CN111394636A - High-strength high-plasticity high-entropy alloy with martensite phase transformation and preparation method thereof - Google Patents

Abstract

The invention relates to a high-strength high-plasticity high-entropy alloy with martensite phase transformation and a preparation method thereof, belonging to the technical field of high-entropy alloys. The high-strength large-plasticity high-entropy alloy with martensite phase transformation consists of Cr, Co, Ni and Si, wherein the molar ratio of the four elements is 1:1:1: x, x is 0.1,0.2, 0.3. The purity of the smelting raw materials of Cr, Co, Ni and Si is more than or equal to 99.9 wt.%. The high-strength large-plasticity high-entropy alloy with martensite phase transformation has the advantages that the work hardening capacity is obviously enhanced, the strength and plasticity are excellent, the application requirements of the modern industry on material impact energy absorption are met, and the application potential is huge; meanwhile, the invention provides a simple and convenient preparation method.

Description

High-strength and high-plastic high-entropy alloy with martensitic transformation and preparation method thereof

technical field

The invention relates to a high-strength and high-plasticity high-entropy alloy with martensitic transformation and a preparation method thereof, belonging to the technical field of high-entropy alloys.

Background technique

Exploring new materials is one of the eternal goals of mankind. As a new research and application field, high-entropy alloy is a new alloy with broad application potential. In recent years, the excellent properties of high-entropy alloy materials have led more and more researchers to develop alloy materials with excellent properties. Many high-entropy alloys have been reported, including NiCoCrFeMn in FCC system and A1CoCr _1.5 in BCC system. Fe _1.5 NiTl _0.5 , all obtained good mechanical properties.

As we all know, entropy is a physical quantity that expresses the degree of disorder in a system, and its size can affect the thermodynamic stability of the system. The most important characteristic of high-entropy alloys is the high-entropy effect. According to the principle of maximum entropy generation, a large entropy value can stabilize the high-entropy phase, and a high mixing entropy makes the alloy tend to form a solid solution rather than an intermetallic compound. High configurational entropy facilitates the formation of a single phase to obtain a solid solution structure.

High-entropy alloys have a typical solid solution phase composed of a variety of components, and it is generally believed that the atoms of each element randomly occupy the lattice positions in the crystal with equal probability, that is, all atoms have no distinction between solute atoms and solvent atoms, and are composed of points of different atomic sizes. The lattice causes lattice distortion, and it is possible to form atomic-level stress to cause lattice distortion, often with special properties. Regarding the lattice distortion in the structure, it can be expressed by the mean square error of the atomic radius difference of the elements contained in the alloy:

δ≤6.6% can be used as the criterion for the formation of solid solution.

At present, the preparation methods of high-entropy alloys are gradually enriched, such as the most widely used vacuum arc melting combined with copper mold casting method, in addition to vacuum induction melting, laser cladding method, magnetron sputtering method, electrochemical deposition method and Mechanical alloying, etc. Generally, the preparation method of bulk high-entropy alloy is mainly prepared by high vacuum arc melting or high vacuum induction melting. The vacuum arc smelting method is firstly to evacuate the electric arc furnace to more than 10 ^-8 Pa, and use tungsten electrode to carry out arc discharge in the environment of argon gas protection, usually using 22~65V arc voltage and 20~50mm arc length for large arc melting. Short-arc operation with current and low voltage, melting in copper crucibles by short-term high temperature. In order to ensure that the composition of the alloy is uniform, the alloy should not be smelted too much each time, about 30 to 70 g, and it needs to be smelted 3 to 5 times. Subsequently, the alloy melt can also be rapidly injected into the water-cooled copper mold by using the gas pressure difference to perform copper mold casting, thereby obtaining a casting with a certain shape.

New high-entropy alloys with excellent properties still need to be extensively developed. In order to develop alloy materials with better performance, researchers are constantly developing and trying.

