EP4667597A1

EP4667597A1 - Nickel-titanium alloy, and preparation method therefor and use thereof

Info

Publication number: EP4667597A1
Application number: EP24762979.3A
Authority: EP
Inventors: Peng Hua; Hongyang LIN; Qingping Sun
Original assignee: Hong Kong University of Science and Technology
Current assignee: Hong Kong University of Science and Technology
Priority date: 2023-02-28
Filing date: 2024-02-07
Publication date: 2025-12-24
Also published as: JP2026508337A; KR20250150142A; CN118563168A; WO2024179296A1

Abstract

A nickel-titanium alloy, and a preparation method therefor and a use thereof. The nickel-titanium alloy comprises titanium, nickel, copper and optional cobalt; based on the total weight of the nickel-titanium alloy, the weight percentages of the titanium, nickel, copper and optional cobalt are respectively: titanium: 38-47%; nickel: 35-50%; copper: 3-20%; and optional cobalt: 0-5%. The nickel-titanium alloy does not suffer from structural fatigue and/or functional fatigue after being subjected to at least ten million loading-unloading cyclic phase transformations.

Description

Cross Reference to the Related Application

The present application claims priority to the Chinese Patent Application No. 2023101853053 entitled "Nickel-titanium-based alloys, methods for manufacturing the same and applications thereof", the entire content of which is incorporated herein by reference.

Technical Field

The present disclosure relates to the field of functional alloys, and particularly relates to a nickel-titanium-based shape memory alloy, a method for manufacturing the same and applications thereof. The present nickel-titanium-based alloy belongs to a functional material with reversible martensitic transformation which could be induced by temperature or stress and provides shape memory effect and superelasticity correspondingly.

Background Art

The main problems of nickel-titanium-based alloys produced industrially at present are high phase transformation stress and poor stability during cyclic phase transformation. Polycrystalline or crystalline-amorphous nickel-titanium-based alloys generally require a high compressive stress of 900-1800 MPa to achieve full martensitic transformation. During cyclic phase transformation, nickel-titanium-based alloys demonstrate functional degradation with the increase of the number of cycles, which is mainly reflected by reductions of transformation stresses and hysteresis loop area, as well as an increase of residual strain. Such functional degradation may eventually lead to a complete loss of superelasticity, i.e., functional fatigue.
The insufficient strength and the incompatibility at phases boundaries between austenite and martensite of conventional nickel-titanium-based alloys can cause the generation and slip of phase transformation induced dislocations, and meanwhile, the dislocations may inhibit reverse phase transformation of martensite, resulting in the generation of residual martensite and leading to functional fatigue of nickel-titanium-based alloys. This is usually accompanied by the nucleation and propagation of cracks which lead to material fracture failure, i.e., structural fatigue, before the complete loss of phase transformation function for nickel-titanium. Taking the background of solid-state elastocaloric cooling application as an example, the required number of phase-transition cycles for an air conditioner using shape memory alloys is about 78 million cycles (12 hours per day and 6 months per year at a working frequency of 1 Hz) calculated based on a ten-year service life. However, most existing shape memory alloys including nickel-titanium cannot meet such a requirement.
Therefore, it is of huge demands to develop nickel-titanium-based alloys with improved properties, especially nickel-titanium-based shape memory alloys with improved cyclic phase transformation stability.

Summary

In order to solve the above problems, the present disclosure provides a novel nickel-titanium-based shape memory alloy, a method for manufacturing the same, and applications thereof in biomedical materials, components for solid-state elastocaloric cooling, for aerospace, for mechanical engineering, for automobiles and/or for buildings, etc. The alloy exhibits high strength, excellent superelasticity and unique combination of high cyclic phase transformation stability and reduced phase transformation stress.
In a particular embodiment, the nickel-titanium-based alloy undergoes at least ten million cycles of cyclic phase transformation under a compressive stress of 350-700 MPa and does not show significant change in the stress-strain curve.
Specifically, the present disclosure provides:

1. A nickel-titanium-based alloy, comprising titanium, nickel, copper and optional cobalt, wherein based on the total weight of the nickel-titanium-based alloy, the weight percentages of the titanium, nickel, copper and optional cobalt are respectively:
- titanium: 38% to 47%;
- nickel: 35% to 50%;
- copper: 3% to 20%; and
- cobalt: 0 to 5%.

Wherein the nickel-titanium-based alloy undergoes at least ten million cycles of loading-unloading cyclic phase transformation and does not show structural fatigue and/or significant functional fatigue.
Preferably, the nickel-titanium-based alloy undergoes at least ten million cycles of loading-unloading cyclic phase transformation under a compressive stress of 350-700 MPa, preferably 550-700 MPa and does not show structural fatigue and/or functional fatigue.
Preferably, the nickel-titanium-based alloy has a microstructure comprising Ti(Ni,Cu)₂ precipitates.
Preferably, the microstructure comprises 5% to 60% by volume of Ti(Ni,Cu)₂ precipitates, based on the total volume.
Preferably, the Ti(Ni,Cu)₂ precipitates has a thickness of 1-20 nm, preferably 1 nm, and a diameter of 10-500 nm.
Preferably, the nickel-titanium-based alloy comprises:

titanium: 38% to 47%;
nickel: 30% to 50%;
copper: 4% to 20%; and
cobalt: 0.1% to 5%.

