Electrolytic double jet sample preparation method for analysis of second phases in nickel-based superalloys

Abstract

The application discloses an electrolytic double-spraying sample preparation method for analyzing second phases in a nickel-based high-temperature alloy, which comprises the following steps: (1) obtaining a sheet with a thickness of 50-100 mu m by pretreating the nickel-based high-temperature alloy; and (2) electrochemically corroding the sheet by using the electrolytic double-spraying method to realize passivation protection of the matrix phase, make the gamma prime phase corrode and dissolve until a SEM observation sample is obtained. The electrochemical corrosion conditions are as follows: the working voltage is 15-20 V; the working temperature is controlled between-20 and-10 DEG C; and the electrolyte is 15-25% concentrated sulfuric acid and 75-85% methanol. The application can avoid the problem that the observation phase such as carbide is lost due to the corrosion of the wrapped matrix, meanwhile, the application will not cause corrosion to the carbide phase and non-metallic inclusions, can clearly retain the outline of the second phase, will not introduce impurity elements, is beneficial to element analysis, and is suitable for the analysis of the second phases in the nickel-based high-temperature alloy.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of metal physical chemistry, and particularly relates to an electrolytic double-spraying sample preparation method for analyzing second phases (γ' phase, carbide phase, oxide phase, etc.) in a nickel-based high-temperature alloy. BACKGROUND

[0002] The nickel-based high-temperature alloy has excellent strength and high-temperature performance, good oxidation resistance and heat corrosion resistance, high organizational stability and reliability, and is used for key components of aerospace engines. In addition to the matrix, the second phase such as the γ' phase and various carbide phases is also included. Since the second phase is small in size (usually below a few hundred nanometers), high-precision scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are usually used for quantitative analysis. In order to accurately perform quantitative analysis and evaluate the influence of the analyzed phase on the material performance, the analyzed phase should be as clear as possible, the boundary should be clear, there should be no omission, and no interference elements should be introduced during the etching process, and therefore the composition of the etching solution and the preparation process for the microstructure characterization sample are required to be strict.

[0003] The γ' phase (Ni3(AL, Ti) phase) is the main strengthening phase of the nickel-based high-temperature alloy, and the distribution and size of the γ' phase are important factors affecting the performance of the alloy. The γ' phase in the nickel-based high-temperature alloy is usually small in size and dense in distribution, and the more accurate the statistical quantification is, the better. Due to the limitation of the preparation process, the carbide phase is an unavoidable second phase in the nickel-based high-temperature alloy, and there are various carbide phases in the nickel-based high-temperature alloy, such as MC, M3C, M6C, M7C3, M 23 C6, etc. Most of the carbide phases are not coherent with the matrix or the γ' phase, have high strength and high brittleness, are considered to be harmful to the performance, and cannot be completely avoided, and can only be strictly controlled in the heat treatment process and crushed during the processing process, and the size and content thereof are strictly limited. In addition, the MC carbide phase formed at the original powder particle boundary will become a nucleation site for the γ' phase, which will cause the formation of coarse γ' phase and weaken the precipitation strengthening effect. However, it is also found that the M 23 C6 phase has a coherent phase interface with the γ' phase, and the size control can be used to improve the material performance. With the further improvement of the high-temperature performance and service performance requirements of the nickel-based high-temperature alloy, the research on the carbide phase is also becoming more and more important. Not only is it limited to the shape and distribution, but the formation mechanism, phase structure, interface state and element segregation also need to be further analyzed, which puts forward higher requirements for TEM characterization and sample preparation. In the TEM electrolytic double-spraying sample preparation method for the γ' phase and the carbide of the nickel-based high-temperature alloy, there are respective limitations in the existing etching solution and process.

