Material Science Manual

7/29/2019 Material Science Manual

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Chapter 1: Introduction-------------------------------------------------------------------------------------------------------------------------------------------------------

Chapter 1

INTRODUCTION

1.1 CLASSIFICATION OF ENGINEERING MATERIALS

Knowledge of engineering materials is an essential requirement for success of the industrializationprocess of any country. In fact, the various stages of human development have always beenassociated to the type of material in use for that particular civilization. Early man used stones formaking hunting tools, and this civilization was called the stone age. The early used metals were inthe form of copper and its alloys bronze and brass, and this civilization is known as the bronze age.It is believed that copper and tin ores were smelt in coal fires, refined and alloyed into bronze as earlyas 3500 BC. At this stage, man had acquired basic skills in metallurgical extraction of these metals,and two thousand years later, man was able to process iron ore into useful metal in Egypt andelsewhere during the iron age. Subsequent generations developed metal alloys with superiormechanical and physical properties to meet the growing demands of the industrialization process.Later, development of polymers and ceramics added to the growing range of engineering materials, tomeet the challenges in the design of more sophisticated machines and processing systems. More andmore of new materials continue to evolve as man seeks to improve comfort and efficiency, while atthe same time man has to face the challenges of dwindling natural resources.

Materials that we use are largely based on the pure sciences of chemistry and physics, obeying thelaws of these basic sciences in their formation, reactions and combinations. All matter is composed ofatoms bonded together in different patterns. Fig. 1.1 shows that all the materials we have can beclassified as organic or inorganic. Organic materials contain the element carbon (and usuallyhydrogen) as a key part of their structure, and are usually derived from living things. Petroleumproducts are organic, because crude oil and coal are derived from plants and animals that died manyyears ago. Sand, rock, water, metals, and inert gases are inorganic materials.

An engineer has a vast range of materials from which to select a material for a particular design.Materials can be classified into four groups: metals, ceramics and glasses, polymers, and composites.Figure 1.2 illustrates these classes of materials and their interaction. Each class consists of a widerange arising from variations in chemical composition, heat treatment, processing etc.

Metals

Metals and alloys, which include steel, aluminium, magnesium, zinc, cast iron, titanium, copper,nickel, and many others, have the general characteristics of good electrical and thermal conductivity,relatively high strength, high stiffness, ductility or formability, and shock resistance. They areparticularly useful for structural or load - bearing applications. Although pure metals are occasionally

used, combinations of metals called alloys are normally designed to provide improvement in aparticular desirable property or permit better combinations of properties.

Ceramics

Ceramics are inorganic, non-metallic materials which consists mainly of silicon chemically combinedwith non metallic elements such as oxygen, carbon and nitrogen. They are normally processed andused at high temperatures. Ceramics, such as brick, glass, tableware, insulators, and abrasives, havepoor electrical and thermal conductivity. Despite their good strength and hardness, ceramics havepoor ductility, formability, and shock resistance. Although unsuitable for structural applications,

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Chapter 1: Introduction-------------------------------------------------------------------------------------------------------------------------------------------------------ceramics posses excellent resistance to high temperatures and certain corrosive media, and have anumber of unusual but desirable optical, electrical, and thermal properties.

Fig.1.1 In our universe, the elements are the building blocks for all materials

Fig. 1.2 The classification of engineering materials

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Chapter 1: Introduction-------------------------------------------------------------------------------------------------------------------------------------------------------Polymers

Polymers include rubber, plastics, and many types of adhesives. They are produced by creating largemolecular structures from organic molecules, obtained from petroleum or agricultural products, in aprocess known as polymerization. Polymers have low electrical and thermal conductivity, have lowstrength, and are not suitable for use at high temperatures. Some polymers (thermoplastics) have

excellent ductility, formability, and shock resistance, while others (thermosets) have oppositeproperties. Polymers are lightweight and frequently have excellent resistance to corrosion.

Composite materials

Composites are formed when two or more materials are combined to produce properties which cannotbe produced by any single material. Concrete, plywood, and fibre-glass are typical examples ofcomposite materials. With composites, one can produce lightweight, strong, ductile, hightemperature -resistant materials that are otherwise unobtainable.

