A nickel-based single-crystal superalloy and its preparation method

Abstract

This invention discloses a low-cost second-generation nickel-based single-crystal superalloy and its preparation method. The alloy composition, by weight percentage, includes: C 0.08–0.15%, Cr 4.3–4.7%, Co 8–10%, W 10.9–11.5%, Mo 0.8–1.1%, Nb 1.2–1.4%, Al 5.75–6.05%, Ti 0.9–1.2%, Hf 0.01–0.1%, B 0.001–0.015%, Y 0.0005–0.025%, Sn 0.0001–0.01%, Re 1.8–2.3%, with Ni as the balance. This alloy has low preparation cost, high creep strength, fatigue resistance, and good microstructural stability, as well as good single-crystal casting performance.

Description

Technical Field

[0001] This invention belongs to the field of nickel-based single-crystal superalloys and their preparation technology, specifically relating to a low-cost nickel-based second-generation single-crystal superalloy and its preparation method. This alloy is suitable for manufacturing turbine rotor blades and guide vanes for aero-engines and other high-temperature components. Background Technology

[0002] Nickel-based superalloys hold a particularly important position in the field of high-temperature alloys. Compared with iron-based and cobalt-based superalloys, nickel-based superalloys possess higher high-temperature strength and structural stability, and are widely used in the manufacture of hot-end components for aero-jet engines and industrial gas turbines. Since transverse grain boundaries perpendicular to the direction of stress are weak points in materials at high temperatures, eliminating transverse grain boundaries can significantly improve the overall performance of the alloy. Therefore, cast superalloys have evolved from equiaxed grains to oriented columnar grains, and to further improve performance, grain boundary-free single-crystal superalloys have been developed.

[0003] Single-crystal superalloys are a development trend for turbine blades in various aero-engines and gas turbines for the next few decades. Since the 1980s, countries have been vying to develop their own single-crystal superalloys to prepare materials for the development of advanced engines with high thrust-to-weight ratios. To date, five generations of single-crystal superalloys have been developed. The second generation of single-crystal superalloys added 3% rhenium (Re) to the first generation. The third generation was marked by the addition of 6% Re. The fourth generation added another 3% Ru to the third generation. The fifth generation increased the Re content to approximately 9%. The addition of Re can significantly improve the high-temperature creep performance of single-crystal superalloys. However, Re is a scarce element with high cost, resulting in high costs for high-generation single-crystal superalloys. Furthermore, Re has a high density, significantly increasing the density of high-generation single-crystal superalloys and adding to the engine's weight. To maintain the excellent high-temperature performance of high-generation single-crystal superalloys while reducing material costs and density, countries are vigorously developing low-Re high-generation single-crystal superalloys.

[0004] Liquid metal cooling (LMC) directional solidification technology has a higher temperature gradient and cooling rate compared to traditional HRS (High Rate Solidification) directional solidification technology. It can significantly refine the alloy microstructure, reduce segregation, and develop single-crystal high-temperature alloys with higher alloying degree. Summary of the Invention

[0005] The purpose of this invention is to provide a low-cost nickel-based second-generation single-crystal superalloy for LMC directional solidification technology and its preparation method. This alloy has a low preparation cost and excellent creep strength, fatigue resistance and good microstructure stability, as well as good single-crystal casting performance, welding performance and coating performance.

[0006] The technical solution of the present invention is as follows:

[0007] A low-cost nickel-based single-crystal superalloy (DD26R), the alloy composition by weight percentage includes:

[0008] C 0.08–0.15%, Cr 4.3–4.7%, Co 8–10%, W 10.9–11.5%, Mo 0.8–1.1%, Nb 1.2–1.4%, Al 5.75–6.05%, Ti 0.9–1.2%, Hf 0.01–0.1%, B 0.001–0.015%, Y 0.0005–0.025%, Sn 0.0001–0.01%, Re 1.8–2.3%, Ni balance.

[0009] The preferred alloy composition of this invention is (by weight percentage):

[0010] C 0.12%, Cr 4.5%, Co 9%, W 11.2%, Mo 0.9%, Nb 1.3%, Al 6.0%, Ti 1.0%, Hf 0.05%, B 0.006%, Y 0.0015%, Sn 0.005%, Re 2%, Ni balance.

[0011] The alloy of this invention can be smelted into a master alloy using pure metallic elements in a vacuum induction melting furnace. The master alloy is then remelted using an LMC directional solidification device with a relatively fast drawing rate, ranging from 6 to 15 mm / min, and directionally solidified into a single-crystal casting using a spiral crystal selection method or a seed crystal method. To further improve its overall performance, the alloy of this invention can be used after heat treatment.

