Multi-scale strain measurement method for metal material at ultralow temperature

Abstract

The invention discloses a multi-scale strain measurement method for a metal material at an ultralow temperature, and the method combines a DIC technology and HR-EBSD analysis, and achieves the precise strain measurement from a macro scale to a micro scale. Macroscopic strain distribution on the surface of a sample is accurately captured through a DIC technology, and HR-EBSD analysis is utilized to further go deep into a microscopic level, so that a microscopic deformation mechanism of a material in an ultralow-temperature environment is disclosed. The overall strain of the material is researched from the macroscopic perspective, the specific region of interest in the macroscopic strain state is further deepened, the microscopic strain distribution of the region of interest is explored, and the deformation behavior and the mechanical property of the material at the ultralow temperature can be more comprehensively understood. Besides, a non-direct contact type heat exchange mode is adopted, so that accurate temperature control is realized, and meanwhile, the influence of water mist or condensation and the like generated by evaporation and condensation of a cooling medium on a test result is avoided.

Description

Technical Field

[0001] The present application relates to the technical field of material strain measurement, and in particular to a multi-scale strain measurement method for metal materials at ultra-low temperatures. Background Art

[0002] With the continuous advancement of science and technology, metal materials are increasingly used in extreme environments, especially in aerospace, deep-sea exploration, cryogenic engineering and other fields. 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 performance and extending their service life.

[0003] Under ultra-low temperature conditions, the microstructure and mechanical properties of metal materials will change significantly. For example, the mechanical properties of materials such as yield strength, tensile strength and toughness are usually improved, but at the same time, they will also face risks such as embrittlement and reduced plasticity. In addition, ultra-low temperature environments may also cause uneven stress distribution inside the material, leading to 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.

[0004] At present, 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 have reduced measurement accuracy due to changes in material properties at low temperatures. These methods can usually only provide macroscopic strain information and cannot deeply reveal the microscopic strain distribution inside the material. Summary of the invention

[0005] The embodiment of the present application solves 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 metal materials under ultra-low temperature conditions.

[0006] In order to achieve the above object, the technical solution of the embodiment of the present invention is:

[0007] In the first aspect, an embodiment of the present invention provides a multi-scale strain measurement method for metal materials at ultra-low temperatures, comprising: preparing a metal material specimen of suitable size according to the sample size requirements of an ultra-low temperature in-situ stretching table; 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 preset refrigerant medium in a non-direct contact heat exchange manner, and performing an in-situ stretching test on the metal material specimen in the ultra-low temperature environment, taking speckle photographs of the specimen at preset gauge lengths at different deformation stages; performing correlation analysis on the speckle photographs before and after deformation of the gauge length segment by DIC technology to obtain a macroscopic strain distribution on the specimen surface; 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; and comprehensively analyzing 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.

[0008] In some possible implementations, before performing electrolytic polishing on the surface of the sample, the method further includes: polishing the surface of the sample in sequence with sandpaper of different mesh sizes so that the surface quality of the sample meets preset quality requirements.

[0009] In some possible implementations, when the sample surface is subjected to electrolytic polishing, the quality and contrast of the speckle pattern are improved by optimizing the electrolyte ratio, voltage setting, current density, and polishing time.

[0010] In some possible implementations, the non-direct contact heat exchange method is to place a refrigerant medium in a cooling chamber outside the scanning electron microscope, and use thermal radiation or thermal convection to transfer the ultra-low temperature to the metal material sample. The refrigerant medium includes but is not limited to liquid nitrogen and liquid helium.

[0011] In some possible implementations, the macroscopic strain distribution and microscopic strain distribution are comprehensively analyzed to determine the strain distribution characteristics of metal material specimens in ultra-low temperature environments, including: spatially mapping and aligning the macroscopic strain distribution obtained by DIC with the microscopic strain distribution obtained by HR-EBSD to establish a coordinate association for cross-scale strain distribution; and fusing macroscopic and microscopic data through a multi-level strain distribution model to obtain the comprehensive strain distribution characteristics of the specimen in an ultra-low temperature environment.

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

[0013] In some possible implementations, the macroscopic strain distribution and the microscopic strain distribution are comprehensively analyzed to determine the strain distribution characteristics of the metal material sample at different scales, including: analyzing the macroscopic strain distribution in the spatial domain and the time domain to obtain the macroscopic strain distribution characteristics of the sample, including but not limited to identifying the strain concentration area and the strain gradient characteristics; extracting the microscopic strain distribution characteristics through image analysis software, including but not limited to the strain value, strain direction, strain concentration point or strain band.