Preferably, the nickel-titanium-based alloy further comprises unavoidable impurities containing: carbon ≤ 0.012% by weight, iron ≤ 0.015% by weight, sulfur ≤ 0.001% by weight, silicon ≤ 0.01% by weight, manganese ≤ 0.008% by weight, zinc ≤ 0.005% by weight, lead + tin + magnesium + bismuth ≤ 0.002% by weight, oxygen + nitrogen ≤ 0.1% by weight, and hydrogen ≤ 0.001% by weight; wherein taking into account small amount of other additive elements and unavoidable impurities, the total amount of the impurities is ≤ 0.5% by weight.
Preferably, the nickel-titanium-based alloy is manufactured by the following method:

melting raw materials of titanium, nickel, copper and optional cobalt into an alloy;
applying a plastic deformation treatment with a cumulative strain of 1% to 60% to the alloy from 1 to 3 directions; and
heat-treating and quenching the alloy undergoing the plastic deformation treatment to obtain the nickel-titanium-based alloy.

2. A method for manufacturing a nickel-titanium-based alloy, comprising:

(i) melting raw materials of titanium, nickel, copper and optional cobalt into an alloy sample;
(ii) applying a plastic deformation treatment with a cumulative strain of 1% to 60% to the alloy sample from 1 to 3 directions; and
(iii) heat-treating and quenching the alloy sample undergoing the plastic deformation treatment to obtain the nickel-titanium-based alloy.

Preferably, the method for manufacturing the nickel-titanium-based alloy further comprises a step of mixing the raw materials of titanium, nickel, copper and optional cobalt prior to step (i).
Preferably, the method for manufacturing the nickel-titanium-based alloy further comprises a step of cutting the melted alloy sample into a cube, a cuboid or a cylinder prior to step (ii), and step (ii) comprises applying a plastic deformation treatment with a cumulative strain of1% to 60% to the cube, the cuboid or the cylinder from 1 to 3 directions.
Optionally, the cutting is at least one selected from wire cutting, diamond electric sawing and laser cutting.
Preferably, step (iii) comprises carrying out the heat treatment at a temperature of 350-700 °C for 0.1-3 hours, and carrying out the quenching in water immediately after the heat treatment.
Preferably, step (i) comprises evacuating an electric arc melting furnace or an induction melting furnace, then introducing high-purity of argon gas with a purity of ≥ 99%, and subsequently melting the raw materials of titanium, nickel, copper and optional cobalt into an alloy.
Preferably, the electric arc melting is carried out under a current of 150-1000 A.
Optionally, the sample obtained by the electric arc melting may be button-shaped with a diameter of 5-100 mm and a thickness of 5-60 mm; or the sample obtained by the electric arc melting and suction casting may have a shape of cylindrical rod with a diameter of 5-20 mm and a height of 100-200 mm.
Preferably, the induction melting furnace has a current of 200-300 A and an oscillation frequency of 1-80 KHz.
Optionally, the sample obtained by the induction melting has a shape of cylindrical rod with a diameter of 5-100 mm and a height of 50-400 mm.
3. An article comprising a nickel-titanium-based alloy, wherein the nickel-titanium-based alloy is any of the nickel-titanium-based alloys described above or the nickel-titanium-based alloy manufactured by any of the methods described above.
Preferably, the article is at least one selected from a biomedical device, a solid-state elastocaloric cooling component, a joint, a fastener, a damping energy dissipation device, an antenna, an electromechanical actuator component, and an automotive component.
Preferably, the solid-state elastocaloric cooling component may generate a temperature drop of at least 10 °C when unloading under a compressive stress of 350-700 MPa.
Use of any of the nickel-titanium-based alloys described above or nickel-titanium-based alloys manufactured by any of the methods described above in biomedical materials, components for solid-state elastocaloric cooling, for aerospace, for mechanical engineering, for automobiles and/or for buildings.
Preferably, in a cyclic stress-strain curve test, the nickel-titanium-based alloy undergoes at least ten million cycles of cyclic phase transformation under a compressive stress of 350-700 MPa and does not generate significant change in the stress-strain curve. That is, compared to the first cycle, the residual strain after the ten millionth cycles of compressive deformation is not more than 1%, preferably not more than 0.2%, the change in phase transformation stress is not more than 25%, preferably not more than 3%, and the change in hysteresis loop area is not more than 30%, preferably not more than 10%.
The novel titanium-nickel-based alloy of the present invention has a microstructure comprising a large amount of nanoscale Ti(Ni,Cu)₂ precipitates. Preferably, the microstructure comprises 5% to 60% by volume of Ti(Ni,Cu)₂ precipitates.
Preferably, the Ti(Ni,Cu)₂ precipitates have a thickness of 1-20 nm, preferably 1 nm, and a diameter of 10-500 nm.
Thus, the dislocation slip is inhibited through the precipitation strengthening, and the martensitic transformation is promoted by the coherency strain and stress between the precipitates and the matrix, thereby, the effects of improving the cyclic phase transformation stability and reducing the phase transformation stress are achieved.
These and other non-limiting features of the present disclosure are disclosed in more detail below.

Brief Description of the Drawings

The following brief description of the drawings is intended to illustrate exemplary embodiments disclosed herein and not to limit the same.

