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Fine-grained alloys by thermomechanical processing
Current Opinion in Solid State and Materials Science 5 (2001) 15–21
Fine-grained alloys by thermomechanical processing F. John Humphreys*, Philip B. Prangnell, Ronald Priestner Manchester Materials Science Centre, UMIST & University of Manchester, Grosvenor Street, Manchester, M1 7 HS, UK
Abstract Grain refinement during thermomechanical processing is conventionally achieved by discontinuous recrystallization. However, techniques involving severe deformation which increase the grain boundary area and can result in a micron-scale grain size by a process of continuous recrystallization have been developed. Various methods which are based on control of the deformation conditions and cooling rate during hot rolling of steels have utilised the a –g phase transformation to produce micron-grained ferrite. Ultra-fine-grain structures exhibit interesting mechanical properties, and in certain cases a large amount of strengthening may be achieved with little or no reduction in ductility. 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction The properties of a metal are strongly influenced by the grain size and the well-known Hall–Petch equation which predicts an increase in yield strength (sy ) with a decrease in grain size (d) has been shown to be applicable to a wide variety of metals.
sy 5 s0 1 kd 21 / 2
(1)
Other examples of the benefits of grain refinement are improvements in the fracture resistance of steels and other alloys, and the phenomenon of superplasticity which can be exhibited in metals with grain sizes less than |10 mm. One of the reasons for the current interest in very fine grained metallic materials is the concept that control of the mechanical properties by processing may be more desirable than the current practice of controlling properties by alloying. This would result in the use of fewer and simpler industrial alloys and would lead to economic benefits as well as improved recyclability. Very small grain sizes may be produced directly from the melt or from the vapour. The grain size decreases as the solidification rate increases and very small grains or even amorphous metals may be produced under extreme conditions. However, because the size of the product is small, i.e. powder, wire or ribbon, further consolidation and processing is required to produce a bulk material. There is currently very great interest in the field of metallic *Corresponding author. Tel.: 144-0161-200-3554; fax: 144-0161200-8877. E-mail address: [email protected] (F. . Humphreys).
and non-metallic nanophase materials which may comprise grains which are only a few atoms in diameter [1]. Such materials often have impressive physical and mechanical properties but because they can generally only be produced in very small quantities, these materials will not be considered in detail in this review.
2. The limits of conventional thermomechanical processing routes There is usually a conflict between the grain size which can be produced in a material, the amount or the dimensions of the material which can be so processed, and the cost of processing. For structural applications, which generally require material of reasonable dimensions in substantial quantities and at low cost, thermomechanical processing, involving the deformation and annealing of bulk alloys is usually considered to be the optimum method for producing fine-grained alloys. Conventional thermomechanical processing of alloys generally results in grain sizes of the order of 30–250 mm [2], and the production of finer grain structures requires careful control of the material and the processing parameters [*3,4] as is discussed in this paper.
2.1. Discontinuous recrystallization Fine-grain microstructures can be produced by the discontinuous recrystallization of a cold worked metal. A small grain size is promoted by a large stored energy resulting from deformation and a large density of sites for nucleating recrystallization [2]. The smallest grain sizes in
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F. J. Humphreys et al. / Current Opinion in Solid State and Materials Science 5 (2001) 15 – 21
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aluminium are typically achieved by deformation to large strains of alloys containing second-phase particles larger than |1 mm, and subsequent annealing to stimulate recrystallization [2]. The commercial Rockwell process which produces |10 mm grains in AA7xxx alloys for superplastic applications is based on this principle [5], and similar grain sizes may be achieved by large strain deformation and annealing of commercial Al–Mg–Fe (AA5xxx) alloys (e.g. Ref. [2]). Ferrite has a high stacking fault energy, so that recovery processes precede and accompany recrystallization of cold rolled ferrite [6] and consequently, it is difficult to obtain grain sizes less than about 10 mm in ferritic steels by discontinuous recrystallization. However, it has recently been shown [**7] that cold rolling and annealing of steel with a pre-refined grain size of |3 mm resulted in a recrystallized grain size of less than 1 mm. This is thought to be attributable to the large number of recrystallization nucleation sites available at pre-existing grain boundaries.
2.2. Continuous recrystallization If a metal is given a large strain at intermediate or high temperatures, a microstructure containing predominantly high angle grain boundaries may evolve with little or no further annealing. Such a microstructure is virtually indistinguishable from one which has been conventionally recrystallized, but because it has evolved gradually and uniformly throughout the material, this phenomenon is generally known as continuous recrystallization. An example of a continuous recrystallization process in a compressed metal is shown schematically in Fig. 1. During hot deformation of a polycrystalline metal, subgrains develop within the grains and the high angle boundaries become corrugated (Fig. 1a). At larger strains (Fig. 1b) the high angle boundaries are pushed together and, because the subgrain size remains approximately constant during deformation, they are separated by fewer subgrains. Eventually, the separation of the high angle boundaries is equal to the subgrain size, the high angle boundaries impinge and a microstructure of small and almost equiaxed grains is formed (Fig. 1c). This process, known as geometric dynamic recrystallization [8] has been extensively studied in aluminium alloys [2]. The conditions under which geometric dynamic recrystallization can occur are a function of the initial grain size and the deformation strain, temperature and strain rate and are shown in Fig. 2 in which the deformation temperature (T ) and strain rate ( ´~ ) have been combined into a single term, the Zener–Hollomon parameter (Z)
S D
Q Z 5 ´~ ? exp ] RT
Fig. 1. Schematic diagram of geometric dynamic recrystallization.
with a large initial grain size (D1 ) is deformed under conditions of low Z (subgrain size d 1 ), then the grain boundary spacing decreases with strain until impingement occurs at A. The size of the grains so formed (D*) is approximately equal to the subgrain size and the strain required is ´A . If the initial grain size were smaller – D2 ,
(2)
The subgrain size is inversely related to Z, being small at low deformation temperatures and high strain rates (large Z) and vice versa. It is seen from Fig. 2 that if an alloy
Fig. 2. Processing conditions for the formation of a stable grain structure during hot deformation.
