Bulk Nanostructured Materials E-Book

Ruslan Z. Valiev

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 765

Veröffentlichungsjahr: 2013

Beschreibung

This book presents the most recent results in the area of bulk nanostructured materials and new trends in their severe plastic deformation (SPD) processing, where these techniques are now emerging from the domain of laboratory-scale research into the commercial production of various bulk nanomaterials. Special emphasis is placed on an analysis of the effect of nanostructures in materials fabricated by SPD on mechanical properties (strength and ductility, fatigue strength and life, superplasticity) and functional behavior (shape memory effects, magnetic and electric properties), as well as the numerous examples of their innovative applications. There is a high innovation potential for industrial applications of bulk nanomaterials for structural use (materials with extreme strength) as well as for functional applications such as nanomagnets, materials for hydrogen storage, thermoelectric materials, superconductors, catalysts, and biomedical implants.

Leseprobe

Contents

Preface

Acknowledgments

Chapter 1: Introduction

REFERENCES

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

2.1 A HISTORICAL RETROSPECTIVE OF SPD PROCESSING

2.2 MAIN TECHNIQUES FOR SEVERE PLASTIC DEFORMATION

2.3 SPD PROCESSING REGIMES FOR GRAIN REFINEMENT

2.4 TYPES OF NANOSTRUCTURES FROM SPD

REFERENCES

Part One: High-Pressure Torsion

Chapter 3: Principles and Technical Parameters of High-Pressure Torsion

3.1 A HISTORY OF HIGH-PRESSURE DEFORMATION

3.2 DEFINITION OF THE STRAIN IMPOSED IN HPT

3.3 PRINCIPLES OF UNCONSTRAINED AND CONSTRAINED HPT

3.4 VARIATION IN HOMOGENEITY ACROSS AN HPT DISK

3.5 INFLUENCE OF APPLIED LOAD AND ACCUMULATED STRAIN ON MICROSTRUCTURAL EVOLUTION

3.6 INFLUENCE OF STRAIN HARDENING AND DYNAMIC RECOVERY

3.7 SIGNIFICANCE OF SLIPPAGE DURING HIGH-PRESSURE TORSIONING

3.8 MODELS FOR THE DEVELOPMENT OF HOMOGENEITY IN HPT

REFERENCES

Chapter 4: HPT Processing of Metals, Alloys, and Composites

4.1 MICROSTRUCTURE EVOLUTION AND GRAIN REFINEMENT IN METALS SUBJECTED TO HPT

4.2 PROCESSING OF SOLID SOLUTIONS AND MULTIPHASE ALLOYS

4.3 PROCESSING OF INTERMETALLICS BY HPT

4.4 PROCESSING OF METAL MATRIX COMPOSITES

REFERENCES

Chapter 5: New Approaches to HPT Processing

5.1 CYCLIC PROCESSING BY REVERSING THE DIRECTION OF TORSIONAL STRAINING

5.2 USING HPT FOR THE COLD CONSOLIDATION OF POWDERS AND MACHINING CHIPS

5.3 EXTENSION OF HPT TO LARGE SAMPLES

REFERENCES

Part Two: Equal Channel Angular Pressing

Chapter 6: Development of Processing Using Equal-Channel Angular Pressing

6.1 CONSTRUCTION OF AN ECAP/ECAE FACILITY

6.2 EQUAL-CHANNEL ANGULAR PRESSING OF RODS,BARS, AND PLATE SAMPLES

6.3 ALTERNATIVE PROCEDURES FOR ACHIEVING ECAP: ROTARY DIES, SIDE-EXTRUSION, AND MULTIPASS DIES

6.4 DEVELOPING ECAP WITH PARALLEL CHANNELS

6.5 CONTINUOUS PROCESSING BY ECAP: FROM CONTINUOUS CONFINED SHEARING, EQUAL-CHANNEL ANGULAR DRAWING AND CONSHEARING, TO CONFORM PROCESS

