A method for improving superelasticity of NiTiHf high temperature shape memory alloy

Abstract

The present invention relates to the technical field of functional property modification of high-temperature shape memory alloys, and in particular to a method for improving the superelasticity of NiTiHf high-temperature shape memory alloys. The method specifically comprises the following steps: sequentially subjecting NiTiHf high-temperature shape memory alloy ingots to homogenization treatment, rolling treatment, and electric pulse treatment. The present invention couples rolling with electric pulses and precisely controls parameters such as temperature, time, and frequency, thereby successfully achieving a NiTiHf high-temperature shape memory alloy with significantly improved superelastic properties without the need for severe plastic deformation treatment and long-term medium and high temperature heat treatment. Compared with the cast NiTiHf high-temperature shape memory alloy, in addition to significantly improving the superelastic properties, the elongation and tensile strength of the alloy are also greatly improved, wherein the recoverable strain increase is more than 200%, the elongation is increased by nearly 3 times, and the tensile strength is increased by more than 10%.

Description

Technical Field

[0001] The invention relates to the technical field of functional property modification of high-temperature shape memory alloys, and in particular to a method for improving the superelasticity of NiTiHf high-temperature shape memory alloys. Background Art

[0002] NiTiHf high temperature shape memory alloy is a smart material with rich martensitic phase transformation characteristics. Compared with alloys such as NiTiPd, NiTiPt and NiTiZr, it has a higher phase transformation temperature, lower preparation cost and relatively excellent functional properties, making it the preferred material for the development of NiTi-based high temperature shape memory alloys. The engineering application of shape memory alloys is closely related to their phase transformation temperature. A higher phase transformation temperature allows them to serve in higher temperature environments. The starting temperature of the martensitic phase transformation of NiTiHf high temperature shape memory alloy M s Greater than 100 ℃, some even exceed 500 ℃, which has preliminarily met the temperature requirements for engineering applications. However, in the process of promotion, it still faces two key problems: poor superelastic performance and high intrinsic brittleness, which seriously restrict the widespread application and further development of NiTiHf high-temperature shape memory alloys.

[0003] In order to solve the problems of poor superelasticity and high intrinsic brittleness of NiTiHf high temperature shape memory alloy, plastic deformation and heat treatment are currently mainly used to improve its functional characteristics and mechanical properties. However, the existing plastic deformation methods and methods are limited in deformation due to the high brittleness of NiTiHf high temperature shape memory alloy. Therefore, the obtained NiTiHf high temperature shape memory alloy has coarse grains, uneven microstructure and extremely poor plasticity; and if heat treatment technology is used, there are disadvantages such as no obvious improvement in plasticity, long heat treatment time, easy oxidation, and poor superelasticity. Summary of the invention

[0004] In view of this, the present invention provides a method for improving the superelasticity of NiTiHf high-temperature shape memory alloy. The method adopts a processing technology of rolling and electric pulse coupling, and can obtain a NiTiHf high-temperature shape memory alloy with significantly improved superelastic performance without severe plastic deformation treatment and long-term medium and high temperature heat treatment.

[0005] To solve the above technical problems, the first aspect of the present invention provides a method for improving the superelasticity of NiTiHf high temperature shape memory alloy, which comprises sequentially subjecting the NiTiHf high temperature shape memory alloy ingot to homogenization treatment, rolling treatment and electric pulse treatment.

[0006] The "rolling + electric pulse" coupling processing technology provided in the first aspect of the present invention ensures that the NiTiHf high temperature shape memory alloy does not undergo unstable cracking after plastic deformation, and uses the "targeting effect" of pulse current on defects such as dislocations and vacancies introduced by deformation to form local stress relaxation and uneven temperature fields in the NiTiHf high temperature shape memory alloy matrix, thereby promoting the microstructure inside the NiTiHf high temperature shape memory alloy to be more uniform and stable. At the same time, considering the "electroplastic effect" of the pulse current, when the pulse current passes through the NiTiHf high temperature shape memory alloy, a large number of directional drifting free electrons (electron wind) will be generated to form a drift electron group. After the drift electron group hits the dislocation, it generates an electronic wind force equivalent to an external thrust, thereby promoting the movement of the dislocation on its slip plane, thereby synergistically improving the superelasticity and plasticity of the NiTiHf high temperature shape memory alloy, and the tensile strength is also greatly improved. Compared with conventional cast NiTiHf high-temperature shape memory alloy, the recoverable strain of the NiTiHf high-temperature shape memory alloy obtained by the present invention increases by more than 200%, the elongation increases by nearly 3 times, and the tensile strength increases by more than 10%.

