CN115386755B - A preparation method of low-cost element-mixed NiTi shape memory alloy by high-temperature homogenization treatment - Google Patents

Abstract

The invention discloses a preparation method of a low-cost element mixed NiTi shape memory alloy through high-temperature homogenization treatment, which comprises the following steps: uniformly mixing Ni-containing powder and Ti-containing powder, and sintering the obtained mixed powder, wherein the sintering parameters are as follows: under high vacuum degree, the temperature is raised to 600 ℃ at 5-20 ℃/min for 0.5-2h,1-2 ℃/min is raised to 700 ℃ for 2-4h,1-2 ℃/min is raised to 1050 ℃ for 1-4h,1-2 ℃/min is raised to 1120 ℃ for 2-4h, and 1-2 ℃/min is raised to 1220-1250 ℃ for 6-10h. According to the invention, the oxygen content of the EPNiTi alloy is reduced to be less than 0.22wt.% through high vacuum sintering, the problems of deflagration reaction and liquid phase loss are overcome, the mechanical property of the EPNiTi is greatly improved, and the tensile elongation is improved to be more than 20%.

Description

A low-cost element-mixed NiTi shape-memory alloy by high-temperature homogenization gold preparation method

technical field

The invention belongs to the technical field of shape memory materials, in particular to a method for preparing a low-cost element-mixed NiTi shape memory alloy through high-temperature homogenization treatment.

Background technique

NiTi alloy is the most concerned and potential shape memory material today. Due to the flexible composition control and low cost of raw materials, the elemental powder method (EP, elemental powder) has always been an important preparation method for powder metallurgy NiTi and porous NiTi alloys. However, due to the difficulty in the preparation of EP, the research on EP NiTi basically has no or only a small amount of tensile properties (elongation less than 5%). Its performance is far from that of pre-alloyed NiTi powder (PP, pre-alloyed powder) products and ingot metallurgical NiTi products (elongation is higher than 15%). Only sufficient tensile properties can meet the requirements of real engineering applications.

The difficulty in preparing low-cost EP NiTi alloys is limited by many factors. First, the impurity content of the raw material mixed with HDH Ti powder is higher than that of pre-alloyed powder. The increase of oxygen impurities can make the Ti ₂ Ni impurity phase exist in a more stable form of Ti ₄ Ni ₂ O, which changes the martensitic transformation temperature. Ti ₄ Ni ₂ O is also an important source of cracks during loading; at the same time, reports have suggested that Oxygen is an important reason for grain boundary amorphization in NiTi alloy, which may be an unfavorable factor leading to mechanical embrittlement. Therefore, reducing the impurity content of raw materials and reducing the oxygen increase in the sintering process are the main control strategies. Ultra-high vacuum sintering is the most effective solution to control the oxygen increase in the process. However, EP NiTi sintering is accompanied by a large amount of exothermic reaction. At a certain background temperature, the sintering can be completed through the propagation of combustion waves at a certain background temperature (self-propagating process). If the reaction is too violent, the central temperature may exceed the melting point of the NiTi alloy (1310°C). As the magnitude of the vacuum increases, the efficiency of radiation heat dissipation of the sintered body may be significantly reduced, and the intense heat release will cause the body to melt and collapse. This detail has not been reported in any detail.

On the other hand, the difficulty in preparing EP NiTi is due to the complex mesophase transformation sequence. Due to the non-uniform particle distribution and the similar energy of the three stable intermediate compounds of NiTi, Ti ₂ Ni and TiNi ₃ during sintering, all intermediate phases have the opportunity to form in large quantities at 720-920°C. Therefore, both the eutectic reaction β-Ti(Ni)+Ti ₂ Ni→L at 942°C and the eutectic reaction TiNi+TiNi ₃ →L at 1118°C may be triggered, forming heterogeneous liquid phases at irregular times and at irregular points . At the same time, due to the large difference in interdiffusion, the rate of Ni diffusion into Ti is much higher than the rate of Ti diffusion into Ni, and Kirkendall pores are easily formed. The aforementioned heterogeneous eutectic liquid phase may be lost and detached from the body, or may be absorbed and aggregated under the capillary action of Kirkendall pores, leaving larger macroscopic pores locally. Just increasing the sintering temperature can only lead to accelerated loss of liquid phase and collapse of green body. Therefore, if the liquid phase cannot be effectively controlled, densification cannot be achieved, and the surface quality and dimensional accuracy of the sample cannot be guaranteed. Because of this, in previous studies, no direct sintering of densified EP NiTi alloys has been reported, and EP NiTi alloys are often studied as porous NiTi.

