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Physics-based modeling of metal additive manufacturing (AM) processes is computationally challenging due to the very fine meshing required in both time and space. State-of-the-art numerical models that offer great insight into the process have been developed, but they require powerful computational resources and weeks of processing time. Thus, it is often more time-effective to fabricate multiple builds within the time it takes to complete one simulation prediction, further reinforcing the current trial-and-error approach to optimizing the build parameters. This study presents a simplified approach to the transient thermal modeling of the AM process. The numerical model is designed to run on a moderate laptop or desktop computer, without use of parallel processing. The method described in this study uses a unique approach to node creation which leverages the simplicity of the finite difference method to provide predictions in less time than it takes to build the part. Coarse meshing in both time and space along with simplifying assumptions about the solidification process are used in this numerical approach. Model predictions track well with experimental measurements. This approach is being developed for use in an industrial setting to inform deposition parameters based on a desired thermal profile.

A two-color pyrometer is aligned co-axially to the laser inside a Laser Engineered Net Shaping machine to gather thermal data during a single layer two-pass deposit. Several molten pool metrics are calculated from the pyrometer data, and anomalies in the data are used to flag potential defect locations. The depositions are scanned using x-ray Computed Tomography (XCT) with a 4.13 micron voxel, and void size and location are recorded. XCT void data is then compared to the thermal analysis to determine the efficacy of the anomaly detection technique. In this build, four voids > 40 microns in diameter were found with XCT. Three defects appeared in the interfacial region between the deposit and substrate and produced signatures that were detected by analyzing the in-situ pyrometer data.

The ability of additive manufacturing to directly fabricate complex shapes provides characterization challenges for part qualification. The orientation of the microstructures produced by these processes will change relative to the surface normal of a complex part. In this work, the microscopy and x-ray tomography of an AlSi10Mg alloy hemispherical shell fabricated using powder bed metal additive manufacturing are used to illustrate some of these challenges. The shell was manufactured using an EOS M280 system in combination with EOS-specified powder and process parameters. The layer-by-layer process of building the shell with the powder bed additive manufacturing approach results in a position-dependent microstructure that continuously changes its orientation relative to the shell surface normal. X-ray tomography was utilized to examine the position-dependent size and distribution of porosity and surface roughness in the 98.6% dense part. Optical and electron microscopy were used to identify global and local position-dependent structures, grain morphologies, chemistry, and precipitate sizes and distributions. The rapid solidification processes within the fusion zone (FZ) after the laser transit results in a small dendrite size. Cell spacings taken from the structure in the middle of the FZ were used with published relationships to estimate a cooling rate of ~9 × 105 K/s. Uniformly-distributed, nanoscale Si precipitates were found within the primary α-Al grains. A thin, distinct boundary layer containing larger α-Al grains and extended regions of the nanocrystalline divorced eutectic material surrounds the FZ. Subtle differences in the composition between the latter layer and the interior of the FZ were noted with scanning transmission electron microscopy (STEM) spectral imaging.

We examine recrystallization and grain growth during processing of accumulative roll-bonded (ARB) Cu-Nb and Zr-Nb composites. Throughout the ARB process, from initial millimeter thick layers down to nanometer thick layers, the mechanism for recrystallization and grain growth is the motion of high-angle grain boundaries (HAGBs). However, the driving forces for these phenomena change as the densities of different types of defects evolve during the process. The creation and redistribution of dislocations, grain boundaries, and phase boundaries has significant effects on recrystallization and grain growth and, thus, on microstructural evolution. Both Cu-Nb and Zr-Nb exhibit a distinct transition in recrystallization and growth behavior at around 500-nm average layer thicknesses. For the thicker layered materials, the microstructure evolution during recrystallization and growth is determined by the density and distribution of dislocations and HAGBs. For layers less than 500 nm, the layers are largely one-grain thick and the grains are nearly dislocation free; coarsening of grains within layers at the nanoscale is due to reduction in phase boundary energy.

Metallic-based multilayered nanocomposites are recognized for their increased plastic flow resistance and indentation hardness, increased ductility, improved radiation damage resistance, improved electrical and magnetic properties, and enhanced fatigue failure resistance compared to conventional metallic materials. One of the ways in which these classes of materials are manufactured is through accumulated roll bonding where the material is produced by several rolling and heat-treatment steps during which the layer thickness is reduced through severe plastic deformation. A single rolling pass of the accumulated roll bonding process in which a Cu/Nb-layered composite with an initial average layer thickness of 24 μm subjected to a 50% height reduction is modeled. A single-crystal model based upon thermally activated dislocation motion is used. Nanohardness tests for both the Cu and Nb layers are used to help initialize the model for each of the two materials. Electron backscatter diffraction (EBSD) data of the heat-treated material is used to characterize the initial state of the composite and to produce 40 combined morphological and crystallographic numerical model realizations of the material. The results suggest very good agreement between the predicted and experimental textures for both the materials. Highly oriented microstructure develops during severe plastic rolling deformation of Cu/Nb nanocomposites. The deformation textures significantly deviate from those expected when rolling Cu or Nb alone, and the Cu/Nb interfaces do not correspond to those with the lowest possible formation energies. We study the interfacial stability of specific Cu/Nb bicrystal configurations under rolling conditions using a finite-element crystal plasticity model. Specifically, we examine how slip activity and lattice reorientation are affected by the kinematic constraint imposed by the interface. Our results show that for certain configurations the slip activity and lattice rotation of the individual crystallites display some sensitivity to the kinematic constraint, yet the overall stability of a given bicrystal can be predicted by the stability of the individual single-crystal orientations. Future work will account for the influence of the bimetal interface on the interface stability and development of enhanced properties.

