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A machine component fabricated by additive manufacturing (AM) of metal powders often possesses steep residual stress gradients with significant magnitudes due to large temperature gradients that transpire within a localized area during fabrication. These processing-induced residual stresses can cause distortion of the part, and if they are of significant magnitude, they could induce cracking of the component, degrade printability, and/or diminish subsequent mechanical performance. The ability to predict these residual stresses imparted by AM is an important step in permeating AM technology for advanced manufacturing. Calibration and validation of AM process models used for prediction are, therefore, a critical step in understanding the origin and mitigating the challenges associated with residual stresses inherent to the AM process. In the present work, the residual strain distributions in components with simple geometries fabricated by a laser powder bed fusion (LBPF) process were characterized non-destructively using energy-dispersive X-ray diffraction in support of the US Air Force Research Laboratory Additive Manufacturing Modeling Challenge Series Groeber et al. (JOM 70:441-448, 2018). The measurement setup and approach are described in detail so that the data can be used as a benchmark to calibrate and validate models for the prediction of macro-scale residual stresses due to the LPBF process.

We describe 3D characterization of an additively manufactured Inconel 625 nickel-base superalloy specimen conducted during a uniaxial tension test using a suite of nondestructive x-ray techniques. High-energy diffraction microscopy in both near- and far-field modalities are employed in situ to track evolution of the material orientation and stress–strain fields at six points during the mechanical test, and these data streams are registered with micro-computed tomography reconstructions which probe the material density. This data volume was matched to a multi-modal serial sectioning characterization of the specimen taken after loading, described in this article’s companion. Twenty-eight grains which were monitored throughout the experiment were selected to form the basis for AFRL AM Modeling Series Challenge 4, Microscale Structure-to-Properties.

High-energy diffraction microscopy (HEDM) in-situ mechanical testing experiments offer unique insight into the evolving deformation state within polycrystalline materials. These experiments rely on a sophisticated analysis of the diffraction data to instantiate a 3D reconstruction of grains and other microstructural features associated with the test volume. For microstructures of engineering alloys that are highly twinned and contain numerous features around the estimated spatial resolution of HEDM reconstructions, the accuracy of the reconstructed microstructure is not known. In this study, we address this uncertainty by characterizing the same HEDM sample volume using destructive serial sectioning (SS) that has higher spatial resolution. The SS experiment was performed on an Inconel 625 alloy sample that had undergone HEDM in-situ mechanical testing to a small amount of plastic strain (~ 0.7%), which was part of the Air Force Research Laboratory Additive Manufacturing (AM) Modeling Series. A custom-built automated multi-modal SS system was used to characterize the entire test volume, with a spatial resolution of approximately 1 µm. Epi-illumination optical microscopy images, backscattered electron images, and electron backscattered diffraction maps were collected on every section. All three data modes were utilized and custom data fusion protocols were developed for 3D reconstruction of the test volume. The grain data were homogenized and downsampled to 2 µm as input for Challenge 4 of the AM Modeling Series, which is available at the Materials Data Facility repository.

The kinetics of the dissolution of the equiaxed alpha phase into the beta matrix in Ti-6Al-4V were established and modeled for both isothermal (constant temperature) and transient/continuous heating conditions. For the isothermal experiments, samples were solution treated at a subtransus temperature to equilibrate the microstructure followed by rapid, direct-resistance heating to a temperature either 25 K or 78 K (25 °C or 78 °C) above the equilibrium beta transus and held for times ranging from 1 to 32 seconds. Dissolution behavior under transient conditions was determined in-situ using an indirect-resistance furnace and X-ray (synchrotron) source; these trials comprised a similar initial subtransus solution heat treatment followed by continuous heating at a constant rate in the range between 15 and 135 K/min (15 and 135 °C/min) to a temperature lying 25 K (25 °C) above the transus. Measurements of the temporal evolution of the volume fraction of alpha were interpreted using numerical simulations based on the Whelan dissolution model modified to treat a distribution of particle sizes and the possible interaction of the concentration gradients developed around adjacent particles; i.e., soft impingement. The isothermal dissolution measurements were bounded by predictions from simulations with and without the soft-impingement assumption. Similar trends were found for continuous-heating behavior. In particular, slow or fast heating-rate observations were replicated by simulation predictions with or without the soft-impingement constraint, respectively.