CN 109355544 discloses a high-entropy alloy added with aluminum and silicon elements and a preparation method thereof. 1.5, 2.0) are prepared by vacuum arc smelting. The invention provides a high-entropy alloy added with aluminum and silicon elements and a preparation method thereof. The aluminum and silicon elements promote the formation of a body-centered cubic phase in a face-centered cubic AlCoCrFeNi system, thereby improving the wear resistance of the high-entropy alloy.

CN 110306186A discloses a silicon-containing high-entropy alloy coating and a preparation method thereof. The coating includes silicon element and main elements, and the main elements include Co element, Cr element, Cu element, Fe element and Mn element, The molar ratio of the Co element, the Cr element, the Cu element, the Fe element, the Mn element and the silicon element is 1:1:1:1:1:0.1-1.0. The prepared high-entropy alloy coating has good softening resistance at high temperature, increased hardness and wear resistance, and improved the macroscopic morphology of the coating surface; the addition of Si makes the BCC phase in the high-entropy alloy transform to the FCC phase, which promotes The FCC phase volume fraction increases.

Although many researchers have carried out research and development, new materials with better combination of strength and toughness need to be further optimized. New materials with good combination of strength and toughness can be used as energy-absorbing materials in aerospace, transportation and other fields. Therefore, the development and design of excellent high-entropy alloys for work hardening are of great significance and value.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a high-strength and high-plastic high-entropy alloy with martensitic transformation, which has significantly enhanced work hardening ability, excellent strength and plasticity, and meets the application requirements of material impact energy absorption in modern industry, It has huge application potential; meanwhile, the present invention provides a simple and convenient preparation method.

The high-strength and high-plasticity high-entropy alloy with martensitic transformation according to the present invention is composed of Cr, Co, Ni and Si elements, and the molar ratio of the four elements is 1:1:1:x, x=0.1, 0.2, 0.3. It is expressed as CrCoNiSi _{x (} x=0.1, 0.2, 0.3).

The purity of the smelting raw materials of Cr, Co, Ni and Si are all ≥99.9wt.%.

The high-entropy alloy has a single-phase fcc structure, which undergoes martensitic transformation during stretching and becomes an hcp phase structure.

The preparation method of the high-strength and high-plastic high-entropy alloy with martensitic transformation is that after purifying the required raw materials, high-vacuum arc melting and copper mold casting are performed; Polishing treatment, followed by heat treatment, rolling and annealing treatment, finally obtains the finished high-entropy alloy.

Specifically, it includes the following steps:

(1) Prepare raw materials, the alloy smelting raw materials used are Cr, Co, Ni and Si elements, which are accurately weighed and proportioned according to the molar ratio, and are used when smelting and preparing alloys;

(2) Purification of pure metals, using Cr, Co, Ni and Si as raw materials for alloy preparation to purify metal surface oxides;

(3) Using a high-vacuum arc melting furnace under the protection of 99.99% high-purity argon, the raw materials are smelted 5 to 8 times to make alloy button ingots and cast into a copper mold, that is, successfully smelted into a cross-sectional area of 2mm*10mm The new type of high-entropy alloy plate; the current is controlled at 170-180A during smelting, 210-220A during suction casting, and the arc length is controlled at 25-30mm;

(4) Cut a sample of 2mm×10mm×10mm on the alloy plate, and mount the sample into a sample with a size of Φ20×10mm using a metallographic mounting machine, and the surface of the 10mm×10mm sample faces upward;

(5) Use 600, 800, 1000, 1500, 2000 and 3000 mesh metallographic sandpapers to polish the surface of the sample respectively, and then use polishing liquid to finely polish the sample;

(6) adopting X-ray diffraction (XRD) to carry out phase composition analysis to the new high-entropy alloy, the scanning angle is 10 °-110 °, and the scanning speed is 3 ° min ^-1 ;