Figure 1 is a partial flow diagram illustrating the preparation of a titanium-nickel-based alloy by a three-axial forging process according to one embodiment of the present disclosure, wherein 11, 12 and 13 are an upper ram, a cuboid sample and a lower ram, respectively.
Figure 2 is a partial flow diagram illustrating the preparation of a titanium-nickel-based alloy by a uniaxial forging process according to another embodiment of the present disclosure, wherein 21, 22 and 23 are an upper ram, a cylindrical sample and a lower ram, respectively.
Figure 3 is a photograph showing the microstructure of a titanium-nickel-based alloy after melting, forging and heat-treating according to an example of the present disclosure, wherein 31 and 32 are a TiNiCuCo matrix and Ti(Ni,Cu)₂ precipitates, respectively.
Figure 4 is a stress-strain curve of the titanium-nickel-based alloy undergoing cyclic deformations after melting, forging and heat-treating according to Example 1 of the present disclosure, wherein 41, 42, 43, 44 and 45 are the phase transformation stress, the residual strain, the hysteresis loop area, the stress-strain curve of the first cycle and the stress-strain curve of the ten millionth cycle, respectively.

Detailed Descriptions

The components, methods, and devices disclosed herein may be more fully understood with reference to the accompanying drawings. For convenience and ease of illustrating the present disclosure, these drawings are merely schematic representations, and are therefore not intended to represent the relative sizes and dimensions of the devices or components thereof, and/or to define or limit the scope of the exemplary embodiments.
Although specific terms are used in the following description for the sake of clarity, these terms are intended to refer only to the particular structure of the embodiments selected for illustration in the drawings, and are not intended to define or limit the scope of the present disclosure. In the drawings and the following description, it is to be understood that the similar numeric designations refer to components of similar function.
The singular forms "a", "an", and "the" include plural referents unless the context clearly dictates otherwise.
Numerical values in the description and claims of the present application should be understood as including the same numerical values when reduced to the same number of significant digits, as well as numerical values differing from the value by less than the experimental error of conventional measurement techniques of the type described in the present application to determine the value.
All ranges disclosed herein are inclusive of the recited endpoints and independently combinable (e.g., the range of "2 g to 10 g" is inclusive of the endpoints 2 g and 10 g, and is inclusive of all intermediate values).
A value modified by a term or terms, such as "about" and "substantially", may not be limited to the precise value specified. The terms used to denote approximation may be consistent with the accuracy of the instrument used to measure the value. The modifier "about" should also be considered as disclosing the range defined by the absolute values of the two endpoints. For example, a statement of "about 2 to about 4" also discloses the range of "2 to 4".
The present disclosure relates to temperature ranges. Notably, these temperatures refer to the temperature of the atmosphere to which the alloy is exposed, or the set temperature of the furnace; the alloy itself need not reach these temperatures.
As used herein, the term "structural fatigue" refers to the fracture failure of a material due to the nucleation and propagation of cracks caused by the lack of strength of the nickel-titanium-based alloy and the incompatibility at boundaries between austenitic and martensitic phases, and the propagation of the cracks before the complete loss of phase transformation function for the alloy.
As used herein, the term "functional fatigue" refers to the degradation of the phase transformation performance of the nickel-titanium-based alloy with an increase in the number of cycles during the cyclic phase transformation, mainly reflected by the decrease of phase transformation stress and hysteresis loop area and the increase of residual strain; such functional degradation with the increase in the number of usage cycles may eventually lead to a complete loss of the superelastic function, i.e., functional fatigue.
As used herein, the term "number of cycles of the cyclic phase transformation" refers to the number of times of a material undergoes a reversible austenite-martensite transformation under stress induction. For example, a material is transformed from austenite to martensite when a proper stress is applied, and then the material is transformed from martensite to austenite after the stress is released. Such a cyclic process is 1 cycle.
As used herein, the term "Ti(Ni,Cu)₂ precipitates" refers to island-shaped nanostructures (a second phase that precipitates from an initial matrix at a specific temperature, and generally does not have a phase transformation function) dispersed in a matrix of a continuous phase (e.g., a TiNiCuCo matrix continuous phase, having a phase transformation function) in a microstructure phase diagram of the nickel-titanium-based alloy. The Ti(Ni,Cu)₂ precipitates generally have a shape of disk, with a thickness of 1-20 nm, preferably 1 nm, and a diameter of 10-500 nm.
As used herein, the term "compressive stress" refers to the value obtained by dividing the compressive load applied to a test specimen by the original cross-sectional area of the test specimen, i.e., the absolute value of the engineering stress in MPa.
As used herein, the term "phase transformation stress" includes the initial stress of martensitic transformation, measured in megapascals (MPa) at the linearly extended intersection of the initial elastic deformation segment and the forward phase transformation segment in a loading stress-strain curve.
As used herein, the term "hysteresis loop area" refers to the area enclosed between the loading curve and the unloading curve in the stress-strain curve, in MPa or MJ/m³.
As used herein, the term "residual strain" refers to the strain that is not recovered after a material undergoing periodic deformation, i.e., the value obtained by dividing the change in length of a sample by the initial length of the sample.
As used herein, the term "superelasticity" refers to the phenomenon that a nickel-titanium-based alloy generates a strain much larger than the elastic limit strain of general metal structure materials under an external force, and the strain can be automatically recovered when unloaded. That is, in the parent phase state, stress-induced martensitic transformation occurs due to the application of external stress, so that the alloy exhibits a mechanical behavior different from that of normal materials, wherein it has an elastic limit far greater than that of normal materials and no longer follows Hooke's law. Superelasticity is force-driven, as compared to shape memory properties. In summary, superelasticity may be classified into linear superelasticity and nonlinear superelasticity. In the stress-strain curve of the former, there is a nearly linear relationship between stress and strain. The nonlinear superelasticity refers to that stress-induced martensitic transformation and reverse transformation thereof occurs respectively during loading and unloading processes within a certain temperature range above the austenite transformation finishing temperature (Af).
The alloys of the present disclosure exhibit excellent superelastic deformation compared to conventional nickel-titanium-based alloys. The alloys of the present disclosure may also exhibit excellent cyclic phase transformation stability and lower phase transformation stress.
In one aspect, the present disclosure provides a nickel-titanium-based alloy comprising titanium, nickel, copper and optional cobalt, wherein based on the total weight of the nickel-titanium-based alloy, the weight percentages of the titanium, nickel, copper and optional cobalt are respectively:

titanium: 38% to 47%;
nickel: 35% to 50%;
copper: 3% to 20%; and
cobalt: 0 to 5%.

