A detection method for the morphological changes of carbide dissolution and precipitation in a superalloy

Abstract

A detection method for the morphological changes of carbide dissolution and precipitation in a superalloy belongs to the research field of precipitated phases in superalloys. In this detection method, after cutting the specimen and removing the oxide scale, a mixture layer of alumina and alkaline silica sol with an appropriate thickness is coated on the surface of the specimen. Then, the specimen is placed in a tube furnace for multiple isothermal solidification experiments. After the specimen is water-cooled, the field emission scanning electron microscope is used to observe the carbide dissolution and precipitation conditions. According to the temperature changes during heating and cooling in the isothermal solidification experiment, the morphological changes of carbide dissolution and precipitation are obtained. This method can simply and accurately observe the process of carbide dissolution and precipitation, which is particularly important for understanding the formation mechanism of carbides, exploring the reasons for elements such as B and Hf to change the carbide morphology, and giving full play to the role of carbides in the alloy.

Description

Technical Field

[0001] The present invention relates to the research field of precipitated phases in superalloys, and specifically to a detection method for the morphological changes of carbide dissolution and precipitation in superalloys. Background Art

[0002] Superalloys refer to highly alloyed iron-based, nickel-based or cobalt-based austenitic metallic materials that can withstand complex stresses at relatively high temperatures (>600°C) and have surface stability. Superalloys have excellent high-temperature mechanical properties, oxidation resistance and hot corrosion resistance, and are widely used in the production of key power transmission components such as aerospace and energy transportation. To achieve high strength and good tissue stability of the alloy, some elements are added to superalloys. The elements added to superalloys can be roughly divided into three categories: one is solid solution strengthening elements such as Cr, W, Mo, Ta, Re, Co, etc., one is precipitation strengthening elements such as Al, Ti, Ta, Nb, etc., and the other is microalloying elements such as C, B, Hf, etc. Among them, as a microalloying element, C is almost added to all superalloys. The C added to the alloy mainly exists in the form of carbides in the solidified structure. During the solidification process of the liquid metal, primary carbides in the form of blocks or cursive shapes precipitate at grain boundaries or between dendrites. The discontinuous granular carbides precipitated at grain boundaries can hinder grain boundary sliding and crack propagation, improve the plastic toughness of the alloy, and increase the creep life. With the development of generations of single crystal superalloys, on the one hand, the total amount of refractory elements (W, Mo, Ta, Re, etc.) gradually increases, and on the other hand, trace elements such as C change from complete removal at the beginning to limited use later, because the addition of C can reduce oxides, improve the purity of the alloy, thereby improving the casting performance of the alloy and reducing the formation of solidification structure defects such as freckles, polycrystals, and small-angle grain boundaries.

[0003] Therefore, as an important trace element, C is added to superalloy materials. With the increase of carbon content, the carbide phase changes from fine blocks to a skeleton shape. In addition, the addition of elements such as B and Hf changes the morphology of carbides. Hf changes the morphology of MC-type carbides from a "Chinese character" shape or a "skeleton" shape to a block shape. Therefore, it is particularly important to explore the process of morphological changes of carbide dissolution and precipitation with temperature changes in superalloys for understanding the formation mechanism of carbides, exploring the reasons for the change of carbide morphology by elements such as B and Hf, and giving full play to the role of carbides in the alloy.

[0004] Currently, the most commonly used method for measuring the dissolution and precipitation temperatures of carbides in alloys is high-temperature differential scanning calorimetry. However, for some single-crystal superalloys with low carbon content, the amount of carbides precipitated during solidification is small, and the sensitivity of DTA is limited. Therefore, it is often difficult to detect the dissolution and precipitation peaks of carbides. With the development of simulation technology, some solidification simulation software such as Thermo-calc and JMatPro are used to guide experiments. The simulation results are based on the equilibrium phase change process and rely on a complete thermodynamics database. However, the actual solidification and precipitation of carbides are non-equilibrium solidification processes, and there are differences between the results obtained by using solidification simulation software and the actual non-equilibrium solidification process. Summary of the Invention

[0005] Based on the problems existing in the prior art, the purpose of the present invention is to provide a detection method for the morphological changes of carbide dissolution and precipitation in superalloys. In this detection method, the morphological evolution process of carbide dissolution and precipitation with temperature changes in superalloys can be detected. First, a mixture layer of alumina and alkaline silica sol with an appropriate thickness is coated on the surface of the specimen, and then the specimen is placed in a tube furnace for multiple isothermal solidification experiments. After the specimen is water-cooled, the field emission scanning electron microscope is used to observe the situation of carbide dissolution and precipitation. According to the temperature changes during heating and cooling in the isothermal solidification experiment, the morphological changes of carbide dissolution and precipitation are obtained. This method can simply and accurately observe the process of carbide dissolution and precipitation.

