1. combination of properties is not desirable for all applications.
Alloying can enhance or suppress these properties rendering an
alloy more suited than the pure metal for a specific application.
5. The use of alloys instead of expensive pure metals promotes
economy in the manufacture and use of components.
1.3 Types of solid solutions
A solid solution occurs when we alloy two metals and they are
completely soluble in each other. If a solid solution alloy is viewed
under a microscope only one type of crystal can be seen just like a
pure and original metal. Solid solution alloys have similar
properties to pure metals but with greater strength but are not as
good as electrical conductors.
The common types of solid solutions are:
1) Substitutional solid solution
2) Interstitial solid solutions
1. Substitutional solid solutions
The name of this solid solution tells you exactly what happens as
atoms of the parent metal are replaced or substituted by atoms of
the alloying metal (solute metal) In this case, the atoms of the two
metals in the alloy are of similar size.
The substitutional solid can be classified into two types
• Ordered solid solution
2. • Disordered solid solution
Ordered solid solution
If the atoms of the solute occupy certain preferred sites in the
lattice of the solvent, an ordered solid solution is formed. It may
occur only at certain fixed ratios of the solute and solvent atoms.
In Cu-Au system, Cu atoms occupying the face-centered sites and
Au atoms occupying the corner sites of the FCC unit cell.
Ordered solid solution
Disordered solid solution
If the atom of the solute are present randomly in the lattice of the
solute, it is known as disordered solid solution.
Most of the solid solutions are disordered solid solutions.
2. Interstitial Solid Solution.
In interstitial solid solution, the solute atoms fit into the space
between the solvent or parent atoms. These spaces or voids are
interstices.
3. Solute atoms located between the atoms of the host metal an
interstitial solution
Solute atoms displacing atoms of the host metal, a substitutional
solution
An orderly structure of Crystal Comprise metals. If the metal is
completely pure, there are no other elements present; it is said to
be single phase.
Under microscopic examination only grain boundaries would be
visible. If we wish to make an alloy by adding another element, the
added element (solute) may go into interstitial or substitutional
solid solution, or it may form another phase. If the added solute
goes completely into solid solution, the host metal may remain
single phase in nature.
4. There is usually a limit to the ability of a host metal to dissolve
solute atoms, and this ability varies with temperature. Chicken
soup is made by dissolving fats and flavoring ingredients in hot
water. When the soup is not, everything is in solution. When the
soup cools to room temperature, the fat will often come out of
solution and form a layer of fat- rich phase on top of the liquid. This
same sort of things happens in metals.
1.3.1 Hume Rotherys rules
Hume Rothery’s Rules
To form an extensive solid solution (i.e., greater than 10 atomic
percent soluble), the solute and solvent elements should obey the
following general rules of Hume Rothery’s.
1. Size factor: The atoms must be of similar size, with less than a
15% difference in atomic radius (in order to minimize the lattice
strain).
2. Crystal structure: The materials must have the same crystal
structure. Otherwise, there is some point at which a transition
occurs from one phase to a second phase with a different
structure.
3. Valence: The atoms must have the same valence†
. Otherwise,
the valence electron difference encourages the formation of
compounds ‡ rather than solutions.
4. Electro negativity: the atoms must have approximately the same
electro negativity. Electro negativity is the ability of the atom to
attract an electron, if electro negativity differs significantly, the
compounds will form.
5. If one or more of the Hume Rothery’s rules are violated, only
partial solubility is possible.
1.4 Intermediate alloy phases
Intermediate Phases
In many binary alloys systems, when the chemical affinity of metals
is great, their mutual solubility becomes limited and intermediate
phases are formed.
The intermediate phase may have either narrow or wide range of
homogeneity and may or may not include a composition having a
simple chemical formula. Intermediate phases may range between
ideal solid solutions and the ideal chemical compound. The
intermediate phases are the phases that form in the intermediate
regions of the equilibrium diagram.
When a compound is formed, the elements loose their individual
identity and characteristic properties to a large extent. Most
compounds, like pure metals, also exhibit a definite melting point
within narrow limits of temperature. Therefore, the cooling curve
for a compound is similar to that for a pure metal.
Intermediate phases may be classed as two types are:
1. Inter metallic compounds of fixed composition.
2. Inter metallic compounds of variable composition.
1.4.1 Electron compounds