The nickel-titanium-based alloy is capable of exhibiting excellent cyclic phase transformation stability, meaning that the alloy undergoes at least ten million cycles of loading-unloading cyclic phase transformation without generating structural fatigue and/or significant functional fatigue.
Preferably, the nickel-titanium-based alloy of the present disclosure undergoes at least ten million cycles of loading-unloading cyclic phase transformation under a compressive stress of 350-700 MPa without generating structural fatigue and/or significant functional fatigue.
In one embodiment, the nickel-titanium-based alloy comprises, in weight percent:

In the above nickel-titanium-based alloy, the sum of the weight percentages of all the components is 100% of the total weight of the alloy.
The nickel-titanium-based alloy may further contain unavoidable impurities. In an embodiment, the impurity content of the nickel-titanium-based alloy is: carbon ≤ 0.012% by weight, iron ≤ 0.015% by weight, sulfur ≤ 0.001% by weight, silicon ≤ 0.01% by weight, manganese ≤ 0.008% by weight, zinc ≤ 0.005% by weight, lead + tin + magnesium + bismuth ≤ 0.002% by weight, oxygen + nitrogen ≤ 0.1% by weight, and hydrogen ≤ 0.001% by weight; wherein taking into account small amount of other additive elements and unavoidable impurities, the total amount of the impurities is ≤ 0.5% by weight.
The nickel and titanium elements are basic constituent elements of the nickel-titanium-based alloy, and these two elements form a phase-changeable shape memory alloy with B2 crystal lattice in nearly equal atomic ratio. The excessive high or low content of any one element of these two elements may cause the formation of excessive precipitates which are rich in nickel (such as Ni₃Ti) or rich in titanium (such as Ti₂Ni) without the phase transformation function, thereby reducing its phase transformation strain and the cooling performance. The copper element is added to replace the position of the nickel element to a certain extent, so that the phase transformation is partially or completely changed from the original incompletely compatible B2-B19' phase transformation into the better compatible B2-B19 phase transformation, and the formation of a Ti(Ni,Cu)₂ precipitates is caused, thereby, the cyclic phase transformation stability of the material is improved. If the copper content is excessively low, it cannot cause the B2-B19 phase transformation and the formation of Ti(Ni,Cu)₂ precipitates. If the copper content is excessively high, the material has poor ductility and is not suitable for forging or other mechanical processing. The cobalt element has a function of effectively reducing the phase transformation temperature of the material, enabling the material to shows superelasticity at room temperature and achieves cooling at room temperature. Excessive cobalt content (>5%) may cause the phase transformation temperature of the material to be too low, resulting in loss of the superelasticity at room temperature and the reduction of the malleability of the material.
Impurities may affect the composition of the precipitates. For example, excessive oxygen and carbon may cause nickel-titanium oxide or titanium-carbon type precipitates, reducing the ductility and malleability of the material, and simultaneously reducing the recoverable strain, adiabatic temperature drop and fatigue resistance of the titanium-nickel-copper-cobalt material.
By improving the chemical components and processing technology, the novel titanium-nickel-based alloy developed by the present disclosure has a microstructure comprising a large amount of nano-sized Ti(Ni,Cu)₂ precipitates which have a thickness of 1-20 nm, preferably 1 nm, and a diameter of 10-500 nm. Figure 3 is a photograph showing the microstructure of a titanium-nickel-based alloy after melting, forging and heat-treating according to an example of the present disclosure.
As shown in Figure 3, the titanium-nickel-based alloy comprises a continuous phase (for example, a TiNiCuCo matrix) 31 and "nano-sized Ti(Ni,Cu)₂ precipitates 32 in the matrix".
Thus, the generation of dislocation is inhibited through the precipitation strengthening, and the martensitic transformation is promoted by the coherency strain and stress between the precipitates and the matrix, thereby, the effects of improving the cyclic phase transformation stability and reducing the phase transformation stress are achieved.
Preferably, the titanium-nickel-based alloy has a microstructure comprising 5% to 60% by volume of Ti(Ni,Cu)₂ precipitates, relative to the total volume.
Preferably, the nickel-titanium-based alloy of the present disclosure is manufactured by the following method:

In another aspect, the present disclosure provides a method for manufacturing a nickel-titanium-based alloy, comprising steps of:

