A method for preparing a pure magnesium metallographic sample

Abstract

This invention belongs to the field of metallographic sample preparation technology and discloses a method for preparing a pure magnesium metallographic sample. The preparation method includes the following steps: (1) polishing the cut pure magnesium sample with sandpaper to remove the surface oxide layer; (2) placing the pure magnesium sample treated in step (1) into an electrolytic polishing solution for polishing, wherein the electrolytic polishing solution is an aqueous solution containing phosphoric acid and ethanol, and then cleaning; (3) observing the metallographic structure of the sample treated in step (2). The method of this invention avoids long-term mechanical polishing, and polishing and etching are carried out simultaneously. The preparation of the metallographic sample can be completed in only one step, without the need for secondary etching, which greatly shortens the preparation time of pure magnesium metallographic samples. It is suitable for metallographic observation of pure magnesium in forged state, extruded state, extruded + hot rolled state and heat-treated state. The electrolytic polishing solution used in this invention has a simple composition, is safe and environmentally friendly, and has a good display effect of grain boundaries after polishing.

Description

Technical Field

[0001] This invention belongs to the field of metallographic sample preparation technology, specifically relating to a method for preparing a pure magnesium metallographic sample. Background Technology

[0002] Metallographic specimens are a powerful tool for studying and evaluating the morphology and microstructure of metallic materials. During the testing and development of new products or processes, metallography helps us quickly analyze the changes in microstructure after different deformations and heat treatments, allowing for timely adjustments to plastic deformation and heat treatment processes. Therefore, the preparation and visualization techniques of metallographic specimens are of paramount importance.

[0003] Magnesium is prone to forming deformation and blurring layers, and pure magnesium, with its lower hardness, will generate twins even with minor deformation, affecting the microstructure display. Therefore, mechanical polishing should be avoided as much as possible during the preparation of metallographic samples. Commonly used metallographic etchants for pure magnesium and magnesium alloys include mixed solutions of oxalic acid and water, mixed solutions of nitric acid and ethanol or methanol, and mixed solutions of picric acid and glacial acetic acid. Picric acid has a good etching effect, but it is difficult to obtain. After etching with these etchants, scratches still remain in the metallographic structure, and slip lines and twins are easily formed, resulting in unclear grain boundaries. For example, patent CN 101985772 B discloses an electrolytic polishing solution for pretreatment of magnesium alloy texture detection, which uses a 20%–40% nitric acid-alcohol solution. This solution is used to improve the flatness and smoothness of the magnesium alloy surface and effectively eliminate the stress layer introduced into the magnesium alloy surface during previous sample preparation. However, verification has shown that the treatment effect on the pure magnesium metallographic samples of this invention is not ideal.

[0004] Patent CN 101591799 B discloses an electrochemical polishing solution containing phosphoric acid, a leveling agent, a brightener, and a polyol for pre-treated magnesium alloys to improve their surface smoothness and corrosion resistance. However, it does not disclose its use for treating pure magnesium metallographic samples to improve the clarity of their metallographic structure. Patent CN 105603500 B discloses an electrochemical polishing solution for pre-treatment of magnesium alloys for residual stress detection, composed of phosphoric acid, glycerol, and anhydrous ethanol. This patent is also used for treating magnesium alloys, and its main function is to facilitate accurate residual stress detection results.

[0005] Due to differences in composition and metallographic structure between pure magnesium and magnesium alloys, the corresponding electropolishing effects will also differ. Currently, there is limited research on treating pure magnesium metallographic samples to improve their grain boundary display.

[0006] Patent CN 107976443 B discloses a method for observing the metallographic structure of pure magnesium. The method involves sequentially grinding the pure magnesium surface with 600#, 1000#, 2000#, and 5000# silicon carbide sandpaper. A first etchant (composed of 5 parts 68% nitric acid and 95 parts water) is then used to etch the surface, followed by a second etchant (composed of 20 parts 85% lactic acid and 80 parts water). This method requires prior mechanical polishing and two consecutive etchant treatments with different etchants to clearly reveal the pure magnesium metallographic structure. It suffers from complex processing procedures and reagents.

