Method for measuring micro-area dislocation density of welded joint based on grain orientation

Abstract

The invention discloses a welding joint micro-region dislocation density measuring method based on grain orientation, and belongs to the field of welding. The method comprises the following steps: firstly, providing a zirconium alloy joint sample, carrying out EBSD analysis on a detection area of the joint sample, and determining a grain structure and grain orientation; crystal grains with orientation are selected, and a TEM sample is prepared through FIB; and respectively representing dislocation parameters of a-type dislocation and c-type dislocation in the TEM sample by using the TEM, and counting to obtain the average dislocation density in a plurality of crystal grains in the detection area as the dislocation density of the detection area. According to the method, the dislocation in the welding joint microcell can be directly represented, the imaging quality when the dislocation density is measured through the TEM method is effectively improved, and the efficiency and accuracy of dislocation density measurement are improved.

Description

Technical Field

[0001] The invention belongs to the field of welding, and in particular relates to a method for measuring dislocation density in micro-regions of welded joints based on grain orientation. Background Art

[0002] During the welding process, the heat-affected zone (HAZ) of metals is affected by the heat input, causing significant changes in its phase structure and microstructure, leading to altered mechanical properties. In some weld structures, the mechanical properties of the HAZ can significantly degrade, and the zone can even become the first weak zone to fail. In-depth research on the microstructure of the weld zone is crucial for further understanding the relationship between microstructural characteristics and weld properties, and for optimizing welding parameters based on these characteristics. Among the various microstructural characteristics of the weld zone, dislocation density is one of the most important parameters affecting weld properties.

[0003] Currently, conventional methods for measuring dislocation density mainly include X-ray diffraction (XRD), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). XRD offers low cost and high efficiency, but its essence is to perform indirect calculations through changes in diffraction peaks and cannot directly determine the microscopic distribution and morphology of dislocations. EBSD offers higher precision and can identify dislocation types, but can only analyze the surface or near-surface region of the sample. TEM can directly provide high-resolution dislocation images, making it suitable for studying the structural characteristics and interactions of dislocations. However, TEM sample preparation is difficult, and the characterization of dislocations is significantly affected by grain orientation and the observed diffraction direction. If the grain orientation of the sample is inappropriate, effective characterization of dislocations is difficult to achieve.

[0004] Zirconium alloy is one of the commonly used structural components in commercial reactor cores. The performance of zirconium alloy under high-temperature irradiation conditions is of great significance to reactor safety. Zirconium alloys contain a-type and c-type dislocations. Among them, a-type dislocations are the main mobile dislocations. Their movement is strongly pinned and hindered by these irradiation defects, which is the primary microscopic cause of radiation hardening and plasticity degradation. The formation and evolution of c-type dislocations and their asymmetric absorption with point defects are the fundamental causes of irradiation growth. Therefore, accurate characterization of a-type and c-type dislocations is of great significance for the reasonable evaluation of the performance degradation of zirconium alloy parts, especially the key areas of zirconium alloy welded joints under irradiation, and for life prediction. However, existing dislocation characterization methods cannot effectively distinguish between a-type and c-type dislocations, which restricts the research on the irradiation performance of zirconium alloys.

[0005] Therefore, a method for measuring the micro-area dislocation density of welded joints based on grain orientation is provided to accurately characterize the a-type dislocations and c-type dislocations in the zirconium alloy structure, which is of positive significance for improving the prediction accuracy of the irradiation performance evolution and service life of zirconium alloys. Summary of the Invention

[0006] The purpose of the present invention is to provide a method for measuring dislocation density in micro-areas of welded joints based on grain orientation, so as to optimize the efficiency of measuring dislocation density by TEM method.

[0007] According to one embodiment of the present invention, a method for measuring dislocation density in micro-regions of a weld joint based on grain orientation comprises the following steps:

[0008] Step a): providing a weld joint sample, wherein the weld joint sample is a zirconium alloy weld joint sample, determining a detection area in the weld joint sample and performing EBSD analysis to obtain a grain structure and grain orientation in the detection area;

[0009] Step b): Select the detection area Oriented grains, TEM samples were prepared by FIB;

[0010] Step c): characterizing the dislocation parameters in each of the TEM samples using TEM, wherein The diffraction vector characterizes the a-type dislocation, and the (0002) diffraction vector characterizes the c-type dislocation;

[0011] Step d): Statistically obtaining an average dislocation density within a plurality of grains within the detection region as the dislocation density of the detection region.

