阅读说明：本技术 用于表征材料局域应变分布特性的sem-dic散斑制备方法 (SEM-DIC speckle preparation method for representing local strain distribution characteristics of material ) 是由 崔喜平 丁浩 郜闹闹 吕洋 王孝君 张圆圆 黄陆军 耿林 于 2020-04-27 设计创作，主要内容包括：本发明公开了用于表征材料局域应变分布特性的SEM-DIC散斑制备方法；属于光测力学、变形测量的技术领域。本发明解决现有技术中散斑分辨率低、散斑点分布不均匀性、适用性差,以及难以实现SEM-DIC应变分析等问题。散斑制备：配制纳米二氧化硅悬浊液；待测样品前处理；待测样品表面散斑制备,散斑图像质量评价。本发明散斑的制备方法简单、快速、适应性强、低成本、实用性强、同时制备的散斑图案分散度好、精度高且不损伤样品表面状态,可以同时准确获得加载过程中样品表面的局部应变分布规律和相对应区域的表面形貌演变规律。(The invention discloses a SEM-DIC speckle preparation method for representing local strain distribution characteristics of a material; belongs to the technical field of optical measurement mechanics and deformation measurement. The invention solves the problems of low speckle resolution, non-uniformity of speckle point distribution, poor applicability, difficulty in SEM-DIC strain analysis and the like in the prior art. Speckle preparation: preparing a nano silicon dioxide suspension; pretreating a sample to be detected; preparing the speckle on the surface of the sample to be measured, and evaluating the quality of the speckle image. The preparation method of the speckle pattern is simple, rapid, strong in adaptability, low in cost and strong in practicability, the prepared speckle pattern is good in dispersity and high in precision, the surface state of the sample is not damaged, and the local strain distribution rule of the surface of the sample in the loading process and the surface appearance evolution rule of the corresponding area can be simultaneously and accurately obtained.)

1. The SEM-DIC speckle preparation method for representing the local strain distribution characteristics of the material is characterized by comprising the following steps of:

step one, mixing KOH, 40% silica sol, polyhydroxy polymer and high-purity distilled water, and performing ultrasonic dispersion to obtain nano SiO₂Suspending liquid;

step two, cutting the sample to be measured to reach the required geometric dimension, and polishing the sample to a bright and scratch-free mirror surface;

step three, keeping the surface to be measured after the treatment of the step two horizontal, and then vertically dropping one drop of the nano SiO obtained in the step one₂Standing the suspension to enable the suspension on the surface to be tested to be freely spread, then sucking the liquid drops on the surface from the edges of the liquid drops by using filter paper, slowly inclining the surface to be tested by 40-80 degrees, rinsing by using absolute ethyl alcohol, and then quickly drying the surface to be tested, namely obtaining speckles on the surface to be tested;

step four, shooting a scanning electron microscope photo of the speckle morphology, if SiO₂The particles are uniformly distributed and SiO in the photo₂The area occupied by the particles is 10% -30%, and SEM-DIC speckles for representing the local strain distribution characteristics of the material are obtained;

if SiO₂Uneven particle distribution or SiO in the photograph₂The occupied area of the particles is less than 10 percent or more than 30 percent, the surface to be measured is polished again to a bright and scratch-free mirror surface, and the speckle pattern is prepared again according to the operation of the third step until SiO₂The particles are uniformly distributed and SiO in the photo₂The area occupied by the particles is 10-30 percent.

2. The speckle manufacturing method according to claim 1, wherein the silica sol of step one is a dispersion of nano-sized silica particles in water of high purity distilled water.

3. The speckle manufacturing method according to claim 1, wherein the sample material to be measured in the second step is a metal or a metal matrix composite.

4. The speckle manufacturing method according to claim 1, wherein the surface to be measured is polished to a bright and scratch-free mirror surface after the steps of mechanically grinding and finely polishing are performed.

