Introduction 3 particularly at high temperatures,and this dynamic recovery plays an important role in the creep and hot working of materials. Recovery generally involves only a partial restoration of properties because the dislocation structure is not completely removed,but reaches a metastable state (fig.1.1b).A further restoration process called recrystallization may occur in which new dislocation-free grains are formed within the deformed or recovered structure (fig.1.Ic).These then grow and consume the old grains,resulting in a new grain structure with a low dislocation density,(fig.1.Id).Recrystallization may take place during deformation at elevated temperatures and this is then termed dynamic recrystallization. Although recrystallization removes the dislocations,the material still contains grain boundaries,which are thermodynamically unstable.Further annealing may result in grain growth,in which the smaller grains are eliminated,the larger grains grow,and the grain boundaries assume a lower energy configuration (fig.1.le).In certain circumstances this normal grain growth may give way to the selective growth of a few large grains (fig.1.1f),a process known as abnormal grain growth or secondary recrystallization. Recent research has shown that borderlines between the various annealing phenomena are often unclear,and it is known that recovery,recrystallization and grain growth may occur in two ways.They occur heterogeneously throughout the material,such that they may be formally described in terms of nucleation and growth stages,and in this case they are described as discontinuous processes.Alternatively,they may occur uniformly,such that the microstructures evolve gradually with no identifiable nucleation and growth stages.In this case,the prefix continuous is used to categorise the phenomena.It should be emphasised that this is a phenomenological categorisa- tion which does not imply the operation of any particular micromechanism.The continuous'phenomena include recovery by subgrain growth,continuous recrys- tallization and normal grain growth and the 'discontinuous'phenomena include discontinuous subgrain growth,primary recrystallization and abnormal grain growth Therefore,as shown in table 1.I there are at least six static annealing phenomena which need to be considered. Although these processes are analysed separately in later chapters,there are circumstances when they can be considered within a unified framework,as discussed in chapter 10.This has the merit of not only emphasising the common features of the Table 1.1 Examples of static annealing phenomena. Recovery Recrystallization Grain growth Continuous Subgrain Continuous Normal grain growth recrystallization growth Discontinuous Discontinuous Primary Abnormal grain subgrain growth recrystallization growth
particularly at high temperatures, and this dynamic recovery plays an important role in the creep and hot working of materials. Recovery generally involves only a partial restoration of properties because the dislocation structure is not completely removed, but reaches a metastable state (fig. 1.1b). A further restoration process called recrystallization may occur in which new dislocation-free grains are formed within the deformed or recovered structure (fig. 1.1c). These then grow and consume the old grains, resulting in a new grain structure with a low dislocation density, (fig. 1.1d). Recrystallization may take place during deformation at elevated temperatures and this is then termed dynamic recrystallization. Although recrystallization removes the dislocations, the material still contains grain boundaries, which are thermodynamically unstable. Further annealing may result in grain growth, in which the smaller grains are eliminated, the larger grains grow, and the grain boundaries assume a lower energy configuration (fig. 1.1e). In certain circumstances this normal grain growth may give way to the selective growth of a few large grains (fig. 1.1f ), a process known as abnormal grain growth or secondary recrystallization. Recent research has shown that borderlines between the various annealing phenomena are often unclear, and it is known that recovery, recrystallization and grain growth may occur in two ways. They occur heterogeneously throughout the material, such that they may be formally described in terms of nucleation and growth stages, and in this case they are described as discontinuous processes. Alternatively, they may occur uniformly, such that the microstructures evolve gradually with no identifiable nucleation and growth stages. In this case, the prefix continuous is used to categorise the phenomena. It should be emphasised that this is a phenomenological categorisation which does not imply the operation of any particular micromechanism. The ‘continuous’ phenomena include recovery by subgrain growth, continuous recrystallization and normal grain growth and the ‘discontinuous’ phenomena include discontinuous subgrain growth, primary recrystallization and abnormal grain growth. Therefore, as shown in table 1.1 there are at least six static annealing phenomena which need to be considered. Although these processes are analysed separately in later chapters, there are circumstances when they can be considered within a unified framework, as discussed in chapter 10. This has the merit of not only emphasising the common features of the Table 1.1 Examples of static annealing phenomena. Recovery Recrystallization Grain growth Continuous Subgrain growth Continuous recrystallization Normal grain growth Discontinuous Discontinuous subgrain growth Primary recrystallization Abnormal grain growth Introduction 3