[0014] In some possible implementations, based on the strain distribution characteristics and combined with the physical and chemical properties of the metal material sample in the ultra-low temperature environment, the plastic deformation mechanism of the metal material sample under the ultra-low temperature condition is determined, including: based on the macroscopic strain distribution characteristics and the microscopic strain distribution characteristics, the strain behavior of the metal material at different scales is analyzed, including the macroscopic plastic deformation mode and the microscopic dislocation movement law; combined with the physical and chemical properties of the material, the strain distribution law of the plastic deformation of the metal material under the ultra-low temperature environment is analyzed; based on the strain distribution law of the plastic deformation of the metal material under the ultra-low temperature environment, combined with microscopic characterization methods, elastic-plastic mechanics theory and / or multi-scale modeling methods, the causes of the local strain concentration during the ultra-low temperature deformation of the metal material are analyzed, and then its plastic deformation mechanism is clarified.

[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 embodiment of the present invention, the DIC technology and HR-EBSD analysis are combined to achieve accurate strain measurement from macro to micro scales. The macroscopic strain distribution on the surface of the sample is accurately captured by DIC technology, and HR-EBSD analysis is further deepened to the micro level to reveal the microscopic deformation mechanism of the material under ultra-low temperature environment. Not only the overall strain of the material is studied from a macroscopic perspective, but also the specific area of ​​interest under the macroscopic strain state is further deepened to explore the microscopic strain distribution of the area of ​​interest, which can more comprehensively understand the deformation behavior and mechanical properties of the material at ultra-low temperature. In addition, the non-direct contact heat exchange method is adopted to achieve precise temperature control while avoiding the influence of water mist or condensation caused by evaporation and condensation of the cooling medium on the test results. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order 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 described below are some embodiments of the present invention. For ordinary technicians in this field, other accompanying drawings can be obtained based on these accompanying drawings without paying any creative work.

[0018] Figure 1 A schematic flow chart of an embodiment of a method for measuring multi-scale strain of metal materials at ultra-low temperatures provided for the implementation of the present invention;

[0019] Figure 2 Schematic diagram of macroscopic strain distribution on the sample surface obtained by DIC in an embodiment of the present invention;

[0020] Figure 3 A schematic diagram of obtaining microscopic strain distribution of a region of interest by using HR-EBSD analysis in an embodiment of the present invention;

[0021] Figure 4 The figure is a schematic flow chart of an embodiment of a method for analyzing the plastic deformation mechanism of metal materials at ultra-low temperatures in an embodiment of the present invention. DETAILED DESCRIPTION

[0022] The technical solutions in the embodiments of the present application will be described clearly and completely below in conjunction with the 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 ordinary technicians in this field without creative work are within the scope of protection of this application.

[0023] In the relevant description of this embodiment, the terms "including, containing, having" and the like are open terms and are generally understood to include but not be limited to; the term "at least one" is generally understood to mean one or more, where "plurality" refers to two or more; the term "at least one of the following" or similar expressions refers to any combination of these items, including any combination of single or plural items, for example, "at least one of a, b or c", or "at least one of a, b and c", can all represent: a, b, c, ab (i.e., a and b), ac, bc, or abc, 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 indicating an "or" relationship before and after.

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

[0025] Those skilled in the art should understand that in the following description of the embodiments of the present application, the order of serial numbers does not mean the order of execution, some or all of the steps can be executed in parallel or sequentially, and the execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present application.

[0026] Those skilled in the art will appreciate that the numerical ranges in the embodiments of the present application are to be construed as also specifically disclosing each intermediate value between the upper and lower limits of the scope. Each smaller range between the intermediate value in any stated value or stated range and any other stated value or intermediate value in the range is also included in the present invention. The upper and lower limits of these smaller ranges may be independently included or excluded in the scope.

[0027] Unless otherwise specified, the technical / scientific terms used herein have the same meanings as those generally understood by those skilled 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 these 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 methods and / or materials related to the documents. In the event of a conflict with any incorporated document, the content of this specification shall prevail.

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

[0029] With the continuous advancement of science and technology, metal materials are increasingly used in extreme environments, especially in aerospace, deep-sea exploration, cryogenic engineering and other fields. 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 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, the mechanical properties of materials such as yield strength, tensile strength and toughness are usually improved, but at the same time, they will also face risks such as embrittlement and reduced plasticity. In addition, ultra-low temperature environments may also cause uneven stress distribution inside the material, leading to 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] At present, 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 have reduced measurement accuracy due to changes in material properties at low temperatures. These methods can usually only provide macroscopic strain information 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 under ultra-low temperature, 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 A schematic diagram of an embodiment of a multi-scale strain measurement method for metal materials at ultra-low temperatures provided by the present invention is shown in FIG. Figure 1 As shown, the multi-scale strain measurement method of the metal material at ultra-low temperature may include:

[0034] S101, preparing a metal material sample with a suitable size according to the sample size requirements of the ultra-low temperature in-situ stretching table;

[0035] Among them, the low-temperature in-situ tensile tester is a device used to perform tensile tests on materials under low-temperature conditions. It combines cryogenic technology and tensile testing technology to test the mechanical properties of materials in a precisely controlled low-temperature environment.

