Matthew M. Nowell

A Review of Strain Analysis Using Electron Backscatter Diffraction

Since the automation of the electron backscatter diffraction (EBSD) technique, EBSD systems have become commonplace in microscopy facilities within materials science and geology research laboratories around the world. The acceptance of the technique is primarily due to the capability of EBSD to aid the research scientist in understanding the crystallographic aspects of microstructure. There has been considerable interest in using EBSD to quantify strain at the submicron scale. To apply EBSD to the characterization of strain, it is important to understand what is practically possible and the underlying assumptions and limitations. This work reviews the current state of technology in terms of strain analysis using EBSD. First, the effects of both elastic and plastic strain on individual EBSD patterns will be considered. Second, the use of EBSD maps for characterizing plastic strain will be explored. Both the potential of the technique and its limitations will be discussed along with the sensitivity of various calculation and mapping parameters.

EBSD Image Quality Mapping

Image quality (IQ) maps constructed from electron backscatter diffraction data provide useful visualizations of microstructure. The contrast in these maps arises from a variety of sources, including phase, strain, topography, and grain boundaries. IQ maps constructed using various IQ metrics are compared to identify the most prominent contrast mechanism for each metric. The conventional IQ metric was found to provide the superior grain boundary and strain contrast, whereas an IQ metric based on the average overall intensity of the diffraction patterns was found to provide better topological and phase contrast.

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).

Introduction and comparison of new EBSD post-processing methodologies

Electron Backscatter Diffraction (EBSD) provides a useful means for characterizing microstructure. However, it can be difficult to obtain index-able diffraction patterns from some samples. This can lead to noisy maps reconstructed from the scan data. Various post-processing methodologies have been developed to improve the scan data generally based on correlating non-indexed or mis-indexed points with the orientations obtained at neighboring points in the scan grid. Two new approaches are introduced (1) a re-scanning approach using local pattern averaging and (2) using the multiple solutions obtained by the triplet indexing method. These methodologies are applied to samples with noise introduced into the patterns artificially and by the operational settings of the EBSD camera. They are also applied to a heavily deformed and a fine-grained sample. In all cases, both techniques provide an improvement in the resulting scan data, the local pattern averaging providing the most improvement of the two. However, the local pattern averaging is most helpful when the noise in the patterns is due to the camera operating conditions as opposed to inherent challenges in the sample itself. A byproduct of this study was insight into the validity of various indexing success rate metrics. A metric based given by the fraction of points with CI values greater than some tolerance value (0.1 in this case) was confirmed to provide an accurate assessment of the indexing success rate.