Recrystallization various processes,but,in breaking down the conventional distinctions between the various annealing phenomena,allows also for the emergence of new phenomena which may not conveniently fit into the traditional categories. 1.1.2 The importance of annealing Many metallic materials are produced initially as large castings which are then further processed in the solid state by forging,rolling,extrusion etc.,to an intermediate or final product.These procedures,which may be carried out hot or cold,and which may involve intermediate anneals,are collectively termed thermomechanical processing. Recovery,recrystallization and grain growth are core elements of this processing To a large extent the mechanical properties and behaviour of a metal depend on the dislocation content and structure,the size of the grains and the orientation or texture of the grains.Of these,the dislocation content and structure are the most important.The mechanical properties depend primarily on the number of dislocations introduced during cold working and their distribution.As this increases from~10m,typical of the annealed state,tomtypical of heavily deformed metals,the yield strength is increased by up to 5-6 times and the ductility decreased.If the strain hardened material is subsequently heated to ~Tm/3 dislocation loss and rearrangement occur and this is manifested by a decrease in strength and increased ductility.There is an enormous literature on the magnitude of these changes and any adequate treatment is beyond the scope of this book.For details of these the reader is referred to the appropriate volumes of Metals Handbook. The grain size and texture after annealing are determined mainly by the recrystallization process,and there are numerous examples of the need to control grain size.For example,a small grain size increases the strength of a steel and may also make it tougher.However,a large grain size may be required in order to reduce creep rates in a nickel-based superalloy for use at high temperatures.Superplastic forming,in which alloys are deformed to large strains at low stresses,is becoming an important technological process for the shaping of advanced materials.Great ingenuity must be exercised in producing the required fine grain size and preventing its growth during high temperature deformation.The control of texture is vital for the successful cold forming of metals,a particularly important example being the deep drawing of aluminium or steel to produce beverage cans. 1.2 HISTORICAL PERSPECTIVE 1.2.1 The early development of the subject Although the art of metalworking including the procedures of deformation and heating has been practised for thousands of years,it is only comparatively recently that some understanding has been gained of the structural changes which accompany these processes.The early history of the annealing of deformed metals has been chronicled very elegantly by Beck(1963),and it is clear that the pace of scientific understanding
various processes, but, in breaking down the conventional distinctions between the various annealing phenomena, allows also for the emergence of new phenomena which may not conveniently fit into the traditional categories. 1.1.2 The importance of annealing Many metallic materials are produced initially as large castings which are then further processed in the solid state by forging, rolling, extrusion etc., to an intermediate or final product. These procedures, which may be carried out hot or cold, and which may involve intermediate anneals, are collectively termed thermomechanical processing. Recovery, recrystallization and grain growth are core elements of this processing. To a large extent the mechanical properties and behaviour of a metal depend on the dislocation content and structure, the size of the grains and the orientation or texture of the grains. Of these, the dislocation content and structure are the most important. The mechanical properties depend primarily on the number of dislocations introduced during cold working and their distribution. As this increases from 1011 m2 , typical of the annealed state, to 1016 m2 , typical of heavily deformed metals, the yield strength is increased by up to 5–6 times and the ductility decreased. If the strain hardened material is subsequently heated to Tm/3 dislocation loss and rearrangement occur and this is manifested by a decrease in strength and increased ductility. There is an enormous literature on the magnitude of these changes and any adequate treatment is beyond the scope of this book. For details of these the reader is referred to the appropriate volumes of Metals Handbook. The grain size and texture after annealing are determined mainly by the recrystallization process, and there are numerous examples of the need to control grain size. For example, a