TEM characterization method for orientation relation between deformed metal crystal orientation and load space orientation

Abstract

The invention discloses a TEM (Transmission Electron Microscope) characterization method of a deformed metal crystal orientation and load space orientation relationship, which comprises the following steps of: marking a load direction of a deformed metal sample to obtain a space vector of a load under a sample coordinate system, and shooting a bright field image and a positive band axis electron diffraction pattern of a to-be-detected area by using a TEM to obtain a positive band axis electron diffraction pattern of the to-be-detected area; and recording the tilt angle of the double-tilt sample rod, and carrying out crystallographic calculation according to the crystal orientation information and the tilt angle transmitted by the diffraction pattern to obtain an orientation matrix of the crystal in the area under a sample coordinate system. According to the method, the crystal orientation is obtained based on the TEM electron diffraction pattern, the spatial orientation relation between the crystal orientation and the load of the deformed metal material is obtained through crystallographic coordinate system conversion, and the microscopic mechanism of deformation and damage of the metal material can be disclosed.

Description

Technical Field

[0001] The invention relates to the field of crystallography and characterization technology, and in particular to a TEM characterization method for the relationship between deformed metal crystal orientation and load spatial orientation. Background Art

[0002] During the deformation process, metal materials will activate plastic deformation mechanisms such as dislocation slip, twinning, and phase transformation to coordinate external loads. The mode and path of plastic deformation are determined by the load direction and crystal orientation. Therefore, the relationship between the crystal orientation of deformed metals and the load spatial orientation is the basis for quantitative analysis of plastic deformation mechanisms, which is of great significance for predicting material mechanical behavior, solving material failure problems, and guiding material design and preparation.

[0003] Scanning electron microscope electron backscatter diffraction (SEM-EBSD) and transmission electron microscope (TEM) techniques are often used to characterize the microstructure and crystal orientation of metal materials after deformation. Since the SEM-EBSD technique uses block samples, it is easy to mark the force direction and can directly obtain the crystal orientation of each pixel point under the sample coordinates. The relationship between the crystal orientation of deformed metals and the load spatial orientation is often characterized by SEM-EBSD technology. However, there are some shortcomings in the characterization of the deformed structure of metal materials by SEM-EBSD technology. First, the spatial resolution of SEM-EBSD technology is limited, and it may not be able to clearly distinguish tiny changes in crystal orientation, especially in microstructures at the nanoscale. Secondly, SEM-EBSD technology is usually difficult to apply to samples with large plastic deformation, because large plastic deformation will cause the crystal structure to break and distort, making it difficult to analyze the electron backscatter signal. TEM, as a microstructure characterization technology with higher resolution, can also obtain the crystal structure and crystal orientation through electron diffraction analysis, but in the process of tilting the sample rod, the load direction of the deformed sample also rotates, and it is impossible to directly obtain the relationship between the crystal orientation and the load spatial orientation in the same coordinate system like SEM-EBSD technology. Nevertheless, the sample rotates around the axis fixed to the sample rod during the tilting process, and the tilting angle can be read from the computer display of the TEM. In theory, the crystal orientation and load direction in the same coordinate system can be obtained through coordinate system conversion. However, there is currently no method that can use any even-numbered rotationally symmetric electron diffraction pattern and its corresponding sample rod tilt angle to obtain the crystal orientation in the same coordinate system, which means that when using TEM to obtain high-resolution microstructure images, it is impossible to provide information on the relationship between the deformed metal crystal orientation and the load spatial position, thereby limiting the understanding of the microscopic mechanism of deformation and damage of metal materials. Summary of the invention

[0004] In view of the above-mentioned deficiencies in the prior art, the object of the present invention is to provide a TEM characterization method for deformed metal crystal orientation and load spatial orientation relationship, so as to solve the problem that the existing TEM characterization technology cannot directly obtain the information on the relationship between crystal orientation and load spatial orientation.

