A Comparison of Solidification Structures and Submicroscale Cellular Segregation in Rapidly Solidified Stainless Steels Produced via Two-Piston Splat Quenching and Laser Powder Bed Fusion.

Fusion-based additive manufacturing techniques leverage rapid solidification (RS) conditions to create parts with complex geometries, unique microscale/nanoscale morphological features, and elemental segregation. Three custom composition stainless steel alloys with varying chromium equivalence to nickel equivalence ratio (Creq/Nieq) between 1.53 and 1.95 were processed using laser powder bed fusion (LPBF) and/or two-piston splat quenching (SQ) to produce solidification rates estimated between 0.4 and 0.8 m/s. Both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were utilized to collect high-resolution images, electron backscatter diffraction (EBSD) phase identification, and measure cellular segregation. Similar features were observed in both LPBF and SQ samples including phase and microstructure, nanoscale oxide particles, cell size, and segregation behavior. However, dislocation pileup was observed along the cell boundaries only in the LPBF austenite solidified microstructure. Targeted adjustment of the SQ feedstock Cr and Ni concentrations, within the ASTM A240 specification for 316L resulted in no observable impact on the cell size, oxide particle size, or magnitude of segregation. Also, the amount of Ni segregation in the ferrite solidified microstructures did not significantly differ, regardless of Cr/Nieq or processing technique. SQ is demonstrated as capable of simulating RS rates and microstructures similar to LPBF for use as an alternative screening tool for new RS alloy compositions.

Energy-filtered electron backscatter diffraction.

The electron backscatter diffraction (EBSD) analytical technique is invaluable for determining the crystallography of bulk alloys, thin films, and nanoparticles. However, our physical understanding of EBSD pattern generation is incomplete, which hinders our ability to push the limits of EBSD analysis. Here, using an energy filter with better than 10 eV resolution, we experimentally demonstrate the energy dependence of EBSD patterns from elements over a large atomic number range. We verify that low-loss electrons are the major contributors to EBSD patterns, but that there is still a diffraction contribution from electrons with only 80% of the incident beam energy. Additionally, the bands in filtered EBSD patterns have contrast that is more than twice the contrast of their unfiltered counterparts. The band contrast reaches a maximum for a cutoff energy in the filter of about 3% below the energy of the incident beam. Different mechanisms are used to explain the drop in contrast on each side of the maximum. With the cutoff set very close to the energy of the incident beam, the patterns become more blurred. We used a Monte Carlo simulation in the analysis of these experiments.