CLASSIFICATION AND SELECTION OF MATERIALS

1. INTRODUCTION

Materials science and engineering plays a vital role in this modern age of science and technology. Various kinds of materials are used in industry, housing, agriculture, transportation, etc. to meet the plant and individual requirements. The rapid developments in the field of quantum theory of solids have opened vast opportunities for better understanding and utilization of various materials. The spectacular success in the field of space is primarily due to the rapid advances in high-temperature and high-strength materials.

The selection of a specific material for a particular use is a very complex process. However, one can simplify the choice if the details about (i) operating parameters, (ii) manufacturing processes, (iii) functional requirements and (iv) cost considerations are known. Factors affecting the selection of materials are sum-marized in Table 1.1.

		Table 1.1		Factors affecting selection of materials
	(i)			(ii)		(iii)		(iv)
Manufacturing  processes		Functional  requirements				Cost  considerations	Operating  parameters
l	Plasticity	l	Strength		l	Raw  material	l	Pressure
l	Malleability	l	Hardness		l	Processing	l	Temperature
l	Ductility	l	Rigidity		l	Storage	l	Flow
l	Machinability	l	Toughness		l	Manpower	l	Type  of  material
l	Casting  properties	l	Thermal  conductivity		l	Special  treatment	l	Corrosion  requirements
l	Weldability	l	Fatigue		l	Inspection	l	Environment
l	Heat	l	Electrical  treatment		l	Packaging  properties	l	Protection  from  fire
l	Tooling	l	Creep		l	Inventory	l	Weathering
l	Surface  finish	l	Aesthetic  look		l	Taxes  and  custom  duty	l	Biological  effects

There are thousands and thousands of materials available and it is very difficult for an engineer to possess a detailed knowledge of all the materials. However, a good grasp of the fundamental principles which control the properties of various materials help one to make the optimum selection of material. In this respect, materials science and engineering draw heavily from the engineering branches, e.g. metallurgy, ceramics and polymer science.

The subject of material science is very vast and unlimited. Broadly speaking, one can sub-divide the field of study into following four branches: (i) Science of metals, (ii) Mechanical behaviour of metals (iii) Engineering metallurgy and (iv) Engineering materials. We shall discuss them in subsequent chapters.

2. ENGINEERING REQUIREMENTS

While selecting materials for engineering purposes, properties such as impact strength, tensile strength, hardness indicate the suitability for selection but the design engineer will have to make sure that the radiography and other properties of the material are as per the specifications. One can dictate the method of production of the component, service life, cost etc. However, due to the varied demands made metallic materials, one may require special surface treatment, e.g. hardening, normalising to cope with the service requires. Besides, chemical properties of materials, e.g. structure, bonding energy, resistance to environ-mental degradation also effect the selection of materials for engineering purposes.

In recent years polymeric materials or plastics have gained considerable popularity as engineering materials. Though inferior to most metallic materials in strength and temperature resistance, these are being used not only in corrosive environment but also in the places where minimum wear is required, e.g. small gear wheels, originally produced from hardened steels, are now manufactured from nylon or teflon. These materials perform satisfactorily, are quiet and do not require lubrication.

Thus, before selecting a material or designing a component, it is essential for one to understand the requirements of the process thoroughly, operating limitations like hazardous or non-hazardous conditions, continuous or non-continuous operation, availability of raw materials as well as spares, availability of alternate materials vis-a-vis life span of the instrument/equipment, cost etc. Different materials possess different properties to meet the various requirement for engineering purposes. The properties of materials which dictate the selection are as follows:

(a)  Mechanical Properties     The important mechanical properties affecting the selection of a material are:

(i)    Tensile Strength: This enables the material to resist the application of a tensile force. To withstand the tensile force, the internal structure of the material provides the internal resistance.

(ii)    Hardness: It is the degree of resistance to indentation or scratching, abrasion and wear. Alloying techniques and heat treatment help to achieve the same.