F. J. Humphreys et al. / Current Opinion in Solid State and Materials Science 5 (2001) 15 – 21
then geometric recrystallization would occur at a lower strain (´B ) but would result in the same grain size. A smaller final grain size (D**) can be achieved by deformation at lower temperatures and strain rates (larger Z), but this will require a larger strain (´C ). Therefore, in order to achieve a small grain size at reasonable strains, a smaller initial grain size (D2 ) is required, together with a larger Z so that the grain boundary spacing and subgrain size intersect at ´D in Fig. 2. A more formal analysis of the conditions under which fine grain structures are expected to be formed has been given by Humphreys [*9]. It has long being known that the development of finegrain superplastic microstructures in Al–Cu–Zr (Supral) alloys occurred by continuous recrystallization [10]. Earlier work suggested that this process involved progressive rotation of subgrains during deformation (see Ref. [2]), but recent research [*11,*12] in which the grain boundaries have been characterised by electron backscatter diffraction (EBSD), has shown that the mechanism is essentially one of high angle boundary accumulation of the type shown in Fig. 1. Very large strain deformation of Al–Fe–Si and similar alloys at room temperature has been shown to result in sub-micron grain structures [*3,*13–*15]. On low temperature annealing, a sub-micron grain structure evolves by a continuous process and the structures are resistant to discontinuous recrystallization. It is thought that the stability of these structures is attributable to the large fraction of high angle grain boundaries in the deformed microstructure [*9,16], and their formation is therefore very similar to the process of geometric dynamic recrystallization discussed above. An important difference between deformation at low and elevated temperatures is that at lower temperatures, grains, particularly large ones, will subdivide or fragment by inhomogeneous deformation [2,*3], thus producing a larger amount of high angle grain boundary than the simple geometric increase discussed above. Additionally, during low temperature deformation, the large secondphase particles present in most commercial aluminium alloys will lead to inhomogeneous local deformation [*3] which will assist the break-up of the grains. Fine-grain microstructures have a large area of grain boundary and therefore a large stored energy, and are intrinsically unstable with respect to grain growth during high temperature annealing or even during the high temperature deformation operation during which they are formed. A dispersion of second-phase particles is generally required to prevent grain growth [1–4,15,17,18], although abnormal or discontinuous grain growth may occur [*15], and the effectiveness of second-phase particles (defined in terms of the volume fraction FV divided by the particle diameter x) in preventing either normal or abnormal grain growth is shown in Fig. 3. However, it has been found that a dispersion of very small particles may, at low deformation temperatures also prevent geometric dynamic re-
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Fig. 3. The predicted regimes of no growth, normal and abnormal growth for fine-grained alloys containing a volume fraction FV of particles of diameter x.
crystallization by impeding the migration of the boundaries which is required for operation of the mechanism [19,*20]. Because of the various constraints on the formation of fine-grain structures by continuous recrystallization and on their stability, there is a restricted process window for their formation as shown schematically in Fig. 4, which is based on an experimental investigation of an Al–2%Mg–Cr alloy [*21] deformed to a von Mises strain of 3 in plane strain. The smallest grains will be formed at low temperatures and large strain rates, but there is a limit (A–B–C) imposed by the condition for geometric dynamic recrystallization discussed above, and at higher temperatures, there are insufficient second-phase particles to prevent grain growth during the processing (C–D). The small process window
Fig. 4. The process window for producing an ultra-fine grain structure in Al–2%Mg by plane strain deformation to a strain of 3. 2000, with permission from IoM Communications Ltd [21].
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means that it would be difficult to process such a material to a 1 mm grain size in a commercial rolling mill.
3. Severe deformation processing The production of micron-grained alloys by conventional thermomechanical processing technology is limited by the achievable strains, which are typically |3–4 if the resultant material is to have a minimum thickness of |1 mm. Several processing routes which allow larger strains have been developed and are discussed below. The mechanisms by which the original grain structure is broken up are thought to be similar to those discussed in the previous section although the more complicated deformation geometries have a significant effect. A technique which increases the available strain is accumulative roll bonding(ARB) in which the material is rolled, stacked and re-rolled, thus maintaining the sample thickness, and this technique has been applied to aluminium alloys [*22] and steels [23] with total strains of up to 8, resulting in sub-micron grain sizes. Critical factors in successful accumulated roll bonding are surface preparation and cleaning, the deformation temperature, and the amount of strain. There has been a substantial recent research effort in developing laboratory scale methods which, by imposing complex and redundant strains, can achieve very large total strains whilst retaining a specimen of reasonable dimensions. One method which has been used with a wide range of materials is that of torsion under hydrostatic pressure, adapted from the Bridgeman anvil [24–26]. In this technique (Fig. 5a), a thin disc is deformed in torsion using the friction provided by the application of a large hydrostatic pressure. The equivalent strains that have been induced with this method are typically of the order of seven, and grain sizes as fine as 0.2 mm have been produced by deformation at room temperature. An alternative method is that of Equal Channel Angular Extrusion (ECAE) developed in the former Soviet Union by
Segal and colleagues [27]. During ECAE the sample is extruded in a closed die which has two intersecting channels of equal size (Fig. 5b) offset at an angle 2f. Assuming ideal conditions and a sharp die corner, the sample will be subjected to a homogenous shear of 2 cotf, and the process may be repeated, thus giving a large cumulative strain to the sample. Alternatively a die which gives several ECAE deformations in a single pass may be used [*28]. There are a number of factors including friction and die shape which make ECAE deformation more complicated than indicated above, and the deformation behaviour in the die has been numerically modelled [29,**30], and the effects of friction and back pressure on the homogeneity of shear have been investigated [**30]. Total strains of |10 are readily achievable and because the sample is constrained, the process can even be used for less ductile materials. The die angle is an important factor in ECAE processing as not only does it determine the strain per pass, but it also affects the geometry of deformation [**31]. In ECAE it is not necessary to keep the work piece orientation the same for each repeated pass, and several permutations are possible, the most common being either no sample rotation or a rotation of 908 between passes. It has been claimed that a 908 rotation between passes is most effective in breaking up the microstructure and producing an equiaxed sub-micron grain structure [32,33] when a 908 die is used. However, it has been found that for a 1208 die, the grain structure is more effectively broken up and a higher fraction of high angle grain boundaries is formed if the sample orientation is maintained during successive passes [**34]. An understanding of the formation of sub-micron grain structures during processing requires detailed quantitative microstructural characterisation. In many investigations (e.g. Refs. [31–33]) the arcing of diffraction patterns in the TEM has been used to obtain a qualitative assessment of the grain structure. However, more detailed characterisation such as the determination of grain boundary character and the percentage of high angle boundaries etc., is only
Fig. 5. Methods of redundant strain processing. (a) Torsion under hydrostatic pressure. (b) Equal channel angular extrusion.
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obtainable with high resolution Electron Backscatter Diffraction (EBSD) using a Field Emission SEM [2–4,34,35].
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ferrite in a duplex a –g microstructure appears to be an important factor.
