Quantitative analysis method for grain boundary precipitated phase in aluminum alloy

Abstract

The invention discloses a quantitative analysis method for a grain boundary precipitated phase in an aluminum alloy. Comprising the following steps: 1, preparing a film sample; step 2, determining image acquisition conditions of each sample; step 3, carrying out chemical component analysis on the precipitated phase; step 4, determining an image acquisition method of each sample; step 5, carrying out image acquisition and thickness measurement on each sample; according to the method, the sample is tilted in the direction perpendicular to the connecting line of the two ends of the grain boundary, the widest grain boundary precipitation-free zone is used as the optimal image acquisition condition, and a proper image acquisition method is selected through chemical component analysis, so that imaging of grain boundary precipitated phases with small sizes or closely arranged grains cannot influence each other, and the imaging accuracy is improved. Therefore, more accurate quantitative analysis can be carried out on the grain boundary precipitated phase.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of testing or analyzing materials by using wave or particle radiation, and particularly relates to a quantitative analysis method of grain boundary precipitated phases in an aluminum alloy. BACKGROUND

[0002] For heat-treatable precipitation-strengthened aluminum alloys, the characteristics of intragranular precipitates, grain boundary precipitates, and grain boundary precipitate-free zones are key factors that determine the performance of the alloys. Among them, the size, morphology, and especially the distribution state of the grain boundary precipitates have a significant impact on the strength, toughness, stress corrosion resistance, and fatigue performance of the material. Continuous network distribution of grain boundary precipitates not only reduces the plasticity and toughness of the aluminum alloy material, but also increases the stress corrosion sensitivity of the alloy and leads to a decrease in the fatigue crack propagation resistance. Therefore, accurately characterizing and analyzing the size and distribution state of the grain boundary precipitates is crucial for the performance evaluation of aluminum alloys. Due to the small size of the grain boundary precipitates in aluminum alloys, transmission electron microscopy is currently used to obtain diffraction contrast images for observation. However, when using transmission electron microscopy to collect images of grain boundaries, the relative orientation relationship between the grain boundary and the electron beam can greatly affect the distribution pattern and size of the precipitates. Therefore, under different actual observation conditions, the size, morphology, and distribution of the precipitates may vary greatly, even for the same grain boundary. It is difficult to judge the grain boundary precipitates, especially when they are small and densely distributed. In some cases, it is even impossible to quantitatively measure the precipitates when they overlap. Therefore, it is currently difficult to accurately measure and quantitatively analyze the grain boundary precipitates. Patent CN113777115 A discloses a quantitative statistical method for precipitates in an alloy, which is suitable for the quantitative characterization of different types and sizes of precipitates. However, it is not suitable for the quantitative analysis of grain boundary precipitates in a specific location and the evaluation of the distribution state of the precipitates. SUMMARY

[0003] To solve the above problems, the present application proposes a quantitative characterization and analysis method for grain boundary precipitates in an aluminum alloy. Transmission electron microscopy is used to quantitatively measure the number, size, and continuity of the grain boundary precipitates in the aluminum alloy, which can realize the quantitative characterization of the grain boundary precipitates in the aluminum alloy and provide a basis for the microstructure and performance evaluation of the aluminum alloy. It is also suitable for the detection and analysis of second phase particles on the grain boundaries of other metal materials. The technical solution includes:

[0004] Step 1, preparing a thin film sample;

[0005] Step 2, determining the image acquisition conditions of each sample:

[0006] The bright field image of the grain boundary position is collected under transmission electron microscopy when a = β = 0°, and the sample is tilted in the direction perpendicular to the line connecting the two ends of the grain boundary, and the widest condition of the grain boundary precipitate-free zone is observed to determine the image collection condition, wherein a is the tilt angle of the sample around the X axis, and β is the tilt angle of the sample around the Y axis;

[0007] Step 3, chemical composition analysis of precipitated phase:

[0008] The chemical elements of the grain boundary precipitated phase are detected and analyzed by an energy spectrometer, the surface scanning image of the grain boundary or the EDS spectrum data of the precipitated phase are collected, and semi-quantitative composition analysis is performed to determine the chemical element content of the precipitated phase;