(iii)    Ductility: This is the property of a metal by virtue of which it can be drawn into wires or elongated before rupture takes place. It depends upon the grain size of the metal crystals.

(iv)    Impact Strength: It is the energy required per unit cross-sectional area to fracture a specimen, i.e., it is a measure of the response of a material to shock loading.

(v)     Wear Resistance: The ability of a material to resist friction wear under particular conditions, i.e. to maintain its physical dimensions when in sliding or rolling contact with a second member.

(vi)    Corrosion Resistance: Those metals and alloys which can withstand the corrosive action of a medium, i.e. corrosion processes proceed in them at a relatively low rate are termed corrosion-resistant.

(vii)    Density: This is an important factor of a material where weight and thus the mass is critical, i.e. aircraft components.

(b)   Thermal Properties The characteristics of a material, which are functions of the temperature, are termed its thermal properties. One can predict the performance of machine components during normal operation, if he has the knowledge of thermal properties. Specific heat, latent heat, thermal conductivity, thermal expansion, thermal stresses, thermal fatigue, etc. are few important thermal properties of materials. These properties play a vital role in selection of material for engineering applications, e.g. when materials are considered for high temperature service. Now, we briefly discuss few of these properties:

(i)   Specific Heat (c): It is the heat capacity of a unit mass of a homogeneous substance. For a homogeneous body, c = C/M, where C is the heat capacity and M is the mass of the body. One can also define it as the quantity of heat required to raise the temperature of a unit mass of the substance through 1°C. Its units are cal/g/°C.

(ii)  Thermal Conductivity (K): This represents the amount of heat conducted per unit time through a unit area perpendicular to the direction of heat conduction when the temperature gradient across the heat conducting element is one unit. Truly speaking the capability of the material to transmit heat through it is termed as the thermal conductivity. Higher the value of thermal conductivity, the greater is the rate at which heat will be transferred through a piece of given size. Copper and aluminium are good conductors of heat and therefore extensively used whenever transfer of heat is desired. Bakelite is a poor conductor of heat and hence used as heat insulator.

Thermal conductivity for  some  of  the  materials  is  given  in  Table  1.2
Table 1.2    Thermal conductivity for some materials
Type  of  the  material			Material	Thermal  conductivity  (K)  (W/m/k)
(i)  Metals			Copper	380
			Aluminium	230
			Cast  iron	52
			Mild  steel	54
			Stainless  steel	16
(ii)  Ceramics			Alumina	2.0
			Titanium  Carbide	3.0
			Glass	1.0
(iii)  Polymers			Bakelite	0.23
				0.0019
(iv)  Composites			Concrete	1.4
			Wood	0.14

(iii) Thermal Expansion: All solids expand on heating and contract on cooling. Thermal expansion may take place either as linear, circumferential or cubical. A solid which expands equally in three mutually orthogo-nal directions is termed as thermally isotropic. The increase in any linear dimension of a solid, e.g. length, width, height on heating is termed as linear expansion. The coefficient of linear expansion is the increase in length per unit length per degree rise in temperature. The increase in volume of a solid on heating is called cubical expansion. The thermal expansion of solids has its origin in the lattice vibration and lattice vibrations increases with the rise in temperature.

(iv) Thermal Resistance (R_T): It is the resistance offered by the conductor when heat flow due to tempera-ture difference between two points of a conductor. It is given by

RT  =			q 1  - q2			second  –  ° C/k  Cal
					H
where  H  ® rate  of  heat  flow  and
		q1  and  q2   are  temperatures  at  two  points  (° C).
(v)  Thermal  Diffusivity  (h):  It  is  given  by
				Thermal conductivity ( K )
h  =							cm3 /s
	Heat capacity ( C p ) ´ density ( r )
=		K			represent  heat  requirement  per  unit  volume
	Cp  r

A material having high heat requirement per unit volume possesses a low thermal diffusivity because more heat must be added to or removed from the material for effecting a temperature change.