REFERENCES

Chapter 7: Fundamental Parameters and Experimental Factors in ECAP

7.1 STRAIN IMPOSED IN ECAP

7.2 PROCESSING ROUTES IN ECAP

7.3 SHEARING PATTERNS ASSOCIATED WITH ECAP

7.4 EXPERIMENTAL FACTORS INFLUENCING ECAP

7.5 ROLE OF INTERNAL HEATING DURING ECAP

7.6 INFLUENCE OF A BACK PRESSURE

REFERENCES

Chapter 8: Grain Refinement in Metallic Systems Processed by ECAP

8.1 MESOSCOPIC CHARACTERISTICS AFTER ECAP

8.2 DEVELOPMENT OF AN ULTRAFINE-GRAINED MICROSTRUCTURE

8.3 FACTORS GOVERNING THE ULTRAFINE GRAIN SIZE IN ECAP

8.4 MICROSTRUCTURAL FEATURES AND TEXTURE AFTER ECAP

8.5 INFLUENCE OF ECAP ON PRECIPITATION

8.6 PRESSING OF MULTIPHASE ALLOYS AND COMPOSITES

8.7 CONSOLIDATION BY ECAP

8.8 POST-ECAP PROCESSING

REFERENCES

Part Three: Fundamentals and Properties of Materials After SPD

Chapter 9: Structural Modeling and Physical Properties of SPD-Processed Materials

9.1 EXPERIMENTAL STUDIES OF GRAIN BOUNDARIES IN BNM

9.2 DEVELOPMENT OF STRUCTURAL MODEL OF BNM

9.3 FUNDAMENTAL PARAMETERS AND PHYSICAL PROPERTIES

REFERENCES

Chapter 10: Mechanical Properties of BNM at Ambient Temperature

10.1 STRENGTH AND “SUPERSTRENGTH”

10.2 PLASTIC DEFORMATION AND DUCTILITY

10.3 FATIGUE BEHAVIOR

10.4 ALTERNATIVE DEFORMATION MECHANISMS AT VERY SMALL GRAIN SIZES

REFERENCES

Chapter 11: Mechanical Properties at High Temperatures

11.1 ACHIEVING SUPERPLASTICITY IN ULTRAFINE-GRAINED METALS

11.2 EFFECTS OF DIFFERENT ECAP PROCESSING ROUTES ON SUPERPLASTICITY

11.3 DEVELOPING A SUPERPLASTIC FORMING CAPABILITY

11.4 CAVITATION IN SUPERPLASTICITY AFTER SPD

11.5 FUTURE PROSPECTS FOR SUPERPLASTICITY IN NANOSTRUCTURED MATERIALS

REFERENCES

Chapter 12: Functional and Multifunctional Properties of Bulk Nanostructured Materials

12.1 CORROSION BEHAVIOR

12.2 WEAR RESISTANCE

12.3 ENHANCED STRENGTH AND CONDUCTIVITY

12.4 BIOMEDICAL BEHAVIOR OF NANOMETALS

12.5 ENHANCED MAGNETIC PROPERTIES

12.6 INELASTICITY AND SHAPE-MEMORY EFFECTS

12.7 OTHER FUNCTIONAL PROPERTIES

REFERENCES

Part Four: Innovation Potential and Prospects for SPD Applications

Chapter 13: Innovation Potential of Bulk Nanostructured Materials

13.1 NANOTITANIUM AND TI ALLOYS FOR MEDICAL IMPLANTS

13.2 NANOSTRUCTURED MG ALLOYS FOR HYDROGEN STORAGE

13.3 MICRO-DEVICES FROM BNM

13.4 INNOVATION POTENTIAL AND APPLICATION OF NANOSTRUCTURED AL ALLOYS

13.5 FABRICATION OF NANOSTRUCTURED STEELS FOR ENGINEERING

REFERENCES

Chapter 14: Conclusions

Supplement Images

Index

Copyright © 2014 by The Minerals, Metals & Materials Society.All rights reserved.

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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.    TA418.9.N35V35 2014    620.1′15–dc23

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

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 attractive for different structural and functional applications. In this research topic, the use of both “bottom-up” and “top-down” approaches for BNM processing/synthesis routes has received considerable attention. In the “bottom-up” approach, bulk nanomaterials are fabricated by assembling individual atoms or by consolidating nanoparticulate solids. The “top-down” 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 atoms/particles has been described in several books, there is at present no international monograph 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.