[0009] Step 4, determining the image collection method of each sample:

[0010] According to the chemical element content of the grain boundary precipitated phase in step 3 and in combination with the high-angle annular dark field contrast, the bright field image or the high-angle annular dark field image is selected as the suitable image collection method;

[0011] Step 5, image collection and thickness measurement of each sample:

[0012] The image collection conditions of step 2 and the image collection method of step 4 are used to collect images under a magnification, and the sample thickness t at the corresponding position is measured by convergent beam electron diffraction or contamination spot method;

[0013] Step 6, statistical measurement of precipitated phase size and grain boundary length:

[0014] The collected pictures are quantitatively analyzed by image analysis software to statistically measure the size L pi of each precipitated phase along the grain boundary direction, the equivalent size L ei of the precipitated phase, the number n of the precipitated phase, the average size L p of the precipitated phase, and the length L b of the grain boundary;

[0015] Step 7, quantitative evaluation of the continuity degree δ of the precipitated phase along the grain boundary:

[0016] Wherein t is the thickness of the thin film sample.

[0017] The thin film sample in step 1 is a thin film sample for transmission electron microscopy observation prepared by cutting, grinding and thinning using an aluminum alloy material.

[0018] The step 2 observes the widest condition of the grain boundary without precipitation zone, and the relative distance of the two precipitation phase particles located at both sides of the grain boundary is the farthest during the tilting process as the image acquisition condition, so that the imaging of the smaller size and closely arranged grain boundary precipitates does not affect each other, thereby enabling quantitative analysis of the grain boundary precipitates.

[0019] In step 2, the method of tilting the sample in the direction perpendicular to the connecting line of the two ends of the grain boundary, first acquires a bright field image of the grain boundary under the condition of α = β = 0°, and then tilts the sample in the direction perpendicular to the connecting line of the two ends of the grain boundary, that is, the tilting method of the sample should satisfy: Where L bx is the projection length of the grain boundary L b in the x direction, L by is the projection length of the grain boundary L b in the y direction.

[0020] In step 4, the atomic number difference between the grain boundary precipitates and the aluminum matrix is used as the basis for selecting the image acquisition method, the larger the atomic number difference, the more suitable it is to use high-angle annular dark field image; the smaller the atomic number difference, the more suitable it is to use bright field image.

[0021] In step 5, at least 5 different grain boundaries are selected in the same sample, and at least 20 grain boundary images of different fields of view are acquired under the corresponding optimal acquisition conditions and magnification.

[0022] The magnification is 1 / 100 to 1 / 10 of the size of the smallest grain boundary precipitates in the length direction of the map, and contains a sufficient number of precipitates for statistics.

[0023] In step 6, Image Pro is used to statistically measure the collected images;

[0024] Where the measurement method of the grain boundary length L b is that when a higher magnification is used for image analysis, the grain boundary can be approximated as a straight line, and the length of the line connecting the two midpoint positions of the image boundary without precipitates is the grain boundary length L b .

[0025] The size of the precipitates is the size L pi of the midpoint connecting line in the long axis direction of the precipitates, and the measurement method of the equivalent size L ei of the precipitates in the direction of the grain boundary is L ei =L pi cosγ, where γ is the included angle between the long axis direction of the precipitates and the length direction of the grain boundary.

[0026] The average size of the precipitates is

[0027] The greater the delta is, the higher the continuity is, and the smaller the delta is, the lower the continuity is.

[0028] The present application has the advantages of:

[0029] 1. The method is suitable for quantitative analysis of grain boundary precipitates in aluminum alloys and other metal materials, and has important application value for precise evaluation of microstructure of materials and research and development of new materials.

[0030] 2. The sample is tilted along the direction perpendicular to the line connecting the two ends of the grain boundary, the widest grain boundary precipitate-free zone is taken as the best image acquisition condition, and the appropriate image acquisition method is selected through chemical composition analysis, so that the imaging of small size or closely arranged grain boundary precipitates does not interfere with each other, thereby enabling more accurate quantitative analysis of grain boundary precipitates.

