Etching liquid for displaying three-dimensional shape of nickel-based metal material and display method

Abstract

The invention discloses an etching solution for displaying the three-dimensional shape of a nickel-based metal material and a display method, and relates to the technical field of etching solutions, the key points of the technical scheme are that the etching solution for displaying the three-dimensional shape of the nickel-based metal material is formed by compounding an alcohol solvent, inorganic strong acid and an organic complexing agent; wherein the organic complexing agent accounts for 1%-3% of the total weight of the etching liquid. The special etching solution formula designed by the invention has a good dissolution rate on a nickel-based superalloy matrix, has excellent protectiveness on various types of carbides by virtue of the synergistic effect of the organic complexing agent (tartaric acid), and is suitable for wide nickel-based superalloy systems.

Description

Technical Field

[0001] This invention relates to the field of etching solution technology, and more specifically, to an etching solution and method for displaying the three-dimensional morphology of nickel-based metal materials. Background Technology

[0002] Nickel-based superalloys are core materials for critical hot-end components in aerospace, energy, and other fields. Their excellent high-temperature strength, creep resistance, and microstructural stability depend not only on coherent reinforcing phases such as γ′ / γ, but also on carbides (such as primary MC and secondary M). 23 C6, M6C, and other carbides also play a crucial role. These carbides precipitate at grain boundaries or within grains, effectively improving the high-temperature mechanical properties and creep rupture life of the alloy through second-phase strengthening and grain boundary pinning effects. However, improper control of the type, size, distribution, and three-dimensional morphology of carbides can also become a source of microcrack initiation and propagation, leading to premature material failure. Therefore, accurately characterizing the true three-dimensional morphology of carbides is a key prerequisite for optimizing alloy design, heat treatment processes, and assessing their service reliability.

[0003] Carbides remain an important reinforcing phase in polycrystalline cast superalloys, exhibiting various morphologies such as granules, strips, blocks, and plates, which are more complex than those of the γ' phase. Furthermore, the γ' phase is primarily composed of Al and Ti, while carbides mainly consist of W, Mo, Cr, and C, showing a significant difference in composition. Existing microstructure display techniques for nickel-based superalloys are mainly optimized for the γ / γ' two-phase system, and their corrosion selectivity and depth are insufficient to address the diverse and chemically inert carbide phases that are widely present in equiaxed cast and directionally solidified superalloys. Moreover, some existing etching solutions typically only achieve shallow surface etching, resulting in an approximately two-dimensional relief morphology that cannot fully and three-dimensionally expose carbides deeply embedded in the matrix, making it difficult to observe their spatial connectivity, interface morphology, and three-dimensional spatial distribution characteristics. For example, Chinese invention patent with publication number CN107488855B discloses a metallographic etchant and its application in corroding Sn-based solder alloys. The metallographic etchant is composed of oxalic acid, hydrochloric acid, acetic acid, methanol and water. Oxalic acid mainly reacts with the metal surface and promotes the oxidation reaction of the metal surface, thus accelerating the corrosion process.

[0004] Currently, there is a lack of a mature method for deep and selective etching of various carbide phases in nickel-based superalloys, making it a technical challenge to clearly and non-destructively observe the three-dimensional morphology of carbides under conventional characterization equipment. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide an etching solution and method for displaying the three-dimensional morphology of nickel-based metal materials. The etching solution is composed of an alcohol solvent, an inorganic strong acid, and an organic complexing agent. Through optimized electrolysis process parameters, selective and deep dissolution of the substrate is achieved, thereby highlighting the carbides in three dimensions.

[0006] The above-mentioned technical objective of the present invention is achieved through the following technical solution:

[0007] In one aspect, an etching solution for displaying the three-dimensional morphology of nickel-based metal materials is provided, which is composed of an alcohol solvent, an inorganic strong acid and an organic complexing agent.

[0008] The organic complexing agent accounts for 1% to 3% of the total weight of the etching solution.

[0009] Preferably, the alcohol solvent is at least one of methanol, ethanol, isopropanol, or ethylene glycol.

[0010] Preferably, the inorganic strong acid is at least one of hydrochloric acid, sulfuric acid, nitric acid, or phosphoric acid.

[0011] Preferably, the alcohol solvent is methanol, the inorganic strong acid is hydrochloric acid, and the volume ratio of methanol to hydrochloric acid is 7:1 to 9:1.

[0012] Preferably, the organic complexing agent is tartaric acid.

