The invention relates to a method based on R-B19apos; method for optimizing super-elastic and elastic-thermal properties of nickel-titanium shape memory alloy based on phase change path

Abstract

The invention discloses a nickel-titanium shape memory alloy super-elastic and elastic-thermal performance optimization method based on an R-to-B19 'phase change path, which comprises the following steps: step 1, performing solid solution heat treatment on a NiTi shape memory alloy plate at 900 DEG C for 1 hour, and then performing water quenching and cooling to room temperature to obtain a solid solution state sample; 2, the solid solution state sample is subjected to cold rolling at the thinning rate of 50%, and a NiTi shape memory alloy thin plate is obtained; and 3, the NiTi shape memory alloy sheet is put into a preheated tubular furnace, heat preservation is conducted for 4 h at the annealing temperature of 300-400 DEG C, water quenching and cooling are conducted to the room temperature, and the nickel-titanium shape memory alloy with the optimized super-elastic and elastic-thermal performance is obtained. According to the method, by controlling the thinning rate and the annealing temperature of cold rolling, collaborative optimization of the superelastic cycle stability and the elastic-thermal refrigeration capacity of the NiTi-based SMAs is achieved, the technological process is simple, the cost is low, and application of the NiTi-based SMAs in the field of solid refrigeration is promoted.

Description

Technical Field

[0001] This invention relates to shape memory alloys, specifically a method for optimizing the hyperelasticity and elastothermal properties of nickel-titanium shape memory alloys based on the R-to-B19′ phase transformation path. Background Technology

[0002] Shape memory alloys (SMAs) have been widely used in aerospace, biomedicine and solid-state refrigeration due to their excellent mechanical properties, biocompatibility and functionality. NiTi alloys, as the most widely used SMAs, have excellent formability and a large amount of latent heat related to martensitic phase transformation. They exhibit superelastic recoverable strain of more than 8% under loading, which is caused by stress-induced reversible martensitic phase transformation. However, the functional fatigue of existing NiTi-based SMAs during cycling due to structural defects has seriously limited their practical application.

[0003] Studies have shown that the B2 (cubic phase) → B19′ (monoclinic phase) phase transition has a significant thermodynamic driving force, resulting in an adiabatic temperature change (ΔT). ad The temperature is approximately 35 K. However, due to the inherent lattice mismatch and significant phase transition energy barrier between the two phases, the ability of NiTi-based SMAs to undergo phase transition and reverse phase transition is hindered. Under cyclic loading, the internal stress around dislocations impedes the reverse phase transition, resulting in the retention of residual martensite after complete unloading, reducing the transformable volume fraction. Moreover, significant residual strain is generated in the early stages of cycling, accelerating hyperelastic degradation. As the number of cycles increases, this microstructural damage accumulates, eventually leading to the complete loss of material function. To address these challenges, various strategies, including cophase precipitation, grain refinement, and composite material design, have been adopted to mitigate the autocatalytic mutation behavior of NiTi-based SMAs and improve their fatigue resistance. Although millions of B2→B19′ reversible transformation cycles have been achieved with minimal degradation, the phase transition capability and elastothermal properties are often neglected. Furthermore, the related manufacturing technologies, such as nanoscale composite structure design, are too costly for the solid-state refrigeration field. Therefore, it is necessary to find an efficient and low-cost method to improve the cyclic stability of NiTi-based SMAs.

[0004] The R phase (rhombohedral phase) in NiTi-based SMAs is an important intermediate phase with excellent lattice compatibility with the B2 phase. It can serve as a transformation buffer to enhance deformation capacity and superelastic recovery. Some researchers have used ternary elements to partially replace Ni atoms to adjust the transformation behavior, establishing a B2→R transformation pathway. Under a constant strain of 1%, it can achieve 100,000 cycles, with an output adiabatic temperature change value (ΔT). adThe fatigue life is 5.8K. However, good cycling stability comes at the cost of recoverable strain, inevitably impairing the elasto-thermal effect. In other words, although the phase transformation path based on B2→R obtained by utilizing the good lattice compatibility between the B2 and R phases can significantly improve fatigue life, the phase transformation strain is only 1% due to insufficient phase transformation, and ΔT ad It only reached 5.8K.

