Metallographic detection method of zinc alloy grain structure

Abstract

The invention provides a metallographic detection method for a zinc alloy grain structure. The metallographic detection method comprises the following steps: S1, immersing the zinc alloy into a corrosive agent for surface treatment, and then carrying out first cleaning and drying to obtain the treated zinc alloy; wherein the corrosive agent is a chromic anhydride-sodium sulfate mixed aqueous solution; and S2, carrying out grain structure detection on the treated zinc alloy in a polarized light mode by adopting a polarized light metallographic microscope so as to measure the grain size. The chromic anhydride-sodium sulfate mixed aqueous solution is used as the corrosive agent, and a layer of film can be formed on the surface of the zinc alloy. The interface between the thin film and alloy crystal grains can generate a specific optical effect, and especially under the action of polarized light, the crystal grains with different orientations present different color contrast. According to the method, the contrast between the grains can be remarkably enhanced, the grain boundary is clearer and distinguishable, and subsequent grain size measurement is facilitated.

Description

Technical Field

[0001] This invention relates to the field of metallographic testing technology for metallic materials, and more specifically, to a metallographic testing method for the grain structure of zinc alloys. Background Technology

[0002] Zinc, as the third most widely used non-ferrous metal after aluminum and copper, has a wide range of applications, including construction, infrastructure (power), transportation, home appliances, and apparel and bags. Due to its excellent casting properties, high economic efficiency, and production effectiveness, zinc alloys are primarily used in the as-cast state for various applications. Furthermore, because the solubility of various alloying elements in zinc alloys is very limited, they mainly exist as precipitates within the matrix of the as-cast zinc alloy.

[0003] Grain size analysis in as-cast zinc alloys helps reveal the correlation between the preparation process and microstructure, optimize production processes, and evaluate the mechanical properties of zinc alloys, guiding material applications and ultimately controlling product quality and ensuring the reliability of zinc alloys. The core purpose of zinc alloy grain size analysis is to establish the correlation between "process-structure-performance" through microstructural quantification, ultimately achieving the goals of process optimization, performance assurance, quality control, and new material development. It is both a quality monitoring method in the production process and a key tool for revealing microscopic mechanisms in materials science research, and is of great significance for the safe application of zinc alloys in the automotive, electronics, and machinery industries. However, current microstructural observation of zinc alloys mainly relies on metallographic methods or scanning electron microscopy to analyze the distribution and size of precipitated phases, with very limited methods for analyzing the orientation and grain size of the zinc matrix. Summary of the Invention

[0004] The main objective of this invention is to provide a metallographic detection method for the grain structure of zinc alloys, so as to solve the problem that it is difficult to accurately measure the grain size in zinc alloys in the prior art.

[0005] To achieve the above objectives, according to one aspect of the present invention, a metallographic inspection method for the grain structure of a zinc alloy is provided. The metallographic inspection method includes: step S1, immersing the zinc alloy in an etchant for surface treatment, followed by a first cleaning and drying to obtain the treated zinc alloy; wherein the etchant is a mixed aqueous solution of chromic anhydride and sodium sulfate; and step S2, using a polarized light metallographic microscope in polarized light mode to inspect the grain structure of the treated zinc alloy to measure the grain size.

[0006] Furthermore, the concentration of chromic anhydride in the corrosive agent is 62.5~500 g / L; and / or, the concentration of sodium sulfate in the corrosive agent is 6.25~50 g / L.

[0007] Furthermore, the concentration of chromic anhydride in the corrosive agent is 150~250 g / L; and / or, the concentration of sodium sulfate in the corrosive agent is 8~16 g / L.

[0008] Furthermore, the mass ratio of chromium anhydride to sodium sulfate in the corrosive agent is (11~30):1.

[0009] Furthermore, the surface treatment time is 3~10 seconds.

[0010] Furthermore, the magnification of the polarized metallurgical microscope is 100~400X.

