A DIC speckle preparation method and a method for characterizing local strain in micro-areas of metal materials

Abstract

The present invention discloses a DIC speckle preparation method and a method for characterizing local strain in a micro-area of ​​a metal material. The DIC speckle preparation method comprises the following steps: polishing the surface of the sample to be tested until the surface is bright and scratch-free; placing the sample in a scanning electron microscope, bombarding the sample with an electron beam to produce uniform and clear speckles on the surface of the sample, thereby obtaining speckles for measuring the local strain distribution characteristics of the material. The present invention only requires a scanning electron microscope to complete the preparation of the speckle, and after the sample is prepared with the speckle, it will not interfere with the collection of other signals on the surface to be tested, and can simultaneously meet the characterization requirements of the material such as SEM and EBSD, thereby realizing the effective combination of EBSD technology and DIC.

Description

Technical Field

[0001] The invention relates to the technical field of solid mechanics, and in particular to a DIC speckle preparation method and a method for characterizing local strain in a micro-region of a metal material. Background Art

[0002] As a non-contact displacement measurement tool, digital image correlation (DIC) is widely used to measure sample displacement and calculate local plastic deformation. By tracking the position change of the speckle in the undeformed and deformed samples, the displacement gradient tensor on the sample surface can be calculated, and then the strain distribution on the sample surface can be calculated. The accuracy of the calculation result is closely related to the quality of the speckle on the sample surface. At the micrometer scale, the commonly used methods include: focused ion beam, chemical physical deposition, chemical corrosion and other methods.

[0003] Combining DIC with other techniques can provide more information during in-situ deformation. Combining electron backscatter diffraction (EBSD) with DIC can obtain the relationship between local strain and grain orientation and microstructure. At present, when using EBSD testing, it is necessary to first highly polish the substrate to be tested, and then set speckles on the substrate to be tested by methods such as focused ion beam, chemical corrosion, and nanofilm reconstruction. However, the above technical processes are complex, costly, and have a long cycle, which limits their widespread application. Since EBSD technology requires a high degree of polishing on the surface of the sample to be tested, it is very easy to damage the surface of the substrate when setting speckles, resulting in an uneven substrate surface, making the substrate after setting speckles blurred when taking pictures; or weakening the electron backscatter signal, interfering with the collection of EBSD data. The above shortcomings make it difficult to combine EBSD technology with DIC.

[0004] Therefore, it is urgent to develop a micro-nano speckle preparation method with high stability and no damage to the sample surface, so as to combine EBSD technology with DIC to obtain more deformation information of the material. Summary of the invention

[0005] In view of the above-mentioned deficiencies in the prior art, the purpose of the present invention is to solve the problem that the existing speckle preparation technology is difficult to synchronously perform EBSD data acquisition, and to provide a DIC speckle preparation method and a method for characterizing local strain in a micro-area of ​​a metal material.

[0006] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0007] A DIC speckle preparation method comprises the following steps:

[0008] Polish the surface of the sample to be tested until it is bright and scratch-free;

[0009] The sample is placed in a scanning electron microscope and the electron beam is used to bombard the sample to produce uniform and clear speckles on the sample surface, that is, DIC speckles are obtained to characterize the distribution characteristics of the local strain of the material.

[0010] The electron beam bombardment voltage is 15-40 KV, the exposure time is 100-1000 ms, and the scanning step is 1-10 μm. During the electron beam bombardment process, the electron beam bombardment parameters can be adjusted in real time through the scanning window until clear speckles are obtained.

[0011] Furthermore, the sample is a metal material.

[0012] Furthermore, the metal material is magnesium alloy, titanium alloy or pure aluminum.

[0013] Furthermore, the bombardment voltage of the electron beam is 20-40 KV, the exposure time is 300-800 ms, and the scanning step length is 1-10 μm.

[0014] Furthermore, the polishing method includes mechanical polishing, electrolytic polishing or chemical polishing.

[0015] The present invention also provides a method for characterizing local strain in a metal material micro region, comprising the following steps:

[0016] S1, preparing speckles from the sample using the DIC speckle preparation method;

[0017] S2, photographing the sample with speckles prepared in S1 through the secondary electron imaging mode of the scanning electron microscope to obtain an undeformed SEM image;

[0018] S3, performing a deformation operation on the sample to deform the sample;

[0019] S4, photographing the deformed sample in S3 through the secondary electron imaging mode of a scanning electron microscope to obtain a SEM image of the deformed sample;

[0020] S5. Calculate the speckles on the undeformed SEM image and the deformed SEM image to characterize the local strain of the sample.

[0021] Furthermore, the deformation in step S3 includes bending, stretching or compression.

[0022] Step S5 specifically includes using software to obtain a grain boundary distribution map of the material, and then calculating the local strain distribution through code based on the speckles on the undeformed SEM image and the deformed SEM image; overlapping the grain boundary distribution map with the local strain distribution to obtain a cloud map containing both grain boundary information and strain distribution.