[0012] This method combines EBSD with TEM technology. First, EBSD is used to screen the grain orientation, and the orientation that is easy for dislocation to form contrast is selected for FIB sample preparation, thereby effectively increasing the number of dislocations that can be observed in the TEM field of view, thereby improving the efficiency and accuracy of dislocation density measurement. For zirconium alloys, when the crystal axis is When, there is The diffraction of the (0002) crystal plane. For the observation of a-type dislocation, the diffraction vector used is For the observation of c-type dislocation, the diffraction vector used is (0002).

[0013] Furthermore, in some embodiments, the inspection area in step a) includes a heat-affected zone of the weld joint sample.

[0014] The structure in the heat-affected zone is significantly affected by thermal stress, and its structure does not undergo significant recrystallization, making it easy to become a weak area of ​​the weld joint.

[0015] Furthermore, in some embodiments, in step c), the dislocation parameters include the number of dislocations and the length of dislocations, and TEM characterization uses a combination of bright field imaging and dark field imaging.

[0016] Furthermore, in some embodiments, in the step d), the dislocation density corresponding to each of the TEM samples is calculated based on the dislocation parameters and the volume of the TEM sample.

[0017] Furthermore, in some embodiments, in step a), the zirconium alloy welded joint sample is obtained by TIG welding.

[0018] Furthermore, in some embodiments, in step a), the zirconium alloy weld joint sample is prepared by the following method: cutting, grinding, and polishing the weld joint to remove surface scratches; and performing vibration polishing and electrolytic polishing to ensure that the surface quality of the zirconium alloy weld joint sample meets the sample requirements of EBSD.

[0019] Furthermore, in some embodiments, electrolytic polishing is performed using a 10% by volume perchloric acid ethanol solution at a voltage of 60V-80V, and the electrolytic polishing time is 10s-30s. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is an IPF image of the EBSD scan of the heat-affected zone of a weld joint in one embodiment;

[0021] Figure 2 This is a TEM photo of a sample in one embodiment;

[0022] Figure 3a This is a bright field image of dislocation in one embodiment;

[0023] Figure 3b for Figure 3a Dark field image of dislocation in the same area;

[0024] Figure 4a This is a bright field image of regional dislocation of the parent material in one embodiment;

[0025] Figure 4b This is a dark field image of regional dislocations in a parent material in one embodiment;

[0026] Figure 5 A pair of TEM images are shown.

[0027] The purpose of the above drawings is to provide a detailed description of the present invention so that those skilled in the art can understand the technical concept of the present invention, and is not intended to limit the present invention. DETAILED DESCRIPTION

[0028] The present invention will be further described in detail below through specific embodiments with reference to the accompanying drawings.

[0029] References to "embodiments" herein mean that a particular feature, structure, or characteristic described in connection with the embodiment may be included in at least one embodiment herein. The appearance of such phrases in various places in the specification does not necessarily refer to the same embodiment, nor does it limit mutually exclusive independent or alternative embodiments. Those skilled in the art will appreciate that an embodiment herein may be combined with other embodiments as long as no structural conflicts arise.

[0030] In the description herein, terms such as "first" and "second" are used only to distinguish different objects and should not be understood to indicate relative importance or to limit the quantity, specific order, or primary and secondary relationship of the described technical features. In the description herein, "plurality" means at least two.

[0031] The weld joint area is subject to significant changes in its structure and properties due to the heat input during the welding process. Generally, in joints formed by fusion welding, the center of the weld is the weld nugget formed by the solidification and crystallization of the material after melting; the heat input in the area connected to the weld nugget is relatively low and the material does not melt, but its microstructure undergoes certain changes due to the heat input, such as the dissolution and precipitation of precipitates, and the increase of dislocations caused by thermal stress. In some weld joints, the dissolution of precipitates and changes in dislocation density in the heat-affected zone may lead to significant changes in their mechanical properties. Under some working conditions, the heat-affected zone will become the weak part of the weld joint, and the risk of failure will increase significantly. Therefore, analyzing and characterizing the microstructure in the heat-affected zone of weld joints plays an important role in deepening the understanding of the law of change in the mechanical properties of weld structure.

