Y.Estrin,A.Vinogradov/Acta Materialia 61 (2013)782-817 783 similar effect is well known in fatigue where the grain size the reviews [21,22]and special issues of Advanced Engineer- of wavy-slip materials has no bearing on the fatigue limit. ing Materials [23],Materials Science and Engineering A [24] These observations can be associated with the vital role and Materials Transactions [25,26]. of the dislocation substructure,which forms during defor- What makes SPD processing techniques so popular is mation(be it monotonic or cyclic),and it is the size of the the possibility of using them to enhance the strength char- substructure which determines the strength characteristics acteristics of conventional metallic materials in a quite of metallic materials.Gow and Cahn [9]emphasized the spectacular way:by a factor of up to eight for pure metals significance of crystallographic texture for the deformation such as copper and by some 30-50%for alloys [7,27]. and recrystallization behaviour of metals and the effect of Despite the impressive property improvement achievable evolving texture on the resultant properties.Bell and Cahn with SPD techniques,their uptake by industry has been [10]outlined many fine features of mechanical twinning, rather sluggish.However,things are now starting to which play an important part in plastic deformation when change,and there is a common feeling in the nanoSPD accommodation by dislocation slip is hindered.Beck [11] community that major breakthroughs in terms of indus- highlighted the possibility of relieving the work-hardening try-scale applications of SPD-based technologies are immi- effects by post-processing recovery.As will be seen in the nent.We have been working in this area for more than a following sections,these ideas have had a great impact decade and have followed its developments closely.In this on the development of the SPD processing and are pivotal article we present our views on what has been achieved, to the modern concepts underlying these techniques.Now- what is possibly achievable,and what future trends are to adays,the subject of SPD processing is represented very be expected from SPD processing technologies.This article prominently on the pages of Acta,as illustrated by the does not represent a full review of the SPD area (one could analysis in Ref.[4].Its revival is due to the work of Segal almost say the discipline of SPD,considering the firm place et al.[12]in the Soviet Union in the mid-1970s.These this group of material processing techniques has taken in authors developed the method of equal-channel angular literature).Rather,it is our personal take on the SPD area pressing (ECAP),which later evolved into what is now and an attempt to foretell its future development.Empha- the most popular SPD technique.It should be mentioned, sis is placed on the scientifically challenging aspects of however,that in the time between the publication of Bridg- SPD,and not so much on technological issues,although man's studies and the reintroduction of this subject in the some insights into the promises and limitations of SPD metal science literature,exploration of the possibilities of technologies will also be given. changing the properties of materials through combined high pressure and shear deformation went on both in the 2.SPD methods Soviet Union and in the West.This less known work has been reviewed in Ref.[13].In particular,credit should be Among the procedures devised for grain refinement. given to the work by the group of N.S.Enikolopian con- SPD techniques are of particular interest and are the ducted mainly on polymers. focus of the present review.These techniques enjoy great A real appreciation for the new possibilities for improv- popularity owing to their ability to produce considerable ing the properties of metallic materials provided by SPD grain refinement in fully dense.bulk-scale work-pieces. techniques came with the work of the group of Valiev thus giving promise for structural applications.The [14,15],which demonstrated the relation between the achievable grain sizes lie within the submicrometer(100- enhanced strength and the extreme grain refinement 1000 nm)and nanometer (<100 nm)ranges.SPD-pro- imparted by SPD processing to a range of metals and cessed materials with such grain sizes are generally alloys.The seminal work of this group,emphasizing the referred to as nanoSPD materials [7],although only the great potential of SPD processing with regard to property latter ones can be regarded as being nanostructured improvement through grain structure modification,has according to the conventional definition.Several compre- heralded what has been described as the "microstructural hensive reviews have focused on various nanoSPD pro- age"of SPD research [4].Over the last decade,the nano- cessing techniques [22,28-33].We refer the reader to the SPD community (www.nanospd.org)has grown to an original works for specific details and only briefly outline impressive group of researchers,and thousands of publica- the general SPD methodology underlining the common tions on ultrafine-grained (UFG)and nanostructured features and the most important differences between the materials produced by SPD have been published.It is nanoSPD processes.By no means do we claim that our probably not surprising that in the year of the Diamond list of currently available manufacturing schemes is Jubilee of this journal,the Acta Materialia Gold Medal exhaustive. goes to Professor Terry Langdon-one of the world leaders After the landmark work by Bridgman mentioned above in the area of nanoSPD materials.A representative collec- [6.341.Langford and Cohen [35]and Rack and Cohen [36] tion of the relevant articles on the subject can be found in demonstrated in the 1960s that the microstructure of Fe- the proceedings of symposia on UFG materials [16,18]and 0.003%C subjected to high strains by wire drawing was nanoSPD conferences [19,20]the most recent ones in a refined to subgrain sizes in the 200-500 nm range.These series of five such forums.Further useful sources include microstructures could not be regarded as UFG proper insimilar effect is well known in