CN115386755A - Preparation method of low-cost element mixed NiTi shape memory alloy through high-temperature homogenization treatment - Google Patents

Preparation method of low-cost element mixed NiTi shape memory alloy through high-temperature homogenization treatment Download PDF

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CN115386755A
CN115386755A CN202210707740.3A CN202210707740A CN115386755A CN 115386755 A CN115386755 A CN 115386755A CN 202210707740 A CN202210707740 A CN 202210707740A CN 115386755 A CN115386755 A CN 115386755A
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CN115386755B (en
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李益民
李东阳
舒畅
何昊
王暾
朱本银
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Central South University
Second Xiangya Hospital of Central South University
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Second Xiangya Hospital of Central South University
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    • C22C1/00—Making non-ferrous alloys
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    • C22C1/0433—Nickel- or cobalt-based alloys
    • C—CHEMISTRY; METALLURGY
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    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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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 to obtain mixed powder, and sintering the mixed powder, wherein the sintering parameters are as follows: under high vacuum degree, heating to 600 ℃ at the temperature of 5-20 ℃/min, preserving heat for 0.5-2h, heating to 700 ℃ at the temperature of 1-2 ℃/min, preserving heat for 2-4h, heating to 1050 ℃ at the temperature of 1-2 ℃/min, preserving heat for 1-4h, heating to 1120 ℃ at the temperature of 1-2 ℃/min, preserving heat for 2-4h, heating to 1220-1250 ℃ at the temperature of 1-2 ℃/min, and preserving heat for 6-10h. The invention reduces the oxygen content of the EPNiTi alloy to less than 0.22 wt% through high vacuum sintering, overcomes the problems of deflagration reaction and liquid phase loss, greatly improves the mechanical property of the EPNiTi and improves the tensile elongation to more than 20%.

Description

Preparation method of low-cost element mixed NiTi shape memory alloy through high-temperature homogenization 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 NiTi shape memory alloy through high-temperature homogenization treatment.
Background
NiTi alloys are the most interesting and potential shape memory materials today. Because of flexible component control and low raw material cost, an element mixing method (EP) is always an important preparation method of powder metallurgy NiTi and porous NiTi alloy. However, due to the difficulties in EP production, the studies of EP NiTi have little or no tensile properties (elongation less than 5%). The performance of the product is far away from that of a pre-alloyed NiTi powder (PP) product and an ingot metallurgy NiTi product (the elongation is higher than 15 percent). Only with sufficient tensile properties can the real engineering application requirements be met.
The difficulty in preparing low cost EP NiTi alloys is limited by a number of factors. First, the impurity content of the raw material obtained by mixing HDH Ti powder is higher than that of the prealloyed powder. Oxygen impurity is increased to enable Ti 2 Ni impurity phase in more stable form Ti 4 Ni 2 Presence of O, changing the martensitic transformation temperature, ti 4 Ni 2 O is also an important crack source during bearing; meanwhile, it is reported that oxygen is an important cause of amorphization of grain boundaries in NiTi alloys, possibly being a negative factor causing mechanical embrittlement. Therefore, reducing the impurity content of the raw materials and reducing the oxygen increase in the sintering process are the main control strategies. Ultra-high vacuum sintering is the most effective scheme for controlling oxygen increase in the process. However, the EP NiTi sintering is accompanied by a large amount of reaction heat release, and under a certain background temperature, the ignition part can be propagated through a combustion wave to complete the sintering (self-propagating process). If the reaction is too severe, the core temperature may exceed the melting point of the NiTi alloy (1310 deg.C). With the increase of the magnitude order of vacuum degree, the efficiency of radiation heat dissipation of the sintered blank can be obviously reducedThe intense heat release melts and collapses the body. This detail has not been reported in any detail.
