Materials for engineering, 3rd Edition - (Malestrom)

Structure of engineering materials 11
1.6 Macrostructure of a cast metal. Large equiaxed grains have
formed at the centre of the ingot. (Courtesy Dr P. A. Withey.)
is too fine to be discerned without the use of a microscope and specimen
preparation is much more critical than for macro-examination. Polishing to
a mirror finish is necessary, usually by holding the specimen against a horizontal
rotating wheel covered with a short-pile cloth fed with a suspension or cream
of a polishing agent. The latter can be magnesium oxide or aluminium oxide
powder, although diamond pastes (of micrometre particle size) are commonly
used. In the case of electrically conducting specimens such as metals, the
final finish is often achieved by electrolytic polishing, where the specimen is
made the anode in a suitable electrolyte. If the current density is correct, a
bright scratch-free surface can be produced.
A much lighter etching treatment is applied for microscopical examination
than for macro-studies. With some etching reagents and very short etching
times, metal is dissolved only at the grain boundaries, giving rise to shallow
grooves there, which are seen as a network of dark lines under the microscope.
A reflecting optical microscope may give magnifications of over 1000 ×,
with a resolution of about 1 µm. The upper limit of magnification of the
optical microscope is often inadequate to resolve structural features which
are important in engineering materials, however, and electron microscopy is
widely employed for this purpose. Field-ion microscopy is a research tool
with a resolving power that permits the resolution of the individual atoms in
crystals and these can be identified by use of the atom-probe technique.
The two most commonly employed techniques of electron microscopy
(EM) are scanning electron microscopy (SEM) and transmission electron
microscopy (TEM).