Heat treatment method for improving strength of refractory high-entropy alloy

Abstract

The invention provides a heat treatment method for improving the strength of a refractory high-entropy alloy, which comprises the following steps: step 1, weighing refractory metal blocks, mixing, and carrying out vacuum melting to obtain a refractory high-entropy alloy casting; 2, the obtained refractory high-entropy alloy casting is cut into columnar samples; 3, placing the columnar sample in the center of a graphite mold, and placing the graphite mold in a spark plasma sintering chamber; 4, the spark plasma sintering chamber is vacuumized till the vacuum degree is 1 * 10 <-2 > Pa or below, then the temperature is increased, the temperature reaches the set temperature, and heat preservation is conducted for 30 min at the temperature and the set pressure; and 5, after heat preservation is finished, the alloy is cooled to 100 DEG C or below along with the furnace, and demolding treatment is conducted.

Description

Technical Field

[0001] The invention relates to an alloy heat treatment method, in particular to a heat treatment method for improving the strength of a refractory high entropy alloy. Background Art

[0002] Refractory high entropy alloy is a new alloy concept, mainly composed of high melting point refractory metal elements Ta, Zr, Nb, Ti, Mo, Hf, W, Cr and V. Due to the high melting point of refractory high entropy alloy, the performance control of refractory high entropy alloy is a difficult problem in the research community, and it is also the fundamental reason that limits the large-scale application of refractory high entropy alloy. Cast refractory high entropy alloy usually shows obvious weaknesses, and microstructural defects limit the mechanical properties of the alloy, and subsequent heat treatment process is necessary. Refractory high entropy alloy usually uses vacuum arc melting and vacuum induction melting technology to prepare bulk samples. During the melting process, due to thermal expansion and contraction, defects such as defects, pores, segregation and chemical inhomogeneity are inevitably generated. These defects essentially limit the mechanical properties, corrosion resistance, wear resistance and other properties of refractory high entropy alloy, hindering the industrial production and application of refractory high entropy alloy. Spark plasma sintering technology (SPS) is a new advanced powder sintering technology, which can achieve material densification by activating powder particles by pulse current, rapidly heating and applying pressure, and greatly improve sintering efficiency. At present, the SPS method has been widely used to synthesize ceramic materials, high entropy alloys and their related composite materials, and to prepare ceramic and metal materials with excellent performance. Summary of the invention

[0003] The object of the present invention is to provide a heat treatment method for improving the mechanical properties of a refractory high entropy alloy, comprising the following steps: Step 1, weighing refractory metal blocks, mixing them and preparing refractory high entropy alloy castings by vacuum melting; Step 2, cutting the obtained refractory high entropy alloy casting into columnar samples; Step 3, placing the columnar sample in the center of the graphite mold and placing it in the spark plasma sintering chamber; Step 4: Evacuate the spark plasma sintering chamber to a vacuum degree of 1×10 -2 After the temperature drops below 0.05 Pa, the temperature is raised to the set temperature and kept at this temperature and set pressure for 30 minutes. Step 5: After the heat preservation is completed, the alloy is cooled to below 100°C in the furnace and demolding is performed.

[0004] Furthermore, the refractory metal is tantalum, niobium, zirconium, titanium, and alloy materials with a melting point higher than 1500°C.

[0005] Furthermore, the purity of the refractory metal block is ≥99.5% (wt%), and the amount of substance of each refractory high entropy alloy casting component is the same.

[0006] Furthermore, in step 4, the SPS heating rate is 80-100°C / min, and after heating to 1200-1400°C, the temperature is kept for 30-40 minutes, and the axial pressure is 30-45MPa.

[0007] Compared with the prior art, the present invention has the following advantages: (1) The method adopted by the present invention effectively solves the problem of insufficient mechanical properties of TaNbZrTi refractory high entropy alloys obtained by traditional smelting methods. The TaNbZrTi refractory high entropy alloy obtained by the treatment of the present invention has improved strength compared with the cast state, can be made into high-strength components, and has a wide range of applications; (2) The method described in the present invention improves the mechanical properties of TaNbZrTi refractory high entropy alloys through SPS, which provides a possibility for the performance enhancement and design of refractory high entropy alloys in the future.