fatigue where the grain size of wavy-slip materials has no bearing on the fatigue limit. These observations can be associated with the vital role of the dislocation substructure, which forms during defor￾mation (be it monotonic or cyclic), and it is the size of the substructure which determines the strength characteristics of metallic materials. Gow and Cahn [9] emphasized the significance of crystallographic texture for the deformation and recrystallization behaviour of metals and the effect of evolving texture on the resultant properties. Bell and Cahn [10] outlined many fine features of mechanical twinning, which play an important part in plastic deformation when accommodation by dislocation slip is hindered. Beck [11] highlighted the possibility of relieving the work-hardening effects by post-processing recovery. As will be seen in the following sections, these ideas have had a great impact on the development of the SPD processing and are pivotal to the modern concepts underlying these techniques. Now￾adays, the subject of SPD processing is represented very prominently on the pages of Acta, as illustrated by the analysis in Ref. [4]. Its revival is due to the work of Segal et al. [12] in the Soviet Union in the mid-1970s. These authors developed the method of equal-channel angular pressing (ECAP), which later evolved into what is now the most popular SPD technique. It should be mentioned, however, that in the time between the publication of Bridg￾man’s studies and the reintroduction of this subject in the metal science literature, exploration of the possibilities of changing the properties of materials through combined high pressure and shear deformation went on both in the Soviet Union and in the West. This less known work has been reviewed in Ref. [13]. In particular, credit should be given to the work by the group of N.S. Enikolopian con￾ducted mainly on polymers. A real appreciation for the new possibilities for improv￾ing the properties of metallic materials provided by SPD techniques came with the work of the group of Valiev [14,15], which demonstrated the relation between the enhanced strength and the extreme grain refinement imparted by SPD processing to a range of metals and alloys. The seminal work of this group, emphasizing the great potential of SPD processing with regard to property improvement through grain structure modification, has heralded what has been described as the “microstructural age” of SPD research [4]. Over the last decade, the nano￾SPD community (www.nanospd.org) has grown to an impressive group of researchers, and thousands of publica￾tions on ultrafine-grained (UFG) and nanostructured materials produced by SPD have been published. It is probably not surprising that in the year of the Diamond Jubilee of this journal, the Acta Materialia Gold Medal goes to Professor Terry Langdon—one of the world leaders in the area of nanoSPD materials. A representative collec￾tion of the relevant articles on the subject can be found in the proceedings of symposia on UFG materials [16,18] and nanoSPD conferences [19,20]—the most recent ones in a series of five such forums. Further useful sources include the reviews [21,22] and special issues of Advanced Engineer￾ing Materials [23], Materials Science and Engineering A [24] and Materials Transactions [25,26]. What makes SPD processing techniques so popular is the possibility of using them to enhance the strength char￾acteristics of conventional metallic materials in a quite spectacular way: by a factor of up to eight for pure metals such as copper and by some 30–50% for alloys [7,27]. Despite the impressive property improvement achievable with SPD techniques, their uptake by industry has been rather sluggish. However, things are now starting to change, and there is a common feeling in the nanoSPD community that major breakthroughs in terms of indus￾try-scale applications of SPD-based technologies are immi￾nent. We have been working in this area for more than a decade and have followed its developments closely. In this article we present our views on what has been achieved, what is possibly achievable, and what future trends are to be expected from SPD processing technologies. This article does not represent a full review of the SPD area (one could almost say the discipline of SPD, considering the firm place this group of material processing techniques has taken in literature). Rather, it is our personal take on the SPD area and an attempt to foretell its future development. Empha￾sis is placed on the scientifically challenging aspects of SPD, and not so much on technological issues, although some insights into the promises and limitations of SPD technologies will also be given. 2. SPD methods Among the procedures devised for grain refinement, SPD techniques are of particular interest and are the focus of the present review. These techniques enjoy great popularity owing to their ability to produce considerable grain refinement in fully dense, bulk-scale work-pieces, thus giving promise for structural applications. The achievable grain sizes lie within the submicrometer (100– 1000 nm) and nanometer (<100 nm) ranges. SPD-pro￾cessed materials with such grain sizes are generally referred to as nanoSPD materials [7], although only the latter ones can be regarded as being nanostructured according to the conventional definition. Several compre￾hensive reviews have focused on various nanoSPD pro￾cessing techniques [22,28–33]. We refer the reader to the original works for specific details and only briefly outline the general SPD methodology underlining the common features and the most important differences between the nanoSPD processes. By no means do we claim that our list of currently available manufacturing schemes is exhaustive. After the landmark work by Bridgman mentioned above [6,34], Langford and Cohen [35] and Rack and Cohen [36] demonstrated in the 1960s that the microstructure of Fe– 0.003% C subjected to high strains by wire drawing was refined to subgrain sizes in the 200–500 nm range. These microstructures could not be regarded as UFG proper in Y. Estrin, A. Vinogradov / Acta Materialia 61 (2013) 782–817 783