(7) The alloy plate is subjected to a homogenization heat treatment for 5h±2min at a temperature of 990-1110℃, then the alloy plate is rolled at room temperature until the thickness is reduced by 69-71%, and then the high-entropy alloy plate is treated at 895-905℃. The semi-finished product is annealed for 1h±2min;

(8) Cut the annealed alloy plate into tensile samples with a gauge length of 0.6 × 4 × 10 mm, and three samples of each composition have ensured the repeatability of the test performance;

(9) A quasi-static tensile test with a strain rate of 1×10 ^-3 s ^-1 was carried out by using the INSTRON mechanical testing machine to stretch the sample;

(10) EBSD and TEM microstructure observations were performed on the stretched new high-entropy alloy samples.

In the present invention, when designing the high-entropy alloy, the parameters of the four metal elements, Cr, Co, Ni and Si, are selected, as shown in Table 1:

Table 1

The invention takes the CrCoNi alloy as the design starting point and combines the design concept of the high-entropy alloy to successfully prepare a new type of CrCoNiSi _x (x=0.1, 0.2, 0.3) high-entropy alloy through a high-vacuum arc melting furnace. The significance of adding Si in the present invention is to reduce the stacking fault energy (SFE) of the alloy, so as to promote the phase transition from fcc to hcp in the process of plastic deformation, thereby realizing the simultaneous improvement of strength and plasticity, and obtaining excellent mechanical properties of high strength and plastic product. At the same time, the short-range order is regulated to a certain extent to optimize the mechanical properties of the alloy.

In the present invention, phase composition analysis and initial microstructure observation are carried out on the CrCoNiSi _x (x=0.1, 0.2, 0.3) high-entropy alloy before stretching. The CrCoNiSi _x (x=0.1, 0.2, 0.3) alloy has a single-phase fcc structure, And the addition of Si increases the grain size of the alloy, and increases the _{recrystallization} rate and degree. The high-entropy alloy has excellent mechanical properties of high strength and high toughness. Through the microscopic characterization of the stretched new high-entropy alloy sample, it is found that the CrCoNiSi _0.2 high-entropy alloy has undergone a transformation from the fcc phase to the hcp phase during the drawing process. The work hardening of the material is greatly improved. The plastic strain of CrCoNiSi _0.3 reaches 92%, the engineering fracture strength reaches 1GPa, and the strong-plastic product reaches 78.8Gpa%, which exceeds most existing alloys. This makes the new CrCoNiSi _x (x = 0.1, 0.2, 0.3) high-entropy alloys expected to be used as energy-absorbing materials in aerospace, new energy and engineering fields.

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

(1) The present invention promotes the reduction of stacking fault energy (SFE) by adding a small amount of Si element, induces the nano-level phase transition from fcc to hcp in the alloy during stretching, and greatly improves the work hardening ability of the alloy. Compared with the medium-entropy alloy CrCoNi under the same process, the toughness and strength are greatly improved at the same time, and the excellent mechanical properties of high strength and high plasticity are obtained;

(2) In the present invention, the addition of Si element makes the alloy have oxidation resistance;

(3) The high-entropy alloy prepared by the present invention meets the application requirements for impact energy absorption of materials in modern industry, and has huge potential application value in the fields of transportation and aerospace structural energy-absorbing materials;

(4) The preparation method of the high-entropy alloy according to the present invention is simple, convenient and widely used.

Description of drawings

Fig. 1 is a vacuum arc melting furnace (Fig. 1a) and a suction-cast high-entropy alloy plate (Fig. 1b) used for preparing high-entropy alloys according to the present invention;

Fig. 2 is the X-ray diffraction pattern of CrCoNiSi _x (x=0.1, 0.2, 0.3) high-entropy alloy before stretching;

Fig. 3 is the IPFx image of EBSD microstructure observation before drawing of CrCoNi and CrCoNiSi _0.3 high-entropy alloys;

Figure 4 is the quasi-static tensile stress-strain curve of CrCoNiSi _x (x=0.1, 0.2, 0.3) high-entropy alloy;

Fig. 5 is the strong-plastic product histogram of CrCoNiSi _x (x=0.1, 0.2, 0.3) high-entropy alloys with different Si contents;

Figure 6 is the work hardening curve of CrCoNiSi _x (x=0.1, 0.2, 0.3) high entropy alloy;

Figure 7 is a TEM image of the CrCoNiSi _0.2 high-entropy alloy after stretching; Figure 7a is a high-resolution image, and Figure 7b is a diffraction spot image of the region corresponding to Figure 7a.