[0031] 3. The present application determines the appropriate magnification and the number of image acquisition under certain conditions, and uses image processing software for analysis, so that the characterization of grain boundary continuity is also quantitatively analyzed, thereby making the detection and analysis results more statistically significant and representative.

[0032] 4. It has the advantages of universality, accuracy, large amount of information and high efficiency. BRIEF DESCRIPTION OF DRAWINGS

[0033] Figure 1 It is a flowchart of the quantitative analysis method of grain boundary precipitates in aluminum alloys according to the present application.

[0034] Figure 2 It is a schematic diagram of the method of tilting the sample used in the broad embodiment of the present application.

[0035] Figure 3 It is a schematic diagram of the measurement method of the length L of precipitates pi and the equivalent size L of precipitates along the grain boundary direction in the broad embodiment of the present application. ei

[0036] Figure 4 It is a schematic diagram of the measurement method of the length L of grain boundaries b in the broad embodiment of the present application.

[0037] Figure 5 It is a grain boundary TEM image of Example 1.

[0038] Figure 6 It is a TEM image of the grain boundary after tilting under the best conditions in Example 1.

[0039] Figure 7 ​HAADF-STEM image of the grain boundary of Example 1 taken under the optimal conditions after tilting.

[0040] Figure 8 CBED diffraction pattern of the area near the grain boundary of Example 1.

[0041] Figure 9 TEM image of the grain boundary of Comparative Example 1. DETAILED DESCRIPTION

[0042] The application will be further described below in conjunction with the accompanying drawings.

[0043] As shown in the broad embodiment of the application, it comprises: Figure 1

[0044] Step 1, preparing thin film samples;

[0045] The aluminum alloy material is processed by cutting, grinding, thinning and other methods to prepare thin film samples (samples) for transmission electron microscope observation;

[0046] Step 2, determining the image acquisition conditions of each sample:

[0047] Adjust the objective lens astigmatism and focus, and collect the bright field image of the grain boundary position when α = β = 0° (α is the tilt angle of the sample around the X axis, and β is the tilt angle of the sample around the Y axis), and the condition of observing the widest precipitate-free zone of the grain boundary can be determined by tilting the sample along the direction perpendicular to the line connecting the two ends of the grain boundary.

[0048] The above-mentioned condition of observing the widest precipitate-free zone of the grain boundary is that the relative distance between the two precipitate particles located on both sides of the grain boundary is the farthest during the tilting process, which is used as the image acquisition condition, so that the imaging of the closely arranged precipitate particles of the grain boundary will not affect each other, thereby enabling accurate quantitative analysis of the grain boundary precipitate.

[0049] In step 2, the method of tilting the sample along the direction perpendicular to the line connecting the two ends of the grain boundary is to collect the bright field image of the grain boundary under the condition of α = β = 0°, wherein L bx is the projection length of the grain boundary L b in the x direction, L by is the projection length of the grain boundary L b in the y direction, and the schematic diagram is shown in Figure 2 To clearly display the closely arranged precipitate particles on the grain boundary in the image, the sample is tilted along the direction perpendicular to the grain boundary, i.e. at two points on the grain boundary with the same x and y but different z, the change of the relative position of the projection after tilting is perpendicular to the direction of the grain boundary. In a three-dimensional coordinate system, the coordinate position of the rotation point P(x, y, z) after rotating by angle α around the x axis and then rotating by angle β around the y axis can be obtained by the following steps: ​

[0050] Rotation matrix R around x axis by angle α x (α) is:

[0051] Rotation matrix R around y axis by angle β y (β) is:

[0052] The coordinates of P(x, y, z) after rotation along x and y axes can be calculated by matrix: P' = PR x (α)R y (β), the coordinates after rotation are: x' = xcosβ + ysinαsinβ + zcosαsinβ, y' = ycosα - zsinα, z' = -xsinβ + ysinαcosβ + zcosαcosβ

