Jay C. Schuren

A rotational and axial motion system load frame insert for in situ high energy x-ray studies

High energy x-ray characterization methods hold great potential for gaining insight into the behavior of materials and providing comparison datasets for the validation and development of mesoscale modeling tools. A suite of techniques have been developed by the x-ray community for characterizing the 3D structure and micromechanical state of polycrystalline materials; however, combining these techniques with in situ mechanical testing under well characterized and controlled boundary conditions has been challenging due to experimental design requirements, which demand new high-precision hardware as well as access to high-energy x-ray beamlines. We describe the design and performance of a load frame insert with a rotational and axial motion system that has been developed to meet these requirements. An example dataset from a deforming titanium alloy demonstrates the new capability.

Quantifying the uncertainty of synchrotron-based lattice strain measurements

Crystallographic lattice strains – measured using diffraction techniques – are the same magnitude as typical macroscopic elastic strains. From a research perspective, the main interest is in measuring changes in lattice strains induced during in-situ loading: either from one macroscopic stress level to another or from one cycle to the next. The hope is to link these measurements to deformation-induced changes in the internal structure of crystals, possibly related to inelastic deformation and damage. These measurements are relatively new – little experimental intuition exists and it is difficult to discern whether observed differences are due to actual micromechanical evolution or to random experimental fluctuations. If the measurements are linked to material evolution on the size scale of the individual crystal, they have the potential to change the ideas about grain scale deformation partitioning processes and can be used to validate crystal-based simulation frameworks. Therefore, understanding the uncertainty associated with the lattice strain experiments is a crucial step in their continued development. If the measured lattice strains are of the same order as the random fluctuations that are part of the measurement process, documenting the strains can create more confusion than understanding. Often lattice strain error is quoted as ±1 × 10−4. This simple value fails to account for the range of factors that contribute to the experimental uncertainty – which, if not properly accounted for, may lead to a false confidence in the measurements. The focus of this paper is the development of a lattice strain uncertainty expression that delineates the contributing factors into terms that vary independently: (i) the contribution from the instrument and (ii) the contribution from the material under investigation. These aspects of uncertainty are described, and it is then possible to employ a calibrant powder method (diffraction from an unstrained material with high-precision lattice constants) to quantify the instrument portion of the lattice strain uncertainty. In these experiments, the instrument contribution to the uncertainty has been found to be a function of the Bragg angle and the intensity of the diffracted peaks. To develop a model for the instrument portion of the lattice strain uncertainty two datasets obtained using a MAR345 online image plate at the Cornell High Energy Synchrotron Source and a GE 41RT amorphous silicon detector at the Advanced Photon Source have been examined.

Experimental measurement of surface strains and local lattice rotations combined with 3D microstructure reconstruction from deformed polycrystalline ensembles at the micro-scale

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.

Crystal Plasticity Model Validation Using Combined High-Energy Diffraction Microscopy Data for a Ti-7Al Specimen

Abstract 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.

High-energy Needs and Capabilities to Study Multiscale Phenomena in Crystalline Materials

High-energy synchrotron X-rays are well suited to study engineering (structural) materials due to their small wavelength, adjustable energy and beam size, high flux, and ability to penetrate bulk polycrystalline samples up to centimeters in thickness. Recent advances in the use of high-speed, high-resolution detectors are making it possible to characterize microstructures at both the single grain and ensemble levels and to characterize the micromechanical responses of polycrystalline aggregates in three dimensions. These capabilities open new avenues of “in-situ” research that augments traditional forensic evidence with real-time data on functioning, evolving systems. X-ray scattering data are extremely rich, but taking the best advantage requires a continued refinement of experimental methods and analysis and a closer coupling of material models to detected intensities.

Integrating experiments and simulations to estimate uncertainty in lattice strain measurements

The investigation of lattice strains has proven to be a viable method for probing the crystal level stress state in deforming polycrystalline samples. Building on the recent availability of high-rate X-ray detectors, we have developed a new technique that combines diffraction data with crystal-based finite element simulations to estimate the lattice strain uncertainty for each measurement. To estimate the uncertainty, we combine the probable number of crystals expected for each measurement with the simulated lattice strain standard deviation. Under this framework, the uncertainty is related not just to the number of diffracting crystals but also to the variability in the lattice strain between these crystals. An estimate of uncertainty for each measurement enables both the investigation of previously inaccessible phenomena where the lattice strain evolution is expectantly small and the application of lattice strain measurement techniques to materials with more complex microstructures. The new approach was demonstrated for an aluminum alloy 7075-T6 sample undergoing uniaxial tensile loading.

