An alloy is a substance having metallic properties. Alloying is done by combining one metallic element with one or more metallic or non-metallic elements. In an alloy, the metallic or non-metallic elements do not exist or operate in distinct layers when solid. An alloy when solid may be composed of eutectic alloy or eutectoid, solid solution, chemical compounds with each other or with pure metals.
Alloying is done by the following four methods
Melting two metals separately and mixing them.
Melting one of the metals having higher melting point than the other and adding those having lower melting point as solids.
Melting one of the metals having lower melting point than the other, and adding those having higher melting point as solids. (The difference of melting point will be very less for the metals selected for such alloying).
The latest process is known as powder metallurgy.
Steel is invariably an alloy of iron and carbon. Alloy steels are produced by alloying metals like chromium, nickel, tungsten, vanadium, cobalt and molybdenum, etc. Chemical elements like Manganese and silicon impart certain properties like hardness, tenacity, heat resistance, fatigue resistance and torsion, etc. Every metal or element mentioned above has its own properties and characteristics, which are imparted to ferrite (iron in pure state).
The alloying elements so alloyed will influence the mechanical and physical properties of the alloys in:
- Increasing tensile strength.
- Increasing depth of penetration for hardening effect, hardenability and toughness (e.g. automobile parts).
- Improving physical properties at either higher or lower temperature.
- Increasing fatigue resistance.
- Increasing work hardening property.
- Increasing or decreasing machinability.
- Improving cutting properties and abrasion resistance.
- Decreasing warping and cracking.
- Resistance to corrosion, wear and tear.
- Improving weldability.
- Decreasing weight.
Physical Properties of Material
Physical properties are employed to describe a material under conditions, in which external forces are not concerned. Physical properties include:
Dimensions Dimension is made by the size of the material. It is measured in terms of length, breadth, width, height, diameter and radius, etc. It is the fundamental physical characteristics of a material.
Density It is the weight of unit volume of a material. Density can be calculated by the general formula.
Density = Mass / Volume
Each material has a specific density. For example, Gold has specific density of 19.32 (heavy metal), Magnesium (light metal) has a specific density of 1.74.
Porosity. Porosity of a material is the ratio of the total volume of pores to the total volume of the material. Porosity in the metal leads to dry corrosion. Porosity in ceramics also leads to weakening of the materials.
Appearance. Appearance is a general characteristic which includes lustre, grains or marks and texture, etc. Lustre is the ability of the material to reflect light. Metals have metallic lustre.
Colour. In the light there are mainly seven spectral compositions briefly known as “VIBGYOR”; each alphabet representing the name of the colour such as violet, indigo, blue, green, yellow, orange and red. The colour of the material depends upon the wavelength of light, that the, material absorbs. It is an important property to identify the originality as well as the quality of the metal.
The magnetic properties of materials arise from spin of the electrons and the orbital motions of electrons around the atomic nuclei. In several atoms the opposite spins neutralise one another, but if there is an excess of electrons spinning in one direction, a magnetic field is produced.
All substances except ferromagnetic materials which can form permanent magnate, exhibit magnetic effects, only when subjected to an external electro –magnetic field. Magnetic properties explain many aspects of the structure and behaviour of the matter. Various magnetic properties are:
Absolute and relative permeability
- Coercive force.
- Super conductivity
Absolute and Relative Permeability. Permeability of a medium is a measure of the conducting power for the magnetic line of force through that medium. Relative permeability of a medium is defined as the ratio of the force between two poles when placed at a fixed distance in the medium to the force between them at the same distance in vacuum.
Coercive Force. It is the opposing magnetising force which is necessary to remove magnetisation or residual magnetisation.
Super Conductivity. Super conductivity is the property of certain specific material such as Tungsten, Tin, Lead, Aluminium, etc. losing their electrical resistance at very low temperature. The temperature at which the material attains super conductivity is called critical temperature.
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. Some of the chemical properties are:
- Corrosion resistance to moist atmosphere.
- Increasing resistance to oxidation at higher temperature.
- Increasing resistance to attack of chemical agents.