So far, there is no research report that considering all the above factors comprehensively, the strength of EP NiTi alloy is increased to more than 700MPa, the elongation is higher than 15%, and the tensile recovery rate of 8% is higher than 80% through vacuum sintering.

Contents of the invention

The technical problem to be solved by the present invention is to propose a new material preparation idea to solve the problems of low-temperature deflagration reaction and high-temperature liquid phase loss in view of the current situation of extremely poor performance of all EP NiTi alloys (tensile elongation is lower than 5%).

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

A preparation method of low-cost element mixed NiTi shape memory alloy processed by high-temperature homogenization, comprising the steps of:

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

(2) The mixed powder 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, then heat at 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, and finally 1 Raise temperature at -2°C/min to 1220-1250°C for 6-10h.

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 hydrogenated dehydrogenation titanium powder is -325 mesh to -200 mesh, more preferably -325 mesh.

Preferably, the Ni atom content in the mixed powder is 49.0-51.0%, more preferably 50.0%.

Preferably, the vacuum degree of the sintering is 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.

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

1. In the present invention, the EP NiTi alloy is sintered with a vacuum system of about 10 ^-4 Pa in the whole process, and the oxygen content of the EP NiTi alloy is reduced to <0.22wt.%.

2. Through process optimization, the problem of deflagration reaction in the low temperature zone (700°C) and liquid phase loss in the high temperature zone (1250°C) is solved at the same time, and the unavoidable process liquid phase and high temperature sintering are used to promote the homogenization of elements. Compared with the existing technology, The engineering performance of EP NiTi is greatly improved, and the strength of EP NiTi alloy is increased to more than 700MPa, the elongation is higher than 15%, and the tensile recovery rate of 8% is higher than 80%.

3. Through detailed process control, an EP NiTi alloy with a relative density of more than 90% is obtained, the tensile strength is increased to more than 700MPa, the elongation is higher than 20%, the tensile recovery rate of 8% is higher than 90%, and the microstructure is uniform; The NiTi alloy is prepared by using low-cost hydrogenated dehydrogenated titanium powder as a raw material, and there is no other scheme that can make the related properties of the NiTi alloy exceed the test results stated in the present invention.

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.

Figure 1 is a schematic diagram of the heating process and the sintered Ti-50.0Ni sample; where (I) when heated to 700 °C at a rate of 5 °C/min, a deflagration reaction occurs to melt the sample; (II) at 1 °C/min (III) Sample II was directly heated to 1250°C with millimeter-scale macroscopic pores; (IV) During continuous heating, sample II was maintained at 1120°C 2h, no macro defects appear.

Figure 2 is the microscopic morphology and EPMA spectrum (distribution of Ni and Ti elements) of the distribution surface of the matrix and the transition ring; there are obvious transition rings in the cross-sectional area of the macroscopic pores.

Figure 3 is an EPMA scan of the element distribution; among them, (a1-a3) kept the sample at 1050 °C for 3 hours; (b1-b3) kept the sample at 1250 °C for 6 hours; the overall average element content is similar, but The samples at 1250°C were significantly more homogeneous.

Figure 4 is an analysis diagram of the element content of EPMA matrix points; among them, (a-b) the sample is kept at 1050 °C for 3 hours; (c-d) the sample is kept at 1250 °C for 6 hours; 9 points are taken for each field of view to calculate the average value , three different fields of view were chosen for each sample.

Figure 5 is a DCS curve diagram with peak width and phase transition temperature changes; among them: (a-c) samples sintered at 1050 °C; (d-f) samples sintered at 1250 °C; (g) DSC curves reflect the uniformity of elements peak width; (h) transition temperature of 1250°C sintered sample.

Fig. 6 is a scanning electron micrograph of a sample sintered at 1050°C; wherein: (a _1-2 ) Ti-49.0Ni, (b _1-2 ) Ti-49.5Ni, (c _1-2 ) Ti-50.0Ni.

Fig. 7 is a scanning electron micrograph of a sample sintered at 1250°C; wherein: (a _1-2 )Ti-49.0Ni, (b _1-2 )Ti-49.5N, (c _1-2 )Ti-50.0Ni.

