CN115261656A

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

Info

Publication number: CN115261656A
Application number: CN202210708983.9A
Authority: CN
Inventors: 李益民; 李东阳; 舒畅; 何昊; 王暾; 杜昌海
Original assignee: Central South University; Second Xiangya Hospital of Central South University
Current assignee: Central South University; Second Xiangya Hospital of Central South University
Priority date: 2022-06-21
Filing date: 2022-06-21
Publication date: 2022-11-01
Anticipated expiration: 2042-06-21
Also published as: CN115261656B

Abstract

The invention discloses a preparation method of a low-cost element mixed porous NiTi shape memory alloy through vacuum high-temperature sintering and aging treatment, which comprises the following steps: mixing Ni-containing powder, ti-containing powder and NaCl powder, cold die pressing the obtained mixed powder to form, desalting, sintering the obtained green blank to obtain a sintered sample, sealing the sintered sample in a quartz tube vacuum manner, carrying out solution treatment at 900-1100 ℃ in a tube furnace muffle furnace, then quenching, and carrying out aging treatment at 350-500 ℃ in Ar gas flow. The invention reduces the oxygen content of the EP NiTi alloy to 0.22-0.36 wt.% by high vacuum sintering, overcomes the difficult problems of deflagration reaction and liquid phase loss,the strength of the EP porous NiTi alloy skeleton matrix is improved to 200MPa cm³·g^‑1Above, when the porosity is higher than 35%, the 8% compression recovery is still higher than 95%.

Description

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

Technical Field

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

Background

On one hand, the porous NiTi alloy has the superelasticity and shape memory effect of a base material, and also has the functional characteristics of most porous materials, so that the porous NiTi alloy is widely concerned in various fields of energy absorption, hydrogen isotope separation, material light weight and the like, and particularly greatly promotes the development of bone implant materials. The human skeleton has about 2 percent recoverable strain, and the superelasticity of the NiTi alloy is beneficial to the strain matching of the implant; meanwhile, compared with structural materials such as Ti alloy, co alloy and the like, the long-term fatigue reliability can be improved under complex and variable external loads. The controllable porous structure effectively reduces the apparent modulus of the implant and reduces the stress shielding phenomenon, and the communicated pore structure is beneficial to tissue growth and nutrition delivery.

Over the last 20 years, powder metallurgy technologies such as self-propagating high-temperature synthesis (SHS), microwave sintering (MWS), spark Plasma Sintering (SPS), hot Isostatic Pressing (HIP), vacuum Sintering (VS), metal Injection Molding (MIM), additive Manufacturing (AM) and the like have been gradually developed as main preparation methods of porous NiTi alloys. Christian Greiner et al (2005) trapped gas using HIP and subsequently expanded at high temperature to give a porous NiTi alloy of 16% porosity, with a strength of 850MPa when compressive strain reaches 11%, and recovered about 6% of the strain after stress relief 8. Bansiddhi et al (2008) used NaCl as a pore-forming agent, and pre-alloyed NiTi powder was subjected to hot isostatic pressing at 1065 ℃ and then to high-temperature homogenization sintering at 1250 ℃ to prepare a porous NiTi alloy with an aperture ratio of 34%, an average pore diameter of about 151 μm, no fracture after 48% compressive strain, a compressive strength of about 1060MPa, and a strain recovery of about 6% after 8% compressive strain and then unloading.

In contrast, low cost elemental NiTi porous materials (EP) have poor performance. Sirikul et al presses Ni and Ti powder to form, and then prepares porous NiTi alloy with 32-58% porosity by self-propagating high-temperature synthesis, wherein the average pore diameter is about 337-497 μm, and the maximum compressive strength is 344MPa; shuilinWu et al by mixing Ni, ti and NH₄HCO₃Powders, porous NiTi alloys with 21% to 48% porosity were prepared using unshelled HIP (CF-HIP), with average pore sizes ranging from about 50 to 800 μm, where samples with 36% and 21% porosity were fully recoverable when strained to 4.2% and 4.7%. Xu et al (2015) uses Ni and Ti powder as raw materials, porous NiTi alloys with different porosities are prepared by adopting a microwave sintering and space occupying method, when the porosity is increased from 22% to 62%, the ultimate compressive strength of the porous NiTi is reduced from 880MPa to 69MPa, the maximum compressive strain is also reduced from 25% to about 10%, and under the strain of 5%, the shape recovery capability is reduced from 4.56% to 2.28%. Khashayar Khanlari et al prepared porous NiTi alloy with porosity of 32-36% by pressing and sintering (1050 ℃ C., different rates) with Ni and Ti powders, sample pores were mostly formed by eutectic liquid phase transfer, the porosity was about 49-71%, the compressive strength was 500-820 MPa, the maximum compressive strain range was about 9-16%, and the samples could be fully recovered after 2.1% strain. Other similar work has been reported.

