29

50 µm

. Fig. 29.8  Electron channeling contrast from grains in polycrystalline Ni deformed by a diamond indentation placed in a single grain; E0 = 20 keV; BSE detector

29.1.3\ Conditions for Detecting Electron Channeling Contrast

Specimen Preparation

Channeling contrast effects are created in a shallow near-­ surface layer that is 10–100 nm in depth, depending on the incident beam energy and the material, where the incident beam has not yet undergone sufficient elastic scatter to destroy the initial beam collimation. Thus, the condition of the sample surface is of extreme importance for a successful channeling contrast measurement. The low level of channeling contrast, 2–5 %, also requires eliminating other competing sources of contrast, especially surface topography. Thus, a typical sample preparation will involve grinding and polishing to achieve a flat, topography-free surface. However, the inevitable surface damage layer produced in many materials, especially metals, by mechanical abrasion, including the “Beilby layer” that remains after the final polishing with the finest scale abrasives, must be removed by chemical or electrochemical polishing or low energy ion beam sputtering. (Note that high energy ion sputtering can produce sub-­ surface damage that can destroy electron channeling contrast.)

Instrument Conditions

Detecting channeling effects that produce weak contrast in the range 2–5 % in an SEM image requires careful choice of operating conditions. The low contrast creates a requirement for a high threshold current, so that typically a beam current of 10 nA or more is needed when scanning at rapid frame rates to find features of interest. After such features have been located, a smaller beam diameter can be used to improve resolution at the inevitable cost of a lower beam current, which then requires a longer frame time to maintain contrast

visibility. Because a small convergence angle is also desirable to maximize channeling contrast, a high electron source brightness is important to maximize the beam current into a focused beam of useful size. A high beam energy in the range from 10 to 30 keV is desirable both to increase source brightness and to create high energy BSEs for efficient detection. Channeling contrast is carried by the high energy fraction of the backscattered electrons, and to maximize that signal, a large solid angle BSE detector (solid state or passive scintillator) is the best choice with the specimen set normal to the beam. The positively biased Everhart–Thornley (E–T) detector, with its high efficiency for BSEs through capture of the SE2 and SE3 signals, is also satisfactory. Because of the weak contrast, the live image display must be enhanced by careful adjustment of the “black level” and amplifier settings to spread the channeling contrast over the full black-to-white range of the final display to render channeling contrast visible. Post-collection image processing by various tools, e.g., CLAHE in ImageJ-Fiji, can be very effective at recovering fine scale details.

Crystallographic contrast by electron channeling provides images of the crystallographic microstructure of materials. For many applications in materials science, it is also important to measure the actual orientation of the microstructure on a local basis with high spatial resolution of 1 micrometer laterally or even finer. The technique of electron backscatter diffraction (EBSD) patterns provides the ideal complement to channeling contrast microscopy.

29.2\ Electron Backscatter Diffraction

in the Scanning Electron Microscope

An understanding of the crystallography of a material is required to fully describe the structure property relationships that control a material’s physical properties. By linking the microstructure to the crystallography of the sample, a full picture of the sample can be developed. The development of electron backscattering diffraction (EBSD) equipment and its placement in commercial tools for phase identification and orientation determination has provided new insights into microstructure, crystallography and materials physical properties.

EBSD in the SEM has been developed for two different purposes. The oldest application of EBSD is for the measurement of texture on a grain by grain basis. Texture determined in this way is called the microtexture of the sample (Randle 2013). An example of this is .  Fig. 29.9, where the point-by-­point orientation of an assembly of ZnO crystals is shown where the color coding indicates the orientation of the crystal. The other use of EBSD is for the identification or discrimination of micrometer or sub-micrometer crystalline phases (Michael 2000; Dingley and Wright 2009).

