4. MECHANICAL BEHAVIOUR
and
TESTING OF MATERIALS
A.K. BHARGAVA and C.P. SHARMA
Department of Metallurgical and Materials Engineering
Malaviya National Institute of Technology
Jaipur
PHI Learning [;)u1ffiG@ llilwlBG@dl
Delhi-110092
2014
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8. Preface
Acknowledgements
Nomenclature
1. Nature of Materials
1.1 Introduction 1
1.2 Interatomic and Intermolecular Bonding 7
1.3 Classification and Combination of Elements JO
1.4 Atomic Arrangement 12
1.5 Engineering Materials 14
1.5.1 Steel 14
1.5.2 Cast Irons 18
1.5.3 Aluminium and Its Alloys 20
1.5.4 Magnesium and Its Alloys 20
1.5.5 Titanium and Its Alloys 20
1.5.6 Copper and Its Alloys 21
1.5.7 Nickel and Its Alloys 22
1.5.8 Cobalt and Its Alloys 24
1.5.9 Ceramic Materials 25
1.5.10 Polymeric Materials 26
1.5.11 Composite Materials 30
2. Crystal Imperfections
2.1 Introduction 32
vii
Contents
xv
xvii
xix
1-31
32-44
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9. viii Contents
2.2 Imperfections 37
2.2.1 Point Imperfections 37
2.2.2 Line Imperfections 40
2.2.3 Surface Imperfections 41
2.2.4 Volume Imperfections 44
3. Mechanical Properties 45-56
3.1 Introduction 45
3.2 Static Mechanical Properties 47
3.2.1 Tensile Strength 47
3.2.2 Compressive Strength 49
3.2.3 Ductility 49
3.2.4 Malleability 50
3.2.5 Stiffness 50
3.2.6 Toughness 51
3.2.7 Creep Strength 52
3.2.8 Hardness 52
3.3 Dynamic Mechanical Properties 53
3.3.1 Impact Strength 53
3.3.2 Fatigue Strength 53
3.3.3 Hardness 54
3.4 Structure-Mechanical Property Relationship 54
4. Dislocation Theory 57-103
4.1 Introduction 57
4.2 The Shear Strength of Ideal and Real Crystals 59
4.3 Geometry of Dislocations 60
4.3.1 Edge Dislocation 61
4.3.2 Screw Dislocation 63
4.4 Burgers Vector, Burgers Circuit and Dislocation Loop 64
4.5 Movement of Dislocations 67
4.5.1 Concept of Slip 67
4.5.2 Dislocations and Slip 70
4.5.3 Slip Plane 72
4.5.4 Cross-Slip 72
4.5.5 Dislocation Climb 73
4.6 Elastic Properties of Dislocations 74
4.6.1 Stress Field and Energy of a Dislocation 74
4.6.2 Forces on Dislocations 77
4.6.3 Line Tension 78
4.7 Forces between Dislocations 80
4.8 Unit Dislocations and Partial Dislocations 84
4.8.1 Dislocations in FCC, BCC and HCP Crystals 85
4.8.2 Dislocations and their Reaction in FCC Crystals 86
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10. Contents ix
4.8.3 Frank Partial Dislocations 91
4.8.4 Lamer-Cottrell Dislocations 92
4.9 Peierls-Nabarro Stress and Dislocation Width 94
4.10 Dislocation Multiplication 96
4.11 Dislocation Intersection 98
4.12 Dislocations in Ceramics 101
5. Deformation of Metals 104-138
5.1 Introduction 104
5.2 Elastic Deformation 104
5.2.1 Significance of Elastic Modulus 106
5.3 Plastic Deformation 107
5.3.1 Deformation by Slip 107
5.3.2 Type of Loading for Plastic Deformation 109
5.3.3 Potential Slip Planes and Directions in Crystals 110
5.3.4 Critical Resolved Shear Stress 113
5.3.5 Strain Hardening in Single Crystal 119
5.3.6 Structural Changes in Cold Worked Polycrystalline
Metals and Alloys 123
5.3.7 Annealing of Cold Worked Metals 128
5.4 Deformation by Twinning 134
5.5 Deformation Behaviour in Ceramics 136
5.6 Deformation Behaviour in Polymers 136
6. Strengthening Mechanisms in Materials 139-205
6.1 Introduction 139
6.2 Grain Boundary Strengthening 141
6.3 Solid Solution Strengthening 145
6.4 Second Phase Particle Strengthening 153
6.4.1 Precipitation Hardening 154
6.4.2 Dispersion Hardening 168
6.5 Strain Hardening 169
6.5.1 Properties Affected by Strain Hardening 171
6.5.2 Industrial Importance of Strain Hardening 172
6.6 Martensitic Strengthening 174
6.7 Composite Strengthening 175
6.7.1 Fibre Strengthened Composites 176
6.7.2 Dispersion Strengthened Composites 188
6.7.3 Particle-Strengthened (or Simply Particulate) Composites 190
6.8 Strengthening of Plastics 190
6.8.1 Strengthening by High Average Molecular Weight 191
6.8.2 Strengthening by Crystallinity 193
6.8.3 Strengthening by Bulky Pendant Atomic Group 195
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11. X Contents
7.
8.
9.
6.8.4 Strengthening Thermoplastics by the Presence of Polar
Atoms or Groups 196
6.8.5 Strengthening Thermoplastics by Introducing Non Carbon
Atoms in the Main Carbon Chain 196
6.8.6 Strengthening Thermoplastics by Introduction of Aromatic
Groups in the Main Chain 197
6.9 Strengthening of Ceramics 197
6.10 Applications of Strengthening Mechanisms to Obtain High
Strength Materials 201
Fracture
7.1 Introduction 206
207
7.2
7.3
7.4
Ductile Fracture
Mechanism of Ductile Fracture
Brittle Fracture 209
209
7.5 Mechanism of Brittle Fracture 210
7.6 Factors Affecting the Type of Fracture 212
Tensile Behaviour
8.1 Introduction 213
8.2 Tension Test and Stress-strain Curves 214
8.3 Tensile Properties 219
8.3.1 Modulus of Elasticity and Stiffness 219
8.4
8.5
8.6
8.7
8.3.2 Yield Strength 221
8.3.3 Tensile Strength 221
8.3.4 Modulus of Resilience 222
8.3.5 Ductility 224
8.3.6 Toughness 225
True Stress-strain Curve 226
Plastic Instability in Tension 232
Discontinuous Yielding (Yield Point Phenomenon)
Important Variables Affecting Tensile Properties
8.7.1 Effect of Gauge Length 239
8.7.2 Effect of Size of the Specimen
8.7.3 Effect of Form of the Specimen
8.7.4 Effect of Strain Rate 243
8.7.5 Effect of Temperature 243
240
241
Hardness Testing
9.1 Introduction 246
9.2 Scratch Hardness 247
9.3 Indentation Hardness 247
9.4 Brinell Hardness Test 249
9.4.1 Precautions 252
233
239
206-212
213-245
246-270
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12. Contents xi
9.4.2 Advantages and Applications of Brinell Hardness Test 253
9.4.3 Disadvantages of Brinell Test 254
9.5 Vickers Hardness Test 254
9.5.1 Derivation of Vickers Formula 255
9.5.2 Sources of Errors 256
9.5.3 Advantages and Applications 257
9.5.4 Disadvantages 257
9.6 Rockwell Hardness Test 258
9.6.1 Principle of Operation 261
9.6.2 Advantages of Rockwell Hardness Test 263
9.6.3 Precautions 263
9.7 Superficial Rockwell Hardness Test 263
9.7.1 Precautions 264
9.8 Microhardness Test 264
9.8.1 Precautions 266
9.8.2 Applications 266
9.8.3 Comparison of Macrohardness and Microhardness Tests 266
9.9 Dynamic Hardness Testing 267
9.9.1 Shore Hardness Testing 267
9.9.2 Poldi Hardness Test 270
10. Ductile-Brittle Transition Behaviour and Fracture
Toughness Test
10.1 Introduction 271
10.2 Ductile-Brittle Transition Behaviour 272
10.3 Transition Temperature and Its Significance 274
10.4 Notch-bar Impact Test 277
10.5 Variable Affecting Impact Values 279
10.6 Behaviour of Polymers under Impact Loading 285
10.7 Fracture Toughness 286
10.7.1 Fracture Stress Test 291
10.7.2 Hardness Indentation Method 292
10.7.3 Importance of Fracture Toughness Determination
10.8 Toughening in Ceramics 294
10.8.1 Crack Deflection Toughening 296
10.8.2 Transformation Toughening 297
10.8.3 Crack Bridging (or Wake) Toughening 299
10.8.4 Microcrack Toughening 302
11. Fatigue Behaviour
11.1 Introduction 304
11.2 Stress Cycles 306
11.3 Macrography of Fatigue Fracture
11.4 Fatigue Test (S-N Curve) 309
308
271-303
294
304-325
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13. xii Contents
12.
11.5 Fatigue Behaviour in Iron and Steel 311
11.6 Mechanisms of Fatigue 312
11.6.1 Orowan's Theory of Fatigue 312
11.6.2 Wood's Theory of Fatigue 315
11.6.3 Fatigue Crack Growth 317
11.7 Low Cycle Fatigue 322
11.8 Variables Affecting Fatigue 322
Creep Behaviour
12.1 Introduction 326
12.2 Creep Curve 327
12.3 Design Curves 331
12.4 Andrade's Analysis of Creep 333
12.5 Creep at Lower Temperature 334
12.6 Activation Energy for Steady-State Creep 335
12.7 Creep at High Temperature 336
12.8 Equicohesive Temperature 336
12.9 Deformation at Elevated Temperature 338
12.9.1 Deformation by Slip 338
12.9.2 Grain Boundary Deformation
12.10 Mechanisms of Creep Deformation
339
339
12.10.1 Dislocation Glide 339
12.10.2 Dislocation Creep 343
12.10.3 Diffusion Creep 345
12.10.4 Grain Boundary Sliding 346
12.11 Metallurgical Factors Affecting Creep Behaviour
12.11.1 Effect of Lattice Structure 347
12.11.2 Effect of Prestrain 348
347
12.11.3 Effect of Soluble Impurities and Alloying Elements
12.11.4 Effect of Second Phase Particles 349
12.11.5 Grain Size 350
12.12 Creep Resistant Materials 350
13. Non-Destructive Testing
13.1 Introduction 357
13.2 Visual Inspection 359
13.3 Liquid Penetrant Inspection (LPI) 360
13.3.1 Procedure 361
13.4 Magnetic Particle Inspection (MPI) 363
13.4.1 Basic Principle 363
13.4.2 Magnetization 364
13.4.3 Magnetization Techniques 366
13.4.4 Procedure for MPI 370
13.4.5 Applications of MPI 372
326-356
348
357-386
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14. Contents xiii
13.5 Eddy Current Inspection (ECI) 372
13.5.1 Basic Principle 373
13.5.2 Operating Variables 375
13.5.3 Applications 377
13.6 Ultrasonic Testing 377
13.6.1 Basic Principle 378
13.6.2 Ultrasonic Waves 378
13.6.3 Ultrasonic Transducers 380
13.6.4 Probes 381
13.6.5 Interaction of Sound Waves at the Interfaces 382
13.6.6 Methods of Ultrasonic Inspection 383
13.6.7 Advantages of Ultrasonic Inspection 385
13.7 Radiographic Inspection 385
Appendix A Hardness Testing 387
Appendix A1 Brine/I Hardness Test 388-391
Appendix A2 Vickers Hardness Test 392-394
Appendix A3 Rockwell Hardness Test 395-397
Appendix B Tensile Testing 398-405
Appendix C Impact Test 406-409
Appendix D Fatigue Test 410-412
Appendix E Sheet Metal Formability Test 413-414
Appendix F Bend Test 415-417
Appendix G Mechanical Properties of Some Representative
Polymer Materials 418-419
Appendix H Table of Hardness Conversion 420-421
Appendix I SI Units 422
Appendix J Conversion Factors 423
Appendix K Unit Conversion 424-426
Appendix L SI Prefixes 427
Appendix M Greek Alphabets 428
Appendix N Table for Conversion of Temperature 429-431
Glossary 433-466
Bibliography 467-468
Questions Bank 469-546
Index 547-562
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16. Preface
Materials have always been the major attraction for the human beings. Significance of materials
is self-evident by the familiar terms-Stone Age, Bronze Age and Iron Age. It is the effective
use of the materials that has been, and is still in use, the criterion for the better living and
economy of a country.
