Observation and quantitative analysis of dislocations in steel using electron channeling contrast imaging method with precise control of electron beam incident direction

Microscopy, 2024, 73(4), 308–317 DOI: https://doi.org/10.1093/jmicro/dfad061 Advance Access Publication Date: 19 December 2023 Article Observation and quantitative analysis of dislocations in steel using electron channeling contrast imaging method with precise control of electron beam incident direction Takashige Mori1,* , Takafumi Amino1 , Chie Yokoyama1 , Shunsuke Taniguchi1 , Takayuki Yonezawa2 and Akira Taniyama1 2 * Research & Development, Nippon Steel Corporation, 1-8 Fuso-Cho, Amagasaki, Hyogo 660-0891, Japan Research & Development, Nippon Steel Corporation, 20-1 Shintomi, Futtsu, Chiba 293-8511, Japan To whom correspondence should be addressed. E-mail: Abstract Electron channeling contrast imaging (ECCI) was applied by precisely controlling the primary electron beam incident direction of the crystal plane in scanning electron microscope (SEM), and the dislocation contrast in steel materials was investigated in detail via SEM/ECCI. The dislocation contrast was observed near a channeling condition, where the incident electron beam direction of the crystal plane varied, and the backscattered electron intensity reached a local minimum. Comparing the dislocation contrasts in the visualized electron channeling contrast (ECC) images and transmission electron microscope (TEM) images, the positions of all dislocation lines were coincident. During the SEM/ECCI observation, the dislocation contrast varied depending on the incident electron beam direction of the crystal plane and accelerating voltages, and optimal conditions existed. When the diffraction condition g and the Burgers vector b of dislocation satisfied the condition g⸱b = 0, the screw dislocation contrast in the ECC image disappeared. An edge dislocation line was wider than a screw dislocation line. Thus, the SEM/ECCI method can be used for dislocation characterization and the strain field evaluation around dislocation, like the TEM method. The depth information of SEM/ECCI, where the channeling condition is strictly satisfied, can be obtained from dislocation contrast deeper than 5𝜉g , typically used for depth of SEM/ECCI. Key words: SEM, ECCI, dislocation, steel, TEM, EBSD Introduction Electron channeling in scanning electron microscopy (SEM) is directly related to crystallographic orientation relative to the incident electron beam. Moreover, the electron channeling SEM images can provide information on the crystallographic properties of a sample [1,2]. Coates [3] first reported that Kikuchi-like reflection patterns were observed in structural images of single-crystal samples obtained by backscattered electrons (BSE) at relatively low magnifications. Subsequently, several qualitative understandings were given by Booker [4] and Hirsch [5], and theoretical interpretations have been discussed so far [6–10]. If electron channeling can observe the local crystal orientation changes, capturing the changes in the BSE intensity should be possible due to lattice defects such as grain boundaries and dislocations in a well-polished bulk sample [5,11]. Several examples are reported regarding the observation of lattice defects in bulk samples [12–23] using electron channeling contrast imaging (ECCI). Furthermore, attempts were made to identify the crystal defect contrast in polycrystalline materials observed by SEM/ECCI by comparing with transmission electron microscopy (TEM); Zauter et al. [24] observed the dislocation structure formed in austenitic stainless steels after fatigue testing by SEM/ECCI and TEM and identified similar characteristic subgrains. Weidner et al. [25] and Sugiyama et al. [26] observed the steel after deformation by SEM/ECCI and TEM, reporting similar characteristics of crystalline defects. Zaefferer et al. [10] directly compared bright-field (BF) TEM and ECC images obtained from the same region of the TEM foil of twining-induced plasticity steel and showed the observation results for the contrast corresponding to dislocation, stacking fault, ε-martensite lamella, and a large slope of stacking fault; distinguishing the dislocation contrast from the surface roughness contrast was difficult. Pang et al. [27] compared SEM/ECCI and TEM dislocation contrasts in the same region of a TEM sample of impacted tantalum. They showed that all the dislocation contrasts observed in SEM/ECCI were also observed in TEM; however, many dislocation lines in the TEM image were absent in the ECC image. Thus, although there are many examples of direct comparison of crystal defect contrasts between SEM/ECCI and TEM, few show a clear correspondence between their dislocation contrast. The orientation relationship between the incident electron beam and crystal must be precisely controlled up to the channeling condition where the rocking curve of the backscattered electron intensity reaches a local minimum [10] to observe dislocation contrast in any region using SEM/ECCI. Mansour et al. [28,29] proposed accurate ECCI (A-ECCI), which accurately measures crystal orientation information Received 1 August 2023; Revised 12 November 2023; Editorial Decision 2 December 2023; Accepted 14 December 2023 © The Author(s) 2023. Published by Oxford University Press on behalf of The Japanese Society of Microscopy. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited. 1 Microscopy, 2024, Vol. 73, No. 4 Experimental methods Fe–0.13C–0.26Si–1.3Mn–0.014P–0.003S (mass%) steel sample was used in this study. By heating the steel ingot to 1100∘ C, hot rolling at a finishing temperature of 750∘ C, and cooling to room temperature, the ferrite/pearlite microstructure was obtained. The surface of the steel sample was mirror-polished by mechanical polishing with emery paper and buffing with diamond slurry and colloidal silica. The crystal orientation of the ferrite grains on the sample surface was determined by EBSD. Then, a Kikuchi map, a simulated electron channeling pattern of the observation area, was calculated from the Euler angles taken by EBSD and the crystal structures representing the crystal orientation of the analyzed points. The channeling conditions suitable for observing dislocations by SEM/ECCI were satisfied near the Bragg conditions [10]. Therefore, simulated pseudo-Kikuchi lines constituting the Kikuchi map were drawn with a width twice than the Bragg angle 𝜃B . The intensity of the simulated pseudo-Kikuchi line is the square of the crystal structure factor F, obtained by assuming that the diffraction intensity of the crystal lattice, follows the dynamical theory and is drawn to be proportional to the brightness of the simulated pseudo-Kikuchi line on each surface. For simplicity, only simulated pseudo-Kikuchi lines with intensities up to the 10th rank were drawn in this study, and the second- and high (...truncated)