A multi-scale strain measurement method for metallic materials at ultra-low temperatures

Abstract

The present application discloses a multi-scale strain measurement method for metallic materials at ultra-low temperatures. This method combines DIC technology and HR-EBSD analysis to achieve precise strain measurement from the macroscopic to the microscopic scale. The macroscopic strain distribution on the specimen surface is accurately captured by DIC technology, and HR-EBSD analysis is used to further delve into the microscopic level to reveal the microscopic deformation mechanism of the material in an ultra-low temperature environment. It not only studies the overall strain of the material from a macroscopic perspective but also further delves into specific regions of interest under this macroscopic strain state to explore the microscopic strain distribution in the regions of interest, enabling a more comprehensive understanding of the deformation behavior and mechanical properties of the material at ultra-low temperatures. Additionally, a non-direct contact heat exchange method is adopted to achieve precise temperature control while avoiding the influence of water mist, condensation, etc. generated by the evaporation and condensation of the cooling medium on the test results.

Description

Technical Field

[0001] This application relates to the technical field of material strain measurement, and particularly to a multi-scale strain measurement method for metallic materials at ultra-low temperatures. Background Art

[0002] With the continuous progress of science and technology, metallic materials are increasingly widely used in extreme environments, especially in fields such as aerospace, deep-sea exploration, and cryogenic engineering. In these applications, metallic materials need to maintain excellent mechanical properties and stability under ultra-low temperature conditions. Therefore, in-depth research on the strain behavior of metallic materials in ultra-low temperature environments is of great significance for improving their service performance and extending their service life.

[0003] Under ultra-low temperature conditions, the microstructure and mechanical properties of metallic materials will change significantly. For example, mechanical property indexes such as the yield strength, tensile strength, and toughness of the material usually increase, but at the same time, risks such as embrittlement and reduced plasticity will also be faced. In addition, the ultra-low temperature environment may also cause non-uniform stress distribution inside the material, leading to stress concentration and early failure. Therefore, accurately measuring the strain distribution of metallic materials at ultra-low temperatures is crucial for evaluating their mechanical properties and predicting their service life.

[0004] Currently, traditional strain measurement methods such as resistance strain gauges and fiber Bragg gratings have many limitations in ultra-low temperature environments. For example, the measurement accuracy of resistance strain gauges may decrease due to changes in material properties at low temperatures. These methods usually can only provide strain information at the macroscopic scale and cannot deeply reveal the microscopic strain distribution inside the material. Summary of the Invention

[0005] The embodiments of this application solve the problem that the prior art cannot measure the microscopic strain distribution inside the material under ultra-low temperature conditions by providing a multi-scale strain measurement method for metallic materials at ultra-low temperatures.

[0006] To achieve the above object, the technical solution of the embodiments of the present invention is as follows:

[0007] First aspect, an embodiment of the present invention provides a multi-scale strain measurement method for metal materials at ultra-low temperatures, including: preparing a metal material specimen with a suitable size according to the sample size requirements of the ultra-low temperature in-situ tensile stage; preparing a speckle pattern for strain measurement on the surface of the specimen by a preset metal surface treatment method; in a scanning electron microscope, constructing a preset ultra-low temperature environment by a non-direct contact heat exchange method with a preset refrigeration medium, and performing an in-situ tensile test on the metal material specimen under the ultra-low temperature environment, and taking speckle photos of a preset gauge section at different deformation stages of the specimen; performing correlation analysis on the speckle photos before and after deformation of the gauge section through DIC technology to obtain the macroscopic strain distribution on the surface of the specimen; determining the region of interest from the macroscopic strain distribution, performing HR-EBSD analysis on the region of interest, and calculating and obtaining the microscopic strain distribution of the region; comprehensively analyzing the macroscopic strain distribution and the microscopic strain distribution to determine the strain distribution characteristics of the metal material specimen under the ultra-low temperature environment.

[0008] In some possible implementation manners, before the electrolytic polishing treatment of the specimen surface, the method further includes: polishing the surface of the specimen successively with sandpapers of different meshes to make the surface quality of the specimen meet the preset quality requirements.

[0009] In some possible implementation manners, when performing the electrolytic polishing treatment on the specimen surface, by optimizing the ratio of the electrolyte, voltage setting, current density, and polishing time, the quality and contrast of the speckle pattern are improved.

[0010] In some possible implementation manners, the non-direct contact heat exchange method is to place the refrigeration medium in a cooling cavity outside the scanning electron microscope, and transfer the ultra-low temperature to the metal material specimen by means of thermal radiation or thermal convection. The refrigeration medium includes but is not limited to liquid nitrogen and liquid helium.

[0011] In some possible implementation manners, comprehensively analyzing the macroscopic strain distribution and the microscopic strain distribution to determine the strain distribution characteristics of the metal material specimen under the ultra-low temperature environment includes: performing spatial mapping alignment on the macroscopic strain distribution obtained by DIC and the microscopic strain distribution obtained by HR-EBSD to establish the coordinate correlation of the cross-scale strain distribution; fusing the macro-micro data through a multi-level strain distribution model to obtain the comprehensive strain distribution characteristics of the specimen under the ultra-low temperature environment.

[0012] In a second aspect, an embodiment of the present invention provides a method for analyzing the plastic deformation mechanism of a metal material at ultra-low temperatures, including: obtaining the strain distribution characteristics of a metal material specimen in an ultra-low temperature environment through the multi-scale strain measurement method provided in the first aspect; determining the plastic deformation mechanism of the metal material specimen under ultra-low temperature conditions based on the strain distribution characteristics and in combination with the physical and chemical properties of the metal material specimen in the ultra-low temperature environment; the plastic deformation mechanism includes, but is not limited to, dislocation slip, grain boundary sliding, twinning deformation, and their interactions.