[0036] In some embodiments, the sample size requirements may include the length, width, thickness, and any specific shape or dimensional tolerance of the specimen. Based on these dimensional requirements, the metal material to be measured is dimensionally processed and prepared. The type of metal material can be determined based on the purpose of the test and the required performance characteristics. For example, if the test is intended to evaluate the strength and toughness of a certain metal at ultra-low temperatures, a metal material with corresponding properties can be selected at this time. During the dimensional processing and preparation process of the metal material specimen, appropriate processing techniques and equipment can be used to ensure that the size of the specimen is accurate and meets the specifications. For example, steps such as cutting, grinding, and polishing are performed to remove any irregular or non-compliant parts.

[0037] S102, preparing a speckle pattern for strain measurement on the surface of the sample by a preset metal surface treatment method;

[0038] In some embodiments, before the above step S102, the above method may further include: polishing the surface of the sample in sequence with sandpaper of different mesh sizes so that the surface quality of the sample meets the preset quality requirements.

[0039] Before electrolytic polishing, in order to ensure that the quality of the sample surface meets the preset requirements, it is usually necessary to grind the sample surface first. Grinding can remove the oxide layer, scratches, stains, uneven parts and other defects on the sample surface, making the metal sample 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 can usually be processed step by step using sandpaper of different mesh sizes. For example, the surface of the sample can be grinded using 320#, 600#, 1200#, 2000#, and 3000# silicon carbide sandpaper in sequence. After each step of grinding, the quality of the sample surface needs to be carefully checked to ensure that there are no obvious scratches and defects. In addition, during the grinding process, it is also necessary to keep the sandpaper moist to avoid overheating of the sandpaper and damage to the sample surface.

[0041] In the above step S102, the speckle pattern is used to more accurately capture and measure the strain of the material in tension, compression or other mechanical tests. In the above metal surface treatment process, the metal sample to be tested is first cleaned and pre-treated to remove surface stains, impurities and oxide layers. For example, by grinding with sandpaper of different meshes as described above. Then, a preset metal surface treatment method, such as electrolytic polishing, spraying Al 2 O 3 Particles, etc., precisely create speckles on the sample surface. These speckles are usually randomly distributed and have different sizes and shapes, so that more deformation details can be captured in the subsequent strain measurement process.

[0042] For example, electrolytic polishing is used as an example: electrolytic polishing forms uniform and tiny holes on the surface of the sample by applying voltage to the sample in a specific electrolyte, using the principle of electrochemical corrosion, and uses the formed holes as speckles to form a speckle image. In the electrolytic polishing method, a suitable electrolyte and polishing parameters, such as voltage, current, polishing time, etc., can be selected to ensure that a speckle pattern that meets the requirements is formed on the surface of the sample. During the polishing process, the sample is placed in an electrolyte and an appropriate voltage is applied for polishing. At the same time, the temperature of the electrolyte and the polishing effect can also be monitored to ensure that an ideal speckle pattern is obtained and that parameters such as the quality and contrast of the speckle pattern meet the requirements.

[0043] It should be noted that the quality of the speckle pattern preparation is crucial to the accuracy of strain measurement. Therefore, in the above step S102, various parameters of the processing process, such as electrolytic polishing time, spraying Al 2 O 3The uniformity of the particles is ensured by the uniformity, stability and repeatability of the speckle pattern. After the speckle pattern is successfully prepared on the sample surface, the macroscopic strain of the metal can be accurately measured by using strain measurement techniques such as Digital Image Correlation (DIC) to capture and analyze the macroscopic changes of the speckle pattern before and after deformation.

[0044] S103, in a scanning electron microscope (SEM), a preset ultra-low temperature environment is established by a preset refrigeration medium in a non-direct contact heat exchange manner, and an in-situ tensile test is performed on a metal material sample in the ultra-low temperature environment, and speckle photographs of the sample at preset gauge lengths 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 in the vicinity of its observation area. This can be achieved by a preset refrigeration medium, which can be a cryogenic liquid such as liquid nitrogen or liquid helium. These refrigeration media have extremely low boiling points and can provide the required ultra-low temperature.

