Materials Engineering Science Processing and Design, Notas de estudo de Engenharia de Materiais

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Materials

Engineering, Science,

Processing and Design

Michael Ashby, Hugh Shercliff and David Cebon

University of Cambridge,

UK

AMSTERDAM • BOSTON • HEIDELBERG • LONDON • NEW YORK • OXFORD PARIS • SAN DIEGO • SAN FRANCISCO • SINGAPORE • SYDNEY • TOKYO Butterworth-Heinemann is an imprint of Elsevier

Contents

Preface ix Acknowledgements xi

Preface

Science-led or Design-led? Two approaches to materials teaching

Most things can be approached in more than one way. In teaching this is especially true. The way to teach a foreign language, for example, depends on the way the student wishes to use it—to read the literature, say, or to find accommodation, order meals and buy beer. So it is with the teaching of this subject. The traditional approach to it starts with fundamentals: the electron, the atom, atomic bonding, and packing, crystallography and crystal defects. Onto this is built alloy theory, the kinetics of phase transformation and the development of microstructure on scales made visible by electron and optical microscopes. This sets the stage for the understanding and control of properties at the mil- limeter or centimeter scale at which they are usually measured. The approach gives little emphasis to the behavior of structures, methods for material selection, and design. The other approach is design-led. The starting point is the need: the requirements that materials must meet if they are to perform properly in a given design. To match materials to designs requires a perspective of the range of properties they offer and the other information that will be needed about them to enable successful selection. Once the importance of a property is established there is good reason to ‘drill down’, so to speak, to examine the science that lies behind it—valuable because an understanding of the fundamentals itself informs material choice and usage. There is sense in both approaches. It depends on the way the student wishes to use the information. If the intent is scientific research, the first is the logical way to go. If it is engineering design, the sec- ond makes better sense. This book follows the second.

What is different about this book?

There are many books about the science of engineering materials and many more about design. What is different about this one? First, a design-led approach specifically developed to guide material selection and manipulation. The approach is systematic, leading from design requirements to a prescription for optimized material choice. The approach is illustrated by numerous case studies. Practice in using it is provided by Exercises. Second, an emphasis on visual communication and a unique graphical presentation of material properties as material property charts. These are a central feature of the approach, helpful both in understanding the origins of properties, their manipulation and their fundamental limits, as well as providing a tool for selection and for understanding the ways in which materials are used. Third, its breadth. We aim here to present the properties of materials, their origins and the way they enter engineering design. A glance at the Contents pages will show sections dealing with:

• Physical properties
• Mechanical characteristics
• Thermal behavior

• Electrical, magnetic and optical response
• Durability
• Processing and the way it influences properties
• Environmental issues

Throughout we aim for a simple, straightforward presentation, developing the materials science as far as is it helpful in guiding engineering design, avoiding detail where this does not contribute to this end. And fourth, synergy with the Cambridge Engineering Selector (CES)^1 —a powerful and widely used PC-based software package that is both a source of material and process information and a tool that implements the methods developed in this book. The book is self-contained: access to the software is not a prerequisite for its use. Availability of the CES EduPack software suite enhances the learning experience. It allows realistic selection studies that properly combine multiple con- straints on material and processes attributes, and it enables the user to explore the ways in which properties are manipulated. The CES EduPack contains an additional tool to allow the science of materials to be explored in more depth. The CES Elements database stores fundamental data for the physical, crystallographic, mechanical, thermal, electrical, magnetic and optical properties of all 111 elements. It allows inter- relationships between properties, developed in the text, to be explored in depth. The approach is developed to a higher level in two further textbooks, the first relating to mechan- ical design^2 , the second to industrial design^3.

x Preface

(^1) The CES EduPack 2007, Granta Design Ltd., Rustat House, 62 Clifton Court, Cambridge CB1 7EG, UK, www.grantadesign.com. (^2) Ashby, M.F. (2005), Materials Selection in Mechanical Design , 3rd edition, Butterworth-Heinemann, Oxford, UK, Chapter 4. ISBN 0-7506-6168-2. ( A more advanced text that develops the ideas presented here in greater depth .) (^3) Ashby, M.F. and Johnson, K. (2002) Materials and Design—The Art and Science of Material Selection in Product Design , Butterworth-Heinemann, Oxford, UK. ISBN 0-7506-5554-2. ( Materials and processes from an aesthetic point of view, emphasizing product design .)

Resources that accompany this book

Exercises

Each chapter ends with exercises of three types: the first rely only on information, diagrams and data contained in the book itself; the second makes use of the CES software in ways that use the methods developed here, and the third explores the science more deeply using the CES Elements database that is part of the CES system.

Instructor’s manual

The book itself contains a comprehensive set of exercises. Worked-out solutions to the exercises are freely available to teachers and lecturers who adopt this book. To access this material online please visit http://textbooks.elsevier.com and follow the instructions on screen.

