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Chapter 2 - Classification of Engineering Materials

Chapter 2 Classification of Engineering Materials _________________________________________________________________________________ 2.1.

Introduction

Engineering materials can be defined as substances of which something is composed or made. They are ubiquitous and are found virtually in every engineering or technical product we use. Common examples of engineering materials include aluminum, copper, steel, concrete, brick, plastic, glass, rubber, paper, and wood (timber). Applications of materials include: (i) metals in automobiles, aircraft, trains, suspension bridges, beverage cans, foils, etc.; (ii) concrete and bricks in houses and highways; (iii) materials in electronic circuit; and (iv) carbide bits on oil-well drills. Each of these products requires materials with specific characteristics such that the materials can be processed into the final products satisfactorily and economically and the products will behave appropriately in service. 2.2.

Origin of Engineering Materials

Most of the substances we deal with in industry and in our everyday life can be classified as organic or inorganic (Figure 2.1). Organic materials are usually derived from living things. They usually contain the elements, carbon and hydrogen. Examples include petroleum products, polymers, and natural resins. Crude oil is essentially the residue of plants that lived many years ago and all plants and animals are organic in nature. Natural resins include gums from trees and rosin from pine trees. They are soluble in organic solvents. In contrast, inorganic materials are substances not derived from living things; for example sand, rock, water, metals, and inert gases. Majority of the engineering materials that are used in engineering applications are obtained by mining their ore from the earth’s crust. The ore is then concentrated to allow the extraction or synthesis of the material from the ore. Few materials, however, such as magnesium, are synthesized from compounds found in the earth’s oceans and atmosphere. Magnesium is produced from salt water. The engineering materials we use are not equally abundant and widespread (Table 2.1 and Figure 2.2).

Figure 2.1: Building blocks for all materials.

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Chapter 2 - Classification of Engineering Materials

Table 2.1 The relative abundance of elements on the Earth’s crust, and in the oceans and atmosphere [1]. Earth’s crust Element Oxygen Silicon Aluminum Iron Calcium Sodium Potassium Magnesium Titanium Hydrogen Phosphorus Manganese Fluorine Barium Strontium Sulfur Carbon

wt.% 47 27 8 5 4 3 3 2 0.4 0.1 0.1 0.1 0.06 0.04 0.04 0.03 0.02

Oceans Element Oxygen Hydrogen Chlorine Sodium Magnesium Sulfur Calcium Potassium Bromine Carbon

wt.% 85 10 2 1 0.1 0.1 0.04 0.04 0.007 0.002

Atmosphere Element Nitrogen Oxygen Argon Carbon dioxide

wt.% 79 19 2 0.04

#The total mass of the crust to a depth of 1 km is x 10 21 kg; the mass of the oceans is 1020 kg; the mass of the atmosphere is 5 x 10 18 kg.

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Percentage of earth’s crust

40

30

20

10 0 O

Si

Al

Fe Ca Element

Na

K

Figure 2.2: Relative abundance of elements in the Earth's crust.

22

Mg

H

Chapter 2 - Classification of Engineering Materials

2.3.

Types of Engineering Materials

For convenience and on the basis of the differences in properties and the common processing procedures, especially in the initial steps that involve raw material processing and primary forming operations, engineering materials can be divided into three main classes, namely metals, ceramics and polymers (Figure 2.3). The general characteristics of the engineering materials are compared in Table 2.2 and Figures 2.4-2.8. The following is a brief description of the characteristics of the three main types of materials: (a) Metals – These are inorganic materials composed of one or more metallic elements such as iron, zinc, titanium, magnesium, aluminum, copper, gold, silver, nickel, etc. They often contain non-metallic elements such as carbon, nitrogen, and oxygen in small amounts. In general, the typical attributes of metals include: (i) crystalline structure - their atoms are arranged in a very orderly manner; (ii) relatively high stiffness (Figure 2.4); (iii) relatively high strength (Figure 2.6), yet are ductile - most pure metals are soft and easily deformed; their strength can be increased by alloying and heat treatment but they remain ductile, allowing them to be formed by deformation processes; (iv) they are tough with fairly high fracture toughness (Figure 2.7) – accounts for their widespread use in structural applications; (v) readily available and easy to fabricate; (vi) generally opaque; (vii) good electrical and thermal conductors (Figure 2.8); (viii) good thermal shock resistance; (ix) moderate temperature resistance (Figure 2.5); and (x) susceptibility to corrosion and oxidation (because of their reactive nature).

Engineering Materials Metals • Ferrous - Cast Iron - Steel

• Non-Ferrous

Polymers • Thermoplastics - nylons, polystyrene, polypropylene, etc.

• Thermosets

- Al, Mg, Cu, Ti, Ni, - epoxies, polyesters etc., alloys • Elastomers • Precious metals - Au - vulcanized rubber

• Superalloys

Ceramics • Traditional - Clay, Silica, Feldspar

• Advanced - Oxides, Nitrides, Carbides, Ferrites, Titanates

• Glass

Metal-Polymer Composites

Polymer-Ceramics Composites

Miscellan. • Advanced materials • Electronic • Magnetic • Construction - Concrete, Wood,

• Biomaterials • Smart materials • Nanomaterials

Metal-Ceramics Composites Figure 2.3: Types of Engineering Materials

(b) Ceramics – These are non-metallic, inorganic solids consisting of compounds formed between metallic and non-metallic elements. They include oxides such as alumina (the material of spark-plug insulators), nitrides, and carbides. Their typical attributes include: (i) resistance to chemical attack; (ii) brittle, but not necessarily weak; (iii) hard and wear resistant (Figure 2.6); (iii) refractory, high melting point, and high temperature strength – they retain their strength at high temperatures (Figure 2.5); (iv) good thermal and electrical insulators; (v) low thermal expansion coefficients; (vi) high stiffness (Figure 2.4); (vi) high compressive strength but low tensile strength; (vii) poor fracture toughness (Figure 2.7) – this gives 23

Chapter 2 - Classification of Engineering Materials

ceramics low tolerance for stress concentrations (like holes or cracks) or for high contact stresses (at clamping points) and makes it more difficult to design with ceramics than metals; and (viii) poor thermal shock resistance. Table 2.2 General comparison of the characteristics of engineering materials Property Mechanical Tensile strength, MPa Yield strength, MPa Elongation Compressive strength, MPa Impact strength Hardness, kg/mm2 Elastic Modulus, MPa Creep strength Fatigue strength Physical Density, Mg/m3 Melting point Thermal conductivity Electrical conductivity Coefficient of thermal expansion Max. use temperature Water absorption Electrical insulation Chemical Crystallinity Chemical resistance Composition (raw materials) Microstructure Fabricability Sheet material Castings Extrusions Machinability Molding

Metals

Ceramics

Polymers

2,069 max. 1,724 max. Up to 50% 2,758 max. Poor to excellent 900 max. Can be 2.76 x 105 Good Good to excellent

690 max. Does not yield Zero 4,137 max. Poor Can be 2,600 Can be 6.21 x 105 Excellent Fair in compression

207 max. 172 max. Can be 100% 276 max. Fair 100 max. Can be 0.69 x 105 Poor Poor

1.66 to 22.14 up to 2,760oC Good to excellent Excellent High 816oC Nil Nil

2.77 to 16.61 >5000oC Poor to fair Nil to some Low 2,760oC Some Good to excellent

0.83 to 2.77...