[0032] Currently, the main methods for measuring dislocation density include X-ray diffraction (XRD), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). The XRD method estimates the grain size and microstrain of the material by analyzing the width of the diffraction peak, thereby calculating the dislocation density. This method is suitable for samples with low dislocation density and can quickly obtain information on the overall dislocation density of the material. Advantages include non-destructive testing, relatively simple operation, and the ability to provide overall crystal structure information. Disadvantages include the inability to distinguish between different types of dislocations, and the presence of microstrain may affect the accuracy of the dislocation density test results.

[0033] The EBSD method determines the crystallographic orientation and crystal structure of a material by analyzing the diffraction pattern of electrons reflected from the sample surface in a scanning electron microscope (SEM). EBSD can provide spatial resolution at the micron level, making it suitable for analyzing microstructures, and can cover a larger sample area to provide comprehensive material structure information. The advantages of EBSD include fast data acquisition, no need to destroy the sample, and the ability to identify and distinguish different types of dislocations. However, EBSD has high requirements for the surface state of the sample, requiring a flat, clean, and contamination-free surface, and mainly analyzes the surface or near-surface area of ​​the sample, with limited analytical capabilities for deep structures. Furthermore, the resolution of EBSD is at the level of 50-100nm, which is much larger than the geometric dimensions of the dislocation structure. The EBSD method is essentially still to indirectly infer the dislocation density by characterizing the lattice distortion within a certain range, and cannot directly characterize the dislocations.

[0034] The TEM method prepares ultra-thin samples, observes the microstructure of dislocations, and calculates the dislocation density based on the line-intersection method. The advantage is that it can provide high-resolution dislocation images, which is suitable for studying the detailed structure and interaction of dislocations. The disadvantages are that sample preparation is difficult, the cost is high, and it is localized. Usually, samples are only taken within a certain grain, and the observed dislocation morphology is affected by the grain orientation and the observed diffraction direction. If the orientation of the selected grains is not good, the adjustment angle is limited when using TEM for dislocation characterization, and it is very likely that the ideal contrast cannot be obtained, resulting in unclear dislocation imaging, which seriously affects the statistics of dislocation density. In conventional TEM sample preparation processes, whether double-spray etching or ion thinning is used, the selection of grain orientation cannot be achieved. The obtained sample grain orientation is random, which restricts the efficiency of TEM measurement of dislocation density and increases the cost of the experiment.

[0035] Some metal materials have a main slip system, and dislocations show obvious orientation during their formation under the influence of the slip system. For example, the main slip system in zirconium alloy is and Dislocations are mainly divided into a-type dislocations (Bergers vector is ) and a+c type dislocation (Bergers vector is ), the two dislocation reactions can generate c-type dislocations (Bergers vector is

[0001] ). For zirconium alloys, when the crystal axis is When, there is The diffraction of the (0002) crystal plane. For the observation of a-type dislocation, the diffraction vector used is For the observation of c-type dislocations, the diffraction vector used is (0002). Therefore, if the grain orientation in the TEM sample used to characterize the dislocation density can be reasonably selected, the imaging quality of the dislocations can be effectively improved, and the efficiency and accuracy of the dislocation density measurement can be improved.

[0036] In order to overcome the shortcomings of the prior art and improve the efficiency and accuracy of TEM method for measuring dislocation density, an embodiment of the present invention provides a method for measuring dislocation density in a micro-region of a weld joint based on grain orientation, the method comprising the following steps:

[0037] Step a): Providing a weld joint sample, cutting, grinding, and polishing the weld joint sample to prepare a sample that meets EBSD analysis standards. In a preferred embodiment, the sample is cut from the heat-affected zone (HAZ) of the weld joint, and the area near the weld toe within the HAZ is used as the inspection area.