[0004] For example, the patent with the patent number ZL202210595866.6 uses 180 mL of concentrated H3PO4 solution, 6 mL of concentrated H2SO4 solution and 8 g of CrO3 to configure the electrolyte. The electrolyte can better expose the γ' phase, but the substrate is corroded too fast, the carbide particles fall off to cause the loss of phase, which is not conducive to analysis. Moreover, CrO3 is highly toxic and can introduce Cr element into the substrate, which is not conducive to element analysis. The patent with the patent number ZL201310110911.5 uses a mixed solution of 10% perchloric acid + 90% anhydrous ethanol, which also has the problem of uncontrollable corrosion rate and the falling off of carbide particles. The patent with the patent number ZL202410776559.7 uses a volume fraction of 3 parts of hydrochloric acid, 1 part of nitric acid, 0.5-1 part of glycerol and 0.5-1 part of phosphoric acid to configure the electrolytic etching solution, which is used for observing the metallographic corrosion of dendrites, but is not conducive to the observation of precipitated phase. The patent with the patent number ZL201310110911.5 uses two kinds of etching solution to prepare samples by electrochemical etching, which is used as the observation of the second phase of the metallographic structure. The first kind of etching solution is a mixed solution of 30-45% sulfuric acid and 55-70% methanol to remove the substrate phase. The second kind of etching solution is a mixed solution of 10-20% sulfuric acid, 5-15% hydrogen peroxide and phosphoric acid to remove the γ' phase to observe the carbide phase. The corrosion mechanism of this method is to remove the substrate phase, but the substrate phase around the second phase is corroded and dissolved, the second phase is lost and falls off, which causes the analysis error of carbide, oxide phase and non-metallic inclusion and other problems. Moreover, the process is more complicated, can only prepare metallographic structure samples, cannot be used for TEM sample preparation, the corrosion process and result are relatively unstable and other problems. SUMMARY

[0005] To solve the problem that the second phase particles of the high-temperature alloy are easy to fall off during the TEM sample preparation, causing analysis error, the purpose of the present application is to provide a sample preparation method for analyzing the second phase in the nickel-based high-temperature alloy. The method can obtain clear second phase particle outline and complete carbide particles, and will not cause etching to the carbide, and solves the problem of analysis error caused by the falling off of the second phase particles during the corrosion of the existing sample preparation method.

[0006] The technical solution adopted by the present application is as follows:

[0007] An electrolytic double-spraying sample preparation method for analyzing the second phase in the nickel-based high-temperature alloy, comprising the following steps:

[0008] (1) obtaining a thin sheet with a thickness of 50-100 μm after pretreatment of the nickel-based high-temperature alloy;

[0009] (2) electrochemically etching the thin sheet of step (1) by the electrolytic double-spraying method to realize the passivation protection of the substrate phase, causing the corrosion and dissolution of the γ' phase, until the SEM observation sample is obtained.

[0010] The electrochemical corrosion conditions are: working voltage of 15-20V; working temperature controlled between -20 and -10℃; electrolyte is 15-25% of concentrated sulfuric acid and 75-85% of methanol in volume percentage.

[0011] Preferably, the electrolytic double jet method in step (2) is to fix the thin slice of step (1) on the anode, place the cathode in the electrolytic cell, spray the electrolyte to the thin slice through the electrolytic double jet instrument, and cause electrolytic dissolution of the thin slice through an applied potential.

[0012] Preferably, the thin slice of step (1) is also double polished; and the corrosion time of step (2) is controlled as follows: first corrosion for 200±50s, take out, clean with anhydrous ethanol, and then observe; if the corrosion degree is not enough, then corrode every 60±30s and observe.

[0013] The sample after electrolysis is sequentially cleaned in three times of anhydrous ethanol solution and then dried.

[0014] Preferably, based on the SEM observation of the sample, electrochemical corrosion is continuously carried out in the same way until a suitable small hole appears in the middle of the sample, to obtain a TEM observation sample. The corrosion time of the TEM observation sample is controlled by the reception of an infrared signal by the photosensitive element of the electrolytic double jet instrument, and when a hole appears in the middle of the sample, the equipment will automatically stop working.

