A medium-entropy alloy with ultra-high low-temperature yield strength and a preparation method thereof

Abstract

The invention discloses a medium entropy alloy with super low temperature yield strength and a preparation method thereof, wherein the molecular formula is Ni2CoCrNb 0.2 , with D0 22 Nano-scale γ″ strengthening phase of the structure. It is manufactured by laser additive manufacturing and then subjected to solid solution and aging treatment. After the alloy was solution treated at 1150℃ for 0.5h, the elements of the alloy were evenly distributed, the dendrites disappeared, and they were transformed into columnar crystals, achieving the effect of solid solution. The yield and tensile strengths dropped to ~380MPa and ~790MPa, respectively, while the tensile elongation increased to ~54%. During the aging treatment, with the increase of time, the strength and hardness of the alloy gradually increased, and the plasticity decreased. When the aging time reached 96 hours, the alloy was tensile tested at 293K, and the strength reached the peak value. The comprehensive mechanical properties were the best, with a yield strength of ~1180MPa, an ultimate strength of ~1135MPa, and a tensile elongation of ~19%. The sample was tensile tested at 77K, and the yield strength of the alloy was ~1320MPa, the ultimate strength was ~1552MPa, and the tensile elongation was ~25%, which is the highest level reported so far, achieving a breakthrough in the yield strength of low-temperature materials.

Description

Technical Field

[0001] The invention belongs to the field of alloys, and in particular relates to a medium-entropy alloy with ultra-high low-temperature yield strength and a preparation method thereof. Background Art

[0002] With the rapid development of my country in the fields of Antarctic scientific research, deep-sea engineering, nuclear fusion and petrochemical industry, the requirements for low-temperature performance of materials are increasing day by day. For example, the low-temperature materials used in nuclear fusion require a yield strength of more than 1100MPa and an elongation of more than 30%. However, most traditional low-temperature alloy materials are austenitic stainless steels, among which the representative 304 stainless steel has a yield strength of only 600MPa at 77K, which greatly limits its industrial application. The country needs to spend a lot of foreign exchange every year to import metal materials used in extremely low temperatures. Therefore, it is urgent to develop low-temperature alloy materials with high yield strength to replace imports.

[0003] In recent years, people have conducted extensive research on low-temperature metal materials, including typical advanced low-temperature steels and low-temperature medium / high entropy alloys. The yield strength of the reported advanced low-temperature steel can reach 800MPa and the elongation can reach 70%. For medium / high entropy alloys, the maximum yield strength of the ternary NiCoCr medium entropy alloy with an equiatomic ratio can reach 700MPa and the elongation exceeds 70%. In order to further improve the low-temperature yield strength of medium / high entropy alloys, researchers have applied second phase strengthening, mainly including L12 phase strengthening, oxide particle strengthening and ceramic particle strengthening. Through second phase strengthening, the reported in-situ nano-oxide reinforced CrMnFeCoNi high entropy alloy exhibits the best low-temperature performance, with a yield strength of 1150MPa and an elongation of 23%. Through second phase strengthening, it provides a good idea for improving the low-temperature yield strength of the alloy, but the yield strength of the material is still not ideal, which limits its application under extreme conditions. Therefore, it is necessary to provide a new way to improve the low-temperature yield strength of medium / high entropy alloys.

[0004] Second phase strengthening (L12 phase, oxide particles and ceramic particles) can significantly improve the low temperature yield strength of metal materials. However, there are few studies on 22 There is no relevant report on strengthening the low-temperature yield strength of metal materials by using the γ″ phase of the structure. For medium / high entropy alloys, the nanoscale γ″ phase usually has a higher lattice mismatch and antiphase interface energy, resulting in high strengthening ability and good strength-ductility balance. Summary of the invention

[0005] In order to solve the above technical problems, the first purpose of the present invention is to provide a medium-entropy alloy with ultra-high low-temperature yield strength, and the second purpose is to provide a method for preparing the alloy. The present invention will focus on how to introduce the γ″ phase with ultra-high strengthening ability into the low-temperature medium-entropy alloy to improve the yield strength of the material at 77K.

