Metals are ubiquitous in engineering, and some like iron, aluminum and titanium particularly
so. But to use them effectively it is important to understand their microstructure, the effects
of alloying, and the different techniques that can be use to strengthen them.
LATTICE DEFECTS
THE CRYSTAL LATTICE AND THE UNIT CELL
The atoms of a pure metal are packed together closely in
a very regular grid. Because of this regular structure,
metal is what we call a crystalline material, and the grid
the atoms are arranged in is called the crystal lattice. The
close packing of the atoms is one of the reasons metals
have much higher densities than most other materials.
We can think of the crystal lattice as a repeating number of identical units, that we call
the unit cell. There are several different ways the atoms of a metal can pack together,
which means that there are several different types of unit cell.
Metals also contain linear defects, called dislocations.
The two main types of dislocations are edge and screw
dislocations, although most real dislocations are a
combination of the two.
Grain Structure
grain
As a metal cools down and solidifies from the molten state,
atoms bond together and a lattice structure begins to form
in several different locations at the same time. Each of
these lattices has its own orientation, so once the metal has
completely cooled it is composed of multiple grains with
different lattice orientations, that are separated by grain
boundaries. Materials made up of a collection of grains are
polycrystalline materials.
STRENGTHENING METALS
grain boundary
A phase diagram shows how the phases within an alloy change for different
temperatures and different amounts of the alloying element. The figure to the
right shows the phase diagram for the iron-carbon alloy. There are four
different phases:
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covering various mechanical and civil engineering topics.
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