[0015] The above SEM observation sample or TEM observation sample is used for analyzing the second phase in the nickel-based high-temperature alloy.

[0016] Preferably, the SEM observation sample or TEM observation sample is used for analyzing the γ' phase, carbide, oxide phase and non-metallic inclusions.

[0017] Preferably, the SEM observation sample is placed under a scanning electron microscope, and the morphology of the γ' phase is photographed and observed according to the γ' phase imprint, and the size and quantity are counted.

[0018] Preferably, the size and quantity of the γ' phase are statistically analyzed: 20 photos of 3000-10000 times are taken, and the plane pixel area of the photos is counted through image analysis software such as Image J.

[0019] An electrolytic double jet sample preparation method for analyzing the second phase in a nickel-based high-temperature alloy, the specific steps are as follows:

[0020] S1. Sample pretreatment: processed by an electric spark wire cutting machine, hand ground or mechanically thinned and double polished, punched into a round sheet by a punching machine, and polished into a round sheet sample with a thickness of 50-100μm.

[0021] ​S2-1: Put the wafer sample with a thickness of 50-100 μm after pretreatment into an electrolytic double spray device for electrochemical thinning sample preparation, the voltage is 15-20 V, and the corrosion time is controlled as follows: first corrosion for 200 s, take out, clean with anhydrous ethanol, blow dry and observe, if the corrosion degree is not enough, corrode every 60 s and observe;

[0022] S2-2: In step S4, select the sample screened by SEM for further thinning, the process parameters are consistent with S2-1, thinning to the middle of the sample until a suitable small hole appears, as a TEM observation sample, the wafer sample after electrolysis needs to be cleaned three times in anhydrous ethanol.

[0023] S2-3: Use liquid nitrogen for cooling during electrolysis, and control the working temperature between-20 and-10℃, to ensure the appropriate sample corrosion rate, prevent the second phase such as γ' phase and carbide phase from being eroded or falling off, and improve the flatness of the sample surface;

[0024] S2-4: Put the wafer into anhydrous ethanol, continue to clean with an ultrasonic cleaner, and finally blow dry the surface residual ethanol with an ear cleaning ball and store;

[0025] S3. Scanning electron microscope observation: after the sample is thinned and corroded by S1-S2, the morphology of γ' phase can be observed and the size and quantity information can be counted under the scanning electron microscope according to the γ' phase imprint. For size and quantity statistical analysis, 20 photos of 3000-10000 times are taken, and the plane pixel area is counted by image analysis software such as Image J. After observation, the sample can be further thinned and perforated as a TEM observation sample according to the same method.

[0026] The purpose of step S3 is: first, the SEM observation has a larger field of view, which can observe and count the distribution of γ' phase in a larger range, and second, it can be used as a preliminary screening of transmission samples, and at least other second phase samples containing target carbide phase are selected.

[0027] S4. Electrolytic double spray thinning and perforation: the wafer selected by S1-S3 is further thinned and perforated by electrolytic double spray method. The process parameters are consistent with S2, and the corrosion time of TEM perforation sample is controlled by the infrared signal received by the photosensitive element of the electrolytic double spray instrument. When the sample is perforated in the middle, the device will automatically stop working.

[0028] S5. Transmission electron microscope observation: the wafer sample after S1-S4 can be further analyzed under the transmission electron microscope. For example, the morphology of carbide can be observed by using higher magnification, and the crystal structure information of carbide below 100 nm can be analyzed by diffraction and high resolution.