[0007] In combination with the first aspect, the method for improving the superelasticity of NiTiHf high temperature shape memory alloy includes the following steps: preparing a NiTiHf high temperature shape memory alloy ingot; subjecting the NiTiHf high temperature shape memory alloy ingot to homogenization treatment, rolling deformation treatment with a total deformation of 50% to 65% and a rolling deformation of 3% to 15% per pass, and electric pulse treatment with a pulse frequency of 100 to 300 Hz and a temperature of 400 to 800 °C.

[0008] During the rolling deformation process, the total deformation is controlled to be 50% to 65%, and the rolling deformation of each pass is 3% to 15%. The plasticity of NiTiHf high-temperature shape memory alloy is still poor after homogenization treatment, so the total deformation should not be too high, and the pressure of each pass should be strictly controlled to avoid cracking during the rolling process. Therefore, the present invention adopts the principle of "small amounts and multiple times" to carry out rolling deformation treatment on the alloy. The above-mentioned total deformation can not only maximize the plasticity of NiTiHf high-temperature shape memory alloy and prevent it from unstable cracking, but also provide a "target target" for subsequent electric pulse treatment. Rolling deformation and electric pulse treatment work together to achieve a synergistic improvement in the superelasticity and plasticity of NiTiHf high-temperature shape memory alloy.

[0009] Preferably, the total deformation amount of the rolling deformation treatment is 56.7% to 58.4%, and more preferably 58.3%.

[0010] Optionally, when preparing the NiTiHf high temperature shape memory alloy ingot, a vacuum arc melting method is used to prepare the ingot.

[0011] Preferably, the pulse frequency is 150-250 Hz.

[0012] During the electric pulse treatment, the surface temperature of the NiTiHf high temperature shape memory alloy is 400-800°C (ie, the temperature of the electric pulse treatment), preferably 500-700°C, and more preferably 550-650°C.

[0013] During the electric pulse treatment process, the surface temperature of the NiTiHf high-temperature shape memory alloy needs to be controlled at 400~800 ℃. If the temperature is too low, the thermal effect of the pulse current is not significant, and the two-phase particles precipitated in the alloy are small in size, small in number, and unevenly distributed, resulting in poor superelasticity of the resulting alloy; if the temperature is too high, the size of the grains increases, the number of dislocations decreases, the two-phase particles coarsen, and the plasticity of the alloy is greatly reduced. The embodiment of the present invention uses an infrared thermal imager to track the surface temperature of the NiTiHf alloy. The thermometer model is FLIR A300, and the measurement range is 0~1000 ℃.

[0014] In combination with the first aspect, in the NiTiHf high temperature shape memory alloy, the Ni element content is 49.0 at%~51.0 at%, the Ti element content is 29.0 at%~36.0 at%, and the Hf element content is 15.0 at%~25.0 at%.

[0015] Preferably, the Ni element content is 49.5 at% to 50.5 at%, the Ti element content is 29.5 at% to 30.5 at%, and the Hf element content is 19.0 at% to 21.0 at%.

[0016] More preferably, the Ni element content is 50.3 at %, the Ti element content is 29.7 at %, and the Hf element content is 20.0 at %.

[0017] In combination with the first aspect, the homogenization treatment is specifically: homogenizing the NiTiHf high temperature shape memory alloy ingot at 1000-1050° C. for 48-84 h.

[0018] During the homogenization process defined by the above parameters, the NiTiHf high temperature shape memory alloy undergoes complete recrystallization, thereby improving the microstructure inside the alloy, eliminating internal stress and reducing component segregation. If the homogenization time is too short, it will lead to uneven microstructure distribution, insufficient internal stress release, and obvious component segregation; if the homogenization time is too long, it will lead to an increase in the size of the crystal grains and cause serious alloy oxidation.

[0019] In combination with the first aspect, the rolling deformation treatment is specifically as follows: the NiTiHf high temperature shape memory alloy ingot after homogenization treatment is kept at 800~900 ℃ for 10~30 min and then rolled for the first time, returned to the furnace and kept at the same temperature for 2~10 min and then rolled for the second time, and then the remaining times are rolled at the same temperature and holding time as the second time, and the total number of rolling passes is 5~10 times.

