A method for improving the microstructure stability of single crystal superalloys by controlling the N content

Abstract

This invention relates to the field of high-temperature alloy technology, specifically to a method for improving the microstructure stability of single-crystal high-temperature alloys by controlling the nitrogen (N) content. The content of refractory elements in single-crystal high-temperature alloys increases with increasing heat resistance, leading to decreased microstructure stability and increased precipitation of topologically close-packed (TCP) phases, resulting in decreased mechanical properties. Based on the influence of N on the TCP phase precipitation behavior in high-temperature alloys, this invention proposes a method to effectively improve the microstructure stability of single-crystal high-temperature alloys by controlling the N content. By adding a certain amount of CrN or AlN during directional solidification to ensure an N mass content of 15-30 ppm in the single-crystal alloy, the amount of TCP phase precipitates in the alloy microstructure is significantly reduced after long-term high-temperature heat exposure, and the microstructure stability of the single-crystal alloy is significantly improved. This invention employs a simple and feasible method to effectively improve the microstructure stability of single-crystal high-temperature alloys.

Description

Technical Field

[0001] This invention relates to the field of high-temperature alloy composition design, specifically a method for improving the microstructure stability of single-crystal high-temperature alloys by controlling the nitrogen content. Background Technology

[0002] As turbine inlet temperatures increase, turbine blade service temperatures also rise. Internationally, turbine blades are generally made from high-generation single-crystal superalloys with high Re content (a rare and precious element). However, the extensive use of rare and precious metals in alloying inevitably leads to high blade costs, significantly increasing the development costs of aero-engines. Due to the high content of refractory elements in high-generation single-crystal superalloys, their microstructure stability decreases during service, making them prone to the precipitation of topologically close-packed (TCP) phases. This results in a significant decline in the alloy's mechanical properties, affecting engine service life. To suppress TCP phase precipitation and improve microstructure stability in single-crystal superalloys, rare and precious metals such as Ru are typically added. Furthermore, with the development of high-generation single-crystal alloys, the Cr content in the alloys has gradually decreased, improving microstructure stability to some extent, but simultaneously reducing the alloy's resistance to oxidation and corrosion. Therefore, there is an urgent need to develop a low-cost method to improve the microstructure stability of single-crystal superalloys.

[0003] Patent CN118389908A discloses a low-cost single-crystal superalloy and its preparation method that balances microstructural stability and performance. It indicates that controlling the sulfur (S) content below 1 ppm can effectively improve the alloy's microstructural stability, but only considers the effect of S on microstructural stability, without addressing the influence of other impurity elements. Patent CN117265329A discloses an in-situ nitride-reinforced additive manufacturing superalloy and its preparation method. It strengthens the superalloy by in-situ nitride generation under a nitrogen-containing atmosphere, but does not mention the effect of nitrogen (N) on the microstructural stability of the superalloy. Patent CN117286359A discloses a nitride-reinforced superalloy and its preparation method. It solves the problem of uneven distribution of nitrides generated in-situ by traditional gaseous nitrogen sources and alloy elements through uniform in-situ self-generation of nitrides in the alloy, but does not address the problem of poor microstructural stability in alloys with high refractory element content.

[0004] Currently, nitrogen (N) in high-temperature alloys is considered an impurity element, and its content should be as low as possible. However, published literature indicates that adding a small amount of N to high-temperature alloys can reduce the amount of porosity and improve the mechanical properties of the alloy. However, there is no research on the effect of N on the microstructure stability of high-temperature alloys. This invention, based on the mechanism of N's effect on the nucleation and growth process of the TCP phase in high-temperature alloys, proposes a method to improve the microstructure stability of single-crystal high-temperature alloys by adding a small amount of N. Summary of the Invention

[0005] The purpose of this invention is to provide a method for controlling the nitrogen content in high-temperature alloys to improve the microstructure stability of single-crystal high-temperature alloys without increasing the cost of the alloy or affecting other properties of the alloy, thereby reducing the tendency of TCP phase precipitation during long-term high-temperature service and ensuring the safety of blades during long-term service.

