Page 123 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Chapter 4 Metal Alloys: Their Structure and Strengthening by Heat Treatment
solute (composed of solute atoms) is the element that is added to the solvent
(composed of host atoms). When the particular crystal structure of the solvent is
maintained during alloying, the alloy is called a solid solution.
Substitutional Solid Solutions. If the size of the solute atom is similar to that of the
solvent atom, the solute atoms can replace solvent atoms and form a substitutional
solid solution (see Fig. 1.7). An example is brass (Section 6.4), an alloy of zinc and
copper, in which zinc (the solute atom) is introduced into the lattice of copper (sol-
vent atoms). The properties of brasses can thus be altered by controlling the amount
of zinc in copper.
Two conditions (known as Hume-Rotlzery rules, after W. Hume-Rothery,
1899-1968) are generally required to form complete substitutional solid solutions:
I. The two metals must have similar crystal structures, and
2. The difference in their atomic radii should be less than 15%.
If these conditions are not satisfied, complete solid solutions will not be obtained and
the amount of solid solution formed will be limited.

Interstitial Solid Solutions. If the size of the solute atom is much smaller than that
of the solvent atom, each solute atom can occupy an interstitial position; such a
process forms an interstitial solid solution. There are two conditions necessary for
forming interstitial solutions:
l. The solvent atom must have more than one valence electron, and
2. The atomic radius of the solute atom must be less than 59% of the atomic
radius for the solvent atom.
If these conditions are not met, interstitial solubility is limited and may not take
place at all.
An important family of interstitial solid solutions is steel (Chapter 5), an alloy
of iron and carbon in which the carbon atoms are present in interstitial positions
between iron atoms. The atomic radius of carbon is 0.071 nm, which is less than
59% of the 0.124-nm radius of the iron atom. The properties of carbon steels can be
varied over a wide range by controlling the proportion of carbon to iron. This con-
trollability is a major reason that steel is such a versatile and useful material with a
wide variety of properties and applications.

4.2.2 lntermetallic Compounds
lntermetallic compounds are complex structures consisting of two metals in which
solute atoms are present among solvent atoms in certain proportions. Some inter-
metallic compounds have solid solubility, and the type of atomic bond may range
from metallic to ionic. lntermetallic compounds are strong, hard, and brittle.
Because of their high melting points, strength at elevated temperatures, good oxida-
tion resistance, and relatively low density, they are candidate materials for appli-
cations such as advanced gas-turbine engines. Typical examples are the aluminides
of titanium (Ti3Al), nickel (Ni3Al), and iron (Fe3Al).

4.2.3 Two-phase Systems
It has been noted that a solid solution is one in which two or more elements in a
solid state form a single homogeneous solid phase in which the elements are uni-
formly distributed throughout the solid mass. Such a system is limited by some max-
imum concentration of solute atoms in the solvent-atom lattice, just as there is a