[0011] Furthermore, the zinc alloy is selected from any one or more of zinc-aluminum alloys, zinc-copper alloys, zinc-aluminum-magnesium alloys, zinc-aluminum-copper-magnesium alloys, and zinc-copper-titanium alloys.

[0012] Furthermore, the cleaning agent used in the first cleaning is water.

[0013] Furthermore, the first cleaning time should be no less than 30 seconds.

[0014] Further, in step S1, the zinc alloy is polished, cleaned and dried in sequence, and then immersed in an etchant for surface treatment.

[0015] Applying the technical solution of this invention, this application uses a mixed aqueous solution of chromic anhydride and sodium sulfate as an etchant to form a thin film on the surface of a zinc alloy. The interface between this thin film and the alloy grains produces specific optical effects, especially under the influence of polarized light, causing grains with different orientations to exhibit different color contrasts. Unlike the principle of nitric acid-ethanol solution generating microstructure contrast imaging by dissolving the matrix or second phase, this method utilizes the interface effect formed between the etchant coating and the sample surface: when polarized light passes through the interface between the film and the sample, it undergoes additional reflection, refraction, or phase changes due to the difference in refractive index, enhancing the degree of polarization state change and thus improving image contrast. This method achieves zinc alloy grain contrast imaging, significantly enhancing the contrast between grains, making grain boundaries clearer and more discernible, facilitating subsequent grain size measurement. Furthermore, the coated film can reduce diffuse reflection from the sample surface through anti-reflection effects, reducing light reflection loss at the interface and allowing more polarized light to interact with the sample along the expected path (such as entering the sample interior and undergoing birefringence), thereby reducing stray light interference and improving the image signal-to-noise ratio. The use of chemical etching to replace the complex electrolytic anodic coating process greatly simplifies sample pretreatment steps, reduces operational difficulty, and improves detection efficiency. This method uses a mixed aqueous solution of chromic anhydride and sodium sulfate, avoiding the direct use of strong acids and reducing potential safety risks during experiments compared to traditional etching solutions such as nitric acid and ethanol. When used with a polarized metallurgical microscope, no additional physical or chemical treatment to enhance contrast is required, further reducing the danger and complexity of sample handling. Attached Figure Description

[0016] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0017] Figure 1 Metallographic images of the zinc alloy microstructure in Embodiment 1 of this application are shown;

[0018] Figure 2 Metallographic images of the zinc alloy microstructure in Embodiment 2 of this application are shown;

[0019] Figure 3 Metallographic images of the zinc alloy microstructure in Embodiment 3 of this application are shown;

[0020] Figure 4 Metallographic images of the zinc alloy microstructure in Comparative Example 1 of this application are shown;

[0021] Figure 5 Metallographic images of the zinc alloy microstructure at various levels are shown. Detailed Implementation

[0022] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0023] As analyzed in the background section of this application, the existing technology has the problem of difficulty in accurately measuring the grain size in zinc alloys. In order to solve the above problem, this application provides a metallographic detection method for the grain structure of zinc alloys.

[0024] In a typical embodiment of this application, a metallographic detection method for the grain structure of a zinc alloy is provided. The metallographic detection method includes: step S1, immersing the zinc alloy in an etchant for surface treatment, followed by a first cleaning and drying to obtain the treated zinc alloy; wherein the etchant is a mixed aqueous solution of chromic anhydride and sodium sulfate; step S2, using a polarized light metallographic microscope in polarized light mode to detect the grain structure of the treated zinc alloy to measure the grain size.

