rate of cooling from a molten state to a solid state. If a
GRAINS (irregularly shaped crystals developed from
the converging lattices). These irregularly shaped
metal cools rapidly, many grains will form, and the size
of the grain will be smaller. The smaller the grain, the
crystals are formed as the metal cools and changes from
a liquid state to a solid state. This change of state is
shorter the slip planes, and the harder and stronger the
called FREEZING or SOLIDIFICATION. As the metal
metal will be. The opposite will occur if the metal is
cooled slowly. Grain size, therefore, directly affects the
freezes, its atoms lose the energy of motion they had in
physical and the mechanical properties of metal and can
the metal's liquid state. The slow-moving atoms
become attached to one another, usually in one of four
be changed by various heat treatment methods.
predictable patterns, depending on the type and
Some grains may be seen with the naked eye. Others
can only be seen under magnification. The study of the
figure 15-1 and will be discussed in greater detail later
microscopic structure of metals is called metallography.
in this chapter.
Instruments used to magnify grains are called
METALLURGICAL MICROSCOPES. These micro-
CRYSTALS AND GRAINS
scopes can magnify grains several hundred times their
actual size. Additional magnification can be gained
The first groups of atoms to form a pattern create
through the use of an ELECTRON MICROSCOPE,
what arc called UNIT CELLS, or GRAIN NUCLEI.
which can magnify up to many thousand times actual
The unit cell is the basic building block of the metal.
size.
Many unit cells connect in the same pattern that makes
Metallurgical microscopes equipped with devices
up the unit cell and forms a CRYSTAL. Crystals then
for photographing the microstructure of metals are
connect to form the metal's CRYSTALLINE STRUC-
known as METALLOGRAPHIC MICROSCOPES.
TURE.
Under ideal conditions, crystals will be perfectly
SPACE LATTICES
shaped. Under real world conditions, however, crystal
shapes are usually distorted. These typically distorted
The arrangement of atoms (the most basic unit of
crystals, or grains, form what is known as the GRAIN
matter) in a crystal can be shown by a graphic
STRUCTURE of the metal.
illustration called a SPACE LATTICE. Each of the four
line-dot drawings in figure 15-l is a space lattice. The
The surface of a metal does not indicate its internal
dots represent atoms, while the lines make the shape
grain structure. However, when the metal is fractured,
easier to visualize.
structure will show on the fractured surface. The size of
the grains depends upon a number of factors, including
The space lattices of most metals can be identified
the nature of the metal, the temperature to which it is
as one of the following four types:
heated, the length of time it is held at a specific
1. Body-centered cubic
temperature, and the rate at which it is cooled from a
liquid to a solid. In general, the quicker a metal
2. Face-centered cubic
solidifies, the smaller the grains will be.
3. Body-centered tetragonal
The line-shaped areas between adjacent grains are
4. Hexagonal close-packed
stress exists at the grain boundaries, due to a mismatch
Body-Centered Cubic Lattice
of each lattice. When a metal is deformed, the atoms in
the structure slide over one another along certain planes
called SLIP PLANES. Slip planes are the planes of least
resistance to an applied force. Metals with large grains
contains nine atoms, one at each corner of the cube and
have long slip planes, allowing deformation to occur
one at the center of the cube. In this arrangement, each
easily. On the other hand, metals with small grains have
atom is held in position by the force of the remaining
short slip planes, making deformation difficult. The
eight atoms.
plane along which a metal separates when subjected to
Metals that have the body-centered cubic arrange-
an applied force is called a CLEAVAGE PLANE.
ment are usually stronger, but harder to work cold,
GRAIN SIZE is determined by the number of grains
than metals that have a face-centered cubic arrange-
per square inch in a metal magnified 100 times normal
ment (discussed next). Some of the body-centered
size. The number of grains that form depends upon the
metals are tungsten, molybdenum (MO), vanadium (V),
15-2