Fe-Ni-Co-Ti-Al medium-entropy invar alloy and preparation method thereof

Abstract

The invention discloses an entropy invar alloy in a Fe-Ni-Co-Ti-Al system and a preparation method of the entropy invar alloy, and belongs to the technical field of metal materials. The element composition general formula of the medium-entropy invar alloy of the Fe-Ni-Co-Ti-Al system is FeaNibCocTidAle, a, b, c, d and e are atomic percentages, 45% < = a < = 55%, 30% < = b < = 45%, 1% < = c < = 10%, 1% < = d < = 5%, 1% < = e < = 5%, and a + b + c + d + e = 100%, the medium-entropy invar alloy has low thermal expansion performance, the thermal expansion coefficient is as low as 5 ppm / DEG C, meanwhile, the medium-entropy invar alloy has good comprehensive mechanical performance, the room-temperature tensile strength reaches up to 1630 MPa, the plastic deformation can reach up to 18% or above, and the medium-entropy invar alloy can be widely applied to the field of high- The alloy can be widely applied to the field of precise instruments, the preparation method of the alloy is simple, machining processes such as smelting and casting under existing production technical conditions can be well met, the steps are simple, and industrial production is easy to achieve.

Description

Technical Field

[0001] The present invention relates to an Fe-Ni-Co-Ti-Al medium-entropy Invar alloy, in particular to an Fe-Ni-Co-Ti-Al medium-entropy Invar alloy with ultra-high strength and plasticity, and also relates to a preparation method thereof, belonging to the technical field of metallic materials. Background Art

[0002] With the development of industry, low thermal expansion materials have become an indispensable part of fields such as energy transportation, precision instruments, and aerospace. The Fe-Ni binary alloy with a nickel content of about 36 at.% is called Invar alloy, which has a low thermal expansion coefficient below the Curie temperature, only one-tenth of that of ordinary steel. During the heating process, the Invar alloy can generate a large spontaneous volume magnetostriction, which offsets the thermal expansion brought by phonons, so as to keep the volume unchanged within a wide temperature range. And because the Invar alloy is a face-centered cubic single phase within a very wide temperature range, it has good plasticity, impact resistance and other properties, but its strength is low, and it can only be applied to precision instruments such as watches, balance rockers, and electron picture tubes, severely limiting its application prospects as a structural-functional integrated material.

[0003] After the 21st century, the application scope of Invar alloy has been continuously expanded. The industrial community has begun to re-evaluate its potential as a structural material and put forward higher requirements for its mechanical properties. However, the strengthening of Invar alloy often cannot take into account many factors such as its plasticity, low thermal expansion and production cost.

[0004] In recent years, the rise of the concept of multi-principal element alloys has provided a new opportunity to solve the above problems. Due to its nearly infinite composition space and stable phase structure, multi-principal element alloys have achieved great success in breaking through the performance limits of traditional materials. Therefore, designing Invar alloy using the concept of multi-principal element alloys has become a very promising research direction. At present, a small number of multi-principal element alloys have been reported to have the Invar effect, and their element compositions all contain 3d transition metal elements, such as Fe-36.2Ni-10.1Co-5.2Cr-2.4Ti, etc., which provides a new idea for the composition design of Invar alloy. Inspired by this idea, researchers began to add elements to traditional Fe 64 Ni 363d transition metal elements are added to invar alloys to develop new types of invar alloys. The literature ("Invar effects in FeNiCo medium entropy alloys: From an Invartreasure map to alloy design", Rao Z, et al. Intermetallics, 2019, 111:106520.) discloses that Co element is added to invar alloys to obtain Fe 63 Ni 32 Co5 super invar alloy, which has a large spontaneous volume magnetostrictive effect and exhibits a near-zero coefficient of thermal expansion from 25 to 260 °C. The literature ("Mechanical and expansion properties of age-hardened Fe-Ni-Ti-Al invar alloys", Lu Jiansheng, et al., Shanghai Research Institute of Metals, 2006(02): 31-34.) adds Ti and Al elements to invar alloys to study the effects of precipitates Ni3Ti and Ni3Al on mechanical properties and thermal expansion properties. The precipitation of intermetallic compounds Ni3Ti and Ni3Al can increase its tensile strength to 1000 MPa at most, but will reduce the content of element Ni in the alloy and increase the coefficient of thermal expansion. It can be seen that it is feasible to design new invar alloys using the concept of multi-principal element alloys. However, there is a trade-off relationship between the Invar effect and high strength and plasticity, and no new invar alloys that break through the limitation of high strength and plasticity have been reported in current related research. Summary of the Invention