[0020] The method for improving the superelasticity of NiTiHf high temperature shape memory alloy provided by the present invention controls the rolling temperature at 800-900°C. During the rolling process, if the rolling temperature is too low, the alloy is prone to unstable cracking during the rolling process; if the holding time is too long, it is easy to form non-coherent Ti2Ni phase and Hf oxide, further increasing the brittleness of the alloy. Secondly, the present invention controls the total rolling pass to 5-10 times. If the number of rolling passes is too small, the amount of pressure on the alloy per pass is too large, and the alloy is prone to unstable cracking; if the number of rolling passes is too large, the alloy is repeatedly heated and kept warm under high temperature conditions, the accumulated strain energy is reduced, the grain size is increased, and the number of dislocations is reduced, which is not conducive to improving the superelasticity of the alloy.

[0021] In combination with the first aspect, in the electrical pulse treatment step, the voltage is set to 8-32 V and the conduction time is set to 20-100 μs.

[0022] In the electric pulse treatment step, the applied voltage is determined according to the temperature rise of the alloy during the current action. The pulse power supply device used in the present invention is SOYI-36750M, the modes are single pulse and normal pulse, and the voltage adjustable range is 0~36V.

[0023] In combination with the first aspect, in the electric pulse treatment step, the off time is set to 3200-9900 μs, and the pulse current action time is set to t 0< t ≤15 min.

[0024] Among them, the sum of the on time (t1) and the off time (t2) is the reciprocal of the pulse frequency, that is, Under the conditions of certain pulse frequency and pulse number, as the on-time increases, the off-time is shortened to ensure that the alloy obtains appropriate temperature rise.

[0025] In combination with the first aspect, in the electric pulse treatment step, the average current density through the alloy cross section is 200-300 A / mm 2 , preferably 230~270 A / mm 2 .

[0026] A second aspect of the present invention provides a NiTiHf high temperature shape memory alloy, which is prepared according to the above method for improving the superelasticity of the NiTiHf high temperature shape memory alloy.

[0027] A third aspect of the present invention provides an application of the above-mentioned high temperature shape memory alloy in the fields of aerospace and aircraft.

[0028] The beneficial effect of the present invention is that the method for improving the superelasticity of NiTiHf high-temperature shape memory alloy provided by the present invention successfully achieves the NiTiHf high-temperature shape memory alloy with significantly improved superelasticity without using severe plastic deformation treatment and long-term medium and high temperature heat treatment by combining rolling and electric pulse processing technology and accurately controlling parameters such as frequency, temperature and time. Compared with traditional heat treatment processes, the electric pulse processing technology used in the present invention has the process advantages of being green, efficient and environmentally friendly.

[0029] In addition to significantly improving the superelastic properties of the alloy, the method provided by the present invention also greatly improves the mechanical properties of the alloy. Compared with the cast NiTiHf high-temperature shape memory alloy, the NiTiHf high-temperature shape memory alloy obtained by the present invention has an increase in recoverable strain of more than 200%, an elongation increased by nearly 3 times, and a tensile strength increased by more than 10%. It has excellent superelasticity, plasticity and tensile strength, and is expected to be used in engineering applications in satellite launch towers, rocket engines, exhaust pipes and air inlets of hypersonic aircraft (where the ambient temperature in service exceeds 200°C). BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 is a TEM image of the NiTiHf high temperature shape memory alloy obtained in Example 1 of the present invention;

[0031] Figure 2 is a TEM image of the NiTiHf high temperature shape memory alloy obtained in Comparative Example 1 of the present invention;

[0032] Figure 3 is the superelastic cyclic loading / unloading stress-strain curve of the NiTiHf high temperature shape memory alloy in Example 1 of the present invention;

[0033] Figure 4 is the superelastic cyclic loading / unloading stress-strain curve of the NiTiHf high temperature shape memory alloy in Example 2 of the present invention;

[0034] Figure 5 is the superelastic cyclic loading / unloading stress-strain curve of the NiTiHf high temperature shape memory alloy in Example 3 of the present invention;

[0035] Figure 6is the superelastic cyclic loading / unloading stress-strain curve of the NiTiHf high temperature shape memory alloy in Example 4 of the present invention;

[0036] Figure 7 It is a summary diagram of the stress-strain curves of the NiTiHf high temperature shape memory alloys obtained in Comparative Example 1 and Example 1 of the present invention. DETAILED DESCRIPTION

[0037] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0038] Studies have shown that pulse current electroplasticity is an effective means to improve the plastic deformation ability of materials, and its application effect has been confirmed in materials such as aluminum alloys, titanium alloys, nickel-based alloys, and binary low-temperature nickel-titanium shape memory alloys. However, there are no reports on the application of pulse current electroplasticity to ternary NiTiHf high-temperature shape memory alloys to improve their superelasticity, and there is no research on coupling plastic deformation with electric pulse treatment to modify ternary NiTiHf high-temperature shape memory alloys.