Fiducial marker application method for position alignment of in situ multimodal X-ray experiments and reconstructions

An evolving suite of X-ray characterization methods are presently available to the materials community, providing a great opportunity to gain new insight into material behavior and provide critical validation data for materials models. Two critical and related issues are sample repositioning during an in situ experiment and registration of multiple data sets after the experiment. To address these issues, a method is described which utilizes a focused ion-beam scanning electron microscope equipped with a micromanipulator to apply gold fiducial markers to samples for X-ray measurements. The method is demonstrated with a synchrotron X-ray experiment involving in situ loading of a titanium alloy tensile specimen.

An experimental system for high temperature X-ray diffraction studies with in situ mechanical loading

An experimental system with in situ thermomechanical loading has been developed to enable high energy synchrotron x-ray diffraction studies of crystalline materials. The system applies and maintains loads of up to 2250 N in uniaxial tension or compression at a frequency of up to 100 Hz. The furnace heats the specimen uniformly up to a maximum temperature of 1200 °C in a variety of atmospheres (oxidizing, inert, reducing) that, combined with in situ mechanical loading, can be used to mimic processing and operating conditions of engineering components. The loaded specimen is reoriented with respect to the incident beam of x-rays using two rotational axes to increase the number of crystal orientations interrogated. The system was used at the Cornell High Energy Synchrotron Source to conduct experiments on single crystal silicon and polycrystalline Low Solvus High Refractory nickel-based superalloy. The data from these experiments provide new insights into how stresses evolve at the crystal scale during thermomechanical loading and complement the development of high-fidelity material models.

Changing the Paradigm for Engineering Design by Merging High Energy X-ray Data with Materials Modeling

The nature of the application of structural materials demands that their performance be relLDEOH��,W∂ s well known that the behavior of such materials is a product of the microstructure, and that failure initiation sites can often be linked to local microstructural features. Yet modern design and sustainment methodologies for structural materials remain reliant upon continuum-level models and large-scale conventional mechanical testing efforts. This is extremely costly, both in the sense that conventional mechanical test databases are expensive to produce, and also that continuum-level design inherently requires unnecessary conservatism in component lifing schemes as local microstructural effects are ignored. Moving forward, the development and validation of a microstructure-sensitive modeling framework that can accurately predict materials behavior (including variability and uncertainty) would allow the maximization of component capability and life, while reducing cost/time to certify and improve safety. Such a model would also open new possibilities for design of components with graded microstructures, where the microstructure at a specific point in a component is tailored to provide optimized properties for that location. Toward this goal, we have endeavored to combine in-situ mechanical testing with advanced characterization methods, including microstructural characterization of the 3D test volume, in order to provide data which can be input and compared to deformation simulations which explicitly represent the 3D microstructure [1,2]. Such information is critical for the validation and further development of microstructure-sensitive modeling tools. In the present work, we describe in-situ tensile tests on polycrystalline metals during integration of three high energy synchrotron x-ray techniques. These techniques include near field orientation microscopy to map the 3D microstructure [3], absorption micro computed tomography to map the presence of voids and/or cracks [4], and far field lattice strain measurements to monitor the internal stress state of individual grains [5]. These experiments have been conducted at Sector 1 of the Advanced Photon Source at Argonne National Lab. An image of the experimental setup is shown in Figure 1.

Modeling Grain Boundary Interfaces in Pure Nickel

This work presents a three tiered modeling approach to examine grain boundary interfaces in a pure Nickel foil material utilizing a crystal plasticity based finite element model (CPFEM). The goal of this work is to calibrate a modeling approach through comparison to experimental data, and then use the models to gain insight into deformation at grain boundaries in Nickel and Nickel-base superalloy polycrystals. The first study utilizes a multi-crystal micro-tension specimen and simulations to calibrate the CPFEM model and examine the development of “hot-spots” or localized plasticity near the grain boundaries. Some orientation combinations exhibit localized plasticity along the boundary (bad-actor boundaries) while others do not. Insight from the deformation of this model is then used to instantiate simulations of Nickel bi-crystals which exhibit localized plasticity near the boundary. The third study embeds the grain boundary interfaces of interest, as determined from the bi-crystal simulations, into a larger polycrystalline simulation utilizing the same CPFEM framework. Using these interfaces we study deformation at these “characteristic” interfaces when subjected to the generalized loading conditions present in a polycrystalline microstructure.