[0005] Aiming at the problems existing in invar alloys in the prior art, the first object of the present invention is to provide a medium-entropy invar alloy of Fe-Ni-Co-Ti-Al system, which has low thermal expansion performance, the coefficient of thermal expansion is as low as 5 ppm / °C, and at the same time has good comprehensive mechanical properties, the tensile strength at room temperature is as high as 1630 MPa, and the plastic deformation amount can reach more than 18%, which has great application potential in the industrial field.

[0006] The second object of the present invention is to provide a preparation method of a high-strength medium-entropy invar alloy of Fe-Ni-Co-Ti-Al system, which is applicable to existing production lines and equipment and has the potential to be easily industrialized.

[0007] In order to achieve the above technical objectives, the present invention provides a medium-entropy invar alloy of Fe-Ni-Co-Ti-Al system, and its general formula of element composition is: Fe a Ni b Co c Ti d Al e, where a, b, c, d, and e are all atomic percentages, 45% ≤ a ≤ 55%, 30% ≤ b ≤ 45%, 1% ≤ c ≤ 10%, 1% ≤ d ≤ 5%, 1% ≤ e ≤ 5%, and a + b + c + d + e = 100%; the medium-entropy Invar alloy matrix is a face-centered cubic structure, and the precipitation phase inside the grains is an ordered face-centered cubic structure phase L12.

[0008] The Fe-Ni-Co-Ti-Al series medium-entropy Invar alloy provided by the present invention has a duplex structure of both FCC phase and L12 phase, and has good comprehensive mechanical properties and low thermal expansion properties. By adding Co, Ti, and Al elements to the FeNi-based alloy, an FeNiCoTiAl series medium-entropy Invar alloy is formed. The yield strength and tensile strength of the matrix FCC are improved by back stress, precipitation strengthening, and transformation-induced plasticity effect (TRIP). At the same time, the ultra-stable L12 and Invar matrix are used to ensure that its coefficient of thermal expansion remains unchanged. The addition of Ti and Al elements in the FeNi-based alloy can promote the precipitation of the Ni3Al-type - L12 phase, improve its strength, and continuous precipitation and discontinuous precipitation occur simultaneously, forming spherical and strip-shaped L12, forming soft zones and hard zones, generating back stress strengthening. Moreover, the contents of Ti and Al cannot be too high, otherwise an ordered body-centered cubic structure (B2 phase) will be formed with Ni element, damaging plasticity. At the same time, they cannot be too little, otherwise the volume fraction of the L12 phase is too small to achieve high strength. During the deformation process, the matrix FCC phase will undergo a martensitic transformation to generate a body-centered cubic (BCC) phase, and the work hardening ability and plasticity of the alloy are improved through the TRIP effect, thereby obtaining higher comprehensive mechanical properties. The addition of Co element in the alloy can increase its Curie temperature, keep its low thermal expansion property in a wide temperature range, and Co element can partially dissolve into the L12 phase to form a multi-principal element (Ni, Co, Fe)3(Ti, Al) precipitation phase, improving the strength of the alloy. It should be noted that the addition amounts of Co, Ti, and Al elements need to be controlled within a reasonable range. When exceeding a certain content, the precipitation of brittle phases such as σ and μ will cause the plasticity of the alloy to decrease. Therefore, the general formula of the element composition is further preferably Fe a Ni b Co c Ti d Al e where 50% ≤ a ≤ 55%, 34% ≤ b ≤ 45%, 2% ≤ c ≤ 6%, 3% ≤ d ≤ 5%, 3% ≤ e ≤ 5%, and a + b + c + d + e = 100%.

[0009] The Fe-Ni-Co-Ti-Al series medium-entropy Invar alloy of the present invention has low thermal expansion properties, with a coefficient of thermal expansion as low as 5 ppm / °C. At the same time, it has good comprehensive mechanical properties, with a room-temperature tensile strength as high as 1630 MPa and a plastic deformation amount of more than 18%, showing great application potential in the industrial field.