[0013] Secondly, a method for displaying the three-dimensional morphology of nickel-based metal materials is provided, comprising the following steps:

[0014] a) Pretreatment of nickel-based superalloy samples, including grinding and polishing to a mirror finish;

[0015] b) Electrolytic etching is performed using the etching solution described in any one of the first aspects, wherein the electrolysis voltage is 3 to 7 V and the electrolysis time is 20 to 60 minutes;

[0016] c) Post-process the electrolyzed sample, including cleaning and drying;

[0017] d) Observe the three-dimensional morphology of the carbides using a scanning electron microscope.

[0018] Preferably, in the electrolytic erosion, the electrolysis voltage is preferably 5V, and the electrolysis time is preferably 30-45 minutes.

[0019] Preferably, the macroscopic criterion for the endpoint of the electrolytic erosion is determined as follows: the sample surface turns grayish-black and shiny particles appear.

[0020] Preferably, the carbide includes MC type, M type 23At least one of C6 type, M6C type or TCP phase carbides.

[0021] Preferably, the nickel-based superalloy is an equiaxed crystal cast superalloy or a directionally solidified superalloy.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] 1. The special etching solution formula designed in this invention has a good dissolution rate for nickel-based superalloy substrates. Simultaneously, thanks to the synergistic effect of the organic complexing agent (tartaric acid), it effectively dissolves various types of carbides (MC, M...). 23 C6 and M6C offer excellent protection and are suitable for a wide range of nickel-based superalloy systems.

[0024] 2. This invention selectively dissolves the matrix to completely "peel out" the carbides that were originally buried inside, allowing direct observation of their three-dimensional morphology under SEM. This completely solves the problem of missing information in two-dimensional cross-sectional observation and achieves true three-dimensional morphology display.

[0025] 3. The macroscopic endpoint criteria (surface blackening, appearance of bright particles) provided by this invention are intuitive and reliable, reducing the reliance on operator experience and facilitating their application in industrial testing.

[0026] 4. This invention provides a direct and reliable characterization method for accurately evaluating the morphology, size, distribution, and evolution of carbides during heat treatment and service, and plays an important supporting role in optimizing alloy composition design, heat treatment processes, and material failure analysis. Attached Figure Description

[0027] The accompanying drawings, which are included to provide a further understanding of embodiments of the invention and form part of this application, do not constitute a limitation thereof. In the drawings:

[0028] Figure 1 This is a SEM image of the M6C carbide morphology of the nickel-based equiaxed crystal cast high-temperature alloy K465 in Embodiment 3 of the present invention. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the embodiments and accompanying drawings. The illustrative embodiments and descriptions of the present invention are only used to explain the present invention and are not intended to limit the present invention.

[0030] Example 1: An etching solution for displaying the three-dimensional morphology of nickel-based metal materials.

[0031] Existing research on microstructure display techniques for nickel-based superalloys largely focuses on revealing the morphology of the γ′ phase in single-crystal superalloys. A typical method involves preparing a specific etching solution (common components include hydrochloric acid, phosphoric acid, nitric acid, perchloric acid, and ferric chloride) and electrolytically or chemically etching the polished surface to preferentially dissolve the γ matrix, causing the cubic γ′ strengthening phase to protrude, thus enabling clear observation of the size, morphology, and distribution of the γ′ phase.

[0032] The etching solution for displaying nickel-based metal materials is formulated from an alcohol solvent, an inorganic strong acid, and an organic complexing agent; wherein the organic complexing agent accounts for 1% to 3% of the total weight of the etching solution, and the volume ratio of the alcohol solvent to the inorganic strong acid is 7:1 to 9:1.

[0033] The role of alcohol solvents is to provide proton solvents, regulate ionization, and slow down the reaction rate to obtain a smooth etching surface. Solvents that can be used in combination or as alternatives include one or more of methanol, ethanol, isopropanol, and ethylene glycol. As low-molecular-weight alcohols, they can adjust the polarity and volatility of the solution, making them suitable for specific alloys or operating environments sensitive to methanol toxicity. For nickel-based metal materials, methanol is the preferred alcohol solvent.

[0034] Inorganic strong acids are highly corrosive to nickel-based alloys. Acids that can be partially substituted or combined include hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, or mixtures thereof. Different acid anions have varying complexing abilities and corrosive properties, potentially producing differentiated corrosion / protection effects on specific types of carbides.

[0035] For example, for MC types rich in Ta and Nb, SO4 can be selected. 2- NO3 - PO4 3- Isoacid anions. For nickel-based metal materials, hydrochloric acid is preferred as the strong inorganic acid, providing H+. + and Cl - Cl - It has a strong corrosive effect on nickel-based alloys.