.  Figure 29.10 is an example of this, showing a dual-phase steel with both ferrite (body-centered cubic) and austenite

(face-centered cubic) present. EBSD can easily discriminate

. Fig. 29.9  Inverse pole figure map of ZnO crystals

a

b

. Fig. 29.10  EBSD of a dual phase steel that contains both austenite and ferrite. EBSD of multiphase samples can discriminate phases with different crystal structures. a This is a band contrast map, basically a measure of the pattern sharpness, which accurately reflects the grain structure of the sample. b Orientation map of the austenite phase while the ferrite is shown in the underlying band contrast image (Bar = 200 µm)

between these two crystal structures. Both applications add an important new tool to the SEM. The SEM now has the capability to study the morphology of a sample through either secondary or backscattered electron imaging, the

chemistry through energy dispersive spectrometry and the crystallography of the sample by electron channeling contrast imaging and EBSD. EBSD techniques have recently been developed that allow both the elastic and plastic strains present in a microstructure to be determined and this has been called high resolution EBSD or HREBSD. A more useful description would be high angular resolution EBSD. This technique is beyond the scope of this chapter (Wilkinson et al. 2006).

Microtexture is a term that means a population of individual orientations that are usually related to some feature of the sample microstructure. A simple example of this is the relationship of the individual grain size to grain orientation. The concept of microtexture may also be extended to include the misorientation between grains, often termed the mesotexture. It is now possible using EBSD to collect and analyze thousands of orientations per minute, thus allowing excellent statistics in various distributions. The ability to link texture and microstructure has enabled significant progress in the understanding of recrystallization, grain boundary structure and properties, grain growth and many other important physical phenomena.

The identification of phases in the SEM has usually been obtained by determining the composition of the phase and then inferring the identity of the phase. This technique is subject to the inherent inaccuracies in quantitative analysis in the SEM using EDS or WDS. In addition, this technique is not useful when one is attempting to identify a phase that has multiple crystal structures, but only one composition. A good example of this concern is TiO2, with three different crystal structures. Another important technological problem is the identification of austenite in ferrite in engineering steels. Austenite has a face centered cubic crystal structure and ferrite is body centered cubic and both phases may have very similar chemistries.

Improved resolution for EBSD has been achieved by utilizing thin samples that allow the transmission and diffraction of electrons using accelerating voltages (20–30 kV) achievable in the SEM. The patterns generated in this way have very similar characteristics to EBSD but are formed in transmission mode and thus, due to the thin sample, have much improved spatial resolution as compared to EBSD performed on bulk samples. Transmission Kikuchi diffraction (TKD) allows the microtexture of ultrafine grained crystalline materials to be studied (Keller and Geiss 2012; Trimby 2012).

In order to best use EBSD, it is helpful if the reader has some knowledge of crystallography and how crystallography is represented. There are a number of excellent books on this subject (McKie and McKie 1986; Rousseau 1998). This module will first describe the origin of the EBSD pattern in the SEM. The detectors or cameras used to detect EBSD patterns will be described. As sample preparation is critical to the success of an EBSD investigation, it is important to understand the methods needed to produce samples of appropriate quality. Finally, details of the actual experiment and the resulting output will be discussed.

29.2.1\ Origin of EBSD Patterns

EBSD patterns are obtained in the SEM by illuminating a highly tilted specimen with a stationary electron beam. The beam electrons interact with the sample and are initially inelastically scattered. There are two current methods that are employed. The first method (conventional EBSD) uses the sample in a mode where backscattered electrons are detected (i.e., from a bulk sample). The second method (often termed TKD or transmission-EBSD although the use

