Constitution of alloys: Solid solutions - substitutional and interstitial. Ferrous and Non Ferrous Metals

1. Prepared By
Er.Navin.H.Yadav
B.E.,M.Tech, Ph.D*,
Asst. professor, Department of Mechanical
engineering,
Kautilya Institute Of Technology Of Engineering ,
Constitution of alloys:
Solid solutions - substitutional and
interstitial.

2.  An Alloy is a substance that has metallic
properties and is composed of two or more
chemical elements , of which at least one is a
metal
 If the system is made up of two elements, it is
called binary alloy system; three elements,
ternary alloy system; etc.

3.  So far, we have discussed about pure metals in
crystalline, polycrystalline and amorphous form. One
important aspect that will need a special attention is
the alloying of metals. Historically, the significance of
alloying was quite known to human civilization.
 Consider for example, the use of Bronze and Brass,
the first one is a metallic alloy of Copper and Tin while
the second one is that of Copper and Zinc. The table
below shows the improvement in mechanical
properties due to such alloying.

4. •You may note that in this case the alloying
has helped to improve the yield strength
substantially. Consequently, the hardness has
also increased, while the fracture toughness
has been sacrificed substantially. Of course all
these aspects were only understood intuitively
by the people of early civilization.

5. Positive Effects
 Remarkable increase in Yield Strength, Impact
Strength and Hardness – eg. alloy steels vs. plain
carbon-steel
 Improvement in corrosion resistance eg.
Chromium in Steel
 Occasional improvement in machinability eg.
Sulphur and Lead in Steel
 Reduces Cost eg. Copper+Cadmium etc.
 Changes basic property

6.  Reduction in Ductility: In most of the cases, the
ductility of an alloy comes down considerably
example, the reduction of ductility in Bronze from
pure Copper.
 Reduction in Conductivity: Most of the times there
are reductions in both thermal and electrical
conductivity; example, Copper + Cadmium.

7.  Dissolving small amounts of one solid substance in
another is a vitally important way of altering the
properties of materials —
The classic example is the semiconductor silicon:
dissolving tiny amounts (less than one part per
million) of phosphorus has a drastic effect on its ability
to conduct electricity (making the material that is
known as an 'n–type' semiconductor); similar amounts
of arsenic have equally large effects but result in
different electrical characteristics (the material
becomes a 'p–type' semiconductor).
 The arrangement and kind of bonding in metals
permits the addition of other elements into the
structure, forming mixtures of metals called alloys.
Even if the added elements are nonmetals, alloys
may still have metallic properties.

8. Alloys
are Solid
Solutions
!

9. •Solute atoms – denotes an element or
compound present in a minor
concentration.
•Solvent atoms – represent the
element or compound present in the
greatest amount.
•Solid solution – the crystal
structure is maintained when solute
atoms are added to the host

10.  There are two types of solid solution in metals:
1. Substitutional
2. Interstitial

11. Substitutional –
In this form of alloying, solute atoms replace or
substitute the host atoms e.g. Au-Ag. The following
parameters are important for substitutional solutions:
•Difference between atomic radii of the solute and
the solvent atoms should be within ± 15 %. The other
important issues are -
•Electronegativity
•Relative valencies of participating atoms
•Crystal structures of metals of both atom types
should be the same.

12. Interstitial – When the difference in radii
between the participating atoms is too
high, the alloying could occur in the form
of interstitial filling. Impurity atoms fill
voids or interstices among host atoms
e.g. Fe-C. The figure below shows
substitutional and interstitial alloying
schematically.

14. Alloy Classification:
Ferrous Alloy:
•High Strength Low Alloy
Steels(Fe+C+Niobium),
•Medium Alloy Nickel-Chromium Steels,
•Manganese Steels,
•High resisting Steels etc.
Nonferrous Alloy:
•Light Alloys like Aluminium, Magnesium,
Titanium & Zinc Alloys
•Heavy Alloys like Copper, Nickel & Cobalt
Alloys