[0013] In some possible implementation manners, by comprehensively analyzing the macroscopic strain distribution and the microscopic strain distribution, the strain distribution characteristics of the metal material specimen at different scales are determined, including: analyzing the macroscopic strain distribution in the spatial domain and the time domain to obtain the macroscopic strain distribution characteristics of the specimen, including but not limited to identifying the strain concentration region and the strain gradient characteristics; extracting the microscopic strain distribution characteristics through image analysis software, including but not limited to strain values, strain directions, strain concentration points, or strain bands.

[0014] In some possible implementation manners, based on the strain distribution characteristics and in combination with the physical and chemical properties of the metal material specimen in the ultra-low temperature environment, the plastic deformation mechanism of the metal material specimen under ultra-low temperature conditions is determined, including: analyzing the strain behavior of the metal material at different scales based on the macroscopic strain distribution characteristics and the microscopic strain distribution characteristics, including macroscopic plastic deformation modes and microscopic dislocation movement laws; analyzing the strain distribution law of the plastic deformation of the metal material in the ultra-low temperature environment in combination with the physical and chemical properties of the material; based on the strain distribution law of the plastic deformation of the metal material in the ultra-low temperature environment, in combination with microscopic characterization methods, elastoplastic mechanics theory, and / or multi-scale modeling and other methods, analyzing the reasons for the formation of local strain concentration during the ultra-low temperature deformation of the metal material, and further clarifying its plastic deformation mechanism.

[0015] One or more technical solutions provided in the embodiments of the present invention have at least the following technical effects or advantages:

[0016] In the embodiments of the present invention, by combining DIC technology and HR-EBSD analysis, accurate strain measurement from the macroscopic to the microscopic scale is achieved. The macroscopic strain distribution on the surface of the specimen is accurately captured through DIC technology, and HR-EBSD analysis is further used to delve into the microscopic level to reveal the microscopic deformation mechanism of the material in the ultra-low temperature environment. It not only studies the overall strain of the material from a macroscopic perspective but also further delves into the specific region of interest under this macroscopic strain state to explore the microscopic strain distribution of the region of interest, enabling a more comprehensive understanding of the deformation behavior and mechanical properties of the material at ultra-low temperatures. In addition, a non-direct contact heat exchange method is adopted, which while achieving precise temperature control, avoids the influence of water mist or condensation generated by the evaporation and condensation of the cooling medium on the test results. Description of the Drawings

[0017] To more clearly illustrate the embodiments of the present invention, the accompanying drawings required for use in the embodiments of the present invention will be briefly introduced below. Obviously, the accompanying drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other accompanying drawings can be obtained based on these drawings.

[0018] Figure 1 Schematic diagram of the embodiment process of a multi-scale strain measurement method for metal materials at ultra-low temperatures provided for the embodiments of the present invention;

[0019] Figure 2 Schematic diagram of obtaining the macroscopic strain distribution on the specimen surface through DIC in the embodiments of the present invention;

[0020] Figure 3 Schematic diagram of obtaining the microscopic strain distribution in the region of interest by using HR-EBSD analysis in the embodiments of the present invention;

[0021] Figure 4 Schematic diagram of the embodiment process of an analysis method for the plastic deformation mechanism of metal materials at ultra-low temperatures in the embodiments of the present invention. Detailed implementation manners

[0022] Next, the technical solutions in the embodiments of the present application will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of protection of the present application.

[0023] In the relevant descriptions of this embodiment, terms such as "include, contain, have" are all open terms, generally preferably understood as including but not limited to; the term "at least one" is generally preferably understood as one or more, where "multiple" means two or more; the term "at least one (item) of the following" or its similar expression refers to any combination of these items, including any combination of single item (item) or plural items (items). For example, "at least one (item) of a, b or c", or, "at least one (item) of a, b and c" can all represent: a, b, c, a-b (that is, a and b), a-c, b-c, or a-b-c, where a, b, c can be single or multiple respectively; the symbol "A / B" is used to describe the selection relationship of associated objects, generally representing an "or" relationship before and after.

[0024] In the following description of this embodiment, the terms used in the embodiments of this application are only for the purpose of describing specific embodiments and are not intended to limit this application. The singular forms "a" and "the" used in the embodiments of this application and the appended claims are also intended to include the plural forms unless the context clearly dictates otherwise.

[0025] Those skilled in the art should understand that in the following description of the embodiments of this application, the sequence numbers do not imply the order of execution, and some or all of the steps may be executed in parallel or sequentially. The execution order of each process should be determined by its function and internal logic and should not constitute any limitation to the implementation process of the embodiments of this application.

[0026] Those skilled in the art should understand that the numerical ranges in the embodiments of this application should be understood as specifically disclosing each intermediate value between the upper and lower limits of the range. Each intermediate value within any stated value or stated range, as well as each smaller range between any other stated value or intermediate value within the range, is also included in the present invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0027] Unless otherwise noted, the technical / scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which this application belongs. Although this application only describes preferred methods and materials, any methods and materials similar or equivalent to those described herein may also be used in the implementation or testing of this application. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials related to the documents. In case of conflict with any incorporated document, the content of this specification shall prevail.

[0028] In order to illustrate the technical solutions of the present invention, specific embodiments are used for illustration below.

[0029] With the continuous progress of science and technology, the application of metal materials in extreme environments is becoming increasingly widespread, especially in the fields of aerospace, deep-sea exploration, cryogenic engineering, etc. In these applications, metal materials need to maintain excellent mechanical properties and stability under ultra-low temperature conditions. Therefore, in-depth research on the strain behavior of metal materials in ultra-low temperature environments is of great significance for improving their service performance and extending their service life.

[0030] Under ultra-low temperature conditions, the microstructure and mechanical properties of metal materials will change significantly. For example, mechanical property indexes such as the yield strength, tensile strength, and toughness of the material usually increase, but at the same time, risks such as embrittlement and reduced plasticity will also be faced. In addition, the ultra-low temperature environment may also cause non-uniformity in the internal stress distribution of the material, resulting in stress concentration and early failure. Therefore, accurately measuring the strain distribution of metal materials at ultra-low temperatures is crucial for evaluating their mechanical properties and predicting their service life.