[0005] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0006] A TEM characterization method for the relationship between the orientation of a deformed metal crystal and the spatial orientation of a load, specifically comprising the following steps:

[0007] Step 1: Obtain a TEM sample of a deformed metal material, mark the force direction of the sample, establish a sample coordinate system S according to a double-tilt sample holder, place the sample on the double-tilt sample holder, and obtain the direction of the load on the sample in the sample coordinate system S;

[0008] Step 2: Place the double-tilt sample holder in the TEM, tilt the sample holder, take a bright field image of the area to be measured and the electron diffraction pattern under the positive zone axis, and record the tilt angle at the same time;

[0009] Step 3: Calibrate the electron diffraction pattern obtained in step 2 to obtain the direction vector of a certain crystal zone axis in the sample and the diffraction vector of a certain diffraction spot of the crystal zone axis in the crystal coordinate system C. Take the transmission spot as the origin and measure the coordinates of the diffraction vector in the electron microscope coordinate system M after tilting the sample rod;

[0010] Step 4: According to the diffraction pattern information, the orientation matrix of the region in the electron microscope coordinate system M is obtained, and the rotation matrix for the conversion between the electron microscope coordinate system M and the sample coordinate system S is obtained according to the tilt angle;

[0011] Step 5: According to the orientation matrix and rotation matrix described in step 4, obtain the orientation matrix of the crystal orientation of the area to be measured in the sample coordinate system S.

[0012] Preferably, in step 1, a sample coordinate system S is established according to a double-tilted sample rod, the length direction of the sample rod is the OX axis direction, the width direction of the sample rod is the OY axis direction, and the normal direction of the sample surface is the OZ axis direction. The deformed metal material is cut into thin slice samples in a specific direction. Optionally, the thin slice surface of the cut thin slice sample is perpendicular to the load direction of the sample, and the load direction of the sample in the sample coordinate system S is parallel to the OZ axis; when the thin slice surface is parallel to the load direction of the sample, the characteristic direction of the load is marked when punching the transmission electron microscope disc sample, and the angle between the force direction and the OX axis direction is recorded when the transmission sample is installed, so that the load direction of the sample in the sample coordinate system S can be obtained.

[0013] Preferably, in step 2, the calibration of the electron diffraction pattern has 180° non-uniqueness, but when the band axis of the electron diffraction pattern is an even-order rotational symmetry axis, the crystal orientation can be uniquely determined, and an even-order rotational symmetry band axis should be selected when tilting the sample to obtain the electron diffraction pattern.

[0014] Preferably, in step 4, the orientation matrix of the crystal in the electron microscope coordinate system M is expressed as:

[0015]

[0016] Among them, [uvw] and [hkl] represent the direction vector of the crystal zone axis in the crystal coordinate system C and the diffraction vector corresponding to a diffraction spot in the diffraction pattern, respectively. The selection of the diffraction spot follows the principle of unique crystal orientation, and the vector [rst] = [uvw] × [hkl]. With the transmission spot of the diffraction pattern as the origin, the horizontal direction is the OX axis direction, the vertical direction is the OY axis direction, and the plane normal is the OZ axis direction, the electron microscope coordinate system M is established, and [xy 0] is the vector of the diffraction spot in the coordinate system M. All vectors are normalized.

[0017] Preferably, in step 4, the rotation matrix is ​​expressed as:

[0018]

[0019] Wherein, α and β represent the tilting angles of the sample around the OX axis and the OY axis of the sample coordinate system S during the tilting of the sample rod, respectively, and counterclockwise rotation is defined as a positive angle.

[0020] Preferably, in step 5, the orientation matrix of the crystal orientation in the sample coordinate system S is expressed as:

[0021]

[0022] The crystal orientation matrix and the direction vector of the load in the sample coordinate system S are obtained, and finally the spatial orientation relationship between the deformed metal crystal orientation and the load in the same coordinate system is obtained.

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

[0024] 1. The existing SEM-EBSD technology can only observe the surface morphology, and the analysis of the internal structure and crystal defects of the material is relatively limited. However, microscopic defects such as dislocations, stacking faults and nano twins will occur in the deformed structure of metal materials. The present invention is based on TEM to characterize the relationship between the crystal orientation and load space orientation of the deformed metal, and can provide more comprehensive information on the microstructure and crystal defects.