[0010] The present invention also provides a preparation method of an Fe-Ni-Co-Ti-Al series medium-entropy Invar alloy. The method comprises the steps of subjecting metal raw materials Fe, Ni, Co, Ti, and Al to surface cleaning pretreatment, and then successively performing vacuum arc melting, casting, solution treatment, cold rolling treatment, recrystallization treatment, and aging treatment to obtain the alloy.

[0011] The preparation method of the Fe-Ni-Co-Ti-Al series medium-entropy Invar alloy of the present invention is simple, can well meet the melting and casting and other processing technologies under the existing production technical conditions, and is easy to realize industrial production.

[0012] During the preparation process of the Fe-Ni-Co-Ti-Al series medium-entropy Invar alloy of the present invention, through treatments such as solution treatment, cold rolling, recrystallization, and aging, the alloy obtains precipitation strengthening, back stress strengthening, and TRIP effect, its strength is significantly improved, good comprehensive mechanical properties can be obtained, its tensile strength is as high as 1630 MPa, the plastic deformation amount can reach more than 18%, and at the same time it has low thermal expansion performance, and the thermal expansion coefficient is as low as 5 ppm / °C.

[0013] As a preferred embodiment, the cleaning pretreatment includes steps such as removing the surface oxide film by sandpaper or grinding wheel, ultrasonic washing, and drying. The cleaning pretreatment process is to first use sandpaper and grinding wheel to remove the surface oxide film of metal raw materials Fe, Ni, Co, Ti, and Al, and then use industrial absolute ethanol and hydrogen chloride for ultrasonic cleaning to remove surface contaminants, etc., to avoid affecting the subsequent melting process, and finally perform conventional drying treatment.

[0014] As a preferred embodiment, the conditions for the vacuum arc melting are: the arc current is 300 - 400 A, the alloy is remelted more than 8 times until the alloy composition is uniform, and the melting time for each time is 90 - 180 s. By regulating the current magnitude, melting times, and melting time during the melting process, etc., the uniformity of alloy melting can be ensured.

[0015] As a preferred embodiment, the conditions for the solution treatment are: holding at a temperature of 1200 - 1300 °C for 2 - 12 hours and then water quenching. Element segregation, dendrites, shrinkage cavities, and other defects generated during the casting process are fully eliminated, and the elements are fully diffused to achieve the purpose of homogenization. If the solution temperature is too high or the time is too long, the alloy is likely to melt or the grain size is too large, affecting the performance. If the solution temperature is too low or the time is too short, it is difficult to achieve the purpose of homogenization.

[0016] As a preferred embodiment, the conditions for cold rolling treatment are as follows: multi-pass cold rolling is carried out, with the rolling reduction per pass being 2 - 5 mm and the total rolling deformation being 75% - 90%. Through rolling treatment, the internal grains are broken, and at the same time, the energy stored in the alloy is made convenient for subsequent recrystallization treatment. If the rolling reduction is too small, the grains will not be completely broken and the stored energy will be too small, resulting in poor mechanical properties of the alloy and inability to reach the required strength. If the rolling reduction is too large, the alloy is prone to generate large internal stresses and thus a large number of cracks will be produced.

[0017] As a preferred embodiment, the conditions for recrystallization treatment are as follows: after holding at a temperature of 800 - 1100 °C for 10 - 30 minutes, water quenching is carried out. By controlling the temperature and time of recrystallization treatment, the fibrous grains after cold deformation are formed into fine equiaxed grains. If the recrystallization temperature is too low or the time is too short, incomplete grain recrystallization and incomplete dissolution of some precipitation phases will occur, thus reducing the plasticity of the alloy. If the recrystallization temperature is too high or the time is too long, the recrystallized grains in the alloy will grow, thus reducing the strength of the alloy.

[0018] As a preferred embodiment, the conditions for aging treatment are as follows: after holding at a temperature of 600 - 750 °C for 2 - 24 hours, water quenching is carried out. If the annealing temperature is too high or the time is too long, the grains will become coarse and the precipitation phases will grow, thus reducing the strength. If the annealing temperature is too low or the time is too short, the precipitation phases will not be completely precipitated, resulting in a reduction in strength.

[0019] As a preferred embodiment, the purities of the metal raw materials Fe, Ni, Co, Ti, and Al are all greater than 99.9 wt.%.