[0053] The sample is tilted along the direction perpendicular to the line connecting the two ends of the grain boundary, the vector direction of the line connecting the two points on the grain boundary with the same x, y and different z after tilting is perpendicular to the vector direction of the grain boundary length, and the relationship between the tilt angles α and β of the sample around the x and y axes respectively should satisfy Δx'·L bx- Δy'·L by +0 = 0, that is, the relationship between the tilt angles α and β of the sample around the x and y axes respectively should satisfy and are mutually restricted according to the specific direction of the grain boundary. In particular, when the grain boundary is parallel to the y axis, α can be 0°, that is, only tilt by angle β; similarly, when the grain boundary is parallel to the x axis, β can be 0°, that is, only tilt by angle α.

[0054] According to the above coordinate formula, the change of the projection relative position of the two points on the grain boundary with the same x, y and different z after tilting is Δx' = (z1-z2)cosαsinβ, Δy' = (z2-z1)sinα, Δz' = (z1-z2)cosαcosβ.

[0055] Therefore, in actual operation, the relationship can be used to ensure that the sample is tilted along the direction perpendicular to the line connecting the two ends of the grain boundary, so as to determine the optimal tilt condition for observing and analyzing the precipitated phase.

[0056] The method of the broad embodiment belongs to the field of quantitative statistical analysis of grain boundary second phase in metal materials, and can accurately characterize the size, number, equivalent size along the grain boundary direction, distribution state and continuity of the second phase on the grain boundary, and is used for analyzing the correlation between the microstructure characteristics of the grain boundary precipitated phase and the preparation process and performance evaluation of aluminum alloy.

[0057] Step 3: Perform chemical composition analysis of the precipitates: Under the image acquisition conditions of Step 2, perform chemical element detection and analysis on the grain boundary precipitates using an energy dispersive spectrometer (EDS). This can acquire surface scan images of the grain boundaries or EDS spectral data of the precipitates, and perform semi-quantitative composition analysis to determine the chemical element content of the precipitates.

[0058] Step 4: Determine the image acquisition method for each sample: Based on the chemical element content of the grain boundary precipitates in Step 3 and combined with the contrast of high-angle annular dark field image (HAADF-STEM), select bright field image or HAADF-STEM as the appropriate image acquisition method.

[0059] In this specific implementation, step 4 uses the difference in atomic number between the grain boundary precipitates and the aluminum matrix as the basis for selecting the image acquisition method. The greater the difference in atomic number, the more suitable it is to use HAADF-STEM images, and vice versa, bright field images are more suitable. Both types of images can also be acquired at the same time.

[0060] Step 5: Image acquisition and thickness measurement for each sample: Using the image acquisition methods and conditions described above, images are acquired at an appropriate magnification. Simultaneously, convergent beam electron diffraction or the contamination spot method is used to measure the sample thickness t at the corresponding locations. At least five different grain boundaries are selected within the same sample, and at least 20 grain boundary images of different fields of view are acquired under the corresponding optimal acquisition conditions and appropriate magnification.

[0061] The appropriate magnification should satisfy the requirement that the size of the smallest grain boundary precipitate occupies 1 / 100 to 1 / 10 of the length of the image to facilitate accurate measurement, while also including a sufficient number of precipitates for statistical purposes.

[0062] Step 6: Statistical measurement of precipitate size and grain boundary length: The acquired images are quantitatively analyzed using image analysis software, and the size L of the precipitate is statistically measured. pi The equivalent size L of the precipitated phase along the grain boundary direction ei The number of precipitated phases, n, and the average size of the precipitated phases, L. p and the length L of the grain boundary b ;

[0063] In step 6, Image Pro is used to perform statistical measurements on the acquired images. The grain boundary length L b When measuring grain boundaries using high magnification for image analysis, the grain boundary can be approximated as a straight line. The length of the line connecting the two midpoints of the image boundary where there are no precipitates is the grain boundary length L. b The size of the precipitated phase is L, which is the length of the line connecting the midpoints of the two narrower ends along the long axis of the precipitated phase. pi The equivalent size L of the precipitated phase along the grain boundary direction ei Measurement method Lei = L pi cos γ, γ is the angle between the long axis of the precipitate phase and the length direction of the grain boundary, as shown in the schematic diagram Figure 3 The average size L p of the precipitate phase can be calculated by the formula .