(vi) Thermal Fatigue: This is the mechanical effect of repeated thermal stresses caused by repeated heating and cooling.

The thermal stresses can be very large, involving considerable plastic flow. We can see that fatigue failures can occur after relatively few cycles. The effect of the high part of the temperature cycle on the strength of material plays an important factor in reducing its life under thermal fatigue.

(c) Electrical Properties Conductivity, resistivity, dielectric strength are few important electrical prop-erties of a material. A material which offers little resistance to the passage of an electric current is said to be a good conductor of electricity.

The  electrical  resistance  of  a  material  depends  on  its  dimensions  and  is  given  by

Length

Resistance  =  Resistivity  ´

Cross-section area

Usually  resistivity  of  a  material  is  quoted  in  the  literature.  Unit  of  resistivity  is  Ohm-metre.

On the basis of electrical resistivity materials are divided as: (i) Conductors (ii) Semiconductors and (iii) Insulators. In general metals are good conductors. Insulators have very high resistivity. Ceramic insu-lators are most common examples and are used on automobile spark plugs, Bakelite handles for electric iron, plastic coverings on cables in domestic wiring.

When a large number of metals and alloys are sufficiently cooled below transition temperature, T_c, enter the state of superconductivity in which the dc resistivity goes to zero. The estimates of the resistivity in the super-conducting phase place it at less than 4 ´ 10^–25 W-m, which is essentially zero for all practical purposes. The highest value of T_c upto 133 K has been reached for mercury cuprate.

(d)   Magnetic Properties Materials in which a state of magnetism can be induced are termed magnetic materials. There are five classes into which magnetic materials may be grouped: (i) diamagnetic (ii) para-magnetic (iii) ferromagnetic (iv) antiferromagnetic and (v) ferrimagnetic. Iron, Cobalt, Nickel and some of their alloys and compounds possess spontaneous magnetization. Magnetic oxides like ferrites and garnets could be used at high frequencies. Because of their excellent magnetic properties alongwith their high electrical resistivity these materials today find use in a variety of applications like magnetic recording tapes, inductors and transformers, memory elements, microwave devices, bubble domain devices, recording hard cores, etc. Hysteresis, permeability and coercive forces are some of the magnetic properties of magnetic substances which are to be considered for the manufacture of transformers and other electronic components.

(e)   Chemical Properties These properties includes atomic weight, molecular weight, atomic number, valency, chemical composition, acidity, alkalinity, etc. These properties govern the selection of materials particularly in Chemical plant.

(f)   Optical Properties The optical properties of materials, e.g. refractive index, reflectivity and absorp-tion coefficient etc. affect the light reflection and transmission.

(g)   Structure of Materials The properties of engineering materials mainly depends on the internal ar-rangement of the atoms on molecules. We must note that in the selection of materials, the awareness regarding differences and similarities between materials is extremely important.

Metals of a single type atom are named pure metals. Metals in actual commercial use are almost exclusively alloys, and not pure metals, since it is possible for the designer to realize an infinite variety of physical properties in the product by varying the metallic composition of the alloy. Alloys are prepared from mixed types of atoms. Alloys are classified as binary alloys, composed of two components, as ternary alloys, composed of three components or as multi component alloys. Most commercial alloys are multicom-ponent. The composition of an alloy is described by giving the percentage (either by weight or by atoms) of each element in it.

The basic atomic arrangement or pattern is not apparent in the final component, e.g. a shaft or a pulley but the properties of the individual crystals within the metallic component, which are controlled by the atomic arrangement, are mainly responsible for their application in industry.

One can determine the strength of a piece of metal by its ability to withstand external loading. The structure of metal or alloy responds internally to the applied load by trying to counteract the magnitude of the applied load and thus tries to keep the constituent atoms in their ordered positions if however the load is higher than the force which holds the atoms in place, the metallic bond becomes ineffective and atoms in the metal are then forced into new displaced positions. The movement of atoms from their original positions in the metal is termed as slip. The ease with which atoms move or slip in a metal is an indication of hardness. We must note that the relative movement of atoms or slip within a material has a direct bearing on the mechanical properties of the material.

