A 3D Reconstruction and Scale Calibration Method for Alloy Fracture Surfaces

Abstract

The present invention provides a method for three-dimensional reconstruction and scale calibration of alloy fracture surfaces, which relates to the technical field of material microscopic characterization and analysis, and includes: processing a sample to obtain an experimental specimen; performing a thermo-mechanical coupling deformation experiment on the specimen to obtain a fracture surface sample; pasting the fracture surface sample on a sample stage and then fixing it in a scanning electron microscope; obtaining fracture surface sample images at different angles by tilting the sample stage, performing three-dimensional reconstruction to obtain a three-dimensional fracture surface model; selecting two marked points on the cross-section of the fracture surface sample, adjusting the height of the sample stage to continuously collect images of the two marked point regions, retaining the clearest image, and recording the three-dimensional coordinates of the sample stage at this time; calculating the distance between the two marked points according to the two sets of three-dimensional coordinates, and scaling the three-dimensional fracture surface model to complete scale calibration. The present invention solves the problem of lack of three-dimensional information during fracture surface analysis, and obtains a three-dimensional fracture surface model with complete features and accurate scale while taking efficiency into account, which can assist in analyzing the fracture process of alloy specimens.

Description

Technical Field

[0001] The present invention relates to the technical field of microscopic characterization and analysis of materials, and particularly to a method for three-dimensional reconstruction and scale calibration of alloy fracture surfaces. Background Art

[0002] Due to their excellent high-temperature strength, fatigue performance, fracture toughness and other advantages, superalloys are widely used in the aerospace and energy fields. During specific use, they generally suffer from external effects such as force and heat, and fracture failure will occur when the damage tolerance is reached. After the alloy sample fractures through a deformation experiment, a pair of matching fracture surfaces will be obtained. The fracture surface records a lot of precious information about the fracture process. By observing and analyzing its morphology, structure and composition, etc., the mechanical and thermal properties of the sample during high-temperature deformation can be understood, which is of great significance for fracture process analysis.

[0003] Conventional fracture surface observation and analysis methods obtain two-dimensional images of the fracture surface morphology and microstructure through an optical microscope or a scanning electron microscope. Among them, the scanning electron microscope image is an intensity map of scattered electrons. Although it seems to have a certain three-dimensional sense, its essence is not a height map and does not contain three-dimensional information. This fact leads to certain limitations when using a scanning electron microscope to observe the fracture surface, which may cause misjudgment by researchers. Therefore, a suitable method is needed to reconstruct the fracture surface to supplement three-dimensional information.

[0004] Three-dimensional reconstruction of the sample in the scanning electron microscope image based on stereophotogrammetry technology is widely used due to its easy operation and low cost. It has the characteristics of not damaging the sample, fast calculation speed, large reconstruction range, etc. This method first obtains images of the sample at multiple angles, identifies and matches the feature points therein, and generates a three-dimensional model after calculating the point cloud.

[0005] The current method for obtaining scanning electron microscope images for three-dimensional reconstruction usually obtains them by rotating the sample stage one week, as shown in Figure 1 However, a large number of features in the alloy fracture surface have strong gradient information. It is difficult to obtain comprehensive feature information by rotating the sample stage, which affects the integrity of the model. At the same time, due to the lack of information such as the shooting angle and the internal and external parameters of the camera, the absolute size of the finally reconstructed three-dimensional model is incorrect and is usually scaled proportionally. Therefore, there is an urgent need for a method that can obtain enough original features for image acquisition and correct the scale of the model while taking efficiency into account, so as to obtain a complete alloy fracture surface model with accurate scale information, and then analyze the fracture process. Summary of the Invention

[0006] The present invention provides a method for three-dimensional reconstruction and scale calibration of alloy fracture surfaces to solve the problem of easy misjudgment caused by the lack of three-dimensional information during existing fracture surface observation, and can effectively supplement the visual information and scale information during fracture surface observation.

[0007] To achieve the above object, the present invention provides the following technical solutions:

[0008] A method for three-dimensional reconstruction and scale calibration of an alloy fracture surface, the steps are as follows:

[0009] S1. Mechanically polish and polish the original alloy sample to obtain an experimental specimen;

[0010] S2. After performing a thermo-mechanical coupling deformation experiment on the specimen, obtain a pair of fracture surface samples;

[0011] S3. Paste the fracture surface sample on the scanning electron microscope sample stage, and fix the sample stage in the scanning electron microscope chamber;

[0012] S4. By tilting the sample stage, obtain images of the fracture surface sample at different angles, perform three-dimensional reconstruction, and obtain a three-dimensional fracture surface model;

[0013] S5. Select two marking points on the cross section of the fracture surface sample, continuously collect images of the two marking point areas by adjusting the height of the sample stage respectively, retain the image with the highest clarity, and record the three-dimensional coordinates of the sample stage at this time;

[0014] S6. Calculate the distance between the two marking points according to the two sets of three-dimensional coordinates, and scale the three-dimensional fracture surface model to complete scale calibration.