Any material prior to its end use is subjected to many processes and treatments. A large
number of materials are known to man and the numbers in the list are increasing gradually with
newer and newer arrivals. This makes the task of selection of the materials very difficult. Widely
differing physical, chemical and mechanical properties, typical processing characteristics such as
castability, machinability, weldability, etc., and the response to various manufacturing methods
further complicate the task of materials selection. The success of selected material is largely
determined by its conformity to the service conditions and the reliability.
Material testing and critical interpretation of the test data play a key role for selection,
designing and manufacturing of a material for end use with a minimum of desired reliability.
Based on this concept, this book has been presented to engineering students and to technical
personnel dealing with materials. The prime aim of the book is to present the subject matter in
most concise, coherent, logical and lucid manner. This book provides an insight into the
mechanical behaviour and testing of metals, polymers, ceramics and composites which are
widely employed for structural applications under varying load, temperature and environments.
The book is designed primarily as a text for the undergraduate and postgraduate students
of Metallurgical and Materials Engineering. Additionally, it will be useful for the undergraduate
and postgraduate students of Mechanical Engineering, Production Engineering, Industrial
Engineering, Automobile Engineering, Chemical Engineering, Polymer and Ceramic
Engineering, Civil Engineering and Structural Engineering. Much care has been taken to cater
the needs of students appearing in the examinations of various professional bodies such as
xv
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17. XVi Preface
The Institution of Engineers (India), Institute of Metals, etc. Besides, the practising engineers
and technical personnel dealing with the materials in general, and dealing with destructive and
non-destructive testing in particular, will also get benefit of the information provided in the text.
The book is organized in thirteen chapters. While Chapter 1, at first, introduces readers
to the fundamentals of materials starting from their basic building units, it gradually rises
through atomic bonding, crystal structure, different classes of engineering materials and their
salient features to some commonly used industrial materials and their engineering applications.
Chapter 2 and Chapter 3 describe role of imperfections on the behaviour of metals and alloys,
and various properties of engineering materials. Chapter 4 deals with dislocation theory in a
simplified, but analytical manner. Chapter 5 speaks about the plastic deformation of the
materials primarily in the light of dislocation theory. Various mechanisms for enhancing strength
of all classes of engineering materials have been discussed in Chapter 6. Chapter 7 has been
exclusively devoted to common aspects of fracture. Out of next six chapters, first five chapters
(Chapters 8-12) describe in detail about destructive tests, whereas Chapter 13 explains
commonly used non-destructive testing methods of materials. Whereas, both theoretical and
practical aspects of destructive/non-destructive testing are covered in these Chapters, the
practical manual is also given at the end in the form of Appendices A to N. A large number of
questions, with solutions, have been incorporated as Question Bank at the end of the book. It
covers about 200 objective type questions appeared in GATE examinations.
For quick reference, glossary of terms has been incorporated in the book.
Every effort is made to avoid repetition in the text. However, readers may find some
repetition which was a compulsion to maintain the coherency of the text.
Authors not only will welcome any constructive suggestion from the readers but also will
acknowledge them in future editions.
A.K. Bhargava and C.P. Sharma
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18. Acknowledgements
Gratitude is the hardest of all emotions to express. There is no word capable of conveying
everything one faces until we reach the world where thoughts can be adequately expressed in
words.
It gives us immense pleasure to put on record our profound sense of indebt to Hon'ble
Prof. T.V. Rajan, Shri Subodh Bhushan Gupta jee, Prof. Surendra Kumar (Department of
Chemical Engineering, IIT Roorkee) and Prof. A.N. Tiwari (Department of Metallurgical
Engineering and Materials Science, IIT Bombay) for their benevolent guidance, constant
encouragement and support in odds. Special thanks to Mrs. & Dr. Ravindra Padalkar,
Mrs. Manju, and Nakul-Taru (daughter's parents-in-law, wife, son-in-law and daughter -
A.K. Bhargava) and to Mrs. Usha, Anand-Avantika, Ayam-Sunita and Avni-Aarini (wife, son-
in-law & daughter, son & daughter-in-law and granddaughters - C.P. Sharma) who not only
maintained pleasant atmosphere during the period of successful completion of this project but
also provided all the cooperation and moral support. Special thanks are due to Mr. Nakul for
visualizing the design of the cover page of the book.
Authors are highly thankful to teachers for their blessings, colleagues and friends for their
constructive suggestions and students for their frequent questions within and outside the
classroom which nucleated the idea of this book.
Thanks are due to highly energetic and committed team of PHI Leaming, New Delhi, for
chasing us and bringing the book in short period.
Last but not the least, we acknowledge each and everyone who directly or indirectly
motivated, helped, and inspired us from time to time.
A.K. Bhargava and C.P. Sharma
xvii
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20. Area of cross-sectional plane (4.5. l)
Constant (6.5)
Surface area of indentation (9.4)
Lateral area of indentation (9.5)
Projected area of indentation (9.8)
Rockwell scale (9.6)
Area of cross-section of composite (6.7.l)
Area of cross-section at fracture (8.3.5)
Area of cross-section of fibre (6.7. l)
Nomenclature
Instantaneous or true area of cross-section (8.4)
Area of cross-section of matrix (6.7.l)
Original area of cross-section (8.2, 8.3.2)
Cross-sectional area of the specimen at notch (10.4)
Area of slip plane (5.3.4)
Axial dimension (1.4)
Shear displacement
Interplanar spacing (4.2)
Lattice parameter (4.5, 4.8, 5.3.3, 6.3, 6.4. l/frequently used)
Side of the projected square indentation (9.5)
Spacing between slip planes (4.9)
Surface crack length or half the interior crack length (7.5, 10.5, 10.7, 10.8, 11.6.2)
Allowable flaw size (10.7.2)
Critical size surface crack (10.5, 10.8)
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21. XX Nomenclature
a Shear angle (5.2)
a Constant (6.2)
a Local order parameter (6.3)
a Interaxial angle (1 .4)
B Constant (6.9)
B Critical thickness (10.7)
B Rockwell scale (9.6)
b Interatomic spacing (4.2)
b Axial dimension (1.4)
b Magnitude of Burgers vector (4.6.1)
b Burgers vector (frequently used)
b1 Burgers vector (4.7, 4.8.1, 4.11)
b2 Burgers vector (4.7, 4.8.1, 4.11)
b3 Burgers vector (4.8.1)
/3 Compressibility (5.2)
/3 Constant (12.4)
/3 Interaxial angle (1 .4)
/3 Load transfer function (6.7.1)
C Constant (11.6.2)
C Rockwell scale (9.6)
Cv Fracture energy (10.3)
c Axial dimension (1.4)
c Height of the unit cell (5.3.3)
c Concentration of solute atoms (6.3)
D Average grain diameter (6.2)
D True diameter (8.4)
D Diameter of ball indenter (9.4)
D Rockwell scale (9.6)
D Self diffusion coefficient (12.11.1)
Dgb Grain boundary diffusion coefficient (12.10.3)
Dv Volume diffusion coefficient (12.10.3)
d Diameter of fibre (6.7.1)
d Grain diameter (6.2, 12.10.3)
d Diameter of indentation (9.4)
d* Critical grain diameter (10.5)
dE Change in energy (4.6.1)
dE Increase in energy (4.6.3)
dl Small distance or displacement (4.6.2)
ds Equilibrium spacing between partial dislocations (4.8.2)
ds Small segment of dislocation line (4.6.2)
d0 Angle subtended at the centre of curvature (4.6.3)
dW Work done (4.6.2)
~ Deformation (11.6.1)
M Increase in energy (4.7)
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22. Ml Activation energy (12.6)
11Uc Interaction energy (6.3)
E Elastic modulus (5.2, 6.7.1, 7.5, 8.3, 10.5, 10.8, 12.10.2)
Elastic strain energy of dislocation (4.6.1)
Energy associated with dislocation (4.8)
Nomenclature xxi
E
E
E' Elastic modulus of the discontinuous and randomly aligned fibre composite
(6.7.1)
E0 Core energy of dislocation (4.6.1)
e Average tensile strain, nominal strain (6.7.1, 8.2, 8.3.1)
e Shear strain (5.3.4)
e Tensile strain (6.7.1)
e Creep strain (12.4)
t:1 The fatigue ductility coefficient (11.7)
Strain rate (8.6)
Creep rate (12.4, 12.5, 12.8)
tdiff Diffusion creep rate (12.10.3)
dis! Dislocation creep rate (12.10.4)
SFE SFE dependent creep rate (12.11.1)
tgbs Grain boundary sliding (GBS) creep (12.10.4)
81 Dislocation glide creep rate (12.10.1)
Steady-state creep rate (12.10.3)
emisfit Misfit parameter (6.3, 6.4.1)
e0 Instantaneous strain (12.4)
t:1 Strain at fracture (8.7.1)
ec Measure of modulus difference between solute and solvent (6.3)
11t:P Plastic strain range (11.7)
er True strain (8.4)
eu Uniform strain (8.7.1)
~· Strain corresponding to yield stress (8.3.4)
F Force (4.6.2, 5.3.4)
F(R) Force of repulsion (4.7)
F(a) Force of attraction (4.7)
f Frequency of alternating current (13.5.1, 13.5.2)
f Volume fraction of precipitate phase (6.4.1)
f Limit frequency (13.5.2)
G Shear modulus (4.1, 4.2, 4.6.1, 4.9, 4.10, 5.2, 6.3, 6.4.1, 10.5, 12.10.1,12.10.2)
r Interaxial angle (1 .4)
r Shear strain (4.2, 5.2)
r Surface energy (10.8)
rm Plastic work done around a crack (10.5)
'l'P Energy required for plastic deformation per unit area (7.5)
rs Stacking fault energy (4.8.2, 12.11.1)
rs Surface energy per unit area (7.5, 10.5)
rs Precipitate-matrix surface energy (6.4.1)
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23. xxii
YAPB
rez
H
HAc
HEc
h
h
h'
K
K
K
K
K
K
Kie
K'
KR*
Kth
K,.