[0064] Step 7, quantitative evaluation of the continuity δ of the precipitate phase along the grain boundary: the sum of the equivalent sizes L ei of the precipitate phase along the grain boundary direction is counted, and the ratio of L ei to the length L b of the grain boundary and the thickness t of the sample is taken as the quantitative evaluation parameter of the continuity of the precipitate phase along the grain boundary. The greater the δ, the higher the continuity, and the smaller the δ, the lower the continuity.

[0065] The more specific steps of the broad embodiment are as follows:

[0066] First, prepare the transmission electron microscope thin film sample, take out a thin piece with a thickness of 0.5 mm-1 mm from the bulk material by slow sawing or wire cutting, and grind the sample to less than 50 μm with metallographic sandpaper from coarse to fine. According to the specific state and surface condition of the material, choose electrolytic double spraying or ion thinning, focused ion beam cutting (FIB) and other methods to prepare a thin film sample that can be observed. When using electron microscope double spraying to prepare the sample, attention should be paid to selecting the electrolytic double spraying liquid suitable for the corresponding material and controlling the appropriate double spraying thinning working conditions. When using ion thinning method, the sample should be fixed on a special sample stage and placed in the ion thinning instrument for thinning until a thin area suitable for transmission electron microscope observation appears. The finally prepared thin film sample should ensure that the thickness of the observed thin area is small enough to ensure its light transmittance (generally less than 100 nm) so that there is enough contrast and clarity in the transmission electron microscope image for observation and measurement, and the surface of the thin film sample is clean without oxidation or corrosion product adhesion.

[0067] Then collect the transmission electron micrograph of the grain boundary, load the prepared thin film sample into a double-inclined transmission electron microscope sample stage, and place it in a transmission electron microscope for observation. First, find the grain boundary and confirm that the position of the grain boundary is clean and free of pollution and has a suitable thickness. Adjust the Z-axis height to adjust the height of the sample, select an appropriate magnification, then place the objective lens diaphragm, fine-tune the objective lens astigmatism and focus, and obtain a clear contrast grain boundary bright field image under the condition of α=β=0°.

[0068] Draw a line between the two midpoint positions of the precipitate-free zone at the grain boundary to determine the grain boundary direction and the length L b of the grain boundary, as shown in the schematic diagram Figure 4 , and measure L bx and L by , wherein L bxFor grain boundaries L b The projected length along the x-direction, L by For grain boundaries L b The projected length along the y-direction, such as Figure 2 As shown. When tilting the sample, control is used... The sample is tilted along a direction perpendicular to the line connecting the two ends of the grain boundary. During the rotation, the relative distance between two precipitate particles located at the same position on both sides of the grain boundary is observed. The condition with the greatest relative distance is taken as the optimal tilting condition so that the imaging of small and closely packed grain boundary precipitates will not affect each other, thus enabling accurate quantitative analysis of grain boundary precipitates.

[0069] Under the aforementioned image acquisition conditions, chemical elemental analysis of the grain boundary second phase can be performed using an energy dispersive spectroscopy (EDS) instrument. This allows for the acquisition of surface scan images of the grain boundaries or EDS spectral data of the precipitated phases, followed by semi-quantitative analysis to determine the chemical elemental content of the precipitated phases. Based on the chemical composition of the grain boundary precipitates and considering the contrast of the HAADF-STEM image, either bright-field imaging or HAADF-STEM imaging can be selected as the appropriate image acquisition method. Alternatively, both methods can be acquired simultaneously to achieve optimal contrast for the grain boundary second phase.