[0025] 2. The prior art requires specifying the band axis of the electron diffraction pattern when obtaining the crystal orientation under TEM characterization. However, due to the angle limitation of the tilting of the transmission sample rod, some crystal orientations are difficult to tilt to the required band axis. The method described in the present invention can uniquely determine the crystal orientation based on any electron diffraction pattern under an even-numbered rotationally symmetric band axis, and has higher flexibility.

[0026] 3. The method of the present invention is simple and easy to implement. It only requires taking a transmission electron diffraction pattern that records the tilt angle. The crystal orientation in the sample coordinate system can be obtained through diffraction pattern calibration and simple coordinate conversion matrix calculation. All operations have clear physical meanings. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 The figure is a schematic diagram of the sample coordinate system S established according to the double-tilt sample holder of the transmission electron microscope in the present invention, wherein α represents the tilt angle of the sample around the OX axis during the tilting of the sample holder, and β represents the tilt angle of the sample around the OY axis during the tilting of the sample holder.

[0028] Figure 2 Flow chart for preparing deformed metal TEM samples with specific loading directions for the present invention.

[0029] Figure 3 It is a schematic diagram of the electron diffraction pattern calibration and the diffraction vector coordinates under the electron microscope coordinate system M in the present invention.

[0030] Figure 4 In the figure: (a) is the bright field image of the area to be measured in Example 1, (b) is the electron diffraction pattern under the

[001] zone axis in Example 1, and (c) is the electron diffraction pattern under the

[110] zone axis in Example 1.

[0031] Figure 5 In: (a) is the {332} in Example 2 <113> (b) is the bright field image of the twin region, (c) is the diffraction pattern of the twin region of Example 2, and (d) is the {332} superimposed pole figure of the parent and twin regions of Example 2. DETAILED DESCRIPTION

[0032] The present invention will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the present invention belong to the scope of protection of the present invention.

[0033] Unless otherwise indicated in specific cases in the present invention, the numerical ranges listed herein include the upper and lower limits, and all integers and fractions within the range, and are not limited to the specific values ​​listed when defining the range.

[0034] 1. A TEM characterization method for the relationship between the orientation of deformed metal crystals and the spatial orientation of load

[0035] The existing technology requires that the band axis be a special low-index band axis when determining the crystal orientation using diffraction patterns under TEM characterization, specifically a cubic structure. <100> With shaft, hexagonal structure <11-20> With shaft, hexagonal structure <10-10> With shaft or hexagonal structure <0002> The band axis, and some crystal orientations are difficult to tilt to the required band axis. Therefore, the present invention considers how to fundamentally solve this problem. During in-depth research, the present invention found that according to the electron microscopy analysis theory, the electron diffraction pattern under the even-order rotationally symmetric band axis can uniquely determine the crystal orientation, and the crystal orientation information in the electron microscope coordinate system can be obtained through the conversion relationship between the crystal coordinate system and the electron microscope coordinate system. During the tilting process, the sample rotates around the axis fixed to the sample rod, and the tilting angle can be read from the computer display of the TEM. In theory, the connection between the electron microscope coordinate system and the sample coordinate system can be established to obtain the crystal orientation in the sample coordinate system. If the direction vector of the load on the sample in the sample coordinate system is known, the spatial orientation relationship between the deformed metal crystal orientation and the load can be obtained. The specific steps are as follows:

[0036] The sample coordinate system S is established based on the double-tilt sample rod, as follows: Figure 1 As shown in the figure, the length direction of the sample rod is the OX axis direction, the width direction of the sample rod is the OY axis direction, and the normal direction of the sample surface is the OZ axis direction. Figure 2 As shown, the deformed metal material is cut into thin slice samples in a specific direction. Optionally, the thin slice surface of the cut thin slice sample is perpendicular to the load direction of the sample, and the load direction of the sample in the sample coordinate system S is parallel to the OZ axis; when the thin slice surface is parallel to the load direction of the sample, the characteristic direction of the load is marked when punching the transmission electron microscope sample, and the angle between the force direction and the OX axis direction is recorded when the transmission sample is installed, so that the load direction of the sample in the sample coordinate system S can be obtained.