[0029] Compared with the prior art method of retaining the gamma prime phase by corroding the matrix phase, when the matrix phase around the second phase is corroded and dissolved, the loss of the second phase wrapped will cause the analysis error problem caused by the loss of carbide, oxide phase and non-metallic inclusion. Although the gamma prime phase in the alloy is relatively large in quantity, the remaining part is also deeply embedded, and the boundary form is not very clear. The application selects 15-25% concentrated sulfuric acid and 75-85% methanol as the double-spray electrolyte, and through controlling the solution temperature and electrolysis parameters in the electrolytic double-spray process, the matrix phase is passivated and protected, and the gamma prime phase has a more negative corrosion potential, so that corrosion and dissolution occur. In the corrosion process, the gamma prime phase is dissolved and becomes smaller, while the matrix wrapped with the gamma prime phase is not corroded, can leave a very clear mark, obtain more morphological detail information, and can be used for the morphology and size statistics of the gamma prime phase. For the carbide, oxide phase and other phases with more positive potential, corrosion damage does not occur, and the problem of the loss of the second phase wrapped due to the corrosion and dissolution of the matrix phase around the second phase is avoided, the analysis error problem caused by the loss of carbide, oxide phase and non-metallic inclusion is avoided, the clear three-dimensional profile of the second phase particles is obtained, and the phase interface with the matrix is retained. In addition, by controlling the electrolytic double-spray parameters and time, the sample is thinned without perforation, at this time the sample is in a corroded state, and the surface is smooth. Due to the thinning of the sample, the contrast of different phases is enhanced, the resolution and imaging quality are improved, and it is beneficial to scanning electron microscope observation. After the sample is observed by the scanning electron microscope, the sample can be further thinned and perforated for transmission electron microscope sample preparation. Since the sample and the preparation conditions are consistent, the continuity and consistency of the two can be ensured, and the advantages of SEM and TEM observation are combined.

[0030] Compared with the prior art, the application has the following advantages and beneficial effects:

[0031] (1) The application provides a corrosion agent, which can corrode the gamma prime phase and protect the matrix phase by controlling the solution temperature and electrolysis parameters in the electrolytic double-spray sample preparation process. After the gamma prime phase is corroded and falls off, the surrounding wrapped matrix is protected and can leave a very clear gamma prime phase mark. Information such as the morphology, size and quantity distribution of the gamma prime phase can be obtained from the mark. The mechanism of the application is opposite to that of the prior art corrosion agent, which corrodes the matrix phase and protects the gamma prime phase, and has complementary effects.

[0032] (2) The application can avoid the problem of the loss of the observed phase such as carbide caused by the corrosion of the wrapped matrix, and will not corrode the carbide phase and non-metallic inclusion, can clearly retain the profile of the second phase, and will not introduce impurity elements, which is beneficial to element analysis and suitable for the analysis and observation of the second phase such as carbide in the nickel-based high-temperature alloy.

[0033] (3) The present application can be used not only to observe the γ' phase and carbide phase, but also to characterize the second phase such as oxide phase formed during alloy preparation, non-metallic inclusions brought by master alloy smelting, etc. Since these phases have higher anode potential than the γ' phase, by controlling electrolysis parameters and corrosion rate, the interface between the phase and the matrix transition phase can be retained.

[0034] (4) The present application can be used for SEM electrolytic corrosion sample and TEM sample preparation at the same time. After SEM electrolytic corrosion sample observation, the same process is used to continue to thin the hole with an electrolytic double spray instrument, and a TEM sample can be obtained. The two sample preparation conditions are the same, which can ensure the continuity and consistency of the two microstructure characterization methods, improve the efficiency and accuracy of microstructure characterization, and the previous SEM observation can also select the best sample for TEM analysis.

[0035] (5) The present application has the advantages of high sample preparation efficiency, high sample quality, and suitability for nanoscale second phase research in nickel-based superalloys, and the electrolytic double spray corrosion solution used does not contain Cr ions, which is environmentally friendly. BRIEF DESCRIPTION OF DRAWINGS

[0036] Figure 1 Scanning electron microscope images of the SEM sample prepared in Example 1 of the present application at different magnifications;

[0037] Figure 2 Transmission electron microscope images of the TEM sample prepared in Example 1 of the present application at different magnifications;

[0038] Figure 3 Nanoscale oxide distribution and morphology images of the TEM sample in Example 1 of the present application at different magnifications;

[0039] Figure 4 Carbide aggregation distribution and morphology images of the TEM sample in Example 1 of the present application.

