A method for in-situ three-dimensional surface reconstruction based on scanning electron microscope

Abstract

The application provides an in-situ three-dimensional surface reconstruction method based on a scanning electron microscope, and relates to the technical field of material microscopic characterization and analysis, and comprises the following steps: preparing a metal sample suitable for in-situ mechanical test, fixing the sample on an in-situ test table, and then fixing the sample as a whole in a scanning electron microscope chamber; performing in-situ mechanical test, and continuously collecting symmetric four-region images of the sample surface at different deformation stages by using a four-region backscattered electron detector; extracting directional gradient information by using a gradient calculation formula, and calculating an X / Y directional gradient matrix of the surface; solving a three-dimensional height field sequence by using a regularization method, and realizing three-dimensional surface reconstruction of a dynamic deformation process. The application innovatively proposes a three-dimensional surface reconstruction algorithm based on multi-detector image fusion, solves the problem that high-precision three-dimensional topography of an arbitrary deformation region cannot be obtained in real time in in-situ scanning electron microscope test, supports cross-scale characterization, and significantly improves the analysis capability of surface microscopic deformation mechanisms of metal materials under complex stress conditions such as tension and fatigue.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of material micro-characterization analysis, and in particular to an in-situ three-dimensional surface reconstruction method based on a scanning electron microscope. BACKGROUND

[0002] In the research of material science, it is crucial to understand the surface morphology change of metal materials during the stress process for revealing the mechanical properties and deformation mechanism. The traditional material characterization method is often difficult to obtain high-precision and high-resolution three-dimensional surface information in the in-situ test process. As a powerful micro-characterization tool, the scanning electron microscope (SEM) can provide high-resolution images of the material surface, but its application in the three-dimensional reconstruction of in-situ test of metal materials faces many challenges.

[0003] Most of the existing three-dimensional reconstruction methods are based on optical microscopes or other non-in-situ techniques, which have limitations in resolution and capturing the true surface state of the material. Although combining SEM with in-situ test devices can observe the micro changes of materials during deformation, how to accurately extract and reconstruct three-dimensional surface height information from SEM images, especially in different deformation stages for precise analysis of any area, is still a problem to be solved. At present, there is a lack of an effective algorithm and method to achieve this goal in the in-situ SEM test process, which limits the in-depth understanding of the surface micro behavior of metal materials under complex stress conditions. SUMMARY

[0004] The present application provides an in-situ three-dimensional surface reconstruction method based on a scanning electron microscope to solve the problem of lack of accurate three-dimensional surface height information of any area in different deformation stages in the existing in-situ SEM test process, which can effectively enhance the in-depth understanding of the surface micro deformation behavior of metal materials under complex stress conditions.

[0005] To achieve the above-mentioned purpose, the present application provides the following technical solutions:

[0006] An in-situ three-dimensional surface reconstruction method based on a scanning electron microscope, the steps are as follows:

[0007] S1, mechanically polishing and polishing the original metal sample, the middle region of the metal sample is provided with a gauge section region, to obtain an in-situ experimental sample; horizontally fixing the in-situ experimental sample on an in-situ test table, and then fixing the in-situ test table in a scanning electron microscope chamber, and performing in-situ mechanical testing along the X direction;

[0008] S2, during the in-situ mechanical testing, using a four-region backscattered electron detector comprising four independent regions A, B, C, D to take four-region images I of the electron intensity distribution of the microstructure of the surface of the in-situ test sample at different deformation stages A B C D wherein I A B and I C D are respectively centrosymmetric about the center of the detector center point;

[0009] S3, based on the four-region images I A B C D , the X-direction and Y-direction gradient matrices G x G y of the region imaged by the images at different deformation stages are calculated

[0010] S4, based on the X-direction and Y-direction gradient matrices G x G y , a regularized three-dimensional reconstruction method is used to reconstruct a three-dimensional height field sequence Z of the region imaged at different deformation stages, to complete the in-situ surface three-dimensional reconstruction.

[0011] Further, the in-situ test bench comprises an in-situ tensile test bench, an in-situ fatigue test bench, and an in-situ creep test bench, corresponding to in-situ tensile testing, in-situ fatigue testing, and in-situ creep testing.