[0031] Currently, traditional strain measurement methods such as resistance strain gauges and fiber Bragg gratings have many limitations in ultra-low temperature environments. For example, resistance strain gauges may experience a decrease in measurement accuracy due to changes in material properties at low temperatures. These methods usually only provide strain information at the macroscopic scale and cannot deeply reveal the microscopic strain distribution inside the material.

[0032] Based on this, an embodiment of the present invention provides a multi-scale strain measurement method for metal materials at ultra-low temperatures, which solves the problem that the prior art cannot measure the microscopic strain distribution inside the material under ultra-low temperature conditions.

[0033] Figure 1 The flowchart of an embodiment of a multi-scale strain measurement method for metal materials at ultra-low temperatures provided for an embodiment of the present invention is shown in Figure 1 As shown, the above multi-scale strain measurement method for metal materials at ultra-low temperatures may include:

[0034] S101, prepare a metal material specimen with appropriate dimensions according to the sample size requirements of the ultra-low temperature in-situ tensile stage;

[0035] Among them, the low-temperature in-situ tensile stage is a device for tensile testing of materials under low-temperature conditions. It combines low-temperature technology and tensile testing technology and can perform mechanical property tests on materials in a precisely controlled low-temperature environment.

[0036] In some embodiments, the sample size requirements may include the length, width, thickness of the specimen, and any specific shape or size tolerances. According to these size requirements, the metal material to be measured is processed and prepared in terms of size. The type of metal material can be determined according to the purpose of the test and the required performance characteristics. For example, if the test aims to evaluate the strength and toughness of a certain metal at ultra-low temperatures, a metal material with corresponding characteristics can be selected at this time. During the process of processing and preparing the dimensions of the metal material specimen, appropriate processing techniques and equipment can be used to ensure that the dimensions of the specimen are precise and meet the specifications. For example, steps such as cutting, grinding, and polishing are used to remove any irregular or non-conforming parts.

[0037] S102, prepare a speckle pattern for strain measurement on the surface of the specimen through a preset metal surface treatment method;

[0038] In some embodiments, before the above step S102, the above method may further include: successively polishing the surface of the specimen with sandpapers of different meshes to make the surface quality of the specimen meet the preset quality requirements.

[0039] Before electrolytic polishing, in order to ensure that the quality of the specimen surface meets the preset requirements, it is usually necessary to first grind the specimen surface. Grinding can remove the oxide layer, scratches, stains, uneven parts and other defects on the specimen surface, making the metal specimen more uniform in the subsequent chemical corrosion process, and improving the effect of electrolytic polishing and the quality of the speckle pattern.

[0040] The grinding process usually can be carried out step by step using sandpapers with different mesh numbers. For example, silicon carbide sandpapers with 320#, 600#, 1200#, 2000#, and 3000# can be used to grind the specimen surface in sequence. After each step of grinding, it is necessary to carefully check the quality of the specimen surface and ensure that there are no obvious scratches and defects, etc. In addition, during the grinding process, it is also necessary to pay attention to keeping the sandpaper moist to avoid overheating of the sandpaper and damage to the specimen surface.

[0041] In the above step S102, the speckle pattern is used to more accurately capture and measure the strain of the material during tensile, compressive or other mechanical tests. During the above metal surface treatment process, the metal specimen to be tested is first cleaned and pre-treated to remove surface stains, impurities, oxide layers, etc. For example, by grinding using sandpapers with different mesh numbers as described above. Then, through a preset metal surface treatment method, such as electrolytic polishing, spraying Al2O3 particles, etc., speckles are precisely manufactured on the specimen surface. These speckles usually show the characteristics of random distribution, different sizes and shapes, so that more deformation details can be captured during the subsequent strain measurement process.

[0042] Exemplarily, taking electrolytic polishing as an example for illustration: Electrolytic polishing forms uniform and tiny pores on the specimen surface by applying a voltage to the specimen in a specific electrolyte, and uses the formed pores as speckles to form a speckle image. In the electrolytic polishing method, by selecting appropriate electrolytes and polishing parameters, such as voltage, current, polishing time, etc., it can be ensured that a speckle pattern meeting the requirements is formed on the specimen surface. During the polishing process, the specimen is placed in the electrolyte and an appropriate voltage is applied for polishing treatment. At the same time, the temperature of the electrolyte and the polishing effect can also be monitored to ensure obtaining an ideal speckle pattern and ensuring that parameters such as the quality and contrast of the speckle pattern meet the requirements.

[0043] It should be noted that the preparation quality of the speckle pattern is crucial for the accuracy of strain measurement. Therefore, in the above step S102, the uniformity, stability, and repeatability of the speckle pattern can be ensured by controlling various parameters of the processing process, such as the electrolytic polishing time and the uniformity of spraying Al2O3 particles. After the speckle pattern is successfully prepared on the specimen surface, strain measurement techniques such as Digital Image Correlation (DIC) can be used to accurately measure the macroscopic strain of the metal by capturing and analyzing the macroscopic changes of the speckle pattern before and after deformation.

[0044] S103, inside a Scanning Electron Microscope (SEM), a preset ultra-low temperature environment is constructed by a preset refrigeration medium in a non-direct contact heat exchange manner, and an in-situ tensile test of the metal material specimen is carried out under the ultra-low temperature environment, and speckle photos of the preset gauge section of the specimen at different deformation stages are taken;

[0045] In the above step S103, first, a stable ultra-low temperature environment needs to be constructed inside the scanning electron microscope or at a position adjacent to its observation area. This can be achieved by a preset refrigeration medium, and the refrigeration medium can be cryogenic liquids such as liquid nitrogen and liquid helium. These refrigeration media have extremely low boiling points and can provide the required ultra-low temperature.

