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Wright, Stuart I.

Publications and source records attributed to Wright, Stuart I..

5 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Reflector Selection for the Indexing of Electron Backscatter Diffraction Patterns

Abstract We propose a new methodology for ranking the reflectors used in traditional Hough-based indexing of electron backscatter diffraction (EBSD) patterns. Instead of kinematic X-ray or electron structure factors ( F hkl ) currently utilized, we propose the integrated Kikuchi band intensity parameter ( β hkl ) based on integrated dynamical electron backscatter intensities. The proposed parameter is compared with the traditional kinematical intensity, $I_{hkl}^{{\rm kin}} $ , as well as the average Hough transform peak intensity, $I_{hkl}^{{\rm HSP}} $ and used to index EBSD patterns for a number of different material systems of varying unit cell complexities including nickel, silicon, rutile, and forsterite. For elemental structures, β hkl closely follows the kinematical ranking. However, significant ranking differences arise for more complex unit cells, with the β hkl parameter showing a better correlation with the integrated Hough intensities. Lastly, Hough-based indexing of a simulated forsterite data set showed an appreciable improvement in the median confidence index (0.15 to 0.35) when β hkl is used instead of $I_{hkl}^{{\rm kin}} $ for ranking the reflectors.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Improved twin detection via tracking of individual Kikuchi band intensity of EBSD patterns

Twin detection via EBSD can be particularly challenging due to the fine scale of certain twin types – for example, compression and double twins in Mg. Even when a grid of sufficient resolution is chosen to ensure scan points within the twins, the interaction volume of the electron beam often encapsulates a region that contains both the parent grain and the twin, confusing the twin identification process. The degradation of the EBSD pattern results in a lower image quality metric, which has long been used to imply potential twins. However, not all bands within the pattern are degraded equally. This paper exploits the fact that parent and twin lattices share common planes that lead to the quality of the associated bands not degrading; i.e. common planes that exist in both grains lead to bands of consistent intensity for scan points adjacent to twin boundaries. Hence, twin boundaries in a microstructure can be recognized, even when they are associated with thin twins. Proof of concept was performed on known twins in Inconel 600, Tantalum, and Magnesium AZ31. This method was then used to search for undetected twins in a Mg AZ31 structure, revealing nearly double the number of twins compared with those initially detected by standard procedures.

47 OTHER INSTRUMENTATION↗
DOE OSTI2017

Influence of Noise-Generating Factors on Cross-Correlation Electron Backscatter Diffraction (EBSD) Measurement of Geometrically Necessary Dislocations (GNDs)

Abstract Studies of dislocation density evolution are fundamental to improved understanding in various areas of deformation mechanics. Recent advances in cross-correlation techniques, applied to electron backscatter diffraction (EBSD) data have particularly shed light on geometrically necessary dislocation (GND) behavior. However, the framework is relatively computationally expensive—patterns are typically saved from the EBSD scan and analyzed offline. A better understanding of the impact of EBSD pattern degradation, such as binning, compression, and various forms of noise, is vital to enable optimization of rapid and low-cost GND analysis. This paper tackles the problem by setting up a set of simulated patterns that mimic real patterns corresponding to a known GND field. The patterns are subsequently degraded in terms of resolution and noise, and the GND densities calculated from the degraded patterns using cross-correlation ESBD are compared with the known values. Some confirmation of validity of the computational degradation of patterns by considering real pattern degradation is also undertaken. The results demonstrate that the EBSD technique is not particularly sensitive to lower levels of binning and image compression, but the precision is sensitive to Poisson-type noise. Some insight is also gained concerning effects of mixed patterns at a grain boundary on measured GND content.

Materials Science↗
NASA NTRS2007

Investigation of Synthetic Mg(1.3)V(1.7)O4 Spinel with MgO Inclusions: Case Study of a Spinel with an Apparently occupied Interstitial Site

A magnesium vanadate spinel crystal, ideally MgV2O4, synthesized at 1 bar, 1200 C and equilibrated under FMQ + 1.3 log f(sub o2) condition, was investigated using single-crystal X-ray diffraction, electron microprobe, and electron backscatter (EBSD). The initial X-ray structure refinements gave tetrahedral and octahedral site occupancies, along with the presence of 0.053 apfu Mg at an interstitial octahedral site . Back-scattered electron (BSE) images and electron microprobe analyses revealed the existence of an Mg-rich phase in the spinel matrix, which was too small (less than or equal to 3microns) for an accurate chemical determination. The EBSD analysis combined with X-ray energy dispersive spectroscop[y (XEDS) suggested that the Mg-rich inclusions are periclase oriented coherently with the spinel matrix. The final structure refinements were optimized by subtracting the X-ray intensity contributions (approx. 9%) of periclase reflections, which eliminated the interstitial Mg. This study provides insight into possible origins of refined interstitial cations reported in the the literature for spinel, and points to the difficulty of using only X-ray diffraction data to distinguish a spinel with interstitial cations from one with coherently oriented MgO inclusions.

Uchida, Hinako↗