The difficulties in the preparation of EP NiTi are on the other hand due to the complex mesophase inversion sequence. EP NiTi alloy in sintering due to non-uniformity of particle distribution, and NiTi, ti 2 Ni、TiNi 3 The three stable intermediates are similar in energy, so that all the intermediate phases have the opportunity to be formed in large quantities at 720-920 ℃. Thus, the eutectic reaction at 942 ℃ of beta-Ti (Ni) + Ti 2 Eutectic reaction of Ni → L and 1118 ℃ TiNi + TiNi 3 Either → L may be triggered to form a non-homogeneous liquid phase at indefinite times and at indefinite points. Meanwhile, due to the large mutual diffusion difference, the speed of Ni diffusing into Ti is much higher than that of Ti diffusing into Ni, and Kirkendall pores are easy to form. The heterogeneous eutectic liquid phase may run off and leave the green body, or may be absorbed and gathered by the capillary action of Kirkendall pores, so that larger macro pores are locally left. Only increasing the sintering temperature results in accelerated loss of the liquid phase and collapse of the green body. Therefore, if the liquid phase cannot be controlled effectively, densification cannot be achieved, and the surface quality and dimensional accuracy of the sample cannot be guaranteed. As such, in the past studies, no directly sintered densified EP NiTi alloy has been reported, and the EP NiTi alloy is often studied as porous NiTi.
To date, no research report comprehensively considers all the factors, and the strength of the EP NiTi alloy is improved to be more than 700MPa by a vacuum sintering method, the elongation is higher than 15%, and the 8% tensile recovery is higher than 80%.
Disclosure of Invention
The invention aims to solve the technical problem of providing a new material preparation idea aiming at the current situation that all EP NiTi alloys have extremely poor performance (the tensile elongation is lower than 5 percent) and solving the problems of low-temperature deflagration reaction and high-temperature liquid phase loss.
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 NiTi shape memory alloy through high-temperature homogenization treatment comprises the following steps:
(1) Uniformly mixing Ni-containing powder and Ti-containing powder to obtain mixed powder;
(2) Sintering the mixed powder, wherein the sintering parameters are as follows: at a vacuum degree of 10 -3 ～10 -5 At 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, then 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.
In the above production method, preferably, the Ni-containing powder is Ni carbonyl powder, and the Ti-containing powder is Ti hydride dehydrogenated powder.
Preferably, the hydrogenated and dehydrogenated titanium powder has a particle size of-325 mesh to-200 mesh, and more preferably-325 mesh.
Preferably, the Ni atom content in the mixed powder material is 49.0 to 51.0%, and more preferably 50.0%.
Preferably, the sintering degree of vacuum is 10 -4 Pa。
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.
Compared with the prior art, the invention has the beneficial effects that:
1. the invention adopts a whole course 10 -4 Carrying out EP NiTi alloy sintering by a vacuum system about Pa to reduce the content of the EP NiTi alloy to be about<0.22wt.％。
2. Through process optimization, the problem of deflagration reaction in a low-temperature region (700 ℃) and liquid phase loss in a high-temperature region (1250 ℃) is solved, the inevitable process liquid phase and high-temperature sintering are utilized to promote element homogenization, and compared with the prior art, the engineering performance of the EP NiTi is greatly improved, so that the strength of the EP NiTi alloy is improved to more than 700MPa, the elongation is higher than 15%, and the 8% tensile recovery rate is higher than 80%.
3. Through detailed process control, the EP NiTi alloy with the relative density of over 90 percent is obtained, the tensile strength is improved to over 700MPa, the elongation is higher than 20 percent, the 8 percent tensile recovery rate is higher than 90 percent, and the microstructure is uniform and consistent; 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.
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 described 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 schematic representation of a heating process and a sample of Ti-50.0Ni in a sintered state; wherein (I) when heated to 700 ℃ at a rate of 5 ℃/min, a deflagration reaction occurs to melt the sample; (II) raising the temperature to 1050 ℃ at a rate of 1 ℃/min and keeping the sample intact; (III) directly heating the sample II to 1250 ℃ and forming a millimeter-sized macroscopic hole; (IV) sample II was held at 1120 deg.C for 2h during the sustained temperature ramp without the occurrence of macroscopic defects.
FIG. 2 is a micro-topography and EPMA map (distribution of Ni and Ti elements) of the substrate and transition ring distribution surfaces; wherein a distinct transition ring is present in the cross-sectional area of the macroscopic pores.