[0038] Specifically, the sample was first ground using metallographic sandpaper of varying roughness, followed by surface polishing using diamond abrasive to remove surface scratches. Next, vibration polishing was used to partially remove the surface stress layer, followed by electrolytic polishing to obtain a sample that met the requirements of EBSD testing. EBSD analysis of the sample revealed the grain structure (including grain size, shape, and integrity) and grain orientation distribution within the test area.

[0039] Step b): In the inspection area, grains with a given grain orientation are selected and a TEM sample is prepared by FIB. Specifically, the given grain orientation is the orientation of the main slip system of the alloy sample.

[0040] Step c): Characterizing the dislocation parameters in the TEM sample using TEM. In a preferred embodiment, the characterization of the dislocations is performed using a combination of bright field imaging and dark field imaging.

[0041] Step d): Count the dislocation parameters from step c), specifically the number and length of dislocations. The sample volume is calculated based on the dimensions of the TEM sample, and the dislocation density in the TEM sample is then calculated. The dislocation density in the TEM sample is used as the dislocation density in the sampled grains. TEM samples sampled from multiple grains are characterized and their dislocation densities are calculated individually. Statistical calculations are then performed to determine the average dislocation density within these grains, which serves as the dislocation density data for the test area.

[0042] In a preferred embodiment, the base metal of the welded joint sample is Zr-2 alloy, produced using TIG welding with the following welding parameters: welding current 59A, welding voltage 9V, and welding speed 1.2mm / s. The welded joint is sectioned, and the heat-affected zone (HAZ) outside the weld toe edge is identified on the cross-section as the inspection area. A 10mm×10mm×1.5mm thin plate is cut using wire cutting to serve as the EBSD sample. The EBSD sample is ground using 400#, 800#, 1500#, 2000#, and 3000# metallographic sandpaper, followed by polishing using a 0.5μm diamond suspension on a woolen polishing cloth to remove surface scratches. Next, vibration polishing is performed for half an hour to partially remove the surface stress layer. Electrolytic polishing is then performed using a 10% by volume perchloric acid solution in ethanol at 70V for 20s. The electrolytic polishing temperature is kept at room temperature to ensure that the surface quality of the EBSD sample meets EBSD testing requirements.

[0043] The EBSD test was performed on the electrolytically polished EBSD sample, and the scanning step size was set to 1.2μm, and the following results were obtained: Figure 1 The EBSD inverse pole figure (Inverse Pole Figure) shown in the figure represents the grain orientation. Five grains with a diameter of more than 10 μm were selected as the orientation. The grains were vertically sampled by FIB to obtain TEM samples.

[0044] Select crystal zone axis TEM characterization was performed at 62000 times, and the tissue morphology was observed as follows Figure 2 As shown. For the observation of a-type dislocation, the diffraction vector used is For the observation of c-type dislocation, the diffraction vector used is (0002). Adjust the appropriate exposure parameters and contrast to obtain 5-10 bright and dark field images of different areas. Among them, the bright field image of the dislocation in one area is as follows: Figure 3a As shown, the dark field image of the dislocation is as follows Figure 3b shown.

[0045] Import the TEM image into the nano measure software, measure the size of the TEM image shooting area, identify and mark the dislocations in the bright field image and dark field image respectively, and measure the number and length of dislocations. After the measurement is completed, the dislocation measurement results are exported; the volume of the shooting area is calculated based on the size of the TEM shooting area and the thickness of the TEM sample; the dislocation density is calculated based on the number and length of dislocations and the volume of the shooting area. Figure 3a and Figure 3b In the region shown, the statistically obtained a-type dislocation number density is about 1.63×10 20 / m 3, with an average length of about 211.4 nm; the number density of c-type dislocations is about 1.64×10 20 / m 3 , with an average length of about 209.2nm.

[0046] The dislocation density of the Zr-2 alloy parent material was characterized in the same way. The dislocation bright field image of one TEM sample is shown in Figure 2. Figure 4a As shown, the dislocation dark field image is Figure 4b As shown. According to statistics, the number density of a-type dislocations is about 3.18×10 20 / m 3 , with an average length of about 55.4 nm; the number density of c-type dislocations is about 4.13×10 20 / m 3 , with an average length of about 65.1nm.