Optionally, the method may further comprise a step of mixing the raw materials of titanium, nickel, copper and cobalt prior to step (i).
Optionally, the method may further comprise a step of cutting the melted alloy sample into a cube, a cuboid or a cylinder prior to step (ii), and the step (ii) comprises applying a plastic deformation treatment with a cumulative strain of 1% to 60% to the cube, the cuboid or the cylinder from 1 to 3 directions.
Optionally, the cutting is at least one selected from wire cutting, diamond electric sawing and laser cutting.
Optionally, in the method, step (iii) comprises carrying out the heat-treating at a temperature of 350-700 °C, preferably 450-700 °C for 0.1-3 hours, and carrying out the quenching in water immediately after the heat-treating.
Optionally, in the method, step (i) comprises evacuating an electric arc melting furnace or an induction melting furnace, then introducing high-purity of argon gas with a purity of ≥ 99%, and subsequently melting the raw materials of titanium, nickel, copper and cobalt into an alloy.
Optionally, in the method, the electric arc melting is carried out under a current of 150-1000 A.
Optionally, the sample obtained by the electric arc melting may be button-shaped with a diameter of 5-100 mm and a thickness of 5-60 mm; or the sample obtained by the electric arc melting and suction casting may have a shape of cylindrical rod with a diameter of 5-20 mm and a height of 100-200 mm.
Further optionally, in the method, the induction melting furnace has a current of 200-300 A and an oscillation frequency of 1-80 KHz.
Optionally, the sample obtained by the induction melting has a shape of cylindrical rod with a diameter of 5-100 mm and a height of 50-400 mm.
Figure 1 is a partial flow diagram illustrating the preparation of a titanium-nickel-based alloy by a three-axial forging process according to a preferred embodiment of the present disclosure.
As shown in Figure 1a, prior to the heat treatment, a plastic deformation treatment (i.e., forging process) of 1% to 20% strain is first applied to a cuboid sample 12 from a direction perpendicular to the C-plane thereof by an upper ram 11 and a lower ram 13. Next, in Figure 1b, the orientation of the sample is rotated centering on the y-axis direction so that the B-plane of the cuboid is perpendicular to the direction along which the stress is applied by the upper ram and the lower ram, thereby, a plastic deformation treatment of 0% to 20% strain to the sample from the direction perpendicular to the B-plane of the cuboid sample is applied. Finally, in Figure 1c, the orientation of the sample is further rotated centering on the x-axis direction so that the A-plane of the cuboid is perpendicular to the direction along which the stress is applied by the upper ram and the lower ram, thereby, a plastic deformation treatment of 0% to 20% strain to the sample from the direction perpendicular to the A-plane of the cuboid sample is applied.
Finally, a plastic deformation process (forging process) with a cumulative strain of 1% to 60% is applied to the sample, and then the forged sample is sent to the step of heat-treating.
Figure 2 is a partial flow diagram illustrating the preparation of a titanium-nickel-based alloy by a uniaxial forging process according to another preferred embodiment of the present disclosure.
As shown in Figure 2, a plastic deformation process (i.e., forging process) of 1% to 60% strain in total is applied to a cylindrical sample 22 along the axial direction thereof by an upper ram 21 and a lower ram 23. In this embodiment, the plastic deformation processing is performed on the sample along only one direction, which is different from the above-described plastic deformation processing performed on the sample along three directions.
Without being bound by theory, it is believed that the plastic deformation treatment with a cumulative strain of 1% to 60% applied to the sample along 1 to 3 directions, as described above, is the key step for forming a large amount of nano-sized Ti(Ni,Cu)₂ precipitates in the microstructure of the titanium-nickel-based alloy. During the plastic deformation process, a large amount of dislocation is generated in the crystal grains so that a structure of nano-sized dislocation cells is formed. During the heat treatment, the dislocation defects promote the nucleation of the precipitates in the dislocation network, and thereby, dense nano-sized precipitates are formed and the phase transformation stability of the material is enhanced.
In a third aspect, the present disclosure provides an article comprising a nickel-titanium-based alloy which is the nickel-titanium-based alloy described above.
Preferably, the article can be at least one selected from a biomedical device, a solid-state elastocaloric cooling component, a joint, a fastener, a damping energy dissipation device, an antenna, an electromechanical actuator component and an automotive component.
Preferably, the solid-state elastocaloric cooling component can generate a temperature drop of at least 10 °C when unloading under a compressive stress of 350-700 MPa.
In a fourth aspect, the present disclosure provides the use of the nickel-titanium-based alloy described above in biomedical materials, components for solid-state elastocaloric cooling, for aerospace, for mechanical engineering, for automobiles and/or for buildings.
According to the present disclosure, in a cyclic stress-strain curve test, the nickel-titanium-based alloy undergoes at least ten million cycles of cyclic phase transformation under a compressive stress of 350-700 MPa and does not generate significant change in the stress-strain curve. In other words, as compared to those of the first cycle, the residual strain of the ten millionth cycle in the test is not more than 1%, preferably not more than 0.2%, the change in phase transformation stress is not more than 25%, preferably not more than 3%, and the change in hysteresis loop area is not more than 30%, preferably not more than 10%.
The following examples are provided to illustrate the alloys, articles and methods of the present disclosure. These examples are illustrative only and are not intended to limit the disclosure to the materials, conditions, or process parameters described therein.

Example 1

Titanium, nickel, copper and cobalt, each having a purity of 99.995 atomic%, were mixed in weight percentages of titanium 43% : nickel 47% : copper 8.5% : cobalt 1.5%, and added into an electric arc melting furnace (DHL 400, SKY Technology Development Co., LTD.) which was evacuated to a pressure of 2×10^-4 Pa and then high-purity of argon with the purity of ≥ 99% was introduced. The current of the melting furnace was adjusted to be 150-700 A. After the raw materials were melted into an alloy, the sample was turned over and re-melted, with a total of 5 times melting and each for 1-4 minutes, thus obtaining an alloy sample. The alloy sample was cut into a cuboid by wire cutting (DWC 90G, Mitsubishi Electric). A plastic deformation treatment (forging process) with a cumulative strain of 60% was gradually applied to the cuboid from three directions using a homemade ceramic upper ram in the manner as shown in Figure 1. The forged alloy material was heat-treated at a temperature of 500 °C for 40 minutes, quenched in water immediately after the heat-treatment, with a quenching time of 1 minute. Thus, the titanium-nickel-copper-cobalt alloy of Example 1 was obtained.