[0046] In some embodiments, the non-direct contact heat exchange method may be to place a refrigerant medium in a circulating flow through a pipeline within a scanning electron microscope to perform heat exchange, thereby allowing the temperature of the metal material sample to reach a preset ultra-low temperature.

[0047] Among them, the ultra-low temperature range is a relative concept, and ultra-low temperature generally refers to a temperature range far below normal temperature or general low temperature. In different scientific and industrial fields, the specific range of ultra-low temperature may be different. In an embodiment of the present invention, the ultra-low temperature may be a refrigeration temperature range of a refrigerant medium. For example, the ultra-low temperature range may be the boiling point of liquid nitrogen, which is approximately -196°C. Liquid nitrogen is often used in experiments that require a low-temperature environment because of its relatively low cost and easy availability. Alternatively, the ultra-low temperature range may also be the boiling point of liquid helium, which has a boiling point of approximately -268.9°C. The specific temperature range can be selected based on the needs in the actual application process.

[0048] It should be noted that in-situ tensile tests in ultra-low temperature environments usually involve condensation, frosting and / or air flow disturbances. When a cooling medium (such as liquid nitrogen) is introduced into a metal material sample for cooling, the surface temperature of the metal material sample will drop sharply, but due to thermal inertia, the temperature inside the scanning electron microscope experimental device will be higher than the surface temperature of the sample. Due to the temperature difference, when moisture encounters a sample 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 sample surface, which may affect the imaging accuracy of the speckle photo of the sample, and thus affect the accuracy in the subsequent strain measurement process. Therefore, in an embodiment of the present invention, the refrigerant medium does not directly contact the metal material to be tested, but the refrigerant medium is placed in a circulating flow through a pipeline to exchange heat with the metal material sample and the surrounding environment, so that the temperature of the metal material sample reaches a preset ultra-low temperature. This method ensures that the sample is tested in a stable ultra-low temperature environment, while avoiding the influence of water mist or condensation generated by the evaporation and condensation of the cooling medium on the test results. In addition, ultra-low temperature achieved by heat exchange can also achieve precise temperature control. For example, precise temperature control can be achieved by controlling the flow rate of the refrigerant medium, the number of cycles, etc.

[0049] In an embodiment of the present invention, after an ultra-low temperature environment is constructed and a metal material sample is installed, an in-situ tensile test can be performed. During the test, a gradually increasing tensile force is applied to the sample by a stretching device of an ultra-low temperature in-situ stretching table until the sample breaks or reaches a predetermined deformation. During the stretching process, the high-resolution imaging function of the SEM is used to capture speckle photos of the sample at different deformation stages, and the entire deformation process of the sample is recorded. When taking speckle photos, ensure that the camera's focus, exposure and other parameters are properly set to obtain clear and accurate images. After shooting, the image processing software can be used to analyze the photos, and the parameters such as the strain distribution and strain value of the sample can be obtained by calculating the displacement and deformation of the speckle.

[0050] S104, using DIC technology to perform correlation analysis on the speckle photographs before and after the gauge section deformation, to obtain the macroscopic strain distribution on the sample surface;

[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 analyzing the correlation of the images before and after the deformation of the object surface. It is widely used in engineering and materials science to analyze structural deformation, stress and strain distribution, etc.

[0052] DIC compares digital images before and after deformation, and uses correlation algorithms to calculate the displacement of different points on the surface of the object, thereby deriving strain. The specific steps include: spraying a random spot 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 markers for DIC measurement and are used to track the displacement of the object surface. Use a high-resolution camera to capture images of the object before and after deformation. Image acquisition can be a single acquisition (suitable for static strain) or multiple continuous acquisitions (suitable for dynamic strain). The image is divided into many small sub-areas (called sub-pixels or subsets), and these sub-areas are analyzed. Each sub-area contains unique speckle features that can be tracked by the algorithm. Import the acquired deformed images into the preset DIC software, such as VIC-2D software, ALDIC, etc. DIC uses a cross-correlation algorithm to compare the images before and after deformation, determine the displacement of each sub-area, and calculate the displacement field of the object surface by comparing the difference between the deformed speckle position and the initial position. The strain field is derived from the displacement field. Strain is the spatial derivative of displacement. DIC can calculate the strain distribution in the plane, including tensile strain, shear strain, etc.

