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Bulk Nanostructured Materials


Fundamentals and Applications

Ruslan Z ValievAlexander P ZhilyaevTerence G Langdon

Copyright copy 2014 by The Minerals Metals amp Materials SocietyAll rights reserved

Published by John Wiley amp Sons Inc Hoboken New JerseyPublished simultaneously in Canada

No part of this publication may be reproduced stored in a retrieval system or transmitted in any form or by any means electronic mechanical photocopying recording scanning or otherwise except as permitted under Section 107 or 108 of the 1976 United States Copyright Act without either the prior written permission of The Minerals Metals amp Materials Society or authorization through payment of the appropriate per-copy fee to the Copyright Clearance Center Inc 222 Rosewood Drive Danvers MA 01923 (978) 750-8400 fax (978) 750-4470 or on the web at wwwcopyrightcom Requests to the Publisher for permission should be addressed to the Permissions Department John Wiley amp Sons Inc 111 River Street Hoboken NJ 07030 (201) 748-6011 fax (201) 748-6008 or online at httpwwwwileycomgopermission

Limit of LiabilityDisclaimer of Warranty While the publisher and author have used their best efforts in preparing this book they make no representations or warranties with respect to the accuracy or completeness of the contents of this book and specifically disclaim any implied warranties of merchantability or fitness for a particular purpose No warranty may be created or extended by sales representatives or written sales materials The advice and strategies contained herein may not be suitable for your situation You should consult with a professional where appropriate Neither the publisher nor author shall be liable for any loss of profit or any other commercial damages including but not limited to special incidental consequential or other damages

Wiley also publishes books in a variety of electronic formats Some content that appears in print may not be available in electronic formats For more information about Wiley products visit the web site at wwwwileycom For general information on other Wiley products and services or for technical support please contact the Wiley Customer Care Department within the United States at (800) 762-2974 outside the United States at (317) 572-3993 or fax (317) 572-4002

Library of Congress Cataloging-in-Publication Data

Valiev Ruslan Z (Ruslan Zufarovich) Bulk nanostructured materials fundamentals and applications by Ruslan Z Valiev Alexander P Zhilyaev Terence G Langdon pages cm Includes bibliographical references and index ISBN 978-1-118-09540-9 (cloth)1 Nanostructured materials I Zhilyaev Alexander P II Langdon Terence G III Title TA4189N35V35 2014 6201prime15ndashdc23 2013016294

Printed in the United States of America

10 9 8 7 6 5 4 3 2 1

To Marina Tatiana and Mady for their support during the writing of this book and throughout our scientific careers

vii

Contents

PRefAce xiii

AcknowLedGmenTs xv

1 Introduction 1

2 Description of Severe Plastic Deformation (SPD) Principles and Techniques 6

21 A Historical Retrospective of SPD Processing 6

22 Main Techniques for Severe Plastic Deformation 8

23 SPD Processing Regimes for Grain Refinement 15

24 Types of Nanostructures from SPD 16

PART one HiGH-PRessuRe ToRsion 23

3 Principles and Technical Parameters of High-Pressure Torsion 25

31 A History of High-Pressure Deformation 25

32 Definition of the Strain Imposed in HPT 28

33 Principles of Unconstrained and Constrained HPT 32

34 Variation in Homogeneity Across an HPT Disk 33

341 Developing a Pictorial Representation of the Microhardness Distributions 33

viii CONTENTS

342 Macroscopic Flow Pattern During HPT 38

343 Occurrence of Nonhomogeneity in the Microstructures Produced by HPT 52

35 Influence of Applied Load and Accumulated Strain on Microstructural Evolution 67

36 Influence of Strain Hardening and Dynamic Recovery 71

37 Significance of Slippage During High-Pressure Torsioning 76

38 Models for the Development of Homogeneity in HPT 81

4 HPT Processing of Metals Alloys and Composites 88

41 Microstructure Evolution and Grain Refinement in Metals Subjected to HPT 88

411 Microstructure and Grain Refinement in fcc and bcc Pure Metals 88

412 Allotropic Transformation in HCP Metals as Mechanism of Grain Refinement 97

413 Significance of the Minimum Grain Size Attained Using HPT 103

414 Microtexture and Grain Boundary Statistics in HPT Metals 107

42 Processing of Solid Solutions and Multiphase Alloys 112

421 High-Pressure Torsion of Solid Solutions 112

422 Grain Refinement During Processing of Multiphase Alloys 119

423 Amorphization and Nanocrystallization of Alloys by HPT 126

43 Processing of Intermetallics by HPT 130

44 Processing of Metal Matrix Composites 136

5 New Approaches to HPT Processing 152

51 Cyclic Processing by Reversing the Direction of Torsional Straining 152

52 Using HPT for the Cold Consolidation of Powders and Machining Chips 173

53 Extension of HPT to Large Samples 180

PART Two equAL cHAnneL AnGuLAR PRessinG 191

6 Development of Processing Using Equal-Channel Angular Pressing 193

61 Construction of an ECAPECAE Facility 193

62 Equal-Channel Angular Pressing of Rods Bars and Plate Samples 195

CONTENTS ix

63 Alternative Procedures for Achieving ECAP Rotary Dies Side-Extrusion and Multipass Dies 198

64 Developing ECAP with Parallel Channels 201

65 Continuous Processing by ECAP From Continuous Confined Shearing Equal-Channel Angular Drawing and Conshearing to Conform Process 204

