The response of polycrystals to plastic deformation is studied at the level of variations within individual grains, and comparisons are made to theoretical calculations using crystal plasticity (CP). We provide a brief overview of CP and a review of the literature, which is dominated by surface observations. The motivating question asks how well CP represents the mesoscale behavior of large populations of dislocations (as carriers of plastic strain). The literature shows consistently that only moderate agreement is found between experiment and calculation. We supplement this with a current example of microstructure evolution in the interior of a copper sample subjected to tensile deformation. Nondestructive measurements of orientation fields were performed using the near-field highenergy X-ray diffraction microscopy (nf-HEDM) technique at the Advanced Photon Source (APS). Starting at highly ordered grains, a single two-dimensional slice of microstructure containing ∼150 grains was followed through multiple strain states, where it tracked lattice rotations and defect accumulation of up to 14% elongation. In accord with the literature, at the scale of individual grains, comparison of observations with CP models indicates reasonable qualitative agreement but significant variations between simulation and experiment are apparent. The conclusion is that in order to be able to quantify the effects of microstructure on the distributions of slip, orientation change, and damage accumulation, the empirically derived constitutive relations used in continuumscale simulations need to be improved. Equally important will be the development of large-scale simulations of polycrystals that directly model dislocations. 317Annu. Rev. Condens. Matter Phys. 2014.5:317-346. Downloaded from www.annualreviews.org by University of Texas -San Antonio on 08/26/14. For personal use only.

How would you… …describe the overall significance of this paper? This paper describes emerging characterization experiments referred to as High Energy Diffraction Microscopy conducted at the Advanced Photon Source (APS) beam line 1-ID-C. "Near field" diffraction is used to quantify three-dimensional orientation maps of polycrystalline samples non-destructively, with incredible detail grain boundary geometry. "Far field" experiments are used to quantify lattice strains and single crystal stress states within large aggregates subjected to in situ loading. …describe this work to a materials science and engineering professional with no experience in your technical specialty? Materials derive their mechanical properties from their internal structure. As engineering moves downscale, it becomes more important to quantify the structure and mechanical response of engineering materials on small size scales. High energy x-ray diffraction methods are rapidly evolving into important microscale characterization tools that can be used together with high fidelity mechanical models. …describe this work to a layperson? Micro-and nano-engineering methods hold enormous promise for a broad spectrum of products and processes. The determination of material attributes and mechanical properties on small size scales is one of the main barriers to moving down scale. Instead of making tiny specimens, we examine deforming test samples using high energy x-rays, created using a special national laboratory facility. This work will enable us to precisely reconstruct the internal structure of engineering alloys and will provide important mechanical data on the micron scale. The status of the High Energy Diffraction Microscopy (HEDM) program at the 1-ID beam line of the Advanced Photon Source is reported. HEDM applies high energy synchrotron radiation for the grain and sub-grain scale structural and mechanical characterization of polycrystalline bulk materials in situ during thermomechanical loading. Case studies demonstrate the mapping of grain boundary topology, the evaluation of stress tensors of individual grains during tensile deformation and comparison to a finite element modeling simulation, and the characterization of evolving dislocation structure. Complementary information is obtained by post mortem electron microscopy on the same sample volume previously investigated by HEDM.

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We present a non-destructive in situ measurement of three-dimensional (3D) microstructure evolution of 99.995% pure polycrystalline copper during tensile loading using synchrotron radiation. Spatially resolved three-dimensional crystallographic orientation fields are reconstructed from the measured diffraction data obtained from a near-field high-energy X-ray diffraction microscopy (nf-HEDM), and the evolution of about 5000 3D bulk grains is tracked through multiple stages of deformation. Spatially resolved observation of macroscopic texture change, anisotropic deformation development, and the correspondence of different crystallographic parameters to defect accumulation are illustrated. Moreover, correlations between different crystallographic parameters, such as crystal rotation evolution, short-and long-range orientation gradient development, microstructural features, and grain size effects are investigated. The current state of data mining tools available to analyze large and complicated diffraction data is presented and challenges associated with extracting meaningful information from these datasets are discussed.

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.