Phase Transformations in Metals and Alloys

528 Pages·1992·17.941 MB·English

by  D. A. Porter, K. E. Easterling (auth.)

Phase Transformations in Metals
and Alloys
This book is dedicated
0/
to the memory
Kenneth E. Easterling
Phase Transformations in Metals
and Alloys
SECOND EDITION
D.A. Porter
Rautaruukki Oy
Research Centre
Raahe
Finland
K.E. Easterling
Formerly School ojEngineering
University oj Exeter
Exeter
UK
I~nl
Springer-Science+Business Media, B.Y.
First edition 1981
Reprinted 1982, 1983, 1984, 1986, 1987, 1988, 1989, 1990, 1991
Second edition 1992
© 1981, 1992 D.A. Porter and K.E. Easterling
Originally pub1ished by Chapman & Hall in 1992.
Typcsct in 1O/12pt Times by Best-set Typesetter LId, Hong Kong
ISBN 978-0-442-31638-9 ISBN 978-1-4899-3051-4 (eBook)
DOI 10.1007/978-1-4899-3051-4
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Contents
Preface to the second edition xi
Preface to the first edition
XII
1 Thermodynamics and Phase Diagrams 1
1.1 Equilibrium 1
1.2 Single Component Systems 4
1.2.1 Gibbs Free Energy as a Function of Temperature 4
1.2.2 Pressure Effects 7
1.2.3 The Driving Force for Solidification 10
1.3 Binary Solutions 11
1.3.1 The Gibbs Free Energy of Binary Solutions 11
1.3.2 Ideal Solutions 13
1.3.3 Chemical Potential 16
1.3.4 Regular Solutions 18
1.3.5 Activity 21
1.3.6 Real Solutions 23
1.3.7 Ordered Phases 24
1.3.8 Intermediate Phases 26
1.4 Equilibrium in Heterogeneous Systems 28
1.5 Binary Phase Diagrams 33
1.5.1 A Simple Phase Diagram 33
1.5.2 Systems with a Miscibility Gap 33
1.5.3 Ordered Alloys 35
1.5.4 Simple Eutectic Systems 36
1.5.5 Phase Diagrams Containing Intermediate Phases 36
1.5.6 The Gibbs Phase Rule 36
1.5.7 The Effect of Temperature on Solid Solubility 41
1.5.8 Equilibrium Vacancy Concentration 43
1.6 The Influence of Interfaces on Equilibrium 44
1.7 Ternary Equilibrium 49
1.8 Additional Thermodynamic Relationships for Binary Solutions 52
1.9 The Kinetics of Phase Transformations 55
References 56
Further Reading 56
Exercises 57
vi Contents
2 Diffusion 60
2.1 Atomic Mechanisms of Diffusion 61
2.2 Interstitial Diffusion 63
2.2.1 Interstitial Diffusion as a Random Jump Process 63
2.2.2 Effect of Temperature-Thermal Activation 66
2.2.3 Steady-State Diffusion 69
2.2.4 Nonsteady-State Diffusion 69
2.2.5 Solutions to the Diffusion Equation 71
Homogenization 71
The Carburization of Steel 73
2.3 Substitutional Diffusion 75
2.3.1 Self-Diffusion 75
2.3.2 Vacancy Diffusion 79
2.3.3 Diffusion in Substitution al Alloys 82
2.3.4 Diffusion in Dilute Substitutional Alloys 91
2.4 Atomic Mobility 92
2.5 Tracer Diffusion in Binary Alloys 94
2.6 Diffusion in Ternary Alloys 96
2.7 High-Diffusivity Paths 98
2.7.1 Diffusion along Grain Boundaries and Free Surfaces 98
2.7.2 Diffusion along Dislocations 102
2.8 Diffusion in Multiphase Binary Systems 103
References 106
Further Reading 106
Exercises 106
3 Crystal Interfaces and Microstructure 110
3.1 Interfacial Free Energy 110
3.2 Solid/Vapour Interfaces 112
3.3 Boundaries in Single-Phase Solids 116
3.3.1 Low-Angle and High-Angle Boundaries 116
3.3.2 Special High-Angle Grain Boundaries 122
3.3.3 Equilibrium in Polycrystalline Materials 124
3.3.4 Thermally Activated Migration of Grain Boundaries 130
3.3.5 The Kinetics of Grain Growth 139
3.4 Interphase Interfaces in Solids 142
3.4.1 Interface Coherence 143
Fully Coherent Interfaces 143
Semicoherent Interfaces 145
Incoherent Interfaces 147
Complex Semicoherent Interfaces 148
3.4.2 Second-Phase Shape: Interfacial Energy Effects 149
Fully Coherent Precipitates 149
Partially Coherent Precipitates 151
Incoherent Precipitates 152
Precipitates on Grain Boundaries 153
Contents Vll
3.4.3 Second-Phase Shape: Misfit Strain Effects 154
Fully Coherent Precipitates 154
Incoherent Inclusions 158
Plate-Like Precipitates 160
3.4.4 Coherency Loss 160
3.4.5 Glissile Interfaces 163
3.4.6 Solid/Liquid Interfaces 168
3.5 Interface Migration 171
3.5.1 Diffusion-Controlled and Interface-Controlled Growth 175
References 180
Further Reading 182
Exercises 182
4 Solidification 185
4.1 Nucleation in Pure Metals 185