k
k
kt
Xo
X;
Nomenclature
Antiphase boundary energy (6.4.1)
Elastic shear strain (4.6.1)
Vickers hardness (10.7.2)
Alternating magnetic field/primary magnetic field (13.5.1)
Secondary magnetic field (13.5.1)
Constant (9.6)
Height of the hammer from its lowest point before its release (10.4)
Height of the hammer from its lowest point after its release (10.4)
Bulk modulus (5.2)
Constant (7.5, 12.4)
Constant called strength coefficient (8.4)
Load factor (9.4)
Major energy shell (1.1)
Stress intensity parameter (10.7, 11.6.2)
Critical stress intensity or fracture toughness (10.7)
Constant called fibre efficiency parameter (6.7.1)
Critical stress intensity factor (10.8)
Threshold stress intensity value (11.6.2)
Measure of extent to which dislocation pile up at barriers (6.2, 10.5)
Constant (6.2)
Boltzman's constant (12.10.3)
Stress concentration at the crack tip (10.7)
Angle that slip plane makes with respect to stress axis before stressing (5.2.4)
Angle of reorientation of slip plane with stress axis at any instant after plastic
deformation (5.3.4)
Inductance (13.5.1)
Length (6.2)
Length of diagonal of projected square shaped indentation (9.5)
Length of longer diagonal of Knoop indentation (9.8)
Length of the specimen at time t (12.4)
Major energy shell (1.1)
Gauge length before plastic deformation (5.3.4)
Length of the specimen just after the load is applied (12.4)
Original gauge length of specimen (8.2, 8.7.1)
Gauge length at any instant after plastic deformation (5.2.4)
Length at fracture (8.7.1)
Azimuthal quantum number (1.1)
Length of cylindrical crystal (4.6.1)
Length of dislocation line segment (4.6.3, 4.10)
Distance between particles (4.6.3, 6.4.1)
Spacing between planes (5.2)
Critical fibre length (6.7.1)
Angle between applied force and slip plane normal (5.3.4)
Constant of proportionality (9.4)
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24. Nomenclature xxiii
A-0 Angle between the axis of tension and slip direction before plastic deformation
(5.3.4)
A; Angle between the axis of tension and slip direction after plastic deformation
(5.3.4)
M Atomic mass number (1.1), Major energy shell (1.1)
m1 Magnetic quantum number (1.1)
ms Spin quantum number (1.1)
M Bending moment (6.7.1)
µ Poisson's ratio (5.2)
µ, Relative permeability of the material (13.5.2)
N Number of grains per square inch (6.2)
N Number of dislocations (6.2)
N Number of stress cycles of failure (11.4)
n ASTM grain size number (6.2)
n Number of vacancies per cubic centimeter (2.2.1)
n Principal quantum number (1.1)
n Strain hardening exponent (8.4)
n Meyer's index (9.4)
v Poisson's ratio (4.6.1, 6.2, 10.7)
m Angular frequency (13.5.1)
P Load or force (8.2, 8.3.4, 8.5, 9.4, 9.5, 9.8)
P Volume fraction porosity (6.9)
Pc Load carried by composite (6.7.1)
P1 Load carried by fibres (6.7.1)
Pm Load carried by matrix (6.7.1)
Pu Maximum load (8.3.3)
Py Load at yield point (8.3.2)
PAB Fraction of A atoms that are nearest to B atoms (6.3)
<p Angle between tensile axis and slip direction (5.3.4)
<p Half the angle subtended by the indentation edge at the centre of the ball (9.4)
Q Energy required to produce a vacancy (2.2.1)
Q Activation energy (12.10.1, 12.10.2)
<p Indenter geometry dependent constant (10.7.2)
q Conventional reduction in area (8.4)
q' True reduction in area (8.4)
R Gas constant (2.2.1)
R Atomic radius (6.3)
R External radius of the cylindrical crystal (4.6.1)
R Ohmic resistance (13.5.1)
R Universal gas constant (12.6)
R Radius of curvature (4.6.3, 4.10, 6.4.1, 6.7.1)
R Range or stress ratio (11.2)
R Reflection coefficient (13.5.1)
r Distance between dislocations (4.7)
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25. xxiv
r
r
r
r
r
r
ro
r1
p
p
p
p
(J
(J
Ga
(Japp
(Je
(Jm
(Jm
<lmax
(Jmin
(JP
(Jc
(Jc
(Jct
<lJ
<lJ
<lJ
<lJ
(Jft
(Jh
<J;
a'm
<lmt
(Jo
(Jo
<Jr
<lx
<J,.
<J,.
Gz
s
s
s
Nomenclature
Distance between solute atom and dislocation (6.3)
Distance from tip of the crack (10.7)
Radius at the tip of the crack (7.5)
Radius of annular ring (4.6.1)
Radius of atom (2.1)
Radius of precipitate particle (6.4.1)
Dislocation core radius (4.6.1, 4.7)
Distance away from dislocation core (4.7)
Density of dislocations (6.2, 8.6)
Density of material (13.6.4)
Radius of curvature at crack tip (10.7)
Resistivity of material (13.5.2)
Normal stress or average stress or nominal stress (4.5.1, 6.7.1, 8.2, 8.3.1)
Applied tensile stress (7.5, 12.10.2)
Alternating stress (11.2)
Applied stress (10.7)
Effective stress (12.10.1)
Matrix stress (6.7.1)
Mean stress (11.2)
Maximum tensile stress (7.5, 11.2)
Minimum compressive stress (11.2)
Strength of the specimen with porosity (6.9)
Critical normal stress (4.5.1, 5.3.4)
Stress in composite (6.7.1)
Tensile strength of composite (6.7.1)
Tensile stress in fibre (6.7.1)
Average stress in fibre (6.7.1)
Fracture stress (10.2, 10.5, 10.7, 10.8)
Critical fracture stress (10.5)
Tensile strength of fibres (6.7.1)
Hydrostatic pressure (5.2)
Lattice frictional stress opposing motion of dislocation (6.2, 10.5)
Matrix stress at strain corresponding to fibre tensile strength (6.7.1)
Matrix tensile strength (6.7.1)
Strength of the specimen without porosity (6.9)
Yield strength/flow stress (6.2, 11.6.1)
True stress (8.4)
Stress component in x-direction (10.7)
Yield stress/yield strength (8.3.2, 10.2, 10.5, 10.7.2)
Stress component in y-direction (10.7)
Stress component in z-direction (10.7)
Cyclic stress (11.4)
Slant height of pyramidal indentation (9.5)
Standard penetration depth (13.5.2)
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26. T
T
Tm
T
t
t
t
t
'l'
'l'
'X'a
'l'b
'l'b
'l';
'X'm
'l'my
'l'cRSS
'l'Rss
'l'p_N
'X'ez
'l'xy
0
0
0"
0'
u
UE
U1
Ur
Us
Ur
/!,.Uc
u
V
Vi, V2, V3
Ve
VJ
Vfcrit
Vm
VJ min
V
w
w
Absolute temperature (2.2.1, 12.6, 12.10.2, 12.10.3)
Line tension (4.6.3)
Nomenclature XXV
Melting temperature in Kelvin (5.3.7, 6.3, 10.2, 12.8, 12.11.2)
Transmission coefficient (13.5.1)
Creep time (12.4, 12.5)
Unit tangent vector (4.4, 4.5.3)
Thickness (6.7.1)
Depth of indentation (9.4, 9.6)
Shear stress (4.2, 4.6.2, 5.2, 6.2, 6.4.1, 11.6.1)
Resolved shear stress (8.6)
Stress to activate the dislocation source (6.2)
Back stress (6.2)
Shear stress corresponding to unit velocity of dislocations (8.6)
Internal stress (6.2)
Maximum shear stress (4.2)
Matrix shear yield strength (6.7.1)
Critical resolved shear stress (5.3.4)
Resolved shear stress (4.5.1, 5.3.4)
Peierls Nabarro shear stress (4.9)
Elastic shear stress (4.6.1)
Shear stress in x-y plane (10.7)
Angle (4.6.3, 10.7)
Stable CuA12 phase (6.4.1)
Coherent phase (6.4.1)
Semicoherent phase (6.4.1)
Elastic strain energy (8.3.4)
Elastic strain energy for unit volume (7.5)
Impact strength (10.4)
Modulus of resilience (8.3.4)
Surface energy associated with a crack of unit width (7.5)
Toughness (8.3.6)
Interaction energy (6.3)
Strain energy per unit volume (8.3.4)
Volume (4.6.1, 5.2)
Vectors (4.12)
Velocity of compressive waves in medium (13.6.4)
Volume fraction of fibres (6.7.1)
Critical volume fraction of fibres (6.7.1)
Volume fraction of matrix (6.7.1)
Minimum volume fraction of fibres (6.7.1)
Velocity of dislocations (8.6)
Weight (10.4)
Dislocation width (4.9)
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27. xxvi Nomenclature
Fraction of A atoms (6.3)
Inductive reactance (13.5.1)
Displacement of atoms (4.2)
Mole fraction of solute (6.3)
Dimensionless constant (10.7)
Atomic number (1.1)
Impedance (13.5.1)
Acoustic impedance (13.6.4)
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28. Nature of Materials
1.1 INTRODUCTION
We find numerous things around us, and use many of them frequently in our day-to-day
activities. The things that can be seen are usually known as materials. However, in true sense,
a material is something that occupies space and has mass. A material, in general, exists in one
of the three states (more frequently called physical states), namely, gaseous, liquid or solid state
depending on its temperature. At a given temperature, physical state having more negative free
energy is thermodynamically stable. For example, only water is stable at room temperature but
ice or water vapour (steam) is not stable.
Every material, irrespective of its physical state, is made of some simple substances which
cannot be further broken down into more simple substances. These simple and pure substances
that cannot be disintegrated by chemical means are referred to as elements. Thus elements are
the basic units of all the materials.
There are about l09 elements known to man out of which majority of elements (say 92)
are gifted to human race by the nature, i.e. these are naturally occurring elements. Remaining
elements have been developed by the man. Aluminium, copper, iron, oxygen, hydrogen and
iodine are some examples of naturally occurring elements. Elements differ from each other and
hence show different characteristic features. An element, though cannot be disintegrated by
chemical means, is not unbreakable. In fact, every element is composed of atoms. While the
characteristic features of atoms are similar for a given element, these are different for different
elements, i.e. all atoms of an element are alike and differ significantly from the atoms of other
elements. The different characteristic features of elements are essentially due to the
characteristic features of these atoms and arrangement of these atoms within the elements.
Initially, an atom was considered as an indivisible smallest unit of the element. Nowadays, it
1
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29. 2 Mechanical Behaviour and Testing of Materials
is well known that an atom is composed of many sub-atomic particles. However, it is true that
an atom is the smallest stable unit of an element which can exhibit all the properties of that
element and is capable of taking part in a chemical reaction. Two or more atoms of an element
or of different elements may combine with each other to form a molecule. In such cases,
molecule is the smallest stable unit exhibiting all the properties of the material under
consideration.
An atom has a well-defined internal structure known as atomic structure. As stated
earlier, an atom is composed of many sub-atomic particles. However, for the sake of
understanding the atomic structure in a simple manner, it is convenient to focus only on three
sub-atomic particles, namely, the proton, neutron and electron. These sub-atomic particles are
usually referred to as elementary particles. The characteristics of these sub-atomic particles are
summarised in Table 1.1.
