WO2024172330A1 - Cr-fe-co-based high-entropy alloy with shape memory characteristic and prediction method for composition range of the alloy - Google Patents

Abstract

The present invention relates to a Cr-Fe-Co-based high-entropy alloy with shape memory characteristics at high temperatures, a method for predicting the composition range of the Cr-Fe-Co-based high-entropy alloy, and a high-entropy alloy manufactured by the method, wherein the high-entropy alloy can be easily manufactured through thermodynamic calculations from existing high-entropy alloys. The invention allows for the development of various types of shape memory alloys that have not been previously reported, by predicting compositions expected to exhibit shape memory characteristics from numerous combinations of high-entropy alloys through thermodynamic calculations. In particular, the Cr-Fe-Co-based high-entropy alloy according to the present invention can be used as a shape memory alloy in high-temperature environments with a phase transformation temperature of 100°C or higher, where the conventional Ti-Ni alloy system cannot be applied. Therefore, the high-entropy alloy can be used as a structural material for components requiring shape memory characteristics at high temperatures, such as actuators. Particularly, the high-temperature shape memory alloy is especially suitable for miniaturizing heavy and bulky actuators and thus is expected to find applications in a wide spectrum of transportation and power generation fields, such as in automobiles, airplanes, and power plants.

Description

CR-FE-CO high-entropy alloy with shape memory properties and method for predicting the composition range of the alloy

The present invention relates to a Cr-Fe-Co high-entropy alloy having shape memory characteristics and a method for predicting the composition range of the alloy, and more specifically, to a Cr-Fe-Co high-entropy alloy having shape memory characteristics at high temperatures, a method for predicting the composition range of the Cr-Fe-Co high-entropy alloy, and a high-entropy alloy manufactured by the method, characterized in that the alloy can be easily manufactured from an existing high-entropy alloy by thermodynamic calculation.

High entropy alloys (HEAs) are alloys made by mixing a number of elements in nearly equal proportions, and are attracting attention as next-generation structural materials due to their excellent mechanical properties.

Recently, many studies have been reported to develop new high-entropy alloys that exhibit twinning-induced plasticity (TWIP) or transformation-induced plasticity (TRIP) by controlling the stacking fault energy (SFE) by adjusting the element ratio constituting the high-entropy alloy in order to develop new alloys with superior mechanical properties. However, there are not many papers or patents related to shape memory characteristics in the development of high-entropy alloys.

Currently commercialized shape memory alloys include Ti-Ni, Cu-Al-Ni, and Fe-Mn-Si, and the number of alloy systems that exhibit shape memory properties among metal materials is very limited. The Ti-Ni shape memory alloy, which is the most widely used in industry, has the advantages of excellent recovery rate and rapid recovery reaction, but its formability is poor, making it expensive to process into products. In addition, the Ti-Ni shape memory alloy has a fatal disadvantage in that it cannot be used as a shape memory alloy at temperatures higher than 100℃ because its phase transformation temperature is low. Research has been conducted to change the Ti/Ni composition in the alloy or to add new elements to increase the phase transformation temperature of the Ti-Ni alloy, but there remains a challenge in that brittle intermetallic compounds that lower the recovery properties are formed during the alloy manufacturing process.

Meanwhile, as a means of improving the problems described above, research results on new shape memory alloys are emerging, but there has been a problem in that the scope of research and development is limited and carried out by mixing different types of elements into commonly known commercial shape memory alloys and controlling the amount of addition.

Therefore, there is an urgent need for the development of various types of high-entropy alloys and methods for expanding the composition range of the alloys to improve the problems described above.

The purpose of the present invention is to provide a Cr-Fe-Co high-entropy alloy having shape memory characteristics at high temperatures, a method for predicting the composition range of a Cr-Fe-Co high-entropy alloy characterized in that the alloy can be easily manufactured by predicting the alloy composition range with low stacking fault energy (SFE) from an existing high-entropy alloy through thermodynamic calculation, and a high-entropy alloy manufactured by the method.