[0047] It can be seen that the dislocation density in the heat-affected zone of the Zr-2 welded joint is lower than that of the base material, and the average length of the dislocation is longer.

[0048] In the above embodiment, a method for measuring the dislocation density of micro-areas of welded joints based on grain orientation is adopted, and the dislocations in the obtained TEM image have good contrast, which effectively improves the statistical efficiency and accuracy, and the obtained dislocation number density result is more accurate.

[0049] In a comparative example, a TIG welded joint of a Zr-2 alloy was directly subjected to TEM characterization. The welding parameters of the welded joint sample were the same as those in the previous embodiment, and a TEM sample was obtained directly in the heat-affected zone by FIB vertical sampling. The TEM image of the sample is shown in FIG. Figure 5 As shown, the dislocation image clarity is poor, and angle adjustment cannot effectively improve the dislocation contrast. It is impossible to distinguish between type A dislocations and type C dislocations, making it difficult to accurately measure and count the dislocations.

[0050] By comparing the above embodiments with the comparative examples, it can be seen that the method for measuring dislocation density in micro-areas of welded joints based on grain orientation provided by the embodiments of the present invention can effectively improve the imaging quality of dislocations in the TEM field of view, realize the separate characterization of type a and type c dislocations by using TEM according to the invisibility criterion, accurately count the dislocation line length and density data in the weld micro-area, and effectively guide the research on the evolution of mechanical properties in the weld micro-area and the optimization of welding parameters.

[0051] The purpose of the above embodiments is to further explain the present invention in detail in conjunction with the accompanying drawings so that those skilled in the art can understand the technical concept of the present invention, and is not intended to limit the present invention. Within the scope of the present invention, optimization or equivalent replacement of the technical features involved, as well as combination of implementation methods in different embodiments without conflict of principle, all fall within the scope of protection of the present invention.

Claims

1. A method for measuring dislocation density in micro-areas of welded joints based on grain orientation, characterized in that: The following steps are involved: Step a): providing a weld joint sample, wherein the weld joint sample is a zirconium alloy weld joint sample, determining a detection area in the weld joint sample and performing EBSD analysis to obtain a grain structure and grain orientation in the detection area; Step b): Select the detection area Oriented grains, TEM samples were prepared by FIB; Step c): characterizing the dislocation parameters in each of the TEM samples using TEM, wherein The diffraction vector characterizes the a-type dislocation, and the (0002) diffraction vector characterizes the c-type dislocation; Step d): Statistically obtaining an average dislocation density within a plurality of grains within the detection region as the dislocation density of the detection region.

2. The method for measuring dislocation density in micro-regions of welded joints based on grain orientation according to claim 1, characterized in that: The inspection area in step a) includes the heat-affected zone of the weld joint sample.

3. The method for measuring dislocation density in micro-regions of welded joints based on grain orientation according to claim 1, characterized in that: In the step c), the dislocation parameters include the number of dislocations and the length of dislocations, and TEM characterization uses a combination of bright field imaging and dark field imaging.

4. The method for measuring dislocation density in micro-regions of welded joints based on grain orientation according to claim 1, characterized in that: In the step d), the dislocation density corresponding to each TEM sample is calculated based on the dislocation parameters and the TEM sample volume.

5. The method for measuring dislocation density in micro-regions of welded joints based on grain orientation according to claim 1, characterized in that: In the step a), the zirconium alloy welded joint sample is obtained by TIG welding.

6. The method for measuring dislocation density in micro-regions of welded joints based on grain orientation according to claim 1, characterized in that: In the step a), the zirconium alloy weld joint sample is prepared by the following methods: cutting, grinding, and polishing the weld joint to remove surface scratches; and performing vibration polishing and electrolytic polishing to ensure that the surface quality of the zirconium alloy weld joint sample meets the sample requirements of EBSD.

7. The method for measuring dislocation density in micro-regions of welded joints based on grain orientation according to claim 6, characterized in that: The electrolytic polishing is performed using a 10% by volume perchloric acid ethanol solution at a voltage of 60V-80V, and the electrolytic polishing time is 10s-30s.

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