A statistical method for the size of nanoscale α2 phase grains in near-α high-temperature titanium alloys

Abstract

The present invention discloses a statistical method for measuring the size of nanoscale α2 phase grains in near-α high-temperature titanium alloys. This method utilizes the specific α2 orientation relationship between the lamellar α phase and the superlattice α2 phase. By rotating the lamellar α phase to the Kikuchi pole, moving the objective lens aperture to select the diffraction beam of the α2 phase for imaging, and using a long exposure of 5 to 20 seconds, the axial characteristics of the nanoscale superlattice α2 phase can be obtained. Grain size statistics are then performed using image processing software with image brightness and shading analysis capabilities to determine the superlattice α2 phase content and diameter in the near-α high-temperature titanium alloy. This method completely solves the difficulty of capturing the axial size of the nanoscale superlattice α2 phase in near-α high-temperature titanium alloys. Furthermore, the method is simple and provides a prerequisite for studying the microstructural evolution of near-α high-temperature titanium alloys under different deformation conditions.

Description

Technical Field

[0001] The present invention relates to a method for statistically analyzing the grain size of nanoscale superlattice α2 phase in a near-α high-temperature titanium alloy, in particular to a method for characterizing the axial characteristics of nanoscale α2 phase in a near-α high-temperature titanium alloy using transmission electron microscopy and statistically analyzing the grain size. Background Art

[0002] As the heat treatment temperature of near-α-type high-temperature titanium alloys increases, the size and content of the superlattice α2 phase gradually change. The diameter of the α2 phase generally ranges from a few nanometers to tens of nanometers, and the size of this α2 phase affects the alloy's strengthening or catalytic effects. During deformation, mobile dislocations will either cut through or bypass the α2 phase, and the interaction between these two mechanisms depends primarily on the specific size of the α2 phase. When the α2 phase is small, the cutting mechanism dominates its strengthening mechanism, while when it is large, the bypass mechanism dominates. When it reaches a critical size, the strengthening effect is strongest. Therefore, determining the size of the α2 phase has a significant impact on the study of the mechanical property evolution of near-α-type high-temperature titanium alloys. However, the α2 phase is typically extremely fine at the nanometer scale and can only be observed and characterized under a transmission electron microscope. However, the α2 phase observed under TEM is often relatively densely distributed, making it difficult to maintain completely consistent light-dark contrast. To date, there is no efficient and accurate method for characterizing and statistically analyzing these extremely fine superlattice α2 phase grains. Summary of the Invention

[0003] The purpose of the present invention is to provide a statistical method for the size of nanoscale α2 phase grains in near-α high-temperature titanium alloys. This method can accurately characterize the grain size of extremely fine superlattice α2 phase and perform efficient statistics.

[0004] The technical solution of the present invention is:

[0005] A statistical method for determining the size of nanoscale α2 phase grains in a near-α titanium alloy, characterized in that the method comprises the following steps:

[0006] (1) Take a titanium alloy sample and find flat lamellar α-phase grains under the bright field image of a transmission electron microscope (TEM);

[0007] (2) Rotate the lamellar α-phase grains found in step (1) to a specific crystal orientation Then, the Kikuchi diffraction spectrum was taken in the diffraction mode to verify whether the orientation contained the superlattice α2 phase;

[0008] (3) moving the objective lens aperture to select the diffraction beam of the α2 phase in the diffraction spectrum obtained in step (2), and photographing at least three different magnifications in the range of 100-300k with an exposure time of 5-20 seconds to obtain a dark field photograph of the phase;

[0009] (4) The α2 phase TEM dark field photograph obtained in step (3) is magnified 8-12 times using image processing software, and then multiple images of the same area size are captured;

[0010] (5) The image obtained in step (4) is processed through image processing software with image brightness and darkness analysis function to perform statistics on the superlattice α2 phase, and a statistical superlattice α2 phase area data table is obtained. The relative content of superlattice α2 is obtained by calculating the ratio of the sum of the superlattice α2 phase area data to the area of ​​the entire image, and the α2 phase diameter is calculated by averaging the area data of each α2 phase.