[0025] This application uses a mixed aqueous solution of chromic anhydride and sodium sulfate as an etchant to form a thin film on the zinc alloy surface. The interface between this film and the alloy grains produces specific optical effects, especially under polarized light, causing grains with different orientations to exhibit different color contrasts. Unlike the principle of nitric acid-ethanol solution generating microstructure contrast imaging by dissolving the matrix or second phase, this method utilizes the interface effect formed between the etchant coating and the sample surface: when polarized light passes through the interface between the film and the sample, it undergoes additional reflection, refraction, or phase changes due to the difference in refractive index, enhancing the degree of polarization state change and thus improving image contrast. This method achieves zinc alloy grain contrast imaging, significantly enhancing the contrast between grains and making grain boundaries clearer and more discernible, facilitating subsequent grain size measurement. Furthermore, the coated film reduces diffuse reflection from the sample surface through an anti-reflection effect, reducing light reflection loss at the interface and allowing more polarized light to interact with the sample along the expected path (such as entering the sample interior and undergoing birefringence), thereby reducing stray light interference and improving the image signal-to-noise ratio. The use of chemical etching to replace the complex electrolytic anodic coating process greatly simplifies sample pretreatment steps, reduces operational difficulty, and improves detection efficiency. This method uses a mixed aqueous solution of chromic anhydride and sodium sulfate, avoiding the direct use of strong acids and reducing potential safety risks during experiments compared to traditional etching solutions such as nitric acid and ethanol. When used with a polarized metallurgical microscope, no additional physical or chemical treatment to enhance contrast is required, further reducing the danger and complexity of sample handling.

[0026] In some embodiments of this application, the concentration of chromic anhydride in the etchant is 62.5~500 g / L; and / or, the concentration of sodium sulfate in the etchant is 6.25~50 g / L.

[0027] Controlling the concentrations of chromic anhydride and sodium sulfate in the etchant within the aforementioned range helps the coating to cover the zinc alloy surface more evenly and produce a more significant difference in refractive index at the interfaces of grains with different orientations. This makes the grain boundaries clearer and significantly improves the contrast between grains, which helps to more accurately identify and measure the grain size in the zinc alloy.

[0028] To further improve the contrast between grains, in some embodiments of this application, the concentration of chromic anhydride in the etchant is 150~250 g / L; and / or, the concentration of sodium sulfate in the etchant is 8~16 g / L.

[0029] In some embodiments of this application, the mass ratio of chromium anhydride to sodium sulfate in the etchant is (1.25~80):1.

[0030] Chromium anhydride, as a strong oxidizing agent, can react with the zinc alloy surface to form an oxide film; while sodium sulfate helps to adjust the pH and stability of the solution, promoting the uniform growth of the oxide film. Controlling the mass ratio of chromium anhydride to sodium sulfate in the etchant within the above-mentioned range helps to form a more uniform and stable coating, improves the consistency of grain contrast, and thus helps to improve the accuracy of detection.

[0031] To further improve the uniformity of the coating and thus further improve the accuracy of the detection, in some embodiments of this application, the mass ratio of chromic anhydride to sodium sulfate in the etchant is (11~30):1, specifically 11:1, 15:1, 20:1, 25:1, 30:1, and any range between two ratios.

[0032] Chromium anhydride, sodium sulfate and water are added to a beaker in sequence, stirred evenly and allowed to stand to obtain a corrosive agent.

[0033] In some embodiments of this application, the surface treatment time is 3 to 10 seconds.

[0034] Controlling the surface treatment time within the above range helps to improve the clarity and integrity of grain boundary contrast while avoiding excessive corrosion and preserving the original morphology of zinc alloy grains.

[0035] In some embodiments of this application, the magnification of the polarized metallurgical microscope is 100~400X.

[0036] In polarized light mode, the microscope can enhance the contrast of grain structure by utilizing the differences in the direction of polarization of light as it passes through the sample. Controlling the magnification of the polarized metallurgical microscope within the aforementioned range helps to clearly observe the detailed features of individual grains while displaying the overall grain distribution, thus improving the accuracy of grain size measurement.

[0037] This testing method is applicable to various types of zinc alloys, including but not limited to, in some embodiments of this application, zinc alloys selected from any one or more of zinc-aluminum alloys, zinc-copper alloys, zinc-aluminum-magnesium alloys, zinc-aluminum-copper-magnesium alloys, and zinc-copper-titanium alloys.

[0038] To effectively remove residual corrosive agents without introducing new impurities, in some embodiments of this application, the cleaning agent used for the first cleaning is water.