A preferred embodiment of the present invention for achieving the above-mentioned task is a Cr-Fe-Co high-entropy alloy having shape memory characteristics, characterized by being a ternary alloy of Cr (chromium), Fe (iron), and Co (cobalt), as a means for solving the task.

In addition, the ternary alloy is characterized by being a Cr-Fe-Co high-entropy alloy having shape memory properties represented by the following [chemical formula].

[chemical formula]

Cr _a Fe _b Co _c

(a+b+c=100, 15≤a≤40 at.%, 10≤b≤60 at.%, 20≤c≤50 at.%)

In addition, the ternary alloy is characterized by being composed of FCC or FCC + HCP or FCC + HCP + σ or FCC + HCP + BCC phases at room temperature when homogenized at 1,473 K, a phase equilibrium temperature (T _O ) of 200 to 1,060 K, and a free energy change amount [ΔG ^(hcp-fcc) ] (based on 300 K) of -2,013 to 228 J/mol.

And another preferred embodiment of the present invention is a method for predicting a composition region of a Cr-Fe-Co high-entropy alloy having a shape memory characteristic, which comprises the steps of: (S1) predicting an alloy composition region having a low stacking fault energy (SFE) by thermodynamic calculation in a high-entropy alloy; (S2) selecting a composition region of a high-entropy alloy in which a stable phase is predicted at a high temperature in the composition region of the high-entropy alloy; and (S3) confirming whether shape memory characteristics are expressed in the selected alloy composition region.

In addition, the alloy composition region prediction step (S1) determines the phase equilibrium temperature T 0 at which the free energy change amount (ΔG ^(HCP-FCC) ) of the γ austenite phase (FCC) and the ε martensite phase (HCP) in the component region of the high-entropy alloy is ₀ , and the phase equilibrium temperature is characterized by T ₀ >0K.

In addition, in the high-entropy alloy composition region selection step (S2), the high temperature is 1,473 K (1,200°C), and the stable phase is the γ austenite phase (FCC).

The present invention has the effect of enabling the simple manufacture of a high-entropy alloy having shape memory characteristics by predicting an alloy composition region having a low stacking fault energy (SFE) through thermodynamic calculations.

And the Cr-Fe-Co high-entropy alloy according to the present invention is a shape memory alloy that can be used in a high-temperature environment with a phase transformation temperature of 100℃ or higher, and can be applied in a temperature range where the existing Ti-Ni alloy cannot be used, and therefore can be applied as a structural material for parts that require shape memory characteristics at high temperatures, such as actuators. In particular, the high-temperature shape memory alloy is a material that is very suitable for miniaturizing heavy and bulky actuators, and can be widely applied in the transportation/power generation fields, such as automobiles, airplanes, and power plants, in the future.

* In particular, the present invention has the effect of enabling the development of various types of shape memory alloys not previously reported by a simple method by predicting, through thermodynamic calculations, the composition expected to exhibit shape memory characteristics among numerous combinations of high-entropy alloys made by mixing a number of elements in nearly equal proportions, by identifying the compositional region in which shape memory characteristics appear in a Cr-Fe-Co ternary alloy through thermodynamic calculations using CALPHAD thermodynamic calculations.

Figure 1 is a block diagram showing a method for predicting the composition range of a Cr-Fe-Co high-entropy alloy having shape memory characteristics according to an embodiment of the present invention.

FIG. 2 is a graph showing the change in Gibbs free energy (ΔG) according to temperature change of a high-entropy alloy having shape memory characteristics according to an embodiment of the present invention and a Cr ₂₀ Mn ₂₀ Fe ₂₀ Co ₂₀ Ni ₂₀ alloy in comparison thereto.