[0005] Given the significant entropy difference (~65 vs ~18 J·K) between the B2→B19′ and B2→R transforms -1 ·kg -1 The ability of the former to effectively reduce temperature through the elastic thermal effect suggests that the phase transition path based on R→B19′ has better phase transition potential. The R→B19′ transition benefits from a lower energy barrier and a smoother phase evolution, providing better superelasticity. Furthermore, the R phase can alleviate stress concentration and inhibit damage accumulation during cycling, thereby extending service life. However, currently, SMAs based on the B2→R→B19′ phase transition path are mainly obtained through thermomechanical means. This technology still needs to address the phase transition barrier and lattice mismatch between the B2 and B19′ phases. Moreover, severe plastic deformation leads to insufficient martensitic phase transformation, inhibiting its superelasticity and elastic-thermal cooling capacity.

[0006] Therefore, it is urgent to construct a novel R→B19′ phase transition path to improve cycle stability while retaining the phase transition capability of NiTi-based SMAs. Summary of the Invention

[0007] The purpose of this invention is to provide a method for optimizing the hyperelasticity and elastothermal properties of nickel-titanium shape memory alloys based on the R-to-B19′ phase transformation path. By means of cold rolling and annealing, the synergistic optimization of the hyperelastic cycle stability and elastothermal cooling capacity of NiTi-based SMAs is achieved.

[0008] This invention is achieved through the following technical solution:

[0009] A method for optimizing the hyperelasticity and elastothermal properties of nickel-titanium shape memory alloys based on the R-to-B19′ phase transformation path includes the following steps:

[0010] Step 1: The NiTi shape memory alloy plate is solution heat treated at 900℃ for 1 hour, and then water quenched to room temperature to obtain a solution-treated sample.

[0011] Step 2: The solution-treated sample is cold-rolled at a thinning rate of 50% to obtain a NiTi shape memory alloy sheet.

[0012] Step 3: Place the NiTi shape memory alloy sheet into a preheated tube furnace, anneal at 300-400℃ for 4 hours, and then water quench to room temperature to obtain a nickel-titanium shape memory alloy with optimized superelasticity and elastothermal properties.

[0013] Furthermore, the atomic percentage of nickel in the NiTi shape memory alloy sheet of step 1 is 50.8%.

[0014] Furthermore, the thickness of the NiTi shape memory alloy plate in step 1 is 2 mm.

[0015] Furthermore, the annealing temperature in step 3 is 400°C.

[0016] The present invention has the following beneficial technical effects:

[0017] This invention controls the thinning rate and annealing temperature during cold rolling to ensure a widespread distribution of the R phase within the B2 matrix at room temperature. This results in NiTi-based SMAs based on the R→B19′ phase transformation path exhibiting both excellent hyperelastic cycling stability and elastothermal cooling capability. This is attributed to two main factors: firstly, the relatively small phase transformation barrier between the B19′ and R phases facilitates martensitic and reverse phase transformations, significantly improving the hyperelastic cycling stability of NiTi-based SMAs; secondly, the large latent heat of phase transformation between the B19′ and R phases is maintained at 12K after adiabatic temperature change, showing no significant decay after 500 cycles, demonstrating superior elastothermal cycling stability. In summary, this invention achieves synergistic optimization of the hyperelastic cycling stability and elastothermal cooling capability of NiTi-based SMAs, solving the problem of traditional phase transformation systems operating under large ΔT... ad The problem of the trade-off between fatigue resistance and performance is addressed, and the process is simple and low-cost, which is of great significance for promoting the application of NiTi-based SMAs in the field of solid-state refrigeration.

[0018] By setting the thickness of the NiTi shape memory alloy sheet to 2mm, the NiTi shape memory alloy sheet can undergo uniform shape deformation during cold rolling, which improves mechanical properties and superelasticity reliability and reduces the risk of processing defects.

[0019] By controlling the annealing temperature at 400℃, NiTi-based SMAs achieved high ΔT values ​​based on the R→B19′ phase transition. ad Synergistic optimization with a long cycle life of more than 500 cycles, and the R→B19′ phase transformation strain of 4.5%, which is significantly higher than the B2→R phase transformation strain of 1.0%, while still maintaining complete reversibility, effectively improves the martensite residue problem commonly found in the traditional B2→B19′ phase transformation. Attached Figure Description

[0020] Figure 1DSC curves of NiTi alloys with different phase transformation paths;

[0021] Figure 2 Tensile test results for NiTi alloys with gradually increasing strain for different phase transformation paths;

[0022] Figure 3 A is obtained from Embodiment 3 and Comparative Example 3 of the present invention. 400 and R 600 Results of a 100-cycle cyclic tensile test at a strain level of 5%;