Acknowledgments

The use of bulk nanostructured metals and alloys as structural and functional materials 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 colleagues and friends from the International NanoSPD Steering Committee—Profs. Yuri Estrin, Zenji Horita, Michael Zehetbauer, Yuntian Zhu (www.nanospd.org), and other members of the materials science community actively working with us to publish 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 “Scientific and Educational Personnel of Innovative Russia” for the years 2009–2013 and by the Russian Federal Ministry for Education and Science (Contract №14.B25.31.0017), and in part by the Russian Foundation for Basic Research as well as the Department of Chemistry, Moscow State University (RZV).

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 Zarema’s hard work and dedication that we were able to overcome all obstacles and complete the manuscript on schedule.

Chapter 1

Introduction

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 Hall–Petch equation, which states that the yield stress, σy, is given by

(1.1)  

where σ0 is termed the friction stress and ky is a constant of yielding [1, 2]. It follows from Equation 1.1 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 (i.e., 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, 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 [4–10].

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 µm 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 submicrometer (100–1000 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 substructures, 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 100–300 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 “nanostructured materials.” In order to qualify as bulk nanostructured materials (BNM), the only additional requirements are that there exists a homogeneous distribution of nanostructural elements in the entire sample and the samples typically have 1000 or more grains/nanostructural 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 “bottom-up” and the “top-down” approaches [11, 12].

As was already noted earlier, in the “bottom-up” 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 applications. 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 “top-down” 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 procedures of metal forming that may be used to impose very high strains on materials 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 achieving 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 “bottom-up” 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 “top-down” 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 refinement 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 different 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 1.1 (redrawn from [22])—the 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 materials [23–27]. 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.

FIGURE 1.1 Principles of nanostructural design of BNM.

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 1951;64:747.

2. Petch NJ. J Iron Steel Inst 1953;174:25.

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

4. Valiev RZ, Islamgaliev RK, Alexandrov IV. Prog Mater Sci 2000;45:103.

5. Valiev RZ, Estrin Y, Horita Z, Langdon TG, Zehetbauer MJ, Zhu YT. JOM 2006;58(4):33.

6. Zehetbauer MJ, Zhu YT, editors. Bulk Nanostructured Materials. Weinheim: Wiley-VCH Verlag GmbH & Co. KGaA; 2009.

7. Valiev RZ, Hahn H, Langdon TG. Special Issue: Bulk Nanostructured Materials. Adv Eng Mater 2010;12:665–815.

8. Horita Z, editor. Nanomaterials by Severe Plastic Deformation. Uetikon-Zürich: Trans Tech; 2006.

9. Estrin Y, Maier HJ, editors. Nanomaterials by Severe Plastic Deformation IV. Uetikon-Zürich: Trans Tech; 2008.

10. Wang JT, Figueiredo RB, Langdon TG, editors. Nanomaterials by Severe Plastic Deformation: NanoSPD5. Uetikon-Zürich: Trans Tech; 2011.

11. Gleiter H. Acta Mater 2000;48:1.

12. Zhu YT, Lowe TC, Langdon TG. Scr Mater 2004;51:825.

13. Erb U, El-Sherik AM, Palumbo G, Aust KT. Nanostruct Mater 1993;2:383.

14. Koch CC, Cho YS. Nanostruct Mater 1992;1:207.

15. Luton MJ, Jayanth CS, Disko MM, Matras S, Vallone J. Mater Res Soc Symp Proc 1989;132:79.

16. Witkin DB, Lavernia EJ. Prog Mater Sci 2006;51:1.

17. Han BQ, Matejczyk D, Zhou F, Zhang Z, Bampton C, Lavernia EJ, Mohamed FA. Metall Mater Trans 2004;35A:947.

18. Valiev RZ, Krasilnikov NA, Tsenev NK. Mat Sci Eng 1991;A137:35.

19. Valiev RZ, Korznikov AV, Mulyukov RR. Mater Sci Eng A 1993;186:141.

20. Valiev RZ. Nat Mater 2004;3:511.

21. Sauvage X, Wilde G, Divinski SV, Horita Z, Valiev RZ. Mater Sci Eng A 2012;540:1.

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; 48:1–384. Oxford: Elsevier Ltd.

