Welding Inspection & Metallurgy API ICP Self Study Notes

10.2 THE STRUCTURE OF METALS AND ALLOYS
Solid metals are crystalline in nature and all have a structure in which the
atoms of each crystal are arranged in a specific geometric pattern. The
physical properties of metallic materials including strength, ductility and
toughness can be attributed to the chemical make-up and orderly
arrangement of these atoms.
Metals in molten or liquid states have no orderly arrangement to the atoms
contained in the melt. As the melt cools, a temperature is reached at which
clusters of atoms bond with each other and start to solidify developing into
solid crystals within the melt. The individual crystals of pure metal are
identical except for their orientation and are called grains. As the temperature
is reduced further, these crystals change in form eventually touch and where
the grains touch an irregular transition layer of atoms is formed, which is
called the grain boundary. Eventually the entire melt solidifies, interlocking
the grains into a solid metallic structure called a casting.