Ultrafine-Grained Materials

Ultrafine-grained (UFG) materials are a new class of metals and alloys characterized by a microstructure with grain sizes less than 1 μm and nanostructural features that provide advanced multifunctional properties. This chapter considers the underlying principles for the formation of UFG materials using severe plastic deformation (SPD) and outlines the conditions for achieving the UFG structures with predominantly high angle grain boundaries, including low temperatures, high degrees of straining, high applied pressures, turbulent, non-monotonic nature of the material flow, achieving higher dislocation density and smaller grain size by using alloys with an ordered structure and materials with low stacking fault energy. The most popular techniques of SPD processing are analyzed, such as high-pressure torsion (HPT) and equal-channel angular pressing (ECAP). The schemes and mechanisms of grain refinement and formation of UFG structures are considered. At the same time, special attention is paid to the analysis of the evolution of microstructures and phase transformation during SPD processing following the results of computer simulation and numerous modern experimental methods of research. Grain boundaries, nanotwins, nanoscale particles, and segregations of alloying elements in nanostructured materials obtained by SPD techniques are described and used for developing a concept of nanostructural design for increasing material properties.