[0037] Place the double-tilt sample holder in the TEM, tilt the sample holder, take a bright field image of the area to be measured and the electron diffraction pattern under the positive zone axis, and record the tilt angle at the same time. The calibration of the electron diffraction pattern is 180° non-unique, but when the zone axis of the electron diffraction pattern is an even-numbered rotational symmetry axis, the crystal orientation can be uniquely determined. When tilting the sample, the even-numbered rotational symmetry zone axis should be selected to obtain the electron diffraction pattern.

[0038] The obtained electron diffraction pattern is calibrated to obtain the direction vector of a certain crystal zone axis in the sample and the diffraction vector of a certain diffraction spot of the crystal zone axis in the crystal coordinate system C. Taking the transmission spot as the origin, the coordinates of the diffraction vector in the electron microscope coordinate system M are measured after the sample rod is tilted.

[0039] According to the diffraction pattern information, the orientation matrix of the region in the electron microscope coordinate system M is obtained, such as Figure 3 As shown in the figure, a diffraction spot in the diffraction pattern under the positive zone axis [U VW] corresponds to the (HKL) plane, and the orientation matrix of the crystal in the electron microscope coordinate system M is expressed as:

[0040]

[0041] Among them, [uvw] and [hkl] represent the direction vector of the crystal zone axis in the crystal coordinate system C and the diffraction vector corresponding to the diffraction spot in the diffraction pattern, respectively. The selection of the diffraction spot follows the principle of unique crystal orientation, and the vector [rst] = [uvw] × [hkl]. With the transmission spot of the diffraction pattern as the origin, the horizontal direction is the OX axis, the vertical direction is the OY axis, and the plane normal is the OZ axis. The electron microscope coordinate system M is established, and [xy 0] is the vector of the diffraction spot in the coordinate system M. All vectors are normalized. The rotation matrix for the conversion between the electron microscope coordinate system M and the sample coordinate system S is obtained according to the tilt angle. The rotation matrix is ​​expressed as:

[0042]

[0043] Wherein, α and β represent the tilting angles of the sample around the OX axis and the OY axis of the sample coordinate system S during the tilting of the sample rod, respectively, and counterclockwise rotation is defined as a positive angle.

[0044] According to the orientation matrix and the rotation matrix, the orientation matrix of the crystal orientation in the test area in the sample coordinate system S is obtained. The orientation matrix of the crystal orientation in the sample coordinate system S is expressed as:

[0045]

[0046] The crystal orientation matrix and the direction vector of the load in the sample coordinate system S are obtained, and finally the spatial orientation relationship between the deformed metal crystal orientation and the load in the same coordinate system is obtained.

[0047] 2. Embodiment

[0048] Embodiment 1:

[0049] The Ti-15Mo alloy was placed in a muffle furnace or a resistance box heating furnace at 900°C, kept warm for 1 hour, and then quenched in water to obtain a full β phase structure. A compression specimen with a size of 9mm×6mm×6mm was cut from the sample and a compression deformation test was carried out on a universal mechanical properties testing machine at a strain rate of 10 -3 s -1, the deformation strain is -2%. A 0.5mm thick slice is cut from the deformed sample, with the slice surface perpendicular to the deformation direction. The slice is ground to a thickness of about 40μm using 400, 800, and 2000 sandpapers, and then a small disc with a diameter of 3mm is punched out from the ground slice for electrolytic double-spray thinning to obtain the final transmission electron microscope sample.