FIG. 3 is a graph showing the results of changes in equilibrium phase fractions according to temperature changes in a Cr-Fe-Co high-entropy alloy according to an embodiment of the present invention and a Cr ₂₀ Mn ₂₀ Fe ₂₀ Co ₂₀ Ni ₂₀ alloy in comparison thereto.

FIG. 4 is a diagram showing a ternary phase diagram according to the Cr-Fe-Co mole fraction of a Cr-Fe-Co high-entropy alloy according to an embodiment of the present invention.

FIG. 5 is a drawing showing an XRD analysis graph of a Cr-Fe-Co high-entropy alloy according to an embodiment of the present invention.

Figure 6 is a photograph taken to check whether the shape of a Cr-Fe-Co high-entropy alloy according to an embodiment of the present invention is restored as a result of a bending test.

Figure 7 is a graph showing the results of the recovery strain rate to show the extent of recovery after deformation for a Cr-Fe-Co high-entropy alloy according to an embodiment of the present invention.

FIGS. 8 to 10 are graphs showing the results of differential thermal analysis performed on a Cr-Fe-Co high-entropy alloy according to an embodiment of the present invention.

It should be noted that a person of ordinary skill in the art can make various modifications to the present invention without departing from the technical spirit of the present invention.

Hereinafter, a Cr-Fe-Co high-entropy alloy (hereinafter referred to as a “high-entropy alloy”) having shape memory characteristics according to a preferred embodiment of the present invention will be described in detail with reference to the attached drawings.

The high-entropy alloy according to the present invention is characterized by being a ternary alloy of Cr (chromium), Fe (iron), and Co (cobalt) that is easily predicted and manufactured through thermodynamic calculations of CALPHAD from an existing high-entropy alloy.

The above high-entropy alloy is specifically represented by the [chemical formula] below.

[chemical formula]

Cr _a Fe _b Co _c

(a+b+c=100, 15≤a≤40 at.%, 10≤b≤60 at.%, 20≤c≤50 at.%)

And the above ternary high-entropy alloy is characterized by being composed of FCC or FCC + HCP or FCC + HCP + σ or FCC + HCP + BCC phase at room temperature.

In addition, it is preferable that the ternary alloy has a phase equilibrium temperature (T _O ) of 200 to 1,060 K and a free energy change amount [ΔG ^(hcp-fcc) ] (based on 300 K) of -2,013 to 228 J/mol.

The phase equilibrium temperature (T _O ) and the free energy change [ΔG ^(hcp-fcc) ] of the above high-entropy alloy must be satisfied at the same time, and if the temperature exceeds the limited range mentioned above, the shape memory is expressed in the case of the FCC single phase or the composite phase FCC + HCP or FCC + HCP + σ or FCC + HCP + BCC phase at 1,473 K (1,200 °C), but there is a concern that the shape memory characteristics may not be expressed properly in the case of the BCC single phase or the composite phase BCC+σ.

Hereinafter, a method for predicting the composition range of a Cr-Fe-Co high-entropy alloy for easily manufacturing a Cr-Fe-Co high-entropy alloy having shape memory characteristics according to the present invention will be specifically described with reference to the attached Drawing 1, as follows.

For reference, FIG. 1 is a block diagram illustrating a method for predicting the composition range of a Cr-Fe-Co high-entropy alloy having shape memory characteristics according to an embodiment of the present invention.

A method for predicting a composition region of a Cr-Fe-Co high-entropy alloy having shape memory characteristics according to the present invention is characterized by including an alloy component region prediction step (S1), a high-entropy alloy composition region selection step (S2), and a step (S3) of confirming whether shape memory characteristics are expressed, as illustrated in FIG. 1.

Hereinafter, the method for predicting the composition range of a Cr-Fe-Co high-entropy alloy according to the present invention will be specifically described step by step as follows.