[0053] In step S104, firstly, a speckle photograph of the gauge segment before deformation is collected and used as a reference image. Subsequently, after the object is deformed by external force or other forms of excitation, a speckle photograph of the gauge segment is collected again to obtain an image after deformation. Among them, the gauge segment is a specific area on the specimen, which is used for correlation analysis of speckle photographs before and after deformation. By comparing the length change of the gauge segment before and after the force, the mechanical properties of the specimen, such as elongation and strain, can be calculated. The position of the gauge segment can be determined according to the purpose of the test and the shape of the specimen. For example, in the above-mentioned tensile test, the gauge segment 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 technique is used to perform correlation analysis on these speckle photos. The analysis process includes identifying characteristic points in the speckle pattern and tracking the position changes of these characteristic points before and after deformation. By calculating the displacement of these characteristic points, the strain distribution on the sample surface can be further derived. The macroscopic strain distribution map can reflect information such as strain concentration areas, crack initiation areas, or specific deformation mode areas that reflect the strain characteristics of the sample.

[0055] Specifically, in the speckle images before and after deformation, the matching of feature points is achieved by comparing the shape, size, grayscale value and other features of the feature points. The matching process needs to take into account the position changes of the feature points that may be caused by deformation, so the elastic matching algorithm can be used. After the feature points are successfully matched, the displacement vector 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 point is only a part of the speckle pattern, in order to obtain the displacement field of the entire gauge section, the displacement of the feature point needs to be interpolated. Commonly used interpolation methods include bilinear interpolation, nearest neighbor interpolation, Gaussian interpolation, etc. Based on the displacement field, the strain distribution on the sample surface can be calculated using numerical differentiation methods. For example, finite difference method, finite element method, etc.

[0056] For example, Figure 2 FIG. 1 is a schematic diagram of the macroscopic strain distribution of the sample surface obtained by DIC in an embodiment of the present invention. Figure 2 The macroscopic strain distribution diagram shown displays the calculated strain field in the form of an image or a chart, so that the macroscopic strain distribution of the sample surface can be obtained, which is convenient for researchers to intuitively observe and analyze the deformation of the sample surface.

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

[0058] S105, 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;

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

[0060] The core of HR-EBSD is to infer strain through the position and shape changes of the Kikuchi Band. When the high-energy electron beam penetrates the surface of the sample, it will be inelastically scattered with the lattice, and then diffracted by the periodically arranged crystal planes to form the Kikuchi pattern. The Kikuchi Band is a set of "atomic scale rulers" that are extremely sensitive to the lattice parameters, and its geometric characteristics correspond to the inter-plane spacing and the angle between the crystal axes.

[0061] When the material deforms elastically, tiny distortions in the lattice trigger three key changes in the Kikuchi bands:

[0062] Translational shift: The change in the direction of the crystal plane normal causes the overall position of the Kikuchi band to move, and the amount of displacement is related to the local strain gradient;

[0063] Bandwidth scaling: Compression or expansion of the interplanar spacing causes nanoscale scaling of the Kikuchi band width, with the scaling directly corresponding to the normal strain component;

[0064] Angled distortion: Shear deformation between adjacent crystal planes will cause the angle of the Kikuchi band to change, and this angle change can be solved as a shear strain tensor.

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

[0066] The DIC technology used in the above-mentioned macroscopic strain distribution measurement takes an image of the sample surface and uses an image processing algorithm to calculate the displacement and strain of the sample at different deformation stages. The advantages of DIC technology are its non-contact nature, full-field measurement capability and high precision. There is no need to physically contact the sample, so there is no impact on the sample, which is suitable for strain measurement of sensitive or fragile materials. However, DIC technology mainly focuses on the macroscopic deformation behavior of the material, and it cannot directly obtain the microstructural information of the material.

[0067] Compared with DIC, EBSD technology can obtain more abundant material information. EBSD is an important means of analyzing the crystal structure and orientation of materials. By measuring the electron backscatter diffraction pattern of the sample, it can obtain information on crystal symmetry, crystal orientation, crystal integrity, lattice constant, etc. However, traditional EBSD uses the pattern calibration result - orientation difference to qualitatively analyze strain, ignoring the strain information corresponding to the pattern distortion. In the embodiment of the present invention, HR-EBSD uses the pattern to directly quantitatively analyze the strain, which can obtain more elastic strain information.

[0068] HR-EBSD relies on the electron backscatter diffraction pattern (EBSD pattern) generated by the SEM and calculates the strain inside the sample by comparing the difference 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 the characteristics of the crystal orientation and lattice distortion. HR-EBSD compares the EBSD patterns of two areas (reference area and deformed area) and uses a cross-correlation algorithm to analyze the tiny displacements and rotations between the patterns. 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 pattern.

[0070] For example, Figure 3 Schematic diagram of obtaining microscopic strain distribution of a region of interest using HR-EBSD analysis in an embodiment of the present invention.