[0020] The preparation method of the Fe-Ni-Co-Ti-Al medium-entropy Invar alloy provided by the present invention includes the following steps:

[0021] Step 1. Use sandpaper or a grinding wheel to grind off the surface oxide layer of the metal raw materials Fe, Ni, Co, Ti, and Al, and perform ultrasonic oscillation cleaning with industrial absolute ethanol and hydrochloric acid solution until there is no obvious precipitate in the solution, and then carry out drying;

[0022] Step 2. Convert the metal raw materials Fe, Ni, Co, Ti, and Al elements into mass percentages according to the atomic percentages in the Fe-Ni-Co-Ti-Al medium-entropy Invar alloy expression, and carry out raw material ratio with a balance with an accuracy of 0.001 g;

[0023] Step 3. Put the raw materials proportioned in Step 2 into a vacuum arc melting furnace, stack them in the melting equipment in the order of melting points from low to high, with Al at the bottom, Ni and Co in the middle, and Fe and Ti at the top. Subsequently, adjust the mechanical pump and molecular pump to reduce the vacuum degree in the melting furnace to 1×10 -4Below Pa, then slowly fill the furnace cavity with argon at about half an atmosphere, and then turn on the electric arc furnace to melt the metal raw materials. The arc current is 300 - 400 A. Repeat the melting more than 8 times until the alloy chemical composition is uniform. Cool for 5 minutes after each melting, and the melting time is 90 - 180 s. After the melting is completed, use a vacuum suction casting device to suck-cast the alloy into a water-cooled copper mold. After cooling for 8 - 10 minutes, an alloy ingot can be obtained;

[0024] Step 4. Alloy solution treatment: Seal the ingot obtained by melting and suction casting in step 3 in a quartz tube. Vacuumize the quartz tube to reduce the vacuum degree to below 2×10 -3 Pa, and fill it with argon protective gas to a pressure of 0.5×10 5 Pa, and then carry out high-temperature solution treatment. The treatment conditions are 1200 - 1300 °C, hold for 2 - 12 hours, and then water quench;

[0025] Step 5. Alloy cold rolling treatment: Cold roll the alloy ingot obtained in step 4. The cold rolling amount each time is about 2 - 5 mm, and the total cold rolling amount is 75% - 90%;

[0026] Step 6. Alloy recrystallization treatment: Carry out recrystallization treatment on the alloy after cold rolling in step 5. The treatment conditions are to hold at a temperature of 800 - 1100 °C for 10 - 30 minutes, and then water quench;

[0027] Step 7. Alloy aging treatment: Carry out aging treatment on the recrystallized alloy in step 6. The treatment conditions are to hold at a temperature of 600 - 750 °C for 2 - 24 hours, and then water quench to obtain the medium-entropy Invar alloy of the present invention.

[0028] Compared with the prior art, the beneficial technical effects brought by the technical solution of the present invention:

[0029] (1) The Fe-Ni-Co-Ti-Al-based medium-entropy Invar alloy provided by the present invention is a five-element alloy containing Fe, Ni, Co, Ti, and Al. The composition general formula is: Fe a Ni b Co c Ti d Al e , where a, b, c, d, and e are all atomic percentages, 45% ≤ a ≤ 55%, 30% ≤ b ≤ 45%, 1% ≤ c ≤ 10%, 1% ≤ d ≤ 5%, 1% ≤ e ≤ 5%, and a + b + c + d + e = 100%. The matrix of the medium-entropy Invar alloy is a face-centered cubic crystal structure, and the precipitation phase inside the grains is the L12 phase. This medium-entropy Invar alloy has good comprehensive mechanical properties. Its tensile strength is as high as 1630 MPa, and the plastic deformation amount can reach more than 18%. At the same time, it has low thermal expansion performance, and the thermal expansion coefficient is as low as 5 ppm / °C.

[0030] (2) The matrix of the Fe-Ni-Co-Ti-Al medium-entropy Invar alloy provided by the present invention has a face-centered cubic crystal structure, and the precipitation phase inside the grains is the L12 phase. After solution treatment, cold rolling, recrystallization and aging treatment, the strength of the FCC phase is significantly improved due to precipitation strengthening, back stress strengthening and TRIP effect. Therefore, good comprehensive mechanical properties can be obtained. At the same time, the ultra-stable L12 phase and the Invar matrix are used to ensure that its coefficient of thermal expansion remains unchanged.