7 Fundamental Parameters and Experimental Factors in ECAP 215

71 Strain Imposed in ECAP 215

72 Processing Routes in ECAP 219

73 Shearing Patterns Associated with ECAP 221

74 Experimental Factors Influencing ECAP 223

741 Influence of the Channel Angle and the Angle of Curvature 223

742 Influence of the Pressing Speed and Temperature 229

75 Role of Internal Heating During ECAP 232

76 Influence of a Back Pressure 234

8 Grain Refinement in Metallic Systems Processed by ECAP 239

81 Mesoscopic Characteristics After ECAP 240

82 Development of an Ultrafine-Grained Microstructure 244

83 Factors Governing the Ultrafine Grain Size in ECAP 253

84 Microstructural Features and Texture After ECAP 256

85 Influence of ECAP on Precipitation 262

86 Pressing of Multiphase Alloys and Composites 266

861 Multiphase Alloys 267

862 Metal Matrix Composites 270

87 Consolidation by ECAP 275

88 Post-ECAP Processing 277

PART THRee fundAmenTALs And PRoPeRTies of mATeRiALs AfTeR sPd 289

9 Structural Modeling and Physical Properties of SPD-Processed Materials 291

91 Experimental Studies of Grain Boundaries in BNM 293

92 Developments of Structural Model of BNM 309

93 Fundamental Parameters and Physical Properties 312

931 Curie Temperature and Magnetic Properties 313

932 Debye Temperature and Diffusivity 315

x CONTENTS

933 Electroconductivity 320

934 Elastic Properties and Internal Friction 323

10 Mechanical Properties of BNM at Ambient Temperature 331

101 Strength and ldquoSuperstrengthrdquo 332

102 Plastic Deformation and Ductility 338

103 Fatigue Behavior 345

104 Alternative Deformation Mechanisms at Very Small Grain Sizes 350

11 Mechanical Properties at High Temperatures 357

111 Achieving Superplasticity in Ultrafine-Grained Metals 359

1111 Superplasticity after Processing by HPT 360

1112 Superplasticity after Processing by ECAP 364

112 Effects of Different ECAP Processing Routes on Superplasticity 370

113 Developing a Superplastic Forming Capability 375

114 Cavitation in Superplasticity After SPD 378

115 Future Prospects for Superplasticity in Nanostructured Materials 380

12 Functional and Multifunctional Properties of Bulk Nanostructured Materials 387

121 Corrosion Behavior 388

122 Wear Resistance 390

123 Enhanced Strength and Conductivity 393

124 Biomedical Behavior of Nanometals 396

125 Enhanced Magnetic Properties 398

126 Inelasticity and Shape-Memory Effects 402

127 Other Functional Properties 405

1271 Enhanced Reaction Kinetics 405

1272 Radiation Resistance 407

1273 Thermoelectric Property 408

CONTENTS xi

PART fouR innoVATion PoTenTiAL And PRosPecTs foR sPd APPLicATions 415

13 Innovation Potential of Bulk Nanostructured Materials 417

131 Nanotitanium and Ti Alloys for Medical Implants 417

132 Nanostructured Mg Alloys for Hydrogen Storage 420

133 Micro-Devices from BNM 423

134 Innovation Potential and Application of Nanostructured Al Alloys 423

135 Fabrication of Nanostructured Steels for Engineering 425

14 Conclusions 434

index 436

xiii

Preface

In recent years the development of bulk nanostructured materials (BNM) has become one of the most topical directions in modern materials science Nanostructuring of various materials paves the way to obtaining unusual properties that are very attrac-tive for different structural and functional applications In this research topic the use of both ldquobottom-uprdquo and ldquotop-downrdquo approaches for BNM processingsynthesis routes has received considerable attention In the ldquobottom-uprdquo approach bulk nano-materials are fabricated by assembling individual atoms or by consolidating nanopar-ticulate solids The ldquotop-downrdquo approach is different because it is based on grain refinement through heavy straining or shock wave loading During the last two decades grain refinement by severe plastic deformation (SPD) techniques has attracted special interest since it offers new opportunities for developing different technologies for the fabrication of commercial nanostructured metals and alloys for various specific applications Very significant progress was made in this area in recent years The generation of new and unusual properties has been demonstrated for a wide range of different metals and alloys examples include very high strength and ductility record-breaking fatigue endurance increased superplastic forming capabilities as well as multifunctional behavior when materials exhibit enhanced functional (electric magnetic corrosion etc) and mechanical properties

The innovation potential of this research area is outstanding and now a transition from laboratory-scale research to industrial applications is starting to emerge In addition the subject of BNM is now entering the textbooks on materials science and related subject areas and therefore it is very important to have a single treatise that comprises the fundamental as well as applied aspects of bulk nanomaterials At the same time although the processing of BNM by assembling individual atomsparticles has been described in several books there is at present no international monograph

xiv PREFACE

devoted exclusively to bulk nanomaterials produced by severe plastic deformation This omission forms the background for the present work Equally it is now apparent that research on BNM has developed so rapidly in recent years that the terminology needs some clarification and it is necessary to provide a clearer definition of the terms widely used within this field This information is given in Chapter 1

xv

Acknowledgments

The use of bulk nanostructured metals and alloys as structural and functional mate-rials of the next generation has remained an open question until recently when a breakthrough has been outlined in this area associated both with the development of new processing routes for the fabrication of bulk nanostructured materials and with investigations of the fundamental mechanisms that lead to novel properties for these materials Our understanding of these issues has naturally developed through our many close interactions with colleagues and associates around the world We would like to express our sincere gratitude and appreciation to all who provided support offered comments discussed allowed quoting of their remarks and publications and otherwise assisted in the preparation of this book particularly our reputable col-leagues and friends from the International NanoSPD Steering CommitteemdashProfs Yuri Estrin Zenji Horita Michael Zehetbauer Yuntian Zhu (wwwnanospdorg) and other members of the materials science community actively working with us to pub-lish many joint works Many of these papers are cited in the reference sections and we take this opportunity to offer our sincere apologies to all collaborators who have been with us over the course of many years and whose contributions we have failed to mention

The preparation of this book was made possible through the support in part by the European Research Council under ERC Grant Agreement No 267464-SPDMETALS (APZ and TGL) in part in the framework of the Federal Target Program ldquoScientific and Educational Personnel of Innovative Russiardquo for the years 2009ndash2013 and by the Russian Federal Ministry for Education and Science (Contract 14B25310017) and in part by the Russian Foundation for Basic Research as well as the Department of Chemistry Moscow State University (RZV)

xvi ACKNOWLEDGMENTS

We are especially grateful to Zarema Safargalina of the Institute of Physics of Advanced Materials Ufa State Aviation Technical University for her outstanding assistance in coordinating all aspects in the preparation of the manuscript It is due to Zaremarsquos hard work and dedication that we were able to overcome all obstacles and complete the manuscript on schedule

Bulk Nanostructured Materials Fundamentals and Applications First Edition Ruslan Z Valiev Alexander P Zhilyaev and Terence G Langdon copy 2014 The Minerals Metals amp Materials Society Published 2014 by John Wiley amp Sons Inc