[0039] Therefore, in view of the poor superelasticity of the current NiTiHf high temperature shape memory alloy and the defects of the current methods for improving its superelasticity (plastic deformation treatment and heat treatment) to a certain extent, the present invention provides a method for improving the superelasticity of the NiTiHf high temperature shape memory alloy. The method adopts a processing technology of rolling and electric pulse coupling, and can obtain a NiTiHf high temperature shape memory alloy with significantly improved superelastic performance without severe plastic deformation treatment and long-term medium and high temperature heat treatment.

[0040] The method for improving the superelasticity of NiTiHf high temperature shape memory alloy provided by the present invention is explained below through specific embodiments.

[0041] In the following embodiments, the pulse power supply device used is SOYI-36750 M, the modes are single pulse and normal pulse, and the voltage adjustable range is 0~36 V; an infrared thermal imager is used to track the surface temperature of the NiTiHf alloy, and the temperature measuring instrument model is FLIR A300, and the measuring range is 0~1000 ℃.

[0042] Example 1

[0043] This embodiment provides a method for improving the superelasticity of a NiTiHf high temperature shape memory alloy, wherein the content of Ni in the alloy ingot is 50.3 at%, the content of Ti is 29.7 at%, and the content of Hf is 20.0 at%. The steps include:

[0044] S1: NiTiHf high temperature shape memory alloy was prepared by vacuum arc melting method, and NiTiHf alloy ingot was obtained after 6 repeated melting.

[0045] S2: The NiTiHf alloy ingot obtained in S1 was kept at 1050 °C in a vacuum tube furnace for 72 h, cooled to room temperature along with the furnace, and the homogenization treatment was completed.

[0046] S3: The NiTiHf high temperature shape memory alloy after S2 homogenization treatment was subjected to rolling deformation treatment, specifically: the first rolling was carried out after keeping at 900 °C for 15 min, and after rolling, it was returned to the furnace and kept at 900 °C for 5 min before the second rolling. Thereafter, the remaining 6 rollings were carried out at the same temperature and holding time as the second rolling. The total deformation was 56.7%, among which the deformation of a single rolling pass was controlled to be 3%~15%.

[0047] S4: The NiTiHf high temperature shape memory alloy after rolling deformation is subjected to electric pulse treatment, specifically: the voltage is set to 10 V, the on time is 50 μs, the off time is 3950 μs, the pulse frequency is 250 Hz, and the cross-sectional area of ​​the NiTiHf alloy is 2 mm 2 , the average current density is 233.45 A / mm 2 , the number of pulses is 1000, and the surface temperature of the alloy is measured to be 521 ℃.

[0048] The TEM image of the obtained NiTiHf high temperature shape memory alloy is shown in Figure 1 shown.

[0049] Example 2

[0050] This embodiment provides a method for improving the superelasticity of a NiTiHf high temperature shape memory alloy, wherein the content of Ni in the alloy ingot is 50.3 at%, the content of Ti is 29.7 at%, and the content of Hf is 20.0 at%. The steps include:

[0051] S1: NiTiHf high temperature shape memory alloy was prepared by vacuum arc melting method, and NiTiHf alloy ingot was obtained after 6 repeated melting.

[0052] S2: The NiTiHf alloy ingot obtained in S1 was kept at 1050 °C in a vacuum tube furnace for 72 h, cooled to room temperature along with the furnace, and the homogenization treatment was completed.

[0053] S3: The NiTiHf high temperature shape memory alloy after S2 homogenization treatment was subjected to rolling deformation treatment, specifically: the first rolling was carried out after keeping at 900 °C for 5 min, and after rolling, it was returned to the furnace and kept at 900 °C for 2 min before the second rolling. Thereafter, the remaining 6 rollings were carried out at the same temperature and holding time as the second rolling. The total deformation was 58.3%, among which the deformation of a single rolling pass was controlled to be 3%~15%.

[0054] S4: The NiTiHf high temperature shape memory alloy after rolling deformation was subjected to electric pulse treatment, specifically: the voltage was set to 30 V, the on time was 40 μs, the off time was 3960 μs, the action time was 10 min, the pulse frequency was 250 Hz, and the cross-sectional area of ​​the NiTiHf alloy was 2 mm 2 , the average current density is 260.1 A / mm 2 The alloy surface temperature was measured to be 559 ℃.