23. Ivanisenko Y, Darbandi A, Dasgupta S, Kruk R, Hahn H. Adv Eng Mater 2010;12:666.

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 2013;560:1.

26. Valiev RZ, Langdon TG. Adv Eng Mater 2010;12:677.

27. Estrin Y, Vinogradov AV. Acta Mater 2013;61:782.

Chapter 2

Description of Severe Plastic Deformation (SPD): Principles and Techniques

2.1 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 laboratories 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 definitions of the relevant terms that are used within this field of research [1]. Processing through the use of SPD is defined as “any method of metal forming under an extensive 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 refinement.”

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 B.C. and the Three States dynasty of 280 A.D., 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 ancient China as early as about 500 B.C. 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 separate 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 B.C. and 300 A.D. 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 technology 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 P.W. 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].

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 application of these devices provided the achievement of very large strains, with true strains exceeding 6–8, which resulted in strong microstructural refinement in metals. This procedure was later named as “high-pressure torsion” (HPT) [12]. Of additional 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 technique—ECAP [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 applications. Therefore, this new approach for grain refinement, based on the achievement of very large true strains with e ≥ 6–8 under high imposed pressures, was termed as “SPD” [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 10–15 years, there appeared a wide diversity of new SPD techniques: for example, accumulative roll bonding (ARB), multiaxial forging, and twist extrusion (TE) (see Section 2.2 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.

2.2 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 0.4 Tm) 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 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.

FIGURE 2.1 Principle of HPT.

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 experiments, processing regimes and routes have been established for many metallic materials, 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.

FIGURE 2.2 Principle of ECAP.

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 2.2, 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, ε, introduced in ECAP is determined by a ­relationship ­incorporating the angle between the two parts of the channel, Φ, and the angle ­representing the outer arc of curvature where the two parts of the channel intersect, Ψ. The relationship is given by [24]

FIGURE 2.3 Schematic illustration of ARB [27].

(2.1)  

where N is the number of passes through the die.

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].

FIGURE 2.4 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].

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 metals/alloys. The principle of MDF is illustrated in Figure 2.4, 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 tooling are relatively low. The choice of the appropriate temperature–strain rate regimes of deformation leads to the desired grain refinement. The operation is usually realized over the temperature interval of 0.1–0.5 Tm, where Tm is the absolute melting temperature, and it is useful for producing relatively large billets with UFG structures [31].

FIGURE 2.5 Principle of CEC [32].

RCS was introduced recently and the principle is illustrated in Figure 2.6 [34]. In a repetitive two-step process, the workpiece is initially deformed to a corrugated shape and then straightened between two flat platens using a processing cycle that may be repeated many times. The RCS facility illustrated in Figure 2.6 subjects the workpiece to both bending and shear, which promotes grain refinement. Processing by RCS was used to produce nanostructures in a copper sample with an average initial grain size of 760 µm [34, 35]. A similar procedure was used later for grain refinement of aluminum [36]. An advantage of RCS is that it can be adapted easily to current industrial rolling facilities. It is not difficult to machine a series of ­corrugating teeth into the rollers of a conventional rolling mill, thus enabling the RCS process, and this has the potential of producing nanostructured materials in a continuous and economical way [37]. The RCS technique is currently in the early stages, and further research is needed to develop the process to a mature SPD technique for producing nanostructured materials. One critical issue is the need to design equipment and processing schedules for improving microstructural homogeneity.

FIGURE 2.6 Principle of RCS [34, 35].

