at 5°C/min and keep it for 0.5h, then raise the temperature to 700°C at 1°C/min Keep warm for 2 hours, then raise the temperature at 1°C/min to 1050°C for 2 hours, then raise the temperature at 1°C/min to 1120°C for 2 hours, and finally raise the temperature to 1240°C at 1°C/min for 8 hours.

The oxygen content of the obtained EPNiTi alloy is 0.29wt.%, the porosity is about 22.0±1.41%, and the average pore diameter is about 91±20 μm. The compressive strength exceeds 1600MPa, the specific strength exceeds 326MPa·cm ³ ·g ^-1 , the ultimate breaking strain exceeds 35%, and the 8% strain recovers 7.96% after compression. The sample did not break after 60 times of loading and unloading under 8% compressive strain, and the final residual strain was 1.55%.

Example 3:

A method for preparing a low-cost element-mixed porous NiTi shape memory alloy through vacuum high-temperature sintering and aging treatment, comprising the following steps:

(1) Hydrodehydrogenation Ti powder (-325 order, 49.5at.%), carbonyl Ni powder and NaCl powder (-200 order, 50.5at.%) are uniformly mixed to obtain mixed powder; the addition amount of NaCl powder is 40% of the volume fraction of the mixed powder;

(2) The mixed powder is cold-formed at 400 MPa, and the pressed green body is desalted in deionized water at a constant temperature of 45 ° C, and the deionized water is replaced every 12 hours, and the desalination time is 3 days;

(3) Sinter the desalted green body, and the sintering parameters are as follows: under a vacuum degree of 10 ^-4 Pa, raise the temperature to 600°C at 5°C/min and keep it for 0.5h, then raise the temperature to 700°C at 1°C/min Keep warm for 2 hours, then raise the temperature at 1°C/min to 1050°C for 2 hours, then raise the temperature at 1°C/min to 1120°C for 2 hours, and finally raise the temperature to 1240°C at 1°C/min for 8 hours.

The oxygen content of the obtained EPNiTi alloy is 0.36wt.%, the porosity is about 37.0±1.15%, and the average pore diameter is about 124±22 μm. The compressive strength is 1236MPa, the specific strength is 307MPa·cm ³ ·g ^-1 , the ultimate breaking strain exceeds 35%, and 7.40% recovery after 8% strain compression. The sample did not break after 60 times of loading and unloading under 8% compressive strain, and the final residual strain was 1.59%.

Example 4:

A method for preparing a low-cost element-mixed porous NiTi shape memory alloy through vacuum high-temperature sintering and aging treatment, comprising the following steps:

(1) Hydrodehydrogenation Ti powder (-325 order, 49.5at.%), carbonyl Ni powder and NaCl powder (-200 order, 50.5at.%) are uniformly mixed to obtain mixed powder; the addition amount of NaCl powder is 15% of the volume fraction of the mixed powder;

(2) The mixed powder is cold-formed at 400 MPa, and the pressed green body is desalted in deionized water at a constant temperature of 45 ° C, and the deionized water is replaced every 12 hours, and the desalination time is 3 days;

(3) The desalted green body is sintered, and the sintering parameters are as follows: under a vacuum degree of 10 ^-4 Pa, first raise the temperature to 600°C at 5°C/min and keep it for 0.5h, then raise the temperature to 700°C at 1°C/min Keep at ℃ for 2 hours, then raise the temperature at 1℃/min to 1050℃ for 2 hours, then raise the temperature at 1℃/min to 1120℃ for 2 hours, and finally raise the temperature at 1℃/min to 1240℃ for 8 hours.

(4) The sintered sample was vacuum-sealed with a quartz tube, solution treated in a tube furnace (1000°C, 1h), quenched in ice water, and then aged at 450°C in an Ar flow for 0.5h.

The oxygen content of the obtained EP porous NiTi alloy is 0.24wt.%, the porosity is about 14.0%, the average pore diameter is about 60 μm, the compressive strength exceeds 1600MPa, the specific strength exceeds 289MPa·cm ³ ·g ^-1 , and the ultimate fracture strain exceeds 16.5%, 8% The strain recovered 7.99% after compression. The sample did not break after loading and unloading for 60 times under 8% compressive strain, and the final residual strain was 0.40%, showing good cycle stability.

