Recent Advances in the Processing and Properties of Ultrafine-Grained Metals Prepared Using Severe Plastic Deformation

Abstract

Conventional thermo-mechanical processing provides the potential for producing materials with very small grain sizes, typically of the order of $\sim$5–10 $\upmu$m. However, recent experiments have demonstrated that much smaller grain sizes may be attained, within the submicrometer or nanometer range, through the application to the material of severe plastic deformation (SPD). In processing by SPD, the material is deformed to a high strain in the presence of a hydrostatic pressure and this introduces a high density of dislocations which re-arrange into a low energy configuration and thereby produce arrays of grain boundaries. Typical SPD processes include equal-channel angular pressing (ECAP) and high-pressure torsion (HPT). Materials processed using SPD have numerous advantages over coarse-grained materials including exceptionally high strength and a potential for use in rapid superplastic forming operations at elevated temperatures. Accordingly, processing by SPD has become a major research area within the field of Materials Science over the last decade. This paper examines some of these more recent developments with an emphasis on processing using HPT.