Although a unified evaluation standard does not exist, the test modes of bearing failure and recoverable strain are not consistent, but it is still not difficult to find that EPporous NiTi alloy matrix skeleton with similar porosity has more pore defects, and the compressive strength, the plasticity and the recoverable strain are far lower than those of the EPporous NiTi alloy. The poor properties of the EP matrix are due, on the one hand, to the high impurity content of the starting material, oxygen-stabilized Ti₄Ni₂O is important for mechanical deformation and high cycle fatigueA source of cracks; on the other hand, the sintering process has a complex intermediate phase evolution process and a non-uniform reaction liquid phase.

Compared to dense NiTi SMAs, porous NiTi alloys are more prone to failure during stress cycling. The main causes of failure include: localized plastic deformation due to stress concentration; the generation of phase transformation dislocation is accompanied in the process of stress-induced martensitic transformation or martensitic de-twinning. The study shows that the Ni is uniformly distributed₄Ti₃The nanometer precipitated phase can effectively inhibit the movement of dislocation and strengthen the resistance of the matrix to plastic deformation in the loading process. Heat treatment is one of the important methods for improving the strength and functional stability of NiTi SMAs matrix, and Chen et al heat treatment introduces Ni into rolled NiTi alloy₄Ti₃The phase of the nano phase, the precipitation strengthening and the fine grain strengthening are combined, so that the stress cycle stability of the alloy is obviously improved. Wang et al introduced dislocation networks inside the grains by repeated phase transformation in rolled NiTi, and then obtained evenly distributed Ni4Ti3 by low temperature aging treatment, which improves the functional stability of NiTi alloy with micron-sized grains. SLM NiTi alloy was heat treated by Lu et al to find spherical Ni₄Ti₃The nanometer precipitation is more helpful to the stress cycle stability of the alloy. Therefore, the framework matrix of the EPporous NiTi alloy is strengthened through the nano precipitation, and the stress cycle stability can be further improved.

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

Disclosure of Invention

The invention aims to solve the technical problem that the strength of a matrix skeleton, namely the specific strength does not exceed 200 MPa-cm under the current situation that all EP porous NiTi alloys have extremely poor performance (the strength of the matrix skeleton is not more than 200 MPa-cm)³·g^-1Compression superelasticity of less than 5%), a high modulus by vacuumA method for preparing a low-cost element mixed porous NiTi shape memory alloy by warm sintering and aging treatment.

In order to solve the technical problems, the technical scheme provided by the invention is as follows:

a preparation method of low-cost element mixed porous NiTi shape memory alloy through vacuum high-temperature sintering and aging treatment comprises the following steps:

(1) Uniformly mixing Ni-containing powder, ti-containing powder and NaCl powder to obtain mixed powder;

(2) Cold die pressing and forming the mixed powder, and desalting the pressed green body in deionized water;

(3) Sintering the desalted green body, wherein the sintering parameters are as follows: at a vacuum degree of 10^-3～10^-5At Pa, firstly heating to 600 ℃ at a speed of 5-20 ℃/min, preserving heat for 0.5-2h, then heating to 700 ℃ at a speed of 1-2 ℃/min, preserving heat for 2-4h, then heating to 1050 ℃ at a speed of 1-2 ℃/min, preserving heat for 1-4h, heating to 1120 ℃ at a speed of 1-2 ℃/min, preserving heat for 2-4h, and finally heating to 1220-1250 ℃ at a speed of 1-2 ℃/min, preserving heat for 6-10h, so as to obtain a sintered sample;

(4) And sealing the sintered sample in a quartz tube vacuum, carrying out solid solution treatment in a tube furnace at the temperature of 900-1100 ℃ for 1-2h, then quenching the sample with ice water, and then carrying out aging treatment at the temperature of 350-500 ℃ in Ar gas flow for 0.5-2h to obtain the low-cost element mixed porous NiTi shape memory alloy.

Different from the prior art, the invention provides that the porosity is regulated and controlled by adding and removing NaCl in high vacuum (10)^-4Pa) at high temperature (1250 ℃) to obtain the EP porous NiTi alloy with the compression performance comparable to the super elasticity of PP. Furthermore, ni precipitation is regulated and controlled through aging₄Ti₃And precipitation is adopted, so that the stress cycle stability is effectively enhanced.

In the above production method, preferably, the Ni-containing powder is Ni carbonyl powder, and the Ti-containing powder is hydrogenated and dehydrogenated Ti powder.

Preferably, the particle size of the hydrogenated and dehydrogenated Ti powder is-325 meshes to-200 meshes, and more preferably-325 meshes.

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

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

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

Preferably, the mixed powder is subjected to cold die pressing forming under 350-550MPa (preferably 400 MPa), and the pressed green body is desalted in constant-temperature deionized water at 37-60 ℃ (preferably 45 ℃).