5. Mechanical properties of fine-grained alloys 4. Grain refinement of steels Sub-micron grain structures have been produced in steels by the severe deformation processes of ECAE [36] and ARB [23] discussed above, and also by the discontinuous recrystallization of pre-refined ferrite (Section 2.1). However, control of the austenite to ferrite transformation can also be used to refine the ferrite grain size during hot rolling, and grain sizes of |5 mm are produced commercially using this method. Deformation during the a –g transformation will affect the phase transformation and may also result in dynamic or static recrystallization of the austenite and ferrite. A summary of recent research which has concentrated on controlling the phase transformation and recrystallization processes so as to produce ultra-fine grained ferrite is given by Priestner and Ibraheem [**7]. Hodgson and colleagues [*37,38] have produced 1 mm ferrite in the surface layers of steel sheet by utilising the large surface strains resulting from roll friction, together with roll chilling, to intergranularly nucleate a high density of ferrite grains during hot rolling of austenite. Priestner and Ibraheem have shown that a refined starting austenite grain size, combined with accelerated cooling, can also produce a 300 mm layer of grains less than 1 mm in a Nb–microalloyed steel [**7].. An alternative approach has been to refine the ferrite grain size by various routes involving warm rolling at |6508C [**7,39–41]. The mechanisms involved in producing micron-grained ferrite by this method are not completely understood, but dynamic recrystallization of the
Although the Hall–Petch relationship of Eq. 1 has been found to apply to most conventional alloys with grain sizes larger than |5 mm, it has been reported that there can be a significant deviation from the d 21 / 2 dependence, leading to lower than predicted yield stresses at submicron grain sizes [42]. However, more recent investigations of aluminium alloys deformed by ECAE have confirmed the validity of the Hall–Petch relationship for grain sizes as small as |0.2 mm [*43,*44] as shown in Fig. 6. The 10-fold increase in yield strength in this alloy, and comparable increases found for AA1100 and AA3004 alloys [*43] due to the grain size reduction, show that such processing can confer on alloys of intrinsic low or moderate strength, properties typical of high strength age-hardened aluminium alloys. Similarly large increases have been found for the hardness [**7] and yield strength [23] of ultra-fine grained steels and strengths of up to |900 MPa have been reported. Hardness results [**7] suggest that the Hall– Petch relationship remains valid for Nb steels of grain sizes down to |0.5 mm, albeit with a lower slope, k in Eq. (1), than at larger grain sizes. It is found that other mechanical properties are also markedly affected by a very small grain size. For example, the work hardening rate is generally lower, leading to a reduced uniform elongation in both aluminium alloys [45] and steels [23]. In aluminium alloys the Luders strain may be more pronounced in very fine-grained material [46] and for the samples of Fig. 6, the materials with the smallest grain size exhibited a Luders strain of |7% [*44]. Although the ductility of the materials with the smallest
Fig. 6. Hall–Petch plot for Al–3%Mg. Copyright 2000, with permission of IoM Communications Ltd [44].
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grain sizes may be only a few percent, low temperature annealing in the range 100–2008C has been shown to restore the ductility whilst still retaining much of the strengthening [*43,*44].
6. Summary Recent research has shown that novel thermomechanical processing routes involving large strain deformation may be used to produce micron or sub-micron grain sizes in aluminium alloys and steels, and in some cases a good combination of mechanical properties may be obtained. If such processes can be developed to operate cost-effectively at a large scale, then the economic and environmental benefits of controlling the properties of commercial alloys by processing rather than alloying can be realised.
References Papers of particular interest, published within the annual period of review, have been highlighted as: * of special interest; ** of outstanding interest. [1] Gleiter H. Nanostructured materials: basic concepts and microstructure. Acta Mater 2000;48:1–30. [2] Humphreys FJ, Hatherly M. Recrystallization and related annealing phenomena, Oxford: Pergamon Press, 1995. [*3] Humphreys FJ, Prangnell PB, Bowen JR, Gholinia A, Harris C. Developing stable fine-grain microstructures by large strain deformation. Philos Trans Royal Soc 1999;A357:1663–80. This paper reviews the methods of achieving fine grain structures by thermomechanical processing and discusses the underlying physical mechanisms. [4] Humphreys FJ, Prangnell PB, Priestner R. In: Sakai T, Suzuki H (editors). Fine-grained alloys by thermomechanical processing. Proc. Rex99, Tsukuba, 1999:69–78. [5] Wert JA, Paton NE, Hamilton CH, Mahoney MW. Grain refinement in 7075 aluminium by thermomechanical processing. Metall Trans 1981;12A:1267–76. [6] Leslie WC, Michalak JT, Aul FW. The annealing of cold-worked iron. In: Spencer CW, Werner FE, editors, Iron and its dilute solid solutions, New York: Interscience, 1963, pp. 119–216. [**7] Priestner R, Ibraheem AK. Processing of steel for ultra-fine ferrite grain structures. Mater Sci Technol 2000. (in press). This work demonstrates three methods of producing sub-micron grained ferrite in a low-carbon, microalloyed steel by rolling: by transformation from hot-worked austenite, employing accelerated cooling; by dynamic recrystallization of ferrite during warm working; and by cold rolling and recrystallization of ferrite pre-processed to an intermediate grain size of 3 mm. [8] McQueen HJ, Knustad O, Ryum N, Solberg JK. Microstructural evolution in Al deformed to strains of 60 at 4008C. Scr Met 1985;19:73–8. [*9] Humphreys FJ. A unified theory of recovery, recrystallization and grain growth, based on the stability and growth of cellular microstructures I–The basic model. Acta Mater 1997;45:4235–40. This paper presents a new analysis of grain stability and explains the stability of heavily cold-worked microstructures.