[0050] A FEI Tecnai G2 F20 transmission electron microscope (acceleration voltage 200KV) was used to take a bright field image of the area to be tested and two electron diffraction patterns under two different positive zones of the area. The tilt angle displayed on the display of this type of transmission electron microscope is clockwise as a positive angle. When calculating the orientation matrix, the opposite number of the tilt angle should be taken. Figure 4 As shown, Figure 4 (a) is the bright field image of the area to be measured, Figure 4 (b) and (c) are electron diffraction patterns under the

[001] and

[110] positive zones, respectively, with tilt angles of α 1 =-2.32, β 1 =13.91 and α 2 =6.03, β 2 =-27.66.

[0051] according to Figure 4 (b) The diffraction pattern, all vectors are normalized to [uvw] = [0 0 1], [hkl] = [0.707, -0.707, 0], [rst] = [0.707, 0.707, 0], [xy 0] = [0.512, 0.859, 0]. Calculate the orientation matrix of the area to be measured:

[0052]

[0053] according to Figure 4 (c) The diffraction pattern, all vectors are normalized to [uvw] = [0.707, 0.707, 0], [hkl] = [0.707, -0.707, 0], [rst] = [0, 0, -1], [xy 0] = [0.978, 0.208, 0]. Calculate the orientation matrix of the area to be measured:

[0054]

[0055] According to two crystal orientation matrices determined by two electron diffraction patterns of the same measured area under different band axes, the orientation difference angle between the two is calculated to be 3.23° considering the crystallographic symmetry, with a small error, which proves the reliability of the method of the present invention.

[0056] Embodiment 2:

[0057] The same electron microscope was used to photograph the {332} <113> The bright field image of the twin region and the corresponding diffraction patterns of the parent and twin regions, such as Figure 5 As shown, Figure 5 (a) is the bright field image of the twin region. Figure 5 (b) and (c) are the electron diffraction patterns of the parent and twin crystals under the

[110] positive zone axis, with tilt angles of α 1 =-14.83, β 1 = -6.32 and α 2 =9.69, β 2 =12.59.

[0058] according to Figure 5 (b) The diffraction pattern, all vectors are normalized to [uvw] = [0.707, 0.707, 0], [hkl] = [0.707, -0.707, 0], [rst] = [0, 0, -1], [xy 0] = [0.971, 0.240, 0]. Calculate the orientation matrix of the parent region:

[0059]

[0060] according to Figure 5 (c) The diffraction pattern, all vectors are normalized to [uvw] = [0.707, 0.707, 0], [hkl] = [0.707, -0.707, 0], [rst] = [0, 0, -1], [xy 0] = [0.749, -0.663, 0]. Calculate the orientation matrix of the twin region:

[0061]

[0062] like Figure 5 As shown in (d), the {332} superimposed pole figure in the sample coordinate system S is drawn from the orientation matrix corresponding to the parent and the twin. The blue circle represents the {332} pole of the parent, and the red triangle represents the {332} pole of the twin. The (233) pole of the parent coincides with one pole of the twin, and the pole of the parent and the pole of the twin are symmetrical about the trace of the (233) plane. The Schmid factors (SF) of the twelve variants are calculated according to the crystal orientation matrix of the parent and the direction of the load, as shown in Table 1. The twin is V9 in the table, and the Schmid factor value is 0.327, which is the maximum value among all variants.

[0063] Table 1: Twelve {332} in Example 2 <113> Twin variants and their Schmid factor values

[0064]

[0065] {332} <113> Twinning is one of the main plastic deformation mechanisms of Ti-15Mo alloy. In this example, the crystal orientation of the matrix and its spatial orientation relationship with the load are obtained by the method of the present invention, the SF value of each {332} twin variant is calculated, and the actual twin variant is determined by the crystal orientation of the twin. It is found that the variant selected in the deformation is theoretically easy to start.

[0066] The above two embodiments illustrate that the method of the present invention has certain advantages in quantitatively analyzing the plastic deformation mechanism. The method can be extended to analyze the relationship between crystal orientation and load spatial orientation of deformation mechanisms such as dislocations, stacking faults and nanotwins that cannot be observed under SEM-EBSD technology.

