High-energy Diffraction Microscopy Research Articles

Evaluating the grain-scale deformation behavior of a single-phase FCC high entropy alloy using synchrotron high energy diffraction microscopy

Although the deformation behavior of high-entropy alloys (HEAs) has been extensively studied at the macroscale, many important properties have yet to be explored for these alloys at the microscale, thus hampering accurate prediction of damage and failure processes. Synchrotron high-energy diffraction microscopy (HEDM) and fast-Fourier transform-based crystal plasticity modeling was conducted to investigate the three-dimensional (3D) grain-resolved micromechanical response for approximately 1,900 constituent grains within a single-phase FCC HEA up to 1% applied strain. The evolution of grain-resolved elastic strains, lattice reorientations, and maximum resolved shear stresses (mRSS) were evaluated to quantify elastic, yield, and fully plastic behavior. Overall, the initial critical resolved shear stress (CRSS), determined via in situ HEDM and companion modeling, was found to be > 20% higher than estimated using the classical polycrystalline Taylor factor (M = 3.06). However, a descriptive parameter based on the average grain-resolved Taylor factor (M¯) was found to show excellent agreement with plastic yielding behavior observed within HEDM datasets. Noticeable deviations in HEDM lattice reorientations compared to both EVP-FFT simulations and classical predictions for FCC polycrystals were discovered, highlighting the complexity in correlating local lattice reorientations, Taylor, and Schmid factors with plastic response for this material at the grain-scale. Therefore, it is anticipated that the overall trends and parameter identification of 3D grain-resolved properties in this study can serve as an important foundation for continued mesoscale investigation on both well-established and newly developed Cantor-like HEAs.

AFRL Additive Manufacturing Modeling Series: Challenge 4, In Situ Mechanical Test of an IN625 Sample with Concurrent High-Energy Diffraction Microscopy Characterization

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.

AFRL Additive Manufacturing Modeling Series: Challenge 4, 3D Reconstruction of an IN625 High-Energy Diffraction Microscopy Sample Using Multi-modal Serial Sectioning

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.

Realization of low-power and high mobility thin film transistors based on MoS2 layers grown by PLD technique

Molybdenum disulfide (MoS2) layers have been successfully grown and optimized for the realization of low power thin film transistors (TFTs) using catalysis free, economical and environment-friendly pulsed laser deposition (PLD) technique on comparably large sized passivated silicon (SiO2/Si) substrate of area 1.2 cm×1.0 cm. Growth of MoS2 monolayer is confirmed by reflection high-energy electron diffraction (RHEED), Raman spectroscopy, photoluminescence (PL) analysis and atomic force microscopy (AFM). The effect of planar electrodes of different metal system (Au/Ag, Pt/Ti, Al, Ni) on the electrical characteristics of MoS2 layers have been inspected in detail. Au/Ag bilayer system proved to be the best electrode for efficient charge injection into the MoS2 layers. A Back-gated field effect transistor (FET) was fabricated with Au/Ag was used as source-drain contacts and MoS2 as the semiconducting channel has been successfully developed. The fabricated transistor shows an accumulation electron mobility around 44.6 cm2 V-1s−1, current ION/OFF ratio of 1.4×105, subthreshold slope of nearly 360 mV/dec, interface trap density of 1.18×1012 /cm2eV and robust current saturation at lower Vds, suggesting that fabricated devices can be used in making low power switching devices.

EXTENDED DEFECTS IN GaAs/Ge/GaAs HETEROSTRUCTURES WITH TURNING GaAs LAYERS

The GaAs layer turn on Ge at a right angle in the substrate plane was studied by high-energy electron diffraction and transmission electron microscopy methods when growing GaAs/Ge/GaAs heterostructures using individual molecular-beam epitaxy systems for GaAs and Ge, with sample transfer through atmosphere.