1

Introduction

Chapter 1

Although the mechanical and physical properties of all crystalline materials are determined by several microstructural parameters the average grain size of the material generally plays a very significant and often a dominant role Thus the strength of all polycrystalline materials is related to the grain size d through the HallndashPetch equation which states that the yield stress s

y is given by

12y 0 y k dσ σ minus= + (11)

where s0 is termed the friction stress and k

y is a constant of yielding [1 2] It follows

from Equation 11 that the strength increases with a reduction in the grain size and this has led to an ever-increasing interest in fabricating materials with extremely small grain sizes

It is now over 25 years since Herbert Gleiter presented the first concepts for developing nanocrystalline (NC) materials (ie materials with a grain size of less than 100 nm) and the potential for producing special properties [3] Since that time the field of nanomaterials has flourished over the last two decades owing to the considerable interest in this topic and the scientific and technological importance

At the same time it is now apparent that research on nanomaterials has developed widely in recent years and the terminology needs some clarification The three terms actively used within this field are ultrafine-grained (UFG) NC

2 iNTRODuCTiON

and nanostructured (NS) materials and it is initially useful to provide a clearer definition of these three terms which have been discussed at several conferences and in reviews [4ndash10]

With reference to the characteristics of polycrystalline materials UFG materials can be defined as polycrystalline materials having very small and reasonably equiaxed grains with average grain sizes less than ~1 microm and grain boundaries with predominantly high angles of misorientation In practice the presence of a large fraction of high-angle grain boundaries is important in order to achieve advanced and unique properties [5] Thus the grain sizes of UFG materials lie both within the sub-micrometer (100ndash1000 nm) and the nanometer (less than 100 nm) ranges For grain sizes below 100 nm the latter are termed nanocrystalline materials or nanocrystals In practice UFG materials also exhibit other structural elements having sizes of less than 100 nm including second-phase particles or precipitates dislocation substruc-tures and pores These nanometer-sized features also have a considerable influence on the properties of the materials For example in severe plastic deformation (SPD) processing nanostructural elements such as nanotwins grain boundary precipitates and dislocation substructures may form within the ultrafine grains of 100ndash300 nm in size and their formation will have a significant effect on the mechanical and functional properties [7] Materials containing these nanostructural elements are designated ldquonanostructured materialsrdquo In order to qualify as bulk nanostructured materials (BNM) the only additional requirements are that there exists a homoge-neous distribution of nanostructural elements in the entire sample and the samples typically have 1000 or more grainsnanostructural elements in at least one direction

To date two basic and complementary approaches have been developed for the synthesis of BNM and these are known as the ldquobottom-uprdquo and the ldquotop-downrdquo approaches [11 12]

As was already noted earlier in the ldquobottom-uprdquo approach BNM are fabricated by assembling individual atoms or by consolidating nanoparticulate solids Examples of these techniques include inert gas condensation [6 11] electrodeposition [13] ball milling with subsequent consolidation [14] and cryomilling with hot isostatic pressing [15 16] where cryomilling essentially denotes mechanical milling in a liquid nitrogen environment In practice these techniques are often limited to the production of fairly small samples that may be useful for applications in fields such as electronic devices but are generally not appropriate for large-scale structural appli-cations Furthermore the final products from these techniques invariably contain some degree of residual porosity and a low level of contamination which is introduced during the fabrication procedure Recent research has shown that large bulk solids in an essentially fully dense state may be produced by combining cryomilling and hot isostatic pressing with subsequent extrusion [17] but the operation of this combined procedure is expensive and at present it is not easily adapted for the production and utilization of structural alloys for large-scale industrial applications

The ldquotop-downrdquo approach is different because it is dependent upon taking a bulk solid with a relatively coarse grain size and processing the solid to produce a UFG microstructure through SPD Processing by SPD refers to various experimental pro-cedures of metal forming that may be used to impose very high strains on materials

iNTRODuCTiON 3

leading to exceptional grain refinement A unique feature of SPD processing is that the high strain is imposed without any significant change in the overall dimensions of the workpiece Another feature is that the shape is retained by using special tool geometries that prevent free flow of the material and thereby produce a significant hydrostatic pressure The presence of this hydrostatic pressure is essential for achiev-ing high strains and introducing the high densities of lattice defects necessary for exceptional grain refinement The SPD processing avoids the small product sizes and the contamination which are inherent features of materials produced using the ldquobot-tom-uprdquo approach and it has the additional advantage that it can be readily and easily applied to a wide range of preselected alloys

The first observations of the production of UFG microstructures in bulk materials using the ldquotop-downrdquo approach appeared in the scientific literature in the early 1990s in several publications dealing with pure metals and alloys [18 19] It is important to note that these early publications provided a direct demonstration of the ability to employ heavy plastic straining in the production of bulk materials having fairly homogeneous and equiaxed microstructures with grain sizes in the submicrometer or nanometer ranges and with a high fraction of high-angle grain boundaries More recently a number of other nanostructural features (twins particles and so on) have been revealed in SPD-processed materials The type and the morphology of such NS elements and their density determine the mechanical chemical and physical properties of NS materials often called the structural and functional properties Over the last few years the studies of BNM tend to be more and more oriented to the development of their advanced and superior properties and in this case the conception of nanostructural design plays a far more important role For example the critical parameter for bulk NS metals and alloys produced by SPD together with the refine-ment of microstructure to the nanosized range is the grain boundary structure because the boundaries can be formed as low- and high-angle special and random and equilibrium and nonequilibrium boundaries depending on the SPD processing regimes [20 21] Furthermore boundaries having different structures exhibit differ-ent transport mechanisms (deformation diffusion etc) that is grain boundary sliding which in turn leads to differences in the properties In such a manner this opens a new way for advancing the properties of UFG materials by appropriately tuning their grain boundary structures

The concept of nanostructural design of materials can be schematically presented in Figure 11 (redrawn from [22])mdashthe scheme modifies and further develops a well-known concept of contemporary creation of novel materials through the integration of theory and modeling structure characterization processing and synthesis as well as the properties studies In addition nanostructuring of bulk materials deals with a far larger number of structural parameters related to the grain size and shape lattice defects in the grain interior as well as with the grain boundary structure and also the presence of segregations and second-phase nanoparticles This provides an opportunity to vary the transport mechanisms and therefore can drastically increase the properties For example nanostructuring of bulk materials by SPD processing permits not only a considerable enhancement of many mechanical and physical properties but also contributes to the appearance of multifunctional

4 iNTRODuCTiON

materials [23ndash27] In this respect it can be anticipated that already in the near future nanostructuring of materials by various processing and synthesis techniques may provide a new breakthrough in the development of materials with superior properties for advanced structural and functional applications

