Nanostructured metallic materials from severe plastic deformation (SPD): physical nature of superstrength and strengthening mechanisms

For the last two decades, processing of ultrafine-grained materials with the grain sizes in the submicron and/or nanoscale range by severe plastic deformation (SPD) techniques has become a popular focus area in modern materials science and engineering. Not only various metallic materials, but also polymers and even some ceramics become the object of research. In particular, the deformation processing of materials using SPD has been a key step that promoted a comprehensive study of the mechanical properties of bulk nanomaterials and is now the basis for numerous studies of various nanostructured metallic materials (see recent reviews and books on this topic). The conducted works are focused on the studies of hardening mechanisms, ductility and the ways of its enhancement, fatigue, impact toughness, and other service properties. In addition, it has been shown in several experimental studies that ultra-fine grained (UFG) metallic materials processed by SPD techniques often exhibit very high-strength properties with values significantly higher than that calculated by the known Hall-Petch equation. The reason for this superstrength may be related to the influence of various nanostructural features observed in SPDed metals and alloys and located both in the grain volume (dislocation substructures, nano-sized second-phase precipitations, nanotwins) and at their boundaries (nanoclusters and segregations). In this connection, the present report analyzes the nature of the superstrength of UFG materials with due consideration of various strengthening mechanisms such as the known mechanisms that are based on the presence of nanoparticles and other above-mentioned nanostructural features, and the novel mechanisms that are related to the influence of grain boundary structures in UFG materials and have been the subject of recent studies performed by the author and his colleagues. Particular emphasis is laid on the use of nanostructured biomaterials for their innovative applications in medicine to produce implants with improved design and biofunctionality.