Laue diffraction pattern9/17/2023 ![]() The presence of Laue oscillations is frequently used as evidence of “the high crystallinity of samples”, “the uniformity of the film and smoothness of the interfaces”, or that “the out-of-plane order is high and coherent over the entire film thickness”. Generally, the presence of Laue oscillations are taken as confirmation that grown films are of high quality, homogenous, contain only the targeted compound, and have smooth and planar top and bottom interfaces, ,, ,, ,,. First predicted by Max von Laue, the Laue interference function relates the number of unit cells in the diffracting crystal to the distribution of diffracted intensity. Typical applications are given.Laue oscillations result from the incomplete destructive interference of a finite number of unit cells and occur when a sample consists of domains with the same number of unit cells across most of the area being probed. Texture components of interest can be highlighted by distinct colours in order to reveal textural inhomogeneities. For a quantitative representation of the spatial distribution of microscale texture, an orientation-to-colour key is used by colour shading inverse pole figures, sections through Euler space or sections through Rodrigues orientation space. Crystal lattice orientation maps (COM) are constructed by assigning to each grid point in the image a colour specific for the grain orientation, the misorientation or the grain boundary character. The automated measurement of lattice orientations takes less than half a second per image point on a Pentium Pro 200 MHz PC. Digital beam scan requires, as a consequence of the high tilt of the specimen with respect to the beam, current calculation of pattern centre and diffraction length (autocalibration) and dynamic focusing by the software. The interactive and automated indexing of patterns and the fully Automated Crystal lattice Orientation Mapping (ACOM) with a computer-controlled SEM are presented in detail. The formation of backscatter Kikuchi patterns and the experimental setup for their acquisition are briefly outlined. Their relation to microstructural features gives insight into many aspects of anisotropic materials properties. A backscatter Kikuchi diffraction attachment to an SEM enables the convenient investigation of individual grain orientations on bulk surfaces. The methodology developed is illustrated using EBSD orientation data obtained from the fatigue crack-tips/wakes in aerospace aluminum alloys 2024-T3-T7451. If one also considers the lattice curvature tensor, five components of the dislocation density tensor-corresponding to a part of the GND content-may be inferred. An advantage of this approach is demonstrated for direct determination of the representative orientation of a grain within an EBSD map by mean, median, or quaternion-based averaging methods that can be further used within analyses or visualization of misorientation or geometrically necessary dislocation (GND) density. Using crystal symmetry operators and the lowest Euclidian-distance criterion, the orientation of each pixel within a grain is redefined. Keywords: micro-Laue indexing template matching misorientation fatigue geometrically necessary dislocations.Ī simple algorithm is developed and implemented to eliminate ambiguities, in both statistical analyses of orientation data (e.g., orientation averaging) and electron backscattered diffraction (EBSD) orientation map visualization, caused by symmetrically equivalent orientations and the wrap-around or umklapp effect. ![]() The methodologies developed are illustrated using micro-Laue diffraction data obtained from the wake of a fatigue crack. The misorientation within a diffracting volume is calculated from `smeared' and/or `split' Laue patterns. A best fit orientation can then be determined by a least-squares fitting approach based on singular value decomposition. The advantages of template matching include (i) elimination of the requirement for extensive peak search/fitting analysis (ii) the ability to index overlapped diffraction patterns obtained from neighboring grains or second phase particles and (iii) the ability to confidently index patterns of low quality. The orientation of a diffracting volume is determined by maximizing a normalized cross correlation index between experimental and theoretical patterns. Experimental diffraction patterns are compared with simulated templates corresponding to crystals of prescribed orientations. A Laue X-ray diffraction pattern indexing scheme, similar to a method previously proposed for convergent beam and backscattered electron diffraction patterns, was implemented. ![]()
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