This book is devoted specifically to BNM produced by SPD In recent years a breakthrough has developed in studies of NS metals and alloys as advanced structural and functional materials associated both with the development of new routes for the fabrication of BNM using SPD and with investigations of the fundamental mechanisms that lead to the new properties of these materials This book describes the new concepts and principles in using SPD processing to fabricate bulk NS metals with advanced properties Special emphasis is placed on the relationships between the microstructural features and properties as well as the innovation potential of SPD-produced nanomaterials

RefeRences

1 Hall EO Proc R Soc B 195164747

2 Petch NJ J Iron Steel Inst 195317425

3 Gleiter H In Hansen N Horsewell A Leffers T Lilholt H editors Proceedings of the 2nd Risoslash International Symposium on Metallurgy and Materials Science Roskilde Risoslash National Laboratory 1981 p 15

4 Valiev RZ Islamgaliev RK Alexandrov IV Prog Mater Sci 200045103

5 Valiev RZ Estrin Y Horita Z Langdon TG Zehetbauer MJ Zhu YT JOM 200658(4)33

PERFORMANCEProcessing

ampsynthesis

Transportmechanisms(deformationdiffusion etc)

Advancedmechanical amp functional

properties

Nanostructuresamp Their

characterization

Figure 11 Principles of nanostructural design of BNM

REFERENCES 5

6 Zehetbauer MJ Zhu YT editors Bulk Nanostructured Materials Weinheim Wiley-VCH Verlag GmbH amp Co KGaA 2009

7 Valiev RZ Hahn H Langdon TG Special Issue Bulk Nanostructured Materials Adv Eng Mater 201012665ndash815

8 Horita Z editor Nanomaterials by Severe Plastic Deformation Uetikon-Zuumlrich Trans Tech 2006

9 Estrin Y Maier HJ editors Nanomaterials by Severe Plastic Deformation IV Uetikon-Zuumlrich Trans Tech 2008

10 Wang JT Figueiredo RB Langdon TG editors Nanomaterials by Severe Plastic Deformation NanoSPD5 Uetikon-Zuumlrich Trans Tech 2011

11 Gleiter H Acta Mater 2000481

12 Zhu YT Lowe TC Langdon TG Scr Mater 200451825

13 Erb U El-Sherik AM Palumbo G Aust KT Nanostruct Mater 19932383

14 Koch CC Cho YS Nanostruct Mater 19921207

15 Luton MJ Jayanth CS Disko MM Matras S Vallone J Mater Res Soc Symp Proc 198913279

16 Witkin DB Lavernia EJ Prog Mater Sci 2006511

17 Han BQ Matejczyk D Zhou F Zhang Z Bampton C Lavernia EJ Mohamed FA Metall Mater Trans 200435A947

18 Valiev RZ Krasilnikov NA Tsenev NK Mat Sci Eng 1991A13735

19 Valiev RZ Korznikov AV Mulyukov RR Mater Sci Eng A 1993186141

20 Valiev RZ Nat Mater 20043511

21 Sauvage X Wilde G Divinski SV Horita Z Valiev RZ Mater Sci Eng A 20125401

22 Suresh S editor The Millennium Special Issue A Selection of Major Topics in Materials Science and Engineering Current Status and Future Directions Acta Materialia 2000 481ndash384 Oxford Elsevier Ltd

23 Ivanisenko Y Darbandi A Dasgupta S Kruk R Hahn H Adv Eng Mater 201012666

24 Horita Z editor Production of multifunctional materials using severe plastic deformation In Proceedings of International Symposium on Giant Straining Process for Advanced Materials (GSAM2010) Fukuoka Kyushu University Press 2011

25 Sabirov I Murashkin MYu Valiev RZ Mater Sci Eng A 20135601

26 Valiev RZ Langdon TG Adv Eng Mater 201012677

27 Estrin Y Vinogradov AV Acta Mater 201361782


6

Description of Severe Plastic Deformation (SPD)

Principles and Techniques

Chapter 2

21 A HisToRicAL ReTRosPecTiVe of sPd PRocessinG

The processing of metallic materials through the application of severe plastic deformation (SPD) has now become of major importance in many research labora-tories around the world In order to establish the terms employed in this research a recent report described the basic principles of SPD processing and provided the def-initions of the relevant terms that are used within this field of research [1] Processing through the use of SPD is defined as ldquoany method of metal forming under an exten-sive hydrostatic pressure that may be used to impose a very high strain on a bulk solid without the introduction of any significant change in the overall dimensions of the sample and having the ability to produce exceptional grain refinementrdquo

Although SPD processing in its modern form is a relatively new development the fundamental principles of this type of metal processing extend back to the work of artisans in ancient times A comprehensive review of these earlier developments was presented at the NanoSPD3 conference in Japan [2] In ancient China during the Han dynasty around 200 bc and the Three States dynasty of 280 ad the local artisans developed and utilized a new and very effective forging technique for the fabrication of steel for use in swords The significant feature of this process was that it consisted of a repetitive forging and folding of the metal which thereby introduced substantial hardening This repetitive forging and folding process became adopted as a viable technique in the production of high-strength products and it forms the basis of the famous Bai-Lian steels Indeed there is evidence for the use of this procedure in

A HiSTORiCAL RETROSPECTiVE OF SPD PROCESSiNG 7

ancient China as early as about 500 bc Numerous archeological artifacts are now available from this early period in the form of steel swords and knives and there are many inscriptions on these ancient objects which provide a concise record of the processing operation For example a 50-Lian steel sword was prepared using 50 sep-arate smeltings or repetitive forging and folding operations Subsequently the processing method spread to Japan and then to India where Wootz steel a special form of ultrahigh carbon steel was developed between approximately 300 bc and 300 ad It is instructive to note that Wootz steel has been specifically designated as an advanced material of the ancient world because of its high impact hardness and superplastic properties at elevated temperatures [3] Further expansion of this tech-nology to the Middle East led to the development of the famous Damascus steel which was manufactured in ancient Syria in the vicinity of Damascus up to the middle of the eighteenth century when the fabrication technique was lost [4] However an important characteristic of all developments in this ancient age is that they lacked scientific rigor and there was no understanding of the effect of these new processing procedures

