The basics of EBSD
Background
The EBSD technique (also known as Backscattered Kikuchi Diffraction, BKD)
was first developed by Alam and co-workers in 1954, who described some
diffraction patterns and called them “wide-angle back-scatter Kikuchi
patterns”, in recognition of related diffraction phenomena reported
by Kikuchi in the 1920’s. However, it was not until the 1970’s
that Venables and co-workers applied EBSD to metallurgical microcrystallography,
paving the way for a more widespread application of EBSD to the materials
sciences in the ensuing 20 years. Rapid developments in both hardware
and software in the past 10 years has made EBSD an easy to use technique
ideal for the rapid analysis of microstructures in a range of crystalline
materials.
EBSD Explained
The collection of an electron backscatter diffraction pattern (EBSP) in
the SEM is relatively straightforward. A polished sample must be tilted
to a relatively high angle (typically 70°) inside the SEM. The electron
beam is then directed at the point of interest on the sample surface:
initial elastic scattering of the incident beam causes the electrons to
diverge from a point just below the sample surface and to impinge upon
crystal planes in all directions. Wherever the Bragg condition for diffraction
is satisfied by a family of atomic lattice planes in the crystal, 2 cones
of diffracted electrons are produced. These cones are produced for each
family of lattice planes.
(Fig 1)
These cones of electrons can be imaged using a phosphor screen attached
to a sensitive camera. the camera is usually positioned horizontally,
so that the phosphor screen is close to the sample in order to capture
a wide angle of the diffraction patterns. Where the cones of electrons
intersect with the phosphor screen, they appear as thin bands. These are
called “Kikuchi bands”, and each one corresponds to a family
of crystal lattice planes. The resulting EBSP is made up of many Kikuchi
bands.
EBSD software automatically locates the positions of individual Kikuchi
bands, compares these to theoretical data about the relevant phase and
rapidly calculates the 3-D crystallographic orientation. The whole process
from start to finish can take less than 0.05 seconds.
Orientation Mapping
This is the most common approach to sample analysis using EBSD. The electron
beam is stepped across the surface of the sample at regularly spaced points;
at each point an EBSP is collected, indexed by the EBSD software and the
orientation and phase information is stored. This information can then
be used to reconstruct the microstructure in the form of an orientation
or a phase map, and the data provide the complete microstructural characteristics
of the sample. Orientation mapping is a quick, easy and comprehensive
approach to characterising microstructures.
EBSD Performance
This table quickly summarises the capability of the EBSD technique, although
the exact values are very dependent on the type of SEM and the material
being analysed.
Spatial resolution 0.05 - 0.5 µm
Angular resolution0.25 - 1°
Acquisition time 0.04 - 1 s
Typical SEM conditions 20kV, 2nA probe
Range of crystal symmetries All 11 Laue groups
Number of phases Unlimited
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Fig 1.
Production of EBSP