3. CLASSIFICATION OF ENGINEERING MATERIALS

The factors which form the basis of various systems of classifications of materials in material science and engineering are: (i) the chemical composition of the material, (ii) the mode of the occurrence of the material in the nature, (iii) the refining and the manufacturing process to which the material is subjected prior it acquires the required properties, (iv) the atomic and crystalline structure of material and (v) the industrial and technical use of the material.

Common engineering materials that falls within the scope of material science and engineering may be classified into one of the following six groups:

(i)    Metals  (ferrous  and  non-ferrous)  and  alloys

(ii)    Ceramics

(iii)    Organic  Polymers

(iv)    Composites

(v)     Semi-conductors

(vi)    Biomaterials

(vii)    Advanced  Materials

(i)   Metals: All the elements are broadly divided into metals and non-metals according to their properties. Metals are element substances which readily give up electrons to form metallic bonds and conduct elec-tricity. Some of the important basic properties of metals are: (a) metals are usually good electrical and thermal conductors, (b) at ordinary temperature metals are usually solid, (c) to some extent metals are malleable and ductile, (d) the freshly cut surfaces of metals are lustrous, (e) when struck metal produce typical sound, and (f) most of the metals form alloys. When two or more pure metals are melted together to form a new metal whose properties are quite different from those of original metals, it is called an alloy.

 

Metallic materials possess specific properties like plasticity and strength. Few favourable characteristics of metallic materials are high lustre, hardness, resistance to corrosion, good thermal and electrical conductivity, malleability, stiffness, the property of magnetism, etc. Metals may be magnetic, non-magnetic in nature. These properties of metallic materials are due to: (i) the atoms of which these metallic materials are composed and (ii) the way in which these atoms are arranged in the space lattice.

Metallic  materials  are  typically  classified  according  to  their  use  in  engineering  as  under:

(i)   Pure Metals: Generally it is very difficult to obtain pure metal. Usually, they are obtained by refining the ore. Mostly, pure metals are not of any use to the engineers. However, by specialised and very expensive techniques, one can obtain pure metals (purity ~ 99.99%), e.g. aluminium, copper etc.

(ii)  Alloyed Metals: Alloys can be formed by blending two or more metals or atleast one being metal. The properties of an alloy can be totally different from its constituent substances, e.g. 18-8 stainless steel, which contains 18%, chromium and 8% nickle, in low carbon steel, carbon is less than 0.15% and this is extremely tough, exceedingly ductile and highly resistant to corrosion. We must note that these properties are quite different from the behaviour of original carbon steel.

(iii)  Ferrous Metals: Iron is the principal constituent of these ferrous metals. Ferrous alloys contain signifi-cant amount of non-ferrous metals. Ferrous alloys are extremely important for engineering purposes. On the basis of the percentage of carbon and their alloying elements present, these can be classified into following groups:

(a)    Mild Steels: The percentage of carbon in these materials range from 0.15% to 0.25%. These are moderately strong and have good weldability. The production cost of these materials is also low.

(b)    Medium Carbon Steels: These contains carbon between 0.3% to 0.6%. The strength of these materials is high but their weldability is comparatively less.

(c)     High Carbon Steels: These contains carbon varying from 0.65% to 1.5%. These materials get hard and tough by heat treatment and their weldability is poor.

The steel formed in which carbon content is upto 1.5%, silica upto 0.5%, and manganese upto 1.5% alongwith traces of other elements is called plain carbon steel.

(d)    Cast Irons: The carbon content in these substances vary between 2% to 4%. The cost of production of these substances is quite low and these are used as ferrous casting alloys.