5. The speckle manufacturing method according to claim 1, wherein step one is SiO in silica sol₂The particle size is selected according to the magnification factor required by SEM shooting, and the selection range is 20nm-80 nm; nano SiO₂The concentration of KOH in the suspension is 2-4 wt.%, the concentration of 40% silica sol is 35-60 wt.%, the concentration of polyhydroxy polymer is 3-8 wt.%, and the ultrasonic dispersion time is 30-120 min.

6. The speckle manufacturing method of claim 3, wherein SiO is at a magnification of 1000 required for SEM photographing₂The particle size is selected to be 60nm-80nm, the concentration of KOH is 2 wt.% per thousand-4 wt.%, the concentration of 40% silica sol is 35 wt.% per thousand, the concentration of polyhydroxy polymer is 3 wt.% per thousand, and the ultrasonic dispersion time is 30 min;

SiO when the magnification required for SEM shooting is 2000₂The particle size is selected to be 40nm-60nm, the concentration of KOH is 2 wt.% per thousand-4 wt.% per thousand, the concentration of 40% silica sol is 40 wt.% per thousand, the concentration of polyhydroxy polymer is 5 wt.% per thousand, and the ultrasonic dispersion time is 60 min;

when the magnification factor required by SEM shooting is 4000, SiO₂The particle size is selected to be 40nm-60nm, the concentration of KOH is 2 wt.% per thousand-4 wt.% per thousand, the concentration of 40% silica sol is 50 wt.% per thousand, the concentration of polyhydroxy polymer is 6 wt.% per thousand, and the ultrasonic dispersion time is 90 min;

when the magnification required by SEM shooting is 8000, SiO₂The particle size is 20nm-40nm, KOH concentration is 2 wt.% per mill-4 wt.%, 40% silica sol concentration is 60 wt.% per mill, and polyhydroxy polymer concentration is 8 wt.% per millThe ultrasonic dispersion time is 120 min.

7. The method for characterizing local deformations of a material according to any of claims 1 to 6, characterized in that said characterization is carried out by:

step 1, performing SEM shooting by adopting secondary electron imaging, finding a speckle region, adjusting brightness and contrast until a clear image is observed, and shooting a shape scanning electron microscope photo of a sample to be detected when the sample is not deformed;

step 2, adjusting the brightness and the contrast until the surface to be measured is black and the silicon dioxide particles are white, and shooting again to obtain a binary image of the sample to be measured when the sample is not deformed;

step 3, carrying out external force loading on the sample to be tested;

step 4, performing SEM shooting by adopting secondary electron imaging to find a speckle area, then adjusting the brightness and the contrast until a clear image is observed, and shooting a shape scanning electron microscope photo of the deformed sample to be detected;

step 5, adjusting the brightness and the contrast until the surface to be measured is black and the silicon dioxide particles are white, and shooting again to obtain a deformed binary image of the sample to be measured;

and 6, analyzing the acquired binary images of the sample to be detected before and after deformation by adopting computer software to obtain displacement and local strain information, and analyzing by combining the appearance scanning electron microscope photos of speckles before and after deformation to obtain the local deformation characteristic of the material.

8. The method for characterizing local deformations of a material according to claim 7, characterized in that the SEM imaging parameters in steps 1 and 4 are: the accelerating voltage is 5-10kV, the probe current is 1.4-3.0nA, the magnification is 1000-10000 times, the collection time (dwell time) is 10 mus, and the resolution is 2048 x 1768.

9. The method for characterizing local deformation of a material as claimed in claim 7, wherein the brightness in step 2 and step 5 is controlled to be 11.5-15, and the contrast is controlled to be 65-73.

10. The method for characterizing local deformation of a material as claimed in claim 7, wherein the computer software of step 6 is VIC-2D software.

Technical Field

The invention belongs to the technical field of optical measurement mechanics and deformation measurement; in particular to a preparation method of material surface speckles, which is used as an identification point and can realize accurate characterization of local strain distribution characteristics of a material by combining a scanning electron microscope with a Digital Image Correlation (DIC).