Image Bank

The Image Bank provides adopting tutors and lecturers with jpegs and gifs of the figures from the book that may be used in lecture slides and class presentations. To access this material please visit http://textbooks.elsevier.com and follow the instructions on screen.

The CES EduPack

CES EduPack is the software-based package to accompany this book, developed by Michael Ashby and Granta Design. Used together, Materials: Engineering, Science, Processing and Design and CES EduPack provide a complete materials, manufacturing and design course. For further information please see the last page of this book, or visit www.grantadesign.com.

Chapter 1

Introduction: materials—

history and character

Chapter contents

1.1 Materials, processes and choice 2 1.2 Material properties 4 1.3 Design-limiting properties 9 1.4 Summary and conclusions 10 1.5 Further reading 10 1.6 Exercises 10

Professor James Stuart, the first Professor of Engineering at Cambridge.

1.1 Materials, processes and choice 3

of an engineer’s training, and it is backed up by widely available packages for solid modeling, finite-element analysis, optimization, and for material and process selection. Software for the last of these—the selection of materials and processes—draws on databases of the attributes of materials and processes, doc- umenting their mutual compatibility, and allows them to be searched and dis- played in ways that enable selections that best meet the requirements of a design. If you travel by foot, bicycle or car, you take a map. The materials landscape, like the terrestrial one, can be complex and confusing; maps, here, are also a good idea. This text presents a design-led approach to materials and manufacturing

10000BC 5000BC 0 1000 1500 1800 1900 1940 1960 1980 1990 2000 2010

10000BC 5000BC 0 1000 1500 1800 1900 1940 1960 1980 1990 2000 2010 Date

Gold Copper Bronze

Iron (^) Cast iron

Wood Skins

Fibers Glues Rubber

Straw-Brick Paper

Bakelite

StoneFlint Pottery

Glass Cement Refractories Portland cement Fusedsilica ceramsPyro-

C-steels Alloy steels

Aluminum Super-alloys

Titanium Zirconium

Nylon PE PMMA

PC Acrylics PS PP

Cermets

Epoxies Polyesters

Technical ceramics: Al 2 O 3 , SiC, Si 3 N 4 , PSZ etc

GFRPCFRP

Kevlar-FRP composites

Metal–matrix

Ceramic composites

High-modulus polymers

High-temperature polymers: PEEK

Glassy metals

Al–Lithium alloys Microalloyed steels Tin Magnesium Metal foams

PTFE

Metals

Polymers & elastomers

Ceramics & glasses

Hybrids

Neanderthal man Julius Caesar Henry VIII of England Queen Victoria Franklin Roosevelt John F. Kennedy George W. Bush

Figure 1.1 The development of materials over time. The materials of pre-history, on the

left, all occur naturally; the challenge for the engineers of that era was one of shaping them. The development of thermochemistry and (later) of polymer chemistry enabled man-made materials, shown in the colored zones. Three— stone, bronze and iron—were of such importance that the era of their dominance is named after them.

processes that makes use of maps: novel graphics to display the world of mate- rials and processes in easily accessible ways. They present the properties of materials in ways that give a global view, that reveal relationships between properties and that enable selection.

1.2 Material properties

So what are these properties? Some, like density (mass per unit volume) and price (the cost per unit volume or weight) are familiar enough, but others are not, and getting them straight is essential. Think first of those that have to do with carrying load safely—the mechanical properties. Mechanical properties A steel ruler is easy to bend elastically —‘elastic’ means that it springs back when released. Its elastic stiffness (here, resistance to bending) is set partly by its shape—thin strips are easy to bend—and partly by a property of the steel itself: its elastic modulus , E. Materials with high E , like steel, are intrinsically stiff; those with low E , like polyethylene, are not. Figure 1.2(b) illustrates the consequences of inadequate stiffness. The steel ruler bends elastically, but if it is a good one, it is hard to give it a permanent bend. Permanent deformation has to do with strength , not stiffness. The ease with which a ruler can be permanently bent depends, again, on its shape and on a different property of the steel—its yield strength , σ y. Materials with large σ y, like titanium alloys, are hard to deform permanently even though their stiffness, coming from E , may not be high; those with low σ y, like lead, can be deformed with ease. When metals deform, they generally get stronger (this is called ‘work hardening’), but there is an ultimate limit, called the tensile strength , σ ts, beyond which the material fails (the amount it stretches before it breaks is called the ductility ). Figure 1.2(c) gives an idea of the consequences of inadequate strength. So far so good. One more. If the ruler were made not of steel but of glass or of PMMA (Plexiglas, Perspex), as transparent rulers are, it is not possible to bend it permanently at all. The ruler will fracture suddenly, without warning, before it acquires a permanent bend. We think of materials that break in this way as brittle, and materials that do not as tough. There is no permanent defor- mation here, so σ y is not the right property. The resistance of materials to cracking and fracture is measured instead by the fracture toughness , K 1c. Steels are tough—well, most are (steels can be made brittle)—they have a high K 1c. Glass epitomizes brittleness; it has a very low K 1c. Figure 1.2(d) suggests conse- quences of inadequate fracture and toughness. We started with the material property density , mass per unit volume, symbol ρ. Density, in a ruler, is irrelevant. But for almost anything that moves, weight carries a fuel penalty, modest for automobiles, greater for trucks and trains, greater still for aircraft, and enormous in space vehicles. Minimizing weight has