Example 2

The titanium-nickel-copper-cobalt alloy of Example 2 was manufactured substantially in the same method as in Example 1, except that the heat-treatment was performed at a temperature of 520 °C.

Example 3

The alloy of Example 3 was manufactured substantially in the same method as in Example 1, except that titanium, nickel, copper and cobalt, each having a purity of 99.995 atomic%, were mixed as a raw material in weight percentages of titanium 42% : nickel 45% : copper 11.5% : cobalt 1.5%, and added into an electric arc melting furnace; and a plastic deformation treatment (forging process) with a cumulative strain of 6% was gradually applied to the cuboid from one direction.

Example 4

The alloy of Example 4 was manufactured substantially in the same method as in Example 3, except that a plastic deformation treatment (forging process) with a cumulative strain of 9% was gradually applied to the cuboid from one direction.

Example 5

The alloy of Example 5 was manufactured substantially in the same method as in Example 3, except that a plastic deformation treatment (forging process) with a cumulative strain of 20% was gradually applied to the cuboid from one direction.

Example 6

The alloy of Example 6 was manufactured substantially in the same method as in Example 5, except that the heat-treatment was performed at a temperature of 600 °C for 2 hours.

Example 7

The alloy of Example 7 was manufactured substantially in the same method as in Example 1, except that a plastic deformation treatment (forging process) with a cumulative strain of 6% was gradually applied to the cuboid from one direction.

Example 8

A titanium-nickel-copper-cobalt alloy of Example 1 was manufactured substantially in the same method as in Example 1, except that a plastic deformation treatment with a cumulative strain of 50% was gradually applied to the cuboid from three directions, and the heat-treatment was performed at a temperature of 400 °C.

Example 9

The titanium-nickel-copper-cobalt alloy of Example 1 was manufactured substantially in the same method as in Example 1, except that cobalt was not added, with weight percentages of titanium 43.7% : nickel 44.5% : copper 12%, and the heat-treatment was performed at a temperature of 400 °C for 10 minutes.

Comparative Example 1

An alloy was manufactured substantially in the same method as in Example 1, except that copper and cobalt were not added, and the raw materials were in weight percentages of titanium 44% and nickel 56%.

Comparative Example 2

An alloy was manufactured substantially in the same method as in Example 1, except that the forging process was not carried out.

Comparative Example 3

An alloy was manufactured substantially in the same method as in Example 3, except that the forging process was not carried out.

Comparative Example 4

An alloy was manufactured substantially in the same method as in Comparative Example 1, except that a plastic deformation treatment (forging process) with a cumulative strain of 50% was gradually applied to the cuboid from one direction.

Test Examples

- Mechanical and thermal properties of the alloys

The prepared samples of Examples and Comparative Examples were subjected to mechanical testing of isothermal loading and unloading (loading at a strain rate of 4 × 10^-4 s^-1 to 550 MPa and unloading at the same strain rate to 50 MPa) using a universal tester (MTS 858) equipped with a displacement meter (COD 632. 02F-20). The force-displacement data were recorded and further converted into stress-strain curves. The phase transformation stress was measured at the linearly extended intersection of the initial elastic deformation segment and the forward phase transformation segment in the loading stress-strain curve. The loading and unloading curves were integrated to obtain the area enclosed by the loading curve and the unloading curve, namely the hysteresis loop area. The heights of the samples before and after the cyclic loading were measured with a micrometer screw gauge and the residual strain after loading was calculated. The adiabatic temperature drop at 550 MPa was measured by using the following method: slowly loading to 550 MPa with a strain rate of 4 × 10^-4 s^-1, maintaining the force for 10 s, and quickly unloading to 0 MPa with a strain rate of 2 s^-1. At the same time, the temperature change of the sample surface at the time of rapid unloading was captured with an infrared camera (FLIR SC 7700M) and the adiabatic temperature drop was calculated.

The results are as shown in Table 1.

Table 1

Test specimen	Phase Transformation stress (MPa)	Hysteresis loop area under a stress of 550 MPa (MPa)	Residual strain (%) after 100 cycles of deformation under a stress of 550 MPa	Adiabatic temperature drop (°C) under 550 MPa
Example 1	413	1.47	0.16	17
Example 2	420	1.6	0.2	18
Example 3	278	1.64	0.15	18
Example 4	315	1.38	0.08	17.3
Example 5	263	2.48	0.27	16.8
Example 6	270	2.8	0.32	16
Example 7	282	1.72	0.19	18.3
Example 8	300	1.9	0.25	16.5
Example 9	290	2.1	0.22	17.2
Comparative Example 1	620	19	7	11
Comparative Example 2	450	6	1.8	9
Comparative Example 3	460	5.3	1.6	8
Comparative Example 4	630	18	7.3	10.5

It can be seen that Examples 1 to 9 have a larger adiabatic temperature drop (greater than 15 °C), lower phase transformation stress, smaller hysteresis loop area (representing lower energy consumption), and smaller residual strain (representing their ability to maintain better performance in cyclic deformation, i.e., higher cycling stability), as compared to Comparative Examples 1 to 4.