(iv)   Non-Ferrous Metals: These substances are composed of metals other than iron. However, these may contain iron in small proportion. Out of several non-ferrous metals only seven are available in sufficient quantity reasonably at low cost and used as common engineering metals. These are aluminium, tin, copper, nickle, zinc and magnesium. Some other non-ferrous metals, about fourteen in number, are produced in relatively small quantities but these are of vital importance in modern industry. These includes, chromium, mercury, cobalt, tungsten, vanadium, molybdenum, antimony, cadmium, zirconium, beryllium, niobium, titanium, tantalum and manganese.

(v)   Sintered Metals: These materials possess very different properties and structures as compared to the metals from which these substances have been cast. Powder metallurgy technique is used to produced sintered metals. The metals to be sintered are first obtained in powered form and then mixed in right calculated proportions. After mixing properly, they are put in the die of desired shape and then processed with certain pressure. Finally, one gets them sintered in the furnace. We must note that the mixture so produced is not the true alloy but it possesses some of the properties of typical alloys.

(vi)  Clad Metals: A ‘sandwich’ of two materials is prepared in order to avail the advantage of the properties of both the materials. This technique is termed as cladding. Using this technique stainless steel is mostly embedded with a thick layer of mild steel, by rolling the two metals together while they are red hot. This technique will not allow corrosion of one surface. Another example of the use of this technique is cladding of duralium with thin sheets of pure aluminium. The surface layers, i.e. outside layers of aluminium resist corrosion, whereas inner layer of duralumin imparts high strength. This technique is relatively cheap to manufacture.

4. ORGANIC, INORGANIC AND BIOLOGICAL MATERIALS

Organic materials are carbon compounds and their derivatives. They are solids composed of long molecular chains. The study of organic compounds is very important because all biological systems are composed of carbon compounds. There are also some materials of biological origin which do not possess organic composition, e.g., limestone.

Organic Materials

These materials are carbon compounds in which carbon is chemically bonded with hydrogen, oxygen and other non-metallic substances. The structure of these compounds is complex. Common organic materials are plastics and synthetic rubbers which are termed as organic polymers. Other examples of organic materials are wood, many types of waxes and petroleum derivatives. Organic polymers are prepared by polymeri-sation reactions, in which simple molecules are chemically combined into long chain molecules or three-dimensional structures. Organic polymers are solids composed of long molecular chains. These materials have low specific gravity and good strength. The two important classes of organic polymers are:

(a)  Thermoplastics: On heating, these materials become soft and hardened again upon cooling, e.g., nylon, polythene, etc.

(b)  Thermosetting plastics: These materials cannot be resoftened after polymerisation, e.g., urea-formalde-hyde, phenol formaldehyde, etc. Due to cross-linking, these materials are hard, tough, non-swelling and brittle. These materials are ideal for moulding and casting into components. They have good corrosion resistance.

The excellent resistance to corrosion, ease of fabrication into desired shape and size, fine lusture, light weight, strength, rigidity have established the polymeric materials and these materials are fast replacing many metallic components. PVC (Polyvinyl Chloride) and polycarbonate polymers are widely used for glazing, roofing and cladding of buildings. Plastics are also used for reducing weight of mobile objects, e.g., cars, aircrafts and rockets. Polypropylenes and polyethylene are used in pipes and manufacturing of tanks. Thermo-plastic films are widely used as lining to avoid seepage of water in canals and lagoons.

To protect metal structure from corrosion, plastics are used as surface coatings. Plastics are also used as main ingredients of adhesives. The lower hardness of plastic materials compared with other materials makes them subjective to attack by insects and rodents.

Because of the presence of carbon, plastics are combustible. The maximum service temperature is of the order of 100° C. These materials are used as thermal insulators because of lower thermal conductivity. Plastic materials have low modulus of rigidity, which can be improved by addition of filters, e.g., glass fibres.

