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Far-Field High-Energy Diffraction Microscopy


completeness and total volume recovered. Post-irradiation annealing removes much of the irradiation-induced damage. T is is indicated by an average completeness of 0.89 and a total reconstructed volume of over 90% in the IRR + ANN sample, values that are close to those measured in the AR sample. T e average grain size of the IRR + ANN sample is 40 μ m, similar to those measured in the other two samples.


Individual grains in an aggregate can be examined in more detail and grains in various material states can be compared. Figure 4b shows a series of diff raction spots from an interior grain in each sample. T eir radii were all approximately 30 μ m, and they all had completeness of 1.0. Compared to the grain from the AR sample, the grain from the IRR sample shows signifi cantly broadened diff raction spots in both the radial direction and the azimuthal direction, indicating that irradiation introduces various types of damage. Furthermore, this indicates that neutron irradiation introduces damage rather uniformly in the material as opposed to the selective smearing seen in crystals that have undergone deformation [ 32 ]. T e spots sharpen aſt er annealing, providing solid evidence that post-irradiation annealing recovers the crystal structure of the grains.


Validation of crystal plasticity


models . Decreasing the need for extensive physical experiments to test and certify new materials is a major goal in materials design and engineering (for example, the aforementioned ICME and MGI eff orts). One key component in these eff orts is being able to validate computer models with experimental data obtained at appropriate length scales. An example of how FF-HEDM can aid this eff ort is provided by Wong et al. [ 23 ] who combined FF-HEDM with in situ thermo- mechanical loading to critically test a crystal plasticity model implemented using a fi nite element formulation. In this work, a uniaxial tension sample made from a high-strength copper alloy (Cu-Cr-Zn) with a 2 mm × 2 mm cross section was subjected to monotonic uniaxial tension. T e material was composed of equiaxed grains approximately 150 µm in diameter. At pre-defi ned macroscopic loads along the uniaxial stress-strain curve, the loading was paused, and FF-HEDM measurements were performed to track the evolution of orientation and stress on a grain- by-grain basis. Figure 5a shows the


44


macroscopic stress strain curve and locations where FF-HEDM measurements were conducted. T e FF-HEDM experiments were conducted at 80.73 keV with a 2 mm × 0.1 mm beam to illuminate a volume of material in the gauge section. T en the sample was translated along Y L to increase the volume of material interrogated. From these FF-HEDM experiments, the crystallographic orientations and the lattice strain tensors of the constituent crystals were obtained. Using these crystallographic orientations, a virtual polycrystal was constructed as shown in Figure 5b . T e virtual polycrystal was used to test the crystal plasticity model. Modelling details are presented in [ 33 ]. T e validation of the model is shown in Figure 6 , which shows the stress and its evolution for a grain determined from FF-HEDM experiments. It also shows the stress and its


Figure 7 : Comparison between (a) the diffraction spots measured using FF-HEDM and corresponding synthetic diffraction spots from fi nite element simulation for (b) grain A and (c) grain B in the polcrystalline aggregate. Here, η is the azimuthal angle where the spot is recorded on the detector. Figure replicated from [ 23 ] with permission of Cambridge University Press.


www.microscopy-today.com • 2017 September


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