Characterization method for directly obtaining fracture cleavage plane orientation by using EBSD technology

Abstract

The invention relates to the field of material science and fracture mechanics, in particular to a technical method for characterizing a fracture microstructure. Specifically, the invention provides a characterization method for directly obtaining fracture cleavage plane orientation by using an EBSD technology, and the characterization method can be used for orientation analysis of a smooth cleavage plane or a quasi-cleavage plane on a fracture of a tensile, fatigue or impact sample. According to the method, direct characterization of a cleavage plane is realized through combination of an EBSD (electron back scattering diffraction) technology and a fracture three-dimensional orientation characterization method, so that the characterization precision of microscopic features of the fracture plane is improved, the characterization cost is greatly reduced, and visual characterization of space orientation information of the facet is realized by utilizing crystallographic calculation.

Description

Technical Field

[0001] The invention relates to the technical field of material fracture behavior analysis, and in particular to a characterization method for directly obtaining the orientation of a fracture cleavage plane by using an EBSD technique. Background Art

[0002] Fractographic analysis is an important means to understand the fracture behavior of metal materials. By observing and analyzing the microscopic characteristics of the fracture, we can deeply understand the fracture mechanism of the material. For fracture surfaces with cleavage characteristics, such as cleavage planes and quasi-cleavage planes in tensile, fatigue or impact specimens, accurately determining their spatial orientation is an important basis for studying fracture behavior.

[0003] At present, EBSD technology has been widely used in the microstructure and orientation analysis of materials. However, due to the complex geometric morphology of the fracture surface, there are certain limitations in directly performing EBSD orientation analysis on the surface grains, especially the need to perform additional surface processing on the sample or destroy the sample itself in order to obtain and analyze EBSD data. For example, one of the common characterization methods is to use FIB technology or laser processing to cut a plane of a certain area from the target characterization grains on the fracture, and further perform orientation characterization on the plane, but the fracture itself is destroyed while obtaining the crystal orientation information. Therefore, the existing methods are not only time-consuming, but also easy to damage the sample, causing the sample to be destroyed and unable to be subsequently characterized, which increases the complexity and error cost of the characterization and consumes a lot of manpower and material resources. Therefore, it is of great significance to develop a method for directly characterizing the spatial orientation of the cleavage plane of the fracture without additional processing for the study of fracture behavior. Summary of the invention

[0004] In view of the above technical problems, the present invention proposes a characterization method for directly obtaining the orientation of the cleavage plane of the fracture surface by using the EBSD technology. The spatial orientation of the cleavage plane or quasi-cleavage plane of the fracture surface of the material is accurately analyzed by combining the three-dimensional orientation calculation of the fracture surface. Without destroying the fracture surface, the orientation analysis of the cleavage plane or quasi-cleavage plane on the fracture surface of the tensile, fatigue, or impact specimen is directly performed.

[0005] To achieve the above object, the present invention adopts the following technical solution: The present invention provides a characterization method for directly obtaining the orientation of the cleavage plane of a fracture surface by using the EBSD technology, comprising the following steps: S1. Cut the fracture surface of the sample after mechanical test along the direction perpendicular to the stress axis to prepare the sample for SEM experiment; S2. Place the sample with the fracture on the sample stage of the scanning electron microscope and fix it in the chamber of the microscope, and select a facet with a relatively smooth and flat surface as the characterization site of the fracture; S3. By adjusting the angle of the sample stage and using the fracture three-dimensional orientation calculation method, calculate the spatial orientation (normal space vector) before the facet angle correction. i +b j +c k ; S4, determining the rotation angle and tilt angle of the sample stage according to the spatial orientation before correction, and rotating and tilting the sample stage to correct the facet normal to be parallel to the electron beam direction; S5. Repeat the fracture three-dimensional orientation calculation method to calculate the spatial orientation a' after the facet angle is corrected i +b' j +c' k ; S6, tilt the sample stage 70° around the X-axis of the electron microscope system coordinate system, so that the angle between the facet normal and the electron beam is 70°, and perform EBSD characterization on the target characterization site; S7, if there is any obstruction, repeat steps 3-5 to avoid obstruction of the diffraction beam and the electron beam; S8. Based on crystallographic calculations and combined with EBSD data, the relationship between facet spatial orientation and crystallographic orientation is established.