Preferably, the deionized water is replaced every 1 to 12 hours during the desalting, and the desalting time is 1 to 7 days.

Preferably, the sintering is performed in a vacuum of 10 degrees^-4Pa.

Preferably, the sintering parameters are as follows: firstly heating to 600 ℃ at a speed of 5 ℃/min, preserving heat for 0.5h, then heating to 700 ℃ at a speed of 1 ℃/min, preserving heat for 2h, heating to 1050 ℃ at a speed of 1 ℃/min, preserving heat for 2h, heating to 1120 ℃ at a speed of 1 ℃/min, preserving heat for 2h, and finally heating to 1240 ℃ at a speed of 1 ℃/min, preserving heat for 8h.

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

Compared with the prior art, the invention has the beneficial effects that:

1. the preparation method of the invention aims at the current situation that all EP porous NiTi alloys have extremely poor performance (the strength of the matrix skeleton, namely the specific strength, is not more than 200 MPa-cm)³·g^-1Compression super elasticity is lower than 5%), provides a new material preparation idea, solves the problems of low-temperature deflagration reaction and high-temperature liquid phase loss, and improves the strength of the EP porous NiTi alloy skeleton matrix to 200 MPa-cm³·g^-1Above, the 8% stretch recovery is higher than 95%.

2. The invention adopts a whole course 10^-4And (3) sintering the porous EPNiTi alloy by using a Pa vacuum system to reduce the oxygen content of the EP porous NiTi alloy to 0.22-0.36 wt.%.

3. Through process optimization, the problems of deflagration reaction in a low-temperature region (700 ℃) and liquid phase loss in a high-temperature region (1250 ℃) are solved, the homogenization of elements is promoted by utilizing unavoidable process liquid phase and high-temperature sintering, and compared with the prior art, the engineering performance of the EP porous NiTi is greatly improved.

4. Through detailed process control, the EPNiTi alloy with uniform components is obtained, the compressive strength of a matrix skeleton is improved to be more than 1200MPa, the fracture strain is higher than 30%, and the 8% tensile recovery rate reaches more than 99%; the method is characterized in that the hydrogenated dehydrogenated titanium powder with low cost is used as a raw material to prepare the NiTi alloy, and no other scheme is available to enable the relevant performance of the NiTi alloy to exceed the test effect stated by the invention.

5. Through solid solution and aging treatment, the plastic deformation capacity of the porous NiTi matrix in the cyclic stress loading-unloading process is strengthened, the residual strain of not more than 0.8 percent can be ensured in the cyclic loading-unloading process for more than 60 times under 8 percent of strain, and the good cyclic stability of compressive stress loading-unloading is shown.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below, and it is obvious that the drawings in the following description are some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to these drawings without creative efforts.

FIG. 1 is a topographical view of a preferred raw material powder; wherein: (a) Ni powder; (b) Ti powder; and (c) NaCl powder.

FIG. 2 is a microstructure and an element distribution pattern (distribution of Ni and Ti elements) of a sintered porous NiTi surface; wherein the method comprises the following steps: (a 1-c 1) the amount of NaCl powder added was 15%; (a 2-c 2) NaCl powder was added in an amount of 25%; (a 3-c 3) NaCl powder was added in an amount of 45%.

FIG. 3 is a porous NiTi alloy pore size distribution plot; wherein: (a) the amount of NaCl powder added was 15%; (b) the amount of NaCl powder added was 25%; (c) the amount of NaCl powder added was 45%.

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

FIG. 5 shows Ni in sintered EP porous NiTi₄Ti₃The appearance and distribution TEM image of precipitated phase; wherein: (a) A₁-a₃) 15% NaCl addition amount; (b)₁-b₃) 25% NaCl addition amount; (c)₁-c₃) 40% NaCl addition amount of the sample; (a)₄、b₄And c₄) Respectively, selected area electron diffraction patterns. Wherein Ni is added with increasing NaCl content₄Ti₃The physical properties of the precipitated phase change significantly.

FIG. 6 is a TEM image of an EP porous NiTi alloy in which 15% NaCl addition amount is added after aging treatment; wherein: (a) And (c) Ni₄Ti₃Morphology and distribution map of precipitated phase; (b) a selected area electron diffraction pattern; (d) is a high resolution map of the local region in (c); (g) is a Fourier transform of (d); (e) is a high resolution plot of the R phase; (f) Is a Fourier transform of (e), in which the arrow indicates the diffraction spot of the R phase.

FIG. 7 is a DCS plot of the phase change temperature change; wherein: (a) a sample in a sintered state; (b) aged specimens.

FIG. 8 is a graph of mechanical properties at room temperature; wherein: (a) a compressive stress strain curve; (b) compression ratio strength comparison map; (c) Compressive load-unload curve of the as-sintered sample at 8% strain, unloading at 120 ℃ for 0.5 hour; (d) The compressive load-unload curve of the aged sample at 8% strain, the unloaded sample being held at 120 ℃ for 0.5 hour; (e) compression recovery compared to other porous NiTi alloys.