Multiscale modeling of cruciform dwell tests with the uncertainty-quantified parametrically homogenized constitutive model

This paper presents a novel multiscale approach for analyzing multi-axial stress-strain evolution in Ti-7Al cruciform specimens under dwell loading, through the use of an uncertainty-quantified, parametrically homogenized constitutive model (UQ-PHCM). The thermodynamically-consistent UQ-PHCM is built from rigorous upscaling of crystal plasticity FE models (CPFEM) using machine learning and uncertainty quantification. They explicitly incorporate microstructural information in the form of representative aggregated microstructural parameters (RAMPs). Uncertainty quantification accounts for uncertainty in model reduction, data sparsity and microstructural descriptors. This paper integrates advanced multiscale, multi-axial experiments with computational modeling at multiple scales to establish the UQ-PHCM as an effective tool for bridging the gap between laboratory specimen-scale experimental observations and micro-scale stresses and strains using CPFEM. The CPFEM is calibrated and validated by experimental data from surface strain measurements using digital image correlation (DIC) and grain-by-grain lattice strain measurements with in-situ far field high energy diffraction microscopy (ff-HEDM). A computational method is developed in CPFEM, to incorporate initial residual stresses consistent with measured lattice strains. The UQ-PHCM is validated with biaxial tensile dwell test results performed on the cruciform specimen with satisfactory prediction of gauge strain evolution in DIC measurements. Uncertainty in the strain field due to microstructural variability is also corroborated by the DIC measurements.

Void coalescence and ductile failure in IN718 investigated via high-energy synchrotron X-ray tomography and diffraction

Ductile failure through the growth and coalescence of voids is of particular relevance for many engineering materials. Yet, the lack of experimental measurements of the mechanical state of the material at an appropriate length scale has limited further understanding of the regime of ductile failure. In this study, local grain-scale experimental measurements are used to determine their relevance for describing and predicting ductile failure. Additive manufactured materials, due to the presence of inherent porosity, as well as the ability to tailor internal porosity, provides a promising avenue to study ductile failure. Selective laser melting is used to manufacture a specially designed specimen with two large, internal voids, in addition to the natural porosity, which is characteristic to the production process. The initial porosity and its evolution upon tensile loading are characterized via micro-tomography. Several locations of void coalescence are captured in the sample, revealing activity of multiple modes of failure. Finite element simulations, with a simplified J2 plasticity model, instantiated with the initial void structure, is deployed, in which geometric localizations of stress concentration corresponded with experimentally observed sites of coalescence but was inadequate in capturing the appropriate failure mechanism. From the experimental results of the far-field high-energy diffraction microscopy, the heterogeneous micromechanical state is identified and tracked around voids due to the local grain interactions. These experiments determine narrow bands of low stress triaxiality, at the onset of failure, which highlight the path of coalescence through intervoid shearing. Diffraction spot spreading analysis aided characterization of intragranular plasticity and strain heterogeneity, which can be coupled with high triaxiality to form the conditions inducive for coalescence through intervoid necking. In this study, the experimentally determined grain-scale description of the micromechanical state provides a physical basis that can accurately capture both the sites and the mechanism of void coalescence at the onset of ductile failure.

High-Energy Coherent X-Ray Diffraction Microscopy of Polycrystal Grains: Steps Toward a Multiscale Approach

We present proof-of-concept imaging measurements of a polycrystalline material that integrate the elements of conventional high-energy X-ray diffraction microscopy with coherent diffraction imaging techniques, and that can enable in-situ strain-sensitive imaging of lattice structure in ensembles of deeply embedded crystals over five decades of length scale upon full realization. Such multi-scale imaging capabilities are critical to addressing important questions in a variety of research areas such as materials science and engineering, chemistry, and solid state physics. Towards this eventual goal, the following key aspects are demonstrated: 1) high-energy Bragg coherent diffraction imaging (HE-BCDI) of sub-micron-scale crystallites at 52 keV at current third-generation synchrotron light sources, 2) HE-BCDI performed in conjunction with far-field high-energy diffraction microscopy (ff-HEDM) on the grains of a polycrystalline sample in an smoothly integrated manner, and 3) the orientation information of an ensemble of grains obtained via ff-HEDM used to perform complementary HE-BCDI on multiple Bragg reflections of a single targeted grain. These steps lay the foundation for integration of HE-BCDI, which typically provides a spatial resolution tens of nanometers, into a broad suite of well-established HEDM methods, extending HEDM beyond the few-micrometer resolution bound and into the nanoscale, and positioning the approach to take full advantage of the orders-of-magnitude improvement of X-ray coherence expected at fourth generation light sources presently being built and commissioned worldwide.