[0067] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit the technical solution. Those skilled in the art should understand that those modifications or equivalent substitutions of the technical solution of the present invention that do not depart from the purpose and scope of the technical solution should be included in the scope of the claims of the present invention.

Claims

1. A TEM characterization method for the relationship between the orientation of deformed metal crystals and the load spatial orientation, characterized in that: The specific steps include: Step 1: Obtain a TEM sample of a deformed metal material, mark the force direction of the sample, establish a sample coordinate system S according to a double-tilt sample holder, place the sample on the double-tilt sample holder, and obtain the direction of the load on the sample in the sample coordinate system S; Step 2: Place the double-tilt sample holder in the TEM, tilt the sample holder, take a bright field image of the area to be measured and the electron diffraction pattern under the positive zone axis, and record the tilt angle at the same time; Step 3: Calibrate the electron diffraction pattern obtained in step 2 to obtain the direction vector of a certain crystal zone axis in the sample and the diffraction vector of a certain diffraction spot of the crystal zone axis in the crystal coordinate system C. Take the transmission spot as the origin and measure the coordinates of the diffraction vector in the electron microscope coordinate system M after tilting the sample rod; Step 4: According to the diffraction pattern information, the orientation matrix of the region in the electron microscope coordinate system M is obtained, and the rotation matrix for the conversion between the electron microscope coordinate system M and the sample coordinate system S is obtained according to the tilt angle; Step 5: According to the orientation matrix and rotation matrix described in step 4, obtain the orientation matrix of the crystal orientation of the area to be measured in the sample coordinate system S.

2. The method according to claim 1, characterized in that: In step 1, a sample coordinate system S is established according to a double-tilted sample rod, the length direction of the sample rod is the OX axis direction, the width direction of the sample rod is the OY axis direction, and the normal direction of the sample surface is the OZ axis direction; the deformed metal material is cut into thin slice samples in a specific direction, and optionally, the thin slice surface of the cut thin slice sample is perpendicular to the load direction of the sample, and the load direction of the sample in the sample coordinate system S is parallel to the OZ axis; when the thin slice surface is parallel to the load direction of the sample, the characteristic direction of the load is marked when punching the transmission electron microscope disc sample, and the angle between the force direction and the OX axis direction is recorded when the transmission sample is installed, so that the load direction of the sample in the sample coordinate system S can be obtained.

3. The method according to claim 1, characterized in that: In step 2, the calibration of the electron diffraction pattern has 180° non-uniqueness. When the band axis of the electron diffraction pattern is an even-numbered rotational symmetry axis, the crystal orientation can be uniquely determined. When tilting the sample, an even-numbered rotational symmetry band axis should be selected to obtain the electron diffraction pattern.

4. The method according to claim 1, characterized in that: In step 4, the orientation matrix of the crystal in the electron microscope coordinate system M is expressed as: Among them, [uvw] and [h kl] represent the direction vector of the crystal zone axis in the crystal coordinate system C and the diffraction vector corresponding to a diffraction spot in the diffraction pattern, respectively. The selection of the diffraction spot follows the principle of unique crystal orientation, and the vector [rst] = [uv w] × [h kl]. With the transmission spot of the diffraction pattern as the origin, the horizontal direction is the OX axis direction, the vertical direction is the OY axis direction, and the plane normal is the OZ axis direction, an electron microscope coordinate system M is established, [xy 0] is the vector of the diffraction spot in the coordinate system M, and all vectors are normalized.

5. The method according to claim 1, characterized in that: In step 4, the rotation matrix is ​​expressed as: Wherein, α and β represent the tilting angles of the sample around the OX axis and the OY axis of the sample coordinate system S during the tilting of the sample rod, respectively, and counterclockwise rotation is defined as a positive angle.

6. The method according to claim 1, characterized in that: In step 5, the orientation matrix of the crystal orientation in the sample coordinate system S is expressed as: The crystal orientation matrix and the direction vector of the load in the sample coordinate system S are obtained, and finally the spatial orientation relationship between the deformed metal crystal orientation and the load in the same coordinate system is obtained.

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