Ti-7Al is a good model material for mimicking the α phase response of near-α and α+β phases of many widely used titanium-based engineering alloys, including Ti-6Al-4V. In this study, three model structures of Ti-7Al are investigated using atomistic simulations by varying the Ti and Al atom positions within the crystalline lattice. These atomic arrangements are based on transmission electron microscopy observations of short-range order. The elastic constants of the three model structures considered are calculated using molecular dynamics simulations. Resonant ultrasound spectroscopy experiments are conducted to obtain the elastic constants at room temperature and a good agreement is found between the simulation and experimental results, providing confidence that the model structures are reasonable. Additionally, energy barriers for crystalline slip are established for these structures by means of calculating the γ-surfaces for different slip systems. Finally, the positions of Al atoms in regards to solid solution strengthening are studied using density functional theory simulations, which demonstrate a higher energy barrier for slip when the Al solute atom is closer to (or at) the fault plane. These results provide quantitative insights into the deformation mechanisms of this alloy.

High-Energy Diffraction Microscopy (HEDM) is a 3-d X-ray characterization method that is uniquely suited to measuring the evolving micro-mechanical state and microstructure of polycrystalline materials during in situ processing. The near-field and far-field configurations provide complementary information; orientation maps computed from the near-field measurements provide grain morphologies, while the high angular resolution of the far-field measurements provides intergranular strain tensors. The ability to measure these data during deformation in situ makes HEDM an ideal tool for validating micro-mechanical deformation models that make their predictions at the scale of individual grains. Crystal Plasticity Finite Element Models (CPFEM) are one such class of micro-mechanical models. While there have been extensive studies validating homogenized CPFEM response at a macroscopic level, a lack of detailed data measured at the level of the microstructure has hindered more stringent model validation efforts. We utilize an HEDM dataset from an alpha-titanium alloy (Ti-7Al), collected at the Advanced Photon Source, Argonne National Laboratory, under in situ tensile deformation. The initial microstructure of the central slab of the gage section, measured via near-field HEDM, is used to inform a CPFEM model. The predicted intergranular stresses for 39 internal grains are then directly compared to data from 4 far-field measurements taken between ~4 and ~80 pct of the macroscopic yield strength. The evolution of the elastic strain state from the CPFEM model and far-field HEDM measurements up to incipient yield are shown to be in good agreement, while residual stress at the individual grain level is found to influence the intergranular stress state even upon loading. Implications for application of such an integrated computational/experimental approach to phenomena such as fatigue are discussed.

Combined application of a near-field High Energy Diffraction Microscopy measurement of crystal lattice orientation fields and a tomographic measurement of pore distributions in a sintered nickel-based superalloy sample allows pore locations to be correlated with microstructural features. Measurements were carried out at the Advanced Photon Source beamline 1-ID using an X-ray energy of 65 keV for each of the measurement modes. The nickel superalloy sample was prepared in such a way as to generate significant thermally induced porosity. A three-dimensionally resolved orientation map is directly overlaid with the tomographically determined pore map through a careful registration procedure. The data are shown to reliably reproduce the expected correlations between specific microstructural features (triple lines and quadruple nodes) and pore positions. With the statistics afforded by the 3D data set, we conclude that within statistical limits, pore formation does not depend on the relative orientations of the grains. The experimental procedures and analysis tools illustrated are being applied to a variety of materials problems in which local heterogeneities can affect materials properties.