The introduction of scientific principles to SPD processing may be traced to the pioneering work by Professor PW Bridgman at Harvard University from the 1930s onwards Bridgman subsequently received the Nobel Prize for his work in high-pressure physics and for his detailed studies on the effects of high pressures on bulk metals From the perspective of this topic it is important to note that Bridgman was the first to propose the studies of metals using a combination of compression and torsional straining and subsequently much of this early work was summarized in a lengthy review that appeared as a book [5]

On the other hand the conception of ldquolarge plastic strainsrdquo or deformations that are characterized by a high value of true accumulated strain e ge 05 and are realized at relatively low temperatures (le04 melting point) has been long used in the branches of physics and mechanics dealing with the problems of plasticity of solids [6ndash8] Large plastic deformations are actively engaged in practice for example in metal forming for shaping in the process of manufacturing of semi-products and parts as well as for materials hardening Traditionally such methods of metal forming as drawing extrusion and rolling are used for these purposes Experimental methods for achieving very large strains are usually based on the rolling of strips or thin foils for example with an initial thickness of ~10 mm up to a final thickness of ~01 mm which is equivalent to 99 reduction in thickness (true strain e = 46) Therefore true strains 4ndash45 are critical for the shaping of bulk billets by conventional metal forming methods and as it was noticed in earlier works that the achievement of even larger strains requires radically different processing techniques [9] On the basis of Bridgmanrsquos idea it was suggested [9 10] that such techniques may comprise the die-sets combining shear (torsion) and compression straining as they make it pos-sible to deform the material without changing its form

In the 1980s these principles were realized by Polish scientists [9] and by Russian scientists from Yekaterinburg (former Sverdlovsk Soviet Union) [11] in creation of experimental die-sets for combined torsion and compression The appli-cation of these devices provided the achievement of very large strains with true

8 DESCRiPTiON OF SEVERE PLASTiC DEFORMATiON (SPD) PRiNCiPLES AND TECHNiQuES

strains exceeding 6ndash8 which resulted in strong microstructural refinement in metals This procedure was later named as ldquohigh-pressure torsionrdquo (HPT) [12] Of addi-tional interest to the topic was the early work by Segal and his collaborators [13] conducted in Minsk in the former Soviet Union (now the capital of Belarus) to develop the process of equal-channel angular pressing (ECAP) This latter procedure represents the most important and most utilized technique for SPD processing at the present time During ECAP the billet shape is also preserved and therefore very large strains are achieved by multipass processing At the same time regarding a starting point of nanostructuring metals by SPD processing the pioneering work was performed by Valiev et al in Ufa in 1988 [14] who for the first time demonstrated the possibility of producing using the HPT procedure ultrafine-grained (UFG) metals and alloys with high-angle grain boundaries that lead to new properties The latter was evidenced by revealing the so-called low-temperature superplasticity in an UFG Al alloy Later in 1991 the UFG materials were first obtained in Ufa by means of another techniquemdashECAP [15] As it will be demonstrated in the following text ECAP allows the production of UFG structures in bulk billets from different metals and alloys Moreover this technique is very promising for practical applica-tions Therefore this new approach for grain refinement based on the achievement of very large true strains with e ge 6ndash8 under high imposed pressures was termed as ldquoSPDrdquo [16] and this attracted very substantial attention with further development of a number of different SPD techniques [1 17 18] Besides SPD processing routes and regimes for grain refinement were established for various metals and alloys thus triggering a new trend in materials science and engineering dealing with the production of bulk nanostructured materials by SPD processing

Since the pioneering work on tailoring of UFG structures by SPD processing [15 16] two SPD techniques namely HPT and ECAP have attracted close attention and recently have been further developed At the same time in the last 10ndash15 years there appeared a wide diversity of new SPD techniques for example accumulative roll bonding (ARB) multiaxial forging and twist extrusion (TE) (see Section 22 for more details) Nevertheless processing by HPT and ECAP has remained the most popular approach and recently this has acquired a new impulse for development through the modification of conventional die-sets and demonstrations that new opportunities are now available for involving these procedures in processing For this reason the very development and application of these two techniques will be the focus of the present book

22 mAin TecHniques foR seVeRe PLAsTic defoRmATion

SPD processing refers to various experimental procedures of metal forming that may be used to impose very high strains on materials leading to exceptional grain refinement Following this objective the unique feature of SPD processing is that high strain is imposed at relatively low temperatures (usually less than 04 T

m)

without any significant change in the overall dimensions of the workpiece Another feature is that the shape is retained due to the use of special tool geometries which

MAiN TECHNiQuES FOR SEVERE PLASTiC DEFORMATiON 9

prevent free flow of material and thereby produce a significant hydrostatic pressure The presence of this hydrostatic pressure is essential for achieving high strains and introducing high densities of lattice defects which are necessary for exceptional grain refinement

As already mentioned HPT and ECAP are the SPD techniques that were used in the early studies to produce nanostructured metals and alloys possessing submicron- or even nanosized grains [15 19] Since the time of the earliest experi-ments processing regimes and routes have been established for many metallic mate-rials including some low-ductility and hard-to-deform materials HPT and ECAP die-sets have also been essentially modernized [1 20] (see also Chapters 3 and 8) Several other techniques of SPD processing are now available as well The major methods for the fabrication of UFG materials that are already established together with HPT and ECAP include multidirectional forging (MDF) ARB cyclic extrusion and compression (CEC) repetitive corrugation and straightening (RCS) and TE These various processes have been intensively studied recently [1] and here their principles are outlined for comparison

High-pressure torsion refers to processing in which the sample generally in the form of a thin disk is subjected to torsional straining under a high hydrostatic pressure the principle of HPT is illustrated schematically in Figure 21 [1 21 22] The disk is located between anvils a hydrostatic pressure is applied and plastic tor-sional straining is achieved by rotation of one of the anvils Usually the applied pressures are higher than 2 GPa If there is no outward flow of material the disk thickness remains constant and the true torsional strain g is given by g = (rh)j where r is the distance from the center of the disk j is the torsional angle in radians and h is the sample thickness An alternative relationship is also available if there is

Figure 21 Principle of HPT


some outward flow of material between the two anvils and a corresponding reduction in the value of h [21] For comparison with other SPD methods the true equivalent strain e can be calculated using the relation e = (1a)∙g where the coefficient a takes either the values from a plastic flow criterion (where a = for Tresca and a = radic3 for von Mises) or from the Taylor theory for polycrystals (where a = 165 for texture-free face-centered cubic (fcc) metals and decreases slightly to lower values during continued deformation) The relatively small disks used in conventional HPT are attractive for products such as small bulk nanomagnets with enhanced soft and hard magnetic properties arterial stents and devices for microelectromechanical system applications There have also been recent attempts to extend HPT to include the processing of larger bulk samples [23]