FIG. 3 is an EPMA surface scan of the distribution of elements; wherein (a 1-a 3) the sample is maintained at 1050 ℃ for 3 hours; (b 1-b 3) holding the sample at 1250 ℃ for 6 hours; the overall average element content was similar, but the samples at 1250 ℃ were significantly more uniform.
FIG. 4 is an EPMA matrix dot element content analysis chart; wherein (a-b) the sample is maintained at 1050 ℃ for 3 hours; (c-d) holding the sample at 1250 ℃ for 6 hours; for each field of view 9 points were taken to calculate the mean, and three different fields of view were selected for each sample.
FIG. 5 is a DCS graph with peak width and phase transition temperature variation; wherein: (a-c) a sample sintered at 1050 ℃; (d-f) a sample sintered at 1250 ℃; (g) a peak width in the DSC curve reflecting the element homogeneity; (h) 1250 ℃ the transition temperature of the sintered sample.
FIG. 6 is a scanning electron micrograph of a 1050 ℃ sintered sample; 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 1250 ℃ sintered sample; 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 pattern of a high temperature homogenizing sintered EP NiTi alloy at 1250 ℃ at room temperature.
FIG. 9 is a graph comparing tensile properties to properties, with a tensile sample having a total length of 46mm, a deformation zone width of 3mm and a thickness of 2mm; wherein: (ii) (a) a tensile stress-strain relationship; (b-d) tensile strain recovery curves of 4% and 8%; (e) literature comparison of ultimate tensile strength and strain; (f) Compared with the tensile and recovery capability of other powder metallurgy nickel-titanium alloys.
FIG. 10 is a scanning electron micrograph of a fracture of the tensile specimen of FIG. 9; wherein: (a) 1-3 )Ti-49.0Ni，(b 1-3 )Ti-49.5Ni，(c 1-3 ) Ti-50.0Ni。
Detailed Description
In order to facilitate an understanding of the invention, reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings, and the scope of the invention is not limited to the following specific embodiments.
Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. 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 indicated, 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:
the invention relates to a preparation method of a low-cost element mixed NiTi shape memory alloy through high-temperature homogenization treatment, which comprises the following steps:
(1) Uniformly mixing carbonyl Ni powder and-325-mesh hydrogenated dehydrotitanium powder to obtain mixed powder, wherein the Ni atom content is 49.0%;
(2) Sintering the mixed powder, wherein the sintering parameters are as follows: at a vacuum degree of 10 -4 And (2) under Pa, firstly heating to 600 ℃ at the speed of 5 ℃/min, preserving heat for 0.5h, then heating to 700 ℃ at the speed of 1 ℃/min, preserving heat for 2h, then heating to 1050 ℃ at the speed of 1 ℃/min, preserving heat for 2h, then heating to 1120 ℃ at the speed of 1 ℃/min, preserving heat for 2h, and finally heating to 1240 ℃ at the speed of 1 ℃/min, preserving heat for 8h, thus obtaining the low-cost element mixed NiTi shape memory alloy (Ti-49 Ni).
The resulting EP NiTi alloy had an oxygen content of 0.22wt.%, a tensile strength of 710MPa, a tensile elongation of 16.5%, and an 8% tensile strain recovery of 85%.
Example 2:
the invention relates to a preparation method of a low-cost element mixed NiTi shape memory alloy through high-temperature homogenization treatment, which comprises the following steps:
(1) Uniformly mixing carbonyl Ni powder and-325-mesh hydrogenated dehydrotitanium powder to obtain mixed powder, wherein the Ni atom content is 49.0%;
(2) Sintering the mixed powder, wherein the sintering parameters are as follows: at a vacuum degree of 10 -4 And (2) under Pa, firstly heating to 600 ℃ at a speed of 10 ℃/min, preserving heat for 1h, then heating to 700 ℃ at a speed of 1.5 ℃/min, preserving heat for 2.5h, then heating to 1050 ℃ at a speed of 1.5 ℃/min, preserving heat for 2h, heating to 1120 ℃ at a speed of 1.5 ℃/min, preserving heat for 3h, and finally heating to 1250 ℃ at a speed of 1.5 ℃/min, preserving heat for 6h to obtain the low-cost element mixed NiTi shape memory alloy (Ti-49 Ni).