1.2 PROPERTIES OF MATERIALS

All engineering products utilize materials. The product demands for a material/materials with specificcharacteristics such that:-(a) the material can be processed into the final product economically,(b) the products satisfy their functional purpose.

To select the optimum material for an engineering application, the understanding of the materialproperties is essential. Table 1.1 is a summary of the different classes of properties that makea material suitable for any engineering application.

1.3 STRUCTURE-PROPERTY PROCESSING RELATIONSHIP

There is a complex three part relationship between the internal structure, the processing of thematerial, and the final properties of the material.

The internal structure constitutes atomic arrangement at one level, crystal structure at the next level,and phase type, size, and distribution at the other level.

Materials can be processed in different ways to produce the desired shape of thecomponent. Metals can be processed by casting, joining (welding, brazing,soldering, adhesive bonding), forming (forging, drawing, extrusion, rolling, bending),

compacting tiny metal powder into a solid mass (powder metallurgy), or machining.Similarly, ceramic materials can be formed into shapes by related processes such ascasting, forming, extrusion, or compaction, often while wet, and sintered at hightemperatures. Polymers are produced by injection of softened plastic into moulds(much like casting), drawing, and forming. The method of processing used largelydepends on the properties and structure of the material, as well as the shape andsize of the final product.

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Chapter 1: Introduction-------------------------------------------------------------------------------------------------------------------------------------------------------The processing of a material affects its structure. A cast copper bar is very differentfrom a formed one. The shape, size, and orientation of the grains is different. Thecast structure has equiaxed and sometimes columnar grains that may contain voidsdue to shrinkage and gas bubbles, and also may have non metallic inclusionstrapped within the structure. The formed material has elongated grains that may

sometimes contain elongated nonmetallic particles and internal defects.

TABLE 1.1: PROPERTIES OF MATERIALS

Economic Properties Price and availability

Bulk mechanical properties DensityModulus and dampingYield strengthTensile strengthHardnessFracture toughness

Fatigue strengthThermal fatigue resistanceCreep strength

Bulk non-mechanical properties Thermal propertiesOptical propertiesMagnetic propertiesElectrical properties

Surface properties Friction, abrasion and wearOxidation and corrosion

Production properties Ease of manufactureFabrication, joining, finishing

Aesthetic properties Appearance, texture, feel

1.4 ENVIRONMENTAL EFFECTS ON MATERIALS BEHAVIOUR

The surroundings to which the material is subjected may also affect the structure-property-processingrelationship.

LoadingThe type of load cycle, and the speed at which it is applied may change the behaviour of the material.

A material that possesses high yield strength may easily fail at lower loads if subjected to cyclicloading (fatigue) or impact.

TemperatureChanges in temperature may dramatically alter the material properties. The strength of mostmaterials decreases as the temperature increases. Metals that have been strengthened by certainheat treatments may suddenly lose their properties when heated. Low temperatures may causecertain ductile metals to fail in a brittle manner even at low loads. High temperature can also lead to

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Chapter 1: Introduction-------------------------------------------------------------------------------------------------------------------------------------------------------changes in the structure of ceramics or cause polymers to melt or char.

AtmosphereMost metals and polymers react with oxygen or other gases particularly at elevated temperatures.Metals and ceramics may catastrophically disintegrate or be chemically attacked, while others may beprotected. Polymers often are hardened or my even depolymerize, char, or burn. steels may react

with hydrogen and become brittle.

CorrosionMetals are attacked by a variety of corrosive liquids. The metal may be uniformly or selectivelyconsumed or may develop cracks or pits leading to premature failure. Ceramics can be attacked byother liquid ceramics, while solvents can dissolve polymers.

RadiationHigh energy radiation, such as neutrons produced in nuclear reactors, can affect the internal structure

of all materials, producing a loss of strength, embrittlement, or critical alteration of physicalproperties.