Equal-channel angular pressing [13] is at present the most developed SPD processing technique The progress in ECAP processing has been discussed quite recently and has been reported elsewhere [20] As illustrated in Figure 22 during ECAP a rod-shaped billet is pressed through a die constrained within a channel which is bent at an abrupt angle

A shear strain is introduced when the billet passes through the point of intersection of the two parts of the channel Since the cross-sectional dimensions of the billet remain unchanged the pressings may be repeated to attain exceptionally high strains The equivalent strain e introduced in ECAP is determined by a relationship incorporating the angle between the two parts of the channel F and

Figure 22 Principle of ECAP


the angle representing the outer arc of curvature where the two parts of the channel intersect Y The relationship is given by [24]

2cot cosec 2 2 2 23

N Φ Ψ Φ Ψε Ψ = + + +

(21)

where N is the number of passes through the dieDuring repetitive pressings the shear strain is accumulated in the billet leading

ultimately to a UFG structure In practice different slip systems may be introduced by rotating the billet about its longitudinal axis between each pass [25] and this leads to four basic processing routes there is no rotation of the billet in route A rotations by 90deg in alternate directions or the same direction in routes B

A and B

C respectively

and rotations by 180deg in route C [26] When using a die with a channel angle of F = 90deg route B

C is generally the most expeditious way to develop a UFG structure

consisting of homogeneous and equiaxed grains with grain boundaries having high angles of misorientation

There have also been numerous recent modifications of conventional ECAP that are designed to yield more efficient grain refinement including the incorporation of a back pressure and the development of continuous processing by ECAP [20]

The technique of ARB makes use of a conventional rolling facility As illustrated in Figure 23 [27] a sheet is rolled so that the thickness is reduced to one-half of the thickness in a pre-rolled condition The rolled sheet is then cut into two halves that are stacked together To achieve good bonding during the rolling operation the two contact faces are degreased and wire brushed before placing them in contact and the stacked sheets are then rolled again to one-half thickness Thus a series of rolling

Surface treatment

Roll-bonding

Stacking Heating

12

Cutting

DegreasingWire-brushing

Figure 23 Schematic illustration of ARB [27]

12 DESCRIPTION OF SEVERE PLASTIC DEFORMATION (SPD) PRINCIPLES AND TECHNIQUES

cutting brushing and stacking operations are repeated so that ultimately a large strain is accumulated in the sheet It is possible to heat the sheet when rolling but at a temperature where there is no recrystallization For the ARB process the equivalent strain after N cycles e

N is given by e

N = 080middotN [27] In practice the UFG structure

produced by ARB is not three-dimensionally equiaxed but rather there is a pancake-like structure which is elongated in the lateral direction This microstructural feature is the same irrespective of the types of metals and alloys The ARB process has recently been applied successfully for the production of metal matrix composites [28]

MDF was applied for the first time in the first half of the 1990s for the formation of UFG structures in bulk billets [29 30] The process of MDF is usually associated with dynamic recrystallization in single-phase metalsalloys The principle of MDF is illustrated in Figure 24 and it assumes multiple repeats of free forging operations including setting and pulling with changes of the axes of the applied load The homogeneity of the strain produced by MDF is lower than in ECAP and HPT However the method can be used to obtain a nanostructured state in rather brittle materials because processing starts at elevated temperatures and the specific loads on

(b)(a) (c)

(d) (e) (f)

(g) (h) (i)

Figure 24 Principle of MDF (a) (b) and (c) show setting and pulling along the first axis (d) (e) and (f) show setting and pulling along the second axis and (g) (h) and (i) show setting and pulling along the third axis [29]













10 9 8 7 6 5 4 3 2 1


vii

Contents

PRefAce xiii

AcknowLedGmenTs xv

1 Introduction 1













viii CONTENTS



























CONTENTS ix































x CONTENTS



























CONTENTS xi








14 Conclusions 434

index 436

xiii

Preface



xiv PREFACE


xv

Acknowledgments



xvi ACKNOWLEDGMENTS



1

Introduction

Chapter 1


y is given by

12y 0 y k dσ σ minus= + (11)






2 iNTRODuCTiON






iNTRODuCTiON 3




4 iNTRODuCTiON



RefeRences







ampsynthesis



properties


characterization


REFERENCES 5
























6



Chapter 2














m)














2cot cosec 2 2 2 23

N Φ Ψ Φ Ψε Ψ = + + +

(21)



A and B

C respectively






Surface treatment

Roll-bonding

Stacking Heating

12

Cutting





N is given by e




(b)(a) (c)

(d) (e) (f)

(g) (h) (i)













10 9 8 7 6 5 4 3 2 1


vii

Contents

PRefAce xiii

AcknowLedGmenTs xv

1 Introduction 1













viii CONTENTS



























CONTENTS ix































x CONTENTS



























CONTENTS xi








14 Conclusions 434

index 436

xiii

Preface



xiv PREFACE


xv

Acknowledgments



xvi ACKNOWLEDGMENTS



1

Introduction

Chapter 1


y is given by

12y 0 y k dσ σ minus= + (11)






2 iNTRODuCTiON






iNTRODuCTiON 3




4 iNTRODuCTiON



RefeRences







ampsynthesis



properties


characterization


REFERENCES 5
























6



Chapter 2














m)














2cot cosec 2 2 2 23

N Φ Ψ Φ Ψε Ψ = + + +

(21)



A and B

C respectively






Surface treatment

Roll-bonding

Stacking Heating

12

Cutting





N is given by e




(b)(a) (c)

(d) (e) (f)

(g) (h) (i)










10 9 8 7 6 5 4 3 2 1


vii

Contents

PRefAce xiii

AcknowLedGmenTs xv

1 Introduction 1













viii CONTENTS



























CONTENTS ix































x CONTENTS



























CONTENTS xi








14 Conclusions 434

index 436

xiii

Preface



xiv PREFACE


xv

Acknowledgments



xvi ACKNOWLEDGMENTS



1

Introduction

Chapter 1


y is given by

12y 0 y k dσ σ minus= + (11)