[0006] In order to achieve the above purpose, the technical method adopted by the present invention is as follows:

[0007] A detection method for the morphological changes of carbide dissolution and precipitation in a superalloy of the present invention includes the following steps:

[0008] S1: Sampling

[0009] Take the superalloy to be measured, cut it into specimens, and remove the oxide scale on the surface of the specimens to obtain specimens without oxide scale.

[0010] S2: Coating a mixture layer of alumina and alkaline silica sol

[0011] According to the mass ratio, alumina powder: alkaline silica sol = 1:(0.34 - 0.38), mix alumina powder and alkaline silica sol, and stir evenly to obtain a mixed solution.

[0012] Apply the mixed solution evenly on the surface of the specimen without oxide scale, heat and dry it, and repeat the application, heating and drying again until a mixture layer of alumina and alkaline silica sol with a thickness of 3 ± 1 mm is coated on the surface of the specimen without oxide scale, obtaining a specimen coated with a mixture layer of alumina and alkaline silica sol.

[0013] S3: Determine the isothermal solidification temperature range

[0014] Use differential scanning calorimetry to measure the DSC curve of the superalloy. According to the DSC curve, determine the solidus temperature and liquidus temperature of the superalloy. Select the range of (temperature of solidus temperature - 30°C) to (temperature of liquidus temperature + 30°C) as the isothermal solidification temperature range, and determine the isothermal solidification experiment temperature at intervals of 5 - 20°C;

[0015] S4: Conduct isothermal solidification experiments

[0016] During the isothermal solidification experiment temperature, perform heating and holding and cooling and holding respectively to obtain multiple specimens after heating heat treatment corresponding to different isothermal solidification experiment temperatures and multiple specimens after cooling heat treatment corresponding to different isothermal solidification experiment temperatures;

[0017] S5: Corrosion observation

[0018] Respectively cut the multiple specimens after heating heat treatment corresponding to different isothermal solidification experiment temperatures and the multiple specimens after cooling heat treatment corresponding to different isothermal solidification experiment temperatures. After making standard metallographic specimens for the cut surfaces, perform chemical corrosion to reveal the carbide morphology, observe the carbide morphology, and obtain the morphological changes of carbide dissolution during the heating process corresponding to the temperature and the morphological changes of carbide precipitation during the temperature cooling process.

[0019] In the above-mentioned S1, the cutting is preferably wire cutting.

[0020] In the above-mentioned S1, the method for removing the oxide scale on the specimen surface is selected from one of sandpaper grinding and grinding wheel grinding.

[0021] In the above-mentioned S2, the particle size of the alumina powder is 300 - 500 mesh.

[0022] In the above-mentioned S2, the number of times of repeated coating and heating and drying is preferably 5 - 7 times.

[0023] In the above-mentioned S3, the test process of the DSC curve is as follows: Use an argon protection atmosphere, quickly heat from room temperature to 800°C, then heat at a heating rate of 10°C / min to 1450°C, then cool at a cooling rate of 10°C / min to 800°C, and then quickly cool to room temperature.

[0024] In the above-mentioned S4,

[0025] (1) Heating process:

[0026] After heating the isothermal heating equipment to the isothermal solidification experiment temperature, place the specimen coated with a mixture layer of alumina and alkaline silica sol in a crucible boat, place it in the constant temperature zone of the isothermal heating equipment, hold for 15 - 30 min, take it out, and cool it with water to obtain the specimen after heating heat treatment;

[0027] Repeat the above isothermal solidification experiment on multiple specimens of the same mixture layer coated with alumina and alkaline silica sol at different isothermal solidification experiment temperatures to obtain multiple specimens after heating treatment at elevated temperatures corresponding to different isothermal solidification experiment temperatures;

[0028] (2) Cooling process:

[0029] Place the specimen coated with the mixture layer of alumina and alkaline silica sol in a crucible boat, place it in the constant temperature zone of an isothermal heating device, heat it above the liquidus temperature and hold for 15 - 30 min, then cool it to the isothermal solidification experiment temperature and hold for 15 - 30 min, take it out and water-cool to obtain the specimen after cooling heat treatment;

[0030] Cool multiple specimens of the same mixture layer coated with alumina and alkaline silica sol to different isothermal solidification experiment temperatures, repeat the above isothermal solidification experiment to obtain multiple specimens after cooling heat treatment corresponding to different isothermal solidification experiment temperatures;

[0031] In the above S4, the isothermal heating device is selected as a tube furnace or a muffle furnace, preferably a tube furnace, and the constant temperature zone of the tube furnace is located at the center of the tube furnace.