The obtained EP NiTi alloy had an oxygen content of 0.21wt.%, a tensile strength of 699MPa, a tensile elongation of 17%, and an 8% tensile strain recovery of 91%.
Example 3:
the invention relates to a preparation method of a low-cost element mixed NiTi shape memory alloy through high-temperature homogenization treatment, which comprises the following steps:
(1) Uniformly mixing carbonyl Ni powder and-325-mesh hydrogenated dehydrotitanium powder to obtain mixed powder, wherein the Ni atom content is 49.0%;
(2) Will be described inSintering the mixed powder, wherein the sintering parameters are as follows: at a vacuum degree of 10 -3 And (2) under Pa, firstly heating to 600 ℃ at the speed of 20 ℃/min, preserving heat for 2h, then heating to 700 ℃ at the speed of 2 ℃/min, preserving heat for 4h, then heating to 1050 ℃ at the speed of 2 ℃/min, preserving heat for 4h, heating to 1120 ℃ at the speed of 2 ℃/min, preserving heat for 6h, and finally heating to 1250 ℃ at the speed of 2 ℃/min, and obtaining the low-cost element mixed NiTi shape memory alloy (Ti-49 Ni).
The obtained EP NiTi alloy has the oxygen content of 0.30 percent, the tensile strength of 650MPa, the tensile elongation of 12 percent and the 8 percent tensile strain recovery of 75 percent. Compared with examples 1 and 2, the oxygen content is obviously increased due to the reduction of vacuum level, the performance is reduced, but the performance is still far higher than the data reported at present.
Example 4:
the invention relates to a preparation method of a low-cost element mixed NiTi shape memory alloy through high-temperature homogenization treatment, which comprises the following steps:
(1) Uniformly mixing carbonyl Ni powder and hydrogenated dehydrogenated titanium powder of-325 meshes to obtain mixed powder, wherein the Ni atom content is 49.5%;
(2) Sintering the mixed powder, wherein the sintering parameters are as follows: at a vacuum degree of 10 -4 And (2) under Pa, firstly heating to 600 ℃ at the speed of 5 ℃/min, preserving heat for 0.5h, then heating to 700 ℃ at the speed of 1 ℃/min, preserving heat for 2h, then heating to 1050 ℃ at the speed of 1 ℃/min, preserving heat for 2h, then heating to 1120 ℃ at the speed of 1 ℃/min, preserving heat for 2h, and finally heating to 1240 ℃ at the speed of 1 ℃/min, preserving heat for 8h, thus obtaining the low-cost element mixed NiTi shape memory alloy (Ti-49.5 Ni).
The obtained EP NiTi alloy has the oxygen content of 0.22wt.%, the tensile strength of 740MPa, the tensile elongation of 21 percent and the 8 percent tensile strain recovery of 95 percent.
Example 5:
the invention relates to a preparation method of a low-cost element mixed NiTi shape memory alloy through high-temperature homogenization treatment, which comprises the following steps:
(1) Uniformly mixing carbonyl Ni powder and hydrogenated dehydrogenated titanium powder of-325 meshes to obtain mixed powder, wherein the Ni atom content is 49.5%;
(2) Subjecting the mixed powder toSintering, wherein the sintering parameters are as follows: at a vacuum degree of 10 -4 And (2) under Pa, firstly heating to 600 ℃ at a speed of 10 ℃/min, preserving heat for 1h, then heating to 700 ℃ at a speed of 1.5 ℃/min, preserving heat for 2.5h, then heating to 1050 ℃ at a speed of 1.5 ℃/min, preserving heat for 2h, heating to 1120 ℃ at a speed of 1.5 ℃/min, preserving heat for 3h, and finally heating to 1250 ℃ at a speed of 1.5 ℃/min, preserving heat for 6h to obtain the low-cost element mixed NiTi shape memory alloy (Ti-49.5 Ni).
