CN116716528B - High-strength plastic nanoparticle precipitation strengthening medium-entropy alloy and preparation method thereof - Google Patents

Abstract

The invention discloses a high-strength plastic nano-particle precipitation-strengthened medium-entropy alloy and a preparation method thereof. The molecular formula is Ni _2.1 CoCrFe _0.5 Nb _0.2 . The alloy has a stable single-phase face-centered cubic structure at high temperature, and a nano-scale γ″ phase with a D0 ₂₂ structure can be precipitated through low-temperature aging. The alloy is manufactured by laser additive manufacturing, followed by solid solution treatment and aging treatment. The prepared medium-entropy alloy has no micropores and microcracks, uniform organization, compact structure, and high elongation. After the alloy is solid solution treated at 1100° C. for 2 hours, the elements of the alloy are evenly distributed, element segregation is eliminated, and the effect of solid solution is achieved. In the aging treatment at 650° C., as time increases, the uniformly precipitated γ″ phase with a D0 ₂₂ structure gradually increases, the strength and hardness of the alloy gradually increase, and the plasticity decreases. When the aging time reaches 120 hours, the strength reaches a peak value, the comprehensive mechanical properties are optimal, the yield strength is 1005MPa, the ultimate strength is 1240MPa, and the tensile elongation is 20%. Provide a theoretical basis for the preparation of high-strength and plastic medium-entropy alloys by additive manufacturing.

Description

A high-strength plastic nanoparticle precipitation-strengthened medium-entropy alloy and preparation method thereof

Technical Field

The invention belongs to the field of alloys, and in particular relates to a high-strength and plastic nano-particle precipitation-strengthened medium-entropy alloy and a preparation method thereof.

Background technique

Medium-entropy alloys are a new type of alloys containing three or more main elements with entropy values of 1.5R≥ΔS≥1R in equimolar ratios or near molar ratios. Medium-entropy alloys composed of multiple main elements not only have unexpectedly simple phase compositions but also have good properties, which have attracted widespread attention. In the past few decades, significant progress has been made in the research on composition design, phase selection, mechanical properties and functional properties, deformation mechanism and processing methods of medium-entropy alloys. Among different processing methods, additive manufacturing, as an advanced manufacturing method, is considered to be a promising technology for preparing medium-entropy alloys. Additive manufacturing is based on the combination of computer science and material processing and forming technology, and shows great potential in the rapid manufacture of large components with complex shapes and the acquisition of alloys with fine structures and excellent mechanical properties. At the same time, the diversity and variability of the composition of medium-entropy alloys have brought more opportunities for the application of additive manufacturing, but the existing limited alloy systems are used for additive manufacturing, and the mechanical properties obtained are still not ideal, such as Ti-6Al-4V, IN718, 316L. Therefore, more high-performance alloy formulation systems for additive manufacturing need to be developed.

Additive manufacturing has significant advantages: (1) high degree of freedom in geometric design and optimization; (2) functional combination and part integration, reducing assembly, improving performance and reliability; (3) high material utilization and energy efficiency; (4) suitable for customization and small batch production; (5) shortening product production and delivery cycle. Therefore, aerospace is a key market driver for the development of additive manufacturing, because its high-value parts often require multi-variety small batch production, highly integrated complex structures and fast and efficient manufacturing processes. Direct laser deposition additive technology, due to the use of coaxial powder feeding, metal powder melts instantly and the alloy solidifies rapidly. Medium entropy alloys are composed of multiple elements, and there are differences in the melting point, shrinkage rate and other properties between the elements. Therefore, in many basic studies that have been carried out, the block high entropy alloys prepared by additive manufacturing have macro cracks and micropores, resulting in poor formability.

A lot of work has been done to improve the strength of single-phase high-entropy alloys prepared by additive manufacturing. For example, a graded metastable microstructure was achieved; a small amount of interstitial atoms were doped into the single-phase matrix; a second nanoparticle was added to the single-phase matrix. However, these methods all showed a moderate strengthening effect. Recently, it has been reported that precipitation strengthening shows a better strengthening effect in medium-entropy alloys compared with other strengthening mechanisms. Al/Ti alloys and Nb alloys have successfully introduced and developed two types of precipitation-hardened medium-entropy alloys, γ′ phase and γ″ phase, respectively.