[0032] In the above S5, wire cutting is used for cutting to avoid the influence of surface oxidation on the experimental results.

[0033] In the above S5, standard metallographic sample preparation is carried out in the order of embedding, rough grinding, fine grinding, and polishing processes.

[0034] In the above S5, the chemical etching solution used for chemical etching is a mixture of CuSO 4 , HCl, H 2 O, and according to the solid-liquid ratio, CuSO 4 : HCl: H 2 O = 4 g: 20 mL: 20 mL. The chemical etching time is preferably 5 - 10 s.

[0035] In the above S5, the carbide morphology is observed by a field emission scanning electron microscope.

[0036] For the detection method of the change in the dissolution and precipitation morphology of carbides in a superalloy of the present invention, its principle and advantages are as follows: Carbides in superalloys often have a relatively high melting point and precipitate at a relatively high temperature below the liquidus temperature. Therefore, the specimen is coated with a mixed solution of alumina powder and alkaline silica sol. When the specimen is heated above the liquidus temperature, the liquid alloy is restricted within the volume wrapped by the mixture layer of alumina and alkaline silica sol, and then the water quenching method is used to explore the change in the dissolution and precipitation morphology of carbides in the superalloy. The present invention has the advantages of simple experimental operation, strong repeatability, and strong reliability of experimental results. Brief Description of the Drawings

[0037] Figure 1 Morphology comparison before and after coating the mixed solution of prepared alumina powder and alkaline silica sol and the superalloy specimens of the embodiments of the present invention.

[0038] Figure 2 Simulation results of the solid-liquid phase line temperature and carbide precipitation temperature of the single-crystal superalloy in Embodiment 1 of the present invention; (a) is the simulation curve of the solid-liquid phase line temperature of the single-crystal superalloy; (b) is the simulation curve of the carbide precipitation temperature of the single-crystal superalloy.

[0039] Figure 3 DSC heating and cooling curves of the single-crystal superalloy in Embodiment 1 of the present invention.

[0040] Figure 4 Evolution process of carbides in the single-crystal superalloy in Embodiment 1 of the present invention with increasing temperature; (a) is the carbide morphology diagram at 1320 °C / 30 min; (b) is the carbide morphology diagram at 1340 °C / 30 min; (c) is the carbide morphology diagram at 1360 °C / 30 min; (d) is the carbide morphology diagram at 1380 °C / 30 min; (e) is the carbide morphology diagram at 1400 °C / 30 min.

[0041] Figure 5 Evolution process of carbides in the single-crystal superalloy in Embodiment 1 of the present invention with decreasing temperature; (a) is the carbide morphology diagram after holding at 1400 °C / 30 min and cooling to 1375 °C / 30 min; (b) is the carbide morphology diagram after holding at 1400 °C / 30 min and cooling to 1360 °C / 30 min; (c) is the carbide morphology diagram after holding at 1400 °C / 30 min and cooling to 1345 °C / 30 min; (d) is the carbide morphology diagram after holding at 1400 °C / 30 min and cooling to 1330 °C / 30 min. Detailed Description of the Invention

[0042] The present invention will be further described in detail below with reference to the drawings and specific embodiments.

[0043] In order to better verify the effectiveness of the method of the present invention, single-crystal superalloys are selected in the embodiments of the present invention, which have a very low C content, indicating the feasibility of using this method for alloys with carbide dissolution and precipitation temperatures that cannot be measured by DSC.

[0044] In the following embodiments, the test process of the DSC curve is as follows: Under an argon protective atmosphere, it is rapidly heated from room temperature to 800 °C, then heated to 1450 °C at a heating rate of 10 °C / min, then cooled to 800 °C at a cooling rate of 10 °C / min, and then rapidly cooled to room temperature.