[0012] Further, in step S2, the center of the four-region backscattered electron detector coincides with the center of the electron beam of the scanning electron microscope, the amplification coefficient of the hardware amplifier connected to the rear of the four-region backscattered electron detector should be calibrated in advance, the field of view of the scanning electron microscope remains fixed during the acquisition process, the working distance WD of the electron beam of the scanning electron microscope is ≤18mm to obtain excellent imaging quality, and the process of imaging the microstructure of the surface of the sample can include any magnification to achieve cross-scale characterization.

[0013] Further, in step S3, the gradient calculation formula for the A, B directions is: The gradient calculation formula for the C, D directions is: wherein k is a proportional coefficient of the first-order term, b is a proportional coefficient of the cubic term and b<0, and ε is a very small constant, and the cubic term is used to eliminate the shadow effect caused by high-slope planes;

[0014] Subsequently, based on the obtained A, B direction gradient G AB ​​​​​​​​With C, D direction gradient G CD Calculate X direction and Y direction gradient matrix Gx, G y , the formula is:

[0015]

[0016] Where θ1, θ2 is the angle between the detector distribution, which is related to the installation angle of the detector, and can be directly measured.

[0017] Further, in step S4, the X direction and Y direction gradient matrix G x ,G y Respectively along the column direction is reconstructed into a column vector, and then spliced into a column vector G, and the three-dimensional height field is solved by using the least square method combined with the regularization method The solving formula is:

[0018]

[0019] Where P is a forward difference operator, L is a central difference operator as a regularization matrix, and λ is a regularization coefficient; The three-dimensional height field at different deformation stages is calculated respectively, a sequence Z is formed, and in-situ surface three-dimensional reconstruction is completed.

[0020] The beneficial effects of the present application are:

[0021] The present application overcomes the problem that accurate three-dimensional surface height information cannot be provided for any area at different deformation stages in in-situ SEM testing, thereby significantly improving the in-depth understanding of the surface microscopic deformation behavior of metal materials under complex stress state. BRIEF DESCRIPTION OF DRAWINGS

[0022] In order to more clearly illustrate the specific embodiments of the present application, the drawings needed in the embodiments will be briefly introduced as follows.

[0023] Figure 1 The in-situ three-dimensional surface reconstruction method flowchart based on a scanning electron microscope provided by the present application.

[0024] Figure 2 It is a schematic diagram of in-situ mechanical test bench and scanning electron microscope imaging.

[0025] Figure 3 It is the force-displacement curve of in-situ tensile test of 5B70 aluminum alloy.

[0026] Figure 4 It is the surface gradient of the center indentation of the gauge section in the initial state of 5B70 aluminum alloy.

[0027] Figure 5 It is the three-dimensional height field of the center indentation of the gauge section in the initial state of 5B70 aluminum alloy.

[0028] Figure 6 The three-dimensional height field of the center indentation of the gauge section of the 5B70 aluminum alloy at different tensile displacements. DETAILED DESCRIPTION

[0029] In order for those skilled in the art to better understand the solutions of the embodiments of the present application, the technical solutions in the embodiments of the present application will be described clearly and completely in combination with the drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, not all. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0030] The purpose of the present application is to provide a scanning electron microscope-based in-situ three-dimensional surface reconstruction method, which solves the problem of lacking accurate three-dimensional surface height information for any area at different deformation stages in the existing in-situ SEM test, and effectively promotes a deeper understanding of the surface microscopic deformation behavior of metal materials under complex stress environment.

[0031] In order to make the above-mentioned purposes, features and advantages of the present application more obvious and easy to understand, the present application will be further described in detail in combination with the drawings and specific embodiments.

[0032] Embodiment:

[0033] In the present embodiment, based on the CIQTEK SEM 5000Pro scanning electron microscope equipped with a four-region backscattered electron detector, a uniaxial in-situ tensile test is performed on an aluminum alloy sample with a 5B70 brand, and four-region images of the center indentation of the gauge section at different deformation stages during the test are obtained and three-dimensional reconstruction is performed, overcoming the difficulty that accurate three-dimensional surface height information cannot be provided for any area at different deformation stages in the in-situ SEM test.