[0006] To achieve the above objectives, the present invention mainly provides the following technical solutions: A method for improving the microstructure stability of single-crystal superalloys by controlling the N content involves melting and casting the single-crystal superalloy together with CrN and / or AlN to prepare a single-crystal casting with the desired N content, subjecting the part to standard heat treatment, and finally obtaining an alloy with an N mass content of 15~30ppm.

[0007] The single-crystal high-temperature alloy and the final alloy obtained are single-crystal high-temperature alloys that meet the same grade composition requirements; The single-crystal superalloy used is a nickel-based single-crystal superalloy with a nitrogen content of 4-6 ppm.

[0008] The process conditions for the standard heat treatment are determined according to the requirements of the alloy grade.

[0009] Includes the following steps: 1) According to the composition requirements of the required single crystal high-temperature alloy grade, weigh the master alloy of the required single crystal high-temperature alloy grade, as well as CrN powder and / or AlN powder. 2) Place the master alloy, CrN powder and / or AlN powder into the crucible of a directional solidification furnace, and melt-cast to prepare single crystal castings with different N contents. Perform standard heat treatment on the parts (determined according to the alloy grade).

[0010] The master alloy used in step 1) is a nickel-based single-crystal superalloy with a nitrogen content of 4-6 ppm. Based on the N content in the master alloy, CrN powder and / or AlN powder are mixed in proportion to finally obtain an alloy with an N mass content of 15~30ppm.

[0011] High-temperature long-term heat exposure tests were conducted on the obtained parts with different N contents to observe the precipitation of TCP phase in the parts with different N contents and to analyze the precipitation tendency of TCP phase in the microstructure of the parts with different N contents. As the N content increased, the precipitation tendency of TCP phase in the microstructure decreased significantly, and the microstructure stability was significantly improved.

[0012] During the heat exposure process, the temperature was 900℃~1100℃, which is close to the service temperature of the parts, and the time was 100h~1000h. After the heat exposure, scanning electron microscopy was performed to observe and analyze the area percentage of TCP phase in alloys with different N contents to determine the microstructure stability of alloys with different N contents.

[0013] In single-crystal superalloys, the content of refractory elements increases with increasing heat resistance, leading to decreased alloy microstructure stability and increased precipitation of topologically close-packed (TCP) phases, resulting in decreased mechanical properties. This invention, based on the influence of nitrogen (N) on TCP phase precipitation behavior in superalloys, proposes a method to effectively improve the microstructure stability of single-crystal superalloys by controlling the N content. By adding a certain amount of CrN or AlN during directional solidification to ensure an N mass content of 15-30 ppm in the single-crystal alloy, the amount of TCP phase precipitates in the alloy microstructure is significantly reduced after long-term high-temperature heat exposure, resulting in a significant improvement in the microstructure stability of the single-crystal alloy. This invention employs a simple and feasible method to effectively improve the microstructure stability of single-crystal superalloys.

[0014] Compared with the prior art, the method of improving the microstructure stability of high-temperature alloys by controlling the nitrogen content of the present invention has at least the following beneficial effects: 1. This invention aims to improve the microstructure stability of high-temperature alloys by adding CrN and AlN during the alloy smelting process, resulting in a nitrogen content of 15-30 ppm in the high-temperature alloy. After heat treatment, the alloy is subjected to prolonged heat exposure at high temperatures, and the microstructure stability of the alloy with higher nitrogen content is significantly improved, with a smaller amount of TCP phase precipitation.

[0015] 2. The design method proposed in this invention adds CrN and AlN to make the N mass content in the alloy 15~30ppm. After the alloy is exposed to heat at 1000℃ for 1000h, the number of TCP phase in the microstructure is significantly reduced compared with the alloy with an N mass content of about 4ppm, which can significantly improve the long service life of the alloy.