[0045] Example 1

[0046] In this example, a heat-resistant corrosion nickel-based single-crystal superalloy with a C content of 0.015% (mass fraction) was selected as the experimental material to study the morphological changes of carbides in the alloy during heating and cooling processes. The specific steps are as follows:

[0047] 1) Sampling:

[0048] Several semi-circular slices with a thickness of 5 mm were cut from the single-crystal superalloy test bar using wire cutting, and the oxide scale on the surface was polished off with 240# sandpaper to obtain the specimens without oxide scale.

[0049] 2) Coating a mixture layer of alumina and alkaline silica sol:

[0050] According to the mass ratio, alumina powder: alkaline silica sol = 1:0.35, a mixed solution of alumina powder and alkaline silica sol was prepared. The prepared mixed solution was evenly coated on the specimens, and the surface was heated and dried using a universal electric furnace. The coating operation was repeated 6 times to make the specimens surface coated with a 3-mm-thick mixture layer of alumina and alkaline silica sol. The comparison of the morphological changes of the prepared mixed solution of alumina powder and alkaline silica sol and the high-temperature alloy specimens before and after coating in this example of the present invention is shown in Figure 1 .

[0051] 4) Determining the isothermal solidification temperature range:

[0052] Differential scanning calorimetry analysis was performed on this single-crystal superalloy. In an argon atmosphere, it was rapidly heated from room temperature to 800 °C, then heated at a heating rate of 10 °C / min to 1450 °C, and then cooled at a cooling rate of 10 °C / min to 800 °C. The heating and cooling DSC curves of this single-crystal superalloy were obtained respectively. The measurement results are as shown in the appendix Figure 3 . According to Figure 3 , the solidus temperature of this single-crystal superalloy was determined to be 1345 °C, the liquidus temperature was 1381 °C, and the isothermal solidification temperature range was determined to be 1315 - 1411 °C.

[0053] 5) Conducting isothermal solidification experiments (heating process):

[0054] According to the isothermal solidification temperature range of 1315 - 1411 °C, at intervals of 20 °C, during the heating process, the isothermal solidification experimental temperatures are respectively 1320 °C, 1340 °C, 1360 °C, 1380 °C, and 1400 °C. A plurality of specimens coated with a mixture layer of alumina and alkaline silica sol are respectively kept warm in a tubular furnace at the above-mentioned isothermal solidification experimental temperatures for 30 min, and then quickly taken out and water-cooled when the time is up. Specifically: heat the tubular furnace to 1320 °C, place a specimen coated with a mixture layer of alumina and alkaline silica sol at the center of the tubular furnace, after keeping warm for 30 min, take it out and water-cool; then raise the temperature of the tubular furnace to 1340 °C, put in another specimen coated with a mixture layer of alumina and alkaline silica sol after reaching the temperature, after keeping warm for 30 min, take it out and water-cool, and so on. Conduct an experiment at each isothermal solidification experimental temperature to obtain a plurality of specimens after heating heat treatment corresponding to different isothermal solidification experimental temperatures.

[0055] 6) Conduct isothermal solidification experiments (cooling process):

[0056] To explore the morphological changes of carbides in the single-crystal superalloy during the cooling process, the specimens coated with a mixture layer of alumina and alkaline silica sol are first kept warm above the liquidus temperature for 30 min, and then kept warm at a certain temperature below the liquidus for 30 min for isothermal solidification experiments. According to the isothermal solidification temperature range of 1315 - 1411 °C, at intervals of 15 °C, select 1400 °C, 1375 °C, 1360 °C, 1345 °C, and 1330 °C as the isothermal solidification experimental temperatures during the cooling process; first keep a specimen coated with a mixture layer of alumina and alkaline silica sol at 1400 °C for 30 min, then cool it in the furnace to 1375 °C and keep warm for 30 min, and quickly take it out and water-cool when the time is up. Then keep another specimen coated with a mixture layer of alumina and alkaline silica sol at 1400 °C for 30 min, then cool it in the furnace to 1360 °C and keep warm for 30 min, and quickly take it out and water-cool when the time is up,... and so on. Keep warm at 1345 °C for 30 min and 1330 °C for 30 min respectively, and quickly take it out and water-cool when the time is up to obtain a plurality of specimens after cooling heat treatment corresponding to different isothermal solidification experimental temperatures.

