Unit 1 Engineering Physics

18 CHAPTER 1. PROPERTIES OF MATTER

on a body, the elasticity of a body decreases.

Eect of temperature:In general, elasticity decreases with increase of temperature.
For example Young’s modulus of pure iron and carbon steels decreases with increase
in temperature. But there are some materials such as Invar steel (FeNi36, an alloy
with 64% iron and 36% nickel) whose elastic properties remain approximately same
for a wide range of temperatures.

Eect of annealing and quenching:Annealing is a heat process whereby a metal is
heated to a specific temperature and then allowed to cool slowly. During annealing
uniform crystalline structure and larger crystal grains are formed. This results in
softening of the materials (or decrease in the rigidness) of materials. Quenching is
a process of cooling a material at a rapid rate. Quenching increases the rigidness of
a material.

Eect of impurities;Presence of impurities alters the elastic properties of materials.
Addition of carbon and other materials in suitable quantities in iron can increase
the Young’s Modulus of steel (alloy of iron with other materials such as carbon,
chromium etc). Another example is brass (alloy of zinc in copper) where the small
percentage of zinc in copper makes the alloy stronger than either copper or zinc.

Eect of hammering and rolling (cold working or work hardening:A bulk spec-
imen of metal or alloy is is made up of a large number of tiny crystal grains. Each
grain is a distinct crystal with its own orientation. When the metal is cold worked
by forging, stamping or rolling, its shape is permanently changed.

This is possible because of defects called in the grain structure which can move

through the crystal structure. These dislocations or slips in the grain structure

allow the overall change in shape of the metal. Each grain can have a very large

number of dislocations which only visible under a powerful microscope. During

hammering and rolling, crystal grains break up into smaller units that increases

their elastic properties. Therefore, Metal samples with smaller crystal grains are

more rigid than those with larger grains. Cold working increases the yield strength

and elastic moduli.

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