Editorial

Abstract

In recent years, the development of bulk nanostructured materials (BNM) has become one of the most topical directions in modern materials science. Nanostructuring of various materials paves the way to obtaining unusual properties that are very attractive for different structural and functional applications. In this research topic, the use of both “bottom-up” and “top-down” approaches for BNM processing/synthesis routes has received considerable attention. In particular, grain refinement by severe plastic deformation (SPD) techniques has attracted special interest since it offers new opportunities for developing different technologies for the fabrication of commercial nanostructured metals and alloys for various specific applications. Very significant progress was made in this area in recent years. The generation of new and unusual properties has been demonstrated for a wide range of different metals and alloys: examples include very high strength and ductility, record-breaking fatigue endurance and the development of superplastic forming capabilities. The innovation potential of this research area is outstanding and now it requires further development for the use of SPD-processed materials in specific applications. In pursuance of this approach, it was proposed to publish a special issue of Advanced Engineering Materials with the general title “Bulk Nanostructured Metals: New Ideas for Innovation.” The contents of this special issue are now complete and the issue consists of two review and seventeen communication papers outlining and formulating the key scientific problems and the most promising approaches towards applications of bulk nanomaterials. At the same time, it is now apparent that research on BNM has developed so rapidly in recent years that the terminology needs some clarification. Three terms widely used within this field are ultrafine-grained (UFG), nanocrystalline (NC) and nanostructured materials and it is necessary to provide a clearer definition of these three terms. With reference to the characteristics of polycrystalline materials, UFG materials are defined as polycrystalline materials having very small grains with average grain sizes less than ∼ 1 µm. Thus, the grain sizes of UFG materials lie both within the sub-micrometer (100–1000 nm) and the nanometer (less than 100 nm) ranges. For grain sizes below 100 nm, the latter are termed “nanocrystalline materials” or “nanocrystals.” In practice, these materials exhibit other structural elements having sizes of less than 100 nm including second phase particles or precipitates, dislocation substructures, pores, etc. These nanometer-sized features also have a considerable influence on the properties of the materials. For example, in SPD processing, nanostructural elements such as nanotwins, grain boundary precipitates and dislocation substructures may form within the ultrafine grains of 100–300 nm in size and this formation will have a significant effect on their mechanical and functional properties. Materials containing these nanostructural elements are designated “nanostructured materials.” In order to qualify as bulk nanostructured materials, the only additional requirements are that there exists a homogeneous distribution of nanostructural elements in the entire sample and the samples typically have 1,000 or more grains/nanostructural elements in at least one direction. These meanings of the aforementioned terms are used in this special issue. In practice, however, the subject of BNM is now entering the textbooks on materials science and related subject areas and therefore it is very important to follow the standard terminology. In our capacity as guest editors of this special issue, we would highly appreciate receiving any comments on the contents of these various papers. Where appropriate, we will gladly continue further discussions on the progress and future directions of this important field.