Voxel-based strain tensors from near-field High Energy Diffraction Microscopy

High energy X-ray diffraction microscopy is one of several techniques that use spatially resolved diffraction to nondestructively map crystal unit cell orientations in three dimensions inside bulk polycrystalline samples. The near-field variant of the technique (nf-HEDM) yields grain shapes, grain boundary character information, and intra-granular orientation variations. Here, we demonstrate, through analysis of simulated and physical data sets, the extension nf-HEDM to include mapping of heterogeneous intra-granular strain states with spatial resolution on the micron scale. Sensitivity to all strain tensor components is <5×10-4. The GPU assisted implementation of an optimization algorithm is described.

X-ray orientation microscopy using topo-tomography and multi-mode diffraction contrast tomography

Polycrystal orientation mapping techniques based on full-field acquisition schemes like X-ray Diffraction Contrast Tomography and certain other variants of 3D X-ray Diffraction or near-field High Energy Diffraction Microscopy enable time efficient mapping of 3D grain microstructures. The spatial resolution obtained with this class of monochromatic beam X-ray diffraction imaging approaches remains typically below the ultimate spatial resolution achievable with X-ray imaging detectors. Introducing a generalised reconstruction framework enabling the combination of acquisitions with different detector pixel size and sample tilt settings provide a pathway towards 3D orientation mapping with a spatial resolution approaching the one of state of the art X-ray imaging detector systems.

High-Energy X-Ray Diffraction Microscopy in Materials Science

High-energy diffraction microscopy (HEDM) is an implementation of three-dimensional X-ray diffraction microscopy. HEDM yields maps of internal crystal orientation fields, strain states, grain shapes and locations as well as intragranular orientation distributions, and grain boundary character. Because it is nondestructive in hard materials, notably metals and ceramics, HEDM has been used to study responses of these materials to external fields including high temperature and mechanical loading. Currently available sources and detectors lead to a spatial resolution of ∼1 μm and an orientation resolution of &lt;0.1○. With the penetration characteristic of high energies ( E ≥ 50 keV), sample cross-section dimensions of ∼1 mm can be studied in materials containing elements across much of the Periodic Table. This review describes hardware and software associated with HEDM as well as examples of applications. These applications include studies of grain growth, recrystallization, texture development, orientation gradients, deformation twinning, annealing twinning, plastic deformation, and additive manufacturing. We also describe relationships to other X-ray-based methods as well as prospects for further development.

Partially embedded Pb chains on a vicinal Si(113) surface

Partially embedded Pb chains are synthesized on a vicinal Si(113) surface in a process of self-organization. The deposition of one monolayer-range amount of Pb orders the substrate over the macroscopic scale and results in extremely long monatomic chains reproducing Si lattice geometry, as demonstrated by reflection high-energy electron diffraction and scanning tunneling microscopy. Angle-resolved photoelectron spectroscopy reveals well-developed one-dimensional metallic bands. Ab initio density functional theory calculations fully support the experimental findings and predict partially embedded zigzag Pb chains with substantially spin-orbit-split one-dimensional electron bands. The present work is experimental evidence of a new type of one-dimensional structures in which one of two chains of atoms building the structure is embedded in the substrate.

MBE Growth and Characterization of Strained HgTe (111) Films on CdTe/GaAs**Supported by the National Natural Science Foundation of China (Grant Nos. 11634009, 61874069, 1177041280 and 11574336), and Shanghai Science and Technology Foundation (Grant No.18JC1420401).