[0057] 7) Corrosion observation: Use wire cutting to cut each specimen after heating heat treatment corresponding to different isothermal solidification experimental temperatures and each specimen after cooling heat treatment corresponding to different isothermal solidification experimental temperatures from the middle part to observe the internal structure. After passing through the standard metallographic sample preparation process and then chemical corrosion, use a field emission scanning electron microscope to observe the morphology of carbides, so as to study the dissolution and precipitation behavior of carbides in the alloy with the change of temperature. Among them, the chemical corrosion solution used in this embodiment is CuSO 4 、HCl、H 2 O mixture, according to the solid-liquid ratio, CuSO 4: HCl:H 2 O = 4 g: 20 mL: 20 mL.

[0058] The experimental results of the examples are analyzed as follows:

[0059] The solidification process of the alloy was simulated using JMatPro software. The starting calculation temperature was set at 1450 °C, and the temperature calculation step size was set at 5 °C. When the mass fraction of the liquid phase reached 0.02%, it was considered to be completely solidified. The simulation results are as shown in Figure 2 the attachment. It can be seen from the JMatPro simulation results that there are two types of carbides in the equilibrium solidification structure of the alloy, namely MC and M 23 C 6 . Among them, the starting precipitation temperature of the MC-type carbide is 1318 °C, and the liquidus temperature of the alloy is 1370 °C, which is close to the liquidus temperature (1381 °C) measured by DSC. However, the solidus temperatures obtained by the two are quite different (the solidus temperature obtained by JMatPro is 1160 °C, and the solidus temperature obtained by DSC is 1345 °C). When the temperature rises to 1400 °C, it can be seen from Figure 4 the attachment that there are still some undissolved granular carbides in the alloy. Some studies have shown that the carbides in superalloys are very stable, and there are still residual MC carbide particles in the alloy melt even above the liquidus temperature. Therefore, it is not difficult to understand that there are still some undissolved carbides in the alloy after holding at 1400 °C for 30 min. The results obtained by JMatPro are based on equilibrium solidification, indicating that there are differences between the actual dissolution behavior of carbides and the JMatPro simulation results.

[0060] The endothermic peaks corresponding to 1193 °C and 1326 °C in the DSC heating curve represent the dissolution temperatures of the γ'-phase and γ / γ'-eutectic in the alloy respectively. The exothermic peak corresponding to 1176 °C in the cooling curve represents the precipitation temperature of the γ'-phase. Since the carbide content in the alloy is very small, there are no obvious endothermic or exothermic peaks shown in the DSC heating and cooling curves.

[0061] As shown in Figure 4 the attachment, from the experimental results of the heating process, it can be seen that the carbides in the alloy are distributed at the interdendritic regions. After chemical etching, the morphology of the primary melting structure is revealed. Obviously, the heat treatment temperature exceeds the primary melting temperature range of the alloy, and the carbides are surrounded by the primary melting zone. As the temperature increases, the size of the carbides gradually decreases, and the amount of carbides also decreases accordingly, indicating that the carbides gradually dissolve with the increase of temperature. Especially when the temperature reaches 1380 °C, the amount of carbides in the alloy decreases significantly.

[0062] As shown in Figure 5As shown, from the experimental results of the cooling process, it can be seen that as the temperature decreases, the number of granular carbides gradually increases and the size also gradually increases. This indicates that during the cooling process, a part of the carbide-forming elements in the melt attach to the undissolved carbides and make them gradually grow. As the temperature further decreases and the degree of supercooling increases, when the carbide nucleation conditions are reached, another part of the carbide-forming elements nucleate independently and gradually grow. Eventually, it is manifested as an increase in the size and number of massive or granular carbides. This shows that the morphological changes of carbide dissolution and precipitation during the heating and cooling processes can be observed by the method of the present invention.

[0063] Example 2

[0064] In this example, a heat-resistant corrosion-resistant nickel-based single-crystal superalloy with a C content of 0.015% (mass fraction) is selected as the experimental material to study the morphological changes of carbides in the alloy during the heating and cooling processes. The specific steps are as follows:

[0065] 1) Sampling: Cut a sample of appropriate size by wire cutting and polish off the oxide scale on the surface with sandpaper.

[0066] 2) Coating a mixture layer of alumina and alkaline silica sol: Prepare a mixed solution of alumina powder and alkaline silica sol, and control the weight ratio of alkaline silica sol to alumina powder between 0.34 and 0.38. Then stir evenly until it becomes viscous. First, evenly coat a layer of the mixed solution on the surface of the sample, and then use a universal electric furnace to heat and dry its surface. Repeat the coating operation 7 times to coat a 4-mm-thick alumina layer on the surface of the sample.