Natural rubber, which is an organic material of biological origin, is an thermoplastic material. It is prepared from a fluid, provided by the rubber trees. Rubber materials are widely used for tyres of automo-biles, insulation of metal components, toys and other rubber products.

Inorganic Materials

These materials include metals, clays, sand rocks, gravels, minerals and ceramics and have mineral origin. These materials are formed due to natural growth and development of living organisms and are not biologi-cal materials.

Rocks  are  the  units  which  form  the  crust  of  the  earth.  The  three  major  groups  of  rocks  are:

(i)   Igneous Rocks: These rocks are formed by the consolidation of semi-liquid of liquid material (magma) and are called Plutonic if their consolidation takes place deep within the earth and volcanic if lava or magma solidifies on the earth’s surface. Basalt is igneous volcanic where as granite is igneous plutonic.

(ii) Sedimentary Rocks: When broken down remains of existing rocks are consolidated under pressure, then the rocks so formed are named as sedimentary rocks, e.g., shale and sandstone rocks. The required pressure for the formation of sedimentary rocks is supplied by the overlying rocky material.

(iii)              (iii) Metamorphic Rocks: These rocks are basically sedimentary rocks which are changed into new rocks by intense heat and pressure, e.g., marble and slates. The structure of these rocks is in between igneous rocks and sedimentary rocks.

Rock materials are widely used for the construction of buildings, houses, bridges, monuments, arches, tombs, etc. The slate, which has got great hardness is still used as roofing material. Basalt, dolerite and rhyolite are crushed into stones and used as concrete aggregate and road construction material.

Another type of materials, i.e. Pozzolanics, are of particular interest to engineers because they are naturally occurring or synthetic silicious materials which hydrate to form cement. Volcanic ash, blast furnace slag, some shales and fly ash are examples of pozzolanic materials. When the cement contains 10-20% ground blast furnace slag, then it is called pozzolans-portland cement, which sets more slowly than ordinary portland cement and has greater resistance to sulphate solutions and sea water.

Rocks, stone, wood, copper, silver, gold etc. are the naturally occurring materials exist in nature in the form in which they are to be used. However, naturally occurring materials are not many in number. Nowadays, most of the materials are manufactured as per requirements. Obviously, the study of engineering materials is also related with the manufacturing process by which the materials are produced to acquire the properties as per requirement.

Copper, silver, gold, etc. metals, which occur in nature, in their free state are mostly chemically inert and highly malleable and ductile as well as extremely corrosion resistant. Alloys of these metals are harder than the basic metals. Carbonates, sulphates and sulphide ores are more reactive metals.

Biological Materials

Leather, limestone, bone, horn, wax, wood etc. are biological materials. Wood is fibrous composition of hydrocarbon, cellulose and lignin and is used for many purposes. Apart from these components a small amount of gum, starch, resins, wax and organic acids are also present in wood. One can classify wood as soft wood and hard wood. Fresh wood contains high percentage of water and to dry out it, seasoning is done. If proper seasoning is not done, defects such as cracks, twist, wrap etc. may occur.

Leather is obtained from the skin of animals after cleaning and tanning operations. Nowadays, it is used for making belts, boxes, shoes, purses etc. To preserve the leather, tanning is used. Following two tanning techniques are widely used:

(a)   Vegetable Tanning: It consist of soaking the skin in tanning liquor for several days and then dried to optimum conditions of leather.

(b)   Chrome Tanning: This technique involves pickling the skin in acid solution and then revolving in a drum which contains chromium salt solution. After that the leather is dried and rolled.

Limestone is an important material which is not organic but has biological origin. It mainly consist of calcium carbonate and limestone. It is widely used to manufacture cement. In Iron and Steel Industries, limestone in pure form is used as flux.

In early days bones of animals were used to make tools and weapons. Nowadays bones are used for the manufacture of glue, gelatin etc. Bones are laminate of organic substances and phosphates and carbonates of calcium. These are stronger in compression as compared to tension.

Table  1.3  lists  typical  examples  from  each  of  the  four  groups  of  materials.

