Travis Rampton

Electron imaging with an EBSD detector

Electron Backscatter Diffraction (EBSD) has proven to be a useful tool for characterizing the crystallographic orientation aspects of microstructures at length scales ranging from tens of nanometers to millimeters in the scanning electron microscope (SEM). With the advent of high-speed digital cameras for EBSD use, it has become practical to use the EBSD detector as an imaging device similar to a backscatter (or forward-scatter) detector. Using the EBSD detector in this manner enables images exhibiting topographic, atomic density and orientation contrast to be obtained at rates similar to slow scanning in the conventional SEM manner. The high-speed acquisition is achieved through extreme binning of the camera-enough to result in a 5 × 5 pixel pattern. At such high binning, the captured patterns are not suitable for indexing. However, no indexing is required for using the detector as an imaging device. Rather, a 5 × 5 array of images is formed by essentially using each pixel in the 5 × 5 pixel pattern as an individual scattered electron detector. The images can also be formed at traditional EBSD scanning rates by recording the image data during a scan or can also be formed through post-processing of patterns recorded at each point in the scan. Such images lend themselves to correlative analysis of image data with the usual orientation data provided by and with chemical data obtained simultaneously via X-Ray Energy Dispersive Spectroscopy (XEDS).

The effect of length scale on the determination of geometrically necessary dislocations via EBSD continuum dislocation microscopy

Electron backscatter diffraction (EBSD) dislocation microscopy is an important, emerging field in metals characterization. Currently, calculation of geometrically necessary dislocation (GND) density is problematic because it has been shown to depend on the step size of the EBSD scan used to investigate the sample. This paper models the change in calculated GND density as a function of step size statistically. The model provides selection criteria for EBSD step size as well as an estimate of the total dislocation content. Evaluation of a heterogeneously deformed tantalum specimen is used to asses the method.

A New Microstructural Imaging Approach through EBSD Pattern Region of Interest Analysis

The requirements of a highly tilted sample and a well-polished surface for optimal EBSD pattern collection can make traditional SEM imaging challenging. With a tilted sample, SEM SE and BSE detectors are often in non-optimal positions for electron detection. For many materials, the polished surfaces produce weak contrast with SE imaging. Traditionally placed BSE detectors must be used with care, as a highly tilted sample can risk collision as the sample size and region of interest increases.

Nanoscale Crystallographic Analysis in FE-SEM Using Transmission Kikuchi Diffraction

Recent developments in SEM column design have led to the ability to produce nm spot sizes even at high probe currents [1], thus pushing the analytical techniques available in the SEM to conduct microanalysis with nanometer resolution. Although the limitations of microanalysis at these spatial resolution requirements stem from the physics of beam-specimen interaction and the volume from which the signal is generated during basic bulk sample observation and microanalysis, use of very thin specimens, similar to TEM, can lead to significant improvements in microanalysis resolution. This approach has been shown to be successful in for EDS analysis [2] and has been gaining prominence for crystallographic analysis using Transmission Kikuchi Diffraction (TKD, also referred to as t-EBSD) with a traditional EBSD camera [3].

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.

Advances in Scattered Electron Intensity Distribution Imaging for Microstructural Visualization and Correlations with EBSD Measurements

Electron backscatter diffraction (EBSD) has evolved into a well-established tool for microstructural characterization by providing quantitative information on crystallographic orientation and phase content and distribution. The rapid acceptance of EBSD as an analytical technique has been driven, in part, by the highly informative contrasts used to create different micrographs which effectively describe the microstructure of the material, including qualitative contrasts such as EBSD image quality measurements [1].

Improving Spatial Detection of Twins Achieved by Measuring Individual Kikuchi Band Intensity in EBSD Patterns

Twin boundaries in a crystalline material can be defined by a particular rotation angle about a particular access or a mirrored crystal orientation about a particular plane [1]. For example, copper twins are typically defined by a 60 rotation about where the associated twin plane is of the {111} family. One critical area of twin research looks at deformation twinning as the limiting factor for formability of Mg alloys, such as AZ31 [2]. In AZ31 there are two basic twin modes: compression twinning and tension twinning. The latter phenomenon forms fairly large, easy to detect twinned regions within parent grains, whereas the former tends to form extremely thin twins that are on the order of 100 nm wide. Additionally, the copper which is frequently seen in many microelectronics contains twins on the order of 10 nm [3]. In both cases these features are within the detectable limits for a modern scanning electron microscope (SEM). However, identifying these twins via crystal orientation relations with electron backscatter diffraction (EBSD) in the SEM relies on a larger spatial resolution which makes detecting these twins from crystallographic information difficult in the SEM. This study presents a method whereby improved spatial resolution of thin twins can be achieved with EBSD.

New Technology and Approaches for the Acceleration and Enhancement of Microstructural Characterization using Electron Backscatter Diffraction

Electron Backscatter Diffraction (EBSD) has developed into a well-established microstructural characterization technique that provides quantifiable information on the grain size and shape, grain boundary character, preferred orientation, and local strain state of crystalline materials. Recent developments in pattern detection technology have enabled EBSD data acquisitions speeds to increase to rates greater than 1,400 analysed EBSD pattern per second.

Hybrid Bishop-Hill model combined finite element analysis for elastic-yield limited design

Purpose – Microstructure-sensitive design (MSD), for optimal performance of engineering components that are sensitive to material anisotropy, has largely been confined to the realm of theory. The purpose of this paper is to insert the MSD framework into a finite element environment in order to arrive at a practical tool for improved selection and design of materials for critical engineering situations. Design/methodology/approach – This study applies the recently developed Hybrid Bishop-Hill (HBH) model to map the yield surface of anisotropic oxygen free electronic copper. Combining this information with the detailed local stresses determined via finite element analysis (FEA), a “configurational yield stress” is determined for the entire component. By varying the material choice/processing conditions and selecting the directionality of anisotropy, an optimal configuration is found. Findings – The paper provides a new FEA-based framework for MSD for yield-limited situations. The approach identified optimal directionality and processing configurations for three engineering situations that are particularly sensitive to material anisotropy. Research limitations/implications – The microstructure design space for this study is limited to a selection of eight copper materials produced by a range of processing methods, but is generalizable to many materials that exhibit anisotropic behavior. Originality/value – The introduction of MSD methodology into a finite element environment is a first step toward a comprehensive designer toolkit for exploiting the anisotropy of general materials (such as metals) in a way that is routinely undertaken in the world of fiber-based composite materials. While the gains are not as sizeable (due to the less-extreme anisotropy), in many applications they may be extremely important.

New Tools for the Study of Deformed and Heat-Treated Materials via Electron Backscatter Diffraction

The commercialization of electron backscatter diffraction (EBSD) systems over 25 years ago has allowed for the routine analysis of crystalline materials ranging from metals to ceramics and composites. Since the first EBSD systems were used they have been used by a variety of research fields to extract an increasing amount of information, but no area of study has been as critical as the ability to understand and quantify deformation in materials. Increased insights into deformed materials are critical to improving processes such as rolling, extrusion, and heating. Traditionally quantifying deformation has been tied to measures of local misorientation such as grain average misorientation or kernel average misorientation [1, 2]. However, no universal measure has been employed to correlate with deformation; thus, it is advantageous to develop new methods for measuring and quantifying deformation until such an all-encompassing technique becomes available.