FIG. 9 is a graph of the effect of porosity on Young's modulus for as-sintered and as-aged samples.

FIG. 10 is a stress strain curve of cyclic compressive loading-unloading for as-sintered and as-aged samples.

FIG. 11 is the residual strain of EP porous NiTi as a function of the number of compression cycles.

Fig. 12 is a graph showing the effect of porosity on yield strength (a) and critical stress (b) of martensitic transformation.

Detailed Description

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

Unless otherwise defined, all terms of art used hereinafter have the same meaning as commonly understood by one of ordinary skill in the art. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of the present invention.

Unless otherwise specifically stated, various raw materials, reagents, instruments, equipment and the like used in the present invention are commercially available or can be prepared by existing methods.

Example 1:

(1) Uniformly mixing hydrogenated and dehydrogenated Ti powder (-325 meshes, 49.5 at.%), carbonyl Ni powder and NaCl powder (-200 meshes, 50.5 at.%) to obtain mixed powder; the addition amount of the NaCl powder is 15 percent of the volume fraction of the mixed powder;

(2) Pressing and forming the mixed powder under 400MPa by a cold die, desalting the pressed green body in deionized water at constant temperature of 45 ℃, and replacing the deionized water once every 12 hours for 3 days;

(3) Sintering the desalted green body, wherein the sintering parameters are as follows: at a vacuum degree of 10^-4And (3) heating to 600 ℃ at the speed of 5 ℃/min under Pa, preserving heat for 0.5h, heating to 700 ℃ at the speed of 1 ℃/min, preserving heat for 2h, heating to 1050 ℃ at the speed of 1 ℃/min, preserving heat for 2h, heating to 1120 ℃ at the speed of 1 ℃/min, preserving heat for 8h, and finally heating to 1240 ℃ at the speed of 1 ℃/min.

The obtained EPNiTi alloy has oxygen content of 0.23wt.%, porosity of 14.0 + -0.52%, average pore diameter of 60 + -15 μm, compressive strength of over 1600MPa, and specific strength of over 289 MPa-cm³·g^-1The ultimate strain at break exceeded 28% and recovery after 8% strain compression was 7.99%. The sample is not broken after being loaded and unloaded for 60 times under 8% compressive strain, and finallyThe residual strain was 1.40%.

Example 2:

(1) Uniformly mixing hydrogenated and dehydrogenated Ti powder (-325 meshes, 49.5 at.%), carbonyl Ni powder and NaCl powder (-200 meshes, 50.5 at.%) to obtain mixed powder; the addition amount of the NaCl powder is 25 percent of the volume fraction of the mixed powder;

(2) Cold die pressing the mixed powder under 400MPa to form, desalting the pressed green body in deionized water at constant temperature of 45 ℃, and replacing the deionized water every 12 hours for 3 days;

The resulting EPNiTi alloy had an oxygen content of 0.29wt.%, a porosity of about 22.0. + -. 1.41%, and a mean pore diameter of about 91. + -. 20 μm. Compressive strength of more than 1600MPa, specific strength of more than 326MPa cm³·g^-1The ultimate strain at break exceeded 35% and recovery after 8% strain compression was 7.96%. The sample did not break after 60 times of loading and unloading at 8% compressive strain, with a final residual strain of 1.55%.

Example 3:

(1) Uniformly mixing hydrogenated and dehydrogenated Ti powder (-325 meshes, 49.5 at.%), carbonyl Ni powder and NaCl powder (-200 meshes, 50.5 at.%) to obtain mixed powder; the addition amount of the NaCl powder is 40 percent of the volume fraction of the mixed powder;

The resulting EPNiTi alloy had an oxygen content of 0.36wt.%, a porosity of about 37.0. + -. 1.15% and a mean pore diameter of about 124. + -. 22 μm. The compressive strength is 1236MPa, and the specific strength is 307MPa cm³·g^-1The ultimate strain at break exceeded 35% and recovery after 8% strain compression was 7.40%. The sample did not break after 60 load unloads at 8% compressive strain, with a final residual strain of 1.59%.

Example 4:

(3) Sintering the desalted green body, wherein the sintering parameters are as follows: at a vacuum degree of 10^-4At Pa, the temperature is firstly raised to 600 ℃ at the speed of 5 ℃/min and is preserved for 0.5h, then raised to 700 ℃ at the speed of 1 ℃/min and is preserved for 2h, then raised to 1050 ℃ at the speed of 1 ℃/min and is preserved for 2h, then raised to 1120 ℃ at the speed of 1 ℃/min and is preserved for 2h, and finally raised to 1240 ℃ at the speed of 1 ℃/min and is preserved for 8h.

