OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 24 No. 4 2022 70 mm, which led to an improvement in the quality of object boundaries due to the appearance of phase contrast. The scanned area had a size of 1 mm3 with a voxel size of 0.7 µm. The exposure time for each projection was 250 ms. The analysis of 1,500 X-ray patterns allows to reconstruct these sizes. An analysis of ~1,000 voids corresponding to each image recorded by laminography was made with the ImageJ software to quantify the size and shape of defects accompanying the initiation of cracks. All measurements were carried out on images parallel to the T–L plane at different sample thicknesses. The analyzed area was 31 mm2 for the RN10 samples and 36 mm2 for the RN2 samples (the geometry of the samples was described in [38]). The area occupied by each void was calculated taking into account its elliptical shape. The total defect fraction was defined as the ratio of the area of voids to the total area analyzed (excluding macrocracks). Geometric parameters of macrocracks were determined individually. An analysis of the results obtained with laminography made it possible to reveal a number of features of the destruction of the AZ31B alloy, which can appear when loading other magnesium-based alloys [36]. Analysis of the behavior of the material in the process of stretching indicates its plasticity. The voids that have arisen during loading are distributed over the entire deformable region of the sample. Damage occurs in the form of flat voids, the size of which is determined by the spatial position, the level of local deformation, and the nature of the stress strain state of the material. The transition from the stage corresponding to the formation of small-sized voids distributed in the volume to the final destruction of the sample occurs through the appearance of macroscopic cracks when the voids merge in the direction of rolling the plate. When several parallel macrofractures occur away from the mouth of the notch and merge together a stepped (corrugated) surface is formed. Based on the studies a conclusion was made about the necessity of further research of the crystallographic aspects of damage development in magnesium alloy samples at a higher spatial resolution. Visualization of fatigue cracks The method of synchrotron computer laminography can also be used to study fatigue cracks that appear in deformable materials. As an example, the results of studying such defects are given here. These defects occurred in a weld joint obtained by friction stir welding. The workpieces were made from aluminum alloy. Table 3 shows the composition of this alloy [38]. The tests were carried out at station BL19B2 of the SPring-8 source. The materials studied in the work were subjected to low-cycle fatigue loading (3.37×105 and 4.8×104 cycles). The function of stress concentrators contributing to the fatigue cracks occurrence was performed by holes with a diameter of 0.3 mm. Figure 15 shows the dimensions of the analyzed samples. During materials testing by laminography the axes of rotation of the samples were tilted by 30° with respect to the X-ray beam. At the output of the monochromator the X-ray energy was 28 keV. An X-ray detector (cooled CCD camera) recorded projection data every 0.5° (with a total rotation of 360°) with an exposure time of 400 ms per image. The sample was removed every 20 exposures and the response caused by the presence of the polymer was recorded to compensate for the attenuation of X-ray radiation caused by the acrylic tube to which the analyzed object was fixed. The projection size fixed by the detector was 1,984×7,680 pixels (after 2×2 binning). Under these conditions there was a compromise between image resolution and simplicity of data processing. The effective size of the detector pixel was 5.7 µm (after binning). The size of the field of view was 11.3 mm (horizontal) × 4.4 mm (vertical). The X-ray beam was parallel at the length between the rotating magnet and the sample (52 m). The distance between the sample and the detector (0.8 m) ensured the manifestation of the phase contrast effect. The fracture images (Fig. 16) were reconstructed using the filtered back projection Ta b l e 3 Composition of aluminum alloy (wt. %) Si Fe Cu Mn Mg Cr Zn Ti Al 0.65 0.2 0.30 0.06 1.04 0.13 0.04 0.02 Rest
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