The alloy composition region prediction step (S1) is a step of predicting an alloy composition region having low stacking fault energy (SFE) by thermodynamic calculation using CALPHAD in a high-entropy alloy, as shown in the attached Drawing 2, which shows a graph showing the change in Gibbs free energy (ΔG) according to temperature change of a high-entropy alloy having shape memory characteristics according to an embodiment of the present invention and a high-entropy alloy of a comparative example, and a description thereof is omitted here since it is specifically described below.

In this step, in order to predict an alloy composition having low stacking fault energy (SFE) in a high-entropy alloy by thermodynamic calculation, the phase equilibrium temperature T ₀ at which the change in free energy [ΔG ^(HCP-FCC) ] is 0 exceeds 0 K. The phase equilibrium temperature should be T ₀ > 0 K so that the stress-induced martensitic transformation occurs at a temperature higher than 0 K, and it is more preferable that it be 200 K < T ₀ < 1,060 K.

The stacking fault energy (SFE) calculation formula for FCC metals uses the known calculation formula as shown in the [mathematical formula] below to calculate the stacking fault energy (SFE).

[Mathematical formula]

γ(SFE) = 2ρΔG ^hcp-fcc + 2σ ^fcc/hcp

In the above equation

γ(SFE) is the stacking fault energy of the alloy, ρ is the molar areal density of the (111) plane, ΔG ^hcp-fcc is the change in Gibbs free energy during the phase transformation from γ austenite (FCC) to ε martensite (HCP), and σ is the interfacial energy between the γ/ε phases.

In the above SFE calculation formula, γ, which represents the stacking fault energy (SFE), is known to depend on the ρ and ΔG ^hcp-fcc and σ ^fcc/hcp values, and while the ρ and σ ^fcc/hcp values are known to be not greatly affected by the composition of the alloy, the ΔG ^hcp-fcc value is known to vary greatly depending on the alloy composition.

Therefore, the process of predicting an alloy with low stacking fault energy (SFE) using CALPHAD calculates ΔG ^hcp-fcc and considers a composition with a sufficiently low value as a composition that exhibits shape memory characteristics.

To present this more specifically, in the present invention, the possibility of implementing shape memory characteristics was predicted using the T ₀ value, which is the phase equilibrium temperature, as in the high-entropy alloy composition region selection step (S2) below, as a more intuitive indicator.

In the present invention, the term 'alloy having low stacking fault energy (SFE)' means that the stacking fault energy (SFE) of the alloy predicted by thermodynamic calculations is lower than the stacking fault energy (SFE) reported in conventional high-entropy alloys.

The high-entropy alloy composition region selection step (S2) is a step of selecting a composition region of a high-entropy alloy in which a stable phase is predicted at high temperatures from the composition region of the high-entropy alloy.

The high-entropy alloy designed from the Cr-Fe-Co ternary alloy according to the present invention is an alloy in which shape memory characteristics are realized by reversible martensitic transformation of the FCC/HCP phase, and has the characteristic that formability and shape memory characteristics are reduced when a phase other than the FCC and HCP phases exists inside the shape memory alloy.

Therefore, it is desirable for high-entropy alloys manufactured using an arc furnace or electric furnace to have a FCC single-phase or FCC+HCP two-phase structure.

To obtain these results, a step of selecting a composition region predicted to form an FCC single phase when heat-treated at 1,473 K (1,200 °C) through CALPHAD calculation is performed. FIG. 3 is an example of this process, and FIG. 4 shows the results of selecting an alloy composition region predicted to have T ₀ and be an FCC single phase at 1,200 °C in a Cr-Fe-Co ternary alloy through this series of processes.

The step (S3) for confirming whether shape memory characteristics are expressed is a step for making a cast material using an arc melting furnace using high-entropy alloys predicted in a selected alloy composition region, and then heat-treating the resulting shape memory alloy specimen at 1,473 K (1,200 °C) for 6 hours to evaluate the crystal structure, shape memory recovery characteristics, and thermal characteristics, as shown in the attached drawings, FIGS. 5 to 9.

