Structure of Metals
Most of the materials used in structural engineering or component fabrication are
metals. Alloying is a common practice because metallic bonds allow joining of
different types of metals.
DEFINE the term alloy.
DESCRIBE an alloy as to the three possible microstructures
and the two general characteristics as compared to pure metals.
IDENTIFY the two desirable properties of type 304 stainless
An alloy is a mixture of two or more materials, at least one of which is a metal. Alloys can
have a microstructure consisting of solid solutions, where secondary atoms are introduced as
substitutionals or interstitials (discussed further in the next chapter and Module 5, Plant
Materials) in a crystal lattice. An alloy might also be a crystal with a metallic compound at each
lattice point. In addition, alloys may be composed of secondary crystals imbedded in a primary
polycrystalline matrix. This type of alloy is called a composite (although the term "composite"
does not necessarily imply that the component materials are metals). Module 2, Properties of
Metals, discusses how different elements change the physical properties of a metal.
Alloys are usually stronger than pure metals, although they generally offer reduced electrical and
thermal conductivity. Strength is the most important criterion by which many structural
materials are judged. Therefore, alloys are used for engineering construction. Steel, probably
the most common structural metal, is a good example of an alloy. It is an alloy of iron and
carbon, with other elements to give it certain desirable properties.
As mentioned in the previous chapter, it is sometimes possible for a material to be composed
of several solid phases. The strengths of these materials are enhanced by allowing a solid
structure to become a form composed of two interspersed phases. When the material in question
is an alloy, it is possible to quench (discussed in more detail in Module 2, Properties of Metals)
the metal from a molten state to form the interspersed phases. The type and rate of quenching
determines the final solid structure and, therefore, its properties.