[0023] Figure 4 A obtained in Embodiment 3 of the present invention 400 Adiabatic temperature changes under cyclic loading and unloading;

[0024] Figure 5 A obtained in Examples 2-3 of the present invention 350 and A 400 Low-cycle fatigue test results;

[0025] Figure 6 A obtained in Examples 2-3 of the present invention 350 and A 400 TEM images;

[0026] Figure 7 A obtained in Embodiment 3 of the present invention 400 XRD patterns during the first loading and unloading process. Detailed Implementation

[0027] The present invention will be further described in detail below with reference to specific embodiments. These descriptions are for explanation purposes only and are not intended to limit the scope of the invention.

[0028] The NiTi shape memory alloy plates selected in Examples 1 to 3 and Comparative Examples 1 to 3 of the present invention are all NiTi-50.8, that is, NiTi alloy plates with an atomic percentage of 50.8% nickel.

[0029] Example 1

[0030] Step 1: A NiTi shape memory alloy plate with a thickness of 2 mm is subjected to solution heat treatment at 900℃ for 1 h, and then water quenched to room temperature to obtain a solution-treated sample.

[0031] Step 2: The solution-treated sample is cold-rolled at a thinning rate of 50% to obtain a NiTi shape memory alloy sheet with a thickness of 1 mm.

[0032] Step 3: Place the NiTi shape memory alloy sheet into a preheated tube furnace, anneal at 300℃ for 4 hours, and quench to room temperature to obtain a nickel-titanium shape memory alloy with optimized superelasticity and elastothermal properties, denoted as A. 300 .

[0033] Example 2

[0034] Step 1: A NiTi shape memory alloy plate with a thickness of 2 mm is subjected to solution heat treatment at 900℃ for 1 h, and then water quenched to room temperature to obtain a solution-treated sample.

[0035] Step 2: The solution-treated sample is cold-rolled at a thinning rate of 50% to obtain a NiTi shape memory alloy sheet with a thickness of 1 mm.

[0036] Step 3: Place the NiTi shape memory alloy sheet into a preheated tube furnace, anneal at 350℃ for 4 hours, and quench to room temperature to obtain a nickel-titanium shape memory alloy with optimized superelasticity and elastothermal properties, denoted as A. 350 .

[0037] Example 3

[0038] Step 1: A NiTi shape memory alloy plate with a thickness of 2 mm is subjected to solution heat treatment at 900℃ for 1 h, and then water quenched to room temperature to obtain a solution-treated sample.

[0039] Step 2: The solution-treated sample is cold-rolled at a thinning rate of 50% to obtain a NiTi shape memory alloy sheet with a thickness of 1 mm.

[0040] Step 3: Place the NiTi shape memory alloy sheet into a preheated tube furnace, anneal at 400℃ for 4 hours, and quench to room temperature to obtain a nickel-titanium shape memory alloy with optimized superelasticity and elastothermal properties, denoted as A. 400 .

[0041] Comparative Example 1

[0042] Step 1: A NiTi shape memory alloy plate with a thickness of 2 mm is subjected to solution heat treatment at 900℃ for 1 h, and then water quenched to room temperature to obtain a solution-treated sample.

[0043] Step 2: The solution-treated sample is cold-rolled at a thinning rate of 50% to obtain a NiTi shape memory alloy sheet with a thickness of 1 mm.

[0044] Step 3: Place the NiTi shape memory alloy sheet into a preheated tube furnace, anneal at 200℃ for 4 hours, and quench to room temperature. The resulting nickel-titanium shape memory alloy is denoted as A. 200 .

[0045] Comparative Example 2

[0046] Step 1: A NiTi shape memory alloy plate with a thickness of 2 mm is subjected to solution heat treatment at 900℃ for 1 h, and then water quenched to room temperature to obtain a solution-treated sample.

[0047] Step 2: The solution-treated sample is cold-rolled at a thinning rate of 50% to obtain a NiTi shape memory alloy sheet with a thickness of 1 mm.

[0048] Step 3: Place the NiTi shape memory alloy sheet into a preheated tube furnace, anneal at 500℃ for 4 hours, and quench to room temperature. The resulting nickel-titanium shape memory alloy is denoted as A. 500 .

[0049] Comparative Example 3

[0050] Step 1: A NiTi shape memory alloy plate with a thickness of 2 mm is subjected to solution heat treatment at 900℃ for 1 h, and then water quenched to room temperature to obtain a solution-treated sample.