[0012] The design principle of the alloy composition of this invention is as follows:

[0013] The invented alloy primarily relies on the refractory elements Re, W, Mo, and Nb to improve the high-temperature strength of the material. However, Re, W, and Mo are also elements that form the harmful TCP phase. Considering all factors, the composition range for Re is determined to be 1.8–2.3%, for W 10.9–11.5%, and for Mo 0.9–1.1%. It is well known that the design philosophy of traditional single-crystal superalloys is to completely eliminate grain boundary strengthening elements (C and B) in first-generation single-crystal superalloys, and to add trace amounts of C and B (generally not exceeding 0.05%) in second- to fourth-generation single-crystal superalloys. This alloy, however, improves its high-temperature strength by adding an appropriate amount of C. Simultaneously, C can effectively purify the alloy (deoxidize) during the alloy melting process, which is beneficial for corrosion resistance and improves the casting processability of the single-crystal alloy. The addition of C can also reduce the tendency for TCP phase formation. Research has shown that when the C content in this alloy exceeds 0.15%, the alloy's creep rupture performance decreases. A C content of 0.08–0.15% yields good overall performance in the alloy. Boron (B) strengthens the unavoidable small-angle grain boundaries in single-crystal alloys; its content is determined to be 0.001–0.015%. Trace amounts of hydrogen sulfide (Hf) are also added to this alloy, significantly improving the compatibility and adhesion between the coating and the substrate, thus extending coating life. It also benefits the processing and mechanical properties of the single-crystal alloy; its content is determined to be 0.01–0.1%. Nitrogen monoxide (Nb) primarily enters the main strengthening phase (γ′ phase) of nickel-based superalloys, improving the stability of the γ′ phase and strengthening the solid solution. Furthermore, it improves the alloy's weldability. Its content is determined to be 1.2–1.4%. Rare earth elements reduce the content of harmful elements such as oxygen (O), sulfur (S), and nitrogen (N) during alloy smelting, purifying the alloy. Simultaneously, rare earth elements improve the alloy's microstructure stability and creep retardation performance after long-term aging. Furthermore, rare earth elements can reduce the viscosity of molten alloys, thereby improving the casting process performance of the alloy. However, rare earth elements volatilize significantly during the smelting process; therefore, the rare earth addition amount of this alloy is specified as 0.0005–0.025% for yttrium, and the rare earth content of single-crystal alloys does not require analysis. Sn is considered a harmful element in traditional high-temperature alloys and needs to be controlled. However, this invention has found that Sn exhibits a small amount of segregation around the eutectic during solidification, which disappears after heat treatment. An appropriate amount of Sn has no adverse effect on the creep rupture and tensile properties of the single-crystal high-temperature alloy of this invention and can improve the low-cycle fatigue performance of the material.

[0014] This alloy has low preparation cost, high creep strength, fatigue resistance and good microstructure stability, and good single crystal casting performance.

[0015] The beneficial technical effects of this invention are as follows:

[0016] 1. The single-crystal alloy developed in this invention contains only about 2% Re element. At the same time, the alloy improves its high-temperature strength by adding appropriate amounts of C and B elements. The high-temperature strength of the alloy reaches the level of a typical second-generation (3% Re) single-crystal high-temperature alloy, which significantly reduces the cost of the alloy.

[0017] 2. The alloy of this invention has good structural stability and single-crystal casting process performance, and its performance is comparable to that of the second-generation single-crystal high-temperature alloys currently available at home and abroad. Attached Figure Description

[0018] Figure 1 The microstructure of DD26R single crystal alloy is shown in (a) as-cast state and (b) heat-treated state.

[0019] Figure 2 EDS surface scan results for the precipitated phase;

[0020] Figure 3 The results are TEM analysis of the precipitated phase, (a) is the transmission analysis result, and (b) is the selected area electron diffraction analysis result.

[0021] Figure 4 Comparison of the durability of DD26R with other typical second-generation single-crystal alloys. Detailed Implementation

[0022] The present invention will be further described below with reference to embodiments and accompanying drawings.

[0023] Example 1

[0024] The composition of the alloy (No.1 alloy) of this invention is shown in Table 1. The preparation process of the alloy is as follows: pure metal elements are melted in a vacuum induction melting furnace to form a master alloy (melting temperature 1500-1600℃); the master alloy is used to prepare single crystal test rods (diameter 16mm, length 200mm) by liquid metal cooling (LMC) directional solidification process (low melting point liquid metal Sn as cooling medium), the holding furnace temperature is 1500℃, the casting temperature is 1550℃, and the pulling speed is 8mm / min.

