A solid solution in a metal is similar to many
solutions you are familiar with. For example: water
dissolves salt. The result is a salty liquid. The taste of
the salt and the wetness of the water have not changed.
As you see, there has been no change of individual
properties. However, you cannot see or distinguish
which is water and which is salt. The loss of individual
identity is apparent. An example of a familiar solid
solution is Monel metal. You know from experience that
Figure 15-4.--Space lattices of two forms of solid solution.
A. Atoms of one element replace atoms of another element.
Monel is tough, and yet soft and plastic; the toughness
B. Atoms of one element fit between atoms of another
element.
combined in the form of a metallic solid solution.
The individual elements lose their identity in a solid
solution. A polished cross section of a material that
characteristics of the metal will be discussed later in this
consists of only one solid solution shows all grains to
chapter.
Ferrite and austenite are two solid solutions that are
ALLOTROPY
important constituents of steel. FERRITE is the name
given to a solid solution of alpha iron and carbon.
AUSTENITE is the term for a solid solution of gamma
Some metals may exist in more than one lattice
iron and carbon. Carbon is only slightly soluble in alpha
form at a particular temperature. When a metal exists in
iron but is quite soluble in gamma iron. Alpha iron at
more than one lattice form it is said to be ALLO-
room temperature can hold only about 0.007 percent
TROPIC in nature. A change from one lattice structure
to another is called an allotropic change. The
carbon in solid solution. At a temperature of 2,065F,
temperature at which allotropic changes take place is
gamma iron can hold up to about 2 percent carbon in
called the TRANSFORMATION TEMPERATURE.
solid solution.
The changes from one form of iron to another are not
As an introduction to compounds, consider
often instantaneous at a specific temperature. Rather,
ordinary table salt. The two poisonous elements,
they generally take place within a range of temperatures
sodium and chlorine, are combined chemically to create
called the TRANSFORMATION TEMPERATURE
a new and different substance, sodium chloride, or table
RANGE. The temperature of the lower end is called the
salt. Salt, with its own identity and properties, does not
lower transformation temperature and the temperature
resemble either sodium or chlorine.
of the upper end is called the upper transformation
Similarly, INTERMETALLIC COMPOUNDS are
temperature.
as carbon or sulfur. Under certain conditions,
intermetallic compounds form and a new substance
INTERNAL STRUCTURE OF METALS
with new properties is created in very much the same
manner but on a more complicated basis. Perhaps the
In alloys (substances composed of two or more
most important thing to remember about the
metals or of a metal and a nonmetal), the internal struc-
intermetallic compounds is the loss of identity and the
ture may be in the form of crystals of pure metals, a solid
change in properties of the combining elements. The
solution, intermetallic compounds, mechanical
heat treater quite often uses the change in properties
mixtures, or some combination of these structures.
offered by compound formations in metals to create
In a solid solution, the elements are completely
compounds with certain desired properties.
dissolved in each other, with the atoms of one element
One intermetallic compound of great importance in
fitting into and forming parts of the space lattice of the
ferrous alloys is known as IRON CARBIDE or
other element. Figure 15-4 illustrates two ways in which
CEMENTITE. This is an extremely hard and brittle
solid solutions may exist. The atoms of one element
compound that is formed by the combination of iron (a
may fit into the spaces between the atoms of another
metal) and carbon (a metalloid). The formula for iron
carbide, or cementite, is Fe3C. This formula shows that
atoms of one element may replace the atoms of another
three atoms of iron combine with one atom of carbon to
view A.
produce one molecule of iron carbide, or cementite
15-5