[0015] Further, in step S3, use conductive glue to vertically paste the fracture surface sample on the edge of the sample stage with the cross section facing up; after placing the sample stage in the scanning electron microscope chamber, fix it with screws.

[0016] Further, in step S4, control the sample stage to first tilt 25° - 45° in the negative X-axis direction, and then tilt in the positive X-axis direction at a step size of ≤ 10° to 25° - 45° in the positive X-axis direction. At each step, it is necessary to adjust the field of view and focal length, take a complete and clear scanning electron microscope image of the fracture surface, and then return the sample stage to the original position;

[0017] Control the sample stage to tilt 25° - 4° in the negative Y-axis direction, and then tilt in the positive Y-axis direction at a step size of ≤ 10° to 25° - 45° in the positive Y-axis direction. At each step, it is necessary to adjust the field of view and focal length, take a complete and clear scanning electron microscope image of the fracture surface, and then return the sample stage to the original position.

[0018] Further, in step S5, at a magnification of ≥ 10k, adjust the scanning electron microscope field of view so that one of the marking points is located at the center of the field of view. Without changing the focal length, continuously adjust the Z-axis of the sample stage at a step size of ≤ 10μm, take at least 10 scanning electron microscope images to form an image sequence of this marking point; perform the same operation on the other marking point to obtain an image sequence of this marking point.

[0019] Further, the steps of scale calibration in step S6 are as follows:

[0020] Evaluate the sharpness of each image in the image sequence using the normalized variance equation to obtain the sharpest image in the image sequence, and derive the three-dimensional coordinates of the sample stage when obtaining this image; the normalized variance equation is as follows:

[0021]

[0022] Where I is the given image, M and N are the width and height of the given image respectively, i(u, v) is the pixel intensity at (u, v), μ is the average pixel intensity of the given image, and s(I) is the normalized variance of the given image;

[0023] Obtain the coordinates of the sample stage when the sharpest image is obtained using two marker points respectively, and calculate the actual distance d between the two marker points R .

[0024] Further, in the three-dimensional fracture model, obtain the coordinates of the two marker points and calculate the distance d between the two points M , and further calculate that the scaling ratio is the actual distance d R divided by the model distance d M , and complete the scale calibration of the three-dimensional fracture model.

[0025] Advantages of the present invention:

[0026] The present invention solves the problem of lack of three-dimensional information in fracture analysis. Under the premise of considering efficiency, a three-dimensional fracture model with complete features and accurate scale is obtained, which can assist in analyzing the fracture process of alloy specimens. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the specific embodiments of the present invention, the drawings required for use in the embodiments will be briefly introduced below.

[0028] Figure 1 Schematic diagram of an existing method for obtaining scanning electron microscope images for three-dimensional reconstruction.

[0029] Figure 2 Flowchart of a method for three-dimensional reconstruction and scale calibration of an alloy fracture provided by the present invention.

[0030] Figure 3 Schematic diagram of an improved method for obtaining scanning electron microscope images for three-dimensional reconstruction provided by the present invention.

[0031] Figure 4 Image of the marker points selected during scale calibration of the IN718 alloy fracture sample.

[0032] Figure 5 It is the image with the highest clarity in the image sequence of the marked points under high magnification of the fracture surface sample of IN718 alloy.

[0033] Figure 6 It is a schematic diagram of the three-dimensional fracture model (unit: micrometer) after scale calibration of the fracture surface sample of IN718 alloy.

[0034] Figure 7 It is an image of the surface topography of the fracture surface sample of TMS138 alloy.

[0035] Figure 8 It is a three-dimensional fracture model diagram after reconstruction of the fracture surface sample of TMS138 alloy. Specific implementation manners

[0036] In order to enable those skilled in the art to better understand the solutions of the embodiments of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention; obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without making creative efforts shall fall within the protection scope of the present invention.