2 iNTRODuCTiON






iNTRODuCTiON 3




4 iNTRODuCTiON



RefeRences







ampsynthesis



properties


characterization


REFERENCES 5
























6



Chapter 2














m)














2cot cosec 2 2 2 23

N Φ Ψ Φ Ψε Ψ = + + +

(21)



A and B

C respectively






Surface treatment

Roll-bonding

Stacking Heating

12

Cutting





N is given by e




(b)(a) (c)

(d) (e) (f)

(g) (h) (i)



vii

Contents

PRefAce xiii

AcknowLedGmenTs xv

1 Introduction 1













viii CONTENTS



























CONTENTS ix































x CONTENTS



























CONTENTS xi








14 Conclusions 434

index 436

xiii

Preface



xiv PREFACE


xv

Acknowledgments



xvi ACKNOWLEDGMENTS



1

Introduction

Chapter 1


y is given by

12y 0 y k dσ σ minus= + (11)






2 iNTRODuCTiON






iNTRODuCTiON 3




4 iNTRODuCTiON



RefeRences







ampsynthesis



properties


characterization


REFERENCES 5
























6



Chapter 2














m)














2cot cosec 2 2 2 23

N Φ Ψ Φ Ψε Ψ = + + +

(21)



A and B

C respectively






Surface treatment

Roll-bonding

Stacking Heating

12

Cutting





N is given by e




(b)(a) (c)

(d) (e) (f)

(g) (h) (i)


vii

Contents

PRefAce xiii

AcknowLedGmenTs xv

1 Introduction 1













viii CONTENTS



























CONTENTS ix































x CONTENTS



























CONTENTS xi








14 Conclusions 434

index 436

xiii

Preface



xiv PREFACE


xv

Acknowledgments



xvi ACKNOWLEDGMENTS



1

Introduction

Chapter 1


y is given by

12y 0 y k dσ σ minus= + (11)






2 iNTRODuCTiON






iNTRODuCTiON 3




4 iNTRODuCTiON



RefeRences







ampsynthesis



properties


characterization


REFERENCES 5
























6



Chapter 2














m)














2cot cosec 2 2 2 23

N Φ Ψ Φ Ψε Ψ = + + +

(21)



A and B

C respectively






Surface treatment

Roll-bonding

Stacking Heating

12

Cutting





N is given by e




(b)(a) (c)

(d) (e) (f)

(g) (h) (i)


viii CONTENTS



























CONTENTS ix































x CONTENTS



























CONTENTS xi








14 Conclusions 434

index 436

xiii

Preface



xiv PREFACE


xv

Acknowledgments



xvi ACKNOWLEDGMENTS



1

Introduction

Chapter 1


y is given by

12y 0 y k dσ σ minus= + (11)






2 iNTRODuCTiON






iNTRODuCTiON 3




4 iNTRODuCTiON



RefeRences







ampsynthesis



properties


characterization


REFERENCES 5
























6



Chapter 2














m)














2cot cosec 2 2 2 23

N Φ Ψ Φ Ψε Ψ = + + +

(21)



A and B

C respectively






Surface treatment

Roll-bonding

Stacking Heating

12

Cutting





N is given by e




(b)(a) (c)

(d) (e) (f)

(g) (h) (i)


CONTENTS ix































x CONTENTS



























CONTENTS xi








14 Conclusions 434

index 436

xiii

Preface



xiv PREFACE


xv

Acknowledgments



xvi ACKNOWLEDGMENTS



1

Introduction

Chapter 1


y is given by

12y 0 y k dσ σ minus= + (11)






2 iNTRODuCTiON






iNTRODuCTiON 3




4 iNTRODuCTiON



RefeRences







ampsynthesis



properties


characterization


REFERENCES 5
























6



Chapter 2














m)














2cot cosec 2 2 2 23

N Φ Ψ Φ Ψε Ψ = + + +

(21)



A and B

C respectively






Surface treatment

Roll-bonding

Stacking Heating

12

Cutting





N is given by e




(b)(a) (c)

(d) (e) (f)

(g) (h) (i)


x CONTENTS



























CONTENTS xi








14 Conclusions 434

index 436

xiii

Preface



xiv PREFACE


xv

Acknowledgments



xvi ACKNOWLEDGMENTS



1

Introduction

Chapter 1


y is given by

12y 0 y k dσ σ minus= + (11)






2 iNTRODuCTiON






iNTRODuCTiON 3




4 iNTRODuCTiON



RefeRences







ampsynthesis



properties


characterization


REFERENCES 5
























6



Chapter 2














m)














2cot cosec 2 2 2 23

N Φ Ψ Φ Ψε Ψ = + + +

(21)



A and B

C respectively






Surface treatment

Roll-bonding

Stacking Heating

12

Cutting





N is given by e




(b)(a) (c)

(d) (e) (f)

(g) (h) (i)


CONTENTS xi








14 Conclusions 434

index 436

xiii

Preface



xiv PREFACE


xv

Acknowledgments



xvi ACKNOWLEDGMENTS



1

Introduction

Chapter 1


y is given by

12y 0 y k dσ σ minus= + (11)






2 iNTRODuCTiON






iNTRODuCTiON 3




4 iNTRODuCTiON



RefeRences







ampsynthesis



properties


characterization


REFERENCES 5
























6



Chapter 2














m)














2cot cosec 2 2 2 23

N Φ Ψ Φ Ψε Ψ = + + +

(21)



A and B

C respectively






Surface treatment

Roll-bonding

Stacking Heating

12

Cutting





N is given by e




(b)(a) (c)

(d) (e) (f)

(g) (h) (i)


xiii

Preface



xiv PREFACE


xv

Acknowledgments



xvi ACKNOWLEDGMENTS



1

Introduction

Chapter 1


y is given by

12y 0 y k dσ σ minus= + (11)






2 iNTRODuCTiON






iNTRODuCTiON 3




4 iNTRODuCTiON



RefeRences







ampsynthesis



properties


characterization


REFERENCES 5
























6



Chapter 2














m)














2cot cosec 2 2 2 23

N Φ Ψ Φ Ψε Ψ = + + +

(21)



A and B

C respectively






Surface treatment

Roll-bonding

Stacking Heating

12

Cutting





N is given by e




(b)(a) (c)

(d) (e) (f)