[0031] (3) The preparation method of the Fe-Ni-Co-Ti-Al medium-entropy Invar alloy provided by the present invention is simple, which can well meet the processing technologies such as melting and casting under the existing production technical conditions. The steps are simple and easy to realize industrial production.

[0032] At the same time, the Fe-Ni-Co-Ti-Al medium-entropy Invar alloy with good mechanical properties and low thermal expansion performance of the present invention is obtained by melting ordinary metal raw materials, and its preparation cost is low, and it has good application prospects in the industrial field. Description of the Drawings

[0033] Figure 1 It is the high-energy X-ray diffraction (HEXRD) pattern of the Fe-Ni-Co-Ti-Al medium-entropy Invar alloy provided in Example 1 of the present invention.

[0034] Figure 2 It is the transmission electron microscope (TEM) image (a) of the Fe-Ni-Co-Ti-Al medium-entropy Invar alloy in Example 1 of the present invention. Figures (b) is the selected area electron diffraction pattern of the L12 phase at the grain boundary, and figure (c) is the selected area electron diffraction pattern of the L12 phase inside the grains.

[0035] Figure 3 It is the room-temperature tensile stress-strain curve of the Fe-Ni-Co-Ti-Al medium-entropy Invar alloy provided in Examples 1 to 3 of the present invention.

[0036] Figure 4 It is the room-temperature tensile stress-strain curve of the Fe-Ni-Co-Ti-Al medium-entropy Invar alloy provided in Comparative Example 1 and Comparative Example 2 of the present invention.

[0037] Figure 5 It is the thermal expansion performance curve of the Fe-Ni-Co-Ti-Al medium-entropy Invar alloy provided in Example 1 of the present invention. Detailed Embodiments

[0038] For the convenience of understanding the present invention, the following combines the drawings with the specific embodiments and examples to describe the content of the present invention more comprehensively and in detail. However, the protection scope of the present invention is not limited to the following specific examples.

[0039] Unless otherwise defined, all technical terms used hereinafter have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing specific embodiments only and are not intended to limit the scope of the present invention.

[0040] Unless otherwise specifically stated, various raw materials, reagents, instruments, and equipment used in the present invention can be obtained through market purchases or can be prepared by existing methods. The test methods are all carried out according to industry standards.

[0041] Example 1

[0042] In this example, the Fe-Ni-Co-Ti-Al medium-entropy Invar alloy with good mechanical properties has a general composition formula: Fe a Ni b Co c Ti d Al e , where a, b, c, d, and e are 54%, 34%, 6%, 3%, and 3% respectively.

[0043] The preparation method of the Fe-Ni-Co-Ti-Al medium-entropy Invar alloy includes the following steps:

[0044] Step 1. Use sandpaper and a grinding wheel to grind off the surface oxide layer of the metal raw materials Fe, Ni, Co, Ti, and Al, and then perform ultrasonic cleaning with industrial absolute ethanol and hydrochloric acid solution, and then dry.

[0045] Step 2. Convert the atomic percentages of the metal raw materials Fe, Ni, Co, Ti, and Al elements in the Fe-Ni-Co-Ti-Al medium-entropy Invar alloy expression into mass percentages and perform raw material proportioning with a balance with an accuracy of 0.001 g.

[0046] Step 3. Put the metal raw materials proportioned in Step 2 into a vacuum arc melting furnace, stack them in the melting equipment in order of melting point from high to low, with Al at the bottom, Ni and Co in the middle, and Fe and Ti at the top. Then adjust the mechanical pump and molecular pump to reduce the vacuum degree in the melting furnace to below 1×10 -4 Pa, then slowly fill the furnace cavity with argon at about half an atmosphere, and then turn on the arc furnace to melt the metal raw materials. The arc current is 350 A, and repeat the melting more than 8 times until the alloy chemical composition is uniform. Cool for 5 min after each melting, and the melting time is 120 s. After the melting is completed, use a vacuum suction casting device to suck-cast the alloy into a water-cooled copper mold, and after cooling for 10 min, an alloy ingot can be obtained to obtain the required medium-entropy Invar alloy material.