Example 5:

A method for preparing a low-cost element-mixed porous NiTi shape memory alloy through vacuum high-temperature sintering and aging treatment, comprising the following steps:

(1) Hydrodehydrogenation Ti powder (-325 order, 49.5at.%), carbonyl Ni powder and NaCl powder (-200 order, 50.5at.%) are uniformly mixed to obtain mixed powder; the addition amount of NaCl powder is 25% of the volume fraction of the mixed powder;

(2) The mixed powder is cold-formed at 400 MPa, and the pressed green body is desalted in deionized water at a constant temperature of 45 ° C, and the deionized water is replaced every 12 hours, and the desalination time is 3 days;

(3) Sinter the desalted green body, and the sintering parameters are as follows: under a vacuum degree of 10 ^-4 Pa, raise the temperature to 600°C at 5°C/min and keep it for 0.5h, then raise the temperature to 700°C at 1°C/min Keep warm for 2 hours, then raise the temperature at 1°C/min to 1050°C for 2 hours, then raise the temperature at 1°C/min to 1120°C for 2 hours, and finally raise the temperature to 1240°C at 1°C/min for 8 hours.

(4) The sintered sample was vacuum-sealed with a quartz tube, solution treated in a tube furnace (1000°C, 1h), quenched in ice water, and then aged in an Ar flow at 450°C for 0.5h to obtain EP Porous NiTi alloy.

The oxygen content of the obtained EP porous NiTi alloy is 0.41wt.%, the porosity is about 22%, the average pore diameter is about 91 μm, the compressive strength exceeds 1600MPa, the specific strength exceeds 326MPa·cm ³ ·g ^-1 , and the ultimate fracture strain exceeds 34%, 8% The strain recovery was 7.96% after compression. The sample did not break after loading and unloading for 60 times under 8% compressive strain, and the final residual strain was 0.85%, showing good cycle stability.

Embodiment 6:

A method for preparing a low-cost element-mixed porous NiTi shape memory alloy through vacuum high-temperature sintering and aging treatment, comprising the following steps:

(1) Hydrodehydrogenation Ti powder (-325 order, 49.5at.%), carbonyl Ni powder and NaCl powder (-200 order, 50.5at.%) are uniformly mixed to obtain mixed powder; the addition amount of NaCl powder is 40% of the volume fraction of the mixed powder;

(2) The mixed powder is cold-formed at 400 MPa, and the pressed green body is desalted in deionized water at a constant temperature of 45 ° C, and the deionized water is replaced every 12 hours, and the desalination time is 3 days;

(3) Sinter the desalted green body, and the sintering parameters are as follows: under a vacuum degree of 10-4Pa, heat up to 600°C at 5°C/min and keep for 0.5h, then heat up to 700°C at 1°C/min 2h, then raise the temperature at 1°C/min to 1050°C and keep it for 2h, then raise the temperature at 1°C/min to 1120°C and keep it for 2h, and finally raise the temperature at 1°C/min to 1240°C and keep it for 8h.

(4) The sintered sample was vacuum-sealed with a quartz tube, solution treated in a tube furnace (1000°C, 1h), quenched in ice water, and then aged in an Ar flow at 450°C for 0.5h to obtain EP Porous NiTi alloy.

The oxygen content of the obtained EP porous NiTi alloy is 0.55wt.%, the porosity is about 38%, the average pore diameter is about 124μm, the compressive strength exceeds 817MPa, the specific strength is 203MPa·cm ³ ·g ^-1 , and the ultimate fracture strain is 32%, 8% The strain recovery was 7.93% after compression. The sample did not break after 60 times of loading and unloading under 8% compressive strain, and the final residual strain was 1.84%.

In order to further verify the impact of the sintering parameters of the present invention on product performance, the present invention also provides the following experimental data:

In the present invention, by combining high-temperature homogeneous sintering and space occupation method, by adding 15vol.%, 25vol.% and 40vol.% NaCl powder, the porosities are respectively 14.0±0.52%, 22.0±1.41% and 37.0±1.0% 1.15% (respectively marked as S1, S2 and S3, and the corresponding aging state samples are marked as A1, A2 and A3), and the opening ratios are 11%, 21.8% and 36.7%, respectively. The average pore sizes were 60±15 μm, 91±20 μm and 124±22 μm, respectively.

Fig. 1 is the topography diagram of raw material powder of the present invention; Wherein: (a) Ni powder, (b) Ti powder, (c) NaCl powder.