[0006] As a further illustration of the present invention, in step S1, a fracture with a height of about 10 mm can be cut by wire cutting along a direction perpendicular to the stress axis to prepare a sample.

[0007] As a further illustration of the present invention, the sample preparation process in step S1 needs to pay attention to the protection of the fracture surface, and a protective layer, such as a special coating or film, can be coated on the surface of the material to prevent oxidation or damage to the fracture surface during the online cutting process. After cutting, it needs to be cleaned and dried to prevent rust on the fracture surface.

[0008] As a further explanation of the present invention, in step S2, the selection of the characterization site must ensure that the facet is relatively smooth and flat, and is not in a hole or pit, and there is no obvious obstruction around it. Only under this condition can the analysis effect of the faceted grains be achieved. The larger the range of surface smoothness, the less obstruction, and the better the analysis effect.

[0009] As a further illustration of the present invention, after the sample is fixed on the SEM sample stage in step S2, it is placed in the electron microscope chamber and is not taken out again, and only the sample stage is rotated and the SEM and EBSD imaging modes are switched.

[0010] As a further explanation of the present invention, based on the principle of EBSD characterization technology, two sets of coordinate systems are required to describe the spatial orientation of the facets, namely the sample coordinate system and the electron microscope system coordinate system. The sample coordinate system is specified as (X1-Y1-Z1), and the electron microscope system coordinate system is specified as (XYZ); the sample rotation process is specified as rotation around the Z-axis direction of the electron microscope system coordinate system (i.e., the rotation angle is the rotation angle around the Z-axis direction of the electron microscope system coordinate system), and the tilting process is specified as rotation around the X-axis direction of the electron microscope system coordinate system (i.e., the tilting angle is the rotation angle around the X-axis direction of the electron microscope system coordinate system). The subsequent rotation and tilting processes will cause the sample coordinate system to change, but the electron microscope system coordinate system is always unique, so the coordinate systems based on the calculation processes of steps S1 to S6 are all electron microscope system coordinate systems.

[0011] As a further illustration of the present invention, the fracture three-dimensional orientation calculation method is an analytical method for determining the spatial orientation of the fracture facets of a material. The method is based on two sets of tissue photos at different inclinations, and locates four feature points of nonlinear distribution in the two sets of photos respectively, and calculates the three-dimensional spatial coordinates of the feature points based on the two-dimensional coordinates of the feature points. In this way, the normal vector of the face where the feature point is located is further determined. The rotation angle and tilt angle of the sample stage required to correct the facet angle are determined according to the normal vector of the facet, and the sample stage is rotated along the X-axis and Z-axis directions respectively, so that the normal direction of the facet coincides with the Z-axis in the electron microscope system coordinate system (XYZ).

[0012] As a further illustration of the present invention, the rotation and tilting process will cause the sample coordinate system to change, and the facet spatial orientation a calculated in step S3 i +b j +c k The sample coordinate system is the sample coordinate system X1-Y1-Z1 before facet angle correction, and the facet spatial orientation a' calculated in step S5 i +b' j +c' k The sample coordinate system is the new sample coordinate system X1*-Y1*-Z1* after the facet angle is corrected.

[0013] As a further explanation of the present invention, step S3 is specifically: quantitatively tilting the fracture surface by adjusting the angle of the sample stage, thereby obtaining two fracture surface facet morphology photos with different tilt angles before facet angle correction under the same magnification, selecting four nonlinearly distributed feature points X, A, B, C on each of the obtained morphology photos, and calculating the spatial orientation a before facet angle correction using the three-dimensional orientation of the fracture surface i +b j +c k .

[0014] As a further illustration of the present invention, the resolution of the topographic photograph obtained in step S3 is greater than 1024*768. The tilt angle used in the characterization method is generally selected in the range of 10°~45°, and the calculated result has an error of 0°~5°. The four feature points selected on the target characterization site facet must be non-collinear to reduce the calculation error.

