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Far-Field High-Energy Diffraction Microscopy


evolution for two grains located in the virtual sample with identical crystallographic orientation as the grain determined from FF-HEDM experiments. Two virtual grains were used to examine the eff ect of the neighborhood on the crystal stresses. T e experimental stresses were computed from the measured lattice strain tensor and anisotropic Hooke’s law, and these were compared to the simulation stresses. Figure 6 shows that the grain is in a multi-axial stress state even though the applied load on the polycrystalline aggregate was uniaxial. Furthermore, the two virtual grains show signifi cant diff erences (xz component of stress for example) indicating that the neighborhood plays a signifi cant role in determining the stress state of a crystal in an aggregate.


Figure 7 shows the evolution of diff raction spots associated with the grains shown in Figure 6 . Similar to the diff raction spots shown in Figure 4b , the experimental diff raction spots for the copper grain also show signifi cant changes. However, unlike the diff raction spots shown in Figure 4b , some diff raction spots smear signifi cantly with applied load, while others do not. T is indicates that smearing from deformation is perhaps more dependent on the crystallographic orientation of the crystal, whereas neutron irradiation and its damage may be less sensitive to the crystallographic orientation. Simulation results also show similar trends to those seen in the experimentally measured diff raction spots with subtle diff erences indicating the eff ect of neighborhood.


Conclusion


HEDM is a unique probe enabling scientists and engineers to view features in materials that are invisible to the naked eye and to traditional microscopy methods. In its near-fi eld confi g- uration, a 3D crystallographic orientation map can be obtained. In the far-fi eld confi guration, the center of mass, crystallo- graphic orientation, and lattice strain tensor of individual grains in a polycrystalline aggregate can be mapped. T e evolution of crystallographic orientation and strain can be tracked on a grain-by-grain basis while the aggregate is subject to a known stimuli. T e formation of substructures, phase transforma- tions, or the introduction of damage from external stimuli can be examined on a grain-by-grain basis through analyzing the evolution of diff raction spots.


In this article, the FF-HEDM technique and its implemen- tation at the APS 1-ID beamline have been described. Two FF-HEDM examples illustrate how this technique can contribute to our understanding of structure-property-processing relationship and provide data for validating complex material models.


Acknowledgments T is research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Offi ce of Science User Facility operated for the DOE Offi ce of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. Work by Zhang et al. [ 19 ] was supported by the U.S. Department of Energy, Office of Nuclear Energy, for the Nuclear Energy Enabling Technology (NEET) Program under Contract DE-AC02-06CH11357. Work by Wong et al. [ 23 ] was provided by the US Department of Energy, Offi ce of Basic Energy Sciences, Materials Sciences and Engineering Division under Grant No. DE-FG02-10ER46758.


2017 September • www.microscopy-today.com


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