Figure 2 shows the pore morphology of porous NiTi SMAs prepared by vacuum sintering. It can be seen that Ni and Ti elements are evenly distributed without obvious segregation. Among them: (a1) The amount of NaCl powder added is 15%, and the porosity after sintering is about 14.0%; (a2) The amount of NaCl powder added is 25%, and the porosity after sintering is 22.0%; (a3) The amount of NaCl powder added The porosity is 45%, and the porosity after sintering is 38%. The opening ratio of all samples is as high as above 95%.

Fig. 3 is a diagram of the pore size distribution of the porous NiTi alloy. Among them: (a) the addition of NaCl powder is 15%, the porosity after sintering is about 14.0%, and the average pore diameter is about 60 μm; (b) the addition amount of NaCl powder is 25%, the porosity after sintering is about 22.0%, and the average pore diameter is about 60 μm. 91 μm; (c) The amount of NaCl powder added is 45%, the porosity after sintering is about 38%, and the average pore diameter is about 124 μm.

The X-ray diffraction results at room temperature are shown in Fig. 4 . The phase is composed of austenite and a small amount of martensite, and Ti ₂ Ni phase is detected. Obvious Ni ₄ Ti ₃ phase peaks were observed in the sintered samples, and the Ni ₄ Ti ₃ peaks disappeared after solid solution and aging treatment. In the previous work, it was reported that the impurity content of C and O leads to the non-equilibrium precipitation of Ni ₄ Ti ₃ in Pre-alloyed Ti-rich NiTi alloy, but the existence of liquid phase in the element NiTi promotes the homogeneity of the composition and the enrichment of Ni Dissolution of the clusters, no precipitation peaks were observed in the sintered samples. In this study, the Ni content of the NiTi alloy is 50.5%, and the corresponding Ni equivalent is further increased, so the existence of Ni4Ti3 is found in the sintered samples. However, the Ni ₄ Ti ₃ peak disappears after solution aging treatment, which may be attributed to the actual volume size reduction or more uniform distribution of Ni ₄ Ti ₃ precipitates.

The morphology and distribution of Ni ₄ Ti ₃ precipitates in sintered EP porous NiTi are shown in Figure 5. Among them: (a ₁ -a ₃ ) sample with 15% NaCl addition; (b ₁ -b ₃ ) sample with 25% NaCl addition; (c ₁ -c ₃ ) sample with 40% NaCl addition; (a ₄ , b ₄ and c ₄ ) are selected area electron diffraction patterns, respectively. Among them, with the increase of NaCl addition, the physical properties of Ni ₄ Ti ₃ precipitates changed significantly. [111] In the selected area electron diffraction (SEAD) pattern of _B2 , the superlattice diffraction point located at the 1/7 position of the <123> reciprocal vector of the B2 austenite matrix and the diffraction point at the 1/3 position of the <110> reciprocal vector respectively confirmed that The existence of Ni ₄ Ti ₃ nano-precipitated phase and R phase was confirmed. By comparing the samples with different porosity, it is found that the shape and size of Ni ₄ Ti ₃ are different. In the 14.0% porosity sample, Ni ₄ Ti ₃ presents a coffee bean shape, and in the 22.0% and 38% porosity samples, it is approximately lens-like , showing an uneven distribution as a whole. The average length of Ni ₄ Ti ₃ nano-precipitated phase in the sintered sample gradually increases with the increase of porosity, which are 20nm, 137nm and 145nm respectively. This difference in morphology and size is mainly attributed to the difference in oxygen content in the matrix. As the oxygen content increases, Ni-rich clusters increase, and the precursor effect on the spontaneous precipitation of Ni ₄ Ti ₃ is enhanced, resulting in larger overall precipitate size.

After solid solution and aging treatment, compared with the sintered state, the Ni ₄ Ti ₃ precipitates in the aged sample are more dispersed and evenly distributed in the matrix (Figure 6a), and the morphology is closer to spherical, with an average size of about 25 μm. The precipitated phase with extremely small size does not appear obvious peaks on the XRD pattern. The smaller size of the dispersed precipitate will have a stronger pinning effect.