The obtained EP NiTi alloy had an oxygen content of 0.23wt.%, a tensile strength of 720MPa, a tensile elongation of 20%, and a recovery of 8% tensile strain of 96%.
Example 6:
the invention relates to a preparation method of a low-cost element mixed NiTi shape memory alloy through high-temperature homogenization treatment, which comprises the following steps:
(1) Uniformly mixing carbonyl Ni powder and-325-mesh hydrogenated dehydrotitanium powder to obtain mixed powder, wherein the Ni atom content is 49.5%;
(2) Sintering the mixed powder, wherein the sintering parameters are as follows: at a vacuum degree of 10 -3 And (2) under Pa, firstly heating to 600 ℃ at the speed of 20 ℃/min, preserving heat for 2h, then heating to 700 ℃ at the speed of 2 ℃/min, preserving heat for 4h, then heating to 1050 ℃ at the speed of 2 ℃/min, preserving heat for 4h, heating to 1120 ℃ at the speed of 2 ℃/min, preserving heat for 6h, and finally heating to 1250 ℃ at the speed of 2 ℃/min, thus obtaining the low-cost element mixed NiTi shape memory alloy (Ti-49.5 Ni).
The resulting EP NiTi alloy had an oxygen content of 0.29wt.%, tensile strength of 677MPa, tensile elongation of 13.5%, and 8% tensile strain recovery of 81%. Compared with examples 4 and 5, the oxygen content is obviously increased and the performance is reduced due to the reduction of the vacuum level, but still far higher than the data reported at present.
Example 7:
the invention relates to a preparation method of a low-cost element mixed NiTi shape memory alloy through high-temperature homogenization treatment, which comprises the following steps:
(1) Uniformly mixing carbonyl Ni powder and-325-mesh hydrogenated dehydrotitanium powder to obtain mixed powder, wherein the Ni atom content is 50.0%;
(2) Burning the mixed powderThe sintering parameters were as follows: at a vacuum degree of 10 -4 And (2) under Pa, firstly heating to 600 ℃ at the speed of 5 ℃/min, preserving heat for 0.5h, then heating to 700 ℃ at the speed of 1 ℃/min, preserving heat for 2h, then heating to 1050 ℃ at the speed of 1 ℃/min, preserving heat for 2h, then heating to 1120 ℃ at the speed of 1 ℃/min, preserving heat for 2h, and finally heating to 1240 ℃ at the speed of 1 ℃/min, preserving heat for 8h, thus obtaining the low-cost element mixed NiTi shape memory alloy (Ti-50 Ni).
The resulting EP NiTi alloy had an oxygen content of 0.22wt.%, a tensile strength of 763MPa, a tensile elongation of 19.8%, and an 8% tensile strain recovery of 94%.
Example 8:
the invention relates to a preparation method of a low-cost element mixed NiTi shape memory alloy through high-temperature homogenization treatment, which comprises the following steps:
(1) Uniformly mixing carbonyl Ni powder and-325-mesh hydrogenated dehydrotitanium powder to obtain mixed powder, wherein the Ni atom content is 50.0%;
(2) Sintering the mixed powder, wherein the sintering parameters are as follows: at a vacuum degree of 10 -4 And (2) under Pa, firstly heating to 600 ℃ at a speed of 10 ℃/min, preserving heat for 1h, then heating to 700 ℃ at a speed of 1.5 ℃/min, preserving heat for 2.5h, then heating to 1050 ℃ at a speed of 1.5 ℃/min, preserving heat for 2h, heating to 1120 ℃ at a speed of 1.5 ℃/min, preserving heat for 3h, and finally heating to 1250 ℃ at a speed of 1.5 ℃/min, preserving heat for 6h to obtain the low-cost element mixed NiTi shape memory alloy (Ti-50 Ni).
The obtained EP NiTi alloy has the oxygen content of 0.22wt.%, the tensile strength of 752MPa, the tensile elongation of 20 percent and the recovery rate of 8 percent tensile strain of 93 percent.
