Review on modified and novel techniques of severe plastic deformation

This review highlights very recent achievements and new developments of severe plastic deformation (SPD) technology for producing bulk ultrafine-grain (UFG) and even nanocrystalline (nc) materials. These numerous modified and novel SPD methods include cyclic forward-backward extrusion, axi-symmetric forward spiral extrusion, vortex extrusion, simple shear extrusion, planar twist extrusion, tubular channel angular pressing, cone-cone method, high-pressure tube twisting, tube channel pressing and elliptical cross-section spiral equal-channel extrusion. According to classification, these new methods are categorized into the extension of equal-channel angle pressing (ECAP), high-pressure torsion (HPT), twist extrusion (TE) and constrained groove pressing (CGP), respectively. The principles of various new SPD technologies are described in detail. In addition, the microstructure revolution characteristics and mechanical properties of materials produced by SPD process, as well as the applications of SPD techniques to UFG materials, are also reported. Furthermore, this article reviews recent progresses in determining the refinement and/or deformation mechanisms, e.g. dislocation deformation mechanism, twin deformation mechanism and grain boundary sliding and torsional deformation mechanism, and further orientation of SPD technology.