分别约为0.3℃、0.2℃和1.5℃。根据R相变热滞后较小

的特点，可以确定S1在冷却过程中的相变顺序分别为A→R、R→M1和A→M2；S2和S3两步相变分别为A→R和R→M)。加热阶段三种孔隙率样品的相变顺序均为M2→A、M1→R和R→A。固溶时效处理后，样品均只发生了单一的R相变。时效态样品的相变峰值温度明显升高，这与致密NiTi合金时效处理后的结果一致。由于Ni4Ti3相的析出，造成基体Ni含量损耗，M相变与R相变温度会随着基体中Ni含量的降低而升高。Figure 7 shows the DSC curves of the phase transition temperature of the samples. The 14% porosity sample exhibits a multi-step phase transition during cooling. 22% and 38% porosity only appear two exothermic peaks during cooling. During the heating stage, three endothermic peaks appeared in all the sintered samples. Compared with the sintered sample, only one phase transition peak appears in the solid solution and aging treated samples during heating and cooling. Among them, S1, S2 and S3 thermal hysteresis temperature

They are about 0.3°C, 0.2°C and 1.5°C respectively. Small thermal hysteresis according to R phase transition

It can be determined that the phase transition sequence of S1 during cooling is A→R, R→M1 and A→M2; the two-step phase transitions of S2 and S3 are A→R and R→M, respectively). The phase transition sequences of the three porosity samples during the heating stage are all M2→A, M1→R and R→A. After solution aging treatment, only a single R phase transition occurred in the samples. The phase transformation peak temperature of the aged sample increases significantly, which is consistent with the result of the aging treatment of the dense NiTi alloy. Due to the precipitation of Ni4Ti3 phase, the Ni content of the matrix is lost, and the M phase transition and R phase transition temperatures will increase with the decrease of Ni content in the matrix.

Figure 8(a) shows the compression deformation behavior of the porous NiTi alloy at room temperature (25 °C). S1 and S2 did not break under the maximum stress of 1600MPa, and the strain reached 28±1.2% and 35±1.4%, respectively, while the ultimate strength of S3 was as high as 1236±40MPa, and the strain was 49±1.1%. After heat treatment, the strength of samples with different porosity at the same strain level has been greatly improved. However, the A3 sample broke after a strain of 31±1.1%, and the corresponding compressive strength was 817±20MPa. Compared with the results of the porous NiTi alloy reported by Khashayar et al., the samples with similar porosity prepared by the present invention have obvious advantages in both strength and elongation. To evaluate the strength of the porous sample matrix, we summarize the mechanical properties of porous NiTi alloys reported in recent years by the specific strength σ _eq :

where σ _eq is the specific strength, σ _max is the ultimate compressive strength, ρ is the density of dense NiTi (different Ni content: 6.45–6.67 g/cm ³ ), and P is the porosity of the porous NiTi alloy. It can be seen from Figure 8(b) that the porous NiTi alloy with a porosity of 38±0.4% prepared in this study has a specific strength as high as 307±6MPa·cm ³ /g, showing excellent resistance to compression deformation.

Figure 8(c) and (d) are compressive stress-strain graphs at 8% strain. As the porosity of the sintered porous NiTi sample increased from 14±0.1% to 38±0.4%, the compressive strength under 8% strain decreased from 615±13MPa to 338±13MPa. After heat treatment, the strength of samples with different porosity increased significantly, A1 The strengths of , A2 and A3 are 851±14MPa, 736±13MPa and 404±10MPa respectively. Due to the coexistence of a small amount of martensite and austenite in the samples tested at room temperature, the volume fractions of austenite and martensite in samples with different pores and corresponding heat treatment states are different at room temperature. Exhibits superelasticity and shape memory properties. After heat treatment at 120 °C for 1 h, the maximum residual strain of various samples was only 0.8%, indicating that while the heat treatment process improved the matrix strength, even if the oxygen content of the matrix increased to a certain extent, it did not significantly damage the functional properties of the porous NiTi alloy. It can be clearly seen from the current results that porous NiTi alloys exhibit excellent superelasticity and shape memory effect at room temperature, and it is feasible to improve the comprehensive properties of porous NiTi alloys by heat treatment.