Strained HgTe thin films are typical three-dimensional topological insulator materials. Most works have focused on HgTe (100) films due to the topological properties resulting from uniaxial strain. In this study, strained HgTe (111) thin films are grown on GaAs (100) substrates with CdTe (111) buffer layers using molecular beam epitaxy (MBE). The optimal growth conditions for HgTe films are determined to be a growth temperature of 160°C and an Hg/Te flux ratio of 200. The strains of HgTe films with different thicknesses are investigated by high-resolution x-ray diffraction, including reciprocal space mapping measurements. The critical thickness of HgTe (111) film on CdTe/GaAs is estimated to be approximately 284 nm by Matthews’ equations, consistent with the experimental results. Reflection high-energy electron diffraction and high-resolution transmission electron microscopy investigations indicate that high-quality HgTe films are obtained. This exploration of the MBE growth of HgTe (111) films provides valuable information for further studies of HgTe-based topological insulators.

Accuracy and precision of near-field high-energy diffraction microscopy forward-model-based microstructure reconstructions

The accuracy of the near-field high-energy diffraction microscopy (nf-HEDM) technique is evaluated by directly comparing an nf-HEDM reconstructed microstructure with an electron backscatter diffraction (EBSD) characterization of the same microstructure. A high-purity gold oligocrystal was chosen for characterization in order to facilitate direct one-to-one comparison between the reconstructions given by each technique. By using the comparatively high spatial resolution of the EBSD reconstruction as the ground truth for the grain-boundary network's morphology, it is determined that nf-HEDM locates internal grain boundaries with an accuracy on average better than the resolution of the imaging detector used or within the reconstruction voxel size, whichever is larger. By taking the intragranular misorientation in well ordered grains as a proxy for orientation resolution, it is determined that standard data collection procedures determine crystallographic orientations to better than 0.1°. The effects of various modified data collection procedures are also examined.

Micromechanical Response of Crystalline Phases in Alternate Cementitious Materials using 3-Dimensional X-ray Techniques

Cementitious materials are complex composites that exhibit significant spatial heterogeneity in their chemical composition and micromechanical response. Modern 3-dimensional characterization techniques using X-rays from synchrotron light sources, such as micro-computed tomography (μCT) and far-field high-energy diffraction microscopy (ff-HEDM), are now capable of probing this micromechanical heterogeneity. In this work, the above mentioned techniques are used to understand the varying micromechanical response of crystalline phases (cubic iron oxide and α-quartz) inherently present within an alkali-activated fly ash (AAF) during in-situ confined compression. A subset of the crystals probed using ff-HEDM are registered with the tomographic reconstructions and tracked through the applied loads, highlighting the combination of μCT and ff-HEDM as a means to examine both elastic strain in the crystalline particles (and by extension local stress response) and plastic strain in the matrix. In this study, significant differences in the load carrying behaviors of the crystalline phases were observed wherein the cubic iron oxide crystals laterally expanded during the confined compression test, while the α-quartz particles laterally contracted and at the final load step, shed load likely due to failure in the surrounding matrix. Finally, the two characterization techniques are discussed in terms of both advantages and associated challenges for analysis of multi-phase cementitious materials.

Exploring new links between crystal plasticity models and high-energy X-ray diffraction microscopy

The advancement and transition of computational materials models requires corresponding developments in experimental methods to provide new phenomenological insights as well as quantification of model performance. In this article, we explore the natural links between crystal plasticity modeling and a suite of high-energy X-ray diffraction experimental techniques referred to as high-energy diffraction microscopy (HEDM). HEDM methods provide information such as the elastic strain state of individual grains and the distribution of crystallographic orientations. As a non-destructive characterization method, pairing HEDM with in situ mechanical testing enables tracking of material state evolution through quantitative measurements that may be directly compared to a simulation. We highlight some recent successes in this area and provide suggestions for future research.