Particle
Proton
Neutron
Electron
TABLE 1.1 Characteristics of sub-atomic particles
Characteristics
These are positively charged particles concentrated in the nucleus. The mass of a
proton is equal to 1.673 x 10-27 kg. The magnitude of charge on a proton is
1.602 x 10-19 coulomb. Relative mass and relative charge are 1 and +1,
respectively.
These are neutral particles, i.e. they do not have any charge. Like protons,
these are also concentrated in the nucleus. The mass of a proton is equal to
1.675 x 10-27 kg. Relative mass and relative charge are 1 and 0, respectively.
These are negatively charged particles moving around the nucleus in various
orbits. The mass of an electron is equal to 0.911 x 10-30 kg. The magnitude of
charge on an electron is 1.602 x 10-19 coulomb. Relative mass and relative
charge are 1/1836 and -1, respectively.
Electrons are negatively charged particles which are much lighter than the both protons
and neutrons. Protons are positively charged particles whereas neutrons are neutral particles.
The mass of a proton is slightly less than the mass of a neutron. An atom, to a first
approximation, can be considered spherical in shape with definite size (diameter). The average
diameter of an atom is of the order of 10-10 metre.
An atom consists of a highly compact central part which is popularly known as nucleus.
This nucleus is comprised of the protons and neutrons. Electrons are revolving around it in
circular or elliptical orbits as shown in Figure 1.1. This distribution of electrons depends on the
nature of the element. The maximum number of electrons that can be present in any shell
(n = 1, 2, 3, ....) is 2n2• For example, first shell, also known as the innermost shell, can have
only 2 [2 x 121electrons and second shell can have 8 [2 x 221electrons only. The last shell,
popularly known as outermost shell, cannot have more than 8 electrons and the shell immediately
preceding the outermost shell cannot have more than 18 electrons. The planetary model, the most
popular model explaining the nature of structure of an atom, is shown in Figure l.2(a). However,
the shells are not as rigid as Figure l.2(a) implies. It is because that no empirical mathematical
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30. Nature of Materials 3
Nucleus (positively charged)
Electrons (negatively charged)
FIGURE 1.1 Structure of an atom showing a central nucleus and electrons
revolving around it.
(a)
containing
nucleons
L-shell
,__+---+--- M-shel
t+--t-- - N-shell
t+-- -0-shell
(b)
FIGURE 1.2 (a) Planetary model of an atom, (b) Electron shell structure around the nucleus.
relationship can be developed to locate the electron in terms of distance parameter from the
nucleus. It is important to emphasize here that electrons have dual nature, i.e. electrons exhibit
properties of a particle as well as of an energy wave. Since electrons are governed by the laws
of quantum mechanics, only certain but definite energy values will be associated with these
electrons and hence electrons will revolve around the nucleus only in some definite orbits with
particular energy levels as shown in Figure l.2(b). According to Heisenberg's uncertainty
principle, it is not possible to determine the position and momentum of an electron
simultaneously. The exact position of an electron can be determined precisely with the help of
four quantum numbers, namely, principal quantum number (n), orbital quantum or azimuthal
quantum number (l), magnetic quantum number (m1) and electron spin quantum number (m5).
The principal quantum number (n) is related to the main energy levels of the electron.
It can be visualised as a shell in the space. In general, each quantum shell is assigned an
alphabet rather than a number. The shells corresponding to n = l, 2, 3, 4, 5, 6 and 7 are
designated by alphabets K, L, M, N, 0, P and Q, respectively. Though principal quantum
number may have any positive integral value from l to infinity, it usually ranges from l to 7.
An electron with higher principal quantum number possesses higher energy and is farther from
the nucleus.
The orbital angular quantum number (l) is related to the angular momentum of the
electrons. It may have any value from 0 to (n - 1). This quantum number speaks about the sub-
energy levels (sub-shells) within the main energy levels (shells). With the help of this number,
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31. 4 Mechanical Behaviour and Testing of Materials
the position of electron can be known with high probability in sub-shells. The sub-energy levels
are denoted by alphabets s, p, d and f The sub-energy level with the lowest energy state is
referred to as s, the next higher is p, and then d and f
The magnetic quantum number (m1) is related to the magnetic moment of the electron
and has values from -l to +l including zero. There are, in general, (21 + 1) values of m1 for a
given l value. The electron spin quantum number (ms) is related to the rotation of the electron
about its own axis. Since there may be two allowed spin directions for an electron spinning
on its own axis (clockwise and anticlockwise), ms may have values +1/2 and -1/2.
Table 1.2, showing distribution of electrons for various elements, helps in understanding
these statements.
Since the atomic nucleus consists of protons and neutrons, it is positively charged. The
average diameter of a nucleus is of the order of 10-14 metre, which is about 1/10,000 of the
diameter of the atom. Hence, while almost the entire mass of an atom is concentrated at the
nucleus, almost all the volume of atom is occupied by the electron cloud moving around the
positively charged nucleus. The electrons, which are moving around the nucleus in various
orbits, are of much significance as these determine the size of the atom, govern the electrical
conductivity and chemical properties, decide the nature of interatomic force of attraction and
affect optical properties. The electrons in the outermost orbit, which are relatively loosely
bonded with the nucleus, are known as valence electrons. Since the number of electrons is
equal to the number of protons in an atom and have equal and opposite electrical charge, an
atom is electrically neutral. Under certain circumstances, there may be an imbalance and the
number of protons may not be equal to the numbers of electrons. In such cases, atom will not
be electrically neutral. A charged atom, due to gain or loss of one or more electrons, is usually
termed as ion. Loss of an electron means that there will be a proton whose positive charge
cannot be neutralised and hence the atom becomes positively charged, i.e. electropositive. On
similar basis, gain of an electron makes the atom electronegative. The terms anion and cation
are used to represent negatively and positively charged ion, respectively.
Atom of an element has definite number of protons in its nucleus and this number differs
from element to element. The number of protons in the nucleus is commonly termed as atomic
number and is frequently denoted by the alphabet Z. For an electrically neutral atom, the
atomic number represents the number of electrons in the atom. As stated earlier, proton and
neutrons contribute to almost all the mass of an atom. The term atomic mass number, generally
expressed by the alphabet M, indicates the total number of protons and neutrons in the nucleus.
It is only the simplest atom of hydrogen which has same value, i.e. 1 for both atomic number and
atomic mass number. While all atoms of an element have the same number of protons, these may
contain different number of neutrons. Atoms of the same element having different atomic mass
numbers are called isotopes. All isotopes of one element have similar chemical properties.
Presence of isotopes is quite common feature for majority of elements. For example, there are two
naturally occurring isotopes of hydrogen (with M as 1 and 2), three isotopes of carbon (with M
as 12, 13 and 14) and three isotopes of oxygen (with M as 16, 17 and 18). It is quite probable
in some cases that, for different elements, the sum of the number of protons and neutrons may
be same, i.e. different elements may have same atomic mass number. In such cases, the number
of protons and neutrons differ in the nuclei. These nuclei are called isobars.
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34. Nature of Materials 7
1.2 INTERATOMIC AND INTERMOLECULAR BONDING
All materials are composed of very large number (several millions) of atoms. These atoms are
joined with each other in one or another way. The word interatomic bonding is most
frequently used to express this joining of atoms. The strength of bonding and the nature of
bonding between atoms differ from material to material and decide the characteristic features
of the material under consideration. In general, the interatomic bonding depends on the
distribution of electrons in the atom and the number of valence electrons. An atom with
completely filled outermost orbit is most stable and hence every atom having incompletely
filled outermost orbit has a tendency to fill the outermost orbit by combining with other atom(s).
Similar to atoms, molecules are also bonded with each other and the bonds between molecules
are called intermolecular bonding. Various bondings present in the materials can be divided
into two major classes, namely, primary and secondary bondings. Primary bondings, the
bondings between atoms, are much stronger than the secondary bondings, which are, in
general, the bondings between the molecules and between the atoms in specific cases. For
example, atoms of inert gases are bonded by secondary bonding in condensed state. Here it is
important to mention that the atoms, from which a molecule is formed, are bonded by strong
primary bonding. There are three types of primary bonds, namely, ionic bonding, covalent
bonding and metallic bonding. These bonds result from electronic orbital interactions. The
manner by which atoms fill their outer 's' and 'p' orbitals differs from one bonding to another.
The weak secondary bonds result from molecular polarization and permanent dipole moment.
van der Waals bond [Figure l.3(a)] and hydrogen bond [Figure l.3(b)] are two common
,,,.,....---......,, ,,.,....---.........,
1+ - , ,+ -,
Dispersio~n
,' ~
,' ~ Dispersion
______.. ~ ------+- I I ------+- I ------+-
/ I /
Dipole / Dipole /
.........___......," ',...___......,"
Dispersion
c::::::=:::,
(a)
~ + Dipole
+ ~ j
- 0 ---~--
H+
-- '
,
+H,'+ -o H+
I H /
I ,
0 _,,
+
+ <l
+ - 0 H
- +<J +
+
(b)
FIGURE 1.3 (a) van der Waals bond, (b) Hydrogen bond.
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35. 8 Mechanical Behaviour and Testing of Materials
examples of the secondary bonds. Materials are bonded by primary bonding, secondary bonding
or by the combination of both. It is not essential that a material will be bonded by only one type
of bonding, i.e. they may have mixed bonding. In fact, most of the materials have mixed
bonding. For examples, pure elements like iron, cobalt, nickel, uranium, iodine and plutonium
as well as intermetallic compounds, and ceramic compounds e.g. silica (SiO2), silicon carbide
(SiC) and titanium nitride (TiN) have mixed bonding.
Ionic bonding results due to transfer of electron(s) from one atom to another (Figure 1.4).
The transfer of electrons takes place in a manner that one atom becomes positively charged and
3s1
0
Sodium atom Chlorine atom
FIGURE 1.4 Ionic bonding showing the formation of a sodium chloride ion pair from sodium and
chlorine atoms. For convenience electrons are shown only in outermost shell of
chlorine atom and ion.
the other negatively charged. The atom which donates electron(s) becomes positively charged
while the atom that accepts electron(s) becomes negatively charged. The tendency for the
formation of ionic bonding is usually found between the atoms having one or two valence
electrons and the atoms having incomplete outermost orbits by one or two electrons. Such a
bond gives a stable structure. Covalent bonding results as a consequence of sharing of
electrons from the outermost orbit by the atoms (Figure 1.5). The sharing of electrons takes
place in such a manner that outermost orbit of each atom gets filled up. Each sharing of
electrons between two atoms results into the formation of one covalent bond. Covalent bonding
occurs only when surrounding atoms are located in a manner that they have some well-defined
directional relationship with each other. This type of bonding exists between the atoms of same
element as well as between the atoms of dissimilar elements. Valence electrons, in certain
elements and solid materials, are so loosely bonded with the nucleus of the atom that these are
capable of moving freely under the influence of an applied voltage. Such electrons are popularly
known as free electrons. Metals and alloys owe their properties essentially due to the presence
of free electrons. Free electrons are shared by all the atoms of the element or the material. These
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36. Nature of Materials 9
0
Silicon atom
FIGURE 1.5 Covalent bonding in silicon crystal.
electrons form electron gas or electron cloud around the atoms which have now become
positively charged ions. The positively charged ions are bonded with one another by mutual
attraction to the nearer electrons in the electron cloud. Such an attraction causes the formation
of metallic bonding (Figure 1.6).
