The data collected with EBSD contains a wealth of sample information which can be processed using a suite of analytical tools to visualise and represent microstructure at the micro and nano scale.
Interrogating the crystallographic orientation and phase information acquired with EBSD can then be processed to deliver information about the sample, which can be linked to the materials processing history and likely performance. Examples include
- Grain Size
- Grain Boundary Characterisation
- Phase Distribution and Fraction
- EBSD Pattern Quality
- Orientation Data
- Orientation Maps
- Pole Figures
- Internal Microstructure
The mechanical and physical properties of metallic materials are closely related to grain size e.g. through the Hall- Petch relationship, where strength is inversely dependent to the square root of grain size . Electron backscatter diffraction (EBSD) on a Scanning Electron Microscope (SEM) is the ideal technique for determining grain size.
A grain is a three-dimensional crystalline volume within a sample that differs in crystallographic orientation from its surroundings but internally has little variation. Grains are identified by defining a critical misorientation angle and grain boundaries are ‘pixel pairs’ which have a misorientation higher than the critical angle. Once individual grains are detected a statistical overview of the grains in the sample can be presented, coupled with a grain map illustrating the individual grains. This data can be linked to the phases present in the map.
In addition, grain measurement parameters can be used to visualise microstructure in a grain measurement map. This representation will highlight, for example, larger grains, or those of a specific size or shape.
The interface between two grains in a polycrystalline material creates a grain boundary. Grain boundaries will influence the properties of the material: typically grain boundaries are a site for the initiation of corrosion and also for the precipitation of new phases from the solid. They are also important in the mechanisms of creep. Grain boundaries can also be beneficial, and disrupt the movement of dislocations through a material, so reducing grain size, and increasing boundaries, is a way to improve mechanical strength. Techniques such as grain boundary engineering (GBE) are applied to improve material properties. As such it is important to identify and characterise different boundary types, and to understand the impact on material behaviour.
Generating a map representation of grain boundaries is powerful when visualising microstructure. Different boundary types are identified by misorientation between the two grains. Typically low angle boundaries or subgrain boundaries are those with a misorientation less than 5 degrees. High angle grain boundaries have a larger misorientation generally greater than 10 degrees. In addition, special boundaries or twin boundaries occur where the crystal lattices share a fraction of the sites on either side of the boundary. These boundaries are called coincident site lattices (CSL), and are denoted by Σ, where Σ is the ratio of the size of the CSL unit cell to the standard unit cell.
A ‘twin limited’ microstructure, i.e. a microstructure composed entirely of special grain boundaries and triple junctions is highly resistant to intergranular degradation. These different grain boundary types are readily identified and displayed with using EBSD.
The identification and distribution of different phases is another important application of EBSD. Phase distribution is represented on a map, with a measure of phase fraction. A phase map is powerful in representing the spatial distribution of phases, for example useful in determining precipitates formation at grain boundaries.
(a) EBSD phase map of a duplex steel sample. The microstructure contains austenite (red) and ferrite (blue) phases; it also includes intermetallic precipitation of both sigma (yellow) and chi (green) phases. These intermetallics are significant as they will degrade mechanical and corrosion properties of the material. Therefore it is important to identify them, determine their distribution and phase fraction.
The EBSD pattern quality parameter assigns a number to the degree of sharpness or band definition in the EBSP. Therefore the pattern quality is influenced by several factors: phase, orientation, contamination, sample preparation and the local crystalline perfection.
Pattern quality maps will often reveal features invisible in the electron image such as grains, grain boundaries, internal grain structure and surface damage such as scratches. The pattern quality map is therefore very useful both during the analysis of the data and as a simple tool for checking the sample before and during analysis – in terms of focus and drift.
In many materials grains do not have a completely random orientation distribution. When orientation is not random the material is said to have preferred orientation or texture. The individual crystal orientation measurements collected by EBSD can be used to show the crystallographic textures developed in the sample. The orientation information acquired from multiple points within each phase enables a statistical check whether that phase has a preferred orientation. This can be achieved by studying orientation maps or by creating pole figures.
The orientation data collected with an EBSD system is spatially displayed in either an Euler Map or a series or inverse pole figure (IPF) maps. The Euler maps give a basic presentation of microstructure. The IPF maps uses the colour from the IPF colour key, in this case the colour assigned is based on the measured orientation and the selected viewing direction. This map is good at representing preferred orientation (or texture), seen as similar or single colours in the map. The orientation data displayed in a map makes it easy to visualise and extract information about how a specific texture is spatially distributed.
In addition those points in the material which have a specific texture in an EBSD map can be identified. This is a useful tool in identifying reference or ideal textures in a sample, such as Cube, GOSS or Fibre.
Pole figures are also applied for displaying texture. They enable 3D orientation data to be plotted in 2D, by converting crystallographic directions into points. This is done automatically with modern EBSD systems, with the pole figure being created being determined by the crystal structure of the phase being displayed.
The orientation data measured by EBSD can be processed to illustrate different aspects of material microstructure. There are many examples of this; probably one of the most frequently used in literature is the Kernel Average Misorientation map (KAM). This is a calculation of the average misorientation between each pixel and its nearest neighbours. This map is used to study subgrain structures, which are an indication of strain which has occurred.