Grain simultaneously on GBE process such as, starting

Grain Boundaries are the important element of
polycrystalline materials and are studied more carefully as compared to
lattice. Their properties are non-uniform and are structure dependent, e.g.
Diffusion, cracking and Intergranular corrosion at grain boundary. In recent
years, research work is focused towards, understanding and improving grain
boundary geometry by a process known as “Grain
Boundary Engineering (GBE)”.25
years, v randle

The aim of
GBE is to achieve a microstructure in which the constituent grain boundaries
should have minimum low free volume and adequately fit together. As Free energy,
mobility, resistivity, diffusivity and segregation are reduced as a result of
low free volume reduction , which in turn imparts resistance intergranular
corrosion, cracking, cavitation and grain growth at grain boundaries. Randle 2010

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In most of
the GBE processes, thermomechanical treatment is carried out by several cycles
of cold working and annealing. But there is no prime GBE process as there is
effect of multifarious variables simultaneously on GBE process such as,
starting microstructure, % deformation, number of cycles,annealing temperature
and time. The main idea is that grain boundary network should contain numerous
good-fitting boundaries, known as ‘special’ boundaries. Several cycles of short
annealing create twin boundaries, principally via the influence of twin–twin
interactions. The further cycle of GBE process are affected by the presence of
prior annealing twins. Stacking fault energy (SFE) directly effect the twinning
tendency and thus it is carried out on materials having low to medium SFE, such
as copper, austenitic
steel and nickel alloys. These interactions result in ?3n misorientations, in
coincidence site lattice (CSL) catalog. An annealing twin is one type of ?3 interface.
The twinning tendency is directly related to the most effective GBE processing
usually involves breaking up the random grain boundary network with ?3n
boundaries or reducing the random network other than ?3n, so that percentage of?3n
increases.  Grain boundary plane
distributions and single-step versus multiple-step grain boundary engineering Valerie
Randle?, Richard Jones 
. Verbatim quote”Example, consider
a microstructure with 100 boundaries, 25 of which are ?3 and 75 of which are
not. If two thirds of the non- ?3 boundaries are eliminated, then the
concentration of ?3 grain boundaries increases from 25% to 50% without
producing any new ?3s” Orientation Distribution of ?3 Grain Boundary Planes in
Ni Before and After Grain Boundary Engineering

Randle-2006The improvement in
properties is attributed to increase in the percentage of low Coincidence site
lattice (CSL) boundaries. It has been reported that percentage of low CSL boundaries
almost doubled as a result of GBE processing. These low CSL are also known as
‘Special Boundaries’. Moreover Special boundaries terminate at low index
planes,( {111} for FCC) and thus they have low energy d772d6d8aab577a69f9ce78e868b3605aff2.
It
is termed as special because properties are significantly superior as compared
to random boundary such as degradation resistance. They are mostly present on high
angle boundaries in bulk materials by the presence of low index planes because usually
there are few low angle boundaries (or STGBs) in a highly twinned GBE material.
kielFaceting is the common way for a crystal to reduce its energy along low
index planes. For example, consider grain boundary on high energy plane (b).
Energy optimization is done by faceting, i.e. small movement of atoms along
{111}, which results in decrease in energy.

Faceted
boundaries denote cusps in the grain boundary energy, and so in a sense all
boundaries which display facets can be considered to be specialRandle2006

Geometric
description of Grain boundaries tells us that Grain boundaries are not flat and
thus they don’t lie on only one plane, also they contains different atomic
steps as well as foreign atoms. Character description of the Grain Boundaries is
more important and five independent variables are required to describe it. Two
variables are associated with misorientations axis, one with misorientations
angle and two with grain boundary plane, or alternatively, four can be
associated with boundary planes in the adjacent grain (two for each) and one is
associated with twist angle.

Mechanisms of grain boundary engineering Valerie Randle *, Gregory
OwenIt
has been found that previously researchers working on GBE of FCC
polycrystalline materials focused mainly on variables related to
misorientations to characterize the interface. Therefore, out of five
variables, only three variables were used and other two related to grain
boundary planes were neglected. But in recent years, evidence shows that the orientation
of grain boundary planes has strong influence on the properties of special
boundaries.

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.