Interfaces, such as grain boundaries, phase boundaries, and surfaces, are important in materials of any microstructural size scale, whether the microstructure is coarse-grained, ultrafine-grained, or nano-grained. In nanostructured materials, however, they dominate material response and as we have seen many times over, can lead to extraordinary and unusual properties that far exceed those of their coarse-grained counterparts. In this article, we focus on bimetal interfaces. To best elucidate interface structure–property–functionality relationships, we focus our studies on simple layered composites composed of an alternating stack of two metals with bimetal interfaces spaced less than 100 nm. We fabricate these nanocomposites by either a bottom–up method (physical vapor deposition) or a top–down method (accumulative roll bonding) to produce two distinct interface types. Atomic-scale differences in interface structure are shown to result in profound effects on bulk-scale properties.

Interfaces and interface/defect interactions increasingly dominate the mechanical response of materials as the dimensions of the grains decrease to the nanoscale. Recently, we reported unusually profuse deformation twinning in Ag-Cu layered eutectic composites with bilayer thicknesses in the submicron regime (~200 nm–400 nm) at room temperature and low strain rates. Using atomistic simulations and dislocation theory, we propose that the Ag-Cu interface facilitated deformation twinning in Cu by permitting the transmission of twinning partials from Ag to Cu. In this way, twins in Ag can provide an ample supply of twinning partials to Cu to support and sustain twin growth in Cu during deformation. Interface-driven twinning as revealed by this study suggests the exciting possibility of altering the roles of dislocation slip and twinning through the design of heterophase interface structure and properties.

Neutrons play a vital role as a powerful tool in basic science and applied research. In this article, the basic properties of neutrons, their generation and detection, as well as some fundamental aspects of neutron instrumentation are introduced. Neutron user facilities, at which the user may obtain more specific information and apply for beam time, are also discussed.

Coherent diffraction imaging (CDI) is an emerging technique that is currently used mainly in the field of physics. An opportunity is present for materials scientists to make use of this technique in order to solve current and future materials-centric problems. In addition to introducing the scope of the section in this article, the readers are also given backgrounds for the authors and how each paper fits together. Finally, a brief review of the inaugural symposium held at the TMS 2013 Annual Meeting is provided along with an invitation for the next symposium scheduled for 2015. Although the concept of CDI has been around since the 1950s, experimental realization of this technique occurred only about 15 years ago. Through a combination of the oversampling method and iterative phase retrieval algorithms, high-resolution images can be obtained through the inversion of diffraction patterns from both crystalline and noncrystalline materials. This allows imaging and characterization through the use of x-rays to occur for a broad range of materials including nanoparticles, strained crystals, biomaterials, and cells. Unique characterization abilities such as threedimensional strain mapping and nondestructive three-dimensional tomographic imaging highlight the emerging importance of CDI in the field of materials science. In this series of papers, current and potential applications of CDI techniques to solve problems in materials science are explored as a way to foster increased interaction between materials scientists and physicists, the predominant developers of CDI. The first paper in this series is titled ‘‘From Grain Boundaries to Single Defects: A Review of Coherent Methods for Materials Imaging in the X-ray Sciences.’’ In this paper, Brian Abbey (La Trobe University, Melbourne, Australia, and Melbourne Centre for Nanofabrication, Melbourne, Australia) provides a broad overview of the various techniques within the field of CDI. Brief descriptions of the governing physics as well as limits of the techniques such as phase contrast tomography, plane wave CDI, ptychographic CDI, diverging beam CDI, reduced coherence CDI, coherent x-ray microbeam diffraction, and Bragg CDI are reported. The author also discusses current and potential uses for each technique within the materials science field. Dr. Abbey, in addition to the previously listed appointments, is also a member of the Micro Materials Characterization Beamline development team for the Australian Synchrotron. The second paper in this series is titled ‘‘X-Ray Coherent Diffraction Imaging of Nanomaterials.’’ Ross Harder (Advanced Photon Source, Argonne, IL, USA) and Ian K. Robinson (University College London, London, England) explore the use of Bragg CDI to image structure in nanosized barium titanate crystals in conjunction with local strain measurements with high sensitivity. Harder manages the 34-ID-C beamline at the Advanced Photon Source and welcomes user proposals that will continue to push the envelope in CDI’s abilities and applications, especially in the field of strain mapping. The final paper in this series is titled ‘‘Studies of Materials at the Nanometer Scale using Coherent X-Ray Diffraction Imaging.’’ Richard Sandberg (Los Alamos National Laboratory, Los Alamos, NM, USA), Zhifeng Huang, Rui Xu, Jose Rodriguez, and John Miao (all from the University of California Los Angeles, Los Angeles, CA, USA) explore the use of CDI in imaging materials as varied as biological cells and GaN quantum dot nanoparticles. The use of CDI is discussed in the context of both beamline-based CDI and table-topsource-based CDI. Sandberg’s primary research focus involves the development of table-top sourJohn Carpenter, chair of and JOM advisor for the Materials Characterization Committee of the TMS Extraction & Processing Division, and Richard Sandberg coordinated the topic Coherent X-ray Diffraction Imaging as a Characterization Tool in this issue. JOM, Vol. 65, No. 9, 2013