[0039] In some embodiments of this application, the first cleaning time is not less than 30 seconds, preferably 30 to 60 seconds.

[0040] Controlling the time of the first cleaning within the above range helps to improve detection efficiency while thoroughly cleaning away residual corrosive agents.

[0041] In some embodiments of this application, in step S1, the zinc alloy is polished, cleaned and dried in sequence, and then immersed in an etchant for surface treatment.

[0042] The polishing step aims to remove machining marks, oxide scale, and micro-defects from the sample surface, creating a flat, smooth surface. A flat surface helps the etchant to cover the sample evenly, ensuring the consistency and stability of the grain structure and avoiding problems such as uneven localized etching and unclear grain display caused by surface roughness.

[0043] The beneficial effects of this application will be further illustrated below with reference to the embodiments.

[0044] Example 1

[0045] The grain structure of the zinc-copper-titanium alloy was analyzed. The alloy, by mass percentage, comprises 1% Cu, 0.2% Ti, and the balance Zn. The specific method was as follows: 1. The zinc-copper-titanium alloy sample was prepared and polished using standard sample preparation procedures; 2. The polished sample underwent a second cleaning and surface drying until no water stains remained; 3. The polished surface of the sample was completely immersed in an etchant for 3 seconds. The etchant was a mixed aqueous solution of chromic anhydride and sodium sulfate, with a chromic anhydride concentration of 200 g / L and a sodium sulfate concentration of 16 g / L, and a mass ratio of 12.5:1. After a first cleaning with deionized water for 1 minute, the sample was dried until no residual water stains remained. The dried sample was placed on the stage of a polarized light metallurgical microscope, and the grain structure was observed under polarized light mode at a magnification of 100X. Figure 1 As shown, the obtained metallographic structure reveals that different grains of the alloy exhibit different color contrasts.

[0046] Example 2

[0047] The grain structure of the zinc-aluminum-magnesium alloy was analyzed. The alloy, by mass percentage, comprises 1% Al, 0.1% Mg, and the balance Zn. The specific method was as follows: 1. The zinc-aluminum-magnesium alloy sample was prepared and polished using standard sample preparation procedures; 2. The polished sample underwent a second cleaning and surface drying until no water stains remained; 3. The polished surface of the sample was completely immersed in an etchant for 5 seconds. The etchant was a mixed aqueous solution of chromic anhydride and sodium sulfate, with a chromic anhydride concentration of 150 g / L and a sodium sulfate concentration of 12.5 g / L, and a mass ratio of 12:1. After a first cleaning with deionized water for 1 minute, the sample was dried until no residual water stains remained. The dried sample was placed on the stage of a polarized light metallurgical microscope, and the grain structure was observed under polarized light mode at a magnification of 100X. Figure 2 As shown, the obtained metallographic structure reveals that different grains of the alloy exhibit different color contrasts.

[0048] Example 3

[0049] The grain structure of the zinc-copper-titanium alloy was analyzed. By mass percentage, the zinc-copper-titanium alloy contains 1% Cu, 0.1% Ti, 0.05% Cr, and the balance Zn. The specific method was as follows: 1. The zinc-copper-titanium alloy sample was prepared and polished using standard sample preparation procedures; 2. The polished sample underwent a second cleaning and surface drying until no water stains remained; 3. The polished surface of the sample was completely immersed in an etchant for 8 seconds. The etchant was a mixed aqueous solution of chromic anhydride and sodium sulfate, with a chromic anhydride concentration of 250 g / L and a sodium sulfate concentration of 8 g / L, and a mass ratio of 30:1. After a first cleaning with deionized water for 1 minute, the sample was dried until no residual water stains remained. The dried sample was placed on the stage of a polarized light metallurgical microscope, and the grain structure was observed under polarized light mode at a magnification of 100X. Figure 3 As shown, the obtained metallographic structure reveals that different grains of the alloy exhibit different color contrasts.

