KR102497922B1 - Shape memory alloy with thermal expansion control through thermo-mechanical treatment, and manufacturing method of the same - Google Patents

Abstract

Various embodiments may provide a controlled thermal expansion shape memory alloy through thermo-mechanical treatment and a manufacturing method thereof. According to various embodiments, the controlled thermal expansion shape memory alloy is obtained by preparing a material having an austenite phase and including a plurality of differently oriented crystal grains, and converting at least a portion of the austenite phase into a martensite phase. It can be made from a material by applying a thermo-mechanical treatment to the material to induce deformation. According to various embodiments, the thermal expansion coefficient of the shape memory alloy is controlled by thermo-mechanical treatment, and the material may be implemented to have anisotropic thermal expansion characteristics exhibiting different thermal expansion characteristics according to different axial directions.

Description

Thermal expansion control type shape memory alloy through thermo-mechanical treatment and method for manufacturing the same

Various embodiments relate to a controlled thermal expansion shape memory alloy through thermo-mechanical treatment and a manufacturing method thereof.

Positive Thermal Expansion (PTE) of materials is an intrinsic property of materials that is very difficult to control, and it plays an important role in determining the stability of various structures and, in particular, the behavior and performance of precision instruments and devices. Therefore, from an industrial point of view, manufacture a material that exhibits negative thermal expansion (NTE) that makes the coefficient of thermal expansion close to zero (ZTE) or enables adjustment of thermal expansion through composite composition with other materials. It is important to do

In the case of the most representative ZTE-type Fe-Ni-based invar alloy commercialized to date, the technology development is limited due to a fixed composition system. Therefore, there is an urgent need to develop a new alloy that can compensate for these limitations and can not only adjust the thermal expansion of the material in a customized manner, but also consider other necessary properties such as diversification of operating temperature, mechanical properties, and electrical/thermal conductivity.

Various embodiments may provide a controlled thermal expansion shape memory alloy through thermo-mechanical treatment and a manufacturing method thereof.

Various embodiments may provide a controlled thermal expansion shape memory alloy and a manufacturing method thereof capable of obtaining at least one of thermal conductivity, electrical conductivity, or mechanical properties depending on the material.

A method for manufacturing a controlled thermal expansion shape memory alloy according to various embodiments includes preparing a material having an austenite phase and including a plurality of differently oriented crystal grains, and marten for at least a portion of the austenite phase. and applying a thermo-mechanical treatment to the material to induce transformation into a martensite phase, whereby a shape memory alloy having a controlled thermal expansion coefficient can be produced from the material by the thermo-mechanical treatment. there is.

A controlled thermal expansion shape memory alloy according to various embodiments may be implemented to have anisotropy exhibiting different thermal expansion characteristics along different axial directions.

According to various embodiments, when an appropriate thermo-mechanical treatment is applied, the in-plane direction of a plate material or the longitudinal direction of a bar or wire material is considered in an important direction when considering the actual utilization of the material. It is possible to control the anisotropy to suppress the thermal expansion of or show negative thermal expansion.

According to various embodiments, a controlled thermal expansion shape memory alloy can be manufactured through thermo-mechanical treatment. That is, through thermo-mechanical treatment, the shape memory alloy is implemented to exhibit different thermal expansion characteristics along different axial directions, and thus it is possible to adjust the thermal expansion coefficient of the shape memory alloy in a form suitable for the final application form. do. Through this, the stability of the structure to which the shape memory alloy is applied may be improved, and the behavior and performance of precision instruments and devices to which the shape memory alloy is applied may be improved. In addition, according to the selection of components of the shape memory alloy, at least one of high thermal conductivity, high electrical conductivity, or excellent mechanical properties may be additionally obtained in addition to thermal expansion control.

1 is a flow chart illustrating a manufacturing method of a controlled thermal expansion type shape memory alloy according to various embodiments.
2A and 2B are exemplary diagrams showing an austenite phase and a martensite phase.
Figure 3 is a view for explaining the thermal expansion adjustment of the shape memory alloy according to one embodiment.
Figure 4 is a view for explaining the thermal expansion adjustment of the shape memory alloy according to another embodiment.
5, 6 and 7 are views for explaining experiments on shape memory alloys according to various embodiments.
8 is a view showing the thermal expansion coefficient of the shape memory alloy according to various embodiments.

