Electron backscatter diffraction (EBSD) is a method to determine the crystal structure, orientation, and related information based on the analysis of diffractive Kikuchi bands excited by electron beams on the surface of inclined samples in a scanning electron microscope. Orientation imaging analysis based on EBSD technology can enable us to obtain more abundant internal information of materials, including quantitative information such as grain size, shape and distribution, grain orientation, and type of grain boundaries. In this paper, we summarize some basic applications of EBSD in twin analysis.
EBSD was used to determine the deformation twin
EBSD technology was used to analyze the microstructure of materials, which could not only obtain the morphological characteristics of the tissues, but also determine the orientation characteristics of different tissues. Due to the unique symmetry between the twins and the matrix, they have a common twin plane and a specific relationship between their orientation, such as the 60°<111> axis relationship between the twins of cubic crystal systems (BCC and FCC) metals and the matrix. Therefore, EBSD technology was used to test the microstructure orientation characteristics of the specimen, and the twin structure inside the specimen could be determined by combining the grain boundary type and pole diagram analysis.
EBSD was used to analyze Fe-65wt.After tensile deformation at medium temperature (400) of %Si alloy, it is found that two groups of band-like structures with obvious orientation differences are formed inside the sample, as shown in Figure 1. The orientation of the two bands presents a 60°<111> axis relationship, and the matching lattice of =3 is obtained according to the 60° rotation of the cubic metal lattice around the [111] axis, and the orientation relationship between the two lattices is a typical twin orientation relationship, so it can be judged that the deformed strips formed inside the alloy are twin relationships. In addition, it can be seen from the corresponding pole diagram (Fig. 1(c)) in Fig. 1(b)) that the matrix and the deformed bands have three overlapping points in the pole diagram, indicating that there is a mirror symmetry relationship between the matrix and the band. For BCC alloy, the twin system is mainly <111 >, and the crystal plane is the twin plane, so it can be further determined that the deformation band is a deformed twin structure.
Figure 1 FE-65 wt.Deformation twins produced by medium-temperature (400) tensile deformation of %Si alloys.
a)sem**；b) Orientation imaging of the local area; (c) The corresponding pole diagram (the colors shown in the diagram correspond to (b)).
EBSD was used to analyze the microstructure of the deformed specimen of high manganese steel with 20% compression deformation, and the orientation imaging and pole maps of the deformed structure were obtained, as shown in Fig. 2. It can be found that two sets of twin variants have formed inside the grains, and the two variants are at an angle of nearly 120°. From the corresponding pole diagrams, it can be seen that there are common planes between the two sets of twin variants and the matrix, and the twin crystals occur on two faces in the matrix grain close to the compression axis, indicating that the crystal orientation of the grain is more prone to twin deformation during compression deformation.
Fig.2. Deformation twins in compression-deformed specimens of high-manganese steel.
a)sem**；b) orientation imaging of constituency B; (c) Corresponding pole diagrams.
EBSD was used to analyze the orientation dependence of twin formation
Grain orientation can significantly affect the twin deformation mechanism, resulting in different combinations of twin systems in the sample. Because the grain has different orientation rotation laws during tension and compression, and the number and dynamics of twin formation are affected by grain orientation and have different characteristics, EBSD technology can easily analyze the orientation dependence of twin formation.
Figure 3 analyzes Fe-6 using EBSD technologyThe twin deformation of the 5 wt%Si alloy (BCC) inside the specimen when it is tensile and compressed to 10% deformation at medium temperature (400). According to the orientation imaging map, it can be found that during the tensile deformation< the grains with directions parallel to the tensile axis 101>> and <111 do not form twins, while a large number of twins are formed inside the grains with the crystal orientation parallel to the tensile axis > <001. During compression deformation< no twins were formed in the grains with the crystal orientation parallel to the compression axis > 001, while a large number of twins were formed in the grains with the crystal orientation parallel to the compression axis > <111.
