Results grain boundary plane orientation. To determine the geometry

Results
obtained from Molecular Dynamics and high resolution electron microscopy have
confirmed the role of grain boundary planes. Grain boundary energy is also
influenced by the tilt and twist characteristics of grain boundary plane, as
they have lower energy compared to the average energy.{A tilt boundary occurs where angle of
misorientation is parallel to the misorientation axis. If the boundary planes
from both interfacing grains are the same It is called symmetrical tilt grain
boundary (STGB), or if there are dissimilar planes from each grain at the
interface an asymmetrical tilt grain boundary (ATGB).Twist boundaries have same
boundary planes in both interfacing grains and the misorientation axis perpendicular
to the boundary plane. Under certain conditions boundary planes can reorient to
lower energy boundary plane without grain rotation or boundary misorientation. Therefore,
considering the grain boundary planes effect on properties, it is necessary to
consider both the orientation distribution of boundary planes as well as to
modify plane orientation towards lowering energy.

a
study it is shown that in iron
bicrystal in
contrast to the coincidence site lattice approach at least one boundary plane was
special, as it was independent of misorientation.

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This
finding raises a new challenge to grain boundary engineering as we have to
consider all five characteristic variables related to grain boundary. Grain
boundary plane engineering is a viable way forward for Grain boundary
engineering. The main reason why grain boundary planes were not taken into
account was because of experimental simplification as there were difficulties
associated in measuring the orientation of Grain boundary planes. Grain boundary plane measurement is not as straightforward
as misorientation measurement because the plane inclination is buried within
the opaque specimen and needs to be accessed somehow. Furthermore, grain
boundary surface is not planar along microfacets but macroscopically curved.

In
recent years several approaches of increasing sophistication are developed
which can overcome the complexities of determining grain boundary plane
orientation.

To determine
the geometry and crystallography of adjoining crystallites, Automated scanning
electron microscope (SEM) mapping is used to record patterns and images of ‘Electron
backscattered diffraction (EBSD). For in depth measurement of grain boundary
geometry, secondary electron images are recorded with submicron resolution.
After the image is recorded, EBSD measurements are made at regular intervals known
as sector. When one sector is characterized, stage automatically moves to the adjacent
sector. As, SEM images are usedto determine grain boundary positions, we can
resolve the positions accurately without gathering redundant orientation data.
After one surface is mapped, serial sectioning is used to remove that surface
layer and the process is repeated. This allows construction of three-dimensional
grain boundary network, specify the misorientation (?g) and the boundary normal direction (n) for a large
number of planar segments. Thus, the grain boundary distribution, ? (?g,n),
is the frequency of occurrence of a specific type of grain boundary, distinguished
on the basis of ?g and n, in units of multiples of a random distribution (MRD).

Another
promising new approach  to determine grain
boundary planes is by the use of ‘dual beam’ instrument, which used focused ion
beam serial sectioning in combination with in situ electron backscatter diffraction.
The three dimensional microstructure is reconstructed from the sections.
Although, data on grain boundary planes are is obtained from this technique but
still there are improvements going on this technique. Diffraction contrast
tomography is also developed to determine grain boundary plane crystallography
in small specimens.

Because it is now possible to measure
these distributions, it is also possible to use them as a metric for
macroscopic materials properties