[0051] Step 2: The solution-treated sample was cold-rolled at a thinning rate of 30% to obtain a NiTi shape memory alloy sheet with a thickness of 1.4 mm.

[0052] Step 3: Place the NiTi shape memory alloy sheet into a preheated tube furnace, anneal at 600℃ for 8 minutes, and quench to room temperature. The resulting nickel-titanium shape memory alloy is denoted as R. 600 .

[0053] 1) Using a TAQ200 differential scanning calorimeter (DSC), at a rate of 10℃ / min, within the range of -150℃ to 150℃, the following samples were tested: solution-treated NiTi shape memory alloy plates, 1mm thick NiTi shape memory alloy sheets (cold rolled), and cold-rolled and annealed NiTi shape memory alloys from Examples 1 to 3 and Comparative Examples 1 to 3 (A). 300 A 350 A 400 A 200 A 500 R 600 Tests were conducted, and the heat flow curves and latent heat of phase transformation were analyzed to reveal the regulatory effect of cold rolling and annealing on the phase transformation path of NiTi alloys. The results are as follows: Figure 1 As shown, where: Figure 1 (a) Reveals that the solid solution sample exhibits typical characteristics Single-step reversible phase transition; Figure 1(b) The phase transformation peak of the cold-rolled sample completely disappeared due to high-density dislocations, and its heat flow curve was smooth, indicating that severe lattice defects have an inhibitory effect on martensite nucleation. Figure 1 (c)~ Figure 1 (g) The phase transformation paths of the nickel-titanium shape memory alloys after cold rolling and annealing in Examples 1-3, Comparative Examples 2 and 3 are shown respectively. It can be seen that when the annealing temperature is in the range of 300-400℃, the cooling process is a single-step B2→R phase transformation, and the heating process is a two-step reverse phase transformation of B19′→R→B2. Moreover, the intensity of the phase transformation peak increases with the increase of the annealing temperature, indicating that the inhibitory effect of dislocation defects on the martensitic phase transformation gradually weakens. When the temperature rises to 500℃, the cooling... The phase transformation path of the process evolved into a two-step phase transformation from B2 to R to B19′, while the heating process evolved into a single-step phase transformation from B19′ to B2. This indicates that the degree of defect recovery is directly related to the thermomechanical treatment. When the temperature rises to 600℃, the defects generated by cold rolling are completely eliminated, exhibiting a phase transformation path consistent with that of the solid solution sample. In summary, the DSC results show that the sample annealed at 300-400℃ has completed the B2→R phase transformation at room temperature, indicating the existence of a stable R phase in the NiTi matrix.

[0054] 2) The solution-treated NiTi shape memory alloy sheet, the 1mm thick NiTi shape memory alloy sheet (cold rolled), and the cold-rolled and annealed NiTi shape memory alloys of Examples 1-3 and Comparative Examples 1-2 (A) were used. 300 A 350 A 400 A 200 A 500 The specimens were cut into dog-bone shapes along the rolling direction and subjected to hyperelastic cyclic testing and elastothermal testing. The strain rate for the hyperelastic cyclic testing was 1 × 10⁻⁶. -3 s -1 The strain rate during the elasto-thermal test was 2 × 10⁻⁶. -3 s -1 The strain rate during unloading was 5.8 × 10⁻⁶. -2 s -1 Adiabatic temperature change value ΔT ad Measured by a type K thermocouple; results are shown below. Figures 2-4 :

[0055] Figure 2 For solid solution specimens, cold-rolled specimens, A 300 A 350 A 400 A 200 and A 500 The tensile test results of the strain variable were gradually increased, among which: Figure 2 (a) solid solution sample and Figure 2 (g) of A 500 The sample exhibits the traditional B2→B19′ phase transformation path. Due to the large phase transformation barrier between the B2 phase and the B19′ martensitic phase, dislocations continuously accumulate during the tensile process, and the martensitic phase transformation and reverse phase transformation are suppressed, resulting in the continuous accumulation of residual strain and eventually functional fatigue. Figure 2 (b) The cold-rolled specimen exhibited large plastic deformation and high internal residual stress, showing characteristics of work hardening, and possessing high strength and low toughness. Figure 2 (c) of A 200 Due to the low annealing temperature, the defects in the sample were difficult to eliminate, and the mechanical properties were similar to those of the cold-rolled sample. Figure 2 (d)~ Figure 2 (f) of A 300 A 350 and A 400 The sample exhibited a complete phase transition plateau and good recoverable deformation, consistent with... Figure 1 The results are consistent with the DSC results, which is attributed to the good lattice compatibility between the R phase and the B19′ phase. Figure 2 (h) shows the elastic modulus of different samples. It can be seen that as the annealing temperature increases, the modulus of the tensile curve first decreases and then increases. The sample obtained at the annealing temperature of 400℃ has the lowest modulus, which is 29.26 GPa. It can be seen that at room temperature, the sample in the B2 state exhibits a larger unrecoverable strain, while the sample in the R phase has better recoverable performance. This indicates that the phase transformation path based on R→B19′ is better than the phase transformation path based on B2→B19′.