29 of the latter implies the impossibility of simultaneous transmission and backscattering of the electrons) relies on a very thin sample and the patterns are then formed by the transmitted electrons. In the case of the bulk sample and backscattered electron detection, the sample is held at a steep angle with respect to the electron beam. When a thin sample is used for TKD, the sample does not need to be tilted at such a high angle, and in fact the electron beam may be used to illuminate a sample which is not tilted. In either case, backscatter or transmission, some of these inelastically scattered electrons that have lost little of their original energy satisfy the diffraction condition with the crystalline planes within the sample. When this interaction happens near the surface of the sample these electrons escape and form the EBSD pattern that is observed. These backscattered electrons appear to originate from a virtual point below the surface of the specimen. These types of patterns were first described by Kikuchi and are often referred to as Kikuchi patterns (Randle 2013). Some of the backscattered electrons satisfy the Bragg conditions for diffraction (+θ and −θ) and are diffracted into cones of intensity with a semi-angle of (90-θ), with the cone axis normal to the diffracting plane. As shown by the Bragg Eq. (29.1), and .  Table 29.1, the short wavelength of the electron (at typical accelerating voltages of 10–30 kV used in the SEM) results in a small Bragg angle of less than 2°. Each plane yields two cones of intensity. The cones are quite flat and when they intercept the imaging plane they are imaged as two nearly straight lines separated by an angle of twice the Bragg angle. An alternative but equivalent description is the single event model. In this model, it is argued that the inelastic and elastic scattering events are intimately related and may be thought of as one event. In this case the electron channels out of the sample and forms the EBSD pattern (Winkelmann 2009; Randle 2013). .  Figure 29.11 are two examples of an EBSD patterns collected from the mineral hematite at 5 kV and 40 kV. Note that the Kikuchi bands appear as nearly straight lines. The effect of the accelerating voltage is clearly seen. As the accelerating voltage is increased the Bragg angle decreases resulting in more narrowly spaced Kikuchi bands in the patterns. It is also important to note that the patterns are fixed with respect to the crystal orientation and so the positions of the line traces in the patterns have not moved. Fortunately we need not understand the exact physics of EBSD pattern formation in order to use these patterns for crystallographic analysis.

a

b

. Fig. 29.11  EBSD patterns acquired from the mineral hematite (trigonal) that demonstrate the pattern changes that result from the acceleration voltage change. Note that as expected from the Bragg equation the band widths decrease with increasing accelerating voltage. a 5 kV, b 40 kV

EBSD patterns consist of what appear to be nearly straight bands (they are actually conic sections) which may have bright or dark centers with respect to the rest of the pattern. These straight bands are the Kikuchi lines discussed previously. The Kikuchi bands intersect in many locations and these are called zone axes which are actual crystallographic directions within the unit cell of the crystal. The angles between the Kikuchi bands and the angles between the zone axes are specific for a given crystal structure. These features can be seen in .  Fig. 29.11b where the Kikuchi bands and the

zone axes are clearly visible. In high quality patterns as shown in .  Fig. 29.11, there are many other features that are visible and may be used to understand the unit cell from which the pattern was collected.

29.2.2\ Cameras for EBSD Pattern Detection

It is relatively easy to collect EBSD patterns in the SEM. The earliest EBSD images were collected by directly exposing photographic emulsions inside the specimen chamber of the SEM. As discussed previously, EBSD patterns are obtained from bulk samples by illuminating a highly tilted surface with the electron beam and then collecting the patterns on a position sensitive detector, sometimes referred to as a camera, that is 1–2 cm from the tilted sample surface. Generally the detector surface is located normal to the sample tilt axis so that tilting of the sample is easily achieved but the detector may also be tilted a few tens of degrees away from the horizontal position. The first electronic capture of EBSD patterns utilized low-light video rate cameras that produced useable but somewhat noisy images requiring the use of very high beam currents in the SEM. Current EBSD cameras consist of a fluorescent screen, either circular or rectangular, that is about 2 cm in diameter if circular or 2 cm on a side if square or rectangular formats are utilized. The screen is coated with a thin Al coating to avoid charging and to exclude light. The fluorescent screen is kept thin so that it can be imaged from the opposite side using either a fiber optic bundle or a lens to transfer the image on the phosphor to a charge coupled device (CCD) or a CMOS type solid state imager. Camera designs strive to minimize the loss of light from the fluorescent screen to increase the speed at which patterns can be collected or to increase the quality of the detected patterns. The transfer optics must also be designed to minimize any optical distortions to the collected patterns (Schwarzer et al. 2009).