Hereinafter, various embodiments of this document will be described with reference to the accompanying drawings.

1 is a flow chart illustrating a manufacturing method of a controlled thermal expansion type shape memory alloy according to various embodiments. 2A and 2B are exemplary diagrams showing an austenite phase and a martensite phase.

Referring to FIG. 1 , in step 110, a polycrystal material may be prepared. In this case, the material may include a plurality of differently oriented grains having an austenite phase as shown in FIG. 2A and having random orientations. Here, the material may be prepared from a shape memory alloy based on thermoelastic martensitic phase transformation such as NiTi, Cu-base, or Fe-base. For example, the material is made of NiTi (nickel titanium) shape memory alloy, and in FIG. 2 , purple may represent nickel (Ni) atoms and green may represent titanium (Ti) atoms.

Next, in steps 120 and 130, a thermo-mechanical treatment may be applied to the prepared shape memory alloy material. Through this, in the material, a phase transformation into martensite phase as shown in FIG. 2B for a part or all fraction of the austenite phase can be induced. At this time, the low-temperature phase martensite phase can be induced to have a deflected orientation. Through this, the material can be implemented to have anisotropic thermal expansion characteristics exhibiting different thermal expansion characteristics along different axial directions. Accordingly, in the material, a stable zero (0) thermal expansion (ZTE) can be secured in a direction perpendicular to the strain axis.

Specifically, in step 120, a mechanical load may be applied to the material under a predetermined temperature. Through this, a phase transformation of part or all of the austenite phase into the martensite phase may be induced in the material, and thus macroscopic anisotropy of thermal expansion may be expressed. At this time, the mechanical load may be applied in a repetitive form, including at least one of a compression load or a tension load. Thereafter, in step 130, a thermal load may be applied to the material. Through this, the ratio of the austenite phase and the martensite phase in the material can be controlled and macroscopic thermal expansion anisotropy can be controlled. At this time, the thermal load may be lower or higher than the temperature when the mechanical load is applied. This will be described in more detail later with reference to FIGS. 3 and 4 .

According to some embodiments, at step 120, a mechanical load may be repeatedly applied to the cell several times under a predetermined temperature (cyclic mechanical loading). At this time, the average anisotropy of thermal expansion of each crystal grain can be finely adjusted. That is, the coefficient of thermal expansion (CTE) according to the macroscopic direction of the shape memory alloy may be finely adjusted.

Figure 3 is a view for explaining the thermal expansion adjustment of the shape memory alloy according to one embodiment.

Referring to FIG. 3 , under a predetermined temperature, for example, 300 K, a compressive type of mechanical load may be repeatedly applied to the material in a specific direction several times. For example, as a compressive load is repeatedly applied to a material having a specific average grain size (12 nm), the material may be deformed as shown in (a) of FIG. 3 . At this time, in the material, as shown in (c) of FIG. 3, a phase transformation may occur (① -> ② -> ③). In (c) of FIG. 3 , blue color may represent an austenite phase, and red color may represent a martensite phase. That is, as a compressive load is repeatedly applied, a part or all of the austenite phase may undergo phase transformation into the martensite phase. Here, as shown in (d) of FIG. 3 so that different colors can represent different crystal directions, a change in the microstructure can be made so that martensite phase crystal grains in the material prefer a specific direction, thereby changing the martensite phase. Orientation of the configured crystal grains may be rearranged. The ratio of the martensite phase and the orientation of each crystal grain can be controlled by selecting the number of repetitions of the compression load and the maximum stress of each load.

After this, repetitive thermal loads may be applied to the material. For example, a thermal load that alternates between a higher temperature and a lower temperature than the predetermined temperature, for example, a temperature of 300 K, may be applied. In this case, as shown in (c) of FIG. 3, a phase transformation may occur (⑥). That is, as a thermal load by repetitive heating and cooling is applied, the ratio of the martensite phase is changed and the orientation of each crystal grain can be adjusted. Here, as shown in (d) of FIG. 3 where different colors represent different crystal directions, a change in the microstructure in the material may be made, and the orientation of crystal grains composed of the martensite phase may be rearranged. Through this, as shown in (b) of FIG. 3, the material exhibits positive (+) thermal expansion characteristics in the rod direction at a predetermined temperature, for example, a temperature lower than 300 K, and in lateral directions orthogonal to the rod direction. It can exhibit negative (-) or zero (0) thermal expansion characteristics.