- Microstructure of the alloys

The microstructure of each samples of Examples and Comparative Examples was observed using a transmission electron microscope (JEOL Co., JEM-ARM 200F) at a voltage of 200 kV. Figure 3 is a photograph showing the microstructure of the titanium-nickel-based alloy after melting, forging and heat-treating according to Example 1. The photograph shows that the alloy contains a TiNiCuCo matrix with a large amount of nano-sized Ti(Ni,Cu)₂ precipitates in the matrix. The Ti(Ni,Cu)₂ precipitates have a thickness of 1 nm, and a diameter of 10-100 nm.

Table 2 shows the data of Ti(Ni,Cu)₂ precipitates in the microstructure of the alloys of the respective Examples and Comparative Examples.

Table 2

Test specimen	Volume proportion (%)	Thickness (nm)	Diameter (nm)
Example 1	35	1	10
Example 2	32	3	15
Example 3	8	5	25
Example 4	15	4	20
Example 5	25	3	13
Example 6	22	5	23
Example 7	7	4	24
Example 8	18	3	19
Example 9	16	5	21
Comparative Example 1	none	none	none
Comparative Example 2	1	80	700
Comparative Example 3	1	100	800
Comparative Example 4	none	none	none

The microstructures of samples of Examples 1 to 9 contain Ti(Ni,Cu)₂ precipitates, or the Ti(Ni,Cu)₂ precipitates have a smaller diameter and a smaller thickness, as well as a suitable proportion (more than 5% and less than 50%), as compared to those of Comparative Examples 1 to 4. The samples of Comparative Examples do not have Ti(Ni,Cu)₂ precipitates, or the precipitates have an excessively small proportion or large size. The precipitates with suitable proportion and proper size are beneficial to the adiabatic temperature drop, the cycling stability, the phase transformation stress, the coefficient of performance (COP, energy efficiency), the fatigue property and the like of the material.

- Loading-unloading cyclic phase transformation properties of the alloys

The loading-unloading curve was measured by a MTS Landmark 370.10 universal tester with a fatigue test frequency of 20 Hz, and the loading magnitude was determined by multiplying the cross-sectional area of the specimen by the designated stress (e.g., 550 MPa). Figure 4 shows the stress-strain curve under the cyclic deformation of the titanium-nickel-based alloy after melting, forging and heat-treating of Example 1. It can be seen that the alloy undergoing at least ten million cycles of cyclic phase transformation under a compressive stress of 550 MPa did not generate significant change in the stress-strain curve.

Table 3 shows changes in phase transformation stress, residual strain, hysteresis loop area and temperature drop after cyclic deformation under 550 MPa for each sample of Examples and Comparative Examples, wherein the sample of Example 1 underwent ten million cycles of cyclic phase transformation, and samples of other Examples underwent ten thousand cycles of cyclic phase transformation.

Table 3

Test specimen	Changes in phase transformation stress (%)	Residual strain (%)	Changes in hysteresis loop area (%)	Changes in temperature drop (%)
Example 1	3.8	0.19	16	1.5
Example 2	4.1	0.23	18	2
Example 3	13	0.5	26	9
Example 4	11	0.4	24	8
Example 5	24	0.9	24	9
Example 6	25	0.92	25	9
Example 7	12	0.45	24	9.1
Example 8	22	0.8	25	7
Example 9	23	0.85	26	9
Comparative Example 1	90	11	98	63
Comparative Example 2	44	10	96	65
Comparative Example 3	43	10.2	96.5	66
Comparative Example 4	91	11	98	63

Table 3 shows that the samples of Examples have better cycling stability than those of the Comparative Examples. Namely, the alloy materials can keep excellent performance in the cyclic loading process. The smaller the residual strain, and the smaller the changes in phase transformation stress, hysteresis loop area and temperature drop, the higher the cycling stability. For these four indicators, the values for Examples 1 to 9 are much smaller than those for the Comparative Examples, indicating that the alloy materials of Examples 1 to 9 have higher cycling stability. The above test results show that a large amount of nano-sized Ti(Ni,Cu)₂ precipitates can be induced in the alloy microstructure by controlling the chemical composition and the processing technique. Therefore, the generation of dislocation is inhibited through the precipitation strengthening, and the martensitic transformation is promoted by the coherency strain and stress between the precipitates and the matrix, thereby, the effects of improving the cyclic phase transformation stability and reducing the phase transformation stress are achieved.
It will be understood that variations of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other various systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by a person skilled in the art, which are also intended to be encompassed by the following claims.

Claims

A nickel-titanium-based alloy, comprising titanium, nickel, copper and optional cobalt, wherein based on the total weight of the nickel-titanium-based alloy, the weight percentages of the titanium, nickel, copper and optional cobalt are respectively:
titanium: 38% to 47%;

nickel: 35% to 50%;

copper: 3% to 20%; and

cobalt: 0 to 5%,

wherein the nickel-titanium-based alloy undergoes at least ten million cycles of loading-unloading cyclic phase transformation without structural fatigue and/or significant functional fatigue.
The nickel-titanium-based alloy according to claim 1, wherein the nickel-titanium-based alloy undergoes at least ten million cycles of loading-unloading cyclic phase transformation under a compressive stress of 350-700 MPa without structural fatigue and/or significant functional fatigue.
The nickel-titanium-based alloy according to claim 1 or 2, wherein the nickel-titanium-based alloy has a microstructure comprising Ti(Ni,Cu)₂ precipitates.
The nickel-titanium-based alloy according to claim 3, wherein the microstructure comprises 5% to 60% by volume of Ti(Ni,Cu)₂ precipitates, relative to the total volume.
The nickel-titanium-based alloy according to claim 3 or 4, wherein the Ti(Ni,Cu)₂ precipitates has a thickness of 1-20 nm, preferably 1 nm, and a diameter of 10-500 nm.
The nickel-titanium-based alloy according to any one of claims 1 to 5, wherein the nickel-titanium-based alloy comprises in weight percentage of:
titanium: 38% to 47%;