[0040] Figure 5 Large-size carbide phase and grain boundary phase shedding observed by SEM in Comparative Example 1.

[0041] Figure 6 Overall microstructure morphology observed by SEM in Comparative Example 1.

[0042] Figure 7 a is an optical microscope photograph after thinning and perforation in Comparative Example 2, Figure 7 b is Figure 7 a is a light transmission photograph, and many small holes of different sizes can be seen in addition to the central hole, Figure 7 c is Figure 7 a is an SEM photograph near the central hole. DETAILED DESCRIPTION

[0043] The application will be described in further detail below with reference to the embodiments and drawings, but the embodiments of the application are not limited thereto.

[0044] Example 1

[0045] In this example, the FGH96 nickel-based superalloy sample is taken as an example, and the main elements of the sample are shown in the following table according to the China Aviation Materials Manual.

[0046] Table 1 Element composition of FGH96 nickel-based superalloy

[0047]

[0048] The specific steps are as follows:

[0049] (1) After taking the sample at a suitable position, a 200-350 μm metal sheet is cut by using an electric spark wire cutting machine as a pretreatment step, and the sheet is polished on both sides by using 400#, 800#, 100#, 1200# and 2000# diamond sandpaper to reduce the thickness to below 100 μm, and then polished on both sides;

[0050] (2) The sample after step (1) is placed in anhydrous ethanol and cleaned by an ultrasonic cleaner for 3 times. The sheet sample is punched into a round sheet by a punching machine, and the burrs around the round sheet are polished off by using 2000# diamond sandpaper; (3) A solution of 20% concentrated sulfuric acid and 80% methanol is selected as the electrolyte;

[0051] (4) The round sheet sample after step (2) is electrolytic double-sprayed thinned, and the solution in (3) is selected as the electrolyte. The voltage of the electrolytic double-sprayed instrument is 15V, and liquid nitrogen is used for cooling during the working process. The working temperature is controlled between -20 and -10℃. After the round sheet sample is electrolyzed for about 300s, it is cleaned by an ultrasonic cleaner for 3 times in anhydrous ethanol. Finally, the surface residual ethanol is blown dry by using an ear cleaning ball;

[0052] (5) The sample is observed by using a scanning electron microscope. The acceleration voltage of the scanning electron microscope is preferably set to 3kV-15kV. The scanning electron microscope observation mode is selected as a backscattering mode. 20 photos of 3000-10000 times are taken, and the plane pixel area is counted by using Image J and other image analysis software. After the observation is completed, the sample can be further thinned and perforated as a TEM observation sample according to the same method;

[0053]

[0054] ​(6) The wafer sample screened in (5) is further electrolytic double-spray thinned to a suitable perforation of the sample, and finally placed under a transmission electron microscope for observation to characterize the phase structure, element distribution and composition, and interface of the nanoscale second phase and non-metallic inclusions.

[0055] By Figure 1 As can be seen, the γ' phase shedding can be observed to leave marks: irregular pits near the grain boundaries are marks of primary γ' phase shedding, and the white particles in the pits are carbide / oxide phases; the smaller square and butterfly-shaped pits in the grains are marks of secondary γ' phase shedding; and the point-like marks below 100 nanometers are marks of primary γ' phase shedding. It can be seen that the γ' phase marks are very clear, with clear edges, and information such as shape, size, and quantity can be intuitively distinguished. The nanoscale carbide / oxide phases are completely retained, and information such as distribution, size, and element distribution can be clearly observed and analyzed. This is conducive to the study of carbide / oxide phases in the alloy.

[0056] By Figure 2 Secondary γ' phase observed under TEM.

[0057] By Figure 3 The nanoscale oxide phase that can be observed has clear boundaries and well-preserved phase interfaces.