[0037] The purpose of the present invention is to provide a method for three-dimensional reconstruction and scale calibration of alloy fracture surfaces, so as to solve the problem that only two-dimensional images can be used for fracture surface analysis and three-dimensional information is lacking, and to provide more complete and comprehensive visual information and more accurate scale information for fracture surface analysis.

[0038] In order to make the above objects, features and advantages of the present invention more obvious and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific implementation manners.

[0039] Embodiment 1:

[0040] In this embodiment, based on the Tescan mira4 scanning electron microscope, a three-dimensional model of the nickel-based polycrystalline fracture surface sample is obtained and the fracture process is analyzed to solve the problem that only two-dimensional information is contained in fracture surface analysis and three-dimensional information is missing.

[0041] The specific steps are as follows:

[0042] As Figure 1-6 shown, a method for three-dimensional reconstruction and scale calibration of alloy fracture surfaces is as follows:

[0043] S1. Prepare an IN718 specimen suitable for high-temperature in-situ tensile testing. The shape is dog-bone-shaped, with a length of 50 mm and an observation gauge section with a length of 1.5 mm and a width of 1.35 mm. Mechanically polish the original tensile specimen successively with 400-mesh, 600-mesh, 800-mesh, and 1200-mesh sandpapers. Use a vibratory polisher to rough-polish and then fine-polish the polished sample. Use a copper chloride solution to corrode the polished sample to focus on the surface morphology during the tensile test.

[0044] S2. Conduct an in-situ tensile test on the specimen at 650 °C until fracture to obtain a pair of fracture samples A and B, and record the experimental data simultaneously.

[0045] S3. Use conductive glue to vertically fix the above-mentioned tensile fracture sample A on the edge of the scanning electron microscope sample stage with the fracture surface facing up. Place the sample stage in the scanning electron microscope chamber and fix it with screws.

[0046] Evacuate the scanning electron microscope chamber to a vacuum, turn on the electron beam, obtain real-time secondary electron imaging, adjust the field of view and focal length so that the fracture sample A is in the center of the field of view and the image is clear. At the same time, adjust the rotation angle of the sample stage and the field of view angle so that the tilted sample stage is along the X-axis direction at this time.

[0047] S4. On the premise of ensuring the integrity of the model, to avoid data redundancy, set the tilt angle range from -30° to 30° and the tilt step size to 5° according to the undulation of the fracture surface morphology.

[0048] Tilt the sample stage 30° along the negative X-axis direction, and tilt the sample stage along the positive X-axis direction in 5° steps to 30° in the positive direction. Adjust the field of view and focal length at each step, and take a total of 13 complete and clear secondary electron images of the fracture sample, and record the tilt angle of the sample stage at each step. After completion, return the tilt angle of the sample stage to 0°.

[0049] Adjust the rotation angle of the sample stage and the field of view angle to increase by 90°. At this time, the tilt of the sample stage will be along the Y-axis direction.

[0050] Tilt the sample stage along the Y-axis. The steps are the same as above, and a total of 13 complete and clear secondary electron images of the fracture sample are obtained. Since the image information obtained when the tilt angle is 0° is the same during the two acquisition processes, a total of 25 effective images of this fracture sample are obtained.

[0051] After the acquisition process is completed, use the secondary electron images of the fracture sample at multiple tilt angles to complete 3D reconstruction to obtain a 3D fracture model. In this embodiment, Agisoft Metashape 3D reconstruction software is used, but it is not limited to this.

[0052] By tilting to collect the original scanning electron microscope images for 3D reconstruction, the problem that the information of some convex and concave parts is incomplete when rotating the sample stage can be solved; within the tilting range of -30° to 30°, the features on the four sides of the fracture sample are complete enough to ensure the integrity of the reconstructed 3D fracture model; collecting images at a step size of 5° can take into account both the reconstruction effect and the collection efficiency, avoiding data redundancy and data insufficiency.

[0053] S5: At a tilting angle of 0°, find two marked points dot1 and dot2 with significant features in the fracture sample; as Figure 4 shown, place dot1 at the center of the field of view, keep the focal length WD at 20 cm unchanged, continuously adjust the Z-axis height of the sample stage at a step size of 10 μm under a magnification of 10k times and collect a total of 20 images in the image sequence. The clarity feature of this image sequence should show gradual clarity to gradual blurring; collect a total of 20 images in the image sequence of dot2 in the same way. It should be noted that the focal length WD also needs to be kept at 20 cm unchanged during this process;

[0054] Repeat the above steps S3 - S5 for another fracture sample B to obtain 25 effective images of fracture sample B and two image sequences of the two selected marked points;