FIGURE 1.6 Two-dimensional schematic diagram illustrating metallic bonding between atoms.
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37. 10 Mechanical Behaviour and Testing of Materials
1.3 CLASSIFICATION AND COMBINATION OF ELEMENTS
It was the year 1869, when Dimitri Mendeleev, a Russian scientist, got success in arranging
the elements in a tabular form and developed the periodic table of the elements. In fact,
while arranging the elements in order of increasing atomic weight, he discovered that
elements showing similar properties are recurring in a periodic manner. A modern periodic
table is shown in Figure 1.7. In this periodic table, elements have been arranged in order of
ascending atomic number. The horizontal rows and the vertical columns, in general, are
called periods and groups, respectively. All the elements of a group exhibit similar
characteristics.
Elements, depending on some common characteristics, are classified into two major
categories, namely, metals and nonmetals. For example, many elements have good thermal
and electrical conductivity, are malleable, ductile and opaque, possess typical lustre usually
referred to as metallic lustre, and are crystalline solids. Such elements are known as metals
or metallic elements and the characteristics stated above, i.e. good thermal conductivity,
electrical conductivity, malleability, ductility coupled with opacity and typical lustre are
called metallic characteristics or properties. All metallic elements are solid at room
temperature except mercury. Nonmetals or nonmetallic elements may exist in solid,
liquid or gaseous state. Solid nonmetallic elements, in contrast to metallic elements, have
poor thermal and electrical conductivity and are brittle in nature. There are certain typical
elements which possess some characteristics of metals and some characteristics of
nonmetals. Hence, these are neither metals nor nonmetals. Such elements constitute a third
category of elements referred to as metalloids. Carbon, boron, silicon, germanium, arsenic,
antimony and tellurium belong to the family of metalloids. Atoms of all metals are bonded
by metallic bonding, whereas the atoms of nonmetals are bonded by bonding other than the
metallic bonding.
A chemical compound is formed when atoms of combining elements are held together
due to the association of electrons of these atoms. A compound, is a homogeneous substance,
has definite composition, fixed properties and a definite representing chemical formula.
Properties of a compound usually differ significantly from properties of elements from which
it is made of. It cannot be separated into constituent elements by simple physical and
mechanical means. Separation into constituent elements is only possible by chemical or electro-
chemical means. Compounds are generally formed between the elements which are far apart
in the periodic table.
A mixture is composed of two or more elements and/or compounds which are in intimate
contact with each other. There is no bonding between these elements or compounds on atomic
level. Bonding is of simple physical or mechanical nature. A mixture, unlike a compound, does
not have definite composition, properties and chemical formula. A mixture can be separated into
its constituent elements by simple physical and mechanical means.
In many cases, combination of two or more elements gives rise to a material which
exhibits metallic characteristics. Such a material is known as alloy. The base element, i.e.
element in major quantity is a metal in all alloys. Nimonics, steels, brasses, bronzes and monels
are some examples of the alloys.
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38. IA
I
H
1.00797 IIA
3 4
Li Be
6.393 9.0 12
II 12
Na Mg VIIIB
22.99 24.3 1 111B IVB VB VIB VIIB ,.-A-..,_ 1B 11B
19 20 21 22 23 24 25 26 27 28 29 30
K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn
39. 102 40.08 44.96 47.90 50.94 52.00 54.94 55.85 58.93 58.71 63.54 65.37
37 38 39 40 41 42 43 44 45 46 47 48
Rb Sr y Zr Nb Mo Tc Ru Rh Pd Ag Cd
85.47 87.62 88.905 91.22 92.9 1 95.94 98 IOI.I 102.90 106.4 107.87 112.4
55 56 72 73 74 75 76 77 78 79 80
Cs Ba
~
Hf Ta w Re Os Ir Pt Au Hg
132.905 137.33 178.49 180.95 183.85 186.2 190.2 192.2 195.09 196.97 200.59
87 88
Fr Ra Lanthanide series
223 226.025 I Actinide series
57 58 59 60 61 62 63 63 65
La Ce Pr Nd Pm Sm Eu Gd Tb
I 138.9 1 140. 12 140.9 1 144.24 147 150.35 152 157.25 158.92
89 90 91 92 93 94 95 96 97
Ac Th Pa u Np Pu Am Cm Bk
227 232.04 23 1 238.03 237 244 243 247 247
FIGURE 1.7 Periodic table.
IIIA IVA VA
5 6 7
B C N
10.8 1 12.0 1] 14.007
13 14 15
Al Si p
26.98 28.09 30.974
31 32 33
Ga Ge As
69.72 72.59 74.92
49 50 51
In Sn Sb
114.82 11 8.69 12 1.75
81 82 83
Tl Pb Bi
204.37 207.19 208.98
66 67 68
Dy Ho Er
162.50 164.93 167.26
98 99 100
Cf Es Fm
25 1 254 257
VIA VIIA
8 9
0 F
15.9994 19.00
16 17
s Cl
32.064 35.453
34 35
Se Br
78.96 79.909
52 53
Te I
127.60 126.90
84 85
Po At
2 10 210
69 70
Tm Yb
168.93 173.04
IOI 102
Md No
258 255
0
-
2
He
4.003
10
Ne
20. 183
18
Ar
39.948
36
Kr
83.80
54
Xe
131.30
86
Rn
222
7 1
Lu
174.97
103
Lw
257
~
C:
<il
C
.....
~
Cb°
55·
iii'
....
....
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39. 12 Mechanical Behaviour and Testing of Materials
1.4 ATOMIC ARRANGEMENT
Atom is the basic unit of an element. In addition to characteristics of the atom/atoms, it is the
arrangement of atoms within the material that controls the properties of the material under
consideration. In a material, atoms may or may not arrange themselves in a well-defined order.
The materials having well-defined arrangement of atoms are referred to as crystalline
materials. All metals and metallic materials are crystalline materials. Certain nonmetallic
materials, e.g. many ceramics also exhibit well-defined arrangement of atoms. Gases are the
example of the materials that do not show any specific arrangement of atoms. A large number
of materials show short-range ordering, i.e. specific arrangement of atoms is restricted to few
atoms (atom's nearest neighbours) only. Such materials are known as noncrystalline or
amorphous materials.
In a crystalline material, atoms are arranged in a well-defined manner and every atom has
identical surroundings. The repetitive three-dimensional arrangement of atoms is known as
crystal structure while the three-dimensional structure comprising of imaginary straight lines
connecting the centres of the atoms is called space lattice or crystal structure lattice. A space
lattice thus obtained contains a large number of small, but equally sized, segments. The smallest
unit of the lattice which on repeating in all the three
directions gives rise to crystal structure lattice is
called unit cell. The unit cell retains the overall
characteristics of the lattice. The size and shape of
unit cell may vary from material to material. A unit
cell can be described completely by six parameters,
namely, three axial dimensions (a, b and c) and three
inter-axial angles (a, f3 and )?. These parameters are
better known as lattice parameters. By convention,
a is the angle between b and c axes, f3 is the angle
between c and a axes and y is the angle between a
and b axes (Figure 1.8).
C
a
FIGURE 1.8 Unit cell showing lattice
constants.
Based on symmetry considerations, there are only seven crystal structure systems.
Table 1.3 provides characteristics of various crystal structure systems.
TABLE 1.3 Characteristics of crystal structure systems
Crystal structure Axes Angles between axes
Cubic a= b = c a= /3 = r= 90°
Tetragonal a = b * c a= /3 = r= 90°
Hexagonal a = b * c a = /3 = 90°, r = 120°
Orthorhombic aCFbCFc a= /3 = r= 90°
Rhombohedral a= b = c a= /3 = Y* 90°
Monoclinic aCFbCFc a = r = 90°, /3 * 90°
Triclinic aCFbCFc a* /3 * Y* 90°
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40. Nature of Materials 13
For these seven crystal structure systems only fourteen (14) space lattices, more popularly
known as Bravais lattices are possible (Figure 1.9). Materials may have more than one crystal
structure. The term allotropy is used for this characteristic property shown by pure elements.
Polymorphism is a more general term for such a characteristic behaviour. Table 1.4 lists crystal
structures of various allotropic forms of some metals.
Simple
cubic
rnJ
Simple
tetragonal
Face-centred Body-centred
cubic cubic
rw m
Body-centred
tetragonal
Hexagonal
Iw rw rm a:B
Simple
orthorhombic
Rhombohedral
Body-centred
orthorhombic
Simple
monoclinic
Base-centred
orthorhombic
Base-centred
monoclinic
Face-centred
orthorhombic
Triclinic
FIGURE 1.9 The fourteen Bravais unit cells. The dots indicate lattice points located at corners,
face-centres or at body centres as the case may be.
TABLE 1.4 Crystal structures of some allotropic forms
Metal Crystal structure Lattice parameters (nm)
Iron BCC (up to 910°C) 0.2866
FCC (910-1402°C) 0.3589
BCC (1402-1539°C) 0.2930
Strontium FCC (up to 557°C) 0.6084
BCC (557-768°C) 0.4840
Titanium HCP (up to 882°C) a = 0.2950, C = 0.4683
BCC (882-1668°C) 0.3320
Zirconium HCP (up to 862°C) a= 0.3231, C = 0.5147
BCC (862-1852°C) 0.3609
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41. 14 Mechanical Behaviour and Testing of Materials
1.5 ENGINEERING MATERIALS
Engineering materials, in general, mean solid materials used by the mankind. These can be
broadly classified into two classes, namely, metallic materials and nonmetallic materials. Metals
and alloys are the examples of metallic materials. Nonmetallic materials are further classified
into two sub-classes, i.e. ceramics (and glasses) and polymers. Ceramics are inorganic
compounds formed by the combination of metals and nonmetals. Glasses are amorphous
inorganic nonmetallic materials. Polymers, also known as polymeric materials, are mostly
organic nonmetallic materials. These are mainly the compounds of carbon and hydrogen with
other atoms such as oxygen, nitrogen, chlorine, fluorine may also be present. Some commonly
used engineering materials are described below.
1.5.1 Steel
Steel is the name given to the family of iron-carbon alloys having carbon content up to about
2%, i.e. up to maximum solid solubility limit of carbon in gamma-iron. Silicon, manganese,
sulphur and phosphorus are always present in all steels. Steels are generally classified into two
major classes, namely, plain carbon steels (or carbon steels) and alloy steels. Plain carbon
steels contain less than 1.65% manganese and 0.60% silicon and do not contain any other
specified element. Properties of plain carbon steels are governed by the carbon content.
According to carbon content, carbon steels are classified as low carbon, medium carbon, and
high carbon steels. Low carbon steels contain less than 0.30% carbon while high carbon steels
contain more than 0.60% carbon. Medium carbon steels have carbon content ranging from
0.30% to 0.60%. Some applications of plain carbon steels have been summarised in Table 1.5.