Example 9:
the invention relates to a preparation method of a low-cost element mixed NiTi shape memory alloy through high-temperature homogenization treatment, which comprises the following steps:
(1) Uniformly mixing carbonyl Ni powder and hydrogenated dehydrogenated titanium powder of-325 meshes to obtain mixed powder, wherein the Ni atom content is 50.0%;
(2) Sintering the mixed powder, wherein the sintering parameters are as follows: at a vacuum degree of 10 -3 At Pa, the temperature is raised to 600 ℃ at 20 ℃/min and kept for 2h, and then the temperature is raised to 700 ℃ at 2 ℃/minAnd (3) preserving heat for 4h, heating to 1050 ℃ at the speed of 2 ℃/min, preserving heat for 4h, heating to 1120 ℃ at the speed of 2 ℃/min, preserving heat for 4h, and finally heating to 1250 ℃ at the speed of 2 ℃/min, preserving heat for 6h, thus obtaining the low-cost element mixed NiTi shape memory alloy (Ti-50 Ni).
The resulting EP NiTi alloy had an oxygen content of 0.29wt.%, a tensile strength of 710MPa, a tensile elongation of 10.5%, and a 8% tensile strain recovery of 88%.
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 Ti-50Ni sintering temperature rise process along with different rates is shown in the attached figure 1. At constant 10 -4 And (4) under Pa high vacuum, heating to 700 ℃ at the heating rate of 5 ℃/min, and melting the sample by deflagration, wherein the deflagration is shown as I. The reaction of simple Ti with Ni powder to release heat is well known, and the Ti gradually reacts to form TiNi and Ti from the dissolution of the outer oxide film of Ti at 600 DEG C 2 Ni、TiNi 3 Three thermodynamically stable intermediate metal compounds release heat of 67kJ/mol, 83kJ/mol and 140kJ/mol. According to the previous research on the sintering process of the EP NiTi alloy at the temperature of 600-1100 ℃, the method comprises the steps of self-propagating sintering, igniting one end, enabling the rest area to gradually finish the reaction of Ni and Ti to generate an intermediate metal compound in a combustion wave propagation mode, controlling the reaction rate in the self-propagating process through the ambient temperature and the ignition temperature, and controlling the whole pore space to be large and irregular and the surface quality to be rough. The essential process for reducing the oxygen content of the EP NiTi and improving the engineering performance is realized by improving the purity of the raw materials and the vacuum degree of vacuum sintering. However, the increased vacuum level also forces the object to have difficulty dissipating heat by conduction, resulting in an increased proportion of black body radiation. It can be speculated that an unknown critical criterion exists for the occurrence of the deflagration reaction, and the unknown critical criterion is closely related to parameters such as the size of a sample, the compact density, the environmental temperature field distribution, the heating rate, the vacuum degree and the like. After the ramp rate decreased from 5 to 1 deg.C/min, the sample no longer exhibited this extreme deflagration reaction, as shown in II. When the temperature of sample II was raised to 1250 ℃ at the same temperature raising rate (III), macro-macropores were observed. This is because, as the temperature continues to rise, not only the original eutectic liquid phase but also the eutectic liquid phase will remainA new eutectic liquid phase is formed (1118 ℃). Figure 2 shows a schematic of the morphology of this macro-porous defect along with the EPMA elemental distribution analysis. Slicing the large-hole area layer by layer, and finding that the obvious transition ring area exists on the linear cutting surface. EPMA mapping analysis is carried out on the interface accessory of the ring region and the EP NiTi substrate, and obvious Ti-rich filler exists in the transition ring region, which is caused by the loss and filling of the original material of the macropore.
In FIG. 1, the temperature is selected to be maintained at about the high temperature eutectic liquid phase reaction temperature (1120 ℃ C., 2 h) so that the liquid phase reaction proceeds at a relatively gentle rate and the region promoting the local segregation of the average component has a sufficient time to achieve homogenization. Finally, the heterogeneous elemental powders in EP NiTi will approach the nominal content when Ti is added 2 Ni and TiNi 3 Is gradually eliminated and the liquid phase is gradually disappeared. Of course, a small amount of Ti2Ni will always be preserved due to the protection of oxygen. After the temperature is continuously increased to 1250 ℃ and sintering is carried out, the sample is complete and has no macro macropore (IV).