Detailed ways

The present invention will be described in detail below through specific examples, but the present invention is not limited to these examples.

Cr, Co, Ni and Si with a purity of 99.9 wt.% are selected as raw materials.

The preparation process of CrCoNiSix ( _x =0.1, 0.2, 0.3) high-entropy alloy is as follows.

Example 1

(1) Prepare raw materials, the alloy smelting raw materials used are Cr, Co, Ni and Si elements, which are accurately weighed and proportioned according to the molar ratio, and are used when smelting and preparing alloys;

(2) Purification of pure metals, using Cr, Co, Ni and Si as raw materials for alloy preparation to purify metal surface oxides;

(3) Using a high-vacuum arc melting furnace under the protection of 99.99% high-purity argon, the raw materials are smelted 5 to 8 times to make alloy button ingots and cast into a copper mold, that is, successfully smelted into a cross-sectional area of 2mm*10mm The new type of high-entropy alloy plate; the current control during smelting is 170-180A, the current during suction casting is controlled at 210-220A, and the arc length is controlled at 25-30mm; see the high-entropy alloy plate for vacuum arc melting furnace and suction casting. figure 1;

(4) Cut a sample of 2mm×10mm×10mm on the alloy plate, and mount the sample into a sample with a size of Φ20×10mm using a metallographic mounting machine, and the surface of the 10mm×10mm sample faces upward;

(5) Use 600, 800, 1000, 1500, 2000 and 3000 mesh metallographic sandpapers to polish the surface of the sample respectively, and then use polishing liquid to finely polish the sample;

(7) The alloy plate is subjected to a homogenization heat treatment at a temperature of 1100 ° C for 5 hours, then the alloy plate is rolled at room temperature to reduce the thickness by 70%, and then the alloy plate is subjected to an annealing treatment at a temperature of 900 ° C for 1 hour;

(8) Cut the annealed alloy plate into tensile samples with a gauge length of 0.6 × 4 × 10 mm, and three samples of each composition have ensured the repeatability of the test performance;

The phase composition of the new high-entropy alloy was analyzed by X-ray diffraction (XRD), the scanning angle was 10°-110°, and the scanning speed was 3°min ^-1 ;

Through the analysis of Jade software, the three new materials are all pure fcc phase structures, as shown in Figure 2.

The EBSD initial microstructure observation of the CrCoNi alloy and CrCoNiSi _0.3 before stretching was carried out. From Figure 3, it can be found that the addition of Si increases the grain size of the alloy, and the new alloy with Si addition is easier to recrystallize, and the rate of crystallization is faster. , higher degree; at the same time, the increase of grain size is beneficial to the occurrence of phase transformation during plastic deformation.

The mechanical properties of the alloy materials prepared above were tested:

Using a precision wire cutting machine, the alloy was cut into tensile samples with a gauge length of 0.6 × 4 × 10 mm (three were prepared for each group of experiments), and then the tensile samples with a gauge length of 0.6 × 4 × 10 mm were stretched by the INSTRON mechanical testing machine. The samples were subjected to quasi-static tensile tests, and the tensile strain rates were all 1×10 ^-3 s ^-1 .