4.1.1 Homogeneous Nucleation 186
4.1.2 The Homogeneous Nucleation Rate 190
4.1.3 Heterogeneous Nucleation 192
4.1.4 Nucleation of Melting 197
4.2 Growth of a Pure Solid 197
4.2.1 Continuous Growth 198
4.2.2 Lateral Growth 198
Surface Nucleation 200
Spiral Growth 201
Growth from Twin Intersections 202
4.2.3 Heat Flow and Interface Stability 203
4.3 Alloy Solidification 207
4.3.1 Solidification of Single-Phase Alloys 208
Equilibrium Solidification 208
No Diffusion in Solid, Perfect Mixing in Liquid 208
No Diffusion in Solid, Diffusional Mixing in Liquid 212
Cellular and Dendritic Solidification 214
4.3.2 Eutectic Solidification 222
Growth of Lamellar Eutectics 223
4.3.3 Off-Eutectic Alloys 229
4.3.4 Peritectic Solidification 231
4.4 Solidification of Ingots and Castings 233
4.4.1 Ingot Structure 233
ChilI Zone 234
Columnar Zone 235
Equiaxed Zone 236
Shrinkage Effects 236
4.4.2 Segregation in Ingots and Castings 237
4.4.3 Continuous Casting 238
Heat Flow in Welding and Continuous
Casting 239
viii Contents
4.5 Solidification of Fusion Welds 243
Inftuence of Welding Speed 245
Geometry of Crystal Growth 248
4.6 Solidification during Quenching from the Melt 249
4.7 Case Studies of some Practical Castings and Welds 249
4.7.1 Casting of Carbon and Low-Alloy Steels 249
4.7.2 Casting of High-Speed Steels 251
4.7.3 Stainless Steel WeId Metal 256
References 259
Further Reading 260
Exercises 260
5 DitTusional Transformations in Solids 263
5.1 Homogeneous Nucleation in Solids 265
5.2 Heterogeneous Nucleation 271
Nucleation on Grain Boundaries 271
Dislocations 274
Excess Vacancies 275
5.2.1 Rate of Heterogeneous Nucleation 276
5.3 Precipitate Growth 279
5.3.1 Growth behind Planar Incoherent Interfaces 279
5.3.2 Diffusion-Controlled Lengthening of Plates or Needles 283
5.3.3 Thickening of Plate-like Precipitates 285
5.4 Overall Transformation Kinetics-TIT Diagrams 287
5.5 Precipitation in Age-Hardening Alloys 291
5.5.1 Precipitation in Aluminium-Copper Alloys 291
GP Zones 291
Transition Phases 292
5.5.2 Precipitation in Aluminium-Silver Alloys 302
5.5.3 Quenched-in Vacancies 303
5.5.4 Age Hardening 304
5.5.5 Spin odal Decomposition 308
5.5.6 Particle Coarsening 314
Lowy 316
LowX 316
e
LowD 317
5.6 The Precipitation of Ferrite from Austenite 317
5.7 Cellular Precipitation 322
5.8 Eutectoid Transformations 326
5.8.1 The Pearlite Reaction in Fe-C Alloys 326
Nucleation of Pearlite 327
Pearlite Growth 330
Pearlite in Off-Eutectoid Fe-C Alloys 333
5.8.2 The Bainite Transformation 334
Contents IX
Upper Bainite 334
Lower Bainite 337
Transformation'Shears 337
5.8.3 The Effeet of Alloying Elements on Hardenability 338
5.8.4 Continuous Cooling Diagrams 344
5.8.5 Fibrous and Interphase Preeipitation in Alloy Steels 349
5.9 Massive Transformations 349
5.10 Ordering Transformations 358
5.11 Case Studies 366
5.11.1 Titanium Forging Alloys 366
5.11.2 The WeIdability of Low-Carbon and Mieroalloyed
Rolled Steels 372
References 377
Further Reading 378
Exercises 379
6 Ditl"usionless Transformations 382
6.1 Characteristies of Diffusionless Transformations 383
6.1.1 The Solid Solution of Carbon in Iron 385
6.2 Martensite Crystallography 389
6.2.1 The Bain Model of the fec ~ bct Transformation 391
6.2.2 Comparison of Crystallographic Theory with
Experimental Results 396
6.3 Theories of Martensite Nucleation 397
6.3.1 Formation of Coherent Nuclei of Martensite 398
6.3.2 Role of Dislocations in Martensite Nucleation 401
6.3.3 Disloeation Strain Energy Assisted Transformation 406
6.4 Martensite Growth 409
6.4.1 Growth of Lath Martensite 410
6.4.2 Plate Martensite 412
6.4.3 Stabilization 415
6.4.4 Effeet of External Stresses 415
6.4.5 Role of Grain Size 416
6.5 Pre-martensite Phenomena 416
6.6 Tempering of Ferrous Martensites 417
Carbon Segregation 420
€-Carbide 421
Cementite 422
Alloy Carbides 422
Effect of Retained Austenite 426
Recovery, Recrystallization and Grain Growth 426
Temper Embrittlement 427
6.7 Case Studies 428
6.7.1 Carbon and Low-Alloy Quenched and Tempered Steels 428
6.7.2 Controlled Transformation Steels 430