[0046] In some embodiments, the above non-direct contact heat exchange manner can be to perform heat exchange by circulating the refrigeration medium through a pipeline inside the scanning electron microscope, so that the temperature of the metal material specimen reaches the preset ultra-low temperature.

[0047] Among them, the ultra-low temperature range is a relative concept. Ultra-low temperature generally refers to a temperature range far lower than normal temperature or general low temperature. In different scientific and industrial fields, the specific ultra-low temperature range may vary. In the embodiments of the present invention, the ultra-low temperature can be the refrigeration temperature range of the refrigeration medium. For example, the ultra-low temperature range can be the boiling point temperature of liquid nitrogen, which is about -196°C. Liquid nitrogen is often used in experiments requiring a low temperature environment due to its relatively low cost and easy availability. Or, the ultra-low temperature range can also be the boiling point temperature of liquid helium, and the boiling point temperature of liquid helium is about -268.9°C. The specific temperature range can be selected based on the requirements in the actual application process.

[0048] It should be noted that condensation, frosting and / or air flow disturbance usually occur during in-situ tensile tests in ultra-low temperature environments. When cooling a metal material specimen by introducing a cooling medium (such as liquid nitrogen), the temperature of the surface of the metal material specimen will drop sharply. However, due to thermal inertia, the temperature inside the scanning electron microscope experimental device will be higher than the surface temperature of the specimen. Due to the temperature difference, when moisture encounters the specimen surface with a temperature lower than the dew point or even the frost point, water vapor will condense to form water mist, or directly form condensation or even frost on the specimen surface, which may affect the imaging accuracy of the speckle photos of the specimen, and further affect the accuracy in the subsequent strain measurement process. Therefore, in the embodiments of the present invention, the refrigeration medium does not directly contact the metal material to be tested, but the refrigeration medium is placed in a circulating flow through a pipeline to exchange heat with the metal material specimen and the surrounding environment, so as to make the temperature of the metal material specimen reach the preset ultra-low temperature. This method ensures that the specimen is tested in a stable ultra-low temperature environment, and at the same time avoids the influence of water mist or condensation generated by the evaporation and condensation of the cooling medium on the test results. Moreover, achieving ultra-low temperature through heat exchange can also achieve precise temperature control. For example, precise temperature control can be achieved by controlling the flow rate of the refrigeration medium, the number of circulation times, etc.

[0049] In the embodiments of the present invention, after constructing an ultra-low temperature environment and installing the metal material specimen, the in-situ tensile test can be carried out. During the test, a gradually increasing tensile force is applied to the specimen through the tensile device of the ultra-low temperature in-situ tensile stage until the specimen fractures or reaches a predetermined deformation amount. During the tensile process, the speckle photos of the specimen at different deformation stages are captured through the high-resolution imaging function of the SEM, and the entire deformation process of the specimen is recorded. When taking the speckle photos, ensure that the parameters such as the focal length and exposure of the camera are set properly to obtain clear and accurate images. After the shooting is completed, the photos can be analyzed using image processing software, and parameters such as the strain distribution and strain value of the specimen can be obtained by calculating the displacement and deformation amount of the speckles.

[0050] S104, perform correlation analysis on the speckle photos before and after deformation of the gauge section through DIC technology to obtain the macroscopic strain distribution on the surface of the specimen;

[0051] Among them, DIC is an optical non-contact strain measurement method based on image analysis, which measures the displacement field and strain field by performing correlation analysis on the images of the object surface before and after deformation. It is widely used in engineering and materials science to analyze structural deformation, stress-strain distribution, etc.

[0052] DIC calculates the displacements of different points on the object surface by comparing digital images before and after deformation, and then derives the strain. The specific steps include: spraying a random speckle pattern, called "speckle", on the surface of the object to be measured, or using the natural texture of the material itself. These speckles are the marked points for DIC measurement and are used to track the surface displacement of the object. Use a high-resolution camera to take images of the object before and after deformation. Image acquisition can be single acquisition (suitable for static strain) or multiple consecutive acquisitions (suitable for dynamic strain). Divide the image into many small sub-regions (called sub-pixels or subsets), and analyze these sub-regions. Each sub-region contains unique speckle features that can be tracked by algorithms. Import the acquired deformed image into a preset DIC software, such as VIC-2D software, ALDIC, etc. DIC uses the cross-correlation algorithm to compare the images before and after deformation, determines the displacement of each sub-region, and calculates the displacement field on the object surface by comparing the difference between the deformed speckle position and the initial position. Derive the strain field from the displacement field. Strain is the spatial derivative of displacement, and through DIC, the in-plane strain distribution, including tensile strain, shear strain, etc., can be calculated.

[0053] In step S104, first, collect the speckle photo of the gauge section before deformation and use it as the reference image. Subsequently, after the object is deformed by an external force or other forms of excitation, collect the speckle photo of the gauge section again to obtain the deformed image. Among them, the gauge section is a specific area on the specimen for the correlation analysis of the speckle photos before and after deformation. By comparing the length changes of the gauge section before and after loading, mechanical property indexes such as the elongation rate and strain of the specimen can be calculated. The position of the gauge section can be determined according to the test purpose and the shape of the specimen. For example, in the above tensile test, the gauge section can be located in the middle of the specimen to capture the main deformation during the tensile process.

[0054] In the embodiment of the present invention, the DIC technology is used to perform the correlation analysis on these speckle photos. The analysis process includes identifying the feature points in the speckle pattern and tracking the position changes of these feature points before and after deformation. By calculating the displacements of these feature points, the strain distribution on the surface of the specimen can be further derived. Information reflecting the strain characteristics of the specimen, such as strain concentration areas, crack initiation areas, or specific deformation mode areas, can be reflected in the macroscopic strain distribution map.