[0055] Example 3

[0056] This embodiment provides a method for improving the superelasticity of a NiTiHf high temperature shape memory alloy, wherein the content of Ni in the alloy ingot is 50.3 at%, the content of Ti is 29.7 at%, and the content of Hf is 20.0 at%. The steps include:

[0057] S1: NiTiHf high temperature shape memory alloy was prepared by vacuum arc melting method, and NiTiHf alloy ingot was obtained after 6 repeated melting.

[0058] S2: The NiTiHf alloy ingot obtained in S1 was kept at 1050 °C in a vacuum tube furnace for 72 h, cooled to room temperature along with the furnace, and the homogenization treatment was completed.

[0059] S3: The NiTiHf high temperature shape memory alloy after S2 homogenization treatment was subjected to rolling deformation treatment, specifically: the first rolling was carried out after keeping at 900 °C for 15 min, and after the rolling was completed, it was returned to the furnace and kept at 900 °C for 5 min before the second rolling, and then the remaining 6 rollings were carried out at the same temperature and holding time as the second rolling. The total deformation was 58.3%, among which the deformation of a single rolling pass was controlled to be 3%~15%.

[0060] S4: The NiTiHf high temperature shape memory alloy after rolling deformation was subjected to electric pulse treatment, specifically: the voltage was set to 30 V, the on time was 40 μs, the off time was 3960 μs, the action time was 5 min, the pulse frequency was 250 Hz, and the cross-sectional area of ​​the NiTiHf alloy was 2 mm2 , the average current density is 260.1 A / mm 2 The alloy surface temperature was measured to be 561 ℃.

[0061] Example 4

[0062] This embodiment provides a method for improving the superelasticity of a NiTiHf high temperature shape memory alloy, wherein the content of Ni in the alloy ingot is 50.3 at%, the content of Ti is 29.7 at%, and the content of Hf is 20.0 at%. The steps include:

[0063] S1: NiTiHf high temperature shape memory alloy was prepared by vacuum arc melting method, and NiTiHf alloy ingot was obtained after 6 repeated melting.

[0064] S2: The NiTiHf alloy ingot obtained in S1 was kept at 1050 °C in a vacuum tube furnace for 72 h, cooled to room temperature along with the furnace, and the homogenization treatment was completed.

[0065] S3: The NiTiHf high temperature shape memory alloy after S2 homogenization treatment was subjected to rolling deformation treatment, specifically: the first rolling was carried out after keeping at 900 °C for 15 min, and after rolling, it was returned to the furnace and kept at 900 °C for 5 min before the second rolling. Thereafter, the remaining 6 rollings were carried out at the same temperature and holding time as the second rolling. The total deformation was 56.7%, among which the deformation of a single rolling pass was controlled to be 3%~15%.

[0066] S4: The NiTiHf high temperature shape memory alloy after rolling deformation was subjected to electric pulse treatment, specifically: the voltage was set to 30 V, the on time was 50 μs, the off time was 3950 μs, the action time was 10 min, the pulse frequency was 250 Hz, and the cross-sectional area of ​​the NiTiHf alloy was 2 mm 2 , the average current density is 250.1 A / mm 2 The surface temperature of the alloy was measured to be 440 ℃.

[0067] Example 5

[0068] This embodiment provides a method for improving the superelasticity of a NiTiHf high temperature shape memory alloy, wherein the content of Ni in the alloy ingot is 49.5 at%, the content of Ti is 29.5 at%, and the content of Hf is 21.0 at%. The steps include:

[0069] S1: NiTiHf high temperature shape memory alloy was prepared by vacuum arc melting method, and NiTiHf alloy ingot was obtained after 6 repeated melting.

[0070] S2: The NiTiHf alloy ingot obtained in S1 was kept at 1050 °C in a vacuum tube furnace for 48 h, cooled to room temperature along with the furnace, and the homogenization treatment was completed.

[0071] S3: The NiTiHf high temperature shape memory alloy after S2 homogenization treatment was subjected to rolling deformation treatment, specifically: the first rolling was carried out after keeping at 900 °C for 10 min, and after the rolling was completed, it was returned to the furnace and kept at 900 °C for 2 min before the second rolling, and then the remaining 8 rollings were carried out under the same conditions as the second rolling, with a total deformation of 55.5%, among which the deformation of a single rolling pass was controlled to be 3%~15%.