[0055] S6: Use the 25 scanning electron microscope effective images of fracture sample A and fracture sample B respectively for 3D reconstruction to obtain the uncalibrated 3D fracture models A* and B*;

[0056] Use the result of the normalized variance equation as the image clarity index to evaluate the image sequences of the marked points dot1 and dot2 of fracture sample A; the normalized variance equation is as follows:

[0057]

[0058] where I is the given image, M and N are the width and height of the given image respectively, i(u, v) is the pixel intensity at (u, v), μ is the average pixel intensity of the given image, and s(I) is the normalized variance of the given image;

[0059] The image with the highest score is considered to have the highest clarity. As Figure 5 shown, record the three-dimensional coordinates of the sample stage when the clearest image is obtained respectively; since the position between the sample stage and the fracture sample is fixed, it can be considered that the distance between the two marked points dot1 and dot2 is the displacement of the sample stage when focusing on these two points; calculate the distance difference between the two points of the sample stage using the distance formula between two points as the distance difference between the two marked points of fracture sample A, which is used to calibrate the scale information of the 3D fracture model A*.

[0060] Repeat step S6 to calculate the distance difference between the two marked points of the fracture surface sample B.

[0061] Find the two marks in the three-dimensional fracture models A* and B* respectively, measure the distance between the points, and calibrate the distance between the two marked points in A* to the distance between the two marked points in A, and the distance between the two marked points in B* to the distance between the two marked points in B by scaling A* and B* respectively, to complete the scale calibration. The calibrated three-dimensional fracture model A* is as Figure 6 shown.

[0062] The tilting angles of the sample stage in the X-axis and Y-axis directions can also be any value between -25° and 45° in the negative direction and between 25° and 45° in the positive direction.

[0063] Example 2:

[0064] As Figure 7 , Figure 8 shown, based on the Tescan clara scanning electron microscope, a three-dimensional model of a nickel-based single crystal fracture sample is obtained using the alloy fracture three-dimensional reconstruction and scale calibration method. Since the scale information of the three-dimensional fracture model is not required in this experiment, only the image acquisition and reconstruction processes are described.

[0065] The specific steps are as follows:

[0066] S1. Prepare a TMS138 specimen suitable for high-temperature in-situ creep experiments, with a dog-bone shape, a length of 46 mm, and an observation gauge section with a length of 1.5 mm and a width of 1.5 mm; mechanically polish the original creep specimen successively with 600-mesh, 800-mesh, 1200-mesh, 2000-mesh, 3000-mesh, and 5000-mesh sandpapers, and then use a vibratory polisher to perform rough polishing and fine polishing on the polished sample successively. Use a solution of HF:HNO3:glycerol = 1:2:3 (volume ratio) to corrode the polished sample for 30 seconds to focus on the surface morphology in the creep experiment;

[0067] S2. Conduct an in-situ creep experiment at 750 °C / 800 MPa until fracture to obtain a pair of fracture samples C and D, and record the experimental data simultaneously;

[0068] S3. Use conductive glue to vertically fix the above creep fracture samples C and D on the edge of the scanning electron microscope sample stage respectively, with the fracture surface facing up; place the sample stage in the scanning electron microscope chamber; fix it with screws.

[0069] Pump the scanning electron microscope chamber into vacuum, turn on the electron beam, obtain real-time secondary electron imaging, adjust the field of view and focal length to make the fracture sample C located at the center of the field of view and the image clear; at the same time, adjust the rotation angle of the sample stage and the field of view angle so that the tilted sample stage is along the X-axis direction at this time;

[0070] S4. Based on the characteristics that the C cross-section of the fracture sample is relatively smooth but has a large slope, set the tilting angle range to -45° to 45°. At the same time, to reduce image redundancy, set the tilting step size to 10°;

[0071] Tilt the sample stage 45° in the negative X-axis direction, and tilt the sample stage along the positive X-axis direction in steps of 10° to 45° in the positive direction. Adjust the field of view and focal length at each step, and take a total of 10 complete and clear secondary electron images of the fracture sample, and record the tilting angle of the sample stage at each step; after completion, return the tilting angle of the sample stage to 0°;

[0072] Since 0° is skipped during the tilting process, after returning the sample stage to the upright position, obtain 1 fracture image of the sample stage without tilting at this time;

[0073] Tilt the sample stage along the Y-axis, and the steps are the same as above, and a total of 10 complete and clear secondary electron images of the fracture sample are obtained; after completion, a total of 21 images of the fracture sample C at different angles are obtained;

[0074] Repeat step S4 for another fracture sample D to obtain a total of 21 effective images of the fracture sample D;

[0075] S5. Use the 21 effective scanning electron microscope images of the fracture sample C and the fracture sample D respectively to perform three-dimensional reconstruction to obtain the three-dimensional fracture models C* and D*, where the fracture model C* is as Figure 8 shown. In this embodiment, Agisoft Metashape three-dimensional reconstruction software is used, but it is not limited to this.