Summary of the invention

In order to solve the above technical problems, the first purpose of the present invention is to provide a high-strength and plastic nanoparticle precipitation-strengthened medium-entropy alloy, and the second purpose is to provide a method for preparing the alloy.

To achieve the above first purpose, the present invention provides the following technical solution: a high-strength plastic nanoparticle precipitation-strengthened medium-entropy alloy, characterized in that the molecular formula is: Ni _2.1 CoCrFe _0.5 Nb _0.2 , the alloy has a stable single-phase face-centered cubic structure at high temperature and a nanoscale γ″ phase with a D0 ₂₂ structure.

The present invention aims to obtain good mechanical properties. The most basic, most thoroughly studied, and structurally stable CoCrFeNi system is used as the matrix, and Nb, a typical precipitation hardening element in nickel-based high-temperature alloys, is added to design a medium-entropy alloy formula for additive manufacturing.

The second object of the present invention is achieved by: a method for preparing a high-strength and plastic nanoparticle precipitation-strengthened medium-entropy alloy, characterized in that the alloy is prepared according to the following method:

1) Powder preparation: spherical powdered raw materials Ni, Co, Cr, Fe and Nb prepared by plasma rotating electrode process are uniformly mixed in a molar ratio of 21:10:10:5:2;

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 120°C for more than 4 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) Solution treatment: the prepared bulk medium entropy alloy is solution treated at 1100°C and directly water quenched.

6) The medium entropy alloy after solid solution is aged at 650°C.

3. The method for preparing high-strength and plastic nanoparticle precipitation-strengthened medium-entropy alloy according to claim 2 is characterized in that the purity of Ni, Co, Cr, Fe and Nb in the alloy raw materials is greater than 99.5wt%.

In the above scheme: the mass of the grinding balls and the mass of the powder are mixed in a ratio of 5:1, the ball milling speed is 350 r/min, and the ball milling is performed for 4 hours.

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.

In the above scheme: in step 4), the laser power is 1200 W, the powder feeding amount is 20 g/min, the scanning rate is 20 mm/s, the layer height is 0.40 mm, and the laser spot diameter is 3.2 mm.

In the above scheme: in step 4), the thickness of the entropy alloy in the deposited state is 0.4-0.5 mm.

In the above scheme: the solution treatment time is 2h.

In the above scheme: the aging treatment time is 24-148h.

The present invention obtains the optimal manufacturing process parameters for additive manufacturing of the medium entropy alloy system by optimizing laser power, spot diameter, powder feeding speed, scanning speed, and Z-direction layer height.

Compared with the prior art, the present invention has the following beneficial effects:

The NiCoCrFe system is the most basic and most thoroughly studied alloy system, having a single and stable FCC structure, and the addition of the Nb element is for the purpose of precipitation strengthening of nanoparticle γ″ phase. The present invention proposes a design strategy combining total valence electron concentration with phase diagram simulation, which is used for the design of γ″ phase in medium-entropy alloys. When the total valence electron concentration value of the medium-entropy alloy is less than 8.4, a brittle Lives phase is easily formed at high temperature, and cracks are easily initiated in the sample under the thermal environment of additive manufacturing. When the total valence electron concentration value of the medium-entropy alloy is greater than 8.4, a single-phase face-centered cubic structure is formed at high temperature, and a γ″ phase with a D0 ₂₂ structure is precipitated at low temperature. The Ni _2.1 CoCrFe _0.5 Nb _0.2 medium-entropy alloy developed by the present invention has a stable single-phase face-centered cubic structure at high temperature, and a nanoscale γ″ phase with a D0 ₂₂ structure can be precipitated by low-temperature aging, thereby improving the strength of the medium-entropy alloy.

The high-strength plastic medium-entropy alloy of the present invention, which has not undergone solid solution and aging treatment, has uniform organization, dense structure, and no micropores and microcracks. The yield strength, tensile strength and elongation are 278MPa, 720MPa and 50% respectively, which exceeds the level of other manufacturing methods and broadens the existing medium-entropy alloy formulation system for additive manufacturing.

After the alloy was solution treated at 1100℃ for 2h, 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 257MPa and 647MPa respectively, while the tensile elongation increased to 58%. 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 120 hours, the strength of the alloy reached its peak, and the comprehensive mechanical properties were the best, with a yield strength of 1005MPa, an ultimate strength of 1240MPa, and a tensile elongation of 20%.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows the calculated phase diagrams of entropy alloys in NiCoCrFNb with different compositions, (a) NiCoCrFeNb _0.2 alloy, (b) Ni _1.6 CoCrFeNb _0.2 alloy and (c) Ni _2.1 CoCrFe _0.5 Nb _0.2 .

