The following properties of materials will be briefly discussed: 1. Physical Properties of Materials 2. Mechanical Properties of Materials 3. Electrical Properties of Materials 4. Magnetic Properties of Materials 5. Chemical Properties of Materials.

1. Physical Properties:

A. The Melting or Freezing Point:

I. The melting or freezing point of pure metal is defined as the temperature at which the solid and liquid phases can exist in stable equilibrium. When a metal is heated to melting point, the liquid phase appears, and if more heat is supplied, the solid melts completely at constant temperature.

II. The freezing of a pure liquid on the other hand, may exhibit the phenomena of supercoiling, the liquid in some cases can be lowered appreciably beyond the melting point without the appearance of crystals. However, when crystals do not appear, the mass rapidly assumes the normal temperature of the melting point.


III. The use of mercury in thermometers, manometers and other instruments arises from its low melting point; the use of tungsten filaments in incandescent high bulbs is possible because of its extremely high melting point.

B. Boiling Point:

The boiling point of a liquid is the temperature at which its vapour pressure equals to one atmosphere. The boiling points of the metals except mercury are high. The boiling point of zinc (907°C) and cadmium (865°C) are sufficiently low so that in recovery of these metals from their ores the metals are vapourised and condensed.

C. Density:


Mass per unit volume is termed as “density.” In metric system it is stated in kg/m3. The low densities of aluminium and magnesium and of their alloys make them particularly valuable in aeronautic and transportation fields.

D. Linear Co-Efficient of Expansion:

The linear coefficient of expansion of a solid is defined as the increase in length, for each degree rise in temperature. These coefficients are important when metals are to be exposed to a considerable range of temperatures as in engine pistons, and other accurately fitting mechanisms.

E. Thermal Conductivity:


I. The thermal conductivity of a metal is defined as the number of kilojoules of heat that would flow per second through a specimen one sq. metre in cross-section and I metre in length when the temperature gradient is 1°C. Silver and copper show the highest thermal conductivities of all metals. Some metals like German silver exhibit very low conductivity and hence find applications where heat losses by metallic conduction should be kept to a minimum.

II. All metals are conductors of electricity; silver is the best conductor and copper is next. It should be noted that while volume aluminium has only 61% of the conductivity of copper, nevertheless weight for weight aluminium because of its low density, shows a conductivity nearly twice that of copper.

F. Electrical Resistivity:

The resistance of a metal is the reciprocal of its conductivity. The electrical resistivity of a metal is the resistance of a specimen of 1 cm in length and 1 sq. cm in cross-section. Since these values for metals are very small if expressed in ohms, they are usually given in micro- ohms, where 1 micro-ohm = 10-6 ohm.

2. Mechanical Properties of Metals:


A. Strength:

The strength of metal is its ability to withstand various forces to which it is subjected during a test or in service. It is usually defined as tensile strength, compressive strength, proof stress, shear strength, etc. Strength of materials is a general expression for the measure of capacity of resistance possessed by solid masses or pieces of various kinds to any cause tending to produce in them a permanent and disabling change of form or positive fracture.

Materials of all kinds owe their strength to the action of the forces residing in and about the molecules of the bodies (the molecular forces) but mainly to that one’s of these known as cohesion; certain modified results of cohesion as toughness or tenacity, hardness, stiffness and elasticity are also important elements, and strength is in relation of the toughness and stiffness combined.

B. Elasticity:


A material is said to be perfectly elastic if the whole of the stress produced by a load disappears completely on the removal of the load, the modulus of elasticity of Young’s modulus (E) is the proportionally constant between stress and strain for elastic materials.

Young’s modulus is the indicative of the property called stiffness; small values of E indicate flexible materials and large value of E reflect stiffness and rigidity. The property of spring back is a function of modulus of elasticity and refers to the extent to which metal springs back when an elastic deforming load is removed. In metal cutting, modulus of elasticity of the cutting tools and tool holder affects their rigidity.

C. Plasticity:

I. Plasticity is the property that enables the formation of permanent deformation in a material. It is reverse of elasticity; a plastic material will retain exactly the shape it takes under load, even after the load is removed. Gold and lead are the highly plastic materials. Plasticity is used in stumping images on coins and ornamental work.

II. During plastic deformation there is the displacement of atoms within metallic grains and consequently the shapes of the metallic components change. It is because of this property that certain synthetic materials are given the name “plastics”. These materials can be changed into required shape easily.

D. Ductility:

It is the ability of a metal to withstand elongation or bending. Due to this property, wires are made by drawing out through a hole. The material shows a considerable amount of plasticity during the ductile extension. This is a valuable property in chains, ropes etc., because they do not snap off, while in service, without giving sufficient warning by elongation.

E. Malleability:

This is the property by virtue of which a material may be hammered or rolled into thin sheets without rupture. This property generally increases with the increase of temperature.

F. Toughness (or Tenacity):

Toughness (or tenacity) is the strength with which the material opposes rupture. It is due to the attraction which the molecules have for each other; giving them power to resist tearing apart.

The area under the stress-strain curve indicates the toughness (i.e., energy which can be absorbed by the material upto the point of rupture). Although the engineering stress-strain curve is often used for this computation, a more realistic result is obtained from a true-stress curve. Toughness is expressed as energy absorbed (Nm) per unit volume of material participating in absorption (m3) or Nm/m3. This result is obtained by multiplying the ordinate by the abscissa (in appropriate units) of stress-strain plot.

G. Brittlenss:

Lack of ductility is brittleness. When a body breaks easily when subjected to shocks it is said to be brittle.

H. Hardness:

I. Hardness is usually defined as resistance of material to penetration. Hard materials resist scratches or being worn out by friction with another body.