(4) The sintered sample is sealed in a vacuum quartz tube, subjected to solution treatment (1000 ℃,1 h) in a tube furnace, quenched by ice water, and then subjected to aging treatment at 450 ℃ in Ar gas flow, wherein the aging time is 0.5h.

The obtained EP porous NiTi alloy has oxygen content of 0.24wt.%, porosity of 14.0%, average pore diameter of 60 μm, and compressive strength of overOver 1600MPa, specific strength over 289MPa cm³·g^-1The ultimate strain at break exceeded 16.5% and recovery after 8% strain compression was 7.99%. The sample did not break after being loaded and unloaded 60 times at 8% compressive strain, and the final residual strain was 0.40%, indicating good cycle stability.

Example 5:

(4) And sealing the sintered sample in a vacuum quartz tube, performing solution treatment (1000 ℃,1 h) in a tube furnace, quenching with ice water, and then performing aging treatment at 450 ℃ in Ar gas flow for 0.5h to obtain the EP porous NiTi alloy.

The obtained EP porous NiTi alloy has an oxygen content of 0.41wt.%, a porosity of about 22%, an average pore diameter of about 91 μm, a compressive strength of over 1600MPa, and a specific strength of over 326 MPa-cm³·g^-1The ultimate strain at break exceeded 34% and recovery after 8% strain compression was 7.96%. The sample did not break after being loaded and unloaded 60 times under 8% compressive strain, and the final residual strain was 0.85%, showing good cycling stability.

Example 6:

(3) Sintering the desalted green body, wherein the sintering parameters are as follows: heating to 600 ℃ at a vacuum degree of 10-4Pa at a speed of 5 ℃/min, preserving heat for 0.5h, heating to 700 ℃ at a speed of 1 ℃/min, preserving heat for 2h, heating to 1050 ℃ at a speed of 1 ℃/min, preserving heat for 2h, heating to 1120 ℃ at a speed of 1 ℃/min, preserving heat for 2h, and finally heating to 1240 ℃ at a speed of 1 ℃/min, preserving heat for 8h.

The obtained EP porous NiTi alloy has the oxygen content of 0.55wt.%, the porosity of about 38 percent, the average pore diameter of about 124 mu m, the compressive strength of more than 817MPa and the specific strength of 203 MPa-cm³·g^-1The ultimate strain at break was 32% and after 8% strain compression it recovered 7.93%. The sample did not break after 60 times loading and unloading at 8% compressive strain, with a final residual strain of 1.84%.

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

the present invention prepares porosities of 14.0 ± 0.52%,22.0 ± 1.41% and 37.0 ± 1.15% (respectively labeled as S1, S2 and S3, respectively for the as-aged samples A1, A2 and A3), respectively, with open porosities of 11%, 21.8% and 36.7%, respectively, by adding NaCl powder of 15vol.%, 25vol.% and 40vol.%, by combining high-temperature homogenization sintering with space occupying. The average pore diameters were 60. + -. 15. Mu.m, 91. + -. 20. Mu.m and 124. + -. 22. Mu.m, respectively.

FIG. 1 is a topographical view of a raw material powder according to the present invention; wherein: ni powder (a), ti powder (b) and NaCl powder (c).

FIG. 2 shows the pore morphology of vacuum sintering to produce porous NiTi SMAs. The distribution of Ni and Ti elements is uniform without obvious segregation. Wherein: (a1) The addition amount of NaCl powder was 15%, and the porosity after sintering was about 14.0%; (a2) The addition amount of NaCl powder is 25%, and the porosity after sintering is 22.0%; (a3) The amount of NaCl powder added was 45% and the porosity after sintering was 38%. The open cell ratio of all samples was as high as 95% or more.

FIG. 3 is a pore size distribution plot for a porous NiTi alloy. Wherein: (a) The added amount of NaCl powder is 15%, the porosity after sintering is about 14.0%, and the average pore diameter is about 60 μm; (b) The added amount of NaCl powder is 25%, the porosity after sintering is about 22.0%, and the average pore diameter is about 91 μm; (c) The NaCl powder was added in an amount of 45%, the porosity after sintering was about 38%, and the average pore diameter was about 124. Mu.m.

The X-ray diffraction results at room temperature are shown in FIG. 4. The phase consisted of austenite and a small amount of martensite, and Ti was detected₂A Ni phase. Significant Ni was observed in the as-sintered sample₄Ti₃Peak of phase, ni after solid solution and aging treatment₄Ti₃The peak disappeared. Previous work reported that C, O impurity levels resulted in Ni in Pre-alloyed Ti-rich NiTi alloys₄Ti₃Nonequilibrium precipitation, but the existence of liquid phase in the sintering process in the NiTi promotes the homogenization of components and the digestion of Ni-rich clusters, and no precipitation peak is observed in the sintered sample. In this study, the Ni content of the NiTi alloy was 50.5%, and the corresponding Ni equivalent was further increased, so that the presence of Ni4Ti3 was found in the sintered sample. But Ni after solution aging treatment₄Ti₃Disappearance of peaks, probably due to Ni₄Ti₃The actual volume size of the precipitate is reduced or more evenly distributed.