[0055] Specifically, in the speckle images before and after deformation, the matching of feature points is achieved by comparing features such as the shape, size, and gray value of the feature points. The matching process needs to consider the possible changes in the positions of the feature points caused by deformation, so an elastic matching algorithm can be used. After the feature points are successfully matched, the displacement vectors of each feature point before and after deformation can be calculated. This displacement vector represents the moving direction and distance of the feature point in space. Since the feature points are only a part of the speckle pattern, in order to obtain the displacement field of the entire gauge section, interpolation of the displacements of the feature points is required. Common interpolation methods include bilinear interpolation, nearest neighbor interpolation, Gaussian interpolation, etc. Based on the displacement field, the strain distribution on the surface of the specimen can be calculated using numerical differentiation methods. For example, the finite difference method, the finite element method, etc.

[0056] Exemplarily, Figure 2 FIG. is a schematic diagram of the macroscopic strain distribution on the surface of the specimen obtained by DIC in the embodiment of the present invention. According to Figure 2 the macroscopic strain distribution diagram shown, by displaying the calculated strain field in the form of an image or a chart, the macroscopic strain distribution on the surface of the specimen can be obtained, which is convenient for researchers to intuitively observe and analyze the deformation of the surface of the specimen.

[0057] In some embodiments, there may be some errors in the obtained macroscopic strain distribution. The errors of the DIC technique mainly come from links such as image acquisition, feature point recognition and matching, and displacement field calculation. These errors may lead to inaccurate strain measurement. In order to reduce the errors, various correction measures can be taken, such as improving the image quality, optimizing the feature point recognition algorithm, using a more accurate interpolation method, etc. In addition, the accuracy and reliability of the DIC technique can also be evaluated through experimental verification and comparison.

[0058] S105, determine the region of interest from the macroscopic strain distribution, perform HR-EBSD analysis on the region of interest, and calculate and obtain the microscopic strain distribution of this region;

[0059] Among them, the strain measurement method based on high-resolution electron backscatter diffraction technology (High-Resolution Electron Backscatter Diffraction, HR-EBSD) improves the spatial resolution on the basis of electron backscatter diffraction (Electron Backscatter Diffraction, EBSD), and can characterize crystal structure information such as the distribution of geometrically necessary dislocations (GND) with a smaller step size. HR-EBSD is mainly used to measure the lattice strain, orientation, and dislocation density of crystal materials. It can obtain strain information at the microscale by analyzing the interaction between electrons and the sample.

[0060] The core of HR-EBSD is to calculate strain by the position and shape changes of Kikuchi Bands. When a high-energy electron beam penetrates the surface layer of a sample, it undergoes inelastic scattering with the lattice and is then diffracted by periodically arranged crystal planes to form Kikuchi patterns. Kikuchi Bands are a set of "atomic-scale rulers" extremely sensitive to lattice parameters, and their geometric features correspond to the interplanar spacing and the angles between crystal axes.

[0061] When the material undergoes elastic deformation, the slight distortion of the lattice will trigger three key changes in Kikuchi Bands:

[0062] Translation offset: The change in the direction of the crystal plane normal leads to the overall movement of the Kikuchi Bands, and the displacement is related to the local strain gradient;

[0063] Bandwidth scaling: The compression or expansion of the interplanar spacing causes the width of the Kikuchi Bands to scale at the nanometer level, and the scaling ratio directly corresponds to the normal strain component;

[0064] Angle distortion: The shear deformation between adjacent crystal planes causes the angles between Kikuchi Bands to change, and this angular change can be resolved into a shear strain tensor.

[0065] In order to achieve sub-pixel accuracy, HR-EBSD adopts a high-precision cross-correlation algorithm: First, the Kikuchi pattern is divided into dozens of micro-regions (such as 32×32 pixels), the cross-correlation function between the experimental pattern and the reference pattern is calculated in the frequency domain using the fast Fourier transform, and then the sub-pixel displacement peak is located through cubic spline interpolation. Finally, the full-field displacement gradient tensor is fitted by the least squares method to decompose the elastic strain tensor and the lattice rotation amount, and its accuracy can reach 0.1 pixel or higher.

[0066] The DIC technology used in the above macroscopic strain distribution measurement calculates the displacement and strain of the specimen at different deformation stages by taking pictures of the surface of the specimen and using image processing algorithms. The advantages of DIC technology lie in its non-contact nature, full-field measurement ability, and high precision. There is no need for physical contact with the specimen, so it will not have any impact on the specimen and is suitable for strain measurement of sensitive or fragile materials. However, DIC technology mainly focuses on the macroscopic deformation behavior of materials and cannot directly obtain the microscopic structure information of materials.

[0067] Compared with DIC, EBSD technology can obtain more abundant material information. EBSD is an important means to analyze the crystal structure and orientation of materials. By measuring the electron backscatter diffraction patterns of samples, it can obtain information in many aspects such as crystal symmetry, crystal orientation, crystal integrity, and lattice constants. However, traditional EBSD uses the orientation difference in the pattern calibration results to qualitatively analyze strain, ignoring the strain information corresponding to pattern distortion. In the embodiments of the present invention, through HR-EBSD, strain can be directly quantitatively analyzed using patterns, and more elastic strain information can be obtained.

[0068] HR-EBSD relies on the electron backscatter diffraction patterns (EBSD patterns) generated by the SEM, and calculates the strain inside the sample by comparing the differences between the EBSD patterns of the undeformed sample (reference state) and the deformed sample.

[0069] When the sample surface is irradiated by a high-energy electron beam, an electron backscatter signal is generated to form an EBSD pattern. The EBSD pattern is a projection of the lattice information inside the sample crystal, reflecting characteristics such as crystal orientation and lattice distortion. HR-EBSD analyzes the small displacements and rotations between the patterns by comparing the EBSD patterns of two regions (reference region and deformed region). The change in the pattern is directly related to the strain, so the elastic strain component of the local lattice can be obtained by calculating the difference in the patterns.

