Improving the Accuracy of Orientation Measurements using EBSD

Abstract

Routine EBSD analysis typically applies a 2D Hough transform technique. This requires collection and background correction of the Kikuchi pattern. A Hough transform is applied to highlight and locate the Kikuchi bands. These bands are revealed as local peaks in the resulting Hough space and their corresponding maxima are found automatically, locating the position and direction of the Kikuchi bands and the corresponding diffraction planes. In this primary band detection routine each maximum is represented by a (rho, theta) position in Hough space. In the subsequent indexing the interplanar angles are matched against a database of known phases, and the phase and orientation of the crystal is determined.Precision and accuracy of this primary band detection is essentially limited by the precision of identifying and locating the Kikuchi bands, which is in turn limited by the Hough transform. Therefore, the precision in the orientation measurement can be improved by increasing the resolution of the Hough transform and the resulting Hough space; however this results in a significant increase in calculation time. In addition, the Hough transform assumes that the Kikuchi bands are straight lines, whereas in reality the Kikuchi bands in a pattern have varying widths and curved (hyperbolic) edges. Therefore the conventional 2D Hough transform method introduces a systematic error which cannot be overcome through increased Hough space resolution.An alternative method [1] describes modifying the Hough transform so that it identifies hyperbolic curves rather than straight lines. This modified Hough transform applies a 3D space where the third dimension is given by the hyperbola variable. This method eliminates the systematic error of the conventional Hough, but it involves a large three dimensional calculation which makes it computationally slow. This paper will present a new analysis routine for EBSPs, which improves the accuracy of band detection and is achievable in real time applications. The method uses the primary band detection as its foundation, and then applies an iterative secondary band detection to improve upon the accuracy of the primary band detection.This accurate determination of band positions enables the calculation of a more accurate orientation matrix and thus improves general EBSD analysis, e.g. ability to distinguish between two phases with a very close crystal structure; or in characterizing low angle boundaries. In addition, this refinement will deliver an accurate solution when the primary band detection is impaired by the presence of excess and deficit lines.The mean angular deviation (MAD) is a measure of the fit between the Kikuchi bands in the measured pattern and the solution. It can be used as figure of merit for the accuracy of band detection and therefore also for phase identification. Many applications of EBSD consider misorientation which is also related to the accuracy and precision of band detection. 724

Deformation of lamellar γ-TiAl below the general yield stress

“…Transmission samples were produced by first dimple grinding electro-discharge 5 machined 3 mm diameter discs with a 1 μm diamond suspension to just 20 μm thick (Dimple Grinder 656, Gatan, USA) and then thinning until perforation in a twinjet electropolisher (Tenupol-5, Struers, Denmark) using a solution of 5% perchloric acid, 35% butanol, 60% methanol by volume held at 32 V and -30 °C. TKD was carried out on a dual beam FIB/SEM (Helios NanoLab, FEI, USA) with an EBSD detector and acquisition software (NordlysNano, Oxford Instruments, UK) employing the refined accuracy routine [20] at 30 kV, 6.4 nA, -20 ° tilt, a working distance of 1.5 mm and a 20 nm stepsize.…”

Fast Granularization of Lath-Like Bainite in FeNiC Alloys During Isothermal Holding at Ms+ 20 K (+20 °C)

“…EBSD data were acquired in 'refined accuracy' mode. [16] This mode improves the angular resolution. Experiments confirmed that low misorientation angles as small as 0.2 deg were reliably detected.…”

Novel remapping approach for HR-EBSD based on demons registration

“…Since EBSD is an orientation measurement technique, improving the angular resolution of the technique is of most interest to researchers. Methods to improve angular resolution have been developed such as the pattern comparison method [2], iterative indexing [3], the 3D-Hough transform [4] and HR-EBSD [5]. In particular, the high resolution (or high angular resolution) electron backscatter diffraction (EBSD) technique is capable of characterizing additional information such as residual stress/strain [6].…”