[0015] As a further illustration of the present invention, in step S6, when performing EBSD characterization on the target characterization site, it is necessary to switch to the EBSD imaging mode, and then perform EBSD characterization on the facet area, subtract the background and obtain a clear and stable Kikuchi pattern, and then start the characterization.

[0016] As a further illustration of the present invention, when the background is subtracted in the EBSD imaging mode, since a good background pattern cannot be obtained on the fracture surface, the background needs to be collected using the same smooth material.

[0017] As a further illustration of the present invention, when obtaining a stable pattern in the EBSD imaging mode, a stable, clear, high-resolution Kikuchi pattern is obtained by adjusting the distance from the fluorescent screen or appropriately correcting the tilt angle (-5°~+5°).

[0018] As a further explanation of the present invention, step S8 is specifically: deriving the Euler angle information of the faceted grain through EBSD data, drawing the inverse pole figure of the grain along the spatial orientation direction based on the Euler angle information, and confirming the relationship between the facet spatial orientation and the crystal orientation based on the pole position.

[0019] As a further illustration of the present invention, the method provided by the present invention is applicable to fracture surfaces with smooth cleavage facet characteristics such as low-cycle, high-cycle, load-holding, impact, creep, and tensile fracture surfaces, but is not applicable to samples with excessive fluctuations in fracture microscopic morphology.

[0020] The above method provided by the present invention, combined with the characterization means of SEM and EBSD, can realize non-destructive analysis of the fracture surface and internal grain spatial orientation obtained after mechanical testing. Specifically, the method first calculates the three-dimensional spatial orientation of the fracture facets, and determines its accurate orientation in the three-dimensional space of the electron microscope cabin by analyzing the morphological characteristics of the cleavage facets or quasi-cleavage facets on the fracture. Then, the spatial orientation information is used to correct the angle of the facets in the EBSD characterization process, and the EBSD technology is used to directly realize the precise orientation characterization of the fracture cleavage facets or quasi-cleavage facets.

[0021] Compared with the prior art, the present invention has the following beneficial technical effects: 1. The present invention mainly utilizes SEM and EBSD analysis and characterization methods to directly obtain the orientation information of faceted grains on the fracture surface with complex geometric morphology, calculates the spatial orientation of the faceted grains through the three-dimensional orientation of the fracture, and rotates the sample stage in the scanning electron microscope chamber to correct the facet normal to be parallel to the electron beam direction, thereby achieving direct characterization of the faceted grains. Compared with other characterization methods, this method does not require destruction or additional processing of the fracture surface, greatly reduces the experimental cost, reduces sample damage and errors in the characterization process, significantly improves the experimental efficiency and characterization accuracy, helps to reveal the fracture mechanism of the material, and provides a reference basis for optimizing the design and manufacturing process of the material.

[0022] 2. The characterization result of the present invention is that the spatial orientation of the cleavage plane or quasi-cleavage plane can be represented on its orientation IPF inverse pole figure through crystallographic calculation, thereby realizing the visual characterization of the microscopic orientation of the fracture surface.

[0023] Other features and advantages of the technical solution will be described in the subsequent description, and partly become apparent from the description, or understood by implementing the technical solution. The purpose and other advantages of the technical solution can be achieved and obtained through the structures specifically pointed out in the written description and the drawings.

[0024] The technical solution of the present technical solution is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The accompanying drawings are used to provide a further understanding of the technical solution and constitute a part of the specification. Together with the embodiments of the technical solution, they are used to explain the technical solution and do not constitute a limitation of the technical solution. In the accompanying drawings: Figure 1 The present invention provides a flow chart of a characterization method for directly obtaining the orientation of a fracture cleavage plane or a quasi-cleavage plane by using the EBSD technology combined with a fracture three-dimensional orientation characterization method.

[0026] Figure 2 The high-resolution SEM / SE photos of the facets before and after tilting and the selected feature point positions at the same magnifications before the facet angle correction in Example 1: (a) tilt angle θ 1 = 0°; (b) tilt angle θ 2=20°.

[0027] Figure 3 The three-dimensional spatial orientation of the facets in the SEM cabin under different conditions: (a) before facet angle correction; (b) after facet angle correction; (c) in EBSD imaging mode, after the sample stage is tilted 70° around the X-axis of the SEM system coordinate system; (d) a real-life photo of the EBSD characterization process in the SEM cabin.