[0070] Exemplarily, Figure 3 FIG. is a schematic diagram for obtaining the microscopic strain distribution of the region of interest by using HR-EBSD analysis in the embodiments of the present invention.

[0071] It should be noted that HR-EBSD is also an analysis for speckle photographs. Therefore, Figure 3 determining the region of interest from the macroscopic strain distribution and performing high-resolution electron backscatter diffraction HR-EBSD analysis on this region is to determine the region of interest from the macroscopic strain distribution, and then determine the corresponding speckle photograph region of the region of interest from the speckle photograph of the metal material specimen. Then, HR-EBSD analysis is used to analyze this speckle photograph region to calculate and obtain the microscopic strain distribution of this region. In this way, it is possible to combine macroscopic deformation observation and microscopic structure analysis simultaneously to achieve multi-scale strain measurement of metal materials.

[0072] In some embodiments, the region of interest may include one or more, and the region of interest to be further studied can be determined according to the macroscopic deformation information shown in the speckle photograph, such as the strain concentration area, crack initiation area or specific deformation mode area mentioned above. This step involves careful observation and analysis of the speckle photograph to identify the key areas related to material properties or deformation mechanisms.

[0073] For example, the region of interest can be a strain concentration area. In the speckle photograph, if it is observed that the strain in a certain region is significantly higher than that in other regions, this region can be determined as the region of interest. Strain concentration is usually related to local weakening of the material, crack initiation or non-uniformity of plastic deformation. By analyzing this region with HR-EBSD, the microscopic mechanism of strain concentration, such as lattice distortion, dislocation accumulation and phase transformation, can be deeply understood.

[0074] Alternatively, the region of interest can also be the crack initiation and propagation region in the strain concentration area. If the speckle photographs show the formation and propagation path of the crack, then these regions are also very important regions of interest. Crack initiation is usually related to microdefects, stress concentration or grain boundary effects of the material. By analyzing the microstructure near the crack through HR-EBSD, the micro-mechanism of crack initiation and how the crack propagates along a specific crystallographic direction or grain boundary can be revealed.

[0075] Select one or more scanning regions within the region of interest and use the HR-EBSD device to collect data. By scanning the electron beam at each point within the scanning region and collecting the diffraction patterns of the backscattered electrons. These diffraction patterns can be used to determine the crystal orientation and microstructure information of each point. The collected EBSD data is processed and analyzed to extract the information of the micro-strain distribution. This includes steps such as the determination of crystal orientation, the measurement of lattice parameters, the calculation and visualization of the strain tensor. Through these analyses, the micro-strain states at different positions within the region of interest can be revealed, such as lattice distortion, grain boundary sliding and phase transformation.

[0076] S106. Comprehensively analyze the macroscopic strain distribution and the microscopic strain distribution to determine the strain distribution characteristics of the metal material specimen in the ultra-low temperature environment.

[0077] In some embodiments, the above step S106 may specifically include:

[0078] S1061. Perform spatial mapping alignment on the macroscopic strain distribution obtained by DIC and the microscopic strain distribution obtained by HR-EBSD to establish the coordinate correlation of the cross-scale strain distribution;

[0079] S1062. Obtain the comprehensive strain distribution characteristics of the specimen in the ultra-low temperature environment by fusing the macro-micro data through a multi-level strain distribution model.

[0080] Specifically, first, a transformation matrix between the DIC image coordinate system and the HR-EBSD scanning coordinate system can be established based on the positioning marks on the specimen surface; then, the SEM image registration algorithm is used to eliminate the coordinate deviation caused by sample displacement or thermal drift to achieve sub-micron spatial alignment of the macro-micro strain fields; after that, the discrete HR-EBSD micro-strain data is mapped to the DIC continuous strain field through an interpolation algorithm to construct a unified strain distribution coordinate framework.

[0081] Then, at the macroscopic scale, the full-field average strain distribution obtained from DIC analysis is retained to characterize the overall deformation behavior of the specimen; in the regions with significant strain gradients (regions of interest), the grain-level strain distribution obtained from HR-EBSD is embedded to characterize the strain inhomogeneity between grains; at the microscopic scale, for the local strain concentration regions, the sub-grain distortion fields resolved by HR-EBSD are integrated to quantify the nano-scale strain concentration effect at dislocation cells / twin boundaries; finally, a comprehensive strain distribution map containing strain information at both macroscopic and microscopic levels is output, thereby obtaining the strain distribution characteristics of the metal material specimen under cryogenic environment.

[0082] In the embodiments of the present invention, by comprehensively analyzing the macroscopic strain distribution (DIC technique) and the microscopic strain distribution (HR-EBSD technique), the cross-scale accurate characterization of the strain characteristics of metal materials under cryogenic environment is realized. The cross-scale strain transfer law from macroscopic continuous deformation to microscopic lattice distortion is fully revealed; the strain concentration positions at key defect regions such as grain boundaries and dislocations are accurately located; through the two-way verification of macro-micro data, the reliability of strain measurement is significantly improved; the cooperative action mechanism between macroscopic plastic flow and microscopic deformation hotspots is intuitively presented, providing a key theoretical basis and engineering guiding parameters for the performance optimization and failure warning of cryogenic materials.

[0083] In addition, by combining the DIC technique and HR-EBSD analysis, accurate strain measurement from macroscopic to microscopic scales is achieved. The macroscopic strain distribution on the specimen surface is accurately captured by the DIC technique, and the HR-EBSD analysis is further used to delve into the microscopic level to reveal the microscopic deformation mechanism of the material under cryogenic environment. Not only the overall strain of the material is studied from a macroscopic perspective, but also the specific region of interest under this macroscopic strain state is further explored to investigate the microscopic strain distribution in the region of interest, which can more comprehensively understand the deformation behavior and mechanical properties of the material under cryogenic conditions. Moreover, by adopting a non-direct contact heat exchange method, while achieving accurate temperature control, the influence of water mist or condensation generated by the evaporation and condensation of the cooling medium on the test results is avoided.