1.5 PRINCIPLES OF MATERIALS SELECTION

The selection of a material for a particular application depends on several factors, as was summarizedin Table 1. The designer may have to answer the following fundamental questions:

Does the material selected meet the strength requirements for the functioning of the design? Is the material available at reasonable cost? Can it be processed to the desired design shape and specifications? Is the material safe to use? Will the material survive the environment in which it is expected to operate? Is it attractive? At the end of its useful life, can the material be recycled?

Materials selection is an important part in the decision making process at each stage of the design.There are many factors that may have to be considered in this selection process, but broadly, onestarts with a broad range of materials, then narrows it down as some materials are eliminated. Thesefactors may be grouped as follows:

(a) Physical factors: These include size, shape, and the weight of the material, while consideringthe space available for the component. The way the particular component may have to beprocessed, such as heat treatment, is constrained by the size and capacity of your heattreatment plant. The weight of the material may add to the energy consumption intransportation systems, making lighter materials more preferable in transport systems suchas aircraft and boats. The shape of the materials is dependent on the production process.

(b) Mechanical factors: These are factors used as design criteria to prevent mechanical failure,such as the tensile strength, the yield strength, the modulus of elasticity, the fracturetoughness, the impact strength, creep strength, bending strength, fatigue strength, plasticity,etc.

(c) Processing and Fabricability: It is important to know how the material will be processed orfabricated to the desired shape and size. The designer ought to know whether the materialwill be cast, forged, machined, welded, etc., and determine also the heat treatment or finishprocessing required before the component is put to use. This includes consideration on how

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Chapter 1: Introduction-------------------------------------------------------------------------------------------------------------------------------------------------------

the material is to be protected from environmental degradation such as oxidation andcorrosion.

(d) Life of Component factors: Manufacturers have to take into account the expected life of thecomponent. These are usually dependent on the type of loading and the environment underwhich the component is operating. So in this group, factors to be considered includecorrosion, oxidation, wear, creep, corrosion fatigue, ultraviolet exposure, and impact loading.Designers often develop life prediction models which are used to estimate the expecteddesign life.

(e) Cost and Availability: We live in a market driven economy, hence it is essential not to ignorethe cost and availability factors, which are interrelated. Costs of transportation from the placeof origin to the place of application may sometimes dictate against the use of a cheapermaterial that is not locally available. Sometimes, the customer may pay a higher cost if thenumber of units ordered are few, or when the items are non standard and thus demandspecial processing.

(f) Codes, Statutory and other factors: Codes are sets of technical requirements that are usually

imposed on the material or the component. These may be set by the customer, or aretechnical requirements of standardization bodies such as ISO, DIN, ASME, ASTM, SAE, BS,etc. Designers must be familiar with these standardization bodies, which in addition, providefor methods of testing and processing materials for particular applications. Statutory factorsrelate to local, state, and federal regulations about the materials and processes used or thedisposal of the material. These relate to health, safety, and environmental requirements.

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Chapter 1: Introduction-------------------------------------------------------------------------------------------------------------------------------------------------------REVIEW PROBLEMS

1. Solid Al2O3 is strong, hard, and wear resistant. Why isn't it used to make sledge hammers?

2. Polyethylene is an inexpensive, easily formed material. Why isn't it used to make paper clips?

3. Classify the following materials as metals, ceramics, polymers, or composite materials:brass, reinforced concrete, rubber, sodium chloride, lead-tin solder, silicon carbide, epoxy,magnesium alloys, concrete, fibreglass, graphite.

4. What mechanical and physical properties are of importance when selecting materials for thefollowing applications?

crankshaft for an automobilepiping to transport hot gases and fluids in a refinerydisposable beverage cansaxle for an automobilefilament in a light bulbwindshield of an automobilepair of scissorsscreen of a television set

5. Describe the type of materials processing techniques that might be used to produce thefollowing products.

automobile engine blockbrickpaper clipwrenchplywoodplastic toyplastic water pipesteel transmission gear

6. Would casting be a good way to form the following materials and products?

aluminium, alumina (Al2O3), beryllium, silicon carbide (SiC), tungsten, glass, titanium,aluminium foil. Explain if casting is not a good choice.

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Material Science Manual