[0047] Step 4. Seal the ingot obtained in Step 3 by melting and suction casting in a quartz tube. Vacuumize the quartz tube to reduce the vacuum degree to below 2×10 -3 Pa, and then fill it with argon protective gas until the air pressure reaches 0.5×10 5 Pa. Then perform high-temperature solution treatment. The treatment conditions are 1200 °C and keep warm for 2 hours, followed by water quenching;

[0048] Step 5. Perform multi-pass cold rolling on the alloy ingot obtained in Step 4. The cold rolling amount each time is about 4 mm, and the total cold rolling amount is 80%;

[0049] Step 6. Perform recrystallization treatment on the alloy cold-rolled in Step 5. Keep warm at 900 °C for 10 minutes, followed by water quenching;

[0050] Step 7. Alloy aging treatment: Perform aging treatment on the recrystallized alloy in Step 6. Keep warm at 675 °C for 4 hours, followed by water quenching to obtain the medium-entropy Invar alloy of the present invention.

[0051] Example 2

[0052] The only difference between this example and Example 1 is that the aging time is 16 h.

[0053] Example 3

[0054] The only difference between this example and Example 1 is that in Fe a Ni b Co c Ti d Al e a, b, c, d, e are 50%, 40%, 2%, 4%, 4% respectively.

[0055] Comparative Example 1

[0056] The only difference between this example and Example 1 is that this example is a medium-entropy alloy not within the scope of the patent protection. The alloy in this example does not add the Co element, and the Ti and Al elements are less, and only (Ni,Fe)3(Ti,Al),Fe can be formed a Ni b Co c Ti d Al e a, b, c, d, e are 50%, 45%, 0%, 2.5%, 2.5% respectively.

[0057] Comparative Example 2

[0058] The only difference between this example and Example 1 is that this example is a medium-entropy alloy not within the scope of the patent protection. The alloy in this example adds excessive Ti and Al elements, resulting in poor plasticity of the FCC phase. Fe aNi b Co c Ti d Al e Among them, a, b, c, d, and e are 50%, 30%, 6%, 7%, and 7% respectively.

[0059] Alloys S1 - S3 prepared in Examples 1 - 3 and alloys S4 - S5 prepared in Comparative Examples 1 - 2

[0060]

[0061] Characterization of the material properties of the materials prepared in Examples 1 - 3 and Comparative Examples 1 - 2:

[0062] (1) Characterization of the crystal structure and microstructure of the Fe-Ni-Co-Ti-Al medium-entropy Invar alloy with comprehensive mechanical properties and low thermal expansion properties:

[0063] Perform HEXRD tests on the Fe-Ni-Co-Ti-Al medium-entropy Invar alloy prepared by the preparation method described in Example 1, and the test results are shown in Figure 1 . As Figure 1 shown, the medium-entropy Invar alloy prepared in the present invention has an FCC and L12 duplex structure after aging treatment (S1).

[0064] Perform TEM tests on the Fe-Ni-Co-Ti-Al medium-entropy Invar alloy prepared by the preparation method described in Example 1. Figure 2 a, Figure 2 b, and Figure 2 c respectively show the FCC phase and L12 precipitation phase of the aged alloy in Example 1 and the corresponding selected area electron diffraction patterns. Diffused L12 precipitation phases can be clearly observed in the TEM dark field phase, and the grain boundaries are strip-shaped ( Figure 2 b), and the intragranular are spherical ( Figure 2 c).

[0065] (2) Room temperature quasi-static tensile properties:

[0066] Cut the samples of the Fe-Ni-Co-Ti-Al medium-entropy Invar alloy (S1, S2, S3, S4, S5) prepared into plate-shaped tensile specimens using a wire electrical discharge machine, and then conduct room temperature tensile tests on a mechanical property testing machine at a tensile rate of 0.572 mm / min. The tensile stress-strain curves of S1, S2, and S3 at room temperature are shown in Figure 3 , and the tensile stress-strain curves of S4 and S5 at room temperature are shown in Figure 4 .

[0067] As the applied stress gradually increases, the sample first undergoes elastic deformation, then plastic deformation after reaching the yield strength, and finally fractures when the plastic strain reaches a certain amount. The yield strengths and tensile strengths of the Fe-Ni-Co-Ti-Al medium-entropy Invar alloys, namely alloys S1, S2, and S3, are 1014 MPa and 1056 MPa, 1365 MPa and 916 MPa, and 1615 MPa and 1728 MPa respectively, and the fracture elongation rates are 22.7%, 23.1%, and 28% respectively. The yield strengths and tensile strengths of alloys S4 and S5, which are not within the scope of the patent protection, are 885 MPa and 1203 MPa, and 868 MPa and 936 MPa respectively, and the fracture elongation rates are 18.3% and 1.41% respectively, showing a brittle fracture mode. Compared with the alloys within the preferred range, the medium-entropy Invar alloy with both precipitation strengthening and transformation-induced plasticity in the present invention has simultaneously improved strength and plasticity.

