PSI - Issue 43

Yoshikazu Nakai et al. / Procedia Structural Integrity 43 (2023) 221–227 Nakai et al./ Structural Integrity Procedia 00 (2022) 000 – 000

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Nomenclature β

total misorientation diffraction angle

θ

Δω IQ

spread of rotation angle of sample satisfying Bragg's condition

image quality in EBSD diffraction contrast tomography refraction contract tomography

DCT RCT

1. Introduction Since metallic materials are often used for machine and structural components subject to high stress, various procedures have been developed to obtain high-strength materials. Among them, grain refinement is particularly effective in increasing the strength of metallic materials; however, it usually causes plastic instability and necking at the early stage of deformation, resulting in the loss of ductility. To solve this trade-off between strength and ductility, Ueno et al. (2012), Zhang et al. (2014, 2015, 2017), Rai et al. (2016, 2017), Park et al. (2018), and Kikuchi et al. (2019) combined grain refinement with powder metallurgy to create “ harmonic structured materials ” in which high strength fine grains are arranged in a network around ductile coarse grains. The three-dimensional periodic structure of the harmonic structured material suppresses necking, as reported by Zhang et al. (2015), and achieves higher strength and ductility. In the present study, to elucidate the unique mechanical properties of the harmonic structured material, its deformation mechanism was clarified by diffraction contrast tomography (DCT) using the ultrabright synchrotron radiation X-ray proposed by Ludwig et al. (2008), which is a nondestructive observation method. DCT enables the continuous observation of the local plastic deformation not only on the sample surface but also on the crystal grains inside the sample, as reported by Shiozawa et al. (2014, 2016) and Nakai et al. (2016, 2017-1, 2017-2). Total misorientation, defined as the spread in the orientation of diffraction spots, which correspond to the number of dislocations in each diffraction plane, was measured by DCT in the tensile test of harmonic structured JIS-SUS304L. The macroscopic deformation of the material in the tensile test was also simultaneously measured by refraction contrast tomography (RCT) imaging.

2. Experimental Procedures 2.1. DCT and RCT imaging

Figure 1 shows an overview of the acquisition geometry of DCT. Diffraction occurs in crystal grains satisfying Bragg's condition when the sample is irradiated with X-rays. Diffraction spots appear in the direction of the diffraction angle, together with dark extinction spots behind the sample. The total misorientation β , which reflects the number of excessive dislocations in each diffraction plane of each grain, can be calculated by measuring the spread of the rotation

Fig. 1. Overview of the acquisition geometry of the DCT, showing the synchrotron beam, the sample, the Debye ring, and both direct and diffraction images on the detector.

angle Δω of the grain satisfying Bragg's condition, the diffraction the angle θ , and the angle ϕ in the Debye ring for each diffraction spot, as reported by Nakai et al. (2016, 2017-2). RCT was used for the measurement of the shape change and local strain of a sample in the tensile test. For DCT, the distance between the sample and the detector should be sufficiently short to detect as many diffraction spots as possible, while it should be sufficiently long to obtain a large effect of refraction of the X-ray at the interface of dissimilar materials by RCT. Consequently, the dual camera system developed by Nakai et al. (2018) was employed in the present study, where the detector for RCT measurement was placed behind the detector for DCT imaging, as