[0028] Figure 4 Schematic diagram of the Kikuchi pattern obtained on the facet after angle correction in the EBSD imaging mode in Example 1: (a) and (b) are the Kikuchi patterns obtained on the facets with and without obstruction on the surface, respectively.

[0029] Figure 5 The high-resolution SEM / SE photos of the facets before and after tilting and the selected feature point positions at two same magnifications after the facet angle correction in Example 1: (a) tilt angle θ 1 = 0°; (b) tilt angle θ 2=20°.

[0030] Figure 6 The good analytical effect of the facet without obstruction in EBSD imaging mode in Example 1: (a) is the selected scanning area; (b) is the corresponding EBSD image (IPF / / Z1*) of the facet in the area.

[0031] Figure 7 It is a triangular inverse pole figure obtained by crystallographic calculation in Example 1, representing the relationship between the facet spatial orientation and the crystal orientation distribution. DETAILED DESCRIPTION

[0032] The preferred embodiments of the present technical solution are described below in conjunction with the accompanying drawings. It should be understood that the preferred embodiments described herein are only used to illustrate and explain the present technical solution, and are not used to limit the present technical solution.

[0033] The material used in the following embodiments is a rod-shaped fracture specimen of Ti150 alloy obtained by forging after load-maintained fatigue failure. The specimen has no anisotropy in other directions except the stress direction, and no other additional surface treatment. The organizational state is a bimodal structure composed of an equiaxed primary α phase and a lamellar secondary α phase. The average size of the primary α grains is about 30 μm, and the average length of the secondary α lamellae is about 100 μm. Example 1

[0034] Step 1: Apply a layer of acrylic varnish on the fracture surface of the Ti150 alloy rod specimen after load-holding fatigue failure, use wire cutting to cut a fracture of about 10 mm in height along the direction perpendicular to the stress axis, immerse it in acetone for ultrasonic cleaning, and finally remove the paint with isopropyl alcohol and dry it to obtain a specimen of suitable size that can be placed in the scanning electron microscope cabin.

[0035] Step 2: Fix the cut fracture surface on the SEM sample stage with conductive glue, place it in the SEM electron microscope chamber, observe the faceted grains near the crack source area on the surface of the sample, and select the facets with relatively smooth and flat surfaces that are closer to the crack source grains as the characterization sites of the fracture surface, and try to avoid surrounding obstructions.

[0036] Step 3: Quantitatively tilt the load-holding fatigue fracture surface by adjusting the angle of the sample stage, so as to obtain two images with the same multiple of facet angle before correction and at different tilt angles. θ The fracture surface facet morphology photos are taken. Four nonlinearly distributed characteristic points X, A, B, C are selected on each of the two high-resolution fracture crack source SEM photos. The position coordinates are shown in Table 1. The spatial orientation (normal space vector) before facet angle correction is calculated using the fracture three-dimensional orientation. i -0.03 j +0.99 k .

[0037] Table 1 Position coordinates and measurement results of the selected feature points before and after quantitative tilting before and after facet angle correction

[0038] Step 4: According to the spatial orientation of the facet, the rotation angle of the sample stage is determined to be 13.02° counterclockwise, and the tilt angle is 7.81° counterclockwise. According to the result, the sample stage is rotated and tilted to correct the normal of the facet to be parallel to the electron beam direction.

[0039] Step 5: As the sample stage rotates, the normal vector of the face changes at this moment. Repeat the fracture three-dimensional orientation calculation method to obtain the spatial orientation after the facet angle is corrected 0.14 i +0 j -0.99 k , the feature point coordinates and calculation results are shown in Table 2.

[0040] Table 2 Position coordinates and measurement results of the selected feature points before and after quantitative tilting after facet angle correction

[0041] Step 6: After correcting the facet angle, tilt the sample stage 70° around the X-axis of the electron microscope system coordinate system, that is, the angle between the facet normal and the electron beam is 70°, switch to EBSD imaging mode, perform EBSD characterization on the facet area, subtract the background and obtain a clear and stable Kikuchi pattern, such as Figure 5 (b) and then start characterization. The characterization results are shown in Figure 6 As shown, most of the smooth areas of the facets have good resolution.

