A method for displaying a metallographic structure of a gold-based alloy

Abstract

The application provides a metallographic structure display method of a gold-based alloy, and the display method comprises the following steps: taking the gold-based alloy as an anode, electrolytic immersion etching the gold-based alloy in an electrolyte composed of hydrochloric acid, nitric acid and anhydrous ethanol, then cleaning the surface of the obtained gold-based alloy to obtain the gold-based alloy after electrolytic immersion etching; soaking the gold-based alloy after electrolytic immersion etching in ammonia water, then cleaning the surface of the obtained gold-based alloy, and obtaining the gold-based alloy after preliminary treatment after the surface is dried; immersing the gold-based alloy after preliminary treatment in a corrosion liquid composed of a saturated aqueous solution of chromium trioxide, hydrochloric acid and distilled water, then cleaning the surface of the obtained gold-based alloy, and completing the display of the metallographic structure of the gold-based alloy after the surface is dried, which greatly facilitates the observation of the metallographic structure of the gold-based alloy and is beneficial to obtaining the gold-based alloy product meeting the performance requirements.

Description

Technical Field

[0001] This invention belongs to the field of metal surface chemistry and electrochemical treatment technology, and specifically relates to a method for displaying the metallographic structure of a gold-based alloy. Background Technology

[0002] Electrical contact materials are widely used in the electrical engineering industry, and their performance directly affects the lifespan and safe operation of switching devices. Current conversion is a fundamental component of the power supply and distribution process in a power system. Any power system must transmit electrical signals or energy from one conductor to another. The connection point between conductors, the electrical contact point, is often the main obstacle to the transmission of electrical signals or energy, and is subject to physical and chemical processes such as mechanical, electrical, thermal, and environmental influences. Therefore, electrical contact materials require alloys with good electrical and thermal conductivity, corrosion resistance, wear resistance, and high chemical stability. Gold-based alloys, as one of the most widely used alloy electrical contact materials, are widely used in precision electrical contact materials or sliding electrical contact materials under low contact pressure and small loads due to their good chemical stability, outstanding electrical contact characteristics, and low yield point and elastic modulus. Currently, the main gold-based alloy electrical contact materials with practical applications are AuAg. 20 AuAg 20 Cu5, AuPt 10 AuAg 25 Pt5 is mainly used in solid and composite rivets, copper alloy overcoatings, and contact rivets. In addition, gold-based alloys have found excellent applications in dental alloys and jewelry gold.

[0003] Gold-based alloys have been widely used in electrical contact materials, jewelry materials, and dental alloys, leading to increasingly stringent requirements for these products. During product manufacturing, differences in alloy composition and adjustments to heat treatment parameters often affect the microstructure of the material, thus influencing its performance. Therefore, to obtain products that meet performance requirements, it is essential to observe the metallographic structure of the gold-based alloy.