EBSD Sample Preparation: Techniques, Tips, and Tricks

Matthew M. Nowell1, Ronald A. Witt2, and Brian W. True1 1 EDAX-TSL, 392 E 12300 S, Draper, UT 84020 2 EBSD Analytical, 2044 N 1100 E, Lehi, UT 84043 matt.nowell@ametek.com Automated analysis of Electron Backscatter Diffraction (EBSD) patterns for orientation imaging and phase identification in materials and earth sciences has become a widely accepted microstructural analysis tool. To briefly review, EBSD is a scanning electron microscope (SEM) based technique where the sample is tilted approximately 70 degrees and the electron beam is positioned in an analytical spot-mode within a selected grain. An EBSD pattern is formed due to the diffraction of the electron beam by select crystallographic planes within the material. The EBSD pattern is representative of both the phase and crystallographic orientation of the selected area. The pattern is imaged by a phosphor screen and recorded with a digital CCD camera and then analyzed. For orientation imaging, EBSD patterns are systematically collected and analyzed from a specified array of measurement points. The microstructure can then be visualized by coloring points on the array according to information derived from the EBSD pattern. For example, orientation imaging maps from a duplex stainless steel sample are shown in Figure 1. In Figure 1a, the colors represent the crystallographic directions aligned with the sample normal direction. The shaded stereographic triangle provides the color key. In Figure 1b, the colors represent the phases within the material; red is BCC ferrite while blue is FCC austenite. In Figure 1c, measurement points of similar crystallographic orientation are grouped together to define the grains within the material. The measured grains are then randomly colored to illustrate the grain size and morphology. Each of these maps is derived from the same acquired data, and many other analytical possibilities exist. In addition, quantitative microstructural and orientation measurements can be made. For example, in this duplex stainless steel the average grain size has a diameter of 32 microns assuming a circular grain shape. However, analysis of the grain shape shows this is not the case. By fitting an ellipse to the points defining a grain, the aspect ratio of the minor to major axes can be calculated. In this example the average aspect ratio is 0.34. The average grain size can then be more accurately defined as an average grain area of 828 square microns. EBSD patterns are generated within a small interaction volume located at the surface of a sample with a penetration depth typically less than 50-100 nm. Because of this, EBSD pattern quality is extremely sensitive to the integrity of the crystallographic lattice order at the surface of the sample. While some samples, such as ECD deposited metal films, require no preparation prior to analysis, often samples must be sectioned and prepared to obtain useable EBSD patterns [1-2]. When considering how to prepare a sample for EBSD analysis, it is important to recognize that in order to obtain highquality patterns the surface deformation that is typically introduced during standard metallographic preparation should be minimized. This deformation will disturb the crystallographic lattice and will result in more diffuse diffraction bands and a loss of intensity and contrast within the pattern. While EBSD patterns can be obtained from rough surfaces, the topography of the surface will often block the diffraction signal from reaching an EBSD detector, and will in turn reduce the yield of usable EBSD patterns obtained across such a surface. For orientation imaging, a flat surface is therefore desirable. Proper sample preparation will result in optimized pattern quality and subsequent high-confidence orientation imaging data. In this work, the sample preparation procedure developed to obtain high-quality EBSD patterns from a nickel-based superalloy will be presented. A rod (6.35mm diameter) of INCONEL 600 (Ni72/Cr16/Fe8) from Goodfellow was selected as a standard sample material. The relatively high effective atomic weight of this alloy provides a strong backscatter signal and pattern intensity. The rod was sectioned with a low-speed diamond saw (Buehler Isomet) at approximately 200 RPMs. This sectioning method was selected as to minimize the damage introduced during cutting. A cutting lubricant (Isocut Fluid) was used to improve cutting rates and minimize frictional heat generated. After sectioning, the samples were then mounted in a copperbased conductive mounting powder using a compressive mounting press (TechPress2 – Allied High Tech Products). This conductive mounting material is preferred for EBSD analysis as it helps minimize electron beam charging effects which can cause intensity blooming in the EBSD patterns and image drift and distortion in orientation maps. Often, even non-conductive samples can be mounted in this material and observed with EBSD without requiring a conductive coating. However, the samples are subjected to both pressure (3800 PSI) and temperature (175°C) during the mounting process. If the samples cannot withstand these influences then

Orientation precision of electron backscatter diffraction measurements near grain boundaries.

Electron backscatter diffraction (EBSD) has become a common technique for measuring crystallographic orientations at spatial resolutions on the order of tens of nanometers and at angular resolutions <0.1°. In a recent search of EBSD papers using Google Scholar™, 60% were found to address some aspect of deformation. Generally, deformation manifests itself in EBSD measurements by small local misorientations. An increase in the local misorientation is often observed near grain boundaries in deformed microstructures. This may be indicative of dislocation pile-up at the boundaries but could also be due to a loss of orientation precision in the EBSD measurements. When the electron beam is positioned at or near a grain boundary, the diffraction volume contains the crystal lattices from the two grains separated by the boundary. Thus, the resulting pattern will contain contributions from both lattices. Such mixed patterns can pose some challenge to the EBSD pattern band detection and indexing algorithms. Through analysis of experimental local misorientation data and simulated pattern mixing, this work shows that some of the rise in local misorientation is an artifact due to the mixed patterns at the boundary but that the rise due to physical phenomena is also observed.