[0034] As shown in Figure 1 The scanning electron microscope-based in-situ three-dimensional surface reconstruction method provided in the present embodiment has the following steps:

[0035] S1, prepare a 5B70 aluminum alloy sample suitable for in-situ tensile test, which is dog-bone-shaped, 50mm in length, contains an observation gauge section with a length of 1.5mm and a width of 1.35mm, and is mechanically polished using 400-mesh, 600-mesh, 800-mesh and 1200-mesh sandpaper in sequence, and then is coarsely polished and finely polished using a vibration polisher; a hardness tester is used to test and leave a cross indentation at the center of the gauge section; the sample is fixed horizontally on an in-situ tensile test bench, and then the in-situ tensile test bench is fixed in a scanning electron microscope chamber, and an in-situ mechanical test along the X direction is performed, as shown in Figure 2 ​

[0036] S2. During the in-situ tensile test, a four-region backscattered electron detector (SEM) containing four independent regions (A, B, C, and D) is used. Regions A, C, B, and D are arranged counter-clockwise and displaced at 100 μm intervals along the tensile direction. Images of the electron intensity distribution in the central indentation region of the gauge length section of the sample at different deformation stages are captured. The center of the four-region SEM coincides with the center of the scanning electron microscope (SEM) electron beam. The current amplification factor of the hardware amplifier connected to the four-region SEM should be pre-calibrated to be consistent. During the acquisition process, the SEM field of view remains fixed, always using the center point of the indentation as the center point of the field of view. The working distance of the SEM electron beam is maintained at approximately 15 mm. After the experiment, the force-displacement curve is output, as shown below. Figure 3 As shown.

[0037] S3, Four-region images based on different deformation stages A ,I B ,I C ,I D Through formula as well as The gradients along directions A, B and C, D of the region captured in the image are calculated at different deformation stages. In this example, k = 1.6, b = -0.2, and ε = 10. -6 ;

[0038] Further, through the formula:

[0039]

[0040] Calculate the gradient matrix G of the surface in the X and Y directions. x G y In this example, θ1 = 40°, θ2 = -50°. The surface gradient of the indentation at the center of the gauge length of the sample is as follows: Figure 4 As shown.

[0041] S4. Combine the gradient matrices G in the X and Y directions. x G y The vectors are reconstructed along the column direction and then concatenated into a column vector G. The three-dimensional height field is then solved using the least squares method combined with regularization. The solution formula is:

[0042]

[0043] Wherein P is a forward difference operator, L is a central difference operator as a regularization matrix, and lambda is a regularization coefficient; in this example, the image size is 1536*1024 pixels, the image width W is 1536 pixels, the image height H is 1024 pixels, and the horizontal field width HFW is 260 mu m. P is a sparse representation of a forward difference matrix, with a size of [2*H*W, H*W], wherein the front [H*W, H*W] is a horizontal difference part, and H*W-H rows are generated, wherein each row has a negative effective value at the diagonal position, and a positive effective value at a position right to the diagonal position by a distance of H; the rear [H*W, H*W] is a vertical difference part, and W*(H-1) rows are generated, and the horizontal adjacent nodes are processed in blocks, and each block has a step length of H, the first H-1 rows have a negative effective value at the diagonal position, and a positive effective value at a position right to the diagonal position by a distance of 1, wherein the size of the effective value is the width W of the image divided by the horizontal field width HFW, to represent the real distance occupied by each pixel. L is a sparse representation of a central difference regularization matrix, with a size of [2*H*W, H*W], wherein the front [H*W, H*W] is a horizontal difference part, and H*(W-2) rows are generated, wherein each row has a positive effective value at the diagonal position, a double negative effective value at a position right to the diagonal position by a distance of H, and a positive effective value at a position right to the diagonal position by a distance of 2H; the rear [H*W, H*W] is a vertical difference part, and W*(H-2) rows are generated, and the horizontal adjacent nodes are processed in blocks, and each block has a step length of H, the first H-2 rows have a positive effective value at the diagonal position, a double negative effective value at a position right to the diagonal position by a distance of 1, and a positive effective value at a position right to the diagonal position by a distance of 2, wherein the size of the effective value is consistent with the size of the effective value in P. The value of lambda is 0.05. The three-dimensional height fields at different deformation stages are calculated respectively to form a sequence Z, and the in-situ three-dimensional reconstruction of the surface is completed, as shown in Figure 5 、 Figure 6

[0044] The results show that after the three-dimensional height field of the microstructure morphology of the sample surface in the in-situ mechanical test process is obtained by using three-dimensional reconstruction, the three-dimensional surface height information of the metal material at different deformation stages can be effectively supplemented, and the in-depth understanding of the surface micro-deformation behavior of the material under complex stress conditions can be effectively strengthened.