Ni in sintered EP porous NiTi₄Ti₃The morphology of the precipitated phase and the distribution TEM morphology are shown in FIG. 5. Wherein: (a)₁-a₃) 15% NaCl addition amount; (b)₁-b₃) 25% NaCl addition amount; (c)₁-c₃)40％NaCl; (a) A₄、b₄And c₄) Respectively, selected area electron diffraction patterns. Wherein Ni is added with increasing NaCl content₄Ti₃The physical properties of the precipitated phase change significantly. [111]_B2In a selected area electron diffraction (SEAD) pattern of (a) located in a B2 austenite matrix<123>Superlattice diffraction point at 1/7 position of reciprocal vector and<110>diffraction points at the 1/3 position of the reciprocal vector respectively prove Ni₄Ti₃Existence of nanometer precipitated phase and R phase. By comparing samples of different porosities, it was found that Ni₄Ti₃Are different in morphology and size, and Ni in the sample with 14.0% porosity₄Ti₃The coffee bean morphology was present, and the 22.0% and 38% porosity samples were approximately lenticular and had an overall non-uniform distribution. Ni in sintered sample₄Ti₃The average length of the nano precipitated phase gradually increases along with the increase of the porosity, and is respectively 20nm,137nm and 145nm. This difference in morphology and size is mainly due to the difference in oxygen content in the matrix. As the oxygen content increases, the Ni-rich clusters increase for Ni₄Ti₃The precursor effect of spontaneous precipitation is enhanced, resulting in an increase in the overall precipitate size.

Ageing Ni in the sample after solution and ageing treatment compared to the as-sintered state₄Ti₃The precipitated phase was more uniformly dispersed in the matrix (FIG. 6 a), and was morphologically closer to spherical with an average size of about 25 μm. The precipitated phase with extremely small size does not have obvious peak on the XRD pattern. Smaller size dispersed precipitates will have a stronger pinning effect.

FIG. 7 shows the DSC curve of the change in phase transition temperature of the sample. The 14% porosity sample showed multiple phase transitions during cooling. The 22% and 38% porosities show only two exothermic peaks upon cooling. Three endothermic peaks appear on the sintered sample during the heating stage. Compared with the sintered sample, the sample subjected to solid solution and aging treatment only shows one phase transition peak in the heating and cooling processes. Wherein, S1, S2 and S3 thermal hysteresis temperature

About 0.3 deg.C, 0.2 deg.C and 1.5 deg.C, respectively. Thermal hysteresis is small according to the R phase change

The phase change sequence of S1 in the cooling process can be determined as A → R, R → M1 and A → M2 respectively; the two-step phase changes of S2 and S3 are A → R and R → M), respectively. Heating stage the phase transition sequence for the three porosity samples was M2 → A, M1 → R and R → A. After the solution aging treatment, only single R phase transformation occurs in all samples. The phase transition peak temperature of the aged sample is obviously increased, which is consistent with the result after the aging treatment of the compact NiTi alloy. Because the Ni content of the matrix is lost due to the precipitation of the Ni4Ti3 phase, the M phase transformation temperature and the R phase transformation temperature can be increased along with the reduction of the Ni content in the matrix.

FIG. 8 (a) shows the compressive deformation behavior of porous NiTi alloys at room temperature (25 deg.C). The S1 and the S2 are not crushed under the maximum stress of 1600MPa, the strain reaches 28 +/-1.2 percent and 35 +/-1.4 percent respectively, and the ultimate strength of the S3 is as high as 1236 +/-40 MPa, and the strain is 49 +/-1.1 percent. After heat treatment, the strength of samples with different porosities is greatly improved under the same strain level. However, the A3 sample is crushed after generating 31 + -1.1% strain, and the corresponding compressive strength is 817 + -20 MPa. Compared with the results reported for the porous NiTi alloy by Khashayar et al, the samples prepared according to the invention with similar porosity, whether strength or elongation, have significant advantages. To evaluate the strength of the porous sample matrix, we used the specific strength σ_eqThe mechanical properties of the porous NiTi alloy reported in recent years are summarized:

wherein sigma_eqRepresenting the specific strength, σ_maxRho is the density of the dense NiTi (different Ni contents: 6.45-6.67 g/cm) for ultimate compressive strength³) And P is the porosity of the porous NiTi alloy. From FIG. 8 (b), it can be seen that the specific strength of the porous NiTi alloy with porosity of 38 + -0.4% prepared by the present study is up to 307 + -6 MPa-cm³(iv)/g, exhibits excellent compression set resistance.