Background

How to accurately characterize macroscopic and microscopic deformation characteristics of materials has become a key scientific problem in the fields of modern material science, solid mechanics and the like. Digital Image Correlation (DIC), also known as digital speckle correlation, first uses an optical electron microscope (OM) or a Scanning Electron Microscope (SEM) to shoot a digital image (speckle pattern) with a certain gray scale distribution, wherein the speckles with high gray scale values in the image can be used as identification points, and the surface deformation is tracked by comparing the position changes of the speckle images, i.e. the speckles, before and after the deformation of the material or the structure surface, so as to obtain the local/full displacement field and the strain field of the material or the structure surface. Generally, randomness of a surface speckle pattern and uniformity of sizes of speckle points are three main factors influencing measurement accuracy of DIC, namely, contrast of the speckle points. Generally, the grains, the inclusions, the second phase particles, the surface scratches and the like have appropriate, random and high-contrast speckle point characteristics, and meet the requirement of DIC analysis on high-quality scattered spots. However, in many cases, natural surfaces often lack appropriately sized or high contrast speckle point features, thereby rendering DIC analysis impossible. In this case, an artificial surface pattern with appropriate characteristics must be made as speckles by surface modification. On a macroscopic scale, surface modification typically includes spraying, grinding, and etching. The speckle patterns obtained by the modification methods have good effect under the condition that the speckle patterns are amplified to 1000 times under a conventional optical microscope. When scanning electron microscope is required to observe micron-scale or even nano-scale features under higher magnification, a more elaborate surface modification method is required to realize DIC analysis, which mainly comprises: chemical and physical deposition, spray coating of micro-nano sized particles such as SiC, diamond, etc.), fluorescent nanoparticle deposition, micro-contact printing, Ultraviolet (UV) lithography, and focused ion beam deposition (FIB), among others. However, the above techniques have complicated processes, high cost and long cycle, which limits their wider application. Therefore, the pending patent (application number: 201811561403.8) proposes a speckle preparation method using settled suspended nanoparticles as a marker, which can successfully prepare speckle patterns on the surface of pure titanium or stainless steel and can realize collection of DIC data, but the defect of this preparation method is that it is difficult to prepare a long-term stable and uniformly dispersed nanoparticle suspension, and this suspension is difficult to regulate and control the concentration in a large range, so that it is impossible to obtain high-density speckle patterns, thereby greatly limiting the spatial resolution improvement in DIC strain analysis. In addition, since the preparation method is to attach non-conductive nanoparticles to all sample surfaces, the conductivity of the sample is deteriorated, and the imaging error is increased. Therefore, a rapid, low-cost, high-adaptability and high-dispersity SEM-DIC speckle preparation method is urgently needed to be developed, so that high-precision and high-resolution DIC data analysis is realized.

Disclosure of Invention

The invention mainly aims to provide a rapid, low-cost, high-adaptability, high-dispersity and high-precision SEM-DIC speckle preparation method, so as to solve the problems of low speckle resolution, non-uniformity of speckles, low applicability, complex process, high cost and the like in the prior art.

In order to solve the technical problem, the SEM-DIC speckle preparation method for characterizing the local strain distribution characteristics of the material is characterized by comprising the following steps of:

step one, mixing KOH, 40% silica sol, polyhydroxy polymer and high-purity distilled water, and performing ultrasonic dispersion to obtain nano SiO₂Suspension liquidIt is uniformly dispersed and stable for a long time;

step two, cutting the sample to be measured to reach the required geometric dimension, and polishing the sample to a bright and scratch-free mirror surface;

step three, keeping the surface to be measured after the treatment of the step two horizontal, and then vertically dropping one drop of the nano SiO obtained in the step one₂Suspending liquid, waiting for 5s to enable the suspending liquid on the surface to be detected to be freely spread, then sucking the liquid drops on the surface from the edges of the liquid drops by using filter paper, slowly inclining the surface to be detected by 40-80 degrees, rinsing by using absolute ethyl alcohol to wash off unsteady floating nano silicon dioxide particles, and then quickly drying the surface to be detected, namely obtaining speckles on the surface to be detected;

step four, shooting a scanning electron microscope photo of the speckle morphology, if SiO₂The particles are uniformly distributed and SiO in the photo₂The area occupied by the particles is 10-30%, and SEM-DIC speckles for representing the local strain distribution characteristics of the material are obtained;

if SiO₂Uneven particle distribution or SiO in the photograph₂The area occupied by the particles is less than 10 percent (or more than 30 percent), the surface to be measured is polished again to a bright and scratch-free mirror surface, and the speckle pattern is prepared again according to the operation of the third step until SiO is achieved₂The particles are uniformly distributed and SiO in the photo₂The area occupied by the particles is 10-30 percent.