[0036] The role of organic complexing agents is to complex dissolved metal ions, preventing their redeposition and contamination of the carbide surface, and optimizing the uniformity of corrosion. In this invention, to effectively complex dissolved matrix metal ions, prevent their redeposition, and inhibit secondary corrosion of the carbide phase, achieving the desired effect of dissolving the matrix and preserving the carbide, tartaric acid is preferably used as the organic complexing agent for nickel-based metal materials. Specifically, tartaric acid can reduce the dissolution effect of the electrolyte on the carbide surface, ensuring the carbide remains intact even during long-term electrolytic corrosion. The complexing ability and selectivity vary for different metal ions, and optimization can be performed for high-temperature alloys with different matrix compositions. For example, for Cr, Co, and Mo, citric acid, ascorbic acid, and ethylenediaminetetraacetic acid can be selected.

[0037] It should be noted that this invention differs from GH series nickel-based superalloys such as GH3536, GH3230, and GH4169 in that: GH series nickel-based superalloys achieve microstructural observation by etching away the γ' phase, with the γ' phase volume in GH series alloys being 10%–15%. In contrast, this invention targets cast alloys where the γ' phase volume is 75%–85%, and this invention involves long-term, deep etching, requiring the complete removal of the surface γ' phase.

[0038] Example 2: A method for displaying the three-dimensional morphology of a nickel-based metal material

[0039] Conventional metallographic observation methods (such as optical microscopy and scanning electron microscopy) can typically only obtain information about the surface of polished or lightly etched samples, presenting a two-dimensional cross-sectional projection of the carbide morphology. The true three-dimensional morphology of carbides—massive, needle-like, skeletal, or lamellar—is obscured by the matrix and difficult to fully reveal. This limitation of two-dimensional morphology prevents researchers from accurately distinguishing, for example, the spatial distribution differences between needle-like and lamellar carbides, thus severely affecting the accurate assessment of carbide growth mechanisms, interactions, and their impact on properties (such as strengthening and embrittlement effects).

[0040] The method for displaying the three-dimensional morphology of nickel-based metal materials according to the present invention is implemented by the following steps.

[0041] Step 1: Sample Pretreatment

[0042] From the nickel-based superalloy ingot or component to be tested, avoiding the surface oxidation and contamination areas, a representative internal sample is taken. The sample undergoes standard metallographic preparation: sequentially using coarse grinding, fine grinding, and polishing to a mirror finish, ensuring the observation surface is free of scratches.

[0043] Step 2: Electrolytic deep etching treatment

[0044] 1) Preparation of etching solution: Prepare a mixture of methanol (CH3OH): hydrochloric acid (HCl) in a volume ratio of 7:1 to 9:1 as the base electrolyte, and then add tartaric acid (C4H6O6) accounting for 1% to 3% of the total weight of the electrolyte and stir evenly.

[0045] 2) Set electrolysis parameters: Use a DC regulated power supply, set the voltage to 3-7 V, preferably 5 V. Use the polished sample as the anode and the stainless steel sheet as the cathode, immerse them in the above etching solution, and keep a certain distance between the two electrodes to avoid short circuit.

[0046] Below 3V, the voltage is too low, which will result in poor erosion effect and make it difficult to achieve three-dimensional morphology; above 7V, the voltage is too high, which will result in too fast erosion speed and strong corrosion ability, causing the carbides to be corroded away.

[0047] 3) Perform electrolytic etching: Apply electricity at room temperature for 20–60 minutes, preferably 30–45 minutes. The macroscopic endpoint of the process is determined when the sample surface changes from a metallic luster to a uniform grayish-black color, and when the sample is slowly rotated under light, numerous fine, shiny particles are observed to stand out on the surface. This phenomenon indicates that the matrix has been effectively dissolved, and the corrosion-resistant carbide phase has been exposed in three dimensions.

[0048] Step 3: Post-processing and observation

[0049] After electrolysis, the sample was immediately removed and ultrasonically cleaned with anhydrous ethanol to remove residual reactants from the surface, then dried with cold air. The treated sample was then observed under a field emission scanning electron microscope (FE-SEM). Backscattered electron (BSE) mode was preferred for observation. Because the carbides are rich in heavy elements such as W, Ta, Nb, and Hf, they exhibit a bright white contrast in BSE mode, forming a striking contrast with the dark, eroded, and recessed matrix areas. This allows for extremely clear visualization of the three-dimensional morphology, spatial distribution, and interconnections of the carbides. Secondary electron (SE) mode can be used in conjunction with BSE mode to observe the γ′ phase morphology, and energy dispersive X-ray spectroscopy (EDS) can be used for qualitative compositional analysis of the exposed carbides.

[0050] In some optional examples, there is a constant voltage (5V) electrolysis mode, which can be extended to precisely controlled “potential-constant electrolysis” or “current-constant electrolysis”, by setting more precise electrochemical parameters to control the selectivity and rate of erosion, with better reproducibility, such as the potential relative to the reference electrode.