The imager should have a sufficient number of pixel elements so that the angular resolution is adequate for detection.

In addition, it is extremely useful to be able to bin the pixels in the detector. Binning pixels simply means that adjacent pixels are summed together (normally in specific patterns like 2 × 2 or 4 × 4) to increase the signal but with a decrease in pattern resolution. Currently, detectors are capable of collecting more than 1000 patterns per second when heavily binned and with sufficient electron beam current.

The entire detector must be mounted on a precision retractable stage. The precision retractable stage allows the detector assembly to be moved into the same position each time the detector is inserted to maintain the geometrical calibration of the system. It is important to position the detector close to the sample, generally the sample to detector distance should be on the order of or even slightly less than the fluorescent screen diameter so that a large portion of the EBSD pattern may be collected. The insertion mechanism is motorized to allow the camera to be moved into position in a matter of few seconds. A typical arrangement of the sample and

Detector

Sample

. Fig. 29.12  An image from the inside of an SEM sample region with the sample tilted for EBSD and the EBSD camera or detector inserted. Also note at the bottom of the detector screen there are small rectangular solid state electron detectors for imaging the sample while it is highly tilted. These detectors result in remarkable crystallographic contrast

the EBSD camera or detector is shown in .  Fig. 29.12. Also visible at the bottom of the EBSD detector are the forescattered electron detectors. These solid state detectors provide excellent crystallographic contrast when the sample is highly tilted and standard backscattered or secondary electron imaging are not optimal.

29.2.3\ EBSD Spatial Resolution

It is important to understand the volume from which the diffraction pattern is generated in the SEM. In many applications the highest resolution is not required to utilize EBSD for the mapping of the texture of a microstructure. However, there are applications where high spatial resolution is required. Examples are the mapping of deformed microstructures or the study of fine grained materials.

The formation of EBSD patterns depends on the scattering and subsequent diffraction of electrons. Bragg’s Law describes the diffraction of specific wavelengths of electrons from the planes in a crystalline solid. If there is a large spread in the energy of the electrons that are exiting the sample surface we would not expect to see the lines that we see in our EBSD patterns due to diffraction. These lines are in nearly the exactly correct position as described by the Bragg diffraction of the electrons based on the beam voltage used to produce the patterns. From this we must conclude that the electrons leaving the surface are of nearly the same energy. Electrons that have lost larger amounts of energy contribute to the background intensity and do not contribute to the diffraction features that we want to observe (Deal et al. 2008).

For EBSD, we tilt the sample at a high angle with respect to the electron beam. This tilt is important to obtaining the high quality patterns that are needed for EBSD. As shown in

.  Fig. 29.13, the tilt angle has a large effect on the pattern quality. The best pattern qualities are obtained at higher tilt angles with some optimal condition between the pattern quality and the ease of imaging of the highly tilted sample.

.  Figure 29.14a shows the backscattered electron energy distribution that is developed from a highly titled sample (70° tilt) compared to 0° tilt. Note that the energy is highly peaked toward the operating voltage of the microscope at 70° tilt when compared to the same distribution from an electron beam that is normal to the sample surface. Further, since the EBSD pattern is clearly visible, they must form from only the electrons that have lost either no or a small amount of their energy. The electrons that have lost only a small amount of their energy are emitted from the sample in regions that are very close to the original beam foot print on the surface of the sample. This is shown in .  Fig. 29.14b, which is a Monte Carlo simulation of the distance from the initial beam impact point that the electrons emerge from the sample surface for all the backscattered electrons, those that lost up to 2 kV and

those that lost only 0.2 kV. The pattern is formed from these low loss electrons and thus the resolution is quite good. Thus, although EBSD involves backscattered electrons, it is a technique with much higher spatial resolution than standard backscattered electron imaging. Typically, resolutions of better than 0.1 μm can be achieved for a beam voltage of 20 kV in the transition metals. The electrons that have lost larger amounts of energy will contribute the background intensity in the EBSD pattern. These electrons are the main reason why there is limited contrast in EBSD patterns without some form of background removal either through division or subtraction or some other computational method of removal (Michael and Goehner 1996).