[0011] Furthermore, in step (1), it is necessary to select the rotation range allowed by the electron microscope according to the Kikuchi diffraction spectrum. The required grains of Kikuchi pole.

[0012] Furthermore, the specific process of step (2) is to rotate the grain along the finest grain band in the Kikuchi spectrum in the selected grain until it is rotated to Kikuchi pole, then return to the diffraction mode to determine whether the diffraction spectrum is consistent with Standard diffraction spectrum, and observe whether superlattice diffraction spots can be observed in it.

[0013] Furthermore, the specific process of step (2) is to rotate the grain along the finest grain band in the Kikuchi spectrum in the selected grain until it is rotated to The process of Kikuchi pole is: take Kikuchi diffraction spectrum in the α phase grain of the layer to determine whether Kikuchi pole, where the ratio of the distance between the transmission spot and the diffraction spot b / a must be close to 1.299; if the diffraction spectrum is not Continue to tilt the sample along the thinnest Kikuchi zone, or change another grain until it is confirmed to be Kikuchi Ji.

[0014] Furthermore, in step (3), the focal length needs to be continuously adjusted at the same position to ensure the accuracy of superlattice α2 phase characterization;

[0015] First, take a picture at the focus position, then fine-tune the electron microscope focus to the underfocus and overfocus directions, and take a picture every 300-600nm to prevent sample shaking from affecting the shooting effect. The focal length range is ±2-5μm, until the picture is clear.

[0016] Furthermore, in step (4), the original α2 phase TEM dark field phase photograph is enlarged and intercepted to ensure the accuracy of the subsequent α2 phase image recognition.

[0017] Furthermore, in step (5), the α2 phase area data can be quickly obtained by using image processing software with image brightness and darkness analysis function, and then the α2 phase content and diameter can be obtained.

[0018] The characteristics of the present invention are:

[0019] 1. The grain size statistical method of TEM photography adopted in the present invention is applicable to the characterization of superlattice α2 phase in all near-α high-temperature titanium alloys;

[0020] 2. The present invention utilizes the specific orientation relationship between the lamellar α phase and the superlattice α2 phase. By rotating the crystal orientation of the lamellar α phase to a specific direction through the TEM operation technology, the microscopic arrangement of the corresponding superlattice α2 phase in that direction can be observed.

[0021] 3. Due to the extremely small size of the superlattice α2 phase, there are extremely stringent requirements on orientation and TEM exposure time. Only when the orientation is correct can the axial arrangement of the needle-like superlattice α2 phase be observed, and the shooting exposure time is determined to be 5 to 20 seconds, so that the diameter size of the phase can be accurately counted.

[0022] 4. The present invention uses a method of intercepting an image after local amplification of the TEM to obtain images with consistent light and dark contrasts for statistical analysis, thereby accurately capturing the contrast of the α2 phase.

[0023] 5. The present invention utilizes image processing software with image brightness and darkness analysis capabilities to rapidly capture α2 phase contrast and provide area data for each identified α2 phase according to the image's inherent scale, thereby reducing the time consumed in manually capturing α2 phase contrast and obtaining the contrast, and also lowering statistical errors.

[0024] This method uses the lamellar α phase and the superlattice α2 phase A specific orientation relationship is achieved by rotating the α phase of the sheet to Finally, Kikuchi moved the objective lens aperture to select the diffraction beam of the α2 phase for imaging. Using a long exposure of 5 to 20 seconds, he was able to obtain the axial characteristics of the nanoscale superlattice α2 phase. Using image processing software with image brightness and shading analysis capabilities, he then performed grain size statistics to determine the superlattice α2 phase content and diameter in the near-α high-temperature titanium alloy. This method completely solved the difficulty of imaging the axial dimensions of the nanoscale superlattice α2 phase in near-α high-temperature titanium alloys. The method is simple and provides a prerequisite for studying the microstructural evolution of near-α high-temperature titanium alloys under different deformation conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 for Kikuchi's extremely positive zone axis diffraction spectrum photo;