[10] Watts BM, Stowell MJ, Baikie BL, Owen DGE. Superplasticity in Al–Cu–Zr alloys – I. Materials preparation and properties. Met Sci J 1976;10:189–97. [*11] McNelley TR, NcMahon ME, Perez-Prado MT. Grain boundary evolution and continuous recrystallization of a superplastic Al–Cu– Zr alloy. Philos Trans Royal Soc Lond 1998;A357:1683–706. [*12] Ridley N, Cullen EM, Humphreys FJ. Effect of Thermomechanical processing on the Evolution of Superplastic Microstructures in Al–Cu–Zr alloys. Mater Sci Technol 2000;16:117–24. This paper demonstrates that the evolution of a fine-grained superplastic microstructure in Supral type alloys occurs by the accumulation of high angle grain boundary during progressive straining. [*13] Oscarsson A, Ekstrom HE, Hutchinson WB. Transition from discontinuous to continuous recrystallization in strip-cast aluminium alloys. Mater Sci Forum 1992;113–115:177–82. [14] Davies RK, Randle V, Marshall GJ. Continuous recrystallization – related phenomena in a commercial Al–Fe–Si alloy. Acta Mater 1998;46:6021–32. [*15] Engler O, Huh M-Y. Evolution of the cube texture in high purity aluminium capacitor foils by continuous recrystallization and subsequent grain growth. Mater Sci Eng 1999;271:371–81. Discusses the texture evolution of aluminium during continuous recrystallization after heavy rolling, and subsequent grain growth. Formation of the cube texture occurs during abnormal grain growth. [16] Oscarsson A, Hutchinson WB, Nicol B, Bate PS, Ekstrom H-E. Misorientation distributions and the transition to continuous recrystallization in a strip cast aluminium alloy. Mater Sci Forum 1994;157–162:1271–6. [17] Humphreys FJ. A unified theory of recovery, recrystallization and grain growth, based on the stability and growth of cellular microstructures II – The effect of second-phase particles. Acta Mater 1997;45:5031–9. [*18] Hasegawa H, Komura S, Utsunomiya A, Horita Z, Furukawa M, Nemoto M, Langdon TG. Thermal stability of ultra-fine grained aluminium in the presence of Mg and Zr additions. Mater Sci Eng 1999;A265:188–96. This paper discusses the effect of alloying additions on the stability of sub-micron grain structures produced by ECAE. [19] Gholinia A, Hulley SI and Prangnell PB. Development of an ultra-fine isotropic grain structure during processing of spray-formed Al–Li–X alloys. In: McNelley T (editor). Proc. Rex96, Monterey, USA, 1997:537–544. [*20] Gholinia A, Sarkar J, Withers PJ, Prangnell PB. Ultrafine grain structures formed by thermomechanical processing of spray cast Al–Li alloys. Mater Sci Technol 1999;15:605–15. This work demonstrates the effect of small second-phase particles on both the formation and the stability of sub-micron grain structures. [*21] Gholinia A, Humphreys FJ, Prangnell PB. Processing ultra-fine grain structures by conventional routes. Mater Sci Technol 2000, (in press). This paper explores and analyses the processing window for producing micron-grained aluminium alloys by warm rolling. [*22] Saito Y, Utsunomiya H, Tsuji N, Sakai T. Novel ultra-high straining process for bulk materials – development of the accumulative roll-bonding (ARB) process. Acta Mater 1999;47:579–83. The paper shows how very large strains leading to fine grain formation, may be produced by successive rolling and sandwiching sheets of aluminium alloy. [23] Tsuji N, Saito Y, Utsunomiya H, Tanigawa S. Ultra-fine grained bulk steel produced by accumulative roll-bonding (ARB) process. Scr Mater 1999;40:795–800. [24] Bridgeman PW. Studies in large plastic flow and fracture, New York: McGraw–Hill, 1952. [25] Valiev RZ, Korznikov AV, Mulyukov RR. Structure and properties of metals with submicrocrystalline structures. The Physics of Metals and Metallography 1992;4:70–86. [26] Horita Z, Smith DJ, Furukawa M, Nemoto M, Valiev RZ, Langdon TG. In: Chandra T, Sakai T (editors). Characterisation of ultra-fine
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[27] [*28]
[29] [**30]
[**31]
[32]
[33]
[**34]
[35]
grained materials produced by torsion straining. Proc. Thermec’97, TMS, 1997:1937–1943. Segal VM. Materials processing by simple shear. Mater Sci Eng 1995;A197:157–64. Nakashima K, Horita Z, Nemoto M, Langdon T. Development of a multi-pass facility for equal-channel angular pressing to high total strains. Mater Sci Eng 2000;A281:82–7. The paper describes the development and operation of a multistage press which can, in a single pass, give a total strain of |5 to a sample. Prangnell PB, Harris C, Roberts SM. Finite element modelling of equal channel angular extrusion. Scr Mater 1997;37:983–8. Bowen JR, Gholinia A, Roberts SM, Prangnell PB. Analysis of the billet deformation behaviour in equal channel angular extrusion. Mater Sci Eng 2000 (in press). A finite element analysis of the deformation of a billet during ECAE is compared with experiments on gridded split samples. Nakashima K, Horita Z, Nemoto M, Langdon TG. Influence of channel die angle on development of ultrafine grains in equal channel angular pressing. Acta Mater 1998;46:1589–99. The paper demonstrates the effect of ECAE die angle and strain path on the development of fine grained microstructures in aluminium alloy.The paper demonstrates the effect of ECAE die angle and strain path on the development of fine grained microstructures in aluminium alloy. Lwahashi Y, Horita Z, Nemoto M, Langdon TG. Equal channel angular pressing for the processing of superplastic materials. Acta Mater 1998;46:3317–25. Ferrasse S, Segal VM, Hartwig KT, Goforth RE. Microstructure and properties of copper and aluminum alloy 3003 heavily worked by equal channel angular extrusion. Metall Mater Trans 1997;29A:1047–57. Gholinia A, Prangnell PB, Markuchev MV. The effect of strain path on the development of deformation structures in severely deformed aluminium alloys processed by ECAE. Acta Mater 2000;48:1115– 30. An experimental investigation and analysis of the effect of sample rotation between passes in an ECAE rig using a 1208 die. It is shown that certain combinations of interpass rotation result in almost complete restoration of the original microstructure. Bowen JR, Prangnell PB, Humphreys FJ. In: Bilde-Sorensen et al. (editors). The effect of deformation temperature on the formation of severely strained microstructures during ECAE processing. Proc. 20th Int Riso Symp. Denmark, 1999:269–276.
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[36] Ibraheem AK, Priestner R, Bowen JR, Prangnell PB, Humphreys FJ. Novel processing routes to ultra-fine-grained steel. In: Proc. thermomechanical processing of steel, London: IOM Communications, 2000, pp. 446–55. [*37] Hodgson PD, Hickson MR, Gibbs RK. Ultrafine ferrite in low carbon steel. Scr Mater 1999;40:1179–87. The paper demonstrates how large surface strains induced by roll friction during hot rolling may lead to the formation of a layer of submicron grains at the surface of steel sheet. [38] Hurley PJ, Kelly GL, Hodgson PD. Ultrafine ferrite formation during hot strip rolling. Mater Sci Technol 2000 (in press). [39] Najafi-Zadeh A, Jonas JJ, Yue S. Effect of dynamic recrystallization on grain refinement of IF steels. Mater Sci Forum 1993;113– 115:441–6. [40] Mabuchi H, Hasegawa T, Ishikawa T. Metallurgical features of steel plates with ultra fine grains in surface layers and their formation mechanism. ISIJ Int 1999;39:477–85. [41] Hayashi T. Creation of equiaxed fine ferrite grain structures by warm deformation of martensite. CAMP-ISIJ 1998;11:1031–4. [42] Morris DG. Mechanical behaviour of nanostructured materials. Mater Sci Foundations 1998;2:1–86. [*43] Horita Z, Fujinami T, Nemoto M, Langdon TG. Equal-channel angular pressing of commercial aluminium alloys: grain refinement, thermal stability and tensile properties. Metall Mater Trans 2000;31A:691–701. The paper deals with several important aspects of fine grains produced by large-strain deformation. A number of aluminium alloys are investigated and the microstructures, annealing behaviour and mechanical properties have been investigated. [*44] Hayes JS, Keyte R and Prangnell PB. Effect of grain size on the tensile behaviour of a sub-micron grained Al–3%Mg alloy produced by severe deformation. Mater Sci Technol 2000. (in press). A detailed analysis of the mechanical properties of an aluminium alloy processed to an ultra-fine grain size by ECAE. The effects of increasing the grain size by subsequent grain growth on the strength and ductility are clearly demonstrated. [45] Mukai T, Kawazoe M, Higashi K. Dynamic mechanical properties of a near-nano aluminium alloy processed by equal-channel-angularextrusion. Nanostructured Mater 1998;10:755–65. [46] Lloyd DJ, Morris LR. Luders band formation in a fine-grained aluminium alloy. Acta Metall Mater 1997;25:857–61.
