18 CHAPTER 1. PROPERTIES OF MATTER
specimen is decided by the stable equilibrium position that every atom finds itself in as
a result of the balance of the interatomic forces. When an external force is applied, this
equilibrium is disturbed and a new equilibrium is reached where the internal atomic forces
and the external force are again balanced. This corresponds to the deformed shape of the
solid. When external forces are removed, the solid returns to the old equilibrium and the
original shape is regained.
Elastic properties properties, such as Young’s Modulus, tensile strength, etc. can be
altered by modifying the internal forces and the arrangement of atoms in the material.
Below is a brief discussion of the factors upon which elasticity of a solid depends.
E ect of stress: Due to large constant stress or repeated application cycles of stresses
on a body, the elasticity of a body decreases.
E ect 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.
E ect 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.
E ect 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.
E ect 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.
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