[0070] Under the optimal image acquisition method and tilt conditions described above, select a suitable magnification. The magnification should ensure that the size of the smallest grain boundary precipitate occupies 1 / 100 to 1 / 10 of the image length. Further fine-tune the focus and astigmatism, then proceed with image acquisition. Simultaneously, acquire CBED patterns or contamination spots in the corresponding areas for calculating the film sample thickness t. Select at least five different grain boundaries in the same sample and acquire at least 20 grain boundary images from different fields of view.

[0071] Next, grain boundary image processing and quantitative statistical analysis of the second phase are performed: Image Pro is used to measure and statistically analyze the acquired images. To facilitate statistical analysis of the second phase at the grain boundary, the corresponding image acquisition magnification is generally high. The grain boundary can be approximated as a straight line. The length of the line connecting the two midpoints of the image boundary where there is no precipitation zone is the grain boundary length L. b The size of the precipitated phase is its length, specifically the dimension L along the line connecting the midpoints of the two smaller precipitated phases. pi The number of precipitates, n, is the number of identifiable second phases distributed on the grain boundaries, from which the average size L of the precipitates is calculated. p for The equivalent size L of the precipitated phase along the grain boundary direction ei The measurement involves measuring the angle γ between the length direction of the precipitated phase and the length direction of the grain boundary, such as... Figure 3 As shown, the projected length of the second phase along the grain boundary direction, L, is calculated based on the included angle γ. ei= L pi cos γ.

[0072] Finally, the quantitative analysis of the continuity of the second phase distribution on the grain boundary is carried out. In order to quantitatively evaluate the continuity of the second phase distribution on the grain boundary, the equivalent size L ei of the precipitated phase along the grain boundary direction in each field of view is counted, and the ratio of the sum to the length L b of the grain boundary and the thickness t of the sample is taken as the quantitative evaluation parameter of the continuity of the precipitated phase along the grain boundary The larger the δ is, the higher the continuity is, and the smaller the δ is, the lower the continuity is. The average value of the δ values of different fields of view of the same grain boundary is taken as the continuity evaluation parameter of the grain boundary, and the average value of the δ values of different grain boundaries of the same sample is taken as the continuity evaluation parameter of the precipitated phase along the grain boundary of the sample. Specific embodiment 1

[0074] 7075 alloy belongs to Al-Zn-Mg-Cu alloy, which is a typical age hardenable aluminum alloy. The η` or η precipitated phase formed during the aging heat treatment process is distributed on the grain boundary. By quantitatively determining the size, morphology, distribution characteristics and the like of the grain boundary precipitated phase, guidance and reference can be provided for the evaluation of the microstructure and performance of the material.

[0075] In this embodiment, the 7075 alloy in the aging heat treatment state is selected as the analysis object. First, the sample is ground on sandpaper to a thickness of 50 microns, and then punched into a disc with a diameter of 3 mm using a punch machine. Subsequently, double-spray electrolytic thinning is carried out in a nitric acid solution with a proportion of 1:3, at a temperature of -30 to -20°C and a voltage of 20V, until the hole is punched, and the sample surface is further cleaned with an ethanol solution to ensure that the thin area of the sample is suitable for transmission electron microscope observation and no contaminants are left. The prepared thin film sample is loaded into a double-inclined transmission electron microscope sample stage, and placed into a transmission electron microscope for observation. Clean and uncontaminated grain boundaries are found, and the magnification is selected to be 58000 times. Subsequently, the objective aperture is inserted, the objective lens astigmatism and focus are adjusted, and the bright field image of the grain boundary is obtained under the condition of α=β=0, as shown in Figure 5 , and the sample position is recorded.

[0076] The two midpoint positions a and b of the precipitate-free zone at the picture boundary in the collected bright field image of the grain boundary Figure 5 are connected to determine the grain boundary direction ab and the length L b of the grain boundary L b = 842 nm, and the length L bx of the grain boundary L b along the x direction L by=293nm. During the tilting process, sinβ = -0.37tanα is maintained to ensure that the sample is tilted along the direction ab perpendicular to the grain boundary. During the rotation, the relative distance between the two precipitated phase particles A and B located on both sides of the grain boundary is observed. The farthest relative distance is 172nm. Figure 6 As shown in the figure. The values ​​of α = 23° and β = -9° under this condition were recorded and used as the optimal image acquisition conditions for this grain boundary. Energy dispersive spectroscopy (EDS) was performed on the precipitates at the grain boundary, and their semi-quantitative composition is shown in Table 1. Since the chemical composition of the precipitates differs significantly from that of the matrix, HAADF-STEM imaging was selected as the image acquisition method for this grain boundary.