Fine-grained alloys by thermomechanical processing F. John Humphreys*, Philip B. Prangnell, Ronald Priestner Manchester Materials Science Centre, UMIST & University of Manchester, Grosvenor Street, Manchester, M1 7 HS, UK
Abstract Grain refinement during thermomechanical processing is conventionally achieved by discontinuous recrystallization. However, techniques involving severe deformation which increase the grain boundary area and can result in a micron-scale grain size by a process of continuous recrystallization have been developed. Various methods which are based on control of the deformation conditions and cooling rate during hot rolling of steels have utilised the a –g phase transformation to produce micron-grained ferrite. Ultra-fine-grain structures exhibit interesting mechanical properties, and in certain cases a large amount of strengthening may be achieved with little or no reduction in ductility. 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction The properties of a metal are strongly influenced by the grain size and the well-known Hall–Petch equation which predicts an increase in yield strength (sy ) with a decrease in grain size (d) has been shown to be applicable to a wide variety of metals.
sy 5 s0 1 kd 21 / 2
(1)
Other examples of the benefits of grain refinement are improvements in the fracture resistance of steels and other alloys, and the phenomenon of superplasticity which can be exhibited in metals with grain sizes less than |10 mm. One of the reasons for the current interest in very fine grained metallic materials is the concept that control of the mechanical properties by processing may be more desirable than the current practice of controlling properties by alloying. This would result in the use of fewer and simpler industrial alloys and would lead to economic benefits as well as improved recyclability. Very small grain sizes may be produced directly from the melt or from the vapour. The grain size decreases as the solidification rate increases and very small grains or even amorphous metals may be produced under extreme conditions. However, because the size of the product is small, i.e. powder, wire or ribbon, further consolidation and processing is required to produce a bulk material. There is currently very great interest in the field of metallic *Corresponding author. Tel.: 144-0161-200-3554; fax: 144-0161200-8877. E-mail address: [email protected] (F. . Humphreys).
and non-metallic nanophase materials which may comprise grains which are only a few atoms in diameter [1]. Such materials often have impressive physical and mechanical properties but because they can generally only be produced in very small quantities, these materials will not be considered in detail in this review.
2. The limits of conventional thermomechanical processing routes There is usually a conflict between the grain size which can be produced in a material, the amount or the dimensions of the material which can be so processed, and the cost of processing. For structural applications, which generally require material of reasonable dimensions in substantial quantities and at low cost, thermomechanical processing, involving the deformation and annealing of bulk alloys is usually considered to be the optimum method for producing fine-grained alloys. Conventional thermomechanical processing of alloys generally results in grain sizes of the order of 30–250 mm [2], and the production of finer grain structures requires careful control of the material and the processing parameters [*3,4] as is discussed in this paper.
2.1. Discontinuous recrystallization Fine-grain microstructures can be produced by the discontinuous recrystallization of a cold worked metal. A small grain size is promoted by a large stored energy resulting from deformation and a large density of sites for nucleating recrystallization [2]. The smallest grain sizes in
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aluminium are typically achieved by deformation to large strains of alloys containing second-phase particles larger than |1 mm, and subsequent annealing to stimulate recrystallization [2]. The commercial Rockwell process which produces |10 mm grains in AA7xxx alloys for superplastic applications is based on this principle [5], and similar grain sizes may be achieved by large strain deformation and annealing of commercial Al–Mg–Fe (AA5xxx) alloys (e.g. Ref. [2]). Ferrite has a high stacking fault energy, so that recovery processes precede and accompany recrystallization of cold rolled ferrite [6] and consequently, it is difficult to obtain grain sizes less than about 10 mm in ferritic steels by discontinuous recrystallization. However, it has recently been shown [**7] that cold rolling and annealing of steel with a pre-refined grain size of |3 mm resulted in a recrystallized grain size of less than 1 mm. This is thought to be attributable to the large number of recrystallization nucleation sites available at pre-existing grain boundaries.
2.2. Continuous recrystallization If a metal is given a large strain at intermediate or high temperatures, a microstructure containing predominantly high angle grain boundaries may evolve with little or no further annealing. Such a microstructure is virtually indistinguishable from one which has been conventionally recrystallized, but because it has evolved gradually and uniformly throughout the material, this phenomenon is generally known as continuous recrystallization. An example of a continuous recrystallization process in a compressed metal is shown schematically in Fig. 1. During hot deformation of a polycrystalline metal, subgrains develop within the grains and the high angle boundaries become corrugated (Fig. 1a). At larger strains (Fig. 1b) the high angle boundaries are pushed together and, because the subgrain size remains approximately constant during deformation, they are separated by fewer subgrains. Eventually, the separation of the high angle boundaries is equal to the subgrain size, the high angle boundaries impinge and a microstructure of small and almost equiaxed grains is formed (Fig. 1c). This process, known as geometric dynamic recrystallization [8] has been extensively studied in aluminium alloys [2]. The conditions under which geometric dynamic recrystallization can occur are a function of the initial grain size and the deformation strain, temperature and strain rate and are shown in Fig. 2 in which the deformation temperature (T ) and strain rate ( ´~ ) have been combined into a single term, the Zener–Hollomon parameter (Z)
S D
Q Z 5 ´~ ? exp ] RT
Fig. 1. Schematic diagram of geometric dynamic recrystallization.
with a large initial grain size (D1 ) is deformed under conditions of low Z (subgrain size d 1 ), then the grain boundary spacing decreases with strain until impingement occurs at A. The size of the grains so formed (D*) is approximately equal to the subgrain size and the strain required is ´A . If the initial grain size were smaller – D2 ,
(2)
The subgrain size is inversely related to Z, being small at low deformation temperatures and high strain rates (large Z) and vice versa. It is seen from Fig. 2 that if an alloy
Fig. 2. Processing conditions for the formation of a stable grain structure during hot deformation.