Fig. 8 is an X-ray diffraction spectrum of EP NiTi alloy sintered at 1250°C for high temperature homogenization at room temperature.

Figure 9 is a comparison chart of tensile properties and properties. The total length of the tensile sample is 46 mm, the width of the deformation zone is 3 mm, and the thickness is 2 mm; where: (a) tensile stress-strain relationship; (b-d) 4% and 8% tensile strain recovery curves; (e) literature comparison of ultimate tensile strength and strain; (f) comparison of tensile and recovery capabilities with other powder metallurgy NiTi alloys.

Fig. 10 is a fracture scanning electron micrograph of the tensile sample in Fig. 9; among them: (a _1-3 )Ti-49.0Ni, (b _1-3 )Ti-49.5Ni, (c _1-3 ) Ti-50.0Ni .

Detailed ways

In order to facilitate 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 kind of preparation method of the low-cost element mixed NiTi shape memory alloy by high temperature homogenization treatment of the present invention, comprises the steps:

(1) Carbonyl Ni powder and -325 purpose hydrogenated dehydrogenation titanium powder are uniformly mixed to obtain a mixed powder, wherein the Ni atom content is 49.0%;

(2) The mixed powder 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 and keep it warm 2h, then raise the temperature at 1°C/min to 1050°C for 2h, then raise the temperature to 1120°C at 1°C/min and hold for 2h, and finally raise the temperature to 1240°C at 1°C/min and hold for 8h to obtain low-cost element mixed NiTi shape memory alloy (Ti-49Ni).

The obtained EP NiTi alloy has an oxygen content of 0.22wt.%, a tensile strength of 710MPa, a tensile elongation of 16.5%, and a 8% tensile strain recovery rate of 85%.

Example 2:

A kind of preparation method of the low-cost element mixed NiTi shape memory alloy by high temperature homogenization treatment of the present invention, comprises the steps:

(1) Carbonyl Ni powder and -325 purpose hydrogenated dehydrogenation titanium powder are uniformly mixed to obtain a mixed powder, wherein the Ni atom content is 49.0%;

(2) The mixed powder is sintered, and the sintering parameters are as follows: under a vacuum degree of 10 ^-4 Pa, first raise the temperature at 10°C/min to 600°C and keep it for 1 hour, then raise the temperature at 1.5°C/min to 700°C and keep it for 2.5 h, then raise the temperature at 1.5°C/min to 1050°C for 2 hours, then raise the temperature at 1.5°C/min to 1120°C for 3 hours, and finally raise the temperature to 1250°C at 1.5°C/min for 6 hours, and obtain the low-cost element mixed NiTi shape memory alloy (Ti-49Ni).

The oxygen content of the obtained EP NiTi alloy is 0.21wt.%, the tensile strength is 699MPa, the tensile elongation is 17%, and the 8% tensile strain recovery rate is 91%.

Example 3:

A kind of preparation method of the low-cost element mixed NiTi shape memory alloy by high temperature homogenization treatment of the present invention, comprises the steps:

(1) Carbonyl Ni powder and -325 purpose hydrogenated dehydrogenation titanium powder are uniformly mixed to obtain a mixed powder, wherein the Ni atom content is 49.0%;

(2) The mixed powder is sintered, and the sintering parameters are as follows: under a vacuum degree of 10 ^-3 Pa, first raise the temperature at 20°C/min to 600°C and keep it for 2h, then raise the temperature at 2°C/min to 700°C and keep it for 4h , and then heated at 2°C/min to 1050°C for 4 hours, then at 2°C/min to 1120°C for 4 hours, and finally at 2°C/min to 1250°C for 6 hours to obtain a low-cost element mixed NiTi shape memory alloy (Ti-49Ni).

The obtained EP NiTi alloy has an oxygen content of 0.30%, a tensile strength of 650 MPa, a tensile elongation of 12%, and an 8% tensile strain recovery rate of 75%. Compared with Examples 1 and 2, due to the decrease of the vacuum level, the oxygen content obviously increases and the performance decreases, but it is still much higher than the current reported data.