nickel: 30% to 50%;

copper: 4% to 20%; and

cobalt: 0.1% to 5%.
The nickel-titanium-based alloy according to any one of claims 1 to 6, wherein the nickel-titanium-based alloy further comprises unavoidable impurities containing: carbon ≤ 0.012% by weight, iron ≤ 0.015% by weight, sulfur ≤ 0.001% by weight, silicon ≤ 0.01% by weight, manganese ≤ 0.008% by weight, zinc ≤ 0.005% by weight, lead + tin + magnesium + bismuth ≤ 0.002% by weight, oxygen + nitrogen ≤ 0.1% by weight, and hydrogen ≤ 0.001% by weight; wherein taking into account small amount of other additive elements and unavoidable impurities, the total amount of the impurities is ≤ 0.5% by weight.
The nickel-titanium-based alloy according to any one of claims 1 to 7, wherein the nickel-titanium-based alloy is manufactured by the following method:
melting raw materials of titanium, nickel, copper and optional cobalt into an alloy;

applying a plastic deformation treatment with a cumulative strain of 1% to 60% to the alloy from 1 to 3 directions; and

heat-treating and quenching the alloy undergoing the plastic deformation treatment to obtain the nickel-titanium-based alloy.
A method for manufacturing a nickel-titanium-based alloy, comprising:
(i) melting raw materials of titanium, nickel, copper and optional cobalt into an alloy sample;

(ii) applying a plastic deformation treatment with a cumulative strain of 1% to 60% to the alloy sample from 1 to 3 directions; and

(iii) heat-treating and quenching the alloy sample undergoing the plastic deformation treatment to obtain the nickel-titanium-based alloy.
The method according to claim 9, further comprising a step of mixing the raw materials of titanium, nickel, copper and optional cobalt prior to step (i).
The method according to claim 9 or 10, further comprising a step of cutting the melted alloy sample into a cube, a cuboid or a cylinder prior to step (ii), and wherein step (ii) comprises applying a plastic deformation treatment with a cumulative strain of 1% to 60% to the cube, the cuboid or the cylinder from 1 to 3 directions; and
optionally, the cutting is at least one selected from wire cutting, diamond electric sawing and laser cutting.
The method according to any one of claims 9 to 11, wherein step (iii) comprises carrying out the heat-treating at a temperature of 350-700 °C for 0.1-3 hours, and carrying out the quenching in water immediately after the heat-treating.
The method according to any one of claims 9 to 12, wherein step (i) comprises evacuating an electric arc melting furnace or an induction melting furnace, then introducing high-purity of argon gas with a purity of ≥ 99%, and subsequently melting the raw materials of titanium, nickel, copper and optional cobalt into an alloy.
The method according to claim 13, wherein the electric arc melting is carried out under a current of 150-1000 A,
optionally, the sample obtained by the electric arc melting is button-shaped with a diameter of 5-100 mm and a thickness of 5-60 mm; or the sample obtained by the electric arc melting and suction casting has a shape of cylindrical rod with a diameter of 5-20 mm and a height of 100-200 mm.
The method according to claim 13, wherein the induction melting furnace has a current of 200-300 A and an oscillation frequency of 1-80 KHz;
optionally, the sample obtained by the induction melting has a shape of cylindrical rod with a diameter of 5-100 mm and a height of 50-400 mm.
An article comprising a nickel-titanium-based alloy, wherein the nickel-titanium-based alloy is the nickel-titanium-based alloy according to any one of claims 1 to 8, or the nickel-titanium-based alloy manufactured by the method according to any one of claims 9 to 15.
The article according to claim 16, wherein the article is at least one selected from a biomedical device, a solid-state elastocaloric cooling component, a joint, a fastener, a damping energy dissipation device, an antenna, an electromechanical actuator component and an automotive component.
The article according to claim 17, wherein the solid-state elastocaloric cooling component generates a temperature drop of at least 10 °C when unloading under a compressive stress of 350-700 MPa.
Use of the nickel-titanium-based alloy according to any one of claims 1 to 8, or the nickel-titanium-based alloy manufactured by the method according to any one of claims 9 to 15 in biomedical materials, components for solid-state elastocaloric cooling, for aerospace, for mechanical engineering, for automobiles and/or for buildings.
The nickel-titanium-based alloy according to any one of claims 1 to 8, the method according to any one of claims 9 to 15, the article according to any one of claims 16 to 18, or the use according to claim 19, characterized in that in a cyclic stress-strain curve test, the nickel-titanium-based alloy undergoes at least ten million cycles of cyclic phase transformation under a compressive stress of 350-700 MPa and does not generate significant change in the stress-strain curve, that is, compared to the first cycle, after the ten millionth cycle of compressive deformation, the residual strain is not more than 1%, preferably not more than 0.2%, the change in phase transformation stress is not more than 25%, preferably not more than 3%, and the change in hysteresis loop area is not more than 30%, preferably not more than 10%.