II. Hardness is primarily a function of the elastic limit (i.e., yield strength) of the material and to a lesser extent a function of the work hardnening co-efficient. The modulus of elasticity also exerts a slight effect on hardness.

III. In the most generally accepted test, an indentor is pressed into the surface of the material by slowly applied known load, and the extent of the resulting impression is measured mechanically or optically. A large impression for a given load and indentor indicates soft material, and the opposite is true for small impression.

IV. The converse of hardness is known as softness.

I. Fatigue:

I. When subjected to fluctuating or repeating loads (or stresses), materials tend to develop a characteristic behaviour which is different from that (or materials) under steady loads. Fatigue is the phenomenon that leads to fracture under such conditions.

Fracture takes place under repeated or fluctuating stresses whose maximum value is less than the tensile strength of the material (under steady load). Fatigue fracture is progressive, beginning as minute cracks that grow under the action of the fluctuating stress.

II. Fatigue fracture starts at the point of highest stress. This point may be determined by the shape of the part; for instant, by stress concentration in a groove. It can also be caused by surface finish, such as tool marks or scratches, and by internal voids such as shrinking cracks and cooling in castings and weldments and defects introduced during mechanical working and by defects, stresses introduced by electroplating.

It must be remembered that surface and internal defects are stress raisers, and the point of highest actual stress may occur at these rather than at the minimum cross-section of highest normal stress. Thus processing methods are extremely important as they affect fatigue behaviour.

J. Creep:

I. “Creep” is the slow plastic deformation of metals under constant stress or under prolonged loading usually at high temperature. It can take place and lead to fracture at static stresses much smaller than those which will break the specimen by loading it quickly. Creep is specially taken care of while designing I.C. engines, boilers and turbines.

II. The creep at a room temperature is known as low temperature creep and occurs in load pipes, roofings, glass as well as in white metal bearings. The creep at high temperatures is known a high temperature creep. It mainly depends upon metal, service temperature to be encountered and the stress involved. For studying its effects, the specimens are put under a constant load; the creep is measured during various time intervals and results then plotted to get a creep curve.

3. Electrical Properties of Materials:

One of the important characteristics of the materials is their ability to permit or resist the flow of electricity.

Materials to be used in electrical equipments can be selected on the basis of their properties, such as:

(i) Resistivity,

(ii) Conductivity,

(iii) Temperature coefficient of resistance,

(iv) Dielectric strength,

(v) Thermoelectricity, and

(vi) Other electrical properties.

(i) Resistivity:

It is a characteristic property of the material of which the conductor is made. It is that electrical property of a material due to which, it impedes or resists the flow of electricity through it.

(ii) Conductivity:

The conductivity (σ) is the reciprocal of electrical resistivity.

(iii) Temperature Coefficient of Resistance:

It is usually employed to specify the variation of resistivity, ρ with temperature.

(iv) Dielectric Strength:

It means the insulating capacity of a material against high voltages. A material having high dielectric-strength can withstand sufficiently high voltage field across it before it will breakdown and conduct. A dielectric is an insulator.

(v) Thermoelectricity:

If two dissimilar metals are joined and this junction is then heated, a small voltage in the millivolt range is produced, and this is known as thermoelectric effect. Thermoelectric effect forms the basis of the thermocouple operation.

Other Electrical Properties of Materials:

Other electrical properties of materials are:

Electrochemical phenomena—as in storage batteries

Electrophysical effects—as in contact potentials

Electro-mechanical effects—as in radars


Some metals and compounds lose their electrical resistance abruptly before absolute zero is reached and become superconductor. Superconductivity, therefore, refers to the phenomenon of abrupt drop of resistivity of some metals at a temperature, called superconducting transition temperature, before absolute zero is reached. This transition temperature is 0.4 K for titanium, 1.17 K for aluminium and 9.2 K for niobium, 14 K for NbH, 1.6 K for Nb4, and 18 K for Nb3S4.

Superconductivity state can be abolished by the application of an external magnetic field or produced by a sufficiently large current flowing through the conductor.

4. Magnetic Properties of Materials:

I. Those materials in which a state of magnetisation can be induced are called “magnetic materials”. Such materials create a magnetic field in the surrounding space.

II. The magnetic properties of materials arise from the spin of electrons and the orbital motion of electrons around the atomic nuclei. In several atoms the opposite spins neutralise one another, but when there is an excess of electrons spinning in one direction, magnetic field is produced. All substances except ferromagnetic material which can form permanent magnets, exhibit magnetic effects only when subjected to an external electromagnetic field.

III. Study of the magnetic properties is necessary because the science of magnetism explains many aspects of the structure and behaviour of the matter.

Some of the important magnetic properties are:

(i) Permeability.

(ii) Coercive Force.

(iii) Magnetic hysteresis.

Absolute Permeability:

It is the ratio of the flux density in a material to the magnetising force producing that flux density and is denoted by μ; μ = μ0 μr where μ0 is the permeability of free space having a value of 4π x 10-7 H/m.

Coercive Force:

It may be defined as the magnetising force which is necessary to neutralise completely the mangetism in an electromagnet after the value of magnetising force becomes zero.

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

5. Chemical Properties of Materials:

A 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, tend to suffer from chemical deterioration.

The chemical properties describe the combining tendencies, corrosion characteristics, reactivities, solubilities, etc., of substances.

Some of the chemical properties are:

(i) Corrosion resistance.

(ii) Chemical composition.

(iii) Acidity or alkalinity.


Corrosion is a gradual, chemical or electrochemical attack on a metal by its surroundings so that the metal is converted into an oxide, salt or some other compound. It may be brought about by almost unlimited number of factors or corrosive media such as air, industrial atmospheres, soils, acids, bases and salt solutions. It may also occur at elevated temperature in media which are inert when near or below room temperature.