Beyond conventional SEM imaging
Scanning Electron Microscopy (SEM) is a well-established technique for the investigation of materials. It provides detailed information on surface morphology, fracture surfaces, defects and microstructural features. Combined with Energy Dispersive X-ray Spectroscopy (EDS/EDX), the chemical composition of individual phases, inclusions and particles can be determined.
A powerful but less widely known extension of SEM is Electron Backscatter Diffraction (EBSD). While SEM primarily provides morphological and EDS chemical information, EBSD reveals the crystallographic structure and orientation of individual grains. This makes it a valuable tool for advanced microstructural characterization and for correlating grain structure with material properties and failure mechanisms.
What is EBSD?
Electron Backscatter Diffraction (EBSD) is an analytical technique used in conjunction with a scanning electron microscope to determine the crystallographic structure and orientation of crystalline materials.
When the electron beam interacts with a crystalline sample, backscattered electrons undergo diffraction at the crystal lattice. The resulting Kikuchi patterns are recorded by an EBSD detector. The geometry of those patterns is characteristic of the crystal structure and orientation. By comparing the measured patterns with reference data, the crystallographic orientation and phase can be determined. Therefore, Kikuchi patterns are considered as crystallographic fingerprints of the material.
How does EBSD work?
High-quality sample preparation is essential for EBSD. The investigated surface must be flat, clean and free of mechanical deformation introduced during sample preparation. Depending on the material, preparation typically includes cutting, mounting, grinding, polishing and, if required, electrolytic polishing.
During the measurement, the sample is positioned at a steep angle towards the EBSD detector. As the electron beam scans across the surface, a diffraction pattern is acquired at each measurement point. The individual measurements are then combined to create a crystallographic map (EBSD map) of the investigated area.
The resulting data can provide information on:
- crystal orientation
- crystallographic phase
- grain size and morphology
- grain boundaries and misorientation
- crystallographic texture
- deformation and microstructural changes
Applications of EBSD
Grain structure and deformation
EBSD allows detailed characterization of the grain structure of polycrystalline materials. Grain size, grain morphology, orientation and grain boundary characteristics can be evaluated and related to manufacturing processes, heat treatments or mechanical deformation.
Phase identification
EBSD can distinguish crystalline phases based on their crystal structure. This is particularly useful when phases have similar chemical compositions and therefore cannot easily be distinguished by EDS.
A relevant example is the determination of retained austenite in heat-treated steels: During transformation hardening, an austenitic phase forms at elevated temperatures and transforms predominantly into martensite during quenching. Depending on the material and heat treatment, a certain amount of austenite remains which has critical impact on the mechanical properties. EBSD provides a direct method to identify and quantify the austenite content and helps to correlate heat treatment, microstructure and material properties.
Crystallographic texture
Manufacturing processes such as rolling, forging, extrusion or additive manufacturing can introduce preferred crystallographic orientations, known as texture. EBSD allows to visualize and to quantify these orientations can help to explain anisotropic material properties.
Failure analysis
EBSD can complement conventional SEM investigations in failure analysis. While SEM provides information about fracture morphology, EBSD can reveal the relationship between the crack path and the underlying microstructure. For example, the preferred propagation path of a crack can be assessed:
- along grain boundaries
- through specific phases
- along particular crystallographic orientations
- through plastically deformed regions
- in association with specific microstructural features
This information can provide valuable input for understanding complex failure mechanisms, including stress-corrosion cracking (SCC).
EBSD investigations at AAC
Novel heat treatment of Cronidur 30
At AAC, a novel heat treatment for Cronidur 30 steel has been developed. In addition to SCC testing, the microstructure was characterized by EBSD to determine the retained austenite content.
Microstructure of drawn tungsten wire
Wire drawing involves severe plastic deformation, which can significantly influence grain size and crystallographic orientation which also impacts the mechanical properties of the wire. EBSD was used to characterize the microstructure of a drawn tungsten wire.
The analysis provided information on grain boundaries, crystallographic texture and grain size. The results show a predominant grain orientation in the drawing direction and an average grain size of ~0.7 µm, with an average aspect ratio of ~0.5.
Conclusion
EBSD significantly extends the capabilities of conventional SEM analysis. In addition to morphology and chemical composition, it provides quantitative information about crystal structure, grain orientation, phases and texture.
By combining SEM, EDS and EBSD, a comprehensive picture of a material’s microstructure can be obtained. This makes EBSD a valuable technique for materials characterization, process development, quality assessment and failure analysis.
