What Is EBSD? | What Is Electron Backscatter Diffraction?

Electron backscatter diffraction and microstructure

Electron backscatter diﬀraction (EBSD) is an SEM-based technique used to analyze crystalline materials for quantitative microstructure characterization. Being a non-destructive technique, EBSD is now used in many research and industrial ﬁelds thanks to its speed and the possibility to complement it with other analytical techniques for in-depth characterization of crystalline samples.

Let’s take a step back, though, and ﬁrst describe what crystallinity and the term “microstructure” mean. Materials such as metals, minerals, and ceramics are crystalline. In crystalline materials, atoms are arranged in an ordered structure, and they repeat periodically in space. The repetition of all the arranged atoms in 3D is called a crystal lattice. In some cases, the same lattice repeats for millimeters, and, in that case, we refer to single crystals.

However, in most cases, materials are formed of aggregates of single crystal “grains” (in these cases, we refer to polycrystalline materials). The crystal lattice orientations vary between the diﬀerent grains, and the interfaces between grains are called boundaries, which constitute a change in the orientation from one grain to another.

Why is microstructure important for materials research, quality control, and new product development?

It’s simple: The microstructure, and possible changes in it, can aﬀect the physical and mechanical properties of a material, including strength, ductility, and toughness, but also corrosion, heat resistance, and electrical properties. An extensive knowledge of the properties is required to tune them to improve products and boost new development. In this view, the characterization of the microstructure and what triggers potential changes in it are crucial information, and we can obtain them with EBSD.

Crystal orientation maps are formed by scanning the electron beam over the sample. Elastically scattered backscattered electrons (BSEs), which have undergone coherent Bragg scattering, are collected by a dedicated detector to form electron backscatter diﬀraction patterns (EBSD patterns) at each point while measuring the orientation from each pattern.

These patterns are used to determine the local crystal structure of the analyzed surface, the crystalline orientation, grain size, phase identiﬁcation and distribution, and texture, and to characterize the grain boundaries from ultraﬁne domains. Additional in-depth analysis of a dataset can provide information about dislocations in the material or details about phase transformations and, with advanced analysis tools, insights about elastic strain and residual stresses.

EBSD provides a wealth of information that aids in a better understanding of the microstructure of crystalline materials.

EBSD datasets containing diﬀraction patterns are used to construct several types of important maps, such as inverse pole ﬁgures (IPF), kernel average misorientation (KAM), grain boundaries, and grain average image quality (IQ). These maps allow the determination of the above-mentioned information and will be discussed in more detail in a following article.

EBSD in microstructure analysis

EBSD’s key role in microstructure analysis makes it an indispensable tool in materials science, allowing researchers to get valuable insights into the internal arrangement of materials. Getting a comprehensive microstructural characterization of the materials under study aids in understanding materials' properties and behaviors, making it an essential component for quality control and product development.

For example, the size of the material’s grains inﬂuences the tensile strength, while the properties of grain boundaries can determine the way in which materials fracture. Optical microscopy and scanning electron microscopy are both extensively used to examine microstructures. However, these techniques may not reveal all the grains and would require time-consuming characterization to provide valuable quantitative information. EBSD, on the other hand, provides complete sets of quantitative information, measuring crystal orientations and unambiguously showing the position and the nature of all grains and grain boundaries. A single EBSD acquisition can cover areas ranging from several μm² to many cm².

EBSD sample preparation

One thing to notice is that the sample preparation is crucial to obtain valuable data and high-quality patterns. The latter, in fact, inﬂuence the conﬁdence of the indexing (process of assigning the diﬀraction pattern to a speciﬁc crystal structure) and the accuracy of the results. While surface preparation for EBSD might be challenging and time-consuming, a polished, scratch-free surface is essential. Because the technique characterizes the material’s topmost surface layers, inhomogeneities or imperfections can aﬀect the quality of the acquired data, even preventing patterns from being visible.

While the quality of the acquired patterns is the focus of sample preparation because indexing success is key to achieving good results, another important consideration must be made. Surface characterization of our material of interest always comes with the assumption that the results are representative of the structure of the bulk below the analyzed area. Nonetheless, if the sample preparation alters the surface through, for example, mechanical deformation or localized heating, the information we get might be wrong or not applicable to the overall characteristics of the material under study.

Regardless of the need for a ﬂat and damage-free surface, EBSD is fast and reliable, and it provides spatially resolved information. For these reasons, this technique is often preferred over other analysis methods. Being versatile and fully automated, EBSD is nowadays a powerful tool in many industries and research ﬁelds.