[0026] Figure 2 for Dark field images of the Kikuchi superlattice α2 phase with extremely positive zone axis diffraction spectrum at different magnifications: (a) at 125k times, (b) at 195k times, and (c) at 250k times;

[0027] Figure 3 This is a magnified photograph of the superlattice α2 phase;

[0028] Figure 4 This is a photo after processing the α2 phase contrast through image brightness and darkness analysis;

[0029] Figure 5 This is the partial area data of the α2 phase obtained by imaging software. DETAILED DESCRIPTION

[0030] The present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0031] A Ti65 alloy sample with a thickness of 50 μm and a diameter of 3 mm was selected for TEM observation and analysis. First, the lamellar α phase grains in the alloy were identified using a bright-field transmission electron microscope (TEM) image. Then, the unique orientation relationship between the lamellar α phase and the superlattice α2 phase was utilized: According to the Kikuchi spectrum, tilting the sample along the finest Kikuchi band causes the lamellar α phase grains to Kikuchi pole, that is, the grain turns to In the positive crystal zone axis direction, three thin bands, two thick bands and one middle band can be observed. Kikuchi pole. Microscopic images of the axial spherical structure of the superlattice α2 phase can be simultaneously obtained. For lamellar α phase grains, focusing is performed in low-magnification imaging mode (10k magnification imaging mode), and then switching to diffraction mode to observe Kikuchi bands.

[0032] Then, take a Kikuchi diffraction spectrum in the α-phase grains of the layer to determine whether Kikuchi pole, where the ratio of the distance between the transmission spot and the diffraction spot b / a needs to be close to 1.299. Figure 1 As shown; if the diffraction spectrum is not Continue to tilt the sample along the thinnest Kikuchi zone, or change another grain until it is confirmed to be Kikuchi is determined After Kikuchi changed the diffraction pattern, he moved the aperture to select the superlattice diffraction spot between the transmission spot and the adjacent diffraction spot, such as Figure 1As shown in the dashed circle, dark-field imaging was performed, with images taken at magnifications of 125k, 195k, and 250k, with an exposure time of 15s. Dark-field images were taken at areas with thin grain edges and no impurities or streaks. First, the image was taken at the positive focus position. The electron microscope focus was then fine-tuned to underfocus and overfocus, with images taken every 500nm to prevent sample shake from affecting the image quality. The focus range was ±2μm until the image was clear.

[0033] Adjacent photos at 125k, 195k and 250k magnifications were taken within the selected grains for data statistics. The TEM photos of the axial grains of the superlattice α2 phase taken are shown in Figure 2. Figure 2 (a), 2(b), 2(c). It can be seen that this method can clearly distinguish the superlattice α2 phase with an average grain size within 0.6-5 nanometers. Then, the clearest TEM photo of the superlattice α2 phase axial grains is selected, here a photo with a magnification of 195k and the photo is magnified 10 times using image processing software. Three photos of the same size (length × width = 9mm × 9mm) at different positions in the photo after magnification 10 times are intercepted as shown in FIG. Figure 3 As shown in (a), 3(b), and 3(c), the brightness and darkness analysis function of the image processing software is used to capture the α2 contrast, which is the white area surrounded by the red line in the figure. Figure 4 (a), 4(b), 4(c), and the software is used to automatically obtain the superlattice α2 phase contrast area data screenshots as shown in Figure 5 (a), 5(b), 5(c) (obj# represents the number of superlattice α2 phases in the three photos, which are 365, 487 and 374 respectively, and area represents the absolute area of ​​the corresponding superlattice α2 phase). The data of the superlattice α2 phase contrast area is processed to obtain the above Figure 4 The α2 phase content (the ratio of the sum of the superlattice α2 phase contrast (or grain) area to the screenshot area (9mm×9mm)) and grain diameter size (since the superlattice α2 phase is spherical, the area of ​​the circle is calculated using the formula, i.e., superlattice α2 phase diameter = 2*√(superlattice α2 phase contrast area / π)) of the three images are listed in Table 1. The data were averaged to obtain the average α2 phase content of 29.45% and the average grain size diameter of 1.055nm in the Ti65 high-temperature titanium alloy at 650℃ for 100h.