[0004] Typically, the corrosion of gold-based alloys involves directly immersing the alloy surface in a corrosive solution. For example, Xia Wen, Wang Shuming, Yang Yin, et al. A novel metallographic etchant for high-purity gold [J]. Physical and Chemical Testing (Physical Section), 2020, 56(05):21-22+26. describes a method for metallographic corrosion of high-purity gold. This method involves immersing the sample surface in aqua regia and a mixed aqueous solution of ferric chloride, hydrochloric acid, and hydrogen peroxide, respectively, and corroding for 20 seconds at room temperature. However, after testing these two corrosive solutions, it was found that the metallographic structure obtained after corrosion was not good. This is because the presence of a certain proportion of silver and copper in the gold-based alloy composition affects the corrosion effect. Xu Kun. Gold, silver, palladium and their alloys in Cr-containing...6+ Metallographic Etching Behavior in Reagents [J]. Physical and Chemical Testing (Physical Section), 1999(07):308-309. The etching process for gold and its alloys involves wiping the sample surface directly at room temperature with cotton soaked in the solution and then immersing it directly in a mixed solution of nitric acid, hydrochloric acid, and saturated chromic anhydride for a few seconds to a few minutes. However, when using this etching method to etch alloys Au-5Ag-20Cu and Au-20Ag-10Cu, their grain boundaries cannot be clearly displayed, even after adjusting the formula. In Yang Yulu and Ma Yonghua's article "Display of Metallographic Structure of AuAgCuMnGd Alloy" [J]. Precious Metals, 1982(03):47-49, a method for displaying the metallographic structure of AuAgCuMnGd alloy was identified, specifically using a high-concentration potassium cyanide and ammonium persulfate mixed solution for heating and etching. In Gangte and Pei Zhuo's "Metallographic Etching Handbook" [M]. Science Press, 1982, electrolytic etching of gold and its alloys in a mixed solution of potassium cyanide and distilled water was also mentioned. However, due to the high toxicity of cyanide, these two methods are difficult to widely apply. Furthermore, in Chinese patent CN110907258A, published on 2020-03-24, entitled "A Method for Electrolytic Etching of Gold," an electrolytic etching method for gold was described. This method uses a mixed solution of hydrogen peroxide and hydrochloric acid as the electrolytic etching solution, but the etching effect is not ideal. Among these methods, adjusting the ratio of the etchant and the voltage during electrolytic etching is helpful. However, unfortunately, after electrolytic etching, a layer of AgCl precipitate always adheres to the surface. It will become mottled with a slight rub, and many white lines will appear. This seriously affects the observation of the microstructure of the gold-based alloy surface and makes it impossible to obtain the microstructure characteristics of the gold-based alloy. Summary of the Invention

[0005] To address the technical problems existing in the current metallographic structure display process of gold-based alloys, this invention provides a method for displaying the metallographic structure of gold-based alloys, which greatly facilitates the observation of the metallographic structure of gold-based alloys and helps to obtain products that meet performance requirements.

[0006] This invention is achieved through the following technical solution:

[0007] A method for displaying the metallographic structure of a gold-based alloy, comprising the following steps:

[0008] S1, using a gold-based alloy as the anode, electrolytically etched it in an electrolyte composed of hydrochloric acid, nitric acid and anhydrous ethanol. The hydrochloric acid accounts for 2.5% to 5% of the electrolyte volume, and the nitric acid accounts for 5% to 10% of the electrolyte volume. After that, the surface of the gold-based alloy is cleaned to obtain the electrolytically etched gold-based alloy.

[0009] S2, the gold-based alloy after electrolytic etching is immersed in ammonia water, then the surface of the gold-based alloy is cleaned, and after the surface is dried, a pre-treated gold-based alloy is obtained.

[0010] S3. The pre-treated gold-based alloy is immersed in an etching solution composed of saturated aqueous solution of chromium trioxide, hydrochloric acid and distilled water. The saturated aqueous solution of chromium trioxide accounts for 5% to 15% of the volume of the etching solution, and the hydrochloric acid accounts for 2.5% to 10% of the volume of the etching solution. After cleaning the surface of the gold-based alloy, the metallographic structure of the gold-based alloy is displayed after the surface is dried.

[0011] Preferably, the gold-based alloy in S1 is a mechanically polished gold-based alloy with a bright surface, free of dirt and scratches, specifically obtained according to the following process:

[0012] Mechanical polishing is automatic polishing with a speed of 1000-1400 rpm. Diamond polishing paste is mixed with tap water for polishing. The polishing cloth is made of flocked fabric, and the time is 5-12 minutes.

[0013] Preferably, in S1, the stainless steel plate serves as the cathode, and the anode and cathode are connected to the positive and negative electrodes of the electrolytic polishing and corrosion instrument, respectively.

[0014] Furthermore, in S1, the electrolyte temperature is 0–30°C, the DC voltage during electrolytic etching is 10–30V, and the time is 1–20s.

[0015] Preferably, in S1, the mass fraction of hydrochloric acid is 37% and the mass fraction of nitric acid is 68%.

[0016] Preferably, water is used for cleaning in S1, S2 and S3.

[0017] Preferably, the ammonia concentration in S2 is 25% to 28%, and the gold-based alloy after electrolytic etching is immersed in the ammonia for 10 to 30 seconds.

[0018] Preferably, in both S2 and S3, the surface of the gold-based alloy is wiped with alcohol and then air-dried.