[0056] To investigate the best mechanical properties of A 400 The evolution of the phase transition path during the cycle will transform A 400 Compared with R, which served as a control sample 600 Under a uniform strain level of 5%, a cyclic loading and unloading tensile test was conducted for 100 cycles, and the results are as follows: Figure 3 As shown, the first circle of the hyperelastic stress-strain curve is represented in black, the 100th circle in red, and the rest in gray. Figure 3 A of (a) 400 The hyperelastic stress-strain curve and the hysteresis curve exhibit a standard flag shape. During the first loading cycle, two phase transition plateaus appear, indicating that a two-step phase transition B2→R→B19′ occurs. The first plateau (located at the purple arrow) corresponds to the B2→R phase transition, and the second plateau corresponds to the R→B19′ phase transition. In subsequent cycles, the loading curve no longer shows the R phase transition plateau, indicating that the R→B19′ transition occurs stably from the second cycle onwards. Figure 3 (b) gives R 600The hyperelastic stress-strain curve shows a large cumulative residual strain in the early stage of the cycle, the phase transformation plateau becomes blurred, the stress hysteresis decreases, and the stress-strain curve gradually becomes quasi-linear after 100 cycles, showing severe functional fatigue. Figure 3 (c) A summary 400 and R 600 Recoverable strain during specimen cycling, A 400 Due to the lower phase transformation barrier between the R phase and the B19′ phase, martensitic and reverse phase transformations are more likely to occur. During subsequent cycling, the recoverable strain rate gradually increases and stabilizes at 99.99%. In contrast, R... 600 It exhibits lower recoverable strain; Figure 3 (d) Statistics on A 400 and R 600 The strain plateaus of the martensitic phase transformation of the sample at cycles 1, 10, 30, 50, and 100, R 600 Due to the accumulation of residual martensite, the B2 component participating in the phase transformation decreases, resulting in a significant drop in the strain plateau. Figure 3 The result of (c) corresponds to this phenomenon reaching saturation during continued cycling, while A 400 The very small drop in strain plateau amplitude is attributed to the R phase, which is widely distributed in the B2 matrix, enabling A phase transition based on the R→B19′ path. 400 Better cycle stability;

[0057] Figure 4 A obtained in Embodiment 3 of the present invention 400 The adiabatic temperature change under cyclic loading and unloading, where: Figure 4 (a) Record A 400 The adiabatic temperature change under cyclic loading and unloading is represented by the adiabatic temperature change value (ΔT) obtained in each cycle. ad Statistics in Figure 4 In (b), due to the first cycle, a two-step phase transition occurs from B2 to R to B19′, ΔT ad Higher, subsequent cycles only involve Phase transition, ΔT ad The temperature drops and quickly stabilizes at around 12K, and shows no significant decay after 500 cycles, exhibiting excellent elastothermal cycling stability. Therefore, this invention represents the first time that a high ΔT based on the R→B19′ phase transformation has been achieved in a NiTi binary alloy. ad Synergistic optimization with long cycle life (>500 cycles) solves the problem of traditional phase transition systems at high ΔT. ad The problem of mutual restraint between fatigue resistance and the phase transformation strain of R→B19′ is 4.5%, which is significantly higher than the phase transformation strain of B2→R (1.0%), and it still maintains complete reversibility, effectively preventing the martensite residue problem commonly found in the traditional B2→B19′ phase transformation.

[0058] 3) Using a fatigue tester at a frequency of 2Hz, A 350 and A 400 The sample underwent low-cycle fatigue testing, and the results are as follows: Figure 5 As shown, where: Figure 5 A of (a) 350 The sample maintained an intact phase transition plateau and good recoverable strain after 3000 cycles, indicating that A 350 It has good cyclic stability, and A 350 It consistently exhibits a single-step phase transition from R to B19′; Figure 5 (b) of A 400 The first platform in the first round of tensile testing represents A. 400 The B2→R phase transition occurs, the second plateau represents the R→B19′ phase transition, and subsequent cycles only exhibit the R→B19′ phase transition, demonstrating good cycle stability compared to A. 350 A 400 It has a larger phase transition plateau and recoverable strain during tensile loading, resulting in better phase transition capability.