[0050] Example 4

[0051] The difference from Example 1 is that the concentration of chromium anhydride in the corrosive agent is 180 g / L, and the mass ratio of chromium anhydride to sodium sulfate is 11:1.

[0052] The grain structure of the zinc-copper-titanium alloy was analyzed. The alloy, by mass percentage, comprises 1% Cu, 0.2% Ti, and the balance Zn. The specific method was as follows: 1. The zinc-copper-titanium alloy sample was prepared and polished using standard sample preparation procedures; 2. The polished sample underwent a second cleaning and surface drying until no water stains remained; 3. The polished surface of the sample was completely immersed in an etchant for 3 seconds. The etchant was a mixed aqueous solution of chromic anhydride and sodium sulfate, with a concentration of 180 g / L for chromic anhydride and 16 g / L for sodium sulfate. After a first cleaning with deionized water for 1 minute, the sample was dried until no residual water stains remained. The dried sample was placed on the stage of a polarized light metallurgical microscope, and the grain structure was observed under polarized light mode at a magnification of 100X.

[0053] Example 5

[0054] The difference from Example 1 is that the concentration of chromium anhydride in the corrosive agent is 150 g / L, and the mass ratio of chromium anhydride to sodium sulfate is 9:1.

[0055] The grain structure of the zinc-copper-titanium alloy was analyzed. The alloy, by mass percentage, comprises 1% Cu, 0.2% Ti, and the balance Zn. The specific method was as follows: 1. The zinc-copper-titanium alloy sample was prepared and polished using standard sample preparation procedures; 2. The polished sample underwent a second cleaning and surface drying until no water stains remained; 3. The polished surface of the sample was completely immersed in an etchant for 3 seconds. The etchant was a mixed aqueous solution of chromic anhydride and sodium sulfate, with a concentration of 150 g / L for chromic anhydride and 16 g / L for sodium sulfate. After a first cleaning with deionized water for 1 minute, the sample was dried until no residual water stains remained. The dried sample was placed on the stage of a polarized light metallurgical microscope, and the grain structure was observed under polarized light mode at a magnification of 100X.

[0056] Example 6

[0057] The difference from Example 1 is that the concentration of chromic anhydride is 500 g / L and the concentration of sodium sulfate is 6.25 g / L;

[0058] The grain structure of the zinc-copper-titanium alloy was analyzed. The alloy, by mass percentage, comprises 1% Cu, 0.2% Ti, and the balance Zn. The specific method was as follows: 1. The zinc-copper-titanium alloy sample was prepared and polished using standard sample preparation procedures; 2. The polished sample underwent a second cleaning and surface drying until no water stains remained; 3. The polished surface of the sample was completely immersed in an etchant for 3 seconds. The etchant was a mixed aqueous solution of chromic anhydride and sodium sulfate, with a concentration of 500 g / L for chromic anhydride and 6.25 g / L for sodium sulfate. After a first cleaning with deionized water for 1 minute, the sample was dried until no residual water stains remained. The dried sample was placed on the stage of a polarized light metallurgical microscope, and the grain structure was observed under polarized light mode at a magnification of 100X.

[0059] Example 7

[0060] The difference from Example 1 is that the concentration of chromic anhydride is 62.5 g / L and the concentration of sodium sulfate is 50 g / L;

[0061] The grain structure of the zinc-copper-titanium alloy was analyzed. The alloy, by mass percentage, comprises 1% Cu, 0.2% Ti, and the balance Zn. The specific method was as follows: 1. The zinc-copper-titanium alloy sample was prepared and polished using standard sample preparation procedures; 2. The polished sample underwent a second cleaning and surface drying until no water stains remained; 3. The polished surface of the sample was completely immersed in an etchant for 3 seconds. The etchant was a mixed aqueous solution of chromic anhydride and sodium sulfate, with a concentration of 6.25 g / L for chromic anhydride and 50 g / L for sodium sulfate. After a first cleaning with deionized water for 1 minute, the sample was dried until no residual water stains remained. The dried sample was placed on the stage of a polarized light metallurgical microscope, and the grain structure was observed under polarized light mode at a magnification of 100X.