Figure 4 is a view for explaining the thermal expansion adjustment of the shape memory alloy according to another embodiment.

Referring to FIG. 4 , under a predetermined temperature, for example, 300 K, a tensile mechanical load may be repeatedly applied several times in a specific direction to the material. For example, as a tensile load is repeatedly applied to a material having a specific average grain size (12 nm), the material may be deformed as shown in FIG. 4(a). At this time, in the material, as shown in (c) of FIG. 4, a phase transformation may occur (① -> ② -> ③). In (c) of FIG. 4 , blue color may represent an austenite phase, and red color may represent a martensite phase. That is, as a tensile load is repeatedly applied, a part of the austenite phase may undergo phase transformation into the martensite phase. Here, as shown in (d) of FIG. 4 where different colors indicate different crystal orientations, a change in the microstructure in the material may be made and the orientation of crystal grains composed of the martensite phase may be rearranged. The ratio of the martensite phase and the orientation of each crystal grain can be controlled by selecting the number of repetitions of the compression load and the maximum stress of each load.

After this, repetitive thermal loads may be applied to the material. For example, a thermal load that alternates between a higher temperature and a lower temperature than the predetermined temperature, for example, a temperature of 300 K, may be applied. In this case, as shown in (c) of FIG. 4, a phase transformation may occur (⑥). That is, as a thermal load by repetitive heating and cooling is applied, the ratio of the martensite phase is changed and the orientation of each crystal grain can be adjusted. Here, as shown in (d) of 4 where different colors represent different crystal directions, a change in the microstructure in the material may be made, and the orientation of crystal grains composed of the martensite phase may be rearranged. Through this, as shown in (b) of FIG. 4, the cell exhibits negative (-) thermal expansion characteristics in the rod direction at a predetermined temperature, for example, a temperature lower than 300 K, and in lateral directions orthogonal to the rod direction. It can exhibit positive (+) thermal expansion characteristics.

3 and 4 show molecular dynamics simulation results considering cyclic compression load and tension load, respectively. Stress-strain curves when cyclic compression or tensile load is applied 5 times at 300 K are shown in FIG. 3(a) and FIG. 4(a), respectively. 3(b) and 4(b) show temperature dependence of atomic volume and strain in each direction. In these results, instead of cooling the material after applying the mechanical load, heating to a high temperature (500 K) was performed immediately (③→⑤ in FIG. 3(b) and FIG. 4(b)). In this process, the martensite phase obtained from the initial mechanical loading completely disappears and causes a phase transformation to the austenite phase. During the subsequent cooling process, the phase transformation from austenite to martensite occurs accompanied by a rapid change in volume (⑤→⑥ in FIG. 3(b) and FIG. 4(b)).

An interesting observation during this cyclic mechanical/thermal loading process is that the anisotropy of thermal expansion is still present during the additional cooling process after heating to completely revert to austenite after application of the mechanical load (Fig. 3(c) and Fig. 4). of (c)). That is, negative (-) or zero (+) thermal expansion in the lateral direction for a single compression rod and negative (-) thermal expansion along the tensile direction by a single tension rod are shown to be the same. As shown in the directional visualization of each crystal grain (Fig. 3(d) and Fig. 4(d)), the color patterns of several grains obtained through the initial mechanical load appear the same in the grains obtained by the subsequent heating and cooling process. . This result suggests that the orientation and twin structure of martensite in several crystal grains are memorized during the phase transformation process induced under the initial cyclic mechanical load and then reproduced during the phase transformation process following the temperature change. This behavior is similar to the well-known two-way shape-memory effect, which remembers not only the shape of the material made of the high-temperature austenite phase, but also the shape of the material made of the martensite phase at low temperatures.

