Metals Microscopic Examination
In this experiment, our aim is examining the microstructure of metals.
By
studying microscopic structures of metals, we determine which material
fits best
to a given application. We used the most common method, optical
technique, to
examine the microstructure. We used a small specimen cut from
the metal to be
examined. To be able to see the structure clearly, we first
cleaned and polished
the specimen. First we start polishing with emery paper
no: 1 and some finer
grades. One should be careful about the coarse abrasive
particles and striations
from them. Cleaning and rotating the specimen 90°
during the transfer can
prevent these. The next step is polishing, yet
washing the sample before
polishing gives a more successful result. Finally,
we polished the specimen on a
rotating cloth covered with an effective
abrasive like Al2O3-Water suspension.
We kept polishing until we obtained
a mirror like face. After we finished
polishing, the crystalline structure of
the specimen, any cracks, seams,
non-metallic inclusions and inhomogenities,
could be revealed. Before start
etching we first applied mounting process. In
this step we used a matched die
set. We placed our sample into the die set in
the way that the rough face of the
specimen was the lower surface and the
polished face looked upward. We filled
the die cavity with Bakelite and then
we transferred our die to a mounpress.
Mounting not only protects our
sample but also by making its base flat and
stable helps us while we are
examining the sample under the microscope. In
etching process, depending upon
chemical composition, energy content and grain
orientation, we determine the
grain boundaries and the presence of chemically
different phases. To reveal
these micro structural details of the polished mount
we used an etchant like
1% Nital. We immersed the mount with the polished face
upward in a small dish
of etching solution. As soon as a bloom appeared we
removed the mount. At
this stage, the first appearance of microstructure can be
observed. After
etching, we rinsed the mount in water and sprayed alcohol over
it. Finally we
dried it in a stream of hot air to get it ready to be examined
under
microscope.