[0045] It can be seen that the in-situ three-dimensional surface reconstruction method based on a scanning electron microscope provided by the application realizes the recovery of the microstructure evolution height information of the material at different deformation stages in the in-situ process by combining the in-situ mechanical test test bench with the imaging of the four-region backscattering electron detector.

[0046] ​The structure, features and effects of the present application are described in detail above according to the embodiments shown in the drawings. The above description is only the preferred embodiments of the present application, but the present application is not limited to the embodiments shown in the drawings. Any changes or modifications made according to the concept of the present application, or equivalent embodiments with equivalent changes, are still within the scope of the present application.

Claims

1. A method for in-situ three-dimensional surface reconstruction based on scanning electron microscopy, characterized in that, The method comprises the following steps: S1, mechanical polishing and polishing are performed on an original metal sample, a middle region of the metal sample is provided with a gauge section region, an in-situ experimental sample is obtained, the in-situ experimental sample is horizontally fixed on an in-situ test bench, then the in-situ test bench is fixed in a scanning electron microscope chamber, and in-situ mechanical testing along an X direction is performed; S2, during the in-situ mechanical testing process, using a four-region backscattered electron detector comprising four independent regions A, B, C, D to take four-region images I of the electron intensity distribution of the microstructure of the surface of the in-situ experimental sample at different deformation stages A , B , C , D , wherein A, C, B, D are arranged in anticlockwise order, and A, B are arranged symmetrically about the center point of the detector, and C, D are arranged symmetrically about the center point of the detector; S3, obtaining the gradient matrix G of the surface in the X direction and the Y direction of the region photographed by the image in different deformation stages based on the four-region image I in the different deformation stages A B C D x y ;​​​​​ The gradient calculation formula of A, B direction is: The gradient calculation formula of C, D direction is: Wherein k is the proportional coefficient of linear term, b is the proportional coefficient of cubic term and b<0, and ε is a very small constant. The cubic term is used to eliminate the shadow effect caused by high slope plane. Subsequently, based on the resulting A, B direction gradients G AB with C, D direction gradients G CD X direction and Y direction gradient matrices Gx, Gy are calculated y The calculation formula is: Wherein θ1, θ2 are distribution angles of the detector, which are related to the installation angle of the detector and can be directly measured, specifically, θ1 is the included angle between the A, B directions and the X direction, and θ2 is the included angle between the C, D directions and the X direction; S4, based on the X direction and Y direction gradient matrix G x ,G y , a regularization three-dimensional reconstruction method is used to reconstruct the three-dimensional height field sequence Z of the photographed region at different deformation stages, to complete in-situ surface three-dimensional reconstruction, specifically: The X direction and Y direction gradient matrix G x ,G y Reshape into column vectors along the column direction respectively, and then splice into a column vector G, and solve the three-dimensional height field by using the least square method combined with the regularization method The solving formula is: Wherein P is a forward difference operator, L is a central difference operator as a regularization matrix, and λ is a regularization coefficient; the three-dimensional height fields in the different deformation stages are calculated respectively to form a sequence Z, and in-situ surface three-dimensional reconstruction is completed.

2. The SEM-based in-situ 3D surface reconstruction method according to claim 1, characterized in that, The in-situ test bench comprises an in-situ tensile test bench, an in-situ fatigue test bench and an in-situ creep test bench, corresponding to in-situ tensile testing, in-situ fatigue testing and in-situ creep testing.

3. The SEM-based in-situ 3D surface reconstruction method according to claim 1, characterized in that, In step S2, the center of the four-region backscattered electron detector coincides with the center of the scanning electron microscope electron beam, the amplification coefficient of the hardware amplifier connected to the rear of the four-region backscattered electron detector should be calibrated in advance, the field of view of the scanning electron microscope remains fixed during the acquisition process, and the working distance WD of the scanning electron microscope electron beam is less than or equal to 18 mm to obtain excellent imaging quality.

Images