This is because plastic deformation due to grain boundaries or dislocation is gradually accumulated by a periodic mechanical load, which affects the anisotropy of martensite grains during the phase transformation process. As can be seen in (e) of FIG. 3 and (e) of FIG. 4, which visualize local plastic deformation, actual plastic deformation is accumulated in the grain boundary region. The plastic deformation accumulated during this periodic mechanical load acts as a factor that allows the nearby martensite phase to adapt to and remember the surrounding environment. During the subsequent thermal loading process, the formation of martensite is guided by this pre-accumulated plastic strain and is induced to favor a particular orientation.

The results shown in FIGS. 3 and 4 indicate that the pre-training process by periodic mechanical/thermal load can be an additional variable that can be adjusted to obtain the desired thermal expansion behavior.

In order to confirm the anisotropic thermal expansion characteristics of the above-described controlled thermal expansion shape memory alloy, experiments were conducted. This will be described later with reference to FIGS. 5 , 6 and 7 .

5, 6 and 7 are views for explaining experiments on shape memory alloys according to various embodiments.

First, as shown in (a) of FIG. 5, the cast heat-treated material was prepared into a regular hexahedron. Here, the material exhibited a phase transformation temperature as shown in FIG. 6(a). Based on the phase transformation temperature of these materials, thermo-mechanical properties to be applied to the material and subsequent heat treatment temperature can be determined. After this, a thermo-mechanical treatment was applied to the material. At this time, as shown in (b) of FIG. 5, the temperature of the material (black line) was changed, and mechanical deformation (orange line) was generated for the material. Here, the material is heated to a temperature at which the austenite phase, which is a high-temperature phase, is stable. Then, repetitive mechanical loads were applied to the material. By cooling the material under an applied mechanical load, martensitic crystal grains biased in a specific orientation were created. As a result of the X-ray diffraction test for the material as shown in FIG. 6 (b) and a confirmation result through a transmission electron microscope as shown in FIG. 6 (c), as the thermo-mechanical treatment is applied, the material is composed of only the martensite phase at room temperature.

At this time, the thermal expansion characteristics of the casting heat-treated material showed the same thermal expansion characteristics in all directions, as shown in FIG. 7 (a). Here, the thermal expansion characteristics of the material exhibited a positive slope, that is, expansion by heating. And, as the thermo-mechanical treatment was applied to the material, the material exhibited anisotropic thermal expansion characteristics as shown in (b), (c) and (d) of FIG. 7 . When the thermo-mechanical treatment was repeated three times at a strain rate of 5%, the material exhibited anisotropic thermal expansion characteristics as shown in FIG. 7(b). When the thermo-mechanical treatment was repeated 10 times at a strain rate of 5%, the material exhibited anisotropic thermal expansion characteristics as shown in FIG. 7(c). When the thermo-mechanical treatment was applied once at a strain rate of 10%, the material exhibited anisotropic thermal expansion characteristics as shown in FIG. 7(d).

In addition, after applying high-temperature thermo-mechanical treatment in the casting heat treatment state (As-homogenized), the degree of deformation and recovery of the material was measured. Here, as shown in (e) of FIG. 7, in the case of the state where the thermo-mechanical treatment is applied (Trained), depending on the strain rate and the number of repetitions of the thermo-mechanical treatment, the material is different compared to the conventional (100%) Strain was measured. Then, after heat treatment at 120 degrees, which completes the austenite phase transformation, with the mechanical load removed, the degree of strain recovery was confirmed through shape memory characteristics. Here, as shown in (e) of FIG. 7, in the state where 120 degree heat treatment is applied once (N = 1), twice (N = 2) and three times (N = 3), the strain rate for the material recovery rate was confirmed.

At this time, the coefficient of thermal expansion in each direction for the material in each state, that is, the casting heat treatment state, the thermo-mechanical treatment applied state, and the 120 degree heat treatment applied state, was measured as shown in (f) of FIG. . As a result, it was confirmed that as the number of repetitions of the thermo-mechanical treatment increased in the direction perpendicular to the strain axis, a more stable ZTE could be secured for the material. Here, in view of the recovery rate as shown in (e) of FIG. 7, the increase in the number of repetitions of the thermo-mechanical treatment causes a two-way shape memory effect, so that the martensite orientation at room temperature It is continuously maintained even after heat treatment at 120 degrees, and it is possible to secure thermal expansion characteristics as shown in (f) of FIG. 7 for the material. Therefore, the thermo-mechanical treatment process may be determined in consideration of the phase transformation temperature of the shape memory alloy, the temperature range to be applied, and the required thermal expansion coefficient.