[0023] Compared with the prior art, the present invention has the following advantages:

[0024] 1. The present invention uses a scanning electron microscope to prepare marking points on the surface of metal materials, without the need for additional speckle preparation tools and materials. Compared with other DIC marking points, the electron beam bombardment traces are selected as marking points, which will not interfere with the acquisition of other signals on the surface to be measured, and can simultaneously meet the characterization requirements of materials such as SEM and EBSD; and it is widely used in a variety of metal materials. Compared with the speckle prepared by traditional corrosion and deposition methods, this method also has the advantages of low cost, short preparation cycle, and no need for specific equipment or specific corrosion chemicals.

[0025] 2. The present invention can accurately control the distribution position, density, and area size of the marking points by adjusting the electron beam bombardment parameters, and can flexibly and accurately meet the DIC analysis requirements.

[0026] 3. The characteristic speckle used to measure the local strain distribution of metal materials described in the present invention is used for non-contact, high-precision measurement of the full-field deformation of metal materials caused by force and thermal loads at room temperature and high temperature environments. It is especially applied to numerical image correlation (DIC), electron backscatter diffraction (EBSD) and in-situ deformation platforms, effectively combining macro deformation, micro-region grain deformation and strain distribution states in specific areas under different deformation amounts, and organically combining mechanical properties with microscopic deformation mechanisms. The deformation mechanism of metal materials can be studied in situ from multiple scales, multiple levels and multiple aspects. The speckle prepared by the method of the present invention can be used to perform multi-faceted test characterization of the test area, avoid repeated test characterization, and reduce test errors. While greatly reducing the test cycle and reducing the test cost, the desired test results and the correlation between specific areas can be obtained simultaneously. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a SEM image of the speckle of the AZ31 magnesium alloy prepared in Example 1 of the present invention.

[0028] Figure 2 It is a schematic flow chart of the method for characterizing local strain in a micro-region of a metal material in the present invention.

[0029] Figure 3 This is a schematic diagram of the bending tool used in Example 2 of the present invention.

[0030] Figure 4 The SEM-EBSD-DIC analysis results of AZ31 magnesium alloy, where a is the inverse pole figure after deformation, b, c, e, and f are Exx, Eyy, Exy, and Eyy, respectively. VM Strain distribution and grain boundary superposition diagram, d is the SEM image after deformation. DETAILED DESCRIPTION

[0031] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0032] 1. Preparation of DIC speckle

[0033] Example 1

[0034] A DIC speckle preparation method comprises the following steps:

[0035] The AZ31 magnesium alloy was polished with 400#, 600#, 800#, 1000# and 1200# metallographic sandpapers in sequence; the test surface of the magnesium alloy was electropolished with commercial magnesium alloy polishing liquid (model: ACII) until the test surface was bright and scratch-free.

[0036] EBSD was taken in a Tescan Mira3 scanning electron microscope equipped with an EBSD probe using an acceleration voltage of 20KV, an exposure time of 364.82 ms, and a scanning step of 4 μm. The electron beam bombardment traces during EBSD shooting showed uniform speckles with an interval of 4 μm on the surface to be tested. That is, the DIC speckle used to characterize the distribution characteristics of local strain of the material was obtained, such as Figure 1 shown.

[0037] The following examples can be prepared

[0038] Example Metal Materials Bombardment voltage KV Exposure time ms Scanning step μm Example 2 Magnesium Alloy 20 364.82 4 Example 3 Titanium Alloy 20 536.21 4 Example 4 Pure aluminum 20 323.14 4

[0039] The method of the present invention solves the problems of high speckle preparation cost, complex process, uneven speckle distribution, poor applicability and interference with other characterizations in the prior art.

[0040] The following embodiment 1 is used to demonstrate that the characteristic speckle prepared by the present invention for measuring the local strain distribution of metal materials will not interfere with the acquisition of other signals on the surface to be measured, and can simultaneously meet the characterization requirements of materials such as SEM and EBSD. In addition, the present invention can accurately control the distribution position, density, and area size of the marking points by adjusting the electron beam bombardment parameters, and can flexibly and accurately meet the DIC analysis requirements.

[0041] 2. Characterization of local strain in micro-regions of metal materials.

[0042] A method for characterizing local strain in a metal micro-region, see Figure 2 , including the following steps,

[0043] S1. Prepare speckles from a sample using the DIC speckle preparation method. This example uses the AZ31 magnesium alloy speckle sample prepared in Example 1.

[0044] S2. The prepared AZ31 magnesium alloy speckle sample was photographed by the secondary electron imaging mode of the scanning electron microscope with an acceleration voltage of 20 KV, an exposure time of 364.82 ms, and a step length of 4 μm to obtain an undeformed SEM image.