[0071] It should be noted that HR-EBSD is also an analysis of speckle photos. Figure 3 In the process of determining the region of interest from the macroscopic strain distribution and performing high-resolution electron backscatter diffraction HR-EBSD analysis on the region, the region of interest is determined from the macroscopic strain distribution, and then the speckle photo area corresponding to the region of interest is determined from the speckle photo of the metal material sample. Then the speckle photo area is analyzed by HR-EBSD analysis to calculate and obtain the microscopic strain distribution of the area. In this way, macroscopic deformation observation and microstructural analysis can be combined at the same time to realize multi-scale strain measurement of metal materials.

[0072] In some embodiments, the region of interest may include one or more regions, and the region of interest that needs further study may be determined based on the macroscopic deformation information displayed in the speckle photograph, such as the strain concentration region, crack initiation region, or specific deformation mode region. This step involves careful observation and analysis of the speckle photograph to identify key regions related to material properties or deformation mechanisms.

[0073] For example, the region of interest can be a strain concentration area. In the speckle photo, if the strain in a certain area is observed to be significantly higher than that in other areas, this area can be identified as the region of interest. Strain concentration is usually related to local weakening of the material, crack initiation, or inhomogeneity of plastic deformation. By analyzing this area with HR-EBSD, we can gain a deeper understanding of the microscopic mechanism of strain concentration, such as lattice distortion, dislocation accumulation, and phase transformation.

[0074] Alternatively, the region of interest can also be the strain concentration area crack initiation and propagation area. If the speckle photo shows the crack formation and propagation path, then these areas are also very important areas of interest. Crack initiation is usually related to microscopic defects, stress concentration or grain boundary effects in the material. Analyzing the microstructure near the crack by HR-EBSD can reveal the microscopic mechanism of crack initiation and how the crack propagates along a specific crystallographic direction or grain boundary.

[0075] Select one or more scanning areas within the region of interest and use the HR-EBSD equipment to collect data. Scan the electron beam at each point in the scanning area and collect the diffraction pattern 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 information about the microscopic strain distribution. It includes steps such as determining the crystal orientation, measuring the lattice parameters, calculating and visualizing the strain tensor. Through these analyses, the microscopic strain states at different locations in the region of interest can be revealed, such as lattice distortion, grain boundary sliding, and phase transformation.

[0076] S106, comprehensively analyzing the macroscopic strain distribution and the microscopic strain distribution to determine the strain distribution characteristics of the metal material sample in the ultra-low temperature environment.

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

[0078] S1061, spatially align the macroscopic strain distribution obtained by DIC with the microscopic strain distribution obtained by HR-EBSD to establish the coordinate association of cross-scale strain distribution;

[0079] S1062, by fusing macro and micro data through a multi-level strain distribution model, the comprehensive strain distribution characteristics of the sample in an ultra-low temperature environment are obtained.

[0080] Specifically, first, the 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 sample surface; then, the SEM image registration algorithm can be used to eliminate the coordinate deviation caused by sample displacement or thermal drift, and achieve submicron spatial alignment of the macro-micro strain field; then, the discrete HR-EBSD micro strain data can be mapped to the DIC continuous strain field through the interpolation algorithm to construct a unified strain distribution coordinate framework.

[0081] Then, at the macroscopic scale, the full-field average strain distribution of the DIC analysis is retained to characterize the overall deformation behavior of the sample; in the area of ​​significant strain gradient (area of ​​interest), the grain-level strain distribution obtained by HR-EBSD is embedded to characterize the strain inhomogeneity between grains; at the microscopic scale, for the local strain concentration area, the subgrain distortion field analyzed by HR-EBSD is integrated to quantify the nanoscale strain concentration effect of dislocation cells / twin boundaries; finally, a comprehensive strain distribution map containing strain information at both the macroscopic and microscopic levels is output, thereby obtaining the strain distribution characteristics of the metal material sample in an ultra-low temperature environment.

[0082] In the embodiment of the present invention, the cross-scale accurate characterization of the strain characteristics of metal materials under ultra-low temperature environment is achieved by comprehensively analyzing the macroscopic strain distribution (DIC technology) and microscopic strain distribution (HR-EBSD technology). The cross-scale strain transfer law from macroscopic continuous deformation to microscopic lattice distortion is fully revealed; the strain concentration position of key defect areas such as grain boundaries and dislocations is accurately located; the reliability of strain measurement is significantly improved through bidirectional verification of macroscopic and microscopic data; the synergistic mechanism of macroscopic plastic flow and microscopic deformation hot spots is intuitively presented, providing key theoretical basis and engineering guidance parameters for ultra-low temperature material performance optimization and failure warning.