8 is a view showing the thermal expansion coefficient of the shape memory alloy according to various embodiments.

Referring to FIG. 8 , the above-described shape memory alloy may have a different coefficient of thermal expansion depending on its material. At this time, the thermal expansion coefficient of the shape memory alloy may be finely adjusted to a desired shape according to the control of the number of applications of at least one of a mechanical load or a thermal load applied during manufacture of the shape memory alloy. For example, the shape memory alloy material is prepared from a shape memory alloy based on thermoelastic martensitic phase transformation such as NiTi, Cu-based, and Fe-based, and satisfies thermal expansion control and, depending on the selected configuration, has high thermal conductivity. At least one of degree, high electrical conductivity, or excellent mechanical properties may be additionally obtained.

According to various embodiments, a controlled thermal expansion shape memory alloy can be manufactured through thermo-mechanical treatment. That is, through thermo-mechanical treatment, it is possible to adjust the thermal expansion coefficient for each direction macroscopically by adjusting the orientation of the martensite phase constituting the shape memory alloy into a desired shape. By applying the material obtained through this process, the stability of the structure can be improved, and the behavior and performance of precision instruments and devices can be improved. In addition, according to the selection of components of the shape memory alloy, at least one of high thermal conductivity, high electrical conductivity, or excellent mechanical properties may be additionally obtained in addition to thermal expansion control.

A method for manufacturing a controlled thermal expansion shape memory alloy according to various embodiments includes preparing a material having an initial austenite phase and including a plurality of differently oriented crystal grains (step 110), and a portion of the austenite phase. or applying a thermo-mechanical treatment to the material to induce a phase transformation to a martensite phase for the majority (steps 120 and 130).

According to various embodiments, when an appropriate thermo-mechanical treatment is applied, the in-plane direction of a plate material or the longitudinal direction of a bar or wire material is considered in an important direction when considering the actual utilization of the material. It is possible to control the anisotropy to suppress the thermal expansion of or show negative thermal expansion.

According to various embodiments, the thermal expansion coefficient of the shape memory alloy may be determined according to the selection (number of repetitions, maximum stress, etc.) of mechanical load and thermal load applied to the material.

According to various embodiments, the step of applying a thermo-mechanical treatment to the material (steps 120 and 130) is performed under a predetermined temperature to induce transformation of some or most of the austenite phase to martensite phase, It may include applying a mechanical load to the workpiece (step 120) and applying a thermal load to the workpiece (step 130).

According to various embodiments, at least one of the step of applying a mechanical load (step 120) or the step of applying a thermal load (step 130) is performed by a predetermined number of repetitions, and the thermal expansion coefficient of the shape memory alloy may be determined according to the number of iterations.

According to various embodiments, a material may be implemented to have anisotropy exhibiting different thermal expansion characteristics along different macroscopic directions.

According to various embodiments, applying a thermal load to the material (step 130) includes applying a thermal load at a temperature lower than the phase transformation temperature to increase the proportion of martensite phase, and increasing the proportion of austenite phase. In order to do so, it may include applying a thermal load at a temperature higher than the phase transformation temperature.

According to various embodiments, mechanical loads may include compression and tension loads applied in a particular direction.

According to one embodiment, the material prepared by the compression rod exhibits positive (+) thermal expansion characteristics in the rod direction at a temperature lower than the phase transformation temperature, and negative (-) or zero (0) thermal expansion characteristics in lateral directions orthogonal to the rod direction. ) can represent the thermal expansion characteristics of

According to another embodiment, a material prepared by a tensile rod exhibits negative (-) thermal expansion characteristics in the rod direction and positive (+) thermal expansion characteristics in lateral directions orthogonal to the rod direction at a temperature lower than the phase transformation temperature. can indicate

According to various embodiments, in a material, a stable zero (0) thermal expansion (ZTE) may be secured in a direction perpendicular to the strain axis.