small grain size increases the strength of a steel and may also make it tougher. However, a large grain size may be required in order to reduce creep rates in a nickel-based superalloy for use at high temperatures. Superplastic forming, in which alloys are deformed to large strains at low stresses, is becoming an important technological process for the shaping of advanced materials. Great ingenuity must be exercised in producing the required fine grain size and preventing its growth during high temperature deformation. The control of texture is vital for the successful cold forming of metals, a particularly important example being the deep drawing of aluminium or steel to produce beverage cans. 1.2 HISTORICAL PERSPECTIVE 1.2.1 The early development of the subject Although the art of metalworking including the procedures of deformation and heating has been practised for thousands of years, it is only comparatively recently that some understanding has been gained of the structural changes which accompany these processes. The early history of the annealing of deformed metals has been chronicled very elegantly by Beck (1963), and it is clear that the pace of scientific understanding 4 Recrystallization
Introduction 5 was largely dictated by the development of techniques for materials characterisation.It should be noted that this constraint still applies. 1.2.1.1 Crystallinity and crystallization In 1829,the French physicist Felix Savart found that specimens from cast ingots of various metals exhibited acoustic anisotropy,and concluded that cast ingots consisted of crystals of different orientations.He also found that although the anisotropy was changed by plastic deformation and subsequent annealing,heating alone produced no change.This is the first recorded evidence for a structural change occurring during the annealing of a cold worked metal. In the mid 19th century,the concept of the crystallization of metals was extensively discussed,and it was widely thought that plastic deformation rendered metals amorphous.This belief arose from the inability to observe grain structures in the deformed metals using visual inspection.On reheating the deformed metal however,the grain structure could sometimes be seen(Percy 1864,Kalischer 1881),and this was then interpreted as crystallization of the metal from its amorphous state. The introduction of metallographic techniques by Sorby,culminating in his paper of 1887 took the subject a step forward.He was able to study the elongated grains in deformed iron and note that on heating,a new equiaxed grain structure was produced,a process which he termed recrystallization.Furthermore,Sorby recognised that the distorted grains must be unstable,and that recrystallization allowed a return to a stable condition.Despite Sorby's work.the idea that cold worked metals were amorphous persisted for some years and was not finally abandoned until the Bakerian lecture by Ewing and Rosenhain in 1900 in which it was clearly shown that plastic deformation took place by slip or twinning,and that in both of these processes the crystal structure was preserved. 1.2.1.2 Recrystallization and grain growth Although recrystallization had been identified by the beginning of the 20th century, recrystallization and grain growth had not clearly been distinguished as separate processes.The outstanding work by Carpenter and Elam(1920)and Altherthum(1922) established that stored energy provided the driving force for recrystallization,and grain boundary energy that for grain growth.This is shown by the terminology for these processes used by Altherthum-Bearbeitungsrekristallisation (cold-work recrystalliza- tion)and Oberflachenrekristallisation (surface tension recrystallization). In 1898 Stead had proposed that grain growth occurred by grain rotation and coalescence, and although Ewing and Rosenhain presented convincing evidence that the mechanism was one of boundary migration,Stead's idea was periodically revived until the work of Carpenter and Elam finally settled the matter in favour of boundary migration. 1.2.1.3 Parameters affecting recrystallization By around 1920,many of the parameters which affected the recrystallization process and the resultant microstructure had been identified
was largely dictated by the development of techniques for materials characterisation. It should be noted that this constraint still applies. 1.2.1.1 Crystallinity and crystallization In 1829, the French physicist Felix Savart found that specimens from cast ingots of various metals exhibited acoustic anisotropy, and concluded that cast ingots consisted of crystals of different orientations. He also found that although the anisotropy was changed by plastic deformation and subsequent annealing, heating alone produced no change. This is the first recorded evidence for a structural change occurring during the annealing of a cold worked metal. In the mid 19th century, the concept of the crystallization of metals was extensively discussed, and it was widely thought that plastic deformation rendered metals amorphous. This belief arose from the inability to observe grain structures in the deformed metals using visual inspection. On reheating the deformed metal however, the grain structure could