[0007] Therefore, providing a rapid and simple method for preparing pure magnesium metallographic samples is of great significance for the microstructure analysis of pure magnesium metal. Summary of the Invention

[0008] To address the shortcomings and deficiencies of the existing technology, the present invention aims to provide a method for preparing pure magnesium metallographic samples. The preparation method of the present invention employs an electrolytic polishing solution of a specific composition and specific processing steps, which can significantly improve the grain boundary display effect of pure magnesium samples.

[0009] The objective of this invention is achieved through the following technical solution:

[0010] A method for preparing a pure magnesium metallographic sample includes the following steps:

[0011] (1) The cut pure magnesium sample was sanded with sandpaper to remove the surface oxide layer;

[0012] (2) The pure magnesium sample after step (1) is placed in an electrolytic polishing solution for polishing, wherein the electrolytic polishing solution is an aqueous solution containing phosphoric acid and ethanol, and then cleaned.

[0013] (3) Observe the metallographic structure of the sample after step (2).

[0014] Furthermore, the pure magnesium sample mentioned in step (1) refers to a pure magnesium sample with a purity of 99.99% or higher.

[0015] Furthermore, the pure magnesium sample mentioned in step (1) includes forged, extruded, extruded + hot rolled, or heat-treated pure magnesium samples.

[0016] Furthermore, the sanding mentioned in step (1) refers to polishing with 2000# sandpaper and water for 10-20 seconds. The polishing time should not be too long; it is sufficient to remove the surface oxide layer completely. Avoid excessive mechanical polishing time, which may cause a deformed layer and affect the observation of the microstructure. At the same time, avoid polishing time that is too short, which may affect the subsequent electrolysis effect.

[0017] Furthermore, the polishing treatment mentioned in step (2) refers to polishing under a DC voltage of 1-3V for 80-120s. Through the above treatment, a white oxide film can be observed on the sample surface, indicating that the metallographic corrosion is complete.

[0018] Further, the electrolytic polishing solution in step (2) is composed of a mixed solution of phosphoric acid solution with a volume fraction of 20% to 40% and the remainder of ethanol, wherein the mass concentration of the phosphoric acid solution is 85%.

[0019] Furthermore, the cleaning described in step (2) refers to ultrasonic cleaning by immersing the sample in an anhydrous ethanol solution.

[0020] Furthermore, the metallographic structure observation mentioned in step (3) refers to observation and photographing using a computer-controlled metallographic microscope.

[0021] Furthermore, the metallographic structure observation described in step (3) is performed immediately after cleaning in step (2), or the cleaned sample is preserved by immersing it in anhydrous ethanol before metallographic structure observation. Preservation by immersion in anhydrous ethanol can avoid prolonged exposure to air, which would cause oxidation of the sample surface and affect the display of the metallographic structure.

[0022] Compared with the prior art, the beneficial effects of the present invention are:

[0023] (1) This invention provides an electrolytic polishing solution for high-purity magnesium and a method for preparing metallographic samples. This method avoids prolonged mechanical polishing, and polishing and etching are carried out simultaneously. The metallographic sample can be prepared in just one step without secondary etching, which greatly shortens the preparation time of metallographic samples of pure magnesium. It is suitable for metallographic observation of pure magnesium in forged, extruded, extruded + hot-rolled, and heat-treated states.

[0024] (2) The electrolytic polishing solution used in this invention has a simple composition, is safe and environmentally friendly, and has a good display effect on grain boundaries after polishing; and the electrolytic polishing solution can be reused after being sealed and stored, which effectively reduces costs. Attached Figure Description

[0025] Figures 1-4 The images show the metallographic structures of pure magnesium obtained in Examples 1-4, respectively.