[0045] S3. Place the AZ31 magnesium alloy sample on a bending tool to perform bending deformation on the sample. The bending tool is as follows: Figure 3 As shown. The AZ31 magnesium alloy sample is placed on the workbench 1 of the bending tool, and the stud 2 is rotated to raise the workbench 1. During the rising process, the upper side of the AZ31 magnesium alloy sample 4 contacts the two pins 3 fixed on the bending tool and deforms. When the deformation reaches the required degree, the stud 2 is rotated in the opposite direction to lower the workbench, and the AZ31 magnesium alloy sample 4 is taken out to obtain the deformed sample.

[0046] S4. The deformed sample in S3 was photographed using the secondary electron imaging mode of a scanning electron microscope with an acceleration voltage of 20 KV, an exposure time of 15.64 ms, and a step size of 1 μm to obtain a SEM image of the deformed sample.

[0047] S5. Use Channel 5 software to obtain the inverse pole figure and grain boundary distribution map, and then calculate the local strain distribution through the code based on the speckle on the undeformed SEM image and the deformed SEM image. Overlap the grain boundary distribution map with the local strain distribution to obtain a cloud map containing both grain boundary information and strain distribution.

[0048] Figure 4 The SEM-EBSD-DIC analysis results of AZ31 magnesium alloy, where a is the inverse pole figure after deformation, which can obtain detailed grain orientation information and grain boundary distribution; b, c, e, and f are Exx, Eyy, Exy, and Eyy, respectively. VM Strain distribution and grain boundary superposition diagram, from Figure 4 The relationship between strain distribution and grain boundary can be obtained from b, c, e, and f in Figure 1. D is the SEM image after deformation, from which the grain boundary distribution and initial local deformation distribution can be roughly observed. Figure 4 It can be seen from b, c, e, and f that the local strain is mainly concentrated in the grain boundary or twin region, which is consistent with the rule that the local strain of magnesium alloy is mainly concentrated near the grain boundary or twin region. In addition, the strain distribution is also consistent with the local deformation result shown in the scanning image (Figure d).

[0049] It can be seen that the calculation results of the local strain distribution of AZ31 magnesium alloy using the DIC speckle prepared by the present invention are consistent with the actual results, indicating that the DIC speckle prepared by the present invention can be used for micro-area strain measurement of materials. At the same time, the details of the inverse pole figure after the speckle is prepared are clear, indicating that the speckle preparation method provided by the present invention can meet the signal acquisition requirements of the EBSD technology.

[0050] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit the technical solution. Those skilled in the art should understand that those modifications or equivalent substitutions of the technical solution of the present invention that do not depart from the purpose and scope of the technical solution should be included in the scope of the claims of the present invention.

Claims

1. A DIC speckle preparation method, characterized in that: The following steps are included: Polish the surface of the sample to be tested until it is bright and scratch-free; The sample is placed in a scanning electron microscope, and a uniform and clear speckle is produced on the surface of the sample by bombarding the sample with an electron beam, that is, a speckle is obtained for measuring the local strain distribution characteristics of the material; The bombardment voltage of the electron beam is 15-40 KV, the exposure time is 100-1000 ms, and the scanning step length is 1-10 μm.

2. The DIC speckle preparation method according to claim 1, characterized in that: The sample is a metal material.

3. The DIC speckle preparation method according to claim 2, characterized in that: The metal material is magnesium alloy, titanium alloy or pure aluminum.

4. The DIC speckle preparation method according to claim 1, characterized in that: The bombardment voltage of the electron beam is 20-40 KV, the exposure time is 300-800 ms, and the scanning step length is 1-10 μm.

5. The DIC speckle preparation method according to claim 1, characterized in that: The polishing method includes mechanical polishing, electrolytic polishing or chemical polishing.

6. A method for characterizing local strain in a metal material micro region, characterized in that: The following steps are included: S1. Using the DIC speckle preparation method as claimed in claim 1 to prepare speckles on the sample; S2, photographing the sample with speckles prepared in S1 through the secondary electron imaging mode of the scanning electron microscope to obtain an undeformed SEM image; S3, performing a deformation operation on the sample to deform the sample; S4, photographing the deformed sample in S3 through the secondary electron imaging mode of a scanning electron microscope to obtain a SEM image of the deformed sample; S5. Calculate the speckles on the undeformed SEM image and the deformed SEM image to characterize the local strain of the sample.

7. The method for characterizing local strain in a metal material micro region according to claim 6, characterized in that: The deformation in step S3 includes bending, stretching or compression.

8. The method for characterizing local strain in a metal material micro region according to claim 6, characterized in that: Step S5 specifically includes using software to obtain a grain boundary distribution map of the material, and then calculating the local strain distribution through code based on the speckles on the undeformed SEM image and the deformed SEM image; overlapping the grain boundary distribution map with the local strain distribution to obtain a cloud map containing both grain boundary information and strain distribution.

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