The high tilt angle limits the spatial resolution attainable due to the elongation of the electron beam foot print on the sample surface. The resolution parallel to the tilt axis is much better than the resolution perpendicular to the tilt axis due to the high sample tilt angles used to acquire EBSD patterns. The resolution perpendicular to the tilt axis is related to the resolution parallel to the tilt axis by

. Fig. 29.14  a Monte Carlo electron trajectory simulations of the backscattered electron distributions for Ni at 20 kV at a tilt of 70° (red) and for a sample at 0° tilt (blue) that is normal to the electron beam. b Backscattered

electron distributions from Ni at 20 kV and a sample tilt of 70° for different levels of energy loss as a function of distance from the beam impact point

where θ is the sample tilt with respect to the horizontal, Lperp is the resolution perpendicular to the tilt axis, and Lpara is the resolution parallel to the tilt axis. The resolution perpendicu-

lar to the tilt axis is roughly three times the resolution parallel to the tilt axis for a tilt angle of 70° and increases to 5.75 times for a tilt angle of 80°. Thus, it is best to work at the lowest sample tilt angles possible consistent with obtaining good EBSD patterns.

29.2.4\ How Does a Modern EBSD System

Index Patterns

Modern EBSD systems all use the same basic steps to go from a collected EBSD pattern to indexing and to the determination of the crystal orientation with respect to a reference frame. Once the pattern is collected, the necessary information for indexing of the pattern (indexing refers to assigning a consistent set of crystallographic directions to the pattern with respect to a given or a set of given candidate structures) must be derived from the diffraction pattern. Once the pattern has been indexed correctly (various vendors use different measures of what constitutes correct indexing), it is a simple matter to determine the crystallographic orientation represented by the EBSD pattern.

The most important factor in obtaining a quality orientation is that the system have good quality patterns for indexing. The quality of the detected patterns needed for a given experimental measurement can be influenced by choices made over how the camera is operated. EBSD patterns are of low inherent contrast mainly due to electrons that have lost

significant amounts of energy contributing to the overall intensity of the pattern background. To compensate for this, methods of removing the background are utilized. Some systems use a software-generated background to compensate for the background and increase the pattern contrast. A more established approach uses a background image obtained by scanning over a large number of grains that produce the background signal. This background is then used to normalize the raw EBSD pattern to produce a pattern with high contrast (Michael and Goehner 1996).

EBSD cameras are generally able to collect patterns at higher angular resolution per pixel than is needed for most orientation mapping. However, if high accuracy is needed, it is possible to use the full resolution of the camera to produce EBSD patterns. The disadvantage of using the full resolution of the EBSD camera is that longer exposure times are needed to produce usable EBSD patterns. This longer exposure can really slow down map acquisition. It is possible to bin the camera resolution, usually by factors of 2, to produce lower resolution patterns but with the ability to collect the patterns at a much higher collection rate. Thus, if the high angular resolution is not required, the camera should be used at a reduced resolution, (2 × 2, 4 × 4, or 8 × 8 binning) to speed up the acquisition. The tradeoff is between orientation accuracy and speed of the measurement.

Once the camera is set correctly for the given experiment, the patterns should be of high quality with good contrast. The next step is for the software to extract the line positions from the collected pattern. In all modern systems this is accomplished with an algorithm called the Hough Transform, which takes straight lines and transfers them into points