[0068] (3)Thermal expansion property:

[0069] The thermal expansion property of the Fe-Ni-Co-Ti-Al medium-entropy Invar alloy prepared by the preparation method described in Example 1 was tested ( Figure 5 ), and it can be seen that it has a low coefficient of thermal expansion in the range of 25 °C to 180 °C, and the average coefficient of thermal expansion is 5 ppm / °C.

Claims

1. A Fe-Ni-Co-Ti-Al medium-entropy Invar alloy, characterized in that: The general formula of the element composition is: Fe a Ni b Co c Ti d Al e , where a, b, c, d, and e are all atomic percentages, 45% ≤ a ≤ 55%, 30% ≤ b ≤ 45%, 1% ≤ c ≤ 10%, 1% ≤ d ≤ 5%, 1% ≤ e ≤ 5%, and a + b + c + d + e = 100%; The matrix of the medium-entropy Invar alloy is a face-centered cubic crystal structure, and the precipitated phase inside the grains is an ordered face-centered cubic structure phase L12.

2. A Fe-Ni-Co-Ti-Al medium-entropy Invar alloy according to claim 1, characterized in that: In the general formula of the element composition, 50% ≤ a ≤ 55%, 34% ≤ b ≤ 45%, 2% ≤ c ≤ 6%, 3% ≤ d ≤ 5%, 3% ≤ e ≤ 5%, and a + b + c + d + e = 100%.

3. The preparation method of a Fe-Ni-Co-Ti-Al medium-entropy Invar alloy as described in claim 1 or 2, characterized by: After the Fe, Ni, Co, Ti, and Al raw materials are subjected to surface cleaning pretreatment, vacuum arc melting, casting, solution treatment, cold rolling treatment, recrystallization treatment, and aging treatment are carried out in sequence to obtain the alloy.

4. The preparation method of a Fe-Ni-Co-Ti-Al series medium-entropy Invar alloy according to claim 3, characterized in that: The cleaning pretreatment includes steps such as removing the surface oxide film by sandpaper or grinding wheel, ultrasonic washing, and drying.

5. The preparation method of a Fe-Ni-Co-Ti-Al series medium-entropy Invar alloy according to claim 3, characterized in that: During the vacuum arc melting process, the metal Fe, Ni, Co, Ti, and Al raw materials are stacked in the melting equipment in the order of increasing melting point, with Al placed at the bottom, Ni and Co placed in the middle, and Fe and Ti placed at the top.

6. The preparation method of a Fe-Ni-Co-Ti-Al series medium-entropy Invar alloy according to claim 3 or 5, characterized in that: The conditions for the vacuum arc melting are: the arc current is 300 - 400 A, and the alloy is melted and homogenized by repeating melting more than 8 times, with each melting time being 90 - 180 s.

7. The preparation method of a Fe-Ni-Co-Ti-Al series medium-entropy Invar alloy according to claim 3, characterized in that: The conditions for the solution treatment are: after holding at a temperature of 1200 - 1300 °C for 2 - 12 hours, water quenching is carried out.

8. The preparation method of a Fe-Ni-Co-Ti-Al series medium-entropy Invar alloy according to claim 3, characterized in that: The conditions for the cold rolling treatment are: multi-pass cold rolling is carried out, with each rolling reduction being 2 - 5 mm and the total rolling deformation being 75% - 90%.

9. The preparation method of a Fe-Ni-Co-Ti-Al series medium entropy Invar alloy according to claim 3, characterized in that: The conditions for the recrystallization treatment are: at a temperature of 800 - 1100 °C, after holding for 10 - 30 minutes, water quenching is carried out.

10. According to the preparation method of a Fe-Ni-Co-Ti-Al medium-entropy Invar alloy described in claim 3, the conditions for the aging treatment are: at a temperature of 600 - 750 °C, after holding for 2 - 24 hours, water quenching is carried out.

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