[0062] Example 8

[0063] The difference from Example 1 is that the surface treatment time is 15 seconds;

[0064] The grain structure of the zinc-copper-titanium alloy was analyzed. The alloy, by mass percentage, comprises 1% Cu, 0.2% Ti, and the balance Zn. The specific method was as follows: 1. The zinc-copper-titanium alloy sample was prepared and polished using standard sample preparation procedures; 2. The polished sample underwent a second cleaning and surface drying until no water stains remained; 3. The polished surface of the sample was completely immersed in an etchant for 15 seconds. The etchant was a mixed aqueous solution of chromic anhydride and sodium sulfate, with a chromic anhydride concentration of 200 g / L and a sodium sulfate concentration of 16 g / L, and a mass ratio of 12.5:1. After a first cleaning with deionized water for 1 minute, the sample was dried until no residual water stains remained. The dried sample was placed on the stage of a polarized light metallurgical microscope, and the grain structure was observed under polarized light mode at a magnification of 100X.

[0065] Example 9

[0066] The difference from Example 1 is that the magnification of the polarized metallurgical microscope is 400X;

[0067] The grain structure of the zinc-copper-titanium alloy was analyzed. The alloy, by mass percentage, comprises 1% Cu, 0.2% Ti, and the balance Zn. The specific method was as follows: 1. The zinc-copper-titanium alloy sample was prepared and polished using standard sample preparation procedures; 2. The polished sample underwent a second cleaning and surface drying until no water stains remained; 3. The polished surface of the sample was completely immersed in an etchant for 3 seconds. The etchant was a mixed aqueous solution of chromic anhydride and sodium sulfate, with a chromic anhydride concentration of 200 g / L and a sodium sulfate concentration of 16 g / L, and a mass ratio of 12.5:1. After a first cleaning with deionized water for 1 minute, the sample was dried until no residual water stains remained. The dried sample was placed on the stage of a polarized light metallurgical microscope, and the grain structure was observed under polarized light mode at a magnification of 400X.

[0068] Example 10

[0069] The difference from Example 1 is that the magnification of the polarized metallurgical microscope is 90X;

[0070] The grain structure of the zinc-copper-titanium alloy was analyzed. The alloy, by mass percentage, comprises 1% Cu, 0.2% Ti, and the balance Zn. The specific method was as follows: 1. The zinc-copper-titanium alloy sample was prepared and polished using standard sample preparation procedures; 2. The polished sample underwent a second cleaning and surface drying until no water stains remained; 3. The polished surface of the sample was completely immersed in an etchant for 3 seconds. The etchant was a mixed aqueous solution of chromic anhydride and sodium sulfate, with a chromic anhydride concentration of 200 g / L and a sodium sulfate concentration of 16 g / L, and a mass ratio of 12.5:1. After a first cleaning with deionized water for 1 minute, the sample was dried until no residual water stains remained. The dried sample was placed on the stage of a polarized light metallurgical microscope, and the grain structure was observed under polarized light mode at a magnification of 90X.

[0071] Example 11

[0072] The difference from Example 1 is that the first cleaning time is 30 seconds;

[0073] The grain structure of the zinc-copper-titanium alloy was analyzed. The alloy, by mass percentage, comprises 1% Cu, 0.2% Ti, and the balance Zn. The specific method was as follows: 1. The zinc-copper-titanium alloy sample was prepared and polished using standard sample preparation procedures; 2. The polished sample underwent a second cleaning and surface drying until no water stains remained; 3. The polished surface of the sample was completely immersed in an etchant for 3 seconds, then removed. The etchant was a mixed aqueous solution of chromic anhydride and sodium sulfate, with a chromic anhydride concentration of 200 g / L and a sodium sulfate concentration of 16 g / L, and a mass ratio of 12.5:1. After a first cleaning with deionized water for 30 seconds, the sample was dried until no residual water stains remained. The dried sample was placed on the stage of a polarized light metallurgical microscope, and the grain structure was observed under polarized light mode at a magnification of 100X.