Therefore, through the steps of S1 to S3, the Cr-Fe-Co high-entropy alloy predicted from the high-entropy alloy was actually manufactured as a ternary Cr-Fe-Co alloy having the composition specified in [Table 1] based on the predicted results in [Table 1] below. At this time, the method of manufacturing the alloy can be either the powder metallurgy method or the casting method, which are conventional alloy manufacturing methods, but in the present invention, the alloy specimen was manufactured by the casting method.

In this way, the present invention compares a high-entropy alloy having a predicted composition ratio as shown in [Table 1] below with an actually manufactured high-entropy alloy, thereby predicting a composition that is expected to exhibit shape memory characteristics among numerous combinations of high-entropy alloys made by mixing a number of elements in nearly equal proportions through thermodynamic calculations, thereby enabling the development of various types of shape-memory alloys that have not been previously reported by a simple method.

Hereinafter, the Cr-Fe-Co high-entropy alloy having shape memory characteristics according to the present invention and the method for predicting the composition range of the alloy will be specifically described through the following examples, and the present invention is not necessarily limited to the following examples.

The Cr-Fe-Co high-entropy alloy having shape memory characteristics according to the present invention was used as a comparative example with the Cr-Mn-Fe-Co-Ni high-entropy alloy known in patent document 3 as shown in [Table 1] below. That is, from the high-entropy alloys of Comparative Examples 1 and 2, Cr ₂₀ Mn ₂₀ Fe ₂₀ Co ₂₀ Ni ₂₀ and Cr ₂₀ Mn ₂₀ Fe ₂₀ Co ₃₅ Ni ₅ , thermodynamic-based calculation software This figure shows the results of organizing the equilibrium phase at T ₀ , ΔG ^(hcp-fcc) and 1,473 K (1,200 °C) and the crystal phase confirmed by actual experiments at room temperature for various alloy compositions of Cr-Fe-Co high-entropy alloys predicted using Thermo-Calc and actual Cr-Fe-Co high-entropy alloys.

[Table 1] below is a table that compares the predicted Cr-Fe-Co high-entropy alloys of Examples 1 to 14 and Comparative Examples 1 to 8, and then manufactures Cr-Fe-Co shape memory alloy specimens obtained by heat-treating at 1,473 K (1,200 °C) for 6 hours, and then organizes the predicted alloys with the actually manufactured alloys.

It was confirmed that Cr-Fe-Co high-entropy alloys have a high _T0 value of 300 K or higher in most compositions except for some compositions, indicating a high possibility of exhibiting shape memory characteristics. In addition, it was confirmed that the crystal structure predicted by thermodynamic calculations and the crystal structure observed in actual experiments were consistent in most alloys.

