PROPERTIES OF METALS: Physical properties, Chemical properties, Thermal properties, Electrical properties, Magnetic properties, Optical properties, Mechanical properties

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The important properties of an engineering
material determine the utility of the material which influences quantitatively
or qualitatively the response of a given material to imposed stimuli and
constraints. The various engineering material properties are given as under.

1. Physical properties

2. Chemical properties

3. Thermal properties

4. Electrical properties

5. Magnetic properties

6. Optical properties, and

7. Mechanical properties

These properties of the material are discussed as under.

Physical Properties

The important physical
properties of the metals are density,
color, size and shape
(dimensions), specific gravity,
porosity, luster
etc. Some of them are defined as under.

1. Density

Mass per unit volume
is called as density.
In metric system
its unit is kg/mm3. Because of very low density, aluminium and magnesium are preferred in aeronautic and
transportation applications.

2. Color

It deals the quality of light reflected from the surface
of metal.

3. Size and shape

Dimensions of any metal reflect
the size and shape of the material. Length, width, height, depth, curvature
diameter etc. determines the size. Shape specifies the rectangular, square,
circular or any other section.

4. Specific Gravity

Specific gravity of any metal is
the ratio of the mass of a given volume of the metal to  the mass of the same volume of water at a specified

5. Porosity

A material is called as porous or permeable if it has
pores within it.

Chemical Properties

The study of chemical properties of materials
is necessary because most of the engineering materials, when they come in
contact with other substances with which they can react, suffer from chemical deterioration of the
surface of the metal. Some of the chemical properties of
  the metals are corrosion resistance, chemical composition and
acidity or
alkalinity. Corrosion is the gradual deterioration of material by chemical reaction
with its environment. 

Thermal Properties

The study of thermal properties is essential
in order to know the response of metal to 
thermal changes i.e. lowering or raising of temperature. Different
thermal properties
are thermal conductivity, thermal expansion, specific
heat, melting point, thermal
diffusivity. Some important
properties are defined
as under.

Melting Point

Melting point is the temperature
at which a pure metal or compound changes its shape from solid to liquid. It is
called as the temperature at which the liquid and solid are in equilibrium. It
can also be said as the transition point between solid and liquid phases. Melting
temperature depends on the nature of inter-atomic and intermolecular bonds. Therefore
higher melting point is exhibited by those materials possessing stronger
bonds. Covalent, ionic, metallic and molecular types of solids have decreasing
order of bonding strength
and melting
point. Melting point of mild steel is
of copper is 1080°C and of Aluminium is 650°C.

Electrical Properties

The various electrical properties of materials
are conductivity, temperature
coefficient of
resistance, dielectric
resistivity, and thermoelectricity. These properties are
defined as

1. Conductivity

Conductivity is defined as the
ability of the material to pass electric current through it easily i.e. the
material which is conductive will provide an easy path for the flow of
electricity through it.

2. Temperature Coefficient of Resistance

It is generally termed as to specify the variation of
resistivity with temperature.

3. Dielectric

It means insulating capacity
of material at high voltage. A material having high dielectric
strength can withstand for longer time for high voltage across it before it
conducts the current through

4. Resistivity

It is the property of a material by which it resists the
flow of electricity through it.

5. Thermoelectricity

If two dissimilar metals are
joined and then this junction is heated, a small voltage (in the milli-volt
range) is produced, and this is known as thermoelectric effect. It is the base
of the thermocouple. Thermo -couples are prepared using the properties of

Magnetic Properties

Magnetic properties of materials arise from
the spin of the electrons and the orbital motion of electrons around the atomic nuclei. In
certain atoms, the opposite spins neutralize
another, but when there is an
excess of electrons spinning in one direction, magnetic field
      is produced. Many materials except ferromagnetic material
which can form permanent magnet, exhibit magnetic affects only
when subjected to an external electro-magnetic field. Magnetic properties of
materials specify many aspects of the structure and
behavior of the matter.
magnetic properties of the materials are magnetic hysteresis,
coercive force and absolute
permeability which are defined as under.