Fig. 8 (c) and (d) are compressive stress-strain plots at 8% strain. As the porosity of the sintered porous NiTi sample is increased from 14 +/-0.1% to 38 +/-0.4%, the compressive strength under 8% strain is reduced from 615 +/-13 MPa to 338 +/-13 MPa, the strength of samples with different porosities is obviously improved after heat treatment, and the strength of A1, A2 and A3 is 851 +/-14 MPa, 736 +/-13 MPa and 404 +/-10 MPa respectively. Because the tested samples at room temperature have a small amount of martensite and austenite, and the volume fractions of the austenite and the martensite at room temperature in different pore samples and corresponding heat treatment state samples are different, the super-elasticity and the shape memory characteristics are simultaneously shown in the compression loading-unloading experimental process. After the heating treatment at 120 ℃ for 1h, the maximum residual strain of each sample is only 0.8%, which shows that the heat treatment process improves the strength of the matrix, and the functional characteristics of the porous NiTi alloy are not obviously damaged even if the oxygen content of the matrix is increased to a certain degree. The current results clearly show that the porous NiTi alloy shows excellent superelasticity and shape memory effect at room temperature, and the improvement of the comprehensive performance of the porous NiTi alloy through heat treatment is feasible.

The modulus of porous NiTi alloys under static and dynamic conditions largely limits their application as orthopedic implants. Due to the existence of pores, the porous NiTi alloy can induce martensite phase transformation even with small stress during the loading process, the martensite phase transformation and the de-twinning process can continuously occur along with the increase of the stress, and generate larger strain than the common alloy, in other words, the stress-induced martensite phase transformation can occur at each stage during the stress loading process, so the low elastic modulus can be shown during the loading process, the alloy is in a complete martensite state during the unloading process, and obviously reversible de-twinning and martensite-austenite transformation can not occur in the initial stage of small strain (0.2 percent), and the measured modulus (E) is measured at the moment_dyn) The Young modulus of the real martensite of the porous NiTi alloy is close to that of the real martensite of the porous NiTi alloy. The widely used model of Gibson and Ashby predicts that the porous alloy young's modulus decreases with the square of the porosity, and we compared the apparent elastic modulus of the porous sample to this model (E = g =)Es_olid×(1-P)²，E_solidThe elastic modulus of the NiTi is 21-69 GPa) of dense martensite, and the experimental value is found to be well in the prediction interval of the model. The apparent elastic modulus of the porous NiTi alloy measured by the research is far lower than the dynamic modulus value, and the rule of the experimental result is consistent with that reported by Bansidhi et al. And the apparent modulus and the dynamic modulus of the porous NiTi alloy both have a linear descending trend along with the increase of the porosity, the porosity of the sintered sample is increased from 14 +/-0.1 percent to 38 +/-0.4 percent, the apparent elastic modulus is reduced from 11.4 +/-1 GPa to 6.0 +/-0.5 GPa, and the dynamic modulus is reduced from 29.7 +/-2 GPa to 15.6 +/-1 GPa. After heat treatment E_app＝7.3～12.4GPa,E_dyn＝15.7～30.5Gpa。

FIG. 9 is the effect of porosity on Young's modulus for as-sintered and as-aged samples. Wherein experimental data for dynamic and apparent Young's modulus are compared to porous solid models of Gibson and Ashby; the experimental data for dynamic and apparent young's modulus are compared to the young's modulus of human bone tissue. As shown in fig. 9, for the porous sample, the heat treatment process resulted in a small increase in the apparent elastic modulus compared to the sintered sample, with no significant effect on the dynamic modulus. This is because the apparent elasticity is sensitive to the heat treatment process. In the process of austenite-martensite transformation, due to Ni₄Ti₃The nano precipitation has pinning effect relative to the phase boundary migration, and the difficulty of stress-induced martensite phase transformation along with Ni₄Ti₃Is increased. Although after heat treatment A_fThe apparent rise may be a certain increase in the volume fraction of martensite in the room temperature aged sample, which may result in a decrease in modulus. These two mutually limiting phenomena lead to a small increase in the elastic modulus after heat treatment, but Ni₄Ti₃The pinning effect on phase boundary migration may be more dominant. The Young's modulus is determined by the electronic structure of the crystal in nature and does not depend on the microstructure, which well explains the reason why the heat treatment process has no obvious influence on the Young's modulus.

In order to avoid the problems of osteoporosis and the like caused by modulus mismatch, the young modulus of the implant is medically required to be within the modulus range of human bones, and E is less than or equal to 17GPa. Therefore, the porous NiTi SMAs with the porosity of 38 +/-0.4% prepared by the invention not only have high strength, large recoverable strain and certain pressure cycle stability, but also meet the requirements of biological application implants on both dynamic modulus and apparent modulus (6-15 GPa), and can effectively avoid adverse risks caused by modulus mismatching.