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then geometric recrystallization would occur at a lower strain (´B ) but would result in the same grain size. A smaller final grain size (D**) can be achieved by deformation at lower temperatures and strain rates (larger Z), but this will require a larger strain (´C ). Therefore, in order to achieve a small grain size at reasonable strains, a smaller initial grain size (D2 ) is required, together with a larger Z so that the grain boundary spacing and subgrain size intersect at ´D in Fig. 2. A more formal analysis of the conditions under which fine grain structures are expected to be formed has been given by Humphreys [*9]. It has long being known that the development of finegrain superplastic microstructures in Al–Cu–Zr (Supral) alloys occurred by continuous recrystallization [10]. Earlier work suggested that this process involved progressive rotation of subgrains during deformation (see Ref. [2]), but recent research [*11,*12] in which the grain boundaries have been characterised by electron backscatter diffraction (EBSD), has shown that the mechanism is essentially one of high angle boundary accumulation of the type shown in Fig. 1. Very large strain deformation of Al–Fe–Si and similar alloys at room temperature has been shown to result in sub-micron grain structures [*3,*13–*15]. On low temperature annealing, a sub-micron grain structure evolves by a continuous process and the structures are resistant to discontinuous recrystallization. It is thought that the stability of these structures is attributable to the large fraction of high angle grain boundaries in the deformed microstructure [*9,16], and their formation is therefore very similar to the process of geometric dynamic recrystallization discussed above. An important difference between deformation at low and elevated temperatures is that at lower temperatures, grains, particularly large ones, will subdivide or fragment by inhomogeneous deformation [2,*3], thus producing a larger amount of high angle grain boundary than the simple geometric increase discussed above. Additionally, during low temperature deformation, the large secondphase particles present in most commercial aluminium alloys will lead to inhomogeneous local deformation [*3] which will assist the break-up of the grains. Fine-grain microstructures have a large area of grain boundary and therefore a large stored energy, and are intrinsically unstable with respect to grain growth during high temperature annealing or even during the high temperature deformation operation during which they are formed. A dispersion of second-phase particles is generally required to prevent grain growth [1–4,15,17,18], although abnormal or discontinuous grain growth may occur [*15], and the effectiveness of second-phase particles (defined in terms of the volume fraction FV divided by the particle diameter x) in preventing either normal or abnormal grain growth is shown in Fig. 3. However, it has been found that a dispersion of very small particles may, at low deformation temperatures also prevent geometric dynamic re-
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Fig. 3. The predicted regimes of no growth, normal and abnormal growth for fine-grained alloys containing a volume fraction FV of particles of diameter x.
crystallization by impeding the migration of the boundaries which is required for operation of the mechanism [19,*20]. Because of the various constraints on the formation of fine-grain structures by continuous recrystallization and on their stability, there is a restricted process window for their formation as shown schematically in Fig. 4, which is based on an experimental investigation of an Al–2%Mg–Cr alloy [*21] deformed to a von Mises strain of 3 in plane strain. The smallest grains will be formed at low temperatures and large strain rates, but there is a limit (A–B–C) imposed by the condition for geometric dynamic recrystallization discussed above, and at higher temperatures, there are insufficient second-phase particles to prevent grain growth during the processing (C–D). The small process window
Fig. 4. The process window for producing an ultra-fine grain structure in Al–2%Mg by plane strain deformation to a strain of 3. 2000, with permission from IoM Communications Ltd [21].
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means that it would be difficult to process such a material to a 1 mm grain size in a commercial rolling mill.
3. Severe deformation processing The production of micron-grained alloys by conventional thermomechanical processing technology is limited by the achievable strains, which are typically |3–4 if the resultant material is to have a minimum thickness of |1 mm. Several processing routes which allow larger strains have been developed and are discussed below. The mechanisms by which the original grain structure is broken up are thought to be similar to those discussed in the previous section although the more complicated deformation geometries have a significant effect. A technique which increases the available strain is accumulative roll bonding(ARB) in which the material is rolled, stacked and re-rolled, thus maintaining the sample thickness, and this technique has been applied to aluminium alloys [*22] and steels [23] with total strains of up to 8, resulting in sub-micron grain sizes. Critical factors in successful accumulated roll bonding are surface preparation and cleaning, the deformation temperature, and the amount of strain. There has been a substantial recent research effort in developing laboratory scale methods which, by imposing complex and redundant strains, can achieve very large total strains whilst retaining a specimen of reasonable dimensions. One method which has been used with a wide range of materials is that of torsion under hydrostatic pressure, adapted from the Bridgeman anvil [24–26]. In this technique (Fig. 5a), a thin disc is deformed in torsion using the friction provided by the application of a large hydrostatic pressure. The equivalent strains that have been induced with this method are typically of the order of seven, and grain sizes as fine as 0.2 mm have been produced by deformation at room temperature. An alternative method is that of Equal Channel Angular Extrusion (ECAE) developed in the former Soviet Union by
Segal and colleagues [27]. During ECAE the sample is extruded in a closed die which has two intersecting channels of equal size (Fig. 5b) offset at an angle 2f. Assuming ideal conditions and a sharp die corner, the sample will be subjected to a homogenous shear of 2 cotf, and the process may be repeated, thus giving a large cumulative strain to the sample. Alternatively a die which gives several ECAE deformations in a single pass may be used [*28]. There are a number of factors including friction and die shape which make ECAE deformation more complicated than indicated above, and the deformation behaviour in the die has been numerically modelled [29,**30], and the effects of friction and back pressure on the homogeneity of shear have been investigated [**30]. Total strains of |10 are readily achievable and because the sample is constrained, the process can even be used for less ductile materials. The die angle is an important factor in ECAE processing as not only does it determine the strain per pass, but it also affects the geometry of deformation [**31]. In ECAE it is not necessary to keep the work piece orientation the same for each repeated pass, and several permutations are possible, the most common being either no sample rotation or a rotation of 908 between passes. It has been claimed that a 908 rotation between passes is most effective in breaking up the microstructure and producing an equiaxed sub-micron grain structure [32,33] when a 908 die is used. However, it has been found that for a 1208 die, the grain structure is more effectively broken up and a higher fraction of high angle grain boundaries is formed if the sample orientation is maintained during successive passes [**34]. An understanding of the formation of sub-micron grain structures during processing requires detailed quantitative microstructural characterisation. In many investigations (e.g. Refs. [31–33]) the arcing of diffraction patterns in the TEM has been used to obtain a qualitative assessment of the grain structure. However, more detailed characterisation such as the determination of grain boundary character and the percentage of high angle boundaries etc., is only
Fig. 5. Methods of redundant strain processing. (a) Torsion under hydrostatic pressure. (b) Equal channel angular extrusion.
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obtainable with high resolution Electron Backscatter Diffraction (EBSD) using a Field Emission SEM [2–4,34,35].
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ferrite in a duplex a –g microstructure appears to be an important factor.