1. Magnetic Hysteresis

Hysteresis is defined as the lagging
of magnetization or induction flux density behind
the magnetizing force or it is
that quality of a magnetic substance due to energy is dissipated in it on
reversal of its magnetism. Below Curie temperature, magnetic hysteresis is the
rising temperature at which the given material
ceases to be ferromagnetic, or the falling
temperature at which it becomes magnetic.
Almost all magnetic materials exhibit the phenomenon called hysteresis.

2.  Coercive Force

It is defined as the magnetizing
force which is essential to neutralize completely the magnetism in an
electromagnet after the value of magnetizing force becomes zero.

3. Absolute Permeability

It is defined as the ratio of
the flux density in a material to the magnetizing force producing that flux
density. Paramagnetic materials possess permeability greater than one whereas
di-magnetic materials have permeability less than one.

Optical Properties

The main optical properties of engineering
materials are refractive index, absorptivity, absorption co-efficient, reflectivity and transmissivity. Refractive index is an important
optical property of metal which is defined
as under.

Refractive Index

It is defined as the ratio of velocity of
light in vacuum to the velocity of a material. It can also be termed as the
ratio of sine of angle of incidence to the sine of refraction.

Mechanical Properties

Under the action of various
kinds of forces, the behavior of the material is studied that measures the
strength and lasting characteristic of a material in service. The mechanical
properties of materials are of great industrial importance in the design of
tools, machines and structures. 

Theses properties are structure
sensitive in the sense that they depend upon the
crystal structure and its bonding forces, and especially upon the nature
and behavior of
the imperfections which-
exist within the crystal itself
or at the grain boundaries. The mechanical properties of the metals are those which are associated with the ability
of the material to resist mechanical forces
and load. The main mechanical properties of the
metal are strength, stiffness, elasticity, plasticity, ductility, malleability, toughness, brittleness, hardness, formability, castability
and weldability. These
properties can be well understood with help of tensile test and
stress strain diagram. The few important and useful mechanical properties
are explained below.

1. Elasticity

It is defined as the property of
a material to regain its original shape after deformation when the external
are removed.  It can also be referred as the power
of material to
  come back to its original
position after deformation when the stress or load is removed. It
     is also called as the tensile property
of the material.

2. Proportional limit

It is defined as the maximum
stress under which a material will maintain a perfectly uniform rate of strain
to stress. Though its value is difficult to measure, yet it can be used   as the important
applications for building
precision instruments, springs,

3. Elastic limit

Many metals can be put under
stress slightly above the proportional limit without taking a permanent set.
The greatest stress that a material can endure without taking up some permanent
set is called elastic limit. Beyond this limit, the metal does not regain its
original form and permanent set will occurs.

4. Yield point

At a specific stress, ductile
metals particularly ceases, offering resistance to tensile forces. This means,
the metals flow and a relatively large permanent set takes place without a noticeable increase in load. This point is called
yield point. Certain
metals such as mild steel exhibit a definite yield point, in which case the yield stress is simply the stress at this point.

5. Strength

Strength is defined as the ability
of a material to resist
the externally applied
forces with breakdown or
yielding. The internal resistance offered by a material to an externally
applied force is called stress. The capacity of bearing load by metal and to
withstand destruction under the action of external loads is known as strength.
The stronger the material the greater the load it can withstand. This property
of material therefore determines the ability to withstand stress without
failure. Strength varies according to the type of loading. It is always
to assess tensile,
compressive, shearing and torsional strengths. The maximum stress
that any material can withstand before destruction is called its ultimate
The tenacity of the material
is its ultimate strength in tension.

