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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Taking into account the effective pixel size of the detector used in the present study, Δβ ave is considered to be evaluated only for grains larger than 10 μm , i.e. , Core structure, indicating that the plastic deformation of the Core structure in the harmonic structured material (MM series) is smaller than that in the homogeneous material (Untreated series) at comparable applied stress, and most of the plastic deformation in the harmonic structured material is localized in the Shell structure. Ameyama et al. reported that for harmonic structured materials, microdeformation mainly occurs in the network of fine grains and suppresses the deformation of the Core structure. Therefore, the present results are considered to indicate a deformation mechanism unique to harmonic structured materials. 3.3. Macroscopic deformation observation by RCT imaging

Fig. 6. Change in average total misorientation in tensile test.

The side view of a specimen of the MM series observed by RCT imaging is shown in Fig. 7, indicating that the deformation is almost uniform, and necking had not occurred except immediately before fracture, corresponding to the results shown in Fig. 5, in which the tensile strength of the Untreated series is shown to be higher than the stress upon fracture, meaning that necking occurred . In contrast, the MM series specimen broke at the maximum stress, i.e. , tensile strength, without necking. The crosssectional images of the MM series specimen observed by RCT are shown in Fig. 8, where three circular (actually, spherical) objects appear. Those are glass beads glued on the surface of the specimen as markers for displacement measurement and to identify the axial position of the crosssection. Inside the specimen, no geometrical defects such as voids can be observed. Although some of the glass beads fell off during the tensile test, the change in the crosssectional area of the specimen could be measured at the same crosssection. The crosssectional area of the MM series specimen measured from the RCT images is shown in Fig. 9, indicating that the crosssectional area is almost constant in the elastic deformation region (from #1 to #5), while it decreases with increasing applied stress in the plastic deformation region Since the harmonic structured stainless steel, whose average grain size of the Shell structure i s larger than 10 μm, has successfully been fabricated recently, and this newly developed material will enable us to measure β ave in the Shell structure. We have already conducted a preliminary measurement of β ave in the Shell structure by DCT. A detailed analysis of the results will be presented in the near future. We also plan to investigate the relationship between the misorientation changes in the harmonic structured material and the geometrical changes in the final fracture area by combining DCT and RCT imaging. 4. Conclusions In the present study, tensile tests were conducted on austenitic stainless steel (JIS-SUS304L) with a harmonic microstructure, and DCT and RCT were conducted simultaneously to continuous observe both the total misorientation of grains and the macroscopic deformation. The following results were obtained. DCT was successfully conducted at SPring-8 in Japan, and the misorientation of the Core structure in the harmonic structure material was measured. 1. The average total misorientation increased with applied stress; however, the total misorientation of the Core structure in the harmonic structured material was lower than that of the conventional material, indicating that the plastic deformation in the harmonic structured material mainly occurs in the Shell structure. 2. The change in cross-sectional area was measured by RCT imaging. (from #6 to #10). 3.4. Future tasks