One intermetallic compound of great importance
ways in which solid solutions may exist. The atoms of
in ferrous alloys is known as IRON CARBON or
one element may fit into the spaces between the atoms
CEMENTITE. This is an extremely hard and brittle
of another element, as indicated in view A; or the
atoms of one element may replace the atoms of another
compound that is formed by the combination of iron
element in the space lattice, as indicated in view B.
(a metal) and carbon (a metalloid). The formula for
iron carbide or cementite is Fe3C. This formula shows
A solid solution in a metal is similar to many
that three atoms of iron combine with one atom of
solutions you are familiar with; for example, water
carbon to produce iron carbide, or cementite.
dissolves salt. The result is a wet salty liquid. The taste
of the salt and the wetness of the water have not
The structure of an alloy is described as being a
changed. As you see, there has been no change of
MECHANICAL MIXTURE when two or more
individual properties. However, you cannot see or
structural forms are mixed together but are still
distinguish which is water or which is salt. The loss of
separately distinguishable. A mechanical mixture of
individual identity is apparent. An example of a
an alloy is comparable, though on a smaller scale, to
familiar solid solution is Monel metal. You know from
the mixture of sand and gravel that may be seen in
experience that Monel is tough, and yet, soft and
concrete.
plastic; the toughness of nickel and the plasticity of
One of the most important mechanical mixtures
copper have been combined in the form of a metallic
that occurs in many steels is known as PEARLITE.
solid solution.
Pearlite, so called because it has a pearly luster when
The individual elements lose their identity in a
seen under a microscope, is an intimate mechanical
solid solution. A polished cross section of a material
mixture of ferrite and cementite in alternate plates or
that consisted of only one solid solution would show
layers. Ferrite is a solid solution, and cementite or iron
all grains to be of the same nominal composition.
carbide is an intermetallic compound; in pearlite the
two are closely mixed to form a characteristic layered
Ferrite and austenite are two solid solutions that
structure.
are important constituents of steel. Ferrite is the name
given to a solid solution of alpha iron and carbon.
Pearlite is formed when steel that contains just
Austenite is the term for a solid solution of gamma iron
about 0.85 percent carbon is heated to a certain
and carbon. Carbon is only slightly soluble in alpha
temperature and then cooled slowly. When the entire
iron but is quite soluble in gamma iron. Alpha iron at
structure of the alloy is in the form of pearlite, this
room temperature can hold only about 0.007 percent
carbon in solid solution. At a temperature of 2065F.
carbon) is often referred to as the eutectoid
gamma iron can hold up to about 1.7 percent carbon
in solid solution.
called the EUTECTOID or the EUTECTOID
INTERMETALLIC COMPOUNDS are com-
STRUCTURE.
pounds formed between a metal and some other sub-
The internal structure of an alloy may show
stance such as carbon or sulfur. There are many
ordinary compounds that we are familiar with in eve-
intermetallic compounds, and mechanical mixtures.
ryday life; common table salt is one. The two poison-
ous elements, sodium and chlorine, when combined,
form sodium chloride or common table salt. Salt does
cooling of the alloy; in other words, they are the result
not resemble either sodium or chlorine, either by iden-
of heat treatment. Figure 6-8 shows, very much
tity or properties. When the two elements are com-
enlarged, a typical combination that occurs when plain
bined chemically, a new and different substance is
carbon steel containing less than 0.85 percent carbon
created. Under certain conditions, intermetallic com-
is heated to a certain temperature and then cooled
pounds form and a new substance with new properties
slowly. As may be seen, this combination consists of
is created in very much the same manner, but on a
the solid solution ferrite and the mechanical mixture
more complicated basis. Perhaps the most important
pearlite, each in crystal form, distributed throughout
thing to remember about the intermetallic compounds
is the loss of identity and the change in properties. The
pearlite in this combination depend largely upon the
heat treater quite often utilizes the change in properties
carbon content of the alloy.
offered by compound formations in metals.
6-6