In order to further evaluate the influence of the heat treatment process on the functional characteristics of the porous NiTi alloy, a strain-fixed cyclic loading test is carried out on the porous NiTi alloy to characterize the cyclic compression stability of the sample. The stress-strain curve results shown in fig. 10 indicate that porous NiTi alloys of varying porosity and corresponding heat treated condition can withstand up to a stress level without fracture under 60 cycle loading tests, exhibiting excellent cyclic plastic resistance. Fig. 11 is a result of measuring the residual strain of the sample every 15 times of heating. The residual strain of the as-sintered sample increased with increasing cycle number, the residual strain of S1 increased from 0.55% to 1.4%, the residual strain of S2 increased from 0.7% to 1.55%, and the residual strain of the S3 sample increased from 0.75% to 1.59%. After 60 cycles of the sample after the solution aging, the residual strain of A1 is only 0.4%, the residual strain of A2 is 0.85%, and the sample shows good compression cycle stability. The results of the cyclic compression test show that the heat treatment process has a significant improvement in the cyclic stability of the samples with porosities of 14 + -0.1% and 24 + -0.2%, but the aging treatment has no significant strengthening effect on the samples with porosities of 38 + -0.4%, and the residual strain of the A3 sample after 60 load-unload cycles reaches 1.84%.

FIG. 12 (a) is a graph representing the yield strength versus porosity for a material, where the yield strength is obtained by the tangent method in FIG. 8 (a); FIG. 12 (b) is represented by the formula σ_SIM＝(1-P)σ_yEstimated critical stress (σ) of martensitic transformation of the sample_SIM) The relationship to porosity. As the porosity of the as-sintered sample increased from 14% to 37%, σ_yThe strain is reduced from 927 +/-30 MPa (10 +/-1 percent of strain) to 386 +/-30 MPa (10.15 +/-1.2 percent of strain), but still higher than the ultimate compressive strength (100-230 MPa) of dense bones of a human body. As porosity increases, σ_SIMFrom 170 +/-7 MPa to 71 +/-8 MPa, the introduction of pores can obviously reduce the yield strength of the material. The porous NiTi samples after solution and aging exhibited higher yield strength and martensitic transformation critical stress during compression. This is due to Ni₄Ti₃Strengthening of the precipitated phase to the matrix, wherein Ni₄Ti₃The distribution of the precipitated phases is a major factor. Compared with the sintered sample, the Ni uniformly dispersed in the sample after heat treatment₄Ti₃The relative dislocation motion is remarkably inhibited, so that the dislocation slip critical shear stress is increased, and the yield strength of the material is improved. Coherent Ni₄Ti₃The precipitated phase boundary also has strong pinning effect, and the existence of coherent stress field inhibits the martensite phase transformation, so that higher stress needs to be provided to induce the martensite phase transformation in the compression process.

Claims

1. A preparation method of low-cost element mixed porous NiTi shape memory alloy through vacuum high-temperature sintering and aging treatment is characterized by comprising the following steps:

2. The production method according to claim 1, wherein the Ni-containing powder is a Ni carbonyl powder and the Ti-containing powder is a hydrogenated dehydrogenated Ti powder.

3. The production method according to claim 2, wherein the hydrogenated dehydrogenated Ti powder has a particle size of-325 to-200 mesh.

4. The method according to claim 1, wherein the atomic ratio of Ni to Ti in the mixed powder is (50.0-51.0): (49.0-50.0).

5. The preparation method according to claim 1, wherein the NaCl powder is added in an amount of 10-50% by volume of the mixed powder; the granularity of the NaCl powder is-325 to-200 meshes.

6. The method according to claim 1, wherein the mixed powder is cold-die pressed at 350 to 550MPa, and the pressed green compact is desalted in deionized water at a constant temperature of 37 to 60 ℃.

7. The method according to claim 6, wherein the deionized water is replaced every 1 to 12 hours for desalting for 1 to 7 days.

8. The method according to claim 1, wherein the degree of vacuum of the sintering is 10^-4Pa。

9. The production method according to any one of claims 1 to 8, wherein the sintering parameters are as follows: firstly heating to 600 ℃ at a speed of 5 ℃/min, preserving heat for 0.5h, then heating to 700 ℃ at a speed of 1 ℃/min, preserving heat for 2h, then heating to 1050 ℃ at a speed of 1 ℃/min, preserving heat for 2h, then heating to 1120 ℃ at a speed of 1 ℃/min, preserving heat for 2h, and finally heating to 1240 ℃ at a speed of 1 ℃/min, preserving heat for 8h.

10. The method according to any one of claims 1 to 8, wherein the solution treatment is carried out at a temperature of 1000 ℃ for a time of 1 hour; the temperature of the aging treatment is 450 ℃, and the time is 0.5h.