Far-Field High-Energy Diffraction Microscopy
Figure 1 : Illustrations of the FF-HEDM setup at the APS 1-ID beamline. The subscript S refers to the translations and rotations in the sample coordinate system. The subscript L refers to the translations and rotations in the laboratory coordinate system. (b) Panoramic view of the FF-HEDM setup at the APS 1-ID-E beamline.
polycrystalline materials. Penetration depth through materials can be on the order of millimeters for most engineering alloys and materials, which is orders of magnitude larger than most laboratory X-ray sources. Small diff raction angles combined with the use of area detectors, strategically placed at various distances downstream from the sample (transmission geometry), allow us to characterize features in a material at various length scales from nanometers (using small- or wide-angle X-ray scattering techniques) to µm (using HEDM). Furthermore, when combined with high-energy X-ray tomography, a wide range of features at various length scales in a material can be characterized. With recent advances in focusing optics suitable for high-energy X-rays, ~1–10 µm spot size is routinely achievable at a wide range of energies [ 13 – 14 ], and sub-µm spot size also has been achieved [ 15 ]. T e spot size is anticipated to improve signifi cantly as emittance of the electron beam in the storage ring is reduced with the introduction of fourth-generation synchrotron sources. [ 16 ]. Finally, vacuum environments, used to minimize air scattering of electron beams and necessary for experiments with low-energy X-rays, are not necessary. T is means that a variety of in situ or in operando environments [ 9 – 12 , 17 ] can be developed with relative ease. Experimental setup . Our setup uses high-energy X-rays in transmission geometry to interrogate a polycrystalline aggregate of material. T e sample is rotated about an axis with respect
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Figure 2 : Example of a FF-HEDM diffraction pattern taken at a wavelength of 0.017286 nm (71.671 keV) using a well-annealed steel sample with face-centered- cubic (fcc) crystal symmetry. The sample-to-detector distance was 755 mm. (a) Example of summed area detector diffraction pattern images taken between -180 and -120 at ω = 0.25 intervals for one illuminated volume. The fi gure only shows a quadrant. Each ring on the image corresponds to a diffracting family of crystallographic planes. Information within individual frames is typically very sparse. (b) Magnifi ed view of a typical frame of an FF-HEDM image set. Green curves illustrate the theoretical location of the Debye-Scherrer rings. Strains in the crystal grains caused the diffraction spots to deviate from the theoretical Debye-Scherrer ring position. The integrated intensity of a diffraction spot is related to the volume of the diffracting grain.
www.microscopy-today.com • 2017 September
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