[0016] The above description is merely an overview of the technical solution of the present invention. In order to better understand the technical means of the present invention and to implement it in accordance with the contents of the specification, the preferred embodiments of the present invention are described in detail below with reference to the accompanying drawings. Attached Figure Description

[0017] Figure 1 The figures show the microstructures of second-generation single-crystal high-temperature alloys with different N contents after 100h heat exposure at 1000℃ in the examples. Figure (a) shows the microstructure of the alloy with 4ppm N content after 1000℃ / 100h heat exposure, Figure (b) shows the microstructure of the alloy with 15ppm N content after 1000℃ / 100h heat exposure, and Figure (c) shows the microstructure of the alloy with 27ppm N content after 1000℃ / 100h heat exposure.

[0018] Figure 2The figures show the microstructures of second-generation single-crystal superalloys with different N contents after 1000h heat exposure at 1000℃ in the examples. Figure (a) shows the microstructure of the alloy with 4ppm N content after 1000℃ / 1000h heat exposure, Figure (b) shows the microstructure of the alloy with 15ppm N content after 1000℃ / 1000h heat exposure, and Figure (c) shows the microstructure of the alloy with 27ppm N content after 1000℃ / 1000h heat exposure.

[0019] Figure 3 The figure shows the area fraction of the TCP phase in the microstructure of second-generation single-crystal superalloys with different N contents after heat exposure at 1000℃ for different times in the examples. In the figure, the horizontal axis represents the heat exposure time (h), and the vertical axis represents the area fraction (%). Detailed Implementation

[0020] The design concept of this invention is as follows: Single-crystal superalloys are prone to precipitating TCP phase during high-temperature service, affecting their mechanical properties. Adding rare and precious metals such as Ru to suppress TCP precipitation significantly increases the alloy's cost. For single-crystal superalloys, reducing the content of TCP-forming elements (W, Mo, Cr, etc.) is typically used to suppress TCP precipitation, but this also affects the alloy's mechanical and oxidation properties. Nitrogen (N) is generally considered an impurity element in single-crystal superalloys, requiring control of its content. However, studies have shown that adding a small amount of N can reduce porosity and improve mechanical properties. Therefore, optimizing the N content is crucial for the long service life of superalloys.

[0021] Based on the above design concept, this invention proposes a method for improving the microstructure stability of single-crystal superalloys by controlling the nitrogen content, the specific steps of which are as follows: 1) Weigh out the second-generation single-crystal high-temperature alloy master alloy ingot (N4 in Table 1) and CrN powder according to the required composition, wherein the CrN powder is wrapped with nickel foil. 2) Place the master alloy ingot and CrN powder of different masses in a directional solidification furnace and melt-cast to prepare single crystal high-temperature alloys with different N contents (N15 and N27 in Table 1, where the yield of N content is about 30%). Directional solidification process: upper zone temperature 1450±10℃, lower zone temperature 1500±10℃, and pulling speed 5mm / min.

[0022] Heat treatment process: 1300℃ / 2h, air cooling; 1120℃ / 4h, air cooling; 1080℃ / 4h, air cooling; 900℃ / 4h, air cooling; 3) The single-crystal high-temperature alloy was subjected to a high-temperature heat exposure test. Specifically, the service temperature of the single-crystal high-temperature alloy was selected as 1000℃, and the heat exposure time was 50h, 100h, 300h, 500h and 1000h respectively. After heat exposure, the sample was directly cooled to room temperature.

[0023] 4) TCP phase separation tendency analysis The samples from step 3) that had undergone heat exposure for different durations were observed using a scanning electron microscope. Figure 1 The microstructure of single-crystal superalloys with different N contents in the examples after 100 hours of heat exposure at 1000°C is shown. It can be seen that the alloys with N mass contents of 4 ppm and 15 ppm began to precipitate TCP phase in their microstructure after 100 hours of heat exposure, while no TCP phase precipitation was observed in the single-crystal superalloy with added N mass content of 27 ppm. Figure 2 The results show that after 1000 h of heat exposure at 1000°C, the three alloys with different N contents precipitated TCP phase. However, the alloy with an N mass content of 27 ppm had the fewest TCP phases, followed by the alloy with an N mass content of 15 ppm, while the alloy with an N mass content of 4 ppm had the most TCP phases.