5. Mechanical properties of fine-grained alloys 4. Grain refinement of steels Sub-micron grain structures have been produced in steels by the severe deformation processes of ECAE [36] and ARB [23] discussed above, and also by the discontinuous recrystallization of pre-refined ferrite (Section 2.1). However, control of the austenite to ferrite transformation can also be used to refine the ferrite grain size during hot rolling, and grain sizes of |5 mm are produced commercially using this method. Deformation during the a –g transformation will affect the phase transformation and may also result in dynamic or static recrystallization of the austenite and ferrite. A summary of recent research which has concentrated on controlling the phase transformation and recrystallization processes so as to produce ultra-fine grained ferrite is given by Priestner and Ibraheem [**7]. Hodgson and colleagues [*37,38] have produced 1 mm ferrite in the surface layers of steel sheet by utilising the large surface strains resulting from roll friction, together with roll chilling, to intergranularly nucleate a high density of ferrite grains during hot rolling of austenite. Priestner and Ibraheem have shown that a refined starting austenite grain size, combined with accelerated cooling, can also produce a 300 mm layer of grains less than 1 mm in a Nb–microalloyed steel [**7].. An alternative approach has been to refine the ferrite grain size by various routes involving warm rolling at |6508C [**7,39–41]. The mechanisms involved in producing micron-grained ferrite by this method are not completely understood, but dynamic recrystallization of the
Although the Hall–Petch relationship of Eq. 1 has been found to apply to most conventional alloys with grain sizes larger than |5 mm, it has been reported that there can be a significant deviation from the d 21 / 2 dependence, leading to lower than predicted yield stresses at submicron grain sizes [42]. However, more recent investigations of aluminium alloys deformed by ECAE have confirmed the validity of the Hall–Petch relationship for grain sizes as small as |0.2 mm [*43,*44] as shown in Fig. 6. The 10-fold increase in yield strength in this alloy, and comparable increases found for AA1100 and AA3004 alloys [*43] due to the grain size reduction, show that such processing can confer on alloys of intrinsic low or moderate strength, properties typical of high strength age-hardened aluminium alloys. Similarly large increases have been found for the hardness [**7] and yield strength [23] of ultra-fine grained steels and strengths of up to |900 MPa have been reported. Hardness results [**7] suggest that the Hall– Petch relationship remains valid for Nb steels of grain sizes down to |0.5 mm, albeit with a lower slope, k in Eq. (1), than at larger grain sizes. It is found that other mechanical properties are also markedly affected by a very small grain size. For example, the work hardening rate is generally lower, leading to a reduced uniform elongation in both aluminium alloys [45] and steels [23]. In aluminium alloys the Luders strain may be more pronounced in very fine-grained material [46] and for the samples of Fig. 6, the materials with the smallest grain size exhibited a Luders strain of |7% [*44]. Although the ductility of the materials with the smallest
Fig. 6. Hall–Petch plot for Al–3%Mg. Copyright 2000, with permission of IoM Communications Ltd [44].
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grain sizes may be only a few percent, low temperature annealing in the range 100–2008C has been shown to restore the ductility whilst still retaining much of the strengthening [*43,*44].
6. Summary Recent research has shown that novel thermomechanical processing routes involving large strain deformation may be used to produce micron or sub-micron grain sizes in aluminium alloys and steels, and in some cases a good combination of mechanical properties may be obtained. If such processes can be developed to operate cost-effectively at a large scale, then the economic and environmental benefits of controlling the properties of commercial alloys by processing rather than alloying can be realised.
References Papers of particular interest, published within the annual period of review, have been highlighted as: * of special interest; ** of outstanding interest. [1] Gleiter H. Nanostructured materials: basic concepts and microstructure. Acta Mater 2000;48:1–30. [2] Humphreys FJ, Hatherly M. Recrystallization and related annealing phenomena, Oxford: Pergamon Press, 1995. [*3] Humphreys FJ, Prangnell PB, Bowen JR, Gholinia A, Harris C. Developing stable fine-grain microstructures by large strain deformation. Philos Trans Royal Soc 1999;A357:1663–80. This paper reviews the methods of achieving fine grain structures by thermomechanical processing and discusses the underlying physical mechanisms. [4] Humphreys FJ, Prangnell PB, Priestner R. In: Sakai T, Suzuki H (editors). Fine-grained alloys by thermomechanical processing. Proc. Rex99, Tsukuba, 1999:69–78. [5] Wert JA, Paton NE, Hamilton CH, Mahoney MW. Grain refinement in 7075 aluminium by thermomechanical processing. Metall Trans 1981;12A:1267–76. [6] Leslie WC, Michalak JT, Aul FW. The annealing of cold-worked iron. In: Spencer CW, Werner FE, editors, Iron and its dilute solid solutions, New York: Interscience, 1963, pp. 119–216. [**7] Priestner R, Ibraheem AK. Processing of steel for ultra-fine ferrite grain structures. Mater Sci Technol 2000. (in press). This work demonstrates three methods of producing sub-micron grained ferrite in a low-carbon, microalloyed steel by rolling: by transformation from hot-worked austenite, employing accelerated cooling; by dynamic recrystallization of ferrite during warm working; and by cold rolling and recrystallization of ferrite pre-processed to an intermediate grain size of 3 mm. [8] McQueen HJ, Knustad O, Ryum N, Solberg JK. Microstructural evolution in Al deformed to strains of 60 at 4008C. Scr Met 1985;19:73–8. [*9] Humphreys FJ. A unified theory of recovery, recrystallization and grain growth, based on the stability and growth of cellular microstructures I–The basic model. Acta Mater 1997;45:4235–40. This paper presents a new analysis of grain stability and explains the stability of heavily cold-worked microstructures.
[10] Watts BM, Stowell MJ, Baikie BL, Owen DGE. Superplasticity in Al–Cu–Zr alloys – I. Materials preparation and properties. Met Sci J 1976;10:189–97. [*11] McNelley TR, NcMahon ME, Perez-Prado MT. Grain boundary evolution and continuous recrystallization of a superplastic Al–Cu– Zr alloy. Philos Trans Royal Soc Lond 1998;A357:1683–706. [*12] Ridley N, Cullen EM, Humphreys FJ. Effect of Thermomechanical processing on the Evolution of Superplastic Microstructures in Al–Cu–Zr alloys. Mater Sci Technol 2000;16:117–24. This paper demonstrates that the evolution of a fine-grained superplastic microstructure in Supral type alloys occurs by the accumulation of high angle grain boundary during progressive straining. [*13] Oscarsson A, Ekstrom HE, Hutchinson WB. Transition from discontinuous to continuous recrystallization in strip-cast aluminium alloys. Mater Sci Forum 1992;113–115:177–82. [14] Davies RK, Randle V, Marshall GJ. Continuous recrystallization – related phenomena in a commercial Al–Fe–Si alloy. Acta Mater 1998;46:6021–32. [*15] Engler O, Huh M-Y. Evolution of the cube texture in high purity aluminium capacitor foils by continuous recrystallization and subsequent grain growth. Mater Sci Eng 1999;271:371–81. Discusses