Example 4:

A kind of preparation method of the low-cost element mixed NiTi shape memory alloy by high temperature homogenization treatment of the present invention, comprises the steps:

(1) Carbonyl Ni powder and -325 purpose hydrogenated dehydrogenation titanium powder are uniformly mixed to obtain a mixed powder, wherein the Ni atom content is 49.5%;

(2) The mixed powder 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 and keep it warm 2h, then raise the temperature at 1°C/min to 1050°C for 2h, then raise the temperature to 1120°C at 1°C/min and hold for 2h, and finally raise the temperature to 1240°C at 1°C/min and hold for 8h to obtain low-cost element mixed NiTi shape memory alloy (Ti-49.5Ni).

The oxygen content of the obtained EP NiTi alloy is 0.22wt.%, the tensile strength is 740MPa, the tensile elongation is 21%, and the 8% tensile strain recovery rate is 95%.

Example 5:

A kind of preparation method of the low-cost element mixed NiTi shape memory alloy by high temperature homogenization treatment of the present invention, comprises the steps:

(1) Carbonyl Ni powder and -325 purpose hydrogenated dehydrogenation titanium powder are uniformly mixed to obtain a mixed powder, wherein the Ni atom content is 49.5%;

(2) The mixed powder is sintered, and the sintering parameters are as follows: under a vacuum degree of 10 ^-4 Pa, first raise the temperature at 10°C/min to 600°C and keep it for 1 hour, then raise the temperature at 1.5°C/min to 700°C and keep it for 2.5 h, then raise the temperature at 1.5°C/min to 1050°C for 2 hours, then raise the temperature at 1.5°C/min to 1120°C for 3 hours, and finally raise the temperature to 1250°C at 1.5°C/min for 6 hours, and obtain the low-cost element mixed NiTi shape memory alloy (Ti-49.5Ni).

The oxygen content of the obtained EP NiTi alloy is 0.23wt.%, the tensile strength is 720MPa, the tensile elongation is 20%, and the 8% tensile strain recovery rate is 96%.

Embodiment 6:

A kind of preparation method of the low-cost element mixed NiTi shape memory alloy by high temperature homogenization treatment of the present invention, comprises the steps:

(1) Carbonyl Ni powder and -325 purpose hydrogenated dehydrogenation titanium powder are uniformly mixed to obtain a mixed powder, wherein the Ni atom content is 49.5%;

(2) The mixed powder is sintered, and the sintering parameters are as follows: under a vacuum degree of 10 ^-3 Pa, first raise the temperature at 20°C/min to 600°C and keep it for 2h, then raise the temperature at 2°C/min to 700°C and keep it for 4h , and then heated at 2°C/min to 1050°C for 4 hours, then at 2°C/min to 1120°C for 4 hours, and finally at 2°C/min to 1250°C for 6 hours to obtain a low-cost element mixed NiTi shape memory alloy (Ti-49.5Ni).

The obtained EP NiTi alloy has an oxygen content of 0.29wt.%, a tensile strength of 677MPa, a tensile elongation of 13.5%, and an 8% tensile strain recovery rate of 81%. Compared with Examples 4 and 5, due to the decrease of the vacuum level, the oxygen content obviously increases and the performance decreases, but it is still much higher than the data reported so far.

Embodiment 7:

A kind of preparation method of the low-cost element mixed NiTi shape memory alloy by high temperature homogenization treatment of the present invention, comprises the steps:

(1) Carbonyl Ni powder and -325 purpose hydrogenated dehydrogenation titanium powder are uniformly mixed to obtain a mixed powder, wherein the Ni atom content is 50.0%;

(2) The mixed powder 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 and keep it warm 2h, then raise the temperature at 1°C/min to 1050°C for 2h, then raise the temperature to 1120°C at 1°C/min and hold for 2h, and finally raise the temperature to 1240°C at 1°C/min and hold for 8h to obtain low-cost element mixed NiTi shape memory alloy (Ti-50Ni).

The obtained EP NiTi alloy has an oxygen content of 0.22wt.%, a tensile strength of 763 MPa, a tensile elongation of 19.8%, and a 8% tensile strain recovery rate of 94%.

Embodiment 8:

A kind of preparation method of the low-cost element mixed NiTi shape memory alloy by high temperature homogenization treatment of the present invention, comprises the steps:

(1) Carbonyl Ni powder and -325 purpose hydrogenated dehydrogenation titanium powder are uniformly mixed to obtain a mixed powder, wherein the Ni atom content is 50.0%;

(2) The mixed powder is sintered, and the sintering parameters are as follows: under a vacuum degree of 10 ^-4 Pa, first raise the temperature at 10°C/min to 600°C and keep it for 1 hour, then raise the temperature at 1.5°C/min to 700°C and keep it for 2.5 h, then raise the temperature at 1.5°C/min to 1050°C for 2 hours, then raise the temperature at 1.5°C/min to 1120°C for 3 hours, and finally raise the temperature to 1250°C at 1.5°C/min for 6 hours, and obtain the low-cost element mixed NiTi shape memory alloy (Ti-50Ni).