[0024] Figure 3 The figure shows the TCP phase area fraction in the microstructure of second-generation single-crystal superalloys with different N contents after heat exposure at 1000℃ for different times in the examples. With increasing heat exposure time (100~1000h), the number of TCP phases increased in all alloys with different N contents, but the growth rate and final area fraction differed significantly among the alloys with different N contents. The alloy with an N mass content of 4ppm had a TCP phase area fraction of approximately 5.04% after 1000h, indicating a high tendency for TCP phase precipitation and poor microstructure stability. As the N content in the alloy increased, TCP phase precipitation was suppressed; the alloy with an N mass content of 27ppm had a TCP phase area fraction of approximately 0.49% after 1000h heat exposure. It is evident that controlling the microstructure stability of single-crystal superalloys with different N contents can be significantly improved throughout the heat exposure process.

[0025] Table 1 Alloy composition and electron vacancies in the examples (content of elements other than N is in wt.%, N content is in ppm)

[0026] According to the composition of the three groups of second-generation single-crystal superalloys with different N contents in Table 1, the electron vacancy concentration (Nv) was calculated according to GB / T 31309-2020, which were 2.17, 2.17, and 2.17, respectively. This shows that the main components of alloys with different N contents have little effect on the precipitation of TCP phase. Under similar Nv values, the precipitation of TCP phase in the microstructure of single-crystal superalloys with high N content is significantly suppressed, indicating that adding a certain amount of N can improve the microstructure stability of the alloy.

[0027] The results show that adding a small amount of nitrogen to single-crystal superalloys can effectively suppress the precipitation of TCP phase during service. Moreover, this method is simple to operate and has low cost, and can provide guidance for the composition design of low-cost, long-life single-crystal superalloys.

Claims

1. A method for improving the microstructure stability of single-crystal superalloys by controlling the nitrogen content, characterized in that, The desired N content single crystal casting is prepared by melting and casting a single crystal superalloy together with CrN and / or AlN. The parts are then subjected to standard heat treatment to finally obtain an alloy with an N mass content of 15~30ppm.

2. The method according to claim 1, characterized in that: The single-crystal high-temperature alloy and the final alloy obtained are single-crystal high-temperature alloys that meet the same grade composition requirements; The single-crystal superalloy used is a nickel-based single-crystal superalloy with a nitrogen content of 4-6 ppm.

3. The method according to claim 1, characterized in that: The process conditions for the standard heat treatment are determined according to the requirements of the alloy grade.

4. The method according to claim 1, 2, or 3, characterized in that: Includes the following steps: 1) According to the composition requirements of the required single crystal high-temperature alloy grade, weigh the master alloy of the required single crystal high-temperature alloy grade, as well as CrN powder and / or AlN powder. 2) Place the master alloy, CrN powder and / or AlN powder into the crucible of a directional solidification furnace, and melt-cast to prepare single crystal castings with different N contents. Perform standard heat treatment on the parts.

5. The method according to claim 4, characterized in that: The master alloy used in step 1) is a nickel-based single-crystal superalloy with a nitrogen content of 4-6 ppm. Based on the N content in the master alloy, CrN powder and / or AlN powder are mixed in proportion to finally obtain an alloy with an N mass content of 15~30ppm.

6. The method according to claim 4, characterized in that: High-temperature long-term heat exposure tests were conducted on the obtained parts with different N contents to observe the precipitation of TCP phase in the parts with different N contents and to analyze the precipitation tendency of TCP phase in the microstructure of the parts with different N contents. As the N content increased, the precipitation tendency of TCP phase in the microstructure decreased significantly, and the microstructure stability was significantly improved.

7. The method according to claim 6, characterized in that: During the heat exposure process, the temperature was 900℃~1100℃, which is close to the service temperature of the parts, and the time was 100h~1000h. After the heat exposure, scanning electron microscopy was performed to observe and analyze the area percentage of TCP phase in alloys with different N contents to determine the microstructure stability of alloys with different N contents.

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