[0074] Example 12

[0075] The difference from Example 1 is that the first cleaning time is 25 seconds;

[0076] The grain structure of the zinc-copper-titanium alloy was analyzed. The alloy, by mass percentage, comprises 1% Cu, 0.2% Ti, and the balance Zn. The specific method was as follows: 1. The zinc-copper-titanium alloy sample was prepared and polished using standard sample preparation procedures; 2. The polished sample underwent a second cleaning and surface drying until no water stains remained; 3. The polished surface of the sample was completely immersed in an etchant for 3 seconds. The etchant was a mixed aqueous solution of chromic anhydride and sodium sulfate, with a chromic anhydride concentration of 200 g / L and a sodium sulfate concentration of 16 g / L, and a mass ratio of 12.5:1. After a first cleaning with deionized water for 25 seconds, the sample was dried until no residual water stains remained. The dried sample was placed on the stage of a polarized light metallurgical microscope, and the grain structure was observed under polarized light mode at a magnification of 100X.

[0077] Comparative Example 1

[0078] The difference from Example 1 is that the grain structure of the zinc-copper-titanium alloy was examined. The zinc-copper-titanium alloy, by mass percentage, comprises 1% Cu, 0.2% Ti, and the balance Zn. The specific method is as follows: 1. The zinc-copper-titanium alloy sample was prepared and polished using a conventional sample preparation process; 2. The polished sample was then cleaned and dried until no water stains remained; 3. The polished surface of the sample was completely immersed in an etchant (nitric acid ethanol solution with a nitric acid concentration of 4%) for 30 seconds, then removed. After rinsing with deionized water for 1 minute, the sample was dried until no residual water stains remained. The dried sample was then placed on the stage of a white light metallographic microscope for 100X grain structure observation. Figure 4As shown, the obtained metallographic structure does not show different colors of different grains of the alloy; only the difference between the alloy precipitates and the matrix can be seen.

[0079] Comparative Example 2

[0080] The difference from Example 1 is that the grain structure of the zinc-copper-titanium alloy was examined. The zinc-copper-titanium alloy, by mass percentage, comprises 1% Cu, 0.2% Ti, and the balance Zn. The specific method is as follows: 1. The zinc-copper-titanium alloy sample was prepared and polished using a conventional sample preparation process; 2. The polished sample was then cleaned and dried until no water stains remained on the surface; 3. The polished surface of the sample was completely immersed in an etchant (nitric acid ethanol solution with a nitric acid mass concentration of 4%) for 30 seconds, then removed. The sample was then rinsed with deionized water for 1 minute and dried until no residual water stains remained on the surface; the dried sample was placed on the stage of a polarized light metallurgical microscope, and the grain structure was observed under polarized light mode at a magnification of 100X.

[0081] The results of grain structure clarity and average grain size observed in the examples and comparative examples are shown in Table 1. The grain structure clarity level diagram is shown in... Figure 5 Divided into Level 1 (see Figure 5 a) Level 2 (see Figure 5 b) Level 3 (see Figure 5 c) and Level 4 (see c) Figure 5 d).

[0082] Table 1

[0083]

[0084] In Table 1, "-" indicates that measurement is not possible. When the grain structure clarity is Level 1, Examples 1, 4, 9, and 11 are the same zinc alloy, and the average grain size measurement is relatively accurate at 115 μm. The significant difference in average grain size among Examples 1, 2, and 3 is due to the different elemental compositions of the zinc alloys in Examples 1, 2, and 3. When the grain structure clarity of the same zinc alloy as Example 1 is Level 2 and Level 3, the measurement accuracy of the average grain size decreases, and the result fluctuates around 115 μm. When the grain structure clarity of the same zinc alloy as Example 1 is Level 4, the grain boundaries are too blurred, making it impossible to measure the average grain size.