Table 1.3    Important grouping of materials

Material  group	Important  characteristics	Typical  examples  of  engineering  use
(1)	(2)	(3)

 

1. Metals and Alloys Lusture, hardness, thermal and electrical conductivity, resistance to corrosion, mal-leability, stiffness and the property of magnetism

Iron and steels, aluminium, copper, silver, gold, zinc, magnesium, brasses, bronzes, manganin, invar, super alloy, boron, rare-earth alloys, conductors, etc.

 

Material  group	Important  characteristics	Typical  examples  of  engineering  use
(1)	(2)		(3)

 

2. Ceramics and Glasses Thermal resistance, hardness, brittleness, opaqueness to light, electrical insulation abrasiveness, high temperature strength and resistance to corrosion

3. Organic Polymers Soft, light in weight, poor conductors of electricity and heat, dimensionally un-stable, ductile, combustible, low thermal resistance

4.  Composites                  They  are  better  than  any  of  the  individual

(i)    Metals and alloys components as regards to their properties and ceramics like strength, stiffness, heat resistance,

(ii)   Metals  and  alloys       etc.

and  organic  poly-

mers

(iii)     Ceramics and organic polymers

Silica, soda-lime-glass, concrete, cement, refractories, Ferrites and garnets, ceramic superconductors, MgO, CdS, Al₂O₃, SiC, BaTiO₃, etc.

Plastics: PVC, PTFE, polyethylene, poly-carbonate

Fibres: terylene, nylon, cotton, natural and synthetic rubbers, leather

Other uses: refrigerants, explosives, insula-tors, lubricants, detergents, fuels, vitamins, medicines for surface treatment, adhesives, fibre-reinforced plastics, etc.

l    Steel-reinforced concrete, dispersion hardened alloys.

l    Vinyl coated steel, whisker-reinforced plastics.

l    Fibre-reinforced plastics, carbon-reinforced rubber.

 

Some  important  properties  for  different  groups  of  materials  are  summarized  in  Table  1.4.

	Table 1.4    Important properties for different groupings of materials
Property		Metals	Ceramics	Polymers	Composites  (wood)
1.	Tensile  strength  (N/mm2)	200–2000	10–400	30–100	20–110
2.	Density  (10N/mm2)	2–8   ´ 103	2–17   ´  103	1–2  ´ 103	0.5   ´ 103
3.	Hardness	medium	high	low	low
4.	Tensile  modulus  (103N/mm2)	100–200	150–450	0.7–3.5	4–20
5.	Melting  point  (° C)	200–3500	2000–4000	70–200	—
6.	Thermal  expansion	medium	low	high	low
7.	Thermal  conductivity	high	medium	low	low
8.	Electrical  conductivity	good  conductors	insulator	insulator	insulator

5. SEMICONDUCTORS

These are the materials which have electrical properties that are intermediate between the electrical con-ductors and insulators. The electrical characteristics of semiconductors are extremely sensitive to the pres-ence of minute concentrations of impurity atoms; these concentrations may be controlled over very small spatial regions. Semiconductors form the backbone of electronic industry. The semiconductors have made possible the advent of integrated circuitary that has totally revolutionized the electronics and computer industries. They affect all walks of life whether it is communications, computers, biomedical, power, aviation, defence, entertainment, etc. The field of semiconductors is rapidly changing and expected to continue in the next decade. Organic semiconductors are expected to play prominent role during this decade. Diamond as semiconductor will also be important. Optoelectronic devices will provide three-dimensional integration of circuits, and optical computing.

 

6. BIOMATERIALS

These are employed in components implanted into the human body for replacement of diseased or damaged body parts. Biomaterials must not produce toxic substances and must be compatible with body tissues (i.e., these materials must not cause adverse biological reactions). All the above materials, i.e., metals, ceramics, polymers, composites, and semiconductors—may be used as biomaterials.