[0026] Figures 5-7 The images show the metallographic structures of pure magnesium obtained in comparative examples 1 to 3. Detailed Implementation

[0027] The present invention will be further described in detail below with reference to the embodiments and accompanying drawings, but the embodiments of the present invention are not limited thereto.

[0028] Example 1

[0029] This embodiment describes a method for preparing a forged pure magnesium metallographic specimen, comprising the following steps:

[0030] (1) The cut forged pure magnesium sample (purity of 99.995%) was lightly polished with water on 2000# sandpaper for 15 seconds to remove the surface oxide layer.

[0031] (2) Place the sample treated in step (1) in an electrolytic polishing solution and polish it for 100 seconds at a DC voltage of 3V. When a white oxide film is attached to the surface of the sample, quickly immerse it in anhydrous ethanol for ultrasonic cleaning. The electrolytic polishing solution consists of a 37% (v / v) phosphoric acid solution (85% (w / v) and a 63% (v / v) ethanol solution.

[0032] (3) Immediately observe and photograph the sample after step (2) under a computer-controlled metallographic microscope.

[0033] The metallographic structure of pure magnesium obtained in this embodiment is shown in the figure below. Figure 1 As shown in the figure, the grain structure of forged pure magnesium is equiaxed, and the grain boundaries are clearly visible.

[0034] Example 2

[0035] This embodiment describes a method for preparing an extruded pure magnesium metallographic sample, comprising the following steps:

[0036] (1) The cut extruded pure magnesium sample (purity of 99.996%) was lightly polished with water on 2000# sandpaper for 15 seconds to remove the surface oxide layer.

[0037] (2) Place the sample treated in step (1) in an electrolytic polishing solution and polish it for 100 seconds at a DC voltage of 3V. When a white oxide film is attached to the surface of the sample, quickly immerse it in anhydrous ethanol for ultrasonic cleaning. The electrolytic polishing solution consists of a 37% (v / v) phosphoric acid solution (85% (w / v) and a 63% (v / v) ethanol solution.

[0038] (3) Immediately observe and photograph the sample after step (2) under a computer-controlled metallographic microscope.

[0039] The metallographic structure of pure magnesium obtained in this embodiment is shown in the figure below. Figure 2 As shown in the figure, the extruded pure magnesium grain structure is equiaxed, with a relatively uneven distribution and clearly visible grain boundaries.

[0040] Example 3

[0041] This embodiment describes a method for preparing an extruded and hot-rolled pure magnesium metallographic specimen, comprising the following steps:

[0042] (1) The cut extruded and hot-rolled pure magnesium sample (purity of 99.996%) was lightly polished with water on 2000# sandpaper for 15 seconds to remove the surface oxide layer.

[0043] (2) Place the sample treated in step (1) in an electrolytic polishing solution and polish it for 100 seconds at a DC voltage of 3V. When a white oxide film is attached to the surface of the sample, quickly immerse it in anhydrous ethanol for ultrasonic cleaning. The electrolytic polishing solution consists of a 37% (v / v) phosphoric acid solution (85% (w / v) and a 63% (v / v) ethanol solution.

[0044] (3) Immediately observe and photograph the sample after step (2) under a computer-controlled metallographic microscope.

[0045] The metallographic structure of pure magnesium obtained in this embodiment is shown in the figure below. Figure 3 As shown in the figure, the grain structure of hot-rolled pure magnesium consists of fine, elongated equiaxed crystals with a relatively uniform distribution and clearly visible grain boundaries.

[0046] Example 4

[0047] This embodiment describes a method for preparing a heat-treated pure magnesium metallographic sample, comprising the following steps:

[0048] (1) The cut heat-treated pure magnesium sample (purity of 99.994%) was lightly polished with water on 2000# sandpaper for 15 seconds to remove the surface oxide layer.

[0049] (2) Place the sample treated in step (1) in an electrolytic polishing solution and polish it for 100 seconds at a DC voltage of 3V. When a white oxide film is attached to the surface of the sample, quickly immerse it in anhydrous ethanol for ultrasonic cleaning. The electrolytic polishing solution consists of a 37% (v / v) phosphoric acid solution (85% (w / v) and a 63% (v / v) ethanol solution.