[0059] 4) At -20℃, using an electrolyte containing 2% H2SO4 and 80% CH3OH (vol%), foils for transmission electron microscopy (TEM) observation were prepared by dual-jet electropolishing. A JEM-2100 TEM was used to examine A... 350 and A 400 The sample was characterized by microstructure and polycrystalline diffraction pattern analysis at a working voltage of 200 kV. The results are as follows: Figure 6 As shown, where: Figure 6 (a) is A 350 The matrix morphology and grain size range from tens to hundreds of nanometers. Figure 6 (b) The presence of R phase diffraction rings can be observed in the polycrystalline diffraction pattern, indicating that the R phase is dispersed in the B2 matrix at room temperature, which also confirms the feasibility of the R→B19′ phase transition path. Figure 6 (c) is A 400 The matrix morphology and grain size range from tens to hundreds of nanometers. Figure 6 (d) shows the presence of R-phase diffraction rings in the polycrystalline diffraction pattern, indicating that the R-phase is widely distributed in the B2 matrix at room temperature. The participation of the R-phase during the stretching process can significantly improve the mechanical properties of NiTi-based SMAs, enabling them to maintain excellent cycle stability under large strain.

[0060] 5) The A obtained in Example 3 was analyzed using a MicroMax-007HF (Rigaku, Japan) equipped with an in-situ stretching scattering system. 400 In-situ XRD tests were performed during the first loading and unloading process, using a wavelength of [wavelength value missing]. The Mo rotating anode target X-ray source has a beam size of approximately 200 μm, and the results are as follows: Figure 7 As shown, since the R phase is diffusely distributed in the B2 phase matrix, no obvious diffraction peaks of the R phase were observed when unloaded. During the loading process from 0 MPa to 300 MPa, B2(221) gradually differentiated into R(412) and R(300), proving that stress-induced R phase transformation corresponds to Figure 3 (a) The first phase transition plateau during loading. Upon further loading to 500 MPa, the R-phase diffraction peak disappears, and a B19′ diffraction peak appears, indicating an R→B19′ martensitic phase transition. Figure 3 The second plateau in (a) shows that when the stress drops from 500 MPa to 100 MPa, the B19′(200) diffraction peak disappears, while the R(412) and R(300) diffraction peaks reappear, indicating that the reverse martensitic transformation from B19′ to R has been completed. When the stress drops to 0 MPa (i.e., completely unloaded), the B19′(200) diffraction peak disappears completely, confirming that there is no residual martensite in the matrix, the reverse martensitic transformation has fully occurred, and the martensitic transformation is completely reversible.

Claims

1. A method for optimizing the hyperelasticity and elastothermal properties of nickel-titanium shape memory alloys based on the R-to-B19' phase transformation path, characterized in that, Includes the following steps: Step 1: The NiTi shape memory alloy plate is solution heat-treated at 900℃ for 1 hour, and then water quenched to room temperature to obtain a solution-treated sample. Step 2: The solution-treated sample is cold-rolled at a thinning rate of 50% to obtain a NiTi shape memory alloy sheet. Step 3: Place the NiTi shape memory alloy sheet into a preheated tube furnace, anneal at 300-400℃ for 4 hours, and then water quench to room temperature to obtain a nickel-titanium shape memory alloy with optimized superelasticity and elastothermal properties.

2. The method for optimizing the hyperelasticity and elastothermal properties of nickel-titanium shape memory alloys based on the R-to-B19' phase transformation path according to claim 1, characterized in that, The atomic percentage of nickel in the NiTi shape memory alloy sheet produced in step 1 is 50.8%.

3. The method for optimizing the hyperelasticity and elastothermal properties of nickel-titanium shape memory alloys based on the R-to-B19' phase transformation path according to claim 1, characterized in that, The thickness of the NiTi shape memory alloy plate in step 1 is 2 mm.

4. The method for optimizing the hyperelasticity and elastothermal properties of nickel-titanium shape memory alloys based on the R-to-B19' phase transformation path according to claim 1, characterized in that, The annealing temperature in step 3 is 400℃.

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