[0019] Preferably, in step S3, the saturated aqueous solution of chromium trioxide, hydrochloric acid, and distilled water are stirred evenly to obtain a corrosion solution.

[0020] Preferably, in S1, hydrochloric acid, nitric acid and anhydrous ethanol are stirred evenly to obtain an electrolyte.

[0021] Compared with the prior art, the present invention has the following beneficial technical effects:

[0022] This invention discloses a method for displaying the metallographic structure of a gold-based alloy. Electrolytic etching of the gold-based alloy removes surface stress and scratches caused by mechanical polishing. The surface precipitate, primarily composed of AgCl, is porous and unstable, resulting in unclear metallographic structure display. AgCl is easily removed by ammonia, and it also removes some obvious contaminants, facilitating the display of the metallographic structure in Cr-containing alloys. 6+ The substrate was etched in a corrosive solution, and ammonia water did not affect the substrate structure. Further etching was then performed using a solution containing strong oxidizing Cr... 6+ The acidic etching solution removes a layer from the surface, which not only clearly reveals the grain boundaries of the surface structure, but also effectively removes surface scratches, further dissolves the metallographic structure, and results in a clear, high-quality metallographic structure without obvious dirt or scratches.

[0023] Furthermore, the etching solution does not need to be prepared and used immediately. As long as the prepared etching solution is sealed and stored, the electrolyte can still be used after about a month. The concentrations of strong oxidants and strong acids in the etching solution are very low, allowing it to be stored for about three months, reducing waste and the inconvenience of preparing the etching solution fresh each time. This invention is quick and simple to operate, generally completing the process within two minutes.

[0024] Furthermore, for gold-based alloys with different compositions, a metallographic structure revealing method combining electrolytic etching and chemical etching can be employed. High-quality surface metallographic structures for each gold-based alloy can be obtained simply by adjusting the ratio of the etching solution and the parameters of the electrolytic etching. This is because in Au-Ag-Cu alloys, Ag preferentially reacts with Cl in hydrochloric acid. - The reaction produces AgCl precipitate, which adheres to the surface and affects the observation of the alloy surface structure. Therefore, it is necessary to reasonably control the volume ratio of hydrochloric acid in the etching solution, and the amount of chemicals used is very small. In the electrolytic etching process, Cu in the Au-Ag-Cu alloy is the most reactive. These strong corrosion reactions that occur through electrolytic etching can effectively remove surface stress and some scratches caused by mechanical polishing. Anhydrous ethanol, as a reducing agent, can reduce some metal ions in the alloy, and the corrosion rate can be accelerated by applying an external current. The electro-bonding strength of gold, silver, and copper is weaker than that of hydrogen. Therefore, in the chemical etching process, this type of alloy cannot be acid-etched without the addition of an oxidant. Chromium trioxide, as a strong oxidant, can play an important role in chemical etching. Through oxidation, the grain boundaries are preferentially and clearly displayed, which not only removes the silver chloride precipitate produced on the alloy surface after electrolytic etching, but also improves the display quality of the surface microstructure. Attached Figure Description

[0025] Figure 1 This is a flowchart showing the metallographic structure of the gold-based alloy described in this invention;

[0026] Figure 2This is a simplified process diagram of the electrolytic etching method in the metallographic structure display method for gold-based alloys described in this invention;

[0027] Figure 3a This is a metallographic image of the surface of Au-20Ag-10Cu after 10s electrolytic etching according to the present invention;

[0028] Figure 3b This is a metallographic image of the surface of Au-20Ag-10Cu after 8s of electrolytic etching according to the present invention;

[0029] Figure 4 This is a metallographic diagram of the surface after electrolytic etching and chemical etching of Au-20Ag-10Cu as described in this invention;

[0030] Figure 5a This is a metallographic diagram of the surface after electrolytic etching of Au-20Ag-30Cu according to the present invention;

[0031] Figure 5b These are surface metallographic images of Au-20Ag-30Cu after electrolytic etching and chemical etching according to the present invention;

[0032] Figure 6a This is a surface metallographic structure diagram of Au-5Ag-20Cu after electrolytic etching at a depth of 100 μm as described in this invention.