[0025] The microstructures of the single crystal test rod in the as-cast state and the heat-treated state (solution treatment 1280℃ / 2h, aging treatment 1120℃ / 2h) are as follows: Figure 1As shown in (a) and (b), both alloy states exhibit dendritic morphology with a primary dendrite spacing of approximately 0.22 mm, significantly lower than that of single-crystal superalloys prepared by conventional directional solidification (HRS) process (holding furnace 1550℃, pulling rate 3 mm / min) (>0.3 mm). Heat treatment reduced elemental segregation, resulting in more uniform dendrite axes and dendrite spacing. The alloy represents the upper limit composition, and the primary focus is on the alloy's microstructural stability. The heat-treated DD26R alloy, after long-term aging at 900℃ for 3000 hours, showed no TCP phase. Rod-like phases in the microstructure, analyzed by EDS and TEM, were identified as M6C type carbides. Figure 2 and Figure 3 As shown, this alloy exhibits good structural stability and is suitable for long-term use.

[0026] Table 1. Composition of Single Crystal Alloy (wt%)

[0027]

[0028] Example 2

[0029] The composition of the alloy of this invention (Alloy No. 2) is shown in Table 2. The master alloy was prepared using a liquid metal cooling (LMC) directional solidification process (using low-melting-point liquid metal Sn as the cooling medium) to produce single-crystal test rods (16 mm in diameter, 200 mm in length). The holding furnace temperature was 1500℃, the casting temperature was 1550℃, and the pulling rate was 10 mm / min. The density of this alloy is 8.65 g / cm³. 3 The thermal conductivity, specific heat capacity, and thermal diffusivity of the alloy are shown in Table 3. The testing standard used is GB / T 22588-2008, which specifies the thermal diffusivity or thermal conductivity by the flash method.

[0030] Table 2. Composition of Single Crystal Alloys (wt%)

[0031]

[0032] Table 3 Thermal conductivity, specific heat capacity, and thermal diffusivity of single-crystal alloys

[0033]

[0034] The results show that the thermal conductivity, specific heat capacity and thermal diffusivity of the alloy increase with increasing temperature, and are comparable to other nickel-based high-temperature alloys.

[0035] Example 3

[0036] The composition of the alloy (Alloy No. 3) of this invention is shown in Table 4. The master alloy was prepared using a liquid metal cooling (LMC) directional solidification process (using low-melting-point liquid metal Sn as the cooling medium) to produce single-crystal test rods (16 mm in diameter, 200 mm in length). The holding furnace temperature was 1500℃, the casting temperature was 1550℃, and the pulling rate was 10 mm / min. The creep rupture properties (test standard: HB5150 High Temperature Tensile Creep Test Method) of the heat-treated alloy (solution treatment 1280℃ / 2h, aging treatment 1120℃ / 2h) are shown in Table 5 and... Figure 4 The addition of a small amount of Re significantly improves the durability of the material, and its performance is comparable to that of other typical second-generation single-crystal superalloys.

[0037] Table 4. Composition of Single Crystal Alloys (wt%)

[0038]

[0039] Table 5. Durability of Single Crystal Alloys

[0040]

[0041]

[0042] Example 4

[0043] The composition of the alloy of this invention (Alloy No. 4) is shown in Table 6. Single crystal specimens (16 mm in diameter, 200 mm in length) were prepared from the master alloy using a liquid metal cooling (LMC) directional solidification process (using low-melting-point liquid metal Sn as the cooling medium). The holding furnace temperature was 1500℃, the casting temperature was 1550℃, and the pulling rate was 10 mm / min. The creep rupture properties (testing standard: HB5150 high-temperature tensile creep rupture test method) of the heat-treated alloy (solution treatment 1280℃ / 2h, aging treatment 1120℃ / 2h) are shown in Table 7, and the tensile properties (testing standard: HB5143 room temperature tensile test method, HB5150 high-temperature tensile creep rupture test method) are shown in Table 8. Comparison of the data with Example 3 shows that the creep rupture properties are comparable, indicating that when the main strengthening element Re is at its lower limit, the performance does not significantly decrease.