sometimes be seen (Percy 1864, Kalischer 1881), and this was then interpreted as crystallization of the metal from its amorphous state. The introduction of metallographic techniques by Sorby, culminating in his paper of 1887 took the subject a step forward. He was able to study the elongated grains in deformed iron and note that on heating, a new equiaxed grain structure was produced, a process which he termed recrystallization. Furthermore, Sorby recognised that the distorted grains must be unstable, and that recrystallization allowed a return to a stable condition. Despite Sorby’s work, the idea that cold worked metals were amorphous persisted for some years and was not finally abandoned until the Bakerian lecture by Ewing and Rosenhain in 1900 in which it was clearly shown that plastic deformation took place by slip or twinning, and that in both of these processes the crystal structure was preserved. 1.2.1.2 Recrystallization and grain growth Although recrystallization had been identified by the beginning of the 20th century, recrystallization and grain growth had not clearly been distinguished as separate processes. The outstanding work by Carpenter and Elam (1920) and Altherthum (1922) established that stored energy provided the driving force for recrystallization, and grain boundary energy that for grain growth. This is shown by the terminology for these processes used by Altherthum – Bearbeitungsrekristallisation (cold-work recrystallization) and Oberfla¨chenrekristallisation (surface tension recrystallization). In 1898 Stead had proposed that grain growth occurred by grain rotation and coalescence, and although Ewing and Rosenhain presented convincing evidence that the mechanism was one of boundary migration, Stead’s idea was periodically revived until the work of Carpenter and Elam finally settled the matter in favour of boundary migration. 1.2.1.3 Parameters affecting recrystallization By around 1920, many of the parameters which affected the recrystallization process and the resultant microstructure had been identified. Introduction 5
Recrystallization Kinetics -The relationship of the recrystallization temperature to the melting temperature had been noted by Ewing and Rosenhain(1900),and Humfrey (1902) showed that the rate of recrystallization increased with an increase in annealing temperature. Strain-Sauveur (1912)found that there was a critical strain for recrystallization,and a relationship between grain size and prior strain was reported by Charpy (1910). Carpenter and Elam (1920)later quantified both of these effects. Grain growth-In a very early paper on the control of microstructure during annealing, Jeffries(1916)showed that abnormal grain growth in thoriated tungsten was promoted in specimens in which normal grain growth had been inhibited. Further developments in the understanding of recrystallization were not possible without a more detailed knowledge of the deformed state.This was provided by the development of the dislocation theory in 1934,and a notable early review of the subject following the advent of the dislocation theory is that of Burgers(1941). From about this period it becomes difficult to distinguish the papers of historical interest from the early key papers which are still relevant to current thinking,and the latter are cited as appropriate within the various chapters of this book.However,it may be helpful to the reader to have a source list of books, reviews and conferences on the subject from the past 50 years,and this is given below. 1.2.2 Some key literature (1952-2003) Monographs on Recrystallization Byrne,J.G.(1965),Recovery,Recrystallization and Grain Growth.McMillan, New York. Cotterill,P.and Mould,P.R.(1976),Recrystallization and Grain Growth in Metals. Surrey Univ.Press,London. Novikov,V.(1997).Grain Growth and Control of Microstructure and Texture in Polycrystalline Materials.CRC Press,Boca Raton. Multi-author,edited compilations on Recrystallization Himmel,L.(ed.),(1963),Recovery and Recrystallization of Metals.Interscience, New York. Margolin,H.(ed.),(1966),Recrystallization,Grain growth and Textures.ASM,Ohio, USA. Haessner,F.(ed.),(1978),Recrystallization of Metallic Materials.Dr.Riederer-Verlag, G.m.b.H Stuttgart
Kinetics – The relationship of the recrystallization temperature to the melting temperature had been noted by Ewing and Rosenhain (1900), and Humfrey (1902) showed that the rate of recrystallization increased with an increase in annealing temperature. Strain – Sauveur (1912) found that there was a critical strain for recrystallization, and a relationship between grain size and prior strain was reported by Charpy (1910). Carpenter and Elam (1920) later quantified both of these effects. Grain growth – In a very early paper on the control of microstructure during annealing, Jeffries (1916) showed that abnormal grain growth in thoriated tungsten was promoted in specimens in which normal grain growth had been inhibited. Further developments in the understanding of recrystallization were not possible without a more detailed knowledge of the deformed state. This was provided by the development of the dislocation theory in 1934, and a notable early review of the subject following the advent of the dislocation theory is that of Burgers (1941). From about this period it becomes difficult to distinguish the papers of historical interest from the early key papers which are still relevant to current thinking, and the latter are cited as appropriate within the various chapters of this book. However, it may be helpful to the reader to have a source list of books, reviews and conferences on the subject from the past 50 years, and this is given below. 