After the EP NiTi alloy is sintered at 1250 ℃, the integral element distribution form, the structure appearance, the phase transition temperature and the mechanical property are greatly changed. FIG. 3 is an EPMA mapping analysis chart after 1050 ℃ low-temperature sintering and 1250 ℃ high-temperature sintering. From the final average Ti and Ni contents, a2 and b2, and a3 and c3 are very similar. Indicating consistency in the total amount of elements across the entire area. This point is the same as the EPMA matrix point analysis in fig. 4. The average Ti and Ni content in fig. 5b and 5d were substantially consistent by statistical analysis of the dot composition of the matrix over different fields of view. However, it can be observed from fig. 3 that a2 and a3 are more mottled and are not as uniform in color as the high temperature sintered sample. This shows that from 1050 ℃ to 1250 ℃, the sample achieves local elemental homogenization with the help of the eutectic liquid phase. Since the statistical Ti and Ni contents in different fields of view (FIG. 4) also do not change much, it can be speculated that the EP NiTi alloy combining long-time heat preservation and slow temperature rise has no large-scale liquid phase flow, which is also a side evidence for avoiding macro macropores.
FIG. 5 is a DSC plot of three Ni content samples sintered at 1050 ℃ low temperature and 1250 ℃ high temperature. Since the phase transition temperature of the NiTi alloy is extremely sensitive to the Ni content, the phase transition temperature decreases by 10 to 15 ℃ every time 0.1at.% Ni is added, so that the peak width of the endothermic and exothermic peaks of the DSC is wide, and the overall component distribution concentration can be actually reflected. Fig. 5g summarizes the peak widths of the martensitic transformation (M) and the reverse transformation (a), and it can be seen that the peak width of the 1050 ℃ sample is much higher than 1250 ℃, which indirectly shows that the compositional distribution on the microscopic region of the high-temperature sintered homogenized EPNiTi alloy is greatly optimized, which creates better conditions for the actual occurrence of the heat-induced martensitic transformation and the stress-induced martensitic transformation. The martensitic transformation temperature of the 1250 ℃ sample is shown in FIG. 5h, and Ms is reduced from 77 ℃ to 7 ℃ along with the adjustment of Ni content, and other characteristic temperature points have similar rules.
Fig. 6 and 7 are SEM topography images (back scattering electron mode) after 1050 ℃ low temperature sintering and 1250 ℃ high temperature sintering, respectively. 1050 ℃ sample has more overall pores but does not have Ti with concentrated distribution 2 A Ni phase. The Ti2Ni is clearer in a sample at 1250 ℃, and is similar to the NiTi alloy sintered by conventional prealloy powder and the NiTi alloy in an ingot casting state. Of course, even after homogenization at high temperature, EPNiTi cannot be densified really, and only can reach about 90% of relative density, and further subsequent improvement is needed. Because EPNiTi alloy sintered at low temperature has more research reports, the invention mainly develops the phase composition and the mechanical property of EPNiTi alloy sintered and homogenized at 1250 ℃. The phase composition of the three Ni content EPNiTi alloys was tested by XRD as shown in figure 8. Due to the large difference between the Ni content and its transformation temperature, at room temperature, 50.0Ni exhibited a B2 matrix, while 49.5Ni and 49.0Ni exhibited a B19' martensitic state. Since the low overall Ni content and the large amount of liquid phase during sintering promote compositional homogenization, no spontaneous formation of Ni was observed 4 Ti 3 And (4) precipitating.