The moduli of porous NiTi alloys under static and dynamic conditions largely limit their application as orthopedic implants. Due to the existence of pores, the porous NiTi alloy can induce martensitic transformation even with a small stress during the loading process. Larger strains, in other words, stress-induced martensitic transformation may occur at various stages during stress loading, thus exhibiting a low elastic modulus during loading, while the alloy is in a fully martensitic state during unloading. In the initial stage of small strain (~0.2%), there is no obvious reversible de-twinning and martensite-to-austenite transformation, and the measured modulus (E _dyn ) is close to that of the real porous NiTi alloy. Young's modulus of martensite. The widely used model of Gibson and Ashby predicts that the Young's modulus of porous alloys decreases with the square of the porosity, and we compare the apparent elastic modulus of porous samples with this model (E= _Esolid ×(1- P) ² , E _solid is dense martensitic NiTi, the elastic modulus is 21 ~ 69GPa), and it is found that the experimental value is well within the prediction interval of this model. The apparent elastic modulus of the porous NiTi alloy measured in this study is much lower than the dynamic modulus value, and the experimental results are consistent with those reported by Bansiddhi et al. Moreover, with the increase of porosity, the apparent modulus and dynamic modulus of the porous NiTi alloy showed a linear decrease trend, the porosity of the sintered sample increased from 14±0.1% to 38±0.4%, and the apparent elastic modulus increased from 11.4 ±1GPa dropped to 6.0±0.5GPa, and the dynamic modulus dropped from 29.7±2GPa to 15.6±1GPa. After heat treatment, E _app =7.3-12.4GPa, E _dyn =15.7-30.5GPa.

Figure 9 shows the effect of porosity on the Young's modulus of the sintered and aged samples. Among them, the experimental data of dynamic and apparent Young's modulus are compared with the porous solid model of Gibson and Ashby; the experimental data of dynamic and apparent Young's modulus are compared with the Young's modulus of human bone tissue. As shown in Figure 9, for porous samples, compared with the sintered samples, the heat treatment process will cause a small increase in the apparent elastic modulus, but the effect on the dynamic modulus is not obvious. This is because the apparent elasticity is sensitive to the heat treatment process. During the transformation from austenite to martensite, due to the pinning effect of Ni ₄ Ti ₃ nano-precipitation on its phase boundary migration, the difficulty of stress-induced martensite transformation increases with the increase of Ni ₄ Ti ₃ . Although A _f increases significantly after heat treatment, the volume fraction of martensite in aged samples at room temperature may increase to some extent, which will lead to a decrease in modulus. These two mutually restrictive phenomena lead to a small increase in the elastic modulus after heat treatment, but the pinning effect of Ni ₄ Ti ₃ on phase boundary migration may be more dominant. The Young's modulus is essentially determined by the electronic structure of the crystal and does not depend on the microstructure, which explains why the heat treatment process has no obvious influence on it.

In order to avoid problems such as osteoporosis caused by modulus mismatch, it is medically required that the Young's modulus of the implant should be within the modulus range of the human bone, E≤17GPa. Therefore, the porous NiTi SMAs prepared by the present invention with a porosity of 38 ± 0.4% not only exhibit high strength, large recoverable strain, and constant pressure cycle stability, but also have both dynamic modulus and apparent modulus (6-15GPa) It meets the requirements of implants for biological applications, and can effectively avoid adverse risks caused by modulus mismatch.

In order to further evaluate the effect of the heat treatment process on the functional properties of the porous NiTi alloy, a constant strain cyclic loading test was carried out on the porous NiTi to characterize the cyclic compression stability of the sample. The stress-strain curve results shown in Figure 10 show that porous NiTi alloys with different porosities and corresponding heat treatment states can withstand 60 cyclic loading tests at high stress levels without fracture, showing excellent cyclic plastic resistance. Figure 11 is the result of measuring the residual strain of the sample every 15 times of heating. The residual strain of the sintered sample increases with the increase of the number of cycles, the residual strain of S1 increases from 0.55% to 1.4%, the residual strain of S2 increases from 0.7% to 1.55%, and the residual strain of S3 sample increases from 0.75% to 1.59%. . After solution aging, after 60 cycles, the residual strain of A1 is only 0.4%, and the residual strain of A2 is 0.85%, showing good compression cycle stability. The results of the cyclic compression test show that the heat treatment process can significantly improve the cycle stability of the samples with porosity of 14±0.1% and 24±0.2%, but the aging treatment has no effect on the strengthening effect of the samples with porosity of 38±0.4%. Obviously, the residual strain of A3 sample reached 1.84% after 60 loading-unloading cycles.