FIG2 is a morphology diagram of the Ni _2.1 CoCrFe _0.5 Nb _0.2 medium entropy alloy prepared by laser additive manufacturing of the present invention.

Figure 3 shows the microstructure morphology and XRD test results of the Ni _2.1 CoCrFe _0.5 Nb _0.2 medium entropy alloy prepared by the present invention, SEM images of (a ₁ ) deposited state, (b ₁ ) solid solution state and (c ₁ ) 120h aged state, and XRD test results of (a ₂ ) deposited state, (b ₂ ) solid solution state and (c ₂ ) 120h aged state.

FIG4 is a surface scan energy spectrum diagram of the Ni _2.1 CoCrFe _0.5 Nb _0.2 medium entropy alloy prepared in the present invention in the deposited state.

Figure 5 shows the EBSD analysis results of the Ni _2.1 CoCrFe _0.5 Nb _0.2 medium entropy alloy prepared by the present invention, IPF diagrams of (a ₁ ) as-deposited (without solution treatment), (b ₁ ) as-solution (without aging treatment), and (c ₁ ) as-aged for 120 h, and grain size distributions of (a ₂ ) as-deposited, (b ₂ ) as-solution, and (c ₂ ) as-aged for 120 h.

FIG6 is a TEM analysis result of the Ni _2.1 CoCrFe _0.5 Nb _0.2 medium entropy alloy prepared by the present invention after aging for 120 h.

FIG. 7 shows the hardness test results of the Ni _2.1 CoCrFe _0.5 Nb _0.2 medium entropy alloy prepared by the present invention at different aging times.

FIG8 shows the tensile test results of the Ni _2.1 CoCrFe _0.5 Nb _0.2 medium entropy alloy prepared by the present invention at different aging times.

Detailed ways

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

Example 1

A high-strength and plastic nanoparticle precipitation-strengthened medium-entropy alloy.

Alloys with molecular formulas of NiCoCrFeNb _0.2 , Ni _1.6 CoCrFeNb _0.2 , and Ni _2.1 CoCrFe _0.5 Nb _0.2 were designed and prepared as follows:

1) Powder preparation: spherical powder raw materials Ni, Co, Cr, Fe and Nb prepared by plasma rotating electrode process are uniformly mixed in a molar ratio of 21:10:10:5:2; the purity of Ni, Co, Cr, Fe and Nb in the alloy raw materials is greater than 99.5wt%.

2) Powder ball milling: place the above mixed powder in a ball mill for ball milling, with the mass of the grinding balls and the mass of the powder being matched in a ratio of 5:1, the ball milling speed being 350 r/min, and the ball milling being performed for 4 h.

3) The ball-milled mixed powder was placed in a vacuum dryer and dried at 120 degrees for more than 4 hours.

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 medium entropy alloy is 0.4-0.5mm. The laser additive manufacturing machine is equipped with a 6KW continuous wave fiber laser and a dual powder feeder automatic feeding device. Laser power: 1200W, powder feeding amount 20g/min, scanning rate: 20mm/s, layer height: 0.40mm, laser spot diameter is 3.2mm.

5) Solution treatment: the prepared bulk Ni _2.1 CoCrFe _0.5 Nb _0.2 medium entropy alloy is directly water quenched at 1100°C for 2h.

6) The Ni _2.1 CoCrFe _0.5 Nb _0.2 medium entropy alloy after solid solution was aged at 650 °C for 24 h, 48 h, 72 h, 96 h, 112 h, 120 h, and 148 h respectively.

The hardness and tensile properties of the samples were tested.