[0033] Figure 6b This is a surface metallographic structure diagram of Au-5Ag-20Cu after electrolytic etching at a depth of 50 μm as described in this invention. Detailed Implementation

[0034] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. The specific embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0035] This invention discloses a method for displaying the metallographic structure of a gold-based alloy, such as... Figure 1 As shown, please follow these steps:

[0036] Step 1: Use the mechanically polished gold-based alloy as the anode. The standard for good mechanical polishing is that the alloy surface is as bright as a mirror, without obvious dirt and scratches. The gold-based alloy is mainly Au-Ag-Cu alloy, which refers to the general term for alloys containing various proportions of gold, silver and copper, including ternary alloys Au-20Ag-10Cu, Au-5Ag-20Cu and Au-20Ag-30Cu, as well as multi-component alloys with it as the basic component, such as Au-Ag-Cu-Pt alloy and Au-Ag-Cu-Pd alloy. Use a stainless steel plate as the cathode, and connect the anode and cathode to the positive and negative electrodes of the electrolytic polishing and etching instrument with wires.

[0037] Mechanical polishing is automatic, with a rotation speed of 1000–1400 rpm. Diamond polishing paste mixed with tap water is used, and the polishing cloth is flocked fabric. The polishing time is 5–12 minutes. Gold-based alloys are relatively soft, so the sample should be gently pressed onto the polishing pad during polishing. The stainless steel plate can be 304 stainless steel. The specific connection method between the gold-based alloy and the electrolytic etching equipment, and a simplified diagram of the electrolytic polishing and etching instrument are shown below. Figure 2 As shown;

[0038] Step 2: Stir hydrochloric acid, nitric acid and anhydrous ethanol in a beaker with a glass rod to obtain an electrolyte. The mass fraction of hydrochloric acid is 37%, the mass fraction of nitric acid is 68%, both of which are strong inorganic acids. Hydrochloric acid accounts for 2.5% to 5% of the electrolyte volume, nitric acid accounts for 5% to 10% of the electrolyte volume, and anhydrous ethanol accounts for 85% to 92.5% of the electrolyte volume.

[0039] First, place the stainless steel plate into the electrolyte and clamp it with the clip at the end of the wire connected to the negative electrode of the electrolytic polishing and etching instrument. Then, clamp the other end of the sample surface to be observed with the clip at the end of the wire connected to the positive electrode of the electrolytic polishing and etching instrument. The clips should not be immersed in the electrolyte. Start timing when the sample surface to be observed is completely immersed in the electrolyte. The electrolyte temperature is 0-30℃, the DC voltage is 10-30V, and the etching time is 1-20s.

[0040] In the electrolytic etching process, Cu is the most reactive component in the Au-Ag-Cu alloy, but Cu mainly reacts with nitric acid: Cu + 4H⁺ + +2NO3 - =Cu 2+ +2NO₂↑+2H₂O; while Ag reacts with Cl in hydrochloric acid. - The reaction produces AgCl precipitate that is deposited on the alloy surface. These strong corrosion reactions that occur through electrolytic etching can effectively remove surface stress and some scratches caused by mechanical polishing. Anhydrous ethanol, as a reducing agent, can reduce some metal ions in the alloy and accelerate the corrosion rate by applying an external current.

[0041] Step 3: Immediately after the electrolytic etching is completed, remove the sample with tweezers, rinse the surface of the sample to be observed with plenty of water, then immerse the sample in ammonia water with a concentration of 25% to 28% for 10 to 30 seconds to remove the AgCl precipitate deposited on the surface during the electrolytic polishing etching process. After that, rinse with tap water and wipe the sample surface with alcohol, and finally air dry.

[0042] Step 4: Stir the saturated aqueous solution of chromium trioxide, hydrochloric acid, and distilled water in a beaker with a glass rod until homogeneous. Chromium trioxide is a red crystalline powder (analytical grade). The resulting etching solution contains 37% hydrochloric acid by mass, 5%–15% saturated aqueous solution of chromium trioxide, 2.5%–10% hydrochloric acid, and 75%–92.5% distilled water. Immerse the surface of the sample to be observed after step 3 in the etching solution for 5–15 seconds.