6. Stiffness

It is defined as the ability
of a material to resist
deformation under stress.
The resistance of a material
to elastic deformation or deflection is called stiffness or
rigidity. A material that suffers slight
or very less deformation under load has a high degree of stiffness or
rigidity.  For
instance suspended beams of steel and aluminium may both be strong enough to
carry the required load but the aluminium beam will “sag”
or deflect further. That means, the
beam is stiffer or more rigid than aluminium
beam. If the material behaves elastically with linear stress-strain
relationship under Hooks law, its stiffness is measured by the Young’s modulus
of elasticity (E). The higher is the value of the Young’s modulus, the stiffer
is the material. In tensile and compressive stress, it is called modulus of
stiffness or “modulus of elasticity”; in shear, the modulus of rigidity, and
this is usually 40% of the value of Young’s modulus for commonly used
materials; in volumetric distortion, the bulk modulus.

7. Plasticity

Plasticity is defined the
mechanical property of a material which retains the deformation produced under
load permanently. This property of
the material is required in
forging, in
stamping images on coins and in ornamental work. It is the ability or tendency
of material
   to undergo some degree of
permanent deformation without its rupture or its failure. Plastic deformation
takes place only after the elastic range of material has been exceeded. Such
property of material is important in forming, shaping, extruding and many other
hot or cold working
processes. Materials
such as clay, lead, etc. are
plastic at room temperature and steel is
at forging temperature. This
property generally increases with increase
in temperature of materials.

8. Ductility

Ductility is termed as the
property of a material enabling it to be drawn into wire with the application
of tensile load. A ductile material must be strong and plastic. The ductility is usually measured by the terms,
percentage elongation and percent reduction in area which
  is often used as empirical measures of ductility. The materials those possess
more than 5% elongation
are called
as ductile materials. The ductile material
commonly used in engineering
practice in order of diminishing ductility are mild steel,
copper, aluminium, nickel, zinc, tin
and lead.

9. Malleability

is the ability of the material to
be flattened into thin sheets under
of heavy compressive forces without cracking by hot or cold
working means. It is a special case of ductility which permits materials to be
rolled or hammered into thin sheets. A malleable material should be plastic but
it is not essential to be so strong. The malleable materials commonly used in
engineering practice in order of diminishing malleability are lead, soft steel, wrought iron, copper
and aluminium. Aluminium, copper, tin,
lead, steel, etc. are recognized as highly malleable metals.

10. Hardness

Hardness is defined as the
ability of a metal to cut another metal. A harder metal can always cut or put impression to the softer metals by virtue
of its hardness. It is a very important property of the metals and has a wide
variety of meanings. It embraces many different properties
such as resistance to wear, scratching, deformation and

11. Brittleness

Brittleness is the property of a
material opposite to ductility. It is the property of breaking of a material
with little permanent distortion. The materials having less than 5% elongation
under loading behavior are said to be brittle materials. Brittle materials when
subjected to tensile loads, snap off without giving any sensible elongation.
Glass, cast iron, brass and ceramics are considered as brittle material.

12. Creep

When a metal part when is
subjected to a high constant stress at high temperature for   a longer period of
time, it will undergo a slow and permanent deformation (in form of a crack
may further propagate further
towards creep failure)
called creep.

13. Formability

It is the property of metals
which denotes the ease in its forming in to various shapes and sizes. The different factors
that affect the formability are crystal structure
of metal, grain size of metal hot and cold working,
alloying element present in the parent metal. Metals with smal1 grain size are
suitable for shallow forming while metal with size are suitable for heavy
forming. Hot working increases formability. Low carbon steel possesses

14. Castability

Castability is defined as the
property of metal, which indicates the ease with it can be casted into
different shapes and sizes. Cast iron, aluminium and brass are possessing good

15. Weldability

is defined as the property of a
metal which indicates the two similar or dissimilar metals are joined by fusion
with or without the application of pressure and with   or without the use of filler metal (welding)
efficiently. Metals having
weldability in the descending order are iron, steel, cast steels and stainless

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