Figure 1 shows the calculated phase diagram of the entropy alloys in NiCoCrFNb with different compositions, (a) NiCoCrFeNb _0.2 alloy, (b) Ni _1.6 CoCrFeNb _0.2 alloy and (c) Ni _2.1 CoCrFe _0.5 Nb _0.2 . It can be seen from Figure 1 that the OVEC values of the entropy alloys in NiCoCrFeNb _0.2 and Ni _1.6 CoCrFeNb _0.2 are 8.09 and 8.33, respectively. The OVEC values of these two alloy compositions are lower than 8.4, so they cannot mainly form the D0 ₂₂ structure. Figure 1 (a) shows the calculated phase diagram of the entropy alloy in NiCoCrFeNb _0.2 , indicating that the Laves phase appears during the laser deposition process. Figure 1 (b) shows the simulation results of the entropy alloy phase diagram in Ni _1.6 CoCrFeNb _0.2 , indicating that the Laves phase decreases with the increase of Ni content. Finally, the composition of Ni _2.1 CoCrFe _0.5 Nb _0.2 medium entropy alloy with OVEC of 8.54 was designed. Figure 1(c) shows the simulation results of Ni _2.1 CoCrFe _0.5 Nb _0.2 medium entropy alloy. A single-phase FCC structure is formed during the laser deposition process, and the γ″ phase is precipitated in the subsequent aging treatment, and the Laves phase disappears. At the same time, Figure 1(c) provides a theoretical reference for the solid solution and aging heat treatment of the deposited alloy.

FIG2 is a morphology diagram of the Ni _2.1 CoCrFe _0.5 Nb _0.2 medium-entropy alloy prepared by laser additive manufacturing of the present invention. As can be seen from FIG2 , the blocky medium-entropy alloy has a regular shape, no collapse, and no cracks or oxidation on the surface.

FIG3 is a microstructure morphology and XRD test results of the Ni _2.1 CoCrFe _0.5 Nb _0.2 medium entropy alloy prepared by the present invention, SEM images of (a ₁ ) deposited state, (b ₁ ) solid solution state and (c ₁ ) 120 hours aging, and XRD test results of (a ₂ ) deposited state, (b ₂ ) solid solution and (c ₂ ) 120 hours aging state. It can be seen from FIG3 that: (a ₁ ) shows that the microstructure of the deposited alloy presents a dendritic microstructure. (b ₁ ) shows the microstructure of the alloy after solid solution treatment, and the dendrites are decomposed. (c ₁ ) shows the microstructure of the alloy after aging, and tiny particles exist in the grains. (a ₂ -c ₂ ) show the XRD patterns of Ni _2.1 CoCrFe _0.5 Nb _0.2 under three treatment conditions, respectively, which clearly shows that all samples have a single FCC crystal structure, and it can be further seen that although there is a particle phase on the matrix, only a single phase is shown after aging treatment. Although the precipitation of the γ″ phase or the dissolution of the Nb-rich phase will lead to changes in the distribution of the Nb element, the particle phase is too fine to be detected by XRD, which ultimately does not lead to significant changes in the lattice parameters.

As can be seen from Figure 4: Figures (b-e) show the element mapping scans of Ni, Co, Cr and Fe, which are uniformly distributed in the microstructure, while Figure (f) shows that the distribution of Nb element is uneven.

FIG5 is the EBSD analysis results of the Ni _2.1 CoCrFe _0.5 Nb _0.2 medium entropy alloy prepared by the present invention, the IPF diagrams of (a ₁ ) the deposited state, (b ₁ ) the solid solution state and (c ₁ ) the 120h aged state, and the grain size distributions of (a ₂ ) the deposited state, (b ₂ ) the solid solution state and (c ₂ ) the 120h aged state. It can be seen from the figure that: FIG (a ₁ ) shows the IPF diagram of the deposited alloy, and FIG (a ₂ ) shows the corresponding grain size distribution, with an average size of 54.42 μm. FIG (b ₁ ) shows the IPF diagram of the solid solution alloy, and FIG (b ₂ ) shows the corresponding grain size distribution, with an average grain size of 58.89 μm. FIG (c ₁ ) shows the IPF diagram of the 120h aged alloy, and FIG (c ₂ ) shows the corresponding grain size distribution, with an average size of 62.08 μm.

FIG6 is the TEM analysis result of the 120h aged state of the Ni _2.1 CoCrFe _0.5 Nb _0.2 medium entropy alloy prepared by the present invention. It can be seen from the figure that the uniformly distributed precipitates with different crystal orientations are mainly disc-shaped. According to the DF image, the volume fraction of the precipitated phase is about 15%, the length is about 9.8nm, and the width is about 3.2nm. (b ₁ ) shows three variants of the γ″ phase, marked with three different yellow symbols, and the crystal orientation relationship between the matrix and the γ″ phase is <001> _m // [001] _γ″ and {100} _m // {100} _γ″ .