The use of TE for grain refinement was introduced in 2004 [38], and the principles are illustrated in Figure 2.7. During TE, a workpiece is pushed through an extrusion die whose cross section maintains its shape and size while it is twisted through a designated angle around its longitudinal axis. As a result, the workpiece regains its shape and size after each TE pass, and thus it is possible to repetitively process a sample for excellent grain refinement. A variety of cross-sectional shapes, but not circular geometries, are possible with this technique. In practice, and by analogy to HPT, the plastic strain is not uniform across the cross section, but the plastic strain increases with the distance from the axis so that the more distant regions have a finer grain size. This microstructural heterogeneity leads to inhomogeneous mechanical properties with the cross-sectional center having the lowest strength. It is anticipated that the microstructural homogeneity may improve with increasing numbers of TE passes.

FIGURE 2.7 Principle of TE [38].

To date, these recently developed SPD techniques have been typically used for laboratory-scale research. The requirement of economically feasible production of UFG metals and alloys that is necessary for successful commercialization raises several new problems in the development of SPD techniques. The most topical tasks are to reduce the material waste, to obtain uniform microstructure and properties in bulk billets and products, and to increase the efficiency of SPD processing. The ways of solving these tasks that have been recently developed on the basis of compre­hensive studies of physics and mechanics of SPD processes will be considered in the ­following sections.

2.3 SPD PROCESSING REGIMES FOR GRAIN REFINEMENT

For the formation of UFG structures with primarily high-angle grain boundaries through SPD processing, five basic rules for grain refinement have been defined [39], four of which are related to the requirements for SPD processing regimes and routes, while the fifth is related to the intrinsic nature of the material under study. These rules are briefly considered later. A detailed description of SPD processing regimes and routes may be found in recent overviews on the subject [20, 40, 41].

SPD processing at low temperatures (as a rule, less than 0.4 Tm) is referred to as a rather important requirement for its realization. Only under these conditions is it ­possible to achieve dislocation densities of 1014 m−2 or higher, up to the limiting values of 1016–1017 m−2 [40, 42], which is necessary for the formation of the UFG structure. Higher processing temperatures result in a lower accumulated dislocation density and an increase in grain size to more than one micron.

The degree of strain during processing (true strain) should exceed 6–8. Although a considerable refinement of the microstructure and the attainment of dislocation densities exceeding 1014 m−2 occur at a strain of 1/2 [40], the formation of UFG structures with a majority of high-angle grain boundaries requires further straining.

High hydrostatic pressures, usually more than 1 GPa, are important for efficient SPD processing. High pressure contributes to the enhancement of deformability of the processed material and therefore provides solidity of the billets even under high strain [1, 20]. Furthermore, the pressure affects the diffusion and thus suppresses the annihilation of deformation-induced lattice defects [43].

The formation of equiaxed ultrafine grains depends on the vorticity of the metal flow. At the macrolevel, the vorticity is related to the non-monotonous character of deformation. For example, the ECAP route BC, in which the billet is rotated by 90° between each pass, is considerably more effective for grain refinement by comparison with route C in which the billet position does not change [20]. At the microlevel, the vorticity is associated with grain rotations and displacements [44].

These five rules are required and typically sufficient conditions for effective grain refinement by SPD processing.

2.4 TYPES OF NANOSTRUCTURES FROM SPD

Although it is possible to achieve a nanocrystalline structure with a grain size less than 100 nm in a number of metals and alloys by means of HPT, for SPD processing, the formation of UFG structures with a mean grain size within the submicron range, that is, with grains 200–500 nm in size, is typical.

However, in processing by SPD, including ECAP, the formation of other structural elements takes place as well—that is, dislocation substructures, twins, grain boundary segregations, and precipitations that also produce a considerable influence on the properties of the materials after processing. Moreover, semi-products in the form of rods, wires, and sheets are produced by deformation and thermal treatment in sequence after ECAP, which additionally refines the microstructure and enhances the properties.

In general, we can single out four types of nanostructural elements in the metals and alloys produced by SPD. It is possible to observe such nanostructures by the application of modern techniques for structural analysis—high-resolution transmission electron microscopy (HRTEM), 3D atom probe, etc. [20, 40, 46, 47]. These four types of structures are as follows:

Nonequilibrium grain boundaries with dislocation arrays. For example, as illustrated in Figure 2.8 [48], an excessively high density of dislocations, facets, and steps is observed at grain boundaries of the UFG Al–3%Mg alloy after HPT, illustrating the nonequilibrium state of the grain boundaries with crystal lattice distortions of 5/7 nm in width near the boundaries [39, 40]. Such nonequilibrium grain boundaries are typical for different materials after SPD processing, and their role in the mechanical behavior of UFG materials has been stressed in a number of works [39, 40, 49].

