NATURE OF MATERIALS - Materials for Civil and Construction Engineers - page 68

agent is simply a chemical that is compounded or in solution in the crys-
talline structure of a metal. Steel, composed primarily of iron and carbon, is
perhaps the most common alloy. The way the alloying agent fits into the crys-
talline structure is extremely important. The alloying atoms can either fit into
the voids between the atoms ( interstitial atoms) or can replace the atoms in
the lattice structure ( substitutional atoms).
Because metals have a close pack structure, the radius of the intersti-
tial atom must be less than 0.6 of the radius of the host element (Derucher
et. al. 1994). Also, the solubility limit of interstitial atoms—less than 6%—
is relatively low. Interstitial atoms can be larger than the size of the void
in the lattice structure, but this will result in a strain of the structure and,
therefore, will limit solubility.
If the characteristics of two metal elements are sufficiently similar, the
metals can have complete miscibility; that is, there is no solubility limit.
The atoms of the elements are completely interchangeable. The similarity
criteria are defined by the Hume-Rothery rules (Shackelford 1996). Under
these rules, the elements must have the following characteristics:

1. less than 15% difference in the atomic radius
2. the same crystal structure
3. similar electronegatives (the ability of the atom to attract an electron)
4. the same valence

Violation of any of the Hume-Rothery rules reduces the solubility of the
atoms. Atoms that are either too large or too small will result in a strain of
the lattice structure.
The arrangement of the alloy atoms in the structure can be either ran-
dom or ordered. In a random arrangement, there is no pattern to the place-
ment of the alloy atoms. An ordered arrangement can develop if the alloy
element has a preference for a certain location in the lattice structure. For
example, in a gold-copper alloy, an FCC structure, the copper preferably
occupies the face positions and the gold preferably occupies the corner
positions.
Frequently, more than one alloying agent is used to modify the char-
acteristics of a metal. Steel is a good example of a multiple element alloy.
By definition, steel contains iron with carbon; however, steel frequently
includes other alloying elements such as chromium, copper, nickel, phos-
phorous, etc.

■ Phase Diagrams

To produce alloys metals, the components are heated to a molten state,
mixed, and then cooled. The temperature at which the material transitions
between a liquid and a solid is a function of the percentages of the compo-
nents. The liquid and solid states of a material are called phases , and a
phase diagram displays the relationship between the percentages of the ele-
ments and the transition temperatures.

2.2.5