The four steps are mainly two standards for judging whether the prepared speckle pattern is qualified: firstly, whether the silicon dioxide particles are uniformly distributed or not; secondly, whether the area of the silicon dioxide particles in the image is within 10% -30% or not, and computer randomly generated speckle images with different proportions (5% -35%) are shown in fig. 10-16, when the proportion is 5%, the silicon dioxide particles are distributed too sparsely, when the proportion is more than 35%, the silicon dioxide particles are distributed too densely, and when the proportion is within 10% -30%, the distribution is proper.

Further, the silica sol in the first step is a dispersion of nano-scale silica particles in water of high purity distilled water.

Further, the sample material to be detected in the second step is a metal or metal matrix composite material.

Further limiting, polishing the surface to be measured to a bright and scratch-free mirror after mechanically grinding and finely polishing the second step; when a sample is ground, metallographic abrasive paper can be used for grinding to 3000 meshes, a mechanical polishing method can be adopted for rough polishing, then chemical mechanical polishing or electrolytic polishing is selected according to the property of the sample, the surface to be measured of the sample is polished to a bright mirror surface without scratches, and the uniform dispersibility of speckle particles on the surface is improved on the high-quality surface.

Further defined, step one SiO in the silica sol₂The particle size is selected to be 20nm-80nm, and silica sol with proper particle size is selected according to the magnification required by SEM shooting; nano SiO₂The concentration of KOH in the suspension is 2-4 wt.%, the concentration of 40% silica sol is 35-60 wt.%, the concentration of polyhydroxy polymer is 3-8 wt.%, and the ultrasonic dispersion time is 30-120 min. The nano SiO was formulated according to Table 1₂The suspension liquid can prepare high-quality speckle patterns under different spatial resolutions.

Table 1 preparation table of silica suspension

Magnification factor	Particle size/nm	KOH	40% silica sol	Polyhydroxy polymers	Time of ultrasound
						1000	60-80	2-4	35	3	30min
2000	40-60	2-4	40	5	60min
						4000	40-60	2-4	50	6	90min
8000	20-40	2-4	60	8	120min

Note: the contents are all per thousand by weight, and the medium is high-purity distilled water.

The method for characterizing the local deformation of the material is completed by the following steps:

step 1, performing SEM shooting by adopting secondary electron imaging, finding a speckle region, adjusting brightness and contrast until a clear image is observed, and shooting a shape scanning electron microscope photo of a sample to be detected when the sample is not deformed;

step 2, adjusting the brightness and the contrast until the surface to be measured is black and the silicon dioxide particles are white, and shooting again to obtain a binary image of the sample to be measured when the sample is not deformed;

step 3, carrying out external force loading on the sample to be tested;

step 4, performing SEM shooting by adopting secondary electron imaging to find a speckle area, then adjusting the brightness and the contrast until a clear image is observed, and shooting a shape scanning electron microscope photo of the deformed sample to be detected;

step 5, adjusting the brightness and the contrast until the surface to be measured is black and the silicon dioxide particles are white, and shooting again to obtain a deformed binary image of the sample to be measured;

and 6, analyzing the acquired binary images of the sample to be detected before and after deformation by adopting computer software to obtain displacement and local strain information, and analyzing by combining the appearance scanning electron microscope photos of speckles before and after deformation to obtain the local deformation characteristic of the material.