division	Composition ratio (at.%)	T 0 (K)	ΔG (hcp-fcc) (J/mol) (Based on 300K)	Predicted stability (Based on 1473K)	Observation (at room temperature)
Comparative Example 1	Cr 20 Mn 20 Fe 20 Co 20 Ni 20	<0	1051	FCC	FCC
Comparative Example 2	Cr 20 Mn 20 Fe 20 Co 35 Ni 5	320	-47	FCC	FCC
Example 1	Cr 40 Fe 10 Co 50	1060	-2013	FCC + σ	FCC + HCP + σ
Example 2	Cr 35 Fe 25 Co 40	780	-1272	FCC + BCC	FCC + HCP + BCC
Example 3	Cr 35 Fe 20 Co 45	830	-1413	FCC	FCC + HCP
Example 4	Cr 35 Fe 15 Co 50	900	-1590	FCC	FCC + HCP
Example 5	Cr 30 Fe 40 Co 30	580	-783	FCC + BCC	FCC + HCP + BCC
Example 6	Cr 30 Fe 30 Co 40	630	-898	FCC	FCC + HCP
Example 7	Cr 30 Fe 20 Co 50	740	-1161	FCC	FCC + HCP
Example 8	Cr 30 Fe 10 Co 60	910	-1566	FCC	FCC + HCP
Example 9	Cr 20 Fe 60 Co 20	410	-331	FCC	FCC + HCP + BCC
Example 10	Cr 20 Fe 50 Co 30	350	-157	FCC	FCC + HCP
Example 11	Cr 20 Fe 40 Co 40	350	-147	FCC	FCC + HCP
Example 12	Cr 20 Fe 30 Co 50	410	-295	FCC	FCC + HCP
Example 13	Cr 15 Fe 45 Co 40	200	228	FCC	FCC + HCP + BCC
Example 14	Cr 15 Fe 35 Co 50	230	159	FCC	FCC + HCP
Comparative Example 3	Cr 15 Fe 65 Co 20	330	-95	FCC	BCC
Comparative Example 4	Cr 15 Fe 55 Co 30	240	149	FCC	BCC
Comparative Example 5	Cr 10 Fe 70 Co 20	250	132	FCC	BCC
Comparative Example 6	Cr 10 Fe 60 Co 30	140	448	FCC	BCC
Comparative Example 7	Cr 10 Fe 50 Co 40	70	624	FCC	BCC
Comparative Example 8	Cr 10 Fe 40 Co 50	50	642	FCC	BCC

When examining the relative stabilities of the γ austenite phase (FCC) and the ε martensite phase (HCP) for the Cr-Fe-Co high-entropy alloy exhibiting shape memory characteristics according to the present invention and the Cr ₂₀ Mn ₂₀ Fe ₂₀ Co ₂₀ Ni ₂₀ alloy, which is a representative and widely known conventional high-entropy alloy, as shown in Fig. 2, the change in Gibbs free energy was found to be ΔG ^(hcp-fcc) > 0 in the range of 0 to 1,000 K. This means that the γ austenite phase (FCC) is more stable than the ε martensite phase (HCP) at all temperatures.

In contrast, Cr ₂₀ Fe ₃₀ Co ₅₀ , a Cr-Fe-Co high-entropy alloy according to the present invention, For Cr ₃₀ Fe ₃₀ Co ₄₀ and Cr ₃₀ Fe ₄₀ Co ₃₀ alloys, ΔG ^(hcp-fcc), which had a positive value at 1000 K, was found to have negative values below 50 K, 410 K, 580 K, and 630 K, respectively. This means that the HCP martensite phase is more stable than the FCC parent phase at the above temperatures.

For reference, FIG. 2 is a graph showing the change in Gibbs free energy (ΔG) according to temperature change of a high-entropy alloy having shape memory characteristics according to an embodiment of the present invention and a conventional high-entropy alloy in comparison thereto.

When examining the change in equilibrium phase fraction according to temperature of the Cr ₃₀ Fe ₃₀ Co ₄₀ high-entropy alloy, which is a Cr-Fe-Co high-entropy alloy according to the present invention, and the Cr ₂₀ Mn ₂₀ Fe ₂₀ Co ₂₀ Ni ₂₀ alloy in comparison therewith, as shown in FIG. 3, there exists a temperature range in which a single FCC phase is predicted to be stable at 1100 to 1560 K for Cr ₂₀ Mn ₂₀ Fe ₂₀ Co ₂₀ Ni ₂₀ and at 1230 to 1678 K for Cr ₃₀ Fe ₃₀ Co _40. This means that when the two alloys are heated to a temperature within this temperature range and heat-treated, a microstructure of a single FCC phase appears. This means that the shape memory characteristics may not be deteriorated after future heat treatment operations such as recrystallization and grain growth.

For reference, FIG. 3 is a graph showing the results of temperature changes in the equilibrium phase fraction of a Cr ₃₀ Fe ₃₀ Co ₄₀ high-entropy alloy according to the present invention and a comparable Cr ₂₀ Mn ₂₀ Fe ₂₀ Co ₂₀ Ni ₂₀ alloy.