[0042] Step 7: When performing EBSD characterization on the facet area, first set a larger step size and perform a rough scan to observe the resolution effect of the facet. If the surrounding obstructions cause poor resolution of a large area, repeat steps 3 to 5, adjust the angle or change the characterization site, and then perform precise characterization.

[0043] Step 8: The Euler angle information of the faceted grain can be derived from the EBSD data. Based on the Euler angle information, the grain is drawn along the spatial orientation direction 0.14 i +0 j -0.99 k The inverse pole figure of . Based on the pole position, the relationship between the facet space orientation and the crystal orientation can be confirmed. In this example, the facet space normal is about 20° to the

[0001] direction, which means that the facets are formed on a plane that is 20° to (0001).

[0044] Figure 1 This is a flow chart of the characterization method for directly obtaining the orientation of the fracture cleavage plane or quasi-cleavage plane using the EBSD technology combined with the fracture three-dimensional orientation calculation method in this embodiment.

[0045] Figure 2 The high-resolution SEM / SE photos of the facets before and after quantitative tilting and the selected feature point positions are shown in the Ti150 alloy load-preserving fatigue fracture facet angle correction of the embodiment. The crack source facet position and its extension path can be seen. The surface in this area is relatively flat, and there is no significant obstruction around it, ensuring that the four selected morphological feature points are not collinear and have unique features that are not easily confused. Among them, X1 and X2, A1 and A2, B1 and B2, C1 and C2 correspond to the same morphological feature points on the two facet scanned photos.

[0046] Figure 3 is the three-dimensional spatial orientation of the facets in the SEM chamber under different states in this embodiment, Figure 3 It can be seen from (a) that before the facet angle is corrected, there is a certain angle between the facet normal and the stress direction, but after the facet angle is corrected, that is, Figure 3 (b) It can be seen that the normal direction of the facet is parallel to the stress direction. After switching to EBSD imaging mode, the sample stage is tilted 70° around the X-axis of the electron microscope system coordinate system, as shown in Figure 3 (c) shows that its actual position in the SEM chamber can be seen from Figure 3 (d)

[0047] Figure 4 Schematic diagram of the Kikuchi pattern obtained on the facet after angle correction in the EBSD imaging mode in this embodiment. It can be seen from the figure that if there is occlusion, the Kikuchi pattern cannot be identified. Figure 4 By rotating the sample and repeating step 3, the sample position is readjusted to avoid the obstruction of the diffraction beam and the electron beam, thus obtaining a clear and stable Kikuchi pattern similar to the one on the facet in this example, as shown in FIG. Figure 4 (b) shown.

[0048] Figure 5 The high-resolution SEM / SE photos of the facets before and after quantitative tilting and the selected characteristic point positions at two same magnifications after the facet angle correction of the Ti150 alloy under load fatigue fracture in this embodiment.

[0049] Figure 6 In this example, the Ti150 alloy fatigue fracture facet has a good analysis effect without obstruction in the EBSD imaging mode. The characterized area and the identifiable sites are marked on the left figure. The analysis results are shown in the right figure, which are representative enough to explain the orientation of this area.

[0050] Figure 7 This is the IPF diagram obtained by crystallographic calculation in Example 1, which shows the relationship between the spatial orientation of the facet and the crystal orientation.

[0051] It can be seen from the experimental results of Example 1 that the present invention achieves non-destructive direct characterization of the orientation of some key areas on the fracture surface without additional processing of the fracture, providing a more convenient, fast and accurate technical means for studying the fracture mechanics behavior of materials and revealing the connection between the material microstructure and the fracture mechanism.

[0052] Obviously, those skilled in the art can make various changes and modifications to the technical solution without departing from the spirit and scope of the technical solution. Thus, if these modifications and variations of the technical solution fall within the scope of the claims of the technical solution and their equivalents, the technical solution is also intended to include these modifications and variations.