[0006] To achieve the above first purpose, the present invention provides the following technical solutions:

[0007] A medium entropy alloy with super low temperature yield strength, characterized in that: the molecular formula is Ni2CoCrNb 0.2 , with D0 22 Nanoscale γ″ strengthening phase of the structure.

[0008] The NiCoCr system is a low-temperature medium-entropy alloy material with excellent performance. It has a single FCC structure. At 77K, deformation twins are formed due to low stacking fault energy, which improves the low-temperature performance. The addition of Nb element is to precipitate the γ″ phase to improve the low-temperature yield strength of the material. When the total valence electron concentration of the medium-entropy alloy is less than 8.4, the brittle Lives phase is easily formed at high temperature, and the sample is prone to crack initiation in the thermal environment of additive manufacturing. When the total valence electron concentration of the medium-entropy alloy is greater than 8.4, a single-phase face-centered cubic structure is formed at high temperature, and a D0 22 The γ" phase of the structure. The Ni2CoCrNb designed by the present invention 0.2 Medium entropy alloy, which has a stable single-phase FCC structure in the thermal environment of additive manufacturing, can precipitate D0 22 The nano-scale γ″ strengthening phase of the structure improves the low-temperature yield strength of metal materials.

[0009] The second object of the present invention is achieved by: a method for preparing a medium-entropy alloy with ultra-high low-temperature yield strength, characterized in that the alloy is prepared according to the following method:

[0010] 1) Powder preparation: spherical powdered raw materials Ni, Co, Cr and Nb prepared by plasma rotating electrode process are uniformly mixed in a molar ratio of 20:10:10:2;

[0011] 2) Powder ball milling: placing the mixed powder in a ball mill for ball milling;

[0012] 3) drying the ball-milled powder in a vacuum dryer at 180° C. for more than 8 hours;

[0013] 4) In a laser additive manufacturing machine, a coaxial nozzle is used to feed powder, and a 304 plate is used as a substrate material to prepare a bulk deposited medium entropy alloy in an argon atmosphere;

[0014] 5) subjecting the prepared bulk medium entropy alloy to a solution treatment at 1150° C. for 0.5 h and a direct water quenching cooling treatment;

[0015] 6) The medium-entropy alloy after solid solution is subjected to aging treatment at 680°C.

[0016] In the above scheme: the mass of the grinding balls and the mass of the powder are matched in a ratio of 4:1, the ball milling speed is 300 r / min, and the ball milling is performed for 3 hours.

[0017] In the above scheme: the purity of Ni, Co, Cr and Nb in the alloy raw materials is greater than 99.5wt%.

[0018] In the above scheme: the laser additive manufacturing machine is equipped with a 6KW continuous wave fiber laser and a dual powder feeder automatic feeding device.

[0019] In the above scheme: laser power: 900W, powder feeding amount: 25g / min, scanning rate: 25mm / s, layer height: 0.30mm, laser spot diameter: 2.5mm.

[0020] In the above scheme: the thickness of the deposited high entropy alloy is 0.3-0.35 mm.

[0021] In the above scheme: the aging treatment time is 24-120h.

[0022] The ultra-high low-temperature yield strength medium-entropy alloy of the present invention, which has not undergone solid solution and aging treatment, has uniform microstructure, dense structure, and no micropores and microcracks. The yield strength, tensile strength and elongation are respectively 420MPa, 800MPa and 48%, which are much higher than other manufacturing methods and broaden the existing low-temperature material formulation system for additive manufacturing.

[0023] After the alloy was solution treated at 1150℃ for 0.5h, the elements of the alloy were evenly distributed, the dendrites disappeared and transformed into columnar crystals, achieving the effect of solid solution. The yield and tensile strengths dropped to ~380MPa and ~790MPa respectively, while the tensile elongation increased to ~54%. During the aging treatment, with the increase of time, the strength and hardness of the alloy gradually increased, and the plasticity decreased. When the aging time reached 96 hours, the alloy was tensile tested at 293K, and the strength of the alloy reached its peak, with the best comprehensive mechanical properties, the yield strength was ~1180MPa, the ultimate strength was ~1135MPa, and the tensile elongation was ~19%, which is the highest level reported so far.