Figure 7 shows the hardness test results of the Ni _2.1 CoCrFe _0.5 Nb _0.2 medium entropy alloy prepared by the present invention at different aging times. It can be seen from the figure that as the aging time increases, the hardness tends to increase, reaching a peak value of 481 HV at 120 h.

FIG8 is the tensile test results of different aging times of the Ni _2.1 CoCrFe _0.5 Nb _0.2 medium entropy alloy prepared by the present invention. It can be seen from the figure that the yield strength of the deposited medium entropy alloy is about 278MPa, the tensile strength is about 720MPa, and the elongation at break is about 50%. After solution treatment, the sample shows better ductility of about 58%, but the strength decreases slightly to about 647MPa. After various aging treatments, the strength of the sample is significantly improved, especially after aging for 120 hours, the yield strength and ultimate tensile strength reach 1005MPa and 1240MPa, respectively.

After the alloy was solution treated at 1100℃ for 2h, 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 257MPa and 647MPa respectively, while the tensile elongation increased to 58%. 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 120 hours, the strength of the alloy reached its peak, and the comprehensive mechanical properties were the best. The yield strength was 1005MPa, the ultimate strength was 1240MPa, and the tensile elongation was 20%, which is the highest level reported so far.

Claims (9)

1. The high-strength plastic nanoparticle precipitation strengthening medium entropy alloy is characterized by having the following molecular formula: ni _2.1CoCrFe_0.5Nb_0.2, the alloy has a stable single-phase face-centered cubic structure at high temperatures, and a nanoscale gamma '' phase with a D0 ₂₂ structure.

2. The preparation method of the high-strength plastic nanoparticle precipitation strengthening medium entropy alloy is characterized by comprising the following steps of:

1) Preparing powder, namely selecting spherical powdery raw materials Ni, co, cr, fe and Nb prepared by a plasma rotating electrode process according to a mole ratio of 21:10:10:5:2, uniformly mixing the materials in proportion;

2) Ball milling of the powder, namely placing the mixed powder into a ball milling tank for ball milling;

3) Placing the powder after ball milling and mixing in a vacuum dryer to be dried for more than 4 hours at 120 ℃;

4) In a laser additive manufacturing machine, adopting a coaxial nozzle to send powder, and preparing a bulk deposition state medium entropy alloy by taking 304 plates as substrate materials in an argon atmosphere;

5) Solid solution treatment, namely solid solution treatment is carried out on the prepared blocky medium-entropy alloy at 1100 ℃, direct water quenching and cooling treatment are carried out,

6) And aging the intermediate entropy alloy after solid solution at 650 ℃.

3. The method for preparing the high-strength plastic nanoparticle precipitation strengthening mid-entropy alloy according to claim 2, which is characterized in that: the purity of Ni, co, cr, fe and Nb in the alloy feedstock is greater than 99.5wt%.

4. The method for preparing the high-strength plastic nanoparticle precipitation strengthening mid-entropy alloy according to claim 3, which is characterized in that: the grinding ball mass and the powder mass are proportioned according to the ratio of 5:1, the ball milling rotating speed is 350 r/min, and the ball milling is 4: 4 h.

5. The method for preparing the high-strength plastic nanoparticle precipitation strengthening mid-entropy alloy according to claim 4, which is characterized in that: the laser additive manufacturing machine is provided with a 6KW continuous wave fiber laser and an automatic feeding device of a double powder feeder.

6. The method for preparing the high-strength plastic nanoparticle precipitation strengthening mid-entropy alloy according to claim 5, which is characterized in that: in step 4), laser power: 1200W, 20g/min of powder feeding amount, and scanning rate: 20mm/s, layer height: 0.40mm, and the laser spot diameter is 3.2mm.

7. The method for preparing the high-strength plastic nanoparticle precipitation strengthening medium entropy alloy according to claim 6, which is characterized in that: in step 4), the thickness of the entropy alloy in the as-deposited state is 0.4-0.5mm.

8. The method for preparing the high-strength plastic nanoparticle precipitation strengthening mid-entropy alloy according to claim 7, which is characterized in that: the solution treatment time is 2h.

9. The method for preparing the high-strength plastic nanoparticle precipitation strengthening mid-entropy alloy according to claim 8, which is characterized in that: aging treatment time is 24-148h.