[0076] The results show that after using three-dimensional reconstruction to obtain the three-dimensional model of the fracture sample, the three-dimensional information missing in the fracture analysis can be effectively supplemented, and the proposed clarity evaluation and calibration method can also effectively supplement the scale information of the three-dimensional fracture model.

[0077] It can be seen that the present invention provides a method for three-dimensional reconstruction and scale calibration of alloy fractures. By proposing an optimized image acquisition method, more features are collected in the original data, improving the integrity of the reconstructed three-dimensional fracture model; calibrating the scale of the three-dimensional fracture model reveals the positional relationship between features with height differences in the sample, and can effectively make up for the deficiencies in fracture analysis.

[0078] The above has detailedly described the structure, characteristics and effects of the present invention according to the illustrated embodiments. The above is only the preferred embodiment of the present invention, but the present invention is not limited to the scope defined by the drawings. Any changes made according to the concept of the present invention, or equivalent embodiments modified into equivalent changes, still within the spirit covered by the specification and the drawings, shall be within the protection scope of the present invention.

Claims

1. A three-dimensional reconstruction and scale calibration method for alloy fracture surfaces, characterized in that, The steps are as follows: S1. Mechanically grind and polish the original alloy sample to obtain an experimental specimen; S2. Conduct a thermo-mechanical coupled deformation experiment on the specimen to obtain a pair of fracture samples; S3. Paste the fracture samples on the scanning electron microscope sample stage and fix the sample stage in the scanning electron microscope chamber; S4. Obtain images of the fracture samples at different angles by tilting the sample stage, perform three-dimensional reconstruction to obtain a three-dimensional fracture model; S5. Select two marking points on the fracture surface of the fracture sample, continuously collect images of the two marking point areas by adjusting the height of the sample stage respectively, retain the image with the highest clarity, and record the three-dimensional coordinates of the sample stage at this time. Specifically: At a magnification of ≥10k, adjust the scanning electron microscope field of view so that one of the marking points is at the center of the field of view. Without changing the focal length, continuously adjust the Z-axis of the sample stage in steps of ≤10μm and take at least 10 scanning electron microscope images to form an image sequence of this marking point; perform the same operation on the other marking point to obtain an image sequence of this marking point; Use the normalized variance equation to evaluate the clarity of each image in the image sequence, obtain the clearest image in the image sequence, and export the three-dimensional coordinates of the sample stage when obtaining this image. The normalized variance equation is as follows: where I is the given image, M and N are the width and height of the given image respectively, i(u,v) is the pixel intensity at (u,v), μ is the average pixel intensity of the given image, and s(I) is the normalized variance of the given image; Obtain the sample stage coordinates when the clearest images are obtained using two marker points respectively, and calculate the actual distance d between the two marker points R ; S6. Obtain the coordinates of two marked points in the three-dimensional fracture model and calculate the distance d between the two marked points M , and further calculate that the scaling ratio is the actual distance d R divided by the model distance d M , and complete the scale calibration of the three-dimensional fracture model.

2. The three-dimensional reconstruction and scale calibration method of the alloy fracture surface according to claim 1, characterized in that In step S3, use conductive glue to vertically paste the fracture samples on the edge of the sample stage with the fracture surface facing up; after placing the sample stage in the scanning electron microscope chamber, fix it with screws.

3. The three-dimensional reconstruction and scale calibration method of the alloy fracture surface according to claim 1, wherein In step S4, control the sample stage to first tilt 25° - 45° in the negative X-axis direction, and then tilt in the positive X-axis direction in steps of ≤10° to 25° - 45° in the positive X-axis direction. Adjust the field of view and focal length at each step and take complete and clear scanning electron microscope images of the fracture. After completion, return the sample stage to the original position; Control the sample stage to tilt 25° - 45° in the negative Y-axis direction, and then tilt in the positive Y-axis direction in steps of ≤10° to 25° - 45° in the positive Y-axis direction. Adjust the field of view and focal length at each step and take complete and clear scanning electron microscope images of the fracture. After completion, return the sample stage to the original position.

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