In order to make the data more statistical, EBSD can be used to analyze the grain orientation of a large number of grains and the formation of twins under the same deformation conditions, and the grain orientation of a large number of twins formed A small number of twins formed and no twins formed are plotted in the same reverse pole diagram, as shown in Fig. 3(c) and (d). It can be seen that the twin has obvious orientation dependence in the process of tensile and compression deformation, and the grain with the grain > <001 parallel to the tensile axis is easier to form twins during the tensile deformation process, while the grain is easy to form twins during the compression deformation process when the grain is oriented parallel to the compression axis near the line <101>-<111 >in the standard triangle. By calculating the Schmid factor of the twin system of the body-centered cubic metal <111 >relative to a given tensile or compression axis, it can be concluded that the crystal direction near <001 > has a large twin Schmid factor when the alloy is tensile and deformed. During compression deformation, the larger twin Schmid factor appears near the > line <101>-<111, indicating that the grains oriented there are more prone to twinning, and the easily twin crystal orientation determined by the experiment is consistent with the crystal orientation of the theoretically calculated twin Schmid factor. It is worth noting that the crystal data collected from the deformed specimen by EBSD has a certain deviation from the initial crystal orientation of the specimen before deformation, but this deviation is negligible for judging the orientation dependence of the deformation twin.
Figure 3 FE-6Twin deformation behavior and orientation dependence of grains with different orientations of 5 WT%Si alloy (BCC).
a) Tensile deformation 10%; (b) Compression set 10%;
c) the orientation dependence of the deformed twins in the tensile specimens; (d) Orientation dependence of deformed twins in compressed specimens.
For FCC metals with a twin system of <112>, the dependence of the twin deformation orientation has another characteristic, and EBSD technology can still be used for analysis. The true strain to the tensile deformation is 03 TWIP steel specimens were subjected to EBSD analysis, and the grain orientations of the twin-producing and non-twinned grains were labeled within the same standard triangle, as shown in Fig. 4(a) and (b). It can be found that when the <001> direction of the grain is parallel to the tensile axis, the grain is not easy to form twins. When the <111> direction of the grain is parallel to the tensile axis, the grain tends to form twins. The above law is significantly different from the twin orientation dependence of BCC alloy during tensile deformation. This variation can be illustrated by the magnitude of the ratio of the Schmid factor of the slip to the twin, as shown in Figure 4(c), for FCC alloys, the grains tend to deform through dislocation slip when the orientation is close to the <001 >, and when the orientation is close to the triangular boundary of <101>-<111>, the grains tend to deform through the twin.
Fig.4. TWIP steel (FE 22wt.)% mn–0.6wt.% c, fcc).
a) The tensile true strain is 03. Imaging image of the longitudinal section orientation of the sample; (b) grain orientation distribution of produced and untwined grains;
c) Distribution of grain orientation that is prone to twin deformation (determined by comparing slip and twin Schmid factor size).
From the above two examples, it can be seen that the EBSD technology can be used to analyze the twin deformation behavior of alloys, and combined with theoretical analysis, the orientation dependence of the formation of twins of different alloys can be determined, and the orientation of the alloys can be controlled to affect the formation of twins and improve the deformation ability of the alloys.
In-situ EBSD was used to observe the changes of twins during deformation
In the process of EBSD sample preparation, a specific position is determined by marking on the surface of the sample, and when the sample is compressed and deformed by different deformations, the microstructure and crystal orientation of the specific position are analyzed by EBSD, and the change law of the twin structure in the deformation process can be obtained. Through the above method, the deformation twin process of AZ31 magnesium alloy can be analyzed [6], as shown in Figure 5. Three different areas were selected for observation, and the compression deformation of the specimen was changed from 16% is gradually increased to 54%。The original specimen is mainly composed of equiaxed grain structure, and no twins appear inside the grain. At lower deformations (16%), with the increase of deformation, the twin gradually expands to the equiaxed grain boundary (the white arrow indicates the direction of expansion), and the deformed twin may pass through the grain boundary. At the same time, the width and length of the same twin increase with the increase of deformation. The above process shows that the formation of twins affects grain boundaries and causes twin nucleation to occur in adjacent grains to coordinate deformation.
Fig.5. Variation of deformation twins of AZ31 magnesium alloy during compression deformation.