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

Distribution of Grain Boundary Planes:

(a)   Commercially
GBE copper: The distribution
of all grain boundary planes in the sample population are plotted as output of
MRD on stereographic projection. It is found out that the distribution feature
is a strong maximum of MRD=4.1 at {111} because 63% of the total interface
length is ? 3 and a large fraction of that is coherent twins. However,
when all ? 3 length is subtracted from the interface length, there is still a
small maximum of MRD=1.30 at {111. This is a
significant point because for FCC material {111} is the lowest energy plane,
which shows that there are additional {111} planes present in the random
boundary network. Seventeen per cent of the total interface
length is misoriented on , including 7% ? 9 and 2% ? 27a.Also it is
found out that when ?3 representation is excluded from stereographic
projection, it showed that for high angle boundaries, there is a strong
tendency to form 011 tilt boundaries, i.e. the plane density is mainly
distributed on the 011 zone. This point is also significant because
tilt boundaries are known to be associated with low energy. Whereas
the ? 9 and ? 27a are present out of geometrical necessity associated with
multiple twinning. Moreover, 1% of other low CSL ?5 and ?7 were also present in
misorientation distribution. However there is no tendency for low index planes
in both of these distributions.

The plane
distribution of other materials like Nickel, brass and Austenitic Stainless
Steel is also generally similar to commercial copper.

Experimental evidence of the role of
the grain boundary plane in GBE :

(a)  
Plane orientation distribution before
and after GBE:

To determine the effect of GBE process on a material, referenced sample
of the material is compared with the GBE sample.

For Example: Research has been done on Ni to
compare the orientation distribution in reference sample vs GBE sample  and how ?3 grain boundaries introduced by the
grain boundary engineering process affect the structure of the grain boundary
network.

It has been found out  that the
grain boundary engineered microstructure has a relatively higher concentration
of ?3 grain boundaries and, when compared to the initial structure, more of
these boundaries have orientations that are inclined by more than 10° from the
(111) orientation of the ideal coherent twin. Brandon Criteria is used to
decide if the boundary is ?3 interface or Random. Also for ?3 interface if the inclined angle is
less than 10° of orientation
then it is Coherent
otherwise incoherent twin.

Also during the GBE process the conventionally measured grain size is not
affected, the average size of the regions containing only ?3n grain boundaries
increases by nearly a factor of two. As, during GBE process, grain boundary surface area is continuously
removed to lower the energy and its associated total interfacial area. The
total interfacial energy is decreased by replacing high energy random grain
boundaries with relatively lower energy ?3 boundaries. For Grain boundary plane orientation
distribution, it is found out that ?3
grain a boundary is strongly peaked at the orientation of the coherent twin
(111). The conventional grain size was found out to be 40 ?m for both samples (Reference
as well as GBE). Interesting point was that if ?3 are removed from GBE sample the grain size becomes
56 ?m, which shows the percentage
presence of ?3 is very
large. Orientation
Distribution of ? 3 Grain Boundary Planes in Ni Before and After Grain Boundary
Engineering

(b)  
New perspectives for study of GBE
material

Application of the five-parameters(grain boundary misorientation and plane
distributions) by GBE done to Brass  highlight
important information for GB plain engineering. Rather than counting proportion
of CSL present, the important parameter is to identify those boundaries that
have low index planes, and how such boundaries connect the grain boundary
network. A specimen
having a high proportion of copper texture {112}, where grain boundary planes were
parallel to {111}, was more crack resistant, whereas a specimen having mostly
brass texture, {11
0} and not sufficient {111} boundaries. Furthermore, connectivity of
grain boundary is very important as boundaries having {111} tended to be grouped
together in the crack-resistant specimen.Randle 2006

(c)   
Grain boundary plane distributions
and single-step versus multiple-step grain

boundary engineering :

The ‘five-parameter’ (grain boundary misorientation and plane
distributions)GBE in type 304 austenitic stainless steel has been measured and
evaluated for an Referenced(as received) sample and samples undergoing both
single-step grain boundary engineering processing (SSGBE) and multiple-step grain
boundary engineering processing (MSGBE).It has been find out that properties of
SSGBE is better than Referenced sample but lower than MGSBE and there is low
occurrence of coherent twins in SSGBE compared to MGSBE. As coherent twins are
generated at migrating boundaries by growth coincidence and fast migrating
boundaries suppress coherent twin growth chances. During MGSBE boundary
migration slow down during successive cycles and condition becomes more
favourable for coherent twin generation. There in Grain Boundary plane engineering it is favourable to
have MGSBE processing. Randle
2009

 

 

There is still
lot of work required to comprehend the manipulation of properties by Grain
boundary plane engineering. Moreover, current research is still biased toward
materials which twin readily, but approaches should be determined for
non-twinning materials. But now with sophisticated techniques we can measure
plane orientation accurately, which was not possible previously and there is motivating
evidence of Grain boundary plane engineering on enhancing material properties,
therefore Grain boundary Plane Engineering should be promoted as a viable way
forward for GBE.