The far-field high-energy diffraction microscopy technique is presented in the context of high-energy synchrotron x-ray diffraction. For each grain in an illuminated polycrystalline volume, the volume-averaged lattice orientations, lattice strain tensors, and centre-of-mass (COM) coordinates may be determined to a high degree of precision: better than 0.05°, 1 × 10−4, and 0.1 pixel, respectively. Because the full lattice strain tensors are available, corresponding mean stress tensors may be calculated unambiguously using single-crystal elastic moduli. A novel formulation for orientation indexing and cell refinement is introduced and demonstrated using two examples: first, sequential indexing and lattice refinement of a single-crystal ruby standard with known COM coordinates; and second, indexing and refinement of simulated diffraction data from an aggregate of 819 individual grains using several sample rotation ranges and including the influence of experimental uncertainties. The speed of acquisition and penetration depth achievable with high-energy (that is, >50 keV) x-rays make this technique ideal for studies of strain/stress evolution in situ, as well as for residual stress analysis.

Articles you may be interested inTwo-dimensional direct numerical simulation evaluation of the flame-surface density model for flames developing from an ignition kernel in lean methane/air mixtures under engine conditions Phys. Fluids 24, 105108 (2012); 10.1063/1.4757655Shear-strain induced decomposition of 1,1-diamino-2,2-dinitroethylene Appl.A numerical model is developed to study the shock wave ignition of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) crystal. The model accounts for the coupling between crystal thermal/ mechanical responses and chemical reactions that are driven by the temperature field. This allows for the direct numerical simulation of decomposition reactions in the hot spots formed by mechanical loading. The model is used to simulate intragranular pore collapse under shock wave loading. In a reference case: (i) shear-enabled micro-jetting is responsible for a modest extent of reaction in the pore collapse region, and (ii) shear banding is found to be an important mode of localization. The shear bands, which are filled with molten HMX, grow out of the pore collapse region and serve as potential ignition sites. The model predictions of shear banding and reactivity are found to be quite sensitive to the respective flow strengths of the solid and liquid phases. In this regard, it is shown that reasonable assumptions of liquid-HMX viscosity can lead to chemical reactions within the shear bands on a nanosecond time scale. V C 2015 AIP Publishing LLC.

We present a multiscale strength model in which strength depends on pressure, strain rate, temperature, and evolving dislocation density. Model construction employs an information passing paradigm to span from the atomistic level to the continuum level. Simulation methods in the overall hierarchy include density functional theory, molecular statics, molecular dynamics, dislocation dynamics, and continuum based approaches. Given the nature of the subcontinuum simulations upon which the strength model is based, the model is particularly appropriate to strain rates in excess of 104 s−1. Strength model parameters are obtained entirely from the hierarchy of simulation methods to obtain a full strength model in a range of loading conditions that so far has been inaccessible to direct measurement of material strength. Model predictions compare favorably with relevant high energy density physics (HEDP) experiments that have bearing on material strength. The model is used to provide insight into HEDP experimental observations and to make predictions of what might be observable using dynamic x-ray diffraction based experimental methods.

This work examines the use of crystal based continuum mechanics in the context of dynamic loading. In particular, we examine model forms and simulations which are relevant to pore collapse in crystalline energetic materials. Strain localization and the associated generation of heat are important for the initiation of chemical reactions in this context. The crystal mechanics based model serves as a convenient testbed for the interactions among wave motion, slip kinetics, defect generation kinetics and physical length scale. After calibration to available molecular dynamics and single crystal gas gun data for HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine), the model is used to predict behaviors for the collapse of pores under various conditions. Implications for experimental observations are discussed.

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.