[0085] As can be seen from the above description, the embodiments of the present invention achieve the following technical effects:

[0086] This application uses a mixed aqueous solution of chromic anhydride and sodium sulfate as an etchant to form a thin film on the zinc alloy surface. The interface between this film and the alloy grains produces specific optical effects, especially under polarized light, causing grains with different orientations to exhibit different color contrasts. Unlike the principle of nitric acid-ethanol solution generating microstructure contrast imaging by dissolving the matrix or second phase, this method utilizes the interface effect formed between the etchant coating and the sample surface: when polarized light passes through the interface between the film and the sample, it undergoes additional reflection, refraction, or phase changes due to the difference in refractive index, enhancing the degree of polarization state change and thus improving image contrast. This method achieves zinc alloy grain contrast imaging, significantly enhancing the contrast between grains and making grain boundaries clearer and more discernible, facilitating subsequent grain size measurement. Furthermore, the coated film reduces diffuse reflection from the sample surface through an anti-reflection effect, reducing light reflection loss at the interface and allowing more polarized light to interact with the sample along the expected path (such as entering the sample interior and undergoing birefringence), thereby reducing stray light interference and improving the image signal-to-noise ratio. The use of chemical etching to replace the complex electrolytic anodic coating process greatly simplifies sample pretreatment steps, reduces operational difficulty, and improves detection efficiency. This method uses a mixed aqueous solution of chromic anhydride and sodium sulfate, avoiding the direct use of strong acids and reducing potential safety risks during experiments compared to traditional etching solutions such as nitric acid and ethanol. When used with a polarized metallurgical microscope, no additional physical or chemical treatment to enhance contrast is required, further reducing the danger and complexity of sample handling.

[0087] The above are merely embodiments of the present invention and are not intended to limit the invention. Those skilled in the art will recognize that the present invention can have various modifications and variations. 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. A metallographic examination method of a zinc alloy grain structure, characterized by, The metallographic detection method comprises: S1, the zinc alloy is immersed in an etchant for surface treatment, and then first cleaning and drying are performed to obtain a treated zinc alloy; wherein the etchant is a mixed aqueous solution of chromic anhydride and sodium sulfate; S2, the treated zinc alloy is detected for grain structure by a polarized light metallographic microscope in a polarized light mode to measure the grain size.

2. The metallographic inspection method according to claim 1, characterized in that, The concentration of the chromic anhydride in the etchant is 62.5-500 g / L; and / or the concentration of the sodium sulfate in the etchant is 6.25-50 g / L.

3. The metallographic inspection method according to claim 2, characterized in that, The concentration of the chromic anhydride in the etchant is 150-250 g / L; and / or the concentration of the sodium sulfate in the etchant is 8-16 g / L.

4. The metallographic inspection method according to claim 1, characterized by The mass ratio of the chromic anhydride to the sodium sulfate in the etchant is (11-30):

1.

5. The metallographic inspection method according to any one of claims 1 to 4, characterized in that, The surface treatment time is 3-10 s.

6. The metallographic inspection method according to any one of claims 1 to 4, characterized in that, The magnification of the polarized light metallographic microscope is 100-400X.

7. The metallographic inspection method according to any one of claims 1 to 4, characterized by, The zinc alloy is selected from any one or more of zinc-aluminum alloy, zinc-copper alloy, zinc-aluminum-magnesium alloy, zinc-aluminum-copper-magnesium alloy and zinc-copper-titanium alloy.

8. The metallographic inspection method according to any one of claims 1 to 4, characterized by, The cleaning agent used in the first cleaning is water.

9. The metallographic inspection method according to any one of claims 1 to 4, characterized by, The first cleaning time is not less than 30 s.

10. The metallographic inspection method according to any one of claims 1 to 4, characterized by, In S1, the zinc alloy is sequentially polished, second cleaned and dried, and then immersed in the etchant for the surface treatment. In S1, the zinc alloy is sequentially polished, second cleaned and dried, and then immersed in the etchant for the surface treatment.

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