The tensile stress-strain curve diagram is made by Origin software, as shown in Figure 4 (in order to compare the properties of CrCoNi alloy, the tensile curve of quasi-static CrCoNi alloy under the same process is also added). With the increase of Si content, the yield strength of the alloy is continuously improved, and the plasticity is greatly improved. The plastic strain of CrCoNi Si _0.3 reaches 92%, and the engineering fracture strength is also greatly improved to 1Gpa.

It can be clearly seen from Figure 5 that with the increase of Si content, the strong-plastic product of the alloy is greatly improved, and the strong-plastic product of CrCoNi Si _0.3 reaches 78.8 Gpa%. This combination of properties exceeds that of most existing alloys.

Microscopic characterization of the stretched new high-entropy alloy samples:

The stretched new high-entropy alloy samples were observed by JEM-2100F transmission electron microscope. Figures 7a and 7b show the high-resolution and corresponding diffraction spots of TEM respectively. In addition to the same deformation twins as CrCoNi alloys, in the new CrCoNi Si _0.2 high-entropy alloys, HCP atomic sequences are also found at the twin boundaries and between stacking faults , the new high-entropy alloy undergoes a transformation from the fcc phase to the hcp phase during the stretching process, which is a highly localized hcp phase transition, which further enhances the work hardening of the material. The work hardening curve is shown in Figure 6.

Claims (10)

1. a high-strength and high-plasticity high-entropy alloy with martensitic transformation, characterized in that: it is composed of Cr, Co, Ni, Si elements, and the molar ratio of the four elements is 1:1:1:x, x = 0.1, 0.2, 0.3. 2 . The high-strength and high-plasticity high-entropy alloy with martensitic transformation according to claim 1 , wherein the smelting raw material purity of Cr, Co, Ni and Si is all ≥99.9wt.%. 3 . 3 . The high-strength and high-plasticity high-entropy alloy with martensitic transformation according to claim 1 , wherein the high-entropy alloy undergoes transformation from fcc phase to hcp phase during the stretching process. 4 . 4. a preparation method of the high-strength high-plasticity high-entropy alloy with martensitic transformation described in any one of claims 1-3, is characterized in that: after required raw material is purified, carry out high-vacuum arc melting and Copper mold casting; then polishing the obtained high-entropy alloy plate semi-finished product, and then performing heat treatment, rolling and annealing treatment, and finally obtaining the high-entropy alloy finished product. 5. The preparation method of the high-strength, high-plasticity high-entropy alloy with martensitic transformation according to claim 4, characterized in that: during high-vacuum arc smelting, the raw materials are smelted under the protection of 99.99% high-purity argon gas Alloy button ingots are made 5 to 8 times and cast into copper molds to obtain semi-finished high-entropy alloy plates. 6 . The method for preparing a high-strength, high-plasticity high-entropy alloy with martensitic transformation according to claim 5 , wherein the current is controlled at 170-180A during smelting, and 210-220A during suction casting, 7 . The arc length is controlled at 25-30mm. 7. The method for preparing a high-strength, high-plasticity, high-entropy alloy with martensitic transformation according to claim 4, characterized in that: when polishing, use 600, 800, 1000, 1500, 2000 and 3000 mesh metallographic structures respectively The surface of the semi-finished high-entropy alloy plate is polished with sandpaper, and then the semi-finished high-entropy alloy plate is finely polished with polishing liquid. 8. The preparation method of high-strength and high-plastic high-entropy alloy with martensitic transformation according to claim 4, characterized in that: during heat treatment, the semi-finished high-entropy alloy plate is subjected to 5h±2min at 990～1110℃ homogenization heat treatment. 9 . The method for preparing a high-strength and high-plastic high-entropy alloy with martensitic transformation according to claim 4 , wherein the semi-finished high-entropy alloy plate is rolled at room temperature until the thickness is reduced by 69-71%. 10 . 10 . The method for preparing a high-strength and high-plastic high-entropy alloy with martensitic transformation according to claim 4 , wherein the semi-finished high-entropy alloy plate is annealed at 895-905° C. for 1h±2min. 11 . .

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