4 Chapter 1 Introduction: materials—history and character

Most materials expand when they are heated, but by differing amounts depending on their thermal expansion coefficient, α. The expansion is small, but its consequences can be large. If, for instance, a rod is constrained, as in Figure 1.3(b), and then heated, expansion forces the rod against the con- straints, causing it to buckle. Railroad track buckles in this way if provision is not made to cope with it. Some materials—metals, for instance—feel cold; others—like woods—feel warm. This feel has to do with two thermal properties of the material: thermal conductivity and heat capacity. The first, thermal conductivity, λ , measures the rate at which heat flows through the material when one side is hot and the other cold. Materials with high λ are what you want if you wish to conduct heat from one place to another, as in cooking pans, radiators and heat exchang- ers; Figure 1.3(c) suggests consequences of high and low λ for the cooking ves- sel. But low λ is useful too—low λ materials insulate homes, reduce the energy consumption of refrigerators and freezers, and enable space vehicles to re-enter the earth’s atmosphere.

6 Chapter 1 Introduction: materials—history and character

(c) High conductivity λ Low conductivity λ

(d) High T-diffusivity a Low T-diffusivity a

(b) High expansion coefficient α Low expansion coefficient α

W (^) W (a) High service temperature T max Low service temperature T max

Figure 1.3 Thermal properties.

These applications have to do with long-time, steady, heat flow. When time is limited, that other property— heat capacity , C (^) p—matters. It measures the amount of heat that it takes to make the temperature of material rise by a given amount. High heat capacity materials—copper, for instance—require a lot of heat to change their temperature; low heat capacity materials, like polymer foams, take much less. Steady heat flow has, as we have said, to do with ther- mal conductivity. There is a subtler property that describes what happens when heat is first applied. Think of lighting the gas under a cold slab of material with a bole of ice-cream on top (here, lime ice-cream) as in Figure 1.3(d). An instant after ignition, the bottom surface is hot but the rest is cold. After a while, the middle gets hot, then later still, the top begins to warm up and the ice-cream first starts to melt. How long does this take? For a given thickness of slab, the time is inversely proportional to the thermal diffusivity, a, of the material of the slab. It differs from the conductivity because materials differ in their heat capacity—in fact, it is proportional to λ /C (^) p. There are other thermal properties—we’ll meet them in Chapters 12 and 13—but this is enough for now. We turn now to matters electrical, magnetic and optical.

Electrical, magnetic and optical properties We start with electrical conduction and insulation (Figure 1.4(a)). Without electrical conduction we would lack the easy access to light, heat, power, con- trol and communication that—today—we take for granted. Metals conduct well—copper and aluminum are the best of those that are affordable. But con- duction is not always a good thing. Fuse boxes, switch casings, the suspensions for transmission lines all require insulators, and in addition those that can carry some load, tolerate some heat and survive a spark if there were one. Here the property we want is resistivity , ρ e, the inverse of electrical conductivity κ e. Most plastics and glass have high resistivity (Figure 1.4(a))—they are used as insulators—though, by special treatment, they can be made to conduct a little. Figure 1.4(b) suggests further electrical properties: the ability to allow the passage of microwave radiation, as in the radome, or to reflect them, as in the passive reflector of the boat. Both have to do with dielectric properties, partic- ularly the dielectric constant ε D. Materials with high ε D respond to an electric field by shifting their electrons about, even reorienting their molecules; those with low ε D are immune to the field and do not respond. We explore this and other electrical properties in Chapter 14. Electricity and magnetism are closely linked. Electric currents induce magnetic fields; a moving magnet induces, in any nearby conductor, an electric current. The response of most materials to magnetic fields is too small to be of practical value. But a few—called ferromagnets and ferrimagnets—have the capacity to trap a magnetic field permanently. These are called ‘hard’ magnetic materials because, once magnetized, they are hard to demagnetize. They are used as permanent magnets in headphones, motors and dynamos. Here the key property is the rema- nence , a measure of the intensity of the retained magnetism. A few others—‘soft’

1.2 Material properties 7