[0077] Table 1. Energy dispersive spectroscopy results of grain boundary precipitates in 7075 alloy

[0078]

[0079] Under the optimal image acquisition method and tilt conditions described above, 125,000x was selected as a suitable magnification. Further fine-tuning of focus and astigmatism was performed before image acquisition. Figure 7 As shown, the CBED convergent beam diffraction pattern at this location was simultaneously acquired as follows: Figure 8 As shown, the thickness of the sample was calculated to be 60.23 nm after calibration.

[0080] Using Image Pro Figure 7 The second phase at the grain boundaries was measured and counted. The number of precipitates, n, was the number of identifiable second phases distributed on the grain boundaries, i.e., 25. The size L of each precipitate was measured along the line connecting the midpoints of its two smaller ends. pi The size of each grain boundary precipitate was measured and statistically analyzed. After calculation, the average size of the precipitate was the equivalent size L of the precipitate along the grain boundary direction. ei The measurement can be performed according to the diagram as shown below. Figure 3 As shown, the angle γ between the length direction of each precipitated phase and the length direction of the grain boundary is measured, and the projected length L of each second phase along the grain boundary direction is calculated based on the angle γ. ei =L pi The measurement results of the cosγ-precipitated phase at grain boundary 1 are shown in Table 2. The continuity evaluation parameter δ can be obtained by calculation using the formula.

[0081] Table 2. Measurement results of grain boundary 1 precipitates in Example 1

[0082]

[0083] Repeat the above steps to measure, statistically analyze, and measure at least 5 grain boundaries of the sample. The quantitative statistical analysis data of the grain boundary precipitates are shown in Table 3.

[0084] Table 3. Quantitative statistical analysis results of grain boundary precipitates in Example 1

[0085]

[0086] Comparative Example 1

[0087] In this comparative example, 7075 alloy in an aged heat-treated state was selected as the analytical object. First, the sample was prepared into an observable thin film using the same sample preparation method and mounted on a double-tilting transmission electron microscope (TEM) stage. The sample was then observed under a TEM to locate clean, uncontaminated grain boundaries. A magnification of 36,000x was selected, and the objective aperture was adjusted for astigmatism and focusing to obtain a bright-field image of the grain boundaries. Figure 9 As shown, the sample location was recorded.

[0088] like Figure 9 To directly acquire images of grain boundary morphology without tilting, it is evident that due to the small size and dense arrangement of precipitates on the grain boundaries, the morphology of individual precipitates cannot be distinguished, thus making it impossible to measure and statistically analyze the size of the precipitates.

Claims

1. A method for quantitatively analyzing grain boundary precipitates in an aluminum alloy, characterized by, The method comprises the following steps: Step 1, preparing a thin film sample; Step 2, determining the image acquisition conditions of each sample: The bright field image of the grain boundary position is acquired under a transmission electron microscope when α=β=0°, and the sample is tilted along the direction perpendicular to the connecting line of the two ends of the grain boundary, and the condition of observing the widest precipitate-free zone of the grain boundary is determined as the image acquisition condition, wherein α is the tilt angle of the sample around the X axis, and β is the tilt angle of the sample around the Y axis; Step 3, performing chemical composition analysis of the precipitate phase: The chemical element detection and analysis of the grain boundary precipitate phase is performed by using an energy spectrometer, the surface scanning image of the grain boundary or the EDS spectrum data of the precipitate phase is acquired, and semi-quantitative composition analysis is performed to determine the chemical element content of the precipitate phase; Step 4, determining the image acquisition method of each sample: The chemical element content of the grain boundary precipitate phase in step 3 is selected in combination with the high-angle annular dark field contrast to select the bright field image or the high-angle annular dark field image as the suitable image acquisition method; Step 5, image acquisition and thickness measurement of each sample: The image acquisition conditions in step 2 and the image acquisition method in step 4 are used to acquire images under a magnification, and the sample thickness t at the corresponding position is measured by using convergent beam electron diffraction or contamination spot method; Step 6, statistical measurement of the size of the precipitate phase and the length of the grain boundary: The collected pictures are quantitatively analyzed by image analysis software to count and measure the size L of precipitated phases pi , the equivalent size L of each precipitated phase along the grain boundary direction ei , the number n of precipitated phases, the average size L of precipitated phases p , and the length L of grain boundaries b ; Step 7, quantitative evaluation of continuity δ of precipitates along grain boundaries is performed. Wherein t is the thickness of the thin film sample.