[0051] In some alternative examples, using pulsed current or pulsed voltage through an "on-off" cycle may be more conducive to the diffusion and removal of corrosion products, resulting in a cleaner carbide surface, especially for deep pores or complex three-dimensional structures.

[0052] This invention uses a relatively low voltage of 3-7 V combined with an electrolysis time of 20-60 minutes to achieve uniform and controllable deep erosion, rather than a violent surface reaction, thus ensuring the integrity of the three-dimensional morphology.

[0053] Example 3: Taking K465 nickel-based superalloy as an example.

[0054] 1) Sampling: Metallographic samples were taken from the center of the K465 nickel-based superalloy ingot.

[0055] 2) Pretreatment: Polished to a mirror finish using standard metallographic grinding.

[0056] 3) Prepare etching solution: Measure 90 mL of methanol and 10 mL of concentrated hydrochloric acid, then add 1 g of tartaric acid and stir until completely dissolved.

[0057] 4) Electrolytic etching: Using the sample as the anode and a stainless steel sheet as the cathode, electrolyze for 30 minutes at a DC voltage of 5 V. Stop the process when the sample surface turns black and a large number of shiny particles appear.

[0058] 5) Post-processing and observation: After ultrasonic cleaning and drying with anhydrous ethanol, the samples were observed under a ZEISS SUPRA 55 scanning electron microscope. The results are as follows: Figure 1 As shown, the carbides exhibit bright contrast, and their spatial three-dimensional morphology (such as blocky and skeletal features) is clearly visible, fully demonstrating the excellent effect of the method of the present invention in revealing the true three-dimensional structure of carbides. The three-dimensional morphology of M6C carbides is perfectly presented.

[0059] This invention establishes an endpoint judgment standard of "surface turning black and the appearance of shiny particles", making the method easy to master and repeat.

[0060] This invention achieves highly selective deep etching, that is, rapidly dissolving the matrix (γ phase) and γ′ phase, while minimizing damage to the chemically stable carbide phase. It breaks through the limitations of two-dimensional characterization and truly and completely exposes the three-dimensional morphology of carbides in nickel-based superalloys.

[0061] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. An etching solution for displaying the three-dimensional morphology of nickel-based metal materials, characterized in that, It is composed of alcohol solvents, inorganic strong acids and organic complexing agents; The organic complexing agent accounts for 1% to 3% of the total weight of the etching solution.

2. The etching solution for displaying the three-dimensional morphology of nickel-based metal materials according to claim 1, characterized in that, The alcohol solvent is at least one of methanol, ethanol, isopropanol, or ethylene glycol.

3. The etching solution for displaying the three-dimensional morphology of nickel-based metal materials according to claim 1, characterized in that, The inorganic strong acid is at least one of hydrochloric acid, sulfuric acid, nitric acid, or phosphoric acid.

4. The etching solution for displaying the three-dimensional morphology of nickel-based metal materials according to claim 1, characterized in that, The alcohol solvent is methanol, the inorganic strong acid is hydrochloric acid, and the volume ratio of methanol to hydrochloric acid is 7:1 to 9:

1.

5. An etching solution for displaying the three-dimensional morphology of nickel-based metal materials according to any one of claims 1-4, characterized in that, The organic complexing agent is tartaric acid.

6. A method for displaying the three-dimensional morphology of a nickel-based metal material, characterized in that, Includes the following steps: a) Pretreatment of nickel-based superalloy samples, including grinding and polishing to a mirror finish; b) Electrolytic etching is performed using the etching solution according to any one of claims 1 to 5, wherein the electrolysis voltage is 3 to 7 V and the electrolysis time is 20 to 60 minutes; c) Post-process the electrolyzed sample, including cleaning and drying; d) Observe the three-dimensional morphology of the carbides using a scanning electron microscope.

7. The method for displaying the three-dimensional morphology of a nickel-based metal material according to claim 6, characterized in that, In the electrolytic erosion process, the preferred electrolysis voltage is 5V, and the preferred electrolysis time is 30-45 minutes.

8. The method for displaying the three-dimensional morphology of a nickel-based metal material according to claim 6, characterized in that, The macroscopic criterion for the endpoint of electrolytic erosion is determined as follows: the sample surface turns grayish-black and shiny particles appear.

9. The method for displaying the three-dimensional morphology of a nickel-based metal material according to claim 6, characterized in that, The carbides include MC type and M type. 23 At least one of C6 type, M6C type or TCP phase carbides.

10. The method for displaying the three-dimensional morphology of a nickel-based metal material according to claim 6, characterized in that, The nickel-based superalloy is an equiaxed crystal cast superalloy or a directionally solidified superalloy.

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