PSI - Issue 43

Yoshikazu Nakai et al. / Procedia Structural Integrity 43 (2023) 221–227 Nakai et al./ Structural Integrity Procedia 00 (2022) 000 – 0 0

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Fig. 2. Schematic of dual camera imaging system (Nakai et al., 2018).

shown in Fig. 2. With this dual camera system, DCT and RCT imaging could be conducted almost simultaneously by moving the detector for DCT imaging just after DCT imaging is completed. DCT and RCT imagings were conducted at the BL46XU beamline of SPring-8 (Super Photon ring-8 GeV), which is the brightest synchrotron radiation facility in Japan and delivers an X-ray beam with ultrahigh brightness and high spatial coherence, allowing microtomography to be performed with high spatial resolution in the μm range. The po lychromatic synchrotron beam was monochromated to an energy of 37 keV using a Si {111} double-crystal monochromator. Two-dimensional projection images were recorded on a high-resolution detector system with a transparent luminescent screen, light optics, and a charge-coupled device (CCD) camera. The sample-to-detector distances were 10 mm and 235 mm for DCT and RCT imaging , respectively. An effective pixel size of 2.7 μm was employed in the projection image for the experiment. The size of the beam at the sample position was limited to 1.0 mm × 1.0 mm by using an X -ray slit in order to detect diffraction spots. Projection images were obtained at intervals of 0.05° over 360° for DCT imaging and 0.5° over 180° for RCT imaging. To conduct the inline test, a stepping-motor-driven tensile machine developed by Nakai et al. (2017) was employed. The machine used was sufficiently small enough to be placed on the rotating stage for tomographic imaging. 2.2. Material and specimen Austenitic stainless steel JIS-SUS304L powder was used to fabricate samples. First, the crystal grains on the surface of the initial powder were refined by mechanical milling (MM) in which JIS-SUJ2 balls and JIS-SUS304L powder were placed in a cemented carbide container of a planetary ball mill. Milling was conducted at room temperature in an argon environment at a pot speed of 200 rpm and a processing time of 180 ks. The MM powder was then pressed by spark plasma sintering (SPS) using a 25-mm-diameter sintering die to produce a harmonic structured material (MM series). The powder was kept at 1223 K for 43.2 ks followed by furnace cooling. For comparison, a coarse-grained material was fabricated by sintering the initial powder without mechanical milling (Untreated series). The relative density of the sintering materials is 99.6%, indicating that there is almost no porosity. The shape and dimensions of tensile test specimens cut from the sintered compact by electrical discharge machining are shown in Fig. 3. Each specimen has a smallest cross section of 0.3 mm × 0.3 mm square and a parallel section length of 0.25 mm. 3. Experimental Results and Discussion 3.1. Microstructural analysis of harmonic materials Prior to DCT and RCT imaging in the tensile test, the microstructure of the harmonic structured material (MM series) was analyzed by electron backscatter diffraction (EBSD). Figure 4 shows the image quality (IQ) map obtained by EBSD analysis, where grain boundaries were defined as boundaries with an orientation difference of 15° or greater and indicated by black lines, showing that fine grains are formed around the coarse-grained structure. The average grain size of the MM series was 2.1 μm for the fine -grained structure (Shell) and 16.8 μm for the coarse-grained structure (Core).