Type of steel
Low carbon steel
Medium carbon steel
High carbon steel
TABLE 1.5 Some applications of plain carbon steels
Applications
Wires, screws, nuts, bolts, rivets, sheets, ship plates, rods, angles, channels,
tubes, shafts, beams, various forgings, tin plate, galvanized plate, stampings,
many structural members, etc.
Stronger nuts, bolts, axles, shafts, high tensile tubes, locomotive tyres, wire
ropes, hammer, agricultural tools, reinforcing bars for cement concrete,
connecting rods, gears, rails, spindles, etc.
Springs, piano wire, wood working tools, metal cutting tools, forging dies,
drills, hand files, handsaw, limit gauges, razors, cold chisels, scissors, blades
of cold shears, sledge hammer, knieves, punches, etc.
Alloy steel contains specified amount(s) of alloying element(s) and/or more than 1.65%
manganese and/or 0.60% silicon. The properties of alloy steels depend on both carbon and
alloying element(s). It is possible to impart specific characteristics in the steels by careful
selection of alloying elements and adding these in controlled amounts. Table 1.6 describes the
effect of various elements on the properties of steels.
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42. Nature of Materials 15
TABLE 1.6 Effects of some elements on steel properties
Alloying elements Effects on steel properties
Aluminium
Chromium
Manganese
Nickel
Molybdenum
Tungsten
Vanadium
Cobalt
Silicon
Copper
Boron
Improves strength and toughness by refining the grain size, forms nitride in
nitriding grade steel (such as 'nitralloy') to impart hardness and wear resistance.
Improves hardenability, when form carbides improves hardness, tensile strength,
wear resistance, and heat resistance. When present in amounts greater than 11%
imparts corrosion and oxidation resistance properties.
Increases hardenability, hardness and tensile strength. Decreases ductile-brittle
transition temperature and thus improves toughness of steel.
Increases hardness, tensile strength and toughness of steel without any loss of
ductility. Imrpoves corrosion and heat resistance of chromium steels.
It promotes grain refinement, increases hardenability and improves high
temperature properties of steels, eliminates temper embrittlement effect of some
steels and enhances corrosion resistance of stainless steels (in particular, the
pitting corrosion resistance).
Improves red hardness, hardenability, tensile strength, resistance to tempering,
wear resistance and high temperature strength of steels.
Refines grain size, prevents grain growth at elevated temperatures, improves
wear resistance, yield strength, ductility and fatigue strength as well as high
temperature strength.
In presence of tungsten and/or molybdenum imparts red hardness of steels as in
super high speed steels.
Increases hardness, and strength without much affecting the ductility when
present in small amount (< 4%). It also imparts springness, oxidation resistance,
and heat resistance to steels. It increases electrical resistivity of steel for that steel
is used for transformers and dynamo applications.
Sometimes added to improve the atmospheric corrosion resistance of steel.
Specifically added in very small amount (0.001-0.005%) to improve
hardenability of steel.
Based on the amount of alloying element(s) present in steel, alloy steel may be referred
to as low alloy steel, medium alloy steel or high alloy steel. In general, low alloy steel contains
up to 5% total alloy content while total alloy content of a high alloy steel exceeds 10%. For
a medium alloy steel, total alloy content lies in between 5 and 10%. Classification into two
classes, namely, low alloy steel and high alloy steel is also quite popular. According to this
classification, low alloy steel contains up to 10% total alloy content while total alloy content
of a high alloy steel exceeds 10%. Some steels of industrial importance have been described
as follows.
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43. 16 Mechanical Behaviour and Testing of Materials
Structural steels
These are the steels widely employed for making engineering components such as springs,
gears, grinding media balls, boiler plates, nuts and bolts, bearings, crankshafts, axles, pistons,
valves and many other components. Such steels are frequently subjected to thermal and
mechanical treatments in order to develop desired mechanical properties. Structural steels
consist of a large number of carbon steels and alloy steels. Structural steels are different from
those steels which are used for specific purposes such as corrosion resistant steels, heat resistant
steels, electrical steels, etc. and steels for constructional applications which are generally used
in as-received condition.
Spring steels
Steels possessing high elastic limit, toughness and fatigue strength are suitable for making
springs. In addition to carbon steels, a large number of low and medium alloy steels, depending
on service conditions, are utilised for making springs. High alloy steels are also used for making
springs where some specific requirement is more important. For example, in highly corrosive
environment or at low temperatures, springs made from stainless steels are used.
Ball bearing steels
Steels suitable for making ball bearings must have high surface hardness to resist wear, tough
core to withstand sudden shocks and good fatigue strength as bearings are subjected to cyclic
loading. Ball bearing steels are essentially high carbon low chromium steels. Carbon content of
these steels ranges from 0.95 to 1.10% while chromium content varies from 0.40-1.60%.
Smaller sized ball bearings are made from the steels having 0.40-0.70% chromium while larger
sized ball bearings are made from the steels having 1.30-1.60% chromium. Medium sized ball
bearings are made from the steels having 0.90-1.20% chromium. Nonmetallic inclusions and
segregations of carbides should be controlled precisely in these steels as their presence impair
properties particularly fatigue strength significantly resulting in premature failure of bearings.
Heat treatment of ball bearing steels consists of hardening and tempering. Hardening is carried
out in the temperature range of 830-840°C. Oil quenched steel is tempered at 150-160°C for
1-2 hours. The minimum hardness of hardened and tempered steels should not be less than 62
on Rockwell C-scale.
High strength low alloy steels (HSLA)
These steels have been developed with main emphasis on better mechanical properties.
Mechanical treatment of these steels is carried out in such a way that desired mechanical
properties are obtained. These steels are not subjected to heat treatment. These steels are
essentially low carbon (C ~ 0.20%) steels with about 1% manganese and small quantities
(< 0.50%) of other elements like aluminium, niobium, vanadium, titanium and chromium.
Copper is added to some grades of these steels to impart resistance to atmospheric corrosion.
Steels containing copper are referred to as weathering steels. The yield strength and tensile
strength of these steels is usually in the range of 290-480 MPa and 415-620 MPa, respectively.
In addition to high strength to weight ratio, these steels exhibit very good formability and
weldability. This is why, better fuel efficiency has been attained by using these steels in
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44. Nature of Materials 17
automobile industry. These steels are used for numerous applications. Major applications
include transmission lines for natural gas, ships, offshore drilling platforms and automotive
components.
Hadfield steel
Also known as austemtlc manganese steel, Hadfield steel contains 1.1-1.4% carbon and
11-14% manganese. Properly heat-treated steel (water quenched from 1000°C) has austenitic
structure and attains high strength, toughness and excellent wear resistance on deformation.
This nonmagnetic steel is used in as-quenched condition. Applications include components in
electromagnetic equipments, rock crusher jaws, crushing and grinding mill liners, railroad
crossing and switch points, dredging equipment, chain for tanks, and excavator buckets.
Stainless steels
Stainless steel is a generic term denoting a large number of chromium and chromium-nickel
steels exhibiting excellent resistance to corrosion and many other useful properties. In addition
to chromium and nickel, sufficient amounts of other alloying elements such as niobium,
molybdenum, titanium, copper, tungsten, selenium, aluminium and tantalum may be present
in these steels. There are five classes of stainless steels, namely, martensitic stainless
steels (l l.5-15.0%Cr), ferritic stainless steels (15.0-30.0%Cr), austenitic stainless steels
(16.0-26.0%Cr, 6.0-22.0%Ni), precipitation hardenable stainless steels (14.0-18.0%Cr,
6.0-8.0%Ni, 2.0-3.0%Mo, 0.75-l.50%Al) and duplex stainless steels (23-30%Cr,
4.5-7.0%Ni, 2.0-4.0%Mo).
Low cost, high hardness, wear resistance and strength coupled with fairly good corrosion
resistance are some of the characteristics of martensitic stainless steels. Some important
applications include cutlery items, surgical instruments, high quality ball bearings, valves and
high quality knives.
Ferritic stainless steels possess fairly high ductility, good hot and cold workability and
excellent resistance to corrosion. Kitchen sinks, decorative trim, annealing baskets, nitric acid
tanks, dairy machinery, motor boat propeller shafts, aeroplane and automobile fittings,
stainless nuts and bolts and furnace parts are some engineering applications of ferritic
stainless steels.
Austenitic stainless steels offer maximum resistance to corrosion and possess excellent
deep drawability, strength and toughness coupled with excellent low-temperature impact
properties. In fact, this class, the most widely used, finds numerous applications. These are the
only stainless steels which are nonmagnetic in nature and hence have an edge over other
stainless steels.
Precipitation hardening stainless steels possess very good combination of corrosion
resistance and strength. These steels derive their strength mainly from precipitation hardening
and martensitic transformation.
Duplex stainless steels have ferrite and austenite in their microstructure attained by
balancing the nickel and chromium contents. Better strength than the austenitic stainless steels,
excellent resistance to corrosion (comparable with austenitic ones), immunity to stress corrosion
cracking and good weldability are some salient features of duplex stainless steels.
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45. 18 Mechanical Behaviour and Testing of Materials
Maraging steels
These steels contain 18-25% nickel, 7-10% cobalt, 3-5% molybdenum, up to 1.75% titanium
and up to 0.20% aluminium. Maraging steels are well known for their good yield strength to
tensile strength ratio, weldability, formability, excellent fracture toughness and resistance to
hydrogen embrittlement. Motor cases for missiles, low temperature structural parts, pressure
vessels and hulls for hydrospace vehicles are some of the major applications of maraging steels.
High-speed steels
These are essentially high carbon-high alloy steels, containing tungsten, chromium, vanadium
and molybdenum as the main alloying elements, are well suited for manufacturing cutting tools.
A typical conventional high-speed steel contains 0.7% carbon, 18% tungsten, 4% chromium
and 1% vanadium. These steels possess high compressive strength, high red hardness and
excellent wear resistance at room temperature as well as at elevated temperatures. Some
important applications are lathe tools, milling cutters, reamers, deep hole drills, taps, blanking
dies, hot forming dies and wearing plates. Tools made from high-speed steels can be safely used
up to "" 600°C. Bearings and springs for high temperature applications and aircraft fasteners are
also made from high-speed steels.
1.5.2 Cast Irons
Cast iron, similar to steel, denotes family of iron-carbon alloys with carbon contents exceeding
2% but less than 6.67%. However, the upper limit of carbon content in cast iron rarely exceeds
4%. Varying quantities of other elements such as silicon, manganese, phosphorus and sulphur
are always present in cast irons. Cast irons offer a wide range of properties, namely, strength,
hardness, wear resistance, corrosion resistance, oxidation resistance and machinability. Various
types of cast irons have been described below.
Grey cast iron
Grey cast iron derives its name from the grey appearance of the fractured surface due to the
presence of graphite. Graphite, in grey cast iron is present in the form of graphite flakes. The
size and distribution of these flakes have considerable impact on mechanical and physical
properties of grey cast irons. The ends of graphite flakes are regions of high stress
concentration. Hence, grey cast irons are brittle and possess low tensile strength and ductility
levels. The widespread engineering applications of grey cast irons are due to many attractive
properties. Some of these attractive properties are high compressive strength, excellent
machinability, good resistance to sliding wear, very good thermal conductivity and
exceptionally good damping capacity.
