Broadly, engineering materials can be classified as:
(a) Metallic materials and
(b) Non-Metallic materials.
Metallic materials could further be classified as:
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(i) Ferrous metals (consisting of iron and alloys of iron) and
(ii) Non-Ferrous materials and alloys.
Non-metallic materials include plastics, asbestos, rubber, wood, concrete, ceramics etc.
The selection of a material for a particular application is governed by the working conditions to which it will be subjected, ease of manufacturing and the cost considerations. Pure metals find few applications in engineering, firstly because they are difficult to produce in pure condition and secondly they generally have poor strength in pure form.
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The various desired and special properties can be achieved by addition of different materials to form alloys. Alloy comprises of a base metal (usually more than 50% content) and one or more alloying elements. The typical properties associated with working conditions are tenacity, elasticity, toughness and hardness and typical properties associated with manufacturing processes are ductility, malleability and plasticity.
The various properties can be determined by testing techniques e.g. tensile strength is determined by tensile test, ductility by bend test, resistance to abrasion by hardness test, toughness by impact test and other special properties like fatigue and creep by fatigue tests and creep tests.
The materials can be manipulated in several ways; the choice being governed by the material, properties desired, shape to be produced, accuracy desired, quantity to be produced and cost aspects.
The mechanical properties of a material determine its usefulness for a particular job. Understanding of mechanical and physical properties of metals and how these are measured help in the selection of materials.
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Understanding of characteristics like creep, scaling, brittle transition, corrosion, fatigue, etc. is of help to be more aware of problems experienced in severe environments. The great utility of metals is due to their elastic behaviour to a certain level of stress followed by a plastic behaviour at higher levels of stress.
It has been established that the life (tool wear) of cutting tools, surface finish, and machinability of metals have no direct relation in any simple way with such physical properties of the work piece as hardness, yield point, or ultimate tensile strength; but these are directly related to the microscopic structure of the metal as revealed by metallographic examination.
However, physical properties of metals are also of much interest in many other ways. Therefore, we will now study the most commonly used metals in respect of their composition, physical properties, metallographic structure and engineering uses.
The structure and characteristics of pure iron are the simplest. How the structure of pure iron will look at a magnification of 1000 x is shown in Fig. 1.7 which shows the grain boundaries. All metals are made up of many individual crystals or grains, the strength and hardness of a given material depending upon its mean grain size.
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Pure metals are relatively weak and soft compared with ordinary structural materials, but their hardness and strength increase with decrease in grain size. It may be noted from Fig. 1.7 for pure iron, that the atoms in any one grain are uniformly aligned whereas the alignment varies from grain to grain. Further the grain boundaries consist of atoms with alignment that gradually change from that of one neighbour to the other.
Two important characteristics of metals are their tendency to strain harden and to recrystallize. Whenever any metal is plastically deformed its grains change in shape from a roughly spherical form to an elongated one. The metal becomes harder and stronger as it is deformed. This effect is known as strain hardening and is most pronounced in the grained metals.
When a strained metal is heated, there is a temperature at which new spherical crystals first begin to form from the old deformed ones. This temperature is known as the Recrystallisation temperature and process is called Strain recrystallisation or Process annealing. For iron it occurs at about 650°C.
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