A Planar Biaxial Experiment Platform for In Situ High-Energy Diffraction Studies

An experimental platform for multiscale studies of materials subjected to plane stress loads is presented. Coupling with far-field high-energy diffraction microscopy for grain-by-grain measurements of elastic strains and rotations provides an additional benefit; it enables the direct assessment of elastic vs. inelastic deformation of the gauge sections of cruciform specimens subjected to plane stress loadings, without any a priori assumptions of the form of constitutive relationships, resolving a long-outstanding challenge of multiaxial mechanical testing. Specifically, a planar biaxial mechanical load frame with four independent hydraulic actuators capable of applying arbitrary loading paths and ratios of tension and compression was designed and built for in situ diffraction experimentation. The load frame is integrated for use at the Argonne National Laboratory Advanced Photon Source (APS) synchrotron, Sector 1, 1-ID-E endstation and the Los Alamos Neutron Science Center (LANSCE) spallation neutron source, Spectrometer for Materials Research at Temperature and Stress (SMARTS) instrument. Cruciform specimen geometries were designed to experience loading ratios in the gauges commensurate with those applied at the grips, and to minimize interference with diffracted X-rays and neutrons. The finite element models used to design the cruciform specimen geometries were experimentally validated using stereo digital image correlation measurements. This complete planar biaxial in situ diffraction platform provides a new capability for studying multiaxial micromechanics of crystalline materials (e.g., elastic, slip, twinning, phase transformation) and their dependencies on grain size, location, texture, and neighborhood characteristics.

Three-dimensional observations of grain volume changes during annealing of polycrystalline Ni

The orientations, locations, and sizes of approximately 2500 grains in a Ni polycrystal were measured at six points in time during an interrupted annealing experiment by synchrotron x-ray based, near field high-energy diffraction microscopy. The volume changes were compared to the geometric characteristics of the grains in both the original microstructures and microstructures in which adjacent twin related domains were merged. Neither the size of a grain nor the number of its nearest neighbors correlates strongly to a grain's volume change over the time scale of this experiment. However, the difference between the number of neighbors a grain has, F, and the average number of neighbors of the neighboring grains have, FNN, is correlated to the volume change. A grain with more (fewer) neighbors than FNN usually grows (shrinks). The correlation between the volume change and F - FNN is obvious only if adjacent twin related domains are merged. The correct sign of the volume change is predicted by F - FNN about two thirds of the time. The results show that the volume change for a given grain is better predicted by comparing a grain's characteristics to its neighbor's characteristics than to the characteristics of the entire ensemble of grains.

Three-dimensional in situ characterization of phase transformation induced austenite grain refinement in nickel-titanium

Near-field and far-field high-energy diffraction microscopy and microcomputed tomography X-ray techniques were used to study a bulk single crystal nickel‑titanium shape memory alloy sample subjected to thermal cycling under a constant applied load. Three-dimensional in situ reconstructions of the austenite microstructure are presented, including the structure and distribution of emergent grain boundaries. After 1 cycle, the subgrain structure is significantly refined, and heterogeneous Σ3 and Σ9 grain boundaries emerge. The low volume and uneven dispersion of the emergent Σ boundaries across the volume show why previous transmission electron microscopy investigations of Σ grain boundary formation were inconsistent.

Intermittent plasticity in individual grains: A study using high energy x-ray diffraction.

Long-standing evidence suggests that plasticity in metals may proceed in an intermittent fashion. While the documentation of intermittency in plastically deforming materials has been achieved in several experimental settings, efforts to draw connections from dislocation motion and structure development to stress relaxation have been limited, especially in the bulk of deforming polycrystals. This work uses high energy x-ray diffraction measurements to build these links by characterizing plastic deformation events inside individual deforming grains in both the titanium alloy, Ti-7Al, and the magnesium alloy, AZ31. This analysis is performed by combining macroscopic stress relaxation data, complete grain stress states found using far-field high energy diffraction microscopy, and rapid x-ray diffraction spot measurements made using a Mixed-Mode Pixel Array Detector. Changes in the dislocation content within the deforming grains are monitored using the evolution of the full 3-D shapes of the diffraction spot intensity distributions in reciprocal space. The results for the Ti-7Al alloy show the presence of large stress fluctuations in contrast to AZ31, which shows a lesser degree of intermittent plastic flow.