White cast iron
In these cast irons, all the carbon is present in combined form, i.e. as cementite. White cast irons
are hard, brittle and difficult to machine and therefore find only limited applications in as-cast
condition. These are mainly used for producing malleable iron castings. In general, most of the
white cast irons are hypoeutectic alloys and have a carbon content of about 3%. High hardness
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46. Nature of Materials 19
of white cast iron is mainly due to the continuous interdendritic network of cementite. This
property (hardness) makes white cast iron a useful wear resistant material.
Chilled cast iron
Chilled cast iron is basically a grey cast iron which on cooling rapidly solidifies as white cast
iron at the surface (case) and as grey cast iron in the interior (core). This type of cast iron is
produced by casting the molten metal of suitable composition, against a metal or graphite
chiller. Due to white cast iron case, these irons are used where high abrasion wear resistance
is desired. Typical applications include railroad freight car wheels, grain-mill rolls, rolls for
crushing ores, grinding balls, hammers, stamp shoes and dies.
Nodular cast iron
Cast irons having nodules/spheroids of graphite instead of graphite flakes possess better
ductility. Such cast irons are popularly known as nodular cast irons. These irons are also
referred to as ductile irons. Nodular cast irons are produced by treating molten iron with
magnesium or cerium. Molten metal must have very low sulphur content.
Malleable cast iron
It is produced by prolonged heat treatment of white cast iron. Due to heat treatment, cementite
gets trans-formed to graphite aggregates. Two important classes of malleable cast iron are
whiteheart and blackheart malleable cast iron. While the former iron is produced by heat
treating white cast iron in an oxidizing atmosphere, the blackheart malleable iron is produced
by heat treating white cast iron in a neutral atmosphere.
Ni-resist cast iron
Ni-resist is a trade name used for a group of high nickel (14 - 36%) austenitic cast irons which
are tough, wear resistant and highly stable under chemical attack. Chromium (1.6 - 6.0%)
imparts hardness, stiffness and good machinability to the cast iron. Carbon is present as graphite
flakes or nodules in an austenitic matrix. Ni-resist cast irons have good resistance to corrosion
and erosion and are able to resist moderately high temperatures. Pumps and valves for
petroleum, power, pulp and paper, and chemical process industries; hot forming dies, textile
rolls, sewage plant castings; piston, seals, valves, gears, impellers, wear rings and cylinder liners
for liquid handling industry are some of the applications of Ni-resist irons.
Ni-hard cast iron
Ni-hard cast irons are basically nickel (3.0 - 7.0%) - chromium (1.5 - 11%) cast irons
possessing outstanding resistance to wear. The wear resistance of Ni-hard cast iron is due to
microstructure which comprises of martensitic matrix and multitude of refined carbides. Ni-hard
cast iron castings find widespread use in the mining, power, cement, ceramic, paint, dredging,
coal, coke, steel and foundry industries. Some typical applications include rolling mill rolls,
grinding media balls, grinding mill liners, slurry pump parts, pulverizer rings, roll heads, pipe
and elbows and mixer blades.
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47. 20 Mechanical Behaviour and Testing of Materials
1.5.3 Aluminium and Its Alloys
Aluminium, the widely used engineering metal, next to iron only, has low specific gravity (2.70)
and melting temperature (660°C), very good electrical (64% IACS) and thermal (240W/mK)
conductivities, high specific strength, good formability, high reflectivity and excellent resistance
to corrosion. The important alloying elements responsible for strengthening of aluminium are
copper, magnesium, manganese, zinc and silicon. Aluminium and its alloys are extensively used
as domestic utensils and containers, electrical and thermal conductors, electrical cables, cylinder
heads and pistons in automobile and aircraft industry, reflectors for vehicle headlamps, flood
lights and telescopes; and foils for heat insulation in buildings, furnaces and food packing.
Duralumin
Duralumin is a commercial heat-treatable alloy based on aluminium-copper-magnesium-
manganese system, has been extensively used in aircraft industries for making frames, ribs,
propeller blades, etc. Other applications include building structures, truck bodies, casing pipes,
etc.
Y-alloy
Y-alloy is an important alloy of aluminium suitable for use at elevated temperature. It contains
4% copper, 2% nickel and 1.5% magnesium. The alloy retains strength at relatively high
temperatures and has good hot workability. Heat treated Y-alloy has NiA13 particles distributed
in the matrix. These particles are responsible for elevated temperature strength of the alloy.
Some applications include pistons, cylinder heads, and general-purpose high strength castings.
1.5.4 Magnesium and Its Alloys
Magnesium, a silvery-white metal has low specific gravity (1.74), low melting point (650°C)
and very good machinability. Low tensile strength (78 MPa), poor ductility and cold
formability, and inferior resistance to corrosion and oxidation restrict the use of magnesium as
a structural material. The main alloying elements added to magnesium are aluminium, zinc, and
manganese. Silicon, tin, zirconium, rare-earth ones, silver and thorium are also added but
usually in small amounts. Wrought magnesium alloys are used as extruded bars, rods and other
sections, forgings, sheets, plates and wires. Fuel tanks, ducts, wing tips, flaps, rudders, etc. are
made from sheets and strips of magnesium alloys. Most of the magnesium alloys have good
machinability and thus may be cast into intricate shapes with a high dimensional accuracy and
good surface finish. A typical magnesium alloy (Mg - 4%Al- 0.5%Zn - 0.3%Mn) is used for
aircraft engine bearing caps, housings, rocker arm supports, doors, hinges, hydraulic cylinders
valve bodies, etc. Susceptibility to stress corrosion cracking, poor fatigue strength, lower
ductility and poor formability, and high chemical reactivity are some major limitations of
magnesium alloys that restrict their widespread applications.
1.5.5 Titanium and Its Alloys
Both tensile strength and specific strength, at room temperature as well as at cryogenic
temperatures, are high for titanium. Titanium has excellent resistance to corrosion. Its corrosion
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48. Nature of Materials 21
resistance, under some atmospheres, is better than the austenitic stainless steel. Titanium is
considered as an ideal metal in marine atmospheres and seawater. Titanium alloys find wide
use in aviation and rocket engineering, ship-building and chemical processing due to their high
room temperature and elevated temperature strength, moderate ductility, good resistance to
corrosion and high specific strength.
Alloys of titanium with aluminium, tin, manganese, molybdenum, vanadium, zirconium,
silicon, chromium or iron are of greater commercial importance. An optimum combination of
strength and ductility is obtained in Ti-5%Al-2.5%Sn alloy. Tin improves creep resistance up
to 500°C. One of the most widely used titanium alloy is Ti-6%Al-4%V. This alloy has high
strength-to-weight ratio, good fatigue and creep strength, and high resistance to oxidation.
Typical applications of this alloy are gas turbine compressors blades, forged and extruded air
frame fittings, sheet metals for high temperature skins and disks and rings.
1.5.6 Copper and Its Alloys
Copper has high ductility and malleability, excellent electrical and thermal conductivity and
good resistance to corrosion. Electrical conductors and cables, heat conductors, domestic
utensils and boilers, water pipes and fittings, tubes, radiators, busbars, switchgears, gaskets, heat
exchangers, etc. are some important applications of copper. Copper alloys can broadly be
divided into three classes, namely, brasses, bronzes and cupronickels. Brass is the name given
to a copper base alloy containing zinc as the main alloying element. Brasses may be binary
alloys or may possess other elements. The term bronze is used to represent a vast family of
copper base alloys with various elements except zinc and nickel. Cupronickel has been a
popular name for the family of binary copper-nickel alloys.
Brasses
These are generally grouped into two main classes, namely, alpha brasses (having zinc up to
39%) and alpha-beta brasses (having 39-45% zinc). While alpha brasses are single-phase
alloys, alpha-beta brasses have microstructures consisting of two phases. Alpha brasses have
better corrosion resistance and cold workability in comparison to alpha-beta brasses which
possess higher strength and hardness coupled with good hot workability. Better ductility of
alpha brasses in comparison to alpha-beta brasses makes them highly suitable for making
components demanding deep drawa-bility. Alpha-beta brasses are relatively brittle and hence
cannot be cold worked. Hence, these brasses are shaped by casting, forging, hot rolling, hot
stamping and extrusion. The Cu-30%Zn alloy, popularly known as cartridge brass, has
optimum combination of strength and ductility and hence is extensively used for making
cartridge cases. Other applications include motor car headlamp reflectors, radiator casings,
fasteners, rivets, springs and plumbing accessories. Admiralty brass, a modified cartridge brass,
has a nominal composition as Cu-29%Zn-l %Sn. It is mainly used for condenser tubes, cold
worked marine parts and heat exchanger tubes. Naval brass, having higher zinc content (37 to
39%) than the admiralty brass, finds applications as welding rods, marine hardware, condenser
plates, propeller shafts, nuts, piston rods and valve stems.
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49. 22 Mechanical Behaviour and Testing of Materials
Bronzes
Springs, bearings, castings, plumbing fixtures, valves, marine-engine parts, connecting
rods, propellers, temple bells and statues are some applications of bronzes. Gunmetal
(Cu-10%Sn-2%Zn), an important tin-bronze, has high strength, hardness and wear resistance.
Its applications include water and steam-pipe fittings and taps, gears, hard castings, bushings
and marine fittings. Aluminium bronzes and silicon bronzes are famous for their excellent
resistance to corrosion coupled with high strength. Silicon bronzes possess good formability,
machinability and weldability. Aluminium bronze finds applications as condenser tubes,
corrosion-resistant vessels, nuts and bolts, protective sheathing in marine applications, gears,
propeller hubs, blades, pump parts, bearings, bushings, nonsparking tools and dies for drawing
and forming purposes. Beryllium bronzes are capable of attaining highest strength amongst all
copper base alloys and are being used for making nonsparking tools, springs, pressure
diaphragms and cells. Various applications of phosphor bronze include springs, gears, bushings,
electrical contacts and wire brushes.
Cupronickels
Cupronickels exhibit high resistance to atmospheric, fresh water and marine corrosion. These are
the most suitable copper alloys resisting corrosive and erosive action of moving seawater. These
alloys have very good cold as well as hot workability. Typical applications include condensers,
evaporators, distillers and heat exchanger tubes for marine vessels and power plants situated near
seashore. Being single-phase alloys, these are strengthened by strain hardening. Addition of nickel
to copper results in improved strength, oxidation resistance and corrosion resistance. Nickel
confers upon copper its characteristic magnetic and electrical properties. A wide range of
copper-nickel alloys is used due to their peculiar physical properties and resistance against
corrosion. Physical properties such as controllable colour, high electrical resistance, low as well
as constant temperature coefficient and typical magnetic properties make copper-nickel alloys
suitable for coinage, ornamental metal work, decorative items, thermocouples, resistance wires
and magnetic tape or wire. Constantan is a copper-nickel alloy having 45% nickel. This alloy
has the maximum as well as constant value of electrical resistance. The temperature coefficient
of resistance is practically zero for this alloy.
1.5.7 Nickel and Its Alloys
Nickel exhibits unique combination of physical properties, mechanical properties and
outstanding corrosion resistance against many aggressive media. It has very good solid
solubility in many metals. Also, many metals have good solid solubility in nickel. It is for this
reason that nickel finds numerous applications as an alloying element in steels, cast irons,
copper alloys, super alloys, etc. and as a base metal of many useful engineering alloys. One
important use of nickel is in electroplating as it can be electroplated on to a number of materials.
