Bulk nanostructured materials from severe plastic deformation
Introduction
In recent years, bulk nanostructured materials (NSM) processed by methods of severe plastic deformation (SPD) have attracted the growing interest of specialists in materials science [1]. This interest is conditioned not only by unique physical and mechanical properties inherent to various nanostructured materials, e.g. processed by gas condensation [2], [3] or ball milling with subsequent consolidation [4], [5], but also by several advantages of SPD materials as compared to other NSM. In particular, SPD methods resulted in overcoming of a number of difficulties connected with residual porosity in compacted samples, impurities from ball milling, processing of large scale billets and practical application of the given materials. The principle of processing of bulk nanostructured materials using SPD methods is an alternative to the existing methods of nanopowder compacting.
It is well known that heavy deformations, for example, by cold rolling or drawing, can result in significant refinement of microstructure at low temperatures [6], [7], [8], [9]. However, the structures formed are usually substructures of a cellular type having boundaries with low angle misorientations. At the same time, the nanostructures formed from SPD are ultra fine-grained structures of a granular type containing mainly high angle grain boundaries.1 Formation of such nanostructures could be realized by SPD methods providing very large deformations at relatively low temperatures under the high pressures imposed [1], [10], [11]. Special methods of mechanical deformation were developed and used for realization of this principle. These methods are as follows: severe torsion straining under high pressure, equal channel angular pressing and others. It was shown that using SPD methods one can fabricate bulk nanostructured samples and billets out of different metals and alloys including a number of commercial alloys and intermetallics.
The first developments and investigations of nanostructured materials processed using SPD methods were fulfilled by Valiev and his co-workers more than 10 years ago [12], [13]. Recent years are characterized by a sharp increase of different publications on this subject. In spite of this, the authors believe that the most interesting and important discoveries are still waiting for us and this scientific direction will find its further more active development due to unusual properties of the materials processed. Many of these properties are unique and rather interesting for applied and fundamental investigations. The present review deals with achievements and difficulties relating to development and investigations of nanostructured materials processed by severe plastic deformation. The review is based essentially on the results obtained by the authors themselves or their colleagues in the process of joint investigations with many Russian and foreign scientists. References on these works are given below. The authors thank all our friends and colleagues for nice cooperation, discussions and useful remarks. We express special gratefulness also to our colleagues N.F. Yunusova and N.A. Enikeev taking part in preparation of the present manuscript.
Section snippets
Methods of severe plastic deformation and formation of nanostructures
Methods of severe plastic deformation should meet a number of requirements which are to be taken into account while developing them for formation of nanostructures in bulk samples and billets. These requirements are as follows. Firstly, it is important to obtain ultra fine-grained structures with prevailing high-angle grain boundaries since only in this case can a qualitative change in properties of materials occur (Section 4). Secondly, the formation of nanostructures uniform within the whole
Structural characterization and modeling of SPD materials
Nanostructured materials processed by SPD methods are characterized by a very high density of grain boundaries and are interface-controlled materials. Thus, it is quite natural that their grain boundaries are in the center of all structural investigations. As shown below, evidence for a non-equilibrium structure of grain boundaries in SPD materials is of special importance. The notions on non-equilibrium grain boundaries were first introduced in the scientific literature somewhere in the 70–80s
Properties of nanostructured SPD materials
As already mentioned, the increased interest in nanostructured materials is associated with foreseeing new advanced physical and mechanical properties attributed to their unusual structure. At present it has already been established that many fundamental properties undergo significant changes both in nanostructured materials processed by SPD methods and in nanocrystals processed by other methods, for example by gas condensation [2]. Among these properties the most interesting are changes in
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