And (5) carrying out different external force loading on the sample to be tested, then carrying out the operations of the steps 4 and 5, obtaining the image information of different external forces, and analyzing according to the step 6 to obtain the local deformation characteristics of the material under different external forces.

Further limiting, the SEM photographing parameters in step 1 and step 4 are: the accelerating voltage is 5-10kV, the probe current is 1.4-3.0nA, the magnification is 1000-10000 times, the collection time (dwell time) is 10 mus, and the resolution is 2048 x 1768.

Further limiting, the brightness is controlled to be 11.5-15 and the contrast is controlled to be 65-73 in the step 2 and the step 5.

Further, the computer software in step 6 is VIC-2D software.

The invention keeps the surface to be measured horizontal to ensure that the suspended liquid can be horizontally and freely spread after being dripped to the surface, thereby keeping the uniform dispersibility of the suspended liquid, and only a small-size rubber head dropper is needed to drip one drop to the surface of a sample during titration, the liquid drop with the diameter of 3-5mm can be formed on the surface of the sample after being spread, and the surrounded area can meet the observation of more than 1000 times of a scanning electron microscope.

The invention uses filter paper to suck the redundant suspension liquid to ensure the uniform distribution of the silicon dioxide nano particles on the surface of the sample, and the filter paper is used to carefully suck the liquid drops on the surface from the edge of the liquid drops after the suspended liquid drops spread to the surface for about 5 seconds, so that no obvious liquid drops exist on the surface of the sample, and the agglomeration of the silicon dioxide particles on the surface caused by the drying of the liquid drops is prevented. After the sample is sucked dry, the surface of the sample is carefully rinsed by using absolute ethyl alcohol, so that redundant floating silicon dioxide nano particles can be cleaned again, redundant liquid on the surface of the sample can be taken away, and the surface of the dried sample is clean. The quality of the marking can be visually checked preliminarily after drying: the dried sample is still a bright mirror surface, the surface speckle area has no obvious change, and the surface is slightly whitened in a hidden and visible mode under the irradiation of strong light.

The invention selects lower accelerating voltage to protect speckles on the surface of the sample and is beneficial to observing the deformation appearance of the surface layer of the sample; the higher probe current is appropriate to increase the charging effect of the silica speckle particles to highlight and reduce image noise. For titanium-based composites, the acceleration voltage and probe current may preferably be 5kV, 1.4 nA; the magnification factor can be selected autonomously according to the required field size, the acquisition time is 10 mus in order to reduce the image noise, and the resolution is 2048 × 1768 in order to improve the spatial resolution of DIC analysis and reduce the image noise. The acquisition time and the resolution can be properly adjusted according to the actual situation, and generally, the longer acquisition time and the higher resolution are favorable for improving the precision; the photographing is performed by first taking the brightness and contrast suitable for observing the appearance of the sample so as to obtain the deformation information (slippage opening, shear band and the like) of the surface of the sample, and then the brightness and contrast are adjusted according to the setting so that the material matrix is basically displayed as black, and the silica particles are displayed as white (similar to a binary image) so as to perform DIC analysis, thereby being beneficial to improving the correlation and reducing the analysis error.

The speckle particles used in the invention are 20-80nm nanometer silicon dioxide particles, and the nanometer silicon dioxide particles are easy to agglomerate in a solvent due to larger surface tension, so that the dispersibility and stability of the suspension are reduced. In the invention, factors such as silicon dioxide nano particles, pH, surfactant, aqueous medium and the like need to be comprehensively considered when preparing the silicon dioxide suspension; the dispersibility and uniformity of the prepared silicon dioxide suspension are ensured by adopting commercial silica sol; KOH is used as a pH regulator to maintain the pH of the suspension at 11-11.5, so that the silicon dioxide particles are uniformly dispersed to form a stable colloid system; the dispersant polyhydroxy polymer is used as a surfactant, so that the dispersibility and stability of the suspension can be effectively improved, and high-purity distilled water is used as a solvent to ensure that the silicon dioxide is not precipitated in a water medium.