[0072] S4: The NiTiHf high temperature shape memory alloy after rolling deformation is subjected to electric pulse treatment, specifically: the voltage is set to 8 V, the on time is 80 μs, the off time is 6320 μs, the pulse frequency is 156 Hz, and the cross-sectional area of ​​the NiTiHf alloy is 2 mm 2 , the average current density is 266.8 A / mm 2 , the number of pulses is 1000, and the surface temperature of the alloy is measured to be 800℃.

[0073] Example 6

[0074] This embodiment provides a method for improving the superelasticity of a NiTiHf high temperature shape memory alloy, wherein the content of Ni in the alloy ingot is 50.5 at%, the content of Ti is 30.5 at%, and the content of Hf is 19.0 at%. The steps include:

[0075] S1: NiTiHf high temperature shape memory alloy was prepared by vacuum arc melting method, and NiTiHf alloy ingot was obtained after 6 repeated melting.

[0076] S2: The NiTiHf alloy ingot obtained in S1 was kept at 1000 °C in a vacuum tube furnace for 84 h, cooled to room temperature along with the furnace to complete the homogenization treatment.

[0077] S3: The NiTiHf high temperature shape memory alloy after S2 homogenization treatment was subjected to rolling deformation treatment, specifically: the first rolling was carried out after keeping at 800 °C for 30 min, and after the rolling was completed, it was returned to the furnace and kept at 800 °C for 5 min before the second rolling, and then the remaining 8 rollings were carried out at the same temperature and holding time as the second rolling. The total deformation was 63.5%, among which the deformation of a single rolling pass was controlled to be 3%~15%.

[0078] S4: The NiTiHf high temperature shape memory alloy after rolling deformation was subjected to electric pulse treatment, specifically: the voltage was set to 30 V, the on time was 100 μs, the off time was 3900 μs, the pulse current action time was 10 min, the pulse frequency was 250 Hz, and the cross-sectional area of ​​the NiTiHf alloy was 2 mm 2 , the average current density is 233.45 A / mm 2 , the number of pulses is 1000, and the surface temperature of the alloy is measured to be 400 ℃.

[0079] Example 7

[0080] This embodiment provides a method for improving the superelasticity of a NiTiHf high temperature shape memory alloy, wherein the content of Ni in the alloy ingot is 49.0 at%, the content of Ti is 36.0 at%, and the content of Hf is 15.0 at%, and the steps include:

[0081] S1: NiTiHf high temperature shape memory alloy was prepared by vacuum arc melting method, and NiTiHf alloy ingot was obtained after 6 repeated melting.

[0082] S2: The NiTiHf alloy ingot obtained in S1 was kept at 1050°C in a vacuum tube furnace for 48 h, cooled to room temperature along with the furnace, and the homogenization treatment was completed.

[0083] S3: The NiTiHf high temperature shape memory alloy after S2 homogenization treatment was subjected to rolling deformation treatment, specifically: the first rolling was carried out after keeping at 800 °C for 30 min, and after rolling, it was returned to the furnace and kept at 800 °C for 2 min before the second rolling. Thereafter, the remaining three rollings were carried out at the same temperature and holding time as the second rolling. The total deformation was 52.3%, among which the deformation of a single rolling pass was controlled to be 3%~15%.

[0084] S4: The NiTiHf high temperature shape memory alloy after rolling deformation was subjected to electric pulse treatment, specifically: the voltage was set to 32 V, the on time was 20 μs, the off time was 8980 μs, the pulse current action time was 5 min, the pulse frequency was 111 Hz, and the cross-sectional area of ​​the NiTiHf alloy was 2 mm 2 , the average current density is 250.125 A / mm 2 , the number of pulses is 1000, and the surface temperature of the alloy is measured to be 782 ℃.

[0085] Example 8

[0086] This embodiment provides a method for improving the superelasticity of a NiTiHf high temperature shape memory alloy, wherein the content of Ni in the alloy ingot is 51.0 at%, the content of Ti is 29.0 at%, and the content of Hf is 20.0 at%. The steps include:

[0087] S1: NiTiHf high temperature shape memory alloy was prepared by vacuum arc melting method, and NiTiHf alloy ingot was obtained after 6 repeated melting.

[0088] S2: The NiTiHf alloy ingot obtained in S1 was kept at 1050 °C in a vacuum tube furnace for 48 h, cooled to room temperature along with the furnace, and the homogenization treatment was completed.

