Microstructure and Solute Segregation around the Melt-Pool Boundary of Orientation-Controlled 316L Austenitic Stainless Steel Produced by Laser Powder Bed Fusion

For this article, we studied the microstructure and solute segregation seen around the melt pool boundary of orientation-controlled 316L austenitic stainless steel produced by laser powder bed fusion, using transmission electron microscopy and energy-dispersive x-ray spectroscopy. We found that the solidification cellular microstructures could be visualized with the aid of solute segregation (Cr and Mo) during solidification. Mn-Si-O inclusions (10-15 nm in diameter) were distributed along the lamellar boundaries, as well as in the dislocation cell walls. It is believed that the grain growth of the inclusions can be effectively suppressed by rapid quenching during the laser powder-bed fusion process. A thin region without cellular microstructures was observed at the melt-pool boundary. The cellular spacing widened near the bottom of the melt-pool boundary, owing to the decrease in the cooling rate. Atomic-structure analysis at the lamellar boundary by high-resolution transmission electron microscopy revealed a local interfacial structure, which is complementary to the results of electron back-scatter diffraction.

Gamma-irradiation-induced micro-structural variations in flame-retardant polyurethane foam using synchrotron X-ray micro-tomography

Flame-retardant polyurethane foams are potential packing materials for the transport casks of highly active nuclear materials for shock absorption and insulation purposes. Exposure of high doses of gamma radiation causes cross-linking and chain sectioning of macromolecules in this polymer foam, which leads to reorganization of their cellular microstructure and thereby variations in physico-mechanical properties. In this study, in-house-developed flame-retardant rigid polyurethane foam samples were exposed to gamma irradiation doses in the 0-20 kGy range and synchrotron radiation X-ray micro-computed tomography (SR-µCT) imaging was employed for the analysis of radiation-induced morphological variations in their cellular microstructure. Qualitative and quantitative analysis of SR-µCT images has revealed significant variations in the average cell size, shape, wall thickness, orientations and spatial anisotropy of the cellular microstructure in polyurethane foam.

3D imaging of a rice pollen grain using transmission X-ray microscopy

For the first time, the three-dimensional (3D) ultrastructure of an intact rice pollen cell has been obtained using a full-field transmission hard X-ray microscope operated in Zernike phase contrast mode. After reconstruction and segmentation from a series of projection images, complete 3D structural information of a 35 µm rice pollen grain is presented at a resolution of ∼100 nm. The reconstruction allows a clear differentiation of various subcellular structures within the rice pollen grain, including aperture, lipid body, mitochondrion, nucleus and vacuole. Furthermore, quantitative information was obtained about the distribution of cytoplasmic organelles and the volume percentage of each kind of organelle. These results demonstrate that transmission X-ray microscopy can be quite powerful for non-destructive investigation of 3D structures of whole eukaryotic cells.