[0043] Au-Ag-Cu is a corrosion-resistant alloy with a high Au content. The electro-bonding strength of gold, silver, and copper is weaker than that of hydrogen. Therefore, without an oxidant, this type of alloy cannot be acid-etched during chemical etching. Chromium trioxide, as a strong oxidant, plays an important role in chemical etching. Through oxidation, it makes the grain boundaries more clearly visible. This not only removes the silver chloride precipitate formed on the surface of the alloy sample after electrolytic etching, but also improves the display quality of the microstructure on the sample surface. At the same time, because the concentration of strong oxidant and strong acid in the prepared etching solution is very low, the chemical etching solution can still be used normally after being stored for about three months. This avoids waste and reduces the trouble of preparing the etching solution every time it is needed.

[0044] Step 5: Finally, rinse the sample with plenty of water, wipe the sample surface with alcohol, and let it air dry to obtain a clear, clean, and scratch-free microstructure.

[0045] All the water used for rinsing can be running tap water, which is readily available and convenient to use.

[0046] The following provides a detailed description of specific embodiments of the present invention. The examples below are for illustrative purposes only and are not intended to limit the scope of the invention. Reagents or instruments used in the examples, unless otherwise specified, are all commercially available conventional products.

[0047] Example 1

[0048] The present invention discloses a method for displaying the metallographic structure of a gold-based alloy, the specific steps of which are as follows:

[0049] Step 1: First, use the mechanically polished Au-20Ag-10Cu as the anode. The standard for mechanical polishing is that the alloy surface is as bright as a mirror, without obvious dirt and scratches. Use 304 stainless steel plate as the cathode.

[0050] The mechanical polishing speed was 1000 rpm and the time was 12 minutes.

[0051] Step 2: Prepare an electrolyte solution by mixing 1.5 ml of 37 wt% concentrated hydrochloric acid, 2.5 ml of 68 wt% concentrated nitric acid, and 46 ml of anhydrous ethanol in a beaker.

[0052] Step 3: Electrolytic etching is carried out in an electrolyte at 5°C. The voltage for electrolytic etching is DC voltage of 10V, and the time is 10s.

[0053] The Au-20Ag-10Cu alloy was electrolytically etched for 8 seconds using the same process.

[0054] Step 4: Remove the electrolytically etched Au-20Ag-10Cu alloy, rinse and dry it. The resulting microstructure images of the alloy surface are shown below. Figure 3a and Figure 3b As shown.

[0055] Figure 3a The alloy surface shown in the image is grayish-black, resembling excessive corrosion. Furthermore, numerous linear scratches on the surface severely impair the visualization of the surface microstructure. Figure 3b The alloy surface shown is not clean, with many large and small contaminants, and importantly, the metallographic microstructure is very unclear. For these cases of insufficient or excessive corrosion, a light chemical etching process should be performed.

[0056] Step 5: Prepare the etching solution. First, weigh 5g of chromium trioxide and stir it in distilled water to make a saturated chromium trioxide solution. Then, add the saturated chromium trioxide solution to a solution in which 5ml of 37wt% concentrated hydrochloric acid and 40ml of distilled water are mixed evenly and stir them evenly.

[0057] Step 6: Immerse the surface of the Au-20Ag-10Cu alloy to be observed after electrolytic polishing in ammonia water for 20 seconds, mainly to remove the AgCl precipitate on the surface. After immersion, rinse and wipe the sample surface with alcohol, and finally dry it. Then immerse it in the freshly prepared etching solution for 8 seconds. At this time, the surface of the alloy sample will change color slightly. After completion, use tweezers to quickly remove the sample.

[0058] Step 7: Rinse the sample with clean water for a few seconds, then wipe the sample surface with a cotton ball soaked in alcohol and let it air dry. Note that chromium trioxide is a heavy metal, and waste liquid containing chromium trioxide should not be discharged at will and should be stored separately.

[0059] like Figure 4 As shown, the microstructure is clean and free of impurities and scratches, with clearly visible grain boundaries; this demonstrates that the metallographic etching method combining electrolytic etching and chemical etching has a significant effect.

[0060] Example 2

[0061] Step 1: First, mechanically polish Au-20Ag-30Cu until the surface is as bright as a mirror, without obvious dirt or scratches;

[0062] The mechanical polishing speed was 1250 rpm and the time was 8 minutes.

