Formation of bulk nanocrystalline hybrid metals under high pressure
Processing of metals through the application of severe plastic deformation (SPD) provides an opportunity for achieving ultrafine grains in the submicrometer or even the nanometer range. Provided these grains in the bulk metals usually show superior mechanical and physical properties. In practice, the strength at ambient temperatures is dictated through the Hall-Petch relationship in which the yield stress varies inversely with the grain size raised to a power of one-half, so that strength increases as the grain size is reduced. At high temperatures, in the regime where diffusion becomes important, there is a potential for achieving superplastic flow and a superplastic forming capability in materials having very small grain sizes. Several SPD methods are now available but the most promising appear to be Equal-Channel Angular Pressing (ECAP) and High-Pressure Torsion (HPT). This presentation examines these procedures with special emphasis on evolution of microstructure and the mechanical properties of the ultrafine-grained (UFG) materials and bulk nanostructured materials (BNM) processed by these SPD techniques.