[0083] In addition, the combination of DIC technology and HR-EBSD analysis has achieved accurate strain measurement from macro to micro scales. The DIC technology is used to accurately capture the macroscopic strain distribution on the surface of the sample, and HR-EBSD analysis is used to further penetrate into the microscopic level to reveal the microscopic deformation mechanism of the material in an ultra-low temperature environment. Not only does it study the overall strain of the material from a macroscopic perspective, but it also goes further into specific areas of interest under the macroscopic strain state to explore the microscopic strain distribution in the area of ​​interest, which can more comprehensively understand the deformation behavior and mechanical properties of the material at ultra-low temperatures. In addition, the use of non-direct contact heat exchange can achieve precise temperature control while avoiding the influence of water mist or condensation caused by the evaporation and condensation of the cooling medium on the test results.

[0084] Based on the same inventive concept, the embodiment of the present invention also provides a method for analyzing the plastic deformation mechanism of metal materials at ultra-low temperatures. Figure 4 FIG. 1 is a flow chart of an embodiment of a method for analyzing the plastic deformation mechanism of a metal material at ultra-low temperature in an embodiment of the present invention, see Figure 4 As shown, including:

[0085] S401, obtaining strain distribution characteristics of a metal material sample in an ultra-low temperature environment by using the multi-scale strain measurement method;

[0086] S402, based on the strain distribution characteristics and combined with the physical and chemical properties of the metal material sample under ultra-low temperature conditions, determine the plastic deformation mechanism of the metal material sample under ultra-low temperature conditions;

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

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

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

[0090] Among them, the overall deformation mode of metal material specimens at ultra-low temperatures may include uniform deformation, local necking, plastic deformation before fracture, etc. The macroscopic strain distribution characteristics of the specimens are extracted from the experimental data, including the size of the strain value, distribution range, strain gradient, and strain concentration area. These characteristics can reflect the overall deformation mode of the specimen at ultra-low temperatures.

[0091] Specifically, if the macroscopic strain distribution of the sample is relatively uniform, there is no obvious strain concentration area, and the strain value is evenly distributed on the surface of the sample, it indicates that the material has undergone uniform deformation at ultra-low temperatures. If there is obvious strain concentration in a certain area of ​​the sample, accompanied by a reduction in the cross-section of the area and an increase in plastic deformation, it indicates that the material has undergone local necking. The necking phenomenon is a sign that the material is about to break, so it is an important indicator for evaluating the fracture toughness of the material and predicting its service life. Before the sample is about to break, it usually goes through a stage of plastic deformation. At this time, the macroscopic strain distribution of the sample 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 law of microscopic dislocation movement includes: combining the microscopic strain distribution characteristics to study the generation, movement and interaction of dislocations inside the material. Using high-resolution electron microscopy and other techniques, we can observe the dislocation morphology, density and distribution, as well as their relationship with microstructural characteristics such as grain boundaries and precipitation phases.

[0093] S4022, combining the physical and chemical properties of materials, analyze the strain distribution law of plastic deformation of metal materials under ultra-low temperature environment;

[0094] Exemplarily, the physical properties of the material may include the effect 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 temperatures, such as phase change, precipitation, diffusion, etc. These chemical reactions will change the microstructure of the material, thereby affecting its strain behavior. Based on a comprehensive analysis of these physical and chemical properties of the metal material to be tested, the strain distribution that may exist in the metal material under an ultra-low temperature environment can be predicted, thereby further summarizing and analyzing the strain distribution law of plastic deformation of the metal material under an ultra-low temperature environment.

[0095] S4023, based on the strain distribution law of plastic deformation of metal materials under ultra-low temperature environment, combined with microscopic characterization methods, elastic-plastic mechanics theory and / or multi-scale modeling methods, analyze the causes of local strain concentration during ultra-low temperature deformation of metal materials, and then clarify its plastic deformation mechanism.

[0096] Among them, the influence of ultra-low temperature environment on the strain behavior of metal materials is mainly manifested as increased brittleness, decreased plastic deformation ability and enhanced strain hardening effect. These effects may be caused by changes in the microstructure inside the material, such as lattice distortion, phase transition, etc. At ultra-low temperatures, the tough-brittle transition temperature of metal materials may be reduced, so that the material exhibits brittle fracture characteristics at lower temperatures. In the embodiment of the present invention, the analysis of the causes of local strain concentration during ultra-low temperature deformation of metal materials is comprehensively considered from multiple dimensions. First, microstructure is the basis for affecting the deformation behavior of materials. At ultra-low temperatures, 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 defective areas are prone to become the source of stress concentration during the deformation process, thereby causing local strain concentration. By using the above-mentioned SEM for microscopic characterization, the microstructural 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, the elastic modulus, yield strength and other mechanical properties of metal materials change, causing the stress-strain relationship of the material to become more complicated. Using the theory of elastic-plastic mechanics, a material constitutive model suitable for ultra-low temperature environments can be established to predict the stress distribution and strain concentration of the material under different deformation conditions. Therefore, the theory of elastic-plastic mechanics provides a theoretical basis for understanding the deformation behavior of materials. Numerical simulation methods, such as finite element analysis, can be used to simulate the microstructural evolution of materials during ultra-low temperature deformation to reveal the formation mechanism of local strain concentration.

