Application Note: PECS Slope Cutting
of Cast Iron
By Richard Mitro, Gatan Inc.
Cast iron with spheroidal
graphite (SG) is made by adding trace amounts of magnesium
to the same raw material as grey iron melt before casting.
The graphite forms in a spherical shape instead of flakes.
The challenges when preparing samples for analysis are retaining
the graphite in its original shape and size and keeping the
graphite nodules retained. A common preparation error in mechanical
polishing is the insufficient removal of smeared matrix metal
after grinding, which can obscure the true shape and size
of graphite.
Gatan PECS with Slope
cutting for preparation of cast iron
The Model 682.40000
SC-Tool in conjunction with the PECS offers a solution to
mechanical polishing problems and was used here to produce
a cross sectional cut across the sample surface while minimizing
mechanical deformation or damage. Since analysis of a specific
area was not required, an angled cut (45o) was
made to provide more surface area for observation of the bulk
sample. Angled cuts also minimize re-deposition of materials
on the cut surface. The cutting head assembly with the sample
was rocked over an angle of 75o to reduce/eliminate
striations in the cut surface.
Preparation: A section
was roughly cut (12mm x 6mm x 1.5mm Thick) from a bulk sample
using a diamond saw with the top as cast surface retained
even after slope cutting and etching. The sample was then
mounted to the 45o cutting stub without additional
preparation steps.
Cutting parameters:
Ion gun voltage 6 keV, Ion beam current
300 uA, 75o rock in right angle orientation, Time
5 hours This resulted in a cut surface measuring 30um –
70um in depth across the entire 12mm width of the sample.
The cut surface revealed the presence of two distinct phases
the dark spherical graphite phase and the lighter matrix as
shown in Fig. 1.
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Fig.
1 SEM image showing the top surface, the cut surface
and front sputtered region. |
An important feature
of the PECS and SC tool is that after cutting to the preferred
depth, the sample may be ion beam polished and selectively
etched. This enables decoration of the cut surface to reveal
internal grain/phase structure and topographical differentiation.
The quality of the graphite matrix clearly shows slope cutting
preserves structures that conventional mechanical polishing
can damage.
The difficulty in the metallographic preparation of cast irons
is to retain the true shape and size of the graphite in its
flake, nodular or tempered form. During grinding the matrix
is smeared over the graphite and unless it is followed by
a very thorough diamond polish, the graphite is not seen in
its true form. Slope cutting and etching eliminate this problem
as shown here. Etching of the sample, after cutting (Fig.
2) revealed the lighter matrix of pearlite microstructure
and also the grain boundaries between the various bi-crystals
of cementite and ferrite (Fig. 3).
Etching parameters:
Etching was done in two steps; first polishing was performed
at a grazing angle to provide a flat surface and second perpendicular
etching to enhance the pearlite microstructure.
Step 1: Ion gun voltage 6 keV, Ion beam current
300 uA, 80o fixed angle, Time 30min
Step2: Ion gun voltage 6 keV, Ion beam current
300 uA, 0o fixed angle, Time 8min
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Fig.
2 SEM image showing the top surface and cut face of
the graphite node, the pearlite microstructure is also
seen. |
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Fig.
3 SEM image showing the grain boundaries between the
various bi-crMMystals of Ferrite and Cementite. |
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Conclusions:
PECS slope cutting along with selective etching resulted in
a clean cut-surface, 30-70 microns deep artifact free and undamaged
graphite nodes. Etching resulted in the emergence of a pearlite
microstructure revealing thin plates of cementite well differentiated
from the ferrite matrix. The grain boundaries between the various
bi-crystals of cementite and ferrite were made evident as well.
Thus, slope cutting combined with selective etching successfully
resulted in revealing the internal structure combining material
contrast with topographical contrast.
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