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Fluid flow within the dendritic structure at the solid–liquid interface in nickel-based superalloys has been studied in two directionally solidified alloy systems. Millimeter-scale, three-dimensional (3D) datasets of dendritic structure have been collected by serial sectioning, and the reconstructed mushy zones have been used as domains for fluid-flow modeling. Flow permeability and the influence of dendritic structure on flow patterns have been investigated. Permeability analyses indicate that the cross flow normal to the withdrawal direction limits the development of flow instabilities. Local Rayleigh numbers calculated using the permeabilities extracted from the 3D dataset are higher than predicted by conventional empirical calculations in the regions of the mushy zone that are prone to the onset of convective instabilities. The ability to measure dendrite surface area in 3D volumes permit improved prediction of permeability as well.

The crystallographic textures produced during additive manufacturing can be understood, predicted, and manipulated by varying the grain nucleation and growth processes. The resultant textures are primarily dictated by the melt pool geometry, which defines the local thermal gradient and thus the preferred crystal growth directions, as well as the scan strategy, which controls the propagation of grain orientations into subsequent layers. This texture can be diluted through heterogeneous nucleation of new grain orientations, which can occur through a variety of mechanisms. This ability to control the texture during additive manufacturing can enable the location-specific control of properties as a function of position in the build.

As an example of data fusion in the context of 3D characterization of materials, this article demonstrates the procedures necessary to align and fuse separate imaging modes, traditional backscattered electron imaging (BSE) and electron backscattered diffraction mapping (EBSD), from serial-sectioning data. The fused data form a unified 3D reconstruction of additively manufactured 316L stainless steel processed by laser powder-bed fusion. We show that, by combining the relatively low-information yet high-fidelity BSE image stack with the more data-rich yet spatially distorted EBSD maps, the 3D reconstruction can leverage the strengths of both imaging techniques. The fully automated alignment procedures and frameworks rely on a number of optimized image warping techniques, with the result that spatial alignment errors are on the order of 0–3  μ m within a region of interest that is > 1  mm.

This paper provides a systematic examination of microstructure and corrosion behavior of additively manufactured 316L stainless steel after post-processing by surface finishing, isothermal heat treating, or hot isostatic pressing. The effects of isothermal heat treatments from 500 °C to 1300 °C and hot isostatic press processing from 1000 °C to 1200 °C were correlated to the evolution of microstructure, pore morphology and volume fraction, microhardness measurements, and corrosion behavior to reveal the different post-processing temperature regimes and their corresponding characteristics. In particular, this study found that the AM 316L microstructures and properties are stable up to nearly 800 °C. Higher temperatures eliminate the fine solidification structure, causing a drop in microhardness and corrosion resistance. Corrosion was primarily driven by the porosity content of the exposed surface and near-surface regions, which is unaffected by isothermal or HIP post-processing, and the corrosion was not significantly affected by recrystallization, which begins around 1050 °C and is nearly complete by 1200 °C. The amount of porosity was not markedly affected by isothermal post-processing, but could be significantly reduced by HIP processing.

While there are many microstructural parameters that can be measured from a planar two-dimensional (2-D) section through a material, there are many measurements that require knowledge of the full three-dimensional (3-D) microstructure, such as true size and shape of individual objects, connectivity and interfacial curvatures. Serial sectioning and reconstruction can reveal the 3-D microstructure but are often considered to be time consuming and labor intensive. However, what is not often realized is that the majority of the time invested in serial sectioning is spent in the image segmentation, wherein individual objects are digitally identified. This article reviews the current state of image segmentation and novel analysis within 3-D materials science. We will also briefly discuss the future possibilities for more efficient segmentation of digital images for a broader range of materials.

This study reveals the material flow that occurs as a function of position across a friction stir weld through an analysis of the deviations between the observed orientations of the shear deformation texture as compared to a model of idealized, geometrically predicted texture orientations at each location. Friction stir welding is a shear-dominated process, and the observed shear textures can indicate the local orientation of material flow during the welding process. Deviations of the observed texture from the geometrically predicted texture orientations reveal previously unreported variations in the actual material flow that occurred during welding as a function of position across the weld. The three-dimensional material flow revealed by these texture orientations shows important variations, particularly on the advancing side of the weld, that can provide critical experimental validation for friction stir welding models.Graphical Abstract

Electron backscatter diffraction (EBSD) has become established as a convenient and accurate method for obtaining texture information. In friction stir welding, however, the complex, three-dimensional curvature of the deposited shear layers causes the textures to vary in orientation across the weld nugget. Only rarely are the EBSD data acquired in the shear deformation frame of reference. Thus, an analysis of those shear textures needs to take into consideration the local orientation of the shear deformation reference frame at the location of the analysis to appropriately identify the resultant texture. This article presents a systematic methodology for the analysis of friction stir weld textures that uses geometry-based rotations to align the analysis orientation to the local shear deformation frame of reference and thereby enable an accurate identification of the textures produced during the friction stir welding process.