Looking at the ternary phase diagram according to the Cr-Fe-Co mole fraction of the Cr-Fe-Co high-entropy alloy according to the present invention, as shown in Fig. 4, ① when the Cr content is 40 at.% or less, the microstructure of the alloy exhibits an FCC or FCC+BCC phase, ② when the Cr content is 40 at.% or more, the formation of the σ phase is expected to become dominant. ③ An increase in the Co content generally means an increase in the T ₀ temperature.

For reference, FIG. 4 is a drawing showing a ternary phase diagram according to the Cr-Fe-Co mole fraction of a Cr-Fe-Co high-entropy alloy according to the present invention.

The crystal structure of some specimens of the Cr-Fe-Co alloy composition according to the present invention presented in the above [Table 1] was confirmed by X-ray diffraction analysis, as shown in Fig. 5. According to Fig. 5, most of the Cr-Fe-Co alloys according to the present invention showed diffraction peaks of the FCC phase and the HCP phase as predicted from the thermodynamic calculation, and in some alloys of the composition, a small peak of the BCC phase was observed in addition to the FCC phase.

Some specimens of the Cr-Fe-Co high-entropy alloy according to the present invention were subjected to a bending test at room temperature and heated at a high temperature to check whether the shape recovered to the initial state. As a result, as shown in the photograph illustrated in FIG. 6, it was confirmed that the specimens of the FCC + HCP phase Cr ₃₀ Fe ₁₀ Co ₆₀ , Cr ₃₀ Fe ₂₀ Co ₅₀ , Cr ₃₀ Fe ₃₀ Co ₄₀ and the FCC + HCP + BCC phase Cr ₃₀ Fe ₄₀ Co ₃₀ of the Cr-Fe-Co high-entropy alloy according to the present invention all recovered their shapes.

Looking at the recovery strain for the Cr-Fe-Co alloy, as shown in FIG. 7, there is a deviation for each specimen, but it was confirmed from the bending test that the FCC + HCP phase Cr ₃₀ Fe ₁₀ Co ₆₀ , Cr ₃₀ Fe ₂₀ Co ₅₀ , Cr ₃₀ Fe ₃₀ Co ₄₀ and FCC + HCP + BCC phase Cr ₃₀ Fe ₄₀ Co ₃₀ specimens of the Cr-Fe-Co high-entropy alloy according to the present invention recovered 0.1 to 1.0% of strain after heating.

And as a result of performing differential thermal analysis on some of the alloy specimens of the predicted composition in the above [Table 1], the exothermic/endothermic reactions occurring in the specimen were recorded while heating to 600℃, cooling to -50℃, and then heating to 600℃ again, as shown in Fig. 8. In this way, when heating and cooling are repeated, it can be seen that heat-induced martensitic transformation and its reverse transformation occur in some materials, in which the matrix phase transforms into martensite.

The operating temperature of a shape memory alloy is ultimately determined by the martensite transformation/reverse transformation temperature. When examining the martensite transformation behavior in detail for Cr ₃₀ Fe ₃₀ Co ₄₀ , a high-entropy alloy based on the Cr-Fe-Co system according to the present invention, since it is restored to austenite, which is a high-temperature stable phase, in the A _s -A _f section (since the shape is restored), the upper limit of the operating temperature is determined as A _s or A _f .

For reference, FIGS. 8 to 10 are graphs showing the results of differential thermal analysis performed on a Cr-Fe-Co high-entropy alloy according to the present invention.

And when differential thermal analysis was repeatedly performed five times on a Cr ₂₀ Fe ₅₀ Co ₃₀ alloy having an FCC + HCP phase of a Cr-Fe-Co high-entropy alloy according to the present invention at 1,473 K, as shown in Fig. 9, there was no significant difference in the phase transformation temperature and enthalpy change even when the heating and cooling processes were repeated several times. This confirmed that the material used in the experiment is a very suitable alloy to be used as an actuator.