1.2.2 Some key literature (1952–2003) Monographs on Recrystallization Byrne, J.G. (1965), Recovery, Recrystallization and Grain Growth. McMillan, New York. Cotterill, P. and Mould, P.R. (1976), Recrystallization and Grain Growth in Metals. Surrey Univ. Press, London. Novikov, V. (1997), Grain Growth and Control of Microstructure and Texture in Polycrystalline Materials. CRC Press, Boca Raton. Multi-author, edited compilations on Recrystallization Himmel, L. (ed.), (1963), Recovery and Recrystallization of Metals. Interscience, New York. Margolin, H. (ed.), (1966), Recrystallization, Grain growth and Textures. ASM, Ohio, USA. Haessner, F. (ed.), (1978), Recrystallization of Metallic Materials. Dr. Riederer-Verlag, G.m.b.H Stuttgart. 6 Recrystallization
Introduction Review articles and books containing chapters on Recrystallization Burke,J.E.and Turnbull,D.(1952),Recrystallization and Grain Growth.Progress in Metal Phys.,3,220. Beck,P.A.(1954),Annealing of Cold-worked Metals.Adv.Phys.,3,245. Leslie,W.C.,Michalak,J.T.and Aul,F.W.(1963),The annealing of cold-worked iron. In:Iron and its Dilute Solid Solutions.(eds.)Spencer and Werner.Interscience.New York.119. Christian,J.W.(2002),The Theory of Transformations in Metals and Alloys.2nd edition, Pergamon,Oxford. Jonas,J.J.,Sellars,C.M.and Tegart,W.J.McG.(1969),Strength and Structure Under Hot Working conditions.Met.Revs.,130,1. Martin,J.W.and Doherty,R.D.(1976),The Stability of Microstructure in Metals. Cambridge University Press. Cahn,R.W.(1996),in Physical Metallurgy.(eds.)Cahn and Haasen.4th edition.North- Holland,Amsterdam. Hutchinson,W.B.(1984),Development and Control of Annealing Textures in Low- Carbon Steels.Int.Met.Rev.,29,25. Honeycombe,R.W.K.(1985),The Plastic Deformation of Metals.Edward Arnold. Humphreys,F.J.(1991).Recrystallization and Recovery.In:Processing of Metals and Alloys.(ed.)R.W.Cahn.VCH.Germany.371. Doherty,R.D.,Hughes,D.A.,Humphreys,F.J.,Jonas,J.J.,Juul Jensen,D.,Kassner, M.E.,King,W.E.,McNelly,T.R..McQueen,H.J.and Rollett,A.D.(1997),Current issues in recrystallization:a review.Mats.Sci.Eng.,A238,219. Proceedings of International Conferences International Recrystallization Conference Series (1990-1999) Chandra,T.(ed.),(1991),Recrystallization'90.TMS,Warrendale,USA. Fuentes,M.and Gil Sevillano,J.(eds.),(1992),Recrystallization'92.Trans.Tech.Pubs. Switzerland. McNelley,T.R.(ed.),(1997).Recrystallization and Related Annealing Phenomena -Rex'96. Sakai,T.and Suzuki,H.G.(1999),Recrystallization and Related Phenomena-Rex'99. Japan Inst.Metal. International Grain Growth Conferences (1991-1998) Abbruzzese,G.and Brozzo,P.(eds.),(1991).Grain Growth in Polycrystalline Materials. Trans.Tech.Publications.Switzerland
Review articles and books containing chapters on Recrystallization Burke, J.E. and Turnbull, D. (1952), Recrystallization and Grain Growth. Progress in Metal Phys., 3, 220. Beck, P.A. (1954), Annealing of Cold-worked Metals. Adv. Phys., 3, 245. Leslie, W.C., Michalak, J.T. and Aul, F.W. (1963), The annealing of cold-worked iron. In: Iron and its Dilute Solid Solutions. (eds.) Spencer and Werner. Interscience. New York. 119. Christian, J.W. (2002), The Theory of Transformations in Metals and Alloys. 2nd edition, Pergamon, Oxford. Jonas, J.J., Sellars, C.M. and Tegart, W.J. McG. (1969), Strength and Structure Under Hot Working conditions. Met. Revs., 130, 1. Martin, J.W. and Doherty, R.D. (1976), The Stability of Microstructure in Metals. Cambridge University Press. Cahn, R.W. (1996), in Physical Metallurgy. (eds.) Cahn and Haasen. 4th edition. NorthHolland, Amsterdam. Hutchinson, W.B. (1984), Development and Control of Annealing Textures in LowCarbon Steels. Int. Met. Rev., 29, 25. Honeycombe, R.W.K. (1985), The Plastic Deformation of Metals. Edward Arnold. Humphreys, F.J. (1991), Recrystallization and Recovery. In: Processing of Metals and Alloys. (ed.) R.W. Cahn. VCH. Germany. 371. Doherty, R.D., Hughes, D.A., Humphreys, F.J., Jonas, J.J., Juul Jensen, D., Kassner, M.E., King, W.E., McNelly, T.R., McQueen, H.J. and Rollett, A.D. (1997), Current issues in recrystallization: a review. Mats. Sci. & Eng., A238, 219. Proceedings of International Conferences International Recrystallization Conference Series (1990–1999) Chandra, T. (ed.), (1991), Recrystallization’90. TMS, Warrendale, USA. Fuentes, M. and Gil Sevillano, J. (eds.), (1992), Recrystallization’92. Trans. Tech. Pubs. Switzerland. McNelley, T.R. (ed.), (1997), Recrystallization and Related Annealing Phenomena – Rex’96. Sakai, T. and Suzuki, H.G. (1999), Recrystallization and Related Phenomena – Rex’99. Japan Inst. Metal. International Grain Growth Conferences (1991–1998) Abbruzzese, G. and Brozzo, P. (eds.), (1991). Grain Growth in Polycrystalline Materials. Trans. Tech. Publications, Switzerland. Introduction 7