[0084] Based on the same inventive concept, the embodiments of the present invention also provide an analysis method for the plastic deformation mechanism of metal materials under cryogenic conditions. Figure 4 It is a schematic flowchart of the embodiment of an analysis method for the plastic deformation mechanism of metal materials under cryogenic conditions in the embodiments of the present invention. Refer to Figure 4 As shown, it includes:

[0085] S401, by using the above multi-scale strain measurement method, obtain the strain distribution characteristics of the metal material specimen under ultra-low temperature environment;

[0086] S402. Based on the strain distribution characteristics and combined with the physical and chemical properties of the metal material specimen in an ultra-low temperature environment, determine the plastic deformation mechanism of the metal material specimen under ultra-low temperature conditions;

[0087] Among them, the plastic deformation mechanism includes but is not limited to dislocation slip, grain boundary sliding, twinning deformation and their interactions.

[0088] In some embodiments, the above step S402 may include:

[0089] S4021. Based on the macroscopic strain distribution characteristics and microscopic strain distribution characteristics, analyze the strain behavior of the metal material at different scales, including at least the macroscopic plastic deformation mode and the microscopic dislocation movement law;

[0090] Among them, the overall deformation mode of the metal material specimen at ultra-low temperature may include, for example, uniform deformation, local necking, plastic deformation before fracture, etc. Extract the macroscopic strain distribution characteristics of the specimen from the experimental data, including the magnitude of the strain value, the distribution range, the strain gradient, and the strain concentration region, etc. These characteristics can reflect the overall deformation mode of the specimen at ultra-low temperature.

[0091] Specifically, if the macroscopic strain distribution of the specimen is relatively uniform, there is no obvious strain concentration region, and the strain values are evenly distributed on the surface of the specimen, it indicates that the material has undergone uniform deformation at ultra-low temperature. If there is an obvious strain concentration in a certain area of the specimen, accompanied by the reduction of the cross-section of this area and the intensification of plastic deformation, it indicates that the material has undergone local necking. The necking phenomenon is a sign that the material is about to fracture, so it is an important indicator for evaluating the fracture toughness of the material and predicting its service life. Before the specimen is about to fracture, it usually undergoes a stage of plastic deformation. At this time, the macroscopic strain distribution of the specimen becomes uneven, and phenomena such as strain concentration and dislocation slip occur. By analyzing these phenomena, the fracture behavior and toughness of the material can be predicted.

[0092] The microscopic dislocation movement law includes: combining the microscopic strain distribution characteristics, studying the generation, movement and interaction of dislocations inside the material. Using techniques such as high-resolution electron microscopy to observe the dislocation morphology, density and distribution, and their relationships with microscopic structural features such as grain boundaries and precipitates.

[0093] S4022. Combining the physical and chemical properties of the material, analyze the strain distribution law of plastic deformation of the metal material in an ultra-low temperature environment;

[0094] Exemplarily, the physical properties of the material may include the effects of ultra-low temperature on the elastic modulus, yield strength, fracture toughness, etc. of the material. Changes in these properties will directly affect the strain behavior and plastic deformation mechanism of the material. The chemical properties of the material may include chemical reactions that may occur inside the material at ultra-low temperature, such as phase transformation, precipitation, diffusion, etc. These chemical reactions will change the microstructure of the material, thereby affecting its strain behavior. Based on the comprehensive analysis of these physical and chemical properties of the metal material to be tested, the possible strain distribution of the metal material in the ultra-low temperature environment can be predicted, and further the strain distribution law of the plastic deformation of the metal material in the ultra-low temperature environment can be summarized and analyzed.

[0095] S4023, based on the strain distribution law of the plastic deformation of the metal material in the ultra-low temperature environment, combined with methods such as microscopic characterization methods, elastoplastic mechanics theory, and / or multi-scale modeling, analyze the reasons for the formation of local strain concentration during the ultra-low temperature deformation of the metal material, and then clarify its plastic deformation mechanism.

[0096] Among them, the influence of the ultra-low temperature environment on the strain behavior of the metal material is mainly manifested as increased brittleness, decreased plastic deformation ability, and enhanced strain hardening effect, etc. These effects may stem from changes in the internal microstructure of the material, such as lattice distortion, phase transformation, etc. At ultra-low temperature, the ductile-brittle transition temperature of the metal material may decrease, causing the material to exhibit brittle fracture characteristics at lower temperatures. In the embodiments of the present invention, the analysis of the reasons for the formation of local strain concentration during the ultra-low temperature deformation of the metal material is considered comprehensively from multiple dimensions. First of all, the microstructure is the basis for affecting the deformation behavior of the material. At ultra-low temperature, the atomic activity inside the metal slows down, and the contribution of microscopic defects such as grain boundaries, dislocations, and inclusions to the mechanical properties of the material becomes more significant. These defect regions are prone to become the source of stress concentration during the deformation process, thereby triggering local strain concentration. By using the above-mentioned SEM for microscopic characterization, the microscopic structure characteristics inside the material can be intuitively observed, providing direct evidence for analyzing the formation of local strain concentration.

[0097] Secondly, under ultra-low temperature conditions, mechanical property parameters such as the elastic modulus and yield strength of the metal material change, resulting in a more complex stress-strain relationship of the material. By using the theory of elastoplastic mechanics, a constitutive model of the material applicable to the ultra-low temperature environment can be established, and then the stress distribution and strain concentration conditions of the material under different deformation conditions can be predicted. Therefore, the theory of elastoplastic mechanics provides a theoretical basis for understanding the deformation behavior of the material. The formation mechanism of local strain concentration can be revealed by numerical simulation methods, such as finite element analysis, by simulating the microscopic structure evolution during the ultra-low temperature deformation of the material.