(g) (h) (i)


xiv PREFACE


xv

Acknowledgments



xvi ACKNOWLEDGMENTS



1

Introduction

Chapter 1


y is given by

12y 0 y k dσ σ minus= + (11)






2 iNTRODuCTiON






iNTRODuCTiON 3




4 iNTRODuCTiON



RefeRences







ampsynthesis



properties


characterization


REFERENCES 5
























6



Chapter 2














m)














2cot cosec 2 2 2 23

N Φ Ψ Φ Ψε Ψ = + + +

(21)



A and B

C respectively






Surface treatment

Roll-bonding

Stacking Heating

12

Cutting





N is given by e




(b)(a) (c)

(d) (e) (f)

(g) (h) (i)


xv

Acknowledgments



xvi ACKNOWLEDGMENTS



1

Introduction

Chapter 1


y is given by

12y 0 y k dσ σ minus= + (11)






2 iNTRODuCTiON






iNTRODuCTiON 3




4 iNTRODuCTiON



RefeRences







ampsynthesis



properties


characterization


REFERENCES 5
























6



Chapter 2














m)














2cot cosec 2 2 2 23

N Φ Ψ Φ Ψε Ψ = + + +

(21)



A and B

C respectively






Surface treatment

Roll-bonding

Stacking Heating

12

Cutting





N is given by e




(b)(a) (c)

(d) (e) (f)

(g) (h) (i)


xvi ACKNOWLEDGMENTS



1

Introduction

Chapter 1


y is given by

12y 0 y k dσ σ minus= + (11)






2 iNTRODuCTiON






iNTRODuCTiON 3




4 iNTRODuCTiON



RefeRences







ampsynthesis



properties


characterization


REFERENCES 5
























6



Chapter 2














m)














2cot cosec 2 2 2 23

N Φ Ψ Φ Ψε Ψ = + + +

(21)



A and B

C respectively






Surface treatment

Roll-bonding

Stacking Heating

12

Cutting





N is given by e




(b)(a) (c)

(d) (e) (f)

(g) (h) (i)



1

Introduction

Chapter 1


y is given by

12y 0 y k dσ σ minus= + (11)






2 iNTRODuCTiON






iNTRODuCTiON 3




4 iNTRODuCTiON



RefeRences







ampsynthesis



properties


characterization


REFERENCES 5
























6



Chapter 2














m)














2cot cosec 2 2 2 23

N Φ Ψ Φ Ψε Ψ = + + +

(21)



A and B

C respectively






Surface treatment

Roll-bonding

Stacking Heating

12

Cutting





N is given by e




(b)(a) (c)

(d) (e) (f)

(g) (h) (i)


2 iNTRODuCTiON






iNTRODuCTiON 3




4 iNTRODuCTiON



RefeRences







ampsynthesis



properties


characterization


REFERENCES 5
























6



Chapter 2














m)














2cot cosec 2 2 2 23

N Φ Ψ Φ Ψε Ψ = + + +

(21)



A and B

C respectively






Surface treatment

Roll-bonding

Stacking Heating

12

Cutting





N is given by e




(b)(a) (c)

(d) (e) (f)

(g) (h) (i)


iNTRODuCTiON 3




4 iNTRODuCTiON



RefeRences







ampsynthesis



properties


characterization


REFERENCES 5
























6



Chapter 2














m)














2cot cosec 2 2 2 23

N Φ Ψ Φ Ψε Ψ = + + +

(21)



A and B

C respectively






Surface treatment

Roll-bonding

Stacking Heating

12

Cutting





N is given by e




(b)(a) (c)

(d) (e) (f)

(g) (h) (i)


4 iNTRODuCTiON



RefeRences







ampsynthesis



properties


characterization


REFERENCES 5
























6



Chapter 2














m)














2cot cosec 2 2 2 23

N Φ Ψ Φ Ψε Ψ = + + +

(21)



A and B

C respectively






Surface treatment

Roll-bonding

Stacking Heating

12

Cutting





N is given by e




(b)(a) (c)

(d) (e) (f)

(g) (h) (i)


REFERENCES 5
























6



Chapter 2














m)














2cot cosec 2 2 2 23

N Φ Ψ Φ Ψε Ψ = + + +

(21)



A and B

C respectively






Surface treatment

Roll-bonding

Stacking Heating

12

Cutting





N is given by e




(b)(a) (c)

(d) (e) (f)

(g) (h) (i)



6



Chapter 2














m)














2cot cosec 2 2 2 23

N Φ Ψ Φ Ψε Ψ = + + +

(21)



A and B

C respectively






Surface treatment

Roll-bonding

Stacking Heating

12

Cutting





N is given by e




(b)(a) (c)

(d) (e) (f)

(g) (h) (i)












m)














2cot cosec 2 2 2 23

N Φ Ψ Φ Ψε Ψ = + + +

(21)



A and B

C respectively






Surface treatment

Roll-bonding

Stacking Heating

12

Cutting





N is given by e




(b)(a) (c)

(d) (e) (f)

(g) (h) (i)







m)














2cot cosec 2 2 2 23

N Φ Ψ Φ Ψε Ψ = + + +

(21)



A and B

C respectively






Surface treatment

Roll-bonding

Stacking Heating

12

Cutting





N is given by e




(b)(a) (c)

(d) (e) (f)

(g) (h) (i)














2cot cosec 2 2 2 23

N Φ Ψ Φ Ψε Ψ = + + +

(21)



A and B

C respectively






Surface treatment

Roll-bonding

Stacking Heating

12

Cutting





N is given by e




(b)(a) (c)

(d) (e) (f)

(g) (h) (i)









2cot cosec 2 2 2 23

N Φ Ψ Φ Ψε Ψ = + + +

(21)



A and B

C respectively






Surface treatment

Roll-bonding

Stacking Heating

12

Cutting





N is given by e




(b)(a) (c)

(d) (e) (f)

(g) (h) (i)




2cot cosec 2 2 2 23

N Φ Ψ Φ Ψε Ψ = + + +

(21)



A and B

C respectively






Surface treatment

Roll-bonding

Stacking Heating

12

Cutting





N is given by e




(b)(a) (c)

(d) (e) (f)

(g) (h) (i)




N is given by e




(b)(a) (c)

(d) (e) (f)

(g) (h) (i)


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9781118095409 - download.e-bookshelf.de · Small Grain Sizes / 350 11. Mechanical Properties at High Temperatures 357 11.1. Achieving Superplasticity in Ultrafine-Grained Metals