Farangis Ram

Error analysis of the crystal orientations and disorientations obtained by the classical electron backscatter diffraction technique

The fidelity – that is, the error, precision and accuracy – of the crystallographic orientations and disorientations obtained by the classical two-dimensional Hough-transform-based analysis of electron backscatter diffraction patterns (EBSPs) is studied. Using EBSPs simulated based on the dynamical electron diffraction theory, the fidelity analysis that has been previously performed using the patterns simulated based on the theory of kinematic electron diffraction is improved. Using the same patterns, the efficacy of a Fisher-distribution-based analytical accuracy measure for orientation and disorientation is verified.

Kikuchi bandlet method for the accurate deconvolution and localization of Kikuchi bands in Kikuchi diffraction patterns

In order to retrieve crystallographic information from an electron backscatter Kikuchi diffraction pattern, its Kikuchi bands have to be localized. One of the main reasons for the limited precision of the present Kikuchi band localization methods is that the diffuse edges of a Kikuchi band are convoluted by many other Kikuchi bands that intersect them. To improve the localization accuracy, Kikuchi bands have to be deconvoluted. In this article, a new method for the deconvolution and localization of Kikuchi bands is presented. The deconvolution is based on the fact that, in a Kikuchi pattern, there are a number of Kikuchi bands that are not parallel to the bands that intersect them. It is performed in Fourier space. After deconvolution, localization is carried out by a quantitative shape analysis of the intensity profiles of the deconvoluted Kikuchi bands. Using the introduced method, for a real electron backscatter Kikuchi diffraction pattern with 45° half capture angle and 0.12° per pixel maximum scale factor, the characteristic hyperbolic features of the Kikuchi bands can be localized with a precision of better than 0.1° in reflection angle.

EMsoft: open source software for electron diffraction/image simulations

EMsoft is an open source package for the simulation of electron diffraction patterns and images for both SEM and TEM modalities. The package has been under development since the late 1990’s and is currently available as version 3.1 from a GitHub repository [1]. The main component of the package is a source code library of functions and subroutines that cover crystallography, symmetry, electron scattering factors, geometry, rotation representations, basic image filters, defect displacement fields, reciprocal space sampling schemes, and so on. The code is mostly written in fortran-90, with occasional routines making use of the C-bindings that are available in fortran-2003. A second library covers routines used to read and write files in the open source HDF5 (Hierarchical Data Format) file format [2], which is the default format for all computational results. At the time of writing of this abstract, more than 200, 000 lines of source code have been written, about half of which are available via the source code repository.

Application of forward models to crystal orientation refinement

Two approaches are proposed for the refinement of electron diffraction pattern indexing. The approaches require two basic ingredients: an accurate physicsbased forward model and an algorithm to search the local orientation neighborhood. Forward models for electron backscatter diffraction (EBSD) and electron channeling pattern (ECP) modalities are coupled with either a multi-resolution brute-force search algorithm or a bound optimization by quadratic approximation algorithm. The efficacy of the methods is evaluated for varying levels of error in the pattern projection center. The EBSD modality shows an orientation improvement when the projection center error is within 1% of the full detector width, whereas the ECP modality shows improvement up to a 5% error. The algorithms are applied to an experimental EBSD scan for partially recrystallized 90/10 brass; the results show that the refinement is necessary to remove the artifacts introduced by the discrete sampling nature of the dictionary indexing method. Finally, a pattern center correction factor is derived for orientations obtained from dictionary indexing for large-area EBSD scans.