[0098] In addition, the multi-scale modeling method can establish a cross-scale model by combining the micro-strain and macro-strain of the material, which can more accurately describe the strain distribution law of the material during the ultra-low temperature deformation process.

[0099] In summary, the analysis of the formation cause of local strain concentration during the ultra-low temperature deformation of metal materials comprehensively uses various means such as microscopic characterization methods, elastic-plastic mechanics theory, and multi-scale modeling. By deeply analyzing the microscopic structural characteristics, mechanical property changes, stress distribution, and strain concentration during the deformation process of the material, the formation mechanism of local strain concentration can be revealed, which can provide theoretical support and technical guidance for optimizing the ultra-low temperature plastic forming process of the material.

[0100] The various embodiments in this specification are described in a progressive manner. For the same or similar parts between the various embodiments, reference can be made to each other. Each embodiment focuses on the differences from other embodiments.

[0101] The above embodiments are only used to illustrate the technical solutions of the present application, rather than limiting the present application; although the present application has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the present application.

Claims

1. A multi-scale strain measurement method for metal materials at ultra-low temperatures, characterized in that Comprising: Preparing a metal material specimen with a suitable size according to the sample size requirements of the cryogenic in-situ tensile stage; Preparing a speckle pattern for strain measurement on the surface of the specimen by a preset metal surface treatment method; Inside a scanning electron microscope, constructing a preset cryogenic environment by a non-direct contact heat exchange method with a preset refrigeration medium, and performing an in-situ tensile test on the metal material specimen under the cryogenic environment, and taking speckle photos of a preset gauge section at different deformation stages of the specimen; Performing correlation analysis on the speckle photos before and after deformation of the gauge section by DIC technology to obtain the macroscopic strain distribution on the surface of the specimen; Determining a region of interest from the macroscopic strain distribution, performing HR-EBSD analysis on the region of interest, and calculating and obtaining the microscopic strain distribution of the region; wherein, the region of interest is a key region related to material properties or deformation mechanisms identified from the macroscopic strain distribution; the performing HR-EBSD analysis on the region of interest, calculating and obtaining the microscopic strain distribution of the region includes processing and analyzing the collected EBSD data to extract information on the microscopic strain distribution, specifically including: dividing the Kikuchi pattern into dozens of micro-regions, calculating the cross-correlation function between the experimental pattern and the reference pattern in the frequency domain using the fast Fourier transform, then positioning the sub-pixel displacement peak through cubic spline interpolation, and finally fitting the full-field displacement gradient tensor by the least squares method to decompose the elastic strain tensor and the lattice rotation amount; Comprehensively analyzing the macroscopic strain distribution and the microscopic strain distribution to determine the strain distribution characteristics of the metal material specimen under the cryogenic environment, including: based on the positioning marks on the surface of the specimen, establishing a transformation matrix between the DIC image coordinate system and the HR-EBSD scanning coordinate system; then using the SEM image registration algorithm to eliminate the coordinate deviation caused by sample displacement or thermal drift to achieve sub-micron spatial alignment of the macro-micro strain fields; then mapping the discrete HR-EBSD microscopic strain data to the DIC continuous strain field through an interpolation algorithm to construct a unified strain distribution coordinate framework; at the macroscopic scale, retaining the full-field average strain distribution analyzed by DIC to characterize the overall deformation behavior of the specimen; in the region of interest, embedding the grain-level strain distribution obtained by HR-EBSD to characterize the strain inhomogeneity between grains; at the microscopic scale, for the local strain concentration area, integrating the sub-grain distortion field analyzed by HR-EBSD to quantify the nano-scale strain concentration effect of dislocation cells / twin boundaries; finally, outputting a comprehensive strain distribution map containing strain information at both the macro-micro levels.

2. The strain measurement method according to claim 1, wherein Before electrolytic polishing the surface of the specimen, the method further includes: Successively polishing the surface of the specimen with sandpapers of different meshes to make the surface quality of the specimen meet the preset quality requirements.

3. The strain measurement method according to claim 2, characterized in that The non-direct contact heat exchange method is to perform heat exchange in a circulating flow manner by placing the refrigeration medium through a pipeline inside the scanning electron microscope to make the temperature of the metal material specimen reach the preset ultra-low temperature; the refrigeration medium includes but is not limited to liquid nitrogen and liquid helium.

4. An analysis method for the plastic deformation mechanism of metal materials at ultra-low temperatures, characterized in that, Comprising: By using the multi-scale strain measurement method according to any one of claims 1 to 3, obtain the strain distribution characteristics of the metal material specimen in the ultra-low temperature environment; Based on the strain distribution characteristics and combined with the physical and chemical properties of the metal material specimen in the ultra-low temperature environment, determine the plastic deformation mechanism of the metal material specimen under ultra-low temperature conditions; the plastic deformation mechanism includes but is not limited to dislocation slip, grain boundary sliding, twinning deformation and their interactions.

5. The analysis method of the plastic deformation mechanism according to claim 4, characterized in that The step of determining the plastic deformation mechanism of the metal material specimen under ultra-low temperature conditions based on the strain distribution characteristics and combined with the physical and chemical properties of the metal material specimen in the ultra-low temperature environment includes: Based on the macroscopic strain distribution characteristics and microscopic strain distribution characteristics, analyze the strain behavior of the metal material at different scales, including macroscopic plastic deformation modes and microscopic dislocation movement laws; Combined with the physical and chemical properties of the material, analyze the strain distribution law of the plastic deformation of the metal material in the ultra-low temperature environment; Based on the strain distribution law of the plastic deformation of the metal material in the ultra-low temperature environment, combined with microscopic characterization methods, elastoplastic mechanics theory and / or multi-scale modeling methods, analyze the reasons for the formation of local strain concentration during the ultra-low temperature deformation of the metal material, and then clarify its plastic deformation mechanism.

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