[0050] (3) Immediately observe and photograph the sample after step (2) under a computer-controlled metallographic microscope.

[0051] The metallographic structure of pure magnesium obtained in this embodiment is shown in the figure below. Figure 4 As shown in the figure, the pure magnesium grain structure after heat treatment is equiaxed, with fine grains, uniform distribution, and clearly visible grain boundaries.

[0052] Comparative Example 1

[0053] This comparative example provides a method for preparing a high-purity magnesium metallographic sample. Compared with Example 1, the electrolytic polishing solution uses a mixed solution of nitric acid and ethanol of the same concentration instead of a mixed solution of phosphoric acid and ethanol, while the rest remains the same.

[0054] The metallographic structure of pure magnesium obtained from this comparative example is shown in the figure. Figure 5 As shown in the figure, the metallographic structure of the forged pure magnesium in this comparative example has poor clarity.

[0055] The comparison with Example 1 shows that the pure magnesium metallographic image obtained by using a mixed solution of nitric acid and ethanol is of poor quality, with oxidation, numerous scratches, and some twins, which affects the display of grain boundaries.

[0056] Comparative Example 2

[0057] This comparative example provides a method for preparing a high-purity magnesium metallographic sample. Compared with Example 4, the polishing time in step (2) is reduced from 100s to 60s, but the rest are the same.

[0058] The metallographic structure of pure magnesium obtained from this comparative example is shown in the figure. Figure 6 As shown in the image, there are some scratches that affect the clarity and aesthetics of the metallographic image.

[0059] Comparative Example 3

[0060] This comparative example provides a method for preparing a high-purity magnesium metallographic sample. Compared with Example 4, step (2) of ultrasonic cleaning by placing the sample in anhydrous ethanol is replaced by ultrasonic cleaning by placing it in water. The rest are the same.

[0061] The metallographic structure of pure magnesium obtained from this comparative example is shown in the figure. Figure 7 As shown in the figure, there is a severe oxidation phenomenon on its surface, which affects the clarity of the tissue.

[0062] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A method for preparing a pure magnesium metallographic sample, characterized in that, Includes the following steps: (1) The cut pure magnesium sample was sanded with sandpaper to remove the surface oxide layer; (2) The pure magnesium sample after step (1) is placed in an electrolytic polishing solution for polishing, wherein the electrolytic polishing solution is an aqueous solution containing phosphoric acid and ethanol, and then cleaned. (3) Observe the metallographic structure of the sample after step (2); The polishing process mentioned in step (2) refers to polishing for 80 to 120 seconds under a DC voltage of 1 to 3V; The electrolytic polishing solution is a mixed solution of 20% to 40% phosphoric acid solution and the balance ethanol, wherein the mass concentration of the phosphoric acid solution is 85%. The cleaning refers to immersing the device in an anhydrous ethanol solution for ultrasonic cleaning. The metallographic structure observation mentioned in step (3) refers to observation and photographing using a computer-controlled metallographic microscope.

2. The method for preparing a pure magnesium metallographic sample according to claim 1, characterized in that, The pure magnesium sample mentioned in step (1) refers to a pure magnesium sample with a purity of 99.99% or higher.

3. The method for preparing a pure magnesium metallographic sample according to claim 1, characterized in that, The pure magnesium sample mentioned in step (1) includes forged, extruded, extruded + hot rolled or heat-treated pure magnesium samples.

4. The method for preparing a pure magnesium metallographic sample according to claim 1, characterized in that, The sanding mentioned in step (1) refers to using 2000# sandpaper and water to polish for 10~20 seconds.

5. The method for preparing a pure magnesium metallographic sample according to claim 1, characterized in that, The metallographic structure observation described in step (3) can be performed immediately after cleaning in step (2), or the cleaned sample can be preserved by immersing it in anhydrous ethanol and then observing the metallographic structure.

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