[0044] Table 6. Composition of Single Crystal Alloys (wt%)

[0045]

[0046] Table 7 Durability

[0047] Temperature (°C) Persistent stress (MPa) Lifespan (h) 700 760 521 700 760 399 850 540 236 850 540 266 850 560 210 850 560 195 850 550 202 900 480 80 975 255 113 1000 225 90 1050 220 22

[0048] Table 8 Tensile Properties

[0049]

[0050] Example 5

[0051] The composition of the alloy (Alloy No. 5) of this invention is shown in Table 9. Single crystal test rods (16 mm in diameter, 200 mm in length) were prepared from the master alloy using a liquid metal cooling (LMC) directional solidification process (using low-melting-point liquid metal Sn as the cooling medium). The holding furnace temperature was 1500℃, the casting temperature was 1550℃, and the pulling rate was 8 mm / min. The high-temperature rotational bending fatigue (GB / T 4337 Rotational Bending Method for Fatigue Testing of Metallic Materials) results of the heat-treated alloy (solution treatment 1280℃ / 2h, aging treatment 1120℃ / 2h) are shown in Table 10. The results show that the rotational bending fatigue strength of this alloy is 380 MPa at 800℃, 340 MPa at 900℃, and 340 MPa at 1000℃.

[0052] Table 9. Composition of Single Crystal Alloys (wt%)

[0053]

[0054] Table 10 Results of High-Temperature Rotational Bending Fatigue Test

[0055]

[0056] Example 6

[0057] The composition of the alloy (Alloy No. 6) of this invention is shown in Table 11. Single crystal specimens (16 mm in diameter, 200 mm in length) were prepared from the master alloy using a liquid metal cooling (LMC) directional solidification process (using low-melting-point liquid metal Sn as the cooling medium). The holding furnace temperature was 1500℃, the casting temperature was 1550℃, and the pulling rate was 8 mm / min. The creep rupture properties of the heat-treated alloy (solution treatment 1280℃ / 2h, aging treatment 1120℃ / 2h) (test standard: HB5150 High Temperature Tensile Creep Test Method) are shown in Table 12. The results show that when the Re element content is below the lower limit of the alloy composition, the creep rupture properties of the alloy decrease significantly, especially with increasing temperature, indicating that Re is an important element for strengthening the high-temperature creep rupture properties of the alloy and needs to be strictly controlled.

[0058] Table 11 Single Crystal Alloy Composition (wt%)

[0059]

[0060] Table 12 Durability

[0061] Temperature (°C) Persistent stress (MPa) Lifespan (h) 700 760 502 700 760 369 850 540 136 850 540 166 850 560 90 850 560 85 900 480 40 975 255 82 1000 225 70 1050 220 12

[0062] Example 7

[0063] The composition of the alloy of this invention (Alloy No. 7) is shown in Table 13. Single crystal test rods (16 mm in diameter, 200 mm in length) were prepared from the master alloy using a liquid metal cooling (LMC) directional solidification process (using low-melting-point liquid metal Sn as the cooling medium). The holding furnace temperature was 1500℃, the casting temperature was 1550℃, and the pulling rate was 8 mm / min. The high-temperature rotational bending fatigue (GB / T 4337 Rotational Bending Method for Fatigue Testing of Metallic Materials) results of the heat-treated alloy (solution treatment 1280℃ / 2h, aging treatment 1120℃ / 2h) are shown in Table 14. The results show that when the alloy does not contain Sn, the rotational bending fatigue strength at 800℃ is 360 MPa, and the rotational bending fatigue strength at 900℃ is 320 MPa, which is slightly lower than that of the alloy of this invention.

[0064] Table 13 Single Crystal Alloy Composition (wt%)

[0065]

[0066] Table 14 Results of High-Temperature Rotary Bending Fatigue Test

[0067]

[0068]

Claims

1. A nickel-based single-crystal superalloy, comprising, by weight percentage: C 0.08–0.15%, Cr 4.3–4.7%, Co 8–10%, W 10.9–11.5%, Mo 0.8–1.1%, Nb 1.2–1.4%, Al 5.75–6.05%, Ti 0.9–1.2%, Hf 0.01–0.1%, B 0.001–0.015%, Y 0.0005–0.025%, Sn 0.0001–0.01%, Re 1.8–2.3%, Ni balance.

2. The nickel-based single-crystal superalloy according to claim 1, characterized in that: The preferred alloy composition is (by weight percentage): C 0.09–0.12%, Cr 4.3–4.5%, Co 8–9%, W 11.0–11.3%, Mo 0.9–1.0%, Nb 1.2–1.3%, Al 5.8–6.0%, Ti 0.9–1.0%, Hf 0.03–0.06%, B 0.005–0.008%, Y 0.001–0.0015%, Sn 0.0005–0.006%, Re 1.9–2.1%, Ni balance.

3. A method for preparing a nickel-based single-crystal superalloy as described in claim 1 or 2, characterized in that: The alloy is remelted using a liquid metal cooling (Sn cold) directional solidification device with a relatively fast drawing rate, ranging from 6 to 15 mm / min.

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