Quantifying the recrystallization texture of tantalum

The utilization of crystallographic texture is gaining acceptance as a commercially viable tool for characterizing the attributes and confirming the quality of engineering materials. Traditionally, x-ray diffraction (XRD) involving the measurement and subsequent analysis of normal-orientation pole figures is used to determine the preferred orientation in sheet metals. Unfortunately, most XRD techniques are incapable of providing quantifying texture information of wrought tantalum. This is because integrative texture measurement methods cannot detect the presence of texture gradients and bands known to be detrimental to the performance of tantalum for select applications. Instead, discrete orientation measurement techniques such as electron backscatter diffraction are required to resolve the microtexture of tantalum. Inverse pole figure maps generated using orientation imaging microscopy have been used to gain qualitative insight into the texture character of wrought tantalum. Recently, a numerical means for quantifying the texture uniformity of tantalum (as well as other materials) from discrete orientation data has been devised and demonstrated.

Angular Precision of Automated Electron Backscatter Diffraction Measurements

Electron backscatter diffraction (EBSD) has become the preferred technique for characterizing the crystallographic orientation of individual grains in polycrystalline microstructures due to its ability to rapidly measure orientations at specific points in the microstructure at resolutions of approximately 20-50nm depending on the capabilities of the scanning electron microscope (SEM) and on the material being characterized. Various authors have studied the angular resolution of the orientations measured using automated EBSD. These studies have stated values ranging from approximately 0.1° to 2° [1-6]. Various factors influence the angular resolution achievable. The two primary factors are the accuracy of the detection of the bands in the EBSD patterns and the accuracy of the pattern center (PC) calibration. The band detection is commonly done using the Hough transform. The effect of varying the Hough transform parameters in order to optimize speed has been explored in a previous work [6]. The present work builds upon the earlier work but with the focus towards achieving the best angular resolution possible regardless of speed. This work first details the methodology used to characterize the angular precision then reports on various approaches to optimizing parameters to improve precision.

Impact of Local Texture on Recrystallization and Grain Growth via In Situ EBSD

While electron backscatter diffraction (EBSD) has become an established technique within materials characterization labs around the world, the technique is still relatively young and new applications are continuing to emerge. Automated EBSD or Orientation Imaging Microscopy (OIM) systems are being used in combination with other equipment within the scanning electron microscope (SEM) to perform in-situ measurements. This includes tensile stages for observing changes in local orientation during deformation and heating stages for studying orientation changes arising during recrystallization and grain growth as well as phase transformations. In addition to these temporally three-dimensional studies, spatially three-dimensional studies can be performed by in-situ serial sectioning in microscopes equipped with both electron and focused ion beams. These in-situ techniques are briefly reviewed. The review is followed by a detailed analysis of in-situ heating experiments on copper. The movement of grain boundaries during recrystallization and subsequent grain growth are tracked. The effect of orientation relationships on grain boundary mobility and nucleation are explored. No special relationship with grain boundary mobility was observed. However, twins appear to play a significant role in the nucleation process.

The correlation of performance in CdTe photovoltaics with grain boundaries

It is assumed that the performance of polycrystalline CdTe photovoltaic thin films is correlated to microstructure, especially aspects of the microstructure related to grain boundaries. However the role of grain boundaries on the electrical performance of these films is not well understood. In an effort to gain understanding into the correlation between the performance and grain boundaries in CdTe thin films, electron backscatter diffraction (EBSD) has been used to characterize the the grain size, grain boundary structure and texture of sputtered CdTe at varying deposition pressures before and after CdCl2 treatment. Weak axisymmetric (or fiber) textures were observed in the as-deposited films. At lower deposition pressures (111) fiber textures were observed and (110) fiber textures were observed at higher lower deposition pressures. Samples treated with CdCl2 exhibited significant grain growth with a high fraction of twin boundaries. A much stronger correlation of performance with grain size was found when the grain size was corrected to exclude the twin boundaries. This observation confirms that the twin boundaries do indeed have different electrical properties than random high-angle boundaries.