The oxygen content of the obtained EP NiTi alloy is 0.22wt.%, the tensile strength is 752MPa, the tensile elongation is 20%, and the 8% tensile strain recovery rate is 93%.

Embodiment 9:

A kind of preparation method of the low-cost element mixed NiTi shape memory alloy by high temperature homogenization treatment of the present invention, comprises the steps:

(1) Carbonyl Ni powder and -325 purpose hydrogenated dehydrogenation titanium powder are uniformly mixed to obtain a mixed powder, wherein the Ni atom content is 50.0%;

(2) The mixed powder is sintered, and the sintering parameters are as follows: under a vacuum degree of 10 ^-3 Pa, first raise the temperature at 20°C/min to 600°C and keep it for 2h, then raise the temperature at 2°C/min to 700°C and keep it for 4h , and then heated at 2°C/min to 1050°C for 4 hours, then at 2°C/min to 1120°C for 4 hours, and finally at 2°C/min to 1250°C for 6 hours to obtain a low-cost element mixed NiTi shape memory alloy (Ti-50Ni).

The obtained EP NiTi alloy has an oxygen content of 0.29wt.%, a tensile strength of 710MPa, a tensile elongation of 10.5%, and an 8% tensile strain recovery rate of 88%.

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:

Ti-50Ni follows the sintering heating process at different rates, as shown in Figure 1. Under a constant 10 ^-4 Pa high vacuum, the temperature was raised to 700°C at a heating rate of 5°C/min, and the sample deflagrated and melted, as shown in I. It is well known that the reaction of elemental Ti and Ni powder is exothermic. Starting from the dissolution of the outer oxide film of Ti at 600 ° C, three thermodynamically stable intermediate metal compounds, TiNi, Ti ₂ Ni, and TiNi ₃ , are gradually reacted, releasing heat of 67 kJ/mol respectively. , 83kJ/mol, 140kJ/mol. According to previous studies on the sintering process of EPNiTi alloys in the range of 600-1100 °C, including self-propagating sintering, one end is ignited, and the rest of the area is gradually completed in the form of combustion wave propagation to form an intermediate metal compound. The self-propagating process The reaction rate is controlled by the ambient temperature and ignition temperature, and the overall pores are large and irregular, and the surface quality is relatively rough. It is necessary to reduce the oxygen content of EP NiTi and improve the engineering performance by improving the purity of raw materials and the vacuum degree of vacuum sintering. However, the increase of the vacuum level will also make it difficult for objects to dissipate heat through conduction, so that the proportion of black body radiation heat dissipation will increase. It can be speculated that there is an unknown critical criterion for the occurrence of this deflagration reaction, which is closely related to parameters such as sample size, compact density, ambient temperature field distribution, heating rate, and vacuum degree. When the heating rate was reduced from 5 to 1 °C/min, the extreme deflagration reaction of the sample no longer occurred, as shown in II. When sample II was heated up to 1250° C. at the same heating rate (III), it was found that macroscopic macropores appeared. This is because as the temperature continues to rise, not only the original eutectic liquid phase exists, but also a new eutectic liquid phase (1118°C). Figure 2 shows the schematic diagram of the morphology of this macroporous defect and the EPMA element distribution analysis. By slicing the macropore area layer by layer, it can be found that there is an obvious transition ring area on the wire-cut surface. The EPMA mapping analysis of the interface between the ring area and the EP NiTi matrix shows that there are obvious Ti-rich fillings in the transition ring area, which is caused by the loss and filling of the original material in the macropores.

In Figure 1, the temperature of the high-temperature eutectic liquid phase reaction (1120°C, 2h) was chosen to keep warm, so that the liquid phase reaction proceeded at a relatively gentle rate, and the area of local segregation and average composition was promoted to have sufficient time to achieve homogenization. Eventually, the heterogeneous elemental powder in EP NiTi will approach the nominal content, and when Ti ₂ Ni and TiNi ₃ are gradually eliminated, the liquid phase will gradually disappear. Of course, a small amount of Ti2Ni will always be preserved due to the protection of oxygen. After continuing to heat up to 1250°C for sintering, the sample is intact without macroscopic macropores (IV).

