Electron backscatter diffraction (EBSD) is a technique for obtaining crystallographic information from samples in the scanning electron microscope.
In this guide to EBSD for beginners we aim to explain both why it is useful to obtain this information and how the EBSD technique works.
30 second overview
This animation shows the basic principle. In this case, an electron beam moves to 5 points on the sample - each point is made from the same material but the grains/crystals are oriented in different ways. Each of the 5 points results in a different backscatter pattern, this is analysed by the software to derive the crystal orientation at that point.
Crystalline Materials and Microstructure
First of all, it's important to understand what crystalline materials are and the concept of microstructure.
Familiar materials such as metals, minerals or ceramics are crystalline. In crystalline materials, the atoms that form the material are arranged to repeat periodically in space. The imaginary three-dimensional grid of points on which the atoms sit is called the crystal lattice. Of course, the size of the atoms and the distances between the repeating groups of atoms are tiny. For example, in aluminium, the atoms are arranged at the corners and face centres of a cube. The length of the edge of the cube is 0.405 nanometre - about 200,000 times smaller than a human hair. (1 nanometre is 10-9m).
At the size scale of atoms, the crystalline structure of material is very regular. Sometimes, atoms can form single crystals where the crystal structure is uniform over many millimetres. Everyone is familiar with the appearance of natural crystals of minerals such as quartz. In these cases the shape and symmetry of the crystal reflects the underlying regularity in the atomic structure. Single crystals can also be fabricated. For example, the single crystal silicon wafers used in microelectronics are up to 300 mm wide.
Left: The unit cell for aluminium. The atoms are at the corners and face centres of a cube.
Right: The unit cell for alumina containing aluminium atoms (red) and oxygen atoms (green).
More often however, the crystal structure is uniform over only short distances. Material is commonly formed of an aggregate of single crystal grains. Such material is called polycrystalline and the size of the grains can range from nanometres to being visible to the naked eye. Even within the single crystal grains the lattice is not perfect and can contain defects which have important effects on the behaviour of the material. The microstructure of a material refers to the assemblage of grains together with other microscopic constituents such as pores and inclusions.
Materials commonly used in engineering such as steel and aluminium are polycrystalline so it is important to have techniques that allow us to analyse their structure in detail. From the perspective of EBSD there are two important characteristics of polycrystalline materials. Firstly, the crystals in the different grains have different orientations. This means the edges of the crystal lattice point in different directions in different grains - if you can't imagine this it will be made clearer in the following sections! Secondly, polycrystalline materials contain regions where the different grains meet called grain boundaries.
Microstructural Analysis with EBSD
Microstructure is important because it determines many of the physical properties of materials.
For example, grain size can influence tensile strength and the properties of grain boundaries can determine the way in which materials fracture. Optical microscopy and scanning electron microscopy are both used to examine microstructure. Polishing and chemical etching can reveal the positions of grains and grain boundaries. However, these techniques may not reveal all the grains. This is where EBSD comes in - it measures crystal orientation and so must be able to show unambiguously the position of all grains and grain boundaries.
EBSD is used to form crystal orientation maps by scanning the electron beam over the sample and measuring the orientation from the diffraction pattern at each point. In a crystal orientation map points with similar crystal orientations are shown in similar colours. In these maps a grain is a region of the sample where the crystal orientation is the same within a certain orientation angle tolerance. The maps can be processed to show with certainty the position of all the grains and grain boundaries.
EBSD is unique in that it provides a link between microstructure and crystallography. It complements conventional analysis techniques by providing definitive information about the crystal orientations present in the sample.
A scanning electron micrograph of the microstructure of a steel sample.
This image shows a crystal orientation map from the same sample. Similar orientations are in similar colours.
This shows grains in the same sample. Different colours are used to identify the grains.
This shows the position of grain boundaries. Different colours show different grain boundary orientations.
Click to see the corresponding crystal orientation data from this sample.
The Diffraction Pattern
In electron backscatter diffraction (EBSD) a beam of electrons is directed onto a tilted crystalline sample in a scanning electron microscope (SEM).
The electrons undergo various interactions with the atoms in the crystal lattice and some of the electrons emerge from the sample. If a fluorescent phosphor screen is placed close to the sample a pattern is formed on the screen because the intensity of the emerging electrons varies slightly with direction. This pattern is called a diffraction pattern and its appearance is very striking.
Unfortunately, there is no simple analogy in the everyday world for the physical phenomena of diffraction that causes the patterns. The symmetry and appearance of the pattern is related intimately to the crystal structure at the point where the beam meets the sample. If the crystal rotates (in other words its orientation changes) the diffraction pattern will be seen to move. If a different type of material is placed under the beam, the diffraction pattern will change completely. So the diffraction pattern can be used to measure crystal orientations and to identify materials.
The top image is a scanning electron micrograph showing the microstructure of a stainless steel. The diffraction pattern and crystal orientation are shown for the point marked by the blue cross. The diffraction pattern is obtained by placing a stationary electron beam at this point.
When the button is pressed, a different point in the microstructure is selected and the corresponding diffraction pattern and crystal orientation shown.
One of the first areas where EBSD was used was in texture analysis.
As you have learnt, many materials are polycrystalline. However, the grains in polycrystalline materials are not usually oriented randomly. The grains often cluster close to certain orientations. The way in which the orientations cluster is called the texture. Textures form because of the ways in which grains form from molten material and from later processing steps.
In a single crystal, physical properties are different depending on the direction in the crystal in which they are measured. So the physical properties of polycrystalline material are dependent on the distribution of crystal orientations present.
Formability of sheet materials is strongly dependent on the material texture and the processing steps in reaching the final material are monitored carefully to ensure the texture is achieved. For example, the texture in aluminium sheets used for beverage cans is carefully controlled - if it were not the tops of the cans would be uneven or the surface not smooth. Many electrical and magnetic material properties also depend on crystal orientation. For example, the cores of electrical transformers are textured in a way to reduce the energy losses in the transformer.
Because EBSD measures crystal orientation the data it provides can also be used to analyse the sample texture. In this way, EBSD is complementary to the standard method of measuring texture using X-ray diffraction. However, EBSD goes further because it can relate the texture (or textures) present to the microstructure of the material. In addition, textures which vary from place to place in a sample can be studied easily. The way in which textures develop can be studied at the level of the microstructure with EBSD.