The crystallization of a large amount of material from a single
point of nucleation results in a single crystal. In engineering
materials, single crystals are produced only under carefully controlled
conditions. The expense of producing single crystal materials
is only justified for special applications, such as turbine engine
blades, solar cells, and piezoelectric materials. Normally when
a material begins to solidify, multiple crystals begin to grow
in the liquid and a polycrystalline (more than one crystal) solid
The moment a crystal begins to grow is know as nucleation and
the point where it occurs is the nucleation point. At the solidification
temperature, atoms of a liquid, such as melted metal, begin to
bond together at the nucleation points and start to form crystals.
The final sizes of the individual crystals depend on the number
of nucleation points. The crystals increase in size by the progressive
addition of atoms and grow until they impinge upon adjacent growing
a) Nucleation of crystals,
b) crystal growth, c) irregular
grains form as crystals grow together, d) grain
boundaries as seen in a microscope.
In engineering materials, a crystal is usually referred to as
a grain. A grain is merely a crystal without smooth faces because
its growth was impeded by contact with another grain or a boundary
surface. The interface formed between grains is called a grain
boundary. The atoms between the grains (at the grain boundaries)
have no crystalline structure and are said to be disordered.
Grains are sometimes large enough to be visible under an ordinary
light microscope or even to the unaided eye. The spangles that
are seen on newly galvanized metals are grains. Rapid cooling
generally results in more nucleation points and smaller grains
(a fine grain structure). Slow cooling generally results in larger
grains which will have lower strength, hardness and ductility.
metals, the crystals that form in the liquid during freezing generally
follow a pattern consisting of a main branch with many appendages.
A crystal with this morphology slightly resembles a pine tree
and is called a dendrite, which means branching. The formation
of dendrites occurs because crystals grow in defined planes due
to the crystal lattice they create. The figure to the right shows
how a cubic crystal can grow in a melt in three dimensions, which
correspond to the six faces of the cube. For clarity of illustration,
the adding of unit cells with continued solidification from the
six faces is shown simply as lines. Secondary dendrite arms branch
off the primary arm, and tertiary arms off the secondary arms
freezing of a polycrystalline material, many dendritic crystals
form and grow until they eventually become large enough to impinge
upon each other. Eventually, the interdendriticspaces between
the dendrite arms crystallize to yield a more regular crystal.
The original dendritic pattern may not be apparent when examining
the microstructure of a material. However, dendrites can often
be seen in solidification voids that sometimes occur in castings
or welds, as shown to the right..
Most materials contract or shrink during solidification and cooling.
Shrinkage is the result of:
- Contraction of the liquid as it cools prior to its solidification
- Contraction during phase change from a liquid to solid
- Contraction of the solid as it continues to cool to ambient
Shrinkage can sometimes cause cracking to occur in component
as it solidifies. Since the coolest area of a volume of liquid
is where it contacts a mold or die, solidification usually begins
first at this surface. As the crystals grow inward, the material
continues to shrink. If the solid surface is too rigid and will
not deform to accommodate the internal shrinkage, the stresses
can become high enough to exceed the tensile strength of the material
and cause a crack to form. Shrinkage cavitation sometimes occurs
because as a material solidifies inward, shrinkage occurred to
such an extent that there is not enough atoms present to fill
the available space and a void is left.