According to various embodiments, the material is prepared from a shape memory alloy based on thermoelastic martensitic phase transformation, such as NiTi, Cu-based (base), or Fe-based, and the shape memory alloy having controlled thermal expansion is composed of components. Depending on the selection, at least one of high thermal conductivity, high electrical conductivity, or excellent mechanical properties may be additionally obtained.

Various embodiments of this document and terms used therein are not intended to limit the technology described in this document to a specific embodiment, and should be understood to include various modifications, equivalents, and/or substitutes of the embodiment. In connection with the description of the drawings, like reference numerals may be used for like elements. Singular expressions may include plural expressions unless the context clearly dictates otherwise. In this document, expressions such as "A or B", "at least one of A and/or B", "A, B or C" or "at least one of A, B and/or C" refer to all of the items listed together. Possible combinations may be included. Expressions such as "first," "second," "first," or "second" may modify the elements in any order or importance, and are used only to distinguish one element from another. The components are not limited. When a (eg, first) component is referred to as being “connected” or “connected” to another (eg, second) component, the certain component is directly connected to the other component, or It may be connected through another component (eg, a third component).

According to various embodiments, each of the components described above may include a single entity or a plurality of entities. According to various embodiments, one or more components or steps among the aforementioned components may be omitted, or one or more other components or steps may be added. Alternatively or additionally, a plurality of components may be integrated into one component. In this case, the integrated component may perform one or more functions of each of the plurality of components identically or similarly to those performed by the corresponding component among the plurality of components prior to integration. According to various embodiments, steps performed by a module, program, or other component are executed sequentially, in parallel, iteratively, or heuristically, or one or more of the steps are executed in a different order, omitted, or , or one or more other steps may be added.

Claims (10)

In the method for producing a controlled thermal expansion shape memory alloy,
Preparing a material having an austenite phase and including a plurality of differently oriented crystal grains; and
Applying a thermo-mechanical treatment to the material to induce transformation of at least a portion of the austenite phase to a martensite phase.
including,
Thus, a shape memory alloy having a controlled thermal expansion coefficient is manufactured from the material by the thermo-mechanical treatment,
The thermal expansion coefficient of the shape memory alloy,
It is determined according to the size of the mechanical load and thermal load applied to the material,
Applying the thermo-mechanical treatment to the material comprises:
applying the mechanical load to the material, under a predetermined temperature, to induce transformation of at least a portion of the austenite phase into the martensite phase; and
applying the thermal load to the workpiece;
including,
manufacturing method.
delete delete According to claim 1,
At least one of applying the mechanical load or applying the thermal load,
It is performed for a predetermined number of iterations,
The thermal expansion coefficient of the shape memory alloy,
Depending on the number of iterations, determined,
manufacturing method.
According to claim 1,
The material is embodied to have anisotropic thermal expansion properties exhibiting different thermal expansion characteristics along different axial directions.
manufacturing method.
According to claim 1,
Applying the thermal load to the material comprises:
applying the thermal load at a temperature lower than the temperature to increase the proportion of the martensite phase; and
Applying the thermal load at a temperature higher than the temperature to increase the proportion of the austenite phase.
including,
manufacturing method.
According to claim 1,
The mechanical load includes a compression load applied in a specific direction;
The material is
As the compression load is applied, exhibiting positive thermal expansion characteristics in the direction and negative or zero thermal expansion characteristics in a direction orthogonal to the direction,
manufacturing method.
According to claim 1,
The mechanical rod includes a tension rod applied in a specific direction,
The material is
As the tension rod is applied, it exhibits negative thermal expansion characteristics in the direction and positive thermal expansion characteristics in a direction orthogonal to the direction.
manufacturing method.
According to claim 1,
The material is
It is prepared from shape memory alloys based on thermoelastic martensitic phase transformation such as NiTi, Cu-base, and Fe-base,
The shape memory alloy,
Depending on the configuration, further obtaining at least one of high thermal conductivity, high electrical conductivity, or excellent mechanical properties,
manufacturing method.
Claim 1, or a shape memory alloy produced according to the manufacturing method according to any one of claims 4 to 9.

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