A micro-tensile test system equipped with in situ monitoring of the in-plane displacements of a surface and an electron backscattered diffraction-based serial-sectioning technique were used to study the deformation (up to 2.4 pct axial plastic strain in tension) of a polycrystalline nickel micro-specimen. The experimental data include the global engineering stress-engineering strain curve, the local mesoscopic in-plane displacement and strain fields, the three-dimensional microstructure of the micro-specimen reconstructed after the tensile test, and the kernel-average misorientation distribution. The crystal plasticity finite element method using elasto-viscoplastic constitutive formulations was used to simulate the global and local deformation responses of the micro-specimen. Three different boundary conditions (BCs) were applied in simulation in order to study the effects of the lateral displacement (perpendicular to the loading direction) of the top and bottom faces of the specimen gage section. The simulation results were compared to the experimental results. The comparison between experiment and simulation results is discussed, based upon their implications for understanding the deformation of micro-specimens and the causes associated with uncertainties embedded in both experimental and numerical approaches. Also, the sensitivity of BCs to near-field and far-field responses of the micro-specimen was systematically studied. Results show that the experimental methodology used in the present study allows for limited but meaningful comparisons to crystal plasticity finite element simulations of the micro-specimen under the small plastic deformation.

This article describes a new approach to characterize the deformation response of polycrystalline metals using a combination of novel micro-scale experimental methodologies. An in-situ scanning electron microscope (SEM)-based tension testing system was used to deform micro-scale polycrystalline samples to modest and moderate plastic strains. These tests included measurement of the local displacement field with nm-scale resolution at the sample surface. After testing, focused ion beam serial sectioning experiments that incorporated electron backscatter diffraction mapping were performed to characterize both the internal 3D grain structure and local lattice rotations that developed within the deformed micro-scale test samples. This combination of experiments enables the local surface displacements and internal lattice rotations to be directly correlated with the underlying 3D polycrystalline microstructure, and such information can be used to validate and guide further development of modeling and simulation methods that predict the local plastic deformation response of polycrystalline ensembles.

A custom in situ testing system has been developed to allow the quantitative study of deformation behavior in microsized specimens with compressive or tensile loading, which allows for ready control of the lateral test frame stiffness. Micromechanical tests have been conducted within the chamber of a scanning electron microscope (SEM), which allows concurrent visualization of plastic flow during the mechanical test. Three distinct experiments are presented for single-crystal nickel-base superalloy Rene N5, each carried out under the same loading orientation on samples of similar size. The variation in mechanical response as reflected in the stress–strain curves for these experiments can be understood by correlating specimen morphology change during plastic flow with the test boundary conditions.

Micro-compression tests allow for the direct measurement of stress-strain behavior in volumes of material that have microscale dimensions. Initial studies worldwide have focused on the exploration of size-scale effects, where sample dimensions at the micrometer-and sub-micrometer scale can dramatically affect the fundamental processes of plastic deformation. Importantly, this scale of test volume can be directly modeled using state-of-the-art discrete dislocation simulations, the results of which have been essential to understanding the changes that can occur to dislocation mechanisms within small volumes. This combination of miniaturized testing and modeling that closely mimics these experiments provides a new pathway to characterize plastic fl ow on a highly localized basis.

To better understand the fundamentals of solid lubrication, microstructural analyses on the wear scar surface and contact interface of Mo–W–S–Se composite films produced by pulsed laser deposition were completed. Focused ion beam (FIB), transmission electron microscopy (TEM), and X-ray energy dispersive spectroscopy were employed to study the cross-sectional microstructure and chemistry of wear scars. In particular, a novel microtribometer was built for in situ tribological measurements within a FIB microscope. The sliding tip was welded in contact to the wear scar surface on the film under load by re-deposition of sputtering materials from the FIB cut of the tip. Using this technique, cross-sectional TEM specimens were prepared precisely at the contact point without tip/film separation. Here, the in situ FIB microtribometer is critically important for retaining the microstructure of lubricant films as formed at the sliding contact interface between the tip and film without separation. It provides the unique ability to stop sliding, section the contact, and reveal microstructural changes to that contact without disrupting the sliding interface. The cross-sectional TEM measurements were performed on the sliding contact interface for both the regions in contact and just past contact, and both the reorientation and recrystallization of lubricant films were revealed.