Monel
Monel is the name given to a nickel-copper alloy containing about 33% copper. This alloy,
having optimum combination of corrosion resistance and strength, retains strength at elevated
temperatures and has good formability. Poor machinability, castability and response to heat
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50. Nature of Materials 23
treatment are some limitations of Monel which can be encountered by the addition of some
alloying elements resulting in the development of a series of alloys called Monels. Monels
contain about 30% copper with small amounts of iron (up to 2%), manganese (up to 5%),
silicon (2 to 4%) or aluminium (2 to 4%) either alone or in combination. These alloys exhibit
high resistance to corrosion in salt water, fresh water, general atmosphere, inorganic acids,
sulphuric acid, nitrates, chlorides and caustic soda. Monels remain unaffected (unattacked) by
hydrogen fluoride, deaerated hydrofluoric acid, sulphurous acid, liquid ammonia, dry chlorine,
neutral salts and organic solvents. Monels find most of applications in chemical and process
industries due to their good corrosion resistance and strength. Applications include vessels,
pipelines, filters, valves, heat exchanger tubes, pump parts, marine propellers, propeller shafts,
nonsparking tools, and containers for handling but not for storage of food products, sulphuric
acid and hydrochloric acid.
Nichrome
Nichrome is the trade name given to nickel-chromium alloy contammg 20% chromium. It
possesses high electrical resistance, high melting point and very good high temperature
oxidation resistance. The alloy is capable of retaining its strength up to 500°C and does not
become brittle on repeated thermal cycling. Typical applications include heating elements for
electric furnaces, kettles, immersion heaters, hair dryers and toasters and as resistances.
Inconels
Inconels are essentially alloys of nickel with 14-17% chromium and 6-10% iron having small
amounts of titanium, aluminium and niobium. These alloys are well known for their superior
resistance to corrosion at elevated temperatures. Inconels resist attack by hydrogen sulphide,
alkalis, dry chlorine and bromine, organic compounds and fatty acids. Some important
applications include furnace heating elements, heat treatment jigs, annealing boxes, fixtures and
trays, valves for internal combustion engine, springs for high temperature use, recuperator
tubes, pyrometer sheaths, heat exchangers for fatty acids and phenols, reaction vessels for
plastics, dying and tanning; and evaporators for sodium sulphite.
Nimonic
Nimonic alloys represent a family of alloys developed by modifying the early 80%Ni-20%Cr
alloy by adding large proportion of cobalt in addition to appreciable amounts of titanium,
aluminium and/or molybdenum. High tensile strength at elevated temperatures, good creep
strength, excellent resistance to high temperature oxidation and good corrosion resistance are
some important properties of Nimonics. Nimonics are used widely for making turbine blades,
exhaust valves, for chemical plant components and equipments which require high strength at
elevated temperatures.
Hastelloys
Hastelloys are nickel base alloys having molybdenum up to 30% and iron around 5%. In
addition to molybdenum and iron, Hastelloys contain tungsten and chromium in appreciable
amounts. These alloys were basically developed for resistance to hydrochloric acid, nitric acid
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51. 24 Mechanical Behaviour and Testing of Materials
and to other nonoxidising acids. These alloys find applications where operating temperatures
are in the range of 700-980°C. These alloys are used for handling and storing hydrochloric
acid, phosphoric acid and other nonoxidising acids. A specific grade of Hastelloys, having about
10% silicon, 3% copper and 1% aluminium is mainly used for handling sulphuric acid of all
concentrations and at all temperatures.
1.5.8 Cobalt and Its Alloys
Cobalt, a strongly ferromagnetic metal with high relative density (8.83) and melting point
(1495°C), finds maximum applications due to its excellent high temperature strength and
magnetic properties. It is mostly used as an alloying element in alloy steels, cemented carbides,
magnetic materials, iron base superalloys and nickel base superalloys. The common alloying
elements added to cobalt are chromium, nickel, molybdenum, iron, tungsten, manganese,
niobium, vanadium, beryllium, titanium and aluminium. Cobalt base alloys offer excellent
resistance against corrosion and oxidation particularly at high temperatures, are resistant to
human body fluid, possess high elevated temperature strength and have high hardness and wear-
resistance. Cobalt alloys have been and are extensively used as high temperature materials, tool
materials, magnetic materials and wear resistant materials. Cobalt base alloys are much
expensive and are used where their high cost is justified on technical grounds.
Stellites
Stellites refer to a group of cobalt base alloys having chromium, tungsten, nickel, molybdenum,
niobium, titanium and iron as alloying elements. High strength and hardness at room and
elevated temperatures, excellent resistance to abrasion at elevated temperatures, high resistance
to corrosion, high resistance to softening during heating and a service temperature range up to
1000°C are some important properties of stellites. These alloys which were initially developed
for gas turbine blades in jet engines, super chargers and afterburners are now used as wear
resistant materials and not as high temperature materials. A typical composition of stellite is as
Co-0.25%C-27%Cr-3%Ni-5%Mo-5%Fe.
Vitallium
Vitallium is the trade name of cobalt base alloy having chromium and molybdenum as the main
alloying elements. This alloy is used for surgical implants. This biocompatible alloy, with
nominal composition as Co-30%Cr-5%Mo, can only be shaped by casting. The alloy lacks
ductility and possesses low yield strength making the alloy unsuitable for many applications.
This alloy is much better than austenitic stainless steel for prosthetic devices.
Vicalloy
Vicalloy is the name given to a cobalt base alloy having a nominal composition as Co-14%V-34%Fe.
By altering amounts of these elements or by adding small amounts of some other alloying
element(s), desired magnetic properties can be obtained. Such Co-V-Fe alloys are known
as vicalloys. These high-energy magnet alloys can be cold worked easily. Magnetic
properties of these alloys are derived from deformable texture and precipitation. Heat-
treated alloy has a high energy product (a million gauss oersted). Energy product may be
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52. Nature of Materials 25
increased anisotropically by heavy cold working to the tune of two to three million gauss
oersted. Applications include magnetic tapes, compass needles and in many electric motors and
devices.
1.5.9 Ceramic Materials
Ceramics are essentially the compounds of metals and nonmetals bonded by ionic, covalent or
mixed bondings. High melting point, low density coupled with high stiffness, high hot strength,
high compressive strength, high hardness, high wear and corrosion resistance, good dielectric
properties, good thermal and electrical insulating properties, etc. are some common charac-
teristics of ceramic materials. Oxides, carbides, nitrides and silicates belong to the family of
ceramic materials. Bricks, refractories, glasses, cement, plaster of paris, abrasives, porcelain
enamel, ferrites, piezoelectrics, dielectrics, garnets, etc. are the examples of some commonly
used ceramic materials.
Ceramics can broadly be grouped as traditional ceramics and modern ceramics.
Traditional ceramics include bricks, tiles, sanitaryware, porcelains, etc. while Al20 3, SiC, MgO,
BeO, B4C, Si3N4, BN, BaTi03, etc. belong to modern ceramics. Television, radio, audio-video
recording equipment, automobiles, modern space vehicles, telecommunication system, etc., are
some typical examples where modern ceramics have been used.
Glass, an amorphous ceramic material, possesses transparency, high hardness and
strength, poor toughness and excellent corrosion resistance to most environments. These
characteristics make glass an important material for many engineering applications such as
construction and vehicle glazing, windows, laboratory ware, beverage container and many
others. Silica is the main constituent of general purpose glasses.
Alumina ceramics
Alumina ceramics are the hardest, strongest and stiffest of the oxide ceramics. Alumina
ceramics are relatively cheap, possess mechanical properties equal to or better than most of the
other oxides, have outstanding electrical resistivity and dielectric strength, exhibit good
resistance to a wide variety of chemicals and remain unaffected by air, sulphurous atmospheres
and water vapour. Alumina ceramics find wide applications in electrical industries as electrical
insulators in spark plugs, chemical industries and in aerospace industries. Due to high hardness
coupled with good wear resistance and close dimensional stability, alumina ceramics also find
applications as abrasion resistant parts such as textile guides, pump plungers, chute linings,
discharge orifices, dies and bearings. Alumina also finds applications in bioimplants because of
its chemical inertness.
Beryllia (BeO)
Beryllia or Beryllium oxide is noted for its excellent high thermal conductivity, high strength
and good dielectric properties. The combination of strength, rigidity and dimensional stability
makes beryllia a useful material in gyroscopes while because of its high thermal conductivity,
it is a widely accepted material for cooling transistors, resistors and substrate in electronic
equipment.
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53. 26 Mechanical Behaviour and Testing of Materials
Silicon carbide
Silicon carbide exhibits an attractive combination of high thermal conductivity, low thermal
expansion and low thermal shock and outstanding resistance against wear, abrasion and
oxidation. The inherent outstanding properties have made silicon carbide useful in applications
such as thrust bearings, ball bearings, pump impellers, extrusion and wire drawing dies, valves
and seals, rocket nozzle throat, and heat exchanger tubes.
Silicon nitride
Silicon nitride (Si3N4) ceramic possesses low thermal expansion, moderate elastic modulus,
high thermal shock resistance, high strength, high wear and oxidation resistance and very good
thermal stability. It is being used as ball and roller bearings, various components such as piston
head, head liner, valve, cylinder liner, valve guide, exhaust port, inner surface of exhaust
manifold, turbine blade of diesel engine of some cars.
Sialons
Sialons has a major drawback with respect to fabrication to shapes with desirable properties and
low density. This difficulty has been overcome by alloying it with metallic oxides (Al203,
Y20 3, MgO, BeO, etc.) and replacement of Si and N by Al and oxygen atoms producing
Si-Al-0-N system. Materials based on this system are referred to as sialons. These materials
are stronger and exhibit excellent thermal shock resistance. Sialons are being used as tool
materials for machining poor machinable materials, welding nozzles, gas shrouds and location
pins for welding machines and rotating shaft seals in hostile environments due to their low
friction, high hardness, wear resistance, thermal shock resistance, electrical insulation and
resistance to molten metal pick up and are being evaluated for roller, shell and ball bearings,
diesel engine components, gas turbine components and many other applications requiring wear
resistance, heat resistance, chemical inertness and thermal shock resistance.
1.5.10 Polymeric Materials
Polymeric materials are predominantly the organic materials made up of long chain molecules.
These can be classified into three main groups, namely, thermosetting plastics (thermosets),
thermoplastics and elastomers. Inorganic or semi-organic polymers are based on silicon and
oxygen and are commonly called silicones.
Thermosets
Thermosets once shaped, cannot be reshaped again as these do not get softened on heating. This
characteristic, i.e. resistance to softening on heating, can be attributed to chemical change that
these materials undergo during the shaping (moulding) process. These are, in general, hard,
rigid and brittle. High strength thermosets can be developed by reinforcing them with fibrous
materials. Examples of thermosets include phenolic resins (bakelite), epoxy resins (araldite),
urea formaldehyde resins, polyurathane, melamine formaldehyde, etc.
The plastics which soften on heating and harden on cooling in a reversible fashion are
called thermoplastics. No chemical reaction occurs during heating. Thus heating and cooling
cycles can be repeated any number of times to obtain a desired shape. These are usually less
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