Jonathan Lind

Three-dimensional plastic response in polycrystalline copper via near-field high-energy X-ray diffraction microscopy

The evolution of the crystallographic orientation field in a polycrystalline sample of copper is mapped in three dimensions as tensile strain is applied. Using forward-modeling analysis of high-energy X-ray diffraction microscopy data collected at the Advanced Photon Source, the ability to track intragranular orientation variations is demonstrated on an ∼2 µm length scale with ∼0.1° orientation precision. Lattice rotations within grains are tracked between states with ∼1° precision. Detailed analysis is presented for a sample cross section before and after ∼6% strain. The voxel-based (0.625 µm triangular mesh) reconstructed structure is used to calculate kernel-averaged misorientation maps, which exhibit complex patterns. Simulated scattering from the reconstructed orientation field is shown to reproduce complex scattering patterns generated by the defected microstructure. Spatial variation of a goodness-of-fit or confidence metric associated with the optimized orientation field indicates regions of relatively high or low orientational disorder. An alignment procedure is used to match sample cross sections in the different strain states. The data and analysis methods point toward the ability to perform detailed comparisons between polycrystal plasticity computational model predictions and experimental observations of macroscopic volumes of material.

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.

Implementation and verification of a microstructure-based capability for modeling microcrack nucleation in LSHR at room temperature

A microstructure-based capability for forecasting microcrack nucleation in the nickel-based superalloy LSHR is proposed, implemented, and partially verified. Specifically, gradient crystal plasticity is applied to finite-element models of the experimentally measured, 3D microstructure wherein a microcrack is known to have nucleated along a coherent Σ3 boundary. The framework is used to analyze this particular nucleation event and conduct an extensive grain boundary analysis study, the results of which underpin the importance that elastic anisotropy and coherency have in the localization of plastic slip.

High-Energy Diffraction Microscopy Characterization of Spall Damage

The emerging characterization technique of high-energy diffraction microscopy (HEDM) was used to investigate ductile dynamic damage evolution in a Cu polycrystal. Experimental efforts were undertaken with the goal of elucidating correlations between microstructural features with preferred damage nucleation sites and the progression of damage at the localization stage. HEDM was used to microstructurally map the initial volume of a 1.2 mm-diameter Cu sample. HEDM in the near-field mode collects diffraction information from high-energy synchrotron radiation to non-destructively probe microstructure and orientation in three dimensions in volumes approaching the bulk scale. The Cu sample was subsequently planar shock-loaded in a plate-on-plate geometry and soft-recovered, using an assembly specially developed for sub-size samples. The ex situ shocked sample was then re-characterized by HEDM, providing data on the location of incipient spall voids with respect to the local microstructural neighborhood. In addition, diffraction quality and misorientation gradient data provide qualitative measures of the spatial distribution of stored work and indicate regions of plastic localization. This provides the potential for unprecedented insight as to the relative preference of spall nucleation sites and correlations between microstructure, damage, and plastic flow.

Tests of microstructure reconstruction by forward modeling of high energy X-ray diffraction microscopy data

Verification tests of the forward modeling technique for near-field high energy X-ray diffraction microscopy are conducted using two simulated microstructures containing uniformly distributed orientations. Comparison between the simulated and reconstructed microstructures is examined with consideration to both crystallographic orientation and spatial geometric accuracy. To probe the dependence of results on experimental parameters, simulated data sets use two different detector configurations and different simulated experimental protocols; in each case, the parameters mimic the experimental geometry used at Advanced Photon Source beamline 1-ID. Results indicate that element orientations are distinguishable to less than 0.1°, while spatial geometric accuracy is limited by the detector resolution.