[0063] Step 2: Prepare the etching solution. First, weigh 5g of chromium trioxide and stir it in distilled water to make a saturated chromium trioxide solution. Then, add the saturated chromium trioxide solution to a solution of 5ml of 37wt% concentrated hydrochloric acid and 30ml of distilled water, and stir them evenly.

[0064] Step 3: Finally, immerse the surface of the Au-20Ag-30Cu alloy to be observed in the etching solution for 12 seconds. After immersion, quickly remove the sample with tweezers.

[0065] Step 4: Rinse the sample with clean water for a few seconds, then wipe the sample surface with a cotton ball soaked in alcohol and let it air dry.

[0066] according to Figure 5a As can be seen from the metallographic image of Au-20Ag-30Cu, there are many black precipitates at the grain boundaries, and the corrosion color on the sample surface is uneven. Therefore, the effectiveness of the combination of electrolytic etching and chemical etching is verified. Based on this, the present invention provides a method for displaying the metallographic structure of gold-based alloys, the specific steps of which are as follows:

[0067] Step 1: First, use the mechanically polished Au-20Ag-30Cu as the anode. The standard for mechanical polishing is that the alloy surface is as bright as a mirror, without obvious dirt and scratches. Use 304 stainless steel plate as the cathode.

[0068] Step 2: Prepare an electrolyte solution by mixing 1.5 ml of 37 wt% concentrated hydrochloric acid, 2.5 ml of 68 wt% concentrated nitric acid, and 46 ml of anhydrous ethanol in a beaker.

[0069] Step 3: Electrolytic etching is carried out in an electrolyte at 7°C. The voltage for electrolytic etching is 10V DC, and the time is 5s.

[0070] Step 4: Take out the Au-20Ag-30Cu alloy after electrolytic etching, rinse and dry it. Then, immerse the surface of the sample to be observed in ammonia water for 10 seconds. After that, take out the sample, rinse it, wipe the sample surface with a cotton ball soaked in alcohol, and finally let it air dry.

[0071] Step 5: Prepare the etching solution. Take 5 ml of saturated chromium trioxide solution and add it to a solution of 3 ml of 37 wt% concentrated hydrochloric acid and 42 ml of distilled water. Stir them thoroughly.

[0072] Step 6: Immerse the alloy surface to be observed in the freshly prepared etching solution for 7 seconds. After immersion, quickly remove the sample with tweezers.

[0073] Step 7: Rinse the sample with clean water for a few seconds, then wipe the sample surface with a cotton ball soaked in alcohol and let it air dry.

[0074] like Figure 5b As shown, the Au-20Ag-30Cu alloy surface obtained by combining electrolytic etching and chemical etching has a clear metallographic structure free of dirt and obvious scratches.

[0075] Example 3

[0076] The present invention discloses a method for displaying the metallographic structure of a gold-based alloy, the specific steps of which are as follows:

[0077] Step 1: First, use the mechanically polished Au-5Ag-20Cu as the anode. The standard for mechanical polishing is that the alloy surface is as bright as a mirror, without obvious dirt and scratches. Use 304 stainless steel plate as the cathode.

[0078] The mechanical polishing speed was 1400 rpm and the time was 5 minutes.

[0079] Step 2: Prepare an electrolyte solution by mixing 1.5 ml of 37 wt% concentrated hydrochloric acid, 2.5 ml of 68 wt% concentrated nitric acid, and 46 ml of anhydrous ethanol in a beaker.

[0080] Step 3: Electrolytic etching is carried out in an electrolyte at 12℃. The voltage for electrolytic etching is DC voltage of 12V, and the time is 10s.

[0081] Step 4: Take out the Au-5Ag-20Cu alloy after electrolytic etching, rinse and dry it. Then, immerse the surface of the sample to be observed in ammonia water for 20 seconds. After that, take it out and rinse the sample. Wipe the sample surface with a cotton ball soaked in alcohol and finally let it air dry.

[0082] Step 5: Prepare the etching solution. Take 5 ml of saturated chromium trioxide solution and add it to a solution of 3 ml of 37 wt% concentrated hydrochloric acid and 42 ml of distilled water. Stir them thoroughly.