[0024] After the alloy was solution treated at 1150℃ for 0.5h, the sample aged at 680℃ for 96h was placed at 77K for tensile test. The strength and plasticity of the alloy were improved. The yield strength of the alloy was ~1320MPa, the ultimate strength was ~1552MPa, and the tensile elongation was ~25%, which is the highest level reported so far and achieved a breakthrough in the yield strength of low-temperature materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 Ni2CoCrNb 0.2 Calculation results of the phase diagram of medium-entropy alloys.

[0026] Figure 2 Ni2CoCrNb prepared by laser additive manufacturing of the present invention 0.2 Morphology of medium-entropy alloy.

[0027] Figure 3 Ni2CoCrNb prepared by the present invention 0.2 The XRD test results of medium-entropy alloys are the test results of samples in the deposited state (without solution treatment), the solution state (without aging treatment) and the sample aged at 680℃ for 96h.

[0028] Figure 4 Ni2CoCrNb prepared by the present invention 0.2 Microstructure morphology of as-deposited medium-entropy alloy.

[0029] Figure 5 Ni2CoCrNb prepared by the present invention 0.2 Hardness test results of medium entropy alloys at different aging times.

[0030] Figure 6 Ni2CoCrNb prepared by the present invention 0.2 TEM analysis results of medium-entropy alloy after aging at 680℃ for 96h.

[0031] Figure 7 These are the tensile test results of the Ni2CoCrNb0.2 medium entropy alloy prepared by the present invention at different aging times.

[0032] Figure 8 The Ni2CoCrNb prepared by the present invention after the final thermal treatment process is determined 0.2 Tensile test results of medium-entropy alloy at 293K and 77K. DETAILED DESCRIPTION

[0033] The present invention will be further described below in conjunction with the embodiments.

[0034] Example 1

[0035] A medium entropy alloy with ultra-high low temperature yield strength, the molecular formula is Ni2CoCrNb 0.2 .

[0036] Prepared as follows:

[0037] 1) Powder preparation: spherical powder raw materials Ni, Co, Cr and Nb prepared by plasma rotating electrode process are selected and evenly mixed in a molar ratio of 20:10:10:2. The purity of Ni, Co, Cr and Nb in the alloy raw materials is greater than 99.5wt%.

[0038] 2) Powder ball milling: the mixed powder is placed in a ball mill for ball milling, the mass of the grinding balls is matched to the mass of the powder in a ratio of 4:1, the ball milling speed is 300 r / min, and the ball milling is performed for 3 h.

[0039] 3) The ball-milled mixed powder is placed in a vacuum dryer and dried at 180° C. for more than 8 hours.

[0040] 4) In the laser additive manufacturing machine, a coaxial nozzle is used to feed powder. In an argon atmosphere, a 304 plate is used as the substrate material to prepare a bulk deposited medium entropy alloy. The thickness of the deposited high entropy alloy is 0.3-0.35mm. The laser additive manufacturing machine is equipped with a 6KW continuous wave fiber laser and a dual powder feeder automatic feeding device. Laser power: 900W, powder feeding amount 25g / min, scanning rate: 25mm / s, layer height: 0.30mm, laser spot diameter is 2.5mm.

[0041] 5) subjecting the prepared bulk medium entropy alloy to a solution treatment at 1150° C. for 0.5 h and a direct water quenching cooling treatment;

[0042] 6) The above-mentioned medium-entropy alloy after solid solution was aged at 680°C for 24h, 48h, 72h, 96h and 120h respectively.

[0043] Figure 1 Ni2CoCrNb 0.2 The phase diagram calculation results of medium entropy alloys are as follows: Figure 1 It can be seen that Ni2CoCrNb 0.2 The simulation results of the medium entropy alloy phase diagram show that a single-phase FCC structure is formed in the high temperature region, and a D0 22 The γ″ phase of the structure does not form the Laves phase. The simulation results prove that the molecular formula Ni2CoCrNb 0.2 The alloy is suitable for additive manufacturing process. At the same time, it provides a temperature reference for the solid solution and aging heat treatment of the deposited medium entropy alloy. Figure 2 It can be seen that the blocky medium-entropy alloy has a regular shape, does not collapse during the printing process, and has no cracks or oxidation on the surface.