[0008] The present invention will be further described below in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 XRD analysis diagrams of the cast TaNbZrTi refractory high entropy alloy, Example 1, and the TaNbZrTi refractory high entropy alloy obtained in Example 2.

[0010] Figure 2 These are the quasi-static tensile and compressive true stress-strain curves of the cast TaNbZrTi refractory high entropy alloy, where (a) is the quasi-static tensile true stress-strain curve, and (b) is the quasi-static compressive true stress-strain curve.

[0011] Figure 3 The quasi-static tensile and compressive true stress-strain curves of the TaNbZrTi refractory high entropy alloy obtained in Example 1, wherein (a) is a quasi-static tensile true stress-strain curve, and (b) is a quasi-static compressive true stress-strain curve.

[0012] Figure 4 The quasi-static tensile and compressive true stress-strain curves of the TaNbZrTi refractory high entropy alloy obtained in Example 2, wherein (a) is a quasi-static tensile true stress-strain curve, and (b) is a quasi-static compressive true stress-strain curve. DETAILED DESCRIPTION

[0013] A heat treatment method for improving the strength of a refractory high entropy alloy, characterized by comprising: Step 1, weighing refractory metal blocks, mixing them and preparing refractory high entropy alloy castings by vacuum melting; Step 2, cutting the obtained refractory high entropy alloy casting into columnar samples; Step 3, placing the columnar sample in the center of the graphite mold and placing it in the spark plasma sintering chamber; Step 4: Evacuate the spark plasma sintering chamber to a vacuum degree of 1×10 -2 After the temperature drops below 0.05 Pa, the temperature is raised to the set temperature and kept at this temperature and set pressure for 30 minutes. Step 5: After the heat preservation is completed, the alloy is cooled to below 100°C in the furnace and demolding is performed.

[0014] The heat treatment principle of SPS to improve the strength of TaNbZrTi refractory high entropy alloy is that: due to the formation of pores and microcracks inside the refractory high entropy alloy during the early smelting process, under the high temperature and pressure of SPS, the pores and microcracks are closed, and the structure is more uniform; SPS has fast heating and cooling rates, which can prevent the growth of grains of refractory high entropy alloys under high temperature conditions. Due to the complex element composition of refractory high entropy alloys and their own high entropy values, the alloys produce dislocations due to supercooling during the rapid heating and cooling process, and then form a grain refinement effect, so the tensile strength and compressive strength will be improved. The improvement of the strength of refractory high entropy alloys is conducive to reducing the defective rate in subsequent processing such as blanking and forging.

[0015] Example 1: A heat treatment method for improving the strength of a refractory high entropy alloy based on SPS, the process steps are as follows: Step 1, weighing tantalum (Ta) blocks, niobium (Nb) blocks, zirconium (Zr) blocks and titanium (Ti) blocks with purity higher than 99.5% (wt%) in equal molar ratios, fully mixing, and preparing TaNbZrTi refractory high entropy alloy by vacuum melting, and repeating reverse melting for more than 6 times to obtain a cast TaNbZrTi refractory high entropy alloy; Step 2, cutting the cast NbZrTiTa refractory high entropy alloy into alloy samples of Φ28×30 mm; Step 3, place the alloy sample in the center of a graphite mold with an inner diameter of Φ30, pad the upper and lower surfaces with two pieces of 1mm thick graphite paper, and put it into the SPS chamber; Step 4: Evacuate the SPS chamber to a vacuum degree of 1×10 -2 Pa, the temperature is raised at 100℃ / min to 1200℃, and kept at this temperature for 30min, with an axial pressure of 45MPa; Step 5, after the heat preservation is completed, the sample is cooled to below 100° C. in the furnace, and the treated sample of this example is obtained after demoulding.

[0016] The treated samples were ground to remove the oxide layer, and the microstructure and mechanical properties were analyzed. The alloy samples obtained in this example were tested, and the test results were as follows: (1) The alloy sample obtained in this example has a single-phase BCC structure; Figure 1 ,From the XRD analysis, it can be seen that the grain size of the sample obtained in Example 1 is reduced and the grains are refined; (2) The sample obtained in this embodiment was tested using a universal material testing machine. Its tensile strength was 919 MPa and its compressive strength was 1584 MPa. The test results were plotted as a true stress-strain curve, see Figure 3 It can be seen that the tensile strength of the alloy sample obtained in Example 1 is increased to twice that before the heat treatment, and the compressive strength is significantly higher than that before the heat treatment.