In addition, the Cr-Fe-Co high-entropy alloy according to the present invention showed a high-temperature martensitic transformation at Ms of 100°C or higher and As of 230°C or higher in all alloys in which a phase transformation was observed as shown in FIGS. 10(a) to 10(d) as a result of differential thermal analysis, and it is expected that the alloys can be manufactured easily since the compositional range in which shape memory characteristics are realized is wide, the martensite temperature control is also easy, and the formability is excellent due to the high-temperature fcc crystal structure, so that they can be manufactured into various shapes.

As described above, the Cr-Fe-Co high-entropy alloy having shape memory characteristics according to preferred embodiments of the present invention and the method for predicting the composition range of the alloy have been described, but this has been described only as an example, and those skilled in the art will readily understand that various changes and modifications are possible within a scope that does not depart from the technical spirit of the present invention.

Claims (10)

1. A Cr-Fe-Co high-entropy alloy having shape memory properties, characterized by being a ternary alloy of Cr (chromium), Fe (iron), and Co (cobalt).
2. In the first paragraph,

   The above ternary alloy is a Cr-Fe-Co high-entropy alloy having shape memory properties, characterized by the following [chemical formula].

   [chemical formula]

   Cr _a Fe _b Co _c

   (a+b+c=100, 15≤a≤40 at.%, 10≤b≤60 at.%, 20≤c≤50 at.%)
3. In either of paragraph 1 or paragraph 2,

   The above ternary alloy is a Cr-Fe-Co high-entropy alloy having shape memory properties, characterized in that it is FCC or FCC + HCP or FCC + HCP + σ or FCC + HCP + BCC at room temperature.
4. In any one of claims 1 to 3,

   The above ternary alloy is a Cr-Fe-Co high-entropy alloy having shape memory properties, characterized by a phase equilibrium temperature (T _O ) of 200 to 1,060 K.
5. In any one of claims 1 to 4,

   The above ternary alloy is a Cr-Fe-Co high-entropy alloy having shape memory properties, characterized by a free energy change amount [ΔG ^(hcp-fcc) ] (based on 300 K) of -2,013 to 228 J/mol.
6. Step (S1) of predicting an alloy composition region having low stacking fault energy (SFE) in a high-entropy alloy by thermodynamic calculation;

   Step (S2) of selecting a composition region of a high-entropy alloy in which a stable phase is predicted at high temperatures from the composition region of the high-entropy alloy; and

   Step (S3) for confirming whether shape memory characteristics are expressed in the selected alloy composition region;

   A method for predicting the composition range of a Cr-Fe-Co high-entropy alloy having shape memory characteristics, characterized by including:
7. In Article 6,

   The above alloy composition region prediction step (S1) is a method for predicting the composition region of a Cr-Fe-Co high-entropy alloy having shape memory characteristics, characterized in that the step of predicting the alloy composition region comprises determining the phase equilibrium temperature T ₀ at which the free energy change amount (ΔG ^(HCP-FCC) ) of the γ austenite phase (FCC) and the ε martensite phase (HCP) in the composition region of the high-entropy alloy is 0.
8. In Article 7,

   A method for predicting the composition range of a Cr-Fe-Co high-entropy alloy having shape memory characteristics, wherein the above-mentioned phase equilibrium temperature is T ₀ 〉0K.
9. In Article 6,

   A method for predicting the composition region of a Cr-Fe-Co high-entropy alloy having shape memory characteristics, characterized in that in the high-entropy alloy composition region selection step (S2), the high temperature is 1,473 K (1,200°C).
10. In Article 6,

    A method for predicting the composition region of a Cr-Fe-Co high-entropy alloy having shape memory characteristics, characterized in that in the step of selecting the composition region of the high-entropy alloy (S2), the stable phase is FCC or FCC + σ or FCC + BCC.

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