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Far-Field High-Energy Diffraction Microscopy

Application Examples of FF-HEDM Data Characterization of neutron-irradiated steel . While

Figure 6 : Stress components of a grain determined from FF-HEDM and simulations. The error bars on the experimental stresses are deduced from experimental strain uncertainty. The error bar on the simulation stresses were obtained by calculating the standard deviation of the stress for all elements that belong to the grain in the virtual polycrystal. Figure replicated from [ 23 ] with permission of Cambridge University Press.

user community, have recently developed a package titled Microstructural Imaging using Diff raction Analysis Soſt ware (MIDAS) that takes advantage of high-performance computing [ 26 – 27 ] so that experimenters can quickly map 3D volumes, sometimes rapidly enough to steer in situ experiments (for example, to optimize the amount of thermo-mechanical load to apply prior to or just aſt er crack initiation). Figure 3 shows some examples of microstructure information obtained from analyzing the raw FF-HEDM data. In this fi gure, the grain COMs are illustrated by the location of the spheres in space. T e colors of the spheres denote information related to the grains. Experimental uncertainty . In a typical confi guration, an illuminated volume that consists of approximately 1,000 grains with a uniform size distribution, uniform crystallographic texture, and moderate levels of plastic deformation can be reliably characterized with relative ease. As the data analysis procedure involves identifying and isolating the diff raction spots, a larger number of grains in the aggregate or strong crystal- lographic texture increase the probability of spot overlap. T e dynamic range of the area detector limits the grain size distri- bution since the integrated intensity of a diff raction spot from a grain is proportional its volume. High levels of deformation typically generate substructures and mosaicity, which cause the diff raction spots to smear, and smeared diff raction spots are more diffi cult to parameterize with XL, YL, and ω . In our typical setup, the spatial and orientation uncertainties are on the order of 10 µm and 0.1 , respectively. T e strain uncertainty is on the order of 0.0001. T e performance of the method is highly dependent on setup and the state of the sample, and these values are provided only as guidance. If the state of the illuminated volume deviates from ideal conditions, these uncertainties will suff er and the analysis can become more challenging.

2017 September •

the eff ects of neutron irradiation on structural steels used in nuclear energy applications have been studied extensively using conventional microscopy techniques, grain-level investigations within bulk samples have been, until recently, impossible because of the diffi culties in handling activated materials. Zhang et al. [ 28 ] investigated the eff ect of neutron irradiation on the microstructure of a high-temperature ultrafi ne-precipitated-strengthened (HT-UPS) austenitic stainless steel ex situ. T is HT-UPS austenitic stainless steel is a candidate for structural and cladding applications in the next-generation nuclear reactors because of its improved creep resistance and potentially better radiation resistance over the traditional Type 316 stainless steels [ 29 – 31 ]. In the work by Zhang et al. [ 28 ], eff ects of neutron irradiation and post-irradiation annealing on grain-scale microstructure of a solution annealed HT-UPS steel were investigated using FF-HEDM technique at the APS 1-ID-E beamline. T ree disk-shaped samples were investigated: unirradiated as-received (AR) steel, neutron irradiated to 3 displace- ments per atom (dpa) at 500 o C (IRR), and the same irradiated condition but subsequently annealed at 600 o C for 1 hour (IRR+ANN). T e FF-HEDM experiments were conducted using X-ray energy of 65.35 keV.

T e beam size was 2 mm (along X L ) × 0.2 mm (along Y L ) to fully illuminate the sample cross section. A set of FF-HEDM diff raction patterns were acquired in sweeping mode with 0.5  increments in ω over the range of -180  to +180  . Aſt er acquiring a set of diff raction patterns for a layer illuminated by the beam, the sample was translated along Y S by 0.2 mm to have another layer measured. For each sample, four layers were measured in total, yielding a total interrogated volume of approximately 0.3 mm 3 . T e total interrogated volume contained several hundreds of grains suitable for statistical analysis. Analysis of the FF-HEDM data was carried out using MIDAS soſt ware and diff raction spots from the {111} and {200} lattice planes. Figure 4a shows the FF-HEDM maps of the three samples. Each grain is represented by a sphere. T e size of a sphere is proportional to the volume of the corresponding grain. Locations of the spheres correspond to the COMs of the grains in the respective samples. T e color of the sphere corresponds to the completeness of the grain. Completeness is defi ned as the ratio of the number of measured diff raction spots during an FF-HEDM experiment and the number of expected diff raction spots for a particular grain. Completeness of 1.0 indicates that all the diff raction spots are correctly found in the measured data. T is metric is used as a measure of how confi dent we are with the identifi ed grain [ 24 ]. In total, 943 grains were found for the AR sample with an average grain radius of 44 μ m and an average completeness of 0.90, contributing to over 90% of the total volume. In contrast, 843 grains were detected in the IRR sample with an average radius of 41 μ m and an average completeness of 0.84, making up to only 75% of the total volume. T is comparison indicates that while the neutron irradiation does not change the grain size signifi cantly, it introduces damage to the material as indicated by the decrease in both


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