7.SELECTION OF MATERIALS

One of the most challenging task of an engineer is the proper selection of the material for a particular job, e.g., a particular component of a machine or structure. An engineer must be in a position to choose the optimum combination of properties in a material at the lowest possible cost without compromising the quality. The properties and behaviour of a material depends upon the several factors, e.g., composition, crystal structure, conditions during service and the interaction among them. The performance of materials may be found satisfactory within certain limitations or conditions. However, beyond these conditions, the performance of materials may not be found satisfactory.

One  can  list  the  major  factors  affecting  the  selection  of  materials  as

(i)	Component  shape	(ii)	Dimensional  tolerance
(iii)	Mechanical  properties	(iv)	Fabrication  requirements
(v)	Service  requirements	(vi)	Cost  of  the  material
(vii)	Cost  of  processing,  and	(viii)	Availability  of  the  material.

All these major factors have a complex effect on the selection of materials. The shape and size of a component has great effect on the choice of the processing unit which ultimately effects the choice of the material. To make it more clear, we consider an example, let the best possible production method is selected, under given conditions, it is die casting, obviously, now the choice of the material becomes limited, i.e. one can only choose materials with lower melting points, e.g. aluminium, zinc, magnesium and thermoplastics.

There are some materials which can be finished to close tolerance while others cannot. Obviously, the required dimensional tolerance for finished components will, influence the choice of materials.

To select a suitable material for specific conditions, all mechanical properties, e.g., hardness, strength, etc. guide us. Method of processing of the material also affects the properties of a component, e.g., forged components can be stronger than the casted components. Different types of working processes may also give different types of fibre structure. However, investment casting can provide precise dimensions at low cost in comparison to machine operations.

Service requirements are dimensional stability, strength, toughness, heat resistance, corrosion resistance, fatigue and creep resistance, electrical and thermal conductivity etc. where as fabrication requirements are castability, i.e., ease in casting a material, weldability-ease in welding the material, machinability-ease to machine a material, formability-ease to form a material, hardenability etc.

In most of the cases, the cost of raw material accounts about 50 per-cent of the finished cost. Obviously, the cost of the material is a major factor which influences the choice of the material or process. We must note that the use of cheaper material will not always reduce the final cost of the component or product. Use of cheaper material may be associated with higher processing cost due to large number of operations to be performed and also more scrap. We can easily see that this sometimes makes the overall cost more than that of expensive raw material in combination with low processing cost due to lesser number of operations and lesser scrap.

The type of material affects the detailed aspect of design and hence the choice of material as well as the process is selected at the early design state, e.g., whether the material is to be joined by spot welding, screws or rivetes, must be decided at the design state.

In most of the industries, the processing cost (labour cost) and other costs such as overhead costs account for about 50% of the production cost. Overhead cost in automatic industries is much more than the other costs. If one can somehow reduce all such costs, the total production cost will automatically reduce. In comparison to conventional processes, sometimes injection moulding process is preferred because the conventional process involves many intermediate stages and several machining processes.

One finds that the cost of production of a component by rolling or forging operations is twice as compared to the production by powered metallurgy process because the rolling or forging are followed by several machining operations for the same finish and tolerances.

We may find that sometimes the availability of the material becomes a governing factor. When the desired material supply is limited, then a costly material which is available in ample quantity may be chosen.

In the light of above factors, sometimes it may be the case that two or more than two materials are found suitable for a particular component or job. Sometimes, it may also happen that the above factors may oppose each other. This shows that there may no exact or true solution and one has to compromise in the final selection. One can compromise by taking into consideration the relative merits and demerits, cost of finished component and its life.

Summarizing, we can say that the selection of material is a dynamic process and change in design may be progressive. Keeping in view, the availability and awareness of latest technological developments, one can always change the material from time to time. While making the selection of material amongst the available large range of different types of materials the wisest choice should be made keeping in view the above factors to achieve the efficient utilisation of materials.