Electron backscatter diffraction (EBSD) is a powerful materials characterization technique that allows us to investigate the microstructural and crystallographic properties of crystalline samples. Operating inside a scanning electron microscope (SEM), EBSD provides key information about phases present, crystal orientations, grain boundaries, lattice defects, and elastic strain.
Definition
Electron backscatter diffraction (EBSD) is a scanning electron microscopy-based technique for microstructural characterization and crystallographic analysis of crystalline materials with high spatial resolution.
The technique relies on the diffraction of electrons from crystalline lattices to form diffraction patterns that contain microstructural and crystallographic information about the sample. As we will explore further, intricate sample preparation and careful data analysis are needed to obtain high-quality results.
When used alongside complementary techniques like energy dispersive X-ray spectroscopy (EDS), EBSD enables comprehensive evaluation of microstructures which supports materials research across diverse fields from geology to advanced manufacturing.
How Does EBSD Work?
To understand how EBSD works, we must first consider the sample preparation methodology. Samples require an extremely smooth surface which involves careful polishing followed by vibropolishing and/or ion milling. This is critical for producing adequate electron backscatter patterns.
Inside the SEM chamber, the polished sample surface is tilted (~70°) and subjected to a focused electron beam. When these electrons interact with the crystalline sample, some are elastically backscattered. These diffuse backscattered electrons form cones that consist of constructive and destructive interference from diffracted electrons.
As the cones intercept a phosphor screen, they generate visible bands known as Kikuchi bands that constitute the electron backscatter diffraction pattern or EBSD pattern.
Each EBSD pattern is unique to the crystal structure and orientation of the diffracting volume of the sample from which it was generated.
Sophisticated software like AZtecCrystal captures and indexes patterns by comparing them to simulated templates, allowing crystal phase and orientation at each measurement point to be determined. Collecting data in a grid-wise manner facilitates the creation of detailed crystal orientation and phase maps.
Applications of EBSD
Armed with an understanding of the fundamentals, we can now explore some of the key capabilities of EBSD mapping:
- Microstructural Characterization: By resolving grain morphology, intragranular misorientations, and relationships between grains, EBSD enables a detailed evaluation of microstructures in materials science. Common applications include extracting grain size statistics, investigating recrystallization and grain growth, and examining phase transformations.
- Phase Discrimination: The technique reliably differentiates phases in multiphase mixtures based on the characteristic Kikuchi patterns generated. This aids identification of constituents in alloy systems, deformation structures, and ceramics.
- Crystal Orientation Mapping: Generating orientation maps is arguably one of the most powerful and useful applications of EBSD. By determining orientations at each scanned point, it reveals grain structures, locations of boundaries, lattice rotations, and more.
- Strain Mapping: Measuring small orientation changes facilitates the analysis of elastic strains and gradients resulting from residual stresses or mechanical deformation.
- Textural Analysis: Assembling thousands of orientation measurements enables detailed quantitative texture analysis. This aids the study of crystal-plastic forming processes and geological deformations.
Advantages of EBSD
Now that we better understand the technique, let’s explore some noteworthy advantages:
- High Spatial Resolution: EBSD routinely achieves resolution of tens of nanometers, enabling analysis of fine-grained and nanostructured materials.
- Large Area Mapping: Automated mapping allows efficient collection of vast orientation datasets across millimeter or centimeter-sized areas.
- Strain Characterization: Unlike X-rays or neutrons, electrons interact with low-angle boundaries, dislocations, and point defects, providing excellent strain sensitivity.
- User-friendly Operation: The ability to leverage existing SEM infrastructure makes EBSD convenient and makes samples quick to set up.
Limitations of EBSD
However, there are still some limitations to consider:
As a surface technique, EBSD analysis is limited to probing the first 50-100 nanometers of material. So results may not represent bulk properties.
Meticulous sample preparation is crucial. Any surface defects or preparation artifacts can obstruct analysis.
Indexing complex patterns accurately relies heavily on robust algorithms and user expertise for reliable results.
There are also challenges in terms of operational constraints – for example, analysis of non-conductive materials requires additional preparation steps.
Conclusion
In summary, electron backscatter diffraction is an increasingly popular microstructural characterization technique that exploits electron diffraction to extract crystallographic information.
With continual improvements in detector technologies, pattern indexing algorithms, and automation, EBSD systems are becoming more powerful, efficient, and convenient to use. We can expect wider adoption across materials research and industrial quality control applications in the years ahead as researchers leverage this technique to unravel microstructural evolution.