FIG. 9 is a graph comparing the tensile rupture and tensile recovery performance of sintered EPTi-49.0/49.5/50.0Ni at 1250 ℃ for a tensile sample having a total length of 46mm, a deformation zone width of 3mm and a thickness of 2mm; before the test, the samples were immersed in liquid nitrogen and each sample was unloaded and kept at 120 ℃ for 0.5 hour; wherein: (a) Tensile stress-strain relationship(ii) a (b-d) tensile strain recovery curves of 4% and 8%. (e) literature comparison of final tensile strength and strain; (f) Compared with the tensile and recovery capability of other powder metallurgy nickel-titanium alloys. After high-temperature sintering, the tensile fracture strength in fig. 10a reaches 701, 725 and 763MPa respectively, the elongation reaches 16.54-21.97%, and the product performance of some prealloyed NiTi powder SLM (fig. 9 e) can be comparable, which is never reached in the research of EPNiTi alloy. Specifically, EPNiTi with three Ni contents are different from each other. The components are different, and on one hand, the martensite phase transformation temperature is adjusted in a large range, so that the phase composition is different from that of the phase composition when the tensile test is carried out at room temperature. However, the samples are soaked in liquid nitrogen for treatment before testing, so that the samples are all subjected to tensile loading in a martensite state, and therefore the three curves are very similar. On the other hand, the content of the impurity phase Ti2Ni is different. Due to Ti [ Ni ] in a plurality of adjacent positions]The inversion defects attract each other, so the solid solubility of Ti in the NiTi phase is not very high, which determines that the Ti-rich region of the NiTi phase region is very narrow in the Ti-Ni phase diagram. Therefore, as the Ti content increases, the Ti content in the NiTi alloy 2 The Ni phase volume fraction will increase. From the SEM of the surface after 8% tensile loading and unloading in FIG. 10, similar to the literature, ti 2 Ni becomes the site of crack initiation, the crack passing directly through Ti 2 Ni region or along Ti 2 The Ni/NiTi interface spreads. Therefore Ti 2 Increasing the volume fraction of Ni gradually decreases the final tensile strength. Meanwhile, 4% and 8% tensile recovery, respectively, were tested, with the final recovery after heating of Ti-49.0Ni being the worst. The data pair of applied strain and recovered strain is shown in fig. 9 f. The high temperature isosintered EPNiTi reported herein exhibits tensile recovery properties approaching 8% full recovery. But due to Ti 2 Ni is widely present and its cracks propagate, with a small amount of permanent set after 8% loading. In the scanning electron microscope image of the fracture of the tensile specimen in fig. 10, it can be seen that the listed EPNiTi alloys with different compositions all have a large number of dimples, which are ductile fractures, and are consistent with higher tensile plasticity.
In general, the invention solves the problems of deflagration reaction in low temperature zone (700 ℃) and liquid phase loss in high temperature zone (1250 ℃) simultaneously through process optimization, and the utilization is inevitableThe process liquid phase and the high temperature sintering of (2) promote element homogenization. Compared with the prior art, the invention adopts the whole process 10 -4 Carrying out EP NiTi alloy sintering by a vacuum system with about Pa, and reducing the oxygen content of the EP NiTi alloy to the range of<0.22wt.%, and greatly improves the engineering performance of the EP NiTi, so that the strength of the EP NiTi alloy is improved to more than 700MPa, the elongation is higher than 15%, and the 8% tensile recovery rate is higher than 80%. Through detailed process control, the EP NiTi alloy with the relative density of over 90 percent is obtained, the tensile strength is improved to over 700MPa, the elongation is higher than 20 percent, the 8 percent tensile recovery rate is higher than 90 percent, and the microstructure is uniform and consistent; 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.

Claims (8)

1. A preparation method of low-cost element mixed NiTi shape memory alloy through high-temperature homogenization treatment is characterized by comprising the following steps:
(1) Uniformly mixing Ni-containing powder and Ti-containing powder to obtain mixed powder;
(2) Sintering the mixed powder, wherein the sintering parameters are as follows: at a vacuum degree of 10 -3 ～10 -5 At 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, then 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.
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 production method according to claim 2, wherein the hydrogenated dehydrogenated Ti powder has a particle size of-325 mesh.
5. The production method according to claim 1, wherein the Ni atom content in the mixed powder is 49.0 to 51.0%.
6. The production method according to claim 5, wherein the Ni atom content in the mixed powder is 50.0%.
7. The method according to claim 1, wherein the degree of vacuum of the sintering is 10 -4 Pa。
8. The production method according to any one of claims 1 to 7, 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.
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