[0067] 3) Measure the DSC curve of the alloy by differential thermal analysis to determine the solidus temperature and liquidus temperature of the superalloy. Select the temperature range from (solidus temperature - 30°C) to (liquidus temperature + 30°C) as the isothermal solidification temperature range, and determine the isothermal solidification experimental temperatures at intervals of 5 - 20°C.

[0068] 4) Conduct isothermal solidification experiments during the heating process

[0069] Use a tube furnace to conduct isothermal solidification experiments. After the tube furnace is heated to the predetermined temperature, place the sample coated with alumina in a crucible boat, and then place it in the constant temperature zone at the center of the tube furnace. Keep it warm for a certain period of time, and immediately take it out and cool it with water when the time is up.

[0070] 5) Conduct isothermal solidification experiments during the cooling process

[0071] Keep the sample coated with a mixture layer of alumina and alkaline silica sol warm at a temperature above the liquidus temperature for 30 minutes first, and then conduct isothermal solidification experiments by keeping it warm at a certain temperature below the liquidus temperature for 30 minutes.

[0072] 6) To avoid the influence of surface oxidation on the experimental results, the heat-treated specimen was cut in half by wire cutting to observe the internal structure. After the specimen went through standard metallographic sample preparation processes such as grinding and polishing, a chemical etching solution was used for etching to reveal the morphology of the carbides. The etching time was 5 - 10 s, and a field emission scanning electron microscope was used to observe the morphology of the carbides. The chemical etching solution used in this example was a mixture of CuSO 4 、HCl、H 2 O. According to the solid-liquid ratio, CuSO 4 : HCl: H 2 O = 4 g: 20 mL: 20 mL.

[0073] Comparative Example 1

[0074] A method for detecting the morphological changes of carbide dissolution and precipitation in a superalloy is the same as Example 1, except that:

[0075] In step 2), the thickness of the mixture layer of alumina and alkaline silica sol was 1 mm, so during the water cooling process, it was prone to cracking and lost the function of the mixture layer of alumina and alkaline silica sol.

[0076] Comparative Example 2

[0077] A method for detecting the morphological changes of carbide dissolution and precipitation in a superalloy is the same as Example 1, except that:

[0078] In step 2), according to the mass ratio, alumina powder: alkaline silica sol = 4:1, then the mixed solution was viscous, not easy to coat, the quality of the crust was poor and not dense.

[0079] It should be understood that the specific order or hierarchy of steps in the disclosed process is an example of an exemplary method. Based on design preferences, it should be understood that the specific order or hierarchy of steps in the process can be rearranged without departing from the protection scope of the present disclosure. The appended method claims present the elements of various steps in an exemplary order and are not intended to be limited to the specific order or hierarchy.

[0080] The above embodiments are only used to illustrate the technology provided by the present invention, so that those familiar with such technologies can understand and apply the content of the present invention, and do not limit the technical solutions. Any modification or equivalent replacement of the technical solutions of the present invention, as long as it does not depart from the scope and purpose of the present invention's solutions, should be covered within the scope of the rights required by the present invention.