Using high energy diffraction microscopy to assess a model for microstructural sensitivity in spall response

We present results from a modeling effort that employs detailed non-destructive three-dimensional microstructure data obtained from X-ray based High Energy Diffraction Microscopy (HEDM) experiments. The emphasis is on validating models that capture microstructural sensitivities so that these models can then be employed in rapid certification procedures. By focusing validation efforts on models that connect directly to experimentally measurable features of the microstructure, we can then build confidence in use of the models for components prepared under different processing routes, with different chemical compositions and attendant impurity distributions, or subjected to different loading conditions. The computational model makes use of a crystal mechanics based constitutive model that includes porosity evolution. The formulation includes nucleation behavior that is fully integrated into a robust numerical procedure, enhancing capabilities for modeling small length scales at which nucleation site potency and volume fraction are more variable. Three-dimensional experimental data are available both pre-shot and post-shot from the same volume of impact-loaded copper. Crystal lattice orientation and porosity data are obtained, respectively, from near-field HEDM and tomography techniques. The availability of such data serves as a primary motivation for the model effort at the microstructural scale.

Quantifying Damage Accumulation Using State-of-the-Art FFT Method

A 3D microstructure, measured by high-energy x-ray diffraction microscopy, is used as an input to a parallelized viscoplastic Fast Fourier Transform code (VPFFT) to simulate a tensile test. Distributions of strain, damage accumulation, neighbor interactions, and Schmid factor mismatch throughout the microstructure are calculated. These results will form the basis of a direct comparison to microstructure maps that track plastic deformation in the real sample.

Shock induced damage in copper: A before and after, three-dimensional study

We report on the microstructural features associated with the formation of incipient spall and damage in a fully recrystallized, high purity copper sample. Before and after ballistic shock loading, approximately 0.8 mm3 of the samples crystal lattice orientation field is mapped using non-destructive near-field High Energy Diffraction Microscopy. Absorption contrast tomography is used to imagevoids after loading. This non-destructive interrogation of damage initiation allows for novel characterization of spall points vis-a-vis microstructural features and a fully 3D examination of microstructural topology and its influence on incipient damage. The spalled region is registered with and mapped back onto the pre-shock orientation field. As expected, the great majority of voids occur at grain boundaries and higher order microstructural features; however, we find no statistical preference for particular grain boundary types. The damaged region contains a large volume of Σ–3 (60°〈111〉) connected domains with a large area fraction of incoherent Σ-3 boundaries.

In Situ Observation of Recovery and Grain Growth in High Purity Aluminum

We have used high energy x-ray diffraction microscopy (HEDM) to study annealing behavior in high purity aluminum. In-situ measurements were carried out at Sector 1 of the Advanced Photon Source. The microstructure in a small sub-volume of a 1 mm diameter wire was mapped in the as-received state and after two differential anneals. Forward modeling analysis reveals three dimensional grain structures and internal orientation distributions inside grains. The analysis demonstrates increased ordering with annealing as well as persistent low angle internal boundaries. Grains that grow from disordered regions are resolution limited single crystals. Together with this recovery behavior, we observe subtle motions of some grain boundaries due to annealing.

Crystallographic character of grain boundaries resistant to hydrogen-assisted fracture in Ni-base alloy 725

Hydrogen embrittlement (HE) causes sudden, costly failures of metal components across a wide range of industries. Yet, despite over a century of research, the physical mechanisms of HE are too poorly understood to predict HE-induced failures with confidence. We use non-destructive, synchrotron-based techniques to investigate the relationship between the crystallographic character of grain boundaries and their susceptibility to hydrogen-assisted fracture in a nickel superalloy. Our data lead us to identify a class of grain boundaries with striking resistance to hydrogen-assisted crack propagation: boundaries with low-index planes (BLIPs). BLIPs are boundaries where at least one of the neighboring grains has a low Miller index facet—{001}, {011}, or {111}—along the grain boundary plane. These boundaries deflect propagating cracks, toughening the material and improving its HE resistance. Our finding paves the way to improved predictions of HE based on the density and distribution of BLIPs in metal microstructures.Exactly how hydrogen renders metals brittle is still unclear, and it remains a challenge to predict component failure due to hydrogen embrittlement. Here, the authors identify a class of grain boundaries in a nickel superalloy that deflects propagating cracks and improves alloy resistance to hydrogen.