[0044] from Figure 3 It can be seen that Ni2CoCrNb under three treatment conditions 0.2 The medium entropy combined XRD patterns clearly show that all samples have a single FCC crystal structure.

[0045] from Figure 4 It can be seen that the grains are columnar and grow along the material stacking direction, and the average grain size is 62 microns.

[0046] from Figure 5 It can be seen that with the increase of aging time, the hardness of the material increases, reaching a peak value of 620HV when reaching 96h, and then showing a downward trend.

[0047] from Figure 6 It can be seen that the uniformly distributed precipitates with different crystal orientations are mainly disc-shaped. According to the DF image, the volume fraction of the precipitate phase is about 15%, the length is about 10nm, the width is about 3nm, and the crystal orientation relationship between the matrix and the γ″ phase is <001> m / /

[001] γ″ and {100} m / / {100} γ″ .

[0048] Figure 7 Ni2CoCrNb prepared by the present invention 0.2 The tensile test results of medium entropy alloys at different aging times are shown in Figure 2. Figure 7 It can be seen that during the aging treatment, as time goes by, the yield strength of the alloy gradually increases and the plasticity decreases. When the aging time reaches 96 hours, the alloy is tensile tested at 293K, and the strength of the alloy reaches its peak, with the best comprehensive mechanical properties, the yield strength is ~1180MPa, the ultimate strength is ~1135MPa, and the tensile elongation is ~19%.

[0049] Figure 8 The Ni2CoCrNb prepared by the present invention after the final thermal treatment process is determined 0.2 The tensile test results of medium entropy alloy at 293K and 77K show that Figure 8 It can be seen that when the sample is placed in a tensile test at 77K, the strength and plasticity of the alloy are improved, the yield strength of the alloy is ~1320MPa, the ultimate strength is ~1552MPa, and the tensile elongation is ~25%.

Claims

1. A medium entropy alloy with ultra-high low temperature yield strength, characterized in that: The molecular formula is Ni2CoCrNb 0.2 , with D0 22 Nanoscale γ″ strengthening phase of the structure.

2. A method for preparing a medium-entropy alloy with ultra-high low-temperature yield strength, characterized in that: Prepared as follows: 1) Powder preparation: spherical powdered raw materials Ni, Co, Cr and Nb prepared by plasma rotating electrode process are uniformly mixed in a molar ratio of 20:10:10:2; 2) Powder ball milling: placing the mixed powder in a ball mill for ball milling; 3) drying the ball-milled powder in a vacuum dryer at 180° C. for more than 8 hours; 4) In a laser additive manufacturing machine, a coaxial nozzle is used to feed powder, and a 304 plate is used as a substrate material to prepare a bulk deposited medium entropy alloy in an argon atmosphere; 5) subjecting the prepared bulk medium entropy alloy to a solution treatment at 1150° C. for 0.5 h and a direct water quenching cooling treatment; 6) The medium-entropy alloy after solid solution is subjected to aging treatment at 680°C.

3. The method for preparing a medium-entropy alloy with ultra-high low-temperature yield strength according to claim 2, characterized in that: The mass of the grinding balls and the mass of the powder are mixed in a ratio of 4:1, the ball milling speed is 300 r / min, and the ball milling is performed for 3 h.

4. The method for preparing a medium-entropy alloy with ultra-high low-temperature yield strength according to claim 3, characterized in that: The purity of Ni, Co, Cr and Nb in the alloy raw materials is greater than 99.5wt%.

5. The method for preparing a medium-entropy alloy with ultra-high low-temperature yield strength according to any one of claims 2 to 4, characterized in that: The laser additive manufacturing machine is equipped with a 6KW continuous wave fiber laser and a dual powder feeder automatic feeding device.

6. The method for preparing a medium-entropy alloy with ultra-high low-temperature yield strength according to claim 5, characterized in that: Laser power: 900W, powder feeding amount 25g / min, scanning rate: 25mm / s, layer height: 0.30mm, laser spot diameter is 2.5mm.

7. The method for preparing a medium-entropy alloy with ultra-high low-temperature yield strength according to claim 2, characterized in that: The aging treatment time is 24-120h.

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