What information does EBSD provide

The basic information provided by Electron Backscatter Diffraction (EBSD) is relatively straightforward:

1. Identification and confirmation of the phase at each analysis point.
2. 3D orientation of the lattice at each analysis point.

EBSD technology, based on these two primary pieces of information, can additionally provide numerous measurements related to microstructure. The performance of EBSD technology depends on various factors, including sample preparation, scanning electron microscope parameters, electron beam characteristics, EBSD detectors and software, as well as the nature of the sample itself.

This article provides an overview of microstructural information provided by EBSD.

Phase

EBSD is often utilized to map the distribution of phases in a sample and measure the volume fractions of these phases. Distinguishing between different phases can be based solely on crystallographic differences or may involve chemical information (from a spectrometer, EDS). The typical output includes phase distribution maps along with corresponding percentage area fractions for each phase, as demonstrated in the example of deformed igneous rock below. EBSD can also be combined with EDS to assist in identifying unknown phases in a sample, such as precipitates. This "phase identification" method is rapid but requires a suitable phase database, thus, on its own, it is not a definitive phase identification.

Figure 1: The EBSD phase distribution map of deformed oxidized gabbro

Texture

Crystal orientation data is the fundamental output of EBSD technology, making it an ideal technique for measuring texture, also known as crystallographic preferred orientation. EBSD is fast and provides spatially-resolved information, allowing us to determine variations in texture within a sample. This gives EBSD a distinct advantage over other texture analysis methods such as X-ray diffraction (XRD) or neutron diffraction. However, it's important to note that EBSD can only provide surface texture measurements unless combined with in situ sectioning analysis methods. Texture measurements are a typical analysis approach for various sample types, especially in industries such as metalworking and geological sciences. In geological sciences, crystallographic preferred orientation (CPO) is used to infer the initiation of specific slip systems. The example below illustrates the alpha-Ti texture in a Ti64 alloy represented in a pole figure.

Figure 2: Alpha-Ti texture in a Ti64 alloy represented in a pole figure

Grains

EBSD orientation maps provide spatially-resolved information about crystallographic orientations, allowing for the deduction of precise grain sizes and shapes. This information includes:

• Grain size
• Grain shape/morphology
• Average grain orientation
• Internal orientation changes within grains
• Twin proportion

All this information can be visualized as orientation distribution maps, as shown in the image below, or used for rigorous statistical data analysis. Grain analysis based on EBSD data finds widespread applications, from quality control in metal and alloy processing to studying grain structures in nanoscale surface coatings. Advanced EBSD post-processing software can reconstruct the grain structures of high-temperature phases before displacive phase transformations, such as the original austenite grains in martensitic steel.

Figure 3: The aspect ratio of austenite grains in the welded zone of duplex stainless steel

Grain Boundaries

From EBSD orientation measurements, detailed crystallographic information about sample grain boundaries can be deduced. This gives EBSD technology a distinct advantage over other techniques, providing comprehensive information about the nature of grain boundaries and perfect statistical data. Information about grain boundaries derived from EBSD orientation maps includes:

• Grain boundary orientation deviation
• Grain boundary rotation axes
• Grain boundary trace lines (complete orientation of grain boundaries can be obtained using 3D-EBSD measurements)
• Special grain boundary identification (e.g., twins or Coincident Site Lattice (CSL) boundaries)
• Complete statistical information about grain boundary lengths

The example images below are from an Al-Mg alloy after deformation and heat treatment. The inverse pole figure shows rotation axes of low-angle grain boundaries (2° ~ 5°), noticeably clustering around the 111 axis. The red markings indicate boundaries with orientation deviations greater than 2° and rotation axes within 5° of the 111 direction. This combination of crystallographic and spatial information highlights a fact: these special low-angle grain boundaries tend to form preferentially in the grains at the bottom of the field of view, possibly influenced by the initial orientation.

Figure 4: Rotation axes of low-angle grain boundaries

Figure 5: Pattern quality and distribution of twin boundaries

Strain

Many EBSD analyses aim to characterize and quantify strain in samples. While elastic strain can be measured through high-resolution EBSD (HR-EBSD) analysis, EBSD is more commonly used to characterize plastic strain. This can be achieved through various methods, including:

• Local lattice orientation gradients (e.g., Kernel Average Misorientation - KAM measurements)
• Geometrically Necessary Dislocation (GND) density
• Intra-granular orientation deviations
• Intra-granular orientation spreads
• Distribution of low-angle grain boundaries

Utilizing EBSD for studying deformation and strain is common in various fields but is particularly effective for researching failure and crack propagation. As an example, the image below shows plastic deformation at the crack tip in a dual-phase steel sample.

Figure 6: The KAM (Kernel Average Misorientation) map highlights the local strain at the crack tip of the dual-phase steel