[0017] Embodiment 2: This embodiment is the same as steps 1, 2, and 3 in Embodiment 1, except that: Step 4: Evacuate the SPS chamber to a vacuum degree of 1×10 -2 Pa, the temperature is raised at 100℃ / min to 1400℃, and kept at this temperature for 30min, with an axial pressure of 45MPa; Step 5, after the heat preservation is completed, the sample is cooled to below 100° C. in the furnace, and the treated sample of this example is obtained after demoulding.

[0018] The treated samples were ground to remove the oxide layer, and the microstructure and mechanical properties were analyzed. The alloy samples obtained in this example were tested, and the test results were as follows: (1) The alloy sample obtained in this example has a single-phase BCC structure; Figure 1 ,From XRD analysis, it can be seen that the grain size of the sample obtained in Example 2 is reduced and the grains are refined; (2) The sample obtained in this embodiment was tested using a universal material testing machine. Its tensile strength was 815 MPa and its compressive strength was 1490 MPa. The test results were plotted as a true stress-strain curve, see Figure 4 It can be seen that the tensile strength of the alloy sample obtained in Example 2 is increased to 1.8 times that before the heat treatment, and the compressive strength is significantly higher than that before the heat treatment.

[0019] Comparative Example: The cast TaNbZrTi refractory high entropy alloy obtained in step 1 of Example 1 or 2 was tested, and the test results were as follows: (1) The alloy sample obtained in this example has a single-phase BCC structure. The XRD analysis results are shown in Figure 1 ; (2) The sample obtained in this embodiment was tested using a universal material testing machine. Its tensile strength was 442 MPa and its compressive strength was 1322 MPa. The test results were plotted as a true stress-strain curve, see Figure 2 .

[0020] Table 1 is a strength comparison of the cast TaNbZrTi refractory high entropy alloy, and the TaNbZrTi refractory high entropy alloys obtained in Example 1 and Example 2. The results show that the tensile strength of the alloy obtained by this method reaches up to 949 MPa, which is 2.1 times the tensile strength (442 MPa) of the comparative cast TaNbZrTi refractory high entropy alloy; the compressive strength reaches up to 1613 MPa, which is 1.2 times the compressive strength (1322 MPa) of the comparative cast TaNbZrTi refractory high entropy alloy.

[0021] Table 1 Comparison of tensile strength and compressive strength of TaNbZrTi refractory high entropy alloy of the present invention sample Tensile strength(MPa) Compressive strength(MPa) Cast 442 1322 Example 1 919 1613 Example 2 949 1435 In summary, the method provided by the present invention is simple and convenient to operate, has a short cycle, is suitable for mass production, and the strength of the obtained finished product is significantly improved compared with the cast TaNbZrTi refractory high entropy alloy.

Claims

1. A heat treatment method for improving the strength of a refractory high entropy alloy, characterized in that: include: Step 1, weighing refractory metal blocks, mixing them and preparing refractory high entropy alloy castings by vacuum melting; Step 2, cutting the obtained refractory high entropy alloy casting into columnar samples; Step 3, placing the columnar sample in the center of the graphite mold and placing it in the spark plasma sintering chamber; Step 4: Evacuate the spark plasma sintering chamber to a vacuum degree of 1×10 -2 After the temperature drops below 0.05 Pa, the temperature is raised to the set temperature and kept at this temperature and set pressure for 30 minutes. Step 5: After the heat preservation is completed, the alloy is cooled to below 100°C in the furnace and demolding is performed.

2. The method according to claim 1, characterized in that Refractory metals include tantalum, niobium, zirconium, titanium, and alloy materials with a melting point above 1500°C.

3. The method according to claim 2, characterized in that The purity of the refractory metal block is ≥99.5% (wt%), and the amount of substances in the components of each refractory high entropy alloy casting is the same.

4. The method according to claim 2, characterized in that: In step 4, the SPS temperature rise rate is 80-100°C / min, and after heating to 1200-1400°C, the temperature is kept for 30-40 minutes, and the axial pressure is 30-45MPa.

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