Figure 12(a) characterizes the corresponding relationship between the yield strength and porosity of the material, where the yield strength is obtained by the tangent method in Figure 8(a); Figure 12(b) is obtained by the formula σ _SIM =(1-P)σ _y Estimated critical stress (σ _SIM ) for martensitic transformation of samples as a function of porosity. When the porosity of the as-sintered sample increases from 14% to 37%, _σy decreases from 927±30MPa (10±1% strain) to 386±30MPa (10.15±1.2% strain), but it is still higher than that of human dense bone Ultimate compressive strength (100~230MPa). As the porosity increases, σ _SIM decreases from 170±7MPa to 71±8MPa, and the introduction of pores will significantly reduce the yield strength of the material. The porous NiTi samples after solution and aging treatment exhibit higher yield strength and critical stress for martensitic transformation during compression. This is attributed to the strengthening effect of Ni ₄ Ti ₃ precipitates on the matrix, and the distribution of Ni ₄ Ti ₃ precipitates is the main factor. Compared with the sintered sample, the uniformly dispersed Ni ₄ Ti ₃ precipitation in the heat-treated sample has a significant inhibitory effect on the relative dislocation movement, resulting in an increase in the critical shear stress of dislocation slip and an increase in the yield strength of the material. The coherent Ni ₄ Ti ₃ precipitation also has a strong pinning effect on the phase boundary. The existence of a coherent stress field inhibits the martensitic transformation, so a higher stress needs to be provided to induce the martensitic transformation during the compression process. .

Claims (10)

1. a method for preparing a low-cost element mixed porous NiTi shape memory alloy by vacuum high-temperature sintering and aging treatment, characterized in that, comprising the steps: (1) uniformly mixing Ni-containing powder, Ti-containing powder and NaCl powder to obtain mixed powder; (2) The mixed powder is cold-formed, and the compacted green body is desalted in deionized water; (3) The desalted green body is sintered, and the sintering parameters are as follows: under a vacuum degree of 10 ^-3 ~ 10 ^-5 Pa, first raise the temperature to 600°C at 5-20°C/min for 0.5-2h, and then 1-2°C/min to 700°C for 2-4h, then 1-2°C/min to 1050°C for 1-4h, then 1-2°C/min to 1120°C for 2-4h, finally Raise the temperature at 1-2°C/min to 1220-1250°C for 6-10 hours to obtain a sintered sample; (4) The sintered sample is vacuum-sealed with a quartz tube, and solid solution treatment is carried out in a tube furnace. The temperature of the solid solution treatment is 900-1100 ° C, and the time is 1-2 h, then quenched in ice water, and then heated in Ar Perform aging treatment at 350-500° C. in air flow, and the aging time is 0.5-2 h, and obtain the low-cost element-mixed porous NiTi shape memory alloy. 2. The preparation method according to claim 1, wherein the Ni-containing powder is carbonyl Ni powder, and the Ti-containing powder is hydrodehydrogenated Ti powder. 3. The preparation method according to claim 2, characterized in that the particle size of the hydrodehydrogenated Ti powder is -325 mesh to -200 mesh. 4. The preparation method according to claim 1, characterized in that the atomic ratio of Ni and Ti in the mixed powder is (50.0-51.0):(49.0-50.0). 5. The preparation method according to claim 1, wherein the added amount of the NaCl powder is 10% to 50% of the volume fraction of the mixed powder; the particle size of the NaCl powder is -325 mesh to - 200 mesh. 6 . The preparation method according to claim 1 , wherein the mixed powder is cold molded and pressed at 350-550 MPa, and the pressed green body is desalted in deionized water at a constant temperature of 37-60° C. 7 . 7. The preparation method according to claim 6, characterized in that the deionized water is replaced every 1-12 hours during the desalination, and the deionization time is 1-7 days. 8. The preparation method according to claim 1, characterized in that the vacuum degree of the sintering is 10 ⁻⁴ Pa. 9. The preparation method according to any one of claims 1-8, characterized in that, the sintering parameters are as follows: first, the temperature is raised to 600°C at 5°C/min and kept for 0.5h, and then the temperature is raised to 600°C at 1°C/min Keep at 700°C for 2 hours, then raise the temperature at 1°C/min to 1050°C for 2 hours, then raise the temperature at 1°C/min to 1120°C for 2 hours, and finally raise the temperature to 1240°C at 1°C/min for 8 hours. 10. The preparation method according to any one of claims 1-8, characterized in that, the temperature of the solution treatment is 1000° C., and the time is 1 h; the temperature of the aging treatment is 450° C., and the time is 0.5 h.

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