metallography

Let us next consider the various techniques for microstructural examination.
It is usually necessary to prepare a section of a material in order to study the
size, shape and distribution of crystals within it. In the case of metallic
materials, this is referred to as metallographic examination (‘materialography’
is sometimes used more generally), and great precautions have to be taken at
every stage to ensure that the method of preparation does not itself alter the
microstructure originally present.

If the section for study is cut from the bulk by milling or sawing, or by the
use of an abrasive cutting wheel, ample cooling and lubrication has to be
provided to prevent its temperature from rising. Gross distortions from the
cutting process are eliminated by grinding the surface with successively
finer abrasives such as emery or silicon carbide. If the grains are coarse
enough to be seen with the naked eye, one can at this stage prepare the
surface for macroscopic examination.

The surface of the specimen is etched, usually in a dilute acid, by immersing
it or swabbing it until the individual grains are revealed. Because of the
different rates of chemical attack along different planes in a crystal, when
the surface is etched, crystallographic terraces are formed upon each grain
and these reflect light in directions which vary with the orientation of the
grain, so that some crystals appear light and some dark. The macrostructure
of a piece of cast metal which has been prepared in this way

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 ìm. 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.