[0089] S3: The NiTiHf high temperature shape memory alloy after S2 homogenization treatment was subjected to rolling deformation treatment, specifically: the first rolling was carried out after keeping at 850 °C for 20 min, and after the rolling was completed, it was returned to the furnace and kept at 800 °C for 2 min before the second rolling, and then the remaining 5 rollings were carried out at the same temperature and holding time as the second rolling. The total deformation was 59.7%, among which the deformation of a single rolling pass was controlled to be 3%~15%.

[0090] S4: The NiTiHf high temperature shape memory alloy after rolling deformation was subjected to electric pulse treatment, specifically: the voltage was set to 12 V, the on time was 50 μs, the off time was 4950 μs, the pulse current action time was 8 min, the pulse frequency was 200 Hz, and the cross-sectional area of ​​the NiTiHf alloy was 2 mm 2 , the average current density is 240.12 A / mm 2 , the number of pulses is 1000, and the surface temperature of the alloy is measured to be 553 ℃.

[0091] Comparative Example 1

[0092] This comparative example provides a method for processing a NiTiHf high temperature shape memory alloy. The composition of the NiTiHf high temperature shape memory alloy is the same as that in Example 1, but the processing method is different. The difference is that only step S1 is performed, and the homogenization treatment, rolling deformation treatment and electric pulse treatment in steps S2, S3 and S4 are not performed, that is, only a cast NiTiHf high temperature shape memory alloy is used.

[0093] The TEM image of the obtained cast NiTiHf high temperature shape memory alloy is shown in Figure 2 shown.

[0094] Comparative Example 2

[0095] This comparative example provides a method for processing a NiTiHf high temperature shape memory alloy. The composition of the NiTiHf high temperature shape memory alloy is the same as that in Example 1, but the processing method is different. The difference is that only steps S1, S2 and S3 are performed, and the electric pulse treatment in S4 is not performed.

[0096] Comparative Example 3

[0097] This comparative example provides a method for treating a NiTiHf high temperature shape memory alloy. The composition of the NiTiHf high temperature shape memory alloy is the same as that in Example 1, but the treatment method is different. The only difference is that the temperature in the electric pulse treatment in step S4 is 950°C.

[0098] Comparative Example 4

[0099] This comparative example provides a method for treating a NiTiHf high temperature shape memory alloy. The composition of the NiTiHf high temperature shape memory alloy is the same as that in Example 1, but the treatment method is different. The only difference is that the temperature in the electric pulse treatment in step S4 is 350°C.

[0100] Comparative Example 5

[0101] This comparative example provides a method for treating a NiTiHf high temperature shape memory alloy. The composition of the NiTiHf high temperature shape memory alloy is the same as that in Example 1, but the treatment method is different. The only difference is that the frequency in the electric pulse treatment in step S4 is 400 Hz.

[0102] Comparative Example 6

[0103] This comparative example provides a method for treating a NiTiHf high temperature shape memory alloy. The composition of the NiTiHf high temperature shape memory alloy is the same as that in Example 1, but the treatment method is different. The only difference is that the frequency in the electric pulse treatment in step S4 is 80 Hz.

[0104] Test Example 1

[0105] The superelasticity and mechanical properties of the NiTiHf high temperature shape memory alloys prepared in Examples 1 to 4 and Comparative Examples 1 to 6 were tested respectively. The superelasticity test method was as follows: after the stress was first loaded to 1% strain, it was unloaded to 0 stress; after loading to 2% strain again, it was unloaded to 0 stress again, and the loading / unloading cycle was repeated until the sample broke. The strain rate during the loading / unloading process of all superelastic cyclic tensile tests was 3.33×10 -4 s -1The maximum strain and cumulative strain of the alloy during the superelastic test, the elongation and tensile strength during the mechanical properties test, and the corresponding increase are recorded. The test results are shown in Table 1. At the same time, the cyclic loading / unloading stress-strain curves of Examples 1 to 4 are shown in Table 1. Figures 3 to 6 As shown, the stress-strain curves of Example 1 and Comparative Example 1 are as follows Figure 7 shown.

[0106] Among them, recoverable strain (%) = maximum strain (%) - cumulative strain (%); recoverable strain increase (%) = (recoverable strain of the embodiment - recoverable strain of the comparative example) / recoverable strain of the comparative example * 100 (excluding comparative example 1). In addition, since comparative example 1 is a cast NiTiHf high temperature shape memory alloy, its recoverable strain is 0, so the recoverable strain increase (%) of embodiments 1 to 4 relative to comparative example 1 = recoverable strain of embodiments 1 to 4 * 100.