the texture evolution of aluminium during continuous recrystallization after heavy rolling, and subsequent grain growth. Formation of the cube texture occurs during abnormal grain growth. [16] Oscarsson A, Hutchinson WB, Nicol B, Bate PS, Ekstrom H-E. Misorientation distributions and the transition to continuous recrystallization in a strip cast aluminium alloy. Mater Sci Forum 1994;157–162:1271–6. [17] Humphreys FJ. A unified theory of recovery, recrystallization and grain growth, based on the stability and growth of cellular microstructures II – The effect of second-phase particles. Acta Mater 1997;45:5031–9. [*18] Hasegawa H, Komura S, Utsunomiya A, Horita Z, Furukawa M, Nemoto M, Langdon TG. Thermal stability of ultra-fine grained aluminium in the presence of Mg and Zr additions. Mater Sci Eng 1999;A265:188–96. This paper discusses the effect of alloying additions on the stability of sub-micron grain structures produced by ECAE. [19] Gholinia A, Hulley SI and Prangnell PB. Development of an ultra-fine isotropic grain structure during processing of spray-formed Al–Li–X alloys. In: McNelley T (editor). Proc. Rex96, Monterey, USA, 1997:537–544. [*20] Gholinia A, Sarkar J, Withers PJ, Prangnell PB. Ultrafine grain structures formed by thermomechanical processing of spray cast Al–Li alloys. Mater Sci Technol 1999;15:605–15. This work demonstrates the effect of small second-phase particles on both the formation and the stability of sub-micron grain structures. [*21] Gholinia A, Humphreys FJ, Prangnell PB. Processing ultra-fine grain structures by conventional routes. Mater Sci Technol 2000, (in press). This paper explores and analyses the processing window for producing micron-grained aluminium alloys by warm rolling. [*22] Saito Y, Utsunomiya H, Tsuji N, Sakai T. Novel ultra-high straining process for bulk materials – development of the accumulative roll-bonding (ARB) process. Acta Mater 1999;47:579–83. The paper shows how very large strains leading to fine grain formation, may be produced by successive rolling and sandwiching sheets of aluminium alloy. [23] Tsuji N, Saito Y, Utsunomiya H, Tanigawa S. Ultra-fine grained bulk steel produced by accumulative roll-bonding (ARB) process. Scr Mater 1999;40:795–800. [24] Bridgeman PW. Studies in large plastic flow and fracture, New York: McGraw–Hill, 1952. [25] Valiev RZ, Korznikov AV, Mulyukov RR. Structure and properties of metals with submicrocrystalline structures. The Physics of Metals and Metallography 1992;4:70–86. [26] Horita Z, Smith DJ, Furukawa M, Nemoto M, Valiev RZ, Langdon TG. In: Chandra T, Sakai T (editors). Characterisation of ultra-fine
F. J. Humphreys et al. / Current Opinion in Solid State and Materials Science 5 (2001) 15 – 21
[27] [*28]
[29] [**30]
[**31]
[32]
[33]
[**34]
[35]
grained materials produced by torsion straining. Proc. Thermec’97, TMS, 1997:1937–1943. Segal VM. Materials processing by simple shear. Mater Sci Eng 1995;A197:157–64. Nakashima K, Horita Z, Nemoto M, Langdon T. Development of a multi-pass facility for equal-channel angular pressing to high total strains. Mater Sci Eng 2000;A281:82–7. The paper describes the development and operation of a multistage press which can, in a single pass, give a total strain of |5 to a sample. Prangnell PB, Harris C, Roberts SM. Finite element modelling of equal channel angular extrusion. Scr Mater 1997;37:983–8. Bowen JR, Gholinia A, Roberts SM, Prangnell PB. Analysis of the billet deformation behaviour in equal channel angular extrusion. Mater Sci Eng 2000 (in press). A finite element analysis of the deformation of a billet during ECAE is compared with experiments on gridded split samples. Nakashima K, Horita Z, Nemoto M, Langdon TG. Influence of channel die angle on development of ultrafine grains in equal channel angular pressing. Acta Mater 1998;46:1589–99. The paper demonstrates the effect of ECAE die angle and strain path on the development of fine grained microstructures in aluminium alloy.The paper demonstrates the effect of ECAE die angle and strain path on the development of fine grained microstructures in aluminium alloy. Lwahashi Y, Horita Z, Nemoto M, Langdon TG. Equal channel angular pressing for the processing of superplastic materials. Acta Mater 1998;46:3317–25. Ferrasse S, Segal VM, Hartwig KT, Goforth RE. Microstructure and properties of copper and aluminum alloy 3003 heavily worked by equal channel angular extrusion. Metall Mater Trans 1997;29A:1047–57. Gholinia A, Prangnell PB, Markuchev MV. The effect of strain path on the development of deformation structures in severely deformed aluminium alloys processed by ECAE. Acta Mater 2000;48:1115– 30. An experimental investigation and analysis of the effect of sample rotation between passes in an ECAE rig using a 1208 die. It is shown that certain combinations of interpass rotation result in almost complete restoration of the original microstructure. Bowen JR, Prangnell PB, Humphreys FJ. In: Bilde-Sorensen et al. (editors). The effect of deformation temperature on the formation of severely strained microstructures during ECAE processing. Proc. 20th Int Riso Symp. Denmark, 1999:269–276.
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[36] Ibraheem AK, Priestner R, Bowen JR, Prangnell PB, Humphreys FJ. Novel processing routes to ultra-fine-grained steel. In: Proc. thermomechanical processing of steel, London: IOM Communications, 2000, pp. 446–55. [*37] Hodgson PD, Hickson MR, Gibbs RK. Ultrafine ferrite in low carbon steel. Scr Mater 1999;40:1179–87. The paper demonstrates how large surface strains induced by roll friction during hot rolling may lead to the formation of a layer of submicron grains at the surface of steel sheet. [38] Hurley PJ, Kelly GL, Hodgson PD. Ultrafine ferrite formation during hot strip rolling. Mater Sci Technol 2000 (in press). [39] Najafi-Zadeh A, Jonas JJ, Yue S. Effect of dynamic recrystallization on grain refinement of IF steels. Mater Sci Forum 1993;113– 115:441–6. [40] Mabuchi H, Hasegawa T, Ishikawa T. Metallurgical features of steel plates with ultra fine grains in surface layers and their formation mechanism. ISIJ Int 1999;39:477–85. [41] Hayashi T. Creation of equiaxed fine ferrite grain structures by warm deformation of martensite. CAMP-ISIJ 1998;11:1031–4. [42] Morris DG. Mechanical behaviour of nanostructured materials. Mater Sci Foundations 1998;2:1–86. [*43] Horita Z, Fujinami T, Nemoto M, Langdon TG. Equal-channel angular pressing of commercial aluminium alloys: grain refinement, thermal stability and tensile properties. Metall Mater Trans 2000;31A:691–701. The paper deals with several important aspects of fine grains produced by large-strain deformation. A number of aluminium alloys are investigated and the microstructures, annealing behaviour and mechanical properties have been investigated. [*44] Hayes JS, Keyte R and Prangnell PB. Effect of grain size on the tensile behaviour of a sub-micron grained Al–3%Mg alloy produced by severe deformation. Mater Sci Technol 2000. (in press). A detailed analysis of the mechanical properties of an aluminium alloy processed to an ultra-fine grain size by ECAE. The effects of increasing the grain size by subsequent grain growth on the strength and ductility are clearly demonstrated. [45] Mukai T, Kawazoe M, Higashi K. Dynamic mechanical properties of a near-nano aluminium alloy processed by equal-channel-angularextrusion. Nanostructured Mater 1998;10:755–65. [46] Lloyd DJ, Morris LR. Luders band formation in a fine-grained aluminium alloy. Acta Metall Mater 1997;25:857–61.