[0083] Step 6: Immerse the alloy surface to be observed in the freshly prepared etching solution for 10 seconds. After immersion, quickly remove the sample with tweezers.

[0084] Step 7: Rinse the sample with clean water for a few seconds, then wipe the sample surface with a cotton ball soaked in alcohol, and let it dry. The resulting microstructure of the alloy surface is shown in the image below. Figure 6a and Figure 6b As shown, Figure 6a This is a low-magnification surface metallographic image at 100 μm. It shows clear grain boundaries, no contamination, and no obvious scratches. Figure 6b This is a surface metallographic image at 50μm. It can be seen that grain boundaries and twin boundaries are clearly visible, and the different grain orientations are indicated by the brightness of the grain color.

[0085] Comparing the metallographic images of these alloys with different compositions reveals slight differences in the level of their microstructure display. This is related to the different compositions of the alloys themselves. Different compositions result in different alloy hardness, and the ratio of silver to copper also affects the corrosion effect, thus leading to slight variations in the final microstructure display. However, this invention still provides a high-quality metallographic display method for gold-based alloys such as Au-20Ag-10Cu, Au-5Ag-20Cu, and Au-20Ag-30Cu through these examples, effectively assisting in the surface microscopic analysis of Au-Ag-Cu alloys.

[0086] Finally, it should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this invention, and these modifications and improvements are all within the scope of protection of this invention.

Claims

1. A method for displaying the metallographic structure of a gold-based alloy, characterized in that, Includes the following steps: S1. A gold-based alloy that has been mechanically polished and has a bright surface free of dirt and scratches is used as the anode. It is electrolytically etched in an electrolyte composed of hydrochloric acid, nitric acid and anhydrous ethanol at 0~30℃. The DC voltage during electrolytic etching is 10~30V and the time is 1~20s. The hydrochloric acid accounts for 2.5%~5% of the electrolyte volume and the nitric acid accounts for 5%~10% of the electrolyte volume. Afterwards, the surface of the gold-based alloy is washed with water to obtain the electrolytically etched gold-based alloy. The gold-based alloy is obtained by the following process: mechanical polishing is automatic polishing with a speed of 1000~1400rpm, polishing is performed with diamond polishing paste and tap water, the polishing cloth is flocked fabric, and the time is 5~12min. S2, the gold-based alloy after electrolytic etching is immersed in ammonia water with a concentration of 25%~28% for 10~30s, and then the surface of the gold-based alloy is washed with water. After the surface is dried, the pre-treated gold-based alloy is obtained. S3. The pre-treated gold-based alloy is immersed in an etching solution composed of saturated aqueous solution of chromium trioxide, hydrochloric acid and distilled water for 5-15 seconds. The saturated aqueous solution of chromium trioxide accounts for 5%-15% of the volume of the etching solution, and the hydrochloric acid accounts for 2.5%-10% of the volume of the etching solution. Then, the surface of the gold-based alloy is washed with water and the metallographic structure of the gold-based alloy is displayed after the surface is dried.

2. The method for displaying the metallographic structure of a gold-based alloy according to claim 1, characterized in that, In S1, the stainless steel plate serves as the cathode, while the anode and cathode are connected to the positive and negative electrodes of the electrolytic polishing and corrosion instrument, respectively.

3. The method for displaying the metallographic structure of a gold-based alloy according to claim 1, characterized in that, In S1, the mass fraction of hydrochloric acid is 37%, and the mass fraction of nitric acid is 68%.

4. The method for displaying the metallographic structure of a gold-based alloy according to claim 1, characterized in that, In both S2 and S3, the gold-based alloy surface was wiped with alcohol and then air-dried.

5. The method for displaying the metallographic structure of a gold-based alloy according to claim 1, characterized in that, In step S3, the saturated aqueous solution of chromium trioxide, hydrochloric acid, and distilled water are stirred evenly to obtain a corrosion solution.

6. The method for displaying the metallographic structure of a gold-based alloy according to claim 1, characterized in that, In S1, hydrochloric acid, nitric acid, and anhydrous ethanol are stirred evenly to obtain an electrolyte.

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