Claims

1. A characterization method for directly obtaining the orientation of the fracture cleavage plane using EBSD technology, characterized in that: The steps include: S1. Cut the fracture surface of the sample after mechanical test along the direction perpendicular to the stress axis to prepare the sample for SEM experiment; S2. Place the sample with the fracture on the sample stage of the scanning electron microscope and fix it in the chamber of the microscope, and select a facet with a relatively smooth and flat surface as the characterization site of the fracture; S3. By adjusting the angle of the sample stage, the spatial orientation a before facet angle correction is calculated using the fracture three-dimensional orientation calculation method. i +b j +c k ; S4, determining the rotation angle and tilt angle of the sample stage according to the spatial orientation before correction, and rotating and tilting the sample stage to correct the facet normal to be parallel to the electron beam direction; S5. Repeat the fracture three-dimensional orientation calculation method to calculate the spatial orientation a' after the facet angle is corrected i +b' j +c' k ; S6, tilt the sample stage 70° around the X-axis of the electron microscope system coordinate system, so that the angle between the facet normal and the electron beam is 70°, and perform EBSD characterization on the target characterization site; S7, if there is any obstruction, repeat steps 3-5 to avoid obstruction of the diffraction beam and the electron beam; S8. Based on crystallographic calculations and combined with EBSD data, the relationship between facet spatial orientation and crystallographic orientation is established.

2. The characterization method for directly obtaining the orientation of the fracture cleavage plane using the EBSD technique as claimed in claim 1, characterized in that: In step S2, the selection of the characterization site must ensure that the facet is relatively smooth and flat, and is not within a hole or pit, and has no obvious obstruction around it.

3. The characterization method for directly obtaining the orientation of the fracture cleavage plane using the EBSD technique as claimed in claim 1, characterized in that: The coordinate systems on which the calculation processes of step S1 to step S6 are based are all the electron microscope system coordinate systems.

4. The characterization method for directly obtaining the orientation of the fracture cleavage plane using the EBSD technique as claimed in claim 1, characterized in that: The facet spatial orientation a calculated in step S3 i +b j +c k The sample coordinate system is the sample coordinate system X1-Y1-Z1 before facet angle correction, and the facet spatial orientation a' calculated in step S5 i +b' j +c' k The sample coordinate system is the new sample coordinate system X1*-Y1*-Z1* after the facet angle is corrected.

5. The characterization method for directly obtaining the orientation of the fracture cleavage plane using the EBSD technique as claimed in claim 1, characterized in that: Step S3 is specifically: By adjusting the angle of the sample stage to quantitatively tilt the fracture, two fracture surface facet morphology photos with different tilt angles before facet angle correction under the same magnification are obtained. Four nonlinearly distributed feature points X, A, B, C are selected on each of the obtained morphology photos, and the spatial orientation a before facet angle correction is calculated using the three-dimensional orientation of the fracture. i +b j +c k .

6. The method for directly obtaining the orientation of the fracture cleavage plane using the EBSD technique as claimed in claim 5, characterized in that: The resolution of the topographic photograph obtained in step S3 is greater than 1024*768.

7. The method for directly obtaining the orientation of the fracture cleavage plane using the EBSD technique as claimed in claim 1, characterized in that: In step S6, when performing EBSD characterization on the target characterization site, it is necessary to switch to the EBSD imaging mode, and then perform EBSD characterization on the facet area, subtract the background and obtain a clear and stable Kikuchi pattern, and then start the characterization.

8. The method for directly obtaining the orientation of the fracture cleavage plane using the EBSD technique as claimed in claim 7, characterized in that: When subtracting the background in EBSD imaging mode, the same smooth material needs to be used for background acquisition.

9. The method for directly obtaining the orientation of the fracture cleavage plane using the EBSD technique as claimed in claim 7, characterized in that: When obtaining stable patterns in EBSD imaging mode, stable, clear, and high-resolution Kikuchi patterns can be obtained by adjusting the distance to the fluorescent screen or by appropriately correcting the tilt angle.

10. The method for directly obtaining the orientation of the fracture cleavage plane using the EBSD technique according to claim 1, characterized in that: Step S8 is specifically as follows: The Euler angle information of the faceted grain is derived from the EBSD data, and the inverse pole figure of the grain along the spatial orientation direction is drawn based on the Euler angle information. The relationship between the facet spatial orientation and the crystal orientation is confirmed based on the pole position.

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