ABSTRACT
E-mail: langdon@usc.eduDespite rosy prospects, the use of nanostructured metals and alloys as advanced structural materials has remained controversial until recently. Only in recent years has a breakthrough been outlined in this area, associated both with development of new routes for the fabrication of bulk nanostructured materials and with investigation of the fundamental mechanisms that lead to the new properties of these materials.This chapter discusses new concepts and principles of using severe plastic deformation (SPD) to fabricate bulk nanostructured metals with advanced properties. Special emphasis is laid on the relationship between microstructural features and properties, as well as the first applications of SPD-produced nanomaterials. 2.1 INTRODUCTION
It is now over 20 years since Herbert Gleiter presented the first concepts for developing nanocrystalline materials (i.e., ultrafine-grained [UFG] materials
with a grain size under 100 nm) with the potential for producing special properties [1]. Since that time, the field of nanostructured materials has developed rapidly, owing to the considerable interest in this topic and the scientific and technological importance.Gleiter’s original idea was that, owing to the very small grain size, nanocrystalline materials will contain an extremely large fraction of grain boundaries having a special atomic structure. As a result, nanomaterials should have unusual properties [2]. As regards the mechanical properties, one may expect very high strength, toughness, fatigue life, and wear resistance. Nanostructuring seemed likely to lead to a revolutionary use of nanomaterials for many functional and structural applications. But in practice these interesting prospects were put in jeopardy. It was shown in numerous investigations [3-5]that, although nanocrystalline materials demonstrated very high strength or hardness, they were of very low ductility or even brittle, thereby producing insuperable problems for use in advanced structural applications. When addressing the reasons for the low ductility, many researchers point out drawbacks in their synthesis on the basis of the compaction of nanopowders obtained using various methods [4, 5]. Nanomaterials produced by compacting usually have residual porosity, contaminations, and, as a rule, small geometric dimensions. All this may lead to a decline in their ductility. Another possible reason is of a fundamental nature: the plastic deformation mechanism associated with the generation and movement of dislocations may not be effective in ultrafine grains (as described later). In this connection, recent findings of extraordinarily high strength and ductility in several bulk UFG metals are especially interesting [6-9]. Various nanomaterials possess microstructural features which are closely linked to the processing methods and regimes. Therefore, it is necessary to initially address the fundamental principles involved in the processing techniques that are currently used to produce these materials and the structural features that are inherent features of these processed materials. 2.2 THE PRINCIPLES OF GRAIN REFINEMENT BY SPD
PROCESSINGIn recent years there has been a growing interest in a new approach to the fabrication of bulk nanostructured metals and alloys, where this new approach forms an alternative to the conventional compaction of nanopowders. This new approach is based on microstructure refinement in bulk billets through the application of SPD; that is, heavy straining under high imposed pressure
[10]. SPD-produced nanomaterials are fully dense, and their large geometric dimensions make it possible to attain excellent properties thorough mechanical testing. The fabrication of bulk nanostructured materials by SPD is now becoming one of the most actively developing areas in the field of nanomaterials [11, 12].