The nano silicon dioxide particles with speckles prepared by the method have large specific surface area, can be effectively attached to the surface of a sample, can correspondingly move along with the deformation of the surface of the sample in the external force loading process without damaging the original state of the surface of the sample, and can simultaneously and accurately obtain the local strain distribution rule of the surface of the sample in the loading process and the surface appearance evolution rule of a corresponding area by adopting SEM combined DIC analysis. Compared with an optical lens, the resolution of the local strain information obtained by the SEM-DIC technology is greatly improved, and the accuracy of DIC data analysis can be ensured by adjusting the photographing parameters of the SEM. The invention provides a simple, rapid, strong-adaptability, low-cost and high-precision SEM-DIC speckle preparation method.

The speckle pattern prepared by the method has good dispersity and high precision, and does not damage the surface state of the sample.

The method for preparing the speckles is simple, rapid, high in adaptability, low in cost and high in practicability.

The characterization method has good correlation and small analysis error, and the accuracy of the micro-area plastic deformation characteristic of the disclosed material is high.

Drawings

FIG. 1 is a SEM photograph of the surface topography of example 1 of the present invention (without external force loading);

FIG. 2 is a SEM photograph of surface speckles of example 1 of the present invention (without external force loading);

FIG. 3 is a SEM photograph of the surface topography of example 2 of the present invention (without external force loading);

FIG. 4 is a SEM photograph of surface speckles of example 2 of the present invention (without external force loading);

FIG. 5 is a SEM photograph of the surface topography of example 3 of the present invention (without external force loading);

FIG. 6 is a SEM photograph of surface speckles of example 3 of the present invention (without external force loading);

FIG. 7 is a SEM photograph of the surface morphology of example 2 of the present invention at a macroscopic strain of 14.0%;

FIG. 8 shows the SEM-DIC analysis results of example 2 of the present invention: exx directional local strain distribution map;

FIG. 9 shows SEM photographs and DIC analysis results of local area surface topography of example 2 of the present invention: local surface deformation of the sample and local strain distribution in the direction of exx at the corresponding position;

FIG. 10 is a computer randomly generated speckle image with 5% speckle pixel ratio;

FIG. 11 is a computer randomly generated speckle image with 10% speckle pixel occupancy;

FIG. 12 is a computer randomly generated speckle image with a 15% ratio of speckle pixels;

FIG. 13 is a computer randomly generated speckle image with 20% speckle pixel ratio;

FIG. 14 is a computer randomly generated speckle image with 25% speckle pixel occupancy;

FIG. 15 is a computer randomly generated speckle image with a 30% ratio of speckle pixels;

FIG. 16 is a computer randomly generated speckle image with a 35% speckle pixel ratio;

Detailed Description

The speckle prepared on the surface of the Titanium Matrix Composite (TMCs) is specifically carried out according to the following steps:

step one, mixing KOH, 40% silica sol, polyhydroxy polymer and high-purity distilled water according to the amplification factor shown in Table 2, and performing ultrasonic dispersion to obtain nano SiO₂Suspending liquid;

TABLE 2

	Magnification factor	Particle size/nm	KOH	40% silica sol	Polyhydroxy polymers	Time of ultrasound
							Example 1	2000	40	2-4	40	5	60min
Example 2	4000	40	2-4	50	6	90min
							Example 3	8000	40	2-4	60	8	120min

Step two, processing a sample to be detected by electric spark, cutting the sample to reach a required geometric size, grinding the sample to 3000 meshes by using metallographic abrasive paper, mechanically polishing the sample until no obvious scratch exists, and then performing electrolytic polishing until a bright mirror surface without scratches exists;

step three, clamping a sample to be detected to enable the surface to be detected processed in the step two to be kept horizontal, then using a rubber head dropper to take the dispersed silicon dioxide suspension in the step one, keeping the silicon dioxide suspension vertical to the surface to be detected, dropping one droplet to a DIC (digital computer) region to be detected of the surface to be detected carefully, waiting for 5s, enabling the suspension on the surface to be detected to spread freely, then using filter paper to suck the droplet on the surface from the edge of the droplet, then slowly inclining the surface to be detected by 60 degrees, taking a proper amount of absolute ethyl alcohol to drop to the original position of the suspension droplet of the silicon dioxide for rinsing so as to fully wash away the redundant suspension and the unsteady nano silicon dioxide particles, and then quickly drying the surface to be detected, namely;

step four, shooting a scanning electron microscope photo of the speckle morphology, if SiO₂The particles are uniformly distributed and SiO in the photo₂The area occupied by the particles is 10% -30%, and the SEM-DIC speckle used for representing the local strain distribution characteristics of the material is obtained.