[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 ultra-low temperature deformation.

[0099] In summary, the analysis of the causes of local strain concentration during ultra-low temperature deformation of metal materials uses a variety of methods, including microscopic characterization methods, elastic-plastic mechanics theory, and multi-scale modeling. By deeply analyzing the microstructural characteristics of the material, changes in mechanical properties, and stress distribution and strain concentration during deformation, 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, and the same or similar parts between the various embodiments can be referenced 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 to limit the present application. Although the present application has been described in detail with reference to the aforementioned embodiments, a person of ordinary skill in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some or all of the technical features thereof may be replaced by equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to 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: include: According to the sample size requirements of the ultra-low temperature in-situ stretching table, prepare metal material samples with appropriate sizes; Preparing a speckle pattern for strain measurement on the surface of the sample by a preset metal surface treatment method; In a scanning electron microscope, a preset ultra-low temperature environment is established by a preset refrigeration medium in a non-direct contact heat exchange manner, and an in-situ tensile test is performed on the metal material sample in the ultra-low temperature environment, and speckle photographs of the sample at preset gauge lengths at different deformation stages are taken; Performing correlation analysis on the speckle photographs before and after the gauge section deformation by DIC technology to obtain the macroscopic strain distribution on the sample surface; Determine a 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 the region; The macroscopic strain distribution and the microscopic strain distribution are comprehensively analyzed to determine the strain distribution characteristics of the metal material sample in an ultra-low temperature environment.

2. The strain measurement method according to claim 1, characterized in that: Before electrolytic polishing the surface of the sample, the method further comprises: The surface of the sample is polished in sequence with sandpaper of different mesh sizes so that the surface quality of the sample meets 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 place the refrigerant medium in a circulating flow through a pipeline in the scanning electron microscope for heat exchange, so that the temperature of the metal material sample reaches a preset ultra-low temperature; the refrigerant medium includes but is not limited to liquid nitrogen and liquid helium.

4. The method according to claim 1, characterized in that The comprehensive analysis of the macroscopic strain distribution and the microscopic strain distribution to determine the strain distribution characteristics of the metal material sample in the ultra-low temperature environment includes: Performing spatial mapping alignment on the macroscopic strain distribution obtained by the DIC and the microscopic strain distribution obtained by the HR-EBSD to establish a coordinate association of cross-scale strain distributions; By fusing macro and micro data with a multi-level strain distribution model, the comprehensive strain distribution characteristics of the sample in an ultra-low temperature environment are obtained.

5. A method for analyzing the plastic deformation mechanism of metal materials at ultra-low temperatures, characterized in that: include: By using the multi-scale strain measurement method according to any one of claims 1 to 4, the strain distribution characteristics of the metal material sample in the ultra-low temperature environment are obtained; Based on the strain distribution characteristics and combined with the physical and chemical properties of the metal material sample under ultra-low temperature conditions, the plastic deformation mechanism of the metal material sample under ultra-low temperature conditions is determined; the plastic deformation mechanism includes but is not limited to dislocation slip, grain boundary sliding, twin deformation and their interactions.

6. The method for analyzing the plastic deformation mechanism according to claim 5, characterized in that: The method of determining the plastic deformation mechanism of the metal material sample under ultra-low temperature conditions based on the strain distribution characteristics and in combination with the physical and chemical properties of the metal material sample under ultra-low temperature conditions includes: Based on the macroscopic strain distribution characteristics and microscopic strain distribution characteristics, the strain behavior of metal materials at different scales is analyzed, including macroscopic plastic deformation mode and microscopic dislocation movement law; Combined with the physical and chemical properties of materials, the strain distribution law of plastic deformation of metal materials in ultra-low temperature environment is analyzed; Based on the strain distribution law of plastic deformation of metal materials under ultra-low temperature environment, combined with microscopic characterization methods, elastic-plastic mechanics theory and / or multi-scale modeling methods, the causes of local strain concentration during ultra-low temperature deformation of metal materials are analyzed, and then the plastic deformation mechanism is clarified.

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