The alloying concept, the necessity of alloying, solid solution types, alloy systems possible

1. A1MET201
Metallurgy and Material Science
Jagadeesh N B.Tech, M. Tech (NIT -W) Materials
Faculty of Materials and Manufacturing Sciences
MVGR College of Engineering-Autonomous
Vizianagaram-535005
njagadeesh@mvgrce.edu.in

2. Jagadeesh N, B. Tech, M. Tech (NIT W),
Assistant Professor
2

3. What is an alloy?
 Alloy is a mixture of two or more elements having metallic
properties.
 The element present in the largest portion is a metal and others
can be metals or non-metals
 The element which is present in the largest amount is called as
the base metal or parent metal or solvent
 Other alloying elements are called as solute

4. Necessity of Alloying
Pure metals possess few important physical and metallic
properties, such as melting point, boiling point, density,
specific gravity, high malleability, ductility, and heat
and electrical conductivity.
These properties can be modified and enhanced by
alloying it with some other metal or nonmetal,
according to the need
Alloys are made to:

5.  Enhance the hardness of a metal
An alloy is harder than its components. Pure metals
are generally soft. The hardness of a metal can be
enhanced by alloying it with another metal or
nonmetal.
EX: Bronze (Cu+ 11% tin) is harder than copper,
making it useful for tools and weapons

6. Lower the melting point
Pure metals have a high melting point. The melting
point lowers when pure metals are alloyed with other
metals or nonmetals.
This makes the metals easily fusible. This property is
utilized to make useful alloys called solders.
EX:
Tin, lead and their alloys, due to their low melting
temperatures and wide availability, are the most
commonly used solder materials

7. Enhance tensile strength
Alloy formation increases the tensile strength of the parent
metal
 Enhance corrosion resistance
Alloys are more resistant to corrosion than pure metals.
Metals in pure form are chemically reactive and can be easily
corroded by the surrounding atmospheric gases and moisture.
Alloying a metal increases the inertness of the metal, which,
in turn, increases corrosion resistance.
EX:
Stainless Steel (alloy of Fe and Cr) is more corrosion resistant
than Iron

8. Modify color
The color of pure metal can be modified by alloying it with
other metals or nonmetals containing suitable color pigments
Provide better cast ability
One of the most essential requirements of getting good
castings is the expansion of the metal on solidification.
Pure molten metals undergo contraction on solidification.
Metals need to be alloyed to obtain good castings because
alloys expand

9. Can you name Different alloys….

10. Classification of alloy systems

11. Solid Solution

12. Solid Solution
It is an alloy in which the solute atoms are
distributed in the solvent and has the same structure
as that of the solvent
Solid solutions have different compositions with
similar structures and are like liquid solutions as
sugar in water

13. Solid Solution
Solid solutions are of two types
 Substitutional Solid Solution
 Interstitial Solid Solution

14. Substitutional Solid Solution
 Substitutional solid solution means the atoms of
solute are substituted at the atomic sites of solvent
Observed generally when the atomic sizes of solute
and solvent are of same order

15. Substitutional Solid Solution
Depending on the distribution of Solute atoms in
solvent these are classified into two types
 Regular or ordered (Ex: Au-Cu Solid solution
below 400°C)
Random or Disordered (Ex: α-Brass)

16. Interstitial Solid Solution
The solute atoms occupy the interstitial sites of
solvent atoms
Generally observed when atomic sizes of solute are
very small as compared to the atomic sizes of
solvent atoms
C, B, O, H, N can form interstitial solid solution with
Iron

17. HUME-ROTHERY’S RULES OF SOLID
SOLUBILITY
In the formation of solid solutions, the solubility
limit of solute in the solvent is governed by certain
factors.
These factors are known as Hume-Rothery’s rules of
Solid Solubility

18. If the atomic sizes of solute and solvent differ by
less than 15%, it is said to have a favorable size
factor for solid solution formation .
If the atomic size difference exceeds 15%, solid
solubility is limited
RRule 1: Atomic Size Factor

19.  The greater the chemical affinity of two metals ,
the more restricted is their solubility and greater is
the tendency of formation of a compound
In general, wider the separation of elements in the
periodic table, greater is their chemical affinity
Rule 2: Chemical Affinity Factor

20.  A metal of higher valency can dissolve only a
small amount of a lower valance metal.
The lower valance metal may have good
solubility for the higher valency metal
Rule 3: Relative Valency factor

21.  metals with same crystal structure will have
greater solubility.
 Differences in crystal structure limits the solid
solubility
* For continuous solubility, atomic size difference should preferably be less
than 8% with other factors favorable
Rule 4: Crystal Structure Factor