When the EP NiTi alloy is sintered at 1250 °C, the overall element distribution, microstructure, phase transition temperature and mechanical properties will have great changes. Figure 3 is the EPMA mapping analysis chart after 1050°C low-temperature sintering and 1250°C high-temperature sintering. Judging from the final average Ti and Ni content, a2 and b2, a3 and c3 are very similar. It shows that the total amount of elements in the whole area is consistent. This point is the same as the EPMA matrix point analysis in Figure 4. Through the statistical analysis of the matrix point composition in different fields of view, the average Ti and Ni contents in Figure 5b and 5d are basically the same. However, it can be observed from Figure 3 that there are obviously more spots in a2 and a3, and the color uniformity is not as good as that of the high-temperature sintered samples. This shows that during the process from 1050 °C to 1250 °C, the sample achieved homogenization of elements in a local area with the help of the eutectic liquid phase. Since the Ti and Ni contents in different fields of view (Figure 4) do not change much, it can be speculated that the EP NiTi alloy combined with long-term heat preservation and slow temperature rise does not have a wide range of liquid phase flow, which is also to avoid macroscopic macropores. The side proof that appeared.

Figure 5 is the DSC curves of samples with three Ni contents sintered at 1050°C at low temperature and at 1250°C at high temperature. Since the phase transition temperature of NiTi alloy is extremely sensitive to the Ni content, the phase transition temperature will drop by 10-15°C for every increase of 0.1at.% Ni, so the peak width of the endothermic and exothermic peaks of DSC can actually reflect the overall composition distribution concentration. Figure 5g summarizes the peak widths of the martensitic phase transition (M) and its reverse phase transition (A). It is not difficult to see that the peak width of the 1050 °C sample is much higher than that of 1250 °C, which can indirectly indicate that the high temperature sintering homogenization In the EPNiTi alloy, the composition distribution in the microscopic area has been greatly optimized, which creates better conditions for the actual occurrence of heat-induced martensitic transformation and stress-induced martensitic transformation. The martensitic transformation temperature of the sample at 1250 °C is shown in Figure 5h. With the adjustment of Ni content, Ms decreases from 77 °C to 7 °C, and other characteristic temperature points have similar laws.

Figure 6 and Figure 7 are the SEM topography images (backscattered electron mode) after low-temperature sintering at 1050°C and high-temperature sintering at 1250°C, respectively. The sample at 1050℃ has more pores overall, but there is no concentrated Ti ₂ Ni phase. However, Ti2Ni is relatively clear in the 1250 °C sample, which is similar to the conventional pre-alloyed powder sintered NiTi alloy and the NiTi alloy in the ingot state. Of course, even after high-temperature homogenization, EPNiTi cannot be truly densified, and can only reach a relative density of about 90%, which needs further improvement in the future. Since there have been many research reports on EPNiTi alloys sintered at low temperatures, the present invention mainly develops the phase composition and mechanical properties of EPNiTi alloys sintered and homogenized at a high temperature of 1250°C. The phase compositions of the three Ni-content EPNiTi alloys were tested by XRD, as shown in Fig. 8. Due to the large difference in Ni content and its phase transition temperature, at room temperature, 50.0Ni presents a B2 matrix, while 49.5Ni and 49.0Ni present a B19' martensite state. Due to the low overall Ni content and the presence of a large amount _of liquid phase during sintering, which facilitates compositional homogenization, no spontaneously formed _Ni4Ti3 precipitates were observed.