[0034] According to the above process, the three selected superlattice α2 phase photos ( Figure 3 The area data obtained by processing with image processing software is 100% repeatable, so the method for statistical analysis of the α2 phase size is very effective and easy to implement.

[0035] Table 1 shows the statistical data of α2 phase content and diameter

[0036] <![CDATA[α2 phase content]]> <![CDATA[α2-phase diameter]]> Figure 4 (a) 26.17％ 0.98nm Figure 4 (b) 31.94％ 1.13nm Figure 4 (c) 30.25％ 1.10nm average value 29.45％ 1.055nm

Claims

1. A statistical method for determining the size of nanoscale α2 phase grains in near-α titanium alloys, characterized by: The method comprises the following steps: (1) Take a titanium alloy sample and find flat lamellar α-phase grains under the bright field image of a transmission electron microscope (TEM); (2) Rotate the lamellar α-phase grains found in step (1) to a specific crystal orientation Then, the Kikuchi diffraction spectrum was taken in the diffraction mode to verify whether the orientation contained the superlattice α2 phase; (3) moving the objective lens aperture to select the diffraction beam of the α2 phase in the diffraction spectrum obtained in step (2), and photographing at least three different magnifications in the range of 100-300k with an exposure time of 5-20 seconds to obtain a dark field photograph of the phase; (4) The α2 phase TEM dark field photograph obtained in step (3) is magnified 8-12 times using image processing software, and then multiple images of the same area size are captured; (5) The image obtained in step (4) is processed through image processing software with image brightness and darkness analysis function to perform statistics on the superlattice α2 phase, and a statistical superlattice α2 phase area data table is obtained. The relative content of superlattice α2 is obtained by calculating the ratio of the sum of the superlattice α2 phase area data to the area of ​​the entire image, and the α2 phase diameter is calculated by averaging the area data of each α2 phase.

2. The method according to claim 1, characterized in that: In step (1), it is necessary to select the rotation range allowed by the electron microscope according to the Kikuchi diffraction spectrum. The required grains of Kikuchi pole.

3. The method according to claim 1, characterized in that: The specific process of step (2) is to rotate the grain along the finest grain band in the Kikuchi spectrum in the selected grain until it is rotated to Kikuchi pole, then return to the diffraction mode to determine whether the diffraction spectrum is consistent with Standard diffraction spectrum, and observe whether superlattice diffraction spots can be observed in it.

4. The method according to claim 3, characterized in that: The specific process of step (2) is to rotate the grain along the finest grain band in the Kikuchi spectrum in the selected grain until it is rotated to The process of Kikuchi pole is: take Kikuchi diffraction spectrum in the α phase grain of the layer to determine whether Kikuchi pole, where the ratio of the distance between the transmission spot and the diffraction spot b / a must be close to 1.299; if the diffraction spectrum is not Continue to tilt the sample along the thinnest Kikuchi zone, or change another grain until it is confirmed to be Kikuchi Ji.

5. The method according to claim 1, characterized in that: In step (3), the focal length needs to be continuously adjusted at the same position to ensure the accuracy of superlattice α2 phase characterization; First, take a picture at the focus position, then fine-tune the electron microscope focus to the underfocus and overfocus directions, and take a picture every 300-600nm to prevent sample shaking from affecting the shooting effect. The focal length range is ±2-5μm, until the picture is clear.

6. The method according to claim 1, characterized in that: In step (4), the original α2 phase TEM dark field phase photograph is enlarged and captured to ensure the accuracy of the subsequent α2 phase image recognition.

7. The method according to claim 1, characterized in that: In step (5), the α2 phase area data can be quickly obtained by using image processing software with image brightness and darkness analysis function, and then the α2 phase content and diameter can be obtained.

Images