2. The method of quantitatively analyzing grain boundary precipitates in an aluminum alloy according to claim 1, characterized by, The thin film sample in step 1 is a thin film sample for transmission electron microscope observation prepared by cutting, grinding and thinning using an aluminum alloy material.

3. The method of quantitatively analyzing grain boundary precipitates in an aluminum alloy according to claim 1, characterized by, In step 2, the condition of observing the widest precipitate-free zone of the grain boundary is selected, and the relative distance of the two precipitate phase particles located at the positions on both sides of the grain boundary is the farthest during the tilting process as the image acquisition condition, so that the imaging of the small-sized and closely arranged grain boundary precipitate phases will not affect each other, thereby enabling quantitative analysis of the grain boundary precipitate phase.

4. The method of quantitatively analyzing grain boundary precipitates in an aluminum alloy according to claim 1 or 3, characterized by, In step 2, first, a bright field image of the grain boundary is acquired under the condition of a = b = 0°, and then the sample is tilted in the direction perpendicular to the line connecting the two ends of the grain boundary, that is, the tilting method of the sample should satisfy: where L bx is the projection length of the grain boundary L b in the x direction, L by is the projection length of the grain boundary L b in the y direction.

5. The method of quantitatively analyzing grain boundary precipitates in an aluminum alloy according to claim 1, characterized by, In step 4, the atomic number difference between the grain boundary precipitate phase and the aluminum matrix is used as the selection basis for the image acquisition method, and the greater the atomic number difference, the more suitable it is to use the high-angle annular dark field image; the smaller the atomic number difference, the more suitable it is to use the bright field image.

6. The method of quantitatively analyzing grain boundary precipitates in an aluminum alloy according to claim 1, characterized by, In step 5, at least 5 different grain boundaries are selected in the same sample, and the grain boundary images of different fields are acquired at least 20 times under the corresponding optimal acquisition conditions and magnification.

7. The method of quantitatively analyzing grain boundary precipitates in an aluminum alloy according to claim 1 or 6, characterized by, The magnification is 1 / 100 to 1 / 10 of the size of the smallest grain boundary precipitate phase in the length direction of the map, and a sufficient number of precipitate phases are contained to facilitate statistics.

8. The method of quantitatively analyzing grain boundary precipitates in an aluminum alloy according to claim 1, characterized by, In step 6, Image Pro is used to statistically measure the acquired images; Wherein the grain boundary length L b The measurement method is as follows: when image analysis is carried out at a higher magnification, the grain boundary can be approximated as a straight line, and the length of the line connecting the two midpoint positions of the precipitate-free zone at the picture boundary is the grain boundary length L b . The size of the precipitate phase is the size L of the line connecting the midpoints of the narrower two ends in the long axis direction of the precipitate phase pi The measurement method of the equivalent size L of the precipitate phase in the grain boundary direction ei L ei = L pi cos γ, where γ is the included angle between the long axis direction of the precipitate phase and the length direction of the grain boundary Average size of precipitate phase where n is the number of precipitate phases.

9. The method of quantitatively analyzing grain boundary precipitates in an aluminum alloy according to claim 1, characterized by, The greater the δ, the higher the continuity, and the smaller the δ, the lower the continuity.

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