Figure 9 is a comparison chart of tensile fracture and tensile recovery properties of EPTi-49.0/49.5/50.0Ni sintered at 1250°C. The total length of the tensile sample is 46mm, the width of the deformation zone is 3mm, and the thickness is 2mm; before the test, the sample was soaked in liquid nitrogen During treatment, each sample was unloaded and kept at 120°C for 0.5 hours; where: (a) tensile stress-strain relationship; (bd) 4% and 8% tensile strain recovery curves. (e) Literature comparison of ultimate tensile strength and strain; (f) Comparison of tensile and recovery capabilities with other powder metallurgy NiTi alloys. After high-temperature sintering, the tensile fracture strengths in Figure 10a reached 701, 725, and 763 MPa, respectively, and the elongation reached 16.54-21.97%, which is already comparable to the performance of some pre-alloyed NiTi powder SLM products (Figure 9e), which is in EPNiTi Alloy research has never been reached. Specifically, EPNiTi with three Ni contents are different from each other. The composition is different. On the one hand, the martensitic transformation temperature is adjusted in a large range, so that the phase composition is different from that when the tensile test is performed at room temperature. However, the samples were soaked in liquid nitrogen before the test, so they were all tensile loaded in the martensitic state, so the three curves were very similar. On the other hand, the content of impurity phase Ti2Ni is different. Since the Ti[Ni] antisite defects in multiple adjacent positions attract each other, the solid solubility of Ti in the NiTi phase will not be very high, which determines that the Ti-rich region in the NiTi phase region is very narrow in the Ti-Ni phase diagram . Therefore, with the increase of Ti content, the volume fraction of Ti ₂ Ni phase in NiTi alloy will increase. From the surface SEM after 8% tensile loading and unloading in Fig. 10, it can be seen that, similar to the literature, Ti ₂ Ni becomes the crack initiation site, and the crack directly passes through the Ti ₂ Ni region or propagates along the Ti ₂ Ni/NiTi interface. So the increase of Ti ₂ Ni volume fraction gradually reduces the ultimate tensile strength. At the same time, 4% and 8% tensile recovery were tested respectively, and the final recovery rate of Ti-49.0Ni after heating was relatively the worst. The data comparison of applied strain and recovery strain is shown in Fig. 9f. EPNiTi sintered by high-temperature homogenization reported in this paper exhibits tensile recovery properties close to 8% full recovery. However, due to the extensive existence of Ti ₂ Ni and its crack propagation, there is a small amount of permanent deformation after 8% loading. It can be seen from the fracture scanning electron microscope image of the tensile sample in Figure 10 that there are a large number of dimples in the EPNiTi alloys with different compositions listed, which are ductile fractures, which is consistent with high tensile plasticity.

In general, the present invention simultaneously solves the problem of deflagration reaction in the low temperature region (700°C) and liquid phase loss in the high temperature region (1250°C) through process optimization, and uses the inevitable process liquid phase and high temperature sintering to promote homogenization of elements. Compared with the prior art, the present invention sinters the EP NiTi alloy with a vacuum system of about 10 ^-4 Pa in the whole process, reduces the oxygen content of the EP NiTi alloy to < 0.22wt.%, and greatly improves the engineering performance of EP NiTi, making EP NiTi The strength of the alloy is increased to more than 700MPa, the elongation is higher than 15%, and the tensile recovery rate of 8% is higher than 80%. Through detailed process control, an EP NiTi alloy with a relative density of more than 90% is obtained, the tensile strength is increased to more than 700MPa, the elongation is higher than 20%, the tensile recovery rate of 8% is higher than 90%, and the microstructure is uniform; The cost hydrogenated dehydrogenated titanium powder is used as a raw material to prepare NiTi alloys, and there is no other scheme that can make the related properties of NiTi alloys exceed the test results stated in the present invention.

Claims (3)

1. A method for preparing a low-cost element mixed NiTi shape memory alloy through high-temperature homogenization treatment, which is characterized by comprising the following steps:

(1) Uniformly mixing Ni-containing powder and Ti-containing powder to obtain mixed powder; the Ni-containing powder is carbonyl Ni powder, and the Ti-containing powder is hydrogenated and dehydrogenated Ti powder; the granularity of the hydrogenated and dehydrogenated Ti powder is minus 325 meshes to minus 200 meshes; the Ni atom content in the mixed powder is 49.0-51.0%;

(2) Sintering the mixed powder, wherein the sintering parameters are as follows: at a vacuum level of 10 ^-4 Under Pa, the temperature is firstly increased to 600 ℃ at 5-10 ℃/min for 0.5-1h, then is increased to 700 ℃ at 1-1.5 ℃/min for 2-2.5h, is increased to 1050 ℃ at 1-1.5 ℃/min for 2h, is increased to 1120 ℃ at 1-1.5 ℃/min for 2-3h, and is increased to 1240-1250 ℃ at 1-1.5 ℃/min for 6-8h.

2. The method according to claim 1, wherein the hydrogenated and dehydrogenated Ti powder has a particle size of-325 mesh.

3. The method according to claim 1, wherein the Ni atom content of the powder mixture is 50.0%.