[0058] By Figure 4 The carbide phases that can be observed are aggregated and distributed at the grain boundaries. These carbide phases are completely retained, and information such as distribution characteristics, quantity, and size can be clearly observed and studied, and the phase interfaces are clear. It can be found that the carbide phases are located inside the γ' phase and can serve as sites for the growth of γ' phase.

[0059] Comparative Example 1

[0060] The difference between this example and Example 1 is that a 15% nitric acid and 85% alcohol electrolyte is used for electrolytic double-spray etching, and the electrolysis conditions are a voltage of 12V and a temperature of -35°C.

[0061] By Figure 5 , 6 As can be seen, the grain boundary phase shedding is severe, and the phase composition cannot be determined; the carbide phase shedding occurs, and small-size carbide phases cannot be observed, and the phase structure and element composition cannot be determined, only large-size carbide phases that are embedded deeper can be observed. The matrix corrosion degree is not enough, the γ' phase density in the material is high, the γ' phase particles overlap each other, which interferes with the contrast, and it is difficult to distinguish the details, which is not conducive to observation and statistics.

[0062] Comparative Example 2

[0063] The difference between this example and Example 1 is that the voltage is 25V.

[0064] By Figure 7 It can be seen that when the voltage is 25V, the corrosion rate is too fast, the central perforation occurs in some areas, and the thin area has multiple small holes, resulting in light transmission, which is not conducive to TEM observation.

[0065] The above examples are the preferred embodiments of the present application, but the embodiments of the present application are not limited to the above examples, and any changes, modifications, substitutions, combinations, simplifications made without departing from the spirit and principles of the present application should be equivalent replacement methods, and are included in the protection scope of the present application.

Claims

1. A method for analyzing a second phase in a nickel-based superalloy by electrolytic double jet specimen preparation, characterized in that It comprises the following steps: (1) obtaining a sheet with a thickness of 50-100 μm from a nickel-based superalloy after pretreatment; (2) electrochemically corroding the sheet of step (1) by using an electrolytic double spraying method to realize passivation protection of the matrix phase and corrosion dissolution of the γ' phase until a SEM observation sample is obtained; The electrochemical corrosion conditions are as follows: the working voltage is 15-20 V; the working temperature is controlled between-20℃ and-10℃; the electrolyte is 15-25% concentrated sulfuric acid and 75-85% methanol in terms of volume percentage.

2. The method of claim 1, wherein, The electrolytic double spraying method of step (2) is to fix the sheet of step (1) on the anode, place the cathode in the electrolytic tank, spray the sheet with the electrolyte through the electrolytic double spraying instrument, and cause the sheet to be electrolytically dissolved through an external potential.

3. The method of claim 1, wherein, The sheet of step (1) is also double-sided polished; the corrosion time of step (2) is controlled as follows: first, corrode for 200±50 s, take out, clean with anhydrous ethanol, and then observe; if the corrosion degree is not enough, corrode again every 60±30 s and observe.

4. The method according to any one of claims 1 to 3, characterized in that, On the basis of the SEM observation sample, continue to electrochemically corrode in the same way until a suitable small hole appears in the middle of the sample to obtain a TEM observation sample.

5. The SEM observation sample prepared by the method of any one of claims 1-3.

6. The TEM observation sample prepared by the method of claim 4.

7. Application of the SEM observation sample of claim 5 or the TEM observation sample of claim 6 to analysis of the second phase in a nickel-based superalloy.

8. Use according to claim 7, characterized in that, The SEM observation sample or the TEM observation sample is used to analyze the γ' phase, carbide, oxide phase, and non-metallic inclusions.

9. Use according to claim 8, characterized in that, Place the SEM observation sample under a scanning electron microscope, observe the morphology of the γ' phase according to the γ' phase imprint, and take photos.

10. Use according to claim 9, characterized in that, Statistical analysis of the size and quantity of the γ' phase: take 20 photos at 3000-10000 times, and complete the statistical analysis of the planar pixel area of the photos through Image J image analysis software.

Images