[0107] Table 1 Summary of the test results of Examples 1 to 4 and Comparative Examples 1 to 6

[0108]

[0109] It can be seen from the data in Table 1 that compared with the cast NiTiHf high-temperature alloy obtained in Comparative Example 1 (cast, without any treatment), the recoverable strain increase of the NiTiHf high-temperature shape memory alloys obtained in Examples 1 to 4 is more than 200%, the elongation is increased by nearly 3 times, and the tensile strength is increased by more than 10%. Compared with the NiTiHf high-temperature alloy obtained in Comparative Example 2 (without the "electric pulse" treatment technology), the recoverable strain increase of the NiTiHf high-temperature shape memory alloys obtained in Examples 1 to 4 is at least 30% and at most 120%, the elongation is increased by more than 1 times, and the tensile strength is increased by more than 5%. It can be seen from this that the method for improving the superelasticity of NiTiHf high-temperature shape memory alloys provided by the present invention not only greatly improves the superelasticity of NiTiHf high-temperature shape memory alloys, but also improves its plasticity and tensile strength, greatly expanding the application field of NiTiHf high-temperature shape memory alloys.

[0110] In addition, it can be seen from the data of Comparative Examples 3 to 6 that when the temperature or frequency during the electric pulse treatment exceeds the temperature range and frequency range specified in the present invention, the recoverable strain of the obtained NiTiHf high temperature shape memory alloy is reduced to below 1.9%, the elongation is reduced to below 5%, and the tensile strength does not reach 650 MPa.

[0111] Depend on Figure 1~2It can be seen that the microstructure of the NiTiHf high temperature shape memory alloy obtained by the present invention is significantly refined, a large number of dislocations are introduced into the matrix, and the microstructure of the alloy becomes uniform and stable under the action of pulse current.

[0112] In summary, the method for improving the superelasticity of NiTiHf high temperature shape memory alloy provided by the present invention can not only greatly improve the superelasticity of NiTiHf high temperature shape memory alloy, but also improve its plasticity (elongation) and tensile strength, that is, the method can synergistically improve the superelasticity, plasticity and tensile strength of NiTiHf high temperature shape memory alloy, which will greatly broaden the application field of NiTiHf high temperature shape memory alloy, and at the same time provide a new idea for the production of NiTiHf high temperature shape memory alloy with superelasticity, which has great economic value and social value.

[0113] What is described above is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A method for improving the superelasticity of NiTiHf high temperature shape memory alloy, characterized in that: The NiTiHf high temperature shape memory alloy ingot is subjected to homogenization treatment, rolling treatment and electric pulse treatment in sequence; The NiTiHf high temperature shape memory alloy ingot is homogenized at 1000-1050°C for 48-84 h; the total deformation of the rolling treatment is 50%-65%, the total rolling passes are 5-10 times, the rolling deformation of each pass is 3%-15%, and the rolling temperature is 800-900°C; the frequency of the electric pulse treatment is 100-300 Hz, and the temperature is 400-800°C; in the NiTiHf high temperature shape memory alloy, the Ni element content is 49.0 at%-51.0 at%, the Ti element content is 29.0 at%-36.0 at%, and the Hf element content is 15.0 at%-25.0 at%.

2. The method for improving the superelasticity of NiTiHf high temperature shape memory alloy according to claim 1, characterized in that: The rolling deformation treatment is specifically as follows: the NiTiHf high temperature shape memory alloy ingot after homogenization treatment is kept at 800-900°C for 10-30 min and then rolled for the first time, returned to the furnace and kept at the same temperature for 2-10 min and then rolled for the second time, and then the remaining times are rolled at the same temperature and holding time as the second time.

3. The method for improving superelasticity of NiTiHf high temperature shape memory alloy according to claim 1, characterized in that: In the electric pulse treatment step, the voltage is set to 8-32 V and the conduction time is set to 20-100 μs.

4. The method for improving superelasticity of NiTiHf high temperature shape memory alloy according to claim 1, characterized in that: In the electric pulse treatment step, the off time is set to 3200~9900 μs, and the pulse current action time is t 0< t ≤15 min.

5. The method for improving superelasticity of NiTiHf high temperature shape memory alloy according to claim 1, characterized in that: In the electric pulse treatment step, the average current density through the alloy cross section is 200-300 A / mm 2 .

6. A NiTiHf high temperature shape memory alloy, characterized in that: It is prepared according to the method for improving the superelasticity of NiTiHf high temperature shape memory alloy according to any one of claims 1 to 5.

7. Application of the NiTiHf high temperature shape memory alloy according to claim 6 in the fields of aerospace and aircraft.

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