Claims

1. A detection method for the morphological changes of carbide dissolution and precipitation in a superalloy at high temperature, characterized in that, it comprises the following steps: S1: Sampling Take the superalloy to be measured, cut it into specimens, and remove the oxide scale on the surface of the specimens to obtain specimens without oxide scale; S2: Coating a mixture layer of alumina and alkaline silica sol According to the mass ratio, alumina powder: alkaline silica sol = 1: (0.34 - 0.38), mix the alumina powder and alkaline silica sol, stir evenly to obtain a mixed solution; Apply the mixed solution evenly on the surface of the specimens without oxide scale, heat and dry it, and repeat the coating, heating and drying again until a mixture layer of alumina and alkaline silica sol with a thickness of 3 ± 1 mm is coated on the surface of the specimens without oxide scale to obtain specimens coated with a mixture layer of alumina and alkaline silica sol; S3: Determine the isothermal solidification temperature range Use differential thermal analysis to measure the DSC curve of the superalloy, and according to the DSC curve, determine the solidus temperature and liquidus temperature of the superalloy, select the range of (the temperature of the solidus temperature - 30°C) to (the temperature of the liquidus temperature + 30°C) as the isothermal solidification temperature range, and determine the isothermal solidification experiment temperature at intervals of 5 - 20°C; S4: Conduct isothermal solidification experiments At the isothermal solidification experiment temperature, carry out heating and holding and cooling and holding respectively, take out and cool to obtain multiple specimens after heating heat treatment corresponding to different isothermal solidification experiment temperatures and multiple specimens after cooling heat treatment corresponding to different isothermal solidification experiment temperatures; (1) Heating process: After heating the isothermal heating equipment to the isothermal solidification experiment temperature, place the specimens coated with a mixture layer of alumina and alkaline silica sol in a crucible boat, place it in the constant temperature zone of the isothermal heating equipment, hold for 15 - 30 min, take out and cool with water to obtain specimens after heating heat treatment; Repeat the above isothermal solidification experiment on multiple specimens coated with the same mixture layer of alumina and alkaline silica sol at different isothermal solidification experiment temperatures to obtain multiple specimens after heating heat treatment corresponding to different isothermal solidification experiment temperatures; (2) Cooling process: Place the specimens coated with a mixture layer of alumina and alkaline silica sol in a crucible boat, place it in the constant temperature zone of the isothermal heating equipment, heat to above the liquidus temperature and hold for 15 - 30 min, then cool to the isothermal solidification experiment temperature and hold for 15 - 30 min, take out and cool with water to obtain specimens after cooling heat treatment; Cool multiple specimens coated with the same mixture layer of alumina and alkaline silica sol to different isothermal solidification experiment temperatures, repeat the above isothermal solidification experiment to obtain multiple specimens after cooling heat treatment corresponding to different isothermal solidification experiment temperatures; S5: Corrosion observation Cut the multiple specimens after heating heat treatment corresponding to different isothermal solidification experiment temperatures and the multiple specimens after cooling heat treatment corresponding to different isothermal solidification experiment temperatures respectively, carry out standard metallographic sample preparation on the cut surface, and then carry out chemical corrosion to show the carbide morphology, observe the carbide morphology, and obtain the morphological changes of carbide dissolution during the heating process and the morphological changes of carbide precipitation during the cooling process corresponding to the temperature.

2. The detection method for the change in the morphology of carbide dissolution and precipitation in superalloys according to claim 1, characterized in that, in S1, wire cutting is used for cutting; and / or, one of sandpaper grinding and grinding wheel grinding is selected as the method for removing the oxide scale on the specimen surface.

3. The detection method for the change in the morphology of carbide dissolution and precipitation in superalloys according to claim 1, characterized in that, in S2, the particle size of the alumina powder is 300 - 500 mesh, and / or, the number of times of repeated coating and heating and drying is 5 - 7 times.

4. The detection method for the change in the morphology of carbide dissolution and precipitation in superalloys according to claim 1, characterized in that, in S3, the test process of the DSC curve is as follows: under an argon protection atmosphere, rapidly heat from room temperature to 800 °C, then increase the temperature to 1450 °C at a heating rate of 10 °C / min, then decrease the temperature to 800 °C at a cooling rate of 10 °C / min, and then rapidly cool to room temperature.

5. The detection method for the change in the morphology of carbide dissolution and precipitation in superalloys according to claim 1, characterized in that, the isothermal heating equipment is selected as a tube furnace or a muffle furnace.

6. The detection method for the change in the morphology of carbide dissolution and precipitation in superalloys according to claim 1, characterized in that, in S5, wire cutting is used for cutting.

7. The detection method for the change in the morphology of carbide dissolution and precipitation in superalloys according to claim 1, characterized in that, in S5, the standard metallographic sample preparation is successively carried out with the processes of embedding, rough grinding, fine grinding, and polishing.

8. The detection method for the change in the morphology of carbide dissolution and precipitation in superalloys according to claim 1, characterized in that, In the above-mentioned S5, the chemical etching solution used for chemical etching is a mixture of CuSO 4 , HCl, and H 2 O. According to the solid-liquid ratio, CuSO 4 : HCl: H 2 O = 4 g: 20 mL: 20 mL, and the chemical etching time is 5 to 10 s.

9. The detection method for the change in the morphology of carbide dissolution and precipitation in superalloys according to claim 1, characterized in that, in S5, the carbide morphology is observed by a field emission scanning electron microscope.

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