FIGURE 2.8 TEM images of nonequilibrium grain boundaries in the UFG Al–3%Mg alloy [48] illustrating high-resolution photographs of regions (a) and (b).

FIGURE 2.9 (a) TEM image of a typical grain with a high density of deformation twins in UFG Cu processed by ECAP with consequent cold rolling. Inset: SAED pattern of the twinned area. (b) HREM image taken from the <110> zone axis. Inset: twin relationship (white lines) and a Frank dislocation at a twin boundary, marked by arrow. This inset is the inverse Fourier transform (FT) of the {111} diffraction spots in the area marked by the white rectangle [50].

Nanotwins, stacking faults, and intragranular cells. These nanostructural elements are typical for materials processed by HPT or ECAP at lower temperatures and/or those subjected to additional cold rolling, extrusion, and drawing. Figure 2.9 shows a TEM image of atomic resolution of UFG Cu after ECAP and cold rolling at liquid nitrogen temperature with clearly observed twins of 10–20 nm in size [50]. Such nanostructured defects also have a considerable effect on material strength [50, 51], for example, increasing the yield stress in UFG Cu from 380 to 510 MPa [50].

FIGURE 2.10 Grain boundary segregations in the HPT-processed 6061 Al alloy (of system Al–Mg–Si): (a) TEM image of the 6061 Al alloy, (b) grain size distribution, and (c) Mg, Cu, and Si distribution in a 3D reconstructed volume analyzed in the 6061 alloy by HPT (6 × 6 × 40 nm3) [46].

Segregation clusters and “clouds.” Recent investigations by a 3D atom probe directly testify to the formation of impurity as well as alloying element segregations at grain boundaries in UFG alloys processed by SPD [46, 47, 52]; see, for example, Figure 2.10 [46]. These segregations form “clouds” and clusters 3–5 nm in size and influence the formation and motion of dislocations, which provides additional strengthening of the alloys, in particular those based on aluminum, by more than 40% [46, 53].

FIGURE 2.11 UFG structure of the alloy Al-6061 after ECAP with parallel channels (4 passes). The formation of nanosized precipitations is clearly visible inside the grain after processing at selected areas (a) and (b) with larger magnification [55].

Nanosized particles: second-phase precipitations. The formation of particles has been observed in many alloys subjected to SPD after solution quenching [20, 54]. Figure 2.11 illustrates an example of such nanoparticles ~10–20 nm in size precipitated in the UFG alloy Al-6061 after ECAP [55]. The presence of nanoparticles originates from the dynamic aging and provides additional precipitation hardening of the alloys [20, 55].

Thus, the UFG metals and alloys processed by SPD techniques and, in particular, ECAP are characterized by a number of nanostructural elements that considerably influence their properties, as will be shown later. That is why these materials are referred to as the class of “bulk nanostructured materials,” and this definition is ­presently accepted by the international community (www. nanospd.org). It is important to note that the strength of such materials as shown in the next section may be considerably higher than anticipated from the Hall–Petch relation.

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Part One

High-Pressure Torsion

Chapter 3

Principles and Technical Parameters of High-Pressure Torsion

3.1 A HISTORY OF HIGH-PRESSURE DEFORMATION

The principle of achieving high strength in metallic alloys through the application of heavy plastic deformation has its origins more than 2000 years ago in the metalworking procedures developed during the Han dynasty (200B.C.) of ancient China [1]. Nevertheless, although this ancient type of processing has similarities with the modern procedures of equal-channel angular pressing (ECAP) and ARB, it was not a true precursor of high-pressure torsion (HPT) because it failed to incorporate any torsional straining. By contrast, the scientific origin of technique by HPT may be traced to a classic work by Bridgman and appearing in his several publications of early 1940s, in particular, the paper in the