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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