The method for characterizing the local deformation of the material is completed by the following steps:

step 1, performing SEM shooting by adopting secondary electron imaging, and controlling SEM shooting parameters to be as follows: the accelerating voltage is 5 kV; the probe current was 1.4 nA; the magnification is controlled to be 2000X, 4000X and 8000X respectively; the acquisition time (dwell) is 10 ms; the resolution ratio is 2048 multiplied by 1768, a speckle area is found, then the brightness and the contrast ratio are adjusted until a clear image is observed, and a shape scanning electron microscope photo of the sample to be measured when the sample is not deformed is shot;

step 2, adjusting the brightness and the contrast (the brightness is adjusted to be 11.5-15, and the contrast is adjusted to be 65-73) until the surface to be measured is black and the silicon dioxide particles are white, and shooting again to obtain a binary image of the sample to be measured when the sample is not deformed;

step 3, carrying out external force loading on the sample to be tested;

and 4, performing SEM shooting by adopting secondary electron imaging, and controlling SEM shooting parameters to be as follows: the accelerating voltage is 5 kV; the probe current was 1.4 nA; the magnification is 2000X, 4000X and 8000X; the acquisition time (dwell) is 10 ms; the resolution ratio is 2048 multiplied by 1768, a speckle area is found, then the brightness and the contrast ratio are adjusted until a clear image is observed, and a shape scanning electron microscope photo of the deformed sample to be detected is shot;

step 5, adjusting the brightness and the contrast (the brightness is adjusted to be 11.5-15, and the contrast is adjusted to be 65-73) until the surface to be detected is black and the silicon dioxide particles are white, and shooting again to obtain a deformed binary image of the sample to be detected;

and 6, analyzing the acquired binary images of the sample to be detected before and after deformation by adopting computer software to obtain displacement and local strain information, and analyzing by combining the appearance scanning electron microscope photos of speckles before and after deformation to obtain the local deformation characteristic of the material.

As can be seen from the corresponding fig. 1-6 of examples 1-3, the speckles obtained by the SEM-DIC speckle preparation method of the present invention have the advantages of randomness, speckle point size consistency, high contrast, etc., the areas occupied by the silicon dioxide speckles are 22%, 12% and 27% in the ranges of fig. 2, 4 and 6, respectively, and the areas are 10% -30%, so as to meet the requirements of speckle preparation, and the nano SiO is simply regulated and controlled₂The content of scattered spots can completely meet the image analysis requirements of different magnifications (1000X-10000X), and simultaneously, the high precision of DIC analysis and the high resolution of surface topography are ensured. The speckle preparation method has the characteristics of simplicity, rapidness, strong adaptability, low cost, high precision and the like, does not damage the surface of the sample in the preparation process, and can simultaneously obtain the local strain distribution rule of the surface of the sample and the surface appearance evolution rule of the corresponding area. FIG. 7 of example 2 is an SEM photograph of the microstructure of a TMCs sample at a macroscopic deformation of 14.0%, wherein the red short line is a slip trace marked at the corresponding position in the sample, and it can be seen from FIG. 7 that the slip bands in the material can be clearly observed in the surface morphology picture collected by the method, and combined with the orientation information or texture composition of the materialThe trace analysis can be carried out on the material, namely the starting condition of the slip system of the deformation of the material microcell is determined. Further, in conjunction with the results of DIC analysis with high resolution of example 2 (see FIGS. 8 and 9), the micro-domain plastic deformation properties of the material can be accurately revealed.